COLUMBIA LIBRARIES OFFSITE HEALTH SCIENCES STANDARD HXOOO 16241 RECAP ;><>i>IH»MrtKHKr{j;; THE LIBRARIES COLUMBIA UNIVERSITY 1 I 1 1 1 I 1 1 1 1 1 i 1 1 1 1 1 1 1 1 i Ej TLn][ruiirruT3lruT3fiiJil|riJii|iinJ^ Digitized by the Internet Archive in 2010 with funding from Open Knowledge Commons http://www.archive.org/details/textbookofanatom01cunn L_«B*=»ARV OP" DR HORATIO B WILLIAMS, NEW YORK C«TY. TEXT-BOOK OF ANATOMY EDITED BY D. J. CUNNINGHAM, F.R.S .D. (eDIN. ET duel.), D.SC, LL.D. (gLASG. ET ST. AND.), D.C.L. (oXON.) PROFESSOR OF ANATOMY, UNIVERSITY OF EDINBURGH ILLUSTRATED WITH 936 WOOD ENGRAVINGS FROM ORIGINAL DRAWINGS, MOSTLY PRINTED IN COLOURS SECOND AND THOROUGHLY REVISED EDITION NEW YORK WILLIAM WOOD AND COMPANY EDINBURGH AND LONDON: YOUNG J. PENTLAND 1905 an \^o^ EDINBUEGH : PRINTED FOR YOUNG J. PENTLAND, 11 TEVIOT PLACE, AND 38 WEST SMITHFIELD, LONDON, EC, BY R. AND R. CLARK, LIMITED. All rights reserved. TO ^tr SEilUam Euxntx, W^MM. F.R.S., M.B., LL.D., D.C.L., D.SC, IN RECOGNITION OF HIS EMINENCE AS AN ANATOMIST AND HIS INFLUENCE AS A TEACHER THIS VOLUME IS DEDICATED BY THOSE OF HIS FORMER PUPILS AND ASSISTANTS WHO HAVE CONTRIBUTED TO ITS PAGES PKEFACE TO THE SECOND EDITION. The gratifying reception given to the Text-book of Anatomy, not only in this country but also in America, has rendered it necessary to prepare a new edition. In carrying out this work the whole book has been carefully revised, and a large number of new illustrations have been added. The sections in which the chief changes and additions have been made are those upon Embryology, the Joints, the Muscles, the Brain and Spinal Cord, the Genito - Urinary Organs, the Lymphatics, and Applied Anatomy. By pruning down in various directions, it has been found possible to incorporate a considerable amount of new matter in these and other sections without materially increasing the bulk of the book. The lamented death of Professor Birmingham was felt by all the contributors to be not only a personal loss, but one which materially affected an important section of the book. Although in very bad health. Professor Birmingham was desirous to undertake his share of the work, and he had the sheets on the Digestive System in his possession for this purpose when he died. There was probably no part of the book which required less revision than his, and the Editor has taken upon himself the duty of making the alterations which seemed essential. In doing this he has been careful to avoid unnecessary changes and to preserve throughout the original character of the article. In the section upon the Muscular System, a series of illustrations has been added in which the areas of muscle attachment are delineated upon the bones. For the preparation of the specimens, and for assistance in the mapping out of these areas, the Editor is indebted to Dr. E. B. Jamieson. Those who have attempted work of this kind will appreciate the amount of labour and judgment entailed, as it is only by taking the average condition in many specimens, and by the close study of the bones selected for the delineations, that a sufficiently accurate result can Ije attained. With the exception of the figures which have been added to the sections of the book dealing with Embryology, Osteology, and the Genito-Urinary Organs, the many new illustrations whicli appear in this edition have been prepared in the Anatomical Department of the University of Edinburgh. In carrying out this work, the Editor has again had the good fortune to secure the co-operation of Mr. vii viii PEEFACE. J. T. Murray, an artist whose ability in the rendering of anatomical subjects is recognised on all hands. Wherever it was felt that colour would increase the artistic effect or the general usefulness of an illustration, it has been freely employed, both in the case of the old and of the new figures. The Editor cannot conclude this preface to the second edition of the Text-book without expressing his grateful acknowledgment of the assistance which has been so freely extended to him by his fellow-contributors at every stage of the work. He has also to thank many readers of the first edition for calling his attention to typographical errors and other imperfections which had escaped his notice in passing the sheets through the press. 18 Grosvenor Crescent, Edinburgh, August 1905. PREFACE TO THE FIRST EDITION. The form which this book has taken expresses the desire of those who have contributed the various sections to produce something which they might dedicate to their former teacher and master, Sir William Turner. With one exception, all the contributors have studied under Sir William Turner, and all but two have for longer or shorter periods acted as his Assistants. Bound together by this common tie, and animated by affection and reverence for their great master, they have sought to produce a book worthy of him whose teaching it so largely reflects, and if this object has not been attained it is not for want of will, but of power, on the part of the writers. In the preparation of a work such as this it is no easy matter to prevent over- lapping of the different articles and to keep the various sections in harmony with each other. Yet in this direction it is believed that a fair amount of success has been attained. Differences of opinion on particular points were bound to arise, but the Editor found in those concerned the greatest readiness to come to a mutual understanding, and he is deeply grateful to his colleagues for the manner in which they endeavoured to lighten his work and assist him in his task. Of course when totally different views were held by two authors on a matter which had to be dealt with in two sections, no serious attempt was made to urge these writers to qualify their statements so as to produce an apparent agreement. It was felt that if this were done the individuality of the author, whi^h forms a characteristic feature of each article as it stands, would thereby be damaged ; and further, it was believed that the same question discussed from two different points of view could not fail to be of advantage to the reader. At the same time it is right to state that the places in which a divergence of opinion appears are very few, and taking into account that nine writers have co-operated with the Editor, a remarkable degree of harmony in the treatment of the different sections has been obtained. The recent introduction of Formalin as a hardening and preserving reagent imposed an especially arduous duty upon those writers who had undertaken the chapters dealing with tlie thoracic and abdominal viscera. The possibilities for establishing a more accurate topography and of improving our conception of the forms assumed by the viscera under different conditions have by this means been greatly extended ; and in preparing the sections which treat of these organs the writers have taken full advantage of the new method. Much, therefore, which appears in this book on the topographical relations of the viscera departs con- siderably from the older and more conventional descriptions hitherto in vogue. The arrangement of the matter treated in the following pages is very much the same as that adopted in the various courses of lectures delivered in the schools from which the different sections of llic work have emanated. The first chapter is a 2 ix X PEEFACE. devoted to the general principles and elementary facts of Embryology. Then follow, in an order best suited for the student, the chapters dealing with the various systems of organs ; whilst the last seventy-five pages are used for the purpose of applying the information conveyed in the preceding part of the book to the practice of medicine and surgery. Each chapter is more or less complete in itself, altliough an effort has been made to weld them all into one consistent whole. The numerous illustrations which appear in the text are all new in tlie sense that in no case has an old drawing or an old block been used. Eurther, the vast majority of the illustrations are new in the sense that they are original. The very few that are not have been taken from monographs dealing with the subjects so illustrated, and in every case the source from which these have been obtained is acknowledged in the text. The drawings for each section were prepared under the personal supervision of its author, and, with the exception of the figures in two chapters, they are the work of Mr. J. T. Murray. This talented artist has devoted much time to the undertaking, and the reader can judge for himself the success which has attended his efforts. The Editor cannot sufficiently express his indebtedness to Mr. Murray for the great technical skill and the patience which he brought to bear upon this extremely trying and difficult work. The chapter on Osteology has been illustrated by Mr. W. C. Stevens ; that upon Embryology by the authors themselves ; whilst the microscopical drawings in the section on the Brain and Cord were executed by Mr. Wm. Cathie. It is also necessary to mention that the coloured outlines representing the attachments of the muscles on the figures of the bones were mapped in by Professor A. M. Paterson. The Editor has to thank his former Assistant, Professor A. F. Dixon of Cardiff, for much help in the correction of the proofs. Trinity College, Dublin, June 1902. LIST OF CONTRIBUTORS. AMBEOSE BIRMINGHAM, M.D., F.R.C.S.I., Formerly Professor of Anatomy, Catliolic University School of Medicine, Dublin. (The Digestive System.) D. J. CUNNINGHAM, M.D., F.R.S., Professor of Anatomy, University of Edinburgh. {The Brain and Sinnal Cord, The Resinratory System, The Ductless Glands.) A. FRANCIS DIXON, M.B., D.Sc. (Dubl), Professor of Anatomy, Trinity College, Dublin. {The Urinogenital System,.) DAVID HEPBURN, M.D., F.R.S.E., Professor of Anatomy, University College, Cardiff. {Arthrology.) ROBERT HOWDEN, M.A., M.B., Professor of Anatomy, University of Durham. {Tlie Organs of Sense and the Integument.) A. M. PATERSON, M.D., Professor of Anatomy, University College, Liverpool. {Myology, The Spinal and Cranial Nerves, The Sympathetic Nervous System.) ARTHUR ROBINSON, M.D., Professor of Anatomy, University of Birmingham. (General Embryology, The Vascular System.) HAROLD J. STILES, M.B., F.R.C.S. Ed., Surgeon to the Royal Hospital for Sick Children, Edinburgh. {Surface and Surgical Anatomy.) ARTHUR THOMSON, M.A., M.B., Profe.s.sor of Human Anatomy, University of Oxford. {Osteology.) A. H. YOUNG, M.B., F.li.C.H., Profe.s.sor of Anatomy, Tlie Owens College, Manchester. {General Embryology, The Vu.icalar System.) The Index la tlie wojk of Di-. T. W. P. Lawi{7:ncio. -xi CONTENTS. INTEODUCTION. GENERAL EMBEYOLOGY. The Animal Cell . Reproduction of Cells The Ovum Its Structure Its Maturation . The Spermatozoon . Fertilisation of the Ovum Segmentation of Ovum Formation of Blastodermic Vesicle Ectoderm and Entoderm . Embryonic Area Neural Groove and Tube . Formation of Notochord . Formation of Ccelom Mesodermic Somites . Folding off of the Embryo The Embryo .... Primitive Alimentary Canal . Pharynx and Stomatodaeum Visceral Clefts and Arches The Skeleton . Composition of Bone Structure of Bone Ossification and Growth of Bones The Vertebral Column . True or Movable Vertebrte . Cervical Vertebrte . Thoracic Vertebrae . Luml^ar Vertebrae False or Fixed Vertebrae Sacrum .... Coccyx .... Vertebral Column as a wliole Cartilaginous Vitrtebral Column OH.sification of Verteljrai Serial Honiologics of tlie Vertebra' Sternum . Ribs Costal Cartilage.4 Thorax as a wdiolt; . BoTies of tlie Skull . Frontal Bone J^arictal JioneH . Occijfilal Jionc . 9 10 10 12 14 16 17 18 19 19 21 24 24 26 26 28 32 34 35 69 70 71 72 74 76 76 80 82 83 83 86 87 90 91 94 94 97 101 101 103 103 107 109 Mouth and Nose PAGE 38 External Ear, Tympanic Cavity, and Eustachian Tube . . . . 43 Hind-gut, Anal Passage, and Post-anal Gut 45 The Limbs 46 Nutrition and Protection of Embryo during Intrauterine Existence 47 Foetal Membranes and Appendages 48 Yolk-Sac 48 Amnion ..... 48 Body-Stalk .... 50 AUantois 51 Umbilical Cord 61 Chorion 52 The Placenta 52 Primitive Vascular System and Fceta [ Circulation .... 60 External Features of Human Embryo a1 different periods . 65 LOGY. Temporal Bones 114 Sphenoid Bone . 122 Ethmoid Bone . 128 Wormian Bones 131 Bones of the Face . 131 Superior Maxillary Bones 131 Malar Bones 135 Nasal Bones 137 Lachrymal Bones 138 Inferior Turbinated Bones 139 Vomer .... 140 Palate Bones 141 Inferior Maxillary Bone . 143 Hyoid Bone 147 Skull as a whole 148 Norma Frontalis 148 Norma Lateralis 152 Norma Occipitalis . 159 Norma Verticalis 159 Norma Basal is . 160 Skull in Section .... . 167 Ui)i)er Surface; of tlie Base of thi Skull. .... \ 167 Mesial Sagittal SectifORMALITIES OP VaSCULAR SySTEM Abnormalities of Heart . Abnormalities of Arteries Branches of Aorta Arteries of Head and Neck Arteries of Upper Limb . Iliac Arteries and their Branches Arteries of Lower Limb . Abnormalities of Veins . Superior Vena Cava . Veins of Upper Extremity Inferior Vena Cava . Veins of Lower Extremity Abnormalities of Lymphatics pa';e 933 933 934 938 938 938 939 942 943 943 946 946 947 947 950 951 952 953 954 954 955 955 955 955 EESPIRATORY SYSTEM. Organs of Eespiration and Voice . Larynx or Organ of Voice Cartilages of Larynx Joints, Ligaments, and Membranes Larynx .... Interior of Larynx .... Laryngeal Muscles .... Trachea of 957 957 958 961 964 968 972 Bronchi Thoracic Cavity Pleural Membranes Mediastinal or Interpleural Space Lungs Root of Lung . Structure of Lung Development of Respiratory Apparatus 975 976 977 982 983 989 990 992 DIGESTIVE SYSTEM. Mouth 995 Palate and Isthmus Faucium . . 998 Tongue 1000 Glands 1007 Salivary Glands .... 1009 Development of Salivary Glands, Palate, and Tongue . . . 1013 Teeth 1014 Permanent Teeth .... 1016 Milk Teeth 1022 Structure of Teeth .... 1023 Development of Teeth . . . 1025 Morphology of Teeth . . . 1029 Pharynx 1029 Development of Pharynx and Tonsil 1037 (Esophagus 1038 Development of QCsophagus . 1042 Abdominal Cavity 1043 Subdivision of Abdominal Cavity . 1045 Peritoneum ...... 1048 Stomach 1050 Structure of Stomach 1058 Intestines 1060 Structure of Intestines 1061 Small Intestine .... 1064 Duodenum 1065 Jejunum and Ileum 1071 Large Intestine .... 1074 Caecum and Appendix . 1076 Colon 1082 Rectum . . . . . 1087 Anal Canal 1091 Peritoneum 1097 Development of Intestinal Canal and Peritoneum .... 1105 Liver 1108 Gail-Bladder and Bile Passages . 1118 Vessels of Liver 1120 Structure of Liver , 1121 Development of Liver 1122 Pancreas. ..... 1124 Development of Pancreas . 1129 UEINOGENITAL SYSTEM. Urinary Oroanb KidiieyH . Bladder . Urethra . 1130 1130 1144 1157 Male Reproductive ORotANS Testis _ Vas D(!f(',r(!ii.s .... Descent of Testis . 1159 1159 1162 1167 xviii CONTENTS. PAOE PAGE Spermatic Cord .... 1168 Fallopian Tubes . 1185 Scrotum 1169 Uterus ..... 1187 Penis ...... 1170 Vagina 1192 Prostate 1173 Female External Genital Organs . 1195 Cowper's Glands .... 1176 Glands of Bartholin . 1198 Male Urethra .... 1177 Development of Urinogenital Organs 1198 Female Reproductive Okuans . . 1181 Mammary Glands . 1207 Ovary 1182 Development of Manimte 1209 DUCTLESS GLANDS. Spleen . 1210 Parathyroids . 1218 Suprarenal Capsules . 1213 Thymus Gland . 1218 Thyroid Body .... . 1216 Carotid and Coccygeal Bodies . 1220 SUEEACE AND SUEGICAL ANATOMY. Head and Neck . 1222 Upper Extremity . 1291 Cranium .... . 1222 Shoulder . . . . 1291 Face . 1236 Axilla 1293 Neck . 1246 Upper Arm 1295 Thorax . 1253 Elbow 1296 Lungs .... . 1255 Forearm and Hand . 1298 Heart and Great Vessels . . 1262 Lower Extremity . 1302 Abdomen .... . 1264 Buttock 1302 Anterior Abdominal Wall . 1264 Back of the Thigh . 1303 Abdominal Cavity . . 1267 Popliteal Space . 1305 Male Perineum . 1277 Front of Thigh . 1305 Prostate . 1278 Knee .... 1307 Female Pelvis . 1282 Leg . 1308 Back ..... . 1284 Foot and Ankle 1310 INDEX 1313 LIST OF ILLUSTRATIONS. GENERAL EMBEYOLOGT. FIG. 1. 2. 3. 4. 5. 9. 10. IL 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25, 26. 27. Horizontal Section through Trunk at Level of First Lumbar Vertebra . Diagram of an Animal Cell Cell Division The Ovum and its Coverings . Maturation of the Ovum : Extrusion of the " Polar Bodies " . Diagram illustrating the Maturation of the Ovum .... Diagram illustrating the Process of Cell-Division resulting in the Formation of Spermatids which are afterwards modified into Sper- matozoa . . . . . Human Spermatozoa Structure of a Human Spermatozoon Fertilisation of the Ovum Segmentation of the Fertilised Ovum in the Eabbit .... Conversion of the Morula to the Blastula Surface View of the Blastodermic Vesicle The Upper Pole of the Blastodermic Vesicle ...... Tran sverse Section of a Ferret Embryo Transverse Section of a Ferret Embryo Surface Areas of the Blastoderm Sections showing the different Areas of the Blastodermic Vesicle Extension of Mesoderm and Forma- tion of Ccelom . . . . Surface View of an Early Embryo . Early Stages in the Folding-off of the Embryo . . . . . The Kelative Positions of the Blastodermic Layers in the Body of the Embryo when the " Fold- ing-off" is completed Transverse Section of a Ferret Ernbiyo . . . . . Further Differentiation of the Meso- derm Coronal Section of a liat Embryo . TraiiBverse Section of a Rat Eiiibryc; Diagivam of a D(;veloj)irigOvum, seen in Longitudinal Section 9 11 12 13 14 15 15 16 17 18 19 20 20 21 23 24 25 25 27 28 29 29 30 31 32 FIG. 28. 29. 30. 3L 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. Diagram representing the Condition of the Alimentary Canal in a Human Embryo about fifteen days old (modified from His) . 33 Further Development of the Aliment- ary Canal, as seen in a Human Embryo about five weeks old . 34 Stages in the Formation of the Tongue and Upper Aperture of the Larynx in the Human Embryo (after His) 36 Head of Human Embryo (four views ; two after His) .... 39 Head of Human Embryo (four views ; two after His) .... 40 Head of Human Embryo (two views ; one after His) .... 41 Vertical Section through Head of Eat Embryo .... 42 Transverse Section through the Head of a Rat Embryo ... 43 Figures, modified from His, illustrat- ing the Formation of the Pinna . 44 Diagrams showing the Separation of the Cloacal Part of the Hind-gut into Genito- urinary Tract and Rectum 46 Transverse Sections of the Uterus and Developing Ovum of a Ferret 49 Very young Human Ovum almost immediately after its entrance into the Decidua .... 53 Relation of the young Human Ovum to the Decidua .... 53 Further Stage of Develoj^ment of the Human Ovum and its Relation to the Decidual Tissues ... 54 Completion of the Decidua Capsu- laris, etc 54 Enlargement of the Blood Sinuses in the Maternal Part of the Placenta and the Closure of the Amnion . 55 Fcjotal Ectoderm surrounding the Maternal Blood Sinuses, etc. . 55 Further Growth of the Placental Sinuses and Villi, etc. . . 56 Later Stage in the Development of tlie Placenta .... 58 XIX LIST OF ILLUSTKATIONS. FIG. 47. Development of Blood-Vessels in the Vascular Area of the Rat The Primitive Blood-Vessels of tlie Eml)ryo ..... Blood - Vessels of a Mammalian Eml)ryo after the Formation of the Heart Diagram of the Foetal Circulation . 51. Human Einhryo at the end of tlie 12th, 13th, and 14th days of De- velopment (after His) . 48, 49 50. \fiE FIG. 52. 61 62 53. 54. 63 64 55. 65 56. Human Embryo at the 21st, 23rd, and 27th days of Development (after His) Human Embryo at the 29th and 32nd days of Development (after His) . Human Fa3tus at the sixth week of Development (after His) Human Fostus six and a half weeks old (after His) .... Human Foetus eight and a half weeks old (after His) . OSTEOLOGY. 57. Fifth Thoracic Vertebra . 58. Fifth Thoracic Vertebra . 59. Fourth Cervical Vertebra 60. The Atlas from Above . 61. Axis from Behind and Above . 62. Axis from the Left Side . 63. First, Ninth, Tenth, Eleventh, and Twelfth Thoracic Vertebrte from the Left Side . . . . 64. Third Lumbar Vertebra from Above and from tlie Left Side 65. The Sacrum (anterior view) 66. The Sacrum (posterior view) . 67. The Coccyx 68. Vertebral Column from the Left Side 69. Vertebral Column as seen from Behind .... 70. Ossification of Vertebrae . 71. Ossification of Vertebrae . 72. Ossification of Sacrum 73. The Sternum (anterior view) . 74. Ossification of the Sternum 75. Fifth Right Rib as seen from Below 76. Fifth Right Rib as seen from Behind .... 77. First and Second Right Ribs a seen from Above . 78. The Thorax as seen from the Front 79. The Thorax as seen from the Right Side 80. Frontal Bone (anterior view) . 81. Frontal Bone as seen from Below 82. Ossification of Frontal Bone 83. Right Parietal Bone (outer side) 84. Right Parietal Bone (inner surface) 85. Occipital Bone as seen from Below 86. Occipital Bone (inner surface) . 87. Ossification of Occipital Bone . 88. Right Temporal Bone as seen from the Outer Side 89. Right Temporal Bone (inner side) 90. Right Temporal Bone as seen from Below 91. Vertical Transverse Section through Left Temporal Bone (anterior half of section) 92. Vertical Transverse Section through Left Temporal Bone (posterior half of section) .... 93. Horizontal Section through Left Temporal Bone (lower half of section) 94. The Outer and Inner Surfaces of the Right Temporal Bone at Birth . 95. Sphenoid as seen from Behind 96. Sphenoid as seen from the Front 75 75 77 78 79 79 81 82 84 85 86 89 91 92 93 95 96 98 99 118 120 120 121 122 123 123 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 99 116. 101 117. 102 118. 104 119. 105 120. 106 107 108 121. 110 122. 111 113 123. 115 124. 116 125. 126. 127. 128. 129. 130. 131. Ossification of Sphenoid . Ethmoid as seen from Behind Ethmoid as seen from the Right Side Section sliowiug Nasal Aspect of Left Lateral Mass of Ethmoid Showing Articulation of Inferior Turbinated Bone with Ethmoid . Ethmoid as seen from Above . Right Superior Maxilla (outer view) Right Superior Maxilla (inner aspect) Ossification of Suj^erior Maxilla Right Malar Bone . Inner Surface of Malar Bone at Birth Right Nasal Bone . Right Lachrymal Bone . Right Inferior Turbinated Bone Vomer as seen from the Right Side Right Palate Bone . Right Palate Bone as seen from Behind .... The Lower Jaw as seen from th Left Side .... The Inner Side of the Right Half of the Lower Jaw Lower Jaw at Birth The Hyoid Bone as seen from the Front Norma Frontalis Norma Lateralis Coronal Section through the Sjjheno - maxillary Fossa of the Right Side Norma Basalis .... Base of the Skull as seen from Above Inner Aspect of Left Half of Skull sagittally divided Nasal Septum as seen from the Left Side Part of the Frontal, Nasal, and Superior Maxillary Bones re- moved in order to display the relation of the various cavities exposed Coronal Section passing inferiorly through interval between First and Second Molar Teeth View of the Cliondro-Cranium of a Human Foetus ... Right Clavicle as seen from Above Right Clavicle as seen from Below Ossification of the Clavicle The Right Scapula as seen from Behind .... LIST OF ILLUSTEATIONS. FIG. PAGE FIO. 132. The Right Scapula as seen from the 168. Front 188 133. Ossification of the Scapula . . 190 169. 134. Anterior View of the Right 170. Humerus ..... 191 135. Posterior View of the Right 171. Humerus 192 172. 136. The Head of the Right Humerus as seen from Above .... 193 173. 137. The Lower Extremity of the Right 174. Humerus as seen from Below . 193 175. 138. The Lower End of the Right Humerus as seen from the Outer 176. Side .193 139. Ossification of the Humerus . . 195 177. 140. The Right Ulna as viewed from the 178. Outer Side 196 141. The Radius and Ulna as seen from 179. the Front 198 142. The Ossification of the Ulna . . 199 180. 143. The Radius and Ulna as seen from 181. Behind 200 144. The Ossification of the Radius . 202 182. 145. The Bones of the Right Wrist and Hand as seen from the Front . 203 183. 146. The Bones of the Right Wrist and 184. Hand as seen from Behind . . 204 185. 147. The Right Scaphoid Bone . . 205 148. The Right Semilunar Bone . . 205 186. 149. The Right Cuneiform Bone . . 206 150. The Right Pisiform Bone . . 206 187. 151. The Right Trapezium . . .206 188. 152. The Right Trapezoid . . .207 189. 153. The Right Os Magnum . . .207 154. The Right Unciform Bone . . 208 190. 155. Radiograph of the Hand at Birth . 209 191. 156. First Right Metacarpal Bone . . 210 192. 157. Second Metacarpal Bone . . 210 193. 158. Third Metacarpal Bone . . . 211 194. 159. Fourth Metacarpal Bone . .211 195. 160. Fifth Metacarpal Bone . . .211 196. 161. The Phalanges of the Fingers . 213 197. 162. Radiographs of Foetal Hands . . 213 198. 163. The Right Innominate Bone as seen from the Outer Side . . . 215 199. 164. The Right Innominate Bone . . 216 165. Ossification of the Innominate Bone 220 166. Male Pelvis as seen from the Front 221 200. 167. Female Pelvis as seen from the 201. Front 221 202. Right Femur as seen from the Front 224 Right Femur as seen from Behind . 225 Back View of Upper End of Right Femur 226 Lower End of Right Femur . . 228 Lower End of Right Femur as seen from below 228 Ossification of Femur . . . 230 Right Patella 230 Upper Surface of Superior Ex- tremity of Right Tibia . .231 Right Tibia and Fibula as seen from the Front .... 232 Ossification of the Tibia . . 235 Right Tibia and Fibula as seen from Behind .... 236 Right Fibula as seen from the Inner Side 237 Ossification of Fibula . . . 239 Bones of the Right Foot as seen from Above 240 Bones of the Right Foot as seen from Below 241 The Right Astragalus . . .242 The Right Astragalus . . . 242 The Right Os Calcis as seen from Above 243 The Right Os Calcis as seen from Below 243 The Right Os Calcis . . .244 Right Navicular Bone . . . 245 Anterior View of the three Cunei- form Bones of the Right Foot . 246 Right Internal Cuneiform . . 246 Right Internal Cuneiform . . 246 Right Middle Cuneiform . . 246 Right Middle Cuneiform . . 246 Right External Cuneiform . . 247 Right External Cuneiform . . 247 Right Cuboid Bone . . .248 Radiographs of the Foetal Foot . 249 The First Metatarsal Bone of the Right Foot 250 View of the Bases and Shafts of the Second, Third, and Fourth Metatarsal Bones of the Right Foot 250 Fifth Right Metatarsal Bone . . 251 The Phalanges of the Toes . . 252 Radiographs of the Foetal Foot . 252 THE AETICULATIONS OK JOINTS. 203. Vertical Section through a Suture . 204. Section through the Occipito- sphenoid Synchondrosis 205. Sutura Serrata .... 206. Diagram of a Diarthrodial Joint . 207. Diagram of a Diarthrodial Joint . 208. Mesial Section through a portion of the Lumbar part of the Spine 209. Anterior Common Ligainent of the Vertebral Column, and the Costo- vertebral Jointsasseen from Front 210. Posterior Common Ligament of the Verteljral Column 211. Ligamenta Subflava 212. Mesial Section through the Occi- pito-atloid and Atlo-axoid Joints 213. Dissection from Behind of the 255 256 256 257 258 262 262 263 264 265 214. 215. 216. 217. 218. 219. 220. Ligaments connecting the Occipi- tal Bone, the Atlas, and the Axis with each other .... 266 Temporo-mandibular Joint . . 267 Section through Temporo-mandi- bular Joint 268 Internal Lateral Ligament of the Temporo-maxillary Joint . . 268 Sterno-clavicular and Costo-sternal Joints 272 Capsule of the Shoulder-joint and Coraco-acromial Ligament . . 276 Capsular Ligament of Shoulder- joint cut across and Humerus removed 277 Vertical SectioTi through the Shoulder-joint .... 278 LIST OF ILLUSTKATIONS. FIO. 221. Anterior View of Elbow-joint 222. Elbow-joint (inner aspect) 223. Vertical Section tlirougli the Trochlear part of the Elbow -joint Orbicular Ligament of the Radius . Carpal Articular Surface of the Radius, and Triangular Fibro- Cartilage of the Wrist Ligaments on Anterior Aspect of Radio-carpal, Carpal, and Carpo- metacarpal Joints Coronal Section through the Radio- carpal, Carpal, Carpo-metacarpal, and Intermetacarpal Joints to show Joint Cavities and Inter- osseous Ligaments (diagram - matic) 228. Metacarpo-phalangeal and Inter- phalangeal Joints Coronal Section of Pelvis Posterior View of the Pelvic Liga- ments and of the Hip-joint . Dissection of the Hip-joint . 224. 225. 226. 227. 229. 230. 23L PAGE 279 280 281 282 282 284 286 288 290 291 294 FIG. 232, Dissection of the Hip-joint from the Front 296 Dissection of the Knee-joint from the Front 298 234. The Knee-joint (2:>osterior view) . 299 235. The Knee-joint opened from behind by the removal of the Posterior Ligament 301 Upper End of Tibia . . .302 Ankle-joint dissected from Behind 305 Articular Surfaces of Tibia and Fibula which are opposed to the Astragalus 306 Ankle and Tarsal-joiuts from the Tibial Aspect .... 307 Ligaments on the Outer Aspect of the Ankle-joint and on the Dorsum of the Tarsus . . . 308 241. The Composite Articular Socket for the Head of the Astragalus . . 309 242. Plantar Aspect of Tarsal and Tarso- metatarsal Joints .... 310 233. 236. 237. 238. 239. 240. THE MUSGULAE SYSTEM. 243. Muscle-Attachments to the Clavicle 244. Superficial Muscles of the Back 245. Muscle-Attachments to the Scapula (posterior surface) 246. Muscle -Attachments to the Front of the Sternum 247. Anterior Muscle? of the Trunk 248. Muscle-Attachments to the Clavicle (under surface) .... 249. The Serratus Magnus Muscle . 250. Muscle-Attachments to the Scapula (anterior aspect) .... 251. Deltoid Region and Back of the Arm ...... 252. Posterior Wall of the Axilla and the Front of the Arm . 253. Muscle-Attachments to the Front of the Humerus .... 254. Muscle-Attachments to the Scapula (posterior surface) 255. Superficial Muscles on the front of the Arm and Forearm . 256. The Muscles on the Back of the Arm, Forearm, and Hand . 257. Muscle-Attachments to the Front of the Humerus .... 258. Muscle-Attachments to the Back of the Humerus .... 259. The Palm of the Hand . 260. The Muscles and Tendons in the Palm of the Hand 261. Section Across the Forearm in the Middle Third . . . . 262. The Muscles and Nerves on the Front of the Forearm and Hand . 263. The Tendons attached to the Index Finger 264. Muscle- Attachments to the Radius and Ulna (Anterior Aspects) 265. Deep Muscles on the Front of the Forearm and Hand 266. Short Muscles of the Hand . 267. Muscle-Attachments to the Palmar Aspect of the Carpus and Meta- carpus ...... 319 268. 320 269. 321 270. 322 271. 323 272. 324 325 273. 274. 325 275. 276. 327 277. 328 329 278. 330 279. 333 280. 334 281. 335 335 282. 337 283. 338 284. 340 285. 341 286. 342 287. 343 288. 344 289. 345 290. 346 The Palmar Interosseous Muscles . 347 Muscle-Attachments to the Dorsal Aspect of the Metacarpus . . 348 Dorsal Interosseous Muscles of the Hand 348 Muscle-Attachments to the Radius and Ulna 351 The Muscles of the Back of the Forearm 352 The Groin . . . . . 356 The Groin 357 Superficial Muscles of the Back . 358 The Muscles on the Front of the Thigh 360 Muscle - Attachments to the An- terior Surface of the Upper Part of the Femur . . . . 361 Transverse Section of the Thigh (Hunter's Canal) .... 362 Muscle-Attachments to the inner side of the Upper Part of the Tibia .363 Muscles and Nerves of the Lumbo- sacral Plexus .... 364 Muscle- Attachments to the Posterior Aspect of the Upper Part of the Femur 365 Muscle-Attachments to the Outer Surface of the Pubis and Ischium 366 Scheme of the Course and Distri- bution of the Obturator Nerve . 367 Posterior Surface of the Thigh . 368 Muscle-Attachments to the Dorsum Ilii and Tuber Ischii . . .369 Muscle-Attachments to the Posterior Aspect of the Upper Part of the Femur 370 The Gluteus Maximus Muscle . 370 The Muscles and Nerves of the Buttock 371 Muscle-Attachments to the Upper Asjject of the Great Trochanter of the Femur . . . .371 Muscle-Attachments to the Dorsum Ilii and Tuber Ischii . . .373 LIST OE ILLUSTEATIONS. XXI 11 FIG. 291. The Muscles on the Back of the Thigh 292. Muscle-Attachments to the Inner Side of the Upper Part of the Tibia 293. Coronal Section through the Left Ankle Joint (Astragalus and Cal- caneum) ..... 294. The Plantar Fascia and Plantar Cutaneous Nerves 295. Muscle -Attachments to Tarsus and Metatarsus 296. Muscles of the Front of the Right Leg and Dorsum of the Foot 297. The Insertions of the Peroneus Longus and Tibialis Posticus Muscles in the Sole of the Right Foot 298. The Soleus Muscle .... 299. Muscle- Attachments to the Posterior Surface of the Tibia . 300. The Deep Muscles on the Back of the Left Leg .... 301. The Muscles of the Right Foot 302. Muscle-Attachments to Tarsus and Metatarsus 303. The Muscles of the Right Foot 304. The Muscles of the Right Foot 305. Interosseous Muscles of the Foot . 306. Transverse Section through the Abdomen, opposite the Second Lumbar Vertebra 307. Schematic Representation of the parts of the Erector Spinae Muscle 308. Scheme of Muscular -Attachments to the Transverse Processes of the Cervical Vertebrae 309. Deeper Muscles of Back 310. The Suboccipital Triangle . 311. Muscle-Attachments to the Sacrum 312. Muscle -Attachments to Occipital Bone 313. Transverse Section in the Cervical Region 314. The Muscles of the Face and Scalp 315. Transverse Vertical Section through the Orbit behind the Eyeball to show the arrangement of Muscles 316. Muscles of the Orbit 317. Muscles of the Orbit 318. Schematic Representation of the Nerves which traverse the Cavity of the Orbit 319. Muscle-Attachments to the Outer Aspect of the Lower Jaw 320. Muscles of Mastication . 321. Muscle-Attachments on the inner side of the Lower Jaw 322. Muscles of Mastication, deeper view ...... PAGE 1 FIG. 323. 374 324. 325. 374 377 326 327 378 328 379 329. 380 330. 381 331. 382 332 383 333. 383 384 334. 335. 386 336. 387 387 388 337. 338. 391 339. 392 340. 341. 342. 393 343. 394 396 344. 396 397 345. 400 402 346. 347. 405 405 406 348. 349. 406 350. 351. 407 408 352. 408 409 PAGE Pterygoid Region .... 409 Muscle -Attachments to Occipital Bone 411 Muscles of the Hyoid Bone and Styloid Process, and the Extrinsic Muscles of the Tongue, with their Nerves 412 Triangles of the Neck . . . 413 Muscle -Attachments on the inner side of the Lower Jaw . . 414 Posterior view of the Pharynx and Constrictor Muscles . . .417 Lateral View of the Wall of the Pharynx 418 Muscle-Attachments to the Upper Surface of the First Rib and the Outer Surface of the Second Rib 420 The Prsevertebral Muscles of the Neck 420 Scheme of Muscular-Attachments to Cervical Vertebrae . . .421 Muscle -Attachments to Occipital Bone 421 Muscles of the Thoracic Wall . 423 The Diaphragm (from below) . 424 View of the Posterior Abdominal Wall, to show the Muscles and the Nerves of the Lumbo -sacral Plexus 425 The Groin 427 Transverse Section through the Abdomen 428 The Groin 428 Anterior Muscles of the Trunk . 429 The Groin 430 The Groin 431 Sheath of the Rectus Abdominis Muscle 432 View of the Posterior Abdominal Wall to show the Muscles and the Nerves of the Lumbo -Sacral Plexus 433 Muscles and Nerves of the Male Perineum 435 Muscles of the Female Perineum (after Peter Thompson) . 436 Triangular Ligament of the Peri- neum, and the Termination of the Pudic Nerve .... 437 Dissection of the Pelvic Fascia from above 438 Oblique Section across the Pelvis . 439 Outer Wall of the Pelvis . . 439 Fascial and Muscular Wall of the Pelvis after removal of part of the Left Innominate Bone . . 440 Scheme to illustrate the disposition of the Myotomes in the Embryo in relation to the Head, Trunk, and Limbs 442 353. Nerve-fibre from v. Kolliker) . 354. Three Nerve-Ceils from the An- terior Horn of Gray Matter of tlio Human Spinal Cord . 355. Two Multipolar Nerve-Cells . THE NERVOUS SYSTEM a Frog (after 356. 444 445 446 Nerve-Cell from Cerebellum (Pro- f(issor Symington) 357. Transverse Section through t])e early Neural Tube (Alfred H. Young) 358. Developmental Stages exhibited by 446 447 LIST OF ILLUSTEATIONS. a Pyramidal Cell of the Brain (after Ram6n y Cajal) . 359. Diagram of the Connexion estab- lished by a Ganglionic and a Motor Neuron (Eam6n y Cajal) . 360. Three Stages in the development of a Cell from a Spinal Ganglion 361. Nerve-Cells as depicted by Bethe . 362. Section through the Central Canal of the Spinal Cord of a Human Embryo (after v. Lenhossek) 363. Human Foetus in the third month of Development, Avith the Brain and Spinal Cord exposed from behind 364. The Conus MeduUaris and tlie Filum Terminale exposed within the Spinal Canal .... 365. The Roots of Origin of the Seventh Dorsal Nerve .... 366. Section through the Conus Medul- laris and the Cauda Equina as they lie in the Spinal Canal 367. Diagram of the Spinal Cord as seen from behind .... 368. Transverse. Section through the Upper Part of the Cervical Region of the Cord of an Orang 369. Section through each of the Four Regions of the Cord 370. Section through the Fifth Cervical Segment of the Cord . 371. Section through the eighth Dorsal Segment of the Spinal Cord 372. Section through the Third Lumbar Segment of the Spinal Cord to show the grouping of the Motor Cells ..... 373. Section through the first Sacral Segment of the Spinal Cord 374. Transverse Section through the White Matter of the Cord . 375. Diagram to show the iG-rangement of the Fibres of the Posterior Nerve -Roots in the Posterior Columns of the Cord . 376. Diagram to show the manner in which the Fibres of the Posterior Nerve-Roots enter and ascend in the Posterior Column of the Cord (from Edinger) .... 377. Diagrammatic Representation of a Transverse Section through the Spinal Cord .... 378. Schema of a Transverse Section through the Early Neural Tube (Young) ..... 379. Three Stages in the Development of the Spinal Cord (His) 380. The Base of the Brain with Cranial Nerves attached .... 381. Schema showing the Connexions of the several parts of the Brain 382. Two Stages in the Development of the Human Brain (after His) . 383. Two Cross Sections through the Fore-Brain 384. The Brain of a Human Embryo in the Fifth Week (after His) . 385. Profile View of the Brain of a Human Embryo of Ten Weeks (His) 447 449 449 450 451 467 FIG. 386. 387. 388. 389. 390. 452 391. 392. 453 453 393. 453 394. 455 456 458 462 395. 396. 397. 463 398. 463 399. 464 400. 465 401. 402. 403. 467 404. 469 405. 471 472 406. 474 476 407. 477 477 478 408. 409. 410. 479 411. Diagrams to illustrate the Alar and Basal Laminae .... 480 Front View of the Medulla, Pons, and Mesencephalon of a full-time Human Foetus . . . .481 Back View of the Medulla, Pons, and Mesencephalon of a full-time Human Foetus .... 482 Diagram of the Decussation of the Pyramids (modified from van Gehuchten) 483 Lateral View of the Medulla, Pons, and Mesencephalon of a full-time Human Foetus .... 484 Floor of the Fourth Ventricle . 487 Section through the Lower End of the Medul-la Oblongata of a Chim- panzee to show the Decussation of the Pyramids .... 489 Transverse Section through Lower End of the Medulla of a full-time Fcetus 491 Section through the Closed Part of Human Medulla immediately above the Decussation of the Pyramids 492 Section through the Lower Part of the Medulla of the Orang . . 492 Transverse Section through the Closed Part of a Foetal Medulla . 493 Transverse Section through the Human Medulla in the Lower Olivary Region .... 493 Transverse Section through the Middle of the Olivary Region of the Human Medulla . . . 495 Inferior Olivary Nucleus as recon- structed and figured by Miss Florence R. Sabin . . . 495 Diagram which shows in part the Fibres which enter into the Constitution of the Restiform Body 496 Section through the Junction be- tween the Cord and Medulla of the Orang 496 Diagram of the Cerebello-olivary Fibres 497 Section through the Lower Part of the Human Pons Varolii imme- diately above the Medulla . . 500 Diagram to show Connexions of the Direct Cerebellar and the Olivo- cerebellar Tracts . . . 501 Transverse Section through the Pons Varolii at the Level of the Nuclei of the Trigeminal Nerve (Orang) 503 Section through the Upper Part of the Pons Varolii of the Orang, above the Level of the Trigeminal Nuclei 504 Two Sections through the Tegmen- tum of the Pons at its Upper Part, close to the Mesencephalon 505 Upper Surface of the Cerebellum . 505 Lower Surface of the Cerebellum . 508 Sagittal Section through the Left Lateral Hemisphere of the Cere- bellum 509 From a Dissection by Dr. E. B. Jamieson, showing Corpus Den- LIST OF ILLUSTEATIONS. XXV Fia. PAGE tatuni and Superior Cerebellar Peduncle, etc 509 412. Mesial Section through the Corpus Callosum, the Mesencephalon, the Pons, Medulla, and Cerebellum . 512 413. Transverse Section through a Cere- bellar Folium (after Kolliker) . 513 414. Section through the Molecular and Granular Layers in the Long Axis of a Cerebellar Folium (after Kolliker) ... .514 415. Diagram of the Spinal Origin of the Spinal Accessory Nerve (after Bruce) ...... 516 416. Section through the Upper Part of the Cervical Region of the Cord (Orang) 516 417. Diagram showing the Brain Con- nexions of the Vagus, Glosso- pharyngeal, Auditory, Facial, Abducent,and Trigeminal Nerves 518 418. Central Connexions of the Cochlear and Vestibular Divisions of the Auditory Nerve .... 520 419. Section through the Pons Varolii of the Orang 522 420. Diagram of the Intrapontine Course pursued by the Facial Nerve . 523 421. Section through the Pons Varolii of the Orang at the Level of the Nuclei of the Trigeminal Nerve 525 422. Three Stages in the Development of the Medulla Oblongata (His — slightly modified) . . . 527 423. Drawings to illustrate the Develop- ment of the Cerebellum (from Kuithan) 529 424. Brain of an Embryo of Eleven Weeks 529 425. Sections throu.gh Cerebellum of Human Foetus .... 530 426. Under Surface of the Cerebellum of a Human Foetus .... 530 427. Cerebellum of a Human Foetus . 530 428. Diagram of the Roots of the Optic Nerve 532 429. Transverse Section through the Upper Part of the Mesencephalon 533 430. Transverse Section through the Human Mesencephalon at the Level of the Inferior Quadri- geminal Body .... 535 431. Transverse Section through the Human Mesencephalon at the Level of the Superior Quadri- geminal Body .... 536 432. Section through the Inferior Quadri- geminal Body and the Tegmentum of the Mesencei^halon below the Level of the Nucleus of the Fourth Nerve in tlie Orang . . . 537 433. Section through the Inferior Quad- rigeminal Body and tlie Tegmen- tum of the Mesencephalon . . 538 434. Diagram of the Connexions of the Posterior Longitudinal Bundle (afier Held— modified) . . 538 435. Diagram of the Connexions of the Mesial Fillet and alscj of certain of the Tha]anio-Corti(-al Fibr(;H . 539 436. Section tliioiigh the Infcj'ior (Jjuad- rigeminai Body and tJie Tegmen- FIO. _ PAGE tum of the Mesencephalon (Orang) 540 437. Section through the Inferior Quad- rigeminal Body and the Tegmen- tum of the Mesencejjhalon (Orang) 541 438. The Two Optic Thalami . . 543 439. Schema 546 440. Coronal Section through the Cere- brum of an Orang . . . 547 441. Mesial Section through the Pituitary Region in a Child of Twelve Months old 549 442. Mesial S ection through the Pituitary Region in the Adult . . . 549 443. Mesial Section through the Corpus Callosum, Diencephalon, etc. . 550 444. Cast of the Ventricles of the Brain (from Retzius) . . . .551 445. Diagram of the Central Connexions of the Opti c Nerve and Optic Tract 552 446. Gyri and Sulci on the Outer Surface of the Cerebral Hemisphere . 555 447. Three Stages in the Development of the Insula and the Insular Opercula 557 448. Development of the Opercula which cover the Insula .... 557 449. Fissure of Rolando fully opened up 558 450. Left Cerebral Hemisphere from a Foetus 559 451. The Gyri and Sulci on the Mesial Aspect of the Cerebral Hemisphere 559 452. Gyri and Sulci on the Tentorial and Orbital Aspects of the Cerebral Hemispheres .... 562 453. Intraparietal Sulcus fuUy opened up 564 454. Internal Parieto-occipital and the Calcarine Fissures fully opened up 565 455. Development of the Parieto-occi- pital and the Calcarine Fissures . 566 456. Coronal Section through the Left Side of the Cerebrum, Mesen- cephalon, and Pons (Chimpanzee) 569 457. The Corpus CaUosum . . .570 458. Profile View of the Fornix . . 572 459. Cast of the Ventricular System of the Brain (after Retzius) . . 574 460. Coronal Section through the Frontal Lobes and the Anterior Horns of the Lateral Ventricles . . .574 461. Dissection to show the Fornix and Lateral Ventricles . . . 575 462. Coronal Section through the Posterior Horns of the Lateral Ventricles 576 463. Dissection to show the Fornix and the Posterior and Descending Cornua of the Lateral Ventricle of the Left Side .... 677 464. Dissection to show the Posterior and Descending Cornua of the Lateral Ventricle . . . 578 465. Horizontal Section through the Right Cerebral Hemisphere . 579 466. Coronal Section through the Cere- bral Hemisplieres . . . 580 467. Coronal Section through the Cere- brum 581 468. Coronal Section through the Left Sid(! of the Cerebrum of an Orang 582 409. Diagram to illustrate Minute Structure of the Cerebral Cortex 586 LIST OF ILLUSTEATIONS. FIG. 470. Diagram of the Minute Structure of the Olfactory Bulb . 471. Two Coronal Sections through the Cerebral Heniisj^heres of an Orang 472. Diagram of the Leading Association Bundles of the Cerebral Hemisi^here 473. Coronal Section through the Left Side of the Cerebrum, Mesen- cephalon and Pons (Chimjjanzee) 474. Diagrams to show Flechsig's Sensory and Association Areas on the sur- face of the Cerebral Hemi8j)here 475. Two Drawings of the Embryonic Brain (by His) .... 476. TwoDrawingsbyHisillustratingthe Develojjment of the Human Brain 477. Sagittal Section through the Skull 478. Diagram to show the Eelations of the Membranes of the Brain to the Cranial Wall, etc. . 479. Membranes of the Spinal Cord, and the Mode of Origin of the Spinal Nerves 480. Mesial Section through the Cranial Vault in the Frontal Region 481. Dissection to show the Velum In- terpositum 482. Diagrammatic Coronal Section through the Optic Thalami 483. Membranes of the Spinal Cord, and the Mode of Origin of the Spinal Nerves 484. Scheme of the Arrangement of the Membranes of the Spinal Cord and the Roots of the Spinal Nerves 485. Diagrammatic Rej^resentation of the Origin of the Spinal Nerves . 486. Scheme of the Distribution of a Typical Spinal Nerve . 487. Distribution of Cutaneous Nerves on the Back of the Trunk . 488. Posterior Cervical Plexus 489. Distribution of Cutaneous Nerves on the Front of the Trunk . 490. The Cervical Plexus 491. Distribution of Cutaneous Nerves to the Head and Neck 492. The Triangles of the Neck . 493. Muscles of the Hyoid Bone and Styloid Process, and the Extrinsic Muscles of the Tongue, with their Nerves 494. Nerves of the Brachial Plexus 495. Diagram of the Origin and Distri- bution of the Nerves to the Pectoral Muscles .... 496. The Posterior Wall of the AxiUa and the Front of the Arm . 497. The Distribution of Cutaneous Nerves on the Front of the Arm and Hand 498. The Distribution of Cutaneous Nerves on the Back of the Arm and Hand 499. Deltoid Region and Back of Arm . 500. Diagrammatic Representation of the Branches of the Musculo- Sjjiral Nerve .... 501. The Muscles of the Back of the Forearm 502. Scheme of the Distribution of a Typical Spinal Nerve . PAGE 587 589 590 592 FIG. 503. 504. 505. 506. 507. 593 , 508. 595 i 509. i 596 ; 510. 598 I I 511. I 512. 601 602 603 604 605 605 607 608 609 611 612 615 616 618 619 620 623 625 626 628 630 631 633 634 636 513. 514. 515. 516. 517. 518. 519. 520. 521. 522. 523. 524. 525. 526. 527. 528. 529. 530. 531. 532. 533. 534. 535. 536. The Distribution of Cutaneous Nerves on the Front of the Trunk 637 Nerves of the Lumbo-Sacral Plexus 640 View of the Posterior Abdominal Wall, to show tlie Muscles and the Nerves of the Lumbo-Sacral Plexus 641 Scheme of the Course and Distri- bution of the Obturator Nerve . 644 Distribution of Cutaneous Nerves on the Front of the Lower Limb . 645 Distribution of Cutaneous Nerves on the Dorsum of the Foot . . 651 Distribution of Cutaneous Nerves on the Back of the Lower Limb . 653 Scheme of Distribution of the Plantar Nerves .... 654 Nerves of the Lumbo-Sacral Plexus 656 The Muscles and Nerves of the Male Perineum .... 658 The Triangular Ligament of the Perineum ..... 659 Scheme of the Innervation of the Hinder Portion of the Trunk and of the Perineum .... 660 Development of the Spinal Nerves 661 Scheme of the Segmental Distribu- tion of the Muscular Nerves of the Upper and Lower Limbs . . 671 View of the Under Surface of the Brain . . . . . .675 Innervation of the Nasal Cavity . 676 Diagram of the Central Connexions of the Optic Nerve and Optic Tract 676 Relations of Structures in the Cav- ernous Sinus and Sphenoidal Fissure 677 Dorsal Surface of the Mid -brain . 677 The Base of the Skull . . .678 Distribution of Sensory Nerves to the Head and Neck . . . 679 Scheme of the Distribution of the OjDhthalmic Nerve . . . 680 Schematic Representation of the Nerves which traverse the Cavity of the Orbit 681 Scheme of the Course and Distri- bution of the Superior Maxillary Nerve ...... 682 Scheme of the Distribution of the Inferior Maxillary Nerve . . 684 The Facial Nerve with its Branches and Communications in the Aque- duct of Fallopius .... 687 Distribution of Facial Nerve out- side the Skull, and Communica- tions with Trigeminal Nerve on the Face . . . • . .688 Scheme of the Origin and Distribu- tion of the Auditory Nerve . . 689 Scheme of the Distribution of the Glosso-jjharjTigeal Nerve . . 690 The Distribution of the Pneumo- gastric Nerve . . . .691 The Constitution of the Cardiac Plexuses 694 The Distribution of the Pneumo- gastric Nerve .... 695 Scheme of the Origin, Connexions, and Distribution of the Spinal Accessory Nerve .... 696 The Muscles of the Hyoid Bone and LIST OF ILLUSTEATIONS. XXVI 1 537. 538. Styloid Process, and the Extrinsic Muscles of the Tongue with their Nerves 697 Coinjjarison of Origins of Nerve- roots from S23inal Cord and Hind- brain (after His) . . . . 700 Scheme to illustrate the Disposition of the Myotomes in the Embryo in relation to the Head, Trunk, and Limbs 702 539. Scheme to illustrate the Embryo- logical Arrangement of the Cranial Nerves 703 Scheme of the Constitution of the White Ramus Communicans of the Sympathetic .... 704 Scheme of the Constitution and Connexions of the Gangliated Cord of the Sympathetic . . 705 540. 541 542. The Distribution of the Sym- pathetic Gangliated Cord in the Neck 707 543. The Constitution of the Cardiac Plexuses 708 544. The Arrangement of the Sym- pathetic System in the Thorax, Abdomen, and Pelvis . . . 709 545. The Lumbar Portion of the Sym- pathetic Gangliated Cord and Lumbar Plexus . . . .711 546. The Arrangement of the Sym- pathetic System in the Thorax, Abdomen, and Pelvis . . .713 547. The Development of the Sym- pathetic Gangliated Cord . . 715 548. Section through the Sympathetic Gangliated Cord of an Embryo . 716 THE OEQANS OF SENSE AND THE INTEGUMENT. 549. Lateral View of Nasal Septum . 718 550. Profile View of the Bony and Carti- laginous Skeleton of the Nose . 719 551. Front View of the Bony and Carti- laginous Skeleton of the Nose . 719 552. Cartilages of Nose from Below . 720 553. Coronal Section through Nasal Fossae ; Anterior Half of Section Viewed from Behind . . . 720 554. Section through Nose of Kitten, showing position of Jacobson's Organ 721 555. View of the Outer Wall of the Nose 721 556. Section through the Olfactory Mucous Membrane . . . 722 557. Olfactory and Supporting Cells . 722 558. Diagram of a Horizontal Section through Left Eyeball and Optic Nerve 724 559. Vertical Section of Cornea . . 726 560. Vertical Section of Chorioid and Inner Part of Sclera . . . 727 561. Diagram of the Circulation in the Eye (Leber) 728 562. Section through Ciliary Region of Eyeball 729 563. Blood-Vessels of Iris and Anterior Part of Chorioid (Arnold) . . 730 564. Diagrammatic Section of the Human Retina (modified from Schultze) 732 565. Perpendicular Sections of Mam- malian Retina (Cajal) . . . 733 j 566. Cone and two Rods from the Human Retina .... 733 I 567. Pigmented Epithelium of Human Retina 734 568. Section through Outer Layers of Retina 734 569. Blood-Vessels of the Retina . . 735 570. Canal of Petit Distended and Viewed from the Front . . 736 571. Lens liardened in Formalin and di.Hsected to show its Conceiiti'ic Lam in if; ..... 730 572. iJiagraiiimatic Representation of llic. Radii Lentis of tlic. I''a;tai LenH . . . . . 737 573. Section through the Equator of the Lens 737 574. Vertical Section through Upper Eyelid 738 575. Sections through Portions of the Heads of Foetal Rabbits . . 741 I 576. Optic Cup and Lens viewed from I Behind and Below . . . 742 i 577. Diagrammatic View of the Organ of Hearing 744 578. View of Outer Surface of Left Pinna 744 579. Outer Surface of Cartilage of Pinna 745 580. Inner Surface of Cartilage of Pinna 745 581. Vertical Transverse S ection of Right Ear 747 582. Horizontal Section through Right Ear 747 583. Section through Left Temporal Bone 749 584. Left Membrana Tympani and Re- cessus Epitympanicus . . . 749 585. Left Tympanic Membrane . . 751 586. Transverse Section of the Cartilag- inous Part of the Eustachian Tube 753 587. Tympanic Ossicles of Left Ear . 755 588. Left Membrana Tympani and Chain of Tympanic Ossicles . . . 755 589. Left Bony Labyrinth . . .759 590. Interior of Left Bony Labyrinth . 759 591. Section of Bony Cochlea . . 761 592. Diagrammatic Representation of the Different Parts of the Mem- branous Labyrinth . . 762 593. Transverse Section of Human Semi- circular Canal (Riidinger) . . 763 594. Section across the Ductus Cochlearis (Retzius) 764 595. Transverse Section through Outer Wall of Ductus Cochlearis (Schwalbe) 765 596. Transverse Section of Corti's Organ from tlie Central Coil of Cochlea (Retzius) 766 597. Membranous Labyrinth of a Five Montlis' Frctus (Retzius) . . 768 598. Part of Cochlear Nerve (Henle) . 768 599. Sections tlirougli tiu; Region of the Hind Brain of Fcjctal Rabbits . 769 XXVIU LIST OF ILLUSTKATIONS. 600. Left Labyrintli of Human Embryo 601. Section tlirougli Papilla Vallata (A) of Human Tongue and (B) of Eabbit. ..... 602. Three-quarter Surface View and Vertical Section of Taste Bud i'rom the Papilla Foliata of a Eabbit 603. Isolated Cells from Taste Bud of Rabbit (Engelmann) . 770 604. Vertical Section of the Skin . 605. Vertical Section of Epidermis and Papillaj of Corium 770 606. Tactile Corpuscles .... 607. Transverse Section of a Nail . 608. Lougitudinal Section througli Root j of Nail 771 609. Transverse Section of Hair Follicle with Contained Hair . 771 PAGE 772 773 775 776 776 777 THE VASCULAE SYSTEM. 612. 613. 610. Structure of Blood-Vessels 611. Transverse Section through Wall of a Large Artery Transverse Section of the Wall of a Vein The Base and Inferior Surface of the Heart 614. The Antero-superior Surface of the Heart 615. The Relation of the Heart to the Anterior Wall of the Thorax 616. The Cavities of the Right Auricle and Right Ventricle of the Heart The Bases of the Ventricles of the Heart The Relations of the Heart and the Auriculo-ventricular, Aortic, and Pulmonary Orifices to the Anterior Thoracic Wall Posterior Wall of the Pericardium after removal of the Heart . The Pulmonary Arteries and Veins and their Relations The Abdominal Aorta and its Branches The Carotidand Subclavian Arteries and their Branches 623. The External Carotid, Internal Maxillary, and Meningeal Ar- teries ...... The Carotid, Subclavian, and Verte- bral Arteries and their Main Branches Distribution of the Cerebral Arteries on the Mesial, Tentorial, and Inferior Surfaces of the Cere- bral Hemispheres Distribution of Cerebral Arteries on the Outer Surface of the Cerebrum The Arteries of the Base of the Brain 628. Dissection of the Back of Shoulder and Upper Arm . 629. The Axillary Artery and Branches and Relations The Brachial Artery and Branches Superficial Dissection of the Front of the Forearm and Hand . 632. Deep Dissection of the Front of the Forearm and Hand The Posterior Interosseous Artery and tlie Second Part of the Radial Artery, with their Branches The Abdominal Aorta and its Branches The Ccfiliac Axis and its Branches 617. 618. 619. 620. 621. 622. 624. 625. 626. 627. 630. 631. 633. 634. the its its 635. 781 781 782 784 785 786 787 788 789 794 796 799 803 810 813 815 816 819 824 827 830 832 833 835 840 844 636. The Superior Mesenteric Artery and its Branches .... 846 637. The Internal Iliac Artery and its Branches in the Female . . 849 638. The Perineal Distribution of the Internal Pudic Artery in the Male 853 639. The Arteries of the Buttock and the Back of the Thigh and Knee . 855 640. The Iliac Arteries and Veins in the Female 857 641. The Femoral Artery and its Branches 858 642. The Femoral Vessels in Scarpa's Triangle 860 643. The Arteries of the Buttock and the Back of the Thigh and Knee . 862 644. The Popliteal and Posterior Tibial Arteries and their Branches . 864 645. The Plantar Arteries and their Branches 866 646. The Anterior Tibial Artery and its Branches 867 647. The Dorsalis Pedis Artery and its Branches 869 648. Superficial Veins of the Head and Neck . . . . . .878 649. The Veins of the Diploe . . 881 650. Dissection of the Head and Neck, showing the Cranial Blood Sinuses and the Upper Part of the Internal Jugular Vein .... 884 651. Basal Blood Sinuses of the Dura Mater 885 652. Superficial Veins on the Dorsum of the Hand and Digits . . 889 653. Superficial Veins on the Flexor Aspect of the Upj)er Extremity . 890 654. Superficial Veins at the Bend of the Elbow 891 655. The Inferior Vena Cava and its Tributaries 893 656. The Femoral Vessels in Scarpa's Triangle 898 657. The Internal or Long Saphenovis Vein and its Tributaries . . 899 658. The External or Short Saphenous Vein and its Tributaries . . 901 659. The Portal Vein and its Tributaries 902 660. The Thoracic Duct and its Tribu- taries 907 661. Lymphatic Vessels and Glands of the Head and Neck . . . 901 662. Superficial Lymphatic Vessels of the Trunk, and the Lymphatic Glands and Vessels Sujaerficial and Deep of the Limbs . . . 912 LIST OF ILLUSTRATIONS. 663. Superficial Lymphatics of the Digits and of the Dorsal Aspect of the Hand 915 664. Deep Lymphatic Glands and Vessels of the Thorax and Abdomen . 919 665. Diagram of the Primitive Vascular System before the Formation of the Heart 926 666. Diagram of the Primitive Blood- Vessels after the Formation of the Heart . . . . .927 667. Diagram of the Primitive Blood- vessels after the Formation of the Heart, but before its Sub- division by Septa into Auricles and Ventricles .... 927 668. Development of the Heart . . 928 669. Development of the Heart and the Main Arteries .... 930 670. Diagram of the Course of the Foetal Circulation . . .931 671. Development of the Venous System (Stage L) 934 672. Development of the Venous System (Stage II.) 935 673. Development of the Venous System (Stage III.) 936 674. Development of the Venous System (Stage IV.) 937 675. Diagram of the Cephalic Aortic Arches, and of the Segmental and Intersegmental Arteries in the Eegion in front of the Umbilicus 939 676. Diagram of the Caudal Aortic Arch, and of the Segmental and Inter- segmental Arteries in the Region behind the Umbilicus . . . 939 677. Diagram showing the Arrangement and Communications of the Seg- mental and Intersegmental Ar- teries at an Early Stage of De- velopment 940 678. Diagram of the Segmental and Intersegmental Arteries at a Later Period of Development than in Fig. 677 . . . . 941 679. Diagram showing the Arrangement and Communications of the Seg- mental Arteries in the Region of the Cephalic Aortic Arches . . 941 THE EESPIRATORY SYSTEM. 680. The Cartilages and Ligaments of the Larynx viewed from the Front 959 681. Profile View of the Cartilages and Ligaments of the Larynx . . 959 682. Cartilages and Ligaments of Larynx from Behind .... 960 683. Dissection to show the Lateral Part of the Crico-thyroid Membrane . 684. Superior Aperture of Larynx 685. Coronal Section through Larynx . 686. Mesial Section through Larynx 687. Diagram of Rima Glottidis . 688. Specimen showing a Great Ex- tension of the Saccule of the Larynx ..... 689. The Crico-Thyroid Muscle . 690. Dissection of the Muscles in the Lateral Wall of the Larynx 691. Dissection of the Muscles on the Posterior Aspect of the Larynx . 692. Cavity of the Larynx 693. The Trachea and Bronchi 694. Transverse Sections through the Trachea and its Immediate Sur- roundings 974 695. 696. 697. 963 698. 964 965 699. 966 966 700. 967 968 701. 969 702 970 972 703 973 704 Diagram showing Arrangement of Pleural Sacs .... 977 Dissection of a Subject hardened by Formalin-Injection, to show the Relations of the two Pleural Sacs 978 Lateral View of the Right Pleural Sac in a Subject hardened by Formalin-Injection . . . 979 Left Pleural Sac in a Subject hardened by Formalin-Injection . 980 Dissection of the Pleural Sacs from Behind 981 Dissection of Thorax and Root of the Neck from the Front to show the Relations of the Lungs, Peri- cardium, and Thymus Gland . 984 Mediastinal Surfaces of the two Lungs of a Subject hardened by Formalin-Injection . . . 985 Outer or Costal Surfaces of the two Lungs 986 Sagittal Section through Left Shoulder and Left Lung . . 987 Sagittal Section through the Left Shoulder, Lung, and Apex of the Heart 988 THE DIGESTIVE SYSTEM. 705. General View of the Digestive System 994 706. Coronal Section through the Closed Moutli 996 707. Open Moutli sliowing Palate and Tonsils 999 708. Sagittal Section through Mouth, Tongue, Larynx, Pliarynx, and Nasal Cavity .... 1001 709. Horizontal Section thioiigli Mfjuth and Pharynx at the Ijevcl of tlie TouhIIh 1002 710. The Anterior Wall of the Phar- ynx with its Orifices, seen from behind 711. The Pa])illai of Tongue . 712. Open Mouth with Tongue raised, and the Sublingual and Apical Glands exposed .... 713. Sections through the Tongue (Krause) ; and Lymjjhoid Follicle from Back Part of Tongue (Mac- alister) 714. Section of a Serous Gland and a 1003 1004 1005 1006 LIST OF ILLUSTEATIONS. FIO. Mucous Gland (Bolim and v. 749. Davidoff) 1008 715. Horizontal Section tlirough Mouth and Pharynx at the Level of the 750. Tonsils 1009 | 716. The Salivary Glands and their \ 751. Ducts 1010 I 717. Teeth of a Child over seven years i 752. old (modified from Testut) . . 1014 753. 718. Vertical Section of Canine Tooth . 1015 719. The Permanent Teeth of the Right 754. Side, Outer or Labial Aspect . 1017 720. The Permanent Teeth of the Right Side, Inner or Lingual Aspect . 1018 755. 721. The Upper Permanent Teeth . 1018 756. 722. The Lower Permanent Teeth . 1020 723. Horizontal Sections through both 757. the Upi^er and Lower Jaws to show the Roots of the Teeth . 1021 758. 724. To show the Relation of the Upper to the Lower Teeth when the Mouth is closed .... 1022 759. 725. The Milk Teeth of the Left Side . 1023 760. 726. Vertical Section of Canine Tooth . 1024 727. Diagram to illustrate the Develop- 761. ment of a Dermal Tooth in the Shark 1025 762. 728. Diagram to illustrate Development 763. of a Tooth . . . . . 1026 764. 729. The Anterior Wall of the Pharynx 765. with its Orifices, seen from behind 1030 730. Sagittal Section through Mouth, Tongue, Larynx, Pharynx, and Nasal Cavity .... 1031 766. 731. The Naso-pharynx from the Front 1032 732. Open Mouth showing Palate and 767. Tonsils 1034 733. Horizontal Section through Mouth 768. and Pharynx at the Level of the 769. Tonsils 1035 734. Diagram to show the Course of the 770. CEsophagus 1038 771. 735. Tracings from Frozen Sections to 772. show the Relations of the (Eso- 773. phagus 1039 736. Dissection to show the Arrangement 774. of the Muscular Fibres on the 775. Back of the CEsophagus and Pharynx 1041 776. 737. The Lower Part of the Pharynx and the Upper Part of the CEsophagus 1042 777. 738. Structure of the CEsophagus . . 1042 739. The Abdominal Viscera in situ . 1044 778. 740. The Front of the Body . . . 1046 741. Diagrammatic Mesial Section of 779. Female Body .... 1048 742. Diagrammatic Transverse Sections 780. of Abdomen 1049 743. Moderately distended Stomach . 1050 744. The Abdominal Viscera after the re- 781. moval of the Jejunum and Ileum 1051 745. The Stomach Chamber and Stomach Bed 1053 782. 746. The Viscera and Vessels on the Posterior Abdominal Wall . . 1054 783. 747. Longitudinal Section through the Pyloric Canal and Commence- ment of the Duodenum in a New- 784. Born Child 1056 785. 748. Abdomen of Female, showing Dis- 786. placements resulting from Tight 787. Lacing 1057 Section through Wall of Stomach, Cardiac Portion (slightly modi- fied from Stijhr) 1058 The Three Layers of the Muscular Coat of the Stomach . . . 1059 Diagram to show Formation of Pylorus 1060 The Mucous Membrane of Stomach 1060 A Portion of Small Intestine, with Mesentery and Vessels . . . 1061 Diagram to show the Structure of the Small and Large Intestine and the Duodenum . . . 1062 Valvuhe Conniventes . . . 1063 Peyer's Patch and Solitary Glands from Intestine of Child . . 1064 The Viscera and Vessels on the Posterior Abdominal Wall . . 1066 The Peritoneal Relations of the Duodenum, Pancreas, Spleen, Kidneys, etc 1067 The Duodenal Fossae and Folds . 1069 The Bile Papilla in the Interior of the Duodenum .... 1069 The Abdominal Viscera after the re- moval of the Jejunum and Ileum 1071 Large Intestine .... 1074 Caecum showing Ileo-csecal Valve . 1076 Three Forms of Ileo-caecal Valve . 1077 Diagrammatic Section through the Junction of Ileum with Caecum, to show the Formation of the Ileo-caecal Valve . . . . 1078 The Blood Supply of the Caecum and Vermiform Appendix . . 1079 Structure of the Vermiform Ap- pendix 1080 The Cgecal Folds and Fossae . . 1081 The Abdominal Viscera after the re- moval of the Jejunum and Ileum 1084 The Iliac and Pelvic Colons . . 1085 The Rectum from Behind . . 1087 Distended Rectum in situ . . 1088 The Peritoneum of the Pelvic Cavity 1090 Diagram of Rectum . . . 1093 The Interior of the Anal Canal and Lower Part of Rectum . . 1094 The Anal Canal and Lower Part of Rectum in the Foetus . . . 1094 Diagrammatic Mesial Section of Female Body .... 1098 The Peritoneum of the Pelvic Cavity 1100 Diagrammatic Transverse Sections of Abdomen .... 1101 The Peritoneal Relations of the Duodenum, Pancreas, Spleen, Kidneys, etc. .... 1103 Two Diagrams to illustrate the Development of the Intestinal Canal 1106 Two Diagrams to illustrate the Development of the Mesenteries . 1 107 Diagrams to illustrate the Develop- ment of the Great Omentum (after Hertwig) . ... . .1107 The Abdominal Viscera in situ . 1109 The Liver from the Front . . 1111 The Liver from Below and Behind 1112 The Abdominal and Thoracic Viscera of a Five Months' Foetus . 1117 LIST OF ILLUSTEATIONS. 788. Structures between the Layers of the Lesser Omentum . 789. Diagram showing the Bile and Pancreatic Ducts piercing the Wall of the Duodenum obliquely 790. Liver of a Pig injected from the Hepatic Vein by T. A. Carter 791 . Diagrams illustrating the Structure of Liver 792. Diagram illustrating the Arrange- ment of the Blood-Vessels and of 1119 1120 1122 1122 the Hepatic Cells and Bile Ducts within a Lobule of the Liver . 1123 793. Two Diagrams to illustrate the De- velopment of the Intestinal Canal 1123 794. The Viscera and Vessels on the Posterior Abdominal Wall . . 1125 795. The Peritoneal Relations of the Duodenum, Pancreas, Sjjleen, Kidneys, etc. . . . .1126 796. The Pancreas and Duodenum from Behind 1128 THE UEINOGENITAL SYSTEM. 797. Dissection to show the Position and Relationships of the Kidneys 798. Transverse Section through the Abdomen at the Level of the Second Lumbar Vertebra . 799. The Posterior Relationships of the Kidneys 800. The Kidneys viewed from Behind . 801. Right Kidney and Duodenum 802. Left Kidney, the Pancreas, the Spleen, and the Descending Colon 803. The Kidneys and Great Vessels viewed from the Front 804. Longitudinal Section through the Kidney 805. Section through a Portion of the Kidney 806. Diagrammatic Representation of the Structures forming a Kidney Lobe 807. Longitudinal Section of the Kidney opening up the Kidney Sinus 808. Mesial Section of an Adult Male Pelvis 809. Mesial Section through the Male Pelvis 810. Mesial Section of the Male Pelvis . 811. Under Aspect of the Empty Male Bladder 812. The Bladder, Prostate, and Seminal Vesicles, viewed from Below 813. The Bladder, Prostate, and Seminal Vesicle, from the Outer Side 814. The Bladder and the Structures traversed by the Urethra in the Male 815. Lateral Aspect of Bladder contain- ing ten ounces of Fluid 816. View looking into the Pelvis from Above and somewhat Behind 817. View of the Interior of the Bladder in the Region of the Urethral Orifice 818. Mesial Section of the Pelvis in an Adult Female .... 819. The Bladder of a Newly -bom Male Child 820. Mesial Section through the Pelvis of Newly-born Child . 821. View looking from Above into the Pelvis and Lower Part of the Abdominal Cavity in a Foetus of about the Seventh Month . 822. View looking into the Male Pelvis Hi-A:]\ from Above and Homcwliat Beliind 823 1131 824 1132 825. 826. 1133 1134 827. 1135 1135 828. 1136 829. 1137 830. 1138 831. 1139 832. 1141 833. 1142 834. 1145 1146 1147 835. 1147 1148 836. 1149 837. 1150 838. 1150 839. 1151 1152 • 840. 1152 841. 1153 ' 842. 843. 1154 844. 845. il54 . Mesial Section of the Pelvis in an Adult Male 1155 . Mesial Section through the Female Pelvis 1158 . The Right Testis and Epididymis . 1159 . Right Testis within Tunica Vaginalis 1160 . Transverse Section of Testis and Epididymis and of Spermatic Cord below External Abdominal Ring 1161 . Diagram to illustrate the Structure of the Testis and Epididymis . 1162 , The Peritoneum of the Pelvic Cavity 1163 Horizontal Section through the Rectum and Bladder . . . 1164 View -of the Base of the Bladder, Prostate, Seminal Vesicles, and Vasa Deferentia from Behind . 1165 The Bladder, Prostate, and Seminal Vesicle viewed from the Side . 1166 The Seminal Vesicle and the Ampulla of Vas Deferens . . 1166 View looking from Above into the Pelvis and Lower Part of the Abdominal Cavity in a Foetus of about the Seventh Month . . 1167 Diagram to illustrate the Descent of the Testis and the manner in which the Tunica Vaginalis is derived 1168 Transverse Sections of Testis and Epididymis, and of Spermatic Cord below External Abdominal Ring 1169 Deep Dissection of the Inguinal Region 1169 Dissection to illustrate the Com- ponent Parts of the Penis . . 1171 A Longitudinal Section of the Terminal Portion of the Penis, and a Tranverse Section through the Body of the Organ . . 1172 Bladder, Prostate, and Seminal Vesicles, from the Outer Side . 1173 Prostate, Bladder, and Seminal Vesicles, seen from Below . . 1174 Transverse Section through the Prostate 1175 Transverse Section through the Prostate 1176 The Bladder and the Structures trav(!rsed by the Urethra . . 1177 Mesial Section of an Adult Male Pelvis 1178 LIST OF ILLUSTEATIONS. FIG. 846 847. 849. 850. 851. 853. 854. FIO. 857. The Prostatic, Membranous, and the Upper Portion of the Spongy Urethra . . . . \ 1178 A Longitudinal Section of the Ter- minal Portion of the Penis, and a ' 858. Transverse Section througli the Body of the Organ . . .1180 859. Mesial Section through the Female Pelvis 1181 860. Side Wall of the Female Pelvis . 1183 The Uterus and Broad Ligament, and Diagrammatic Representa- tion of the Uterine Cavity . . 1183 861. Graafian Follicle . . . .1185 852. The Uterus and Broad Ligament, ' and Diagrammatic Representa- tion of the Uterine Cavity . . 1188 862. Mesial Section of the Pelvis in an Adult Female . . . .1193 863. The Vagina, the Base of Bladder, and the Recto-vaginal Pouch of i 864. Peritoneum . . . .1194 855. Female External Genital Organs . 1196 | 865. 856. Dissection of the Female External 866. Genital Organs . . . . 1197 | Dissection of Female Perineum to show the Clitoris, the Bulb of the Vestibule, and Bartholin's Glands. .... 1198 Development of the Bladder, Ureter, and Kidney . . . 1199 Transverse Section through the Body of a Fowl Embryo . .1200 Diagram to illustrate the Manner in which the Ureter, the Vas Deferens, and the Bladder arise in the Embryo 1201 Diagrammatic Representation and Comparison of the Manner in which the Urinogenital Passages arise in the two Sexes . . . 1202 Transverse Section through the Body of a Rat Embryo . . 1203 Development of the External Genital Organs .... 1205 External Genital Organs in Male Embryo 1206 Dissection of the Mammary Gland 1207 Section through a Mammary Gland 1208 THE DUCTLESS GLANDS. 867. Dissection of the Spleen, Liver, and Kidneys from Behind . 868. The Spleen 869. Anterior Surfaces of Suprarenal Capsules . . . . . 870. Posterior Surfaces of Suprarenal Capsules . . . . . 871. Transverse Section through the Suprarenal Capsule of a New- born Child in situ 872. Dissection of the Thyroid Body 1211 1212 1214 1214 1215 and of the Parts in immediate re- lation to it . 873. Thymus Gland ina FuU-timeFcetus hardened by Formalin-injection . 874. Dissection to show the Thymus Gland in an Adult Female . 875. Deej) Surface of Thymus Gland 876. Section through Carotid Body 877. Schema of the Relation presented by the Carotid Gland and its ac- cessory outlying Parts to Branches of middle Sacral Artery 1216 1218 1219 1220 1221 1221 SUEFACE AND SURGICAL ANATOMY. 878. Diagrammatic Representation of a Coronal Section through the Scalp, Cranium, Meninges, and Cortex Cerebri .... 1223 879. Cranio-Cerebral Topography . 1226 880. Scheme showing Relative Topo- graphy of the Chief Subdivi- sions of the Motor Area (adapted from Griinbaum and Sherring- ton) 1227 881. Cranio-Cerebral Topography . 1228 882. View of the Outer Wall of the Middle Ear .... 1230 Left Tympanic Membrane . . 1230 View of the Inner Wall of the Middle Ear .... 1231 Section through Left Temporal Bone, showing Outer Wall of Tymj)anic Cavity, etc. . . . 1231 Section through Petrous Portion of Temporal Bone of Adult . . 1232 887. Frontal Sinuses of Average Dimen- sions, with a Mesial Septum (Logan Turner) .... 1234 888. A Large Right Frontal Sinus with Septum Oblique to the Left (Logan Turner) .... 1234 883. 884. 885. 886. 889. Right Frontal Sinus of very large Dimensions ; Left Sinus unopened (Logan Turner) .... 1235 890. Vertical Coronal Section through the Nose and Frontal Sinuses . 1236 891. Head of Human Embryo about 29 days old 1240 892. Coronal Section through the Face of a Human Embryo at the Seventh Week 1241 893. From a Photograph showingDouble Complete Hare - LijJ and Cleft Palate .....'. 1241 894. Shows arrangement of Bones in Double Cleft Palate . . . 1242 895. Coronal Section through the Tongue and Submaxillary Region in a Plane behind the Molar Teeth . 1243 896. Open Mouth with Tongue raised and the Sublingual and Apical Glands exposed .... 1243 897. Horizontal Section through Mouth and Pharynx at the Level of the Tonsils 1245 898. Anterior Aspect of Neck and Shoulders 1247 899. Dissection of the Front of the Neck 1248 LIST OF ILLUSTEATIONS. XXXIU FIO. 900. 901. 902. 903. 904. 905. 906. 907. 908. 909. 910. 911. 912. 913. 914. 915. 916. PAOE Lateral Aspect of Neck . . . 1250 Anterior Aspect of Trunk, showing Surface Topography of Viscera . 1254 Dissection of a Subject to show the relations of the two Pleural Sacs viewed from the Front . . 1256 Anterior Aspect of Trunk, showing Surface Topography of Viscera . 1257 Lateral View of the Right Pleural Sac 1258 Dissection of the Pleural Sacs from Behind 1259 Dissection from Behind to show the relation of the two Pleural Sacs to the Kidneys .... 1260 Posterior Aspect of Trunk, showing Surface Topography of Viscera . 1261 1 Relations of the Cavities and Valves Y of the Heart to Anterior Wall of I Thorax 1263 The Groin 1265 Anterior Aspect of Trunk, showing Surface Topography of Viscera . 1268 Lateral Aspect of Trunk, showing Surface Topography of Viscera . 1269 The Cffical Folds and Fossae . . 1273 The Blood Supply of the Ceecum and Vermiform Appendix . . 1274 FIO. PAOf: 917. Anterior Aspect of Trunk, showing Surface Topography of Viscera . 1275 918. Dissection of the Perineum . . 1277 919. The Interior of the Anal Canal and Lower Part of Rectum . . . 1280 920. The Rectum from Behind . .1282 921. Dissection of the Spleen, Liver, aiid Kidneys from Behind . . . 1286 922. Posterior Aspect of Trunk, showing Surface Topography of Viscera . 1287 923. Dissection of the Left Hypochon- drium 1288 924. Axilla, Inner Aspect of Upper Arm and Elbow 1294 925. Extensor Aspect of Upper Limb . 1295 926. Transverse Section through the Bend of the Elbow . . . 1297 927. Bend of Elbow, Front of Forearm, and Palm of Hand . . .1298 928. Palm of Hand .... 1299 929. Dorsal Aspect of Hand . . . 1300 930. Section through Thigh at the Level of the Upper Part of Hunter's Canal 1303 931. Section through the Thigh immedi- ately above the Patella . . 1304 932. The Thigh and Groin . . . 1305 933. Anterior Aspect of Knee . . 1307 934. Outer Aspect of Knee and Leg . 1309 935. Outer Aspect of Foot and Ankle . 1310 936. Inner Aspect of Foot and Ankle . 1311 TEXT-BOOK OF ANATOMY. TEXT-BOOK OF ANATOMY INTRODUCTION. Anatomy is a comprehensive term, which includes several closely related branches of study. Primarily it is employed to indicate the study of the several parts which build up the body, and the relationship which these present to each other. But during the period of its existence the individual exhibits many structural chanoes: its structure is not the same at all stages of its life. The ovum or starting-point of every individual is very different from the finished organism as represented by the adult, and the series of changes through which the organism passes until its structure is perfected and full growth is attained constitute the study of development. The general term " development " includes not only the various and striking structural changes which occur during the intrauterine life of the individual, to the study of which the term embryology is more specially applied, but also many growth processes which occur after birth, such as the later stages in the ossification and growth of the bones, the eruption of the two series of teeth, the adjustment of the vascular system to its new requirements, etc. The actual obser- vation of the processes by which the parts of the body are gradually formed, and of the structural arrangements by means of which a temporary connexion is estab- lished between the ovum and the mother, through which an interchange of nutritive and other matters between the two takes place, renders embryology one of the most interesting of all the departments of anatomy. The term ontogeny is also used to denote the development of the individual. There is, however, another form of development, slower, but just as certain in its processes, which affects not only the individual, but every member of the animal group collectively to which it belongs. The theory of descent or evolution leads us to believe that between man of the present day and his remote ancestors there is a wide structural gap, which, if the geological record were perfect, would be seen to be completely occupied by long-lost intermediate forms. In the process of evolution, therefore, structural changes have gradually taken place which have modified the entire race. A more or less close or remote blood-relationship links together all the members of the animal kingdom. These evolutionary phases constitute the ancestral history or phylogeny of the individual. Ontogeny and phylogeny are intertwined in a remarkable manner, and present certain extraordinary relationships. In other words, the ancestral evolutionary development appears to be so stamj^ed upon an iri(]ividual that it repeats certain of the phylogenctic stages witli more or less clearness during the process of its own individual development. Thus at an early l>oriod in the embryology of man we recognise evanescent gill-slits comparable with those of a fish, wliilst a study of the development of his heart sliows that it passes 2 TEXT-BOOK OF ANATOMY. through transitory structural conditions in many respects similar to the permanent condition of tlie heart in certain of the lower animals. It is in connexion with this that the phrase has arisen that every animal in its individual development or ontogeny climbs up its own genealogical tree — a saying which, taking it even in the Ijroadest sense, is only partially true. The higher conceptions of anatomy, which are obtained by taking a general survey of the structural aspects of the entire animal kingdom, constitute morpliology. The morphologist investigates the laws of form and structure, and in his generalisa- tions he gives attention to detail only in so far as this is necessary for the proper establishment of his views. The knowledge of anatomy which is required by the student of medicine is different. It is essentially one of detail, and often details important from the practical and utilitarian points of view have little or no morphological value. This want of balance in the interest attached to anatomical facts, according to the aspect from which they are examined, so I'ar from being unfortunate, affords the teacher the means of making the study of anatomy at once fascinating and attractive. Almost every fact which is brought under the notice of the student cari be accompanied by a morphological or a practical application. This it is that lightens a study which, presented to the student of medicine in any other way, would be at once dry and tedious. Certain terms employed in morphology require early and definite explanation. These are homology, serial homology, and homoplasy. The same organ repeated in two different animals is said to present a case of homology. But this morphological identity between these two organs must be proved beyond dispute before the homology between them can be allowed. In deciding this identity the great and essential test is that the two organs in question should have a similar develop- mental origin. Thus the fore-limb of a quadruped is homologous with the upper limb of man ; the puny collar-bone of a tiger, the fibrous thread which is the only representative of this bone in the horse, and the strongly marked clavicle of the ape or man, are all, strictly speaking, homologous with each other. Homologous oro-ans in different animals usually present a similar position and a similar structure, but not invariably so. It is not uncommon for a muscle to wander somewhat from its original position, and many cases could be quoted in which parts have become completely transformed in structure, either from disuse or for the purpose of meeting some special demand in the animal economy. In the study of the muscles and ligaments instances of this will be brought under the notice of tlie reader. Identity or correspondence in the function performed by two organs in two different animals is not taken into consideration in deciding questions of homology. The gills of a fish and the lungs of a higher vertebrate perform very much the same physiological office, and yet they are not homologous. The term analogy is often used to express functional correspondence of this kind. Often organs which perform totally different functions are yet perfectly homologous. Thus the wing of a bat or the wing of a bird, both of which are subservient to flight, are homologous with the upper limb of man, the office of which is the different one of prehension. In the construction of vertebrates and certain other animal groups a series of similar parts are repeated along a longitudinal axis, one after the other. Thus the series of vertebrae which build up the backbone, the series of ribs which gird round either side of the chest, the series of intercostal muscles which fill up the intervals between the ribs, the series of nerves which arise from the brain and spinal cord, are all examples of this. An animal exhibiting such a condition of parts is said to present the segmental type of organisation, and in the early stages of development this segmentation is much more strongly marked, and is to be seen in parts which INTIUJDUCTION. 3 siihsoqueiitly lose all trace of such a subdivisiou. The parts thus repeated are said to be serially homologous. But there are other instances of serial homology besides those which are manifestly produced by segmentation. The upper limb is serially homologous with the lower limb : each is composed of parts which, to a large extent, are repeated in the other, and the correct adjustment of this comparison between the several parts of the upper and lower limbs constitutes one of the most difficult and yet interesting problems of morphology. Homoplasy is a term which has been introduced to express a form of corre- spondence between organs in different animals which cannot be included under the term homology. Two animal groups, which originally have sprung from the same stem-form, may independently develop a similar structural character which is altogether absent in the ancestor common to both. Thus the common ancestor of man and the carnivora in all probability possessed a smooth brain, and yet the human brain and the carnivore brain are both richly convoluted. Not only this, but certain anatomists seek to reconcile the convolutionary pattern of the one with the convolutionary pattern of the other. What correspondence there is does not, in every instance, constitute a case of homology, because there is not in every case a community of origin. Correspondence of this kind is included under the term " homoplasy." Another example is afforded by the heart of the mammal and that of the bird. In both of these groups the ventricular portion of the heart consists of a right and a left chamber, and yet the ventricular septum in the one is not homologous with the corresponding septum in the other, because the common ancestor from which both have sprung possessed a heart with a single ventricular cavity, and the double-chambered condition has been a subsequent and independent development in the two groups. Systematic Anatomy. — The human body is composed of a combination of several systems of organs, and the several parts of each system not only present a certain similarity in structure, but also fulfil special functions. Thus we have — • 1. The skeletal system, composed of the bones and certain cartilaginous and membranous parts associated with them, the study of which is known as osteology. 2. The articulatory system, which includes the joints or articulations, the study of which is termed arthrology. 3. The muscular system, comprising the muscles, the study of which constitutes myology. 4. The nervous system, in which are included the brain, the spinal cord, the spinal and cranial ganglia, the sympathetic ganglia, and the various nerves proceeding from and entering these. The study of these parts is expressed by the term neurology. In this system the organs of sense may also be included. 5. The vascular and lymphatic system, including the heart, blood-vessels, the lymphatic vessels, and the lymphatic glands. Angeiology is the term applied to the study of this system. 6. The resjdralory system, in which we place the lungs, windpipe, and larynx. 7. The digestive system, which consists of the alimentary canal and its associated glands, and parts such as the tongue, teeth, liver, pancreas, etc. 8. The urogenital system, composed of the urinary organs and the reprcxluctive organs — tlie latter differing in the two sexes. The term splanchnology denotes the study of the organs includcul in the respiratory, digestive, and genito-urinary systems. 9. The integumentary system, consisting of the skin, nails, hair, etc. These numerous organs which form the various systems are themselves built up of tissues, the ultimat(j tdenients of which can only Ijc studied by the aid of the 4 TEXT-BOOK OF ANATOMY. microscope. The study of these elements and the manner in which they are grouped together to form the various tissues of the body forms an important branch of anatomy, which is termed histology. The structure of the human body may be studied in two different ways. The several parts may be considered with reference to their relative positions, either as they are met with in the course of an ordinary dissection, or as they are seen on the surface of a section through the body. This is the to2wgra2')Jiical method. On the other hand, the several systems of organs may be treated separately and in sequence. This constitutes the systematic method, and it is the plan which is adhered to in this treatise. Descriptive Terms. — Anatomy is a descriptive science founded on observation, and in order that precision and accuracy may be attained it is absolutely necessary that we should be provided with a series of well-defined descriptive terms. It must be clearly understood that all descriptions are framed on the supposition that the body is in the erect position, with the arms by the side, and the hands held so that Pig. 1. Horizontal Section through the Trunk at the Level op the First Lumbar Vertebra. the palms look forwards and the thumbs outwards. An imaginary plane of section, passing longitudinally through the body so as to divide it accurately into a right and left half, is called the mesial plane, Fig. 1 (M.P.). When the right and left halves of the body are studied it will be found that both are to a large extent formed of similar parts. The right and left limbs are alike ; the right and left halves of the brain are the same ; there are a right and left kidney and a right and left lung, and so on. So far the organs are said to be symmetrically arranged. But still a large amount of asymmetry may be observed. Thus the chief bulk of the liver lies to the right side of the mesial plane, and the spleen is an organ which belongs wholly to the left half of the body. Indeed, it is well to state that perfect symmetry never does exist. There always will be, and always must be, a certain want of balance between symmetrically placed parts of the body. Thus the right upper limb is, as a rule, constructed upon a heavier and more massive plan than the left, and even in those organs where the symmetry appears most perfect, as for instance the brain and spinal cord, it only requires a closer study to reveal many points of difference between the right and left halves. The line on the front of the body along which the mesial plane reaches the surface is INTRODUCTION. 5 termed the anterior median line ; whilst the corresponding line Ijehind is called tlie posterior median line. It is convenient to employ other terms to indicate other imaginary planes of section through the hody. The term sagittal is, therefore, used to denote any plane which cuts through the body along a path which is parallel to the mesial plane (S S') ; and the term coronal or frontal is given to any vertical plane which passes through the body in a path which cuts the mesial plane at right angles (C C). The term horizontal as applied to a plane of section requires no explanation. Any structure which lies nearer to the mesial plane than another is said to be internal or mesial to it ; and any structure placed further from the mesial plane than another is said to lie external or lateral to it. Thus in Fig. 1, A is external to B ; whilst B is internal to A. The terms anterior and ventral are synonymous, and are used to indicate a structure (D) which lies nearer to the front or ventral surface of the body than another structure (E) which is placed nearer to the back or dorsal surface of the body, and which is thus said to be posterior or dorsal. In some respects it would be well to discard the terms " anterior " and " posterior " in favour of " ventral " and " dorsal," seeing that the former are only applicable to man in the erect attitude, and cannot be applied to an animal in the prone or quadrupedal position. They have, however, become so deeply ingrained into the descriptive language of the human anatomist that it would hardly be advisable at the present moment to adopt this course. A similar objection may be raised to the terms superior and inferior, which are employed to indicate the relative levels at which two structures lie with reference to the upper and lower ends of the body. The equivalent terms of cephalic and preaxial are, therefore, frequently used in place of "superior," and caudal and postaxial in place of " inferior." The terms proximal and distal should only be applied in the description of the limbs. They denote relative nearness to or distance from the trunk. Thus the hand is distal to the fore-arm, whilst the upper arm or brachium is proximal to the fore-arm. GENERAL EMBRYOLOGY. By Alfeed H. Young and Arthur Eobixson. Although the tissues and organs of the body when fully formed differ greatly not only in respect of their functional characteristics, hut also with regard to their structural features, they are developed from cell elements so similar in appear- ance at first that they cannot be distinguished from one another. They are all the offspring of parent cells — the female cell or ovum, and the male cell or spermatozoon. Developmental processes apparently take place in the female cell alone, but they cannot occur unless the essential elements of a sperm or male cell previously unite with it. Like all animal cells, the ovum is a mass of protoplasm {cytoplasm) containing a nucleus. In many cells the cytoplasm or cell body is itself enclosed by an ex- ternal investing membrane, the cell wall, and such a membrane is present in the ovum. Speaking generally, animal cells are minute structures, those of the human body rarely attaining a diameter of more than about '083 mm., but they vary somewhat in size, they assume different forms, and they acquire characteristic peculiarities associated with their positions and functions ; thus, whilst the majority of the constituent cells of an individual form the various tissues and organs of the body, others are reproductive or germinal cells. Ova are simply specialised cells modified and adapted for the purpose of repro- duction and the continuance of the species. They are enclosed, and partially or entirely matured, in the ovaries, or female generative glands, in the cell-lined spaces know^n as Graafian follicles. When an ovum has reached a certain stage of development it is discharged from the ovary, and passing along the oviduct or Fallopian tube it eventually reaches the cavity of the uterus. Though mature and capable of being fertilised it may not be impregnated, in which case it does not remain in the uterus, but is cast out from that organ. If, however, it becomes fertilised, by union with the male germinal element, it is retained in the uterus, and develops into an embryo which possesses all the characteristic features of the species to which it belongs and most of the special j)eculiarities of its parents. When the embryo, or the foetus as it is termed after it has assumed definite form, is capable of independent existence, its intrauterine life terminates, and it is separated from the rest of the ovum and is born. The development of the individual, however, is not complete, nor does it become complete until the new being reaches the adult condition. The term embryology is sometimes used to include the consideration ol' all the developmental changes and j)rocesses which take place in the ovum from the begin- ning up to the final adult stage. It is more convenient, however, to restrict its a])plication to the study of those changes which take place during the development and growth of the organism before the ffjctus is separated i'rom the rest of the ovum, or, in other words, during its intrauterine existence. r.riefiy epitomised, the sequence of changes is as follows : — Inqn-eguatioji of the mature ovum is followed by segmentation or cleavage. By a series of successive divisions the egg-cell is divided into two, four, eight, and ultimately into a large 7 8 GENERAL EMBEYOLOGY. number of cells, and so is transformed into a multicellular mass, the morula. The majority of the " segmentatinn masses " or cells, or blastomeres, as they are termed, are differentiated into tissue elements, but a certain number retain the characters of the original germ-cells and become ova or sperm-cells, which form the "points of departure " of succeeding generations. Every germ-cell is derived, therefore, " by a continuous and unbroken series of cell-divisions " which have extended through the past from the most primitive ancestor, and it forms a point from which, under ordinary circumstances, all future generations will commence. It is in this sense that the changes through which a living being passes in the course of its life " may, in their completest form, be considered as constituting a morpho- logical cycle, beginning with the ovum and ending with the ovum again." To follow these changes it is necessary that the characters and capabilities of the constructive elements should be clearly understood. The animal cell, which plays an all-important part in the life-history of the individual, and the modified germ-cells must be carefully studied, and as far as possible the exact nature of their constituent parts ascertained. The phenomena of impregnation and segmentation, and the subsequent develop- mental processes and morphological changes which result in the formation of the embryo, and, finally, the arrangements for the nutrition and protection of the ovum during its intrauterine existence, will then he considered. THE ANIMAL CELL. Cells are the structural units of the body. Each cell has an individual life- history within the tissue or organ to which it belongs, it is produced by a pre- existing cell, it develops and grows, is modified by circumstances, reproduces other cells similar to itself, or it dies. A cell possesses a body and a nucleus. An external investing membrane or cell wall may or may not be differentiated. The cell body consists of proto- Nucieoiu', - „ ^ ^ ^ spongiopiasm plasui — an Unstable, highly complex (cyto-reticuium) Qrgauic substaucc, the constitution of which is approximately repre- sented by the formula C^QH^ogN^gOcf,. It is colourless, semi-fluid, viscous, insoluble in water, capable of osmosis, and it is contractile and irritable. In the living condition it always contains a certain amount of water and various inorganic matters. It is to be observed, how- ever, that there are many varieties Nucleai ^- membrane — ' Hyaloplasm Attraction sphere Centrosome Fig. 2. — Diagram op an Animal Cell. of protoplasm, differing somewhat in nature and qualities, The protoplasm of the cell body is called cytoplasm. Under low powers of the microscope it is homogeneous or slightly granular, but with higher magnification, and especially after the application of staining agents, it is possible to distinguish — (1) A highly refractile, elastic, and extensile netv^ork — the cy to -reticulum or spongiopiasm — the meshes of which are filled with (2) A clear, semi-fluid substance — the cytolymph or hyaloplasm. The fibres of the reticulum present some few minute rounded bodies of doubtful nature, which are termed microsomes. The nucleus is a spherical vesicle embedded in the cell body. It is surrounded by a distinct nuclear membrane, and usually contains nucleoli. It consists of modified protoplasm, which is termed karyoplasm, the precise rela- tion of which to the cytoplasm is not clear. Structurally it resembles cytoplasm in that it presents a fine reticulum, the fibres of which seem to be continuous with the cyto-reticulum through the nuclear membrane, whilst its meshes are occupied by nuclear juice. THE ANJMAL CELL. 9 The reticulum forms a fine network composed of linin fibres (acliromatic substance). There is also a coarser network, more readily stainable, consisting of chromatin, granular portions of which may also be embedded in the linin. Instead of forming a coarse network the chromatin may be arranged in the form of a con- voluted cord, or as a number of separate filaments, and in certain cases it constitutes a series of loops from which secondary branches are projected, the apices of the loops being grouped together at one pole of the nucleus round a clear area known as the " polar field." The nuclear membrane consists of both chromatin and linin. Nucleoli are of two kinds, true and false. A true nucleolus is a small, retractile particle, of spherical outline, embedded in the reticulum. It stains deeply, and is said to consist of a special modification of the karyoplasm which is called pyrenin. False nucleoli are simply the nodes of the chromatin reticulum. The nucleus is capable of motion ; it has been seen to alter its shape in the living cell, and it undoubtedly plays an active part in the process of cell reproduction. In addition to the nucleus many cells contain one or more small rounded bodies called centrosomes. The centrosomes are modified portions of the protoplasm, and they lie, as a rule, in a modified, clear-looking area of the cytoplasm, which is known as the attraction sphere. This is generally situated close to the nucleus, and from its surface a number of fine radiating lines project into the adjacent cytoplasm. Centrosomes become very evident when reproduction commences, but are not so distinct at other times. The attraction sphere also becomes more evident when cell-division commences, and the radii which project from it, as well as the contained centrosome, appear to play important parts in the reproductive process. Reproduction of Cells. — Cell division or reproduction may take place either — 1. By direct division — amitosis ; 2. By indirect division — mitosis or karyokinesis. In the amitotic or direct form of division the nucleus, and then the cell body, are equatorially constricted, the constrictions deepen until both are completely divided, and so two daughter cells are produced. Apparently the attraction sphere Fio. .3. — Cell Division. Successive stages of mitosis or karyokinesis (diagrammatic, modified I'mni liiiiiicffny). A, B, C, D, iind K illustrate, till; plieiioiiiciia of tlin piopliasu ; F tliose of the metai)lia.sc'. ; (J and I! tliosu of tlic aiuijiliase ; .J, K, aud I> tlio.so of till! t(',lo|)lia.sc. and centrosome Jjlay some part in this ])rocess, Init wbetlior thciir innnence is initiative or directive is unknown. Mitosis, the process of indirect division, is by r;i,r the iiiosl, coninidn nioih; of 10 GENEEAL EMBEYOLOGY. cell-division. It is a complex process, and the phenomena observable during its progress are classified into four groups : (1) the pro])hase, (2) the metaphase, (3) the anaphase, and (4) the telophase. The phenomena of the p)i'ophase commence with the division of the centrosome and attraction spliere into two parts which travel to opposite poles of the nucleus. At tlie same time the reticulum of the nucleus disappears, and in its place a con- voluted cord of chromatin, tlie skein or spirem, is formed (Fig. 3, A, B, and C) ; this is afterwards broken up into a number of segments which may be mere rods, but which more frequently have the form of V-shaped loops (Fig. 3, D). The nucleoli disappear, and some of the filaments which radiate I'rom the newly-formed atti'action spheres seem to penetrate the nuclear membrane at the poles of the nucleus. The nuclear membrane subsequently disappears, and the filaments passing from the attraction spheres into the nucleus form two cones, the bases of which meet at the equator of the nucleus, where they fuse together, forming an achromatic spindle which extends between the two attraction spheres (Fig. 3, E). The loops, or rods, of chromatin are gradually grouped at the equator of the spindle, each rod, or chromosome, being apparently connected with one of the achromatic fibrils ; and the prophase is completed. In the metaphase each chromosome is split longitudinally into two halves — daughter chromosomes — which separate from one another ; the separation com- mences at the apex of each V-shaped chromosome, which appears to be attached to an achromatic fibril (Fig. 3, F). In the anap)hase the daughter chromosomes pass to the opposite poles of the spindle. It is suggested that this movement is brought about by the contraction of the spindle fibrils, but this is doubtful, though it is noteworthy that in some cases fine achromatic fibrils connecting the separated daughter chromosomes are present (Fig. 3, G and H). Slightly before, or simultaneously with, the completion of the anaphase the cell body is equatorially constricted. During the telophase the constriction deepens and the cell is divided into two daughter cells. Whilst this division is taking place the daughter chromosomes, which are grouped in the neighbourhood of each attraction sphere at opposite ends of the spindle, unite into a convoluted cord, round which a nuclear membrane is formed, whilst the cord is converted into a reticulum, and nucleoli appear (Fig. 3, J, K, L). Therefore when the separation of the daughter cells is completed, at the end of the telophase, each possesses all the characteristic features of the mother cell. Reproductive Cells. — The germinal elements, the union of which is essential to the formation of a new being, are the ovum or female element, and the sperma- tozoon or male element. THE OVUM. Structurally an ovum presents all the characteristic features of a typical cell. It is peculiar because of the large size of the nucleus and nucleolus and in the possession of two investing membranes, an inner one, the vitelline membrane, which corresponds to the cell wall, and an outer one, the oolemma or zona pellucida. Moreover, the nucleus always occupies an excentric position in the cytoplasm, and the cell body contains nutritive material in the form of yolk granules. The constituent parts of an ovum have received distinctive names, however ; thus the cell body is known as the yolk or vitellus, the nucleus is termed the germinal vesicle, and the true nucleolus is called the germinal spot. Vitellus or Yolk. — The body of the ovum, consisting as in an ordinary cell of cytoplasm resolvable into reticulum and cytolymph, contains also numerous granules of small but varying size called yolk granules. These are highly retractile, fatty, and albuminoid bodies containing phosphorus and mineral salts ; collectively they constitute the deutoplasm or nutritive yolk, in contradistinction to the cytoplasm or formative yolk. Xutritive or food yolk plays an important part in development. In some animals it is the only means of support for the embryo in the early stages of development ; in most mammals, on the other hand, the embryo is supplied almost from the first with food not from the egg itself, but directly from the motb.er THE OVUM. 11 through the placenta. The amount of deutoplasm present in the ova ol' different animals therefore varies greatly. Ova in which there is no deutoplasm are spoken of as alecithal. Such ova, if they exist, are very rare ; most of those usually classed under this head undoubtedly contain a certain amount of deutoplasm granules scattered throughout the cyto- plasm, and are better described by the term oligolecithal. The size of an ovum is determined by the amount of food yolk present, and all oligolecithal ova are small; the human ovum, which may be taken as a type of the class, is about '2 mm., or -j-i^th of an inch in diameter. As the deutoplasm is increased in amount the ovum is increased in size. The deutoplasm also tends to accumulate in certain situations ; if the accumulation is at one extremity of the cell the ovum is described as telolecithal ; such ova are naturally divisible into two areas or poles, a cytoplasmic or formative pole, and a deutoplasmic or nutritive pole. In eutelolecithal ova the deutoplasn] almost entirely displaces the cytoplasm from one pole, as in the egg of the fowl, in which the cytoplasm is represented by a disc spread over one pole of a large deutoplasmic mass. In many of the arthropoda the deutoplasm accumulates at the centre of the ovum, which is there- fore termed centrolecithal. The germinal vesicle or nucleus of the human ovum is about '05 mm. or --o~oth of an inch in diameter, i.e. \ the diameter of the whole ovum. It lies excentrically in the yolk, and has the usual characters of a cell nucleus, i.e. it possesses a nuclear membrane within which is the karyoplasm, divisible into reticulum or nucleoplasm, and nuclear juice. The nucleoplasm consists of chromatin and achromatic fibres (linin), and the nuclear juice contains one or more spherical and highly refractile true nucleoli or germinal spots ; the nodes of the reticulum constitute false nucleoli. In addition to the nucleus, the vitelhis, at certain periods, also contains a structure known as the vitelline body, body of Balbiani, or accessory nucleus. It is readily seen in young OA'a lying near the nucleus. It contains one or more centrosomes, and probably represents an attraction sphere. Vitelline Membrane. — - The vitelline membrane is simply the peripheral portion of the vitellus, modified and transformed into a fine structureless envelope which covers the outer surface of the yolk. It is usually closely applied to the inner aspect of the outer membrane, the zona pellucida, and is best seen in the dead ovum and after treatment by reagents. It is therefore thought by some to be merely a condensation of the outer part of the vitellus produced by the action of the reagents. There is evidence, however, to show that it is present in the normal living ovum. Zona Pellucida or Oolemma. — This membrane is thick, tough, and refractile. It serves as a protective covering f(jr the ovum, and persists for a considerable liuK; after fertilisati(m, only disappearing when the ovum becomes attached to the uterus. It is perforated by numerous fine canals, which give to the broad clear . membrane a finely striated aytpearance, from which circumstance it has been called i,li(! "zona striata." The zona ]Killucida is not formed by the ovum, but is secreted i)y th(j cells of tin; Graafian folli(;le in which tin; ovum lies; it is consequently r(!gurded as a secondary membrane, and is altogether different irom the vitelline Fig. 4. — The Ovum and its Coverinc!S (Diagrammatic). The corona radiata, which completely surrounds the ovum, is ouly represented in the lower part of the figure. 1. Corona radiata. 5. Vitellus or Yolk. 2. Granular layer. ti. Germinal vesicle (nucleus). 3. Vitelline membrane. 7. Germinal spot (nncleolus). 4. Zona pellucida (oolemma). 8. Nuclear membrane. 12 GENEEAL EMBEYOLOGY. membrane. The perforatious in the zona serve for the passage of nutritive material to the ovum. When the ovum leaves the Graafian follicle it is surrounded by several layers of cells, the innermost of vi^hich are columnar. They are derived from the cells of the follicle, and collectively constitute the corona radiata ; the cells gradually diminish in size, and ultimately disappear. Their function is unknown, but between them and the zona pellucida there is a layer of granular matter, proljably formed by the cells of the corona radiata, which rapidly swells up when the ovum is liberated from the follicle, and forms a gelatinous elastic layer called the albumen ; this increases in thickness as the ovum passes along the oviduct, and persists for some time after it enters the uterine cavity. The function of the albumen has not been definitely ascertained ; it may act merely as a protective covering against undue pressure, possibly it may be nutritive, whilst in the dog it apparently helps to fix the ovum to the wall of the uterus. It has not been found in all mammalian ova, and it has not been seen round the human ovum ; still it may be present, for human ova at the stage when it might be expected to develop have not yet been observed. Special Characters of the Ovum. — The ovum as it lies in the Graafian follicle presents no obvious structural modifications when compared with an ordinary animal cell, but undoubtedly differs greatly in its capabilities and life history. Unlike an ordinary cell, it has no inherent power of division into two equal parts, but before it is capable of fertilisation it twice undergoes unequal division during the period of ripening or maturation ; again, its history is different from that of ordinary tissue cells, though it corresponds closely with that of the male germinal elements or spermatozoa. Maturation of the Ovum. — As it lies in the Graafian follicle before nuitura- D E P Fio. 5. — The Maturation of the Ovum : Extrusion of the " Polar Bodies " (Diagrammatic). A, An ovum at the commencement of the process ; B, After the formation of the spindle. The chromosomes are gathered at the equator of the spindle. C, One apex of the spindle has projected into a bud on the surface, and half of the divided dyads have pas.sed to each pole ; D, The se]iaration of the first polar hody ; E, The commencement of the second polar body ; F, The completion of the second polar body. tion commences, the ovum is, strictly speaking, an oocyte of the first order, derived by a process of cell -division and growth from a primitive germinal cell which became embedded in the ovary. The more direct descendants of this primitive germ cell are called oogonia, and from them oocytes of the first order are developed. THE OVUM. 13 The oocyte of the first order is at first small, but as the Graafian follicle in which it lies grows the oocyte also increases in size, and either before or immediately after its extrusion from the follicle it undergoes the changes which constitute maturation or preparation for fertilisation ; in other words, it divides twice into unequal parts by a modified process of mitotic division. It is a well-known fact that all animal cells contain in their nuclei a definite number of chromosomes, the number varying in different groups of animals, but being constant in any given species, and both the primitive germ cells and the oogonia descended from them contain the same number of chromosomes as ordinary tissue cells. But when the oocyte of the first order begins to prepare for the first division during the process of maturation, it is seen that the number of chromosomes it contains is only half that of the ordinary tissue cells. Moreover, the chromosomes are not slender V-shaped loops, but short, thick rods in rings or groups of four granules, and if they are rod-like they do not lie in the prophase with their long axes at an angle with the achromatic spindle, as in ordinary mitosis, but parallel with the filaments of the spindle. During the metaphase the chromosomes do not split longitudinally as in ordinary mitosis, but transversely. This modifica- Oocyte of 1st order Oocyte of 2iid order l«t polar body Descendant of 1st polar body Mature ovum Fig. 6. — Diagram illustrating the Maturation of the Ovum. It must be remembered that iu some cases only one polar body is formed, and in others the first polar body does not divide into two parts. tion of ordinary or homotype mitosis is known as heterotype mitosis ; it takes place not only in oocytes of the first order at the commencement of maturation, but also in the cells of malignant tumours, and its exact signification is not understood. If we imagine that the nuclei of the tissue cells of the animal with which we are dealing contain six chromosomes each, then at the commencement of the first maturation division of the oocyte only three chromosomes will be seen on the spindle which forms. The spindle at its appearance is parallel with one surface of the ovum, but it gradually rotates till it stands at right angles to the surface upon which it impinges, pushing a small part of the cytoplasm before it in the form of a small bud (Fig. 5, B and C). In the anaphase the chromosomes divide transversely, and three half chromosomes pass to the inner end of the spindle and three to the outer end, that is, into the bud-like projection. When the teloi)haso is compk;ted the oocyte of the first order is divided into a small ]);i,rt, the first polar body, and a larger part, th(! occyte of the second order, and each contains three f:hroinoHomcs fKig. 5, \)). Almost inimediHtely the hoterotype division is followed in the oocyte of the second order by a homotype division, no resting stage interven- ing. A n(!W spindle appears, the throe {;hromosomes radiate from its equator in the usual way, and tliey (h'vidc hjngitudinally. Three of the daughter 14 GENEEAL EMBEYOLOGY. chromosomes pass to the inner end of the spindle and three to the outer, which projects into a second polar bud. Division u(av occurs, and the oocyte of the second order divides into the mature ovum and the second polar body, each of which contains three chromosomes in its nucleus (Fig. 5, E and ¥). Thus at the end of maturation there lie within the zona pellucida the mature ovum and two polar bodies, and the nucleus of the mature ovum, or female pronucleus as it is called, contains half the number of chromosomes which were present in the primitive germ cell.^ THE SPEEMATOZOON. Spermatozoa are modified cells produced in the testicles or male generative glands. They are formed from the spermatogonia or sperm mother cells wliich are derived from the primitive germ cells of the testicle, and which ultimately produce the spermatocytes of the first order (Fig. 7), the latter constituting the immediate point of departure in the production of a spermatozoon, and corresponding therefore with the ovum or oocyte of the first order immediately before its maturation commences. The daughter cells of the spermatocytes of the first order are spermatocytes of the second order, and their descendants the spermatids are the granddaugliter cells of the spermatocytes of the first order. The spermatids become closely associated with special sustentacular or nurse cells, and during this association are converted into spermatozoa. Each spermatid is a cell possessing a cell body, a nucleus, two centrosomes, and a structure known as an idiosome. The latter is believed by some authorities to Spermatocyte of 1st order Spermatid Spermatid Spermatid Spermati Fig. 7. — Diagram illustratinc; the Process of Cell-Division resulting in the Formation of Spermatids which are afterwards modified into Spermatozoa. represent the attraction sphere, whilst others look upon it as a special modification of the protoplasm only established during the process of mitosis, and distinct from the attraction sphere. However this may be, the nucleus of the spermatid becomes the liead of the spermatozoon (Fig. 9). The idiosome forms the head cap, a tail filament grows out from the region of the centrosomes, and the body of the spermatid forms part if not all of the neck, body, tail, and end piece of the spermatozoon. The centrosomes become embedded in the neck, one at its cephalic and the other at its caudal end. The axial filament is closely connected with the posterior centrosome, which has possibly united with the cytoplasm in its formation. ^ In the mouse, and probably also in some other mammals, only one polar body is formed in mnny cases. THE SPERMATOZOON. 15 There is no doubt that the mature ovum and the spermatozoon, so far as their development is concerned, are very similar. Each is derived from a primitive germ cell, which becomes embedded in the generative gland. From the primitive germ cell in the one case oogonia are formed and in the other spermatogonia, and from the oogonia and spermatogonia respectively oocytes and spermatocytes of the first order are derived. The oocyte of the first order divides by heterotype mitosis into an oocyte of the second order and the first polar body, and the spermatocyte of the first order divides, also by heterotype mitosis, into two spermatocytes of the second order. By a second and homotype mitosis the oocyte of the second order divides into a mature ovum and a second polar body, and the spermatocyte of the second order divides into two spermatids. Thus the final result of the division of the spermatocyte of the first order is the formation of four granddaughter cells or spermatids, each of which contains half the number of chromatic particles present in the primitive germ cell, whilst the final result of the division of the oocyte of the first order is the production in some cases of only three granddaughter cells, the mature ovum and two polar bodies, but in many cases the first polar body divides in the homotype manner simultaneously with the division of the oocyte of the second order, and thus the final result of the division of the oocyte is four granddaughter cells, the mature ovum and three polar bodies, each containing half the number of chromosomes present in the primitive germ cell. There are, however, two differences of importance between the male and the female elements. The final result of the division of the female element is one mature ovum immediately capable of fertilisation and further develop- ment, and two or three polar bodies which are incapable of further development and which ultimately disappear. On the other hand, the result of the division of the male element is the production of four equal parts, the spermatids, each of which undergoes further modi- fication, and is transformed into a spermatozoon capable of fertil- ising a mature ovum. A spermatozoon, like an ovum, is a nucleated mass of cytoplasm, but it presents strik- ing modifications in structure. It is very small, and possesses a head, a neck, a body, a tail, and ^). In addition it is provided with covers more than the anterior half This cap is modified over the apex Head Body Neck Body. — End piece- iHead cap Ant. centrosome Post, centrosome Axial fibre Spiral sheath Mitochondrial sheath Terminal disc Fi( 8. — Human Spermatozoa (after Retzius). \, Side view ; B, Front view. End piece . Sheath of axial fibre an end piece (Fig. a head cap which of tlie head fFig. 9) of the head into a sharp cutting edge, by means of which the spermatozoon, driven forward by the movement of the tail, pierces its way through the oolemma of the ovum. In the short neck are an anterior and a posterior ' I'ntro.some separated by an intermediate disc, and from the posterior centrosome an axial filament extends through the body and tail, and terminates ])Osteriorly as the end piece. The axial filament is surrounded by a sheath which is thicker in tlie body than in the tail. Outside the Hxial filament, in the body, is a spiral sheath, and this is enclosed pnnctiform siiljstance, the iiiitochondria,! sheath, whicl) rests a,t the lower end the body on an annulus or terininal disc. Ffo. 9. -S'niDCTUHE OK A Human Spkumatozoon (after Meeves). sheath of the )y a sheath of of 16 GENERAL EMBEYOLOGY. A transverse striation of the head, a spiral filament, a spiral sheatli associated with the hody and tail, and a terminal spear connected witii the head have been described by Barclleben and others, but apparently they do not exist normally as parts of the human spermatozoon. The head of the spermatozoon is ovoid and laterally compressed, so that when viewed from the side it appears pointed ; it is about 4"5 /x long, 2-5 fx broad, and 1-5 /x, thick. The body is somewhat longer than the head, and the tail is six times as long as the body, therefore the total length of tlie spermatozoon is about one- fifth of the diameter of the ovum. FERTILISATION OF THE OVUM AND THE RESULTS THAT ENSUE. Fertilisation. — The mature ovum is fertilised by a spermatozoon. The two generative elements meet, and fertilisation takes place as a rule in the upper part of the Fallopian tube. The spermatozoon penetrates the zona pellucida of the ovum, cutting through it by means of the sharp edge of its head cap. At the same time a conical projection, the cone of attraction, appears on the surface of the ovum, within the zona pellucida, directly beneath the point at which the _MP \ c. i-C -SN Fig. 10. — Fertilisation of the Ovum (Diagrammatic). A, The entrance of the spermatozoon and the formation of the cone of attraction ; B, The appearance of the centrosome ; C, The approachment of the male ami female jironuclei ; D, The first segmentation nncleus. C. CA. Centrosome. Cone of attraction. FP. Female pronucleus. MP. Male pronucleus. P. Polar body. SN. Segmentation nucleus. spermatozoon is entering. The head, and probably a portion of the body of the spermatozoon, plunge into the cone of attraction ; the remainder of the body and the tail are cast off and disappear. The portion of the spermatozoon which enters the cytoplasm of the ovum is converted into a nucleus, the male pronucleus, which is accompanied by its attraction sphere and centrosome. When the male pronucleus is distinctly formed the granules of the cytoplasm in its neighbourhood begin to radiate around it, as if under its influence, and the pronucleus itself travels inwards. As the male pronucleus approaches the female pronucleus the latter shows FEETILISATION OF THE OVUM. 17 signs of activity, it undergoes changes of form, and moves to meet the male pronucleus. For a time the two pronuclei lie in juxtaposition, and ultimately they fuse together, forming the first segmentation nucleus. The first segmentation nucleus is accompanied by two centrosomes which lie at its opposite poles, and are the products of the male centrosome which divides as the pronuclei fuse. The fertilised ovum, the product of the fusion of the mature ovum and the spermatozoon, contains in its nucleus, the first segmentation nucleus, the same number of chromosomes as the primitive ovum or the sperm-mother cell, but the chromosomes of the segmentation nucleus are derived partly from a male and partly from a female individual. According to some authorities, both the male and female pronuclei are accompanied by centrosomes, aild at the moment of imion of the pronuclei each centrosome divides ; thus four half-centrosornes are formed, two male and two female. From the foiu' half-centrosomes two new centrosomes are formed by the union of half a male centrosome with half a female centrosome. If this view be correct, each of the two centrosomes which accomj^any the first segmentation nucleus contains both male and female elements. Segmentation.- — Segmentation is the division of the fertilised ovum (oosperm) into a number of cells. These cells are afterwards arranged in layers — the germinal layers or layers of the blastoderm ; ultimately they are differentiated into the tissue elements of the body. A B c Fig. 11. — Segmentation of the Fertilised Ovum in the Rabbit. Formation of blastomeres and morula (Diagrammatic). A, Division into two segments ; B, Division into four segments ; C, Morula ; P, Polar bodies. All the phenomena of segmentation have not been observed in the human ovum, and it is to be understood that the following description is based chiefly upon the conditions met with in rodents, more especially in the rabbit, an animal well adapted for the study of these phenomena. After a period of quiescence, which succeeds the fusion of the male and female pronuclei, a period of activity supervenes, during which repeated divisions of the impregnated ovum result in the production of a solid mass of cells called a morula. The divisions are mitotic, and all the phenomena associated with mitosis are readily observable in properly prepared specimens. The planes which separate the several segments of the divided ovum in its various stages are termed the " planes of segmentation," and in some animals the first plane by which the ovum is divided into the first two daughter cells coincides with the future mid-axial or mesial plane of the body, the descendants of the cell lying to the right of it being developed into the right half of the body, and those of the cell to the left into the left half. There is no proof, however, that this occurs in mammals; all that is definitely known is that the first division separates the ovum into two parts of unequal size but of similar colour and structure. The second plane of segmentation is at right angles to the first, and it separates the two daughter cells into four granddaughter cells, of which, in some cases, two may be larger and two smaller. The subsequent divisions occur irregularly, and they result in the formation of numerous cells (blastomeres) which a])parontly only differ in size in the rabbit, but which also differ in appearance in many mammals. They are mixed together so irregularly that it is impossil)](; to distinguish the 2 18 GENEEAL EMBEYOLOGY. Fig. 12. — Conversion op the Morula to THE Blastula. Formation of blastodermic vesicle and membrane. A, Appearance of segmentation cavity and attachment of inner cell mass to ectoderm at upper pole of ovum ; B-', Extension and flattening of inner cell mass as it occurs in rabbit and some other mammals ; B", Ex- tension of entoderm as it occurs in insec- tivora, monkeys, apes, and man ; C, Com- pletion of bilaminar blastodermic vesicle. BC, Blastodermic cavity ; EC, Ectoderm ; EE, Embryonic ectoderm ; EN, Entoderm ; I, Inner cell mass ; SC, Segmentation cavity ; ZP, Zona pellucida. descendants of one daughter cell from those of the other, and in this, the morula stage, there is frequently no indication of any separation of the cells into layers. In the meantime the polar bodies have disappeared. The next phenomenon of importance is the appearance of a cavity — the segmentation cavity — in the morula ; the ovum assumes a vesicular character, and is now termed a blastula. Simul- taneously with the appearance of the cavity the cells of the morula are arranged in two groups — an outer and an inner. The cells of the outer group form a layer, the primitive' ectoderm or epiblast ; those of the inner group remain massed together and constitute the inner cell mass. The two groups are in contact at one pole of the ovum, and it is in this region that the embryo develops (Fig. 12, A). In the rabbit and in some other mammals the outer cells of the inner mass at the embryonic pole of the ovum blend with the superjacent primitive ectoderm (Eauber's cells) to form the embryonic ectoderm. This is merely a modi- fication of the more general plan, by which the inner cell mass becomes the inner layer of the now vesicular ovum, but the conversion of the inner mass into the entoderm may take place in two different ways. (1) In the rabbit and in many other mam- mals the inner mass gradually flattens out till its cells form a layer at the embryonic pole, and the wall of the vesicular ovum, which is now called a blastodermic vesicle, is partly unilaminar and partly bilaminar (B^, Eig. 12). Gradually, however, the margins of the entodermal layer extend, and ultimately the cavity of the vesicle is surrounded by two complete layers, ectoderm and entoderm. In the hedgehog, in monkeys, apes, and the human subject, a cavity appears in the inner cell mass (B^, Fig. 12), and the cells around it assume a laminar character, constituting the en- toderm, which is separated from the ectoderm, except in the embryonic area, by the original cavity of the blastodermic vesicle. In the case of the hedgehog the cavity in the entoderm expands until the entoderm is forced into contact with the ectoderm, and a condition is attained similar to that met with in the rabbit (C, Fig. 12), but in monkeys, apes, and the human subject the expansion of the entoderm cavity is not so great, and the entoderm does not attain contact with the ectoderm except in the embryonic area. In amphiosus and many of the invertebrata the results of the segmentation are nOt quite the same as in mammals, for at a very early period, without the definite formation of a moridamass, the segmenta- tion cells arrange themselves in alayer round a central THE EMBEYONIC AEEA. 19 cavity and form a complete unilaminar blastoderm. In these cases the bilaminar condition is produced by the invagination of a portion of the wall of the vesicle. The opening at which the invagination occurs is called tlie blastopore. The cavity enclosed Vjy the invaginated cells is the archenteron, or primitive alimentary cavity or gastrula cavity. Except at the blastopore the cavity is surrounded by two layers of cells, an outer the ectoderm, and an inner the entoderm, and the animal at this period of its development is a gastrula. In the mammal the cells which become invaginated in amphioxus to form the entoderm are enclosed in the interior of the morula mass at a very early period of the segmentation, before the vesicular condition is attained, but eventually, as already pointed out, they form a layer inside the ectoderm and they enclose a cavity, the blastodermic cavity, which is homologous with the archenteron or gastrula cavity of the lower forms, but the cavity is closed and the blastopore is not obvious. In amphioxus, however, the blastopore becomes elongated antero-posteriorly, and along the margins a third layer, the mesoderm, grows out between the two primitive layers. In the mammal, on the other hand, after the entoderm and ectoderm are definitely established, a linear streak, called the primitive streak, to which further reference will be made, appears on the surface of the ovum ; this becomes perforated at its anterior end, and from its margins and mesoderm extends outwards ; clearly, therefore, it represents the blastopore of amphioxus though an actual perforation is only present for a short time, and the mammalian ovum at this period may be looked upon as a gastrula. The ectoderm and entoderm together constitute the blastoderm or blastodermic membrane, which is bilaminar, and the vesicle of which they form the wall is no longer spoken of as the blastula, but as the blastodermic vesicle. Structure of the Ectoderm and Entoderm. — The cells of the ectoderm are at first irregular in size and shape, and their outlines are indistinct ; but after a short time the ectoderm cells at one pole of the blastodermic vesicle become cubical or slightly columnar, whilst the remaining cells of the outer layer are flattened and have irregular outlines. The columnar cells form the ectoderm of the embryo, and the flattened cells are util- ised in the formation of nutritive and protec- tive structures- known as the placenta and foetal membranes. The cells of the en- toderm are also, at first, very irregular in shape and size, but after- wards, as they are spread out into a layer, ^ they become more or less rounded, and they anastomose together by filamentous processes. Mesoderm A Fig. 13. — Surface View op the Blastodermic Vesicle. Showing the embryonic area and the commencenient of the mesoderm. Before the appearance of the primitive streak — the embryonic area is circular in form and bilaminar tliroiighout ; B, After the appearance of the primitive streak. The posterior end of the primitive streak shows a crescentic thickening, wliich indicates the commencement of the meso- derm or middle layer of the blastodermic membrane. At a still later period they are transformed into polygonal plates which appear spindle-shaped in section (Fig. 12). Embryonic Area. — When the upper pole of the bilaminar blastodermic vesicle is examined in surface view from above, a dark, somewhat opaque circular area is visible ; this is known as the embryonic area. It is coextensive with the columnar portion of the ectoderm. Very soon after it appears the embryonic area becomes ovoid ; tlie small end of the ovoid area is posterior, that is, it lies in the region which is afterwards converted into tlie posterior part of the embryo. At the hinder end of the ovoid area a still da,rkor ])atcli of triangular form is developed ; this soon becomes crescentic, and is th(! first indication of tliu ])rimitive streak atul of the formation of a third blastodermic layer termed the. mesoderm or mesoblast. The primitive streak consists of thickened ectoderm which is seen in transverse sections projcicting downwards, and resting upon the entoderm in the form of a ridge. 20 GENERAL EMBEYOLOGY. From the sides and the posterior extremity of the ectodermal ridge a lamina of cells projects outwards, and gradually insinuates itself between the ectoderm and the entoderm over the whole area of the vesicle, except in certain regions to be afterwards described. This lamina is the rudiment of the mesoderm. With the formation of the mesoderm the blastodermic membrane becomes trilaminar. Embryonic area -' Nenriil groove Ectoderm Entoderm Embryonic area Primitive groove Ectoderm Fig. 14.— The Upper Pole of the Blastodermic Vesicle. Showing the embryonic area, the primitive streak with the extension of the mesoderm from its sides and posterior end, and the commencement of the neural groove. A, Surface view (diagrammatic) ; B and C, Transverse sections through the blastoderm of the ferret at the stage represented in A and along the lines h and c respectively. The majority of the cells of the mesoderm are derived from those of the primitive streak, but it is said that cells from the entoderm also take part in its formation. Young mesodermal cells are round or ovoid, and some give off numerous processes. In later stages they may assume various shapes, and many closely resemble the cells of the ectoderm or those of the entoderm. As the blastodermic vesicle grows, the embryonic or germinal area becomes pyriform and increases in length, principally in the posterior part of its extent where the primitive streak is situated ; at the same time the streak lengthens and EG ^ >SoP SoMJ Fig. 15. — Transverse Section of a Ferret Embryo. Showing neural groove before the separation of the para.xial from the lateral mesoderm. C. Coelom. GC. Germinal cell. PM. Paraxial mesoderm. SoM. Somatic mesoderm. EC. Ectoderm. N. Notochord. SB. Spongioblast. SoP. Somatopleure. EN. Entoderm. NG. Neural groove. SG. Spinal ganglion. SpP. Splanchnopleure. SpM. Splanchnic mesoderm. becomes more linear. For a short time a groove, the primitive groove, appears on the surface of the streak. It is deepest in front, where in some mammals, includ- ing man, a small transitory perforation is formed, the neurenteric canal. A second broader and shallower groove then appears in the embryonic area immediately in front of the primitive streak ; this is the neural groove, the rudiment of the nervous system. The neural groove, its bounding folds, and the nervous system subsequently developed from them are formed entirely of ectodermal elements, which at first are continuous with those forming the outer layer of the embryo. The posterior eiid of the neural groove embraces the anterior end of the primitive streak and groove, and at this period the neurenteric canal forms a communication THE NEURAL OR MEDULLARY GROOVE. 21 between the interior of t]ie ovum and the bottom of the neural groove, which latter afterwards becomes the closed canal of the central nervous system. In some vertebrates the neurenteric canal persists for a considerable period, and upon the development of the alimentary canal it constitutes a communicating channel between it and the cavity of the neural tube. As the neural groove grows backwards the anterior part of the primitive streak is absorbed, and although the posterior part continues to grow, the primitive streak as a whole diminishes in length ; ultimately the greater part of the primitive streak disappears, but a portion is recognisable for a considerable time extending from the base of the tail, a transitory structure in the human embryo, to the ventral wall of the body. This portion forms the posterior boundary of the primitive alimentary canal ; it remains bilaminar, and is called the cloacal membrane. The primitive streak is of great morphological importance ; recent researches have shown that from it and the cells in its neighbourhood the greater part of the body of the embryo, with the exception of the anterior jjart of the head and heart region, is developed. It possibly represents the mouth of a remote (pre-vertebrate) ancestor, the fused lips of which formed the body of a primitive vertebrate animal. The aperture of this mouth is still represented in lower vertebrates by an opening known as the blastopore. The neurenteric canal is the only represen- tative of the opening in the human subject. The neural or medullary groove is bounded laterally by medullary folds which are continuous in front of the groove, but separate behind where they SoP SpP — a^tiiv^ f*'^, ,e|<3p>^ ,-^ SpP r IMC N PA Fig. 16 — Transverse Section of Ferret Embryo. Showing the closiire of the neural groove, the formation of the neural crest, the outgrowth of the spinal ganglia, the commencement of the separation of the paraxial mesoderm from the lateral plates, and the differentiation of the intermediate cell mass. C. Ccelom. GC. Germinal cell. PA. Primitive aorta. CC. Central canal. IMC. Intermediate cell mass. PS. Mesodermic somite. EC. Ectoderm. N. Notochord. SB. Spongioblast. EN. Entoderm. NC. Neural crest. SC. Spinal cord. SpP. Splanchnopleure. SG. Spinal ganglion. SoM. Somatic mesoderm. SoP. Somatopleure. SpM. Splanchnic mesoderm. embrace the anterior end of the primitive streak. The neural groove increases in length both in front and behind. The backward increase takes place at the expense of the primitive streak, whilst the anterior increase is due to the rapid growth of the anterior part of the embryonic area; at the same time, not because of, though coincident with, an increase of the mesoderm which has grown beneath them, the medullary folds are gradually elevated, and their apices bending inwards unite together over the neural groove, which is thus converted into a tube or canal — the neural tube. The medullary folds unite, in the first place, in the region which afterwards becomes the neck ; and subsequently they unite progressively, forwards and backwards. Along the line of union the neural tube is connected, for a time, with the surface ectoderm by a ridge of cells, the neural crest. The crest soon separates from tiie surface, but it remains connected with the neural tube, and is utilised in the formation of the cranial and spinal nerve ganglia, the sympatiietic ganglia, the carotid and coccygeal bodies, and the medullary parts of the suprarenal bodies, whilst the walls of the neural tube are converted into the nervous and sustentacular tissue elements of the whole of the central nervous system (brain and spinal cord). 22 GENERAL EMBEYOLOGY. B.efore the tube is closed the neural groove is dilated at each end (see Fig. 20). The posterior dilatation is single ; it constitutes the rhomboidal sinus, which under ordinary circumstances soon disappears. Anteriorly, numerous dilatations are distinguishable at first. The exact nunil>er of these dilatations (neuromeres) is said to be eleven. As the tulje closes they resolve themselves into three distinct vesicles termed the primary cerebral vesicles. These constitute the rudiments of the fore- mid- and hind-brains, and their respective ventricular cavities. The remainder of the cavity of the tube becomes the central canal of the spinal cord. After the separation of the neural crest from the surface the mesoderm com- pletely surrounds the whole of the neural tujje, and from it are formed the meml)ranes of the Ijrain and spinal cord and their skeletal environments. The ectodermal cells which form the wall of the primitive neural tube are ill- defined, but they soon differentiate into two sets, spongioblasts and germinal cells. The spongioblasts are the more numerous, they are columnar in form, and all extend from an internal limiting membrane which is developed round the periphery of the central canal to an external limiting membrane which forms the outer limit of the neural tube. There is frequently considerable difficulty in recognising their columnar character, even in the early stages, partly because their nuclei do not all lie at the same level, and partly because they are so closely opposed. The spongioblasts are converted into the sustentacular tissue, or myelospongium, of the brain and spinal cord, but all do not undergo precisely the same transformations. The inner portions of those spongioblasts whose nuclei lie near the central canal retain a columnar form, and cilia grow from their free surfaces into the lumen of the canal; in other words, they are converted into the ciliated epithelium of the central canal, but the outer portions of the same cells are transformed into fibrillar processes which terminate externally by fusing with the external limiting membrane. The remaining spongioblasts entirely lose their columnar form, they become much branched, and their branches interlace with the fibrillar processes of the ciliated epithelial cells, and with similar branches of neighbouring cells, forming the reticular sustentacular tissue or myelospongium ; the external limiting membrane is produced by the close interweaving of the peripheral myelospongial fibrils. The germinal cells are spherical in outline, and contain clear protoplasm and darkly-staining nuclei. They lie between the inner ends of the spongioblasts close to the central canal where, at the fourth or fifth week, they form an^ irregular layer, and it is believed that they very soon give rise to a new generation of cells, the neuroblasts, or young nerve-cells — at all events neuroblasts appear as the germinal cells disappear. Each neuroblast rapidly becomes pyriform by the outgrowth of an axial process or axon, which projects from its outer end towards the peripliery of the tube. Shortly after their formation the neuroblasts migrate outwards, and ultimately those of the cord are arranged in longer or shorter columns in the myelospongium, whilst those of the brain are grouped together in definite areas to form the cerebral nuclei. Each neuroblast as it develops gives off many processes, which vary in length and thickness. The first formed of these is the axial process or axon already referred to. It carries impulses from the cell, gives off lateral branches, and terminates either in association with a special end-organ or by ramifying amidst other nerve-processes or round a nerve-cell of the central or peripheral nervous system. The remaining processes of the neuroblast are called dendrites or protoplasmic processes. They are usually shorter and more branched than the axon, and they carry impulses to the cell. The whole system of cell body, axon, and dendrites into which a neuroblast develops is termed a neuron. Every neuron is probably a separate and distinct entity. Its processes neither anastomose together nor with the processes of other neurons. They lie, however, in close contiguity with either the Ijody or processes of other neurons, or with special end -organs, and it is possible for impulses to pass from one neuron to another although there is no structural continuity between them. Extension of the Mesoderm and Division of the Blastodermic Membrane into Areas. — It has already been pointed out that when the primitive streak first EXTENSION OF THE MESODERM. Proamniotic area Bucco- pharyngeal area Pericardial area appears it consists of a thickened ridge of ectoderm situated in the posterior part of the embryonic area and resting upon the entoderm. The anterior end of the ridge soon fuses with the entoderm beneath it, and from its sides and posterior extremity a lamelliform outgrowth, the mesoderm, is projected between the ectoderm and entoderm. At its commencement the mesoderm is an outgrowth from the primitive streak, but during its subsequent extension it is probably added to by cells proliferated from the entoderm. As it extends the mesoderm forms a semilunar sheet of cells, the concavity of the semilune being turned forwards, whilst the convexity is gradually projected beyond the margins of the embryonic area. The cornua of the semilunar sheet grow forwards on either side of, and at some little distance from the middle line, immediately beneath the medullary folds. Each cornu on reaching the anterior end of the embryonic area bifurcates, and the resulting processes unite with their fellows of the opposite side. At the same time the mesoderm grows from its convex margin, and extends over the rest of the ovum as a continuous sheet. But even when the extension is completed, in the majority of mammals, three areas on the upper aspect of the ovum remain devoid of mesoderm, and consist only of ectoderm and entoderm. The largest of these areas lies directly in front of the embryonic region. In many mammals it is folded Notochordai upwards and backwards in ^^^'^ front of the head of the embryo, when this becomes distinguishable, and it takes part in the formation of one of the protecting fcetal membranes, viz. the amnion ; it is therefore called the proamnion, and the area from which it is developed constitutes the proamniotic area. Probably it is not present in the human blasto- derm, or if it exists it is very transitory. The second of the areas into which the mesoderm does not extend lies in the embryonic region. It is separated from the proamniotic area by a bar of mesoderm in which the pericardial cavity afterwards appears. The anterior part of this second area is situated in front of the neural groove, and as, at a later period, it forms a septum between the primitive mouth and the pharynx, it may be termed the bucco-pharyngeal area. The posterior part of the area forms the floor of the medullary groove, and as the notochord is formed from its entodermal layer we have named it the notochordai area. The third area corresponds to the posterior part of the primitive streak. It extends from the base of the tail towards the umbilicus, forming the cloacal mem- brane, which itself forms the posterior boundary of the primitive alimentary canal. It is eventually perforated by the genito-urinary and anal apertures. Except in the areas just described, tlie blastodermic membrane is trilaminar, and at an early period it is possible to distinguish the regions in which the heart and pericardium, tlKi amnion, and the })lacental and non-placental parts of the chorion are subsequently developed. These regions form fairly well-dclined areas, to the relative positions of which reference may now be made. I'he anterior part of the embryonic area in front of tiie Ijucco-pharyngeal area is the region in whicli the pericardium and heart are developed, and it may therefore be termed tlie pericardial area, "■J'lie blaHtodeniiic membrane iiimicdiately surroimding tlie cmljiyonic area, Fig. 17. — Surface Areas of the Blastoderm. 24 GENEEAL EMBEYOLOGY. including the proamniotic part in those animals in which it exists, is the amniotic area, and this is bounded externally by a band of elevated and thickened ectoderm which indicates the placental area. Tlie latter, together with the blastoderm over the rest of the ovum, forms the chorionic area, which is separable, therefore, into placental and non-placental portions. These areas are further referred to in the description of the folding ofl" of the embryo and the formation of the foetal membranes and placenta. Formation of the Notochord. — The notochord is the primitive skeletal axis of the embryo. When difi'erentiated it forms a rod which intervenes between the ectodermal neural tube and the entoderm of the primitive alimentary canal. It is developed from the entoderm beneath the neural groove in the notochordal area. A linear strip of entoderm thickens and then separates as a solid rod of cells, the continuity of the entodermal layer being restored beneath it. "When it is completed the notochord extends from a point immediately behind the primitive fore-brain, and Embryonic arfia NG Amniotic area Placental area Chorionic area Notochordal area Pericardial area I Primitive streak Amniotic area Placental area Chorionic area Fig. 18. — Sections showing the different Areas of the Blastodermic Vesicle (Diagrammatic). I. Transverse section ; IT. Longitudinal section. EC. Ectoderm. M. Mesoderm. EN. Entoderm. N. Notochordal thickening. NGr. Neural groove. beneath the anterior end of the mid-brain, to the anterior end of the primitive streak, and in later stages, as the skeleton is formed, the notochord can be traced from the post-sphenoid section of the base of the skull, which is situated beneath the mid-brain, to the tip of the coccyx. The separation of the notochord from the entoderm commences in the cervical region, and extends forwards and backwards. The anterior extremity is the last part to be detached, the separation occurring shortly after the perforation and disappear- ance of the bucco-pharyngeal membrane. The cellular notochord develops a cuticular sheath ; it is subsequently surrounded by mesoderm which separates it both from the neural tube and the entoderm, and which is ultimately transformed into the vertebrae and their ligaments, the intervertebral discs, the basi-sphenoid and basi-occipital parts of the skull, and the membranes of the brain and cord. As the surrounding mesoderm is differentiated the notochord becomes nodulated ; the thickened portions are situated in the regions of the intervertebral discs, and the intermediate constricted portions in the regions of the vertebral bodies. The vertebral portions gradually disappear, and the intervertebral parts are converted into a kind of mucoid tissue, the pulp of the intervertebral discs. Formation of the Coelom. — In all animals in which the entoderm and ectoderm FOKMATION OF THE CGELOM. 25 lie in close relation with each other over the whole surface of the ovum, the r I' Pericardial area Fig. 19. — Extension of Mesoderm and Formation of C(elom (Diagrammatic). A. Mesoderm spreading from the sides of the ectodermal primitive streak, and extending between the ectoderm and entoderm. B. Further extension of the mesoderm and appearance of coelomic cleft-like spaces. C. Complete delamination of the mesoderm and formation of ccelom. BC. Blastodermic cavity. C. Coelom. EC. Ectoderm. EN. Entoderm. M. Mesoderm. P. Primitive Streak. SoF. Somatopleure. SpP. Splanchnoplenre. mesoderm as it extends from the primitive streak forms a single layer, and where this exists the blastoderm is _«_ Proamniotic area trilaminar, but in monkeys, apes, and the human subject, as the ectoderm and entoderm are separated from each other by the segmentation cavity (Fig. 12, B-), the mesoderm al- most from its commencement extends in two layers, one the splanchnic on the entoderm, and the other the somatic on the inner surface of the ecto- derm. In other mammals before the extending meso- derm entirely separates the ectoderm from the entoderm, a cavity is formed in it by the union of a series of cleft- like spaces which appear near the margin of the embryonic area and rapidly fuse together, formin^^ the CCaloin or body F'"- 20.— Suhfack View of an Eaui,v Emhhvu (Diagrammatic). cavity. Thus in the majority Sliowlng the neural groove, dilated in tlie head region but still un- of mammalH tlie CfP.lom is closed, and the first protovertebral somites. The n.argius of the , , ,. , cttlomic space are indicated by dotted lines. lormed by trie cleavage ot the mesoderm, but in the higlier forms it represents the segmentation cavity which has become surrounded by two extending layers of mesoderm. In those animals Rhomboidal sinus 26 GENERAL EMBRYOLOGY. in which it appears by cleavage of the mesoderm, it extends towards both poles of the ovum, but in the higher forms only towards the embryonic pole, and in both groups its extension in the eml^ryonic area is arrested before it quite reaches the side of the notochord and the mesial plane of the primitive streak. It extends across the pericardial area, however, and forms the rudiment of the pericardial cavity, which appears as a transverse tubular passage continuous on each side with the general body cavity. The outer or parietal layer of the mesoderm becomes more or less closely attached to the ectoderm, and with it forms the somatopleure, whilst tlie inner or visceral layer is similarly associated with the entoderm to form the splanchnopleure. When the ccelom is fully formed the blastoderm contains two cavities, one, the coelom or body cavity, situated between the two layers of the mesoderm, and the other the cavity of the blastodermic vesicle, usually called the vitelline cavity, which lies inside the entoderm. Mesodermic or Protovertebral Somites. — During the formation of the coelom the undivided mesoderm at each side of the notochord thickens, principally by a dorsal upgrowth which is coincident with the uprising of the ectodermal medullary folds which bound the neural groove. There are thus formed two thickened bars of mesodermal tissue, one on each side of the neural tube, and they together constitute the paraxial mesoderm (Fig. 15) ; the more laterally situated portions of the mesoderm are known as the lateral plates. The paraxial mesoderm is soon divided, except in the head region, by a number of transverse clefts into a series of cubical masses termed the mesodermic somites. These are at first partially, and afterwards more completely separated from the lateral plates by longitudinal grooves. After the longitudinal grooves are formed, the mesodermic somites of each side are connected with the lateral mesoderm by a somewhat contracted strand of cells, the intermediate cell mass (Fig. 16). This strand is represented in lower vertebrates by a series of separate cords of cells, the stalks of the somites, each somite possessing one stalk. The separation of the paraxial mesoderm from the lateral plates and the segmentation of the former into somites extends forwards to the region of the hind brain, where the first protovertebral somite is formed. In front of this the mesoderm, in mammals at least, does not become segmented. The cavity of the coelom may extend into the paraxial mesoderm before it is segmented into protovertebral somites, or it may stop just outside the limits of the paraxial mesoderm. In the former case each somite, when separated from the lateral plate, contains a cavity, and the intermediate cell mass is also hollow for a time. In the latter case the protovertebral somites and the intermediate cell masses are solid ; at a later period, however, a cavity which contains a few spherical cells appears temporarily in each somite. Folding Off of the Embryo from the Blastodermic Vesicle. — Although so many rudiments of the embryo become distinguishable at an early period in its development (the embryonic area, the primitive streak and groove, the neural groove, the notochord, and the protovertebral somites), the body of the embryo does not assume its characteristic form until it becomes raised and folded off from the general surface of the blastodermic vesicle. The main cause of the folding off of the embryo from the surface of the vesicle is the more rapid growth of the embryonic area as contrasted with the slower growth and expansion of the remainder of the wall of the vesicle ; and the moulding of the increasing embryonic area into the form of the embryo is due to differences in the rate of growth of the various parts of the area itself. The manner in which the area is folded, and the changes in the relative positions of its various parts which necessarily result, will be easily understood by reference to Figs. 21 and 27. The embryonic area at an early period increases rapidly, especially in length. Its margins, however, appear to remain comparatively fixed, and hence as the area increases it must fold upon itself. It becomes more convex externally, and is raised slightly above the general surface, but at the same time it apparently sinks into the interior of the ovum, and the amnion folds close over it. FOLDING OFF OF THE EMBEYO. 27 The antero-posterior growth is greater than the lateral; consequently the folding of the emhryonic area is most marked in front and behind. Anterior and posterior, or cephalic and caudal folds are formed, which indicate the head and tail extremities of the embryo. Similarly, lateral folds define the lateral limits of the body. When the body of the embryo thus becomes folded off it contains a portion of the blastodermic cavity and of the ccelom ; the former is the primitive alimentary canal, and the latter is the rudiment of the pericardial, pleural, and peritoneal cavities. The communication between the pleuro-peritoneal and the extra-em?jryonic SoM _ SoM Early Stages in the Foldixg Off of the Embryo (Diagrammatic). I. Longitudinal section of a developing ovum. The folding off of the embryo has commenced, and the head fold, bending down in front, has invaginated the amniotic area. The tail fold is j^artly formed, and the primitive alimentary canal, closed in front, communicates freely with the yolk-sac by a wide umbilical aperture. II. Transverse section of a developing ovum showing the commencement of the " folding off. " The thickened embryonic area is convex externally, and it already appears to sink below the surface of the ovum. III. Tran.sverse section showing the " folding off " more advanced. The changes seen in II. are more marked, and by apparent constriction at the junction of the embryonic area with the rest of the blastodermic vesicle the embryo is still further nipped off, and distinct lateral folds are formed. The division of the cavity of the blastodermic vesicle into that of the primitive alimentary canal and that of the yolk-sac is shown in all tlie figures. The amniotic area, directed upwards and inwards in II., forms with the placental area the amniotic fold, and in III, tlie amniotic folds of opposite sides are approaching one another over the back of the embryo to enclose the cavity of the amnion. The relative iiositions of the dilierent areas of tlie blastoderm are correspondingly modified. PA. Placental area. SpM. Splanchnic niesodertn. PAC Primitive alimentary canal. YS. Yolk-sac. SoM. Somatic mesoderm. V. Villi. portion of the cadom is obviously iKnnided Ijy the margins of tlie emljryonic area, which constitutes tlie limits of the umbilical orifice. The margins of the embryonic area retain approximately their original positions, and in its further growth the embryo extends beyond them in all directions. AA. Amnion fold. EC. Ectoderm. C. Ccelom. EN. Entoderm. EA. Embryonic area. N. Notochord. 28 GENERAL EMBRYOLOGY. THE EMBRYO. The embryo, now easily distinguishaljle from the rest of the ovum, is ah'eady sufficiently developed to give some indication of the general plan of its organisation, and of the ultimate relation and fate of the three layers of the blastoderm which enter into its constitution. There are as yet no limbs, but the general contour of the head and body are defined. It possesses a notoehord, afterwards replaced by the permanent vertebral column, which constitutes a longitudinal central axis. On the dorsal aspect of the notoehord the neural groove is closing to form the neural canal, or primitive cerebro-spinal nervous system, whilst on its ventral side a portion of the blastodermic cavity is being included as a primitive tubular alimentary canal, which freely communicates with the remainder of the blastodermic cavity now called the cavity of the yolk-sac. The formation of the mesodermic somites has commenced, and this is the first indication of that segmenta- tion which is such a char- acteristic feature in the structure of the vertebrate body. The general relations of the three layers of the blas- toderm remain unaltered. Thus, externally, there is a layer of ectoderm f ormiu g the surface of the body ; inter- nally, a layer of entoderm I i^ Lining the primitive aliment- FiG. 22.— The Relative Positions of the Blastodermic Layers ary canal; and between them IN the Body of the Embryo when the "Folding Off" is is the mesoderm enclosing completed (Diagrammatic). f'kg coelom The surface ectoderm forms the epithelial ele- ments ^ of the skin and its appendages, and of the glands which open on it. Thus the hairs and hair-follicles, the nails, the enamel of the teeth, the epithelium of the sebaceous glands, of the sweat glands, and of the mammary glands are all ectodermal. The epithelium of the conjunctivse and of the lachrymal glands is also derived from ectoderm. The roof of the mouth, the inner surfaces of the cheeks, the nasal passages and their associated cavities, together with the adjacent part of the pharynx and the anterior lobe of the pituitary body, as well as the external auditory canal and the outer layer of the tympanic membrane, are all developed from the surface, and their epithelium, with that of their glands, is ectodermal in origin. The epithelium of the sense organs, except that of taste (the tongue), is derived from ectoderm ; the auditory and olfactory epithelial elements, and those of the lens and cornea, are from surface ectoderm ; whilst the epithelial elements of the retina are from neural ectoderm. The neural ectoderm is removed from the surface to form the neural tube and neural crest, from which the cells and fibres of the whole of the nervous system,"-^ ^ The term "Epithelium" is applied to tissues consisting of cells which are united with oue another by means of a small amount of intercellular substance. The cells constituting epithelium are always arranged in one or more layers ; they cover free surfaces and line the various cavities of the body, including the vascular and lymphatic systems ; they also form the active elements in secretory glands and line their ducts. Epithelium is always non-vascular, and the cells receive their nourishment from blood-vessels which are in their vicinity. Epithelial cells are modified in accordance with the particular functions they are called upon to serve, and they present many variations in shape, size, and structure, e.g. the neuro-epithelial cells of the central nervous system and of the peripheral sense organs differ considerably from the more ordinary epithelial type ; but they are simply more specialised, and therefore more modified. " It seems jiossible that this statement may, before long, require modification, for evidence is being brought forward to show that some portions of the system are developed from peripherally situated ectodermal cells I. Transverse section through the umbilical aperture. II. Similar section in front of or behind the umbilicus. AC. Alimentary canal. EN. Entoderm. N. Notoehord. EC. Ectoderm. M. Mesoderm. SC. Spinal cord. VI. Vitello-intestinal duct and umbilical aperture. THE EMBEYO. 29 both central and peripheral, and the sustentacular tissue of tlie brain and spinal Fig. 23. — Transverse Section of a Ferret Embryo, Showing further differentiation of the mesoderm. SC. Spinal cord. SG. Spinal ganglion. SL. Scleratogenoiis layer of pro- tovertebral somite. SoM. Somatic mesoderm. SoP. Somatoplenre. SpM. Splanchnic mesoderm. cc. Central canal. ML. Muscular layer of mesoder- CL. Cutaneous lamella of proto- mic somite. vertebral somite. N. Notochovd. CO. CcElom. NC. Neural crest. EC. Ectoderm. PA. Primitive aorta. EN. Entoderm. PS. Mesodermic somite. GC. Germinal cell. SB. Spongioblast. SpP. Splanchnopleure. AMC Proto- vertebral- somite Cutaneons lamella Muscle platP Sclerato genons layer cord, are developed. The neural ectoderm also furnishes the epithelial elements of the retinae, the pineal gland, and of the posterior lobe of the pituitary body. It forms a large ^m part of the vitre- ous humour of the eye, and con- tributes to the formation of the carotid, coccy- geal and supra- renal bodies. The entoderm lines the alimen- tary canal and the spaces and glands which open into it, ex- cept the upper parts of the mouth and phar- ynx and the terminal portion of the rectum ; thus the eyji- thelium of the Eustachian tube and tympanic cavity, the trachea, the bronchi, the air- vesicles of the lungs, the gall- bladder, the urinary Ijladder, and ]jart of ihe urethra is entodermal. It Ibrms the e])ithelial constituents of the taste buds or organs of taste, the liver and the pancreas, the Somatopleuie 'splanchnopleuie Fig. 24. — Further Differentiation of the Mesoderm. Transverse section of a rat embryo, showing the transformation of the cells of the scleratogenous layer of a protovertel jral somite and their extension round the notochord and spinal cord. AM. Amnion. AMC. Amnion cavity. C. Cf/doiii. K.C. Ectoderm. N. Notochord. PA. Primitive aorta. PAC. Primitive alimentaiy canal. VD. Vitello-intestinal duct. SC. Spinal cord. SG. Spinal ganglion. SoM. Somatic mesoderm. SpM. Splanchnic mesoderm. 30 GENEEAL EMBEYOLOGY. epithelium lining the vesicles of the thyroid body and the cell nests of the thymus gland. From the mesoderm all the remaining structures which intervene between the surface ectoderm and the entodermal lining of the primitive alimentary tube are formed. Mesodermic Somites and the Lateral Plates. — Each mesodermal somite consists of numerous cells arranged radially round a central cavity — the myeloccele \ this latter, however, quickly disappears. The cells of the somites are gradually grouped into three sets, two to the inner and lower side of the cavity, and one to its upper and outer side. The two groups. on the lower and inner side are an outer, next the ca\dty, the muscle plate, and an inner, the scleratogenous layer. The group on the upper and outer side of the cavity is the sulj-epithelial or cutaneous lamella. Scleratogenous Layer. — The cells of Scleratogenous layer Muscle plate Blood-vessel Spinal cord Ectoderm Mesoderm llw--. Blood- r)^-^ vessels Fig. 25. — Coronal Section op a Eat Embeyo. Showing the relationship of the extending scleratogenous tissue to the sjnnal cord and to the muscle plates. this layer proliferate rapidly and migrate inwards, surrounding the notochord, and passing both be- neath the neural tube and up- wards along its lateral walls to- its dorsal aspect ; they intermingle above and below with the cells of the corresponding layer of the opposite side, and in front and behind with the cells of the sclerato- genous layers of adjacent somites. In this way the neural tube and the notochord are gradually en- veloped by a continuous sheath of mesodermal tissue, which forms the membranous vertebral column. This is perforated at regular in- tervals by the nerve-roots issuing from the spinal cord and brain, and by the vessels of supply to> those structures. From its sub- stance the vertebrae and ligaments, the greater part of the interverte- bral discs, and the investing mem- branes of the brain and cord are afterwards developed. Muscle Plates. — The cells of the muscle plate layer lose their original epithelial-like characters; they elongate antero-posteriorly, become- spindle-shaped and striated, and they give rise to the striped muscles of the body. For a long time the fibres developed from each muscle plate remain localised and quite distinct from the fibres developed from neighbouring segments ; the masses they form are called myotomes. After a time, however, the fibres of neighbouring myotomes are more or less intermingled, and in the adult, except in certain situa- tions, the intermyotomic intervals are no longer recognisable. The main portions of the myotomes are converted into the muscle masses situated in the dorsal part of the body wall, that is, into the erectores spinie and their main subdivisions, and the other muscles which occupy the vertebral grooves. In the lower vertebrates the ventral ends of the myotomes descend in the- somatopleure almost to the mid-ventral line, and are transformed into the muscles of the ventro-lateral walls of the body. A similar descent of the ventral ends of the myotomes into the lateral walls of the body has not been proved in the highest vertebrates. In mammals, including man, the ventral ends of the myotomes only descend for a short distance in the somatopleure, and then all trace of their char- acteristic structure is lost. It is presumed, however, that cells budded off from the- THE EMBEYO. 31 myotomes descend to a lower level, and that they take part in the formation of the ventro-lateral muscles. In lower vertebrates bud-like projections pass from the myotomes in the thoracic and pelvic regions into the limb rudiments, and from these the muscles of the limbs are developed. In the highest vertebrates distinct buds from the myo- tomes have not been observed, but it is said that outgrowths of cells pass from the myotomes into the limb buds, where they proliferate and form the limb-muscles. The occurrence of these outgrowths into the limbs, like the descent of the lower ends of the myotomes into the ventral part of the body- wall, has not been proved in mammals; possibly it occurs, but if not, the ventral and limb-muscles of mammals must be developed from the somatopleural mesoderm. Cutaneous Lamellae of the Mesodermic Somites. — The cells which form Hind -brain Auditory ganglion ,Eudiment of otic vesicle Paraxial mescderm^^ ^ — v~,^ "■ . ^'V^''^°l£ ^^ P^.^^'^ SoM SpMl First cephalic aortic arch. -Transverse Section of a Rat Embryo. Showing the relation of the paraxial mesoderm of the head to the lateral plates, the commencement of the formation of the otic vesicles and hyomandibular clefts, and the relation of the primitive heart to the pericardium and fore-gut. EC. Ectoderm. SoM. Somatic mesoderm. SpM. Splanchnic mesoderm. the outer and dorsal walls of the cavities of the mesodermal somites retain their epithelial-hke characters for a longer period than those of other portions of the somites, and at the borders of the lamellae they pass by gradual transition into the cells of the muscle plates. After a time they undergo histological differentia- tion, and they are utilised in the formation of the subcutaneous tissues and fascise on the dorsal aspect of the body, and outgrowths, which descend with the offsets of the muscle plates, enter into the formation of the ventro-lateral walls of the body. Mesodermic Somites of the Head. — It ]ias already been jwinted out (p. 26) that proto vertebral somites are not recognisable in mammals further forwards than the occipital region ; but, from the evidence obtained by examination of lower vertebrates, it is believed that originally nine somites were present in the cephalic region. From the first, second, and third of these, muscle j)lates form which ai-e developed into the muscles of the eyeballs. If any muscle plates are formed in connexion with the fourth, fifth, and sixth somites they disapjjear, leaving no traces, and the muscles develojjed fi'om tlie remaining cej)halic somites are those of the tongue and those connecting the head with the shoulder girdle. Lateral Plates. — At an (jariy stage, before its separation from the paraxial iijfjsodcriii, each lateral plate is divided into an outer or somatic and an inner or splanchnic layer. 'J'iic somatic layer is concerned with the i'ormatiou of the parietal layers of the pleural and ])critoii(!al uieiultrancs, and with the development 32 GENEEAL EMBEYOLOGY. of the connective tissues, fasciae, and vessels of the ventro-lateral walls of the body ; and in mamnials, apparently, it also gives origin to the ventro-lateral body muscles and the muscles of the limljs. The splanchnic portions of the lateral plates form the fascise, the connective tissues, the smooth muscles of the walls of tlie ahmentary canal, the heart and great blood-vessels, the visceral layers of the pleural and peritoneal membranes, the spleen, and the germinal epithelium, which becomes transformed into the mother cells of the ova and spermatozoa. In the cephalic region, in higher vertebrates, lateral plates are not recognisable, exceyjt so far as they may Ije represented by the walls of the pericardium; but in some lower vertebrates lateral plates can be distinguished, corresponding in number with the cephalic somites, and it has been asserted that the muscles of the face and the muscles of mastication are developed from the lateral plates associated with the second and third cephahc somites. The subject, however, is one which is still obscure, and requires further investigation before any very positive conclusion can be arrived at. Intermediate Cell Mass. — As already mentioned, the lateral plates and the mesodermic somites are connected by the intermediate cell masses, which are intimately associated with the development of the ducts and tubules of the genital and urinary organs in man and other mammals. On each side the mass soon separates from the mesodermic somites, and is transformed by rapid proliferation of its cells into an elongated body, the Wolffian body or primitive kidney, which projects downwards into the dorsal angle of the body cavity. In early stages it extends from the fifth somite of the body, backwards to its posterior end, but is most clearly differentiated in the middle portion. The Wolffian duct and tubules and the Mlillerian duct are developed in connexion with it; after the second month of intrauterine hfe it degenerates, and is replaced by the permanent kidney, which is formed dorsal to its posterior extremity. THE DEVELOPMENT OF THE PEIMITIVE ALIMENTAEY CANAL. When the cephalic, caudal, and lateral folds are established, and the general outline of the embryo is clearly defined, its dorsal and lateral surfaces and its Spinal cord Xotocliord Placental area Rliomboidal^iuus Primitive streak Cloacal membrane Placental area SpM SoM Bucco-pharyngeal membrane Fig. 27. — Diagram of a Developing Ovum, seen in Longitudinal Section. Tlie folding off of the embryo has commenced, and the do^vn■^vard bend of the head fold in front lias invagi- nated the amniotic area. The tail fold is partly formed, and the primitive alimentary canal, closed in front by the bucco-pharyngeal memljrane and behind by the cloacal membrane, is distinguishable ; it communicates freely with the yolk sac by a wide umbilical a]ierture. C. Ccelom. EN. Entoderm. SoM. Somatic mesoderm. EC. Ectoderm. M. Mesoderm. SpM. Splanchnic mesoderm. anterior and posterior extremities are easily recognisable, and, as' the embryo is folded off from the surface of the blastodermic vesicle, a portion of the blastodermic cavity is enclosed within it ; this is the primitive alimentary canal. It is simply an incomplete tubular cavity, situated beneath the notochorcl, which is bounded in THE PRIMITIVE ALIMENTARY CANAL. front by the head fold, beliind by the tail fold, and laterally by the lateral foldB, but is widely open below and continuous with the cavity of the yolk-sac. As the head of the embryo grows more rapidly than any other part, the head fold is more marked than the other folds, and with its formation the pericardial area is bent round until it becomes ventral in position, its original upper and lower surfaces being reversed (Fig. 27). It is owing to this change of relative position that the ventral wall of the alimentary canal is completed in front, and it is obvious that its anterior limit corresponds to the bucco-pharyngeal area of the blastoderm. Tiie part of the blastodermic cavity enclosed in the head fold con- stitutes the fore-gut. The tail fold at this period is small, but it limits the primitive gut behind. The ventral closing of the posterior end of the primitive alimentary canal to form a hind-gut is produced, as in the case of the fore-gut, by bending of the embryonic area. This takes place in the region of the tail fold ; but the posterior part of the em- bryonic area retains for a con- siderable time its original position, and forms a connect- ing stalk, termed the body- stalk, between the embryo and the chorionic area of the blastoderm. Ultimately, how- ever, this terminal section of the embryonic area is re- versed in position, its posterior end being carried forwards till it forms the posterior boundary of the umbilical orifice, and the ventral wall of the hind-gut is thus completed. The rest of the primitive alimentary canal constitutes the mid -gut. It remains for some time in free communica- tion with the cavity of the yolk-sac, and this communica- tion between the alimentary canal and the yolk-sac at a later stage forms a tubular passage, the vitello- intestinal duct. The entoderm forms the lining epithelium of the ali- mentary canal, but this is invested by the splanchnic layer of the mesoderm, which is separated from the somatopleure or body wall by the coelom or body cavity. As the splanchnic mesoderm passes on each side to its continuity with the somatic mesoderm it forms a fold, by whicli the gut is suspended from the under surface of the primitive vertebral column ; this fold is the mesentery. When the diaphragm is formed at a later period it separates tlie thorax from the abdomen, and divides the coelom into pleural and peritoneal portions. The y»rimitive alimentary canal is almost a straight tube, blind at both its extremities, and communicating only with tlie cavity of the yolk-sac. As yet there is no mouth and no anal passage or aperture. 'I'lie simple tuljular canal is divisible into fore-gut, mid-gut, and hind-gut, parts which are conveniently associated developrnentally with definite portions of the fully-formed alimentary canal. Yolk-sac Cloacal meuibraue Body stalk Fig. 28. — Diagram representing the Condition of the Ali- mentary Canal in a Human Embryo about Fifteen Days Old (modified from His). The visceral clefts are formed, and the subdivisions of the fore-gut, together with the rudimeuts of the bronchi and liver, are distinct. GENEEAL EMBEYOLOGY. Thus the fore-gut is converted into the pharynx, cesophagus, stomach, and the greater part of the duodenum ; whilst from the mid-gut and the hind-gut the rest of the small intestine (jejunum and ileum), and the whole length of the large intestine (caecum, colon, and rectum), are formed. There is no sharp limit Ijetween the mid-gut and the hind-gut, or between the portions of the intestinal canal which develop from them. Diverticular outgrowths from the entoderm of the primitive alimentary canal form the rudiments of the intestinal glands, including the liver and pancreas ; of the respiratory apparatus ; and of the thyroid and thymus glands. Details of the formation of these structures are given in the special description of the develop- ment of the system to which each belongs. 20 28 Fig. 29. — Further Development of the Alimentary Canal, as seen in a Human Embryo ABOUT Five Weeks Old (Diagrammatic). The tongue is well formed, the trachea and cesophagus are separated, the bronchi have commenced to branch ; the duodenal curve is well formed, and the Cfficum has appeared in the loop of the mid-gut. The cloaca is partially separated into genito-urinary and rectal portions. 1 . Hind-brain. 2. Month. 3. Tongue. 4. Pericardium. 5. Pharynx. 6. Heart. 7. Trachea. 8. (Esophagus. 9. Lung. 10. Liver. 11. Bile duct. 12. Stomach. 13. Pancreas. 14. Small intestine. 15. Coecum. 16. Intestinal loop. 17. Large intestine. 18. Notochord. 19. Vertebra. 20. Spinal cord. 21. Bladder. 22. Wolffian duct. 23. Kidney. 24. Ureter. 25. Rectum. 26. Proctodfeum. 27. Allantoic diverticulum. 28. Vitello-intestinal duct. 29. Fore-brain. 30. Mid-brain. Except with respect to the anterior part of the fore-gut, the changes in shape and position which the originally simple alimentary tube undergoes during its conversion into its final or adult form are described in the account of the development of the digestive organs ; but the development of the pharynx and the structures associated with it, and the formation of the mouth and anus, may be considered now^ Development of the Pharynx and Stomatodaeum. — The development of the anterior part of the fore-gut into the pharynx and the floor of the mouth is so inti- mately associated with the formation of a primitive mouth, the stomatodteum, that the two must to a certain extent be considered simultaneously. The stomatodaeum first appears as a depression between the head and the peri- VISCEEAL CLEFTS AND VISCERAL ARCHES. 35 cardial region. It is produced by the downward growth of the fore-part of the head in front and the bulging forward of the pericardium behind, and it is separated from the anterior end of the fore-gut by the bilaminar bucco-pharyngeal membrane. When the stomatodteurn first appears it is not enclosed laterally ; but at a later period side boundaries are formed, and the space is developed into the upper part of the mouth and the nasal cavities. The fore-gut, a relatively wide space, continuous posteriorly with the mid-gut, is at first closed anteriorly by the bucco-pharyngeal membrane, which separates it from the stomatodseum. About the fifteenth day, in the human embryo, the bucco- pharyngeal membrane disappears, the fore -gut is then thrown into continuity with the stomatodseal space, and the anterior opening of the alimentary canal is formed. As development proceeds the cavity of the fore-gut is gradually compressed dorso-ventrally until its transverse section assumes a triangular outline ; but in the earliest stages there are no indications of the various organs which are ulti- mately developed from its walls. After a short interval, however, two elevations appear in its ventral wall. The anterior of these is a rounded elevation, termed the tuberculum impar. It is situated directly behind the lower ends of two raised bars or arches, called the mandibular arches, which are growing down into the floor of the fore-gut from the anterior parts of its lateral walls. The tuberculum impar is the rudiment of the anterior two-thirds of the tongue, which is thus formed in the floor of the entodermal portion of the alimentary canal. The more posterior elevation, termed the furcula, is a curved ridge, which bounds a mesial longitudinal depression. It is separated from the lateral walls of the fore-gut and from the tuberculum impar by a groove, the sinus arcuatus. The anterior part of the furcula is transformed into the epiglottis and the margins of the upper aperture of the larynx ; the median depression becomes the cavity of the larynx and of the trachea, and from its posterior end hollow outgrowths extend and form the rudiments of the epithelial lining of the bronchi and lungs. Still more posteriorly, behind the region of the furcula, a dilatation of the fore-gut is formed, which projects forwards and downwards towards the pericardium. This is the first indication of the stomach. Visceral Clefts and Visceral Arches. — In the lateral wall of the anterior part of the fore-gut, on each side, four incomplete and more or less transverse clefts, the visceral clefts, appear. They are due to outward linear pouchings of the entoderm, and corresponding, but less marked, inward depressions of the ectoderm. The anterior cleft is the best marked, and the rest diminish in size from before back- wards. At the bottoms of the clefts the ectoderm and the entoderm are in contact, but the thin membranes thus formed, which intervene between the cavity of the fore-gut and the exterior, are only exceptionally and abnormally perforated in the human subject, though in lower vertebrates they invariably disappear, and the pharyngeal or anterior part of the fore-gut is thrown into continuity, laterally, with the exterior by a number of narrow slits, the gill slits, which are used for respiratory purposes. In man and other mammals, however, the floors of the second, third, and fourth clefts are utilised in the formation of the sides of the neck ; that of the first cleft is transformed into the tympanic membrane, which separates the external auditory meatus from the cavity of the tympanum. In the further consideration of the fate of the visceral clefts, it must be borne in mind that each consists of an inner or entodermal portion and an outer or ecto- dermal portion. The inner yjart of the first cleft is converted into the tympanum and the Eustachian tulje, and the outer part becomes the external auditory meatus. No traces of the outer part of the second cleft are left, but a portion of the inner part can be recognised as a slight depression above the tonsil in the lateral wall of the pharynx and in a recess, the fossa of Kosenmiiller, behind the pharyngeal end of the Eustachian tube. Both the outer and inner portions of the third and fourth clefts diHa]jp(;ar, but from their inner parts diverticula are given off which form the ruchments oi' the tliymus and the lateral lohes of the tliyroid l)ody. The diverticula from whicfi the thymus is developed are (Uirived from the third clefts, wiiilst each lateral lobe of the thyroid body, in the majoi'ity of mammals, is formed I'jy a diverticulum i'rom the fourth cleft, but in some, mammals the lateral lobes are GENERAL EMBRYOLOGY. derived from the median diverticulum, and the outgrowths from the posterior parts of the fourth clefts constitute the post-hranchial bodies. The margins of the visceral clefts are thickened by the growth of the mesoderm between the entodermal and ectodermal layers, and they are moulded into a series of five rounded bars, the visceral arches, of which the fifth is not recognisable externally, though it is easily seen internally. The dorsal extremities of the arches terminate at the sides of the head below the level of the neural tube, and in the early stages the vential ends rest upon the pericardial region. When the neck is formed, it grows forwards from the pericardial region and carries with it the lower ends of the visceral arches, whicli henceforth terminate in its ventral wall. As the visceral arches are carried forwards the head is strongly curved towards the ventral aspect, and the lower ends of the visceral arches are pushed backwards over each other till the ^ ^^ fourtli is over- lapped by the third, and the third by the second. The first arch is the mandi- bular, the second the hyoid, the third the thyro- hyoid; the fourth and fitth have no special designa- tions. Each arch is covered — ex- ternally by ecto- derm, internally by entoderm, and its core is formed of mesoderm, in which there is developed a bar of cartilage and a blood - vessel called a cephalic aortic arch. At first each arch is limited to the side wall of the fore-gut; but after a time it is prolonged into the ventral wall, encroaching, with the exception of the first, upon the sinus arcuatus. The first, or mandibular arch, is formed between the first visceral cleft and the bucco-pharyngeal membrane. As it develops it forms the lateral and lower boundaries of the stomatodseal space, and it grows downwards till it meets its fellow of the opposite side in the ventral middle line, immediately in front of the tuberculum impar. The greater part of this arch is converted into the lower jaw and the soft tissues which invest it. From its upper part a process grows forwards, the maxillary process, from which the upper lateral part of the face, between the orbit and the mouth, is developed, and in which the superior maxillary, the malar, and the palate bones, and possibly the internal pterygoid plate also, are developed and ossified. From the posterior border of the outer aspect of the mandibular arch the tragus and a portion of the helix of the pinna of the external ear are formed. The carti- laginous bar in its interior is known as Meckel's cartilage. It forms the primitive Pig. 30. Stages in the Formation of the Tongue and Upper Aperture of the Larynx in the Human Embryo (alter His). Embryo 14 days old. II. Embryo 23 days old. III. Embryo 28 to 30 days old. IV. Embryo 2 months old. Coelom. G. Glottis. Epiglottis. SA. Sinus arcuatus. Furcula. T. Tongue. Foramen cajcum. TI. Tuberculum impar. 1] 2 Visceral 2 I , (arches. C. E. F. FC. VENTEAL WALL OF THE FORE-GUT. 37 skeleton of the arch. Its upper and lower extremities are ossified and remain in the adult, the former as the malleus, and possibly the incus, and the latter as the symphysial part of the lower jaw. The remainder of the cartilaginous bar dis- appears, but the fibrous membrane which surrounds the lower section of the inter- mediate part is ossified and converted into the main part of the lower jaw, whilst that round the upper section of the intermediate portion persists as the spheno- mandibular ligament. The blood-vessel developed in the mandibular arch is, for the main part, a transitory structure, but its ventral section is converted into the internal maxillary, superficial temporal, facial and lingual arteries. The second and third arches are continued downwards into the floor of the pharyngeal portion of the fore-gut. There, converging, they insinuate themselves between the tuberculum impar and the furcula, across the anterior part of the sinus arcuatus, and uniting together form a transverse bar. This rapidly changes into a semilunar ridge which first embraces, and afterwards fuses with the posterior part of the tuberculum impar, and it forms the posterior third of the tongue. The second arch takes part in the formation of the side and anterior part of the neck. From its anterior border externally a part of the helix, the antihelix, the antitragus, and the lobule of the pinna of the external ear are developed. The lower and upper portions of its cartilaginous bar- — the hyoid bar — are ossified ; the lower portion forms part of the body and the small cornu of the hyoid bone on its own side, and the upper portion is converted into the intra- and extra-temporal sections of the styloid process (the tympano-hyal and stylo-hyal portions of the styloid process of the temporal bone). The fibrous tissue of the intermediate part of the hyoidean bar persists in the adult as the stylo-hyoid ligament. The blood- vessel of the hyoid arch, the second cephalic aortic arch, almost entirely disappears, but from its ventral extremity the ascending pharyngeal, occipital, and posterior auricular arteries are probably developed. The third visceral arch forms part of the neck posterior to the region of the second arch, and, as already pointed out, its lower end takes part in the formation of the posterior part of the tongue. The upper and middle parts of its cartilaginous bar disappear, but the lower part persists, and is converted into the posterior part of the body and the great cornu of the hyoid bone on its own side. Its blood- vessel, the third cephalic aortic arch, becomes the lower part of the stem of the internal carotid artery. The fourth and fifth visceral arches also enter into the formation of the neck, but their exact limits in the adult cannot be defined. Of the upper sections of their cartilaginous bars no trace remains in the adult, but their lower portions are beheved to enter into the formation of the thyroid cartilage of the larynx. The blood-vessel of the fourth arch on the right side becomes part of the right sub- clavian artery, that on the left side is converted into the arch of the aorta. The vessels of the fifth arches form portions of the pulmonary arteries, and that on the left side forms also the ductus arteriosus. Further Development of the Ventral Wall of the Fore-gut in the Region of the Furcula. — ^The sinus arcuatus which surrounds the furcula disappears to a great extent as development proceeds, but certain parts of it remain and are recog- nisable in the adult. The anterior portion immediately in front of the furcula is divided into two parts as the lower ends of the second and third arches of the two sides converge and fuse in the ventral wall of the pharyngeal portion of the fore-gut ; the middle portion of the sinus, in front of the transverse bar formed by this fusion, persists in the adult as the foramen caecum of the tongue, and at a very early period a diverticulum grows backwards from it in the fioor of the pharynx, dorsal to the cartilage bars which form the hyoid bone, but ventral to the rudi- ments of the tliyroid cartilage. This diverticulum is the thyro-glossal duct. As soon as it reaches the level of the fcjurth visceral clefts it enlarges, unites with the diverticula from those clefts which form the lateral lobes of the thyroid body, and is itself converted into the isthmus of the thyroid, its pyramidal process and the thyro-glossal duct or the fibrous cord into which that duct becomes converted in the adult. Occasionally the thyro-glossal duct is not wholly transformed into a fibrous cord, but portions of it remain in the form of isolated vesicles, lined with 38 GENEEAL EMBEYOLOGY. columnar (n' cul)ical epithelium, or as cords of cells, and these occasionally undergo abnormal development, forming tumours at the base of the tongue or in the upper part of the neck. The portion of the sinus arcuatus which lies behind the conjoined lower extremities of the second and third arches of opposite sides, and in front of the furcula, persists in a modified form in the adult, and is recognisable as glosso- epiglottidean pouches or valleculge at the base of the tongue. The furcula and the groove in the ventral wall of the fore-gut, which it embraces antero-laterally, are both of considerable importance. The anterior part of the furcula is situated in the ventral wall of the pharyngeal portion of the fore-gut, but its backward prolongations and the furrow between them lie in what may be termed the intermediate part of the fore-gut, that is, in that part of the fore-gut which intervenes between the pharyngeal and stomach regions. Gradually the furrow deepens, and its posterior extremity dilates on each side. Afterwards the margins of the furrow coalesce from behind forwards, and in this manner the cavity of the furrow is separated from the fore-gut, its walls are converted into the trachea and the lower part of the larynx, whilst the diverticula which are projected from its posterior end form the rudiments of the bronchi. The fusion of the margin of the furrow ceases a short distance behind its anterior extremity, which latter persists as the superior aperture of the larynx. The anterior part of the furcula, which bounds this aperture in front, becomes the epiglottis, and its lateral extensions, which form the margins of the aperture, are converted into the aryteno- epiglottidean folds in the substance of which the arytenoid cartilages and the cartilages of Santorini and Wrisberg (cuneiform cartilages) are formed. DEVELOPMENT OF THE MOUTH AND THE NOSE. The nose is formed entirely from the stomatodseum. The mouth has a double origin ; the roof and fore-part, including the teeth, are developed from the stomato- dseum, whilst the floor and the tongue are developed from the pharyngeal portion of the fore-gut. It has already been pointed out (p. 34) that the stomatodseal depression lies between the anterior part of the head {i.e. the tissues forming the base of the primary fore-brain) and the pericardial region, and that it is separated posteriorly from the fore-gut by the bucco-pharyngeal membrane. At first it has no distinct lateral boundaries, but subsequently the mandibular arches, which are developed at the sides of the bucco-pharyngeal membrane, project forward beyond the membrane and form lateral limits of the depression. If the stomatodseal space is examined from the front at this period the following boundaries are recognisable : — Above and in front is the projecting anterior part of the head which is termed the fronto-nasal process, laterally are the mandibular arches, and below and posteriorly is the anterior part of the pericardial region. After a short time the lower ends of the mandibular arches meet in front of the pericardial region, and, fusing together, form the posterior or lower margin of the aperture ; simultaneously the lateral boundaries of the space are still further completed by the forward growth of a nodular projection, the maxillary process, from the upper end of each mandibular arch. About the fifteenth day the bucco-pharyngeal membrane disappears, and the stomatodeeal space and pharynx are thenceforth continuous. No trace of the bucco-pharyngeal mem- brane is recognisable in the adult, but its position may be represented by an imaginary plane extending from the anterior parb of the basi-sphenoid above to the base of the alveolar process of the lower jaw, on its lingual surface, below. Whilst the boundaries of the stomatodwal space are being defined, two oval depressions, lined with thickened epithelium, appear in its upper boundary on the lower and anterior surfaces of the fronto-nasal process ; these are the olfactory pits or depressions. A portion of the epithelium of their walls is separated off and takes part in the formation of the olfactory bulbs, whilst the remainder is transformed into the olfactory epithelium, from which the olfactory nerve-fibres grow" inwards to the olfactory bulbs. As the olfactory pits deepen they grow backwards into the roof of the stomatodseal space, and at the same time they separate the lower portion THE MOUTH AND NOSE. 39 of the fronto-nasal process into three parts, constituting a median and two lateral nasal processes. At each lateral angle of the median nasal process a spheroidal elevation, the globular process, appears. The part of the median nasal process which intervenes between the two globular processes is divided into two areas, an upper triangular and a lower quadrilateral, by the appearance of a transverse ridge, which is afterwards moulded into the tip of the nose. The upper triangular area becomes the dorsum of the nose, and the lower quadrilateral area forms the columella, i.e. the lower and anterior part of the septum between the anterior Mesencephalon Maxillary Eye process Mandibular arch Prosencephalon Stomatodaeum Fig. 31. I. Side view of the head of human embryo about 27 days old, showing the olfactory pit and the visceral arches and clefts (from His). II. Transverse section through the head of an embryo, showing the relation of the olfactory pits to the fore- braiu and to the roof of the stomatodteal space. III. Head of human embryo about 29 days old, showing the division of the lower part of the mesial frontal process into the two globular processes, the intervention of the olfactory pits between the mesial and lateral nasal processes, and the approximation of the maxillary and lateral nasal processes, which, however, are separated by the oculo-uasal sulcus (from His). IV. Transverse section of head of embryo, showing the deepening of the olfactory pits and their relation to the hemisphere vesicles of the fore-brain. nasal ajjertures. The globular processes are utilised in the formation of the philtrum or middle part of tlie upy^er lip, and the lateral nasal processes form the ala3 of the nose or lateral boundaries of tlie anterior nasal apertures. As the olfactory pits deepen and grow backwards into the roof of the stomatodceum the maxillary pro- cesses grow forwards from the lateral boundaries of that space, that is from the upper ends of the mandibular arches, and pass beneath the eyes, which now form distinct prominences on the sides of the head. The upper borders of the maxillary processes come into contact with the lateral nasal ])rocesses from which they are temporarily separated by grooves, the oculo-nasal sulci. These latter pass from the depressions round th(; eyeballs, tlie rudimentary conjunctival sacs, to the margins of the nasal pits. The anterior extremities (jf the maxillary processes impinge 40 GENEEAL EMBEYOLOGY. upon the globular processes, and ultimately their upper borders and anterior ex- tremities fuse with the lateral nasal and globular processes, completing the lower boundaries of the anterior nasal orifices and the lateral parts of the primitive upper lip. At the same time the oculo-nasal sulci are converted first into solid cords of cells, and afterwards into the lachrymal sacs and the nasal ducts, which henceforth constitute the channels of communication between the conjunctival sacs and the nose. The result of the ingrowth of the maxillary processes and their fusion with tlie Cerpbral lieiiiisijlieres Olfactory pit Globular process Maxillary proces; Olfactory pit ^ Maxillary process Globular process Mouth Aut. nasal orifice !;5 Globular process Maxillary process Lower jaw Cerebral lieuiisphere Nasal cavity Jacobson's organ Globular process Maxillary process Lower jaw ir III Mouth Fir.. 32. I. Portion of tlie head and neck of a human embryo 32 days old. The floor of the mouth and pharynx and the ventral part of the anterior portion of the body have been removed. By the approximation of the globular and maxillary processes the boundaries of the anterior nares are almost complete, bnt the olfactory pits still open in the whole of their lengths into the roof of the mouth (from His). Transverse section of the head of an embryo, showing the close apposition of the globular and maxillary processes. Head of human embryo about 2 months old, showing the union of the globular processes and their fusion, with the maxillary processes. The anterior nasal apertures are now completely defined (Irom His). IV. Transverse section of the head of an embryo, showing tlie fusion of the maxillary processes with the globular processes, and the separation anteriorly of the nose from the mouth. lateral nasal and globular processes is the division of the large orifice which led into the stomatodgeal space into three parts — a large lower, and two smaller upper aper- tures. The lower opening is the aperture of the mouth ; it is bounded below by the united mandibular arches, and above by the fused mesial nasal and maxillary processes. The smaller upper openings are the anterior nares, which on their first formation are merely foramina of communication between the exterior and the upper part of the stomatodseal space ; the latter is not yet separated into nasal and oral chambers. Formation of the Palate and the Separation of the Nasal and Buccal Cavities. — This separation is effected by the formation of the palate, which is developed to a slight extent by the backward growth of the globular processes rOKMATION OF THE PALATE. 41 along the roof of the space as a pair of ridges, termed the nasal laminse, which fuse together to form a small anterior portion of the palate, viz. the intermaxillary pro- cess, in which the intermaxillary parts of the superior maxillae are formed. The remaining and greater part of the palate is formed by two ledge-like ingrowths, one from the inner surface of each maxillary process, which meet and fuse anteriorly with the intermaxillary process, and behind this with each other. In these pro- jections the palatal processes of the superior maxillse and the horizontal plates of the palate bones are formed, and by their fusion the upper part of the stomatodaeal space is separated off from the remainder as a common nasal chamber which com- municates in front with the exterior by the anterior narial orifices, and behind with the pharyngeal portion of the fore-gut by the choanal apertures or posterior nares. The lower part of the stomatodseal space and the front part of the fore-gut together form the mouth or buccal cavity ; this opens anteriorly by a transverse aperture, the boundaries of which have already been described, and posteriorly it is in direct continuity with the pharynx. The division of the common nasal chamber into two parts commences before its separation from the mouth is completed, and it is brought about by the development Anterior nasal orifice Ethmo-vomerine plate Nasal cavity Jacobson's organ Mouth Palatal, process Pituitary depression Fig. 33. Mecliel's cartilage I. Portion of the head of a human embryo about 2^ mouths old (His). The lips are sejjarated from the gums, and the line of the common dental germ is visible in the latter. The palatal processes are growing inwards from the maxillary processes. II. Transverse section of the head of an embryo after the fusion of the palatal processes of the maxillary pro- cesses with the nasal septum, which grows backwards from the fused globular processes. of a septum which is continuous anteriorly with the fused nasal laminee, and which grows downwards and backwards from the mesial part of the under aspect of the fronto-nasal process. This septum fuses below with the conjoined margins of the palatal ledges of the maxillary processes, and a vertical plate of cartilage soon develops in its interior, which is continuous above with the cartilaginous base of the cranium (basi-cranial axis). A portion of this septal cartilage remains in the adult as the septal cartilage of the nose, and the remainder is more or less completely replaced by the vertical plate of the ethmoid bone and by the vomer. The lateral wall of each nasal chamber is formed, in the lower part of its extent, by the maxillary process of the mandibular arch, in which the superior maxillary, malar, and palate bones, and possibly the internal pterygoid plate, are developed, and in the upper part by the outer boundary of the original nasal pit, which now forms only the upper part of the nasal cavity. In this upper section of the outer wall an outgrowth of the basi-cranial axis projects downwards, and is developed into the lateral mass of the ethmoid bone ; probably it also takes part in tlie formation of the inferior turbiual bone. The fusion of the three segments of the palate commences anteriorly at the eighth week by the union of the maxillary and globular processes ; it passes back- wards and is completed by the fusion of the posterior parts of the palatal ledges of the maxillary processes about the tenth week. To the non-completion of this fusion the various cases of hare-lip and cleft palate are due. Organ of Jacobson. — 'I'lie organs of .facobsou arc rudinicntary structures in num. 42 GENEEAL EMBEYOLOGY. They lie in the lower and anterior part of the nasal septum, one upon each side. They are developed as small diverticula which grow backwards and upwards in the substance of the septum, and their points of commencement are situated immediately above the intermaxillary segment of the palate. Each diverticulum is partially sur- rounded, on its inner side, by a cartilaginous capsule, it ends blindly behind, and it opens anteriorly close to the floor of the nose in the region of Stenson's foramen — a small aperture left between the premaxillary and maxillary sections of the bony palate. Pituitary Body. — The pituitary body is formed partly from the lloor of the first primary cerebral vesicle, and partly from the roof of the stomatodteal space. The stomatodccal portion appears as a small pouch, Eathke's pouch, which grows upwards into the base of the head immediately in front of the dorsal margin of the bucco-pharyngeal membrane and the anterior end of the notochord, and behind the fore-brain. It is lined by ectoderm, and soon becomes a conical vesicle which lies beneath the base of the fore-brain. Its orifice of communica- tion with the stomatodaeal space is gradu- ally constricted until the lumen dis- appears, and then for a time the vesicle is connected with the surface by a solid ^^ cord of ectodermal cells. This also dis- appears, and the vesicle is embedded in the base of the head in a region above and between those parts of the basal axis which afterwards are transformed into the basi- and pre-sphenoid elements of the sphenoid bone. During the period of its formation and separation the ingrowth from the stomatodgeum comes into relation pos- teriorly with a small diverticulum from the floor of the fore-brain, which dilates at its lower end to form the posterior or cerebral lobe of the pituitary body, whilst its upper part remains as the infundibulum, the connecting stalk be- tween the pituitary body and the floor of the third ventricle of the brain. The anterior or stomatoda^al lobe of the pituitary body is much larger than the Fig. 34.- - Vertical Section through Head of Rat Embryo. Showing the formation of the two parts of the pituitary body (diagrammatic). (Ectoderm is represented in black, entoderm in bhie, and mesoderm in red.) At. Ax. B. Atlas. Axis. Cartilaginous basi- cranial axis. Heart. HB. Hind-brain. MB. Mid-brain. N. Part of nasal cavity H. P. Pineal body. PR. Cerebral hemisphere. Pfi. Cerebral part ot pituit- posterior lobe, wliich it surrounds and ary body. ^ , , , . „ , . . , Pt^. Buccal part of jDituitary body. SG. Spinal ganglion. T. Tonsrue. Th. Thalamencephalon. conceals both in front and at the sides. It is evident that in the early stages the pituitary body consists of two ecto- dermal vesicles, the cavity of the pos- terior vesicle is continuous with the cerebral tube, and that of the anterior vesicle with the cavity of the primitive mouth. The cavity of the posterior vesicle is generally obliterated, and though nervous structures are for a time developed in its walls they entirely disappear in man and are replaced by vascular connective tissue. Occasionally a small part of the cavity remains as a minute vesicle lined with columnar ciliated epithelium. The cavity of the anterior vesicle persists, it sends out numerous diverticula, and is gradually converted into a number of tubular spaces, lined with cubical or columnar cells, united together by vascular connective tissue which has grown amidst the tubules from the surroundiuo- mesoderm. THE EXTEENAL EAR AND EUSTACHIAN TUBE. 43 HM THE EXTERNAL EAR, THE TYMPANIC CAVITY, AND THE EUSTACHIAN TUBE. The external ear, the tympanic cavity, and the Eustachian tube are all developed from the first visceral cleft and its boundaries. The cleft lies between the mandibular (first) and the hyoid (second) visceral arch in the side wall of the pharyngeal portion of the fore-gut, and, before a neck is developed, it extends from just ventral to the otic vesicle, which lies at the side of the hind-brain, above, to the pericardial region below. The membrane which lies at the bottom of the cleft consists in the early stages of ectoderm and entoderm, but in a short time a thin layer of mesoderm grows between the two primary layers, and the trilaminar septum is ultimately converted into hb the tympanic membrane which separates the external from the middle ear. The differentiation of the outer part of the cleft is initi- ated by the appearance of six tubercles round its margins, which are afterwards trans- formed into the several parts by" of the pinna. Two tubercles are formed anteriorly on the mandibular arch, one at the dorsal end of the cleft and three posteriorly on the hyoid arch. The two tubercles on the mandibular arch are a small lower, the tuberculum tragicum, and a larger upper, the tuberculum anterius helicis. The tubercle at the upper end of the cleft is the tuberculum intermedium helicis. The upper tubercle on the hyoid arch is the tuber- culum anthelicis, the middle is the tuberculum antitragicum, and the lowest is the tuber- culum lobulare. Shortly after the appearance of the tubercles a process, the caudal process, grows backwards and downwards, from the posterior part of the tuberculum intermedium helicis, behind the tuberculum anthelicis and the tuberculum anti- tragicum to the tuberculum lobulare, with which it fuses. The tuberculum tragicum remains more or less distinct, and it forms the prominence called the tragus which lies in front of the concha and external auditory meatus. The two tubercles of the helix and the caudal process unite to form the helix or marginal portion of the pinna ; this terminates below in the lobule which is developed from the tuberculum lobulare. The tuberculum anthelicis and the tuberculum antitragicum are the rudiments respectively of the antihelix and the antitragus, and tlie latter unites below the lower part of the cleft with the rudiment of the tragus, forming the lower boundary of the outer part of the external meatus. It should be noted that in the early stages the tuberculum anterius helicis lies in front of the outer part of tlie first visceral cleft, but it does not retain this position in the later stages during which the cleft is relatively reduced in size, and when development is completed and the outer part of tlie cleft is transformed into the external auditory meatus the commencement of the helix, which is developed from the tuberculum anterius helicis, is situated just a})0vc tlie outer extremity of the external meatus. Fig. 35. -Transverse Section through the Head OF A Rat Embryo. Showing the rudiments of the three parts of the ear and their relation to the hyo-mandibular cleft. BV. Blood-vessels. C. Cochlea. EM. Ext. auditory meatus. ET. Eustachian tube. HB. Hind-brain. HM. Hyo-mandibular cleft. N. Notochord. OV. Otic vesicle. P. Pharynx. EL. Recessus labyrinthii. SC. Semicircular canal. T. Tympanum. 44 GENEEAL EMBEYOLOGY. The outer part of the cleft is moulded into the external auditory passage. It remains relatively shallow and devoid of bony boundaries till after birth, but in the subcutaneous tissue round the lower margin of the tympanic membrane an incomplete ring of bone is formed during the third month, and at an earlier period, above the upper part of that membrane, the rudiment of the squamous jjart of the temporal bone appears. To the outer side of the tympanic ring in the suljcutaueous tissue of the pinna and the outer part of the external auditory passage three pieces of cartilage appear, and they afterwards join to form the cartilage of the pinna and the external auditory meatus. After birth the external meatus is deepened by the outgrowth of the tympanic ring below and of the squamous part of the temporal bone above, together with a coincident in- crease of the outer part of the canal. The tympanic cavity and the Eustachian tube are both formed from the inner part of the first visceral cleft, and consequently they are both lined by entoderm. The tympanic cavity is de\ eloped from the dorsal or upper end of this portion of the cleft, and it is prolonged upwards on the outer side of the otic vesicle which simultaneously descends in the tissues of the head. Thus the upper end of the inner portion of the cleft, which is somewhat dilated, comes to lie between the otic vesicle, which is developed into the internal ear on the inner side, and the tympanic membrane which separates it from the external auditory meatus on the outer side, and it remains in the adult as a laterally compressed space, the tympanic cavity, which is continuous through the Eustachian tube with the upper part of the pharynx. In the mesoderm round the inner, upper, and back part of the cavity the petrous part of the temporal bone is developed and ossified, and in the lower and anterior part the tympanic ossification extends outwards during the formation of the tympanic plate. The upper part of the tympanic space is prolonged backwards between the ossifying petrous and sq\iamous parts of the temporal bone, where it forms a recess known in the adult as the mastoid antrum, from which at a later period diverticuhi are projected into the mastoid portion of the temporal bone, forming the mastoid air cells. '> The lower portion of the inner part of the cleft is moved obliquely forwards. As development proceeds it is contracted and carried downwards and forwards in front of the developing otic vesicle. It is the rudiment of the Eustachian tube, and, as the septum which separates the nasal chambers from the mouth is formed^ Fiu. 36.- -Figures, modified from His, illustrating the Formation of THE Pinna. Tuberculuiu tragicura = Tragus. ,, anterius helicis "j ,, intermedium helicis ^ Helix. Cauda helicis j Tuberculum iinthelicis = Autihelix. 6. Tuberculum autitragicum = Anti- tragus. 7. Tubeiculum lobulare = Lobule. HM. Hyo-mandibular cleft. OV. Otic vesicle. THE HIND-GUT AND ANAL PASSAGE. 45 its lower end attains a position just Vjehind and at the side of the posterior narial orifice in the upper and lateral part of the pharynx. Apparently, therefore, the lower end of the Eustachian tube has a much higher position than that originally occupied by the lower end of the cleft from which it is formed, for it will be remembered that the lower end of the first visceral cleft is situated, in the early stages, at the side of the tuberculum impar from which the anterior two-thirds of the tongue is formed. This alteration in relative position is due, however, not to elevation of the lower end of the first visceral cleft during its transformation into the Eustachian passage, but to the enormous downgrowth of the mandibular arches, which carry with them the tongue, as they enlarge to form the lower jaw. THE HIND-GUT, THE ANAL PASSAGE, AND THE POST-ANAL OE TAIL-GUT. By the formation of the mouth the primitive alimentary canal opens anteriorly ; it remains closed posteriorly until a later date, when the anal passage and orifice are developed. The posterior end of the hind-gut which is enclosed in the tail- fold is termed the cloaca. The cloaca is dilated, and, assuming a conical form, receives the terminations of the genito-urinary ducts. It is bounded postero-inferiorly by the cloacal membrane which extends from the root of the tail to the body stalk by whicli the embryo is attached to the chorion. The cloacal membrane is modified from the posterior part of the primitive streak ; this remains on the surface of the body after the anterior part has been separated and enclosed during the completion of the posterior part of the neural canal, and it forms a septum between the cavity of the cloaca and the exterior. It consists at first of ectoderm and entoderm alone, and it is only at its lower and anterior part that it is subsequently invaded to a slight extent by mesoderm. During the second month of intrauterine life the cloaca is divided into a ventral or genito-urinary, and a dorsal or rectal section, by the formation and fusion of lateral folds, which gradually unite, from before backwards, till finally the posterior end of the septum approaches and fuses with the cloacal membrane, and the rectum is separated from the genito-urinary chamber. Before this separation is completed an eminence appears in the region of the anterior part of the cloacal membrane at the junction of the ventral surface with the posterior extremity of the body, i.e. in that part which afterwards becomes the region of the symphysis pubis. This eminence is the genital eminence, and from it are formed the penis in the male and the clitoris in the female. The genital eminence is surrounded by an oval fold of skin, genital fold, which extends from the front of the eminence to the root of the tail and encloses a shallow fossa, the cloacal fossa, at the bottom of which is the cloacal membrane. The posterior part of the cloacal fossa is afterwards separated from the anterior part by a transverse fold, the perineal fold, which crosses the external surface of the cloacal membrane in a position which corresponds internally with that occupied by the lower end of the septum separating the genito-urinary from the rectal portions of the cloacal. The posterior part of the cloacal fossa, behind the transverse fold, is the proctodseal depression or proctodaeum ; at first its long axis lies transversely, afterwards it assumes a triangular and then a circular form, the sphincter ani muscle develops in its walls, and it is transformed into the greater part, if not the whole, of the anal canal of the adult. It is separated from the rectum by the posterior part of the cloacal membrane, Init when that disappciars, at a date which has not yet been definitely ascertained, but probably about the third month, the anal passage forms the canal by wliich the rectum communicates with the exterior of the body. 1'he orifices of the alimentary canal are thus completed. The Post-anal or Tail-Gut. — ^Whcn the hind-gut is first enclosed there is no tail, but a rudimentary tail is .subse()ueHtly developed as an outgrowth from the dorsal end of the tail-fold, i.e. from the posterior extremity of the body of the embryo. As the tail is formed, a narrow tube, which comminiicafces in front with the hiiid-gut, is developed within it. This is called the post-anal or tail-gut. As a rul(! it only exists for a short 46 GENEEAL EMBEYOLOGY. time, disappearing from before l)ack wards about the period when cartilage begins to be formed in the body and limbs, and before the cloaca is divided into its rectal and genito- urinary portions. In the few cases in which it persists it retains its continuity with the rectum, which is formed from the dorsal part of the cloaca. The tail-gut appears in the human subject when the embryo is 3 mm. long, and the rudimeiitary tail is just visible as a small nodule. When the embryo attains the length of 4 '8 mm. the anterior part of the tail-gut begins to degenerate, its cavity disappears, and it is converted into a solid cord of cells which is still attached in front to the hind-gut. In embryos 11 "5 mm. long, when the tail has been enclosed in the posterior part of the body, the connexion of the tail- gut with the hind -gut is lost, and the tail -gut is represented by a small vesicle with a short cord of degenerating cells attached to its anterior part. In larger embryos the tail-gut entirely disappears. When, as in the human siibject, the rudimentary tail is eventually embedded in the posterior end of the body, any rudiments of the tail-gut which persist will be found in this situation ; it is stated that such rudi- ments occasionally develop into tumour formations. In mammals with free tails, rudiments of the tail-gut may be met with in any part of the tail, and apparently the anterior portioii occasionally persists and maintains its connexion with the rectum, from which it extends backwards as a narrow and blind diverticulum. THE LIMBS. Though the body of the embryo begins to assume definite form as soon as it is folded and nipped off from the rest of the ovum, it does not present any distinguishable human characteristics until the anterior and posterior limbs are formed. There are no traces of these before the third week of in- trauterine life when two longitudinal ridges, the Wolffian ridges, are developed, one on each lateral tion of the cioacai part of the hind- surfacc of the body, just external to the outer gut^ into genito- urinary tract and niargins of the mesodcrmic somitcs, and opposite the line of the intermediate cell mass. The rudi- ments of the fore- and hind-limbs are discernible, almost from the first, as slight prominehces of the Wolffian ridges, and in the fourth week they project as bud-like outgrowths in the thoracic and pelvic regions respectively. The development of the fore-limb or arm is throughout slightly in advance of that of the hind-limb or leg. At the fourth week each limb-bud is a flattened semilunar projection, as long as it is broad, with a dorsal and a ventral surface and an anterior or preaxial, and a posterior or postaxial border. As growth proceeds the elongating limb-buds are bent ventrally, and in the fifth week two transverse furrows, on the ventral aspect of each, indicate the positions of the joints and the division of each limb into three segments — distal, middle, and proximal — representing the hand, fore-arm, and arm in the upper limb, and the foot, leg, and thigh in the lower limb. The terminal or distal segments are broad, flat plates with rounded margins, but each is soon divided into a some- what enlarged basal part, and a thinner and more flattened marginal part. It is where these two parts are continuous that the rudiments of the digits appear. They become distinguishable about the end of the fifth week as small lobes which gradually extend outwards. In the fore-limb the fingers project beyond the margin of the hand-segments in the sixth week, but the toes do not reach the margins of Fig. 37. — Diagrams showing the separa- rectum. A. Allantoic stalk. B. Bladder. C. Cloaca. CM. Cloacal membrane K. Kidney. II. Rectum. U. Ureter. Ur. Urethra. VD. Vas deferens. VS. Vesicula semi- nalis. WD. Wolffian duct. THE LIMBS. 47 the foot till the early part of the seventh week. The nails appear at the third month, and reach the ends of the digits at the sixth month. In the primary position of the limbs the elbow and the knee appear alike to be directed outwards, but this is soon altered. At the end of the sixth week each limb undergoes a partial rotation, the direction of which is different in the fore- and hind-limbs respectively. In the former the elbow is turned backwards, the ventral surface therefore becomes anterior, and the preaxial (thumb) margin is directed outwards ; in the hind-limb the knee is turned forwards, and the ventral surface of the limb becomes posterior, whilst the preaxial (great toe) margin is directed inwards ; thus in the adult the anterior surface and outer border of the upper extremity correspond with the posterior surface and inner border of the lower extremity, whilst obviously the posterior surface and inner border of the former are homologous with the anterior surface and outer border of the latter. Each limb-bud may be regarded as an extension from a definite number of the segments of the body ; it contains a core of mesoderm, and the anterior or ventral primary divisions of the corresponding spinal nerve segments are apparently prolonged into it. The central part of the mesoderm, except in the regions of the joints where cavities appear, is condensed and then converted first into cartilage, and afterwards into bone. The proximal part of the bony skeleton of each limb, the limb girdle, is not, however, developed in the limb-bud, but in the body- wall at its base. The more superficially situated mesoderm is transformed into muscles and subcutaneous tissues, the extensor muscles appearing on the dorsal and the flexor muscles on the ventral aspect. As the nerve trunks pass into the free portion of the limb they bifurcate, the branches passing respectively to the dorsal or extensor aspect of the limb, and to the ventral or flexor aspect. Apparently in mammals the whole of the mesodermal core of each limb-bud is formed from the somatic mesoderm of the lateral plates. If this is the case the muscles of the limbs differ in origin from those of the back, for the latter are developed from the muscle plates of the protovertebral somites. In lower verte- brates (cartilaginous fishes) buds are given off to the limbs from the muscle plates and cutaneous lamellae in the thoracic and pelvic regions, and as the muscle plates pass downwards in the somatopleure towards the ventral aspect of the body, these buds grow outwards into the limb-rudiments and develop into the muscles of the limbs. Presumably this is the more primitive arrangement, and that met with in man and other mammals is secondary, and it is stated that although no distinct buds from the muscle plates pass into the limbs of mammals, nevertheless the limb-muscles are formed by cells, proliferated from the muscle plates, which have migrated into the somatopleural mesoderm of the limbs. THE NUTRITION AND PROTECTION OF THE EMBRYO DURING ITS INTRAUTERINE EXISTENCE. The impregnated ovum during its passage down the Fallopian tube, and for a brief period also after it enters the uterus, lives either on the yolk gTanules (deuto- plasm; embedded in its own cytoplasm, or upon material absorljed from the fluids by which it is surrounded. The human ovum is very small, and consequently it is almost from the first dependent for its nutrition upon sources of supply outside itself. The urgent necessity for adequate arrangements whereby this may be effected leads to that early establishment of an intimate vascular connexion between the embryo and the mother which is so characteristic a feature in the development of the human (jvum. At the end of the second week, after fertilisa- tion of tin; ovum, tlie embryo is separated by a slight constriction from the rest of the Idastodermic vesicle, and iiJn^udy a primitive heart and rudimentary blood- vessels are distinguishable. The development oi" tin; vascular system, and the cstaltlishmcnt of the fcetal 48 GENERAL EMBRYOLOGY. circulatiou, however, cannot well be understood initil the formation and structural features of the group of closely associated extra-embryonic organs or appendages have been considered. This group includes the yolk-sac, the chorion, the amnion, the allantois, and the placenta. THE FCETAL MEMBRANES AND APPENDAGES. Yolk-Sac or Umbilical Vesicle. — That portion of the Ijlastodermic cavity and its wall which is not included in the body of the eml^ryo to form the primitive alimentary canal constitutes the umbilical vesicle or yolk-sac. Its walls, like its cavity, are continuous with the corresponding parts of the intestine, and their structural features are identical, there being an inner layer of entodermal cells and an outer layer which is formed by the splanchnic layer of the mesoderm. In the human embryo the yolk-sac is a small iiask-like body, suspended from the ventral wall of the alimentary canal by a hollow stalk, the vitello-intestinal duct, which passes through the umbilical orifice. It lies in the extra-embryonic continuation of the body-cavity (ccelom), aud is filled with fluid. Possibly the contents of the yolk-sac are utilised in the nutrition of the embryo in its earliest stages, and the first rudiments of the blood vascular system, viz. blood corpuscles and vessels, appear in its walls. In the human embryo, however, it is of little nutritional importance ; it soon atrophies and almost entirely disappears, but leaves traces of its existence in the umbilical cord. Amnion. — The amnion is a protective sac which surrounds the embryo. It is formed, after the development of the ccelom, from the amniotic area of the blastoderm, and its wall is continuous, at the margins of the umbilical orifice, with the body-wall of the embryo. Both walls consist of a layer of ectoderm and a layer of somatic mesoderm, but whilst in the body-wall the ectoderm is external and the mesoderm internal, the relative positions of the layers are reversed in the amnion, the mesoderm being external and the ectoderm internal. The cavity enclosed between the amnion and the embryo, the amniotic cavity, is filled with fluid, the amniotic fluid, in which the embryo floats. The amniotic cavity is quite shut off for some time from all the neighbouring spaces, but after the disappearance of the bucco-pharyngeal and cloacal membranes it commimicates, both anteriorly and posteriorly, with the alimentary canal of the embryo. The development of the amnion in mammals is closely associated with the attachment of the ovum to the uterine wall and with the subsequent formation of the placenta. Thus in the carnivora, before the ccelom is formed, the ectoderm in the chorionic area becomes attached to the uterine tissues by small villous out- growths which invade the uterine mucous membrane. This attachment is most complete in the placental region, that is, around the margins of the amniotic area. As the embryo is folded off from the blastoderm and the ccelom develops, both the embryo and the amniotic area remain quite free from the uterine tissues, indeed, it may be said that, at this period, the embryo is suspended from the margins of the placental area by the amniotic membrane. As development proceeds the amniotic area increases in extent by interstitial growth, and thereupon the embryo, the membrane which suspends it being relaxed, sinks more and more into the interior of the ovum, or, to be more precise, into the coelomic space, which, in the meantime, has considerably increased. At the same time the growth of the placental area causes all parts of its inner margin to converge, and as the inner margins of the placental area are continuous with the outer margins of the amniotic membrane, the amnion is gradually carried over the dorsal surface of the embryo till its margins meet and fuse. After the fusion of its margins the amnion separates entirely from the chorionic area, henceforth known as the chorion, and forms a closed sac which completely surrounds the embryo. On reference to Figs. 21 and 27 it will be seen that as the wall of the blasto- dermic vesicle is carried inwards over the dorsal surface of the embryo it is folded ; the outer part of the fold consists of the chorionic portion, and the inner part of the THE FCETAL MEMBEANES AND APPENDAGES. 49 amniotic portion of the blastoderm. The fold is called the amnion fold ; it is quite continuous round the whole margin of the embryo, but some ^jarts of it are more advanced than others, or in other words the convergence of the inner margin of the placental area of the blastoderm over the dorsal surface of the embryo does not take place at the same rate or to the same extent in all parts. For convenience of description it is usual to divide the amnion fold into four parts — the cephalic, the caudal, and the two lateral amnion folds ; these, however, are all continuous with one another. The inner part of the fold, which is formed from the amniotic area, is termed the true amnion, and the outer part, formed from the chorionic area, the false amnion. The latter term is, however, synonymous with chorion, and as it is misleading, it should be avoided. As the amnion is formed from the amniotic area of the blastoderm after the extension of the coelom, it must consist, as previously mentioned, of ectoderm and somatic mesoderm, 'and as the sur- face of the amniotic area is reversed during the formation of the amnion folds, it is obvious that in the fully-formed amnion the ectodermal layer is internal and the somatic mesoderm external. In the case of the human ovum the phe- nomena of amnion forma- tion are probably practi- cally similar, except that the ovum develops not in •showing the formation and closure of tlie amnion folds, the completion of , •. p .-I i the amnion, and the coincident ingrowth of the inner margins of the the cavity Ot tne uterus, placental area of the blastoderm. but in the substance of the mucous membrane into which it has penetrated. It is therefore surrounded by the mucous membrane on all sides, and the chorionic part of the surface of the ovum is closely attached to the surrounding tissue, but the amniotic and embryonic areas are free. If this is the case the process of amnion folding can take place in the human ovum exactly as in the ovum of a carnivorous animal, the inner margin of the chorionic area growing inwards over the amniotic and embryonic areas, the only difference being that the process takes place in a cavity in tlie mucous membrane and not on its surface. This conclusion is su])ported by Spec's observations on human ova and by tt)ose of Selenka on the ova of monkeys and apes, but it is, however, possible that, as in some rodents and inscctivora, the amnion cavity appears in a mass of ectoderm which lies at the embryonic yjole of the ovuin, tlie mass being cleft by the appearance of the cavity into CMibryonic and amniotic sections. I'lio two parts are then continuous at the 4 Fig. 38.- -Transveesb Sections of the Uterus and Developing Ovum of a Ferret. A. Amnion. EN. Entoderm. SC. Stratum compactum. AF. Amnion fold M. Muscular wall SS. Stratum spongiosum. C. Cojloni. of uterus. SoM. Somatic mesoderm. Ch. Chorion. NG. Neural groove. Sp.M. Splanchnic mesoderm. EC. Ectoderm. PV. Placental villus. UL. Unchanged layer of uterine mucosa. 50 GENEKAL EMBKYOLOGY. margin of the embryonic area, and the mesoderm growing round the amniotic ectoderm separates it from the chorionic ectoderm. After the amnion is completed its cavity is distended with fluid. As it expands it gradually obliterates the extra-embryonic part of the coelomic cavity, and finally its outer surface, of somatic mesoderm, comes into contact and fuses with the somatic mesoderm on the inner surface of the chorion. At this period the cavities in the ovum are the aumiotic cavity, the remains of the yolk-sac, and those portions of the orio-inal blastodermic and coelomic spaces which have been included in the embryo. In the human ovum, when the amnion folds unite and the true amnion separates from the chorion, the embryo and its enclosing amnion would be free within the cavity of the chorion, or extra-embryonic ccelom, were it not that a very short cord of somatic mesoderm and ectoderm, the body-stalk, connects the posterior end of the embryo with the somatic mesoderm on the inner surface of the chorion. Body-Stalk. — To thoroughly understand how this union is effected in the human ovum, and to comprehend the nature of the body-stalk, it is necessary to refer to some striking peculiarities which are to be observed in the earlier stages in the development of the human embryo. When segmentation is completed, and the morula is converted into a blastula by the appearance of a cavity in its interior, the human ovum consists of an outer layer, the ectoderm, and an inner cell-mass (Figs. 12 and 39). The latter, however, which is attached to a small area of the ectoderm, does not, as in many mammals, extend itself by migration round the inner surface of that layer, and so transform the unilaminar into a bilaminar blastoderm and convert the cavity of the blastula into the blastodermic cavity. The sequence of events is different : a cavity or space appears in the inner cell-mass itself (Figs. 41, 42, and 44), and this expanding rapidly, is ultimately converted into the yolk- sac and the alimentary canal of the embryo ; it corresponds, therefore, with the blastodermic cavity of other mammals. Thus the entoderm, though derived from the inner cell-mass, never lines the inner surface of the ectoderm except in the embryonic area, for soon after the appearance of the cavity of the inner cell-mass the mesoderm grows rapidly from the primitive streak and extends, not in a single layer, as in the majority of mammals, but as two layers, one over the outer surface of the entoderm, the splanchnic layer, and the other, the somatic layer, over the inner surface of the ectoderm. The cavity of the blastula is thus ultimately enclosed between the somatic and splanchnic layers of the mesoderm, and so becomes converted into the coelomic space (Fig. 42). As the mesoderm extends, the several areas of the blastoderm are differentiated as in other mammals, but the embryonic and amniotic areas remain of relatively small size. The separation of the amnion from the chorion is effected at a very early period, before the folding off of the embryo has commenced, but the somatic mesoderm growing from the posterior end of the embryonic area still retains its connexion with the similar layer on the inner surface of the chorion, and it forms a short, and for a time a broad stalk which unites the embryo, and consequently the amnion and the blastodermic cavity, with which the embryo is connected, to the chorion (Fig. 42). In addition to forming a bond of union between the embryo and the chorion the mesodermal stalk conducts blood-vessels from the embryo to the chorion, and more especially to its placental part. At an early period a pouch-like diverticulum projects from the posterior part of the entodermal sac. This is the allantoic diverticulum ; it lies beneath the posterior part of the embryonic area, and the area is curved upon itself so that its convexity looks towards the entodermal sac, and its concavity towards the amnion. After the embryonic area has increased in extent, and when the folding off of the embryo has commenced, the anterior end of the area and the posterior end of the primitive streak remain relatively stationary as in other mammals, the cephalic and caudal folds appear, and the curvature of the greater part of the area is reversed, but the most posterior part retains its original position, lying for a time parallel with the caudal fold ; afterwards, however, it assumes a more horizontal position. This posterior section of the embryonic area contains the diverticular process of the entodermal sac which is called the allantois ; it also contains the THE ALLANTOIS AND UMBILICAL COED. 51 blood-vessels, allantoic arteries and veins, which pass between the embryo and the placenta. It is in relation at first with the amnion, it appears to be entirely behind the embryo, and it is called the " body-stalk." At a later period, when the stalk of mesoderm— the allantoic stalk — which connects it with the inner surface of the chorion is elongated, this part of the embryonic area is reversed in position, its anterior end is carried forwards till it forms the posterior boundary of the umbilical orifice, and the area in question forms the ventral wall of the body from the umbilical to the genital region. AUantois. — The allantois plays an important part in the formation of the placenta. It consists of two portions, an entodermic diverticulum irom the ventral wall of the cloacal part of the hind-gut, and a mesodermal covering. The ento- dermic diverticulum appears in the human subject, before the hind-gut is defined, as a hollow blind protrusion from the blastodermic cavity ; it extends behind the primitive streak into the mesoderm of the body-stalk, but as the folding off of the embryo proceeds, and the body-stalk is carried forward into the ventral wall of the embryo, the position of the diverticulum is altered, and ultimately, when the folding off is completed, it springs from the ventral part of the cloaca, runs forward to the umbilical orifice, and passing through it, projects for a short distance still invested with the mesodermal covering primarily obtained from the body-stalk. The ventral part of the cloaca is afterwards converted into the bladder, while the rectum is formed from the dorsal part. The mesodermal sheath which surrounds the entodermic diverticulum extends beyond it to the inner surface of the chorion ; the part which extends beyond the diverticulum is at first extremely short, indeed it is only recognisable as a layer of mesoderm uniting the body-stalk and chorion, but as development proceeds and the body-stalk is folded forward to form the ventral wall of the body of the embryo, posterior to the umbilicus, this portion of the mesoderm is elongated, and it forms the allantoic stalk by which the embryo retains its connexion with the chorion, and along which pass the allantoic or umbilical arteries to, and the corresponding veins from, the chorionic villi. After the separation of the cloaca into bladder and rectum, that portion of the allantois which lies in the body of the embryo, between the apex of the bladder and the umbilical orifice, is gradually converted into a fibrous cord, the urachus. The entodermal diverticulum disappears, and after birth, when the placental circu- lation ceases, the umbilical arteries are transformed into fine fibrous strands. The remainder of the allantois which lies outside the body of the embryo, and which takes part in the formation of the umbilical cord and placenta, is separated from the embryo at birth. Umbilical Cord. — The umbilical cord is essentially a mesodermal structure which connects the embryo with the placenta, serving as a passage for the allantoic vessels to and from the foetal portion of the latter organ. It replaces, functionally, the body-stalk and the allantoic stalk, which were earlier provisions for the same purpose, and it is formed by the fusion of the allantoic stalk with part of the vitello-intestinal duct and the remains of the yolk-sac. The vitello-intestinal duct is at first a relatively wide channel which connects the primitive gut with the yolk-sac ; it passes through the umbilical orifice. In later stages, as the body-stalk is swung round into the ventral wall of the body, the allantoic stalk, which projects from the end of the body-stalk, is brought into close relation with the distal end of the vitello-intestinal duct and the remains of the yolk-sac ; the mesodermal constituents of the three structures then fuse together, and the whole is surrounded by the expanding amnion. In this way the umbilical cord is formed. It includes, therefore, the allantoic stalk and its blood- vessels, together with the remains of the yolk-sac and its stalk, the vitello-intestinal duct, all of' which arc invested and bound together by the amnion. The mesodermal core of the cord forms a fibro-mucoid tissue known as "Wharton's jelly," which consists of stellate and irregular cells embedded in a gelatinous matrix. The blood-vessels of the cord are situated in the core, and include two allantoic or umbilical arteries which run spirally round a single umbilical vein. The terminal portion of the allantoic diverticulum projects into 52 GENEEAL EMBEYOLOGY. the embryonic end of the cord, and at first a loop of intestine protrudes into it for a short distance ; the gut, however, soon recedes into the abdominal cavity. The umbilical cord, which extends from the umbilical oriJ&ce to the centre of the placenta, is at first short and straight. As the amnion expands the length of the umbilical cord increases until, at the time of birth it measures, on an average, about 20 inches. This increase in the length of the cord allows the fcetus to float freely in the amniotic fluid, and prevents traction on the placenta. After the middle of the second month the umbilical cord is twisted spirally, usually from right to left. It is suggested that this is due either to the great elongation of the allantoic arteries or to muscular movements of the fo-tus, and it involves a rotation of the foetus in the amniotic fluid. Chorion. — The chorionic area, by far the largest of the areas into which the blastoderm is divisible, lies external to the amniotic area. In most mammals it consists at first of ectoderm and entoderm, but after the extension and cleavage of the mesoderm has taken place, it is formed by ectoderm and somatic mesoderm. In man, however, it consists in the earliest stages of ectoderm alone, but on the formation and extension of the mesoderm it also acquires an inner layer of somatic mesoderm. In all cases, therefore, it eventually consists of the same two layers. The ectoderm of the chorionic area which immediately surrounds the amniotic area thickens to form the annular placental area, and in this way the chorionic area becomes divisible into placental and non-placental regions. When the blastodermic vesicle enters the uterus numerous ectodermal villous processes grow from the surface of the chorionic area, both in its placental and non- placental parts, and attach themselves to the uterine mucous membrane. As already pointed out in the description of the formation of the amnion, the embryonic and amniotic areas do not become attached to the viterus, but remain free from it, whilst by the approximation and fusion of its inner margins, the rapidly growing ring-like placental area is converted into a disc which intervenes between the amnion and the uterine wall. The chorionic area after the separation of the amnion is known as the chorion or chorionic membrane. The chorion forms the outer wall of a vesicle, the chorionic vesicle, which is the modified remains of the blastodermic vesicle, and which contains the embryo, the yolk-sac, the amnion, and the allantois. It consists of an outer layer of ectoderm and an inner layer of somatic mesoderm. The cavity of the chorion is the extra- embryonic portion of the coelom. For a time it remains distinct, and is traversed by the allantoic stalk which unites the embryo to the inner or mesodermal layer of the placental area. The cavity is ultimately obliterated by the growth of the amnion, the latter sac expanding rapidly till its outer surface is in contact and intimately blended with the inner surface of the chorion. Chorionic Villi. — The villous processes which begin to grow from the surface of the chorionic area before it is separated from the amnion continue to develop after the separation of the two membranes is completed. They penetrate the surrounding uterine tissues. At first each consists of ectoderm only, but a core of vascular mesoderm is soon acquired. The villi increase in size and in com- plexity also, but ultimately only those in the placental area persist and continue to grow ; the remainder atrophy and disappear. Thus the placental region of the chorion eventually constitutes the main bond of union between the ovum and the mother, and it forms the foetal part of the placenta. THE PLACENTA. The placenta is the organ of foetal nutrition and respiration. In it the blood-vessels of the foetus and those of the uterus are brought into such close relationship with one another that free interchanges readily take place between the blood of the mother and that of the fcetus. In the simplest form of placenta the foetal villi are merely embedded in the maternal mucous membrane, and the relationship between foetal and maternal blood is not very close. In other THE PLACENTA. 53 forms, e.g. the human placenta, the relation of foetal to maternal blood is much more intimate ; this involves marked complications in the elements of the placenta, and its structure becomes correspondingly more complex. In all forms, however, the placenta consists of foetal and maternal portions. Before the impregnated ovum reaches the uterine cavity the mucous membrane of the uterus undergoes certain changes in preparation for its reception and reten- Decidua Jiasalis Gland Cavity which becomes coelom Unchanged layer itum spongiosum -v uterine Stratum compactum/'""°°''^ Ectodermal villus enclosiijg space containing maternal blood Inner mass (Entoderm) Decidua vera Fig. 39. — Diagram representing a very young human ovum almost immediately after its entrance into the decidua, and whilst the place of its entrance is still covered with a plug of fibrin. Tlie ectoderm has already proliferated and embraced spaces which contain maternal blood and are continuous with the maternal blood-vessels. tion, and the modified mucous membrane is known as the uterine decidua. These changes are, for the most part, hypertrophic ; the vascularity of the mucous mem- brane is increased mainly by the dilatation of its veins and caxDillaries, the tubular uterine glands become Decidua basalis , C Unchanged part Gland i -r> i ^ i ^ L Dilated pait- ^"W-.-J — -i Decidua vera elongated, irregular, and tortuous, and they dilate both at their orifices and in the deeper part of the mucous membrane ; at the same time the inter - glandular connective tis- sue proliferates, and as a result the decidua is thicker, softer, and more spongy than the unaltered mucous membrane. After the developing ovum enters the uterus it comes into contact with the decidua, into which it forces its way destroying the surface epithelium, and destroy- ing, or pushing aside, the superficial portions of the glands. In this way it becomes embedded in the vascular interglandular tissue Unchanged layer stratum spongiosum Stiatum compactum, Blnod-vessel Uterine 'mucosa Ectodermal villus Inner cell-mass (Entoderm) Cavity of uterus Fig. 40. — Diagram, showing Decidua vera the relation of the young the decidua. human ovum to The ectoderm is distinct from the inner cell-mass, but as yet there is no entodermal cavity in the latter. Tlie orifice which it makes in the superficial surface of the decidua is plugged, for a time, by a mass ot fibrin, but all traces of the aperture which the fibrin closes eventually disappear and the ovum is entirely (incapsuled in the decidua, which again presents an unbroken surface towards the uterine cavity. 4a 54 GENERAL EMBRYOLOGY. Immediately after the ovum becomes embedded the superficial ectoderm cells of its surface ^jroliferate rapidly and form numerous branching processes or villi which invade the surrounding decidua, destroying its cells and gradually enclosing the maternal blood spaces Unchanged layer Deciduii basalis Maternal vessel Stratum spongiosum Stratum com pactum Placental villus Primitive streak Mesoderm Placental villus Cavity whicli becomes ccelom Decidua vera JUecidua vera with which they come into relation. After a time the maternal tissues which at first surrounded the maternal blood spaces entirely disappear, and from this period onwards the maternal blood circu- lates in the spaces in the foetal ectoderm. After a time the proliferation of the ectodermal vilh be- comes most marked im- mediately around the margin of the amniotic area of the ovum, and thus a placental area of the chorion is differ- entiated. In the mean- time the expanding ovum has forced the decidua on its superficial surface to- wards the cavity of the uterus, and at this period three areas of the decidua can be distinguished. The part outside the ovum, that is between the ovum and the muscular wall of the uterus, is known as the decidua basalis ; the part pushed into the cavity of the uterus around the growing ovum is the decidua capsularis ; and the remainder of the decidua is the decidua vera. The decidua cap- sularis is not formed, as was formerly be- lieved, by folds of the decidua which have grown up and sur- rounded an ovum merely attached to the surface of the decidua basalis. On the contrary it is merely the superficial part of that portion of the decidua into Fig. 41. — Diagram, showing a ftirther stage of development of the human ovum and its relation to the decidual tissues. The entodermal cavity or yolk-sac has appeared in the inner cell -mass, and the mesoderm has commenced to extend from the primitive streak in two layers, splanchnic on the yolk-sac and somatic on the ectoderm. Unchanged layer stratum spongiosum. Stratum compactuof^ ^Placental villus Maternal vessel Body-stalk /"^Ectoderiii Somatic mesoderm Entoderm Splanchnic mesoderm Decidua vera Decidua vera Fig. 42. — Diagram, .showing the completion of the decidna capsularis, the enlargement of the maternal blood-vessels in the stratum conipactum of the decidua basali.s, the increase of the placental villi, the formation of the amnion folds, and the appearance of the allantoic diverticnlum. which the ovum has penetrated, therefore it may contain glands or the remains of glands which open on its superficial surface into the uterine cavity, but no glands open on its inner surface. The decidua basalis lies in contact with the placental area of the chorion, i.e. the fcetal part of the placenta, and it forms the maternal part of the organ. In the fully developed human placenta, the foetal and maternal tissues of which it THE PLACENTA. 55 DeciduaAbasalis Unchanged layer Stratum spongiosum Stratum compactum Placpntal villus Amnion Maternal blood-vessol Foital ectoderm villus, enclosing space filled with matPinal blood Allantoic diverticulum Ectodeim Somatic mesoderm Splanchnic mesoderm Entoderm Decidua vera' Decidua Yem is formed are so intimately mingled and blended together that it is impossible to say where one ends and the other begins. By a careful study, however, of a series of placentse of different ages a fairly clear and satisfactory idea of the i^art played by the maternal and the foetal elements respectively, as well as of their relations to each other, may be obtained. The structural char- acters of the completed organ will be best under- stood if the two con- stituent parts are first considered separately. Foetal Part of the Placenta. — The villi of the placental portion of the chorion invade and penetrate the decidua basalis, whilst the villi of the non-placental chorionic area of the ovum enter the decidua capsularis. As previously ex- plained, in connexion Fig. 43. — Diagram, showing enlargement of the blood sinuses in the maternal with the formation both P^'^*' °f ^^® placenta and the closure of the amnion. of the amnion and of the chorion, the annular placental area is converted into a circular disc. It consists, like the rest of the chorion, of ectoderm and mesoderm, and it contains Decidua basalis ramificatious of the allantoic vessels ; but the ectoderm is thickened and increased, its villi are larger than those of the non- placental region of the chorion, and it is directly con- nected with the allantoic stalk. The early villi are merely ecto- dermal buds which penetrate the sur- rounding decidual tissues, destroying and replacing the uterine elements. These villi grow and branch, and their branches an- astomose together Fio. 44. — DiAGUA.M, showing the Itctal ectoderm surrounding the maternal blood SUrrOUllding the sinuses, the commencement of secondary fmtal villi which project into the l)lood SDaCCS of the sinuses, and the disappearance of the supcrliiial iiOTtions oftlie glands. , . , ' ^ i • i decidua which are in the immediate neighliourhood oi' the ovum. As development proceeds every ectodermal villus is penetrated Ijy an outgrowth of the subjacent mesoderm Unchanged layer Stiatum spongiosum Placental -s illus Maternal blood sinus Maternal vessel Placental villus ^Allantoic stalk Foetal villus Decidua \eia Ectoderm Somatic mesoderm Splanchnic mesoderm Entodeim Decidua vera 56 GENEEAL EMBEYOLOGY. Unclianged lay Stratum spoiigiosuni Stratum compactin Maternal blood sinus Foetal villus Allantoic diverticulum ^Ectodenii which carries branches of the allantoic vessels, and so the villi become vascularised with fcetal blood. For some time all the villi, placental and non-placental, grow and absorb nutriment from the maternal tissues, pro})ably utilising as food the tissues which they destroy and replace ; but when the decidua capsularis is thinned by the expansion of the growing ovum, the villi of the non-placental region which have penetrated it are no longer able to obtain nutrient matter, and they con- sequently atrophy and disappear. The placental villi, on the contrary, continue to increase ; they grow in size and become more complex, and secondary branches growing from them project into the maternal blood spaces which they have surrounded and float in the maternal blood. When the formation of the placenta is completed, its foetal part consists of villi, each of which possesses an external covering of two layers of ectodermal cells and a vascular mesodermal core ; the villi project into the interior of large blood spaces which are surrounded more or less completely by foetal ectoderm, and they are bathed by maternal Vjlood from which they ob- tain the materials necessary for the nutrition and growth of the em- bryo, and into which they trans- mit the effete ex- cretory matters from the embryo. Maternal Part of the Pla- centa and the Changes in the Decidua. — The occurrence of further changes in the decidua, after the developing ovum enters the uterus, is dependent upon the retention of the ovum in the substance of the decidua. These changes only occur, therefore, in ' what may be termed the decidua of pregnancy. They are intimately associated with and essential to the development of the maternal part of the placenta, and a more detailed and complete account of the decidua and the modifications of its several parts is therefore necessary. The decidua is formed by the mucous membrane of the uterus, which is a hollow, thick-walled muscular organ, situated in the pelvic cavity. The mucous membrane contains numerous tubular glands embedded in an interglandular tissue formed of round and irregular cells. The uterine glands are lined by a columnar or cubical epithelium, and they open into the cavity of the uterus on a surface which is also covered by columnar cells. The whole of the nmcous membrane is plentifully supplied with blood-vessels which pass into it from the surrounding muscular walls, and it is transformed into the decidua by proliferation and hypertrophy of all its parts. The interglandular tissue increases in amount and its blood-vessels dilate, especially near the surface of the membrane; but the most striking of the early changes occur in the glands — they become longer, more tortuous, their Coelom Somatic mesoderm Splanchnic mesoderm Entoderm Decidua capsularis Decidua vera Fig. 45. — Diagram, showing further growth of the placental sinuses and villi ; the fusion of the decidua capsularis with the decidua vera, and the obliteration of the uterine cavity. THE PLACENTA. 57 apertures enlarge and assume a funnel-like appearance, and they dilate a short distance from their terminations into large iiTegular spaces. The increase of the interglandular tissue is most marked in the intervals between the dilated portions of the glands and their apertures, and when all the changes are fully established it is possible to recognise three layers of the decidual tissue. (1) A superficial relatively thick layer in which the interglandular tissue xjreponderates, the stratum compactum ; (2) A layer formed principally by the dilated portions of the glands, the stratum spongiosum ; and (3) a thin deep part of the membrane which contains the outer extremities of the glands which are practically unchanged, the unchanged layer. The decidua capsularis differs from the other portions of the decidua in that it represents only the superficial portion of the other parts of the decidua, and therefore contains no spongy layer or deep unchanged layer. This is obviously the case, for after the ovum has penetrated through the surface epithelium it becomes embedded in the stratum compactum of that portion of the decidua which it has invaded, and consequently the portion of the stratum which closes over it cannot contain more than the outer parts of some of the uterine glands with their orifices and the intervening interglandular tissue. The changes which occur in the decidua capsularis are due, first, to its connexion with and invasion by the chorionic villi ; and, secondly, to the pressure exerted upon it by the enlarging ovum. The former influence is brought to bear whilst the decidua is still increasing ; the latter after it has reached its full development. The changes which result from its union with the chorion are the destruction and absorption of some of the interglandular tissue ; they are due to the activity of the ectodermic cells of the chorion, which attack and invade the uterine tissues. The changes due to pressure exerted by the enlarging ovum are diminution of vascularity, disappearance of the lumina of such portions of the glands as remain in the decidua capsularis, and the removal of their epithelium, together with the co- incident atrophy of the fcetal villi which have penetrated this portion of the placental decidua. All these changes result in the reduction of the decidua capsularis to a thin membrane in which no traces of the original structure are recognisable, in the fusion of the altered decidua capsularis with the decidua vera, and the consequent obliteration of the uterine cavity. After the fifth month the decidua vera also undergoes atrophic changes, but they do not proceed so far as in the decidua capsularis ; nevertheless the stratum com- pactum is greatly reduced, the superficial epithelium and the superficial parts of the glands entirely disappear from it, the interglandular tissue becomes less vascular, and it diminishes very considerably in thickness. The epithelium dis- appears from the spaces in the spongy layer, and the spaces themselves are flattened out into long slit -like clefts, in which condition they remain till the period of pregnancy is completed. The decidua vera is thus reduced to the condition of a relatively thin membrane, and its inner surface is fused with the remains of the decidua capsularis. Decidua Basalis. — This portion of the decidua is constituted by the deeper part of the stratum compactum in which the ovum is embedded together with the more externally situated spongy and unchanged layers, and, apart from the changes due to the invasion of the foetal villi, the most important transformations in this part of the decidua occur in the stratum compactum. The alterations in the spongy layer are similar to those which occur in the same layer of the decidua vera, viz. the lining epithelium disajjpears and the spaces are flattened out into a layer of cleft-like slits. In the stratum compactum, however, much more striking clianges occur ; all traces of the glands disappear, Ijut the blood-vessels become greatly dilated, and, consequently, tlie layer increases considerably in thickness. The small blood-vessels wliJcli lie in the immediate neighljourhood of tlie ovum beconie converted into enormous blood sinuses, Ijut in the deeper part of the stratum a thin layer, which lies next the stratum spongiosum, remains relatively unchanged; this deepc-r part is called the basal layer, and through it the blood-vessels pass to and from the 58 GENERAL EMBRYOLOGY. blood sinuses in the more snperticial portion of the membrane. When it is completed, therefore, the maternal portion of the placenta, which is the transformed decidua basalis, no longer consists of the stratum compactum, the stratum spongiosum, and the unchanged layer, but it is formed from within outwards of — (1) a layer of blood sinuses, (2) the basal layer, (3) the modified spongy layer, and (4) the unchanged layer. The difference between the decidua Ijasalis and the maternal part of the placenta may be tabulated as follows : — Decidua basalis. Deep part of stratum compuctuni Stratum spongiosum Unchanged layer Maternal placenta. Layer of blood sinuses. Basal layer. Modified stratum spongiosum. Unchanged layer. Unchanged layer- Stratum spongiosiini^-^ Stiatnm com pact 111 II PliCfntal villu It must not be forgotten, however, that whilst the changes which result in the formation of the maternal placenta out of the decidua Ijasalis are taking place the stratum compactum has been invaded by the placental villi. The first result of this invasion is the destruction of much of the decidual tissue by Placenta the ectodemi of the foetal villi. Gradually the ec- toderm of theA'illi, always in advance of the main body, reaches and sur- rounds the dilated decidual vessels, destroys the in- tervening tissues, and ultimately re- places the endo- thelial walls of the vessels, which by this time have dilated into enor- mous spaces. Into these spaces the ramifications of the vilh project, and, as the endo- thelial walls are destroyed, they lie directly within the cavities of the spaces, and are surrounded on all sides by maternal blood. The most peculiar feature of this part of the placenta, when fully developed, is that the whole of the maternal por- tion of it, except the blood, has been removed and replaced by foetal tissues, so that, although the maternal blood continues to circulate in the same spaces which it has occupied from the first, viz. the blood sinuses in the more superficial part of the stratum compactum of the maternal decidua, yet the walls of these spaces have been replaced more or less Peri- cardium Fused decidua cap- sulaiis and decidua \eia I''used mesoderm of amnion and cliorion Fig. 46. — Diagraji. Later stage in the development of the placenta, showing the relations of the fcetal villi to the placental sinuses, the fusion of the amnion with the inner surface of the chorion, and the thinning of the fused deciduaj (cap- sularis and vera). THE PLACENTA. 59 completely by foetal ectoderm, and, consequently, the spaces now lie in the midst of the fcetal tissues. The invasion of the maternal by the fcetal part of the placenta proceeds as far as the basal layer, and in this region the foetal ectoderm is directly continuous with the walls of the maternal blood-vessels at the points where they enter the sinuses. Although the invasion of the deeidua basalis is so complete, some portions of the maternal tissues persist ; thus the basal layer and many strands of the stratum compactum escape destruction. The latter extend from the basal layer to the outer surface of the chorion, and they are eventually converted into fibrous strands, which divide the superficial part of the completed placenta into lobular areas. The completed placenta consists, therefore, of closely intermingled and fused foetal and maternal tissues, through which both the foetal and maternal blood streams circulate. It is well adapted, on account of its peculiarities of structure, to fulfil the nutritive and respiratory requirements of the embryo. The fcetal blood stream which flows through the placental villi and the maternal blood stream in the placental sinuses are only separated from each other by two layers of foetal ectodermal epithelium and a small amount of foetal mesoderm, the latter being practically reduced to the single layer of endothelial cells which form the walls of the foetal vessels. Through these layers, by osmosis, and possibly by secretion, materials are passed both from mother to embryo and from embryo to mother, the placenta serving not only for purposes of nutrition and respiration, but also as an excretory organ. A^hilst the placenta is attaining its full development the amnion is expanding, and finally its outer surface fuses with the inner surface of the chorion, consequently, the innermost portion of the placenta is provided with a covering of amnion. The full-time placenta is a discoid mass about 20 or 25 inches (50 to 60 cm.) in circumference and 1^ in thickness at its centre ; it is much thinner, however, at its margins, where it is continuous with the membranes formed by the fused chorion, deeidua vera, and deeidua capsularis. Its weight is about one pound, and it consists from within outwards of the following layers : — Foetal Maternal Amnion ( Ectoderm. ( Mesoderm. Allantois with fcetal vessels . . Mesoderm. Chorion \ Mesoderm. I xLctoderm. Layer of maternal blood sinuses and remains of the intergiandular tissue of the stratum compactum. Basal layer. Modified spongy layer. Unchanged layer. When the period of intrauterine life is completed the muscular walls of the uterus contract and the lower orifice of the uterine cavity is dilated, the fused chorion and amnion, which close the upper part of the orifice, rupture and the amniotic fluid escapes, the foetus is then expelled, but it remains attached to the placenta by tlie umbilical cord. The cord is divided artificially, and after a short period the jjlacenta and membranes are expelled. The membranes attached' to the ])lacenta consist of tlie fused anmion, chorion, deeidua capsularis, and also the deeidua vera internal to the altered spongy layer ; therefore botli the placenta and the jnembranes consist of maternal and foetal tissues.. IBefore tlie placenta and membranes are expelled the uterine deeidua is separated into two parts l)y a cleavage which takes place in the modified stratum spongiosum. The inner portion which includes the placenta and membranes is cast off. The outer portion remains in the uterus; it consists almost entirely of the deep unchanged layer of the deeidua, and from it the uterine mucous membrane is reconstructed. 60 GENEEAL EMBRYOLOGY., THE PRIMITIVE VASCULAR SYSTEM AND THE FGETAL CIRCULATION. It has already been said that the ovum during its passage down the Fallopian tube lives either on its own yolk particles or upon substances absorbed from the fluids by which it is surrounded. For a time after it enters the uterus its nutrition must be provided for in a similar manner, but as soon as the chorionic villi are formed it is probable that the ectodermal cells, of which in the earliest stages they entirely consist, and wluch cover their surfaces in the later stages, actually eat up the decidual tissues which they invade and use them for food. This source of nutrition, however, is only sufficient for the short period during which the ovum remains relatively small, and substances absorbed through the surface cells can be readily transmitted to all its parts. In addition to the solid decidual tissues devoured by the ectodermal cells it is evident that fluids from the mother are also absorbed, for the yolk-sac and crelom enlarge and are filled with fluid. The only sources from which this can have been derived are the uterine glands or the blood and lymph vessels of the decidua. In all probability the fluids absorbed into the ovum contain nutritive material, and so long as the embryo is constituted by the thin layers of the early blastoderm sufficient food material can easily be absorbed. When, however, the various parts of the embryo increase in thickness and become moulded into the form of organs they are no longer in such intimate relation with the surrounding nutritive fluids, whilst, further, as their development progresses they require a greater amount of food and oxygen than they can obtain from these fluids. There is, therefore, an imperative necessity for a further supply of nutritive material by which their re- quirements may be satisfied, faihng which, development must cease and death ensue. To meet this necessity the vascular system is formed. It is essentially an irrigation system consisting of a propulsive organ, the heart, and of tubular vessels, the blood-vessels, all of which contain blood. The heart propels the blood through the blood-vessels to all parts of the embryo, but the blood which is at first formed from the mesoderm of the ovum must, at least so far as its fluid part is concerned, be supplemented largely from maternal sources. It is necessary, there- fore, that the foetal blood-vessels be brought into close relation with the maternal blood-vessels at an early period. It is for this purpose, amongst others, that the large blood sinuses are formed in the maternal portion of the placenta, and that they are surrounded and invaded by the fcetal villi, carrying in their interior branches of the foetal blood-vessels, and as previously shown, the foetal blood- vessels in the placenta are only separated from the maternal blood in the sinuses by their own thin mesodermal walls, and by one or two layers of ectodermal cells. When the placenta is fully formed fluids can readily pass from the maternal to the foetal vessels, and there can be no doubt that both food and oxygen pass from the maternal blood to the foetal blood through and by the agency of the intervening cells, whilst at the same time the waste products which are formed in the embryo pass outwards to the maternal blood. Ob\dously, however, a system of vessels filled with fluid would be of little use in the general economy unless there w^ere some means by which the fluid could be kept in constant movement. In the first instance this is accomplished by rhythmical contractions of the vessel walls, but in a short time portions of the two primitive stem-vessels which appear in the embryo are modified into a single propulsive organ, the heart, which forces the fluid, or blood, in a definite direction both through the body of the embryo, along the body-stalk or umbihcal cord, accord- ing to the age of the embryo, and through the vessels in the placental villi. We have now to consider how the blood-vessels and blood are formed. Where, or how, the first blood-vessels appear in the human subject is not definitely known, but in other mammals they are first seen outside the body of the embryo in the wall of the yolk-sac. The outer layer of the wall of the yolk- sac consists of splanchnic mesoderm, and in that part of this layer which lies nearest the primitive alimentary canal a large number of the cells proliferate PEIMITIVE VASCULAR SYSTEM AND FCETAL CIRCULATION. 61 rapidly and, fusing together, form nmlti-nucleated masses of protoplasm, the " blood islands " of Pander. Soon after their appearance the blood islands anastomose together by means of nucleated processes which they throw out on all sides, and thus a nucleated proto- plasmic reticulum is formed in the substance of the splanchnic mesoderm. The region in which this occurs is known as the vascular area. The solid nucleated reticulum is soon converted into a system of anastomosing canals, the primitive blood-vessels, by the ap- pearance within it of numerous vacuoles which soon fuse together, whilst at the same time the nu- cleated protoplasm is trans- formed into cells. The cells which lie nearest the in- terior separate from each other and form the primi- tive blood -corpuscles, whilst those situated ex- ternally remain connected by their margins and form the endothelial walls of the embryonic vessels. The fluid which fills these first- formed vessels in the vas- cular area is probably de- rived either from thecoelom or from the yolk-sac. The primitive blood - corpuscles are nucleated cells of a reddish colour ; white or colourless blood- corpuscles appear later, and it is stated that those first formed are developed in the thymus gland. Nucleated red cor- puscles persist and increase in number till the end of the second month of in- trauterine life ; they are then gradually replaced by non-nucleated red corpus- cles. The majority of the nucleated red corpuscles disappear long before birth, but a few can usually be found in the blood of the new-born child. There is some doubt about their ultimate fate, but it is generally believed that their nuclei disappear, and that they are converted into non-nucleated corpuscles. Directly after the appearance of the blood islands in the vascular area of the yolk-sac, and just as the folding off of the embryo commences, two short tubular vessels appear in the splanchnic layer of the pericardial mesoderm. These vessels at once extend forwards and outwards into the extra-embryonic region where they become connected with the vessels of the vascular area ; they also extend back- wards in the body of the embryo beneath the protovertebral somites. In the majority of mammals they at first terminate behind, as in front, on the wall of the yolk-sac, but after a time the main stems appear to be continued along the allantoic stalk to tlie placenta, whilst they give olf branches to the yolk-sac. It is ])robable that in the htmian cnd^ryo also, tliough this has not a])])arently been actually ob.sorved, these main stem vessels, the jirimitive aorttu, end at first on the wall of EN Fig. 47. BV2 -Development of Blood-Vessels in the Vascular Area op THE Rat. " blood III. Entoderm aud splanchnic mesoderm. Proliferation of cells of mesoderm and formation of islands." Commencing differentiation of islands to form blood-vessels and blood-corpuscles. IV. Completed vessels. BC Blood-corpuscles. BVo Blood-vessels. BI Blood islands. EVj Blood islands being trans- EN Entoderm, formed into blood-vessels. M Mesoderm. 62 GENEEAL EMBEYOLOGY. the yolk-sac, but on the fourteenth day of intrauterine life, before the heart is formed, the two primitive stem vessels pass backwards along the body-stalk to the chorion, their terminal branches entering the chorionic villi. As they pass back- wards the primitive aortse give off branches to the wall of the yolk-sac. Thus, at this period the vascular system of the human embryo consists of two longitudinal vessels which run parallel with each other, one on each side of the middle line, throughout the whole length of the embryo. They comnumicate anteriorly with the vessels on the yolk-sac, and terminate posteriorly in the chorion. When the circulation commences the blood flows from the anterior part of the vascular area into the anterior ends of the primitive aortse, and passes backwards through the embryo. Some of it is returned to the vascular area by the branches which are given off to the walls of the yolk-sac ; but the greater part is carried to the chorion, whence it returns by venous channels, the allantoic veins, which have been developed in the meantime, to the anterior ends of the primitive aort^. As the cephalic and caudal folds are developed the anterior and posterior parts of the primitive aortse are carried into the ventral wall of the body of the embryo, and thus each primitive vessel is divisible into three parts : (1) a dorsal part, the primitive dorsal aorta, which extends from the dorsal end of the mandibular arch to the cloaca, and runs beneath the protovertebral somites ; (2) an anterior ventral part, situated in the dorsal wall of the pericardium and extending from the umbilicus to the ventral end of the man- dibular arch ; and (3) a posterior ventral part, which at first runs in the ventral wall at the side of the cloaca, and then turns backwards in the body -stalk to the placenta, but afterwards, when the posterior part of the ventral wall of the body is completed, it extends forwards from the pelvic region to the umbihcal orifice, through which it passes to the umbilical cord. The three sections are united together by two arches — an anterior arch, the first cephalic aortic arch, which passes through the mandibular arch, and a posterior arch, the caudal arch, which lies at the side of the cloaca. In a short time four additional communica- tions are formed between the anterior ventral and the dorsal part of each primitive aorta; they are the second, third, fourth, and fifth cephalic aortic arches, each of which lies in the substance of the corresponding visceral arch. As soon as the last cephalic aortic arch is developed the rudiments of the main vessels of the embryo are established; and by a series of transformations, for a full account of which the chapter which deals with the Vascular System must be consulted, there are formed from the vessels which have been mentioned the heart, the aorta, the main vessels of the head and neck, the pulmonary artery and its primary branches, the common and internal iliac arteries, and the hypogastric arteries. The blood distributed by the various arteries is returned to the heart by vessels called veins, 'which are developed in the substance of the mesoderm in the same manner as the arteries. From the yolk-sac the blood returns by the vitelline veins ; from the alimentary canal and its appendages, through the portal and hepatic veins ; from the head and neck, by the jugular veins and the superior vena cava ; and from the body and lower limbs, first by the cardinal veins, and afterwards by the inferior vena cava and the azygos veins. The heart is formed by the fusion of portions of the anterior ventral sections of the primitive aortse behind the origins of the cephalic aortic arches, and, therefore, it is primitively a bilateral organ. Subsequently it possesses for a time a single chamber, but this is afterwards divided. During the greater part of foetal life the Cephalic aottic arch Anterior vential aorta Primitive dorsal aorta Vitelline vein Umbilical vein Splanchnic arteries to vascular area Vitelline artery Poster or ventral aorta ■ Primitive caudal arch Hypogastric artery .Chorioiiie vessels Fig. 48. — The Primitive Blood- Vessels OF THE Embryo. PRIMITIVE VASCULAR SYSTEM AND FGETAL CIRCULATION. 6J heart, as in the adult, possesses four cham?jers — two auricles or upper chambers, and two ventricles or lower chambers, right and left. The two auricles communicate with the corresponding ventricles through auriculo- ventricular apertures, and with each other through a foramen, the foramen ovale, in the septum between them. In the adult the blood enters the right auricle by the superior and inferior venae cavse and the coronary sinus ; from the right auricle it passes into the right ventricle, by which it is propelled through the pulmonary arteries and lungs ; re- turning to the heart by the pulmonary veins it passes into the left auricle, and then into the left ventricle, by the contraction of which it is forced into the systemic aorta. From the aorta, by various branches, it traverses the organs and tissues of the body, and is returned again to the right auricle. The course of the foetal circulation differs from that of the adult ; the blood passes out of the body into the placenta, to be oxygenated and purified, the lungs of the fcetus remaining functionless until the time of birth. Very little of the blood which is ejected from the right ventricle at every contraction of that chamber reaches the lungs; the greater part is transferred from the pulmonary Caudal arches Dorsal aortse 7th pair of segmental arteries Vertebral arteries 1st pair of segniental arteries Umbilical vein Splanchnic arteries Hypogastric artei-y 1st ceplialio aortic arch 2ncl cephalic aortic arch 3rd cephalic aortic arch 4th cephalic aortic arch 5th cephalic aortic arch Aortic bulb Ventricle Yolk-sac Auricle Sinus venosus Vitelliue vein Fig. 49. — Diagram of the Blood-Vessels of a Mammalian Embeto after the formation of the Heart. artery to the aorta by an anastomosing channel, the ductus arteriosus, which disappears after the pulmonary circulation is established. During the later months of fcetal life, blood enters the right auricle by the superior and inferior venae cavse and through the coronary sinus; only a small amount of blood, viz. that returning from the walls of the heart, enters the right auricle through the coronary sinus. The blood poured into the right auricle by the superior vena cava is returned from the head, neck, upper extremities, and the thoracic walls; passing from the auricle by the right auriculo-ventricular opening it enters the right ventricle ; from the right ventricle it is forced into the pulmonary artery, and a small part of it traverses the lungs and returns to the left auricle Ijy the pulmonary veins ; the main part, however, is conducted by the ductus arteriosus into the aorta at a point beyond where the main vessel of supply to the left upper extremity, the left subclavian artery, rises. The IjIoocI wliicli enters the right auricle by the inferior vena cava is mixed ; it consists partly of purified blood from the placenta, and partly of impure blood returning i'rom the abdomen and lower extremities. The blood Irom the placenta is returned to the embryo })y the umbilical vein. From the umbilical vein it passes along a channel called the ductus venosus, which terminates in the upper part of the inferior vena cava. I'he mixed l>lood from the inferior vena cava ])asses through tlie right auricle, traverses the foramen ovale in the interauricular septum, and 64 GENEKAL EMBEYOLOGY. enters the left auricle ; from the left auricle it is transferred to the left ventricle through the left auriculo-ventricular opening, and the left ventricle ejects it into the aorta. From the first part of the aorta some of the blood passes into the vessels which supply the head and neck and upper extremities, the remainder Internal jugular vein External jugular vein Right vertebral artery , Right subclavian artery Right subclavian vein Innominate veins Pulmonary artery Superior vena cava Vena azygos major Right auricle Right ventricle Hepatic vein Inferior vena cava Intercostal veins 1st cephalic aortic arch Internal carotid artery 2ncl cephalic aortic arch Kxterual carotid artery 3rd cejjhalic aortic arch Vertebral artery ~= — Subclavian artery 4th cephalic aortic arch Superior intercostal vein 5tli cephalic aortic arch ■Pulmonary artery Vena azygos minor superior ft auricle Vena azygos minor inf'eri Atrophied cardinal vein Poi'tal vein Renal vein Lumbar vein Common iliac arfcer External iliac artery Internal iliac artery _1 fVorta Atrophied cardinal vein Placenta pcgastric arteries Middle sacral vein 'Fig. 50. — Diagram of the Fcetal Circulation. mixes with the blood conveyed to the aorta by the ductus arteriosus, and the blood, thus further mixed, is in part distributed to the walls of the thorax and abdomen to the abdominal viscera and to the lower extremities, and in part it passes to the placenta. Before birth, therefore, there is no pure arterial or fully oxygenated blood in the arteries of the foetus. The blood entering the heart by the superior vena cava IS venous blood from the head, neck, upper extremities, and thorax ; that entering by the inferior vena cava is mixed blood, consisting of venous blood from the lower PEIMITIVE VASCULAR SVSTEM AND F(KTAJ. (JIRCULATION. 65 part of the body and the lower extremities, and arterial blood from the placenta. The two streams do not mix in the right auricle, but the mixed or more arterial stream passes directly through the right into the left auricle, thence into the left ventricle, and from the left ventricle into the aorta or main systemic vessel, which conveys it to all parts of the body. The different parts of the body do not, however, receive equally oxygenated blood, for the venous stream which enters the right auricle by the superior vena cava, passes through that cavity into the right ventricle ; by the right ventricle it is forced into the pulmonary artery, from which some small part passes into the lungs, and so back to the left auricle by the pulmonary veins, but by far the greater part is carried by the ductus arteriosus to the aorta, which it enters beyond the origins of the vessels which supply the head, neck, and upper extremities ; therefore the blood in the lower part of the aorta, which is distributed to the abdomen, the abdominal viscera including the liver, the lower limbs, and the placenta, is much more mixed or impure (less oxygenated) than that which is dis- tributed to the head, neck, and upper extremities from the upper part of the aorta. SUMMARY OF THE EXTERNAL FEATURES OF THE HUMAN EMBRYO AT DIFFERENT PERIODS OF DEVELOPMENT. First week. — The phenomena of fertilisation and segmentation have not been observed in the liuman ovum, but there is no reason to beheve that they differ in any essential respect from those met witli in the ova of other mammals. Fertilisation probably occurs in the upper part of the Fallopian tube, and segmentation is completed in the lower part of the same canal by the sixth or eighth day, when, presumably, the ovum becomes a morula, and passes, either as such or as a blastula, into the cavity of the uterus. Second week. — At the twelfth day the ovum is embedded in the uterine wall ; it is a lenticular vesicle, which measures 5-5 mm. ^^ ^ (i of an inch) in length and 3"3 mm. (|- of an inch) in breadth. Its upper and lower surfaces are smooth and convex, the latter being somewhat flatter than the former, and it is surrounded equatorially by a broad band of villi, some of which are slightly branched. The wall of the vesicle and the villous processes which project from it consist of ecto- AM A B Fig. 51. Human embryo at the end of the 12th day of development ; B. At the end of the 13th day of development ; C. At the end of the 14th day of development. (After His.) Amnion ; AS. Allantoic stalk ; BS. Body-stalk ; CV. Chorionic villi on a segment of the chorion ; E. Embryo ; H. Head of emVjryo ; PR. Peri- cardial region ; SS. Stomatodasal depression ; YS. Yolk-sac. dermal cells, and in the embryonic area, which is clearlj^ marked on the upper surface, there is an inner layer of granular nucleated corpuscles. By the end of the twelfth or the beginning of the thirteenth day the length of the ovum has increased to 6 mm. (| inch), and its breadth to 4-5 mm. (,l inch). The embryonic area is no longer on the surface of the ovum, for the amnion folds have closed. The yolk- sac is formed, and the rudiment of the allantoic duct projects backwards from the upper and posterior part of the embryonic area. Mesoderm has formed, and it has extended round the yolk-sac and over the inner siirface of the chorion. The embryonic area, with the yolk-sac and the amnion, are enclosed within the blastoderm, but they remain attached to the inner surface of the chorion by a relatively thick stalk of ectodermal and mesodermal tissue, the body-stalk, which is subsequently replaced by the umbilical cord. The outer surface of the ovum, which now consists entirely of chorion, is covered witli small villi into some of which mesodermal cores are projecting. Oliviously the ovum of the latter part of the twelfth day dift'crs considerably from tliat of the earlier part of the same day, but the transitional stages which intervene between the two have not yet been observed. Probably, however, the iiuier granular layer of cells in the embryonic area, which rejjresent the ent(;derm, increase and form a solid mass in which a cavity soon appears. Directly after the formation of the cavity in the entoderm the primitive streak appears, and the mesoderm, growing from it, rapidly 5 66 GENERAL EMBRYOLOGY. oovers the entodermal sac and spreads over the inner surface of the chorionic area. At the same time the amniotic folds form and separate from the chorion, but tiiis separation is not effected till the mesoderm, extending backwards from the posterior end of the embryonic area, has reached and becomes connected with the inner surface of the chorion. Consequently, when the amniotic folds fuse together and separate from the remainder of the blastoderm, the embryonic area with the yolk-sac and amnion still remain attached to the inner surface of the chorion. During the thirteenth day the embryonic area is elevated, the cephalic and caudal folds are developed, and the pericardial region becomes prominent between the head extremity of the embryo and the upper and anterior part of the yolk-sac. The neural groove and the neural folds appear ; the posterior ends of the folds embrace tlie anterior extremity of. the primitive streak on which the primitive groove is formed. At the anterior end of the primitive groove a neurenteric canal appears, forming a communication between the neural groove and the posterior end of the primitive alimentary canal On tlie fourteenth day the embryo is more distinctly separated from the yolk-sac ; the head increases considerably in size, and its anterior part is bent downwards. The posterior part of the neural canal is completed, except at the extreme end, by the meeting and fusion of the neural folds, but it is still open anteriorly, where traces of the cerebral vesicles are present. The two halves of the heart fuse together ; the single tube thus formed is sliglitly bent upon itself, and its outline is visible from the exterior. The pericardial region increases in size, and a distinct stomatodeeal sp)ace appears between it and the anterior part of the head. The outlines of fourteen protovertebral somites are visible on the outer surface of the body. Fig. 52. D. AS. Human embryo at tlie '21st day of development ; E. At the 23rd day of development ; F. At the 27th day of development. (After His. ) Allantoic stalk ; BS. Body-stalk ; CV. Chorionic villi on a segment of the chorion ; EY. Eye ; FL, Fore- limb ; H. Head ; HA. Hyoid arch ; HL. Hind-lirnb ; MA. Mandibular arch ; MB. Mid-brain ; MP. Maxillary process ; OP. Olfactory pit ; OV. Otic vesicle ; PR. Pericardial region ; PS. Protovertebral somite ; SS. Stomatodseal space ; UC. Umbilical cord ; VC. Visceral cleft ; WR. Wolffian ridge ; YS. Yolk-sac. Third week. — On the fifteenth day the auditory pits and two visceral clefts appear. The head and pericardial region enlarge, and the stomatodaeal space, which increases simultaneously, becomes more defined laterally by the forward growth of tlie maxillary processes. By the end of the third week Wolffian ridges appear below the ventral ends of the protovertebral somites ; they are most marked in the thoracic and pelvic regions, where bud-like projections form the rudiments of the limbs. Four visceral clefts ai-e visible, and there is a distinct tail. Fourth week. — The embi-yo is curved upon itself, and its outline is almost circular. The visceral arches begin to overlap each other. The rudiments of the external ear are just visible as small nodules. The limb rudiments are flat, oval buds. Fifth week. — The curvature is diminished, and the head and neck form about half the embryo. The eye is recognisable externally. The nose begins to grow forwards, but it is still broad and flat, and the nostrils are widely apart. The nodular elements of the external ear fuse together. The segments of the limbs are defined, but the digits do not project beyond the ends of the limb-buds. The genital tubercle, the rudiment of the external generative organs, is formed. Sixth week. — During the sixth week the increase in size is less rapid than in previous stages, but the embrj^o begins to assume a more distinctly human form. The HUMAN EMBRYO AT DIFFERENT STAGES. 67 head remains relatively large and it is bent at right angles to the body. The neck is better defined and appears as a constricted region between the head and trunk. The G H Fig. 5-3. G. Human embryo at the 29tli day of development ; H. At the 32ud day of development. (After His.) EB. Rudiment of ear ; EY. Eye ; FL. Fore-limb ; HA. Hyoid arch ; HL. Hind-limb ; MA. Mandibular arch ; MB. Mid-brain ; MP. Maxillary process ; OP. Olfactory pit ; PR. Pericardial region ; UC. Umljilical cord. maxillary processes fuse with the lateral nasal processes, and the lips and eyelids begin to assume their characteristic form. The outer parts of all the visceral clefts except the hyo-mandibular disappear. The external ear acquires its adult form. The rotation of Fl(i. 54. — HU.MAN i<'(KTUS AT THK SIXTH WEKK FiG. 55. — HUMAN FCETL'S SIX AND A 01' Devei.oi'.ment. (After Elis.) half weeks old. (After His.) D. Digits ; KE. Rudiment of ear ; FIj. Fore-limb ; HL. Hind-limb ; MP. Ma-xillary process ; N. Nose ; (JO. Umbilical cord. the limbs commences, and the fingers reach tiie extremity of tht; hand ; the tail is beginninj/ to disappear as an external yjrojection. Seventh week. -The flexure of the head upon the body is reduced. The nose fjrojccts more than in the preceding stage, and the chin a])])earH. The toes reach the margins of the feet, a)id the projecting ])ortion of the tail is still further reduced in length. Eighth week. — 'I'hc flexure of the licad disappears. The forehead projects. 68 GENERAL EMBRYOLOGY. The nose narrows and becomes more prominent, but the anterior nasal orifices are still directed forwards. The upper lip is completed by the fusion of the globular processes. The thumb is widely separated from the fingers, and the hand assumes a distinctly human appearance. The tail is reduced to a small nodule, and the umbilical coi-d is attached to the lower part of the abdominal wall. At the end of the second month the total length of the foetus, excluding the legs, is 28 mm. (1^ in.). Third month. — The head grows less rapidly and, though it is still large, it is relatively smaller in proportion to the whole body. The eyelids close, and their margins fuse together. The neck increases in length. The various parts of the limbs assume theirdefinite proportions, and nails appear on the fingers and toes. The procto- dseum is formed and the external generative organs are differentiated, so that the sex can be distin- guished on external examination. The skin is a rosy colour, thin and delicate, but more consistent than in the preceding stages. By the end of the third month the total length of the foetus, ex- cluding the legs, is 7 cm. (2 -| in.), including the legs, 9-10 cm. (34-4 in.), and it weighs from 100-125 grammes (31-41 oz.). Fourth month. — In the fourth month the skin becomes firmer, and fine hairs are de- veloped. The disproportion between the fore and hind limbs disappears. If the foetus is born at this period it may live for a fcAV hours. Its total length from vertex to heels is 16-20cm. (6|-8 in.), from vertex to coccyx 12-13 cm. (44-5i in.), and it weighs from 230-260 grammes (S^r-Ol oz.). Fifth month. — The skin becomes firmer, the hairs are more developed, and sebaceous matter appears on the surface of the body. The legs are longer than the arms, and the umbilicus is farther from the pubis. At the end of the month the total length of the foetus, from vertex to heels, is 25-27 cm. (10-104 in.), from vertex to coccyx 20 cm. (8 in.), and its average weight is about half a kilogramme (IjV lbs.). Sixth month. — The skin is wrinkled and of a dirty reddish colour. The hairs are stronger and darker. The deposit of sebaceous matter is greater, especially in the axillae and groins. The eyelashes and eyebrows appear. At the end of the month the total length of the foetus, from vertex to heels, is from 30-32 cm. (12-12f in.), and its average weight is about one kilogramme (2i lbs.). Seventh month. — The skin is still a dirty red colour, but it is lighter than in the previous month. The body is more plump on account of a greater deposit of sub- cutaneous fat. The eyelids reopen, and the foetus is capable of living if born at this period. Its total length at the end of the month, measured from vertex to heels, is 35-36 cm. (14-14| in.), and its weight is about one and a half kilogrammes (3| lbs.). Eighth month. — The skin is comj)letely covered with sebaceous deposit which is thickest on the head and in the axillae and groins, and its colour changes to a bright flesh tint. The umbilicus is farther from the pubis, but it is not yet at the centre of the body. The total length of the foetus, from vertex to heels, is 40 cm. (16 in.), and its weight varies from 2 to 2h kilogrammes (4^-5^ lbs.). Ninth month. — The hair begins to disappear from the body, but it remains long and abundant on the head. The skin becomes paler, the plumpness increases, and the umbilicus reaches the centre of the body. At the end of the ninth month, when the foetus is born, it measures about 50 cm. from vertex to heels (20 in.), and it weighs from 3-3-^ kilogrammes {^to''^tw lbs.). Tlie age of a foetus may be estimated approximately by Hasse's rule, viz. Uj) to the fifth month the length in centimeters, the lower limbs being included, equals the square of the age in months, and after the fifth month the length in centimeters equals the age multiplied by five. Fig. 56. — Human Fcetus eight and a halb^ WEEKS OLD. (After His.) GE. Genital eminence ; UC. Umbilical cord. OSTEOLOGY. THE SKELETON. By Aethue Thomson. The term skeleton (from the Greek, cT-KeAero?, dried) is applied to the parts which remain after the softer tissues of the body have been disintegrated or removed, and includes not only the bones, but also the cartilages and ligaments which bind them together. In the restricted sense of the word the skeleton denotes the osseous framework of the body. It is in this sense that it is generally employed in human anatomy. The skeleton serves to support the softer structures which are grouped around it, and also affords protection to many of the delicate organs which are lodged within its cavities. By the articulation of its several parts, its segments are con- verted into levers which constitute the passive portion of the locomotory system. Eecent research has also proved that certain cells found in bone -marrow are intimately associated with the development and production of some of the corpuscles of the blood. Bone may be regarded as white fibrous tissue which, having become calcified, has undergone subsequent changes, so as to be converted into true osseous tissue. Most probably all bone is of membranous origin, but it may pass through a stage in which cartilage plays an important part in its development. In many instances the cartilage persists, and is not converted into bone, as in the case of the articular cartilage which clothes the joint surfaces, the nasal septum, the cartilages of the nose, and the cartilages of the ribs. A persistence of the membranous condition is met with in man in the case of the tentorium cerebelli, which in some groups of animals (Carnivora) is converted into a bony partition. Skeletal structures may be derived from each of the three layers of the trilaminar blastoderm. The exo-skeleton includes structures of ectodermal, and some of mesodermal origin in the shape of hair, nails, feathers, teeth, scales, armour- plates, etc., whilst the endo-skeleton, with which we are more particularly concerned, is largely derived from the mesoblastic tissue, but also includes the notocliord, an entodermal structure which forms the primitive endo-skeleton, around which the axial skeleton is subsequently developed in the Vertebrata. The endo-skeleton is divisible into an axial portion, appertaining to the trunk and head, and an appen- dicular part associated with the limbs. It also includes the splanchnic skeleton, which comprises certain bones developed in the substance of some of the viscera, such as the os cordis and os penis of certain mammals. In man, perhaps, the cartilaginous framework of the trachea and bronchi may be referred to this system. The number of the bones of the skeleton of man varies according to age. Owing to a process of fusion taking place during growth, the number in the adult is less than the number in the child. The following table does not include the sesamoid bones which are frequently developed in tendons, the most constant ossicles of this description being those in relation to the metacarpo-phalangeal joint of the thumb, and the metatarso-phalangeal joint of the great toe. 70 OSTEOLOG-Y. life The table represents the iiuml)er oi hones distinct and separa])le during' adult Siiij^le 15 jues. Pairs. Total. The verteltral c(jluiiiu 2G 26 The skull . 6 8 22 Axial skeleton The sternum 1 1 The rihs . 12 24 The hyoid bone 1 1 A|)|)eii(licular skeletnii Tlie upper limbs . 'I'he lowei' liud>s . 32 31 64 62 ri)e ossicles of the ear 3 6 34 86 206 Bones are often classitied according to their shape. Thus long bones, that is to say, hones of elongated cylindrical form, are more or less characteristic of the limbs. Broad or flat bones are plate-like, and serve as protective coverings to the structures they overlie ; the bones of the cranial vault display this particular form. Other bones, such as the carpus and tarsus, are termed short bones ; whilst the bones of the cranial base, the face, and the vertebrae, are frequently referred to as irregular bones. Various descriptive terms are applied to the prominences commonly met with on a bone, such as tuberosity, eminence, 'protuberance, process, tubercle, spine, ridge, crest, and line. These may be articular in their nature, or may serve as points or lines of muscular and ligamentous attachment. The surface of the bone may be excavated into pits, depressiofis, fossm, cavities, furrows, grooves, and notches. These may be articular or non-articular, the latter serving for the recep- tion of organs, tendons, ligaments, vessels, and nerves. In some instances the substance of the bone is hollowed out to form an air space, sinus, or antrum. Bones are traversed hj foramina and canals ; these may be for the entrance and exit of nutrient vessels, or for the transmission of vessels and nerves from one region to another. A cleft, hiatus, ov fissure serves the same purpose. Channels of this kind are usually placed in the line of a suture, or correspond to the line of fusion of the primitive portions of the bone which they pierce. Composition of Bone. — Bone is composed of a combination of organic and inorganic substances in about the proportion of one to two. Organic matter (Fat Collagen) Mineral matter — Calcic phosphate . Calcic carbonate . Calcic fluoride Magnesic phosphate Sodic chloride 58-23^ 7-32 1-41 1-32 •69 31-04 68-97 100-00 The animal matter may be removed by boiling or charring. According to the completeness with which the fibrous elements have been withdrawn, so the brittle- ness of the bone increases. When subjected to high temperatures the earthy matter alone remains. By soaking a bone in acid the salts may be dissolved out, leaving only the organic part. The shape of the bone is still retained, though it has now become soft, and can be bent about in any direction. Bone may be examined either in the fresh or dry condition. In the former, state it retains all its organic parts, which include the fibrous tissue in and around it, the blood-vessels and their contents, together with the cellular elements found within the substance of the bone itself, and the marrow which occupies the lacunar spaces and marrow cavity. In the dried or macerated bone most 'of these have disappeared, though a considerable portion of the organic matter still remains, even in bones of great antiquity and in a more or less fossil condition. Con- sidering its nature and the amount of material employed, bone possesses a remark- STKUCTUKE OF T30NE. 71 able strength, equal to nearly twice that of oak, whilst it is capable of resisting a greater crushing strain ; it is stated that a cubic inch of bone will support a weight of over two tons. Its elasticity is remarkable, and is of the greatest service in enabling it to withstand the shocks to which it is so frequently subjected. In regions where wood is scarce the natives use the ribs of large mammals as a sub- stitute in the construction of their bows. Its hardness and density vary in different parts of the skeleton, and its permanency and durability exceed that of any other tissue of the body, except the enamel and dentine of the teeth. The osseous remains of a race over eighty centuries old is now being excavated in Egypt. Structure of Bone. — The structure of the bone varies with the form of the bone examined. If a long bone be studied in section, the shaft or diaphysis is seen to be hollow, displaying a cavity of elongated shape, which contains the soft cellular marrow. Around this, the bone is deposited in spicules so as to form a loose osseous meshwork, which becomes denser as it reaches the circumference, and gradually merges with the compact layer which forms the outer investing envelope. The extremities of the bone, usually developed from separate or secondary centres called epiphyses, are composed of cancellous tissue, usually finer in the grain and not, as a rule, displaying any medullary cavity. Here the confining shell of bone is thin, and displays none of the stoutness which is so characteristic of the outer layers in the shaft. In the recent condition, the extremities are cartilage-covered where they enter into the formation of joints. In flat bones the osseous tissue is disposed in two compact layers, with a layer of softer cancellous bone, here called the diploe, sandwiched in between. There is no medullary cavity, although in certain regions the substance of the diploe may be absorbed, thus forming air-spaces or air- sinuses. True bone differs from calcified cartilage or membrane in that it not merely consists of the deposition of earthy salts within its matrix, but displays a definite arrangement of its organic and inorganic parts. Dense bone merely differs from loose or cancellous bone in the compactness of its tissue, the characteristic feature of which is the arrangement of the osseous lamellae to form what are called Haversian systems. These consist of a central or Haversian canal which contains the vessels of the bone. Around this the osseous lamellae are arranged con- centrically, separated here and there by interspaces called lacunae, in which the bone corpuscles are lodged. Passing from these lacunae are many fine channels called canaliculi. These are disposed radially to the Haversian canal, and pass through the osseous lamellae. They are occupied by the slender processes of the bone corpuscles. Each Haversian system consists of from three to ten concentric rings of osseous lamellae. In addition to the lamellae of the Haversian systems there are others which are termed the interstitial lamellae ; these occupy the intervals between adjoining Haversian systems, and consist of Haversian systems which have undergone a process of partial absorption. Towards the surface of the bone, and subjacent to the peri- osteal membrane which surrounds the shaft, there are lamellae arranged circum- ferentially; these are sometimes referred to as the outer fundamental lamellae. The periosteal membrane which surrounds the bone, and which plays so important a part in its development, sends in processes through the various Haversian systems, which carry with them vessels and cells, thus forming an organic meshwork around which the earthy salts are deposited. The interior of the bone, viz. the marrow cavity, and the interspaces within the cancellous tissue, as well as some of the larger Haversian canals, are occupied by the marrow or medulla of the bone. This varies considerably in its composition in different bones. In the medullary cavity of the shafts of the long bones it consists mainly of fat cells, together with a few marrow cells proper, supported by a kind of retiform tissue, and is known as the yellow marrow. In other situations, viz. in the diploe of the cranial bones, in the cancellated tissue of the epiphyses of the long bones, tbe vertebrae, the sternum, and the ribs, the marrow is more fluid in its consistence, contains less fat, bub is characterised by the presence of marrow-cells proper, which resemble; in some respects colourless blood corpuscles. In addition to tiiese, however, there are smaller reddish coloured cells, akin to the nucleated red 72 OSTEOLOGY. corpuscles of the blood of the embryo. It is these cells (erythroblasts) which are concerned in the formation of the coloured corpuscles of the blood. Marrow which displays these characteristic appearances is distingidshed from the yellow variety already described by being called the red marrow. In the diploe of the cranial bones of aged individuals the marrow, which has undergone degenerative changes, is sometimes referred to as the gelatinous marrow. Apart from the adaptation of form rendered necessary by the use to which the bone is put, external influences are seen to react upon the intimate structure of the bone itself. Thus, if sections of different bones be made, the structural arrange- ment of their cancellous and dense tissue is seen to vary. In long bones the walls of the shaft are thick and strong, more particularly towards the concave side if the shaft happens to be bent. The marrow cavity — ^largest towards the centre — gradually tapers towards the extremities, being encroached upon by the surround- ing cancellous tissue, which is disposed in lines converging towards the extremities like the sides of a vaulted arch, thereby forming platforms on which the epiphyses are supported. The surfaces of these platforms are not smooth, but so arranged as best to withstand the strain to which the epiphyses are habitually subjected. Such provision is necessary in order to obviate the tendency to separation, which might otherwise occur prior to the complete osseous union of the diaphysis with the epiphysis. In the epiphysis itself the arrangement of the fibres of the cancellous tissue is determined by the disposition of the articular surfaces. The osseous lamellae, as a rule, are disposed at right angles to the planes of the articular facets, whilst they are bound together by other lamellae arranged conformably with these articular planes. The former correspond to the direction of greatest pressure, whilst the latter agree with the lines of greatest tension. In cases where there is an outstanding process projecting from the shaft, as, for example, the head and neck of the femur, a section of the bone displays a bracket-like arrangement of the osseous fibres of the cancellous tissue, which assists materially in strengthening the bone. Ossification and Growth of Bones. — For an account of the earlier development of the skeleton the reader is referred to the section on Embryology. Concerning the subsequent changes which take place, these are dependent on the conversion of the scleratogenous tissue into membrane and cartilage. A characteristic of this tissue is that it contains elements which become formed into bone-producing cells, called osteoblasts. These are met with in the connective tissue from which the membrane bones are formed, whilst they also appear in the deeper layers of the investing tissue of the cartilage (perichondrium), and so lead to its conversion into the bone- producing layer or periosteum. All true bone, therefore, may probably be regarded as of membranous origin, though its appearance is preceded in some instances by the deposition of cartilage ; in this case calcification of the cartilage is an essential stage in the process of bone formation, but the ultimate conversion into true bone, with characteristic Haversian systems, leads to the absorption and disappearance of this primitive calcified cartilage. Membrane bones are such as have developed from fibrous tissue without having passed through a cartilaginous stage. Of this nature are the bones of the cranial vault and the majority of the bones of the face, viz. the superior maxillae, malars, nasals, lachrymals, and palate bones, as well as the vomer. The internal pterygoid plate is also of membranous origin. Cartilage bones are those which are preformed in cartilage, and include most of the bones of the skeleton. Their growth is often described as endochondral and ectochondral, the former term implying the deposition of membrane bone in the centre of the cartilage, while the latter signifies a deposit of membrane bone on the surface of the cartilage, the osteogenetic layer on the surface of the cartilage being called the perichondrium till once bone has been formed, when it is called the periosteum. In the course of the development of a bone from membrane, as, for example, the parietal bone, the fibrous tissue corresponding to the position of the primary centre becomes osteogenetic, because here appear the bone-forming cells (osteoblasts), which rapidly surround themselves with a bony deposit more or less spicular in OSSIFICATION AND GEOWTH OF BONES. 73 arrangement. As growth goes on these osteoblasts become embedded in the ossify- ing matrix, and remain as the corpuscles of the future bone, the spaces in which they are lodged corresponding to the lacunte and canaliculi of the fully developed osseous tissue. From the primary centre ossification spreads eccentrically towards the margins of the bone, where ultimately the sutures are formed. Here the growth rendered necessary by the expansion of the cranium takes place through the agency of an intervening layer of vascular connective tissue rich in osteoblasts ; but in course of time the activity of this is reduced until only a thin layer of intermediate tissue persists along the line of the suture ; this may eventu- ally become absorbed, leading to the obliteration of the suture by the osseous union of the contiguous bones. Whilst the expansion of the bone in all directions is thus provided for, its increase in thickness is determined by the activity of the underlying and overlying strata. These form the periosteum, and furnish the lamellae which constitute the inner and outer compact osseous layers. In a cartilage bone changes of a similar nature occur. The cartilage, which may be regarded histologically as white fibrous tissue + chondro-sulphuric acid and a certain amount of lime salts, undergoes the following changes : — First, the cartilage cells being arranged in rows, become enlarged ; secondly, the matrix between the cartilage cells becomes calcified by the deposition of an additional amount of lime salts ; thirdly, the rows of cells become confluent ; and, fourthly, into the spaces so formed extend the blood-vessels derived from the vascular layer of the periosteum. Accompanying these vessels are osteoblasts and osteoclasts, the former building up true bone at the expense of the calcified cartilage, the latter causing an absorption of the newly-formed bone, and leading to its conversion into a marrow cavity, so that in due course all the cartilage or its products disappear. At the same time that this is taking place within the cartilage, the perichondrium is undergoing conversion into the periosteum, an investing membrane, the deeper stratum of which, highly vascular, furnishes a layer of osteoblast cells which serve to develop the circumferential lamellae of the bone. It is by the accrescence of these layers externally, and their absorption internally through the action of the osteoclast cells, that growth takes place transversely. A transverse section of the shaft of a long bone shows this very clearly. Centrally there is the marrow cavity, formed primarily by the absorption of the calcified cartilage; around this the cancellous tissue produced by the partial erosion of the primary periosteal bone is disposed, whilst externally there is the dense envelope made up of the more recent periosteal growth. Such a description, whilst explaining the growth of bone circumferentially, fails to account for its growth in length ; hence the necessity in long bones for some arrangement whereby ossification may take place at one or both extremities of the shaft. This zone of growth is situated where the ossified shaft becomes continuous with the cartilaginous epiphysis. In addition, within these epiphysial cartilages calcification of the cartilage takes place centrally, just as in the diaphysis. The two parts of the bone, viz. the diaphysis and epiphysis, are thus separated by a layer of cartilage, as yet uncalcified, but extremely active in growth owing to the invasion of vessels and cells from a vascular zone which surrounds the epiphysis. The nucleus of the epiphysis becomes converted into true bone, which grows eccentrically. This arrangement provides for the growth of the shaft towards the epiphysis, and the growth of the epiphysis towards the shaft ; so that as long as the active intervening layer of cartilage persists, extension of growth in a longitudinal direction is possible. Subsequently, however, at variable periods the intervening layer of cartilage becomes calcified, and true bony growth occurs within it, thus leading to complete osseous union between the shaft and epiphysis. When this has taken place all further growth in a longitudinal direction ceases. In cases where the epiphysis enters into the formation of a joint, tlie cartilage over the articular area persists and undergoes neither calcification nor ossification. In long bones the ossific nucleus for the shaft or diaphysis is the first to appear, and is hence often called the primary centre of ossification. The centres for the epiphyses appear subsequently at variable periods, and are referred to as the secondary centres of ossification. From what has been said it will ])G gathered that the vascular supply of the bone is derived irom the vessels of the periosteum. These consist of fine arteries 74 OSTEOLOGY. which enter the surface of the shaft and epiphysis; but in addition there is a larger trunk which enters the diaphysis and reaches the medullary cavity. This is called the. nutrient artery of the bone. The direction taken by this vessel varies in different bones. In the upper limb the artery runs downwards in the case of the humerus and upwards in the radius and ulna ; in the lower limb the nutrient vessel of the femur is directed towards the upper extremity of the shaft, whilst in the tibia and fibula it follows a downward course. It is difficult to account for this difference in the arrangement of the vessels ; but it has been pointed out tliat when all the joints are flexed, as in the position occupied by the foetus in utero, the direction taken by the vessels is the same, and corresponds to a line passing from the head towards the tail-end of the embryo. Consequently, in the upper limb the vessels flow towards the elbow, whilst in the lower limb they pass from the knee. The attention of anatomists has long been directed to the elucidation of the laws which regulate bone-growth. Our present knowledge of the subject may be briefly summarised in the following generalisations : — 1. In bones with a shaft and two epiphyses, the epiphysis towards which the nutrient artery is directed is the first to unite with the shaft. 2. In bones with a shaft and two epiphyses, as a rule the epiphysis which com- mences to ossify latest unites soonest with the shaft. (The fibula is a notable exception to this rule. See p. 239.) 3. In bones with a shaft and one epiphysis the nutrient artery is directed towards the end of the bone which has no epiphysis. (This arrangement holds good in the case of the clavicle, the metacarpus, metatarsus, and phalanges.) 4. When an epiphysis is ossified from more than one centre, coalescence takes place between the separate ossific nuclei before the epiphysis unites with the shaft. Highly suggestive, too, are the following propositions — That ossification first commences in the epiphysis which ultimately acquires the largest relative propor- tion to the rest of the bone, and that the ossification of the epiphysis is also correlated with its functional importance. In cases of long bones with -only one epiphysis, the epiphysis is placed at the end of the bone where there is most movement. The veins which permeate the cancellous texture of the bone are large and thin- walled. They do not accompany the arteries, and, as a rule, in long bones they escape through large openings near the articular surface. In flat bones they occupy channels within the diploe, and drain into an adjacent sinus, or form communica- tions with the superficial veins of the scalp. The lymphatics are mainly periosteal, but enter the bone along with the vessels and become perivascular. The nerves which accompany the arteries are probably destined for the supply of the coats of these vessels. Whether they end in the bony tissue or not is unknown. THE VERTEBRAL COLUMN. The vertebral column (columna vertebralis) of man consists of thirty-three segments or vertebrae, placed one on the top of the other. In the adult, certain of these vertebrae have become fused together in the process of growth to form bones, the segmental arrangement of which is somewhat obscured, though even in their fully-developed condition sufficient evidence remains to demonstrate their com- pound nature. The vertebrte so blended are termed the fixed or false vertebrae, whilst those Ijetween which osseous union has not taken place are described as the movable or true vertebrae. This fusion of the vertebral sesiments is met with at either extremity of the vertebral column, more particularly below% where the column is modified to adapt it for union with the girdle of the lower limb, and also in the region of man's degenerated caudal appendage. But a partial union of the vertebral segments also takes place above, between the two highest vertebrae, in association with the mechanism necessary to provide for the movements of the head on the column. THE VEETEBRAL COLUMN. 75 For descriptive purposes the vertebral column is subdivided according to the regions thi-ough which it passes. Thus the vertebrte are described as cervical (vertebrtc cervicales), dorsal or thoracic (vertebrae thoracales), lumbar (vertebra; Jumbales), sacral (vertebrae sacrales), and coccygeal (vertebrae caudales;, according as they lie in the regions of the neck, back, loins, pelvis, and tail. The number of vertebrae met with in these regions is fairly constant, though, as will be hereafter pointed out, variations may occur in the number of the members of the different series. The vertebrae in man are thus apportioned — 7 cervical, 12 dorsal, 5 lumljar, 5 sacral, and 4 or 5 coccygeal ; the three former groups comprise the true or mov- able vertebrae, the two latter the false or fixed vertebrae. The vertebral formula may be thus expressed : — Movable or True Vertebrae. Cervical. Dorsal. Lumbar. 7 12 5 Fixed or False Vertebrse. Sacral. Coccygeal. 5 4 = 33. Sufierior articular process Pedicle Demi-facet for head of lib Bony Facet for tubercle of rib Demi-facet for head of rib (As viewed from the right side.) Spinous process The vertebrae of which the column is built up, though displaying great diversity of characters in the regions above enumerated, yet preserve certain features in common. All possess a solid part, centrum or body (corpus vertebrae) ; all have articular processes by which they articulate with their fellows ; most have muscular processes de- veloped in connexion with them ; whilst the majority display a verte- bral or spinal foramen (foramen verte- brale) formed by the union of a bony arch (arcus vertebrae) with the body. These common characters ruay best be studied by selecting for descrip- tion an intermediate member of the series. For this purpose one of the middle or lower thoracic vertebrae may be chosen. A typical vertebra may be de- scribed as consisting of a body or centrum (corpus vertebrae) composed of a mass of spongy bone, more or less cylindrical in form. The size and shape of the body is liable to considei'able variation according to the vertebra examined. The upper and lower surfaces of the body are very slightly concave from before backwards and from side to side, due to the thickening of the bone around its margins. Intherecentcon- dition these surfaces afford attach- ment for the intervertebral discs which are placed like pads between the bodies of the movable members of the series. The circumference of the body, formed as it is of more compact bone tlian the interior, is usually slightly concave from above downwards, though it becomes flat behind, where the body forms the anterior boundary of the spinal or vertebral foramen, at wliich ]joint it is usually slightly concave from side to side. The vertical surfaces of the body are pierced liere and there by foramina for the Facet for tubercle of rib Superior articular process Demi-facet for head of rilj (As viewed from above.) FiG.s. .57, 58. — Fifth Thoracic Vkhtehka. 76 OSTEOLOGY. passage of nutrient vessels, more particularly on the posterior surface, where a depression of considerable size receives the openings of tlie canals through which some of the veins which drain the body of the bone escape. Connected with the body posteriorly there is a bony arch (arcus vertebrae), which, by its union with the body, encloses a foramen of variable size, the spinal or vertebral foramen (foramen vertebrale). When the vertebrte are placed on the top of each other these foramina form with the uniting ligaments a continuous canal — spinal or neural canal — in which the spinal cord with its coverings is lodged. The arch, which is formed by the union of the pedicles and laminae, besides enclosing the spinal foramen, also supports a certain number of processes ; of these, some are outstanding, and may be regarded as a series of levers to which muscles are attached, whilst others are articular and assist in uniting the different vertebrte together by means of a series of movable joints. The pedicles are the bars of bone which pass from the back of the body of the vertebrte on either side to the points where the articular processes are united to the arch. The pedicles are compressed laterally, and have rounded superior and inferior borders. Since the vertical breadth of the pedicles is not as great as the thickness of the body to which they are attached, it follows that when the vertebrae are placed one above the other a series of intervals is left between the pedicles of the different vertebrae. These spaces, enclosed in front by the bodies of the vertebrae and their inter- vertebral discs, and behind by the coaptation of the articular processes, form a series of holes communicating with the neural or spinal canal ; these are called the intervertebral foramina (foramina intervertebralia), and allow of the transmission of spinal nerves and vessels. As each intervertebral foramen is bounded al)Ove and below by a pedicle, the grooved surfaces in correspondence with the upper and lower borders of the pedicles are called the upper and lower intervertebral grooves or notches (incisura vertebralis superior et inferior). Posteriorly, the two pedicles are united by two somewhat flattened plates of bone — the laminae, which converge towards the middle line, and become fused with the root of the projecting spinous process (processus spinosus). The breadth of the laminse and their sloping arrange- ment are such, that when the vertebrae, are articulated together they leave little space between them, thus enclosing fairly completely the neural canal, of which they form the posterior wall. The edges and inner surfaces of the laminae are rough for the attachment of the ligaments which bind them together. The muscular processes are three in number, viz. two transverse processes — one on either side — and one central or median, the spinous process. The former (processus transversus) project outwards on either side from the arch at the point where the pedicle joins the lamina. The latter (processus spinosus) extends back- wards in the middle line from the point of fusion of the laminae. The spinous processes display much variety of length and form. The articular processes (zygapophyses), four in number, are arranged in pairs — one superior, the other inferior ; the former are placed on the upper surface of the arch where the pedicles and laminae join, the latter below the arch in correspondence with the superior. Whilst differing much in the direction of their articular surfaces, the upper have generally a backward tendency, whilst the lower incline forwards. THE TRUE OR MOVABLE VERTEBRAE. The Cervical Vertebrae. The cervical vertebrae (vertebrae cervicales), seven in number, can be readily distinguished from all the other vertebrae by the fact that their transverse pro- cesses are pierced by a foramen. The two highest, and the lowest, require special description ; the remaining four conform to a common type. Their bodies, the smallest of all the true vertebrae, are oblong in shape, the transverse diameter being much longer than the antero-posterior width. The upper surface, which slopes from behind forwards and downwards, is concave from side to side, owing to the marked projection of its lateral margins. Its anterior lip is rounded off, whilst its posterior edge is sharply defined. THE CEEVICAL VERTEBEiE. 77 The inferior surface, which is more or less saddle-shaped, is directed downwards and backwards. It is convex from side to side, and concave from before backwards, with a slight rounding off of the projecting anterior lip. The vertical diameter of the body is small in proportion to its width. The anterior surface is flat in the middle line, but furrowed laterally. The posterior surface, which is rough and pierced by many small foramina, is flat from side to side and above downwards ; it forms in its entire extent the anterior wall of the spinal foramen. The lateral aspects of the body, particularly in their upper parts, are fused with the costal parts of the transverse processes, and form the inner wall of the vertebrarterial foramen (foramen transversarium). The pedicles which spring from the posterior half of the lateral aspects of the body, about equidistant from their upper and lower margins, are directed hori- zontally backwards and outwards. The superior and inferior notches are nearly equal in depth. The laminae are long, and about as wide as the body of the bone is thick. The spinal foramen is larger than in the thoracic and lumbar regions ; its shape is triangular, or more nearly semilunar. The transverse processes, so called, are pierced by the vertebrarterial foramen Bifid spine Superior articular process Superior notch Vertebrarterial foramen Infeuoi notch Inferior articular process Spinous process Vertebrarterial foramen Anterior tubercle Fig. 59. — Foctrth Cervical Vertebra from Above and from the Right Side. (foramen transversarium). They consist of two parts — the part behind the foramen, which springs from the neural arch and is the true transverse process, and the part in front, which is homologous with the ribs in the thoracic portion of the column. These two processes, united externally by a bridge of bone, which thus converts the interval between them into a foramen, terminate in two tubercles, known respectively as the anterior and posterior tubercles. The general direction of these processes is outwards, slightly forwards, and a little downwards, the anterior tubercles lying internal to the posterior. The two tubercles are separated above by a groove directed outwards, downwards, and forwards ; along this the spinal nerve trunk passes. The vertebrarterial foramen (foramen transversarium), often sub- divided by a spicule of bone, is traversed by the vertebral artery and vein in the upper six vertebrae. The spinous processes, which are directed downwards, are short, compressed vertically, and bifid. The articular processes are supported on cylindrical masses of bone fused with the arch where the pedicles and laminte join. These cylinders are sliced away obliquely above and below, so that the superior articular facets, more or less circular in form, are directed upwards and backwards, whilst tlie corresponding inferior surfaces are turned downwards and forwards. Variations. — Hzawlowski records the presence of an iiidependeiit rib element in tlic Irans- ver.«f, procc-H of flic (V)iirlli (■(■rvicii] vfvUihvd. (Anat. Anz. Jena, vol. xx. ji. 306.) First Cervical Vertebra or Atlas. — This bone may be readily recognised by tl)e absence of the Itody und spinous process. It consists of two lateral masses, which 8uy)port the articular and transverse processes. The lateral masses are tlieiiiselvcH united by two curved l)ars of bone, tlio anterior and posterior arches, of wliich the former is tlie stonter and sliorter. Each lateral mass is irr((gula,rly six- sided, and HO placed that it lies closer to its fellow of tin; oppositti side in front than 78 OSTEOLOGY. Fig. 60. — The Atlas prom Above. 1. Posterior arch. "2. Transverse process. 3. Tuliercle for transverse ligament. 4. Anterior arch. 5. Anterior tubercle. process. 7. Superior articular process. 8. Foramen for vertebral artery. 9. Groove for vertebral artery. 10. Posterior tubercle. behind. Its upper surface is excavated to form an elongated oval facet, concave from before backwards, and inclined obliquely inwards ; not infrequently this articular surface displays indications of division into two parts. These facets are for the recejitiou of the condyles of the occipital bone. The inferior articular facets are placed on the under surfaces of the lateral ^ masses. Of circular form, they display a slight side- to -side concavity, though flat in the antero-posterior direction. Their disposition is such tliat their surfaces incline downwards and slightly inwards. They rest on the superior articular processes of the second cer- vical vertebra. Springing from the anterior and inner aspects of the lateral masses, and uniting them in front, is a curved bar of bone, the anterior arch (arcus 6. Surface for articulation with odontoid interior) : compressed on process. < :; , .-'■ . , • i either side, this is thick- ened centrally so as to form on its anterior aspect the rounded anterior tubercle (tuberculum anterius). In correspondence with this on the posterior surface of this arch is a circular facet (fovea dentis) for articulation with the odontoid process of the second cervical vertebra (axis). The inner surface of the lateral mass is rough and irregular, displaying a little tubercle for the attachment of the tranverse ligament which passes across the space included between the two lateral masses and the anterior arch, thus holding the odontoid process of the axis in position. Behind each tubercle there is usually a deep pit, opening into the bottom of which are the canals for the nutrient vessels. External to the lateral mass, and principally from its upper half, the transverse process arises by two roots which include between them the vertebrarterial foramen. The transverse process is long, obliquely compressed, and down-turned ; the anterior and posterior tubercles are no longer distinguishable, as they have fused to form one mass. The posterior arch arises in part from the posterior surface of the lateral mass, and in part from the posterior root of the transverse process. Compressed from above downwards anteriorly, where it bounds a groove which curves around the posterior aspect of the superior articular process, which groove is also continuous externally with the vertebrarterial foramen, the posterior arch becomes thicker mesially, at which point it displays posteriorly a rough irregular projection — the posterior tubercle (tuberculum posterius), the feeble representative of the spinous process. A prominent little tubercle, arising from the posterior extremity of the superior articular process, overhangs the groove above mentioned, and not in- frequently becomes developed so as to form a bridge of bone across it, converting the groove into a canal through which the vertebral artery and the first cervical or suboccipital nerve pass — a condition normally met with in many animals. It is noteworthy that the grooves traversed by the two highest spinal nerves lie behind the articular processes, in place of in front, as in other parts of the column. The ring formed by the lateral masses and the anterior and posterior arches is of irregular outline. The anterior part, cut off from the rest by the transverse ligament, serves for the lodgment of the odontoid process of the axis; the larger part behind corresponds to the upper part of the neural or spinal canal. Variations. — Tlie vertebrarterial foramen is often deficient in front. Imperfect ossification occasionally leads to tlie anterior and posterior arches being incomjilete. The superior articular THE CERVICAL VERTEBRiE. 79 surfaces are occasionally partially or conii^letely divided into antei-ior and posterior iiortions. In some instances tlie extremity of the transverse process has two tubercles. The transverse jirocess may, in rare cases, articulate with a projecting process (paroccipital) from the under surface of the jugular process of the occipital bone (see p. 113). An upward extension from the median jjart of the anterior arch, due probably to an ossification of the anterior occipito-atlantal ligament, may articulate with the anterior surface of the summit of the odontoid jjrocess of the axis. Allen has noticed the articulation of the superior border of the posterior arch with the posterior border of the foramen magnum. Cases of jjartial or complete fusion of the atlas with the occipital bone are not uncommon (see p. 113). Second Cervical Vertebra, Axis, or Epistropheus. — This is characterised by the presence of the tootli-like odontoid process (dens) whicli projects upwards from the superior surface of the body. Slightly constricted where it joins the body, the odontoid process tapers to a blunt point superiorly, on the sides of which there are surfaces for the attachment of the odontoid or check ligaments. When the atlas and axis are articulated together this process hes behind the anterior arcli of the atlas, and displays on its anterior surface an oval or circular facet which articu- lates with that on the posterior surface of the anterior arch of the atlas. On the Odoutoid process Groove for transverse ligament Superior articular surface Odontoid process Articular surface for anterior arch of atlas Foramen ten vertebi al arteiy Infenoi aitiLuLii process Spine Fig. 61. — Axis prom Behind and Above. Inferior articular jwocess bral artery Transverse process Pig. 62. — Axis from the Left Side. posterior aspect of the neck of the odontoid process there is a shallow groove which receives the transverse ligament which holds it in position. The anterior surface of the body has a raised triangular surface, wdiich ends superiorly in a ridge passing upwards to the neck of the odontoid process. The pedicles are concealed above by the superior articular processes ; interiorly, they are deeply grooved. The laminae — prismatic on section — are thick and strong, ending in a stout, broad, and bifid spinous process, the under surface of which is deeply grooved, whilst its sides meet superiorly in a ridge. Placed over the pedicles and the anterior root of the transverse processes are the superior articular surfaces. These are more or less circular in shape, slightly convex from before backwards, flat from side to side, and have a direction upwards and a little outwards. They are channelled interiorly by the vertebrarterial foramina which turn outwards beneath them. The grooves by which the second cervical nerves leave the neural canal cross the laminte immediately behind the superior articular processes. The inferior articular processes agree in form and position with those of the remaining njcmbors of the scries, and are placed behind the inferior intervertebral notches. I'he transverse process is markedly down-turned, with a single pointed extremity. Variations. In somi' iuslanfi-s t|]c .suinmit of till' (idoiiluid jiroccss iirticulates with a- |ii'(iiiii- iiciil tiiljcrcle oil tlic, aiit(!i'ior Ijorder of the foramen magiiuiii (tljird o(;r,i])iial condyle, see ]). J 13;. I>(;imi;t {Trann. J'ath. Soc. JJuhtin,, vo]. vii.) records a case in whi(;li tlici odontoid jjrocess was doul)l<;, due to the jtersiHteiice of the primitive condition in which it is d(!velo])ed fi'om two centres. Occasionally tJie odontoid ])rocess fails to be united with tlie body of \\\v. axis, forming an OS odoritoideiiiii comparable to that met witli in the crocodilia ((iliaconiini, Komiti, and 80 OSTEOLOGY. Turner). The vertebrarterial foramen is not infrequently incomplete, owing to the imperfect ossification of the posterior root of the transverse process. The seventh cervical vertebra, or vertebra prominens, receives the latter name from the outstandiug nature of its spinous process, which ends in a single hroad tubercle. This forms a well-marked surface projection at the back of the root of the neck. The transverse processes are broad, being flattened from above downwards ; they project considerably beyond those of the sixth. The maximum width between their extremities agrees with that between the transverse processes of the atlas, these two constituting the widest members of the cervical series. The verte- brarterial foramen is small. Not infrequently the costal element is separate from the true transverse process, thus constituting a cervical rib. Variations. — The vertebrarterial foramen may be absent on one or other side. Thokacic Vertebra*;. The thoracic or dorsal vertebrae (vertebrse thoracales), twelve in number, are distinguished by having facets on the sides of their bodies for the heads of the ribs, and in most instances also articular surfaces on their transverse processes for the tubercles of the ribs (Figs. 57 and 58, p. 75). The body is described as characteristically heart-shaped, though in the upper and lower members of the series it undergoes transition to the typical forms of the cervical and lumbar vertebrae respectively. Its antero -posterior and transverse widths are nearly equal ; the latter is greatest in line with the facets for the heads of the rib. The bodies are slightly thicker behind than in front, thus adapting themselves to the anterior concavity which the column displays in this region. The bodies of the second to the ninth thoracic vertebrae inclusive, each possess four costal demi-facets — a superior pair placed on the upper margin of the body, close to the junction of the pedicle with the centrum, and an inferior pair situated on the lower edge, close to and in front of the inferior intervertebral grooves. When contiguous vertebne are articulated, the upper pair of demi-facets of the lower vertebra coincide with the lower demi-facets of the higher vertebra, and, together with the intervening intervertebral disc, form an articular cup for the reception of the head of a rib. Of these facets on the body the upper pair are the primary articular surfaces for the head of the rib ; the lower are only acquired secondarily. Moreover, these facets, though apparently placed on the body, are in reality developed on the sides of the pedicles behind the line of union of the pedicles with the centrum (neuro-central synchondrosis), as will be explained hereafter. The pedicles are short and thick, and directed backwards and slightly upwards. The superior notch is faintly marked ; the inferior notch is deep. The laminae are broad, flat, and sloping, having sharp upper and lower margins. When the vertebrae are superposed the latter overlap the former so as to form an imbricated arrangement. The spinal foramen is smaller than in the cervical and lumbar regions, and nearly circular in shape. The spinous processes vary in length and direction, being shorter and more horizontal in the upper and lower members of the series, longest and most oblique in direction towards the middle of this part of the column. Nearly all have a down- ward inclination, and are so arranged that they overlap one another. Triangular in section where they spring from the neural arch, they become laterally compressed towards their extremities, which are capped l)y more or less distinct tubercles. The transverse processes are directed backwards and outwards, and a little upwards. They gradually decrease in size and length from above downwards. Each has a somewhat expanded extremity, the anterior surface of which, in the case of the upper ten vertebrae, is hollowed out in the form of a circular facet for articulation with the tubercle of the rib which rests in the upper demi-facet of the vertebra to which the transverse process belongs. The superior articular processes are vertical and have their surfaces directed backwards, slightly upwards, and a little outwards; the inferior, correspondingly forwards, downwards, and inwards. Certain of the thoracic vertebrae display characters by which they can readily THOKACIC VEBTEBEiE. 81 be recognised. These are the first, tenth, eleventh, and twelfth, and sometimes the ninth. The first thoracic verte- bra resembles the seventh cervical in the shape of its body, and the length and direction of its spine. There is an entire facet on either side of the bodj for the head • of the first rib, and one demi- facet on each side at the lower border of its body, to complete the socket for the head of the second rib. Its transverse processes are long, and the superior inter- vertebral notch is better marked than in other mem- bers of the thoracic series. The superior articular sur- faces are directed backwards and upwards, not outwards as in the lower members of the series. The ninth thoracic vertebra occasionally has only the upper pair of demi- facets on its body ; at other times it conforms to the usual type. The tenth thoracic vertebra may have only one complete costal facet on each side for the X. rib, though sometimes the articular socket may be completed by the ninth dorsal vertebra. The facet on the transverse process is generally small, and sometimes absent. The eleventh thoracic vertebra has a complete circular facet on the outer side of each pedicle for pia, 63. articulation with the XI. rib. Its transverse processes are j inferior articular process with short and stunted, and have out-turned facet. no facets ^' ^'"s^® ^^"^^^ ^°^ ^^^^'^ °^ ^^^- ^^^ The twelfth thoracic 3 vertebra has a single facet on the pedicle on each side '*• for the XII. rib. Its trans- 5 processes, which have verse First, Ninth, Tenth, Eleventh, and Twelfth Thoracic Vertebra from the Left Side. 8. Single facet for head of I. rib. 9. Facet on transverse process for tul)erosity of I. rib. 10. Facet on transverse process for tuberosity of IX. rib. 11. Facet on transverse process for tuberosity of X. rib, in tliis particular instance well marked. S. Superior^ Tubercles rMiiinmillary, r , „ ■ L corre- J Accossory. '• f'f™"'' r spondiM, \ Tnn.svorso 10. Kxtoi-iial J l(j V ol' luiiib.-ir. no facet on transverse process. Single facet for head of XI. rib ; no facet on transverse process. Single or demi-facet for head of X. rib. Occasional demi-facet for head of X. rib. Demi - facet for head of IX. no facets, are broken up into smaller tubercles, called 7. B^I.'^faoet for head of 11. vi resp(!ctively the external, 8ii])Crior, and inferior tubercles. These are homologous with the transverse mam- niillary and accessory ])r(K;(!Sses of tin; lumbar verteljnc Indications of these ])rocess(;s may also bo met with in the tenth and eleventh thoracic vertobrtB. The G 82 OSTEOLOaY. twelfth thoracic vertebra may usually l:)e distinguished from the eleventh by the arrangement of its inferior articular processes, which resemble those of the lumbar series in being out-turned; but the eleventh occasionally displays the same arrangement, in which case it is not always easy to distinguish between them. -Barclay Smith {Journ. Anat. and Physiul. Lond. 1902, p. 372) records five cases irior articular processes of the twelfth thoracic vertebra displayed thoracic and Variations, in which the superior articular processes lumbar ciiaracteri sties on the opposite sides, Inferioi aiticulai procpss MaminiUary process Lumbar Vertebra. The lumbar vertebrae (vertebrae lumbales), five in number, are the largest of the movable vertebrae. They have no costal articular facets, nor are their trans- verse processes pierced by ®P'"^ a foramen. In this way they can be readily dis- tinguished from the mem- bers of the cervical and thoracic series. The body is kidney- shaped in outline, and of large size. The transverse diameter is usually about a half greater than the antero ~ posterior width. The anterior thickness is slightly greater than the posterior, being thus adapted to the anterior convex curve of the column in this region. The pedicles, directed horizontally backwards, are short and stout; the superior notches are shallow, but deeper than in the thoracic region; the inferior grooves are deep. The laminse are broad and nearly vertical, sloping but little. They support on their lower margins the inferior articular pro- cesses. The spinal foramen is large and triangular. The spinous processes, spatula shaped, with a thickened posterior margin, project backwards and slightly downwards. The transverse processes, more slender than in the dorsal region, pass hori- zontally outwards, with a slight backward inclina- tion. Arising from the junction of the pedicles with the laminse. in the higher members of the series, they tend to advance so as to become fused with the outer side of the pedicle and back of the body in the two lower lumbar vertebrte. In these latter vertebrae the superior intervertebral grooves are carried obliquely across the upper surfaces of the bases of the transverse processes. The transverse processes Body Superior articular process Mamiiulliiy piocess Tians\erse process Fig Inferior articular process 64. — Third Lumbar Vertebra from Above, and from the Left Side. THE FALSE OR FIXED VERTEBRA. 83 lie in line with the external tubercles of the lower thoracic vertebrae, with whicli they are serially homologous, and are to be regarded as representing the costal element. Placed on their base posteriorly, and just external to and below the superior articular processes, are the small accessory tubercles (processus accessorii) which are in series with the inferior tubercles of the lower thoracic vertebrai. The superior articular processes are stout, oval, curved plates of bone, fused in front with the pedicles and laniinse, and having their concave articular surfaces vertical and in-turned. Externally, and on their posterior edge, the bone rises in the form of an elongated oval tubercle, the mammillary process (processus mammillaris) ; these are in correspondence with the superior tubercles of the lower thoracic transverse pro- cesses. The inferior articular processes lie on either side of the root of the spinous process, supported on the inferior margin of the lamina. Their articular surfaces, oval in outline, convex from side to side, and plane from above downwards, are out-turned. The inferior articular processes are much closer together than the superior ; so that when the vertebrae are articulated the superior articular processes of the lower vertebra embrace the inferior articular processes of the higher vertebra. The fifth lumbar vertebra is characterised by the size of its body, which is the largest of all the vertebrse. Further, the under surface of the body is cut away at the expense of its posterior part : hence the thickness of the centrum in front much exceeds that of the vertical diameter behind. The transverse process is pyramidal in form, and stouter than those of the other lumbar vertebree. It arises by a broad base from the side of the back of the body, as well as from the pedicle, and is directed outwards and a little backwards and upwards. Its upper surface is slightly grooved by the superior intervertebral notch. A deep notch separates it posteriorly from the superior articular processes, which are less in-turned than in the other members of the series, their articular surfaces being directed more backwards than inwards, and displaying less concavity. The inferior articular processes are further apart than is the case with the other members of the series, they lie in line with the superior. The spinous process is shorter and narrower than the other lumbar spines, particularly so in the female. Variations,— The mammillary and accessory processes are sometimes unduly developed. The neural arch of the fifth lumbar vertebra is occasionally interrupted on either side by a synchon- drosis which runs between the upper and lower articular processes. In macerated specimens the two parts of the bone are thus separate and independent. The anterior includes the centrum, to- gether with the pedicles, transverse and superior articular processes ; the posterior comprises the inferior articular processes, the laminae, and the spine. — (Turner, Challenger Reports, vol. xvi.) Szawlowski and Dwight record instances of the occurrence of a foramen in the transverse jjrocess of the V. lumbar vertebra {Anat. Anz. Jena, vol. xx.), and Ramsay Smith describes a case in which the right transverse process of the IV. lumbar vertebra of an Australian sprang from the side of the body in front of the pedicle, being unconnected either with the pedicle or articular process. THE FALSE OR FIXED VERTEBRA. The Saceum. The sacrum (os sacrum), of roughly triangular shape, is formed by the fusion normally, of five vertebrse. The anterior surface of the bone is slightly hollow from side to side and concave from above downwards, the curve being usually most pronounced opposite the third sacral segment. The central part corresponds to the bodies of the sacral vertebrte, the lines of fusion of which are indicated by a series of four parallel ridges which cross the central part of the bone at gradually diminishing intervals from above downwards; externally, these ridges disappear on either side on the inner walls of the four anterior sacral foramina (foramina sacralia anterioi-a). The size of these holes decreases from above downwards. The upper and under border of each foramen is formed by a stout bar of bone, of which there are five on each side, corresponding in number with the vertebrie present. These unite externally so as to i'orm the. lateral mass (])ars lateralis), and tlnis enclose the foramina to the outer side, though here the edge is not abrupt, but 84 OSTEOLOGY. sloped so as to pass gradually into the canal. The large anterior divisions of the sacral nerves pass through these foramina and occupy the shallow grooves. The bone is broadest across the first sacral vertebra, tends to narrow opposite the second, and again usually increases in width opposite the third. When this condition is well marked, the edge has a notched appearance (sacral notch) which assists in the interlocking of the sacro-iliac joint ; this feature is common in the Simiidse and some of the lower races of mankind (Paterson). The surface of bone between and external to the first, second, third, and fourth foramina affords attachment to the fibres of origin of the pyriformis which may in some instances extend on to the bodies of the II. and III. segments (Adolphi), whilst on the edge external to and below the fourth foramen the coccygeus is inserted. The posterior surface is rough and irregular. Convex from above downwards it displays mesially a crest (crista sacralis media) whereon are seen four elongated Supei'ior articular processes Transverse process of first sacral vertebra Anterior sacial foramen Inferior lateral antjle, Groove for fifth sacral nei\e Coccygeal articular surface Fig. 65. — The Sacrum (anterior view). tubercles — the spines of the upper four sacral vertebrte. External to these the bone forms a groove — the sacral groove — the floor of which is made up of the con- fluent laminae of the corresponding vertebrte. In line with the intervals between the spines, and wider apart above than below, another series of tubercles is to be seen. These are due to the fusion of the articular processes of the sacral vertebrae, and together they form faint irregular ridges on the bone (cristse sacrales articulares). Normally, the spine of the lowest sacral segment is absent, and the laminse do not coalesce mesially, thus leaving a gap in which the spinal canal is exposed (hiatus sacralis) ; whilst inferiorly the tubercles corresponding to the inferior articular processes of the last sacral vertebra form little down-projecting processes — the sacral cornua (cornua sacralia) — by means of which the sacrum is in part united to the coccyx. Just wide of the articular tubercles are the posterior sacral foramina (foramina sacralia posteriora), for the transmission of the posterior divisions of the sacral nerves. These are in correspondence with the anterior foramina, so that a THE SACRUM. 85 probe can be passed directly tlirough both openings ; but it is to he noted that the posterior are much smaller, and their margins much sharper, than is the case with the anterior. The surface of the lateral mass external to the posterior sacral foramina is rough and irregular, owing to the presence of four more or less elevated tubercles, which constitute the lateral ridges on either side of the bone (cristse sacrales laterales), and which are serially homologous with the true transverse processes of the lumbar vertebra. The posterior surface of the bone furnishes an extensive surface for the origin of the erector spinse muscles, whilst the edge of the bone external to the third and fourth foramen gives attachment to the gluteus maximus. The base of the bone displays features more in accordance with a tyi:)ical Superior aperture of sacral canal Superior articular process Posterior sacral foramen Infpiior lateial angle Inferior a^ierture of sacial canal Groove for fifth sacral ner\ e Coceygeul articular surface Fig. 66. — The Sacrum (posterior view). verteVjra. Centrally, and in front, is placed the body, the upper surface of which articulates with the last lumbar vertebra through the medium of an intervertebral disc. The anterior margin is thin and projecting, overhanging the general con- cavity of the front of the bone, and forming what is called the promontory (promon- torium). Behind the Ijody, the spinal canal, of triangular form with slightly appressed sides, is seen, whilst posteriorly is the short spinous process forming the highest tubercle of the median crest. Spreading out from the sides, and partly from the back of the body on either side, is a fan -shaped mass of bone, the upper surface of which is sliglitly concave from side to side, and convex from above and behind downwards and forwards. I'his, th(3 ala (ala sacralis), corresponds to the thick upper border of the lateral mass, and is forined, as will be explained hereafter, by elements wliich correspond to the pedicles and transverse processes of the sacral vertebne, tf>g(;ther with superadded structures — the sacral ribs. The external margin of the lateral mass, as seen from above, is sharp and outwardly convex, terminating behind in a prominent tubercle — the highest of the series of elevations seen on the posterior surface of the bone, wliich have been already described as serially homologous with 8Q OSTEOLOGY. the true transverse processes of the kimbar vertebrse. Fused with the back of each lateral mass, and separated from it externally hj a narrow but deep notch, is the superior articular process. This supports a vertical articular surface, which is of circular or oval form, and concave from side to side, having a general direction backwards and a little inwards. The borders of the l^one are thick above, where they articulate with the ilia, thin and tapering below, where they furnish attachments for the powerful sacro- sciatic ligaments. The iliac articular surfaces are described as auricular in shape (facies auriculares), and overlie the lateral masses formed by the first three sacral vertebrse, though this arrangement is liable to considerable variation. Behind the auricular surface the bone is rough and pitted by three distinct depressions for the attachment of the strong sacro- iliac ligaments. Inferiorly, the edge formed by the lateral masses of the fourth and fifth sacral vertebrai becomes gradually thinner, and at the inferior lateral angle changes its direction and sweeps inwards towards the body of the fifth sacral segment. The apex, or lower end of the sacrum, is formed by the small oval body of the fifth sacral vertebra, which articulates with the coccyx. The sacral canal follows the curve of the bone ; more or less triangular in shape above, it becomes appressed and flattened below. Inferiorly, its posterior wall is deficient owing to the imperfect ossification of the laminee of the fifth, and, it may be, of the fourth sacral segments. Passing obliquely outwards and down- wards from this canal into the lateral masses on either side are the four pairs of intervertebral foramina, each of which is connected externally with a V-shaped canal which terminates in front and behind in the anterior and posterior sacral foramina. The hinder limb of the V is shorter and narrower than the anterior. The female sacrum is proportionately broader than the male. Its curves are liable to great individual variation, though the absolute depth of the curve is less than in the male. Variations. — Tlie number of sacral segments may be increased to six or rediiced to four (see p. 90). Transition forms are occasionally met with, in wbich. the first sacral segment displays on one side purely sacral characters, i.e. it articulates witli the innominate bone, whilst on the opposite side it may present all the features of a lumbar vertebra. Through deficiency in the develojjment of the laminae, the neural canal may be exposed throughout its entire length, or to a greater extent than is normally the case. (Paterson, Roy. Dublin ^Soc. Scientific Trans. vol. v. Series II.) Szawlowski and Barclay Smith record the occurrence of a foramen in the lateral mass of the I. sacral vertebra {Journ. of Anat. and Physiol. Lond. voL xxxvi. p. 372). Coccyx. The coccyx consists of four — sometimes five, less frequently three — rudimentary vertebrse, which tend to become fused. The first piece is larger than the others ; it has an oval 7 hollow facet on its upper surface, which articulates with the body of the last sacral segment. Pos- teriorly, two processes, cornua coccygea, which He in series with the articular processes of the sacrum, extend up- wards and unite with the sacral cornua, thus bridging over the notch 7. Transverse process, f^^, ^|^g ^^^^ ^f ^j^g gf^j^ 8. Transverse process. , _ sacral nerve, and con- verting it into a foramen, the last of the intervertebral series. From the outer sides of the body project rudimentary transverse processes which may, Fig. 67.- A. Posterior Surface. 1. Transverse process. 3. Sacrum. 2. Transverse process. 4. Cornu. The Coccyx. B. Anterior Surface, 5. Sacrum. 6. C'ornu. VERTEBEAL COLUMN AS A WHOLE. 87 or may not, unite with the sacrum close to the lower lateral angles ; in the latter case the fifth anterior sacral foramina are enclosed. Inferiorly, the body of the bone articulates with the succeeding vertebra. The second coccygeal vertebra displays slight traces of a transverse process and the rudiments of pedicles. The following segments are mere rounded or oval-shaped nodules of bone. Fusion between the lower elements occurs normally in middle life, whilst union between the first and second segments occurs somewhat later. It is not unusual, however, to find that the first coccygeal vertebra remains separate from the others. Though very variable, as a rule, fusion occurs more commonly in the male, and at an earlier age, than in the female. Szawlowski has recorded a case in which a curved process arose from the ventral surface of the first coccygeal segment. He regards this as possibly the homologue of a ventral arch (Anat. Am. Jena, vol. xx. p. 320). From the posterior surface of the coccyx the gluteus maximus arises. To its external borders are attached the coccygei and levatores ani muscles, and from its tip spring the fibres of the sphincter ani. VERTEBRAL COLUMN AS A WHOLE. When all the vertebrae are articulated together, the resulting column displays certain characteristic features. The division of the column into a true or movable part, comprising the members of the cervical, thoracic, and lumbar series, and a false or fixed portion, including the sacrum and coccyx, can now be readily recognised. The vertebrse are so disposed that the centra or bodies form an interrupted column of solid parts in front, which constitutes the axis of support for the head and trunk ; whilst the neural arches behind constitute a canal for the lodgment and protection of the spinal cord and its membranes. In the movable part of the column both the anterior supporting axis and the neural canal are liable to changes in their disposition owing to the movements of the head and trunk. Like the bodies and neural arches, the spinous and transverse processes are also superposed, and fall in line, forming three series of interrupted ridges — one (the spinous) placed centrally and behind, the others (the transverse) placed laterally. In this way two vertebral grooves are formed which lie between the central and lateral ridges. The floor of each groove is formed by the laminse and articular processes, and in these grooves are lodged the muscles which serve to support and control the movements of the column. Further, the column so constituted is seen to display certain curves in an antero- posterior direction. These curves are, of course, subject to very great variation according to the position of the trunk and head, and can only be satis- factorily studied in a fresh specimen ; but if care be exercised in the articulation of the vertebrae, the following characteristic features may be observed, assuming, of course, that the column is erect and the head so placed that the axis of vision is directed towards the horizon. There is a forward curve in the cervical region, which gradually merges with the backward thoracic curve ; this becomes con- tinuous below with an anterior convexity in the lumbar region, which ends more or less abruptly at the union of the fifth lumbar with the first sacral vertebra, where the sacrum slopes suddenly backwards, causing the column to form a marked projection — the sacro-vertebral angle. Below this, the anterior concavity of the front of the sacrum is directed downwards as well as forwards. Of these four curves, two — tlie tlioracic and sacral — are primary, they alone exist during fcetal life ; whilst the cervical and lumbar forward curves only make their appearance after birth — the former being associated with the extension and elevation of the head, whilst the latter is developed in connexion with the use of the hind limb in tbe hyper-extended position, which in man is correlated with tbe assumption of tlie erect yjosture ; tliis curve, therefore, only appears after the child has begun to walk. For tbese reasons the cervical and lumbar curves are described as secondary and comp(!nsatory. Not infrequently there is a slight lateral curvature in the thoracic region, the f'onvexity of the curve being usually directed towards the right side. This may OSTEOLOGY. be associated with a greater use of the muscles of tlie right upper limb, or may depend on the pressure exercised by the upper part of the thoracic aorta on the vertebrae of the thoracic region, thus causing a slight lateral displacement, together with a flattening of the side of the five thoracic vertebrse (impressio aortica) as was first pointed out by Wood (Journ. Anat. and Phy- siol, vol. iii.) Above and below this curve there are slight compensatory curves in the opposite direction. The line which unites the tips of the spines is not a repetition of the curves formed by the bodies. This is due to the fact that the length and direction of the spines vary much in different regions ; thus in the neck, with the exception of the second, sixth, and seventh, the spines are all short (absent in the case of the atlas). In the thoracic region the spines, though long, are obliquely placed — a circumstance which much reduces their prominence ; that of the seventh thoracic vertebra is usually the longest and most slanting. Below this point the length of the spines gradually decreases, and their position more nearly approaches the horizontal. In the loins the spines have all a slight downward direction. Taken as a wliole, the sjiines of tlie movable vertebras in man have a downward inclination — a character which he shares with the anthropoid apes and a few other animals. This character serves to distinguish his column from those of lower mammals in which the sjjines of the lumbar vertebrjB are directed headwards towards the " centre of motion," which is usually situated near the hinder extremity of the thorax, where a vertebra is placed the direction of whose sj^ine is vertical to the horizontally disjDosed column ; this vertebra is often referred to as the anticlinal vertebra. The spines of the upper three or four sacral verte- brfe form an osseous ridge with interrupted tubercles. The ridge formed by the vertel^ral spines is an important determinant of the surface form, as it cor- responds to the median furrow of the back, and here the individual spines may be felt and counted from the seventh cervical down to the sacral region. This is best done when the back is well bent forwards. As viewed from the front, the vertebral bodies increase in width from the second cervical to the first thoracic ; thence a reduction in breadth takes place to the level of the fourth thoracic, below which there is a gradual increase in their transverse diameters until the sacrum is reached. Here a rapid reduction in width takes place, terminating inferiorly in the nodules of the coccyx. The transverse processes of the atlas are wide and outstanding. The succeeding four cervical verte- brae have transverse processes of nearly equal width ; the seventh, however, displays a marked increase in its transverse diameter, and is about equal in width to the first thoracic vertebra. Below this a gradual and regular diminution in width characterises the transverse processes of the thoracic vertebrae, until in the case of the eleventh and twelfth they are merely represented by the small external tubercles. In the lumbar region the transverse processes again appear outstanding, and of nearly equal length. The transverse diameter of the lateral mass of the first sacral vertebra forms Fig. 68. — Vertebral Column FROM THE Left Side. VEETEBRAL COLUMN AS A WHOLE. 89 until the diameter ~.^M^^ n (u* the widest part of the column. Below this a decrease in width occurs level of the third sacral segment is reached, at which point the transverse is somewhat abruptly diminished, a reduction in width which is further suddenly accentuated opposite the fifth sacral segment. As viewed from the side, the bodies display a gradual increase in their antero-posterior width until the second lumbar vertebra is reached, below which this diameter is slightly reduced. In the sacral region the reduction in this diameter is great in the first and second sacral segments, more gradual and less marked in the last three segments. The facets for the heads of the ribs in the upper thoracic region lie on the sides of the bodies ; those for the tenth, eleventh, and twelfth are placed farther back on the pedicles. The intervertebral foramina increase in size from above downwards in the movable part of the column, being largest in the lumbar region. In the sacral region they decrease in size from above downwards. In the cervical region the two highest cervical nerves pass out behind the articular processes of the atlas and axis, and lie, therefore, behind the corresponding transverse processes of these vertebrae. The succeeding cervical nerves pass out through the inter- vertebral foramina which are placed between the transverse processes and in front of the articular processes. In the thoracic and lumbar vertebrae the intervertebral foramina lie in front of both the articular and transverse processes. The arrangement of these foramina in the sacrum has been already sufficiently explained. The neural canal for the lodgment of the spinal cord and its meninges is largest in the cervical and lumbar regions, in both of which it assumes a triangular form ; whilst it is narrow and circular in the thoracic region. These facts are correlated with the movements of the column which are most free in those regions where the canal is largest, i.e. the neck and loins. The average length of the vertebral column IkS from 70 to 73 centimeti'es, or from '11\ to 28f inches. Of this the cervical part measures from 13 to 14 cm. ; the thoracic, 27 to 29 cm. ; lumbar, 17 to 18 cm. ; and the sacro-coccygeal, 12 to 15 cm. The individual differences in the length of the column are less than one might expect, the variation in height of different individuals being often largely dependent on the length of the lower limbs. In the female the average length of the column is about 60 centimetres, or 23| inches, and the curve in the lumbar region is iisually more pronounced. Architecture.— The vertebrae are formed of sjjongy bone confined ^ ijO ll^ within a tliin and dense envelope. In the bodies the arrangement of ^ \ the cuncellou.s tissue, which is traversed by venous channels, is such V -Q as to display a vertical striation with lamellag arranged horizontally. The external, superior, and inferior walls arc. very thin — that directed to the neural canal being usually thicker and denser than the others. In the pedicles and roots of the transverse processes tlie cancellous tissue is much more open. The outer envelope is much thicker where 1''^"- ^^- — Vkrtebral Column it hounds the neurab ring, and Avhei'e it forms the bottom of the '^^ ^^'^'^ '^^^^^ Behind. superior and inferior interverteljral notches. In the laminaj the snongy tissue is confined ]>etween two compact layers, of which that directed to the spinal canal is the thicker, hi the spiiujus ))rocesses the upjier edge is always the more compact Variations. Numerical Variations of the Column as a Whole. —increase in the number of vertehrai segnicnts is usually du<; to diirriitnccs in the nunihei' of the coccygeal vertebrae ; these Ol \ 90 OSTEOLOGY. may vary from four — wliicli may be regarded as the normal number — t(j six. Tlie number of presacral or movable vertebras is normally 24 (7 C, 12 D, and 5 L). In which case the 25th vertebra forms the first sacral segment (vertebra fulcralis of Welcker). Tlie number of presacral vertebr;e may be increased by the intercalation of a segment either in the thoracic or luml)ar region without any alteration in the number of the sacral or coccygeal elements : thus we may have 7 C, 13 D, and 5 L, or 7 C, 12 D, and 6 L, or may be reduced by the disajipearance of a A-ertebral seg- ment — thus, 7 C, 12 D, and 4 L. Such an arrangement presupposes developmental errors either of excess or default in the segmentation of the column. On the other liand, the total number of vertebral segments remaining tlie same (24 or 25), we may have variations in the uumbei' of those assigned to different regions due to the addition of a vertebral segment to one, and its conseq^uent subtraction from another region. Thus, in the 24 presacral vertel)r8e, in cases of the occurrence of cervical ribs the formula is reari'anged thus — 6 C, 13 D, and 5 L, or, in the case of a thirteenth rib being present, the formula would be 7 C, 13 D, 4 L, as happens normally in the gorilla and chimpanzee. Similarly, the number of the presacral vertebr;e (24) may be increased by the withdrawal of a segment from the sacral region — 7 C, 12 D, 6 L, and 4 S — or diminished by an increase in the number of the sacral vertebrae, as in the formula 7 C, 12 D, 4 L, and 6 S. In- crease in the numljer of sacral segments may be due to fusion with a lumlmr vertebrae, or by the addition of a coccygeal element : the latter is more frequently tlie case. This varialjility in the constitution of the sacrum is necessarily correlated with a shifting liackAvards and forwards of the pelvic girdle along the vertebral column. Rosenburg considers that the 26th, 27th, and 28th vertebrae are the primitive sacral segments, and that the sacral characters of the 25th vertebrte (the first sacral segment in the normal adult column) are only secondarily acquired. He thus sujjposes that during development there is a forward shifting of the sacrum and pelvic girdle, with a consequent reduction in the length of the jsresacral portion of the column. This view is ojjposed by Paterson {Roy. Dublin Soe. Scientific Trans, vol. v. Ser. II.), who found that ossification took place in the ahe of the 25tli vertebra (first adult sacral segment) before it made its ap^Jear- ance in the alse of the 26tli vertebra. He thus assumes that the ahe of the 25th vertebra may be regarded as the main and primary attachment with the ilium. His conclusions, based on a large number of oljservations, are at variance with Rosenburg's views, for, according to his opinion, liberation of the first sacral segment is more common than assimilation with the fifth lumbar vertebra, and assimilation of the first coccygeal vertebra with the sacrum is more common than liberation of the fifth sacral, thus leading to the inference that the sacrum tends to shift back- wards more often than forwards. (See also T. D wight, Anat. Anz. Jena, vol. xix. j)p. 321, 337.) THE CAKTILAGINOUS VEETEBKAL COLUMN. As has been already stated (p. 30), the neural tube and the notochord are enveloped by a continuous sheath of mesodermal tissue which forms the membranous vertebral column. It is by the chondrification of this that the cartilaginous column is developed. This process commences about the end of the first or the beginning of the second month of foetal life. In correspondence with each vertebral segment, two symmetrical nodules of cartilage appear on either side of the notochord ; these rapidly surround and constrict it. By their fusion they constitute the body of a cartilaginous vertebra, and are so disposed that they alternate in position with the muscle plates which are lying on either side. In this way a vertebral body corre- sponds in position to the posterior half of the anterior myotome, and the anterior half of the posterior myotome, the intermyotomic intervals, which contain the connective tissue plates separating the muscle segments, lie in line laterally with the mid -points of the sides of the cartilaginous vertebrae. It is by chondri- fication of these intersegmental layers that in certain regions the ribs are ultimately developed. Meanwhile, the. scleratogenous tissue between the chondri- fying vertebral bodies undergoes little change and persists as the intervertebral disc. Here the embedded chorda undergoes but slight compression and enlarges, so that if a length of the column be examined in longitudinal section the noto- chord displays a moniliform appearance, the constricted parts of the chorda corresponding to the bodies, the enlarged portions to the discs. The former disappear at a later stage when ossification begins, but the latter persist in the adult as the pulpy core in the centre of the intervertebral disc. The portions of the scleratogenous tissue which lie lateral to the chorda have next to be considered ; these extend dorsalwards around the neural canal, and ventralwards beneath the chorda. The former is sometimes called the vertebral bow, the latter the hypochordal bow. The vertebral ■ bow begins to chondrify on either side, and forms the lateral portions of the cartilaginous neural arch, the extremities of which usually unite dorsally about the fourth month of fcetal life ; if from defective development this union should fail to occur, a deformity known as spina bifida is the result. OSSIFICATION OF THE VERTEBE^. 91 From the cartilaginous neural arch so formed arise the chondrified rudiments of the spinous, transverse, and articular processes. The chondrification of the neural arch is variously described as being in- dependent of the body or an extension from it ; in any case, union between it and the body is rapidly effected. The scleratogenous tissue between the cartilaginous neural arches which does not undergo chondrification persists as the ligaments uniting the neural laminai. As regards the so-called hypochordal bow, for the most part it disappears. By some it is regarded as being represented by a fibrous strand in the inter- vertebral disc in front of the vertebra to which it belongs. It is, however, noteworthy that in the case of the atlas vertebra there is an exception to this arrangement ; for here the hypochordal bow chrondrifies and subsequently by ossification forms the anterior arch of that bone — an arch which lies ventral to, and embraces the odontoid process of the axis {q.v. p. 92). It is only in the thoracic region that the ribs, developed as stated above by the chondrification of the intersegmental septa, attain their full dimensions. In the cervical, lumbar, and sacral regions they exist only in a rudimentary or modified form, as has been described elsewhere. In the construction of the chest wall the ribs are supported ventrally by the sternum, as to the development of which there is some difference of opinion. Enge has described this bone as formed by the fusion of two cartilaginous bands produced by the coalescence of the expanded ends of the first five or seven cartilaginous ribs. Paterson, on the other hand, regards the sternum as arising independently of the ribs by the union of a right and left sternal bar in the median ventral line. There are also reasons for supposing that the presternum is intimately associated with the development of the ventral part of the shoulder girdle. Ossification of the Vertebrae. — The vertebrae are developed by ossification of the cartilage which surrounds the notochord and which passes backwards over the sides of the neural canal. The centres for the bodies first appear in the lower thoracic vertebrae about the tenth week. An oval nucleus /^fc\ develops in each body. At first it is placed dorsal to the noto- f-^llf^^^V^is^^^'^.''^^'^ chord, but subsequently surrounds and causes the disappearance \m |i^"*^"'^^i of that structure. Occasionally, however, the primitive centre |m^ yjr ^^'^^^ appears to be formed by the coalescence of two primary nuclei. ^^M^^^m Support is given to this view by the occasional occurrence of ^kg^^^mCmti-e for vertebrae in which the body is developed in two lateral halves, or ^^^^^^^'^'^^ in cases where only one-half of the body persists (Turner) ; nor- mally, however, it is impossible to make out this division. From ^^^ /O. —Ossification these single nuclei the bodies are developed, the process extending up and down the column until, by the fifth month, all the centra possess ossific deposits, except the coccygeal segments. About the seventh week a single centre appears in the neural arch on either side. These commence first to ossify in the upper cervical region and extend rapidly downwai'ds throughout the column. They first appear near the bases of the superior articular processes, and extend backwards into the laminae, outwards into the transverse processes, and forwards into the pedicles. These latter project anteriorly, and form a considerable portion of the postero-lateral aspects of the body, from which, however, they ai'e separated by a cartilaginous strip — the neuro-central synchondrosis — which does not entirely disappear until about the fifth or sixth year. It is important to note that in the thoracic region the costal facets lie behind the neuro-central synchondrosis, and are therefore borne on the lateral aspects of the pedicles. Fusion of the laminae in the mesial plane beliind begins, after birth, in the lumbar region and extends upwards, so that Vjy the fifteenth month or thereabouts the arches in the cervical region are com- pleted behind. fn the sacral region ossification is slower, the spinal canal not being enclosed till the seventh to the tenth year. The spinous processes are cartilaginous at birth, but these y>ecome ossified ])y tlie extension iiito them of the bony luminfc. At puljcrty certain secondary centres or epiphyses make their appearance ; these are five in number. One caps the summit of the spinous process, except in the cervical region. A single epiphysis on either side apjjcars at the extremity of the transverse process, and in the tliomcic region assists in forming the articular surface for the tul)crclc of the riV). Two epiphysial plates are formed — one for the upper, and the second for the 92 OSTEOLOG-Y. lower surface of the body, including also that part which lies behind the neuro-central synchondrosis and formed by the pedicle ; from these the thickened circumference of both upper and lower aspects of the body are derived. Fusion of these centres with the rest of the bone is not complete till the twenty-fifth year. In the cervical region independent centres are described as occurring in the anterior roots of the transverse processes of the sixth and seventh vertebraj. These correspond to the costal element, and may occasionally persist in the form of cervical ribs. Elsewhere they are formed by lateral extensions from the pedicle. In the lumbar region the transverse process of the first lumbar vertebra is occasionally associated with an independent costal centre, which may blend with it, or persist as a Fig. 71. — Ossification of Vertebrae. Cervical vertebra. appears 22 23 1. Centre for body. 2. Sujjerior epiphysial plate. 3. Anterior bar of transverse process developed by lateral extension from pedicle. 4. Neuro-central synchondrosis. 5. Inferior epiphysial plate. Limibar vertebra. 6. Body. 7. Superior epiphysial plate. 8. Epiphysis for niammillary process. 9. Epiphysis for transverse process. 10. Epiphysis for spine. 11. Neuro-central synchondrosis. 12. Inferior epiphysial plate. Dorsal vertebra. 13. Centre for body. 14. Superior epiphysial plate, appears about puberty ; unites at 25th year. 15. Neuro-central synchondrosis does not ossify till 5th or 6th year. 16. Appears at puberty ; unites at 25th year. 17. Appears at puberty ; unites at 25th year. 18.- Ai:)pears about 6th week. Axis. 19. Centre for transverse process and neural arch ; appears about 8th week. lumbar rib. The mammillary processes are derived from separate epiphyses. The neural arch of the fifth lumbar vertebra is occasionally developed from two centres on either side, as is demonstrated by the fact that the arch is sometimes divided b}^ a synchondrodial joint running obliquely across between the superior and inferior articular processes on either side. (See ante, p. 83 ; also Fortschritte anf dem Gebiete der Rontgenstrahlen. Erganzungsheft i. ; "die Entwickelung des menschlichen Knochengertistes wiihrend des fotalen Lebens," von Lambertz.) Atlas. — The lateral masses and posterior arch are developed from twO' centres — one on either side — which correspond with the centres from which the neural arches of the other members of the series are developed. These make their appearance about the seventh week, and do not unite posteriorly till after the third year. Their point of union is sometimes preceded by the formation of a distinct spinal nucleus (Quain). The anterior 20. Synchondroses close about 3rd year. 21. Centre for summit of odontoid process ; 3rd to 5th year, fuses 8th to 12th year. Appears about 5th or 6th month ; unites with opposite side 7th to 8th month. Synchondrosis closes from 4th to 6th year. 24. Inferior epiphysial plate ; appears about puberty, unites about 25th year. 25. Single or double centre for body ; appears about 5th month. Atlas. 26. Posterior arch and lateral masses developed from a single centre on either side, ■which appears about 7th week. 27. Anterior arch and jiortion of superior articular surface developed from single or double centre, ajDpearing during 1st year. Dorsal vertebra. 28. Epiphysis for transverse jirocess ; appears about puberty, unites about 25th year. 29. Ejsiphysis apj^ears about puberty ; unites about 25th or 27th year. 30. Centre for neural arch on either side ; appears about 6th or 7th week, the laminte unite from birth to 15th month. 31. Centre for body ; appears about 6th week, unites with neural arch from 5th to 6th year. OSSIFICATION OF THE VEETEBR^. 93 arch is devcloijecl from centres variously described as single or double, which appear in one of the hypochordal arches of cartilage described by Froriep {Arch. f. Anat. u. Physiol., Anat. Ahth. 1886) which here persists. In this cartilage ossification commences during the first year of life. Union with the lateral masses is delayed till six or eight years after birth. The external extremities of the anterior arch assist in forming the fore part of the superior articular processes. Axis. — The axis ossifies from five primitive centres. Of these, two — one on either side — appear about the seventh week, and form the articular and transverse processes, together with the laminae and spine. One, or it may be two, nuclei appear in the lower part of the body about the fifth month. The upper part of the body, including a small part of the superior articular process, and the base of the odontoid process, are developed from two laterally-placed nuclei which appear shortly after, and fuse together at the seventh or eighth month, so that at birth the bone consists of four pieces. Fusion between these parts takes place in the following order : — The odontoid unites with the body and lateral parts about the third or fourth year ; union between the two lateral portions posteriorly and the body and lateral parts in front, is complete at from four to six years. The summit of the odontoid process is developed from a separate centre, occasionally double, which appears from the third to the fifth year, and fuses with the rest of the bone from the eighth to the twelfth year. About puberty an annular epiphysis is developed on the under surface of the body, with which it is comjDletely united during the twentieth to the twenty-fifth year. Some authorities state that a few granules between the base of the odontoid and the upper surface of the body represent the superior epiphysial plate ; but as fusion between the odontoid and the body occurs before the time for the appearance of these secondary epiphysial plates, this can hardly be regarded as correct. The line of fusion of the odontoid with the body is defined by a small disc of cartilage which persists within the substance of the bone till an advanced period of life. A pair of epiphyses placed over the tubercles of the spine, if not always present, are at least frequent. Sacrum. — Each of the sacral segments is ossified from three centres : one for the body, and two for the neural arch— that for the body, which makes its appearance in the first three sacral vertebrae about the end of the third ''v'''''5Essfesar\\ /^ month, about the fifth to the eighth month for the last two segments. From the two centres for the neural arches, which make their appearance ___ ^ about the fifth or sixth month ^^^^^^^^^ ^^°- 72.— Ossification of Sacrum. in the higher segments, the ^^^^^^Bfj^KJ' *•«• Centres for bodies ; h.b. Epiphysial lamina3, articular processes, ^^^^^^T Pj^^«^ °" b°^^i«« ' '^■''- Centres for costal 1,1 i. ■ \ ^s c ^S& '^ff^^ elements ; d.d. Centres for neural arches : and the posterior half of ^^^^^^^^^ e ^. Late^l epiphyses, the alae on either side are developed. The spinal canal is not enclosed till the seventh to the tenth year, the laminae usually failing to meet in the lowest segment, and occasionally^ to a greater or less extent, in some of the higher segments. The anterior part of the lateral masses is developed from separate centres which represent the costal elements (Gegenbauer). These appear about the sixth to the eighth month, and may develop in relation to the upper four sacral segments ; more usually they are met with in connexion with the first three, and exceptionally they may be found only in the upper two. It is by fusion of these with the posterior arches that the lateral masses which support the innominate bones are formed. The costal elements fuse about the second to the fifth year with the neural arches, prior to their union with the centra ; and the segments of the lateral masses unite with each other sooner than the union of the bodies is efl^ected. The latter only takes place after puberty by the fusion of the epiphysial plates, a pair of which make their appearance between the centra of each segment. The lower segments begin to unite together about the eighteenth year, bxit fusion between the first and second sacral verte- bne is not completed till the twenty-fifth year or after. In addition to the foregoing, two thin osseous laminfo are developed in tlic cartilage covering the outer surface of the alar mass. The upper of these overspreads the auricular surface, whilst the lower forms the sharp edge below. The extremities of the upper spinous processes arc occasionally developefl from independent epiphyses. On making a mesial section of an adult bone the por.sistence of the intervertebral discs lictween the centra is indicated by a scries of oval cavities. 94 OSTEOLOGY. Coccygeal Vertebras. — Tiiese arc cartilaginous at birth. Each has a separate centre ; the first a];)pears from the first to the fourth year, the second from tlie sixth to the tenth year, the third and fourth segments at or about puberty. Secondary centres, for tlie coccygeal cornua and epiphysial plates for the bodies are also described. Fusion of the various segments begins below and proceeds upwards, but is liable to great indi- vidual variation. In advanced life the coccyx is often ossified to the sacrum. SERIAL HOMOLOGIES OF THE VERTEBRAE. It is a self-evident fact that the vertebral column consists of a number of segments or verte- brae all possessing some characters in common. These vertebrae or segments undergo modifications according to the region they occupy and the functions they are called ujjon to serve, so that their correspondence and identity is thereby obscured. There is no difficulty in recognising the homology of the bodies and neural arches throughout the colunui. According to some anatomists the neural arch is the more primitive element in the formation of a vertebra, whilst others hold that the centra are the foundation of the column. Be that as it may, we find that in the higher vertebrates, at least, the bodies are the j^arts Avhicli most persist. They are, however, subject to modifications dependent on their fusion with one another. This occurs in the cervical part of the column where the centrum of the first cervical or atlas vertebra has for functional reasons become fused with the bod}' of the second or axis vertebra to form the odontoid process of that segment. For similar reasons, and in association with the union of the girdle of the hind-limb with the column, the bodies of the vertebrae which corresj^ond to the sacral segment become fused together to form a solid mass. In the terminal portion of the caudal region the centra alone represent the vertebral segments. As regards the neural arch, this in man becomes deficient in the lower sacral region, and absent altogether in the lower coccygeal segments. The sj)inous processes are absent in the case of the first cervical, lower sacral, and all the coccygeal A'ertebrte, and display characteristic difterences in the cervical, thoracic, and luml»ar regions, which have been already described. The articular processes (zygaj^ophyses) are secondary developments, and disj^lay great diversity of form, deter- mined l)y their functional requirements. It is noteworthy that, in the case of the uppei' two cervical vertebra?, they are so disposed as to lie in front of the foramina of exit of the upper two spinal nerves, and by this arrangement the weight of the head is transmitted to the solid column formed by the vertebral bodies, and not on to the series of neural arches. It is in regard to the homology of the transverse processes, so called, that most ditticulty arises. In the thoracic region they can best be studied in their simplest form ; here the ribs — which Qegenbauer regards as a differentiation from the inferior or hfemal arches, in ojDposition to the view advanced by others that they are a secondary development from the fibrous intermuscular septa — articulate with the transverse processes and bodies of the thoracic vertebrae through the agency of the tubercular (diapophysis) and capitular (parapophysis) jjrocesses respectively, the latter being placed, strictly sjDeaking, on the neui^al arch behind the line of the neuro-centi-al synchondrosis. An interval is thus left between the neck of the rib and the front of the transverse j^rocess ; this forms an arterial passage which corresj^onds to the vertebrarterial canal in the transverse l^rocesses of the cervical vertebrae, the anterior bar of which is homologous with the head and tubercle of the thoracic rib, whilst the jjosterior part lies in series with the thoracic transverse 231'ocess. These homologies are further emphasised by the fact that in the case of the seventh cervical vertebra the anterior limb of the so-called transverse jsrocess is develojDed from an independent ossific centre, which occasionally persists in an independent form as a cervical rib. In the lumbar region the external or transverse jirocess is serially homologous with the thoracic ribs, though here, owing to the coalescence of the contiguous j)arts, there is no arterial channel between the rib element and the true transverse process, which is represented by the accessory processes (anaj)ophysis), jjlaced posteriorly at the root of the so-called transverse process of human anatomy. Support is given to this view by the presence of a distinct costal element in connexion with the transverse process of the first lumbar vertebra, which accounts for the occasional formation of a supernumerary rib in this region. The cases of foramina in the transverse processes of the lumbar vertebrte (see p. 83) are also noteworthy as supporting this view. In the sacrum the lateral mass of the bone is made itp of comliined transverse and costal elements, with only very exceptionally an intervening arterial channel (see p. 86). In the case of the upjser three sacral segments the costal elements are largely developed, assist in supporting the ilia, and are called the true sacral vertebra; ; whilst the lower sacral segments, which are not in contact with the ilia, are referred to as the j^seudo-sacral vertebrae. The anterior arch of the atlas vertebra is, according to Froriep, develojied from a hypochordal strip of cartilage (hypochordal spange). THE STERNUM. The sternum or breast bone occupies the middle of the upper part of the thoracic wall anteriorly. It is connected laterally with the cartilages of the first seven ribs, and supports, superiorly, the clavicles. It consists of three parts, named respectively the manubrium or presternum ; the body (corpus sterni), gladiolus or mesostemum ; and the ensiform. or xiphoid cartilage (processus xiphoideus) or the THE STERNUM. 95 Interclavicular notch CI tviculai facial Eib car- tilage Manubrium II. Ril) caitila, III. Rib cartila: metasternum. Of these the body is formed by the fusion in early life of four segments or sternebrte. The manubrium, usually separate throughout life from the rest of the bone, though occasionally fused with it, is of a flattened triangular form. The anterior surface, slightly saddle-shaped, affords attachment to the fibres of the pectoralis major and sterno- mastoid muscles. It is bounded above by a thick border, the lateral parts of which are hollowed out obliquely to form the facets (incisurse claviculares) for the sternal ends of the clavicles ; around the facets, which have an upward, outward, and slightly backward direction, the bone is faintly lipped. In the in- terval between these two facets there is a slight notch (incisura jugularis) which forms the floor of the characteristic hollow seen at the root of the neck anteriorly- the suprasternal notch, or pit of the neck. The lateral borders are excavated im- mediately below the clavicular facets for the reception of the cartilages of the first ribs. Below this, the margin of the l^one slopes inwards, and is sharp, except interiorly, where it presents a facet which supports a part of the second costal cartilage. Around this the bone is usually lipped anteriorly. The upper angles correspond to the ridge separating the clavicular facets from the first costal facets : whilst the lower angle, which may be regarded as cut across trans- versely, forms the surface which is united by cartilage to the body of the sternum, iv. Ribcaitiiage- The anterior edge of this surface is usually [)rominent. The posterior aspect of the manubrium is smoother than the anterior, is pierced by numerous foramina, and is slightly concave from side to side and above downwards. Here are attached some of the fibres of the sterno-hyoid and sterno-thyroid muscles. The body (corpus sterni), usually ^^- ^^^^^^'^^^^sf twice the length and from half to two- thirds the width of the manubrium, dis- plays evidence of its composite nature. If the anterior surface, which is slightly convex from above down wards, and I'aintly concave from side to side, be carefully examined, three ill-marked ridges may be seen crossing it transversely ; these cor- respond to the lines of fusion between the four primitive segments. To this surface of the Ijone the great pectoral muscles are extensively attached on eith(!r side of the middle line. The lateral borders are thick and interruyjted at YX)ints corresponding to the transverse lines already mentioned by U-shaped hollows, the edges of which are more or less projecting. These are for the reception of the cartilages of the third, fourth, and fifth ribs. 'J'he upper harder is united to the manubrium ;i,l)Ove, and forms with it an angle of variable degree — the sternal angle ra-ngiilus sterni). A small facet is formed at the i'X])(iiiH('. of the outer extniuiity of this border, and in conjunction with the I'acet on the lower edge of the manubrium forms a recess on either side, in line with V. Rib cartilai VII Rib cartila, Ensifonn procesi Fig. 73. — The Sternum (anterior view). 96 OSTEOLOGY. the angle, into which the cartilage of the second rib fits. The lower border of the body is curved, and is united in the middle line with the xiphoid cartilage, whilst on either side it is pitted to receive the cartilages of the sixth and seventh ribs, the latter being in part supported by the xiphoid cartilage. The middle line of the body of the sternum anteriorly corresponds to the Hoor of the median surface furrow, which runs down the front of the chest in the interval between the two great pectoral muscles. The j^osterior surface is slightly concave from above downwards, and displays faint indications of three transverse lines in correspondence with those placed anteriorly. It is in relation with the pleura and pericardium, and affords attachment at its lower extremity to the triangularis sterni muscle. The xiphi- sternum (processus xiphoideus) displays many varieties of i'orm and structure. It is a pointed process of cartilage, supported by a core of bone con- nected above with the lower end of the body of the sternum, and having its lower extremity, to which the linea alba is attached, free. It lies somewhat posterior to the plane of the anterior surface of the manubrium, and forms a floor to the V- shaped interval between the cartilages of the seventh ribs. In this way a depression is formed, the surface hollow in correspondence with which is called the pit of the stomach or infrasternal depression. To the sides of this process are attached the aponeuroses of the abdominal muscles, whilst posteriorly the fibres of the diaphragm and triangularis sterni muscles derive attachment from it. It remains partly cartilaginous until middle life, at which time it generally undergoes ossification, particularly at its upper part, which becomes fused with the body. Of varied form, it may be met with of spatula-shape, bifid, circular, pierced in the centre, or twisted and deflected to one or other side, or turned forward. The sternum as a whole is broadest above where the first rib cartilages are attached. It becomes narrow opposite the second rib cartilages, but again expands until the level of the fifth rib cartilage is reached, below which it is rapidly reduced in width and ends below in the pointed xiphoid cartilage. Its position in the body is oblique from above downwards and forwards ; its axis, if prolonged upwards, would touch the column opposite the third or fourth cervical vertebra. }^^^ii.,=;^.^-f ^^^ Though liable to changes in position by the rising and fall- ing of the chest wall, its upper extremity corresponds to the level of the lower border of the second dorsal vertebra, whilst the lower end of the xiphoid cartilage usually falls in hue with the disc between the tenth and eleventh dorsal vertebrse. In women tlie sternum is usually narrower and shorter than in men, and its position less oblique. Architecture. — It consists of large-celled spongy bone, which is highly vascular, and is contained between two layers of thin comiaact tissue. At birth. Fig. 74. At 3 years. -Ossification of the Sternum. In this figure the second as well as the third segmeut of the body jjossesses two centres. 1. Appears about 5th or sixth mouth. 2. Appear about 7th mouth ; iinite from 20 to 25. 3. Appear about Sth or ninth month ; III. segment unites with II. about pul:>erty ; IV. segment unites with III. iu early childhood. 4. Appears about 3rd year or later. Ossification. — The carti- laginous sternum, developed from the fusion mesially of two cartilaginous bands uniting the anterior extremities of the carti- lages of the first eight ribs, according to the researches of Ruge and more recently of Eggeling, begins to ossify about the sixth month of fa?tal life. About this time a single centi-e appears in the manubrium; at birth this is well developed. Secondary epiphyses have been described in connexion with the clavicular facets ; these do not unite with tlae rest of THE EIBS. 97 the manubrium till adult life is reached. The body formed by the fusion of four segments is ossified from independent centres, either single or double, for each segment. These appear — the highest as early as the sixth month of intrauterine life — in some cases even before the manubrium has begun to ossify (Lambertz), the lowest towards the end of full term. The common arrangement met with at birth is a single centre for the first, and double centres for each of the succeeding segments. Union between these segments occurs rather irregularly, and is liable to much variation. The fourth unites with the third segment in early childhood, the third with the second about puberty, whilst the fusion of the second with the first segment may not be complete till the twentieth or twenty-fifth year. The xiphi-sternum usually ossifies from a single centre, which may appear as early as the third year, though often very much later. The xiphi-sternum usually unites with the body about forty or fifty, and in exceptional cases osseous imion between the body and manubrium may occur in advanced life. According to Paterson the presternum is developed in association with the shoulder girdle and becomes only secondarily associated with the ventrally growing ribs. Variations. — The sternum is liable to considerable individual variations affecting its length and direction. The majority of bones are asymmetrical, displaying irregularities in the levels of the clavicular facets. The higher costal facets may be closer together on one, iisually the right side, than the other, whilst the pre-mesosternal joint is often oblique, sloping somewhat to the right. According to Birmingham, these are the result of the strain thrown on the shoulder by pressure either directly applied or through the pull of a weight carried in the hand. Sometimes the sternum articulates with eight rib cartilages. This may happen on one or both sides, but when unilateral, much more frequently on the right side — a condition by some associated with right-handedness. It is, however, more probably a persistence of the primitive condition of the cartilaginous sternum, in which each half is connected with the anterior extremities of the first eight costal arches. In some rare cases only six pairs of ribs articulate by means of their costal cartilages with the sternum. Recently Lickley has brought forward evidence to show that the seventh rib is undergoing regressive changes. {Anat Anz. vol. xxiv. p. 326.) Occasionally the presternum supports the first three ribs ; in other words, the manubrium has absorbed the highest segment of the body. Keith has pointed out that this is the condition most commonly met with in the gibbon, and regards its occurrence in man as a reversion to the simian type. As far as is at present known, its occurrence seems more common in the lower races. Through errors of develojament the sternum may be fissured throughout, due to failure of fusion of the cartilaginous hemisterna. The two ossified halves are usually widely separated above, but united together below by an arthrodial joint. The heart and pericardium are thus uncovered by the bone. Occasionally this condition is associated with ectopia cordis, under which circumstances life is rendered impossible. Through defects in ossification the mesosternum may be pierced by a hole, usually in its lower part, or through failure of fusion of the lateral centres one or more of the segments of the body may be divided longitudinally. Occasionally small ossicles are found in the ligaments of the sterno-clavicular articulation. These are the so-called episternal bones, the morphological significance of which, however, has not yet been satisfactorily determined. They are by some regarded as the homologues of the interclavicle or episternal bone of monotremes. THE EIBS. The ribs (costse) of which there are twelve pairs, form a series of curved osseous bands which support the thoracic wall; posteriorly they articulate with the thoracic or dorsal vertebras, anteriorly each rib is provided with a costal cartilage. The first seven ribs articulate with the sternum by means of their cartilages, and are termed the true (costse verse) or vertebro-sternal ribs. The lower five ribs are not so supported, and are described as the false ribs (costse spurise). Of these the eighth, ninth, and tenth are united by their cartilages to the cartilage of the seventh rib, and are called the vertebro-chondral ribs, whilst the last two ribs are free at their anterior extremities, and are named the floating or vertebral ribs. A typical rib consists of a head (capitulum oostse), a neck (collum costte), a tubercle rtul^crciilum costae), and a shaft (corpus costtc), on which, near its posterior iTid, is th(; angle Cangulus costcc). 'J'he head, ])laced on the posterior or vertebral end of the bone, is somewhat expanded. Internally its articular surface is wedge-shaped and divided into two parts, an upper and lower, by a ridge or crest (crista (;apituli), to which the intcr- articiibir ligaincnt is attatjlied. Of these two facets tlic lower is usually the larger, and articidatCH witli the up])cr f;i,cet on the body of the vcrtcibra in numerical correspondence with it, whilst the upper facet is for the corresponding area on the 8 98 OSTEOLOGY. Head Tubercle lower part of the body of the vertebra above. The head is supported by a more or less constricted bar of bone, the neck. This becomes continuous with the shaft externally, at which point there is a well-marked tubercle on its posterior surface. The anterior surface of the neck is smooth; its posterior aspect is rough, and pierced by numerous small holes for vessels. Here is attached the middle costo- trans- verse ligament. Not uncommonly the upper border of the neck is lipped and ridged (crista colli costse), and affords attachment to the anterior costo- transverse lio-ament. The tubercle consists of an articular and a non-articular part ; the former is internal to and below the latter. Its articular surface, of rounded or oval shape, is directed downwards, backwards, and a little inwards, and rests upon a facet on the transverse process of the vertebra in numerical correspondence with the rib. The non-articular part, most prominent in the upper ribs, has the fibres of the posterior costo-transverse ligament attached to it. It is usually separated from the upper border of the neck and shaft by a groove, in which lies the external branch of the posterior division of the thoracic nerve. The shaft (corpus costse) is thin, flattened, and band- like. Its length varies much ; the seventh and eighth, which are usually the longest, are from two and a half to three times the length of the first and twelfth ribs respect- ively. The shafts are curved so as to adapt them to the form of the thoracic wall. More acute in the upper members of the series, where the shafts are shorter, the curve opens out in the middle and lower parts of the thorax, where the diameters of that cavity are greater. The curve, however, is not uniform. Including the whole length of the bone, it will be seen to be most accentuated towards the hinder part, where, in correspondence with the point at which the bend is most pronounced, there is a rough ridge placed obliquely across the outer surface of the shaft ; this is the angle (angulus cost?e). The distance between the angle and the tubercle is greatest on the eighth rib ; above that, the width between these two points gradually decreases until, in the case of the first rib, the two coincide. Below the level of the eighth rib the distance slightly diminishes in conformity with the general narrowing of the thorax below that level. Combined with this curve, there is in many of the ribs a twist. This may best be understood if the student will take a strip of stiff paper and bend it in the form of the curve of the rib. If, after he has done this, he pulls down the fore end and turns up the hind end of the strip, he will have imparted to the strip of paper a twist similar to that met with in the rib. This appearance is best seen in the middle members of the series, notably in the seventh and eighth ribs, above and below which it gradually becomes less marked. It. is the occurrence of this twist which prevents the extremities of the ribs, together with the shaft, from resting on the same plane surface. To this rule there are certain notable exceptions, viz. the first and second, the twelfth, and not infrequently the eleventh. The shaft has two surfaces, an internal and an external, and two borders, a For costal cartilai; Fig. 75. — Fifth Eight Rib AS SEEN FROM BeLOW. THE KIBS. 99 Facets on head Articular part of tubercle for transverse process of vertebra Fig. 76. — Fifth PaoHT Kib as seen from Behind. superior and an inferior. The external surface, which is smootli, conforms to the general vertical convexity of the thorax, being directed upwards in tlie first rib, upwards and outwards in the higher ribs, outwards in the middle series, and out- wards and slightly down- wards in the tenth, eleventh, and twelfth. The internal surfaces are arranged con- versely and are covered by the parietal pleura. Towards the sternal end of the middle ribs, where the downward twist is most marked, there is often an oblique line across the outer surface. This is sometimes referred to as the anterior angle. The upper border of the shaft is thick and rounded behind, thinner and sharper in front ; to it are attached the fibres of the internal and external intercostal muscles. The lower border is grooved behind at the expense of the inner surface, and is overhuug externally by a sharp margin. Anteriorly this subcostal groove (sulcus costalis) fades away, and its lips coalesce to form a rounded edge. The inter- costal vessels and nerve are lodged in this groove, whilst its lips afford at- tachment to the external and internal intercostal muscles respectively. On the floor of the groove may also be seen the openings of the canals for the transmission of the nutrient vessels, which are directed towards the vertebral end of the rib. The anterior or sternal extremity of the shaft, often slightly enlarged, displays an elongated oval pit into which the costal cartilage is sunk. Peculiar Ribs. — The first, second, tenth, eleventh, and tweKth ribs all display characters by which they can be readily recognised. The first rib can be easily distin- guished from the others by its size, curvature, and flattened form. The head, which is of small size, has a single oval or circular facet, which is directed inwards and slightly back- wards for articulation with the side of the body of the first thoracic verte- bra. The neck is flattened from above downwards, and is slightly down-turned towards the end which supports the head. Its anterior border is rounded and smooth ; its posterior edge rough for the attachment of ligaments. At the ])oii)t where tlie neck joins the shaft posteriorly, a prominent tubercle curves upwards and backwards. The inner and under surface of this process has a small circular facet which rests on a corresponding articular surface on the transverse process of the first thoracic vertelira. The angle coiucidos with tlie tubercle, and thus assists in cmpliasising its ])rominence. Tlic surfaccis of tlio body of the rib are directed u])wardH and downwards, its Ijorders inwards and outwards. If the linger be run along the thin inner border, a distinct spine or tubercle can be readily Fio. 77. — First and Second Rioht Ribs as seen FROM Above. 100 OSTEOLOGY. felt about an inch or an inch and a quarter from its anterior extremity. This is the scalene tubercle (tubercuhim scaleni) for the attachment of the scalenus anticus muscle. There is a shallow, oblique groove crossing the upper surface of the shaft in front of this for the lodgment of the subclavian vein ; whilst behind the tubercle there is another groove, usually better marked, and passing obliquely forwards for the subclavian artery (sulcus subclavise). The space on the upper surface of the rib between this latter groove and the tubercle posteriorly is some- what rough, and affords attachment to the jfibres of the scalenus medius muscle. The anterior extremity of the rib is thickened and often expanded for the reception of its costal cartilage, which is not infrequently ossified. The under surface of the rib is smooth and is covered by pleura. The outer convex border, thin in front, is usually thick and rough behind the subclavian groove, where it has attached to it the fibres of the first digitation of the serratus magnus. Along this edge, also, are attached the external and internal intercostal muscles of the first intercostal space. The inner concave border is thin, and has connected with it the aponeurotic expansion known as Sibson's fascia. The second rib may be distinguished by the size of its curve ; the absence of any twist on its shaft, so that it can be laid flat on the table ; the oblique direction of the surfaces of its shaft, the outer being directed upwards and outwards, whilst the inner is turned downwards and inwards ; and the presence of a well-marked, rough, oval area about the middle of its outer surface and lower border for part of the first, and the whole of the second digitation of the serratus magnus muscle. The head has two facets, and the angle is close to the tubercle posteriorly. The tenth rib has usually only a single articular facet on the head, and may or may not have a facet on the tubercle. The- eleventh and twelfth ribs are recognised by their length. Their heads, usually large in proportion to their shafts, support a single facet for articulation with the eleventh and twelfth dorsal vertebrae respectively. The tubercles are ill- developed and have no articular facets. The angle is faintly marked on the eleventh, scarcely perceptible on the twelfth. Their anterior extremities are narrow and pointed and tipped with cartilage. The subcostal groove is absent in the twelfth, and but slightly seen in the eleventh. The twelfth is considerably shorter than the eleventh rib. Architecture. — Each, rib consists of a curved and flattened bar of bone, tbe interior of wbicli is loose and cancellous, whilst tbe investing envelope is comimct. Tbe inner table is mucb the stronger, attaining its maximum thickness opposite tbe angle — in front and behind which it becomes gradually reduced. The outer table, much thinner, is stoutest opposite the angle ; on the posterior surface of tbe tubercle and neck it forms but a thin layer. Tbe compact layers forming the upper and lower borders are not so thick as those forming the inner and outer surfaces. Tbe cancellous tissue, loose and open in tbe shaft, is most compact in tbe region of tbe bead and towards the anterior extremity. Variations. — The number of ribs may be increased or diminished. Increase may occur by the addition of a cervical rib due to the independent development of the costal element in the transverse process of the seventh cervical vertebra. This may happen on one or both sides. Tbe range of development of these cervical ribs varies ; they may unite in front with the sternum, or they may be fused anteriorly with tbe cartilage of tbe first rib, or the cervical rib may be free. It may in some instances be represented mainly by a ligamentous band, or its vertebral and sternal ends may be alone developed, the intermediate part being fibrous. At times tbe vertebral end only may be formed and may be fused with the first rib, thus leading to the formation of a bicipital rib such as occurs in many cetaceans. Increase in the number of ribs may also be due to tbe ossification of the costal element which is normally present in the embryo in connexion witb the first lumbar vertebra. (Eosenberg, Mvrph. Jahrb. i.) Keduction in the number of ribs is less common. The twelfth rib rarely aborts ; in some cases tbe first rib is rudimentary. Cases of congenital absence of some of tbe ribs have been recorded by Hutchinson, Murray, and Ludeke. Fusion of adjacent ribs may occur. (Lane, Gmfs Hosp. Rex)orts, 1883.) Variations in form may be in great part due to tbe occupation of the individual and tbe con- stricting influence of corsets. Independently of these influences, tbe fore part of the shaft is sometimes cleft so as to appear double ; at other times the cleft may be incomplete so as to form a perforation. Occasionally adjacent ribs are imited towards their posterior part by processes having an intermediate ossicle between (Meckel), thus recalling the condition normally met with in birds ; more usually, however, the bony projections are not in contact. The number of true or vertebro-sternal ribs may be reduced to six, or increased to eight (see ante, p. 97). THE THOEAX AS A WHOLE. 101 Ossification. — Ossification begins in the cartilaginous ribs about the sixth week, and rapidly extends along the shaft, so that by the end of the third month it has reached the permanent costal cartilage. The sixth and seventh ribs are the earliest to ossify ; the first rib being the last (Lambertz). At puberty, or before, secondary centres appear. Of these there are three — an epiphysis for the articular surface of the tubercle, one for the non-articular part of the same process, and one for the head. By the twenty-fifth year fusion between these and the shaft is complete. THE COSTAL CAETILAGES. The costal cartilages, of which there are twelve pairs, are bars of hyaline cartilage united to the anterior extremities of the ribs, into which they are recessed and held in position by the periosteum. Through these cartilages the first seven ribs are connected directly with the sternum by means of synovial joints corresponding to the notches along the margins of the breast bone. To this there is an exception in the case of the first rib, the cartilage of which is directly blended with the manubrium sterni. The eighth, ninth, and tenth are connected indirectly with the sternum by their union with each other, and their articulation, through the medium of the eighth, with the seventh rib cartilage, whilst the eleventh and twelfth cartilages tip the ribs to which they belong, and lie free in the muscles of the flank. The costal cartilages increase in length from the first to the seventh, below which they become shorter. The first inclines obliquely downwards and inwards to unite with the upper angles of the manubrium. The second lies more or less horizontally. The third to the seventh gradually become more and more curved, inclin- ing downward from the extremities of their re- spective ribs, and then turning upwards to reach the sternum. The tenth cartilage articulates by means of a synovial joint with the ninth, the ninth with the eighth, and the eighth with the seventh. There are also surfaces for the articulation of the seventh with the sixth, and sometimes for the sixth with the fifth. Variations. — Occa- sionally a costal cartilage is unduly broad, and may be pierced by a foramen. The number of costal cartilages connected with the sternum may be re- duced to six or increased to eight (see p. 97). In advanced life there is a tendency towards ossifica- tion in the layers under- lying the perichondrium, more particidarly in the case of the first rib cartilage YiQ. 78. — Thk Thouax as skbn kuom Tiiii Fhont. vvliich it may be regarded as a more or less normal occurrence. TWK THORAX AS A WJIOLK. The bony and cartilaginous thorax is barrel-shaped, being narrower al)ove than below, and compressed from before ba(;kwards. Its posterior wall is longer than its 8a 102 OSTEOLOGY. anterior, and its transverse width, which reaches its maximum opposite the eighth or ninth rib, is much in excess of its sagittal diameter. This is largely owing to the forward projection of the thoracic part of the vertebral column into the thoracic cavity. The anterior wall is formed by the ribs and rib cartilages, together with the sternum. The posterior wall comprises the thoracic part of the vertebral column and the ribs as far as their angles. Owing to the back- ward curve of the ribs, and the projection forwards of the vertebral bodies, the. antero- posterior diameter of the thoracic cavity is considerably greater on either side of the middle line than in the mesial plane, thus allowing for the lodgment of the rounded pos- terior borders of the lungs. For the same reason the furrow on either side of the spinous processes of the thoracic verte- brae is converted into a broad groove (vertebral groove), the floor of which is in part formed by the ribs as far as their angles. The grooves so formed are each occupied by the fleshy mass of the erector spinee muscle. The lateral walls are formed by the costal arches. The ribs which run obliquely from above downwards and forwards do not lie parallel to each other, but spread somewhat, so that the intervals between them (intercostal spaces) are wider in front than behind. The superior aperture or inlet formed by the body of the first thoracic vertebra be- hind, the arches of the first rib on either side, and the upper border of the manubrium sterni in front, is contracted and of reniform shape. The plane of the inlet is oblique from behind downwards and forwards, so that in expiration the upper border of the sternum lies on a level with the disc between the second and third thoracic vertebrae. The lower aperture, of large size, is bounded in the middle line behind by the twelfth thoracic vertebra ; passing thence the twelfth ribs slope outwards, down- wards, and forwards. From these a line carried horizontally forwards from their tips touches the end of the eleventh rib, and then curving slightly upward reaches the cartilage of the tenth rib. Here it follows the confluent margins of the car- tilages of the tenth, ninth, eighth, and seventh ribs, finally reaching the xiphoid cartilage, where it forms with the costal margin of the opiDosite side the subcostal angle, the summit of which coincides with the xiphi-sternal articulation ; in expiration this joint usually lies on a level with the intervertebral disc between the ninth and tenth thoracic vertebrae, and corresponds with the surface depression familiarly known as the pit of the stomach. The inferior aperture of the thorax is occupied by the vault of the diaphragm. Fig. 79. — The Thorax as seen from the Right Side. THE FEONTAL BONE. 103 In the foetal condition the form of the thorax differs from that of the adult. It is laterally compressed — in this respect resembling the simian type. Its antero-posterior diameter is relatively greater than in the adult. At birth changes in form take place dependent on the expansion of the lungs ; during subsequent growth, the further ex- pansion of the thoracic cavity in a transverse direction is correlated with the assumption of the erect posture, and the use of the fore-limbs as prehensile organs. Sexual Differences. — The thorax of the female is usually described as being proportionately shorter and rounder than the male. It also tends to narrowness in the lower segment. It is hardly necessary to point out that the natural form is often modified by the use of tight or ill-fitting corsets. THE BONES OF THE SKULL (Ossa Ceanii). The term skull (cranium) is commonly employed to signify the entire skeleton of the head. This comprises the bony envelope which surrounds the brain (cranium cerebrale), and the osseous structures which support the face (cranium viscerale, ossa faciei). In catalogues of craniological collections the terms used are as follows : — Skull = entii-e skeleton of head, including the mandible. Cranium = the skull, minus the mandible. Calvaria =that part of the skull which remains after the bones of the face have been removed or destroyed. The cranium cerebrale is composed of the occipital (os occipitale), the sphenoid (os sphenoidale), the ethmoid (os ethmoidale), and the frontal (os frontale), the two parietals (ossa parietalia), and the two temporals (ossa temporalia) — eight bones in all. The bones of the face (cranium viscerale, ossa faciei) include the following : — Two single, viz. the vomer (vomer), and the inferior maxilla or mandible (mandibula), and twelve bones, arranged in pairs, viz. the superior maxillary (maxillse), malar (ossa zygomatica), palate (ossa palatina), together with the lachrjmial (ossa lacrymalia), nasal (ossa nasalia), and inferior turbinated (conchee inferiores) — fourteen bones in all. According to the scheme of international nomenclature, the inferior turbinals, the lachrymals, the nasals, and the vomer are included under the cranium cerebrale, and not with the cranium viscerale. The hyoid bone is usually described along with the skull. If, in addition, the bones of the middle ear, three on each side (malleus, incus, and stapes), be in- cluded, the skeleton of the head consists of twenty-nine bones. The separate bones will first be described, and then the skull will be considered as a whole and in section. THE SEPARATE BONES OF THE SKULL. The Frontal Bone. The frontal bone (os frontale), situated in the fore part of the cranium, is a single bone formed by the fusion in early life of two symmetrical halves. It con- sists of a frontal part, which corresponds to the region of the forehead ; an orbital part, which enters in the structure of the roof of the orbits ; and a nasal part, which assists in forming the roof of the nasal fossae. The frontal part (pars frontalis) is the shell-like portion of the bone which rises uj^wards above the orbital arches. Its external surface is rounded from side to side and from above downwards. This convexity is most pronounced about 1^ inches above the orbital margins on either side of the middle line, constituting what are known as the frontal eminences (tubera frontalia). These mark the original sites of the centres from which the bone ossifies. The lower margin of til is })art is formed on either side of the middle line by the curved orbital margins (margines supraorbi tales), the outer and inner extremities of wliich constitute the external and internal angular processes respectively. The latter descend to a lower lever than the I'ornier, and articulate with the lachrymal 8& 104 OSTEOLOGY. bones, being separated from each other by a rough articulate surface — the nasal notch for the nasal and superior maxillary bones. The curve of the orbital margin varies in different individuals and races ; towards its inner third it is crossed by a groove, not unfrequeutly converted into a foramen — the supraorbital notch or foramen (incisura sive foramen supraorbitalis). Through this there pass the supraorl)ital nerve and artery. Above the supraorljital margin the character of the bone displays marked differences in the two sexes : in the male, above the interval between the two internal angular processes, there is usually a well-marked prominence, called the glabella, from this the fulness extends outwards above the orbital margin, varying in degree and extent, and forming the elevations known as the supraorbital or superciliary ridges (arcus superciliares). The prominence of these naturally reacts on the character of the supraorbital margins, which are thicker and more rounded in the male than in the female. Passing upwards over the glabella, the remains of the suture which originally separated the two halves of the frontal bone can usually be seen ; above this point all trace of the suture is generally obliterated. Frontal eminences External angular process -4?*,^ Temporal crest Superciliary ridge Orlabella and remains of frontal suture Internal angular process Supraorbital notch. For articulation •\vitli nasal bone l^lf Nasal spine Fig. 80. — Frontal Bone (Anterior View). Extending upwards from the external angular process is a well-marked ridge, which curves upwards and slightly inwards, then turning backwards it arches across the lateral aspect of the bone. This is the temporal ridge or crest (linea tem- poralis), which serves to separate the anterior surface of the frontal portion^f the bone from its temporal aspect. The latter (facies temporalis) forms the flooMF the upper and anterior part of the temporal fossa, and serves for the attachm§|MPf the temporal muscle. The orbital part of the bone (pars orbitalis) consists of two transversely-curved plates, each having the form of a sextant ; their inner edges, which are cellular, lie parallel to each other, and are separated in their posterior half by the ethmoidal notch (incisura ethmoidalis), in which the ethmoid bone is lodged. The edges of the notch on either side are grooved in front and behind by the anterior and posterior ethmoidal foramina, which are completed when the ethmoid is in situ. The anterior transmits the internal branch of the nasal nerve and the anterior ethmoidal vessels ; the posterior, the posterior ethmoidal vessels. In front of the ethmoidal notch is the nasal notch, from the centre of which the nasal process projects downwards and for- wards to terminate in the nasal spine (spina nasalis), which lies between, and articu- lates with the nasal bones and perpendicular plate of the ethmoid. On either side THE FEONTAL BONE. 105 of the root of this process the bone is grooved obliquely from above downwards and forwards, and enters into the formation of the narrow roof (pars nasalis) of the nasal fossse. Anteriorly the nasal notch is limited by a rough U-shaped articular surface, the median part of which articulates with the nasal bones, whilst on either side the nasal processes of the superior maxillae are united with it. Behind this, amid the broken cells, the passages leading into the frontal sinuses are readily distinguished, and here the inner edges of the orbital plates articulate with the lachrymal bones. The orbital plate is thin and brittle. In front it is bounded by the superior orbital margin, just within wliich, midway between the internal angular process and the supraorbital notch there is a small shallow depression (fovea trochlearis), often displaying a spicule of bone arising from its edge (spina trochlearis), which affords attachment to the pulley of the superior oblique mnscle of the eyeball. Externally the orbital plate is overhung by the orbital margin and the external angular process, and in the hollow so produced (fossa glandules lachrymalis) the lachrymal gland is lodged. The extremity of the external angular process (pro- cessus zygomaticus) articulates with the frontal process of the malar bone. Behind For articulation with lesser wing of sphenoid Superior longitudinal sinus and falx cerebri ..X"^"^*!- Meningeal groove Orbital plate Temporal suiface External angular pioce&s Surface for articula- • tion with great wing of sphenoid Lachrymal fossa ^^"^^ Internal orbital canals \"5upiaoibital notch Iiochlear fosba Ethmoidal notch Frontal sinus / -^ \ Nasal suitacc r Nasal notch Nasal spine Fig. 81. — Frontal Bone as seen from Below. ^ this the irregular edge of the orbital plate is united with the great wing of the sphenoid by a triangular area, which also extends on to the inferior aspect of the temporal surface of the frontal bone. The apex of the orbital plate, for the space of about half an inch, articulates with the lesser wing of the sphenoid. The cerebral surface of the bone forms a fossa in which lie the fore and under parts of the frontal lobes of the cerebrum, the convolutions of which impress their form on tlie inner aspect of the bone. Here, too, on either side of the middle line, may be seen depressions for the lodgment of Pacchionian bodies. Descending from the centre of the upper margin of the bone is a vertical groove, the frontal sulcus ; narrowing below, this ends in a ridge — the frontal crest — which nearly reached the fore yj^irt of the ethmoidal notch, wliere it terminates in a small orifice, the foramen Ccccum, placed usually in the suture between the fore part of the ethmois and the frontal. This foramen may, or may not, transmit a small vein from the nose to the commencement (d" the superior longitudinal sinus. This sinus, which is interposed between the layers of the falx cerebri, is at first attached to the frontal crest, but subsequently occupies the frontal sulcus. Deeply concave from side to side and from above downwards, th(! lateral aspects of the fossa are seen to be traversed by snifdl grooves for tii(^ anterior branches of the middle meningeal artfirie.s. Below, the orbital plates bulge into the floor of the fossa, so that the 106 OSTEOLOGY. ethmoidal notch appears recessed between them. On either side of the notch faint grooves for the meningeal branches of the ethmoidal vessels may be seen. The circumference of the fossa is formed by the serrated edges of the bone which articulate with the parietals above, and on either side below with the great and lesser wings of the sphenoid. Connexions. — The frontal articulates with twelve bones, viz. posteriorly with the parietals and sphenoid ; externally with the malars ; inferiorly and internally with the nasals, superior maxillae, lachrymals, and ethmoid. Architecture. — The frontal bone is comj^osed, like the other bones of the cranial vault, of two layers of compact tissiie, enclosing between them a layer of sjjongy cancellou.'^ texture — the diploe. In certain definite situations, owing to the absorption of this intermediate layer, the bone is hollow, forming the frontal air sinuses. The position and extent of these is to some extent indicated by the degree of projection of the su2:)erciliary ridges, though this must not be taken as an absolutely reliable guide, for cases are recorded where the ridges were low and the sinuses large, and vice versa. Of much surgical importance, these air-spaces only attain their full develoj)ment after the age of puberty, being of larger size in the male than in the female, a circumstance which accounts for the more vertical aj)pearance of the forehead in woman as con- trasted with man. Usually two in number, they are placed one on either side of the middle line, and communicate by means of the infundibulum with the nasal fossa of the same side. It is excejDtional to find the sinuses of opposite sides in communication with each other, as they are generally separated by a comjjlete jjartition which, however, is occasionally much deflected to one or other side. Logan Turner (" On the Illumination of the Air Sinuses of the Skull, with some Obsei'vations uj^on the Surgical Anatomy of the Frontal Sinus," Edin. Med. Jour., May 1898) gives the average dimensions of these sinuses as follows : — Height, 31 mm., i.e. from the fronto- nasal aj^erture upwards ; breadth, 30 mm., i.e. from the septum horizontally outwards ; depth, 17 mm., from the anterior wall at the level of fronto-nasal suture backwards along the orbital roof Exceptionally large sinuses are sometimes met with extending backwards oA^er the orbit so as to form a double roof to that space. There is a specimen in the Oxford collection in which the sinus is so large, and extends so far back, that the optic nerve is carried through it in a bony tube. Another point of some practical importance is that the sinuses are hardly ever sym- metrical. It is rare to meet with cases of their complete absence, although sometimes the sinus on one or other side may be wanting. The external angular process, from the arrangement of its surfaces and the density of its structure, is particularly well adapted to resist the pi'essure to which it is subjected when the jaws are firmly closed. Variations. — That most frequently met with is a persistence of the suture which unites the two halves of the bone in the infantile condition : skulLs disj)laying this peculiarity are termed metopic. The researches of various observers — Broca, Eanke, Gruber, Manouvrier, Anoutchine, and Papillault {Rev. mens, de I'ecole d' Anthro2)ol. de Paris, annee 6, n. 3) — ^^oint to the more frequent occurrence of this metopic suture in the higher than in the lower races of man ; and Calmette asserts its greater frequence in the brachycephalic than the dolichocejjhalic type. Separate ossicles (Wormian bones) may occur in the region of the anterior fontanelle. The fusion of these with one or other half of the frontal explains how the metopic suture is not always in line with the sagittal suture (Stieda, Anat. Anz. 1897, p. 227) ; they occasionally jjersist, however, and form by their coalescence a bregmatic bone (G. Zoja, Bull. Scientifico, xvii. -p. 76, Pavia). Turner {Ghallenger Reports, part xxix.) records an instance of direct articulation of the frontal with the orbital plate of the sujjerior maxilla in a Bush skull, and other examples of the same anomaly, which obtains normally in the skulls of the chimpanzee and gorilla, have been observed {Jour. Anat. and Physiol. vol. xxiv. p. 349). Schwalbe (1901) records the jjresence of small inde- j)endent ossicles (supranasal bones) in the anterior part of the metopic suture. The same anatomist has also recently directed attention {Zeit. f. Morpli. und Anthr. vol. iii. p. 93) to the existence of the metoj)ic fontanelle, first described by Gerdy, and the occurrence of metopic ossicles (ossa interfrontalia) and canals. Ossification. — Ossification begins in membrane from two centres, which appear about tlie sixth or seventh Aveek, one on either side immediately above the orbital margin. From these the two halves of the frontal part of the bone are developed, and by extension inwards and backwards from their lower part the orbital plates are also formed. Serres, Rambaud, and Renault and v. Ihering describe the occurrence of three pairs of secondary centres somewhat later : one pair for the nasal spine on either side of the foramen caecum ; a centre on either side in Fig. 82. — Ossification of Frontal Bone. a, Metopic suture still open, b, Position of primary centre. c, Centre for e.Yternal angular iDrocess. d, Centre for region of trochlea, e, Centres for nasal spine. THE PAEIETAL BONES. 107 correspondence with the position of each trochlear fossa ; and a centre for each external angular process. Fusion between these secondary and the primary centres is usually complete about the sixth or seventh month of fojtal life. At birth the two symmetrical halves of the bone are separated by the metopic suture, obliteration of which gradually takes place, so that about the fifth or sixth year it is more or less completely closed, traces only of the suture being left above and below. In about eight per cent of Europeans, however, the suture persists in the adult (see ante). At birth the supraorbital notches lie near the middle of the supraorbital arches. Traces of the frontal sinuses may be met with about the second year, but it is only about the age of seven that they can be definitely recognised. From that time they increase in size till the age of puberty, subsequent to which time they attain their maxi- mum development. The Paeietal Bones. The parietal bones (ossa parietalia), two in number, are placed on either side of the vault of the cranium, articulating with the frontal anteriorly, the occipital Parietal eminence Parietal foramen / \ Superior temporal line Inferior temporal line For articulation with great wing of sphenoid For articulation with squamous temporal ]5'or articulation with mastoid-temporal Fig. 83. — Eight Parietal Bone (Outer Side). posteriorly, and the temporals and sphenoid inferiorly. Each bone possesses an external and internal surface, four borders, and four angles. The external surface, convex from above downwards and from before backwards, displays towards'its centre a more or less pronounced elevation, the parietal eminence (tuber parietale). This marks the position of the primitive ossific centre, and not unfrequently corresponds to the point of maximum width of the head. At a variable distance from the lower bordcjr of the bone, and more or less parallel to it, two curved lines can usually be distinguished ; these together constitute the temporal crest. The superior temporal line (\mvAi temporalis superior) serves for the attachment of the temporal fascia ; the inferior temporal line (linea temporalis inferior) delines the attachment of the temporal muscle, the extent and development of which necessarily determinfis the position of tlie crest. Tlie surface below the crest enters into the formation of the floor of the temporal fossa, and is called the planum temporals; it also affords origin to tin; tem]joral muscl(!, find is often faintly marked by grooves wliich indicate the course of the middle temporal artery. 108 OSTEOLOGY. Above the superior temporal line the bone is covered only by the tissues of the scalp. Near its upper border, and about an inch from its posterior superior' angle, is the small parietal foramen (foramen parietale), through which y^ass a small arteriole and an emissary vein. The inner or cerebral aspect is concave from side to side and from above down- wards, moulded over the surface of portions of the frontal, parietal, occipital, and temporal lobes of the cerebrum, it displays impressions corresponding to the arrange- ment of the convolutions of these portions of the brain. It also presents a series of well-marked grooves for the lodgment of the branches of the middle meningeal artery ; these radiate from the anterior inferior angle of the bone, the best marked running upwards at some little distance behind and parallel to its anterior border. Within the upper margin are a series of depressions for Pacchionian bodies, and here also the bone is channelled so as to form a groove (sulcus sagittalis), which is completed by articulation with its fellow of the opposite side. Within Depressions for Paccliioiiian bodiijs Anterior interior angle Grooves for middle meningeal artery- Groove for lateral sinus Fig. 84. — Right Parietal Bone (Inner Surface). this groove lies the superior longitudinal venous sinus, and to its edges the falx cerebri is attached. Close to the inferior posterior angle there is also a curved groove, the lateral sulcus, in which the lateral venous sinus is lodged. The anterior, superior, and -posterior horders are deeply serrated. The anterior articulates with the frontal bone, and constitutes the coronal suture ; the posterior is united with the occipital bone, and forms the lambdoid suture. The superior border articulates with its fellow of the opposite side by means of the sagittal suture ; in the interval between the two parietal foramina this suture is usually simple in its outline. The anterior superior angle (angulus frontalis) is almost rectangular, and corresponds to the site of the anterior fontanelle. The -posterior superior angle (angulus occipitalis), usually more or less rounded, corresponds in position to the posterior fontanelle. The inferior border is curved, and shorter than the others ; it lies between the anterior and posterior inferior angles. Sharp and bevelled at the expense of its outer table, it displays a fluted arraugement, and articulates with the squamous part of the temporal bone. The anterior inferior angle (angulu^ THE OCCIPITAL BONE. 109 sphenoiclalis), pointed and prominent, articulates with the great wing of the sphenoid. It is wedged into the angle formed by the union of that bone with the frontal, and is bevelled at the expense of its inner table anteriorly, whilst inferiorly it is thinned at the expense of its outer table. The posterior inferior angle (angulus mastoideus) is a truncated angle lying between the inferior and posterior Ijorders. It is deeply serrated, and articulates with the mastoid process of the temporal bone. Not unfrequently there is a channel in this suture which transmits an emissary vein. Connexions. — The parietal bone articulates with its fellow, with the frontal, occij^ital, mastoid and squamous temporal, and with the sphenoid. Occasionally the inferior angle may not reach the great wing of the sphenoid, being separated from it by the articulation of the squamous temporal with the frontal (see also p. 119). Architecture. — Thin towards its lower part, where it enters into the formation of the temporal fossa, it is thickest along the superior border and in the neighbourhood of the posterior superior angle. Variations. — A number of cases have been recorded in which the parietal is divided into an upper and lower part by an an tero -posterior suture parallel to the sagittal suture. Coraini {AUi. d. XI. Oongr. Med. Internaz. Boma, 1894, vol. v.) records a case in which the parietal was in- completely divided into an anterior and posterior part by a vertical suture. The parietal foramina vary greatly in size, and to some extent in position. They are sometimes absent on one or other side, or both. They correspond in position to the sagittal fontanelle. Sometimes the ossification of this fontanelle is incomplete and a small transverse fissure remains. The parietal foramen represents the patent external extremity of this fissure after its edges have coalesced. Occasionally in the region of the anterior fontanelle an ossicle of variable size may be met with. This is the so-called prae-interparietal bone. According to its fusion with adjacent bones it may disturb the direction of the sagittal suture. Ossification. — Ossification takes place in membrane by the deposition of earthy matter, the centre for which, most probably formed by the coalescence of two nuclei, appears over the parietal eminence about the sixth or seventh week of foetal life ; from this it spreads in a radial manner towards the edges of the bone, where, however, the mem- branous condition still for some time persists constituting the fontanelles. These corre- spond in position to the angles of the bone. Ossification is also somewhat delayed in the region of the parietal foramina, constituting what is known as the sagittal fontanelle, a membranous interval which is not unfrequently apparent even at birth. The Occipital Bone. The occipital bone (os occipitale), placed at the back and lower part of the cranium, consists of three parts, arranged around a large oval hole, called the occipital foramen or foramen magnum. The expanded curved plate behind the foramen is the tabular or squamous part. The thick rod-like portion in front of the foramen is the basilar process. On either side the foramen is bounded by the lateral or condylic portions. The tabular or squamous part (squama occipitalis) in shape somewhat resembles a Gothic arch, and is curved from side to side and from above downwards. It forms inferiorly a small portion of the middle of the posterior boundary of the foramen magnum, and unites on either side of that with the lateral parts of the bone. About the centre of the external surface of the squama there is a prominence — the external occipital protuberance (protuberantia occipitalis externa), which varies considerably in its distinctness and projection, and serves for the attachment of the ligamentum nuchse. From the protuberance on either side two lines curve out towards the external angles of the bone. These are known respectively as the highest and superior curved lines (linea nuchse suprema and linea nuchse superior). To the upper of these the epicranial aponeurosis is attached, whilst the lower serves for the origin of the trapezius and occipitalis muscles and the insertion of the sterno- mastoid and splenius capitis muscles. The two lines together serve to divide the external surface of the tabular part into an upper or occipital portion (planum occipitale), covered by the hairy seal]) and a lower or nuchal part (planum nuchale) serving for the attachment of the fleshy muscles of the back of the neck. As a rule the occi]Htal part bulges backwards beyond tlie external occipital protuberance ; exceptionally, however, the latter process is the most outstanding part of the bone. The nuchal plane, irregular and rough, is divided into two lateral lialves by a median ridge — tlie external occipital crest (linea nuchte mediana), which stretches 110 OSTEOLOGY. from the external occipital protuberance above to the posterior border of the foramen magnum below. Crossing tlie nuchal plane transversely, about its middle, is the inferior curved line (linea nuchte inferior), which passes outwards and forwards on either side towards the lateral margins of the bone. The areas thus marked out serve for the attachment of the complexus, obliquus superior, and rectus capitis posticus major and minor muscles. The internal surface of the squama, concave from side to side and from above downwards, is subdivided into four fossai l)y a crucial arrangement of ridges and grooves. The upper pair of fossae lodges the occipital lobes of the cerebrum, the lower pair the lobes of the cerebellum. Near the centre of this aspect of the bone is the internal occipital protuberance (protuberantia occipitalis interna), an irregular elevation, the sides of which are variously channelled according to the disposition of the grooves. Leading from this to the hinder margin of the foramen magnum Higliest curved lino External occipital protuberance Supei'jor curved line Inferior curved line Jugular process Jugular notch Condyle, Pharyngeal tubercle Fig. 85. — Occipital Bone as seen from Below. is a sharp and well-defined ridge, the internal occipital crest (crista occipitalis interna), which serves for the attachment of the falx cerebelli, a process of dura mater which separates the two cerebellar hendspheres. Passing upwards from the internal occipital protuberance there is usually a well-marked ridge, to one or other side of which, more frequently the right (with the bone in the normal position and viewed from behind), there is a well-defined groove, the sulcus sagittalis, the outer lip of which is generally less prominent. Placed in this groove is the superior longitudinal venous sinus, and attached to the lips is the falx cerebri. At right angles to the foregoing, and at the level of the internal occipital protuberance, with which they become confluent, are two transverse grooves, the sulci transversi. These grooves, which have more or less prominent edges, lie between the upper and lower pairs of fossae, and serve for the attachment of the tentorium cerebelli as well as the lodgment of the lateral blood-sinuses. Commonly the right lateral groove is confluent with the groove to the right side of the median ridge, but exceptions to this rule are not infrequent. The angle formed by the union of the venous sinuses THE OCCIPITAL BONE. Ill lodged in these grooves constitutes the torcular Herophili, which may accordingly be placed to one or other side of the internal occipital protuberance, more frequently the right ; in some cases, however, it may occupy a central position. The superior angle, more or less sharp and pointed, is wedged in between the two parietal bones, its position corresponding to the site of the posterior fontanelle. The lateral angle articulates on either side with the posterior extremity of the mastoid portion of the temporal bone. The superior borders, much serrated, articu- late with the parietal bones forming the lambdoid suture ; and the lateral edges, extending from the external angles to the jugular process inferiorly, are connected with the inner sides of the mastoid portions of the temporals. The lateral or condylic parts of the occipital bone (partes laterales) are placed For superior longitudinal sinus and falx cerebri Cerebral fossa Superior angle Internal occipital jHotuberance For lateral sinus and tentorium Lateral angle Cerebellar fossa Internal occipital crest 1 ~^ Jugular process Posterior condylic foramen Jugular notch Groove for inferior petrosal sinus Basilar process Fio. 86. — Occipital Bone (Inner Surface). Basilar groove on either side of the foramen magnum; on their under surface they bear the condyles (condyli occipitales) by means of which the skull articulates with the atlas vertebra. Of elongated oval form, the condyles are so disposed that their anterior extremities, in line with the anterior margin of the foramen magnum, lie closer togetlier than tlieir posterior ends, which extend as far back as the middle of the external Itorders of the foramen. Convex from before backwards, they are skewed so that their surfaces, which are nearly plane from side to side, are directed slightly outwards. Each is supported on a boss of bone, pierced by the anterior condylic foramen (canalis hyp(jglossi), which opens obliquely from within outwards and forwards on the floor of a fossa called the anterior condylic fossa, situated just external to the I'orc part of the condyle. The foramen transmits the hypoglossal or XII. cranial nerve;, together with a meningeal brancli oF the ascending ]>haryngeal art(!ry and its coni])anion veins. 15(!liind the; cundyh; is ])la(;ed the posterior condylic foBsa, in the fiofjr of whicli tlie posterior condylic foramen lieipKiiitly opcnis. Through 112 OSTEOLOGY. this a vein passes which joins the lateral sinus. The edge of the foramen magnum immediately behind the condyle is often grooved for the passage of the vertebral artery around it. Jutting out from the posterior half of the condyle is a stout bar of bone, serially homologous with the vertebral transverse process — this is the jugular process (processus jugularis) ; deeply notched in front, its anterior border is free and rounded, and forms the posterior boundary of the jugular foramen. Curving outwards from this margin, in line with the anterior condylic foramen, there is often a small pointed projection, the processus intra-jugulare, which serves to divide the jugular foramen into two compartments. Externally the jugular process articulates by means of a synchondrosis with the jugular surface of the petrous part of the temporal bone. Its posterior border is confluent with the lower and lateral portion of the occipital squama, and its under surface is rough and tubercular for the attachment of the rectus capitis lateralis muscle. The superior aspect of the lateral part displays on either side of the foramen magnum an elevated surface of oval form, the tuberculum jugulare ; this corresponds to the part of the bone which bridges over the canal for the hypoglossal nerve. Its upper surface in many instances displays an oblique groove running across it ; in this are lodged the glosso-pharyngeal, vagus, and accessory nerves. The jugular process is deeply grooved superiorly for the lower part of the lateral blood sinus, which here turns round the anterior free edge of the process into the jugular foramen. Joining this, close to its inner edge, is the opening of the posterior condyhc foramen when that canal exists. The basilar part of the occipital bone (pars basilaris) extends forwards and upwards from the foramen magnum. Its anterior extremity is usually sawn across, as, after adult life, it is necessary to sever it in this way from the sphenoid, the cartilage uniting the two bones having by that time become completely ossified. Broad and thin behind, it narrows laterally and thickens vertically in front, where on section it displays a quadrilateral form. Projecting from its under surface some little distance in front of the foramen magnum is the pharyngeal tubercle (tuber- culum pharyngeum) to which the fibrous raphe of the pharynx is attached ; on either side of this the rectus capitis anticus major and minor muscles are inserted. The upper surface forms a broad and shallow groove which slopes upwards and forwards from the thin anterior margin of the foramen magnum ; in this rests the medulla oblongata. On either side its lateral edges are faintly grooved for the inferior petrosal venous sinuses, below which the lateral aspect of the bone is rough for the cartilage which unites it to the sides and apex of the petrous part of the temporal bone. The foramen magnum, of oval shape, so disposed that its long axis lies in the sagittal plane, is of variable size and form. The plane of its outlet differs somewhat in individual skulls ; in most instances it is directed downwards and slightly forwards. In front the condyles encroach upon it, and narrow to some extent its transverse diameter. To its margins are attached the ligaments which unite it with the atlas and axis. Through it pass the lower part of the medulla oblongata where it becomes continuous with the spinal cord, the two vertebral arteries, the spinal accessory nerves, and the blood-vessels of the meninges of the upper part of the cord. Connexions. — The occipital bone articulates witli tlie two jaarietals in front and above, witli tbe sphenoid in front and below, with the two temporals on either side, and with the atlas vertebra by means of its condyles. Architecture. — The squamous part displays thickenings in the position of the various ridges and crests, the stoutest part corresponding to the internal and external occij^ital protuberances, though it should be noted that the two protuberances do not necessarily coincide, the internal being, as a rule, placed at a higher level than the external. If the bone be held up to the light it Avill be at once apparent that it is much thinner where it forms the floor of the inferior fossae than in the uj^per part. The basilar portion consists of a sj^ongy core surroimded by a more compact outer envelope, thickest on its lower surface. In the condyles the spongy tissue is arranged radially to their convex articular surfaces, the hypoglossal canal being surrounded by particularly dense and compact bone. Variations. — The most striking of the many A^ariations to which this bone is subject is the separation of the upper part of the occipital squama to form an indejjendent bone — the inter- parietal bone, called also, from the frequency of its occurrence in Peruvian skulls, the os Incse THE OCCIPITAL BONE. 113 As will be seen below (see Ossification), the occurrence of this anomaly is explained development- ally. In place of forming a single bone the interjjarietal is occasionally met with in two symmetrical halves, and instances have been recorded of its occurrence in three or even four pieces. In the latter cases the two anterior parts form the pre-interparietals. Instances are recorded of the j^resence of a separate ejiiphysis between the basi-occipital and the sphenoid, the OS basioticum (Albrecht) or the os prte-basi-occipital. The articular surface of the condyles is sometimes divided into an anterior and posterior part. The so-called third occipital condyle is an outstanding j^rocess rising from the anterior border of tlie foramen magnum, the extremity of which articulates with the odontoid process of the axis. Guerri has recorded a case, in which in a foetal skull, there were two outstanding tubercles in the position of the third occipital condyle, indejoendent of the basi-occipital jaortions of the condyles (Anat. Anz. vol. xix. p. 42). This appears to confirm the view of Macalister that there are two different structures included under this name — one a mesial ossification in the sheath of the notochord, and the second, a lateral, usually paired process, caused by the deficiency of the mesial part of the hypochordal element of the hindmost occipital vertebra, with thickenings of the lateral parts of the arch. Springing from the under surface of the extremity of the jugular process, a rough or smooth elevated surface, or else a projecting process, the extremity of which may articulate with the transverse process of the atlas, is sometimes met with. This is the paroccipital or paramastoid process. Numerous instances of fusion of the atlas with the occipital bone have been recorded. Many are, no doubt, i^athological in their origin ; others are associated with errors in development. Interesting anomalies are these in which there is evidence of the intercalation of a new vertebral element between the atlas and occipital, constituting what is termed a proatlas. Ossification. — The major part of the bone ossifies in cartilage, the upper part of the squama (interparietal), alone developing in membrane. The basilar part begins to ossify about the sixth week of foetal life by the appeai'ance of two centres, one in front of the other; the anterior, according to Albrecht, constitutes the basiotic, the posterior the basi-occipital. These two centres — which there is some reason to believe — may themselves be formed by the fusion of pairs placed laterally, rapidly tmite, so that the occurrence of one centre alone is frequently described. From this the fore part of the margin of the foramen magnum is formed, together with a portion of the anterior end of the occipital condyle on either side. It helps also to close up the front of the anterior condylic canal. Union with the condylic parts is complete about the fourth or fifth year. Ankylosis between the basi-occipital and the sphenoid takes place about the twenty-fifth year. The lateral, condylic, or exoccipital parts begin to ossify from a single centre about the end of the second month of foetal life. The notch for the hypoglossal canal appears about the third month. From this centre is formed the posterior three- fourths of the occipital condyle. The exoccipital is usually completely fused with the squama by the third year or earlier. As already noted, the squama consists of two parts — the one above the occipital crest, the other below it : the former develops in membrane, the latter in cartilage. In a three-months' foetus this difference is very characteristic. The cartilaginous part (supra-occipital) begins to ossify from two centres about the sixth or seventh week, which rapidly join to form an elongated strip placed transversely in the region of the occipital protuberance. The centres for the upper part (interparietal) appear later. According to Maggi {Arch. Ital. Biol, tome '^' 26, fas. 2, p. 301), they are four in number, of Fio. 87.— Ossification of Occipital Bone which two placed on either side of the middle line appear about the second month. The other pair, placed laterally, are seen about the third month ; fu.siou between these takes place early, but their dispo.sition and arrangement explain the anomalies to which this part of the bone is subject. The mesial pair may persist as separate ossicles, or fuse to form the pre-interparietals, whilst the lateral pair may remain independent of the supra-occipital as a single or double interparietal bone. Union between the supra-occipital and the interparietal elements occurs about the third or fourth niontli ; but evidence of their separation is frequently Basilar centre ; b, Exoccipital ; c, Ossicle of Kerkring ; d, Supra-occiiiitid (froiri car- tilage) ; e. Fissure between siqtra-oecipital and interparietal ; ,/; Interparietal (from iiienibraiie) ; //, Fissure between inter- parietals. 114 OSTEOLOGY. met with even in the adult by the persistence of a transverse suture running inwards from each ' external angle of the squama, or, as above mentioned, there may be an os Incse. The supra-occipital forms a small part of the middle of the hinder border of the foramen ■ magnum, though here a small independent centre, known as the ossicle of Kerkring, is occasionally met with. Other independent centres ai-e sometimes seen between the supra-occipital and the exoccipitals. At birth the occipital consists of four parts — the interparietal and supra-occipital combined, the basi-occipital, and the exoccijjitals — one on either side. The Temporal Bones. The temporal bone (us temporale) lies about the centre of the lower half of either side of the skull, and enters largely into the formation of the cranial base. It is placed betv^een the occipital Ijehind, the parietal above, the sphenoid in front, and the occipital and sphenoid internally and below. At birth it consists of three parts — an iipper and outer part, the squamous or squamo-zygomatic portion ; an inner and posterior portion, the petro-mastoid, which contains the parts specially associated with the sense of hearing, together with the organ associated with equilibration; and an under or tympanic part, from which the floor aud anterior wall of the external auditory meatus is formed. The squamous part (pars squamosa) consists of a thin shell-like plate of bone placed vertically, having an inner (cerebral) and an outer (temporal) surface and a semicircular upper border. Inferiorly, behind, and internally it is fused in early life with the petro-mastoid portion by means of the squamoso-mastoid and the petro-squamosal sutures, traces of which are often met with in the adult bone ; whilst below and in front it is separated from the tympanic and petrous parts by the Griaserian fissure. Its external surface, smooth and slightly convex, enters into the formation of the floor of the temporal fossa, and affords attachment to the temporal muscle. Near its hinder part it is crossed by one or more ascending grooves for the branches of the middle temporal artery. In front and below there springs from it the zygomatic process (processus zygomaticus). This arises by a broad attachment, the surfaces of which are inferior and superior ; curving outwards and forwards, it then becomes twisted and narrow so that its sides are turned inwards and outwards and its edges directed upwards and downwards. Anteriorly it ends in an obhque serrated extremity which articulates with the zygomatic pro- cess of the malar bone. Posteriorly the edges of the zygomatic process separate and are termed its roots. The upper edge, which becomes the posterior root, sweeps back over the external auditory meatus, and is confluent with a ridge, the supra-mastoid crest, which curves backwards and slightly upwards, and serves to define the limit of the temporal fossa posteriorly. The inferior edge turns inwards and constitutes the anterior root ; the under surface of this forms a transversely-disposed rounded ridge, the articular eminence (tuberculum articulare), behind which there is a deep hollow, the glenoid fossa (fossa mandibularis), limited posteriorly by the tympanic plate, and crossed at its deepest part by an oblique fissure, the Glaserian fissure (fissura petro-tympanica). This cleft, which is closed externally, transmits about its middle the tympanic branches of the internal maxillary artery, and lodges the slender process of the malleus. At its inner end the lips of this fissure are frequently separated by a thin scale of bone, a downgrowth from the tegmen tympani of the petrous part, which here separates the tympanic from the squamous elements, forming in its descent the major part of the outer wall of the osseous Eustachian canal, which lies immediately internal to it. Between this scale of bone and the posterior edge of the fissure there is a small canal (canal of Huguier), which transmits the chorda tympani nerve. The part of the glenoid fossa in front of the fissure articulates with the condyle of the inferior maxilla, through the medium of the interarticular cartilage, which is here interposed and rests as well on the tuberculum articulare. Posteriorly the part of the fossa behind the fissure is non- articular and lodges a portion of the parotid gland. At the angle formed by the divergence of the two roots of the zygoma, in correspondence with the outer part of the articular eminence, there is a rounded tubercle ; to this are attached the THE TEMPOEAL BONES. 115 fibres of the external lateral ligament of the temporo-mandibular joint. In front of the inner end of the articular eminence there is a small triangular surface, limited in front by the edge of the anterior root, and internally by a thick serrated margin which articulates with the outer side of the great wing of the sphenoid ; this area forms part of the roof of the zygomatic fossa. Just anterior to the external auditory meatus and projecting downwards from the under surface of the posterior root there is a conical process, called the post-glenoid tubercle, which forms a prominent anterior lip to the external extremity of tlie Glaserian fissure ; it is the representative in man of a process which occurs in some mammals and prevents the backward displacement of the lower jaw. By some anatomists it is referred to as the middle root of the zygoma. The zygomatic process by its lower margin and inner surface gives origin to the masseter muscle, whilst attached to its upper edge are the layers of the temporal fascia. Behind the external auditory meatus, and below the supramastoid crest, the Groove for middle temporal artery' Supramastoid ^-j crest^c^ Parietal notch Temporal surface Zygoma Remains of masto-squamosal suture Eminentia articularis Glenoid fossa Mastoid piocess External auditory meatus Auricular External Process fissure auditory process Styloid process^ Fig. 88.— Right Temporal Bone as seen from the Outer Side. squamous element extends downwards as a pointed process, which assists in forming the roof and posterior wall of the external auditory meatus, where it unites inferiorly with the tympanic plate. In the adult this process is occasionally sharply defined posteriorly by an oblique irregular fissure, the remams of the masto-squamosal suture. Professor Macewen has pointed out that this suture frequently remains open till puberty and occasionally after, and may be of iinportance as a channel along which in- fective processes may extend. The inner surface of the squamous part, less extensive than the outer aspect owing to the bevelling of the superior border, is marked by the impression of the convolutions of the temporal lobe of the cerebrum, and is limited below by tlie petro-squamosal suture, the remains of which can frequently be seen. It is crossed in front by an ascending groove for the middle meningeal artery, branches from \vhi(;h course backwards over the })ono in grooves more or less parallel to its upper border. The superior harder of tb(; squamous part is curved, shar]), and scalo-libs, being bevelled at the expense of its inner table, except in front, where the margin is thick \)a 116 OSTEOLOGY. and stout. Here it articulates with the great wing of the sphenoid, its union with that hone extending to near the fore part of the summit of the curve, behind which it is united to the parietal overlapping the lower border of that bone ; posteriorly the free margin of the squamous part ends at an angle formed between it and the mastoid process called the incisura parietalis. The tympanic part (pars tympanica) of the temporal bone forms the anterior, lower and part of the posterior wall of the external auditory meatus. Bounded in front and above by the Glaserian fissure, it forms the hinder wall ot the non- articular part of the glenoid fossa. Fused internally with the petrous part, its lower edge, sharp and well defined internally, splits to enclose the root ot the proiectincf styloid process, and is hence called the vaginal process. Externally it unites wfth the fore part of the mastoid process, and higher up with the descending process of the squamous part, from both of which it is separated by the auricular fissure (fissura tympano-mastoidea) through which the auricular branch oi the Eminence of superior ' / M'nuciieular canal Groove for middle meningeal artery ~"~~;;~r^~^9 ^^la^.'s,.,-' Parietal notch Groove for superior XJetrosal sinus Petro-squaraous suture- Groove for inferior petrosal sinus Groove for lateral sinus ^Aqueduct of the vestibule 1 ii'i : ii.'il auditory meatus Aqueduct of the cochlea Inner surface of mastoid process Styloid process Fig. 89.— Right Tempokal Bone (Inner Side). vagus escapes. Its free border, which forms the anterior, lower, and part of the posterior border of the external auditory meatus, is usually somewhat thickened and rough, and serves for the attachment of the cartilaginous part of the canal. The external auditory meatus (meatus acusticus externus) is directed obliquely inwards and a little forwards, and describes a slight curve, the convexity of which is directed upwards ; of oval form, its long axis, close to its orifice, is nearly vertical, but, as it passes inwards, inclines somewhat forwards so as to give a twist to the canal. The depth of the canal to the attachment of the membrana tympani averages from 14 to 16 mm. The upper margin of the outer orifice overhangs considerably the lower edge, but owing to the obliquity of the inner aperture, to which the membrana tympani is attached, the upper wall of the osseous canal only exceeds the length of the lower wall by one or two millimetres. The petro-mastoid part (pars petrosa et mastoidea) of the temporal bone, of pyramidal form, is fused to the inner aspect of the tympanic and squamosal portions, extending behind them, however, to form the well-marked and prominent mastoid process, which lies posterior to the external auditory meatus. This process (pars mastoidea) forms a nipple-like projection, the size of which differs considerably in different individuals. Usually larger in the male than in the female, its rough THE TEMPOEAL BONES. 117 outer surface and lower border serve for the insertions of the sterno-mastoid, splenius capitis, and trachelo-mastoid muscles. Within and below its pointed extremity there is a deep groove (incisura mastoidea), usually well marked, which gives origin to the posterior belly of the digastric muscle ; whilst lying to the inner side of this, and separated from it by a more or less well-defined rough ridge, there can oftentimes be seen a narrow, shallow furrow, which indicates the course of the occipital artery. The inner surface of the mastoid portion forms, in part, the lateral wall of the posterior cranial fossa, in which the cerebellar lobes are lodged. Coursing across this aspect of the bone there is a broad curved groove, the con- vexity of which is directed forwards and lies in the angle formed by the base of the petrous part and its fusion with the mastoid portion. The depth to which the bone is here channelled varies considerably, and is important from a surgical standpoint, as herein lies the sigmoid portion of the lateral venous sinus. Anteriorly the mastoid is fused with the descending process of the squamosal above, and below, where it is united with the tympanic, it enters into the formation of the posterior wall of the external auditory meatus and the cavity of the tympanum. Above, its free margin is rough and serrated, and articulates with the posterior inferior angle of the parietal; behind and below it articulates by a jagged suture with the occipital. Traversing this suture, or near it, is the mastoid foramen (foramen mastoideum), which transmits a vein from the lateral sinus to the cutaneous occipital vein, together with a small branch of the occipital artery. The petrous part of the petro-mastoid is of the form of an elongated three-sided pyramid. By its base it is united obliquely to the inner sides of the squamosal and tympanic parts. Its apex is directed inwards, forwards, and a little upwards. Its three surfaces are arranged as follows : — The superior or anterior looks upwards, slightly forwards, and a little outwards, and forms part of the floor of the middle cranial fossa. The posterior is directed backwards and inwards, and forms part of the anterior wall of the posterior cranial fossa. The inferior is seen on the under surface of the base of the skull, and is directed downwards. The borders are named respectively anterior, superior, and posterior. The anterior border is short, and forms an acute angle with the fore part of the squamous part; within this angle is wedged the spinous part of the great wing of the sphenoid. Here, too, the osseous Eustachian canal (canalis musculotubarius) may be seen leading backwards and outwards from the summit of the angle to reach the fore part of the cavity of the tympanum. On looking into it, the canal is seen to be divided into two unequal parts by an osseous partition, the cochleariform process (septum tubse). The upper compartment, the smaller of the two (semicanalis m. tensoris tympani), lodges the tensor tympani muscle, whilst the lower (semi- canalis tubse auditivse) forms the osseous part of a channel (the Eustachian tube), which serves to conduct air from the pharynx to the tympanum. The posterior border is in part articular and in part non-articular. Posteriorly and externally it corresponds to the upper margin of an area on the inferior surface with which the extremity of the jugular process of the exoccipital articulates. In front of that it is irregularly notched, and forms the free anterior edge of the jugular foramen, internal to- which it has a sharp curved border, often grooved, reaching to the apex. This groove, which is completed by articulation with the outer side of the basi-occipital, lodges the inferior petrosal venus sinus. The superior harder is a twisted edge which is continuous with the upper margin of the groove for the lateral sinus posteriorly, and anteriorly and internally reaches the apex of the bone. Eunning along it there is usually a well-marked groove for the superior petrosal venous sinus, and near its inner extremity it is slightly notched for the passage of the trigeminal nerve. Along the entire length of this border the tentorium cerebelli is attached. On the inferior surface of the petrous part, which is bounded in front by the anterior border intfsrnally, and the tympanic plate externally, and behind by the ]>ost(;rior }>order, the following structures are to be noted : — tSpringing from and ensheathed by the vaginal process is the slender and pointed styloid process (pro- cessus styloideus), the length of which varies mucli. Projecting downwards and slightly forwards and inwards, it affords attachments for the stylo-glossus, 9& 118 OSTEOLOGY. Zygoma Zygomatic surface Canal for chorda tympani Eustachian canal Carotid canal Groove for inferior jjetrosal sinus stylo-hyoicl, and stylo-pharyngeus muscles as well as the stylo-hyoid and stylo- mandilnilar ligaments. Just behind it, and between it and the mastoid process, is the stylo-mastoid foramen (foramen stylomastoideum), which lies at the anterior end of the digastric groove, and transmits the facial nerve and the stylo-mastoid artery. Immediately internal to the styloid process there is a deep, smooth, excavated hollow, tlie jugular fossa (fossa jugnlaris), which is converted into a foramen (jugular) by articulation with the occipital bone. Behind and external to the fossa there is a small quadrilateral surface of variable size, which is united to the extremity of the jugular process of the exoccipital by a synchondrosis. Inside the fossa, on its outer aspect, or placed on its external border, is the opening of a small canal (canaliculus mastoideus), which passes outwards to open into the can alls faciahs, and transmits the auricular branch of the vagus (Arnold's nerve), which ultimately escapes through the auricular fissure (see ante). In front of the jugular fossa and separated Temporal surface IVom it by a sliarp crcst, and just internal to the tympanic plate, is the circular opening of the inferior orifice of the carotid canal (canalis caroti- cus). Directed at first up- wards, this canal bends at a right angle and turns for- wards and inwards, lying parallel to the anterior border ; reaching the fore part of the apex of the bone, it opens in front by an oblique ragged orifice. -Aqueduct of cochlea ^hrough the caual the in- janal for Jacobson s o ternal carotid artery, accom- panied by a plexus of sym- pathetic nerves, passes into the cranium. On the ridge of bone separating the jugular fossa from the carotid canal is the opening of a small canal (canaliculus tympanicus), through which the tympanic branch of the glosso-pharyu- geal (nerve of Jacobson) passes to reach the tym- panum. Within the orifice of the carotid canal another small opening or openings (canaliculi carotici tympanici) may be noticed which afford passage to the tympanic branches of the internal carotid artery and carotid sympathetic plexus. Occupying the interval between the jugular fossa and the carotid canal on the inner side is a V-shaped depression (fossula fenestrte cochlese), on the floor of which and close to the posterior border is the orifice of the aqueduct of the cochlea (apertura externa aquseductus cochleae). In the fossa is lodged the petrous ganglion of the glosso-pharyngeal nerve, and the aqueduct transmits a tubular prolongation of the dura mater, wliich forms a channel of communication between the perilymph of the cochlea and the subarachnoid space. A small vein also passes through it. In front of and internal to the orifice of the carotid canal the under surface of the apex of the bone corresponds to a rough quadrilateral surface which forms the floor of the carotid canal, and also serves for the attachment of the cartilaginous part of the Eustachian tube as well as the origin of the levator palati muscle ; elsewhere it has attached to it the dense fibrous tissue which fills up the cleft (petro-basilar fissure) between it and the basilar process of the occipital bone. Tubercle Eminentia articularr Glenoid fobsa Glasenan fissure Tympanic plate Ext. auditoiy meatus Styloid process Vaginal process Auricular Assure Stylo-mastoiil foramen Mastoid process Digastric groove Groove foi occipital artery nerve Jugular fossa Canal for Ai-nold nerve ■Jugular surface Fig. 90. — Ric4ht Temporal Bone as seen from Below THE TEMPOEAL BONES. 119 The superior or anterior surface bears the impress of the convolutions of the under surface of the . temporal lobe of the cerebrum, which rests upon it ; in addition, there is a distinct hut shallow depression (impressio trigemini) near the apex, corresponding to the roof of the carotid canal ; in this is lodged the Gasserian ganglion on the sensory root of the V. cranial nerve. External to the middle of the upper surface, and close to its posterior border, is the elevation (eminentia arcuata), more or less pronounced, which marks the position of the superior semicircular canal here lodged within the bone. A little in front of this, and in line with the angle formed by the anterior border and the squamous part, is the slit-like opening of the hiatus Fallopii (hiatus canalis facialis), within the projecting lip of which two small orifices can usually be seen. These are the openings of the aciuseductus Fallopii (canalis faciahs) ; if a bristle be passed through the inner of the two openings it will be observed to pass into the bottom of the internal auditory meatus, if into the outer, it will pass through the aqueduct of Fallopius, and, provided the channel be clear, will appear on the under surface of the bone at the stylo- mastoid foramen. Leading forwards and inwards from the hiatus towards the anterior border is a groove ; in this lies the great superficial petrosal nerve which passes out of the hiatus. A small branch of the middle meningeal artery also enters the bone here. A little external to the hiatus is another small opening (apertura superior canahs tympanici), often difficult to see ; from this a groove runs forwards which channels the upper surface of the roof of the canal for the tensor tympani muscle. Through this foramen and along this groove passes the lesser superficial petrosal nerve. Behind this, and in front of the arcuate eminence, the bone is usually thin (as may be seen by holding it up to the light falling through the external auditory meatus), roofing in the cavity of the tympanum and forming the tegmen tympani. Externally the line of fusion of the petrous with the squamous part is often indicated by a faint and irregular petro-squamous fissure. The most conspicuous object on the posterior surface of the petrous part of the bone is the internal auditory meatus (meatus acusticus internus) about 8 mm. deep in the adult, which has an obhque oval aperture and leads outwards and slightly downwards into the substance of the bone, giving passage to the auditory and facial nerves, together with the pars intermedia and the auditory branch of the basilar artery. The canal appears to end blindly ; but if it be large, or still better, if part of it be cut away, its fundus will be seen to be crossed by a horizontal ridge, the falciform crest, which divides it into two fossse, the floors of which (laminae cribrosse) are pierced by numerous small foramina for the branches of the auditory nerve and the vessels passing to the membranous labyrinth, whilst in the fore and upper part of the higher fossa the orifice of the Fallopian aqueduct (canalis facialis), through which the facial nerve passes, is seen leading in the direction of the hiatus Fallopii (see ante). External to the internal auditory meatus and above it, close to the superior border, an irregular depression, often faintly marked, with one or two small foramina opening into it, is to be noticed. This is the floccular fossa (fossa subarcuata), best seen in young bones, where it forms a distinct recess, which is bounded above by the bulging caused by the superior semicircular canal, within the concavity of which it is placed ; it lodges a process of the dura mater. Below and external to this, separated from it by a smooth, elevated curved ridge, is the opening of the aqueduct of the vestibule (apertura externa aqu^eductus vestibuli), often concealed in a narrow curved fissure overhung by a sharp scale of bone. In this is lodged the saccus endolymphaticus. The ridge above it corresponds to the upper half of the posterior semicircular canal. Connexions. — Tlie temporal lK))ie tu'ticulates witli tlie malar, sphenoid, parietal, and occipital liones, and by a movable joint witli the inferior maxilla. Occasionally the temporal articulates with the frontal, as happens noi'inally in tlie anthropoid apes ; although the region of the plerion (see p. 1.04) is cliaracterised by an X-like form in the lower races of man there is no evidence that the occurrence of a fronto-squamosal suture is more frerinent in the lower than the liiglier races, its occurnnice being due to the manner of fusion of the so-called ejiipteric ossicles with the surrounding Ijones. Architecture. — Tlie temporal bone is remarkable for the liardn(!ss and d(;nsity of its petrous part, wlierein is lodgr;d the osseous hibyi'intli wliirli contains tlie delicate organs associated with the senses ni lieariiig and efpiilibration. Tlie middle ear or tympanum is a cavity which contains 120 OSTEOLOGY. the small auditory ossicles, and is separated from the external auditory meatus by the membrana tympani. In front it communicates witli tlie pharynx by the Eustachian tube; behind, it opens into the mastoid antrum and mastoid air-cells by the aditus ad antrum. Siq^eriorhj, it is separated from the middle cranial fossa by a thin plate of bone called the tegmen tympani. Inferiorly, its lioor is formed in jjart by the roof of the jugular fossa and the carotid canal. Internally, it is related to the structures wliich form the inner ear, notably the cochlea and vestibule, in front of which it is separated by a thin plate of bone from the caiotid canal. Curving OA'er the cavity of the tymj^anum is the aquDeductus Fallopii, the thin walls of which are occasionally deficient. These details will be further dealt with in the section de- voted to the Organs of Sense. Variations. — The occurrence of a deficiency in the floor of the external auditory meatus is not uncommon in the adult. It is met M'ith commonly in the child till about the age of five, and is due to incom- plete ossification of the tjanpanic plate. The line of the petro- squamosal suture is occasionally grooved for the lodgment of a sinus (petro-squamosal) ; sometimes the posterior end of this is continuous with a canal which pierces the superior border of the bone and opens into the lateral sinus. An- teriorly the groove may pass into a canal which pierces the root of the zygoma and ajsj^ears externally above the external extremity of the Glaserian fissure. These are the remains of channels through which the blood passed in the foetal condition (see cmte). Symington has described a case in which the squamous part was distinct and separate from the rest of the temjjoral bone in an adult ; whilst Hyrtl has observed the division of the temjjoral squama into two by a transverse suture. P. P. Laidlaw {Journ. Anat. and Physiol, vol. xxxvii. p. 364) describes a temporal bone in which there was absence of the internal auditory meatus and of the stylo-mastoid foramen. The jugular fossa also was absent, and there was partial absence of the groove for the lateral sinus associated with the presence of a large mastoid foramen. An instance of a rudimentary condition of the carotid canal is also referred to in the same volume by G. H. K. Macalister. G. Caribbe {Anat. Anz. vol. XX. External semicircular canal p. 81) notes the occurrence in idiots and imbeciles of a more pronounced form of j)ost-glenoid tubercle, and associates it with regressive changes in the develoj^ment of the temporal bone. External auditoiy meatus Osseous Eustachian canal Superior opening of the canal for the tympanic branch of glosso-pharyngeal \ estibule Aqu.Liluctus Fallopii Fenestia o^alls cut across Fenestra rotunda cut across Fig. 91. — Vertical Transverse Section through Left Temporal Bone (Anterior Half of Section). Superior semicircular canal Ossification. — The petro- Fenestra ovalis. cut across P'enestra rotunda cut across Vestibule into openings ol semicircular canals mastoid portion of the bone- is auditory meatus developed by the deposition of earthy matter in the cartila- ginous ear capsule and the peri- chondrium lining the labyrinth. The squamous and tympanic parts are ossified in membrane. Ossification commences in the ear capsule in the fifth month, and proceeds so rapidly that by the end of the sixth month the individual centres are more or less fused. Of these, one which appears ni the vicinity of the eminentia arcuata is the most definite in position' and form; from this a lamina of bone of spiral form is developed, which covers in the inner limb of the superior semicircular canal, and forms the roof of the internal auditory meatus, together with the commencement of the Fallopian aqueduct. Reaching forwards, it extends to the apex of the petrous part; whilst externally it forms part of the inner Opening leading into mastoid antrum. „. Aquicductus /' Fallopii J — ^Canalis stapedii Tympanum External auditory nieat\is Fig. 92.— Vertical Transverse Section through Left Temporal Bone (Posterior Half of Sectiou). THE TEMPOEAL BONES. 121 styloid process broken otf Glenoid fossa Groove for membrana tympani Extei'ual auditory meatus Mastoid air-cells Carotid canal Tyinpanuni Cochlea Internal auditory meatus Vestibule, fenestra ovalis cut across Superior semicircular canals AqucCductus Falloxni External semicircular canal Fig. 93. — Horizontal Section through Left Temporal Bone (Lower Half of Sectiou). wall of the tympanum, surrounds the fenestra ovalis, and encloses within its substance portions of the cochlea, vestibule, and superior semicircular canal. Another centre appears in the vicinity of the promontory on the inner wall /^' of the tympanum, surrounds the / / Osseous Eustacliian canal fenestra rotunda, forms the floor ■' ' of the vestibule, and extends inwards to complete the floor of the internal auditory meatus. Surrounding the cochlea in- feriorly and externally, it com- pletes the floor of the tympanum, and ultimately blends with the fore and under part of the tympanic ring. The carotid canal at first grooves it, and is then subsequently surrounded by it. According to Lambertz the lamina spiralis of the cochlea ossifies in membrane. The roof of the tympanum is formed from a separate centre, which extends backwards towards the superior semicircular canal, and encloses the tympanic part of the aque- duct of Fallopius ; externally this centre unites by suture with the squamosal, and sends down a thin process, which appears between the lips of the Glaserian fissure, and forms the outer wall of the Eustachian tube. Nuclei, either single or multiple, appear in the base of the petrous part, and envelop the posterior and external semicircular canals. It is by extension from this part that the mastoid process is ultimately developed. To these centres the terms pro-otic, opisthotic, pterotic, and epiotic, respectively, have been applied by Huxley and others. The styloid process, an independent development from the upper end of the cartilage of the second visceral arch, is ossified from two centres. The upper or basal appears before birth, and rapidly unites with the petro-mastoid, the tympanic plate encircling it in front. This represents the tympanohyal of comparative anatomy. At birth, or subsequent to it, another centre appears in the cartilage below the above : this is the stylohyal. Ankylosis usually occurs in adult life between the tympanohyal and stylohyal, the union of the two constituting the so-called styloid process of human anatomy. The centre from which the squamo-zygomatic develops appears in membrane about the end of the second month. Situated near the I'oot of the zygoma, it extends forwards and outwards into that process, inwards to form the floor of the glenoid fossa, and upwards into the squamosal. From this latter there is a downward and backward exten- sion, which forms the post-auditory process ; this ultimately blends with the posterior limb of the tympanic ring, being separated from it in the adult by the auricular fissure. It forms the outer wall of the mastoid antrum, and constitutes the fore and upper part of the mastoid process in the adult. About the third month a centre appears in the outer membranous wall of the tympanum : from this the tympanic ring is developed. Incom- plete above, it displays two free extremities. Of these, the anterior is somewhat enlarged, and unites in front with the glenoid portion of the squamo-zygomatic, being separated from it by the Glaserian fissure and the downgrov/th from the tegmen tympani ; the posterior joins the post-auditory process of the squamo-zygomatic above mentioned. Below, it blends internally with the portion of the pctro-mastoid which forms the floor of the tympanum, and enshcathcs the tympanohyal behind. From the outer side of the lower part of this ring two tubercles arise ; these grow outwards, and so form the floor of the external auditory meatus. '^J'he interval between them remains vuiossificd till aljoiit the age of five or six, after which closure takes place. This deficiency may, how- ever, persist even in adult life (see ante, Variations). At birth the temporal bone can usually be separated into its component parts. The 122 OSTEOLOGY. outer surface of the petrous part not only forms the inner wall of the tympanum, but is hollowed out behind and above to form the inner side of the mastoid antrum, the outer wall of which is completed by the post-auditoiy process of the squamo-zj^gomatic. As yet the mastoid process is undeveloped. It only assumes its nipple-like form about the second year. Towards puberty its cancellous tissue becomes permeated with air spaces, Fig. 94. — a. The Outer SaRFACE of the Eight Temporal Bone at Birth. B. The same with Squamo-zygomatic Portion Removed. (The lettering is tlie same in both A and B.) a, Tympanic ring, b, Inner wall of tympanum, c, Fenestra rotunda, d, Foramen ovale, e, Mastoid. /, Mastoid process. g. Masto- squamosal suture, with foramen for transmission of vessels. A, Squamo-zygomatic, removed in figure B to show how its descending process forms the outer wall of the mastoid antrum. Fig. 94. — C. Inner Surface of the Eight Temporal Bone at Birth. a, Squamo-zygomatic. b, Petro- squamosal suture and foramen (just above the end of the lead line). f, Subarcuate fossa, d, Aqufeductus vestibuli. e, Aquseductus cochlete. /, Internal auditory meatus. g. Upper end of carotid canal. which are in communication with and extensions from the mastoid antrum. The external auditory meatus is unossified in front and below, the outgrowth from the tympanic ring occurring subsequent to birth. The glenoid fossa is shallow and everted ; the jugular fossa is ill-marked ; whilst the subarcuate fossa is represented by a deep pit, the so-called floccular fossa of comparative anatomy. The hiatus Fallopii is an open groove, displaying at either end the openings of the inner and outer portions of the Fallopian aqueduct. The Sphenoid Bone. The sphenoid bone (os sphenoidale) lies in front of the basi-occipital mesially, and the temporals on either side. It enters into the formation of the cranial, orbital, and nasal cavities, as well as the temporal, zygomatic, pterygoid, and spheno-maxillary fossae. It consists of a body with three pairs of expanded pro- cesses, the great wings, the lesser wings, and the pterygoid processes. The body (corpus), more or less cubical in form, is hollow, and contains within it the two large sphenoidal air sinuses. These are separated by a partition, which is usually deflected to one or other side of the middle line. Each sinus extends outwards for a short distance into the root of the great wing, and downwards and outwards towards the base of the pterygoid process of the same side. They communicate by apertures with the upper and back part of the nasal fossae. In the adult the j^osterior aspect of the body displays a sawn surface due to its separation from the basi-occipital with which in the adult it is firmly ankylosed. The sujperior surface, from the fore angles of which the lesser wings arise, displays an appearance comparal:)le to that of an oriental saddle, over its middle there is a deep depression, the sella turcica or pituitary fossa (fossa hypophyseos), in which is lodged the pituitary l;)ody. Behind, this is overhung by a sloping ridge, the dorsum sella, the posterior surface of which is inclined upwards, and is in continuation with the basilar groove of the occipital bone. Anteriorly and externally the angles of this ridge project over the pituitary fossa in the form of prominent tubercles, called the posterior clinoid processes (processus clinoidei posteriores). In front of the pituitary fossa there is a transverse elevation, the olivary eminence (tuberculum sellae), towards the outer extremities of which, and somewhat behind, there are often- THE SPHENOID BONE. 123 times little spurs of bone, the middle clinoid processes rprocessus clinoidei ruediij. In front of the olivary eminence is the optic groove (sulcus chiasmatis), which passes outwards on either side to become continuous, between the roots of the lesser wings, with the optic foramina. Foramen rotunduni Groove for Eubtachiaii tube Vidian canal External pterygoid plate - Internal pterygoid plate Pterygoid notch Hainular process Fig. 95. — Sphenoid as seen from Behind. Tliis groove is Hal ile to considerable variations, and apparently does not always serve for the lodgment of the optic chiasma. (Lawrence, " Proc. Soc. Anat.," Journ. Anat. and Physiol. vol. xxviii. p. 18.) In front of the optic groove, from which it is often separated by a thin sharp edge, the superior surface continues forwards on the same plane as the upper surfaces of Oi bital surface Infra temporal crest ^ Spine Spheno-maxiUary surface External pterygoid plate laiiiiilar process of nternal jjteiygoid plato Pterygoid notcl Fig. 96. — Sphenoid as shen khom the Front. the IcHHfir wings, and terminates anteriorly in a ragged edge, wliich articulates with the cribriform plate of the ethmoid, and has often projecting from it, mesially, a pointed process, the ethmoidal spine. The lateral aspects of the body are fused with the great wings, and in ])art also with the roots of the pterygoid processes. Curving along the side of the body, sii])erior to its attachment to the great wing, is an jT-sliaped groove, the carotid groove (sulcus caroticus), which marks the 124 OSTEOLOGY. position and course of the internal carotid artery. Posteriorly, the hinder margin of this groove, formed by the salient outer edge of the posterior surface of the body, articulates with the apex of the petrous portion of the temporal bone, and is hence called the petrosal process ; just above this, on the lateral border of the dorsum sellae, there is often a groove for the sixth nerve. The anterior surface of the body displays a vertical mesial sphenoidal crest (crista sphenoidalis), continuous above with the ethmoidal syjine, and below with the pointed projection called the rostrum. This crest articulates in front with the perpendicular plate of the ethmoid. On either side of the middle line are seen the irregular openings leading into the sphenoidal air sinuses, the thin anterior walls of which are in part formed by the absorption of the sphenoidal turbinated bones with which in early life they are in contact. With the exception of a broad groove leading downwards from the apertures above mentioned, which enters into the formation of the roof of the nasal fossa of the corresponding side, the lateral aspects of this surface of the bone are elsewhere in articulation with the lateral masses of the ethmoid and the orbital processes of the palate bones. The rostrum is continued mesially for some distance along the inferior surface of the body, where it forms a prominent keel which fits into the recess formed by the al£e of the vomer. The edges of the latter serve to separate the rostrum from the incurved vaginal processes at the roots of the internal pterygoid plates. Posteriorly the under surface of the body of the sphenoid is rougher, and covered by the mucous membrane of the roof of the pharynx ; here, occasionally, a median depression may be seen which marks the position of the inferior extremity of a foetal channel, called the canalis cranio -pharyngeus. The lesser or orbital wings (alee parvse) are two flattened triangular plates of bone which project forwards and outwards from the fore and upper part of the body of the bone, with which they are united by two roots which enclose between them the optic foramina (foramina optica) for the transmission of the optic nerves and ophthalmic arteries. Of these roots, the posterior springs from the body just wide of the olivary eminence, separating the carotid groove behind from the optic foramen in front ; externally this root is confluent with the recurved posterior angle of the lesser wing, forming the projection known as the anterior clinoid process (processus clinoideus anterior), which overhangs the fore part of the body of the bone. The anterior root, broad and compressed, unites the upper surface of the lesser wing with the fore and upper part of the body. Externally the outer angle terminates in a pointed process which reaches the region of the pterion and there articulates with the frontal, and may come in contact with the great wing. The superior aspect is smooth, and forms in part the floor of the anterior cranial fossa. The inferior surface constitutes part of the posterior portion of the upper wall of the orbit, and also serves to roof in the sphenoidal fissure which separates the lesser from the greater wings below. The anterior edge is ragged and irregular, and articulates with the orbital plates of the frontal. The posterior margin, sharp and sickle-shaped, separates the anterior from the middle cranial fossa, and corresponds to the position of the Sylvian fissure on the surface of the cerebrum. The greater or temporal wings (alee magnse), as seen from above, are of a somewhat crescentic form. If the inner convex edge of the crescent be divided into fifths, the posterior fifth extends backwards and outwards beyond the body of the bone, presenting a free posterior edge, which forms the anterior boundary of the foramen lacerum medium. This border ends behind in the horn of the crescent, from which a pointed process projects downwards, called the alar or sphenoidal spine (spina angularis), this is wedged into the angle between the petrous and squamous parts of the temporal bone. The inner surface of the posterior border and spine is furrowed for the cartilaginous Eustachian tube (sulcus tubse), whilst on the inner side of the spine the course of the chorda tympani nerve is indicated by a groove (Lucas). The second fifth of the convex border of the crescent is fused to the side of the body and united below with the root of the pterygoid process. The angle formed by the union of the great wing with the side of the body posteriorly corre- sponds to the hinder end of the carotid groove, the outer lip of which is formed by a projecting lamina called the lingula. The remaining three-fifths of the convex THE SPHENOID BONE. 125 border is divisible into two nearly equal ]3arts ; the inner is a free, curved, sharp margin, which forms the inferior margin of the sphenoidal fissure (fissura orbitalis superior), the cleft which separates the great wing from the lesser wing, and which establishes a wide channel of communication between the middle cranial fossa and the cavity of the orbit, transmitting the third, fourth, ophthalmic division of the fifth, and the sixth cranial nerves, together with the ophthalmic veins. Wide of the sphenoidal fissure this edge becomes broad and serraj;ed, articulating with the frontal bone internally, and at the part corresponding to the anterior horn of the crescent, by a surface of variable width, it unites with the anterior inferior angle of the parietal bone. The external lorder corresponds to the concave side of the crescent, and is serrated for articulation with the squamous temporal, being thin and bevelled at the expense of its outer surface above and externally, and broad and thick behind as it passes towards the alar spine. The swperior or cerebral surface is concave from behind forwards, and in its fore part from side to side also ; it forms a considerable part of the floor of the middle cranial fossa, and bears the impress of the convolutions of the extremity of the temporal lobe of the cerebrum which rests upon it ; towards its outer side it is grooved obliquely by an anterior branch of the middle meningeal artery. The following foramina pierce the great wing : close to and in front of the alar spine is the foramen spinosum for the transmission of the middle meningeal artery and its companion vein, together with a recurrent branch from the third division of the V. nerve. In front of and internal to this, and close to the posterior free border, is the foramen ovale, of large size and elongated form. This gives passage to the motor root and inferior sensory division of the V. nerve, and admits the small meningeal branch of the middle meningeal artery ; a small emissary vein from the cavernous sinus usually passes through this foramen, and occasionally also the small superficial petrosal nerve. Near the fore part of the root of the great wing, and just below the sphenoidal fissure, is the foramen rotundum, of smaller size and circular form. Through this the second division of the V. nerve escapes from the cranium. Occasionally there is a small canal — the foramen of Vesalius — which pierces the root of the great wing to the inner side of the foramen ovale. This opens below into the scaphoid fossa at the base of the internal pterygoid plate, and transmits a small vein. Occasionally there is a small foramen (canaliculus innomiuatus) to the inner side of the foramen spinosum for the transmission of the small superficial petrosal nerve. The outer surface of the great wing is divided into three well-marked areas ; of these the upper two are separated by an oblique jagged ridge, the malar crest (margo zygomaticus), for articulation with the orbital process of the malar bone. The lower part of this ridge may occasionally articulate with the malar process of the superior maxilla. Of these two areas the orbital (facies orbitalis) is directed forwards and a little inwards ; of quadrilateral shape, it forms the back and outer wall of the orbit ; plane and smooth, it is bounded behind by the sharp inferior free margin of the sphenoidal fissure, towards the inner extremity of which a pointed spine for the attachment of the inferior common ligament of origin of the ocular muscles can usually be seen. It is limited superiorly by the edge of a rough triangular area which articulates with the frontal bone ; anteriorly by the malar crest ; whilst inferiorly a free, well-defined horizontal margin constitutes the posterior and external boundary of the spheno-maxillary fissure (fissura orbitalis ini'erior), which separates this part of the bone from the orbital plate of the superior maxilla. Below this border there is a grooved surface which leads inwards toward the orifice of the foramen rotundum. In the articulated skull this forms part of the posterior wall of the spheno-maxillary fossa. To the outer side of the malar crest, which })ounds it in front, is the temporal area (facies temporalis), concavo-convex from Ijcforc; backwards. It slopes inwards below, where it is se])arated from the zygomatic area by a well-marked muscular ridge, the infra-temporal crest or pterygoid ridge (crista infratemporalis). Behind, the tem])oral surface is l^ounded by the margin of the great wing which articulates with the sfinamous tem])oral, and above by the edge which unites it with the anterior inferior angle of the parietal and the frontal bone. The temporal surface 126 OSTEOLOGY. enters into the formation of the floor of the fossa of the same name, and affords an extensive attachment to the fibres of origin of the temporal muscle. The zygomatic surface (facies infratemporalis), situated below the infra-temporal crest, corresponds to the under surface of the posterior half of the great wing ; it extends as far back as the alar spine and posterior border. Opening on it are seen the orifices of the foramen spinosum and ovale. It is slightly concave from side to side, and is confluent internally with the outer surface of the external pterygoid plate. In front it is bounded by a ridge which curves upwards and outwards from the fore part of the external pterygoid plate to join the infratemporal crest. In the articulated skull this ridge forms the posterior boundary of the pterygo-maxillary fissure. The zygomatic surface overhangs the zygomatic fossa, and affords an origin for the upper head of the external pterygoid muscle. The pterygoid processes (processus pterygoidei) spring from the inferior surface of the lateral aspect of the body as well as the under side of the root of the great wings, and pass vertically downwards. Each consists of two laminse, the external and internal pterygoid plates, fused together anteriorly, and enclosing between them posteriorly the pterygoid fossa (fossa pterygoidea). The external pterygoid plate (lamina lateralis processus pterygoidei), thin and expanded, is directed obliquely backwards and outwards, its lower part being often somewhat everted. Its hinder edge is sharp, and often has projecting from it one or two spines, to one of which (processus pterygo-spinosus) the pterygo-spinous ligament which stretches towards the alar spine is attached. Externally it furnishes an origin for the lower head of the external pterygoid muscle, and on its inner side, where it forms the lateral wall of the pterygoid fossa, it supplies an attachment for the internal pterygoid muscle. ' The internal pterygoid plate (lamina medialis processus pterygoidei) is narrower and somewhat stouter. By its inner aspect it forms the posterior part of the lateral wall of the nasal fossse; externally it is directed towards the pterygoid fossa. Its posterior edge ends below in the hook -like hamular process (hamulus pterygoidei), which, reaching a lower level than the external plate, curves back- wards and outwards, furnishing a groove in which the tendon of the tensor palati muscle glides ; superiorly, the sharp posterior margin of the inner plate bifurcates, so as to enclose the shallow scaphoid fossa from which the tensor palati muscle arises, and wherein may occasionally be seen the inferior aperture, of the foramen Vesalii. To the inner edge of this fossa, as well as to the posterior border of the internal pterygoid plate, the pharyngeal aponeurosis is attached. Here, too, the cartilage of the Eustachian tube is supported on a slight projection, and the palato- pharyngeus muscle receives an origin, whilst the superior constrictor of the pharynx arises from the lower third of the same border and from the hamular process. Superiorly and internally the inner plate forms an incurved lamina of bone, the vaginal process (processus vaginalis), which is applied to the under surface of the lateral aspect of the body reaching inwards, towards the root of the rostrum, from which, however, it is separated by a groove, in which, in the articulated skull, the ala of the vomer is lodged. The angle formed by the vaginal process and the internal edge of the scaphoid fossa forms a projection called the pterygoid tubercle, immediately above which is the posterior aperture of the Vidian canal (canalis pterygoideus), through which the Vidian nerve and artery are transmitted. On its under surface the vaginal process displays a groove (sulcus pterygo-palatinus) which in the articulated skull is converted into the pterygo-palatine canal by its union with the palate bone. In front, at its root, the pterygoid process displays a broad smooth surface (facies spheno-maxillaris), which is confluent above with the root of the great wing around the foramen rotundum, and forms the posterior wall of the spheno-maxillary fossa. Here, to the inner side of the foramen rotundum, is seen the anterior opening of the Vidian canal. Below, the pterygoid plates are separated by an angular cleft, the pterygoid notch (fissura pterygoidea) ; in this is lodged the tuberosity of the palate bone, the margins of which articulate with the serrated edges of the recess. Connexions. — The spLenoid articulates with the occij^ital, temporals, parietals, frontal, THE SPHENOID BONE. 127 ethmoid, sphenoidal turbinals, vomer, palate and malar bones, and occasionally v/ith the superior maxillae. Architecture. — In the adult the body of the bone is hollow and encloses the sphenoidal air cells, usually two in number, sejjarated by a septum. The arrangement and extent of these air cells vary ; sometimes they are multilocular, at other times simple, while occasionally they extend backwards into the basi-occipital and outwards and downwards into the roots of the great wings and pterygoid processes. Cases are on record in which in the adult the body of the bone was not pneumatic. Variations. — Through imperfect ossification the foramen spinosum and foramen ovale are sometimes incomplete posteriorly. Le Double (Bull, et mim. de la Soc. d'Anth. de Paris, 5*^ ser. vol. iii. p. 550), records a case in which the foramen rotundum and the sphenoidal fissure were united so as to form a single cleft. Through deficiency of its external wall, the optic foramen, in rare instances, communicates with the sphenoidal fissure. Duplication of the optic foramen is also recorded as a rare occurrence, the artery passing through one canal, the nerve through the other. Persistence of the cranio -pharyngeal canal is also occasionally met with. On the other hand, owing to the ossification of fibrous bands connecting the several bony points, anomalous foramina are frequently met with. Cases of persistence of the cranio -pharyngeal canal have been recorded. Ossification. — The sphenoid of man is formed by the fusion of two parts, the pre- sphenoid and the post-sphenoid, each associated with certain processes. In most manmials the orbito-sphenoids or lesser wings fuse with the pre-sphenoid, whilst the alisphenoids or greater Avings, together with the internal pterygoid plate, ankylose with the post-sphenoid. The ossification of these several parts takes place in cartilage, with the exception of the internal pterygoid plate, which is developed from an independent centre in the connective tissue of the lateral wall of the oral cavity (Hertwig). At the end of the second month a centre appears in the root of the great wing between the foramen ovale and foramen rotundum; from this the ossification spreads outwards and backwards and also downwards into the external pterygoid plate. Meanwhile two centres appear about the same time in the basi-sphenoid in relation to the floor of the sella turcica and on either side of the cranio-pharyngeal canal, around which they ossify, ultimately leading to the obliteration of this channel. Somewhat later a centre appears on either side, from which the lateral aspect of the body and the lingula are developed. Fusion between these four centres is usually complete by the sixth month. In the pre-sphenoid a pair of lateral nuclei make their appearance about the middle of the third month, just external to the optic foramina ; from each of these the orbito- sphenoids (lesser wings) and their roots are developed. About the same time another pair of centres, placed mesial to the optic foramina, constitute the body of the pre- sphenoid. By the coalescence of these in front, and their ultimate union with the basi- sphenoid behind, a cartilaginous interval is enclosed, of triangular shape, which, however, becomes gradually reduced in size by the ingrowth of its margins so as to form two mesially - placed foramina, as may be fre- quently observed in young bones — one open- ing on the surface of the olivary eminence, the other being placed anteriorly. " Proc. Soc. Anat.," Journ. Anat. and Physiol, vol. xxviii. p. 19.) As has been seen, the internal pterygoid plates are developed in membrane and are the first parts of the sphenoid to ossify. (Fawcett, Anat. Auz., vol. xxvi. 1905, p. 280.) Each is derived from a single nucleus which appears about the ninth or tenth week, and fuses with the under surface of the great wing, there forming a groove which is converted into the Vidian canal when the alisplienoid and internal pterygoid plates fuse later with the body of the post- sphenoid. The hamular process, however, chondrifies before it ossifies during the third month. Fawcett also regards the external pterygoid plate as of membranous origin. At birth the sphenoid consists of three parts : one comprising the orbito-sphenoids together with the body of tlie pre-sphcu(jid and the basi-sphenoid, the others consisting of the alisphenoids, one on eitlier side. Fusion of the latter witli the former occurs near the end of the first year. The dorsum sella at birth consists of a cartilaginous plate which M!parates the body of the f)Ost-sphcnoid from the basi-occipital. This slowly ossifies, but the curtilage does not entirely disappear till the age of twenty-five, by which time bony anky- losis of the basi-craniul axis is complete. For a considerable time the under surface of the Fig. 97. — Ossification of Sphenoid. «, Pre-spheuoid ; b, Orbito-spheuoids ; c, Alisphenoids ; d, Internal pterygoid plates ; e, Basi-sphenoid. (Lawrence, 128 OSTEOLOaY. body of the pre-sphenoid displays a bullate appearance, with the sides of whicli the sphenoidal turbinated bones articulate. It is only after the seventh or eighth year is reached that the cancellous tissue within this part of the bone becomes absorbed to form the sphenoidal sinuses. The sphenoidal turbinals (conchte sphenoidales), or bones of Bertin, best studied in childhood, are formed by tlie fusion of four distinct ossicles (Cleland), the centres for which appear in the later months of utero-gestation. Each bone consists of a hollow, three- sided pyramid, the apex of which is in contact with the fore part of the vaginal process of the internal pterygoid, whilst the base fits on to the posterior surface of the lateral mass of the ethmoid. The inferior surface of each forms the roof of the corresponding nasal fossa, and completes the formation of the spheuo-palatine foramen, whilst the external aspect is united with the palate bone and forms the inner wall of the spheno-maxillaiy fossa, and occasionally constitutes a part of the orbital wall posterior to the os planum of the ethmoid. The superior surface of the sphenoidal turbinal is applied to the fore and under surface of the body of the pre-sphenoid on either side of the rostrum. It is by the absorption of this wall that the sphenoidal sinuses are ultimately opened up. The base of the pyramid forms the aperture through which each of these sinuses opens into the nasal fossse in the adult. Owing to their firm ankylosis with the surrounding bones, these ossicles are merely represented in the adult disarticulated skull by the irregular fragments adherent to the separated borders of the ethmoid, palate, and sphenoid bones. The Ethmoid Bone. The ethmoid bone (os ethmoidale) lies in front of the sphenoid, and occupies the interval between the orbital plates of the frontal, thus entering into the forma- tion of the anterior cranial fossa as well as the inner walls of the orbits and the roof and inner and outer walls of the nasal fossse. The bone, which is extremely light, consists of two cellular parts — the lateral masses, which are united superiorly to a mesial vertical plate by a thin horizontal lamina which, from its perforated condition, is called the cribriform plate. The study of this bone will be much facilitated by cutting through the cribriform plate on one side of the vertical plate, thus removing the lateral mass of one .side and exposing more fully the central perpendicular lamina. The vertical plate (lamina perpendicularis), of irregular pentagonal shape, forms the upper part of the nasal septum. Its superior border projects above the level of the cribriform plate so as to form a crest, which is much elevated an- teriorly, where it terminates in a bullate process called the crista galli, the upper edge of which is sharp and pointed, and affords attachment to the falx cerebri. In front of this process there is a groove which separates the alar processes (pro- cessus alares) which project from the crista galH on either side. By ar- ticulation with the frontal bone this groove is converted into a canal, the foramen cascum ; this, however, is not always bhnd, but frequently transmits a vein to the roof of the nose. The posterior margin of the vertical plate is thin, and articulates with the crest of the sphenoid. The posterior inferior border in the adult is ankylosed with the vomer; and the anterior inferior edge, which is usually thicker than the others, unites with the cartilaginous nasal septum. The a7iterior sitperior border articulates with the nasal spine of the frontal bone and with the median crest formed by the union of the two nasal bones. The vertical plate, which is usually deflected to one or other side, has generally smooth surfaces, except above, where they are channelled by Alar process Crista sfalli Os planum piocess 98. — ETHMOm AS SEEN FROM BEHIND. THE ETHMOID BONE. 129 Ciista fralli Anterior and ijosterior ethmoidal grooves Alar process Os planum (orbital surface) Middle meatus Infuudibuluii] Vertical plate Middle turbinated bone. Uncinate process Fig. 99. — Ethmoid as seen from the PiTght Side. short and shallow grooves leading to the foramina which pierce the cribriform plate ; these are for the lodgment of the olfactory nerves. The lateral mass or labyrinth (labyrinthus) is composed of papery bone, enclosing a large number of air-cells ; these are arranged in three groups — an anterior, a middle, and a posterior, the walls of which have been broken in front, above, behind, and below, in the pro- cess of disarticulation. Externally they are closed in by a thin, ob- long lamina, the orbital plate or os planum (lamina papyracea), which forms a part of the inner wall of the orbit, and articulates above with the orbital plate of the frontal, which here roofs in the ethmoidal cells. The line of this suture is pierced by two canals, the anterior and posterior ethmoidal canals, both of which transmit small ethmoidal vessels, whilst the anterior also gives passage to the nasal nerve. In front the os planum articu- lates with the lachrymal bone ; whilst heloio, by its union with the orbital surface of the superior maxillary bone, the air-sinuses in both situations are completed. Posteriorly the os planum articulates with the sphenoid, and at its posterior inferior angle for a variable distance with the orbital process of the palate bone, both of which serve to close in the air-cells. The mesial aspect of the lateral mass displays the convoluted turbinated processes, usually two in number, though occasionally there may be three — rarely more. In cases where there are two ethmo-turbinals they are separated posteriorly by a deep groove. A channel is thus formed in the back part of the lateral and upper aspect of the nasal fossae, called the superior meatus, which is roofed in by the superior turbinated process (concha superior), whilst its floor is formed by the upper surface of the middle turbinated process (concha media). The posterior ethmoidal cells open into this meatus. In front of the superior meatus, which only grooves the posterior half of this aspect of the bone, the surface is rounded from above downwards and before backwards, and forms the inner wall of the anterior and middle ethmoidal ceUs. Eunning obliquely from above downwards and backwards over the mesial surface of the superior concha, are a number of fine grooves continuous above with the fora- mina in the cribriform plate ; these are fewer and more scattered in front, do not pass on to the middle concha, and are for the olfactory nerves. The middle turbinated process (concha media) is nearly twice the length of the superior. Its anterior extremity is united for a short distance to the superior turbinated crest on the inner side of the frontal process of the superior maxilla. By its thickened, free convoluted border it overhangs a deep grorjve which runs along the under surface of the lateral mass. Til is is the middle meatus of the nose. It r<}ceives the openings of the middle ethmoidal cells and a passage which runs upwards and forwards from it, the infundibulum. This communicates with the anterior ethmoidal cells and the froiiUiI sinus. 'I'he out(!r side of the niiddle meatus is formed by tlie thin inner 10 Suijerior turbinated bone Anterior ethmoidal groove Uncinate process Fig. 100. — Section showing Na.sal Aspect of Left Lateral Mass of Ethmoid. 130 OSTEOLOGY. Os pliinmii Alar process Lacliryiiial process walls of the ethmoidal cells. Curving downwards, backwards, and a little out- wards from the roof of the fore-part of this meatus is the uncinate process (pro- cessus unciuatus). This bridges across the irregular opening on the inner wall of the maxillary sinus, and articu- Cristagaiii lates iuferiorly with the eth- moidal process of the inferior turbinated bone. The hinder extremity of the middle turbin- ated bone articulates with the ethmoidal crest on the vertical plate of palate bone. The cribriform plate (lamina cribrosa) is the horizontal lamina which connects the lateral masses with the vertical plate, much in the same manner as the cross limb of a capital T is arranged. It occupies the interval between the orbital plates of the frontal bone, roofinff in the nasal fossae in- ^■^^ Iiiierior turbinated bone fcriorly, and Superiorly forming on either side of the crista galli FIG. lOi.-SHowiNG ARTICULATION OF Inpekioh TURBINATED ^^^o shallow olfactory grooves in Bone with Ethmoid. which, m the recent condition, the olfactory lobes of the cere- brum are lodged. Numerous foramina for the transmission of the olfactory nerves pierce this part of the bone ; those to the inner and outer sides of the groove are the largest and most regular in their arrangement. Along the outer edges of the cribriform plate two notches can usually be distinguished ; when articulated with the frontal bone these form the inner openings of the ethmoidal canals. Leading forward from the anterior of these there is often a groove which crosses to the side of the crista galli, where it ends in a slit which allows of the transmission of the nasal nerve to the nose. Posteriorly the cribriform plate articulates with the ethmoidal spine of the sphenoid. MaxiUaiy process Ethinoidal process Vertical plate Alar process Crista aalli t for nasal nerve Us planum Lateral mass Fig. 102. — Ethmoid as seen from Above. Connexions. — The ethmoid articulates with the sphenoid and sphenoidal turbinals, the frontal, the two nasals, two superior maxillte, two lachrymals, two inferior turbinals, two palates, and the vomer. Variations. — The size of the os planum is liable to considerable variations. In the lower races it tends „ , „ to be narrower from above downwards than in the ii'nni'upae higher, in this resjiect resembling the condition met with in the anthropoids. The os planum may fail to articulate with the lachrymal owing to the union of the frontal with the orbital process of the superior maxilla in front of it. (Orbito-maxillary frontal suture. A. Thomson, Journ. Anat. and Physiol, vol. xxiv. p. 349.) Division of the os planum by a vertical suture into an anterior and posterior part has been frequently recorded. The number of the turliinals may be increased from two to four, or may be reduced to one. (Report of Committee of Collect. Invest., Journ. Anat. and Physiol., vol. xxviii. p. 74.) Ossification takes place in the cartilage of the nasal capsule. Each lateral mass has one centre, Avhich appears about the fourth or tifth month in the neighbourhood of the OS planum. From this the laminae around the ethmoidal air cells are formed which are complete at bii'th, the air-sinuses in this instance not being formed by the absorption of cancellous bone. From these centres the turbinals are also developed, and these, too, are ossified at the ninth month. BONES OF THE FACE. 131 At birth the ossified lateral masses are united to the central cartilaginous plate by a fibrous layer. Two centres make their appearance in the mesial cartilage on either side of the root of the crista galli about the end of the first year ; from these, the crista galli and the vertical plate are ossified as well as the mesial part of the cribriform plate, the lateral portions of which are derived from an inward extension of the lateral mass. Ossification is usually complete about the fifth or sixth year. About the twenty-fifth year bony union has taken place between the cribriform plate and the sphenoid, but ankylosis between the vertical plate and the vomer is not usual till the fortieth or forty- fifth year. Wormian Bones. Along the line of the cranial sutures and in the region of the fontanelles, isolated bones of irregular form and variable size are occasionally met with. These are the so-called Wormian bones, named after the Danish anatomist Wormius. They are also called sutural or epactal bones. Their presence depends on the fact that they are either developed from distinct ossific nuclei, or it may be from a division of the primary ossific deposit. Their occurrence may also be associated with certain pathological conditions which modify the development of the bone. They usually include the whole thickness of the cranial wall, or it may be only involve the outer or inner tables of the cranial bones. They are most frequent in the region of the lambda and the lambdoid suture. They occur commonly about the pterion, and in this situation are called epipteric bones (Flower). By their fusion with one or other of the adjacent bones they here lead to the occurrence of a fronto-squamosal suture. Their presence has also been noted along the line of the sagittal suture, and sometimes in metopic skulls in the inter-frontal suture. They are occasionally met with at the asterion and more rarely at the obelion. They appear less frequently in the face, but their presence has been noted around the lachrymal bone, and also at the extremity of the spheno-maxillary fissure, where they may form an independent nodule wedged in between the great wing of the sphenoid, the malar, and the superior maxillary bones. BONES OF THE FACE. The bones of the face (ossa faciei), fourteen in number, comprise two superior maxillee, two palates, two malars, two lachrymals, and two nasals, together with the vomer and inferior maxilla. The Supeeior Maxillary Bones. The superior maxillae (maxillse), of which there are two, unite to form the upper jaw. Each consists of a body, with which are connected four projections, named respectively the zygomatic, frontal, alveolar, and palatal processes. The body (corpus) is of pyramidal form, and contains within it a hollow called the antrum or maxillary air-sinus. It has four surfaces — an antero-external or facial, a postero-external or zygomatic, a supero-external or orbital, and an internal or nasal. The antero-external or facial surface (facies anterior) is confluent below with the alveolar process. Above, it is separated from the orbital aspect by the infraorbital margin (margo infraorbitalis), whilst internally it is limited by the free margin of the nasal notch, which ends below in the pointed anterior nasal spine (spina nasalis anterior). Posteriorly it is separated from the zygomatic surface by the inferior border of the zygomatic process. The facial aspect of the bone is ridged by the sockets of the teeth (juga alveolaria). The ridge corresponding to the root of the canine tooth is usually the most pronounced ; internal to this, and overlying the roots of the incisor teeth, is the shallow incisive or myrtiform fossa, whilst placed externally, on a higher level, is the deeper canine fossa, the floor of which is formed in part by the projecting zygomatic process. Above this, and near the infraorbital margin, is the infraorbital foramen, the external opening of the infraorbital canal, which transmits the infraorbital nerve and artery. The postero- external, or zygomatic surface is separated above from the orbital asyicct l)y a rounded free edge, which f'ornis the anterior margin of the spheno-maxillary fissure in th<; articulated skull. Inferioi'ly and in fnnit it is scjparated from the facial surface by the zygomatic process and its free lower border. Internally it is limited 132 OSTEOLOGY, Nasal process LiohtM nl gioo\e liilVaorbital foramen Incisor fossa Tuberositj Right superior Maxilla (Outer View). by a sharp, irregular margin with which the palate bone articulates. This surface is more or less convex, and is directed towards the zygomatic and spheno-maxillary fossae. It is pierced in a downward direction by the apertures of the posterior dental canals (foramina alveolaria), two or more in number, which transmit the corre- sponding nerves and vessels to the molar teeth. Its lower part, slightly more proDii- nentwhere it overhangs the root of the wisdom molar, is often called the tuberosity (tuber maxillare). Thesi'/pero- external or orbital sur- face (planum orbitale), smooth and plane, is triangular in shape and forms part of the floor of the orbit. Its an- terior edge corresponds to the iniraorbital margin ; its posterior border coincides with the anterior boundary of the spheno maxillary fissure. Its thin inner edge, which may be regarded as the base of the triangle, is notched in front to form the lachrymal groove (sulcus lacri- malis), behind which it articulates with the lachrymal bone for a short distance, then for a greater length with the OS planum of the ^^"'"' i'™'-'*'-^' ethmoid, and terminates posteriorly in a surface for articulation with the orbital process of the palate bone. Its ex- ternal angle corresponds to the outspring of the zygomatic process. Tra- versing its substance is the infraorbital canal, the anterior opening of which has been already noticed on the facial aspect of the body. Behind, however, owing to deficiency of its roof, the canal forms a groove which lips the edge of the bone which constitutes the anterior boundary of the spheno- maxillary fissure. If this canal be laid open, the orifices of the middle and anterior dental canals will be seen, which transmit the corresponding vessels and nerves to the bicuspid and incisor teeth. The inner or nasal surface (facies nasalis) of the body is directed inwards towards the nasal fosscC. Below it is confluent with the upper Ridge for middle turbinated bone Middle meatus- Ridge for inferior turbinated bone Inferior meatus Incisor crest Anterior na spi Alveolar process Nasal crest Fk;. 104. — RicHT Superior Maxilla (Inner Aspect). THE SUPERIOK MAXILLAE Y BONES. 133 surface of the palatal process ; in front it is linjited by the sharp edge of the nasal notch ; above and in front it is continuous with the inner surface of tlie frontal process ; behind this it is deeply channelled by the lachrymal groove, which is converted into a canal by articulation with the lachrymal and inferior turbinated bones. The channel so formed conveys the nasal duct from the orbital cavity above to the inferior nasal meatus below. Behind this groove the upper edge of this area corresponds to the inner margin of the orbital surface, and articulates from before backwards with the lachrymal, os planum of the ethmoid, and the orbital process of the palate bone. The posterior border, rough for articulation with the palate bone, is traversed obliquely from above downwards and slightly inwards by a groove, which, by articulation with the palate bone, is converted into the posterior palatine or palato-maxillary canal which transmits the descending pala- tine artery and great palatine nerve. Towards its upper and hinder part the nasal surface of the body displays the irregular, more or less triangular, opening of the antrum (sinus maxillaris). This aperture, which, in the articulated skull opens into the middle meatus of the nose, is much reduced in size by articulation with the lachrymal, ethmoid, palate, and inferior turbinal bones. In front of the lachrymal groove the inner surface is ridged horizontally by the inferior turbinated crest (crista conchalis), to which the inferior turbinated bone is attached. Below this the bone forms the outer wall of the inferior nasal meatus, receiving the termination of the lachrymal groove. Above, and for some little distance also on the inner side of the frontal process, it constitutes the smooth outer wall of the atrium of the middle meatus. The zygomatic or malar process (processus zygomaticus), which is placed on the outer surface of the body, is confluent anteriorly with the facial surface of the body ; posteriorly, where it is concave from side to side, with the zygomatic surface ; whilst superiorly, where it is rough and articular, it forms the apex of the triangular orbital plate, and supports the malar bone. Inferiorly, its anterior and posterior surfaces meet to form an arched border, which fuses with the alveolar process opposite the root of the first molar tooth, and serves to separate the facial from the zygomatic aspects of the body. The frontal or nasal process (processus frontalis) rises from the upper and fore-part of the body. It has two surfaces — one external, the other internal. The external is divided into two by a vertical ridge (crista lachrymalis anterior), which is the upward extension of the infraorbital margin. The narrow strip of bone behind this ridge is hollowed out, and leads into the lachrymal groove below. Posteriorly the edge of the frontal process here articulates with the lachrymal, and so forms the fossa for the lodgment of the lachrymal sac (fossa sacci lacrimalis). In front of the vertical crest, to which the tendo oculi is attached, the external surface is confluent below with the facial aspect of the body, and forms the side of the root of the nose. Its anterior edge is rough, or grooved, for articulation with the nasal bone. Superiorly the summit of the process is serrated for articulation with the nasal notch of the frontal bone. The inner surface of the nasal process is directed towards the nasal fossse. It is crossed obliquely from below upwards and backwards by a ridge — the agger nasi or superior turbinated crest (crista ethmoidalis). Below this the bone is smooth and forms the upper part of the atrium, whilst the ridge itself articulates posteriorly with the fore-part of the middle turbinated bone, formed by the inferior turbinated process of the ethmoid bone. The alveolar process (processus alveolaris) projects from the under surface of the body of the bone below the level of the palatal process. (Jf curved form, it completes, with its fellow of the opposite side, the alveolar arch, in which are embedded in sockets or alveoli the roots of the teeth of the upper jaw ; ordinarily in the adult, when dentition is complete, each alveolar process supports eight teeth. Piercing the inner surface of the alveolar border behind the incisor teeth two small vascular foramina are usually visible. When any or all the teeth are shed the alveoli become absorbed, and the process may under these circumstances be reduced to tbe level of the plane of the ])alatal process. Posteriorly the alveolar jjroccHS cuds below the tuberosity of the body ; anteriorly it shares in the formation of the intermaxillary suture. 134 OSTEOLOGY. The palatal process (processus palatinus), of the form of a quadrant, lies in the horizontal plane; it has two surfaces — upper and under — and three borders, a straight internal, a more or less straight posterior, and a curved external,, by which latter it is attached to the inner side of the body and alveolar process as far back as the interval between the second and third molar teeth. Its under surface, together with that of its fellow, forms the anterior three-fourths of the vaulted hard palate; it is rough and pitted for the glands of the mucous membrane of the roof of the mouth, and is grooved on either side near the alveolar margin by a channel which passes forward from the posterior palatine canal and transmits the great palatine nerve and descending palatine artery. Its superior surface, smooth and concave from side to side, forms the floor of the corresponding nasal fossa. Its internal or mesial border, broad and serrated, rises in a ridge superiorly, so as to form with its fellow of the opposite side the nasal crest (crista nasalis), which is grooved superiorly to receive the lower border of the vomer. In front of its articulation with the vomer this ridge rises somewhat higher, being named the incisor crest, anterior to which it projects beyond the free border of the nasal notch, and together with its fellow forms the pointed projection called the anterior nasal spine (spina nasalis anterior). These parts support the septal cartilage of the nose. Immediately to the outer side of the incisor crest the superior surface of the palatal process is pierced by a foramen which leads downwards, forward, and a little inwards, to open into a broad groove on the mesial border of the bone immediately behind the central incisor tooth. When the two maxillse are articulated, the two grooves form the oval anterior palatine canal or fossa, into which the two aforementioned foramina open like the limbs of a Y ; these are called the incisor foramina, or the foramina of Stensen, and contain the remains of uhe organs of Jacobson. In front and behind these, and lying within the fossa and in the line of the suture, are the smaller foramina of Scarpa, which transmit the naso-palatine nerves, the right nerve usually passing through the posterior foramen, the left through the anterior. The posterior border of the palatal process, which is sharp and thin, falls in line with the interval between the second and third molar, and articulates with the horizontal plate of the palate bone. The maxillary sinus or antrum of Highmore (sinus maxillaris) lies within the body of the bone, and is of corresponding pyramidal form, its base being directed towards the nasal fossa, with the middle meatus of which it communicates, its summit extending outwards into the root of the zygomatic process. It is closed in externally and above by the thin walls which form the facial, zygomatic, and orbital surfaces of the body. Inferiorly it overlies the alveolar process in which the molar teeth are implanted, the sockets of which are separated from it by a thin layer of bone. Advantage is taken of this circumstance to pierce the floor of the antrum in such conditions as necessitate its tliorough drainage, as its natural outlet into the middle meatus is of the nature of an overflow aj^erture, and so preveiats purulent fluids, which may here accumulate, from being readily discharged. The angles and corners of this cavity are frequently groined by narrow ridges of bone, one superiorly corresponds to the relief formed by the infraorbital canal. A vascular and nervous groove is often exposed, curving along the floor of the antrum just above the alveoli of the teeth. The interior of the cavity is lined by an extension from the mucous membrane of the nose. Connexions. — The superior maxillary bone articulates with the nasal, frontal, lachrymal, and ethmoid bones above, externally with the malar, and occasionally with the siDlienoid, posteriorly and internallj^ with the 'pala.te, whilst on its inner side it unites with its fellow of the op23osite side, and also sujij^orts the inferior turl)inated bone and tlie A^omer. Architecture. — The disjiosition of the maxillary sinus within the body of the bone has been already referred to. In union with its fellow, the A'aulted arrangement of the hard palate is well displayed, and the arched arrangement of the suj^erior alveolar processes is obA'ious. It is in these latter processes around the sockets for the reception of the teeth that the cancellous tissue of the bone is seen ; elsewhere its walls are formed by thin and dense bone. Variations. — Not unfrequently there is a suture running vertically through the bar of bone which separates the infraorl:)ital foramen from the infraorbital margin. Through imperfections in ossification the infraoi'bital canal may form an open groove along the floor of the orbit. THE MALAR BONES. 135 Ossification. — The superior maxilla; are developed in the connective tissue around the oral aperture of the embryo. The centres from wliich the bone ossifies are not preceded by a cartilaginous stage. Their number is uncertain, as early fusion occurs between them. They first make their appearance in the second month of intrauterine life, shortly after the clavicle has begun to ossify. By the sixth montli they are so united that their independent character is obscured. Five centres are described — an external or malar, which forms the bone to the outer side of the infraorbital canal ; an inner or orbito-nasal, from which is developed the iimer part of the floor of the orbit, the frontal process, and the wall of the antrum ; a palatine, for the posterior three-fourths of the palatal process ; a nasal, situated between the frontal process and the canine tooth ; and within this and nearer the middle line and below, an incisive centre, from which the pre- masillse are developed, thus forming the anterior fourth of the palatal process in the adult. In the early stages of the development of the bone the alveolar groove, in which the teeth are developed, lies close below the infraorbital groove, and it is not till later that they become separated by the growth of the antrum, which first makes its appearance as a shallow fossa to the inner side of the orbito-nasal element about the fourth month. In the adult bone the course of the infraorbital canal and foramen indicates the line of fusion of the orbito-nasal and malar elements, whilst the position of the anterior palatine canal serves to determine the line of union of the incisive with the palatal elements. In addition to the foregoing centres, Rambaud and Renault describe another which, together with its fellow, is wedged in between the incisive and the palatal elements beneath the vomer, thus explaining the Y-shaped arrangement of the foramina of Stensen, which open into the anterior palatine canal. The premaxillae, which in most vertebrates are in- dependent bones lying in front of the superior maxillae, constitute in man and apes the portions of the upper jaw which lie in front of the anterior palatine foramen, and support the superior incisor teeth. They are de- veloped from the incisive centres above described ; the line of fusion of these elements with the maxillse proper can readily be seen in young skulls, and occasionally also in the adult. It corresponds to a suture which passes on the palate obliquely outwards and forwards, from the anterior palatine foramen to the interval between the lateral incisor and the canine tooth. In cases of alveolar cleft palate the adjacent bones fail to unite along the line of the suture. In some instances, however, the cleft passes outwards between the central and lateral incisor teeth, and this condition sviggests the explanation that the preniaxillary element is derived from two centres — a lateral and a mesial. The researches of Albrecht and Warinski have confirmed this view. The latter anatomist further observes that the lateral cleavage may lead to a division of the dental germ of the lateral incisor tooth, and so explain the occurrence of the supernumer- ary incisor which is occasionally met with. In this way the different varieties of cleft palate are x'eadily explained ; mesial cleft palate being due to failure of union between the two preniaxillary bones. Lateral cleft palate may be of two types : the cleft in one case passing forward between the central and lateral incisor, and being due to the non-union of the two elements from which the premaxilla is primarily developed ; the other, in which the cleft passes between the lateral incisor and the canine, or between the lateral incisor and a supernumerary incisor, owing to the imperfect fusion of the premaxilla laterally with the maxilla. Fig. 105. — Ossification of Superior Maxilla. A, Outer side ; B, luuer side ; C, Under side, a, Nasal process ; 6, Orbital plate ; c, Anterior nasal spine ; d, Infraorbital groove ; e. Infraorbital foramen ; /, Anterior l^alatine groove ; g, Palatal process ; h, Premaxillary suture ; i. Alveolar process. The Malar Bones. The malar bone fos zygomaticum) underlies the most prominent part of the cheek, and is hence often called the cheek-bone. Placed to the outer side of the orbital cavity, it forms the sharp external border of that hollow, and serves to 136 OSTEOLOGY. separate that space from the temporal and zygomatic foss» which lie behind; below, it rests upon and is united to the superior maxilla ; behind, it enters into the formation of the zygomatic arch which bridges across the temporal fossa. As viewed from the outer side, the bone is convex from side to side, and has four angles, of which three are prominent. These are the ascending or frontal (processus fronto-sphenoidalis), the anterior or pointed extremity of the maxillary border, and the posterior or temporal (processus temporalis). The most elevated part of the convex outer surfaa (facies anterior) forms the malar tuberosity. The processus temporalis, sometimes called the zygomatic process, ends posteriorly in an oblique edge, which articulates with the extremity of the zygomatic process of the temporal bone. The frontal, the most prominent of its processes, is united superiorly to the external angular process of the frontal bone. The edo-e between the frontal and temporal processes is thin and sharp ; it affords attachment to the temporal fascia, and near its upper end there is usually a pronounced angle (processus marginalis), formed by a sudden change in the direc- tion of the border of the bone. It is just below this point that the temporal branch of the orbital nerve becomes cutaneous. The lower margin of the temporal process is somewhat thicker and rounded ; it extends downwards and forwards towards the inferior angle, where the bone articulates with the superior maxilla. Frontal process PYontal process Temporal border Temporal canal Orbital surface Temporal process ■p' Masseteric border For articulation with superior maxilla Orbital process Temporal process Temporo- zygomatic surface Fig. 106.— Right Malar Bone. A, Outer Bide ; B, Inner Side. and is there confluent with the ridge which separates the facial from the zygomatic aspect of the upper jaw. This edge of the bone is sometimes called the masseteric border, since it affords attachment to the fibres of origin of the masseter muscle. Sweeping downwards in front of the frontal process is a curved edge which terminates inferiorly in a pointed process. This border forms the outer and, in part, the inferior margin of the orbital cavity. Between the anterior extremity of the masseteric edge and the pointed anterior angle there is an irregular suture by which the bone is joined to the maxilla. The opening of the malar canal (foramen zygomatico-faciale) is seen on the outer surface of the bone ; its size and position are very variable. The mesial as'pect of the bone is distinguished by a curved elevated crest, called the orbital process, which extends inwards and backwards, and is confluent externally with the orbital margin. This process has two surfaces — one anterior, which forms a part of the outer and lower wall of the orbit, and one posterior, which is directed towards the temporal fossa above and the zygomatic fossa below. The free edge of the orbital process is thin and serrated ; a little below its middle it is usually interrupted by a non-articular notch, which corresponds to the anterior extremity of the spheno-maxillary fissure. The part above this articulates with the great wing of the sphenoid, the portion below with the orbital plate of the superior maxilla. Behind the orbital process the inner surface of the bone is concave from side to side, and extends backwards along the mesial aspect of the temporal process and upwards over the posterior half of the inner side of the frontal process, thus entering into the formation of the zygomatic and temporal fossae respectively. THE NASAL BONE.S. 137 The orbital surface of the orljital process usually displays the openings of two canals (foramina zygomatic -orbitalia) — one which traverses the bone below the orbital margin and appears on the front of the bone as already described, the other which passes obliquely upwards and outwards through the orbital process and appears in the temporal fossa, to the inner side of the frontal process (foramen zygoma tico-temporale). The former transmits the ramus subcutaneous malai ; the latter the temporal branch of the orbital nerve. Below the orbital process there is a rough triangular area, bounded externally by the maxillary border. This articulates with the malar process of the superior maxilla, and occasionally forms the outer wall of the antrum. Connexions.- — The malar bone articulates with the frontal, sphenoid, superior maxilla, and temporal bones. Architecture. — In structure the bone is compact, witli little cancellous tissue. Together with the zygomatic process of the temporal bone it forms the buttress which supports the superior maxilla and the outer orbital wall externally. Additional strength is imparted to the bone by the angular mode of union of its orbital and facial parts. Variations.' — Cases of division of the malar bone by a horizontal suture have been recorded, as well as instances of its separation into two parts by a vertical suture. Owing to the supposed more frequent occurrence of this divided condition in Asiatics the malar has been named the os Japonicum. Barclay Smith (" Proc. Anat. Soc," Joihr. Anat. and Phijsiol, April 1898, p. 40) describes a case in v/hich the malar bone was divided into two parts, an upper and lower, by a backward extension of the maxilla, which articulated with the zygomatic process of the temporal, thus forming a temporo-maxillary arch. Varieties of a like kind have also been described by Gruber and others. Cases have been noted where, owing to deficiency in the development of the malar, the continuity of the zygomatic arch has been incomplete. Ossification. — The malar ossifies in membrane. Its basis appears about tlie tenth week as a thin ossifying lamina which corresponds to the orbital margin, attached to which there is a backward expansion corresponding to the body of the bone ; from this posteriorly there extends the element of the temporal process. On the inner side, and above the angle formed by the orbital and temporal margins, there appears a secondary thickening, which develops into a cap-shaped layer which fits into the recess and ultimately forms the surface of the bone directed to the temporal fossa. Below the orbital ciub-shap'ed process margin on the inner side, and extending backwards ^j^^ 107.— Inner Surface op towards the temporal process, is another secondary Malar Bone at Birth. thickening which forms a club-shaped nodule, the thick end of which is directed forwards, whilst posteriorly it forms, in part, the lower margin of the body and temporal process. The overlap of these several parts leads to the formation of grooves which may persist in the adult as sutures. (Karl Toldt, junr., Sitzshr. des Akad. des Wiss., Wien. July 1902.) The Nasal Bones. The nasal bones (ossa nasalia), two in number, lie in the interval between the frontal processes of the superior maxillae, there forming the root or bridge of the nose. Each bone is of elongated quadrangular form, having two surfaces — an inner and outer — and four borders. The external surface, somewhat constricted about its middle, is convex from side to side, and slightly concavo-convex from above downwards. Near its centre there is usually the opening of a nutrient canal. The internal surface is not so extensive as the external, as the superior and anterior articular borders encroach somewhat upon it above. Concave from side to side, and also from above downwards, it is covered, in the recent condition, by the mucous membrane of the nose. Eunning downwards along this surface is a narrow groove Tsulcus ethmoidalis) which transmits the internal nasal nerve. The anterior or internal })order, narrow below, is thick above, and, in conjuncjtion with its fellow at the opj>osite side, with which it articulates, forms a median crest posteriorly, which is united to the nasal spine of the frontal, the vertical plate of the ethmoid, and the septal cartilage of the nose, in that order frou) above down- wards. The posterior or external border, usually the longest, is serrated and 138 OSTEOLOGY. bevelled to fit on to the anterior edge of the frontal process of the superior maxilla. The superior border forms a wide toothed surface, which articulates with the inner part of the nasal notch of the frontal bone anteriorly ; whilst, behind, it rests in contact with the root of the nasal process of the same bone. The inferior border is thin and sharp, and is con- nected below with the lateral cartilage of the nose, and is usually deeply notched near its mesial extremity. Connexions. — The nasal bone articulates witli its fellow of the opposite side, with the frontal above, behind with the mesial jDlate of the ethmoid and with the frontal process of the suijerior maxilla. It is also A, Outer side ; united to the septal and upj^er lateral carti- lages of tlie nose. Architecture. — Formed of dense and compact bone ; the strength of the nasal bones is increased by their mode of union and the formation of a median crest jjosteriorly. Variations. — The size and configuration of the nasal bones vary greatly in different races, being, as a rule, large and prominent in the white races, and flat and reduced in size, as well as depressed, in the Mongolian and Negro stock. Obliteration of the internasal suture is unusual ; it is stated to occur more frequently in negroes, and is the recognised condition in adult apes. Duckworth has recorded a case {Journ. Anat. and Physiol., vol. xxxvi. j). 257) of undue extension downwards of the nasal bone, which may be perhaps accounted for on the supposition that the lower part is a persistent portion of the premaxilla. Ossification. — The nasal bones are each developed from a single centre, which makes its appearance about the end of the second month in the membrane covering the fore- part of the cartilaginous nasal capsule. Subsequent to birth the underlying cartilaginous stratum disappears, persisting, however, below in the form of the lateral nasal cartilage, and behind as the septal cartilage of the nose. Fig. 108.- -RiGHT Nasal Bone. B, Inner side. Orbital surface The Lachrymal Bones. The lachrymal bone (os lachrymale), or os unguis, a thin scale of bone about the size of a finger-nail, forms part of the inner orbital wall behind the frontal pro- cess of the superior maxilla. Irregularly quadrangular, it has two surfaces — an inner and outer — and four borders. Its external or orbital surface has a vertical ridge, the lachrymal crest (crista lachrymalis posterior), running downwards upon it. In front of this is the lachrymal groove (sulcus lachrymalis) for the lodgment of the lachry- mal sac. The floor of this groove descends below the level of the bulk of the bone, and forms the descending process, which helps to complete the osseous canal for the nasal duct, and articulates inferiorly with the inferior turbinal. The lower end of the lachrymal crest terminates in a hook- like projection, the hamular process (hamulus lachry- malis), which curves round the posterior and outer edge of the lachrymal notch of the superior maxilla, and thus defines the upper aperture of the canal for the nasal duct. To the free edge of the crest behind the lachrymal groove are attached the reflected portion of the tendo oculi, and the tensor tarsi muscle. The part of the bone behind the lachrymal crest is smooth and continuous with the surface of the os planum of the ethmoid. The inner surface is irregular and cellular above ; it closes in some of the anterior ethmoidal cells. Where it is smoother it forms a part of the lateral wall of the middle meatus of the nose immediately behind the frontal process of the superior maxilla, and above the inferior turbinated bone. The superior border articulates with the orbital plate of the frontal ; the anterior edge with the posterior border of the frontal process of the superior maxilla, with Fig. 109. — Right Lachrymal Bone (Orbital Surface). THE INFERIOE TURBINATED BONES. 139 which it completes the lachrymal groove for the lodgment of the lachrymal sac. The inferior margin articulates with the orbital surface of the superior maxilla, and in front by its descending process with the inferior turbinal. Posteriorly the bone articulates with the anterior border of the os planum of the ethmoid. Connexions. — The lachrymal bone articulates with four bones — the frontal, ethmoid, inferior turbinal, and the suj^erior maxilla. Architecture. — The bone consists of a thin pajaery translucent lamina, somewhat strength- ened by the addition of the vertical crest. Variations. — The lachrymal is occasionally absent. In some cases it is divided into two parts ; in others replaced by a number of smaller ossicles. In rare instances the hamular process may extend forwards to reach the orbital margin, and so bear a share in the formation of the face, as in lemurs (Gegenbauer). In other instances the hamulus is much reduced in size. Occasion- ally the lachrymal is separated from the os planum of the ethmoid by a down-growth from the frontal, which articulates with the orbital process of the superior maxilla, as is the normal disposition in the gorilla and chimpanzee. (Turner, Challenger Reports, " Zoology," voL x. Part IV. Plate I. ; and A. Thomson, Journ. Anat. and Physiol., London, vol. xxiv. p. 349.) Ossification. — The lachrynaal is developed from a single centre, which makes its appearance about the end of the second or the beginning of the third month of intra- uterine life in the membrane around the cartilaginous nasal capsule. The Infeeior Turbinated Bones. The inferior turbinated or spongy bone (concha inferior) is a shell-like lamina of bone lying along the lower part of the outer wall of the nasal fossa. Of elongated form, the bone displays two curved borders enclosing an internal and external surface. The superior or attached border is thin and sharp in front and behind, where Lachrymal process Ethmoidal process Lachrymal process Ethmoidal process A Maxillary process B Fig. 110. — PiiGHT INFEKIOE TURBINATED BoNE. A, luiier Surface ; B, Outer Surface. it articulates with the inferior turbinal crests on the inner surface of the body of the superior maxilla and the vertical plate of the palate bone respectively. Between these two borders the central part of the upper edge rises in the form of a sharp crest, the fore-part of which forms the upstanding lachrymal process (pro- cessus lachrymalis), which articulates above with the descending process of the lachrymal bone, as well as with the edges of the nasal groove of the superior maxilla, thus completing the osseous canal of the nasal duct. The posterior end of this crest is elevated in the form of an irregular projection called the ethmoidal process (processus ethmoidalis). This unites with the uncinate process of the ethmoid bone (see Fig. 101). Spreading downwards from the middle of the superior border, on its outer side, is a thin, irregular plate of bone, the maxillary process (processus maxillaris), which partially conceals the outer concave surface of the bone, and, by its union with the inner wall of the maxillary sinus, assists in the completion of the partition which separates that cavity from the inferior nasal meatus. The inferior or free border, gently curved from before backwards and slightly out-turned, is roundfid and full,-and formed of bone which is deeply pitted and of a somewhat cellular cliaracter. The anterior and posterior extremities of tlie bone, formed by the convergcmce of the upper and lower borders, are thin and sharp; as a rule the hinder end is the mon; pointed of the two. The internal surface projects into the nasal fossa ; convcix from above downwards, and slightly 140 OSTEOLOGY. curved from before backwards, it forms the floor of the middle meatus. It is rough and pitted, and displays some scattered and longitudinally directed vascular grooves. The outer surface overhangs the inferior nasal meatus. Concave from above downwards, and to some extent from before backwards, it is directed towards the outer wall of the nasal fossa. It is smooth in front, where it corresponds to the opening of the canal for the nasal duct ; behind and towards its lower border it is irregular and pitted. In the disarticulated bone this surface is in part con- cealed by the downward projecting maxillary process. Connexions. — The inferior turbinal articulates with the su2:'erior maxilla, lachrymal, ethmoid, and palate bones. Variation. — A case in which the inferior turbinals were absent has been recorded by Hyrtl. Ossification. — The infei-ior turbinate bone, the maxillo- turbinal of comparative anatomy, is derived from the cartilage forming the outer wall of the nasal capsule, the upper portion of which forms the ethmo-turbinals. It ossifies, however, from a separate centre, which appears about the fifth month of foetal life, and later contracts a union by a horizontal lamella on its outer side with the superior maxillary bone. Fm. 111. Palate Superior maxilla -Vomer as seen from the Right Side. The Vomer. The vomer, a bone of irregular quadrilateral shape, is placed mesially in the hinder part of the nasal septum. It has four borders and two surfaces. The superior border, which can readily be distinguished by the presence on either side of an everted lip or ala, slopes from behind upwards and forwards, and articulates with the under surface of the body of the sphenoid, the pointed rostrum of which is received into the groove formed by the projecting alee. Laterally these alae are wedged in between the sphenoidal pro- cesses of the palate bone in front, and the vaginal processes at the root of the internal pterygoid plates behind. The posterior harder, which slopes from behind downwards and forwards, is free, and forms a sharp, slightly curved edge ; this constitutes the posterior margin of the nasal septum, and serves to separate the openings of the posterior nares. The inferior border, more or less horizontal in direction, articulates with the nasal crest formed by the superior maxillary and palate bones. The anterior edge is the longest ; it slopes obliquely from above downwards and forwards. In its upper half it is ankylosed to the perpendicular plate of the ethmoid ; in its lower half this margin is grooved for the reception of the septal cartilage of the nose. The anterior extremity of the bone forms a truncated angle, which articulates with the hinder border of the incisor crest of the superior maxillse, and sends downwards a pointed process which passes between the incisor foramina. The right and left surfaces of the bone are smooth, and covered by mucous membrane. It is not uncommon to find them deflected to one or other side. A few vascular grooves may be noticed scattered over these surfaces, and one, usually more distinct than the others, running obliquely down- wards and forwards, indicates the course of the naso-palatine nerve. Connexions. — The vomer articulates with the sjjhenoid, the ethmoid, the palates, and the superior maxillpe. In front it supports the septal cartilage. Architecture. — The bone is composed of two comi^act layers fused below, but separated above by the groove for the lodgment of the rostrum of the sphenoid behind, and the sejDtal cartilage in front. The lamellae are also sej^arated from each other by a canal which runs horizontally from behind forwards in the substance of the bone, and which transmits the nutrient vessel of the bone. Variations. — Owing to imperfect ossification there may be a deficiency in the boue, filled up during life by cartilage. The se23aration of the two lamellae along the anterior border varies considerably, and instances are recorded where they were separated by a considerable cavity within the substance of the bone. The spheno-vomerine canal is a minute opening behind THE PALATE BONES. 141 the rostrum of the sphenoid, and between it and the alas of the vomer, by which the nutrient artery enters the bone. Ossification. — The vomer, primitively double, begins to ossify about the end of the second month of foetal life. A nucleus appears on either side in the membrane overlying the back and lower part of the vomerine cartilage ; these form the primitive lamellge developed on either side of, and not from, the cartilage. About the third month these laminae become fused behind and below, thus forming a deep groove in which the cartilage is lodged. As growth goes on the groove becomes reduced by the further fusion of the lateral plates and the absorption of the cartilage, until the age of puberty, by which time the lateral laminae have united to form a mesial plate, the primitively divided con- dition of which is now only represented by the eversion of the alse and the grooving along- the anterior border. The Palate Bones. The palate bone (os palatinum), of irregular shape, assists in the formation of the outer wall of the back part of the nasal fossae, the posterior portion of the hard palate, the orbit, the spheno-maxillary, zygomatic, and the pterygoid fossae. It Sphenoid Spheno- maxillary fossa Sphenoidal process ,>-^ Orbital process ^\ ^Ethmoid Orbital surface For superior maxilla Orbital process Orbital surface Ethmoid Sphenoid Ethmoidal crest — -* Spheno-palatine notch Middle meatus Inferior meatus Pterygoid fossa J ]\ Tuberosity Surface for attach.' Surface Posterior palatine canal ""'""plate" Posteuor Internal of pterygoideus for superior nasal spine pterygoid plate extemus maxilla A Fig. 112.— Right Palate Bone. A, As seen from the Outer Side ; B, As viewed from the Inuer Side. consists of a horizontal and a vertical plate, united to each other like the hmbs of the letter L. At their point of union there is an irregular outstanding process called the tuberosity, whilst capping the summit of the vertical plate and separated by a deep cleft are two irregular pieces^ of bone, called the sphenoidal and orbital processes. ^ j r -u a The horizontal plate (pars horizontalis) has two surfaces and four borders As its name implies, it is horizontal in position, and forms the posterior third ot the hard palate. Its upper surface, which is smooth, is slightly concave from side to side, and forms the floor of the hinder part of the nasal fossae. Its inferior surface, rougher, is directed towards the mouth, and near its posterior edge otten displays a transverse ridge for the attachment of a part of the aponeurosis ot the tensor palati muscle. The anterior border articulates by means ot an irregular suture with the hinder edge of the palatal process of the superior maxilla. Ihe posterior raan/m is i'rce and concave from side to side ; by its sharp edge it furnishes attachment to the aponeurosis of the soft palate. The internal border is upturned, and when it articulates with its fellow of the opposite side it iorms superiorly a central crest continuous in front with the nasal crest of the superior maxiiia ; it supports the hinder part of the lower border oi' the vomer, and j)ro.jectmg beyond the line of the i;ost(;rior border forms tlu; posterior nasal or palatine spine (spina iiasalis posterior;. Tlie external larder fuses with the vertical plate, forming with 142 OSTEOLOGY. Orbital process Sphenoid Sphenoidal process Pterygo-palatme groo\ e Orbital /'surface it a right angle. The liinder extremity of this edge is grooved bj the lower end of the posterior palatine canal. The vertical plate (pars perpeudicularis) is very much broader below than above. Composed of thin bone, it is liable to be broken in the process of disarticu- lation, particularly at its upper part, so that it is somewhat uncommon to meet with a perfect specimen. It may be described as possessing two surfaces and four borders. Its inner surface, which is directed towards tbe cavity of the nose, is crossed horizontally about its middle by the inferior turbinated crest (crista turbinalis) with which the hinder end of the superior border of the inferior turbinated bone articulates ; above and below this it enters into the formation of the outer wall of the middle and inferior meatuses of the nose respectively. Near the upper extremity of the vertical plate, and below the processes which spring from it, there is another ridge more or less parallel to that already described. This is the superior turbinated or ethmoidal crest (crista ethmoidalis), and with this the hinder extremity of the middle turljinated Ijone is united. The external surface, which forms the inner wall of the spheno- maxillary fossa, is channelled by a vertical groove (sulcus pterygo-palatinus), converted into a canal by articulation with the superior maxillary bone. This canal, called the posterior palatine canal transmits the large palatine nerve and descending palatine vessels. Anteriorly the external surface projects forwards to a variable extent, and helps to close in the antrum of the maxilla by its maxillary Middle meatus Mpi ""''^" process. The anterior border is a thin edge of irregular outline which articu- lates above with the ethmoid, with the posterior edge of the maxillary process of the inferior turbinated bone about its middle, and below with the superior maxilla. The posterior border, thin above, where it articulates with the fore-part of the internal pterygoid plate, expands below into a pyramidal process called the tuberosity. The inferior border of the vertical plate is confluent with the outer edge of the horizontal plate ; posteriorly, and immediately in front of the tuberosity, it is notched by the lower extremity of the posterior palatine canal. The superior border supports the orbital and sphenoidal processes ; the former — the anterior — is separated from the latter by a notch (incisura spheno-palatina), which is converted into the spheno -palatine foramen by the articulation of the palate bone with the under surface of the sphenoid. Through this communication between the spheno-maxillary and nasal fossae pass the spheno-palatine artery and the nasal branches of the spheno-palatine ganglion. The tuberosity (processus pyramidalis) is directed backwards and outwards from the angle formed by the vertical and horizontal plates, and presents on its posterior stirface a central smooth vertical groove, bounded on either side by rough articular furrows which unite above in a V-shaped manner with the upper thin posterior edge. These latter articulate with the fore-parts of the lower portions of the internal and external pterygoid plates, while the central groove fits into the wedge- like interval between the two pterygoid plates, thus entering into the formation of the pterygoid fossa. The outer surface of the tuberosity is rough above, where it is confluent with the outer surface of the vertical plate which articulates with the tuberosity of the superior maxilla ; beloiv, there is a small, smooth, triangular area which appears between the tuberosity of the superior maxilla and the outer surface of the external pterygoid plate, and so enters into the floor of the zygomatic fossa. Passing through the tuberosity in a vertical direction are the posterior and external accessory palatine canals (foramina palatina minora) for the transmission of the smaller palatine nerves and vessels. Inferior turbinated crest Inferior meatus Nasal crest Posterior nasal spine Horizontal plate For internal pterygoid plate Fig. 113. — Right Palate Bone. As seen from Behind. THE INFERIOE MAXILLAEY BONE. 143 The orbital process (processus orbitalis), shaped like a hollow cube, surmounts the fore-part of the vertical plate. The open mouth of the cube is usually directed backwards and inwards towards the fore-part of the body of the sphenoid, with the cavity of which it commonly communicates ; the fore-part of the cube articulates with the inner end of the angle formed by the orbital plate and zygomatic surface of the superior maxilla. Of the remaining four surfaces, one directed forwards and inwards articulates with the ethmoid. The others are non-articular : the superior enters into the formation of the floor of the orbit ; the external is directed towards the spheno-maxillary fossa ; whilst the inferior, which is confluent with the inner surface of the vertical plate, is of variable extent, and overhangs the superior meatus of the nose. The sphenoidal process (processus sphenoidalis), much smaller than the orbital, curves upwards, inwards, and backwards from the hinder part of the summit of the vertical plate. Its superior surface, which is grooved, articulates with the fore-part of the under surface of the body of the sphenoid and the root of the internal pterygoid plate, thereby converting the groove into the pterygo-palatine canal, which transmits an artery of the same name together with a pharyngeal branch from the spheno-palatine ganglion. Its outer side enters into the formation of part of the inner wall of the spheno-maxillary fossa. Its internal curved aspect is directed towards the nasal fossa, whilst its inner edge is in contact with the ala of the vomer. Connexions. — The palate bone articulates with its fellow of tlie opj^osite side, with the ethmoid, vomer, sphenoid, sujjerior maxilla, and inferior turbinated bones. Ossification. — The palate bones are developed from the ossification of the membrane covering the sides of the oral cavity. According to Rambaud and Renault, two primitive centres appear about the sixth week of foetal life. From one of these the tuberosity and the part of the vertical plate behind the posterior palatine groove is developed ; from the other the remainder of the bone is formed, with the exception of the orbital and sphenoidal processes which are derived from secondary centres that make their appearance some- what later. Other authorities describe the bone as ossifying from a single centre which appears about the end of the second month in the angle between the vertical and hori- zontal plates. At birth the bone is much longer in its antero-posterior diameter than in its vertical height, the converse of its typical adult form. The Infeeior Maxillary Bone. The inferior maxilla (rnandibula) or mandible, of horse-shoe shape, with the extremities upturned, is the only movable bone of the face. Stout and strong, it supports the teeth of the lower dental arch, and articulates with the base of the cranium, by the joints, on either side, between its condyles and the glenoid fossse of the temporal bones. The anterior or horizontal part, which contains the teeth, is called the body (corpus) ; the posterior or vertical portions constitute the rami (rami mandibulse). The body (corpus mandibulse) displays in the middle line in front a faint vertical ridge, the symphysis, which indicates the line of fusion of the two symmetrical halves from which the bone is primarily developed. Inferiorly this ridge divides so as to enclose, in well-marked specimens, a triangular area — the mental protuberance Tprotuberantia mentalis), the centre of which is somewhat depressed, thus emphasising the inferior angles, which are known as the mental tubercles ftubera mentalia). The outer surface is crossed by a faint, elevated ridge, the external oblique line (linea obliqua), which runs upwards and backwards from the mental tubercle to the fore-part of the anterior border of the ramus, with which it is confluent. A little ab(jve this, midway between the upper and lower borders of the jaw, and in line with the root of the second bicuspid tooth, the bone is pierced by the mental foramen (foramen mentale) ; this is the anterior opening of the inferior dental canal, wliifib traverses the body of the bone. Through this aperture the mental vessels and nerves reach the surface. The up'per harder 144 OSTEOLOGY, supports the sixteen teeth of the lower jaw. It is thick behind and thinner in front, in correspondence with the size of the roots of the teeth. Anteriorly the sockets of the incisor and canine teeth produce a series of vertical elevations (juga alveolaria), of which that corresponding to the canine tooth is the most prominent. When this is outstanding it gives rise to a hollowing of the surface between it and the symphysis, often referred to as the incisor fossa ; frequently, however, this is only faintly marked. Below the external ol)li(jue line the bone is full and rounded, and ends below in the inferior horde?' or base. This slopes outwards at the sides, and forwards in front, where it is thick and hollowed out on either side of the symphysis to form the digastric fossae (fossai digastricije), to which the anterior bellies of the digastric muscles are attached; narrowing somewhat behind this, the inferior border again expands opposite the molar teeth, and finally becoming reduced in width, terminates posteriorly at the angle formed between it and the posterior border of the ramus. The deep or inner surface of the body is crossed by the internal oblique line or mylo-hyoid ridge (linea mylo-hyoidea). This slants from above downwards and forwards towards the lower part of the symphysis. It serves for the origin of the mylo-hyoid muscle, and also furnishes an attachment to the superior con- strictor of the pharynx just behind the last molar tooth. Below the hinder part of this ridge the surface is hollowed to form a fossa for the lodgment of the submaxil- lary gland. Above the fore -part of the internal ob- lique line the bone is smooth and usually con- vex. Here the sublingual gland lies in relation to it. In the angle formed by the convergence of i the two internal oblique lines, and in correspondence with the back of the lower part of the symphysis, there is a raised tubercle surmounted by two laterally placed spines, the mental or genial spines (spinse meutales). Occasionally these are ao-ain subdivided into an upper and lower pair, or it may be that the lower pair may fuse to form a rough median ridge. To the upper pair of spines the genio-hyoglossi muscles are attached, whilst the lower pair serve for the origin of the genio-hyoid muscles. Immediately above the tubercle there is a median foramen for the transmission of a nutrient vessel, and close to the alveolar border opposite the intervals between the central and lateral incisors, there are two little vascular canals. The ramus (ramus mandibulse) passes upwards from the back part of the bone, forming by the junction of its posterior border with the base of the body tlie angle (angulus mandibulse), which is usually rounded and more or less everted. The outer surface of the ramus affords attachment to the masseter muscle, and when that muscle is powerfully developed the bone is usually marked by a series of oblique curved ridges, best seen towards the angle. About the middle of the deep . or inner surface is the large opening (foramen mandibulare) of the inferior dental Fig. 114. — The Lower .Jaw as seen from the Left Side. 1. Meutal tubercle. 2. Mental prominence. 3. Symphysis. 4. Coronoid i^rocess. 5. Condyles. 6. Neck. 7. Angle. 8. E.xternal oblique line. 9. Mental foramen. THE INFERIOE MAXILLAEY BONE. 145 canal, which runs downwards and forwards to reach the body, and transmits the inferior dental vessels and nerves. This aperture is overhung in front by a jjointed scale of bone, the lingula, to the edges of which the internal lateral ligament of the temporo-maxillary articulation is attached. Behind the lingula and leading down- wards and forwards for an inch or so from the opening of the inferior dental canal is the mylo-hyoid groove (sulcus mylo-hyoideus), along which the mylo-hyoid artery and nerve pass. Behind and below this groove the inner surface of the angle is rough for the attachment of the internal pterygoid muscle. Superiorly the ramus supports the coronoid process in front, and the condyle behind, the two being separated by the wide sigmoid notch (incisura mandibul&e), over which there X-»as8 in the recent condition the vessels and nerve to e the masseter muscle. The coronoid process, of variable length and beak-shaped, is limited behind by a thin curved margin, which forms the anterior boundary of the sigmoid notch. In front its anterior edge is convex from above downwards and forwards, and becomes confluent below with the anterior border of the ramus and the external oblique line. To the inner side of this edge there is a grooved elongated triangular surface, the inner margin of which, commencing above near the summit of the coronoid process, leads downwards along the inner 3 side of the root of the last molar tooth towards the internal oblique line. Behind this ridge the thick- ness of the ramus is much reduced. The temporal muscle is inserted into the margins and inner surface of the coronoid process. The pos- terior border of the ramus is con- tinued upwards to support the con- dyle (capitulum mandibulse), below which it is somewhat constricted to form the neck (collum mandibulse), which is compressed from before backwards, and bounds the sigmoid hollow posteriorly. To the inner side of the neck, im- mediately below the condyle, there is a little depression (fovea pterygoidea) for the insertion of the external pterygoid muscle. The convex surface of the condyle is transversely elongated, and so disposed that its long axis is inclined nearly horizontally from within outwards and a little forwards. The convexity of the condyle is more marked in its antero-posterior than in its transverse diameter. Fig. 115. — The Inneu Side of the Eight Half of the Lower Jaw. 1. Mental spines. 2. Surface in relation to the sublingual gland. 3. Alveolar border. 4. Lingula. 5. Coronoid process. 6. Condyles. 7. Inferior dental foramen. 8. Mylo-hyoid groove. 9. Angle. 10. Fossa for submaxillary gland. 11. Internal oblique line. 12. Digastric fossa. Architecture. — The mandiljle is rcanai'kable for the density and thickness of its inner and outer walls. Wliere these coalesce below at the base of the body, the bone is i^articularly stout. Superiorly, w'here they form the walls of the alveoli, they gradually thin, being thicker, however, on the inner than the outer side, except in the region of the last molar tooth, where the inner wall is the thinner. The cancellous substance is open-jneshed below, finei' and more condensed where it surrounds the alveoli. The inferior dental canal is large and lias no very definite wall ; it is prolonged beyond the mental foramen t(j reach the incisor teeth. From it nume,roiis channels j)ass upwards to the sockets of the teeth, and it com- municates freely with the surronrifling cancellous tissue. Above the canal the substance of the bone is broken up by the alveoli for the recejjtion of the roots of the teeth. In the substance 11 146 OSTEOLOGY. of the condyle the cancellous tissue is more compact, with a general striation vertical to the articular surface. The mental prominence is an essentially human characteristic; by some it is associated with the development of speech in man, others regard it as due to the reduction in the size of the teeth. Variations.^Considerable differences are met with in the height of the coronoid process: usually its sumiuit reaches the same level as the condyle, or slightly above it ; occasionally, how- ever, it rises to a much liigher level ; in other cases it is much reduced. These difi'erences naturally react on the form of the sigmoid notch. The projection of the mental protuberance is also liable to vary. Occasionally the mental foramen is douljle, and sometimes the mylo-hyoid groove is for a short distance converted into a canal. Ossification. — The development of the lower jaw is intimately associated with Meckel's cartilage, the cartilaginous bar of the first visceral or mandibular arch. Meckel's cartilages, of which there are two, are connected proximally with the periotic capsule and cranial base. Their distal ends are united in the region of the symphysis. It is in the connective tissue overlying the outer surface of this cartilaginous arch that the bulk of the lower jaw is developed. The cartilage itself is not converted into bone, but undergoes resorption, except its h £S!Wh anterior extremity, which is stated to undergo ossification to form the part of the jaw lying be- tween the mental foramen and the symphysis. Inathird or fourth month foetus the cartilage can be traced from the under surface of the fore-part of the tympanic ring down- Avards and forwai'ds to reach the jaw, to which it is attached at the opening of the inferior dental canal ; from this it may be traced forwards as a narrow strip applied to the inner surface of the mandible, which it sensibly grooves. The proximal end of this furrow remains permanently as the mylo-hyoid groove. The part of the cartilage between the tympanic ring and the jaw becomes converted into fibrous tissue, and persists in the adult as the so-called internal lateral ligament of the temporo-maxillary articulation, its proximal end through the Glaserian fissure being con- tinuous with the slender process of the malleus. J. Chain e (Cmnptes Rendus. Biologie, 1903) takes exception to this view and regards the internal lateral ligament as the remnant of a muscular slip. The part which is applied to the inner surface of the lower jaw disappears. In the tissue overlying the cartilage ossification begins by several centres as early as the sixth or seventh week of foetal life, in this respect resembling the clavicle, by which it is alone preceded. The dentary or basal centre forms the outer wall and lower border. With this is united the splenial portion, which appears somewhat later, forming the inner table from near the symphysis backwards towards the opening of the inferior dental canal where it terminates in the lingula. By the union of these two parts a groove is formed, which ultimately becomes covered in, and in which the inferior dental nerve and vessels are lodged. As has been already stated, the part of the body between the symphysis and the mental foramen is regarded as directly developed by the ossification of the fore-part of the Meckelian cartilage. As Avill have been gathered from the above description, the upper part of the I'amus and its processes have no connexion with Meckel's cartilage. The condyle and the coronoid process arc each developed from a separate centre, pi-eceded by a cartilaginous matrix. These several centres are all united about the fourth month. At birth the lower jaw consists of two lateral halves united at the symphysis by fibrous Fig. 116. -Lower Jaw at Birth. A, As seen from a1>ove B, Outer side : C, Inner side. a, Mental foramen ; &, Inferior dental canal ; c, Lingula ; d, Sockets for the dental sacs THE HYOID BONE. 147 tissue ; towards the end of the first, or during the second year, osseous union between the two halves is complete. In infancy the jaw is shallow and the rami proportionately small; fiirthei', owing to the obliquity of the ramus the angle is large, averaging about 150". The mental foramen lies near the lower border of the bone. Coincident with the eruption of the teeth and the use of the jaw in mastication, the rami rapidly increase in size, and the angle becomes more acute. After the completion of the permanent dentition it approaches more nearly a right angle varying from 110° to 120'. The body of the bone is stout and deep, and the mental foramen xisually lies midway between the upper and lower borders. As age advances, owing to the loss of the teeth and the consequent shrinkage and absorption of the alveolar border of the bone, the body becomes narrow and attenuated, and the mental foramen now lies close to the upper border. At the same time the angle opens out again (1.30° to 140°), in this respect resembling the infantile condition. In old age the coronoid process and the condyle form a more open angle with each other than in the adult. The Hyoid Bone. The hyoid bone (os hyoideum), or os linguae, though placed in the neck, is developmentally connected with the skull. It lies between the mandible above and the larynx below, and is connected with the root of the tongue. Of U-shaped form, as its name implies (Greek v and e'l8o<;, like), it consists in the adult of a central part, or body, with which on either side are united two long processes extending backwards — the great cornua. At the point where these are ossified with the body, the lesser cornua, which project upwards and backwards, are placed. The body (basis) is arched from side to side and compressed from before back- wards, so that its surfaces slope downwards and forwards. Its cmterior surface displays a slight median ridge, on either side of which the bone is marked by the attachment of the mylo-hyoid muscles. Its 'posterior surface, deeply hollowed, is concave from side to side and from above downwards. Herein lie a quantity of fat and a bursa which separates this aspect from the thyro-hyoid membrane. The upper border is broad ; it is separated from the anterior surface by a transverse ridge, behind which are the impressions for the attachment of the genio- hyoid muscles. Its hinder edge is thin and sharp; to this, above, are attached the genio-glossi, whilst behind and below the thyro-hyoid membrane is connected with it. The inferior border is well defined and narrow ; it serves for the attachment of the omo-hyoid, sterno-hyoid, thyro-hyoid, and stylo-hyoid muscles. The great cornua are connected on either side with the lateral parts of the body. At first, union is effected by synchondroses, which, however, ultimately ossify. These cornua curve backwards as well as upwards, and terminate ia more or less rounded and expanded extremities. Compre.-sed laterally, they serve for the attachments externally of the thyro-hyoid and hyo-glossi muscles, and the middle constrictor of the pharynx from below upwards, whilst internally they are con- nected with the lateral expansions of the thyro-hyoid membrane, the free edges of which are somewhat thickened, and connect the extremities of the great cornua with the ends of the superior cornua of the thyroid cartilage below. The lesser cornua, frequently cartilaginous in part, are about the size of grains of wheat. They rest upon the upper surface of the bone at the junctions of the great cornua with the body. In youth they are separated from, but in advanced life become ossified with, the rest of the bone, from which they are directed upwards, backwards, and a little outwards. Their summits are connected with the stylo- hyoid ligaments ; they also serve for the attachment of muscles. Connexions. Tlie liyoid jh Hhnig from the styloid proce.sse.s of the temporal bones by the fltylo-liyoi'l lif,'aiiic,iits. Inferiorly it is connected with tlie thyi'oid cartilage of the larynx by the ihyro-liyoid ligariients and niiMiil)iane. Posteriorly it is intimately associated with the epiglottis. Fig. 117. — The Hyoid Bone as seen FROM THE Front. 148 OSTEOLOGY. Ossification. — In considering the development of the hyoid bone it is necessary to refer to the arrangement and disposition of the cartilaginous laars of the second and third visceral arches. That of the second visceral arch, the hyoid bar — or Reichert's cartilage, as it is sometimes called — is united above to the petrous tempoi'al, whilst ventrally it is joined to its fellow of the opposite side by an independent mesial cartilage. Chondrifica- tion of the third visceral arch only occurs towards its ventral extremity, forming what is known as the thyro-hyoid bar. This also unites with the mesial cartilage above mentioned. In these cartilaginous processes ossific centres apj^ear in certain definite situations. Towards the end of fcctal life a single centre (by some authorities regarded as primarily double) appears in the mesial cartilage, and forms the body of the bone (basihyal). About the same time ossification begins in the lower ends of the thyro-hyoid bars, and from these the great cornua are developed (thyro-hyals). During the first year the lower ends of the hyoid bars begin to ossify and form the lesser cornua (cerato-hyals). The cephalic ends of the same cartilages meanwhile ossify to form the styloid processes (stylo- hyals) (see p. 121), whilst the intervening portions of cartilage undergo resorption and become converted into the fibrous tissue of the stylo-hyoid ligaments, which in the adult connect the lesser cornua with the styloid processes of the temporal bone. The great cornua fuse with the body in middle life ; the lesser cornua only at a more advanced period. Variations in the course of development lead to interesting anomalies of the hyoid apparatus. The lesser cornua may be unduly long or the stylo-hyoid ligament may be bony ; in this case the cartilage has not undergone resorption, but has passed on to the further stage of ossification, thus forming an epihyal element comparable to that in the dog. The ossified stylo-hyoid ligament, as felt thi'ough the pharyngeal wall, may be mistaken for a foreign body. (Farmer, G. W. S., Brit. Med. Journ. 1900, vol. i. p. 1405.) THE SKULL AS A WHOLE. The skull as a whole may be studied as seen from the front (norma frontahs), from the side (norma lateralis), from the back (norma occipitalis), from above (norma verticalis), and from below (norma basalis). NOKMA FkONTALIS. In front, the smooth convexity of the frontal bone limits this region above, whilst inferiorly, when the lower jaw is disarticulated, the teeth of the upper jaw form its lower boundary. The large openings of the orbits are seen on either side ; whilst placed mesially and at a somewhat lower level is the anterior nasal aperture (apertura pyriformis) leading into the nasal fossae. The frontal region, convex from above downwards and from side to side, is limited externally by two ridges, which are the anterior extremities of the temporal lines. Superiorly the fulness of the bone blends with the convexity of the vertex. Inferiorly the frontal bone forms on either side the arched superior border of the orbit (margo supraorbitalis). The space between these borders corresponds to the root of the nose, an 1 here are seen the sutures which unite the frontal with the nasal bones in the middle line, and with the nasal process of the superior maxilla on either side, called the naso-frontal and fronto-maxillary sutures respectively. The orbital arch is thin and sharp externally, but becomes thick and more rounded towards its inner side, where it forms the internal angular process and unites with the frontal process of the superior maxilla and the lachrymal bone on the inner wall of the orbit. This arched border is interrupted towards the inner side by a notch (incisura supraorl:)italis), sometimes converted into a foramen, for the transmission of the supraorbital nerve and artery. In the middle line, just above the naso-frontal suture, there is often the remains of a median suture (sutura frontalis), which marks the fusion of the two halves from which the bone is primarily ossified. Here also a prominence, of variable extent — the glabella — is met with ; from this there passes out on either side above and over the orbital margin a projection called the superciliary ridge (arcus superciliaris). The orbital fossae, of more or less conical form, display a tendency to assume the shape of four-sided pyramids by the flattening of the superior, inferior, and lateral walls. The base, which is directed forwards and a little outwards, NOEMA FEONTALIS OF THE SKULL. 149 corresponds to the orbital aperture. Tlie shape of this is liable to individual and racial variations, being nearly circular in the Mongoloid type, whilst it displays a more or less quadrangular Ibrm in Australoid skulls. The upper margin, as has been already stated, is formed by the frontal bone between the internal and external angular processes. The outer, and about half the lower, margin are formed by the sharp curved edge between the facial and orbital surfaces of the malar bone. The internal border and the remainder of the lower margin are determined by the outer surface of the frontal process of the superior maxilla, and the sharp edge separating the facial from the orbital surface of the same bone. Three sutures interrupt the continuity of the orbital margin — the fronto-malar (sutura zygomatico- frontalis) externally, the fronto-maxillary (sutura fronto-maxillaris) internally, both lying about the same level, and the malo-maxillary (sutura zygomatico-maxillaris) inferiorly. The apex of the space is directed backwards and inwards, so that the inner walls of the two orbits lie nearly parallel to each other, whilst the outer walls are so disposed as to form nearly a right angle with each other. The depth of the orbit measures, on an average, about two inches (5 cm.). At the apex there are two openings ; the larger, known as the sphenoidal fissure (fissura orbitalis superior), passes from the apex of the space outwards and a little upwards for the distance of three-quarters of an inch or so, between the roof and outer wall of the orbit. The inner third of this fissure is broad and of circular form. Externally it is consider- ably reduced in width. Through this the third, fourth, ophthalmic division of the fifth, and the sixth nerves enter the orbit, whilst the ophthalmic veins pass backwards through it. Above and internal to the inner end of the sphenoidal fissure there is a smaller circular opening, the optic foramen (foramen opticum), for the transmission of the optic nerve and ophthalmic artery. The roof of the orhit, which is very thin and brittle towards its centre, is formed in front by the orbital plate of the frontal bone (pars orbitalis) and behind by a small triangular piece of the lesser wing of the sphenoid, which surrounds the optic foramen and forms the upper border of the sphenoidal fissure. Externally this surface is separated from the outer wall by the sphenoidal fissure posteriorly, anteriorly by an irregular suture between the orbital part of the frontal and the upper margin of the orbital surface of the great wing of the sphenoid, ex- ternal to which the external angular process of the frontal articulates with the malar. Internally the roof is marked off from the inner wall by a suture, more or less horizontal in direction, between the orbital plate of the frontal and the following bones in order from before backwards, viz. the frontal process of the superior maxilla, the lachrymal bone, and the os planum of ethmoid. In the suture between the last-mentioned bone and the frontal there are two foramina, the anterior and posterior internal orbital or ethmoidal canals (foramen ethmoidale anterius et posterius); both transmit ethmoidal vessels — the anterior affording passage to the nasal nerve as well. The roof is concave from side to side, and to some extent also from before backwards. About midway between the fronto - maxillary suture and the supraorbital notch or foramen, but within the margin of the orbit, there is a small depression, occasionally replaced by a spine (fovea vel. spina trochlearis), for the attachment of the cartilaginous pulley of the superior oblique muscle of the eyeball. Under cover of the external angular process the roof is more deeply excavated, forming a shallow fossa for the lodgment of the lachrymal gland (fossa glandulte lachrymalis). In front, the roof separates the orbit from the frontal sinus, and along its inner border it is in relation with the ethmoidal air-cells. The relation to these air spaces is variable, depending on the development and size of the sinuses. The rest of the roof, which is very thin, forms by its upper surface the floor of the anterior cranial fossa, in which are lodged the frontal lobes of the cerebrum. The floor of the orlit is Ibrmed by the orbital plate of the superior maxilla, together with part of the orbital surface of tluj malar bone, and a small triiingular ])iece of bone, the orbital process of the palate, which is wedged in posteriorly. Externally, for three-quarters of its length posteriorly, it is s(iymrated I'rom the outer wall, wliich is here ffjrmed by the great wing of the sphenoid, by a cleft called the spheno- maxillary fissure (fissura orbitalis inferior). Through this there pass the 150 OSTEOLOGY. superior maxillary division of the fifth nerve on its way to the infraorbital canal, the orbital or temporo-malar branch of the same nerve, the infraorbital vessels, and some 1. Mental protuberance. 2. Body of lower jaw. 3. Eamus of lower jaw. 4. Anterior nasal spine 5. Canine fossa. 6. Infraorbital canal. 7. Malar canal. 8. Orbital surface of superior maxilla. 9. Temporal fossa. 10. Os planum of ethmoid. 11. Sphenoidal fissure. 12. Lachrymal bone and groove. Fig. 118. — Norma Frontalis. 13. Optic foramen. 14. Orbital foramina. 15. Temporal ridge. 16. Supraorbital notch. 17. Glabella. 18. Frontal eminence. 19. Superciliary ridge. 20. Parietal bone. 21. Fronto-nasal suture. 22. Pterion. 23. Great wing of sphenoid. 24. Orbital surface of great wino of sphenoid. 25. Squamoiis temporal. 26. Left nasal bone. 27. Malar bone. 28. Siiheno-maxillary fissure. 29. Zygomatic arch. Anterior nasal aperture, displaying nasal septum and inferior and middle turbinated bones. Mastoid process. Incisor fossa. Angle of jaw. Mental foramen. Symphysis. 30 31. 32. 33. 34. 35. twigs from Meckel's (spheno-palatine) ganglion. By means of this fissure the orbit ■communicates with the spheno-maxillary fossa behind and the zygomatic fossa to the outer side. Internally the floor is limited from behind forwards by the suture NOKMA FRONTALIS OF THE SKULL. 151 between the i'ollowing bones, viz. the orbital process of the palate below with the body of the sphenoid above and behind, and the os planum of the ethmoid above and in front — anterior to which the orbital plate of the superior maxilla below articulat(i8 with the OS planum of the ethmoid and the lachrymal above and in front. At the anterior extremity of this line of sutures the inner edge of the orbital plate of the superior maxilla is notched and free between the point where it articulates with the lachrymal posteriorly and the part from which its frontal process rises. Here it forms the outer edge of a canal, down which the membranous nasal duct passes to the nose. The floor of the orbit is thin behind and at the sides, but thicker in front where it blends with the orbital margin. Passing in a sagittal direction through its substance is the infraorbital canal, the roof of which is usually deficient behind, where it becomes continuous with a broad, shallow groove, which leads forwards from the anterior margin of the spheno-maxillary fissure. This canal (canalis infraorbitalis) opens on the facial surface of the superior maxillary imme- diately below the orbital margin (foramen infraorbitale) and transmits the superior maxillary division of the fifth nerve, together with the infraorbital vessels. The floor forms a thin partition which separates the orbit from the antrum or sinus of the superior maxilla, which lies below. Internally it completes the lower ethmoidal air-cells, and separates the orbit from the middle meatus of the nasal fossae. The outer loall of the orbit, which is the strongest, is formed by the orbital surface of the great wing of the sphenoid and the upper part of the orbital surface of the malar bone. Above it, behind, is the sphenoidal fissure, whilst below, and extending much farther forward, is the spheno-maxillary fissure. The anterior margin of the outer wall is stout and formed by the malar bone, behind which, formed in part by the orbital process of the malar bone and the malar edge (margo- zygomaticus) of the great wing of the sphenoid, it forms a fairly thick partition between the orbit in front and the temporal fossa behind. Crossing this surface from above downwards, close to the anterior extremity of the spheno-maxillary fissure, is the suture between the malar bone and the great wing of the sphenoid (sutura spheno - zygomatica). This wall is pierced in front by one or two small canals (foramen zygomatico-orbitale), which traverse the malar bone and allow of the transmission of the temporal and malar branches of the orbital portion of the superior maxillary division of the fifth nerve. The inner wall of the orbit is formed from before backwards by a small part of the frontal process of the superior maxilla, by the lachrymal, and by the os planum or orbital plate of the ethmoid (lamina papyracea ossis ethmoidalis), posterior to which is a small part of the lateral aspect of the body of the sphenoid in front of the optic foramen. Above, the orbital plate of the frontal bone forms a continuous suture from before backwards with the bones just enumerated ; whilst below, the lachrymal and the os planum of the ethmoid articulate with the orbital plate of the superior maxilla ; posteriorly the hinder extremity of the os planum and the fore-part of the body of the sphenoid articulate with the orbital process of the palate. The orbital surface of the lachrymal bone is divided into two by a vertical ridge — the lachrymal crest (crista lachrymalis posterior) — which forms in front the posterior half of a hollow, the lachrymal groove (sulcus lachrymalis), the anterior part of which is completed by the channelled posterior border of the frontal process of the superior maxilla. In the lachrymal groove or fossa (fossa sacci lachrymalis) is lodged the lachrymal sac, whilst passing from it and occupying the canal, of which the upper opening is at present seen, is the membranous nasal duct. The extremely thin wall of the lower part of the lachrymal fossa separates the orbit from the fore-part of the middle meatus of the nasal fossa. To the inner side of the upper and fore j>art of the lachrymal bone, and separated from the orbit merely by the thickness of that bone, is the passage leading from the nose to the frontal sinus (infundibulum ethmoidale), whilst the part of the bone behind the lachrymal crest forms the thin partition between the orbit and the anterior ethmoidal cells. Behind, wliore tlie body of the 8])henoid forms part of the inner wall of the orbit, the sphenoidal air sinus is in relation to the apex of that space, though here the jjartitioti wall l)(jtvvc(!ii the two cavities is much thicker. The skeleton of the face on its anterior surface is formed by the two superior 152 OSTEOLOGY. maxillffi, the frontal processes of which have been ah'eady seen to pass up to articu- late with the internal angular processes of the frontal bone, thus forming the lower halves of the inner margins of the orbit. Joined to the upper jaws externally are the malar or cheek bones (ossa zygomatica), which are supported by their union with the temporal bones posteriorly through the medium of the zygomatic arches. The suture which separates the malar from the superior maxilla (sutura zygomatico- maxillaris) commences above about the centre of the lower orbital margin and passes obliquely downward and outward, its lower end lying in vertical line with the outer orbital margin. The two superior maxillee are separated by the nasal fossse, which here open anteriorly. Above, the two nasal bones are wedged in between the frontal processes of the maxillaB ; whilst below the nasal aperture, the maxillfe themselves are united in the middle line by the intermaxillary suture (sutura intermaxillaris). The nasal aperture (apertura pyriformis), which lies below and in part between the orbits, is of variable shape and size — usually pyriform, it tends to be long and narrow in Europeans, as contrasted with the shorter and wider form met with in the negroid races. Its edges are formed below and on either side by the free curved margin of the body and the frontal process of the superior maxilla ; and above, and partly at the sides, by the free border of the nasal bones. In the middle line, inferiorly, corresponding to the upper end of the intermaxillary suture there is an outstanding process — the anterior nasal spine (spina nasalis anterior) formed by the coalescence of spicules from both maxillae ; arising from this, and passing backwards and upwards, is a thin bony partition — the osseous septum of the nose. Often deflected to one or other side, it divides the cavity of the nose (cavum nasi) into a right and left half. Projecting into these chambers from their outer walls can be seen the inner surfaces and free borders of the middle (concha media) and inferior (concha inferior) turbinated bones, the spaces below and between which form the inferior and middle meatuses of the nose respectively. Below the orbit, and to the outer side of the nasal aperture, the anterior or facial surface of the body of the superior maxilla (corpus maxillte) is seen ; this is continuous inferiorly with the outer surface of the alveolar process (process alveo- laris), in which are embedded the roots of the upper teeth. A horizontal line drawn round the jaw on the level of a point midway between the lower border of the nasal aperture and the alveolar edge corresponds to the plane of the bard palate. Below that the alveolar process separates the cavity of the mouth from the front of the face ; whilst above, the large air space, the maxil- lary sinus (sinus maxillaris), or antrum of Highmore, lies within the body of the superior maxilla. The malar or cheek bone (os zygomaticum) forms the lower half of the outer, and outer half of the lower border of the orbit. Its external aspect corresponds to the point of greatest width of the face, the modelling of which depends on the flatness or projection of this bone. When the lower jaw (mandibula) is in position, and the teeth in both jaws are complete, the lower dental arch will be seen to be smaller in all its diameters than the upper, so that when the jaws are closed the upper teeth slightly overlap the lower both in front and at the sides. Exceptionally a departure from this arrange- ment is met with. Norma Lateralis. Viewing this aspect of the skull, in the first instance, without the lower jaw^, it is seen to be formed in part by the bones of the cranium, and in part by the bones of the face. A line drawn from the fronto-nasal suture to the tip of the mastoid process serves to define roughly the boundary between these portions of the skull. Of ovoid shape, the cranium is formed above by the frontal, parietal, and occipital bones from before backwards; whilst below, included within these are. the sphenoid and temporal bones. The sutures between these several bones are arranged as follows : Commencing at the external angular process of the frontal, the suture between that bone and the malar is first seen ; tracing this backwards and a little NOEMA LATEEALIS OF THE SKULL. 15J upwards, the lower edge of the frontal next articulates with the upper margin of the great wing of the sphenoid for a distance varying from three-quarters of an inch to one inch. Here the posterior border of the frontal turns upwards and slightly back- wards, forming with the parietal the coronal suture (sutura coronalis). The lower border of the parietal bone, which is placed immediately behind the frontal, articulates Fig. 119. — Norma Lateralis. 12. Lambcloid suture. 13. Occipital 1:10116. 14. Lambda. 15. Obelion placed between the two parietal foramina. 16. Parietal bone. 17. Lower temporal ridge. 18. Upper temporal ridge. 19. Squamous j^art of temporal bone. 20. Bregma. 21. Coronal suture. 22. Stejiliaiiiou. 23. Frontal bone. 24. Pterion. 25. Temporal fossa. 26. GreM; wing of sphenoid. 27. Malar bone. 28. Malar canal. 29. Lachrymal bone. 30. Nasal bone. 31. InfraorVjital canal. 32. Anterior nasal aperture. 1. Mental foramen. 2. Body of low£r jaw. 3. Superior maxilla. 4. Eamus of lower jaw. 5. Zygomatic arch.- 6. Styloid process. 7. External auditory meatus. 8. Mastoid process. 9. Asterion. 10. Superior curved line of occipital bone. 11. External occipital protuberance. anteriorly with the hinder part of the upper edge of the great wing of the sphenoid. The extent of this suture (sutura spheno-parietalis) is liable to very great indi- vidual variation — at times being broad, in other instances being pointed and narrow, whilst occasionally the parietal does not articulate with the sphenoid at all. JJehind the spheno-pari(;tal suture Uw, parietal articulates witli the squamous part of the temporal THutura Hquamf)sa). This re])oats to a certain extent the curve formed l^y tfie outline oi' the calvaria, and ends posteriorly about one inch beliind the external auditory meatus. Here the suture alters its character aiui direction, and in jdace of being scaly, becomes toothed and irregular, uniting for the space of 154 OSTEOLOGY. an inch or so the posterior inferior angle of the parietal with the mastoid process of the temporal bone. This suture (sutura parieto-mastoid) is more or less hori- zontal in direction, and lies in line and on a level with the upper border of the zygomatic arch. At a point about two inches behind the external auditory meatus the posterior border of the parietal bone turns obliquely upwards and backwards, and forms with the tabular part of the occipital bone the strongly-denticulated lambdoid suture (sutura lambdoidea). Inferiorly this suture is continued obliquely downwards between the occipital bone and the hinder border of the mastoid portion of the temporal, where it forms the occipito- mastoid suture (sutura occipito- mastoidea), much simpler and less serrated than the two previously mentioned. These three sutures just described meet in tri-radiate fashion at a point called the asterion. Anteriorly the curve of the squamous suture is continued downward between the anterior edge of the squamous part of the temporal and the posterior border of the great wing of the sphenoid ; inferiorly it lies in plane with the middle of the zygomatic arch. The sutures around the summit of the great wing of the sphenoid are arranged like the letter H placed obliquely, the cross-piece of the H corresponding to the spheno-parietal suture. When this is short, and becomes a mere point of contact, the arrangement then resembles the letter X- This region is named the pterion. Curving over the lateral region of the calvaria in a longitudinal direction is the temporal crest (linea temporalis). This is often double. The lower line marks the limit of the attachment of the temporal muscle, whilst the upper ridge defines the attachment of the temporal fascia. Commencing in front at the external angular process of the frontal, the crest sweeps upwards and backwards across the lower part of that bone, and then crossing the coronal suture — a point called the stephanion — it passes on to the parietal, over which it curves in the direction of its posterior inferior angle. Here it is continued on to the temporal bone, where it sweeps forward to form the supramastoid crest, which serves to separate the squamous from the mastoid portion of the temporal bone externally. Carried forward, this ridge is seen to become continuous with the upper border of the zygomatic arch over the external auditory meatus. In front, the temporal ridge separates the temporal fossa from the region of the forehead ; above and behind, it bounds the temporal fossa which lies within its concavity, and serves to separate that hollow from the surface of the calvaria which is overlain by the scalp. Above the level of the temporal Knes the surfaces of the frontal and parietal bones are smooth, the latter exhibiting an elevation of varying prominence and position, but usually situated about the centre of the bone, called the parietal eminence (tuber parietale). A slight hollowing of the surface of the parietal behind and parallel to the coronal suture is not uncommon, and is referred to as the post -coronal depression. As seen in profile, the part of the calvaria behind and below the lambdoid suture is formed by the tabular part of the occipital bone. In line with the zygomatic arch this outline is interrupted by the external occipital protuberance or inion (protuber- antia occipitalis externa). The projection of this point is variable ; but its position can usually be easily determined in the living. Passing forwards from it, and blending anteriorly with the posterior border of the mastoid process of the temporal bone, is a rough crest, the superior curved line (linea nuchse. superior), a little above which there is often a much fainter line, the highest curved line (linea nuchse suprema) ; this affords attachment to the epicranial aponeurosis. These two lines serve to separate the part of the cranium above, which is covered by scalp, from thab below, which serves for the attachment of the fleshy muscles of the back of the neck, the latter surface (planum nuchale) being rough and irregular as contrasted with the smooth superior part (planum occipitale). The fulness of these two parts of the occipital bone varies much. There is frequently a pronounced bulging of the planum occipitale, and the position of the lambda can often be easily determined in the living ; similarly the planum nuchale may be either com- paratively flat or else full and rounded. These differences are of course associated with corresponding differences in the development of the cerebral and cerebellar lobes which are lodged in relation to the internal aspect of these parts of the bone. NOEMA LATERALIS OF THE SKULL. 155 The further description of the planum nuchale is best deferred till the base of the skull (norma basalis) is studied. Temporal Fossa. — Within the limits of the temporal lines the side of the cranium slopes forwards, inwards, and downwards, thus leaving a considerable interval between its lower part and the zygomatic arch. This space or hollow is called the temporal fossa (fossa temporalis) ; bounded above and behind by the temporal lines, its inferior limit is defined by the level of the zygomatic arch. Deepest opposite the angle formed by the frontal and temporal processes of the malar bone, the fossa becomes shallow towards its circumference. Its floor, which is slightly concavo-convex from before backwards about mid-level, is formed above by the temporal surface (facies temporalis) of the frontal, behind by the anterior inferior angle (angulis sphenoidalis) of the parietal, as well as the lower portion of that bone, below the temporal crest ; below and in front by the temporal surface of the great wing of the sphenoid, and behind and below by the squamous portion of the temporal bone. Inferiorly the floor is limited in front by the free inferior border of the great wing of the sphenoid, which forms the upper boundary of the spheno-maxillary fissure ; behind that, by a rough ridge, the infratemporal crest or pterygoid ridge (crista infratemporalis), which crosses the external surface of the great wing of the sphenoid, to become con- tinuous posteriorly with a ridge on the lower surface of the squamous temporal, from which the anterior root of the zygomatic process springs. In front the temporal fossa is separated from the orbit by the external angular process of the frontal above, and by the orbital process of the malar and its junction with the external border of the great wing of the sphenoid between the orbital and temporal surfaces of that process. Externally and in front, the fossa is overhung by the backward projection of the frontal process of the malar bone, and it is under cover of this, and within the angle formed by the frontal and orbital processes of the malar, that we see the opening of the temporal canal, which pierces the orbital plate of the malar and transmits the temporal branch of the orbital nerve — a filament of the superior maxillary division of the V nerve. The fore-part of the spheno -maxillary fissure (fissura orbita inferior) opens into the lower part of the temporal fossa, and thus establishes a communication between it and the orbit. If the floor of the fossa be carefully examined, some more or less distinct vascular grooves may be seen. One passing upwards over the posterior part of the squamous temporal, immediately in front of and above the external auditory meatus, is for the middle temporal artery ; two others, usually less distinct, pass up, one over the temporal surface of the great wing of the sphenoid, the other over the fore-part of the squamous temporal; these are for the anterior and posterior deep temporal branches of the internal maxillary artery. The fossa contains the temporal muscle with its vessels and nerves, together with the temporal branch of the orbital nerve .and some fat ; all of which are enclosed by the fascia which stretches over the space from the upper temporal line above to the superior border of the zygomatic arch below. The extent of the fossa depends on the size of the temporal muscle, the development of which is correlated with the size and weight of the lower jaw. Springing from the front and lower part of the squamous temporal is the zygomatic process of that bone ; it has two roots, an anterior and a posterior, between and below which are placed the glenoid fossa (fossa mandibularis) in front, and the •opening of the external auditory meatus behind. Of compressed triangular form, the process at first has its surfaces directed upwards and downwards, but curving out- wards and forwards, it twists on itself, so that its narrowed surfaces are now turned outwards and inwards, and its edges upwards and downwards ; passing forwards, it •expands somewhat, and ends in an oblique serrated surface, which unites with the temporal process of the malar bone and comj^ietes the zygomatic arch. It is the upper edge of this bridge of bone which forms the posterior root. The lower border, turning inwards, forms the anterior root, and serves to separate the temporal from the zygomatic surface of the squamous temporal, blending in front with tlie infra- temporal crest on the outer surface of the great wing of the sphenoid. The under ■surface of this root is convex from })efore backwards, and is thrown into relief by the glenoid hollow, which ])asse.s u]) }>ehind it. In this way a downward projection, whicii is called the eminentia articularis, is formed. 156 OSTEOLOGY. The alar spine of the sphenoid (spina angularis) lies immediately to the inner side of the articular part of the glenoid fossa. Its size and projection vary. It is well to remember its relation to the condyle of the lower jaw when that bone is in position ; lying, as it does, to the inner side and a little in front of that pro- cess, it affords attachment to the so-called long internal lateral ligament (spheno- mandibular) of the temporo-maxillary articulation. As will be seen hereafter, the anterior extremity of the osseous Eustachian canal lies immediately to its inner side (p. 164). A noteworthy feature about the articular part of the glenoid fossa is the thinness of the bony plate which serves to separate it from the middle cranial fossa above. The vaginal process is a crest of bone which runs obHquely forwards from the front and inner side of the mastoid process, just below the external auditory meatus, to the alar spine of the sphenoid. Passing downwards and slightly forwards from the centre of this, and ensheathed by it in front and at the sides, is the pointed styloid process, the length of which is extremely variable. In the recess between the posterior root of the zygoma and the upper curved edge of the meatus there is usually a depression, though in some instances this may be replaced by a slight bulging of the bone. If from the posterior root of the zygoma a vertical line be let fall, tangential to the posterior edge of the meatus, a small triangular area is mapped off which has been named Ijy Macewen the supra- meatal triangle. Surgically this is of importance, as it is the spot selected in which to trephine the bone to reach the mastoid antrum (see p. 120). In the suture between the posterior border of the mastoid-temporal and the tabular plate of the occipital, there is usually a foramen (mastoid) for the trans- mission of an emissary vein from the lateral sinus within the cranium to the cutaneous occipital vein of the scalp ; this opening, which may be double, varies greatly in size, and is usually placed on a level with the external auditory meatus. Zygomatic Fossa. — The side of the cranium in front of the anterior root of the zygomatic process of the temporal bone is deeply hollowed, forming the zygomatic or infratemporal fossa (fossa infratemporalis) ; this in topographical anatomy corresponds to the pterygo-maxillary region. The student must bear in mind that, in examining this space, the ramus and coronoid process of the lower jaw form its outer wall ; but this bone for the present being withdrawn, enables us to get a better view of the boundaries of the space. In front its anterior luall is formed by the convex posterior or zygomatic surface (facies infratemporalis) of the superior maxilla, which rises behind the socket for the last molar tooth to form the tuberosity (tuber-maxillare). Anteriorly the zygomatic surface of the upper jaw is separated from its facial aspect by the rounded inferior margin of the malar or zygomatic process which supports the malar bone. This latter curves outwards and backwards, forming part of the upper and anterior wall of the fossa. On the inner surface of this wall will be seen the suture uniting the malar and superior maxillary bones (sutura zygomatico-maxillaris), which runs obliquely upwards and inwards to reach the external extremity of the spheno-maxillary fissure, the lower border of which forms the superior boundary of the zygomatic surface of the upper jaw. On this aspect of the bone are to be seen the openings of the posterior dental canals (foramina alveolaria) two or more in number, which transmit the nerves and vessels to the upper molar teeth. The inner ivall of the zygomatic fossa is formed by the outer surface of the external pterygoid plate (lamina lateralis processus pterygoidei), the width and shape of which varies greatly ; its posterior border is thin and sharp, and often furnished with spiny points, to one of which the pterygo- spinous ligament, which stretches from this border to the alar spine of the sphenoid, is attached. It occasionally happens that this ligament becomes ossified. Anteriorly the external pterygoid plate is separated from the superior maxilla above by an interval called the pterygo-maxillary fissure. Below this the bones are apparently fused, but a careful inspection of the skull, together with an examina- tion of the disarticulated bones, will enable the student to realise that, wedged in between the two bones at this point, is a part of one of the smaller bones of the face, the tuberosity of the palate bone (processus pyramidalis ossis palatini). The lower border of the external pterygoid plate is usually curved and slightly ' NORMA LATERALIS OF THE SKULL. L57 everted. Superiorly, where the external pterygoid plate is generally narrower, it sweeps upwards to become continuous with the broad under surface of the great wing of the sphenoid ; this, which overhangs in part the zygomatic fossa superiorly, is limited above by the infratemporal crest which separates its zygomatic from its temporal surface. The zygomatic surface of the great wing of the sphenoid is limited in front and below by the edge which forms the upper boundary of the spheno-maxillary fissure, whilst behind it reaches as far back as the inner extremity of the Glaserian fissure, where it terminates in the alar spine. It is from this point that the suture (sutura spheno-squamosa) curves forward and upwards to reach the region of the pterion. The infratemporal or zygomatic ■surface of the great wing of the sphenoid, and the outer surface of the external pterygoid plate, alike afford extensive attachments for the external j)terygoid muscle, whilst the former is pierced by minute canals for the transmission of emissary veins. Occasionally a larger vascular foramen is present (foramen Vesalii), through which a vein runs from the cavernous sinus within the cranium to the pterygoid venous plexus situated in the pterygo-maxillary region. Immediately behind the root of the external pterygoid plate there is a large oval hole, the foramen ovale, and behind that, and in line with the alar spine, is the smaller foramen spinosum. These two foramina cannot usually be seen in a side view of the skull, and are better studied when the base is examined ; they are mentioned, however, because they transmit structures which here pass from and enter the cranium, viz. the inferior maxillary division of the fifth nerve, together with its motor root, and the small meningeal artery through the foramen ovale, and the middle meningeal artery and its companion veins through the foramen spinosum. A part of the squamous temporal also forms a small portion of the roof of this fossa ; it consists of a triangular area immediately in front of the eminentia articularis, and between it and the anterior root of the zygomatic process of the temporal, which is here curving inwards and forwards, to become continuous with the infratemporal crest. Internally this surface is continuous with the zygomatic surface of the great wing of the sphenoid, separated from it, however, by the hinder part of the spheno- squamosal suture. When the lower jaw is in position, the zygomatic fossa is concealed by the ramus of the mandible, the inner surface of which, in its upper half, forms the outer wall of that space. Viewed from the outer side, the ramus of the inferior maxilla displays considerable differences in different skulls. These are mainly due to varia- tions in its width and in the nature of the angle which it forms at its fusion with the body of the bone. A considerable interval separates the posterior border of the ramus from the front of the mastoid process. Within this space may be seen the free inferior edge of the tympanic plate (vaginal process), from which, just below the external auditory meatus, the styloid process of the temporal bone is observed passing downwards and slightly forwards. The width and height of the coronoid process vary much, oftentimes reaching the level of the top of the condyle. Its extremity, when the lower jaw is closed, lies just within the fore-part of the zygo- matic arch ; at other times it rises to a much higher level, so that its point may be seen above the level of the upper border of the zygomatic arch. The posterior edge of the coronoid process forms the anterior border of the sigmoid notch, and limits in front the interval left between the lower border of the posterior half of the zygomatic arch and the upper hollowed edge of the ramus. On looking into this interval, the floor of the zygomatic fossa may be seen, formed anteriorly by the external jjterygoid plate ; whilst posteriorly it is possible to pass a prol)e right across the base of the skull from one sigmoid notch to the other, the shaft of the probe lying immediately behind the pterygoid processes of the sphenoid, and cross- ing the foramina ovalia, tlirough which the inferior maxillary divisions of the fifth nerves ])ass. The ramus and coronoid process are so placed as to occupy a position inter- mediate between the zygomatic arc!) externally and the external pterygoid plate internally ; their inner surface, therefore, forms the outer wall of the zygomatic fossa. On a level with the surface of the crowns of the teeth of the lower jaw, and situated about the middle of this aspect of the ramus, is the inferior dental foramen 158 OSTEOLOGY. (foramen mandibulare), the superior opening of the inferior dental canal (canalis mandibulse), which traverses the body of the bone. Through this foramen there pass the inferior dental branch of the inferior maxillary division of the fifth nerve, together with the inferior dental artery and its companion veins. As will now be seen, when the lower jaw is in position, the zygomatic fossa is closed in externally by the ramus of the mandible. In front there is an interval between the anterior border of the ramus and the zygomatic surface of the superior maxilla, through which pass the buccal branch of the fifth nerve and the communicating vein between the pterygoid plexus and the facial vein. Above, in the interval between the sigmoid edge and the lower border of the zygomatic arch, there pass from the fossa the vessels and nerves which supply the masseter muscle. Between the posterior border of the ramus and the styloid process there enter and leave the large vessels which are found within the space. Superiorly, under cover of the zygomatic arch, the zygomatic fossa communicates with the temporal fossa, whilst inferiorly it is continuous with the inframaxillary region. Internally, on the floor of the fossa there is an f-shaped fissure, the horizontal limb of which corresponds to the spheno-maxillary fissure, forming a channel of communication between the fossa and the orbit, through which passes the orbital branch of the superior maxillary division of the fifth nerve ; whilst the vertical cleft is the pterygo- maxillary fissure, which leads into a small fossa placed between the front of the root of the pterygoid process of the sphenoid and the back of the superior maxilla, called the spheno-maxillary fossa (fossa pterygo-palatina). The following foramina open into the zygomatic fossa — the foramen ovale. 24 23 22 Fig. 120. — Coronal Section through the Spheno- Maxillary Fossa of the Right Side. A. Anterior Wall. B. Posterior Wall. C. Diagrammatic representation of a horizontal section across the fossa. 1. Spheno-palatiiie foramen. 2. Apex of orbital cavity. 3. Spheno-maxillary fissure. 4. Spheno-maxillary fissure. 5. Pterygo-niaxillary fissure. 6. Dental foramina. 7. Part of pterygoid fossa. S, 9, 10. Posterior palatine and ac- cessory palatine canals. 11. Foramen rotunduin. 12. Sphenoidal fissure. 13. Optic foramen. 14. Sphenoidal sinus. 15. Pterygo-palatine canal. 16. Vidian canal. IT. Spheiio-palatine foramen. 18. Spheno-maxillary fossa. 19. Infra-orbital groove. 20. Spheno-maxillary fissure. 21. Pterygo-niaxillary fissure. 22. Foramen rotundum. 23. Vidian canal. 24. Pterygo-palatine canal. foramen spinosum, posterior dental foramina, inferior dental foramen, minute foramina for the transmission of emissary veins ; of these one of large size is occasionally present, the foramen of Vesalius. Spheno-Maxillary Fossa. — This space, which corresponds to the angular interval .NOEMA OCGiriTALIS OF THE SKULL. L59 between the pterygo-maxillary and spheno-inaxillary fissures, and which lies between the maxilla in front and the root of the pterygoid process behind, is bounded internally by the vertical plate of the palate bone, which separates it from the nasal cavity, with which, however, it communicates by means of the spheno-palatine foramen, which lies between the orbital and sphenoidal processes of the palate bone and the under surface of the body of the sphenoid. Opening into this fossa, above and behind, are the foramen rotundum, the Vidian canal and the pterygo-palatine canal from without inwards, whilst below is the superior orifice of the posterior palatine canal, together with openings of the accessory posterior palatine canals. Its roof is formed by the under surface of the body of the sphenoid and the orbital plate of the palate bone. Anteriorly it lies in relation to the apex of the orbit, with which it communicates by means of the spheno- maxillary fissure ; whilst externally, as already stated, it communicates with the zygomatic fossa through the pterygo-maxillary fissure. Norma Occipitalis. This view of the cranium includes the posterior halves of the two parietal bones above, the tabular part of the occipital bone below, and the mastoid portions of the temporal bones on either side inferiorly. The shape of this aspect of the skull varies much, but ordinarily the greatest width corresponds to the level of the parietal eminences. The sutures on this view of the calvaria display a tri-radiate arrangement, one limb of which is vertical, and corresponds to the posterior part of the interparietal or sagittal suture (sutura sagittalis). The other two limbs pass outwards and downwards in the direction of the mastoid processes, uniting the two parietal bones in front with the occipital bone behind ; these constitute the A-shaped lambdoid suture (sutura lambdoidea). The point of confluence of the sagittal and lambdoid sutures is called the lambda. This can generally be felt in the living, owing to the tendency of the tabular part of the occipital to project slightly immediately below this spot. About one inch and a quarter above the lambda the two small parietal foramina (foramina parietalia) are seen, through which pass the small emissary veins of Santorini, which connect the intracranial venous system with the superficial veins of scalp. These small holes lie about y^g- of an inch apart on either side of the sagittal suture, which here, for the space of about an inch, displays a simplicity of outline in striking contrast with its serrated arrangement elsewhere. The term obelion is applied to a point on the sagittal suture in line with the two parietal foramina. The lambdoid suture is characterised by great irregularity of outline, and not unfrequejitly chains of separated ossicles are met with in it, the so-called Wormian bones. The tabular part of the occipital bone is divided into two parts by the superior curved line (linea nuchse superior), the central part of which forms the external occipital protuberance (protuberantia occipitalis exterior). The part above, called the occipital surface (planum occipitale), comes within our present consideration ; the part below, the nuchal surface (planum nuchale), though seen in perspective, had best be considered when the base is examined. A little above the level of the superior curved line the occipital surface is crossed on either side by a faint lunated line, the highest curved line (linea nuchse suprema) to which are attached the occipitales muscles and the epicranial aponeurosis. The projection of the occipital surface varies much in individual skulls ; most frequently it overhangs the external occipital protuberance, forming a distinct boss ; exceptionally, however, the latter may be the most projecting part of the bone. The extremity of the superior curved line on either side corresponds to the position of the asterion (jj. 154). External to these points the outline of the skull is determined by the downward projection of the mastoid processes, the inner surfaces of whicli are deeply grooved for tlie attachment of the posterior bellies of the digastric muscles, thus causing these processes to appear more pointed when viewed from this aspect. Norma Verticalis. This is the view of the (;alvaria as seen from above. It is liable to great 160 OSTEOLOGY. diversities of form. Thus its shape may vary from an elongated oval to an outline more nearly circular. These differences have been classified, and form important distinctions from a craDiometrical standpoint (p. 178), the rounder varieties being termed tlie brachycephalic, whilst the elongated belong to the dolichocephalic group. Another noteworthy point in this view is the fact that in some instances the zygomatic arches are seen, whilst in others tliey are concealed by the overhang and l:)ulge of the sides of the fore-part of the cranium. The former condition is described as phoenozygous, the latter as cryptozygous, and each is more or less closely associated with the long or round varieties of head-form respectively. The sutures displayed have a T-shaped arrangement. Placed mesially between the two parietal bones is the sagittal suture. This is finely denticulated, except in the region of the obelion, though, of course, this will not be apparent if obliteration of the suture has taken place through fusion of the two parietal l)ones. Posteriorly the sagittal suture unites with the lambdoid suture at the lambda, which marks in the adult the position of the posterior fontanelle of the foetus. Anteriorly it terminates by joining the transverse suture which separates the frontal bone anteriorly from the parietals behind ; this latter is called the coronal suture, and the point of junction between the sagittal and coronal sutures is known as the bregma which corresponds in position to the anterior fontanelle of the foetus. The summit of the vault of the calvaria corresponds to a variable point in the line of the sagittal suture, and is named the vertex. The coronal suture is less denticulated centrally than laterally. Occasionally there is a persistence of the suture which unites the two halves of the frontal bone; under these conditions the line of the sagittal suture is carried forward to the fronto-nasal suture, and a skull displaying this peculiarity is described as metopic. Behind the coronal suture may occasionally be seen the post-coronal depression (p. 154), and in some instances the vault of the calvaria forms a broad, slightly elevated crest along the line of the sagittal suture. On either side, the temporal ridges can be seen curving over the lateral and superior aspects of the parietal bones. As the lower of these crosses the coronal suture in front it marks a spot known as the stephanion, useful as affording a fixed point from which to estimate the bi-stephanic diameter. The interval between the temporal ridges on either side will vary according to the form of the skull and the development of the temporal muscle. In this view of the calvaria a small part of the lambdoid suture on either side of the lambda is visible posteriorly. Norma Basalis. The base of the cranium — i.e. the skull without the mandible — includes a descrip- tion of the under surfaces of the skeleton of the face (cranium viscerale) and the cranium (cranium cerebrale). The former includes the hard palate formed by the superior maxillae and palate bones, the superior dental arch, and the bodies of the superior maxillae as seen from below ; whilst externally, and united with the bodies of the superior maxilla, the malar bones are displayed, curving backwards to form the anterior halves of the zygomatic arches. In the middle line, passing from the upper surface of the hard palate, is the osseous septum of the nose, here formed by the vomer, which is united above to the under surface of the body of the sphenoid. The under surface of the cranium is pierced by the foramen magnum for the transmission of the spinal cord and its membranes. In front of this a stout bar of bone extends forwards in the middle line, formed by the union of the body of the sphenoid in front with the basilar process of the occipital bone behind. In adult skulls all trace of the fusion of these two bones has disappeared ; when union is in- complete, it indicates that the skull is that of a person below the age of twenty-five. The sphenoid comprises that part of the calvaria which forms the roof and sides of the apertvires which lie on either side of the nasal septum above, the hard palate — the choanse or posterior nares. Laterally the under surfaces of the great sphenoidal wings extend as far forward as the posterior border of the spheno- maxillary fissure ; whilst posteriorly they reach as far as the alar spine, external to which the spheno-squamosal suture, separating the great wing of the sphenoid from NORMA BASALIS OF THE SKULL. IGl 1. External occipital crest. 2. Superior curved line of the occipital bone. 3. Foramen magnum. 4. Occipital condyle. 5. Digastric groove. 6. Mastoid process. 7. External auditory meatus. 8. Styloid process. 9. Glenoid fossa. 10. Foramen spinosum. 11. Sphenoidal spine. 12. Foramen ovah;. 13. External pterygoid filate. Fig. 121. — Norma Basalis. 14. Haniular process of internal 27. pterj'goid i:ilate. 28. 1.5. Nasal septum. 29. 16. Posterior nasal spine. 30. 17. Horizontal plate of palate bone. 18. Palatal process of superior maxilla. 31. 19. Anterior jjalatine canal. 32. 20. Intermaxillary suture. 33. 21. Posterior palatine canal. 34. 22. Malar process of superior maxilla. 35. 23. Spheno-niaxillary fissure. 36. 24. Zygomatic fossa. 37. 25. Zygomatic arch. 38. 26. Posterior open! iig of left nasal fossa. 39. Pterygoid fossa. Scaphoid fossa. Foramen lacerum medium. Opening of osseous Eu.stachian canal. Carotid canal. Jugular fossa. Stylo-niastoid foramen. Jugular process of occipital bone. Groove for occipital artery. Mastoid foramen. Posterior condylic foramen. Inferior curved lineof occii)ital bone External occipital protuberance. the squamous portion of the temporal, curves forwards and upwards, internal to the eminentia articularis, to reach the fiecomes more obtuse. 13 178 OSTEOLOGY. CEANIOLOGY. The various groiq^s of mankind display in their physical attributes certain features which are more or less characteristic of the stock to which they belong. Craniology deals with these differences so far as they affect the skull. The method whereby these differences are recorded involves the accurate measurement of the skull in most of its details. Such procedure is included under the term craniometry. Here only the outlines of the subject are briefly referred to ; for such as desire fuller information on tlie subject, the works of Broca, Topinard, Flower, and Turner may be consulted. The races of man display great variations in regard to the size of the skull. Apart altogether from individual differences and the proportion of head-size to body -height, it may be generally assumed that the size of the skull in the more highly civilised races is much in excess of that displayed in lower tj-pes. The size of the head is intimately correlated with the develop- ment of the brain. By measuring the capacity of that part of the skull occupied by the encephalon, we are enabled to form some estimate of the size of the brain. The cranial capacity is determined by filling the cranial cavity with some suitable material and then taking the cubage of its contents. Various methods are employed, each of which has its advantage. The use of fluids, which of course would be the most accurate, is rendered im^jracti cable, without special precautions, owing to the fact that the macerated skull is pierced by so many foramina. As a matter of j)ractice, it is found that leaden shot, glass beads, or seeds of various sorts are the most serviceable. The results obtained display a considerable range of variation. For purposes of classification and comparison, skulls are grouped according to their cranial capacity into the following varieties : — Microcephalic skulls are those with a capacity below 1350 c.c, and include such well-known races as Andamanese, Veddahs, Australians, Bushmen, Tasmanians, etc. Mesoceplialic skulls range from 1350 c.c. to 1450 c.c, and embrace examples of the following varieties : American Indians, Chinese, some African Negroes. Megacephalic skulls are those with a capacity over 1450 cc, and are most commonly met with in the more highly civilised races : Mixed Europeans, Japanese, Eskimo, etc. Apart from its size, the form of the cranium has been regarded as an important factor in the classification of skulls ; though whether these differences in shape have not been unduly em- phasised in the past is open to question. The relation of the breadth to the length of the skull is expressed by means of the cephalic index which records the proportion of the maximum breadth to the maximum length of the skull, assuming the latter equal 100, or — Max. breadth X 100 ~ , ,. . ■, ^ — ^^f T Ti = Cephalic index. Max. length The results are classified into three groups : — 1. Dolichocephalic, with an index below 75 : Australians, Kaffirs, Zulus, Eskimo, Fijians. 2. Mesaticephalic, ranging from 75 to 80 : Europeans (mixed), Chinese, Polynesians (mixed). 3. Brachycephalic, with an index over 80 : Malays, Burmese, American Indians, Anda- manese. In order to provide for uniformity in the results of different observers, some system is neces- sary by which the various jjoints from which the measurements are taken must correspond. AVhilst there is much difference in the value of the measurements insisted on by indiAddual anatomists, all agree in endeavouring to select such jDoints on the skull as may be readily deter- mined, and which have a fairly fixed anatomical position. The more important of these " fixed points " are included in the subjoined table : — Nasion. — The middle of the naso-frontal suture. Glabella. — A point midway between the two superciliary ridges. Ophryon. — The central jooint of the narrowest transverse diameter of the forehead, measured from one temporal line to the other. Inion. — The external occipital protuberance. Maximum Occipital Point. — The point on the occipital squama in the sagittal plane most distant from the glal;)ella. Opisthion. — The middle of the j^osterior margin of the foramen magnum. Basion. — The middle of the anterior margin of the foramen magnum. Bregma. — The point of junction of the coronal and sagittal sutures. Alveolar Point. — The centre of the anterior margin of the ujiper alveolar margin. Subnasal Point. — The middle of the inferior border of the anterior nasal aperture at the centre of the nasal spine. Vertex. — The summit of the cranial vault, Obelion. — A ^^oint over the sagittal suture, on a line with the parietal foramina. Lambda. — The meeting-point of the sagittal and lambdoid sutures. Pterion. — The region of the antero -lateral fontanelle whei-e the angles of the frontal, parietal, squamous temporal, and alisphenoid lie in relation to one another. As a CRANIOLOGY. 179 rule, the sutures are arranged like the letter H, the parietal and alisphenoid separating the frontal from the squamous temporal. In other cases the form of the suture is like an X ; whilst in a third variety the frontal and squamous temporal articulate with each other, thus separating the alisphenoid from the parietal. Asterion is the region of the postero-lateral fontanelle where the lambdoid, parieto-mastoid, and occipito-mastoid sutures meet. Stephanion. — The point where the coronal suture crosses the temporal crest. Dacryon. — The point where the vertical lachrymo-maxillary suture meets the fronto-nasal suture at the inner angle of the orbit. Jugal Point. — Corresponds to the angle between the vertical border and the margin of the zygomatic process of the malar bone. Gonion. — The outer side of the angle of the inferior maxilla. The measurements of the length of the skull may be taken between a variety of points — the nasion, glabella, or ophryon in front, and the inion or maximum occipital point behind. Or the maximum length alone may be taken without reference to any fixed points. In all cases it is better to state precisely where the measurement is taken. The maximum breadth of the head is very variable as regards its position ; it is advisable to note whether it occurs above or below the j)arieto-squamosal suture. The inter-relation of these measurements as expressed by the cephalic index has been already referred to. The width of the head may also be measured from one asterion to the other, biasterionic width, or by taking the bistephanic diameter. The height of the cranium is usually ascertained by measuring the distance from the basion to the bregma. The relation of the height to the length may be expressed by the height or vertical index, thus — Height X 100 ,^ ,. ... — $ -, — = Vertical index. Length Skulls are classified in accordance with the relations of length and height as follows : — Tapeinocephalic index below 72. Chamaecephalic index up to 70. Metriocephalic index between 72 and 77. , Orthocephalic index from 70-1 to 75. Akrocephalic index above 77 (Turner). Hypsicephalic index 75'1 and upwards (KoUmann, Eanke, and Virchow). The horizontal circumference of the cranium, which ranges from 450 mm. to 550 mm., is measured around a plane cutting the glabella or ophryon anteriorly, and the maximum occipital point i^osterioiiy. The longitudinal arc is measured from the nasion in front to the opisthion behind ; if to this be added the basi-nasal length and the distance between the basion and the opisthion, we have a record of the vertico-mesial circumference of the cranium. This may further be divided by measuring the lengths of the frontal, parietal, and occipital jDortions of the superior longitudinal arc. In this way the relative proportions of these bones may be expressed. The measurements of the skeleton of the face are more complex, but, on the whole, of greater value than the measurements of the cranium. It is in the face that the characteristic features of race are best observed, and it is here that osseous structure most accurately records the form and projjortions of the living. The form of the face varies like that of the cranium in the relative proportions of its length and breadth. Generally speaking, a dolichocephalic cranium is associated with a long face, whilst the brachycephalic type of head is correlated with a rounder and shorter face. This rule, how- ever, is not universal, and there are many exceptions to it. The determination of the facial index varies according to whether the measurements are made with or without the mandible in position. In the former case the length is measured from the ophryon or nasion above to the mental tubercle below, and compared with the maximum bizygomatic width. This is referred to as the total facial index, and is obtained by the formula — Ophryo-mentallen gthxlOO ^^^^^^ ^^^.^^ .^^^^ iiizygomatic width More usually, however, owing to the loss of the lower jaw, the proportions of the face are expressed by the superior facial index. This is determined by comparing the ophryo-alveolar or naso-alveolar length with the bizygomatic width, thus — Ophryo-alveolar length x 100 „ . ~ . , . , — -1. ^. ?,-r = Superior facial index. Jiizygomatic width The terms dolichofacial or leptoprosope and brachyfacial or chamoeprosope have been employed to express the differences thus recorded. Uniformity in tliese measurements, however, is far from complete since many anthropologists compare the widtli witli the lengtli = 100. Tlie jiiojiortiori of the facc-vvidlli to tbe width of the calvaria is roughly expressed by the use of the t<;rms cryptozygous and phaenozygous as applied to the skull. In the former case the zygomatic arches are concealed, when the .skull is viewed from above, by the overhanging and 13 a 180 OSTEOLOGY. projection of the sides of the cranial box ; in the latter instance, owing to the narrowness of the calvaria, the zygomatic arches are clearly visible. The projection of the face, so characteristic of certain races (Negroes for example), may be estimated on the living by measuring the angle formed by two straight lines, the one passing from the middle of the external auditory meatus to the lower margin of the septum of the nose ; the other drawn from the most prominent part of the forehead above to touch the incisor teeth below. The angle formed by the intersection of these two lines is called the facial angle (Camper), and ranges from 62° to 85°. The smaller angle is characteristic of a muzzle-like projection of tlie lower part of the face. The larger angle is the concomitant of a more vertical profile. The degree of projection of the upjier jaw in the macerated cranium is most commonly expressed by emj)loying the gnathic or alveolar index of Flower. This records the relative proportions of the basi-alveolar and basi-nasal lengths, the latter being regarded as = 100, thus — Basi-alveolar length x 100 „,,..•, ^ — ^ p = Gnathic index. Uasi-nasal length The results are conveniently grouped into three classes : — Orthognathous, index below 98 : including mixed Europeans, ancient Egyptians, etc. Mesognathous, index from 98 to 103 : Chinese, Japanese, Eskimo, Polynesians (mixed). Prognathous, index aboA^e 103 : Tasmanians, Australians, Melanesians, various African Negroes. Unfortunately, however", little reliance can be placed on the results obtained by this method, since it takes no account of the projiortion of the third or facial side of the gnathic triangle. For a further discussion of this matter see Thomson and Maclver, Races of the Thebaid (Oxford : Clarendon Press, 1905). The form of the nasal aperture in the macerated skull is of much value from an ethnic standpoint, as it is so intimately associated with the shape of the nose in the living. The greatest width of the nasal aperture is compared with the nasal height (measured from the nasion to the lower border of the nasal aperture) and the nasal index is thus determined : — Nasal width x 100 „ , • ■■ — ^r^^ — ^-T — ^^f- — = Nasal index. ISlasal height Skulls are — Leptorhine, with a nasal index below 48 : as in mixed Europeans, ancient Egyptians, American Indians, etc. Mesorhine, with an index ranging from 48 to 53 : as in Chinese, Japanese, Malays, etc. Platyrhine, with an index above 53 : as in Australians, Negroes, Kafi&rs, Zulus, etc. The form of the orbit varies considerably in different races, but is of much less value from the standj)oint of classification. The orbital index expresses the proportion of the orbital height to the orbital width, and is obtained by the following formula : — Orbital height x 100 « t,-+ i • a — 7^; — ^— r- — ^ — • 1 ,^ ^ Uruitai inctex. Orbital width The orbital height is the distance between the upper and lower margins of the orbit at the middle ; whilst the orbital width is measured from a point where the ridge which forms the posterior boundary of the lachrymal groove meets the fronto - lachrymal suture (Flower), or from the dacrj^on (Broca) to the most distant point from these on the anterior edge of the outer border of the orljit. The form of the orbital aperture is referred to as — Megaseme, if the index be over 89 ; Mesoseme, if the index be between 89 and 84 ; Microseme, if the index be below 84. The variations met with in the form of the palate and dentary arcade may be expressed by the palato-maxillary index of Flower. The length is measured from the alveolar point to a line drawn across the hinder borders of the sujjerior maxillary bones, whilst the width is taken between the outer borders of the alveolar arch immediately above the middle of the second molar tooth. To obtain the index, the following formula is employed : — Palato-maxillary width x 100 _ , . .„ . , — =,^, ^ — .tT , ^, = Palato-maxillary index. Palato-maxillary length For purj)oses of classification Turner has introduced the following terms : — Dolichuranic, index below 110. Mesuranic, index between 110 and 115. Brachyuranic, index above 115. As is elsewhere stated (p. 183), the size of the teeth has an important influence on the aKihitecture of the skull. Considered from a racial standpoint, the relative size of the teeth to DEVELOPMENT OF THE CHONDEO-ORANIUM. 181 the length of the cranio-facial axis has been found hy Flower to be a character of much value. The dental length is taken by measuring the distance betweeii the anterior surface of the first pre- molar and the posterior surface of tlie third molar of the upper jaw. To obtain the dental index the following formula is used : — Dental length x 100 ^ . , • ■, - =R — ; ri n — = Dental index. Easi-nasal length Following the convenient method of division adopted with other indices, the dental indices may be divided into three series, called respectively — Microdont, index below 42 : including the so-called Caucasian or white races. Mesodont, index between 42 and 44 : including the Mongolian or yellow races. Megadont, index above 44 : comprising the black races, including the Australians. Many complicated instruments have been devised to take the various measurements required, but for all jsractical purposes the calipers designed by Flower or the comjxis glissih-e of Broca are sufficient. As an aid to calculating the indices, the tables published in the Osteological Catalogue of the Royal College of Surgeons of England, Part I., Man.; Index- Tahellen zum Anthropometrisclien Gebrauche, C. M. Furst, Jena, 1902 ; or the index calculator invented by Dr. Waterston will be found of much service in saving time. DEVELOPMENT OF THE CHONDRO-CRANIUM AND MORPHOLOGY OF THE SKULL. As has been already stated (p. 24), the chorda dorsalis extends forwards to a point immediately beneath the anterior end of the mid-brain. Li front of this the head takes a bend so that the large fore-brain overlaps the anterior extremity of the notochord. At this stage of development the cerebral vesicles are enclosed in a membranous covering derived from the mesenchyme surrounding the notochord ; this differentiated mesodermal layer is called the primordial membranous cranium. From it the meninges which invest the brain are derived. In lower vertebrates this membranous capsule becomes converted into a thick -walled cartilaginous envelope, the primordial cartilaginous cranium. In mammals, however, only the basal part of this capsule becomes chondrified, the roof and part of the sides remaining membranous. In considering the chondrification of the skull in mammals, it must be noted that part only of the base is traversed by the notochord, viz. that portion which extends from the foramen magnum to the dorsxim sellse of the sphenoid. It is, therefore, conveniently divided into two parts — one posterior, surrounding the notochord, and hence called chordal, and one in front, into which the notochord does not extend, and hence termed prechordal. These correspond respectively to the vertebral and evertebral regions of Gegenbauer. In the chordal region a pair of elongated cartilages, called the parachordal cartilages, appear one on either side of the notochord ; these soon all but completely envelop the chorda, and expand so as to form the basilar or occipital plate, which ossifies later to form the basilar process of the occipital bone, and the dorsum sellee of the sphenoid. By backward extension from the occipital plate on either side and aromid the foramen magnum the cartilaginous ex-occipitals and supra-occipitals are also formed. In the prechordal region two curved strips of cartilage, the trabeculse cranii of Rathke, arise and pass forwards from the anterior extremity of the notochord, one on either side of the cranio -pharyngeal canal. In front these trabeculse spread out and ultimately fuse to form the ethmoidal plate, which constitutes the fore part of the chondro-cranium. Posteriorly the trabeculse unite with the basilar plate and thus surround the cranio-pharyngeal canal, the lumen of which is subsequently closed to foi-m the floor of the pituitary fossa, in which rests the hypophysis cerebri. Owing to the presence of the nasal capsules, the foi-e-part of the ethmoidal plate becomes differentiated into an ethmo- vomerine region, from which the nasal septum and its associated cartilages are derived, whilst the remainder of the ethmoidal plate by expansion and subsequent ossifica- tion develops to form the pre-sphenoid, the orbito-sphenoids, and the ali-sphenoids, which latter assist in completing the orbital cavity for the lodgment of the eyeball. The membranous ear capsules wliich lie lateral to the parachordal cartilages become chondrified and form the cartilaginous ear capsules. These soon unite with the lateral aspects of the basilar plate, but are separated in front from the cartilaginous alisphenoid of the ethmoidal plate by a inembranous interval, which is subsequently occupied by the squamosal, a bone of dermic origin. This disappearance of the cartilage under the squa- mosal was regarded by Parker as the diagnostic mark of the mammalian chondro-cranium. From the ventral surface of this cartilaginous platform — formed, as described, by the 13 6 182 OSTEOLOGY. union of the trabeculte, parachordal cartilages, and cartilaginous ear capsules — is suspended the cartilaginous framework of the visceral arches, which play so important a part in the development of the face, an account of which is elsewhere given (p. 36). A consideration of the facts of comparative anatomy and embryology appears to justifiy the assumption that the mammalian skull is of two -fold origin — that, in fact, it is composed of two envelopes, an outer and an inner, primarily distinct, but which in the process of evolution have become intimately fused together. The inner, called the primordial skull, is that which has just been described, and consists of the chondro- cranium and the branchial skeleton. The outer, which is of dermic origin, includes the bones of the cranial vault and face which are developed in membrane. This secondary skull, which first appears in higher fishes as ossified dermal plates overlying the primary skull, acquires a great importance in the mammalia, as owing to the expansion of the Pars ethmoidali Orbito-.sx)henoi Sphenoidal fissure r j Alisplienoid Carotid canal Meatus auditoriu; internus Floccular fossa Foramen jugulars Canalis hypoglossi Foramen magnum Lamina ciibrosa \ ^ Oibital portion of orbito-sphenoid Optic- foramen — Olivary process Sella turcica ^^ --- T, y ^- T-' — Dorsum sella; Pars petrosa 111..., ,^,," ' Superior semicircular canal Pars niastoidea Occipital fontauelle Fig. 127. — View of the Chondro- Cranium of a Human Fcetus 5 cm. in length from Vertex to Coccyx (about the middle of tlie third inonth) ; the cartilage is coloured blue. The line to the right of the drawing shows the actual size. brain and the pi-ogressive reduction of the chondro-cranium, these dermal bones become engrafted on and incorporated with the primordial skull, and act as covering bones to the cavities of the cranium and face; for it may be well to point out that these dermal or membrane bones are not necessarily external in position, as over the cranial vault, but also develop in the tissues underlying the mucous membrane of the cavities of the face. Advantage is taken of this difference in the mode of development of the bones of the skull to classify them according to their origin into cartilage or primordial bones, and membrane or secondary bones. These differences in the growth of the bone must not be too much insisted on in determining the homologies of the bones of the skull, as it is now generally recognised that all bone is of membranous origin, and that whilst in some cases cartilage may become calcified, it never undergoes conversion into true bone, but is replaced by ossific deposit derived from a membranous source. In the subsequent growth of the skull parts of the cartilaginous, cranium persist as the septal and alar cartilages of the nose, whilst for a considerable period the basi-sphenoid and basi-occipital are still united by cai'tilage. The cartilage also which blocks the foramen lacerum medium may be regarded as a remnant of the chondro-cranium. Till two years after birth there are membranous intervals between the edges and MORPHOLOGY OF THE SKULL. 183 angles of the bones of the cranial vault. These are termed tlie fontanelles. Normally they are six in number, and correspond in the adult to the position of the bregma and lambda in the middle line and the pterion and asterion on either side. The anterior or bregmatic fontanelle is diamond-shaped, and corresponds to the converging angles of the parietals and two halves of the frontal bone. The posterior fontanelle is triangular in form, and lies between the two parietals and the summit of the occipital squama. The antero-lateral fontanelle lies between the contiguous margins of the frontal, parietal, squamous temporal, and great wing of the sphenoid, whilst the poster o -lateral fontanelle is situated between the adjacent borders of the parietal, occipital, and mastoid portion of the temporal. The term occipital fontanelle is applied to a membranous interval which occurs in the foetal condition posterior to the foramen magnum, and between the lateral halves of the cartilaginous supra- occipital. Its persistence accounts for the occasional occurrence of a cerebral meningocele. ^ Whilst in many instances the primordial and secondary bones remain distinct in the fully-developed condition, they sometimes fuse to form complex bones, such as the temporal and sphenoid (see pp. 120 and 127). Various theories have been advanced to account for the mode of formation of the skull. The earliest of these was called the vertebrate theory, which assumed that the cranium was built up of a series of modified vertebrae, the centra of which corresponded to the basi-cranial axis, whilst the neural arches were represented by the covering bones of the cranium. In view of the more recent researches regarding the composite origin of the skull above referred to, this theory was necessarily abandoned. It gave way to the suggestion of Gegenbauer that the primordial cranium has arisen by the fusion of several segments equivalent to vertebrae, the number of which he determined by noting the metameric arrangement of the cranial nerves, of which he concluded there were nine pairs, arranged much like spinal nerves, both as to their origin and distribution. The olfactory and optic nerves, though frequently referred to as cranial nerves, are excluded, since from the nature of their development they are to be regarded as metamorphosed parts of the brain itself. Gegenbauer therefore assumed that that portion of the cranial base which is traversed by the nine pairs of segmentally arranged cranial nerves must be formed by the fusion of nine vertebral segments ; and as the region where the nerves escape corresponds to the part of the chondro-cranium traversed by the notochord, he calls it the vertebral portion of the cranial base, in contradistinction to the trabecular or non-vertebral part which lies in front. This latter he regards as a new formation adapted to receive the greatly-developed brain and afford protection to the organs of sight and smell. As has been pointed out by Hertwig, there is an essential difference between the development of the axial cartilaginous skeleton of the trunk and head. The former becomes segmented into distinct vertebrae alternating with intervertebral ligaments ; whilst the latter, in order to attain the rigidity necessary in this part of the skeleton, is never so divided. It follows from this that the original segmentation of the head is only expressed in three ways, viz. in the appearance of several primitive segments (myotomes), in the arrangement of the cranial nerves, and in the fundament of the visceral skeleton (visceral arches). According to Froriep the mammalian occipital corresponds to the fusion of four vertebrse, and there is some reason for supposing that in some classes of vertebrates the occipital region of the primordial cranium is increased by fusion with the higher cervical segments. The form of skull characteristic of man is dependent on the large proportionate development of the cranial part, which contains the brain, and the reduction in size of the visceral part (face), which protects the organs of special sense. This leads to a decrease in the mass and projection of the jaws, as well as a reduction in the size of the teeth. Associated with the smaller mandible there is a feebler musculature, with a reduced area of attachment to the sides of the skull. In this way the disappearance of the muscular crests and fossa3, so characteristic of lower forms, is accounted for. At the same time the fact that the skull is poised on the summit of a vertical column, leads to important modifications in its structure. The disposition of parts is such that the occipito-vertebral articulation is so placed that the fore and hind parts of the head nearly balance each other, thus obviating the necessity for a powerful muscular and ligamentous mechanism to hold the head erect. Another noticeable feature in connexion with man's skeleton is the prolonged period during which growth may occur before maturity is reached ; this is associated with a more complete consolidation of the skull, since bones, which in lower forms remain throughout ' For furtlier iiiforniatioii regfinling iuiomalous conditions of the fontanollos ami tlie occurrence of Wormian or sutural osHicles, Hce V. Frassetto, Aim. dns ,Hi:i. Nat. ZdoI. 8" sidv. xviii. 190:3. Vi c 184 OSTEOLOGY. life distinct, are in man fused witli each other, as exemplified in the case of the presphenoid and postsphenoid, the occipital and the interparietal, to mention one or two instances among many. It is noteworthy, however, that during ontogeny the morphological signifi- cance of these bones is clearly demonstrated by their independent ossification. The points of exit of the various cranial nerves remain remarkably constant, and in their primitive condition serve to suggest the segmental arrangement of the cartilaginous chondro-cranium already referred to. Owing to the very great modifications which the mammalian skull has undergone in the process of its evolution, it may be pointed out that the passage of the nerves through the dura mater — a derivative, the readers may be reminded, of the primordial membranous cranium (see ante) — alone represents the primitive disposition of the nerves. Their subsequent escape through the bony base is a later and secondary development. In some cases the two, membranous or primary and the osseous or secondary foramina, correspond. In other instances the exit of the nerves through the dura mater does not coincide with the passage through the bone. Of interest in this connexion it may be pointed out that the foramina and canals which traverse the skull are either situated in the line of suture between adjacent bones or in the line of fusion of the constituent parts of which the bone pierced is made up. For example, the sphenoidal fissure is situated between the ox'bito and alisphenoids ; the anterior condylic between the basi and exoccipitals ; the jugular between the petrous, basi, and exoccipital ; the optic between the orbito-sphenoid and the presphenoid ; the Vidian between the alisphenoid, internal pterygoid plate, and the lingula. THE BONES OF THE UPPEE EXTEEMITY. The Clavicle. The clavicle (clavicula), or collar bone, one of the elements in the formation of the shoulder girdle, consists of a curved shaft, the extremities of which are enlarged. The inner end, since it articulates with the sternum, is called the sternal end ; the outer extremity, from its union with the acromion process of the scapula, is known as the acromial end. The sternal end (extremitas sternalis) is enlarged, and rests upon the meniscus Fig. 128. — Right Clavicle as seen from Above. of fibro-cartilage which is interposed between it and the clavicular facet on the upper and external angle of the manubrium sterni. It is also supported by a small part of the inner end of the cartilage of the first rib. Its articular surface, usually broader from above downwards than from side to side, displays an antero-posterior convexity, whilst tending to be slightly concave in a vertical direction. The edge around the articular area which serves for the attachment of the capsule of the sterno-clavicular articulation is sharp and well defined, except below where it is rounded. The shaft is so curved that its anterior outline is convex in its inner two-thirds, whilst concave in the outer third of its length. Eounded or prismatic in form towards its sternal extremity, the shaft becomes compressed and flattened towards its outer end. Its superior surface, which is smooth and subcutaneous throughout its whole length, is directed upwards and forwards; the inferior surface is inclined downwards and backwards. The anterior border, which separates the upper from the under surface in front, is rough and tubercular towards its inner end for the attachment of the clavicular fik'es of the pectoralis major, whilst externally THE CLAVICLE. 185 where it becomes continuous with the anterior margin of the acromial end, it is better defined, and bears the imprint of the origin of the fibres of the deltoid muscle ; here, not uncommonly, a projecting spur of bone, called the deltoid tubercle, may be seen. The 'posterior harder is broad internally, where it is lipped Acromial FACET Rhomboid impebssios Fig. 129. — Right Clavicle as seen fbom Below. superiorly to furnish an attachment for the clavicular fibres of the sterno- mastoid muscle ; behind and below this the sterno - hyoid and sterno - thyroid muscles are attached to the bone. Externally, the posterior border becomes more rounded, and is confluent with the posterior edge of the acromial end at a point where there is a marked outgrowth of bone from its under surface, the conoid tubercle (tuberositas coracoidea). Into the outer third of this border are inserted the upper and anterior fibres of the trapezius muscle. The inferior surface of the shaft close to the sternal end is marked by an irregular elongated impression (tuberositas costalis), often deeply pitted, for the attachment of the rhomboid ligament, which unites it to the cartilage of the first rib. External to this the shaft is channelled by a groove which terminates close to the conoid tubercle ; into this groove the subclavius muscle is inserted. The acromial end of the bone is flattened and compressed from above down- wards, and expanded from before backwards ; its anterior edge is sharp and well defined, and gives attachment to the deltoid muscle, which also spreads over part of its upper surface. Its posterior margin is rougher and more tubercular, and provides a surface for the insertion of the trapezius. The area between these two muscular attachments is smooth and subcutaneous. The outer edge of this forward-turned part of the bone is provided with an oval facet (facies articularis acromiaiis) for articulation with the acromion process of the scapula ; the margins around this articular area serve for the attachment of the capsule of the joint. The inferior surface of the acromial end of the bone is traversed obliquely from behind forwards and outwards by a rough ridge or line called the trapezoid or oblique ridge. The posterior extremity of this ridge, as it abuts on the posterior border of the bone, forms a prominent process, the conoid tubercle (tuberositas coracoidea) ; to each of these, respectively, are attached the trapezoid and conoid portions of the coraco-clavicular ligament. Nutrient Foramina.— The forainina for the larger nutrient vessels, of which there may be one or two directed outwards, are usually found about the middle of the posterior border, or it may be opening into the floor of the groove for the subclavius muscle. Architecture. — The shaft consists of an outer layer of compact bone, thickest towards the centre, Init gradually thinning towards the extremities, the investing envelope of which consists merely of a thin shell. Within the shaft the cancellous tissue displays a longitudinal striation, which internally assumes a more cellular appearance. At the acromial end the general arrange- ment of the fibres resembles the appearance of the sides of a Gothic arch. The curves of the Ijone impart an elasticity to it, which is of much service in reducing the effects of the shocks to which it is- so frequ(;ntly subjected. Variations.- -The clavicles of women are more slender, less curved, and shorter than those of men. In the latter the bone is so inclined that its outer end lies slightly higher or on the same level with tlie sternal end. In women the bone usually slopes a little downward and outward. The more i)i'on(junced cni-ves of some bones are probably associated with a more powerful development rocess is the honio- logue in a rudimentary form of a canal present in many animals, notably in the carnivora and marsupials. In addition to the broad musculo-spiral groove already described, and which is no doubt produced by the twisting or torsion of the shaft, there is occasionally a distinct narrow groove posterior to it, which marks precisely the course of the musculo-spiral nerve as it turns round the outer side of the shaft of the bone. Ossification. — At birth the shaft of the humerus is usually the only part of the bone ossified, if we except the occasional presence (22 per cent) of an ossific centre in the head. (H. R. Spencer, Journ. Anat. and Physiol, vol. xxv. p. 552.) The centre for the shaft makes its appearance early in the second month of intrauterine life. Within the first six months after birth a centre usually appears for the head ; this is succeeded by one for the great tuberosity during the second or third year. These soon coalesce ; and a thii'd centre for the small tuberosity begins to appear about the end of the third year, or may be delayed till the fourth or fifth year. These three centres are all blended by the seventh year, and form an epiphysis, which ultimately unites with the shaft about the age of twenty-five. It may be noticed that the superior end of the diaphysis is conical and pointed in the centre, over which the epiphysis fits as a cap, an arrangement which thus tends to prevent its displacement before union has occurred. The first centre to appear in the loiver extremity is that for the capitellum about the second or third THE ULNA. 195 year. This extends inwards, and forms the outer half of the trochlear surface, the centre for the inner half not making its appearance till the eleventh or twelfth year. Separate centres are developed in connexion with the epicondyles ; that for the external appears about the twelfth year, and rapidly coalescing with the centres for the capitellum and trochlea forms an epiphysis, which unites with the shaft about the sixteenth or seventeenth year. The centre for the internal epicondyle appears about the fifth year ; it forms a separate epiphysis, which unites with the shaft about eighteen or nineteen. These two epiphyses at the lower end of the bone are separated by a down-growth of the At iDirth. About 5 years. About 12 years. Fig. 139. — Ossification of the Humerus. About 16 years. 8. Centre for small tulDerosity fuses with other centres about 7 years. 9. Appears about 11 or 12 years. 10. Inferior epiphysis fuses with shaft about 16 to 17 years. 11. Superior epiphysis fuses with shaft about 25 years. 12. Puses Vith shaft about 17 to 18 years. 1. Appears early in 2nd month fojtal life. 2. For tuberosity, appears 2 to 3 years. 3. For head, appears within first 6 months. 4. For internal condyle, appears about 5 years. 5. For capitellum, appears 2 to 3 years. 6. Appears about 12 years. 7. Centres for head and great tuberosity, coalesce about 5 years. shaft, which lies between the internal epicondyle and the trochlea, and forms part of the base and inner side of the latter process. The epicondylic process when present is developed from the diaphysis, and has been observed to be already well ossified by the third year. ("Proc. Anat Soc." Journ. Anat. and Physiol. 1898.) The Ulna. Of the two bones of the forearm, the ulna, which is placed internally, is the longer. It consists of a large superior extremity supporting the olecranon and coronoid processes ; a shaft tapering from above downwards ; and a small rounded inferior end called the head. Superior Extremity. — The olecranon process (olecranon) lies in line with the shaft. Its posterior surface, more or less triangular in form, is smooth and subciitaneoiis and covered by a bursa. Its superior aspect, which forms with the jjosterior surface a nearly rectangular projection — the tip of the elbow — furnishes a surface for the insertion of the tendon of the triceps muscle, together with a smooth area which is overlain by the same tendon, but separated from it by a bursal sac. To the anterior crescentic border of this X'rocess are attached the fibres of the jxjsterior part of the capsule and portion of the internal lateral ligament of tl)e el bow- joint. The anterior surface is articular, and enters into the foruiation of tfie great sigmoid cavity. 196 OSTEOLOGY. Olecranon process Greai sigmoid CAVITY Small sigmoid CAVITY Bicipital hollow Posterior border "Interosseous border The coronoid process (processus coronoideus) is a bracket-like process, which juts forwards from the fore and upper part of the shaft, and is fused with the olecranon process superiorly. By its upper surface it enters into the formation of the great sigmoid cavity, whilst its anterior aspect, which is separated from its upper side by a sharp irregular margin, slopes downwards and backwards to become confluent with the anterior surface of the shaft. Of tri- angular shape, this area, which is rough and -Coronoid process tubcrcular, terminates inferiorly in an oval elevated tubercle (tuberositas ulnae), into which the tendon of the brachialis anticus muscle is inserted. Of the lateral margins of the coronoid process, the inner is usually the better defined. Above, where it joins the sujjerior border, there is generally a salient tubercle, to which one of the heads of origin of the flexor sublimis digitorum muscle is attached, whilst below this point the inner border furnishes origins for the pronator radii teres, and occasionally for the flexor longus pollicis muscles, from above downwards. The smooth inner surface of the coronoid process merges with the olecranon behind, and with the internal surface of the shaft below. The great sigmoid cavity (incisura semilunaris), for articulation with the trochlea of the humerus, is a semicircular notch, the upper part of which is formed by the anterior surface of the olecranon, whilst below it is completed by the upper surface of the coronoid process. Constricted towards its deepest part by the notching of its lateral borders, the articular surface is occasionally crossed by a narrow impression which serves to define the olecranon process above from the coronoid below. The articular area is divided into an inner portion, slightly concave transversely, and an outer part, transversely convex to a slight degree, by a longi- tudinal smooth ridge which extends from the most prominent part of the border of the olecranon above to the most outstanding point of the coronoid process below. The margins of the great sigmoid cavity are sharp and well defined, and serve, with the exception of the area occupied by the small sigmoid cavity, for the attachment of the capsule of the elbow-joint. The small sigmoid cavity, placed on the outer side of the coronoid process, is an oblong articular surface for the reception of the head of the radius. It encroaches on the lower and outer part of the great sigmoid notch, so as to narrow it con- siderably. Separated from it by a rectangular curved edge, it displays a surface which is plane from above downwards, and concave from before backwards. Its anterior extremity is narrower and more pointed than its posterior, and becomes confluent with the anterior edge ' of the coronoid process, at which point the orbicular Ugament, which retains the head of the radius in position, is attached in front. Its posterior border, wider and more outstanding, hes in hne, and is continuous with the interosseous margin ai -Head Articular surface 'for radius ^Groove for ext. carpi ULNARIS Styloid process Fig. 140. — The Right Ulna as viewed FROM THE Outer Side. THE ULNA. 197 of the shaft. Behind this border, the orbicular ligament is attached pos- teriorly. The shaft of the ulna (corpus ulnse), which is nearly straight, or but slightly curved; is stout and thick above, gradually tapering towards its lower extremity. It may be divided into two surfaces, a flexor and an extensor, by two well-defined borders, an external or interosseous (crista interossea), and a posterior (margo dorsalis), which latter is subcutaneous throughout its whole length. The outer, or interosseous border (crista interossea), is crisp and sharp in the upper three-fourths of the shaft, but becomes faint and ill-defined in the lower fourth. To this, with the exception only of the part which forms the posterior boundary of the bicipital hollow, is attached the interosseous membrane which connects the two bones of the forearm. The posterior border (margo dorsalis), of sinuous outline, curving outwards above, and slightly inwards below, is continuous superiorly with the triangular subcutaneous area on the back of the olecranon, being formed by the confluence of the borders which bound that surface ; well marked above, it becomes faint and more rounded below, but may be traced down- wards to the posterior surface of the base of the styloid process. To this border is attached an aponeurosis common to the flexor carpi ulnaris, extensor carpi ulnaris, and flexor profundus digitorum muscles. A noteworthy feature in connexion with this part of the shaft is the fact that it is subcutaneous, and can easily be felt beneath the skin throughout its whole length. The flexor surface corresponds to the front and inner side of the shaft. It is frequently described as consisting of two surfaces, an anterior and an internal, which are separated by a rounded anterior border (margo volaris), which extends from the tubercle above towards the styloid process below. The prominence of this ridge varies in different bones, being well marked in bones of a pronounced type, but corresponding merely to the rounding of the surfaces in poorly developed specimens. The flexor aspect of the bone affords an extensive origin to the flexor profundus digitorum muscle, which clothes its anterior and inner sides in its upper three-fourths, reaching as far back as the posterior border, and extending upwards as high as the inner side of the coronoid process. Immediately below the small sigmoid cavity there is a hollow triangular area, limited behind by the upper part of the interosseous crest, and defined in front by an oblique hne which extends downwards and backwards from the outer margin of the coronoid process. In this hollow the bicipital tubercle of the radius rests when the forearm is in the prone position, and to its floor are attached the fibres of origin of the supinator radii brevis muscle. The lower fourth of the shaft is crossed by the fibres of the pronator quadratus muscle, which derives its origin from a more or less weU-defined crest, which winds spirally downwards and backwards towards the front of the root of the styloid yjrocess, and is continuous above with the so-called anterior border. The extensor aspect of the shaft lies posteriorly between the posterior border and the interosseous crest. At its upper part it is placed behind the great and small sigmoid cavities, extending on to the outer side of the olecranon. Here an area corresponding to the upper third of the length of the bone is marked off" inferiorly by an oblique ridge which leaves the interosseous crest about an inch or more below the hinder edge of the small sigmoid cavity. Into this somewhat triangular surface the fibres of the anconeus are inserted. Below this the posterior surface is subdivided by a faint longitudinal ridge, the bone between which and the interosseous crest furnishes origins for the extensor ossis metacarpi pollicis, extensor longus pollicis, and extensor indicis muscles, in order from above downwards. The surface of bone between the posterior border and the afore-men- tioned longitudinal line is smooth and overlain by the extensor carpi ulnaris muscle. The inferior extremity of the ulna presents a rounded head (capitulum ulnie), from which, on its iiinor and ]>osterior aspect, there projects downwards a cylindrical pointed ]jroccss called the styloid process f processus styloideus). To the extremity of this latter is attaclicd the internal lateral ligament, whilst in front it has con- nected with it the antero-internal y)ortion of the capsule of the wrist-joint. The antero-extcrnal half of the circumference of the head is furnished with a smooth narrow convex articular surface, which fits into the sigmoid cavity of the radius. Uh 198 OSTEOLOGY. Dl ; ( H \NON PROC LSS Great suimoid cavity Small sigmoid ( AVITV — Bicipital hollow Tm-ebosseods U BDER Its inferior surface, fiat and semilunar in shape, and separated from the root of the styloid process by a well-marked groove, rests on the upper surface of the triangular fibro- cartilage of the wrist, the apex of which is attached to the groove just mentioned. The margins of the head in front and behind theradialarticular surface have attached to them the an- terior and posterior inferior radio - ulnar ligaments. The hinder and outer surface of the styloid process is channelled by a groove which separates it from the posterior surface of the head, and extends some little way up the posterior aspect of the lower end of the shaft. In this is lodged the tendon of the extensor carpi ulnaris muscle. The proportion- ate length of the ulna to the body height is as 1 is to 6'26-6'66. Nutrient Foramina. — A fora- men, liaving an upward direction for the nutrient artery of the shaft, opens on the anterior surface of the bone from two to three inches laelow the tuberosity. Vascular canals of large size are seen above and behind the small sigmoid cavity, just posterior to the notched external border of the great sigmoid cavity. At the lower end of the bone similar openings are seen in the groove between the styloid process and the inferior articular sur- face of the head. Connexions. — The ulna articu- lates above with the trochlea of the humerus. On the outer side it is in contact with the radius above and below, the suj)erior radio -ulnar ar- ticulation being formed by the head of the radius and the small sigmoid cavity of the ulna, the inferior radio- ulnar joint comjj rising the head of the ulna, which hts into the sigmoid cavity of the radius. Between these two joints the shafts of the bones are united by the interosseous membrane. The inferior surface of the head of the ulna does not articulate with the carjDus, but rests on the ujjjjer surface of the interposed triangular fibro - cartilage. The ulna is sujDcrficial throughout its entire extent. Su- jjeriorly the olecranon process can be readily recognised, particularly when the elbow is bent, as in this jjosition the olecranon is withdrawn from the olecranon fossa of the humerus in which it rests when the joint is ex- tended. Below this the subcutaneous triangular area on the back of the olecranon can be easily determined, and from it the posterior border of the bone can readily be traced along the line of the " ulnar furrow " to the styloid process below. With the hand supine this latter process can be felt to the inner side and slightly behind the wrist. When the hand is pronated, the lower end of the radius rolls round the lower extremity of the ulna, and the antero-external surface of the head of Interosseous BORDER Fig. 141.— The Radius and Ulna as seen from the Fbont. THE RADIUS. ]99 the latter hone now forms a well-marked projection on the Lack of the wrist in line witli the cleft between tlie little and ring fingers. Architecture, — The weakest parts of the bone are the constricted portion of the great sigmoid cavity, and the shaft in its lower third, the bone being most lialjle to fracture at these points. On section the medullary cavity is seen to extend upwards as high as the base of the coronoid process ; inferiorly it reaches as low as the upper end of the lower fifth of the bone. The walls of the shaft, which are formed of dense bone, are much thicker posteriorly than anteriorly. Above they are continuous with the front of the coronoid process and the back of the olecranon, where they are composed of layers of looser texture, which, however, gradually become thinner as the points of these processes are reached. Inferiorly they gradually taper until the head and styloid j)rocess are reached, round which they form a thin shell, considerably thickened, however, in the region of the groove for the extensor carpi ulnaris muscle. The bulk of the upper extremity is formed of loose cellular bone, arranged in a series of arcades, stretching from the anterior to the posterior wall over the upper end of the medullary canal. Above the constricted part of the great sigmoid cavity the bone displays a difl'erent structure ; here it is formed of spongy bone, of closer texture, arranged generally in lines radial to the articular surface. At the point of constriction of the great sigmoid cavity the layer immediately subjacent is much denser and more compact. The lower fifth of the bone is formed of loose spongy bone, the fibres of which have a general longitudinal arrangement ; towards its extremity the meshes become smaller. Variations. — Cases of partial or complete absence of the ulna through congenital defect have been recorded. Eosenmliller has described a case in which the olecranon was separated from the upper end of the bone, resembling thus in some respects the patella. In powerfully developed bones there is a tendency to the formation of a sharp projecting crest corresponding to the inser- tion of the triceps. Ossification. — The ulna is ossified from one primary and two or more secondary centres. The centre for the shaft appears Fuses with shaft about 16 years Appears about 10 years early in the second month of foetal life. At birth the shaft and a considerable part of the upper extremity, including the coronoid process, are ossified, as well as part of the lower extremity. The olecranon process and the inferior surface of the head and the styloid process are cartilaginous. About ten years of age a secondary centre appears in the cartilage at the upper end of the bone, and forms an epiphysis which unites with the shaft about sixteen. A scale-like centre appears in the cartilage of the head about the sixth year, from which the under surface of that part of the bone is developed, and by the extension of which the styloid process is also ossified ; this epiphysis does not unite with the shaft till the twentieth or twenty -third year. Independent centres for the styloid process and for the extreme edge of the olecranon have also been described. The student may here be warned that the epiphysial line between the shaft and superior or olecranon epiphysis does not correspond to the constricted part of the great sigmoid cavity, but lies considerably above it. Appears about G years Puses with shaft 20-23 years At Birth. About 12 years. About 16 years. Fig. 142. — The Ossification op the Ulna. The Eadius. The radius, or outer bone of the forearm, is shorter than the ulna, with which it is united on the inner side. Superiorly it articulates with the humerus, and below suyjports the carpus. It consists of a head, a neck, a tubercle, a shaft, and an expanded lower extremity. The shaft is narrow above, but increases in all its diameters below. Upper Extremity. — The head fcapitulum) is disc-shaped and provided with a sliallovv conciavc surface ffovea capital] radii) superiorly for articulation with the ca])itelhim of the humerus. I'he circumference of the head (circumfenmtia articu- laris) is smooth and is embraced by the orbicular ligament. On the inner side it is usually much broader, and displays an articular surface, plane from above down- 14 c 200 OSTEOLOG-Y. Subcutaneous SURFACE Head Neck INTEEOSSEOUS— BORDER Post, oblique LINE wards, which rolls within the small sigmoid cavity of the ulna in the movements of pronation and supination. The character of the outer half of the circumference Olecranon diffcrs froui the inner in being narrower, and rounded from above downwards. The neck (collum radii) is the constricted part of the shaft which supports the head, the overhang of the latter being greatest towards the outer and posterior side. Below the neck, on the inner side, there is an outstanding oval prominence, the bi- cipital tuberosity (tuber- ositas radii). The posterior part of this is rough for the insertion of the biceps tendon, whilst the anterior half is smooth and covered by a bursa which intervenes between it and the tendon. The shaft (corpus radii), which has an outward curve and is narrow above and broad below, is wedge-shaped on section. The edge of the wedge forms the sharp inner interosseous margin of the bone (crista interossea), whilst its base corresponds to the thick and rounded outer border over which the anterior or flexor surface becomes cohflaent with the posterior or extensor surface. The internal or interos- seous border, faint above where it lies in line with the posterior border of the bicipital tubercle, becomes sharp and prominent in the middle third of the bone. Below this it splits into two faint lines, which lead to either side of the sigmoid cavity on the lower eud of the bone, thus including between them a narrow triangular area into which the deeper fibres of the pronator radii quadratus muscle are inserted. To this border, as well as to the posterior of the two Groove for ext. . carpi ulnaris Ext. ossis metacarp. poll. Ext. brevis poll. Ext. carpi rad. ongior Ext, carpi rad. brevior Styloid pboc. Fig. 143. — The Radius and Ulna as seen from Behind. divergent lilies, the interos- seous membrane is attached. The external border (oftentimes described as the external surface) is thick and rounded above, but becomes thinner and more prominent below, where it merges THE EADIUS. 201 with the base of the styloid process. About its middle the anterior and posterior obli(|ue Hnes become contlueut with it, and here, placed between them, is a rough elongated impression which marks the insertion of the pronator radii teres muscle. Above this, and on the outer surface of the neck, the supinator radii brevis muscle is inserted, whilst inferiorly this border is overlain by the tendons of the brachio- radialis and the extensor carpi radialis longior and brevior muscles. The anterior or flexor surface (facies volaris) is crossed obliquely by a line which runs from the bicipital tubercle above, downwards, and outw^ards towards the middle of the outer border of the shaft. This, oftentimes called the anterior oblique line, serves for the attachment of the radial head of origin of the flexor sublimis digitorum muscle. Above it, the front of the bone has the fibres of the supinator radii brevis muscle inserted into it, whilst below and internal to it, extending as far down as the inferior limit of the middle third of the bone, is an extensive surface for the origin of the flexor longus poUicis muscle. In the lower fourth of the bone, where the shaft is broad and flat in front, there is a surface for the insertion of the pronator quadratus muscle which also extends back to tlie inter- osseous ridge. The extensor or posterior surface (facies dorsalis) is also crossed by an oblique line, less distinct than the anterior. This serves to define the superior limit of the origin of the extensor ossis metacarpi pollicis muscle. Above this, the back of the neck and upper part of the shaft is overlain by the fibres of the supinator radii brevis, which become attached to this surface of the bone in its outer half. Below the posterior oblique line the posterior surface in the upper part of its inner half gives origin to the extensor ossis metacarpi pollicis, and the extensor brevis pollicis muscles in order from above downwards. The lower extremity, which tends to be turned sKghtly forward when viewed from below, has a somewhat triangular form. Its inferior carpal articular surface, concave from before backwards, and slightly so from side to side, is divided into two facets by a slight antero-posterior ridge, best marked at its extremities where the anterior and posterior margins are notched ; the external of these areas, of tri- angular shape, is for articulation with the scaphoid, whilst the inner, quadrilateral in form, is for the semilunar bone. The anterior border, prominent and turned forward, is rough at its edge, where it serves for the attachment of the anterior part of the capsule of the wrist joint. The posterior border is rough, rounded, and tubercular, and is grooved by many tendons ; of these grooves the best marked is one which passes obliquely across its posterior surface. This is for the tendon of the extensor longus pollicis muscle. The outer Up of this groove is often very prominent, and forms an outstanding tubercle. To the ulnar side of this oblique groove there is a broad shallow furrow in which the tendons of the extensor communis digitorum and extensor indicis muscles are lodged, whilst to its outer side and between it and the styloid process, there is another broad groove, subdivided by a faint ridge into two, for the passage of the tendons of the extensor carpi radialis brevior and the extensor carpi radialis longior in that order from within outwards. The styloid process (pro- cessus styloideus) lies to the outer side of the inferior extremity ; broad at its base, it becomes narrow and pointed below where by its inner cartilage-covered surface it forms the summit of the inferior triangular articular area. The outer surface of this process is crossed obliquely from above downwards and forwards by a shallow groove, the anterior lip of which is sharp and well marked, and serves to separate it from the anterior surface of the bone, whilst the posterior lip is often emphasised by a small tubercle above. The tendon of the brachio-radialis muscle is inserted into the upper part of either lip, and also spreads out on to the floor of the groove, whilst the tendons of the extensor ossis metacarpi pollicis and the extensor brevis pollicis muscles lie within the groove. To the tip of the styloid process is attached the external laUsral ligament of the wrist. On the inner side of the lower extremity is yjlaccd the sigmoid cavity Hncisura idnaris) for the reception of the head of the ulna. Concave frnm befcjrc backwards, and yjlane from above downwards, it forms by its inferior margin a rectangular edge which separates it from the inferior carpal surface. To this edge the base of the tri.ingular fibro-cartilage is attached, a structure which serves to separate the inferior articular surface of the head of 202 OSTEOLOGY. the ulna from the carpus. The anterior and posterior edges of the sigmoid cavity, more or less prominent, serve for the attachment of ligaments. The proportionate length of the radius to the body height is as 1 is to 6-70-7-11. Nutrient Foramina. — Tlie openings of several small nutrient canals may be seen in the region of the neck. That for the shaft, which has an uj^ward direction, is usually placed on the front of the l)one, internal to the anterior oblique line, and from an inch and a half to two inches below the bicipital tubercle. The back of the lower e.xtremity of the bone is pierced by many small foramina. Connexions. — The radius articulates with the capitellum of the humerus in the flexed position of the elbow, with the ulna to its inner side by the superior and inferior radio-ulnar joints, and with the scaphoid and semilunar ])ones of the carjjus below. AboA^e, the head of the bone can be felt in the intermuscular depression on the outer side of the back of the elbow ; here the bone is only covered by the skin, superficial fascia, and the thin common tendinous origin of the extensor muscles, as well as the ligaments which support it. Its jwsition can best be ascer- tained by pronating and supinating the bones of the forearm, when the head will be felt rotating beneath the finger. The lower end of the bone is overlain in front and behind by the flexor and extensor tendons, but its general form can be readily made out. The styloid process lying to the outer side of the wrist in line with the extended thumb can easily be recognised ; note that it reaches a lower level than the corresponding process of the ulna. The outer border of the lower third of the shaft can be distinctly felt, as here the bone is only overlain by tendons. Architecture. — The neck is the narrowest part of the bone ; here fracture may occur, though not commonly. The point at which the bone is usually broken is about one inch above the lower extremity. This is accounted for by the fact that the radius sujjports the hand at the radio-car j)al articulation, and the shocks to which the latter is subjected, as in endeavouring to save oneself from falling, are naturally transmitted to the radius. On section, the medullary canal is seen to extend as high as the neck ; below, it reaches to the level of the inferior fifth of the bone. Its walls are thick as compared with the diameters of the bone, particularly along the interosseous border, thus imparting rigidity to the curve of the shaft ; these walls thin out above and below. Superiorly, the surface of the bicipital tubercle is formed of a thin shell of bone, which, however, thickens again where it passes on to the neck. The upper extremity is formed of spongy bone arranged in the form of arcades, reaching below the level of the bicipital tubercle internally, but not extending below the level of the neck ex- ternally. Beneath the capitellar articular surface there is a dense layer, thickest in the centre, and thinning towards the circumference ; this is overlain by a very thin layer of less compact bone. The inferior fifth of the shaft and lower extremity are formed of loose sjaongy bone ^ arranged more or less longitudinally. Ini- Z A i?„ „ -fi i,„ff TQ on ,<.„ mediately subjacent to the carpal articular 2^ rt Appears Fuses with shaft 18-20 years ,. ,'',,■•' ■ ^ , i t about 5-7 I suriace the tissue is more coinjDact, and dis- j)lays a striation parallel to the articular plane. The nutrient canal of the ^haft j^ierces the anterior wall of the upjaer jDart of the medul- lary cavity obliquely li'om below uj)wards for the space of half an inch. Variations. — Cases of congenital absence of the radius are recorded ; in these the thumb is not infrequently wanting as well. Ossification. — The centre for the shaft makes its appearance early in the second month of intrauterine life. At birth the shaft is well formed ; its upper and lower extremities are capped with cartilage, and the bicipital tubercle is beginning to appear. A secondary centre appears in the cartilage of the lower extremity about the second or third year ; this does not unite with the shaft until the twentieth or twenty-fifth year, somewliat earlier in the female. From this the carpal and ulnar articular surfaces are formed. The centre for the head appears from the fifth to the seventh year, and fuses with the neck about the age of eighteen or twenty. It forms the capitellar articular surface and combines with the neck to form the area for articulation with the small sigmoid cavity of the ulna. A scale-like epiphysis capping the summit of the bicipital tubercle has been described ; this appears about the fourteenth or fifteenth year, and rapidly fuses with that process. Appears about 2-3 years Unites witli shaft 20-25 years At Birth. About 12 years. About 16 years. Fig. 144. — The Ossification of the Kadios. THE BONES OF THE HAND. 203 THE BONES OF THE HAND. The bones of the hand, twenty-seven in number, may be conveniently divided into three groups : — (1) The bones of the wrist or carjjus — eight in number. (2) The bones of the palm or metacarpus — five in number. (3) The bones of the fingers and thumb or ^^Yidl&ngen— fourteen in number. U^CIIOEM Cuneiform Pl'^IFOPM The Carpus. The carpal bones (ossa carpi) are arranged in two rows : the first, or proximal row, comprises from without inwards, the scaphoid (os naviculare), semi- lunar (os lunatum), cuneiform (os tri- quetrum), and pisi- form (os pisiforme) ; the second or distal row includes the trapezium (os mul- tangulum majus), trapezoid (os mul- tangulum minus), OS magnum (os capi- tatum), and unciform sesamoid bones (os hamatum). Ir- regularly six-sided, each of these bones possesses non- ar- ticular palmar and dorsal surfaces. In addition, the mar- ginal bones are non- articular along their ulnar and radial aspects according as they form the inner or outer members of the series. I. Metacarpal - , — ^ Metacarpal Via. 14.'». — 'J'he Bones of the Right Wrist and Hand as seen from THE Front. Scaphoid Bone Tos naviculare;. — This is the largest as well as the outermost Ijoric of trio first row. Its 'palmar surface, rough for the attachment of ligaments, is irregularly triangular. 'I'he inrcrior external angle forms a projection called 204 OSTEOLOG-Y. Os MAGNUM Semilunar Scaphoid Trapezoid 1r VPEZIUM Cuneiform Pisiform . Mft^carpai. V. Metacarpal radial above below. the tuberosity ; this can be felt at the base of the root of the thumb. Its swperior surface is convex from side to side and before backwards for articulation with the radius. This area extends considerably over the posterior surface of the bone. Its inferior surface is convex from before backwards, and extends on to the dorsal aspect of the bone, slightly convex from side to side ; it is divisible into two areas, the outer for articulation with the trapezium, the inner for the trapezoid. The outer surface is narrow and rounded and forms a non - articular border, which extends from the articular surface to the tuberosity The inner surface is hollowed out in front for articulation with the head of the os magnum. Above this it displays a small semilunar - shaped facet for the semilunar bone. The dorsal non- artictdar surface lies be- tween the radial articular surface above and the surface for the trapezium and trapezoid below. It is obliquely grooved for the attachment of the posterior ligaments of the wrist. The scaphoid artic- ulates with five bones — the radius, the semilunar, the OS magnum, the trape- zoid, and the trapezium. Semilunar Bone (os lunatum). — So called from its deeply excavated form, the semilunar bone lies between the scaphoid on the outer side and the cuneiform on the inner. Its 'palmar surface, of rhombic form and con- siderable size, is rough for the attachment of liga- ments ; its superior sur- face, convex from side to side and from before backwards, articulates with the radius and in part with the under surface of the triangular fibro-cartilage of the wrist. Its inferior aspect, deeply hollowed from before backwards, is divided into two articular areas, of which the outer is the larger ; this is for the head of the os magnum ; the inner, narrow from side to side, articulates with the unciform. Its external surface, crescentic in shape, serves for articulation with the scaphoid, and also for the attachment of the interosseous ligaments which connect it with that bone. Its inner surface, of quadrilateral form, is cartilage-covered for articulation with the cuneiform, and the edge which separates this from the superior surface has attached to it the interosseous ligament which unites these two bones. The rough dorsal non- articular surface is much smaller than the palmar ; by this means the front and back of the bone can readily be determined. The semilunar articulates Second PHALANX Third PHALANX Fig. 146.- -The Bones of the Right Wrist and Hand as seen FROM Behind. THE CAEPUS. 20r Trape^iium Racliub Radius Fig. 147. — The Right Scaphoid Bone. Note. — The bone is represented in the centre of the figure in the position which it occupies in the hand viewed from the front. The views on either side, and above and below, represent respectively the corresponding surfaces of the bone turned towards the spectator. with five bones — the scaphoid, the radius, the cuneiform, the unciform, and tlie os magnum. Cuneiform or Pyramidal Bone (os triquetrum). — This bone may be recognised by the small oval or circular facet on its anterior surface for the pisiform. This is placed towards the lower part of the ])almar surface, which is elsewhere rough for ligaments. The bone is placed obliquely, so that its surfaces cannot be ac- curately described as inferior, superior, etc. ; but for convenience of description, the method already adopted is adhered to. The sitperior surface has a convex rhombic sur- face for articulation with the under surface of the triangular fibre - cartilage in adduction of the hand, though ordinarily it does not ap- pear to be in contact with that structure. To the ulnar side of this it is rough for ligaments. The inferior surface is elongated and concavo-convex from without inwards ; here the bone articulates with the unciform. The external surface, broader in front than behind, articulates with the semilunar. The inner surface, rounded and rough, is confluent above and behind with the superior and dorsal aspects of the bone. The dorsal surface, rounded and smooth externally, is ridged and grooved internally for the attachment of ligaments. The cuneiform articulates with three bones, viz. the pisiform, the unciform, and the semilunar. Pisiform bone (os pisiforme). — About the size and shape of a large pea, the pisiform bone rests on the anterior surface of the fore end of the cuneiform, with which it articulates by an oval or circular facet on its dorsal aspect. The rounded mass of the rest of the bone is non-articular, and inclines downwards and outwards so as to overhang the articular facet in front and externally. The mass of the bone is usually separated ^,o ,„ „ ,. , from the articular surface by a 148.— 'Jhe Right Semilunar Bone. nii.Ti.-i. t , small but distinct groove. Into Note.— The bone is represented in the centre of the figure in the the summit of the bone the f™nl'°"n/f views I'TitT r"«n" ^'f V"^"^ ?T/^' tendon of the flexor carpi ulnaris iront. ine views on either side, and above and below, , . . , J^, represent respectively the corresponding surfaces of the bone lIllJ''^Cle IS inserted, and here also turned towards the spectator. the anterior annular ligament is attached. Trapezium ^os multanguliim majus).— The trapezium is the outermost l)ono of the secon.J i-.AV u\- the carpus. It may be readily recognised by the oval saddle-shaped facet on its inferior surface for articulation with the metacarpal bone of the thumb. Os iiia -shaped part to the inner side, and in front and below, forms the pubis. The two latter portions of the bone enclose between them a large aperture of irregular outline, called the thyroid or obturator foramen (foramen obturatorum), which is placed in front and below, and to the inner side of the acetabulum. The ilium, almost a quadrant in form, consists of an expanded plate of bone, having a curved superior border, the iliac crest (crista iliaca). Viewed from the side, this forms a curve corresponding to the circumference of the circle of which the bone is the quadrant ; viewed from above, however, it will be seen to display a double bend — convex anteriorly and externally, and concave posteriorly and externally. The iliac crest is stout and thick, and for descriptive purposes is divided into an outer lip (labium externum), an inner lip (labium internum), and an intermediate surface (linea intermedia) which is broad behind, narrowest about its middle, and wider again in front. About 2| inches from the anterior extremity of the crest the outer lip is usually markedly prominent and forms a projecting tubercle, which can readily be felt in the living. Attached to these surfaces and lips anteriorly are the muscles of the flank, whilst from them posteriorly the latissimus dorsi, quadratus lumborum, and erector spinse muscles derive their origins. The crest ends in front in a pointed process, the anterior superior iliac spine (spina iliaca anterior superior). To this the outer extremity of Poupart's ligament is attached, as well as the sartorius muscle, which also arises from the edge of bone immediately below it, whilst from the same process and from the anterior end of the outer lip of the iliac crest externally the tensor fasciae femoris muscle takes origin. The anterior border of the ilium stretches from the anterior superior iliac spine to the margin of the acetabulum below. Above, it is thin ; but below, it forms a thick tubercular process, the anterior inferior iliac spine (spina iliaca anterior in- ferior). From this the rectus femoris muscle arises, whilst the stout fibres of the ilio-femoral ligament of the hip-joint are attached to it immediately above the acetabular margin. Posteriorly, the crest terminates in the posterior superior iliac spine (spina iliaca posterior superior). Below this, the posterior border of the bone is sharp and irregularly notched terminating in a prominent angle, the posterior THE INNOMINATE BONE. 215 inferior iliac spine (spina iliaca posterior inferior), in front of which the edge of the bone becomes thick and rounded, and forms a wide notch which sweeps forwards and downwards to join the mass of bone behind the acetabuhim, where it becomes fused with the ischium ; this is called the ilio-sciatic or great sciatic notch (incisura ischiadica major). The ilium has two surfaces, an inner and an outer. The external surface is THF, ILTTIM Middle gluteal linf Posterior gluteal line' Posterior SUPEllIOR spine Posterior inferior spine Groove for tendon of obturatui extern us- Ischial spine Small sciatic notlh Anterior SUPERIOR spine Inferior gluteal line Interior inferior spine Acetabulum Ilio-pectineal eminence [schial tuberosity Superior ramus -of pubis Pubic spine ^ Crest of pubis Body of pubis- jNtERIOK RAMUb OF PUBIS Ramus of Ischium Fio. 163. — The RifiHT Innominate Bone as seen from the Outer Side. divided into two parts, viz. a lower acetabular, and an upper gluteal part. The lower frjrms a little less tiian the upper two-fifths of the acetabular hollow, and is separated from the larger gluteal surface above by the upper prominent margin of the articular cavity. TIk; gluteal surface, broa,d and expand(!d, is concavo-convex from behind forward. It is tni,vcrs(!d hy three rough curved lines, well seen in strongly develop(;d hon(;H, hut oi'ten faint and indistinct in feebly marked speci- mens. Of these the inferior curved line (linea glut;ea inferior) curves backwards from a point immediately above the anterior inferior spine towards the ilio-sciatic 15 & 216 OSTEOLOGY. notch posteriorly ; the bone between this and the acetabular margin is marked by a rough shallow groove, from which the reflected head of the rectus femoris muscle arises. The middle curved line (linea glutsea anterior) commences at the crest of the ilium, alxjut one inch and a half behind the anterior superior iliac spine, and Crest of the ilium For posterior sacro-iliac lioamknt Anterior superior- SPINE Small sciatic notch Symphysis pubis PUBIS ISCHIUM Ischial tuberosity Inferior ramus of pubis Ramus of Ischium Fig. 164. — The Right Innominate Bone (Inner Aspect). sweeps backwards and downwards towards the upper and posterior part of the ilio- sciatic notch. The surface between this line and the preceding furnishes an exten- sive origin for the gluteus minimus muscle. The posterior or superior curved line (linea glutsea posterior) leaves the iliac crest about two and a half inches in front of the posterior superior iliac spine, and bends downwards and slightly forwards in a direction anterior to the posterior inferior spine. The area between this and the middle curved line is for the origin of the gluteus medius muscle, whilst the rough THE INNOMINATE BONE. 217 surface immediately above and behind it is for some of the fibres of origin of the gluteus maximus muscle. The inner surface of the ilium is divided into two areas which present very characteristic differences. The posterior or sacral part, which is rough, displays in front a somewhat smooth, auricular surface (facies auricularis) which is cartilage- coated in the recent condition, and articulates with the sacrum. Above and behind this there is an elevated irregular area, the tuberosity (tuberositas iliaca), which is here and there deeply pitted for the attachment of the strong posterior sacro-iliac ligaments. Above this the bone becomes confluent with the inner lip of the iliac crest, and here it affords an origin to ^he erector spinse and multifidus spinse diiuscles. The anterior part of the inner aspect of the bone is smooth and exten- sive ; it is subdivided by an oblique ridge, called the ilio- pectineal line (linea arcuata), which passes forwards and downwards from the most prominent point of the auricular surface towards the inner side of the ilio-pectineal eminence which is placed just above and in front of the acetabulum, and marks the fusion of the ilium with the pubis. Above this the bone forms the shallow iliac fossa (fossa iliaca), from the floor of which the iliacus muscle arises, whilst leading from the fossa, below and in front, there is a shallow furrow, passing over the superior acetabular margin, between the anterior inferior iliac spine on the outer side and the ilio-pectineal eminence internally, for. the lodgment of the tendinous and fleshy part of the ilio-psoas muscle. If held up to the light the floor of the deepest part of the iliac fossa will be seen to be formed of but a thin layer of bone. A nutrient foramen of large size is seen piercing the bone towards the hinder part of the fossa. Below and behind the ilio-pectineal line the inner surface of the ilium forms a small portion of the wall of the true pelvis ; the bone here is smooth, and rounded off posteriorly into the ilio-sciatic notch, where it becomes confluent with the inner aspect of the ischium. Just anterior to the ilio-sciatic notch there are usually the openings of one or two large vascular foramina. From this surface arise some of the posterior fibres of the obturator internus muscle. The ischium constitutes the lower and hinder part of the innominate bone. Superiorly its body (corpus) forms somewhat more than the inferior two-fifths of the acetabulum together with the bone supporting it behind and within. Below this, the superior ramus passes downwards and backwards as a stout three-sided piece of bone, from the inferior extremity of which a compressed bar of bone, called the inferior ramus, extends forwards at an acute angle. This latter unites in front and above with the descending ramus of the pubis, and encloses the aperture called the obturator foramen. Superiorly, and on the outer aspect of the ischium, the acetabular surface is separated from the bone below by a sharp and prominent margin, which is, however, deficient in front, where it corresponds to the cotyloid notch (incisura acetabuli) leading into the articular hollow ; the floor of this notch is entirely formed by the ischium. Below the prominent acetabular margin there is a well- marked groove in which the tendon of the obturator externus lies. Beneath this tlie autero-external surface of the superior and inferior rami furnish surfaces for the attachments of the obturator externus, quadratus femoris, and adductor magnus muscles. The postero-external surface of the ischium forms the convex surface on the back of the acetabulum. The inner border of this is sharp and well defined, and is confluent above with the border of the ilium, which sweeps round the great or ilio-sciatic notch. From this border, on a level with the lower edge of the acetabulum, there springs a pointed process, the spine (spina ischiadica), to which is attached the lesser sacro-sciatic ligament and the superior gemellus muscle. Inferior to this, the postero-external surface narrows rapidly, its inner border just below the spine being hollowed out to form the small sciatic notch, (incisura ischiadica minor). The lower part of this surface and the angle formed l)y the two rami are capped by an irregularly rough pyriform mass called tlu; tuberosity (tuber ischii). This is divided by an oblique ridge into tw(j an^as, the iqipttr and outer for the tendon of origin of tlie, seiuimeinl^raiiosus muscle,, the lower and inncsr i'or the conj(jincd lu^ads of the biceps and semitendinosus muscles. Its prominent inner lip serves for the attachment of the great sacro-sciatic ligament, whilst its outer edge furnishes an 218 OSTEOLOGY. origin for the quadratus feinoris muscle ; in front and below, the adductor magnus muscle is attached to it. The inner surface of the body and superior ramus of the ischium form in part the wall of the true pelvis. Smooth and slightly concave from Ijefore backwards, and nearly plane i'rom above downwards, it is widest opposite the level of the ischial spine. Below this, its posterior edge is rounded and forms a groove leading to the small sciatic notch, along and over which the tendon of the obturator internus passes. To part of this surface the fibres of the obturator internus are attached, whilst the inner aspect of the spine supplies points of origin for the coccygeus and levator ani muscles, as well as furnishing^fin attachment to the " wiiite line " of the pelvic fascia. The inner surface of the inferior ramus of the ischium is smooth^ and so rounded that its inferior edge tends to be everted. To this, as well as to its margin, is attached the crus penis, together with the ischio-cavernosus, obturator internus, transversus perinei, and compressor urethrse muscles. In the female, structures in correspondence with these are also found. The fore-part of the innominate bone is formed by the pubis ; it is by means of the union of this bone with its fellow of the opposite side that the pelvic girdle is completed in front. The pubis (os pubis) consists of two rami — a superior, ascending, or horizontal (ramus superior ossis pubis), and an inferior or descending (ramus inferior ossis puljis). The broad part of the bone formed by the fusion of these two rami is the body. The body is sometimes described as that part of the bone which enters into the formation of tlie acetabulum, but the English nomenclature has here been followed. The body of the pubis has two surfaces. Of these the inner or posterior is smooth, and forms the fore-part of the wall of the true pelvis ; hereto are attached the levator ani muscle and puboprostatic ligaments. The anterior or external surface is rougher, and furnishes origins for the gracilis, adductor longus, adductor brevis, and some of the fibres of the obturator externus muscles. The inner border is provided with an elongated oval cartilage -covered surface by means of which it is united to its fellow of the opposite side, the joint being called the symphysis pubis. The upper border, thick and rounded, projects somewhat, so as to overhang the anterior surface. It is called the crest. Internally this forms with the inner border or symphysis the angle, whilst externally it terminates in a pointed process, the spine (tuberculum pubicum). From the crest arise the rectus abdominis and pyramidalis muscles, and to the spine is attached the inner end of Poupart's ligament. Passing upwards and outwards from the outer side of the body towards the acetabulum, of which it forms about the anterior fifth, is the superior ramus (ramus superior). This has three surfaces : an antero-superior, an antero- inferior, and an internal or posterior. The antero-superior surface is triangular in form. Its apex corresponds to the pubic spine ; its anterior inferior border to the obturator crest (crista obturatoria), leading from the spine to the upper border of the cotyloid notch ; whilst its sharp postero-superior border trends upwards and outwards from the spine, and is continuous with the iliac portion of the ilio-pectineal line just internal to the ilio-pectineal eminence, forming as it passes along the superior ramus the pubic portion of that same line (pecten ossis pubis). On this line, just within the ilio-pectineal eminence, there is often a short, sharp crest which marks the insertion of the psoas parvus. The base of the triangle corresponds to the ilio-pectineal eminence above and the upper margin of the cotyloid notch below. Slightly hollow from side to side, and convex from before backwards, this surface provides an origin for, and is in part overlain by, the pectineus muscle. The internal or posterior surface of the superior ramus is smooth, concave from side to side, and slightly rounded from above downwards ; by its sharp inferior curved border it completes the thyroid foramen, as seen from behind. The antero-inferior surface forms the roof of the broad obturator groove (sulcus obtaratorius) which passes obliquely downwards and forwards between the lower margin of the antero-superior surface in front and the inferior sharp border of the internal surface behind. The inferior or descending ramus of the pubis (ramus inferior) passes downwards and outwards from the lower part of the body. THE INNOMINATE BONE. 219 Flattened and compressed, it unites with the inferior ramus of the ischium, and thus encloses the thyroid foramen, whilst in correspondence with its fellow of the opposite side it completes the formation of the pubic arch. Anteriorly it furnishes origins for the gracilis, adductor brevis, and adductor magnus muscles, as well as some of the fibres of the obturator externus muscle. Its inner surface is smooth, whilst its lower border, rounded or more or less everted, has attached to it the fore-part of the crus penis and the subpubic ligament. The acetabulum or cotyloid cavity is the nearly circular hollow in which the head of the thigh bone fits. As has been already stated, it is formed by the fusion of the ilium and ischium and pubis in the following proportions : the ilium a little less than two-fifths, the ischium somewhat more than two-fifths, the pubis con- stituting the remaining one-fifth. It is so placed as to be directed downwards, outwards, and forwards, and is surrounded by a prominent margin, to which the capsule and cotyloid ligament of the hip - joint are attached. Opposite the obturator foramen this margin is interrupted by the cotyloid notch (incisura acetabuli) ; immediately external to the ilio- pectineal eminence the margin is slightly hollowed, whilst occasionally there is a feeble notching of the border above and behind. These irregularities in the outline of the margin correspond to the lines of fusion of the ilium and pubis, and the ilium and ischium respectively. The floor of the acetabulum is furnished with a horseshoe-shaped articular surface, which lines the circumference of the hollow, except in front, where it is interrupted by the cotyloid notch. It is broad above ; narrower in front and below. Within this articular surface there is a more or less circular rough area (fossa acetabuli) continuous in front and below with the floor of the cotyloid notch. This, some- what depressed below the surface of the articular area, lodges a quantity of fat, and provides accommodation for the interarticular ligament of the joint. As may be seen by holding the bone up to the light, the floor of this part of the acetabulum is not usually of great thickness. The major part of the non-articular area is formed by the ischium, which also forms the floor of the cotyloid notch. . The thyroid or obturator foramen (foramen obturatum) Hes in front of, below, and internal to the acetabulum. The margins of this opening, which are formed in front and above by the pubis, and behind and below by the ischium, are sharp and thin, except above, where the under surface of the superior ramus of the pubis is channelled by the obturator groove. Below, and on either side of this groove, two tubercles can usually be seen. The one, situated on the edge of the ischium, just in front of the cotyloid notch, is named the posterior obturator tubercle (tuberculum obturatorium posterius) ; the other, placed on the lower border of the inner surface of the superior ramus of the pubis, is called the anterior obturator tubercle (tuberculum obturatorium anterius). Between these two tubercles there passes a ligamentous band, which converts the groove into a canal along which the obturator vessels and nerve pass. Elsewhere in the fresh condition the obturator or thyroid membrane stretches across the opening from margin to margin. The form of the foramen varies much, being oval in some specimens, in others more nearly triangular ; its relative width in the female is greater than the male. Nutrient foramina for the ilium are seen on the floor of the iliac fossa, just in front of the sacro-auricular surface ; on the pelvic aspect of the bone, close to the great sciatic notch ; and on the gluteal surface externally, near the centre of the middle curved line. For the ischiiim, on its pelvic surface, and also externally on the groove below the acetabulum. For the pubis, on the surface of the body, and deeply also from the acetabular fossa. Connexions. — The innominate bone articulates with the sacrum Ijehind, with the femur to tlie outer side and below, and with its fellow of the opposite side internally and in front. Each of its thi'ee parts comes into direct relation with the surface. Above, the iliac crest assists in forming the iliac furrow, which serves to separate the region of the flank from that of the buttock. In front, the anterior superior iliac spine forms a definite landmark ; whilst behind, the posterior superior iliac spines will be found to correspond with dimples situated on either side of the middle line of the root of the back. The symphysis, thcs crest, and spine of the jjubis can all be distinguished in front, though overlain by a considerable quantity of fat, whilst the position of the tuberosities of the; ischia, when uncovei'ed by the great gluteal muscles in the flexed ]josition of the thigh, can readily be ascertained. Tn the perineal region the outline of the pubic and ischial rami can easily be determined by digital examination. 220 OSTEOLOGY. Appears about later end of 2nd m. o1' foetal life Architecture.— As a flat bone tlie os innominatitm consists of spongy tissue between two compact external layers. These latter vary nuicli in thickness, being exceptionally stout along the ilio-pectinal line and the floor of the iliac fossa innuediately above it. The gluteal aspect of the ilium is also formed by a layer of considerable thickness. The spongy tissue is loose and cellular in tlie thick part of the ilium and in the body of the ischium ; absent where the floor of the iliac fossa is formed by the coalescence of the thin dense confining layers ; fine grained and more compact in the tuberosity of the ischium, the iliac crest, and the floor of the acetabulum, in which latter situation it is striated by fibres which are directed radially to the surface of that hollow, these again being crossed at right angles by others Avhich are arranged circumferentially. This spongy tissue forms a more compact layer over the surface of the upper and back portion of the acetal)ular articular area. The bottom of the floor of the acetabulum varies in thickness ; in most cases it is thin, and in exceptional instances the bone is here deficient. The same condition has been met with in the iliac fossa, where absorption of the thin bony plate has taken place. Variations.— Some of the anomalies met with in the haunch bone are due to ossification of the ligaments connected with it ; in other cases they depend on errors of development. Failure of union between the pubic and ischial rami has also been recorded. Cases have occurred where the obturator groove has been bridged across by bone, and one case is noted of absence of the cotyloid notch on the acetabular margin. In rare cases the os acetabuli (see Ossification) remains as a separate bone. Ossification commences in the ilium about the ninth week of intrauterine life; about the fourth month a centre appears below the acetabulum for the ischium, the pubis being developed from a Appears about 15 centre which appears in years; fuses 22-25 ^ i- ,i I i i front of the acetabulum about the fifth or sixth month. At birth the form of the ilium is well defined ; the body and part of the tuberosity of the ischium are ossified, as well as the horizontal ramusand partof the body of the pubis. All three parts enter into the forma- tion of the sides of the acetabulum, and by the third year have converged to form the bottom of that hollow, ^ being separated from each other by a tri- radiate piece of cartilage, in which, about the twelfth year, independent ossific centres make their ap- pearance, which may or may not become fused with the adjacent bones. In the latter case they unite to form an independent ossicle, the OS acetabuli, which subsequently fuses with and forms the acetabular part of the pubis. By the age of sixteen the ossification of the acetabulum is usually completed, whilst the rami of the ischium and pubis commonly unite about the tenth year. Secondary centres, six in number, make their appearance about the age of puberty, and are found in the following situations : one for the anterior inferior iliac spine, one for the iliac crest and the anterior and posterior superior iliac spines, a scale-like epiphysis over the tuberosity of the ischium, a separate epiphysis for the spine of the ischium, (?) a point for the spine and another for the angle of the pubis. Fusion between these and the primary centres is usually complete between the twenty-second and twenty-fifth years. Parsons (Journ. Anat. and Physiol., vol. xxxvii. p. 315) regards the ischial epiphysis as the homologue of the hypo-ischium in reptiles, and suggests that the epiphysis over the angle of the pubis may represent the epipubic bone of marsupials. Appears about 4th m. of foetal life Appears about 15 years ; fuses 22- 25 years years Vppears bout 18 % ears Unite about 10 years At Birth. About 12 or 13 years. Fig. 165. — Ossification op the Innominate Bone. The Pelvis. The pelvis is formed by the union of the innominate bones with each other in front, and with the sacrum behind. In man the dwarfed caudal vertebrae THE PELVIS. 221 (coccygeal) are curved forwards and so encroach upon the limits of the pelvic cavity inferiorly. The pelvis is divided into two parts by the ilio-pectineal lines, which curve forwards from the upper part of the lateral masses of the sacrum behind, to the roots of the spines of the pubes in front. The part above is called the false pelvis (pelvis major), and serves by the expanded iliac fossee to support the Fig. 166. — Male Pelvis as seen from the Front. abdominal contents; the part below, the true pelvis (pelvis minor) contains the pelvic viscera, and in the female forms the bony canal through which, at full term, the fcetus is expelled. The true pelvis is bounded in front by the symphysis pubis in the middle line, and by the body and rami of the pubis on either side, laterally by the smooth inner surfaces of the ischia and ischial rami, together with a small part of the ilium Female Pei-vis as sh;h:n from thk Front. below the iliac ])ortioii of the i]io-])ectincal lino. Springing from the posterior margin of th(; ischium are the intuiried ischial spines. ]iehind, the broad curved anterior surface of the sacrum, and below it, the small and irregular coccyx, form its posterior wall. I'otweeii tlu; sides of the sacrum behind, and the ischium and ilium in front and above, t-iien; is a wide interval, culled the sacro-sciatic notch, 222 OSTEOLOaY. which is, however, bridged across in tlie recent condition by the great and small sacro-sciatic hgaments, which thus convert it into two foramina, the larger above the spine of the ischium — the great sacro-sciatic foramen, the lower and smaller below the spine, called the small sacro-sciatic foramen. The inlet (apertura pelvis superior) of the pelvis is bounded in front by the symphysis pubis, witb the body of the pubis on either side ; laterally by the ilio- pectineal lines ; and behind by the sacral prominence. The circumference of this aperture is often called the brim of the pelvis ; in the male its shape is cordate, in the female more oval. The aritero-posterior or conjugate diameter is measured from the sacro- vertebral angle to the symphysis pubis ; the oblique diameter from the sacro-iliac joint of one side to the ilio-pectineal eminence of the other ; whilst the transverse diameter is taken across the point of greatest width. The outlet (apertura pelvis inferior) is bounded anteriorly by the pubic arch (arcus pubis), formed in front and above by the bodies of the puljis, with the symphysis between them, and the inferior puljic rami below and on either side. These latter are continuous with the ischial rami which pass backwards and outwards to the ischial tuberosities, which are placed on either side of this aperture. In the middle line and behind, the tip of the coccyx projects forward, and in the recent condition the interval between this and the ischial tuberosities is bridged across by the great sacro-sciatic ligament, the inferior edge of which necessarily assists in determining the shape of the outlet. As the anterior wall of the cavity, formed by the symphysis pubis, measures from IJ to 2 inches, whilst the posterior wall, made up of the sacrum and coccyx, is from 5 to 6 inches in length, it follows that the planes of the inlet and outlet are not parallel, but placed at an angle to each other. The term axis of the pelvis is given to lines drawn at right angles to the centre of these planes. Thus, with the pelvis in its true position, when the figure is erect, the axis of the inlet corresponds to a line drawn downwards and backwards from the umbilicus towards the tip of the coccyx below, whilst the axis of the outlet is directed downwards and slightly backwards, or downwards and a little forwards, varying according to the length of the coccyx. Between these two planes the axis of the cavity, as it passes through planes of varying degrees of obliquity, describes a curve repeating pretty closely the curve of the sacrum and coccyx. Position of the Pelvis. — The position of the pelvis in the living when the figure is erect may be approximately represented by placing it so that the anterior superior iliac spines and the sj^mphysis pubis lie in the same vertical plane. Under these conditions the plane of the inlet is oblique, and forms with a horizontal line an angle of from 50° to 60°. The position of the pelvis depends upon the length of the ilio-femoral ligaments of the hip-joint, being more oblique when these are short, as usually happens in women in whom the anterior superior iliac spines tend to lie in a plane slightly in advance of that occupied by the symphysis pubis. In cases where the ilio-femoral ligament is long, a greater amount of extension of the hip-joint is permitted, and this leads to a lessening of the obliquity of the pelvis. This condition, Avhicli is more typical of men, I'esults in the anterior superior iliac spines lying in a plane slightly posterior to the plane of the sym- physis, whilst the angle formed by the plane of the inlet and the horizontal is theref)y reduced. Bearing in mind the oblique position of the pelvis, it will now^ be seen that the front of the sacrum is directed downwards more than forwards, and that the sacral pro- montory is raised as much as from 3^ to 4 inches above the upper border of the symphysis pubis, lying higher than the level of a line connecting the two anterior superior iliac spines. Fi'om the manner in which the sacrum articulates with the ilia, it will be noticed that the weight of the trunk is transmitted downwards through the thickest and strongest part of the bone (see Architecture) to the upper part of the acetabula, where these rest on the heads of the femora. Sexual Differences. — The female pelvis is lighter in its construction than that of the male ; its surfaces are smoother, and the indications of muscular attachments less marked. Its height is less and the splay of its walls not so pronounced as in the male, so that the female pelvis has been well described as a short segment of a long cone as contrasted with the male pelvis, which is a long segment of a short cone. The cavity of the true pelvis in the female is more roomy, and the ischial spines not so nmch inturned. The pubic arch is wide and rounded, and will usually admit a right-angled-set square THE PELVIS. 223 being placed within, so that the summit touches the under surface of the symphysis pubis, whilst the sides lie in contact with the ischial rami. In the male tlie arch is narrow and angular, forming an angle of from 65° to 70°. The sacro-sciatic notch in tiie female is wide and shallow. The distance from the postei'ior edge of the body of the ischium to the posterior inferior iliac spine is longer, measuring on an average 50 mm. (2 inches) in tlie female, as contrasted with 40 mm. (1-| inches) in the male. The inlet in the female is large and oval or reniform, as compared with the cribbed and heart-shaped aperture in the male. The sacro-vertebral angle is more pronounced in the female, and the obliquity of the inlet greater. The sacrum is shorter and wider. The posterior superior iliac spines lie wider apart ; the pubic crests are longer ; and the pubic spines are separated by a greater interval than in man. The outlet is larger ; the tubero- sities of the ischia ai"e farther apart; and the coccyx does not project forward so much. The curve of the sacrum is liable to veiy great individual variation. As a rule the curve is more uniform in the male, whilst in the female it tends to be flatter above and more accentuated below. There is a greater proportionate width between the acetabular hollows in the female than in the male. Of much importance from the standpoint of the obstetrician ai'e the various diameters of the true pelvis. In regard to this it is worthy of note that the plane of " greatest pelvic expansion " extends from the union between the second and third sacral vertebrae behind, to the middle of the symphysis pubis in front, its lateral boundaries on either side corresponding with the mid-point of the inner surface of the acetabulum ; whilst the plane of "least pelvic diameter" lies somewhat lower, and is defined by lines passing through the sacro-coccygeal articulation, the ischial spines, and the lower third of the symphysis pubis (Norris). Subjoined is a table showing the principal average measurements in the two sexes : — Males. Females. Maximum distance between the iliac crests Distance between the anterior superior iliac spines Distance between the last lumbar sj)ine and the front of the symj)hysis pubis ] 1| in., or 282 mm. 9| in., or 240 mm. 7 in., or 176 mm. 10| in., or 273 mm. 9| in., or 250 mm. 7| in., or 180 mm. True Pelvis. Males. Females. lulet. Outlet. Inlet. Cavity. Outlet. Greatest. Least. Antero -posterior (conju- gate) diameter Oblique diameter . Transverse diameter 4 in., or 101 mm. 4| in., or 120 mm. 5 in., or 127 mm. 3| in., or 95 mm. 3i in., or 88 mm. 3^ in., or 88 mm. 4f in., or 110 mm. 5 in., or 125 mm. 5^ in., or 135 mm. 5 in., or 127 mm. 4§ in., or 125 mm. 4f in., or 110 mm. 4| in., or 110 mm. 4i in., or 115 mm. 4| in., or 115 mm. 4§ in., or 110 mm. Growth of the Pelvis. — From the close association of the pelvic girdle with the lower limb we find tliat its growth takes place concurrently with the develojiment of that member. At birth the lower limits measure but a fourth of the entire body length ; consequently at that time the pelvis, as compared with the head and trunk, is relatively small. At this jDeriod of life the bladder in both sexes is in gi'eater part an abdominal organ, whilst in the female the uterus has not yet sunk into the true pelvic cavity, and the ovaries and Fallopian tubes rest in the iliac fo88;t. The sacro-vertebral angle, though readily recognised, is as yet but faintly marked. Coincident with the remarkaljle growth of the lower lindas and the assumjition of the erect position when the child begins to walk, striking changes take place in the form and size of the pelvis. Tlie.se consist in a greater expansion of the iliac bones necessai'ily associated with the growth of the luusclcs which control the movements of the hip, together with a marked increase in the sacro-vertel^ral angle due to the develojuuent of a forward lumbar curve ; at the same time, the weight of the trimk l)(;irig thrown on the sacrum causes the elements of that bone to sink to a lower level h(!twt;eii the inuoiiiiriate bones. The cavity of the true pelvis increases in size proportional! J', and the vi.scera afore-mentioned now begin to sink down and have assumed a po.sitiou within the pelviH by the fifth or sixth yeai'. Tin; extension of the thighs in the 224 OSTEOLOGY. Head Geeat trochanter Spiral lisk ujj light position necessarily brings about a more pronounced pelvic obliquity, whilst the stoutness and thickness of the ilium over the upper part of the acetabulum is much increased to withstand the pressure to which it is obviously subjected. Coincident with this is the gradual development of the iliac portion of the ilio-pectineal line, which serves in the adult to separate sharply the false from the true pelvis. This part of the bone is remarkably strong, as has been shown (see Architecture), and serves to transmit the body weight from the sacrum to the thigh bone. The sexual differences of the pelvis, so far as they refer to the general configuration of this I^art of the skeleton, are as jjronounced at the third or fourth month of fostal life as they are in the adult (Fehling, Ztschr. f. Geburtsh. u. Gynaek. Bd. ix. and x. ; A. Thomson, Journ. Anat. and Physiol, vol. xxxiii. p. 359). The rougher appearance of the male type is cor- related with the more powerful muscular de- velopment. The Femur. The femur or thigh bone is remark- able for its length, being the longest bone in the body. Superiorly the femora are separated by the width of the pelvis. Inferiorly they articulate with the tibia3 and patellae. In the military position of attention, with the knees close together, the shafts of the thigh bones occupy an oblique position. For descriptive purposes the bone is divided into an upper ex- tremity, comprising the head, neck, and two trochanters ; a shaft ; and a lower extremity, forming the expansions known as the condyles. The head (caput femoris) is the hemi- spherical articular surface which, when coated with cartilage, fits into the aceta- bular hollow. Its pole is directed upwards and inwards and slightly forwards. A little below the summit, and usually somewhat behind it, is a hollow oval pit (fovea capitis femoris) for the attachment of the ligamentum teres. The circum- ference of the head forms a lip with a wavy outline, more prominent above and behind than in front. The head is sup- ported by a stout compressed bar of bone, the neck (collum fem- oris), which forms with the upper end of the shaft an angle of about 125 degrees, and is di- rected upwards, inwards, and a little forwards. Its vertical width ex- ceeds its antero- pos- terior thickness. Con- stricted about its middle, it expands internally to support the head, whilst externally, where it joins the shaft, its vertical diameter is External xuBEROsiTy — Adductoh tubercle Internal tuberosity External condyle Patellar surface Internal condyle Fig. 168. — Right Femur as seen prom the Front. THE FEMUE. 221 Digital fossa Great trochanter 1 ubercle of quadratus Imertroch- AlNTERIC RIDGE Gluteai ridge Apierial foramen much increased. Anteriorly it is clearly defined from the shaft by a rough ridge which commences above on a prominence, sometimes called the tubercle of the femur, and passes obliquely downwards and inwards. head. This constitutes the upper possa fob lig. teres- part of the spiral line (linea inter - trochanterica), and serves for the attachment of the ilio-femoral ligament of the hip-joint. Posteriorly where the neck unites with the shaft, there is a full rounded ridge passing from the trochanter major above to the trochanter minor below ; this is the posterior intertrochanteric line or ridge (crista intertro- chanterica). A little above the middle of this ridge there is usually a fulness which serves to indicate the upper limit of attach- ment of the quadratus fem- oris muscle, and is called the tubercle for the quad- ratus. Externally the neck is embedded in the inner surface of the trochanter major, by which, at its upper and back part, it is to some extent overhung. Here is situated the digital fossa (fossa trochanterica), into which the tendon of the obtm^ator externus is inserted. Passing nearly horizontally across the back of the neck there is a faint groove leading into this de- pression ; in this the tendon of the obturator externus muscle lies. Inferiorly the neck becomesconfiuent with the trochanter minor be- hind, and is continuous with the inner surface of the shaft in front. The neck is pierced by many vascular canals, most numerous at the upper and back part. Some are directed upwards towards the head, whilst others pass in the direction of the trochanter major. The trochanter major is a larg(;quadi7uigiilur pi'o- cess which caps the ujii^er and outer part of the shaft, and overhiings the root of the neck above and behind. Its outer sv/rface, of rounded irregxilar form, slopes upwards 16 Ext. epicondtlic line Popliteal surface Internal, TaiiEKOSITY Intrrnal condvlk o Surface for JL attacli merit of posterior crucial ligaminit -External tuberosity —Surface for attacliment of .'uit. crucial ligament RXTEBNAL condyle In 1 1 l.( ONDYLIC NOTCH Fro. 109. — IlifjitT Kkmub as seen from Biciiind. 226 OSTEOLOGY. Great trochanter Gluteal ridge and inwards, and is separated from the external surface of the shaft below by a more or less horizontal ridge. Crossing it obliquely from the posterior superior to the anterior inferior angle is a rough line which serves for the insertion of the gluteus medius muscle ; above and below this the surface of the bone is smoother and is overlain by bursae. The Hi? ATI ^ t/ anterior surf ace, somewhat oblong in shape, and inclined obliquely from ])elow upwards and inwards, is elevated from the general aspect of the shaft below, from which it is separated in front by an oblique line leading upwards and inwards to the tubercle at the upper end of the superior part of the spiral line. This surface serves for the insertion of the gluteus minimus. The superior border is curved and elevated ; into it are inserted the tendons of the obturator internus and gemelli muscles within and in front, and the pyriformis muscle above and behind. The posterior border is thick and rounded, and forms the upper part of the posterior intertrochanteric ridge. The angle formed by the superior and posterior borders is sharp and pointed, and forms the tip of the trochanter overhanging the digital fossa, which lies im- mediately below and within its inner surface. The trochanter minor is an elevated pyramidal process situ- ated at the back of the inner and upper part of the shaft where that becomes continuous with the lower and posterior part of the neck. Confluent above with the posterior intertrochanteric ridge, it gradually fades away into the back of the shaft below. The combined tendon of the ilio-psoas is inserted into this process and the bone immediately below^ it. The shaft (corpus femoris), which is characterised by its great length, is cylin- drical in form. As viewed from the front, it is straight or but slightly curved ; as seen in profile, it is bent forwards, the curve being most pronounced in its upper part. The shaft is thinnest at some little distance above its middle ; below this it gradually increases in width to support the condyles inferiorly ; its antero-posterior diameter, however, is not much increased below. Its surfaces are generally smooth and rounded, except behind, where, running longitudinally down the centre of its curved posterior aspect, there is a rough-lipped ridge, the hnea aspera (linea aspera). ' Most s alient towards the middle of the shaft, the linea aspera consists of an inner lip (labium mediale) and an outer lip (labium laterale), with a narrow intervening rough surface. Above, about 2 to 2h inches from the trochanter minor, the linea aspera is formed by the convergence of three lines. Of these the outer is a rough, somewhat elevated, ridge, which commences above, on the back of the shaft, external to and on a level with the trochanter minor, and becomes continuous below with the outer lip of the linea aspera. This serves for the bony insertion of the gluteus maximus, and is occasionally developed into an outstanding process called the trochanter tertius. Internally the inner lip of the linea aspera is confluent above with a line which winds round the shaft upwards and forwards in front of the ■Vrterial foramen LiXEA ASPERA Fig. 170.— Back View of Upper End of Eioht Femdr. THE FEMUE. 227 trochanter minor to become continuous with the rough ridge which serves to detiue the neck from the shaft anteriorly (see ante). The whole constitutes what is known as the spiral line, and extends from the fore and upper part of the trochanter major above to the linea aspera below. Intermediate in position between the spiral line in front and internally, and the gluteal ridge externally, there is a third line, the pectineal line (linea pectinea), which passes down from the trochanter minor and fades away interiorly into the surface between the two lips of the linea aspera. Into this the pectineus muscle is inserted. About the junction of the middle with the lower third of the shaft the two lips of the linea aspera separate from one another, each passing in the direction of the condyle of the corresponding side. The lines so formed are called the inner and outer epicondylic lines respectively, and enclose between them a smooth triangular area corresponding to the back of the lower third of the shaft ; this, called the popliteal surface (planum popliteum), forms the floor of the upper part of the popliteal space. The continuity of the upper part of the internal epicondylic line is but faintly marked, being interrupted by a wide and faint groove along which the popliteal artery passes to enter the space of that name. Below, where the line ends on the upper and inner surface of the internal condyle, there is a little spur of bone called the adductor tubercle, to which the tendon of the adductor magnus is attached, and behind which the inner head of the gastrocnemius muscle takes origin. The linea aspera affords extensive linear attachments to many of the muscles of the thigh. The vastus internus arises from the spiral line above and the inner lijD of the linea aspera below. The adductor longus is inserted into the inner lip about the middle third of the length of the shaft. The adductor magnus is inserted into the intermediate part of the line, extending as high as the level of the trochanter minor, where it lies internal to the insertion of the gluteus maximus. Below, its insertioii 2:)asses on the internal epicondylic ridge, reaching as low as the adductor tubercle. The adductor brevis muscle is inserted into the linea aspera above, between the pec- tineus and adductor longus muscles internally and the adductor magnus externally. Below the insertion of the gluteus maximus the short head of the biceps arises from tlie outer lip as well as from the external epicondylic line ; in front these also serve for the origin of the vastus externus muscle. The canals for the nutrient arteries of the shaft, which have an upward direction, are usually two in number, and are placed on or near the linea aspera — the upper one about the level of the junction of the middle and upper third of the bone, the lower some three or four inches below — usually on the inner side of the shaft, immediately in front of the inner lip of the linea aspera. The front and lateral aspects of the shaft are covered by, and furnish surfaces for, the origins of the vasti and crureus muscles. The lower extremity of the femur comprises the two condyles. These are two recurved processes of bone, each provided with an articular surface, and separated behind by a deep intercondylic notch. United in front, where their combined articular surfaces form an area on which the patella rests, the two condyles differ from each other in the following respects : If the shaft of the bone be held vertically, the internal condyle is seen to reach a lower level than the external ; but, as the femur lies ojjliquely in the thigh, the condyles are so placed that their inferior sur- faces lie in the same horizontal plane. Viewed from below, the internal condyle is seen to be the narrower and shorter of the two. The external condyle is broader, and advances farther forward and higher up on the anterior surface of the shaft. The intercondylic notch (fossa intercondyloidea) reaches forwards as far as a trans- verse line drawn through the centre of the external condyle. Its sides are formed by the inner and outer surfaces of the outer and inner condyles respectively, the latter bfung more deeply excavated, and displaying an oval surface near its lower and ant(irior part for the attachment of the posterior crucial ligament of the knee- joint. Placed high up, on the posterior part of the inner surface of the external condyle, there is a corresponding surface for the attachment of the anterior crucial ligament. I'he floor of the notch, which is pierced by numerous vascular canals, slop(!s upwards and backwards towards the ]joplit(!al surface on the back of the shaft, from which it is H(']>aiated by a slight ridge (linea intcrcondyloide;!,) to which the ])OHterior jtart (jf the ca])sule of the knee-joint is attached. 'i lie cutaneous asp(!ct of each condyle {i.e. the outer surface of the external condyle and the iriner. surface of the intf^rnal condyle) pi'csents an elevated rough 16a 228 OSTEOLOGY. surface, called the tuberosity (epicondylus), that of the internal (epicondylus medialis) being the more prouounced and outstanding from the line of the shaft ; capped above by the adductor tubercle, it affords attachment near its most pro- minent point to the fibres of tlie internal lateral ligament of the knee-joint. The external tuberosity (epicondylus lateralis), less pronounced and lying more in line with the outer surface of the shaft, is channelled behind by a curved groove, the lower rounded lip of which serves to separate it from the inferior articular surface. This groove ends in front in a pit which is placed just below the most salient point of the tuberosity ; hereto is attached the tendon of the popliteus muscle, which overlies the lower lip of the groove in the extended position of the joint, but slips into and occupies the groove when the joint is flexed. Behind the most prominent part of the external tuberosity, and just above the pit for the attachment of the popliteus, the external lateral Hgament of the knee- joint is attached, whilst superior to that there is a circumscribed area for the origin of the tendinous part of the outer head of the gastrocnemius muscle. The articular surface on the lower extremity is divisible into three parts — that which corresponds to the inferior surface of the shaft and which is formed by the coalescence of the two condyles in front, and those which overlie the under and hinder aspects of each of those processes. The former is separated from the latter by two shallow oblique grooves which traverse the articular surface from before backwards, on either side, in the direction of the anterior part of the intercondylic notch. These furrows are the impressions in which fit the fore-parts of the internal and external semilunar cartilages of the knee-joint respectively, when the knee-joint is extended. The anterior articular area or trochlea (facies patellaris) is adapted for articulation with the patella. Convex from above downwards, it displays a broad and shallow central groove, bounded on either side by two slightly convex surfaces. Of the two sides, Surface for the attachment of- ext. lateral li Groove for tendon of- popliteus Fig. 171. — LowEE End of Right Femur (Outer Side). Patellar surface Impression of internal semi- lunar cartilage Impression of external semi lunar cartilage External tuberosity Semilunar patellar facet Internal tibial surface External tibial surface the external is the wider and more prominent, and rises on the front of the bone to a higher level than the internal. The con- dyloid or tibial surfaces are convex from side to side, and convex from before backwards. Sweeping round the under surface and posterior extremities of the condyles, they de- scribe a spiral curve more open in front than behind. The inner condyloid ar- ticular surface is narrower than the outer, and when viewed from below is also seen to describe a curve around a vertical axis. The articular surface of the external condyle is inclined obliquely from before backwards and slightly outwards. The surfaces of the condyles above the articular area posteriorly are confluent superiorly with the popliteal surface of the shaft ; from these areas the heacis of the gastro- cnemius muscles arise. The bone from which the inner head of the muscle springs is often elevated in the form of a tubercle placed on the lower part of the popliteal surface of the shaft, just above the internal condyle. The proportionate length of the femur to the body height is as 1 is to 3-53-3'92. Ext. condyle Intercondylic notch Int. condyle Surface of attachment of posterior crucial ligament Fig. 172. — Lower End of Right Femur as seen from Below. THE FEMUE. 229 Arterial Foramina. — Numerous vascular canals are seen in the region of the neck, at the bottom of the digital fossa, on the posterior intercondylic ridge and on the external surface of the great trochanter. The nutrient arteries for the shaft pierce the bone on or near the linea aspera. Both back and front of the lower end of the shaft display the openings of numerous vascular canals, and the floor of the intercondylic notch is also similarly pierced. Connexions. — The femur articulates witli the os innominatum above and the tibia and patella below. The external surface of the great trochanter determines the point of greatest hip width in the male, being covered only by the skin and sujjerlicial fascia and the aponeurotic insertion of the gluteus maximus. In the erect position the tijj of the trochanter corresponds to the level of the centre of the hip -joint. When the thigh is flexed the trochanter major sinks under cover of the anterior fibres of the gluteus maximus. In women the hip width is usually greatest at some little distance below the trochanter, due to the accumulation of fat in this region. The shaft of the bone is surrounded on all sides by muscles. Its forward curve, however, is account- able to some extent for the fulness of the front of the thigh. The exposed surfaces of the condyles determine to a large extent the form of the knee. In flexion the articular edges can easily be recognised on either side of and below the patella. Architecture. — The shaft has a medullary cavity which reaches as high as the root of the small trochanter. Inferiorly it extends to within 3^ inches of the lower articular surface. In the upper half the outer compact wall is very thick, but below the middle of the shaft it gradually thins until it reaches the condyles inferiorly, over which it passes as a thin, hardly definable external layer. Above, it is especially thick along the line of the linea aspera, and here the large nutrient canal may be seen passing obliq^uely upwards in the substance of the dense bone for the space of two inches. In the upper end of the shaft the osseous lamellae springing from the sides of the medullary cavity arch inwards towards the centre, intersecting each other in a manner comparable to the tracery of a Gothic window. The lower wall of the neck is thick below, near the trochanter minor, but thins rapidly before it reaches the head. From this aspect of the neck there spring a series of oblique, lamellae which pass upwards and inwards, spreading in fan-shaped manner into the under surface of the head. These are intersected above by lamellae which arch inwards from the outer side of the shaft below the great trochanter, as well as from the under surface of the thin but compact outer shell of the upper surface of the neck, the whole forming a bracket-like arrangement which assists materially in adding to the strength of the neck. Further support is given by the addition of a spur of dense bone which springs from the inner surface of the under side of the neck, just in front of and above the trochanter minor : this is called the calcar femorale. From it stout lamellai having a vertical direction arise. The spongy tissue of the head and great trochanter is finely reticulated, that of the lower part of the neck and upper part of the shaft being more open in its texture. Passing vertically downwards through this tissue there is a vascular canal, the orifice of which opens externally on the floor of the digital fossa. The spongy tissue of the lower part of the shaft is more delicate and uniform in its arrangement, displaying a more or less parallel striation in a longitudinal direction. Subjacent to the articular surface the tissue is rendered more compact by the addition of lamellae disposed in curves in harmony with the external aspect of the bone. Variations. — Absence of the pit on the head of the femur for the attachment of the ligamentum teres has been recorded. This corresponds with the condition met with in the orang. Not infrequently there is an extension of the articular surface of the head on to the fore and upper aspect of the neck ; this is a " pressure facet " caused by the contact of the iliac portion of the acetabular margin with the neck of the bone, when the limb is maintained for long periods in the flexed position, as in tailors, and also in those races who habitually squat (Lane, Journ. Anat. and Physiol., vol. xxii. p. 606). The occurrence of a trochanter tertius has been already referred to. Its presence is not con- fined to individuals of powerful physique, but may occur in those of slender build, so far suggesting that it is not to be regarded merely as an indication of excessive muscular develop- ment. The observations of Dixon {Journ. Anat. and Physiol., vol. xxx. p. 502), who noted the occurrence of a separate epiphysis in three cases in connexion with it, seem to point to its possessing some morphological significance. Occasionally the gluteal ridge may be replaced by a hollow, the fossa hypotrochanterica, or in some cases the two may co-exist. The angle of the neck is more open in the child than in the adult, and tends to be less when the femoral length is short and the pelvic width great — conditions which particularly appertain to the female. There is no evidence to show that after growth is cpmpleted any alteration takes place in the angle with advancing years (Humphry). The curvature of the shaft may undergo considerable variations, and the appearance of the posterior surface of the Vjone may )je modified by an absence of the linea aspera, a condition re.seni];ling that seen in apes ; or by an unusual elevation of the bone which supjjorts the ridge (Jemur a piluHtre), produced, as Manouvrier has suggested, by the excessive development of the muscles here attached. Under tlie term '■^ jilafymerie," Manouvrier descril:)es an antero-posterior compression of the U2)pei' part of the shaft, frequently met with in the femora of prehistoric races. Ossification. — 'I'lie sluU't begins to ossify early in the second mouth of foetal life, and at Inrth disphiys enlargements ut Vjoth ends, which are capped with cartilage. If the inferior cartilaginous end be sliced away, a small ossific nucleus for the inferior epiphysis will usually be seen. This, as a rule, makes its appearance towards the latter end of the ninth month IGi 230 OSTEOLOGY. of foetal life, and is at service from a medico-legal standpoint in determining the age of the foetus. According to Hartman, it is absent in about 12 per cent of children at term, and may appear as early as the eighth month of foetal life in about 7 per cent. The superior extremity, entii'ely carti- ,„„ „i^„„f Fuse-) -with slialt Appears about , . ^ ^ . „ early part of *"°'r ^ early part first year Appears about 2-3 vpar Usually appears in the 9th month of foetal life laginous at birth, comprises the head, neck, and tro- chanter major. A centre appeai-s for the head during the early part of the first year. That for the tro- chanter major begins to ossify about the second or third year, whilst the neck is developed as an upward extension of the shaft, which is, however, not confined to the neck alone, but forms the lower circumference of the articular head, as may be seen in bones up to the age of twelve or sixteen ; after that, the separate epiphysis of the head begins to over- lap it so as to cover it en- tirely when fusion is com- plete at the age of eighteen or twenty. The epiphysisof thegreat trochanter unites with the shaft and neck about eigh- teen or nineteen, whilst the epiphysis for the trochanter minor, which usually makes its appearance about the twelfth or thirteenth year, is usually completely fused with the shaft about the age of eighteen. The epiphysis for the lower end, although the first to ossify, is not completely united to the shaft until from about the twentieth to the twenty-second year. It is worthy of note that the line of fusion of the shaft and inferior epiphysis passes through the adductor tubercle, a point which can easily be determined in the living. Usually appear before birth At birth. Fuses with shaft about 20-22 years About 12 years. About 16 years. Fig. 173. — Ossification of Femue. The Patella. The patella, the largest of the sesamoid bones, overlies the front of the knee- joint in the tendon of the quadriceps extensor. Of compressed form and somewhat triangular shape, its 1 1 • 4. Extern \i \RTirrLAR facet lower angle projects downwards and forms a peak, called the apex (apex patellse), whilst its upper edge, or "base (basis patellse), broad, thick, and sloping forwards and a little downwards, is divided into two areas by a transverse line or groove ; the an- terior area so defined serves for the attach- ment of the common tendon of the quad- riceps extensor muscle, whilst the posterior, of compressed triangular shape, is covered by synovial membrane. The inner and outer borders, of curved outline, receive the insertions of the vastus internus and externus muscles respectively, the Fig. a. Anterior surface. Surface for the liganientum patellfe 174, — Eight Patella. B. Posterior surface. THE TIBIA. 231 attachment of the vastus internus being more extensive than that of the vastus externus. The anterior surface of the bone, slightly convex in both diameters, has a fibrous appearance, due to its longitudinal striation, and is pierced here and there by the openings of vascular canals. The posterior or femoral articular surface is divided into two unequal parts (of which the external is the wider) by a vertical elevation which glides in the furrow of the trochlear surface of the femur, and in extreme flexion passes to occupy the intercondylic notch. The outer of the two femoral surfaces is slightly concave in both its diameters ; the inner, though slightly concave from above downwards, is usually plane, or somewhat convex trans- versely. Occasionally, in the macerated bone, indications of a third vertical area are to be noted along the inner edge of the internal aspect. This defines the X->art of the articular surface which rests on the border of the internal condyle in extreme flexion. Below the femoral articular area the deep surface of the apex is rough and irregular ; the greater part of this is covered by synovial membrane, the liga- mentum patellse being attached to its summit and margins, reaching some little distance round the borders on to the anterior aspect of this part of the bone. Architecture. — The bone consists of a tliick dense layer anteriorly, which, thins towards the edges on either side and below ; above, it corresponds to the area of insertion of the quadriceps. The femoral articular surface is composed of a layer of compact bone, thickest in correspondence with the vertical elevation. Sandwiched between these two layers is a varying thickness of spongy tissue of fairly close grain, the striation of which on cross section runs in parallel lines from back to front ; on vertical section the tissue appears to be arranged in lines passing radially from the deep surface of the femoral area to the more extensive anterior dense plate. Variations. — Cases of congenital absence of the patella have been recorded. F. C. Kempson {Joum. Anat. and Physiol, vol. xxxvi.) has recently drawn attention to the condition described as emargination of the patella. In specimens displaying this appearance the margin of the bones is concave from a point about half an inch to the outer side of the middle line, to a point half-way down the outer margin of the bone, here there is usually a pointed spine directed upwards and outwards. The condition appears to be associated with the insertion of the tendon of the vastus externus. G. Joachimstal {Archiv u. Atlas der nomalen und xiatholo- gischen Anatomie in typischen Rontgenbildern, Bd. 8) figures a case in which on both sides the patella was double in an adult, the lower and much the smaller portion was embedded in the ligamentum patellse. Ossification. — The patella is laid down in cartilage about the third month of foetal life. At birth it is cartilaginous, and the tendon of the quadriceps is continuous with the ligamentum patellee over its anterior surface, and can easily be dissected off. About the third year an ossific centre appears in it and spreads more particularly over its deeper surface. Ossification is usually completed by the age of puberty. The Tibia It is much stouter and stronger than its The tibia is the inner bone of the leg. neighbour the fibula, with which it is united above and be- low. By its superior expanded extremity it supports the con- dyles of the femur, while inferiorly it shares in the forma- tion of the ankle-joint, articulating with the upper surface and inner side of the as- tragalus. The superior ex- tremity com firisfis the inner and outer tuberosities, the spine, and the tubercle. Each tuberosity is provided on its upper aspect with an articular surface (facies articularis superior), which supports the corresponding femoral condyle, as well as the V>c Surface for attachment of anterior extremity of internal semilunar fartilage Anterior crucial ligament Spine Internal tuberosity Synovial ruRvrn SURKAf 1 Surface for attacli of post pxHpmi of internal semilunar cartilage .'01 I ITI Al NO 1 1 II Post, crucial ligament Surf for attachment of ant extremity of external semilunar cart. External tuberosity iifice for attach, of post ( \tremity of ( \t( III il semilunar caitil i„( Fk;. 17.5. — Ul'I'ER SURKACK OK SUPERIOH EXTREMITY OK RKJHT TIBFA. 232 OSTEOLOGY. Ilio-tibial baiifl Spine External tubeeohitv Internal tuberosity Subcutaneous internal SURFACE interposed semilunar cartilage. Of these two condylic surfaces the inner is the larger; of oval shape, its long axis is placed antero-posteriorly. Slightly concave from before backwards and from side to side, its circumference rises in the form of a sharp and well-defined edge. The outer condylic surface is smaller and rounder. Slightly concave from side to side, and gently convex from before back- wards, its circumference is well de- fined in front, but is rounded off behind, thus markedly increasing the convexity of its posterior part. Between the two condylic surfaces the bone is raised in the centre to form the spine (eminentia intercon- dyloidea), the summit of which is grooved and capped on either side by tubercles which spring from and are formed by the upward extension of the neighbouring condylic areas. Of these tubercles the inner (tuber- culum intercondyloideum mediale) is the higher, and longer in an antero-posterior direction, the outer (tuberculum intercondyloid- eum laterale) being more pointed and not so elevated. In front and behind the spine the articular areas are separated by two irregular V-shaped surfaces, the intercondylic fossse. The anterior fossa (fossa inter- condyloidea anterior), the larger and wider, furnishes areas for the attach- ment of the semilunar cartilages on either side, and for the anterior crucial ligament immediately in front of the spine. The floor of this space is pierced by many nutrient foramina. The posterior intercondylic fossa (fossa intercondyloidea posterior) is concave from side to side, and slopes downwards and backwards. The external semilunar cartilage is at- tached near its apex to a surface which rises on to the back of the spine ; the internal semilunar carti- lage is fixed to a groove which runs along its inner edge, and the pos- terior crucial ligament derives an attachment from the smooth posterior rounded surface. The external tuberosity (condylus lateralis) is the smaller of the two. It overhangs the shaft to a greater extent than the internal, though this is obscured in the living by its articulation ^^•ith the fibula. The facet for the fibula, often small and indistinct, is placed postero-externally on the under surface of its most projecting part. Interosseous RIDGE Surface foe ex- tensors OF ANKLE Surface for flexors of ankle Internal malleolus 176. — Right Tibia and Fibula as seen from THE Front. THE TIBIA. 233 Antero-externally the imprint caused by the attachment of the ilio-tibial band is often quite distinct. The circumference of the internal tuberosity (condylus medialis) is grooved postero-internally for tlie insertion of the tendon of the semi- membranosus. In front of the tuberosities, and about an inch below the level of the condylic sur- faces, there is an oval elevation called the tubercle of the tibia,.or the anterior tuberosity (tuberositas tibiae). The upper half of this is smooth and covered by a bursa, while the lower part is rough and serves for the attachment of the ligamentum patellte. Considered in its entirety, the upper extremity of the tibia is broader trans- versely than antero-posteriorly, and is inclined backwards so as to overhang the shaft posteriorly. The shaft (corpus tibiai) is irregularly three-sided. It is narrowest about the junction of its middle and lower thirds, and expands above and below to support the extremities, liunning down the front of the bone there is a gently-curved, prominent margin confluent above with the tubercle, but fading away inferiorly on the anterior surface of the lower third of the bone, where it may be traced in the direction of the anterior border of the internal malleolus. This is the crest or sMn (crista anterior), which is subcutaneous throughout its entire length. To the inner side of this is a smooth, slightly convex surface, which reaches as high as the internal tuberosity above, and inferiorly becomes continuous with the inner surface of the internal malleolus. This is the internal or subcutaneous surface (facies medialis) of the shaft, which is covered only by skin and superficial fascia, except in its upper fourth, where the tendons of the sartorius, gracilis, and semitendinosus muscles overlie it, as they pass towards their insertions. This surface is limited posteriorly by the internal border (margo medialis) which passes from the inner and under surface of the internal tuberosity above to the hinder border of the internal malleolus below. This border is rounded and indefinite above and below, beins; usually best marked about its middle third. To the outer side of the tibial crest is the external surface of the bone (facies lateralis); it is limited behind by a straight vertical ridge, the crista interossea, to which the interosseous membrane, which occupies the interval between the tibia and the fibula, is attached. This ridge commences above, near the middle of the outer and under surface of the external tuberosity, and terminates below about two inches above the lower extremity by dividing into two lines, which separate and enclose between them the surface for articulation with the lower end of the fibula, and the area of attachment of the inferior interosseous ligament, which here unites the two bones. In its upper two-thirds the external surface provides an extensive origin for the tibialis anticus. Inferiorly, where the tibial crest is no longer well defined, the external surface turns forward on to the front of the shaft, and is limited inferiorly by the anterior margin of the inferior articular surface. Over this the tendon of the tibialis anticus, and the combined fleshy and tendinous parts of the extensor proprius hallucis and extensor com- munis digitorum muscles pass obliquely downwards. The posterior surface (facies posterior) of the shaft lies between the interosseous ridge externally and the in- ternal border on the inner side. Its contours are liable to considerable variation according to the degree of lateral compression of the bone. It is usually fuU and rounded above, and flat below. Superiorly it is crossed by the oblique or popliteal line (linea poplitea), which runs downwards and inwards, from the fibular facet above, to the internal border on a level with the junction of the middle with the upper third of the shaft. To this line, as well as to the internal border for some distance below it, the soleus muscle is attached. Into the bulk of the triangular area above it the popliteus muscle is inserted. Arising from the middle of the popliteal line there is a vertical ridge, which passes downwards and divides the posterior asj^ect of the shaft into two surfaces — an external for the tibial origin of the tibialis posticus muscle, and an internal for the flexor longus digitorum muscle. The inferior third of this surface of the shaft is free I'rom muscular attachments, but is overlain Ity the tendons of the above muscles, together with that of the Hexor longus hallucis. A large nutrient canal, having a downward direction, opens on the posterior surface of the shaft a little })elow the ])opliteal line and just external to the vertical ridge which springs from it. 234 OSTEOLOGY. The inferior extremity of the tibia displays an expanded quadrangular form. It is famished with a saddle-shaped articular surface on its under surface (facias articularis inferior), which is concave from before backwards and slightly convex from side to side. This rests upon the superior articular surface of the body of the astragalus, and is bounded in front and behind by well-defined borders. The anterior border is the rounder and thicker, and is oftentimes channelled by a groove for the attachment of the anterior ligament of the joint ; further, it is occasionally provided with a pressure facet caused by the locking of the bone against the neck of the astragalus in extreme flexion. Externally the edge of the articular urea corresponds to the base of the triangle formed by the splitting of the interosseous ridge into two parts. Where these two lines join it, both in front and behind, the bone is elevated into the form of tubercles, in the hollow between which" (iiicisura fibularis) the lower end of the fibula is lodged, being held in position by powerful ligaments. The cartilage-covered surface occasionally extends for some little distance above the base of the triangle. Internally there is a down-projecting process, called the internal malleolus (malleolus medialis), the inner aspect of which is subcutaneous and forms the projection of the inner ankle. Its external surface is furnished with a pyriform facet (facies articularis malleolaris), confluent above with the cartilage -covered area on the inferior extremity of the shaft; this articulates with a corresponding area on the inner surface of the body of the astragalus. Inferiorly the malleolus is pointed in front, but notched behind for the attachment of the internal lateral ligament of the ankle. Running obliquely along the posterior surface of the malleolus there is a broad groove (sulcus malleolaris) in which the tendons of the tibialis posticus and flexor longus digitorum muscles are lodged ; whilst a little to the fibular side of this, and running downwards over the posterior surface of the lower extremity of the bone, there is another groove, often faintly marked, for the lodgment of the tendon of the flexor longus hallucis muscle. The proportionate length of the tibia to the body height is as 1 is to 4-32-4-80. Arterial Foramina. — Nutrient canals are seen piercing tlie upper extremity of the bone around its circumference and above the tubercle. The floors of the intercondylic fosste are also similarly pierced, and there is usually a canal of large size opening on the summit of the spine. Two or three foramina of fair size are seen running upwards into the substance of the bone a little below and to the inner side of the tubercle, while the principal vessel for the shaft passes down- wards into the bone on its posterior surface, about the level of the junction -of the iqjper and middle thirds. The inner surface of the internal malleoluis, as well as the anterior and jDosterior borders of the inferior extremity, are likewise pitted by the orifices of small vascular channels. Connexions. — Superiorly the tibia supports the condyles of the femur, and is connected in front with the patella by means of the patellar ligament. Articulating externally with the fibula above and below, it is united to that bone throughout nearly its entire length by the inter- osseous membrane. The crest and internal surface can be readily examined, as they are sub- cutaneous, except above where the internal surface is overlain by the thin tendinous aponeurosis of the muscles passing over the inner side of the knee. The form of the lower part of the knee in front is determined by the tuberosities on either side crossed mesially by the ligamentum patellae. Inferiorly the internal malleolus forms the jirojection of the inner ankle, Avhich is wider, not so low, less 23ointed, and placed in advance of the projection of the outer ankle. The front and back of the lower end of the bone are crossed by tendons, which mask to a certain extent its form. Architecture. — The shaft of the bone is remarkable for the thickness and density of the osseous tissue which underlies the crest. The posterior wall is stout, but the internal and external walls are thinner. The several walls are thickest oj^j^osite the middle of the shaft, and thin out above and below where the shaft unites with the epiphyses. The medullary canal, narrow and circular in the middle of the bone, increases in all its diameters above and below, and reaches to within 2^ to 3 inches of either extremity. Superiorly the arrangement of the lamellse of the spongy tissue resembles a series of arches sjiringing from the dense outer walls. These form a jalatform on which the superior epijjhysis rests, tlie sjjongy tissue of which disjDlays a more or less vertical striation. This is much more compact under the condylic surfaces, the superficial aspect of which is formed by a thin layer of dense bone. The spine and tubercle are also formed of compact tissue, whilst the circumference of the tuberosities is covered by a thinner and less dense wall. In the lower end of the shaft the spongy tissue, of a loose and cellular character, is arranged in vertical fibres, blending inferiorly with the closer tissue of the inferior epiphysis, the articular surface of which is covered by a thin but dense layer. In the adult bone the nutrient canal for the shaft is embedded in the dense posterior wall for the space of two inches. Variations — The tibia is often unduly laterally conqjressed, leading to an increase in its THE FIBULA. Appears before birth May appeal independently about 11 } ears antero-posterior diameter as compared with its transverse width. This condition is more commonly met with in the bones of i^rehistoric and savage races than in modern Europeans. Attention was first directed to this particular form by Busk, who named the condition platycnemia. The general appearance of such tibite resembles that seen in the apes, and depends on an exceptional development of the tibialis posticus muscle, though, as Manouvrier has pointed out, in apes this is associated with the direct action of the muscle on the foot, as in climbing, whereas in man, as a consequence of the bipedal mode of progression, the muscle is employed in an inverse sense, viz. by steadying the tibia on the foot, and thus providing a fixed base on which the femur can move. Such platycnemic tibise are occasionally met with in the more highly civilised races, and are, according to Manouvrier, associated with habits of great activity among the inhabitants of rough and mountainous districts. Another interesting condition is one in which the upper extremity is more strongly recurved than is usuaL This retroversion of the head of the tibia was at one time supposed to represent an intermediate condition in which the knee could not be fully extended so as to bring the axis of the leg in line with the thigh ; but such opinion has now been upset by the researches of Manouvrier, who claims that it is the outcome of a habit not uncommon amongst peasants and countrymen, viz. that of walking habitually with the knees slightly bent. Habitual posture also leaves its impi'ess on the form of the tibia, and in races in which the use of the chair is unknown, the extreme degree of flexion of the knee and ankle necessitated by the adoption of the squatting position as an attitude of habitual rest is associated with an increase in the convexity of the external condylic surface, and the appearance, not infrequently, of a pressure facet on the anterior border of the lower extremity, which rests in that position on the neck of the astragalus. Cases of con- ^ .^-u -^ ^^ ^ x «^ ^- .,11 ? ±1 j_-T • ^ 1 Fuses With shaft about 20-24 years genital absence oi the tibia have been frequently described, amongst the most recent being those recorded by Glutton, Joachimsthal, Bland Sutton, and Waitz. Ossification. — The shaft begins ^ to ossify early in the second month | ; of intrauterine life. At birth it is o well formed, and capped above and f below by pieces of cartilage, in the upper of Avhich the centre for the superior epiphysis has already usually made its appearance. From this the tuberosities and tubercle are de- veloped, though sometimes an inde- pendent centre for the latter appears about the eleventh or twelfth years, rapidly joining with the already well- developed mass of the I'est of the epiphysis. Complete fusion between the superior epiphysis and the shaft doesnot take place until the twentieth or the twenty-fourth year. The centre for the lower ai*ticular surface and the internal malleolus makes its appearance about the end of the second year, and union with the shaft is usually complete by the age of eighteen. Lambertz notes the occasional presence of an accessory nucleus in the malleolus. Appears about 1^ yeais Fubes about Ibth yeai At birth. About 12 years. About 16 years. Fig. 177. — Ossification op the Tibia. The Fibula. The fibula, or peroneal bone, is a slender bone with two enlarged ends. It lies to the outer side of the tibia, with which it is firmly united by ligaments, and nearly equals that hone in length. The first difficulty which the student has to overcome is to determine which is the upper and which the lower extremity of the bone. This can easily be done by recognising the fact that there is a deep pit on the inner aspect of the lower extremity immediately behind the triangular articular surface. Holding the bone vertically with the lower extremity downwards and so turned that the triangular articular area lies in front of the notch already spoken of, the subcutaneou.s non-articular aspect of the inferior extremity will point to the side to which the bone bslongs. The superior extremity or head of the fi})ula (capitulum fihiila3), of irregular rounded form, is bcvolled on its innc^r surface so as to adapt it to the form of the under surface of tlie external tuberosity ol' the tibia. At the border, where this 236 OSTEOLOGY. IXT. CON'DYLIC SURFACF oxdylic surfaci:: Popliteal NOTCH Oblique li\ SUEFACE FOE EX- surface becomes confluent with the outer aspect of the head, there is a pointed upstanding eminence called the styloid process (apex capituli fibuhe) ; to this the short external lateral ligament is attached, as well as a piece of the tendon of the biceps, which is inserted into its fore - part. Immediately to the inner side of this, and occupying the summit of the internal sloping- surface, there is an articular area (facies articularis capituli), of vari- able size and more or less triangular shape. This serves for articulation with the external tuberosity of the tibia. The long external lateral ligament, together with the re- mainder of the tendon of the biceps muscle which surrounds it, is at- tached to the outer and upper side of the head in front of the styloid process. In front and behind the head there are usually prominent tubercles. The anterior of these is associated with the origin of the peroneus longus muscle ; the TENsoEs OF ANKLE postcrlor, whllst fumlshing an origin for the upper fibres of the soleus, serves to deepen the groove, behind the superior tibio- fibular articulation, in which the tendon and fleshy part of the popliteus muscle play. The constricted por- tion of the shaft below the head is often referred to as the neck ; around the outer side of this the external popliteal nerve winds. The shaft of the fibula (corpus fibulse) presents many varieties of shape and form, being ridged and channelled in such a way as greatly to increase the difficulties of the student in recognising the various surfaces described. The most im- portant point is first to determine the position of the interosseous ridge. Holding the bone in the position which it normally occupies in the leg, it will be noticed that the external surface of the lower extremity is limited in front and behind by two lines, which, con- verging above, enclose between them a triangular subcutaneous area which lies immediately above the outer ankle. From the summit of the triangle so formed a well- defined ridge may be traced up the front of the shaft to reach the anterior aspect of the head. This is the anterior lorder, and must not be mistaken for the inter- POSTERIOE &IRF\CE Groove foe/ flexor lonoi7s hallucis Geoo\-e for tendons of peroneus longus and P.REVIS External malleolus Fig. 178.- -RiGHT Tibia axd Fibula as seen from Behind. THE FIBULA. 237 STVr.OID PROCESS Neck- Interosseous ridge - osseous ridge, which is now easy to find, for the next ridge which lies immediately internal to the anterior border, or towards the tibial side on the anterior aspect of the bone, is the line to which the interosseous membrane is attached. As a rule these ^^^t for tibia. two lines are separated by a considerable interval in the lower half of the bone, but tend to run much closer together above ; indeed it is not uncommon to find that they coalesce to form a single crest. Let it therefore be clear that the interosseous line is that which lies im- mediately internal to the ridge which springs inferiorly from the malleolar subcutaneous triangular surface, not- withstanding the differences in width of the surface which separates the lines, or their occasional coalescence above. The position of the interosseous ridge enables us at once to separate the flexor aspect of the bone from its extensor surface, using these terms in relation to the movements of the ankle. The use of these terms is not, strictly speaking, correct, and they are here used in a physiological and not in a morphological sense. The anterior surface of the leg is the true extensor surface, and is comparable with the posterior surface of the forearm, the change in position having been brought about developmentally by difference in the rotation of the limbs. Flexion of the ankle, so called, is in reality an extensor movement, and corresjDonds to extension at the wrist. (See Humphry, Journ. Anat. and Physiol., vol. xxviii. p. 15.) In addition, there is the peroneal surface, which corresponds to the outer side of the shaft. Starting then at the interosseous ridge, and passing forwards round the outer side of the shaft, the flexor surface is the first met with ; this is bounded externally by the anterior border, and, as has been said, may be either of considerable width or almost linear. From this arises the extensor communis digitorum, together with the peroneus tertius and the extensor proprius hallucis muscles, which, though extensors of the toes, are also flexors of the ankle. The anterior border serves for the attachment of the intermuscular septum, which separates the foregoing group of muscles from that which lies along the outer side of the shaft, viz. the peroneus longus and brevis muscles. The surface from which these arise is limited behind by the posterior border, which is usually sharp and well defined below, where it is continuous with the bone immediately above the pit on the inner surface of the lower extremity, whilst it tends to be less distinct and more rounded above where it runs into the base of the styloid y^rocess. In its upper third or fourth this border is often rough and tubercular where it serves for the origin of the soleus. The outer or peroneal surface is somewhat twisted, being directed rather forwards above, but tending to turn backwards below where it Vjecomes continuous with the groove which courses along the back of the external malleolus and which lodges the tendons of the peroneus longus and brevis muscles. The remainder of the shaft, included between the posterior border behind and the inttsrosseous ridge in front and internally, is the extensor surface, for here arise the several muscles whose action in part is to extend tlie ankle. Tliis surface is cut up by a curved ridge often the most prominent and outstanding on the bone. Interosseous_ RIDGE ffi Rough surface for inter-_ osseous" ligament Facet for astragalus- 03 OJ O S ° H H m 5 < o H S !J EXTEIINAL MALLEOLUS Fro. 179.— RioHT FjiiULA as SKKN FROM THU iNNHIl SiDK. 238 OSTEOLOGY. aud hence frequently mistaken by the student Tor the interosseous ridge ; it serves to define the area for the origin of the tibiahs posticus, and arises below from the posterior border of the interosseous ridge at the junction of the middle and inferior thirds of the shaft, curves a little backwards, and passing upwards and oljliquely forwards again joins the interosseous ridge once more in the region of the neck. This is oftentimes called the internal border (crista mediahs) ; and the surface so mapped off, the internal surface. The ridge itself serves for the attachment of the aponeurosis which covers the tibialis posticus muscle. The remainder of the extensor aspect of the shaft, which, above, is directed backwards, is so twisted that inferiorly it is directed inwards. From this, in its upper part, the soleus muscle arises ; whilst lower down, the iiexor longus hallucis muscle derives an extensive origin. Both of these muscles, together with the tibialis posticus, act as extensors of the ankle. On this aspect of the bone, at or near the middle of the shaft, and just behind the prominent tibial ridge, is the opening of the nutrient canal, which has a downward direction. The inferior extremity of the fibula, or external malleolus (malleolus lateralis), is of pyramidal form. Its inner surface is i'urnished with a triangular articular area (facies articularis malleoli), plane from before backwards, and slightly convex from above downwards, which articulates with a corresponding surface on the outer side of the body of the astragalus. Behind this there is a deep pit, to which the posterior fasciculus of the external lateral ligament is attached. Above the articular facet there is a rough triangular area on the extensor surface of the shaft, from the summit of which the interosseous ridge arises ; hereto are attached the strong fibres of the inferior interosseous ligament which binds together the opposed surfaces of the tibia and fibula. The external surface of the inferior extremity forms the elevation of the external malleolus which determines the shape of the projection of the outer ankle. Eounded from side to side and from above down- wards, it terminates below in a pointed process, which reaches a lower level than the corresponding process of the tibia, from which it also differs in being narrower and more pointed and being placed in a plane nearer the heel. Superiorly, tliis surface, which is subcutaneous, is continuous with the triangular subcutaneous area so clearly defined by the convergence above of the lines which unite to form the anterior border. The anterior border and tip of the external malleolus furnish attachments to the anterior and middle bands of the external lateral ligament of the ankle. The posterior surface of the external malleolus, broad above, where it is confiuent with the peroneal or external surface, is reduced in width below by the presence of the pit which lies to its inner side. This aspect of the bone is grooved (sulcus malleolaris) by the tendons of the peroneus longus and brevis muscles, which curve round the posterior and lower-pointed aspect of the malleolus. The proportionate length of the fibula to the body height is as 1 is to 4-37-4-82. Arterial Foramina. — Nvimerous minute vascular canals are seen piercing the outer surface of the head, and one or two of larger size are seen on the inner surface immediately in front of the superior articular facet. The canal for the nutrient artery of the shaft, which has a downward direction, is situated on the back of the bone about its middle. The outer surface of the external malleolus displays the openings of many small canals, and one or two larger openings are to be noted at the bottom of the pit behind the inferior articular surface. Connexions. — The head and external malleolus, and jjart of the shaft immediately above the latter, are subcutaneous. The remainder of the shaft is covered on all sides by the muscles which surround it. Superiorly the bone plays no part in the formation of the knee-joint, but inferiorly assists materially in strengthening the ankle-joint by its union with the tibia and its articulation with the astragalus. In position the bone is not parallel to the axis of the tibia, but oblique to it, its ujjper extremity lying posterior and external to a vertical line passing through the external malleolus. Architecture. — A medullary canal luns throughout the length of the shaft, reaching as high as the neck above, and extending as low as a point about 2^ inches above the inferior extremity of the external malleolus. The oiiter wall of the shaft is usually considerably thicker than the inner. The head is formed of loose cellular bone, enclosed within a very thin dense enveloj^e. The spongy tissue of the lower extremity is more compact, and acquires considerable density on the surfaces underlying the articular area and the pit behind it. The canal for the nutrient artery of the shaft ojiens into the medullary cavity about an inch below its external aperture. THE ASTEAGALUS. 239 Appears about 3-4 years Fuses with shaft about 20-24 years Ossification. — The shaft begins to ossify about the middle of the second month of foetal life. At the end of the third month there is but little difference in size between it and the tibia, and at birth the fibula is much larger in projDortion to the size of the tibia than in the adult. Its extremities are cartilaginous, the lower extremity not being as long as the er surface of the body is provided with a saddle-shaped articular surface (trochlea tali), broader in front than behind, for articulation with the under surface of the tibia. The inner edge of the trochlea is straight ; whilst the outer border, which is sharp in front and more rounded behind, is curved inwards posteriorly' wliere it is Ijevelled to form a narrow, elon- gated, triangular facet, which is in contact with the transverse or in- ferior tibio-fibular liga- ment during flexion of the ankle (Fawcett, Ed. Med. Journ., 1895). Over the outer border the cartilage - covered surface is continuous externally with an ex- tensive area of the form of a quadrant. This is concave from above downwards, and articu- lates with the inner surface of the filjular malleolus. The inferior angle of this area is prominent and some- what everted, and some- times referred to as the external process (proces- sus lateralis tali). The inner aspect of the body has a comma -shaped facet, confluent with the superior articular surface, over the inner edge of the trochlea ; this articulates with the outer surface of the tibial malleolus. In- ferior to this facet the bone isroughand pitted hj numerous small openings, and just below the tail of the comma there is a circular im- pression for the attach- ment of the deep fibres of the internal lateral ligament. On the in- ferior surface of the body there is a deep concave facet, called the posterior calcanean facet (facies calcanea articularis posterior), which is of more or less oval or oblong form and is placed obliquely from behind forwards and outwards; this rests upon a corresponding surface on the upper aspect of the os calcis. In front of this, and crossing the bone from within outwards and forwards, is a deep furrow (sulcus tali), the floor of which is pierced by numerous large canals. It serves for the attachment of the V. Metataesa Sesamoid bone First phalanx Third or terminal phalanx Fig. 181. — Bones of the Eight Foot as seen from Above. THE ASTRAGALUS. 241 OS CALCIS Sustentaculum tali Astragalus SCAPHOIlJ or NAVICULAR Cuboid Middle cuNEif gem External cuneiform strong interosseous ligament which unites the astragalus with the os calcis, and separates the facet already described from a smaller oval articular area having a slightly convex surface, which lies immediately in front of it. This is called the middle calcanean facet (facies articularis calcanea media), and articulates with the upper surface of the sustentaculum tali of the OS calcis. Posteriorly the body is provided with two tubercles, separated by a groove ; the external of these (processus posterior tali) is usually the larger, and is occa- sionally a separate ossicle (ostrigonum). To it is attached the posterior fasciculus of the external lateral ligament of the ankle-joint. The groove, which winds obliquely from above downwards and in- wards over the pos- terior surface of the bone, lodges the tendon of the flexor longus hallucis muscle. The head (caput tali), of oval form, is directed forwards and inwards. Its anterior surface is convex from side to side and from above downwards, and ar- ticulates with the navicular bone (facies articularis navicularis). In - feriorly this surface is confluent with the middle calcanean facet, but in well- marked specimens, or when the bones are articulated, it will be seen that a small area in front of, and external to, the middle calcanean facet rests upon an articular surface on the u])per part of the fore portion of the OS calcis, and is called the anterior calcanean facet (facies articularis calcanea anterior). To the inner and under surface of the head there is a cartilage-covered Hurfac;e which does not articulate with any bone, but rests on the upper surface of the inferior calcaneo- navicular ligam(3nt, and is supported on the inner First phalanx , I. Metatarsal Sesamoid bones Metatarsal Fig. 182.- SrcoND PHALANX^ ThiKD or terminal I'lIALANX -Bones of the Right Foot as seen fkom Below. 242 OSTEOLOGY. side by the tendon of the tibialis posticus muscle (Fawcett, Eil. Med. Jonrn. 1895, p. 987). Fig. 183. — The Right Astragalus. A. Upper Surface. B. Uniler Surface. Groove for flex. long, hallucis. Internal tubercle. Trochlear surface fob tibia. Body. For articulation with internal malleolus. Head. For ARTICULATION with NAVICULAR. Neck. 9. For ARTICULATION WITH EXTERNAL malleolus. 10. Surface against which the INFERIOR TIBIO-riBULAR LIGA- ment rests. 11. External tubercle. 12. External tubercle. 13. Posterior, middle, and anterior facets for OS CALCIS. 14. For articulation with navi- cular. 15. Surface resting on inferior CALCANEO -navicular LIGA- MENT. 16. Interosseous groove. 17. Internal tubercle. 18. Groove for flexor longus hallucis. The neck (coUum tali), best seen above, passes from the front of the body and inclines towards the inner side. It is confluent with the inner surface in front of Fig. 184. — The Eight Astragalus A. As seen from tlie Outer Side. External tubercle. Groove for flexor longus hallucis. Internal tubercle. Surface against which the inferior tibio-fibular liga- ment RESTS. Trochlea for tibia. Fob articulation with external malleolus. Neck. S.^ Head. For abticulation with navicular. B. As seen from the Inner Side. 10. Interosseous groove. 18. 11. Anterior middle, and posterior facets for OS IP. CALCIS. •20. 12. Body. 13. Surface resting on internal 21. CALCANEO - navicular LIGA- 22. MENT. 14. For ARTICULATION WITH NAVI- 23. CULAR. 24. 15. Head. 25. l(i. Neck. 17. Trochlea for tibia. For ABTICUL-iTION WITH INTERNAL malleolus. Body. Impression for intebn.\l lateral ligament. Internal tubercle. GR00^^; for flexor longus hallucis. B.xternal tubercle. Interosseous groove. Posterior and middle facets for os calcis. the internal malleolar facet, and externally forms a wide groove, which becomes continuous inferiorly with the outer end of the interosseous groove. 1 THE OS CALCI8. 243 Variations. — The anterior is sometimes separated from tlie middle calcaiiean facet by a non- articular furrow. The posterior external tubercle, often largely developed, is occasionally (2-6 per cent) a separate ossicle forming what is known as the os trigonum (Bardeleben) ; or it may be united to the body of the astragalus by a distinct synchondrosis. A smooth articular surface may occasionally be found on the outer side of the upper surface of the neck. This is a pressure facet dependent on the frequent use of the ankle-joint in a condition of extreme flexion, and is caused by the opposition of the bone against the anterior edge of the lower end of the tibia. For a detailed study of the varieties of this bone, see E. B. S. Sewell {Journ. Anat. and Phydol, vol. xxxAdii.) The Os Calcis. The OS calcis (calcaneus) is the largest of the tarsal bones. It supports the astragalus above and articulates with the cuboid in front. Inferiorly and behind, its posterior extremity or tuberosity forms the heel on which so large a proportion of the body weight rests. The long axis of the bone inchnes forwards and a little outwards. The iipper surface of the os calcis is divisible into two parts— a posterior non- [ Ant. rn:il latrral li.naniPiit Greater sigmoirt avity Orbicular ligament Coronoid process Fig. 224. — Orbicular Ligament of the Radius. of the elbow-join b, and tlierefore, in a sense, it lies mthin the cover of the capsule of the elbow-joint ; but its special feature is the — Orbicular ligament (lig. annulare radii, Figs. 221 and 224), which lias formerly been mentioned as the inferior line of attachment of the external lateral ligament and the ligaments on the front and back of the elbow- joint. It is a strong,well-deJ&ned struc- ture, attached by its extremities to the anterior and posterior margins of the lesser sigmoid cavity, and thus it forms nearly four-fifths of an osseo-tendinous circle or ring. This circle is some- what wider at the upper than at the lower margin of the orbicular ligament, which, by encircling the upper part of the neck of the radius, tends to prevent displace- ment of the head of that bone in a downward direction. The lower margin of this ligament is not directly attached to the radius. The synovial membrane is continuous with that which lines the elbow-joint. It closes the joint cavity at the inferior unattached margin of the orbicular ligament, where it is somewhat loosely arranged in its reflexion from the ligament to the neck of the radius. Inferior Radio-ulnar Joint (articulatio radio-ulnaris distalis). — This joint is situated between the sigmoid cavity on the inner side of the lower end of the radius and the lateral aspect of the head of the ulna. In addition, it includes the inferior surface of the head of the ulna, which articulates with the superior surface of a triangular plate of fibro-cartilage, by means of which the joint is excluded from the radio-carpal articulation The triangular interarticular fibro-cartilage (discus articularis. Figs. 225 and 227), besides presenting articular surfaces to two separate joints, is an important ligament concerned in binding together the lower ends of the radius and ulna.. It is attached by its apex to the de- pression at the outer side of the root of the styloid process of the ulna, and by its base to the sharp line of de- marcation between the sigmoid cavity and the carpal articular surface of the radius. The ligamentous cap- sule is very imperfect, and consists of scattered fibres, termed the anterior and posterior radio-ulnar ligaments (Fig. 226). These ligaments pass transversely between adjoining non-articular surfaces on the radius and ulna, and are of sufficient length to permit of the movements of the radius in pronation and supination. The synovial membrane completes the closure of the joint cavity. It forms a loose bulging projection (recessus sacciformis), passing vipwards between the lower ends of the shafts of the radius and ulna, and it also clothes the upper surface of the triangular fibro-cartilage (Fig. 227). The cavity of this joint is quite distinct from that of the radio-carpal articulation, except when the triangular fibro-cartilage presents a perforation. Between the foregoing articulations there are two accessory ligaments, viz. the interosseous membrane and the oblique ligament, which connect together the shafts of the radius and ulna. Su]face scaphoid bon Groove for temlw of extensor longu pollicis Styloid process of ulna triangular fibi'o-cartilage Ti langnlar fibro-cartilage Surface for semilunar bone Fig 225. — Carpal Articular Surface of the Radius, and Triangular Fibro-cartilage of the Wrist. THE EADIO-CAEPAL JOINT. 283 The interosseous membrane (Fig. 222) of the fore-arm (memljrana interossea inter- brachii) is a strong filn-ous membrane which stretches across the interval between the radius and ulna, and is firmly attached to the interosseous border of each. Below it extends downwards to the lower limit of the space between the bones, whilst above it does not reach higher than a point about one inch Ijelow the tuberosity of the radius. A gap, called the hiatus interosseus, is thus left above its upper margin, and through this the posterior interosseous vessels pass backwards between the bones to reach the dorsal aspect of the fore-arm. This gap is bounded above by the oblique ligament. The fibres which compose the interosseous membrane run for the most part downwards and inwards from the radius to the ulna, although on its dorsal aspect several bands may be observed stretching in an opposite direc- tion. The interosseous membrane augments the surface available for the origin of the muscles of the fore-arm ; it braces the radius and ulna together ; and when shocks are communicated from the hand to the radius, owing to the direction of its fibres, the interosseous membrane transmits these, to some slight extent, to the ulna. The oblique ligament (Fig. 222) is a slender tendinous band of very varying strength which springs from the outer part of the coronoid process of the ulna, and stretches obliquely downwards and outwards to the radius where it is attached immediately below the bicipital tuberosity. Movements of the Radius on the Ulna. — The axis about wliicli the radius moves is a longitudinal cue, haviug one end passing through the centre of the head of the radius and the other through the styloid process of the rdna and the line of the ring-finger. In this axis the liead of the radius is so secured that it can only rotate upon the lesser sigmoid cavity of the ulna within the orbicular ligament, and consequently the radial head remains upon the same j)lane as the ulna ; but the lower end of the radius being merely restrained by the triangular fibro -cartilage, is able to describe nearly a half-circle, of which the apex of this ligament is the centre. In this movement the radius carries the hand from a position in which the palm is directed forwards, and in which the radius and ulna lie parallel to each other (supination), to one in which the palm is directed backwards, and the radius lies diagonally across the front of the ulna (pronation). The ulna is unable to rotate upon a long axis, but while the radius is travelling through the arc of a circle from without inwards in front of the ulna, it will usually be seen that the ulna appears to move through the arc of a smaller circle in the reverse direction, viz. from within outwards. If the humerus be prevented from moving at the shoulder-joint, a very large propor- tion, if not the entire amount, of this apparent movement of the ulna will disappear. At the same time some observers maintain that it really occurs at the elbow-joint, associated with lateral moA'ement during slight degrees of flexion and extension at that joint. THE RADIO-CARPAL JOINT. This joint (articulatio radiocarpea) is a bi-axial diarthrosis, frequently called a condyloid joint. The articular elements which enter into its formation are: on its 2}roximal side, the inferior surface of the lower end of the radius, together with the inferior surface of the triangular fibro-cartilage ; on its distcd side, the superior articular surfaces of the scaphoid, semilunar, and cuneiform bones. The articular surface of the radius is concave both in its antero-posterior and transverse diameters, in order to adapt itself to the opposing surfaces of the scaphoid and semilunar, which are convex in the two axes named. In the ordinary straight position of the hand the triangular fibro-cartilage is in contact with the semilunar bone, and the upper articular surface of the cuneiform bone is in contact with the capsule of the joint. When, however, the hand is bent towards the ulna, the cuneiform bone is carried outwards as well as the semilunar and scaphoid, and the triangular fibro-cartilage comes into contact ■with the cuneiform. The articular surface of the radius is subdivided by an antero- posterior, slightly elevated ridge, into an outer triangular facet which usually arti- culates witli the scayjhoid, and an inner quadrilateral facet for articulation with a portion of LJie s(;iniluriar bone. In the intervals between the scaphoid, sendlunar, and cuneiform bones, the con- tinuity of the articular surfaces is usually maintained by the presence of interosseous h'gamerits which are situated upon the same level as the articular cartilage. Ligaments. — A caytsular ligament completely surrounds the joint. It is some- what loosely iirranged, and injnuits of subdivision into the following portions: — "J'he external lateral ligament TFig. 22G; is a well-defined band which is attached 284 THE AETICULATIONS OR JOINTS. Anterior radio ulnar lijiamenl Internal lateral ligament Pisiform bone Evternal lateral lU'ament Os magnum, with ligaments radiat Unciform process by one end to the tip of the styloid process of the radius, and by the other to a rough area at the base of the tubercle of the scaphoid bone, i.e. external to its radial articular surface. The internal lateral ligament (Fig. 226) is also a distinct rounded structure, having one end attached to the tip of the styloid process of the ulna, and the other to the rough non- articular border of the cuneiform bone, some of its fibres being prolonged to the pisiform bone. The anterior ligament (Fig. 226) is attached superiorly to the anterior margin of the lower end of the radius, as well as i uhercie of scaphoid slightly to the base of the styloid process of the Riilge on i a i trapezium ulna. tSomc transversc fibres may be seen, but Trapezium the greater number pass obliquely downwards and inwards to the palmar non-articular surfaces of the scaphoid, semilunar, and cuneiform bones, while some of them may e^'en be continued as far as the OS magnum. Those fibres from the ulna run obliquely out- wards. On its deeper aspect this ligament is closely adherent to the anterior border of the triangular fibro-cartilage of the inferior radio-ulnar articulation. The posterior ligament extends from the posterior margin of the lower end of the radius, obliquely downwards and inwards, to the dorsal non -articular areas on the proximal row of the carpal bones. The shp to the latter assists in forming the fibrous sheath through which the tendon of the extensor carpi ulnaris muscle travels to its insertion. The principal bundle of fibres is connected with the cuneiform bone. The synovial membrane (Fig. 227) is simple, and is confined to the articulation, except in those cases in which the triangular fibro-cartilage is perforated, or in which one of the interosseous ligaments between the carpal bones of the first row is absent. Movements at the Radio-carpal Joint. — Tke radio -carpal jomt ajffords an excellent example (if a biaxial articulation, in which a long transverse axis of movement is situated more or less at right angles to a short axis placed in the antero-jjosterior direction. The nature of the move- ments which are possible about these two axes is essentially the same in both cases, viz. flexion and extension. The movements about the longer transverse axis are anterior or jsalmar flexion, extension, and its continuation into dorsi-flexion. About the shorter antero-posterior axis we get movements which result from combined action by certain flexor and extensor nmscles, whereby the radial or ulnar borders of the hand may be a^^proximated towards the corresjionding borders of the fore-arm. Lateral movement may also be possible to a slight extent. The range of move- ment iir connexion with either of the principal axes is largely a matter of individual peculiarity, for, with the excei^tion of the lateral ligaments, there is no serious obstacle to the cultivation of greater mobility at the radio-carpal joint. Fig. 226.- -LlGAMENTS ON ANTEHIOE ASPECT OF RaDIO-CARPAL, Carpal, and Carpo-metacarpal Joints. CARPAL JOINTS. The articulations subsisting between the individual carpal bones (articulationes intercarpece) are all diarthroses, and although the total amount of movement throughout the series is considerable, yet the extent of movement which is possible ARTICULATIONS OF THE OARPU>S. 285 between the two rows or between any two carpal bones is extremely limited. For this reason, as well as because of the nature of the movement, these articula- tions are called gliding joints (arthrodia). It is advisable to Gonsider, first, the articulations between individual bones of the proximal row ; second, the articulations between the separate bones of the distal row ; third, the articulation of the proximal and distal rows with each otlier ; fourth, the pisiform articulation. The proximal row of carpal articulations (Fig. 226) comprises the joints between the scaphoid, semilunar, and cuneiform bones. On their adjacent lateral aspects these bones are partly articular and partly non-articular. Three sets of simple but strong, although short ligamentous bands bind these three carpal bones together, and form an investment for three sides of their inter- carpal joints. These are — (1) the anterior or palmar ligaments, two in number, which consist of transverse fibres passing between the adjacent rough palmar surfaces of the bones ; (2) the posterior or dorsal ligaments, also two in number, and composed of similar short transverse fibres passing between the adjacent dorsal surfaces ; (3) the interosseous ligaments (Fig. 227), again two in number, and transverse in direction, situated on a level with the superior articular surfaces, and extending from the palmar to the dorsal aspect of the bones, while attached to non-articular areas of the opposing surfaces. The radio-carpal joint is entirely shut off from the inter- carpal joints, and also from the joint between the two rows of carpal bones, except in rare cases, when an interosseous ligament is wanting. The distal row of carpal articulations (Fig. 226) includes the joints between the trapezium, trapezoid, os magnum, and unciform bones. Articular facets occur on the opposing lateral faces of the individual bones. Associated with this row there are again simple bands of considerable strength, and presenting an arrangement similar to that seen in the proximal row. As in the former case, they invest the intercarpal articulations, except on the superior aspect, where they communicate with the transverse carpal joint, and on the inferior aspect, where they communicate with the carpo- metacarpal joint cavity. The anterior or palmar ligaments are three in number. They extend in a trans- verse direction between contiguous portions of the rough palmar surfaces of the bones. The posterior or dorsal ligaments, also three in number, are similarly dis- posed on the dorsal aspect. The interosseous ligaments (Fig. 227) are two or three in number. That which joins os magnum to unciform is the strongest ; that between the trapezoid and os magnum is situated towards the dorsal parts of their opposing surfaces ; the third, situated between contiguous non-articular surfaces of the trapezium and trapezoid, is always the feeblest, and is frequently absent. The transverse carpal articulation (Fig. 227) is situated between the proximal and distal rows of the carpus. The bones of the proximal row present the following characters on iheir inferior or distal aspect. The outer part of the articular surface is deeply concave, both in the antero-posterior and in the transverse directions, but the inner part of the same surface is concavo-convex, more especially in the trans- verse direction. Superiorly, the articular surfaces of the distal row of carpal bones present an irregular outline. That part pertaining to the trapezium and trapezoid is concave in the antero-posterior and transverse directions, and lies at a considerably lower level than the yjortion belonging to the os magnum and unciform, which is, more- over, markedly convex in tlie antero-posterior and transverse directions, with the exception of the innermost Y)art of the unciform, where it is concavo-convex in l)0th of these directions. This articulation is invested by a complete short capsule (Fig. 226) which binds the two rows of the carpus together, and sends ]:)rolongations to the investing capsules of the y)ro.ximal and distal articulations. The ligament as a whole is very strong, and individual b;uids an; not n>adily defined, althougli certain special bands may Ijc described. TIk; palmar ligaments r;i,diate from the os magnum to the scaphoid, cuncdffjrm, and ])iHiform. The interval between the os magnum and sendlunar is occu])ied by oblique fibres, some of whicli pass from scaphoid to cuneiform, while these are joined by others, prolonged o])liquely downwards and inwards, from the 286 THE AETICULATIONS OR JOINTS. radial end of the anterior radio-carpal ligament. By these different bands the palmar aspect of the joint is completely closed. The dorsal ligaments are more feeble than the palmar. They form a thin, loosely- arranged stratum, in which the only noteworthy bands are one which joins the scaphoid to os magnum, and another which joins cuneiform to unciform. The external lateral ligament (lig. collaterale carpi radiale, Fig. 227) extends between contiguous rough areas on the radial aspects of the scaphoid and trapezium. By its margins it is continuous both with the palmar and dorsal ligaments. The internal lateral ligament (lig. collaterale carpi ulnare. Fig. 227) is arranged like the former in regard to its margins, and by its ends it is attached to the con- tiguous rough ulnar surfaces of the cuneiform and unciform bones. Both of these lateral ligaments are directly continuous with the corresponding lateral ligaments of the radio-carpal joint. An interosseous ligament (Fig. 227) is occasionally found within the capsule, extending across the joint cavity between the os magnum and the scaphoid. The pisi-cuneiform articulation is an arthroidal diarthrosis. The mutual articular surfaces of the two bones are flattened and circular, and only permit of a small amount of gliding movement. The joint is provided with a thin but complete capsule of fibrous tissue, which is specially strengthened inferiorly by two strong bands, viz. pisi-unciform (lig. piso- hamatum) and pisi-metacarpal (lig. pisometacarpeum. Fig. 226). Both of these bands extend from the lower and inner aspect of the pisiform to adjoining parts of the hook of the unciform and base of the fifth metacarpal bone respectively. To a great extent these ligamentous bands may be regarded as extensions of the in- sertion of the tendon of the flexor carpi ulnaris muscle which is attached to the upper part of the pisiform bone. Looked at as ligaments, however, they are specially strong to prevent the displacement of the pisiform bone during contrac- tion of the muscle inserted into it. The synovial membranes (Fig. 227) of the carpal joints are two in number. Of these, one is restricted to the pisi-cuneiform articula- tion, and is correspondingly simple, although occasionally the joint cavity may com- municate with that of the radio-carpal joint. The other synovial mem- brane is associated with the transverse carpal joint which extends transversely be- tween the two rows of carpal bones, with prolongations into the vertical intervals between the adjoining bones of each row, i.e. the inter- carpal articulations. It is, therefore, an elaborate cavity, which may be still further extended, by the absence of interosseous ligaments, so as to reach the radio-carpal and carpo- metacarpal series of joints. The first condition is rare, but the second is not uncommon, and results from the absence of the inter- osseous ligament between trapezium and trapezoid, or of that between trapezoid and OS mag-num. Trajjezoid Trapezium Fig. 227. — Coronal Section through the radio-carpal, carpal, carpo- metacarpal, aud intermetacarpal joints, to show joint cavities and interosseous ligaments (diagrammatic). CAEPO-METAGAEPAL JOINTS. 287 INTERMETACARPAL JOINTS. The four inner metacarpal bones articulate with each other at their proximal ends or bases, between the opposing surfaces of which joint cavities are found — arthrodial diarthroses. These cavities are continuous with the carpo-metacarpal joint (not yet described), and hence the ligamentous arrangements only enclose three aspects of each joint. Three strong transverse ligaments (Figs. 226 and 227j bind adjacent palmar, dorsal, and interosseous areas of the bases of the metacarpal bones, and hence they are called ligamenta basium (oss. metacarp.) volaria, dorsalia et interossea. A synovial membrane is associated with each of these joints, but it may be regarded as a prolongation from the carpo-metacarpal articulation. CARPO-METACARPAL JOINTS. The articulation of the metacarpal bone of the thumb with the trapezium differs in so many respects from the articulation between the other metacarpal bones and the carpus, that it must be considered separately. (A) The articulatio carpo-metacarpea pollicis (Figs. 226 and 227) is the joint between the infero-external surface of the trapezium and the superior surface of the base of the first metacarpal bone. Both of these surfaces are saddle -shaped, and they articulate by mutual co-aptation. The joint-cavity is surrounded by a fibrous capsule, in which we may recognise palmar, dorsal, external, and internal lateral bands, the last being the strongest and most important. Synovial membrane lines the capsule, and the joint-cavity is isolated and quite separate from the other carpal and carpo-metacarpal articulations. At this joint movements occur about at least three axes. Thus, around a more or less trans- verse axis, flexion and extension take jAsLce ; in an antero -posterior axis abduction and adduction (movements which have reference to the middle line of the hand) are found ; while a certain amount of rotation is possible in the longitudinal axis of the digit. The very characteristic movement of opposition, m which the tip of the thumb may be applied to the tips of all the fingers, results from a combination of flexion, adduction, and rotation, and by combining all the movements possible at the various axes a considerable degree of circumduction may be produced, in spite of the fact that this is not a ball-and-socket jointi (B) The articulationes carpo-metacarpeae digitorum are the joints between the bases of the four inner metacarpal bones and the four bones of the distal row of the carpus. They are all arthrodial diarthroses, and the opposed articular surfaces present alternate elevations and depressions which form a series of interlocking joints. The joint cavities between the carpal bones of the distal row, and also the more extensive intermetacarpal joint cavities, open into this articulation. This series of joints is invested by a common capsule which is weakest on its radial side, but is otherwise well defined. Its fibres arrange themselves in small slips, which pass obliquely in different directions, and vary in number for each metacarpal bone. Thus the oblique palmar ligaments (ligamenta carpo-metacarpea volaria, Pig. 225) usually consist of one slip for each metacarpal bone, but there may be two slips, and the third metacarpal bone frequently has three, of which one lies obliquely in front of the tendon of the flexor carpi radialis muscle. Tlui oblique dorsal ligaments (ligamenta carpo-metacarpea dorsalia) are similar short bands, of greater strength and clearer definition, by which the index meta- carpal is bound to the trapezium and trapezoid ; the middle metacarpal to the os magnum, and frequently to the trapezoid ; the ring metacarpal to the os magnum and unciform, and the metacarpal of the minimus to the unciform. Interosseous ligaments, one or sometimes two in number, occur within the capsule. They are usually situated in relation to one or both of the contiguous margins of the bases of the third and fourth metacar[)al bones, from which they extend upwards to adjacent niargins of the os magnum arul unciform. Occasionally they are sulliciently dev(;lo])(;(l to dividliteus muscle (Fig. 234). The tendon of insertion of the semi - )ueml)ranosus muscle contril)utes an important expansion which augments the ])osteri<)r ligament on its superficial aHi)ect. This <;xpausion — ligamentum posticum Winslowii — passes obliquely upwards and outwards to lose itself in the general ligauienl, but it is most distinct in the region between the femoral condyles, where it may present upper and lower arcuate 300 THE ARTICULATIONS OR JOINTS. borders. A number of vessels aud nerves perforate this ligament, and hence it presents a number of apertures. The internal lateral ligament (lig. collaterale tibiale, Figs. 233 and 235) is a well- defined, strong, fiat band which is applied to the inner side of the knee-joint, and is rather wider in the middle than at either end. It is frequently regarded as consisting of two portions — an anterior or long portion, and a posterior or short one. The two parts arise close together from the non-articular inner surface of the inner condyle, immediately lielow the adductor tubercle. The short or posterior portion descends slightly backwards, to be attaclied to the postero-internal aspect of tlie inner part of the tibia above the groove for the semi-membranosus tendon. The long or anterior portion inclines somewhat forwards, and descending superficially to the tendon of the semi-membranosus, it is continued downwards, to be attached to the upper part of the inner surface of the shaft of the tibia below the level of the anterior tuberosity. On its superficial aspect the internal lateral ligament is augmented by prolonga- tions from the tendons of the semi-membranosus and sartorius muscles, but is separated by a bursa from the tendons of adductor-gracilis, semi-tendinosus, and sartorius. Its deep surface is adherent to the convex edge of the internal semilunar cartilage, but lower down the inferior internal articular vessels intervene between the ligament and the shaft of the tibia. The external lateral ligament (lig. collaterale fibulare,Figs. 233 and 235), sometimes called the ligamentum laterale externum longum, is a distinct rounded band which is under cover of the ordinary capsule, and yet well separated from the joint cavity by intervening objects. It is attached superiorly to a tubercle on the outer surface of the external condyle, immediately above the groove occupied by the tendon of the popliteus muscle, superficial to which the ligament descends. By its lower end it is attached to the outer side of the head of the fibula, in front of the styloid process. In its course vertically downwards it splits the tendon of insertion of the biceps flexor cruris (Fig. 233), the portions of which are fixed to the head of the fibula on either side of the ligament, and a bursa may intervene between the tendon and the ligament. The inferior external articular vessels pass forwards subjacent to this ligament and above the head of the fibula. Unlike the internal ligament, it is not attached to the corresponding semilunar cartilage. The ligamentum laterale externum breve seu posticum (Fig. 234) is an iuconstant structure wliicli is attached by its upper end immediately behind the preceding, and subjacent to the outer head of the gastrocnemius muscle. It likewise descends superficial to the popliteal tendon, and is affixed inferiorly into the styloid process of the fibula. The intra-articular structures of the knee-joint are more important and more numerous than in any other joint of the body. The crucial ligaments (ligamenta cruciata genu) are two strong, rounded, tendinous bands, which extend from the non-articular area on the upper surface of the head of the tibia to the non-articular sides of the intercondyloid notch of the femur. These interarticular ligaments are distinguished from each other as the anterior or external and the posterior or internal. They cross each other like the limbs of an X, yet they remain distinct throughout, and each has its own partial synovial covering. They lie within the capsule of the joint, and extend between non- articular surfaces in relation to the longitudinal axis of the limb. The ligamentum cruciatum anterius (Figs. 233, 235, and 236) is attached inferiorly to the inner part of the rough, depressed area in front of and close to the spine of the tibia. It passes obliquely upwards, outwards, and backwards to the inner non-articular surface of the external condyle, where it finds attachment far back in the posterior part of the intercondyloid notch. This ligament is tense in the position of extension, and therefore it assists in maintaining the erect attitude. The ligamentum cruciatum posterius (Figs. 233, 235, and 236) is somewhat shorter than the preceding. It is attached inferiorly to the hinder part of the depressed surface behind the spine of the tibia and close to the popliteal notch. Its fibres pass obliquely upwards, forwards, and inwards, to be inserted into the outer non- articular surface of the inner condyle, far forwards towards the anterior margin of the intercondyloid notch. It is rendered tense in the position of flexion. THE KNEE-JOINT. 501 The semilunar interarticular fibro-cartilages are two in number — an inner and an outer— placed horizontally Ijetween the articular surfaces of the femur and tibia. In general outline they correspond to the circumferential portions of the tibial i'acets upon which they rest. Each has a thick, convex, fixed border in relation to the periphery of the joint, and a thin, concave, free border directed towards the interior of the joint. Neither of them is sufficiently large to cover the whole of the tibial articular surface upon which it rests. The upper and lower surfaces of each semilune are smooth and free, and each cartilage terminates in an anterior and a posterior fibrous horn or coriiu. The internal semilunar fibro - cartilage Cmeniscus medialis. Figs. 2-35 and 236) Tendon of insertion ot adductor niagnn muscle (cut) Popliteal surface of femur Anterior crucial ligament Tendon of popliteus muscle cut) Accessoryattaclimeiit y r , of external semilun n J \j cartiku Hi" Internal semilunar caitilascp External semilunar cartilage Posterior crucial ligament Tendon of semi-memhianus' mus( le (cu , Internal lateial ligament Popliteal surface of tibia Groove on tibia for tendon of popliteus muscle Superior portion of cap- .sule of superior tibio- fibular articulation .External lateral ligament of knee-joint Posterior superior tibio- fibular ligament "•Head of fibula Fig. 23.5. — The Knee-Joint opened kkom Behind by the Removal of the Posterior Ligament. forms very nearly a semicircle. It is attached by its anterior horn to the non- articular surface on the head of the tibia, in front of the tibial attachment of the anterior crucial ligament, and by its posterior horn to the non-articular surface immediately in front of the tibial attachment of the posterior crucial ligament. The deep or hinder part of the internal lateral ligament is attached to its |)eriphery. The external semilunar fibro-cartilage (meniscus later.ilis, Figs. 235 and 236) is attached by its anterior hf)rn to the non-articular surface of the tibia in front of the tiftial S])ine, whore it is yilaced to the out(!r side, and ])artly under cover of the tibial end of the anterior crucial ligament. By its ])OSterior horn it' is attached to the interval between the two tubercles which surmount the tibial spine, i.e. in front of the attachment of the posterior horn of the internal semilunar cartilage. 1'his fibro- cartilagfi, with its two horns, therefore forms almost a comijlete circle. Posteriorly it is attached ])y its yjerijihery to the jiosterior ligament, 1)ut on the outer side it 502 THE AETICULATIONS OE JOINTS. Transverse lip;rirneiit Anterior cornu of internal semilunar cartila Anterior cornn of external seniihiiiar cartilage Internal semilunar^^v |S fibro- 1 ^ cartilage \ Posterior cornu of internal semi- lunar cartilaKB Posterior crucial lit^ament Posterior cornu of exter- semilunar cartilage Fasciculus from external semilunar cartilage to posterior crucial ligament i'lG. 236. — Upper E^'D■OF Tibta with Semilunar Cartilages and Attached Portions of Crucial Ligaments. is separated from the external lateral ligaineut by the tendon of the popliteus muscle, and on this aspect its periphery is free. The two horns of the external semilune are endjraced by the two horns of the internal one, and, while the anterior crucial ligament has its tibial attachment almost between the anterior horns of the two semilunes, the tibial attachment of the posterior crucial ligament is situated behind the posterior horns of the two semilunes. Both semilunes possess certain accessory attachments. Thus the external semi- lune sends a large bundle of fibres from its convex posterior border to augment the posterioraspect of the posterior crucial liga- ment, by which these fibres are conducted to the f e in u r. Again, the con- vex or periph- eral margins of each semilune possess certain attachments to the deep surface of the capsule on its inner and posterior as- pects, as has already been explained, but, in addition, they are attached to the non-articular circumference of the tibial head by short fibrous bands known as the ligamenta coronaria. Lastly, a rounded band which varies in strength, the transverse ligament (lig. transversum genu. Figs. 233 and 236), stretches between the anterior convex margins of the two semilunes, crossing the front part of the non-articular area on the tibial head in its course. The synovial membrane of the knee-joint is not only the largest, but the most elaborately arranged of its kind in the body. It not only lines the capsule, but it forms a more or less extensive covering for the intracapsular ligaments and the free surface of the infra-patellar pad of fat. This pad acts as a wedge which fits into the interval lietween the patella, tibia, and femoral condyles, and the synovial membrane upon its surface forms a band or fold which extends from below the level of the patellar articular surface to the anterior part of the intercondyloid notch. This is in no sense a ligament, although it is named the ligramentum mucosum, or plica syno\'ialispatellaris. At its femoral eud it is narrow and attenu- ated, but at its patellar end it expands laterally to form wing-like fringes or mem- branes — the alar ligaments fplicse alares) — wdiich are often distinguished from each other as the inner (plica aliformis medialis) and the outer (plica aliformis lateralis). These folds are more or less loaded with fat. Apart from these special foldings, the synovial membrane lines the deep surface of the common extensor tendon, and extends upwards for a variable distance above the patella. This extension of the joint cavity almost ahvays communicates with a large bursa situated still higher on the front of the femur. Tracing the synovial membrane downwards, it wall be found to cover both surfaces of the semilunar fibro- cartilages. The peripheral or convex margins of these cartilages are only covered by this membrane where they are unattached to the capsule. A prolongation invests the intracapsular portion of the tendon of the popliteus muscle, and separates this tendon from the back part of the tibial head, besides intervening between the external semilune and the head of the til)ia. From the back part of the joint cavity the synovial membrane extends forwards, THE KNEE- JOINT. 803 and provides a partial covering for the crucial ligaments Ijetween which a bursa may be found. This somewhat complicated arrangement of the synovial membrane may be readily comprehended if it be borne in mind that it really represents the fusion of three separate synovial cavities, which in some animals are permanently distinct. These are indicated in the two femoro-tibial and the single femoro-patellar parts of the articulation. The joint cavity may communicate with bursas situated in relation to the inner head of the gastrocnemius muscle and the tendon of the semi-membranosus muscle, besides the large supra-patellar bursa already described. Lastly, there may be intercommunication between this joint cavity and that of the superior tibio-fibular articulation. Movements at the Knee- Joint. — In studying the movements whicli may occur at the human knee-joint, it is necessary to bear in mind that the lower limb of man is primarily required for purposes of support and locomotion. The principal requirement of the former function is stability accompanied by rigidity, whereas in the latter function the special desideratum is regu- lated and controlled mobility. Thus, in the same joint, two entirely oi:)posite conditions have to be provided. The stable conditions of support are chiefly concerned in the maintenance of the erect attitude, and the mechanism associated therewith does not call for the exertion of a large degree of sustained muscular effort. In standing erect the attitude of the limb is that of extension, which mainly concerns the femoro-tibial parts of the joint. In this position the force of gravity acts along a vertical line which falls in front of the transverse axis of the joint, and therefore any tendency to flexion, i.e. bending backwards, is mechanically counteracted by the application of a force which tends to j^roduce bending forwards (so-called over-extension). This, however, is absolutely prohibited in normal states of the joint, by the tension of the posterior and lateral ligaments aided by the anterior crucial ligament. The value of this fact may be seen by observing the eftect produced by giving the joint a sudden push from behind, which causes an immediate reversal of the positions of the transverse and vertical axes, whereby the body weight at once produces flexion of the joint. The semilunar cartilages and the infra-patellar pad of fat also assist in maintaining extension, by reason of their close adaptation to, and packing round the condyles as these rest upon the tibia. The anterior margin of the intercondyloid fossa is also brought into contact with the front of the anterior crucial ligament. In the position of extension the patella is retained at a high level in relation to the trochlear sur- face of the femur, so that the lower articular facets of the patella are in contact with the trochlea. During locomotion the movements of the knee-joint are somewhat intricate, for both the femoro-tibial and the femoro-patellar sections of the joint are brought into action. The principal movement which results is flexion, with which there is associated, both at its beginning and ending, a certain amount of screw movement or rotation. Flexion and rotation occur at the femoro-tibial sections of the joint, whereas the movement at the femoro-patellar portion produces a regulating and controlling influence uj)on flexion. Taking these factors separately, we observe that each condyle adapts itself to a shallow cup formed by the head of the tibia and the corresponding semilunar cartilage, and as the two condyles move simultaneously and parallel to each other, there is more than the characteristic hinge-joint action, for each condyle glides and rolls in its cup " like a wheel restrained by a drag " (Goodsir) when the movement of bending occurs. Thus the different parts of the condyles are successively brought into relation with the transverse axis of the joint while it passes from extension to flexion and vice versa. From the fact that the internal condyle is longer than the external, it is believed that extension is completed by a movement of rotation whereby the joint becomes locked, and the anterior crucial, the posterior and the lateral ligaments, become tense. A sinular rotation initiates the movement of flexion, and unlocks the joint by relaxing the liga- ments just mentioned. Since the tibia and foot are fixed in the act of walking, it is the femur which rotates upon the tibia in passing from extension to flexion and vice versa ; and as relaxation of the ilio-femoral ligament is essential for this rotation, some observers are of opinion that the body weight falls beliind the transverse axis of the knee-joint, as in the case of the Lip-joint, and consequently that extension of the knee-joint is maintained l)y the ilio-femoral ligament, as it is not possible to bend the knee witljout first having bent the hip-joint. During flexion and extension the semilunar cartilages g]id(; along with the condyles, so as to maintain their close adaptation anrl preserve tlieir value as packing agents. When the movement of flexion is completed, the condyles are r(;tain(;d upon the tibia, and prevented from slipj)ing off by the tt;nsion of tin; posterior crucial ligament. In this position a small degree of rotation of the tibia, both inwanls and outwards, is also permissible. TIk! i-egulafing and controlling in(luenc(! of the femoro-patellar portion of tlu; articulation is brought into j»lay during the movements of fl(r.\iou and extension. In the latter position the infi^rior pair of patellar facets is in aj)position with the upper part of the femoral trochlea. As (I'-xion advance.^, the niiddie ])air of facets adapt thciniselves to a deeper area of the trochlea, into which the jiafellar keel fits. When flexion is still further advanced, the upper pair of patellar facets will he found fitting into that j)art of tlie trochlea adjoining the intercondyloid notch ; 304 THE AETICULATIONS OR JOINTS. and finally, when flexion is complete, the i)atel]a lies op])Osite the intercondyloid notch, while the forward thrust of the longer internal condyle brings its semilunar facet (Groodsir) into ajiposition witli the somewhat vertical facet at the inner border of the j)atella. The wedge-like inliuence of the patella is most marked, for it is only in the position of extension that it can be moved from side to side. The movements of the patella may be described as gliding and co-aptation, as it slips or rocks from one pair of facets to another in its progress along the trough of the femoral trochlea. THE TIBIO-FIBULAR JOINTS. The upper and lower ends of the fibula articulate with the tibia. Primarily, the fibula is required to form a strong lateral support for the ankle-joint, and therefore its articulations are so arranged as to provide a certain amount of elasticity without any sacrifice of the rigidity necessary for security. Hence the amount of movement is very small, but what there is enables these joints to be classified as arthrodial diarthroses. The superior tibio-fibular joint (articulatio tibio-fibularis) is formed, on tlie one hand, by a flat oval or circular facet which is situated upon the postero-external aspect of the outer tuberosity of the head of the tibia, and is directed downwards and backwards ; on the other, by a similar facet on the upper surface of the head of the fibula in front of the styloid process. A fibrous capsule (capsula articularis, Tig. 234) invests the joint, and it may be regarded as holding the articular surfaces in apposition, although certain special bands receive separate designations. Occasionally there is an opening in the capsule by which communication is established between the joint cavity and the knee-joint through the intermediation of the synovial prolongation, subjacent to the tendon of the popliteus muscle. The anterior superior tibio-fibular ligament (lig. capituli fibul?e anterius. Fig. 233) is a strong flat band whose fibres extend from the anterior aspect of the fibular head, upwards and inwards, to the adjoining part of the tuberosity of the tibia. The posterior superior tibio-fibular ligament (lig. capituli filjulte anterius. Fig. 234) is a similar, but weaker band, passing upwards and inwards from the posterior aspect of the fibular head to the posterior aspect of the outer tuberosity of the tibia, where they are attached immediately below the opening in the capsule of the knee-joint, from which the tendon of the popliteus muscle escapes. Equally strong but much shorter bands are found on the superior and inferior aspects of the joint. The former is intimately associated with the tendon of the biceps flexor cruris muscle which strengthens the upper aspect of the joint, and here also is found the occasional opening by which it communicates with the knee- joint. The synovial membrane is in certain cases continuous with that of the knee- joint in the manner already described. The interosseous membrane (membrana interossea cruris, Figs. 234 and 237) plays the part of an accessory ligament both for the upper and the lower tibio-fibular joint. It is attached to the interosseous borders on the shafts of the tibia and fibula, and binds them together. The general direction of its fibres is from the tibia downwards and outwards to the fibula, but many fibres pass in the opposite direction. The membrane may extend upwards until it comes into contact with the ligaments of the superior tibio-fibular joint, but there is always a vertical oval aperture in its upper part for the forward passage of the anterior tibial vessels. This aperture (Fig. 234), which is about one inch long, adjoins the shaft of the fibula at a point rather less than one inch below its head. Towards the lower end of the leg the distance between the tibia and the fibula rapidly diminishes, and consequently the width of the interosseous membrane is correspondingly reduced, so that it is tense throughout its entire length. In the lower part of the membrane there is a small opening ibr the passage of the anterior perforating vessels. There is no sharply-marked demarcation between the interosseous membrane and the interosseous ligament which connects the lower ends of the tibia and fibula — the one, indeed, may be said to run into the other. The inferior tibio-fibular joint (syndesmosis tibio-fibulare) is not on all occasions pro^'ided with articular cartilage, so that it may either be a separate articu- THE TIBIO-FIBULAK JOINTS. 505 lation, or it may merely preseut a series of ligaments which are accessory to the uukle- ioint, because it is clear that, under any circumstances, the object aimed at in this articulation is to obtain additional security for the ankle-joint. The articular surface on the tibia, when present, constitutes a narrow articular strip on the outer side of the lower end of the bone, and the joint-cavity is practically an upward extension of the ankle-joint. The corresponding fibular facet is continuous with the ex- tensive articular area, by means of which the fibula articulates with the astragalus. By far the greater part of the opposing surfaces of tibia and fibula are, however, non-articular and rough. The supporting ligaments are of great strength. Lower end of shaft uf tibi Groove on internal malleolui for tendon of tibialis posticus— nr-r tendon "' ' Trochlear surface of astragalus Internal lateral ligament Fibrous sheath for tendon of flexoi longus hallucis Sustentaculum tab Flexor longus hallucis tendon (cut) Posterior caleaneo-astragaloid ligament 1_- Tibio-fibular interosseous meiiiVjrane Tjower end of shaft of libula Posterior inferior tibio- tibular ligament Transverse inferior tibio- fibular ligament Facet on astragalus for transverse inferior tibio- iiliular ligament Posterior talo-iibular ligament (posterior fasciculus of external lateral ligament) Calcaneo-ftbuiar ligament (middle fasciculus of external lateral ligament) Tuberosity of os calcis Fig. 237. — Ankle-Joint Dissected vhou Behind with Part ok the Capsular Ligament Removed. The anterior inferior tibio-fibular ligament (lig. malleoli lateralis anterius. Fig. 240) consists of strong fibres which pass obliquely downwards and outwards from the front of the lower end of the tibia to the front of the external malleolus. The posterior inferior tibio-fibular ligament (lig. malleoli lateralis posterius, Figs. 237 and 2.':i8; is equally strong, and passes in a similar direction between corre- sponding posterior surfaces. A transverse inferior tibio-fibular ligament (Figs. 237 and 238) stretches, in the direction indicated by its name, between the posterior inferior border of the tibia and the upper end of the pit on tlie inner and posterior aspect of the external malleolus. An interosseous ligament, jxnverful and somewhat extensive, connects the con- tiguous rough ntjii-artioular surfaces. Sujjeriorly, as already mentioned, it is con- tinuous with the intercsseous membrane. Anteriorly and postericjrly it comes into contact with the more superficial ligaments. Inferiorly it descends until it comes into intimate asHOciation with the joint-cavity. 21 306 THE AETICULATIONS OE JOINTS. A synovial membrane is found lining the small joint-cavity, but it is always a direct prolongation from that which lines the ankle-joint. JOINTS OF THE FOOT. THE ANKLE-JOINT. The ankle-joint (articulatio talo-cruralis) is a ginglymus variety of a diarthrosis. The bones which enter into its formation are the lower ends of the tibia and fibula, with the articular areas on the upper, lateral, and mesial surfaces of the astragalus. The tibia and fibula, aided by the transverse inferior tibio-fibular ligament, form a three-sided socket within which the astragalus is accommodated. The roof or highest part of the socket, which is wider in front than behind, is formed chiefly by the quadrilateral articular surface which characterises the lower end of the tibia, but towards its postero-external margin the transverse inferior tibio-fibular liga- ment assists in its formation. Here also the tibial articular surface is continuous with the narrow articidar facet already described as forming part of the inferior tibio-fibular joint. The inner wall of the socket is formed by the articular facet on the outer side of the internal malleolus, and there is no interruption of the articular cartilage between the roof and inner wall. The outer wall of the socket is quite separate from the foregoing parts, and consists of a large triangular facet upon the inner side of the external malleolus. This facet is situated immediately in front of the deep pit which characterises the posterior part of this surface of the fibula. A small lunated facet is frequently found upon the anterior surface of tlie lower end of the tibia, particularly among those races characterised by the adoption of the " squatting '" posture. When this facet exists it is continuous with the anterior margin of the roof of the socket, and it articulates with a similar facet upon the upper surface of the neck of the astragalus in the extreme flexion of the ankle-joint which "squatting" entails. The articular surface upon the body of the astragalus adapts itself to the tibio- fibular socket, and presents articular facets corresponding to the roof and sides of the socket. Thus the superior surface of the astragalus possesses a quadrilateral articular area, wider in front than behind, distinctly convex in the antero-posterior direction, and slightly concave transversely. In addition, towards its postero- external margin, there is also a narrow antero-posterior facet corresponding to the transverse inferior tibio-fibular ligament. The articular cartilage of this upper surface is continued without interruption to the tibial and fibular sides of the bone, although the margins of the superior area are sharply defined from the lateral facets, the outer of which is triangular in outline, while the inner is pyriform, but in each case the surface is vertical. Ligaments. — The ligaments form a complete investment for the joint, i.e. a capsule in which the individual parts vary considerably in strength, and are described under separate names. The anterior ligament is an extremely thin membrane, containing very few longitudinal fibres. It extends from the lower border of the tibia to the upper border of the head of the astragalus, passing in lateral or fj^-Qnt of a pad of fat which deltoid Tin ligament of fills up the liollow abovc the aiikle-ioint i n ,-\ , i neck 01 that bone. The posterior ligament is attached to contigu- ous non- articular borders of the tibia and astragalus. Many of its fibres radiate inwards from the external malleolus. This aspect of the joint is strengthened by the strong, well-defined, trans- verse ligament already described in connexion with the inferior tibio-fibular joint. All tenor talo fibular lij;amei Articular facet on external malleoU Anteiioi mfeiior tibio fibular ligament Internal Calcaneo-fibular ligament' Posterior inferioi tibio-fibnlai ligament Posterior talo-fibular ligament -Articular Surfaces of Tibia and Fibula which are opposed to the astragalus. Transverse inferior tibio-fibular ligament Synovial pad of fat Fig. 238. THE ANKLE-JOINT. 307 The external lateral ligament (Figs. 237, 238, and 240) is very powerful, and is divisible into three fasciculi, which are distinguished from each other by names descriptive of their chief points of attachment. The anterior fasciculus (lig. talo-fibulare anterius) is the shortest. It extends from the anterior border of the external malleolus to the astragalus immediately in front of its external articular surface. The middle fasciculus (lig. calcaneo-fibulare) is a strong and rounded cord. It is attached by one end to the front of the tip of the external malleolus, and by the other to the outer side of the os calcis, immediately above the groove for the peroneal tendons. The 2^osterior fasciculus (lig. talo-fibulare posterius) is the strongest. It runs transversely between the lower part of the fibular or digital fossa on the inner aspect of the malleolus and the posterior surface of the astragalus, where it is attached Internal malleolus Internal lateral or deltoid lijjanient of the ankle Tiochlear surface of astragalus Groove for tendon of tibialis ]josticus muscle on inferior ' ilcaiieo-scaphoid ligament / Groove and tunnel for the tendon of flexor longus hallucis muscle Inner tarso- ^ ^^^^^^ - -.x. - .»., . - . -...>^ ,..-. _/0s calcis metatarsal .joint (opened) Long plantar ligament Tendon of tibialis posticus muscle (cut) Sustentaculum tali Fig. 239. — Ankle and Tarsal-Joints from the Tibial Aspect. to the external tubercle and the adjoining rough surface. Sometimes this tubercle is detached from the astragalus, and represents a separate bone — the os trigonum. ^^The internal lateral ligament (lig. deltoideum, Figs. 238 and 239) has the general shape of a delta,and is even stronger than the external ligament. It is attached above to a m-irked impression on the lower part of the internal malleolus, and below, in a continuous layer, to the scaphoid, astragalus, and os calcis. In it we may recog- nise the following special bands — {a) the lig. talo-tihicde anterius, which extends from the front of the inner malleolus to the neck of the astragalus ; (&) the lig. talo-tihiale 'posterius, stretching between the back of the inner malleolus and the postero-internal rough surface of the astragalus ; (c) the lig. tihio -navicular e, which extends from the tiyj of the inner malleolus to the inner side of the scaphoid ; {d) the lig. calcaneo-tihicde, which extends between the tip of the inner malleolus and the inner side of the Hiistentaculum tali; (e) lig. talo-tihiale profundum, which consists of deeyxjr fibres exttiuding I'roiii the tip of the internal malleolus ti3 the inner side of the ;istragaluH. Synovial membrane lines tbe capsular ligament, and, as already described, the joint-cavity communicateB directly with the inferior tibio-fibular joint. Both at the front and \y,)Mk of the ankle-joint, as well as superiorly in the angle formed })y the three bones, the synovial membrane covers pads of fat. 308 THE ARTICULATIONS OR JOINTS. Movements at the Ankle-Joint. — In the erect attitude the foot is placed at right angles to the leg; in other words, tlic ncniiiul position of the ankle-joint is flexion. Those movements which tend to diminish the angle so formed by the dorsum of the foot and the front of the leg are called dorsiflexion, while those which tend to increase the angle, i.e. to straighten the foot upon the leg, are called extension. As a matter of fact neither dorsiflexion nor extension is ever completcdy carried out, and the range of movement of which the foot is capable is limited to about 90". These movements occur about an oljliquely transverse axis, as is indicated by the natural outward pointing of the toes. The weight of the body falls slightly anterior to the ankle- joint, so that a certain amount of muscular action is necessitated in order to maintain the foot at right angles to the leg ; but additional stability is olitained fi'om the obliquity above mentioned. When the foot is raised from the ground, muscular action tends naturally to produce a certain amount of extension. When the foot is extended, as in standing on the toes, the hinder narrow part of the astragalus moves forM'ards into the wider part of the interval ljet\\een the tibia and tibula, whereas in dorsiflexion, as in raising the fore part of the foot from the ground, the widest part of the astragalus is forced back between the tibia and fibula ; but notwithstanding the dif- ference between these two movements, the fibula remains in close contact with the astragalus by reason of the action of the transverse inferior tibio-fibular ligament and the posterior talo-fibular ligament, so that lateral movement is ^u'evented. It is doubtful whether lateral movement at the ankle-joint can be obtained by any natural movement of the foot, although it is generally believed that in the position of jiartial extension a small amount of lateral movement may be produced by the application of external force. " This apparent play " of the ankle-joint cluring extension " is really due to oscillation of the small bones of the foot on each other, largely of the scaphoid on the astragalus, but also of the cuboid on the calcaneum. Excessive mobility of these latter is restrained by an important function of the posterior tubercle of the cuboid which locks into a notch in the os calcis" (Blake). INTERTARSAL JOINTS. These joints (articulationes intertarsese) are all diarthroses in which the gliding movement is characteristic, as in the carpus. With the view of obtaining a proper con- Posterior infpii tibio-fibular h^aim Articwlav surface of astnr,alu Posterior fasciculus of extern i lateral ligainoiit of ankle Anteiioi lufeii i tibio-libuhir ligament Aiticul u -^urt ice of astragalus ■Vnterior fasciculus of external lateral ligament ot lukle 1 01 sal astiagalo navicular ligauiont y_ \stia^ ilo na\iculai joint ^:^ Evt L ill aneo navicalar ligament Middle fasciculus of external lateral ligament of ankle^ Posterior talo-calcaneal ligament Os calcis I) isil scapho-cuneiform qilio-cuboid ligaments Ml Idle cuneiform 1 xternal cuneiform Cuboid iJorsal calcaiieo-cuboiil ligament L ikaueo-cuboid joint lendon of peroneus longus ' Interosseous tilo calcaneU ligament Tilo calcineal joint External tilo cakaueil ligament Fig. 240.— Ligaments on the Outkr Asi'ect of the Ankle-Joixt and on the Dorsum of the Tarsus. ception of the many beautiful mechanical principles involved in the construction of the foot, it is necessary to study these articulations with considerable attention to detail. Articulatio Talo-calcanea. — The astragalus and os calcis articulate with each other in the articulatio talo-calcanea or calcaneo-astragaloid joint. This joint is situated between the inferior facet on the body of the astragalus INTEETAESAL JOINTS. 309 Scaplioid bom Inferior calcaneo- scaphoid ligament Internal calcaneo- scaphoid ligament Tendon of tibialis posticus muscl (cut) Sustentaculum tali ; articular surface for astragalus Articular surface on scaphoid for head of astragalus Inner surface of external calcaneo-scaphoid ligament External calcaneo-scaphoid Interosseous cal- caneo-astragaloid ligament Articular surface on OS calcis for body of astragalus Os calcis and a corresponding facet on the upper aspect of the hinder part of the os calcis. On each bone the articulation is limited in front by a wide, deep groove which runs obliquely across each bone from within outwards and forwards. The supporting and investing ligaments form a capsule, consisting for the most part of short fibres, but the joint derives additional strength from the external and internal lateral ligaments of the ankle-joint. The capsule is subdivided into the following astragalo-calcaneal or talo-calcaneal bands : — The anterior talo-calcaneal ligament consists of a band of short fibres placed immediately in relation to the anterior end of the deep groove which bounds the articular facets. They are attached to the antero - external aspect of the neck of the astragalus, from which they extend downwards to the adjacent superior surface of the os calcis. The external talo-cal- caneal ligament (Fig. 240) is in continuity with the hinder border of the pre- ceding ligament, and it is placed parallel to, but on a deeper plane than, the middle fasciculus of the external lateral ligament of the ankle-joint. It con- sists of short fibres passing between the adjacent rough outer margins of the two bones. The posterior talo-cal- caneal ligament (Fig. 240) closes the joint-cavity on its posterior aspect. It consists of fibres which radiate from the posterior aspect of the external tubercle of the astragalus to the upper surface of the os calcis, immediately behind the articular facet. The internal talo-calcaneal ligament lies oldiquely on the inner side of the joint, and consists of fibres which extend from the inner posterior tubercle of the astragalus to the hinder roughened border of the sustentaculum tali. Some of its fibres become continuous with the internal calcaneo-scaphoid ligament. The interosseous talo-calcaneal ligament (Fig. 240) closes the antero-internal iispect of the joint. It is the strongest of the series of ligaments entering into the capsule. Compared with it the other bands are, comparatively speaking, insigni- ficant. Its attachments are to the bottom of each groove, so that it occupies the tarsal canal formed by these opposing grooves. A synovial membrane lines the capsule, and it is distinct from other tarsal synovial inem l)ranes. Articulatio Talo-calcaneo-navicularis. — This is one of the most important of the joints of the foot, not only because the astragalus is here situated in relation to the summit of the antero-posterior arch of the foot, but because the head of the astragalus is received into a composite socket made up of sustentaculum tali, scaphoid, and the inferior or internal calcaneo-scaphoid ligament. The articular suri'ace on the head of the astragalus presents anteriorly a convex rounded facet for articulation with the scaphoid, inferiorly a convex facet which rests upon the sustentaculum tali, and intermediate between these two there is a triangular facet which articulates with the inferior calcaneo-scaphoid ligament. All the.se facets are in continuity with each other, and are in front of the tarsal groove on the under surface of the astragalus. Occasionally a fourth narrow I'acet is Fig. 241. — The Composite Articular Socket for the Head oi-- THE Astragalus. 310 THE AETICULATIONS OK JOINTS. found along the outer and hinder part of the articular surface of the head of the astragalus, whereby it articulates with superior or external calcaneo-scaphoid ligament. The scaphoid or navicular bone presents a shallow, cup-shaped, articular cavity towards the head of the astragalus. The articular surface of the sustentaculum tali is concave, and is usually marked off into two fleets. Two ligaments play an iniY)ortant part in binding together the os calcis and the scaplioid, although these bones do not directly articulate ; and further, these ligaments provide additional articular surfaces for the head of the astragalus. These are the two following : — (a) The inferior or internal calcaneo-scaphoid ligament (Figs. 239 and 242) is an Tendon of insortion of pproneus longus muscle Hasp of metatarsal bone of A I Inllux Plantar Inter-metatarsal ligaments Plantar cuboid iid Plantar cubo-cuneifonn ligament ■"' "1 Tendon of peroneus longus musclt— Long plantai ligament Tendon of insertion of tibialis aiiticus muscle Internal cuneiform bone Plantar scaplio-cuneifonn ligament X \ Tendon of tibialis posticus ^' muscle dnove for tendon of tibialis "y posticus muscle Inferior calcaneo-scaphoid li 'anient- '' ^s^j^^Intprnal lateral or deltoid isanient of ankle Internal malleolus Groove for tendon of flexor longus hallucis muscle Os calcis Fig. 242. — Plantaij Aspect of Tarsal amd Tarso-metataesal Joints. extremely powerful fibro-cartilaginous tie-band. It extends between the anterior margin of the sustentaculum tali and the inferior surface of thescaphoid bone. Certain of its upper fibres radiate upwards on the inner surface of the scaphoid, and become continuous with the tibio-navicular portion of the deltoid ligament of the ankle- joint. The plantar aspect of this ligament is in contact with the tendon of the tibialis posticus muscle, through which the head of the talus receives great support. Superiorly it contributes an articular surface which forms a triangular portion of the floor of the composite socket in which the head of the talus is received. (6) The superior or external calcaneo-scaphoid ligament (Fig. 241) lies deeply in the front part of the sinus tarsi, i.e. the interval between the astragalus and os calcis. Its fibres are short, and extend from the dorsal surface of the front part of the OS calcis, immediately to the outer side of the sustentacular facet, forwards to the outer side of the scaphoid bone. Frequently the ligament presents a surface INTEETARSAL JOIiNTS. 311 which articulates with the head of the astragalus, and in these cases it forms a part of the composite socket. The cavity of the talo-calcaneo-navicular joint is closed posteriorly by the interosseous talo-calcaneal ligament already described. On its inner and outer inferior aspects it is closed by the calcaneo-scaphoid ligaments. The superior and lateral aspects are covered by an astragalo-scaphoid membrane or ligament. This ligament is thin, and extends from the upper non-articular area on the head of the astragalus to the dorsal surface of the scaphoid bone. It may be subdivided into dorsal (superiorj, lateral (external), and medial (internal), astragalo-scaphoid ligaments (Fig. 239), which, with the calcaneo-scaphoid and interosseous talo-calcaneal ligaments, complete the capsular investment of the joint. A distinct synovial membrane lines all parts of the capsule of the joint. Articulatio Calcaneo-cuboidea. — This is situated between the anterior concavo- convex surface of the os calcis and the posterior similar surface of the cuboid. The ligaments which invest this joint constitute a calcaneo-cuboid capsule, whose parts are arranged in relation to the four non-articular sides of the cuboid bone, and are especially strong upon the plantar aspect, in relation to their great import- ance in resisting strains. The internal calcaneo-cuboid ligament occupies part of the interval between the astragalus and os calcis — sinus tarsi. It is sometimes called the interosseous calcaneo-cuboid ligament, and, in conjunction with the superior or external calcaneo-scaphoid ligament, it forms a V-shaped structure, of which the single end is attached to the os calcis, and the double ends separate to reach contiguous areas on the scaphoid and cuboid respectively. The dorsal calcaneo-cuboid ligament (Fig. 240) is a broad portion of the capsule extending between the dorsal surfaces of the two bones. The external calcaneo-cuboid ligament is another but narrower part of the capsule which extends from the outer aspect of the os calcis to the outer side of the cuboid, immediately behind the facet on the tuberosity. The inferior calcaneo-cuboid ligaments are two in number — a superficial and a deep. The superficial series of fibres, the long plantar ligament (Fig. "242), is attached to the under surface of the os calcis in front of its tuberosities. It forms a long powerful structure which runs forwards to be fixed to the under surface of the cuboid ridge, but many of its fibres pass superficial to the tendon of the peroneus longus, and extend to the bases of the third, fourth, and fifth metatarsal bones. The deep series of fibres, the short plantar ligament (Fig. 242), is distinctly separated from the former by a layer of areolar tissue. It forms a broad t)ut short band of great strength, which is attached to the under surface of the front end of the os calcis, and extends to the under surface of the cuboid just behind the ridge. Both of these ligaments are of great importance in maintaining the longitudinal arch of the foot, and in this respect are only second to the inferior calcaneo-scaphoid ligaments. A synovial membrane lines the capsule. Transverse Tarsal Articulation. — This is a term sometimes applied to the astragalo-scaphoid and calcaneo-cuboid joints. These articulations do not com- municate with each other ; and although there is an occasional direct articula- tion between the scaphoid and cuboid, it does not constitute an extension of the transverse tarsal joint, but is a prolongation from the series of scapho-cuneiform and cuneo-cuboid articulations. Nevertheless there is always a eet of ligaments which bind the scaphoid and cuboid bones together, and these may be regarded as accessory to the various transverse tarsal joints. The dorsal scapho-cuboid ligament (Fig. 240) consists of short oblique fibres which attach tbe contiguous dorsal surfaces of the cuboid and scaphoid bones. The plantar scapho-cuboid ligament is transverse in direction, and extends between adjaccMiL ]»laiila,r an;a,s oi' the culioid and scaphoid bones. The interosseous scapho-cuboid ligament int(!rvorieH between contiguous surfaces of t lie same bones. When there is an extension of the scapho-cuneiform joint back- wards l>etweeri the scaphoid and cuboid, it is situated in front of the last-men- tioned ligament, and is called tlie articulatio scapho-cuboidea. Around this joint 312 THE AKTICULATIONS OE JOINTS. the preceding ligaments are grouped. Since, however, the joint is inconstant while the ligaments are always present, it is preferable to consider them as above indicated. Scapho-cuneiform Articulation (articulatio cuneo-navicularis). — This joint is situated between the scaplioid and the three cuneiform Ijones. The anterior surface of the scaphoid presents facets for each of the cuneiform bones, but its articular surface is not interrupted. These facets form a somewhat convex anterior surface which fits into the shallow articular concavity presented by the proximal ends of the three cuneiform bones. This joint may be extended by the occasioual scapho-cuboid articulation already referred to. The capsule is composed of short, strong bands which are distinctly visible on all sides except towards the cuboid bone, where the joint may connnunicate with the cuneo-cuboid and scapho-cuboid joints. Anteriorly the joint communi- cates with the intercuneiform articulations. The dorsal parts of the capsule are short longitudinal bands termed dorsal scapho-cuneiform ligaments (Figs. 239 and 240). These extend without interruption to the inuer aspect of the joint. Interiorly there are similar bands, known as plantar scapho-cuneiform ligaments, also longitudinal in direction, but intimately associated with offsets Irom the tendon of the tibialis posticus muscle. The synovial membrane which lines the capsule sends prolongations forwards on each side of the middle cuneiform bone, and in addition it often communicates witii the cuneo-cuboid joint cavity, and it always communicates with the scapho- cuboid cavity when that joint exists. Intercuneiform Articulations. — These are two in number, and exist between adjacent contiguous surfaces of the three cuneiform bones. These surfaces are partly articular and partly non-articular. The small size of the middle cuneiform bone allows the internal cuneiform as well as the external cuneiform to project forwards beyond it on both sides, and therefore the articular surfaces turned towards the middle cuneiform are not entirely occupied by that bone. They form a recess towards the metatarsus, into which the base of the second metatarsal bone is thrust. Dorsal intercuneiform ligaments constitute fairly strong transverse ' bands which extend between adjacent dorsal surfaces and invest the joint cavities in this direction. The plantar or interosseous intercuneiform ligaments are two strong bands which pass from the rough non-articular areas on opposite sides of the middle cuneiform to the opposing surfaces of the inner and outer cuneiform bones. These ligaments shut in the joint cavities inferiorly, and also anteriorly in the case of the outer of the two joints. The synovial membrane is an extension of that which lines the scapho-cuneiform joint ; but while it is restricted to the outer of the two joints, in the case of the inner one it is prolonged still farther forward to the tarso-metatarsal series of joints. Cubo-cuneiform Articulation. — This occurs between the rounded or oval facets on the opposing surfaces of tlie cuboid and external cuneiform. The dorsal cubo-cuneiform ligament is a flat, somewhat transverse band which closes the joint on its superior aspect, and extends between the dorsal surfaces of the two bones. The plantar cubo-cuneiform ligament is diificult to determine. It is situated subjacent to the long plantar ligament, and extends between adjacent rough svirfaces of the two bones. The interosseous cubo-cuneiform ligament is the strongest. It closes the joint cavity anteriorly, and is attached to the contiguous non-articular surfaces of the two bones. The synovial membrane is frequently distinct, but at other times the joint cavity communicates with those of the scapho-cuneiform and scapho-cuboid articulations. Synovial Membranes of the Intertarsal Joints. — Four and sometimes five distinct and separate synovial membranes may thus be enumerated in connexion with the tarsal articulations, viz. : (1) talo-calcaneal; (2) talo-calcaneo-navicularis ; (3) calcaneo-cuboid ; (4) ^ scapho-cuneiform and its extensions ; (5) occasionally cubo-cuneiform. TAESO-METATARSAL JOINTS. 313 TARSO-METATARSAL JOINTS. The tarso-metatarsal joints are found between certain articular facets on the cuboid and three cuneiform bones on the one hand, and others on the bases of the five metatarsal bones. These articulations are associated with three distinct synovial cavities — namely, an inner, middle, and outer. (1) The inner tarso-metatarsal articulation occurs between the distal convex reuiform surface of the internal cuneiform bone and the concavo-reniform surface on the proximal aspect of the base of the first metatarsal bone. Ligaments which form a capsule surround the articulation. In the capsule the dorsal and plantar tarso-metatarsal bands are its strongest parts, but it is not deficient either on the inuer or on the outer aspects. A separate synovial membrane lines the capsule. (2) The middle tarso-metatarsal articulation is an elaborate joint. It involves the three cuneiform bones and the bases of the second, third, and part of the fourth metatarsal bones. The articulation presents the outline of an indented parapet both on its tarsal and its metatarsal aspects. Thus, on its tarsal side, the inner and the outer cunei- form bones project in front of the middle cuneiform, so that the latter only presents a distal surface to the articulation ; while the internal cuneiform presents a portion of its external surface, and the external cuneiform presents ?joth its distal and portions of its outer and inner surfaces, since it projects in front of the cuboid bone. On its metatarsal side the base of the second metatarsal bone fits into the indentation between the outer and inner cuneiforms, to which it presents external and internal articular facets, but its proximal facet rests upon the distal facet of the middle cuneiform. The base of the third metatarsal bone rests its proximal facet upon the outer cuneiform. The fourth metatarsal base presents part of its internal facet to the external side of the outer cuneiform. In this way the indentations alternate on the two sides of the articulation, and an extremely powerful interlock- ing of parts is provided, which places any marked independent movement of these metatarsal iDones entirely out of the question. The dorsal tarso-metatarsal ligaments are broad, flat bands which represent the most distinct part of an investing capsule. They pass from behind forwards, and while the second metatarsal bone receives three, i.e. one from each cuneiform, the third metatarsal only receives one — from the external cuneiform. The plantar tarso-metatarsal ligaments correspond with the foregoing in their general arrangement, but they are weaker. That for the second metatarsal is the strongest. Oblique bands extend from the inner cuneiform bone to the second and third metatarsals. The interosseous cuneo-metatarsal ligaments are three in number. The inner connects the outer side of the internal cuneiform with the inner side of the base of the second metatarsal bone. The middle connects the inner side of the external cuneiform with the outer side of the base of the second metatarsal. The outer connects the adjacent outer sides of the external cuneiform and third metatarsal. The synovial membrane, which lines this articulation, sends a prolongation back- wards between the inner and middle cuneiform bones, where it opens into the scayjho-cuneiform joint. It is likewise prolonged forwards upon both sides of each of the bases of the second and third metatarsal bones. (3) The external tarso-metatarsal articulation is found between the proximal surfaces of the bases of the fourth and fifth metatarsal bones and the distal surface of the cuboid. The investing capsule may bo resolved into the following ligaments : — The dorsal tarso -metatarsal ligaments rcsein))le those already descriljed. The base of the fourth metatarsal receives one froin tlie external cuneiform and one from the cuboid. The base of the fifth metatarsal receives one from the cuboid. The plantar tarso-metatarsal ligaments are the wealcest bands of the series, and corisiHt of scattered filjrcs ])aHsing from the cuboid to the bases of the two metatarsals. Some fibres, which are almost transverse, extend from the 22 314 THE ARTICULATIONS OE JOINTS. external cuneiform to the fifth metatarsal, and additional filjres reach the meta- tarsals in question from the long plantar ligament (calcaneo-cuboid). Occasionally the tarsal end of the external interosseous (cuneo-metatarsal) liga- ment is attached to the inner margin of the culjoid. The synovial membrane is restricted to this articulation, and merely sends a prolongation forwards between the opposing articulate aspects of the fourth and fifth metatarsal bases. INTERMETATARSAL JOINTS. The intermetatarsal articulations are found between adjacent lateral aspects of the bases of the four outer metatarsal bones. The articular facets are small, oval, or rounded surfaces which occupy only a limited portion of the flattened contiguous surfaces of the bones. Each joint is provided with a capsule, which, however, is not a complete investment, because the three joint cavities are in free communication on their proximal aspects with the tarso-metatarsal joint cavities — one with the outer and two with the middle. The definite fibres of each capsule are situated chiefly in the transverse direction. The dorsal ligaments are short bands which extend from one base to the other. The plantar and interosseous ligaments are similarly arranged, but the latter are the strongest and most important members of this series. The synovial membranes are extensions from those which line the outer and middle tarso-metatarsal joint cavities. Frequently a bursa is found between the bases of the first and second metatarsal bones. It produces an apj^earance of indistinct facetting upon these bones, and it may communicate with the inner tarso-metatarsal (cuneo-metatarsal) joint. The transverse metatarsal ligament lies upon, and is attached to, the non- articular plantar aspects of the heads of all the metatarsal bones. It differs from the corresponding ligament in the palm in the fact that it binds all the metatarsal bones together, whereas in the palm the thumb is left free. It is closely associated with the plantar fibrous plates of the metatarso-phalangeal joints, to the plantar surfaces of which it contributes prolongations termed ligamenta accessoria plantaria. METATARSO-PHALANGEAL JOINTS. Each of these joints is a modified ball-and-socket in which a shallow cup upon the bases of the first phalanges receives the somewhat globular head of a metatarsal bone. Each joint retains a modified capsule which invests the joint. Its only distinct bands are the ligamenta collateralia. These are strong cord-like bands which are situated on the inner and outer sides of each joint, where they extend between adjacent rough surfaces. On the dorsal aspect ligaments distinct from the dorsal expansion of the ex- tensor tendons can hardly be said to exist. The plantar aspect of the capsule consists of a thick fibrous plate, which in the case of the great toe presents developed within it two large sesamoid bones. In the other toes this plate remains fibrous throughout, and is grooved on its plantar aspect for the accommodation of the long flexor tendons. It will thus be seen that the metatarso-phalangeal joints are constructed upon a plan very similar to that of the corresponding joints in the hand. A synovial membrane lines the capsule of each articulation. INTERPHALANGEAL JOINTS. Each toe possesses two interphalangeal joints except the great toe, which has only one. Not unfrequently in the little toe the distal joint is obliterated through ankylosis. All the joints of this series are uniaxial or hinge joints. The nature of the articular surfaces closely resembles the corresponding joints in the fingers. Each joint possesses a capsule which is either very thin or limited to synovial membrane on the dorsal aspect. The plantar surface of the capsule is strength- INTERPHALANGEAL JOINTS. 315 ened by a fibrous plate. The lateral ligaments (ligamenta collateralia) are well- defined bands similar to those already described in connexion with the metatarso- phalangeal joints. A synovial membrane lines each capsule in the series. Mechanism of the Foot. — The bones of tbe foot are arranged in the form of a longitudinal and a transverse arch. The longitudinal arch is built on a very remarkable plan. Posteriorly the mass of the os calcis constitutes a rigid and stable pier of support, while anterioi'ly, by increasing the number of component parts, the anterior pier acquires great flexibility and elasticity without sacrificing strength or stability. The summit of the arch is formed by the astragalus, which receives the weight of the body from the tibia, and the resilience of the arch is assured by the calcaneo-scaphoid and calcaneo-cuboid ligaments, together with the plantar fascia, which act as powerful braces or tie bands, preventing undue separation of the piers of the arch, and consequent flattening of the foot. The weight of the body is distributed over all the five digits, owing to the arrangement of the bones of the foot in two parallel columns, an inner and an outer. The former, consisting of the astragalus, scaphoid, and the three cuneiforms, with the three inner metatarsal bones, distributes weight through the talo-scaphoid joint, while the latter {i.e. the outer column), comprising the calcaneum, cuboid, and the two outer metatarsal bones, acts in a similar manner through the talo-calcanean joint. The main line of immobility of this arch passes from the heel forwards through the middle toe, but its anterior section, which is slender, is supported on either side by two metatarsal bones, with their proximal tarsal associations, in all of which greater freedom of movement is found. The transverse arch is most marked at the level of tarso-metatarsal articulations. The intersection of these two arches at right angles to each other introduces an architectural feature of great importance in connexion with the support of heavy weights. These longitudinal and transverse arches of the foot are in effect " vaults " intersecting each other at right angles, and in relation to the area which is common to both " vaults " the body weight is superposed exactly as the dome of a cathedral is carried upon two intersecting vaults. Movements at the Joints of Tarsus, Metatarsus, and Phalanges. — Considered in detail, the amount of movement which takes place between any two of these bones is extremely small, and, so far as the tarsus and metatarsus are concerned, it is mostly of the nature of a gliding motion. At the metatarso -phalangeal and interphalangeal joints movement is much more free, and is of the nature of flexion (bending of the toes towards the sole of the foot, i.e. plantar flexion) and extension. The latter movement when continued so as to raise the toes from the ground, and bend or approximate them towards the front of the leg, is termed dorsiflexion. Coincident with dorsiflexion there is always associated a certain amount of spreading of the toes, which is called abduction, and similarly with prolonged flexion there follows a diminution or narrowing of the transverse diameter of the front part of the foot by drawing the toes together — a move- ment termed adduction. In the foot the movements of abduction and adduction take place in regard to a plane which bisects the foot antero-posteriorly through the second toe, for this toe carries the first and second dorsal interosseous muscles. Notwithstanding the small amount of possible movement in connexion with individual tarsal and metatarsal joints, yet the sum total of these movements is considerable as regards the entire foot. In this way the movements of inversion and eversion of the foot result. By inversion we mean the raising of the inner border of the foot so that the sole looks inwards, while the toes are depressed towards the ground, and the outer border of the foot remains down- wards. This takes place chiefly at the talo-calcanean joint, but the transverse tarsal joints also participate. Eversion is chiefly the opposite of inversion, and the return of the foot to the normal position of the erect attitude ; but under certain conditions it may be carried further, so that the outer border of the foot is raised from the ground, while the inner border is depressed. In both of these movements thei-e is rotation between the astragalus and os calcis about an oblique axis which passes from the inner side of the neck of the astragalus to the outer and lower part of the os calcis. Of course all the movements of the foot are subordinated to its primary functions as an organ of support and progression. For these purposes its longitudinal and transverse arches are of extreme importance. The longitudinal arch resting on the os calcis behind and the heads of the metatarsal bones in front receives the weight of the body, as already exjalained, on the summit of the astragalus in the line of the third toe. Hence it is that the inner malleolus ajii^ears to be unduly proMiinent on the inner side of the ankle. The transverse arch buttresses the longitudinal one, and tlierefore, whetlier the body weight fall to the outer or tlie inner side of the longitudinal arch, it is supported Ijy a mechanism at once stable, flexible, and elastic, or resilient, and capable of reducing to a minimum all jars that may be received by the fore-part of the foot. As the heel is raised in the act of walking, the weight is gradually transferred from the outer to the inner side of the foot, until the foot finally leaves the ground with a propulsive movement, which results froiri flexion of the phalanges of the great toe. In this connexion it is worthy of note that the longitudinal line of greatest strength is on the inner side of the longitudinal arch, i.e. in I'elution to the great toe. 22 a THE MUSCULAR SYSTEM. MYOLOGY. By A. M. Paterson. The movements of the various parts and organs of the body are brought about by the agency of muscle-cells, which are characterised by a special histological structure and by the special function of contracting in length under the influence of a proper stimulus. There are three classes of muscle-cells : (1) the striated, and usually voluntary muscle-cells, out of which the skeletal muscular system is constructed ; (2) the non- striated, involuntary muscle-cells, occurring in the walls of vessels and hollow viscera, etc. ; and (3) the cardial muscle-cells, striated but involuntary, of which the substance of the heart is composed. The following section deals solely with the skeletal muscles, the structure, arrangement, and mechanical action of which are based upon a common plan. The cells of which the skeletal muscles are composed are long, narrow, and characterised by a peculiar striation, which is different from the striation of the muscle-cells of the heart ; they also differ both in structure and function from the non-striated muscle-cells which occur in viscera and vessels. A typical skeletal muscle consists of a fleshy mass enveloped in a membranous aponeurosis or fascia, and provided at its extremities or borders with membranous or tendinous attachments to bone, cartilage, or fascia. Each muscle is made up of a number of fasciculi or bundles, arranged together in different muscles in different ways, so as to give rise to the particular form of the muscle in question. These fasciculi are connected together by a dehcate connective tissue, the perimysium externum, continuous externally with the aponeurosis enclosing the muscle. Each muscular bundle or fasciculus is composed of a number of narrow, elon- gated muscle-cells or fibres, held together by a still more delicate connective tissue, the perimysium internum. This tissue is connected on the one hand with the sarco- lemma or cell-wall of the muscle-cell^ and on the other hand with the coarser tissue of the perimysium externum enclosing the muscular bundles. By means of these connective tissue envelopes the muscle-cells, the essential agents of motor activity, are brought into firm and intimate relation with the osseous or other attachments of the muscle. Through the agency of sarcolemma, perimysium internum, perimysium externum, aponeurosis, and tendon, the muscle- cell when it contracts can produce a precise and definite effect upon the weight to be moved. Each muscle is supplied by one or more nerves, which, in their course through the muscle, separate into smaller and smaller branches, ultimately, by their terminal filaments (axons), forming special end-organs in relation to each muscle-cell. While a muscle may thus be looked upon as an organ endowed with particular properties, and executing a definite movement in response to a stimulus by the simultaneous contraction of its constituent cells, the various muscles may further be considered in groups, associated together by mode of development, nerve-supply, and co-ordination of action. For example, we speak of the hamstring muscles of 316 MYOLOGY. 317 the thigh, the muscles of the back, and the pr&evertebral muscles — groups iu which separate muscles are associated together by development, nerve-supply, and action. In their development the separate muscles arise from the subdivision of a larger stratum, as in the limbs, or from the fusion of segmental elements (myotomes), as in the case of the axial muscles ; the peripheral nerves supplying skeletal muscles are distributed, through the plexuses or directly, so as to associate particular muscles morphologically and physiologically, and to secure a co-ordinated movement by the simultaneous contraction of several muscles together. FASCI.E. Beneath the skin there are two (or in some regions three) layers of tissue which require consideration : the superficial fascia (panniculus adiposus), the deep fascia (fascia lata), and, in animals, the panniculus carnosus (rudimentary in man, and represented chiefly by the platysma myoides in the neck). Superficial Fascia. — The superficial fascia is a continuous sheet of areolar tissue which underlies the skin of the whole body. It is closely adherent to the cutis vera, and is sometimes termed panniculus adiposus, from the fact that, except beneath the skin of the eyelids, penis, and scrotum, it is always more or less impregnated with fat. It is traversed by the cutaneous vessels and nerves ; and its deep surface, membranous in character, is in loose connexion with the subjacent deep fascia. It is in this layer that dropsical effusions chiefly occur. Deep Fascia. — Underneath the skin and superficial fascia is a fibrous membrane, bluish white in colour, devoid of fat, and in closest relation to skeleton, ligaments, and muscles. This is the deep fascia. It covers, invests, and in some cases forms the means of attachment of the various muscles. It has a special tendency to become attached to all subcutaneous bony prominences, and to be continuous with the connecting ligaments. It forms septal processes, which separate groups of muscles and individual muscles ; enclose glands and viscera ; and form sheaths for vessels and nerves. Around joints it gives rise to bands which strengthen the capsule or limit the mobility of the joint, or, as in the case of annular ligaments, bind down the tendons passing over the joint. It not only ensheathes vessels and nerves, but is perforated by those which pass between super- ficial and deeper parts. The term aponeurosis is used in relation to muscles. It is synonymous with deep fascia, either as an investing fascia, or as a membranous layer which {e.g. vertebral aponeurosis) performs at one and the same time the purpose of a deep fascia and the expanded membranous attachment of a muscle. The panniculus carnosus is a thin muscular layer enveloping the trunk of animals with a hairy or furry coat. It is strongly developed in the hedgehog. In man it is represented mainly in the (rudimentary) platysma myoides. It is placed between the superficial and the deep fascia. Bursae. — ^Where a tendon passes over a bony surface, or where the superficial fascia and skin cover a bony prominence, there is generally formed a synovial sac, or bursa, containing fluid, for the purpose of lubricating the surface over which the tendon or fascia glides. Allied to these are the synovial sheaths which envelop tendons beneath the annular ligaments in relation to the several joints. Description of Muscles. — In studying the muscular system it is necessary to note the following characters in reference to each individual muscle : (1) The shape of the muscle — flat, cylindrical, triangular, rhomboidal, etc. — and the character of its extremities — membranous, tendinous, or fleshy. (2) The attachments of the muscle. The orir/in is the more fixed or central attachment : the insertion is the more movaljle or peri])])oral attachment. (3) The relations of the surfaces and borders of the musch; to bones, joints, other muscles, and other important structures. (4) Its vascular iiud nervous supply ; and (5) its action. It must be borne in mind that hardly any single muscle acts alone. Each muscle, as a rule, forms one of a group acting more or less in harmony with, and antagonised by, other and opposite groups. 22 & 318 THE MUSCULAE SYSTEM. DESCRIPTION OF THE MUSCLES. The skeletal muscles may be divided into two series : axial and appendicular. The axial muscles comprise the muscles of the trunk, head, and face, including the panniculus carnosus (platysma myoides). These muscles are more or less segmental in arrangement, grouped around the axial skeleton. The appendicular muscles, the muscles of the limbs, are grouped around the appendicular skeleton. They are not segmental in arrangement, they are clearly separate from the axial muscles, and they are arranged in definite strata in relation to the bones of the limbs. APPENDICULAR MUSCLES. THE UPPER LIMB. FASCIA AND SUPERFICIAL MUSCLES OF THE BACK. Fascia. The superficial fascia of the back presents no peculiarity. It is usually of considerable thickness, and contains a quantity of fat. The deep fascia closely invests the muscles. It is attached in the middle line of the back to the ligamentum nuchse, supraspinous ligaments, and vertebral spines ; laterally it is attached to the spine of the scapula and the clavicle, and is continued over the deltoid region to the arm. In the neck it is attached above to the superior curved line of the occipital bone, and is continuous laterally with the deep cervical fascia. Below the level of the arm it is continuous round the border of the latissimus dorsi muscle, with the fascia of the axilla and of the abdominal wall. In the back and loin it constitutes the vertebral aponeurosis or aponeurosis of the latissimus dorsi. It conceals the erector spiufe muscle, forming the posterior layer of the lumbar fascia, and it is attached internally to the vertebral spines, and externally to the angles of the ribs, to the lumbar fascia, and to the iliac crest. The Superficial Muscles of the Back. The muscles of the back are arranged in four series according to their attach- ments: (1) vertebro-scapularand vertebro-humeral; (2) vertebro-costal; (3)vertebro- cranial ; and (4) vertebral. The first of this series consists of the posterior muscles connecting the upper limb to the trunk, and comprises the first two layers of the muscles of the back — (1) trapezius and latissimus dorsi, and (2) levator anguli scapuhe and rhomboidei (major and minor). The deeper (axial) muscles of the back are dealt with later. The trapezius (m. trapezius, cucullaris) is a large triangular muscle occupying the upper part of the back. It arises from the superior curved line of the occipital bone in its inner third, from the external occipital protuberance (Fig. 312, p. 397), from the ligamentum nuchse, from the spines of the seventh cervical and all the thoracic vertebrae, and the corresponding supra-spinous ligaments. The origin is by direct fleshy attachment, except in relation to the occipital bone, the lower part of the neck, and the lower thoracic vertebrse, in which places the origins are tendinous. From their origin the muscular fibres converge towards the bones of the shoulder, to be inserted conti-nously from before backwards as follows: (1) The occipital and upper cervical fibres — into the posterior surface of the clavicle in its outer third (Figs. 243, p. 319, and 248, p. 324); (2) the lower cervical and upper thoracic fibres— into the inner border of the acromion process, and the upper border of the spine of the scapula ; and (3) the lower thoracic fibres, by a triangular flat tendon, beneath which a bursa is placed — into a rough tuberosity at the base of the spine of the scapula (Fig. 299, p. 383). The fibres inserted into the clavicle, acromion, and the upper border of the spine of the scapula, spread over the adjacent subcvitaneous surfaces of these bones for a variable distance. FASCIA AND SUPEIiFICIAL MUSCLES OF THE BACK. 319 The occipital portion of tlie muscle may be in the form of a separate slip, or may be entirely absent. The trapezius is superficial in its whole extent. Its upper lateral border forms the posterior limit of the posterior triangle of the neck. The lower lateral border, passing over the upper edge of the latissimus dorsi and the vertebral border of the scapula, forms a boundary of the so-called triangle of auscultation completed below by the latissimus dorsi, and externally by the vertebral border of the scapula. This space is partly filled up by the rhomboideus major. The deep surface of the muscle is in contact with the complexus, splenius capitis, omohyoid, supraspinatus, levator anguli scapulae, rhomboidei, serratus posticus superior, erector spinse, and latissimus dorsi muscles. The spinal accessory nerve, branches of the cervical plexus (C. 3. 4.), and of the superficial cervical and posterior scapular arteries, are situated beneath the muscle. The latissimus dorsi is a large triangular muscle occupying the lower part of the back. It has a triple origin. The greater part of the muscle arises — (1) from Pectoralis major (origin) Trapezius (insertion), Fig. 243. — Muscle- Attachments to the Clavicle (Upper Surface). the vertehral aponewosis (posterior layer of the lumbar fascia or aponeurosis of the latissimus dorsi). This is a thick membrane which conceals the erector spinee in the lower part of the back. Through it the latissimus dorsi gains attachment to the spines of the lower six thoracic vertebrae, the spines of the lumbar vertebrae, and the tendon of the erector spinse with which the aponeurosis blends below. It also arises more externally by fleshy fibres from the posterior part of the iliac crest From this origin the muscle is directed upwards and outwards, its fibres converging to the lower angle of the scapula. In relation to its upper and outer borders additional fibres arise. (2) Along the outer harder muscular slips arise from the lower three or four ribs, inter-digitating with the origins of the obliquus externus abdominis. (3) As the upper border of the muscle passes horizontally over the lower angle of the scapula, an additional fleshy slip usually takes origin from this part of the bone to join the muscle on its deep surface (Fig. 245, p. 321). Beyond the lower angle of the scapula the latissimus dorsi, greatly narrowed, curves spirally round the teres major muscle, and forms the prominence of the posterior axillary fold. It ends in a ribbon-like tendon, closely adherent at first to the teres major, which is inserted into the floor of the bicipital groove of the humerus, extending for about three inches below the lower and outer part of the lesser tuberosity (Fig. 253, p. 329). It is placed, behind the coraco-brachialis and biceps, and the axillary vessels and nerves, and in front of the insertion of the teres major, from which it is separated by a bursa. In the back the latissimus dorsi is superficial, except in its upper part, which is concealed by the trapezius. It lies upon the lumbar fascia, serratus posticus inferior, the ribs, and lower angle of the scapula, and at its borders two triangular spaces are formed ; at the upper border is the so-called triangle of auscultation ; at the outer border is the triangle of Petit, a small space bounded by the iliac crest, the latissimus dorsi, and the obliquus externus abdominis. This is the site of an occasiona] luml^ar licrnia. Th(; levator anguli scapulae (levator scapulae) is a strap-like muscle, arising by tendinous sli|:)S from the posterior tubercles of the transverse processes of the first three or four cervical vertebrae, between the attachments of the scalenus medius in front and the splenius colli behind. It is directed downwards along the 320 THE MUSCULAE SYSTEM. side of the neck, to be inserted into the vertebral border of the scapula in its upper fourth, from the superior angle to the spine (Fig. 245, p. 321). It is concealed in its upper third by the sterno-mastoid and deep muscles of the neck. In its middle third it occupies the floor of the posterior triangle. In its lower third it is again complexus Stebn'o-mastoid- Splenios capitis SpLENIUS COLLI' Serratus posticus superior Levator anguli scAPUL,y- Rhomboxdeus minor Rhomboideus MAJOR ■Sterno-mastoid Deltoid Rhomboideus Teres major' Vertebral aponeurosis Mfp- Teres major Latissimus dorsi Serratus posticus inferior Obliquus externus abdominis Obliquus internus Fascia over gluteus ina\imus Latissimus DORSI Fascia over gluteus maximui. (cut along upper m border of the muscle) Gluteus maximus Fig. 244. — Superficial Muscles of the Back. hidden from view by the trapezius. It conceals the splenius colli and cerAdcalis asceudens muscles. The rhomboideus minor may be regarded as a separated slip of the rhom- boideus major, with which it is often continuous. It arises from the ligamentum nuchce and the spines of the seventh cervical and first thoracic vertebra. Passing obliquely downwards and outwards, it is inserted into the vertebral border of the FASCIiE AND MUSCLES OF THE PECTOKAL REGION. 521 Deltoid (origin) Triceps (origin of long head) Teres minor (origin) with gap for dorsal scapular artery scapula below the levator anguli scapulae muscle, and opposite to the base of the spine (Fig. 245, p. 321). The rhomboideus major arises from the spinous processes of the thoracic vertebrse from the second to the fifth inclusive, and from the corresponding supra- spinous ligaments. Pass- ing downwards and out- wards, it is inserted below the rhomboideus minor into the vertebral border of the scapula, between the spine and the lower angle (Fig. 245, p. 321). The muscle is only in- serted directly into the scapula by means of its lower fibres. Its upper part is attached to a membranous band, which, connected for the most part by loose areolar tissue to the vertebral border of the scapula, is fixed to the bone at its extremities, above near the base of the spine, and below at the inferior angle. The rhomboid muscles are concealed for the most part by the trapezius. The lower part of the rhomboideus major is superficial in the triangle of auscultation. The muscles cover the serratus posticus superior and vertebral aponeurosis. Teres major (origin) Latib.simus dorsi (oi igin) Pig. 245. — Muscle-Attachments to the Scapula (Posterior Surface). THE FASCIA AND MUSCLES OF THE PECTOEAL EEGION. FASCIiE. The superficial fascia of the chest usually contains a quantity of fat, in which the mamma is embedded. The origin of the platysma myoides muscle lies beneath its upper part. The deep fascia is attached above to the clavicle, and internally to the sternum. Below it is continuous with the fascia of the abdominal wall. It gives origin to the platysma myoides, and invests the pectoralis major. At the outer border of the great pectoral muscle it is thickened, and forms the floor of the axillary sjjace (axillary fascia), continued posteriorly on to the posterior fold of the axilla (teres major and latissmus dorsi), and externally into connexion with the deep fascia of the arm. Costo-Coracoid Membrane. — Beneath the pectoralis major a deeper stratum of fascia invests the pectoralis minor muscle. At the up])er border of this muscle it forms the costo-coracoid membrane, which passes upwards to the lower border of the subclaviijs muscle, where it splits into two layers, attached in front of and behind that muscle to the borders of the under surface of the clavicle. The membrane traced inwards along the subclavius muscle is attached to the first costal cartilage ; yjassing outwards along the upp(;r border of the pectoralis minor it roaches the coracoid j)rocess. The part of the membrane extending directly between the first 322 THE MUSCULAE SYSTEM. costal cartilage and the coracoid process is thickened and forms the costo-coracoid ligament. The costo-coracoid memljrane is otherwise thin and of com]^>aratively small importance. It is pierced by the cephalic vein, thoracic axis artery and vein, and branches of the external anterior thoracic nerve. By its deep surface it is connected to the sheath of the axillary vessels. At the lower border of the pectoralis minor there is a further extension of the deep fascia beneath the pectoralis major. It passes downwards to join the fascia forming the floor of the axilla, and is continued externally into the fascia covering the biceps and coraco-brachialis muscles. MUSCLES OF THE PECTORAL REGION. The anterior muscles connecting the upper limb to the axial skeleton comprise the pectoralis major, pectoralis minor, subclavius, serratus magnus, and sterno- cleido-mastoid. The last is described in a later section. The pectoralis major is a large fan-shaped muscle arising in three parts : (1) a clavicular "portion arising from the front of the clavicle in its inner half or two- thirds (Figs. 248, p. 324, and 243, p. 319) ; (2) a costo-sternal portion, the largest sterno-cieido-mastoicKorigin) P^^t of the muscle, arising from the anterior surface of the pre-sternum and meso-sternum by tendinous fibres decussating with those of the opposite muscle (Fig. 246, p. 322) ; and more deeply from the cartilages of the iirst six ribs; (3) an ahdominal portion, a small and separate slip, arising from the aponeurosis of the obliquus externus muscle. The abdominal slip, at first separate, soon merges with the costo-sternal portion, but a distinct interval usually remains between the two first -named parts of the muscle. The fibres converge towards the upper part of the arm, and are inseparably blended at a point half an inch from their in- sertion into the humerus. The muscle is inserted into (1) the outer border of the bicipital groove of the humerus, extending upwards to the great tuberosity, and blending externally with the insertion of the deltoid, internally with the insertion of the latissimus dorsi (Fig. 253, p. 329) ; (2) from the upper border of the insertion a membranous band extends upwards to the capsule of the shoulder -joint, enveloping at the same time the tendon of the biceps ; and (3) from the lower border a band of fascia passes downwards to join the fascia of the arm. The arrangement of the fibres of the muscle at its insertion is peculiar. The muscle is twisted on itself, so that the lower (costo-sternal) fibres are directed upwards and outwards behind the upper (clavicular) part of the muscle ; in conse- quence the clavicular part is attached to the humerus lower down than the costo-sternal portion, and is inserted also into the fascia of the arm. The twisting of the fibres is specially found in the lower costo-sternal fibres of the muscle and the abdominal fibres. These curve upwards behind the upper costo-sternal fibres, and are inserted highest to the shaft of the humerus, helping to form the fascial expansion which extends upwards over the biceps tendon to the capsule of the shoulder-joint. In this way a bilaminar tendon is produced united Rectus abdominis (insertion) Fig. 246. — Muscle- Attach jient.s to the Front of the Sternum. MUSCLES OF THE PECTORAL KEGION. 323 alono- its lower border ; and consisting of a superficial lamina formed by the upper Gosto-sternal fibres, blending for the most part with the tendon of the clavicular portion ; and a deep lamina, composed of the twisted lower costo-sternal and abdominal fibres. The disposition of the muscular fibres at their insertions is the Sterno-mastoid' Trapezius SUBf LAVirS Coracoid process Pectoralis MAJOR (divided) Pectoralis MINOR Pectoralis MAJOR (divided) Serratus \\ \ J MAGNUS LmecC transversse Obliquus externus abdominis Lmea spmilunaris Lmea alba Pvramidalis abdominis Poupart s hgaiaeiit External abdominal nng- TrianKiilai fa->cia Fio. 247. — Anterior Muscles of the Trunk. reason for tlie application of the terms ijortio attollens to the clavicular portion, and 'portio deprimens to the costo-sternal and abdominal portions of the muscle. I 'laced superficially on the chest wall, the pectoralis major forms the front wall of the axilla, and by its lower or outer border, the anterior fold of the space. Its upper border is sei>arated from the edge of the deltoid muscle by an interval in which lie the cephalic vein and humeral artery. Its deep surface is in relation nvith the ribs and intercostal muscles, the costo-coracoid membrane and 324 THE MUSCULAE SYSTEM. the structures piercing it, the pectoralis minor, the axillary vessels, and the nerves of the brachial plexus. Sternalis muscle. — The sternalis is an occasional muscle placed, wlien present, parallel to the sternum upon the sterno-costal origin of the pectoralis major. It has attachments which are very variable both above and below, to the costal cartilages, sternum, rectus sheath, sterno- mastoid, and pectoralis major. Its nerve-supply is from one or both of the anterior thoracic nerves. In certain rare cases it has been said to be innervated by intercostal nerves. It is present in 4'4 cases out of 100, and it is slightly more frequent in the male than in the female. It has been regarded by different observers as (1) a vestige of the panuiculus carnosus, (2) a homologue of the sterno-mastoid, or (3) a displaced slip of the pectoralis major. Chondro-epitrochlearis, dorso-epitrochlearis, axillary arches, costo-coracoideus. — One or other of tlie above-iiauied slips is occasionally j^resent, crossing the floor (jf the axilla in the interval between the latissimus dorsi and the j^ectoralis major. They take origin from the costal cartilages, ribs, or borders of the pectoralis major {chondro-epitroddearis, axillary arches, costo- coracoicleas), or from the border of the latissimus dorsi {dorso-ejntrochlearis, axillary arches, costo- coracoideus). Their insertion is variable. The chondro-epitrochlearis and dorso-epitrochlearis are inserted into the fascia of the arm on the inner side, the internal intermuscular septum, or the internal condyle of the humerus. The axillary arches are inserted into the border of the pectoralis major, the fascia of the arm, or the coraco-brachialis or biceps muscle. The costo-coracoideus, arising from the ribs or the aponeurosis of the obliquus externus, or detaching itself from the border of the pectoralis major or latissimus dorsi, is attached to the coracoid process, alone or along with one of the muscles attached to that bone. These variable slips of muscle are supplied by the internal anterior thoracic nerve, the lesser internal cutaneous nerve, or the intercosto- humeral. The pectoralis minor is a narrow, flat, triangular muscle. It arises from (1) the outer surfaces and upper borders of the third, fourth, and fifth ribs near their anterior ends, and (2) from the fascia covering the third and fourth intercostal spaces between these ribs. It may have an additional origin from the second rib (Fig. 830, p. 420) ; and that from the fifth rib is often absent. Directed obliquely outwards and upwards, it is inserted by a short, fiat tendon into the outer half of the anterior (inner) border and upper surface of the coracoid process (Fig. 250, p. 325), and usually also into the conjoint origin of the biceps and coraco-brachialis. It is wholly concealed by the pectoralis major, except when the arm is raised, when the outer border of the muscle becomes superficial. It enters into the formation of the front wall of the axilla, and gives attachment along its upper border to the costo-coracoid membrane. It crosses the axillary vessels and the cords of the brachial plexus, and is pierced by branches of the anterior thoracic nerves. Either in part or wholly the jDectoralis minor may pass over the coracoid process of the scapula, sejDarated from it by a bursa, to be inserted into the coraco-acromial ligament, or the acromion process ; or piercing the coraco-acromial ligament, it may be attached to the capsule of the shoulder-joint (coraco-humeral ligament). Pectoralis minimus. — This is a slender slip, rarely present, which extends between the first costal cartilage and the coracoid j^rocess. The subclavius muscle arises from the upper surface of the first costal cartilage in front of the costo - clavicular ligament, and from the upper surface of the Pectoralis major (origin) Trapezius (insertion) Subt;lavius (insertion) Fig. 248. — Muscle-Attachments to the Clavicle (Under Surface). distal end of the first rib (Fig. 330, p. 420). It is inserted into a groove in the middle third of the under surface of the clavicle (Fig. 248, p. 324). The muscle is invested by the fascia which forms the costo-coracoid membrane, and is concealed by the clavicle and the clavicular origin of the pectoralis major. MUSCLES OF THE PECTOKAL EEGION. 325 The sterno-clavicularis is a small separate slip, rarely j^resent, extending beneath the pector- alis major from the upper j^art of the sternum to the clavicle. The serratus magnus (m. serratus anterior) is a large curved quadrilateral muscle occupying the side of the chest and inner wall of the axilla. It arises by fleshy slips from the external aspect of the upper eight and occasionally (as in the figure) from nine ribs. The first slip is a double one, arising from the first two ribs and the fascia covering the inter- vening space (Fig. 330, p. 420). The insertion of the muscle is threefold. (1) }s The first portion of the muscle (from the first and second ribs) is directed back- wards to be inserted into the ventral aspect of the upper angle of the scapula. (2) The next three slips of the muscle (from the second, third, and fourth ribs) are inserted into the vertebral border of the scapula. (3) The last four slips (from the fifth, sixth, seventh, and eighth ribs) are directed obliquely upwards and backwards, to be inserted on the ventral Deltoid (origin) Biceps and coraco biachialis (origin) I Ppctoialis ramor (insertion) Omohyoid (origin) Triceps (origin ot long head) Fio. 250.- -Ml'scle-Attach.mk.n'I'.s to 'I'he Scapula (Anterior Aspect). Fig. 249.— The Serratus Magnus Muscle. aspect of the lower angle of the scapula (Fig. 250, p. 325). The external surface of the muscle is partly superficial below the axillary space, on the side wall of the chest, where its slips of origin are seen inter - digitating with those of the obliquus externus abdominis. Higher up it forms . the inner wall of the axilla, and is in contact with the pectoral muscles in front and the subscapularis behind. Its upper border ap- pears in the floor of the pos- terior triangle, and over it the axillary artery and the cords of the brachial plexus pass in their course through the aimpit. The lower border is oblique, and is in contact with the latissimus dorsi muscle. The deep surface of the muscle is in contact with the chest wall, so that the serratus magnus along with the sul)scapularis muHcL^ separates the scapula from the ribs. The muscle may extend higher than usual, so as to be continuous in the neck with th(; levfitor angiili scapulae. 326 THE MUSCULAE SYSTEM. Nerve-Supply. The nerves supplying the muscles connecting the upper limb to tlie trunk are given in the following table :■ — Muscles. Trapezius Latissimus dorsi Levator scapulae Rhomboidei Pectorales Subclavius Serratus magnus Nerves. (Spinal accessory nerve I Cervical plexus Long subscapular / Cervical plexus I Posterior scapular Posterior scapular / External anterior thoracic llnternal „ „ Brachial plexus Posterior thoracic Origin. Spinal Cord C. 3. 4. C. 6. 7. 8. C. 3. 4. C. 5. C. 5. C. 5. 6. 7 C. 8. T. 1 C. 5. 6. C. 5. 6. 7 Actions. The muscles of this group (together with the sterno-cleido-mastoid and omohyoid muscles) act for the most part in the movements of the shoulder girdle at the sterno-clavicular and acromio- clavicular joints. At the former joint they jsroduce the various moA^ements of the clavicle on the sternum, and cause the shoulder to move in an arc the centre of which is the sterno-clavicular joint. At the acromio-clavicular articulation they produce a rotation of the scapula on the clavicle, and a consequent alteration in the direction of the glenoid fossa. At the same time the several muscles are agents in other equally important movements, when the shoulder girdle is fixed ; movements of the head and neck ; movements of the trunk and ribs ; and, in addition in the case of the pectoralis major and latissimus dorsi, important movements of the arm at the shoulder-joint. 1. Movements of the Shoulder Girdle, — The action of this group of muscles on the shoulder girdle (mainly corresponding to movements at the sterno-clavicular joint) may be expressed in the following table : — a. Movement in a Vertical Plane. b. Movement in a Horizontal Plane. Elevation. Depression. Forwards. Backwards. Trapezius (upper Trapezius (lower fibres) fibres) Levator scapulae j Subclavius Rhomboidei I Pectoralis minor Sterno-mastoid , Latissimus dorsi Omohyoid ' Pectoralis major (lower fibres) Serratus magnus Pectoralis major Pectoralis minor Trapezius Rhomboidei Latissimus dorsi j c. Rotation — a combination of these muscles. 2. Movements of the scapula on the clavicle produce an alteration of the direction of the glenoid fossa of the scaj)ula, and are accompanied by movements, inwards or outwards, forwards or backwards, of the inferior angle of the scapula. By the combined action of the muscles acting upon the shoulder girdle a rotatory movement of the scapula at the acromio-clavicular joint is effected, by which the relation of the glenoid fossa to the head of the humerus is preserved in movements of the arm. 3. In forced inspiration, the sterno-mastoid, trapezius, levator scapulae, rhomboidei, sub- clavius, omohyoid, serratus magnus, pectoral muscles, and latissimus dorsi, acting together, raise and fix the shoulder girdle ; while those of them which have costal attachments — subclavius, pectoral muscles, serratus magnus, and latissimus dorsi, simultaneously elevate the ribs and expand the thorax. 4. Lateral flexion and rotation of the spine in the neck is effected partly by the action of the trajsezius, levator scapula, and rhomboid muscles (with the shoulder fixed). The latissimus dorsi and pectoralis major act in climbing in a similar way, raising up the trunk towards the shoulder. 5. Action on the Upper Limb. — By reason of their insertion into the humerus the pectoralis major and latissimus dorsi muscles assist the movements of the upper limb. Acting together, the two muscles depress the shoulder, and draw the arm to the side of the body, at the same time rotating the humerus inwards. The two parts of the pectoralis major have slightly different actions on the humerus. The clavicular part of the muscle {portio attollens) draws the arm in- wards and upwards ; the costo-sternal part of the muscle (j)ortio cUprimens) draws it inwards and downwards. The latissimus dorsi acting alone, besides rotating the limb, draws it inwards and backwards, as in the act of swimming. FASCIiE AND MUSCLES OF THE SHOULDER. 327 FASCIA AND MUSCLES OF THE SHOULDER. The deep fascia covering the scapular muscles presents no feature of special importance. Attached to the clavicle, acromion, and scapular spine, it is thin over the deltoid muscle. Belovs^ the deltoid it is thicker ; it encases and gives origin to the infraspinatus muscle, and is continuous with the fasciae of the axilla and the back. Muscles. The muscles proper to the shoulder comprise the deltoid, supraspinatus, infra- IjEvator anguli scapula.. SOPR^SPINATUS Scapular spine (cut) Infraspinatus ' __Teres minor Dorsal scapular artery (branch of)' Latissimus dorsi ^ ^head)