Corr- QM 105.P22 The morphology of the skull / 3 1924 003 112 004 Cornell University Library The original of tliis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31 9240031 1 2004 THE MORPHOLOGY OF THE SKULL. THE MOEPHOLOGY OF THE SKULL BY W. K; PARKEE, F.RS., HUNTEEIAN PKOPESSO^, EOTAL COLLEGE .OP SnBGEONS ; AND G.- T. BETTANY, M.A., B.Sc, SHUTTLEWOETH SCHOiAH, CAIUS COLLEOE, CAMBEIDOE ; LEOTDKER ON BOTAUT IN aUy'S HOSPITAL MEDICAL SCHOOL. ILontJon : MACMILLAN AND CO. 1877. -r [All Bights reserved.'] PEIKTED BY C. J. CLAY, M.A. AT THE UNIVEKSITY PKESS. PEEFACE. A SKETCH of the history of the skull in the principal ' types of vertebrates is here for the first time presented. It has been attempted to narrate the facts by means of a consistent terminology, amplifying what Prof Huxley has admirably developed; but the descriptions involve as few theoretical opinions as possible. The convenience of students has been considered throughout; summaries of nearly every stage, and of each Chapter from the second to the eighth, have been carefully drawn up. By the help of the index the history of individual bones or tracts can be examined comparatively. Many points of interest have necessarily been omitted in a work treating of anatomy especially in a develop- mental aspect. It is not expected that the book will be thoroughly intelligible after mere reading. The limita- tions of space and of illustration will in some cases account for this; but frequently the complexity of the structures described is such that a brief description may easily lack clearness. The student should consult the very much larger series of figures to be found in original memoirs in the Philosophical Transactions and elsewhere'; 1 Fowl, Phil. Trans., 1869; Prog, 1871; Salmon, 1873; Pig, 1874; Axolotl, 1877 (in the press); Sharks and Bays, Zool. Trans., 1877. A memoir on the Snake is in preparation. VI PREFACE. but personal dissection will in many cases be required for a full comprehension. It is hoped that the book may be found useful as a help to practical work. The types chosen are as accessible as any (the Newt or Salamandei', which will do as well, being substituted for the Axolotl). The adult skull should first be mastered as far as possi- ble ; and then the earlier stages should be worked out both by ordinary dissection, and by transverse and longi- tudinal sections. So far as interpretations are put forward, they are given merely as honest endeavours, not as final judgments. It has been our desire neither to exaggerate the value of what is known, nor to force facts to bear more than legiti- mate inferences. To assist many students to learn mor- phology for themselves is far more our object than to persuade them to accept our momentary ideas. Conse- quently we have not discussed the views of the great anatomists of the days before embryology had illuminated the dark problems of animal structure. Our omission to mention their names is due to no undervaluation of their vast labours and the treasures they have bequeathed to us. One name, however, will remain ever connected with the insight which realised the bearing on the vertebrate skull of its developmental history. Prof. Huxley, since delivering his Croonian lecture (Proc. Roy. Soc. 1858), has never rested in his efforts to throw light upon this subject; and he has had a very great share in the re- searches and elucidations which have made this book possible. His papers in various transactions and journals', 1 Proc. Eoy. Soc. ; Proc. Zool. Soc. ; Jour. Geol. Soc. ; Decades Geol. Sin-yey ; Jour. Anat. and Phys. PREFACE. Vll and his larger works*, bear the most emphatic testimony to his labours. We express our cordial acknowledgments to Mr F. M. Balfour, M.A., Fellow and Lecturer of Trinity College, Cambridge, for reading many pages of proof-sheets, and especially for his kind help in reference to the first Chapter. The illustrations are, with very few exceptions, repro- ductions, mediate or immediate, of Mr Parker's original drawings; some of them have appeared previously in other works. They have been drawn on wood by Mr T. P. CoUings, and engraved by Mr J. D. Cooper. In concluding our difficult task, we cannot but ask in- diilgence for the errors or imperfections which must neces- sarily be associated with the first production of a work on what may be called almost a new subject. 1 Elements of Comparative Anatomy: Anatomy of Vertebrated Animals. London, SepUmber, 1877. CONTENTS. CHAPTER I. PAGE PeELIMINARY EMBBYOLOaY ... 1 CHAPTER II. The Skulls of the Dogfish and the Skate . 14 CHAPTER III. The Skdll of the Salmon ... 43 Appendix on tlie Skulls of Fishes 83 MursBnoids 83, Siluroids 84, Ganoids 84, Ceratodus 87, Lepido- eiren 88, ChimEera 89, Elasmobranohs 89. CHAPTER IV. The Skull of the Axolotl . . 91 Appendix on the Skulls of Urodeles 129 Proteus 129, Siren 130, Menopoma 131, Menohranchus 133, the Salamandrine Skull 134. CHAPTER V. The Skull of the Common Frog . . . 136 Appendix on the Skulls of Anura 176 Bana pipiens 176, Pseudis 176, Bufo 177, Daotylethra 177, Pipa 181. X CONTENTS. CHAPTER VI. PAGR The Skull op the CoHiiou Snake . . 167 Appendix on the Skulls of Eeptilea 213 Chelone 213, Lizards 215, Crocodiles 217. CHAPTER VII. The Skull op the Common Fowl . . 219 Appendix on the Skulls of Birds . . . . 262 Struthionid^ 262, schizognaihae 262, desmognathse 263, ^gi- thognatha3 264, saurognath* 265. CHAPTER Vni. The Skull oj? the Pig . 267 Appendix on the Skulls of Mammalia . . . 301 The Human Skull . . . 301 CHAPTER IX. The Mohphology of the Skull . 310 The Cartilaginous SkuU 810; the Sense Capsules 320; the Arches 326 ; the Cranial Nerves 332 ; plan and segmentation of the Cartilaginous Skull 336 ; the Osseous Skull 343; Evolution 858. LIST OF ILLUSTEATIONS. FIG. PAGE 1. Head of embryo Dogfish, 11 lines long 15 2. Head of embryo Dogfish, 11 lines long, median longitudinal section 17 3. Head of embryo Skate, 1-J in. long 20 4. Head of embryo Dogfish, 1\ in. long 22 5. Head of embryo Dogfish, second stage ; basal view of cranium 24 6. Head of embryo Dogfish, third stage; median longitudmal section 28 7. Skull of adult Dogfish, side view 36 8. Skull of Skate, nearly adult 40 9. Embryo Salmon, about | inch long ; side view of head within chorion . .......... 44 10. Embryo Salmon, about f inch long; upper view of head, dis- sected, the neural tissue having been removed ... 45 11. Embryo Salmon, about f inch long; lower view of head, with the arches shining through 47 12. Embryo Salmon, partly hatched ; median longitudinal section of head 49 13. Embryo Salmon, not long before hatching; under view of head, with arches seen through 50 14. Embryo Salmon, not long before hatching; lower view of skull dissected, the branchial arches having been removed . 52 Xll LIST OF ILLUSTRATIONS. FIG. PAGE 15. Salmon JFry, second week after hatching; upper view of sliuU (iisseoted 54 16. Salmon Fry, second wfiek after hatching; transverse section of head, through forebrain and eyeballs ..... 56 17. Salmon Fry, second week after hatching ; side view of skull . 59 18. Young Salmon of the first summer, about 2 inches long ; side view of skull, excluding branchial arches .... 62 19. Adult Salmon; lateral view of chondrooranium with its ec- tosteal bones 69 20. Adult Salmon: median longitudinal section through skull, after removal of jaws and arches 70 21. Adult Salmon : side view of skull with all bones attached . 75 22. Head of Axolotl, just after hatching, side view ... 95 23. Larval Axolotl, about five lines long; upper view of skull, dissected 99 24. Larval Axolotl, about five lines long ; transverse vertical section of head, through eyeballs 100 25. Larval Axolotl, three-quarters of an inch long; upper view of basis oranii and lower jaw, dissected ..... 104 26. Young Axolotl, 2J inches long; under view of skull, dissected, the lower jaw and gill-arches having been removed . . 109 27. Young Axolotl, 2J inches long; upper view of skull; lower jaw removed Ill 28. Adult Axolotl ; under view of skull, the lower jaw and arches being removed, and also the investing bones on the right side 115 29. Axolotl, nearly adult; side view of skuU 120 30. SkuU of Amblystoma, side view ...... 124 31. Tadpole four Hnes long, four or five days after hatching; side view of head . 137 32. Embryo Frog, just before hatching ; side view of head, with skin removed *'■...... 133 33. Tadpole of Common Toad, one-third of an inch long ; cranial and mandibular cartUages seen from above .... 141 34. Tadpole about one inch long; view efface and cranial floor from above, the brain having been removed . . . 144 LIST OF ILLUSTRATIONS, XIU FIG. PAGE 35. Tadpole one inch long; side view of skull dissected . . 146 36. Tadpole ■with tail beginning to shrink; side view of skull without branchial arches , 150 37. Tonng Frog, with tail just absorbed ; side view of skull . . 155 38. Young Frog, near end of first summer; upper view of skull, with left mandible removed, and. the right extended outwards 158 39. Adult Frog; side view of auditory region, with semicircular canals and parts of middle ear displayed .... 161 40. Adult Frog; upper view of skull with investing bones and lower jaw removed 163 41. Adult Frog; median longitudinal section of skuU, lower jaw removed 165 42. Adult Frog J upper view of skull ,..,,, 166 43. Adult Frog; under view of skull .,..•. 168 44. Adult (edible) Frog: under view of skull; investing bones re- moved on right side (Huxley) 169 45. Adult Frog; side view of skull, dissected. .... 171 46. Adtilt Frog ; columella auris 172 47. Adult Frog ; hyobranchial plate 173 48. Embryo Snake, If inch long; side view of head with facial arches seen through 190 49. Embryo Snake, about If inch long ; chondrocranium seen from above, the brain and jaws having been removed . . . 191 50. Embryo Snake, about 2J inches long; side view of head, dissected 198 51. Adult Snake; side view of skull, with jaws removed , . 206 52. Adult Snake ; under view of skuU . . . t . • 208 63. Adult Snake ; skuU seen from above 210 54. Embryo Chick of the fourth day of incubation: head viewed from below as an opacLue object (Foster and Balfour) . . 220 55. Embryo Chick, fifth day of incubation ; view of cranial struc- tures from above 221 XIV LIST OF ILLUSTEATIONS. FIG. PAGE 56. Embryo Chick, fifth day of inonbation; head viewed from below, with skeletal parts seen through .... 222 57. Embryo Chick, sixth day of incubation ; head seen from below (Huxley) 224 58. Embryo Chick, seventh day of incubation; side view of skull. 226 59. Embryo Chick, middle of second week of incubation; under view of skull, with arches removed 232 60. Embryo Chick, end of second week of incubation ; posterior view of cranium ........ 235 61. Embryo Chick, end of second week of incubation; inner view of hinder part of cranium ....... 236 62. Embryo Chick, end of second week of incubation; upper view of skull, the brain and parostoses being removed . . . 238 63. Chick two days old; median longitudinal section of skull, the brain being removed . 242 64. Chick two days old ; external lateral view of skuU . . 244 65. Chick two days old; under view of skull, with lower jaw re- moved 246 66. Fowl of first winter ; median longitudinal section of skull . 250 67 — 71. Views of nasal structures of Fowl of first winter . . 252 72. Fowl several years old ; side view of skull .... 254 73. Fowl several yeais old; under view of skuU with lower jaw removed 256 74. Adult Fowl ; side view of columella auris .... 258 75. Fowl several years old : hyobranchial apparatus from above . 259 76. Embryo Pig, two-thirds of an inch long; side view of head and neck .......... 268 77. Embryo Pig, two-thirds of an inch long ; elements of the skuU seei> somewhat diagrammatically from below . . . 270 78. Embryo Pig, two-thirds of an inch long ; palatal view . . 272 79. Embryo Pig, an inch and a third long; median longitudinal section of head, with nasal septum removed .... 281 80. Embryo Pig, an inch and a third long; posterior view of a section through the basal region of the skull . . _ 284 LIST OF ILLUSTRATIONS. XV FIG. PAGE 81. Embryo Pig, an inch and a thii-d long; side view of man- dibular and hyoid arcbes ....... 285 82. Embryo Pig, 2| inches long ; under view of skull with lower jaw removed 287 83. Embryo Pig, 2J inches long ; vertical section of head, showing structures between and beneath the orbits .... 290 84. Embryo Pig, six inches long; outer view of occipital and auditory regions 29.5 85. The Pig at birth ; outer view of auditory capsule, &c. . . 296 86. Auditory chain of bones i^- THE MORPHOLOGY OF THE SKULL. CHAPTER I. PRELIMINARY EMBRYOLOGY. 1. In the study of the morphology of the skull, it is necessary to take into account all the forms which the skull assumes, from its origin in the embryo to its adult condition. Inasmuch as skeletal elements are apparent in the head at a very early period of development, no student will thoroughly apprehend their nature without Some knowledge of the processes by which the body of a vertebrated animal is evolved trum its germ. The following summary is intended to leiresh the memory of those who have studied embiyology practically, or by reading special treatises, and also tor the information of others who may not have time for such study. 2. The fertilised and developing ovum of all ver- tebrates contains a membrane called the blastoderm or germinal membrane, which is the rudiment of the future animal. It is produced by a process of segmentation, varying greatly in details, affecting the whole or part of the primary ovum. 3. The blastoderm, at an early stage after its forma- tion, consists of three layers of ceils. The upper or ex- ternal layer is called the epiblast ; the middle layer, tlie mesohlant ; and the lower or inner, the hypoblast. The epiblast gives rise (1) to all the epidermis and epidermic appendages of the body, (2) to the epithehal lining of B. M. 1 2 MORPHOLOGY OF THE SKULL. [CHAP. the cerebro-spinal canal and its derivatives, the ventricles of the brain, (3) to the cerebro-spinal nervous centres, and (4) to various parts of the organs of special sense. The hypoblast is the source of the epithelium of the alimentary canal and all the organs developed as diver- ticula from it. From the mesoblast tbe remaining parts of the body are derived, including muscles, bones, con- nective tissue, and blood-vessels ; the generative and urinary organs ; the dermis, and the mucous membranes of the alimentary canal and of the organs connected therewith. 4. To produce this diversity of parts, the layers of the blastoderm become thickened in various regions; folds and pits arise in many situations, and form pouches, sacs, tubes, and processes ; the mesoblast especially splits into distinct strata. At the same time the cells com- posing the layers gradually alter their character as they multiply, and, instead of being very much alike, become more and more unlike, and specialised for the functions they have to perform in the growing organism. In most cases the blastoderm sooner or later grows round the mass of nutritive material constituting the yelk, and encloses it\ .5. A portion of the blastoderm is directly developed into the embryo : among the functions of the remainder is the absorption and transmission of nutriment to the embryo. The locality of the latter is early marked out by the formation of a trench bounding an oval region in the blastoderm. The part enclosed becomes thence- forth the embryo ; and the demarcation is made more definite by the trench, as it grows, continually under- mining the enclosed parts. As this trench extends inwards, the embryo rides upon the rest of the blastoderm, now called the umbilical vesicle, and its enclosed yelk. The 1 This description will not hold good for Mammals, where there is no proper yelk ; but the main facts, independently of those due to the exist- ence of the yelk, are true for them. It is not intended to include the development of Amphioxus in this general view. The account in § 6 requires modification in its application to Osseous Fishes. I.] PRELIMINARY EMBRYOLOaY. 3 head of the creature is from the first distinguishable as being included within the earliest developed part of the trench, at one end of the oval. Afterwards, certain mem- branous appendages, the amnion and the allantois, arise in the development of several classes of vertebrates : for their description the reader is referred to general embryo- logical works. 6. We have described the formation of a simple longitudinally tubular embryo by gradual constriction of a portion of the blastoderm from the remainder. Coinci- dent with this process is another, by which a second longitudinal tube is formed parallel with and above the first. This arises by the growth of two parallel folds from the upper surface of the embryo along its axis, having a shallow longitudinal groove between them. These medullary folds, after growing to a certain height, arch over towards each other and coalesce, for a greater or less distance, so as to form a tube open at both ends; it is the neural tube, in connection with which the cerebro-spinal nervous system takes its rise. The an- terior and posterior openings of the tube become closed at a variable period after its formation. 7. The neural tube is necessarily lined with epiblast, since it originates on the upper or epiblastic surface of the embryo. When it is closed, there is an external epiblastic surface as before, and an internal epiblast lining the tube. " Between these two strata the growing mesoblast gradually penetrates until it has completely separated the outer from the inner epiblastic layer. The neural canal marks the dorsal region of the embryo ; while the parts beneath it belong to the ventral aspect of the body. 8. The original embryonic cavity, or space con- stricted off by the trench as it grows inwards, is con- verted into a double tube by a process of splitting, which divides the mesoblast into two layers. The inner of these layers, where it comes into relation with the hypoblast, forms with it a membrane called the 1—2 4 MORPHOLOGY OF THE SKULL. [CHAP. splat) clinopleure, or visceral wall : while the outer layer of mesoblast with the adherent epiblast is known as the somatopleure, or body-wall. The splanchnopleure grows inwards on the ventral side of the embryo more rapidly than the somatopleure ; and it early forms a very distinct alimentary tube, shut off from the umbilical vesicle. A space is thus left all round the digestive canal between the somatopleure and the splanchnopleure : this is the pleuroperitoueal cavity. The somatopleure of either side, growing towards the middle ventral line at a slower rate, meets its fellow eventually and coalesces with it to form the ventral wall. 9. The main organs of the body which surround the alimentary canal are developed, (1) as pouches or tubular protrusions from the wall of the digestive tube in different situations : these tubes becoming branched, and sur- rounded by differentiated mesoblast, gradually assume characteristic forms, and fill up the larger portion of the original pleuroperitoneal space, which however every- where surrounds them ; the lungs, liver, and pancreas are good representatives of this class ; of course all these organs contain hypoblast; (2j as thickenings and develop- ments of the mesol)last, independently of the hypoblast or epiblast ; such are the Wolffian bodies, kidneys, and sexual organs. 10. A structure of great importance arises early, along the axis of the embryo, between the neural and the alimentary tube. One or other of the principal germinal layers' gives origin to a median longitudinal string of cells, forming an axial rod beneath the floor of the neural tube. Anteriorly this rod, called the noto- chord, does not extend quite so lar forwards as does the neural canal. The formation of the limbs, as paired out- growths from lateral ridges on the sides of the embryo, must not engage our attention here. 1 The controversy in whicli the subject of the origin of the noto- chord IB at present inTolved lenders this indecision necessary. I,] PRELIMINARY EMBRYOLOGY. 5 11. The neural tube and the rudiments of the nervous system in the anterior region of the body very early manifest differentiation from the corresponding parts behind them. The cephalic end of the tube is dilated in a pyriform manner: this dilatation increases in all di- mensions, and its walls show a series of constrictions. Ultimately, out of the fore end of the tube, three cerebral vesicles are formed, one behind another, having their cavities freely continuous with the rest of the tube behind, which is thenceforth distinguished as the spinal canal. The notochord, underlying the neural tube, stops short of its cephalic end, terminating underneath the middle or the hinder cerebral vesicle. The nervous tissue becoming differentiated in the walls of these vesicles constitutes three main divisions of the brain, viz. the fore, the mid, and the hindbrain. The mesoblast ultimately encircles the cranial cavity as it does the spinal canal. 12. As the cerebral vesicles enlarge in all dimensions, their increase is not followed by any high development of the anterior part of the lower or visceral tube. Conse- quently, at an early period the head consists mainly of the large neural protuberances, and these bend downwards around the anterior end of the notochord and of the alimentary tube. This flexure, taking place about a point beneath the middle or hinder part of the middle cerebral vesicle, is called the mesocephalic (less precisely the cranial) flexure. The first cerebral vesicle comes to occupy an infero-anterior, the second a supero-anterior position. In addition to the definite meso- cephalic flexure, there is in many cases a considerable ventral curvature of the body, and especially of the neck. One of the most remarkable things in vertebrate develop- ment is the way in which the cranial axis ultimately becomes once more approximately straight; while the ventral appendages of the skull acquire a high degree of development. 13. The first cerebral vesicle enlarges transversely, and the lateral protuberances after a time become con- 6 MORPHOLOGY OF THE SKULL. [cHAP. stricted from the axial region. Thus arise stalked optic vesicles, which at a later period, being met by involuted epiblast from the exterior, are pitted so that the external ■wall is opposed to the inner concave wall, just as part of a hollow indiarubber ball may be pushed in till it meets the opposite surface of the ball. The stalks of the optic vesicles are subsequently known as the optic nerves. These nerves soon appear to proceed from the under part of the brain; for the constriction by which the optic vesicles are distinguished is much deeper above than below, and consequently the stalk is carried downwards towards the base of the brain. 14. The first cerebral vesicle retains its distinctive appellation of forebrain, and gives origin to the cerebral hemispheres, which are not constricted from the axis by a stalk, and which in higher vertebrates achieve a pre- ponderance over all other cranial structures. The hemi- spheres themselves bud out anteriorly into smaller bulbs constituting the pair of olfactory lobes, from which the olfactory nerves are subsequently derived. The remains of the primary cerebral vesicle form what is known as the vesicle of the third ventricle, the lateral cavities or ven- tricles of the hemispheres having been originally con- sidered in cerebral topography as the first two ventricles. From the centre of the floor of the third ventricle, which usually becomes the hindmost of the elements produced from the original first vesicle, arises a funnel-shaped pro- cess, the infundihulum, which is extended downwards to join the pituitary body situated at the extreme fore end of the alimentary canal. 1.5. The remaining principal parts of the brain, as related to the primary vesicles, may be briefly mentioned. The middle vesicle gives rise to the optic lobes (corpora quadrigemina of mammals) and to the crura cerebri where they are distinct; its cavity in higher forms becomes re- duced to a narrow channel between the third and the fourth ventricles, or the primary cavities of the fore and the hindbrain. In the latter, the roof developes into the I.] PRELIMINARY EMBRYOLOGY. 7 eerebellum in front, the floor behind forms the medulla oblongata. 16. The chief cranial nerves other than the olfactory and optic, which Nhave already been referred to, arise on either side of the floor of the hindbrain, and in some cases certainly by outgrowths from it. There are four principal masses, two of which are near the anterior end of the notochord, two near the posterior limit of the head. The first is the rudiment of the trigeminal nerve ; it early becomes forked peripherallj', one branch passing forwards to the eyeball, the other downwards to the mandible. The second becomes the facial nerve; the third and fourth, the glossopharyngeal and vagus (or pneumogastricj. All these nerves, at or near their cerebral termination, possess special ganglia. 17. Three pairs of pouch-like ingrowths of epiblast originate on the outside of the primary membranous cranial cavity, and contribute to form organs of special sense. The mesoblast becomes distinctively aggregated around these pouches, and more or less completely cuts off communication between the pouch and the external epiblast. 18. The anterior of these organs are the nasal sacs, which are always related at first to the forebrain, usually lying under it. Their involution is distinguished by the prominent rim bounding it. This border is in certain cases deficient in its lower part, and when the process which bounds the outer side of the mouth grows over it to unite with the fore lip, this deficiency is con- verted into a short canal opening on the roof of the mouth; while the orifice of the original involution be- comes definitely located on the face. Thus two narial orifices are formed, anterior and posterior, or superior and inferior. The inferior openings may sometimes be carried backwards in the mouth by supplementary plates de- veloped beneath them, closing their access to the anterior part of the mouth, and only allowing communication posteriorly. 8 MORPHOLOGY OF THE SKULL. [CHAP. 19. In its later development, each nasal pouch be- comes a cavity lined by a sensory membrane, part of which receives special nerves from the olfactory lobes of the brain. Mostly, the nasal cavities are elaborated in a labyrinthic manner ; that is, the surface of the lining membrane is extended by the projection of complex folds into the cavity; and these are supported by corresponding growths of hard parts. The latter are derived from or are continuous with the cranial skeleton, and play a great part in the morphology of the skull. 20. The second pair of involutions, the optic sacs, are usually much larger than the nasal. Their primary posi- tion is above and behind the olfactory sacs, at the sides of the midbrain; but this location is frequently modified afterwards, and they become placed at the sides of and often below the forebrain — almost entirely anterior to it in some cases. The optic pouches of epiblast grow in- wards until they indent the optic vesicles of the brain, and, pushing their outer part inwards till it comes in contact with their inner surface, give rise to the cup which subsequently constitutes the retina. The epiblastic mass gets completely cut off from the exterior, and forms the crystalline lens. Mesoblastic growths form all the other structures of the eyeballs. The shape of the eye is mainly tletermined by a strong fibrous membrane, the sclerotic, which often becomes cartilaginous, and even acquires ossi- fications, called sclerotic plates; but inasmuch as the eye- balls must be mobile, the sclerotic plates never enter into combination with the cranial skeleton, and will scarcely receive any mention in the following pages. 21. A cavity called the orbit, open externally and botmded by skeletal elements, is in most cases formed, to lodge and protect the mobile eyeballs. The study of the orbit is of high interest in cranial morphology; but it is rendered comparatively simple by the fact that the sense- organ, with its special skeletal investment, lies complete within the orbit; there is no interlacement of parts or complication of regions. A primary cleft arises between I.] PRELIMINARY EMBRYOLOGY. 9 the optic involution and the maxillopalatine process which bounds the mouth. This opens on the roof of the mouth where no secondary palate is developed, and into the nasal passage where there is such a palate. It is variously called the lachrymal or orbitonasal canal. 22. The third pair of special sense organs, the ear- sacs, begin as large involutions of epibiast at the sides of the hindbrain. The epiblastic pouch speedily becomes hollow, and the cavity early expands to large dimensions, and gives off processes which constitute the auditory laby- rinth. Mesoblastic tissue engirths these processes, and forms a hard investment to the whole organ. At first this investment is usually distinct from the proper cranium ; but they soon coalesce, and the regions of both become sometimes almost inextricably intertwined. 23. The main part of the cavity of the auditory labyrinth is denominated the vestibule; it has an upward and backward process, the aqueductus vestibuli, towards the situation of the primary involution and the long persistent membranous space or fenestra in that position. Hollow pro- cesses arise from the vestibule,- anteriorly, posteriorly, and externally ; and as they grow, mesoblast penetrates into their concavity, leaving the periphery pervious as a tube, which forms a curve sometimes bearing a close resem- blance to a semicircle; hence these three tubes are called semicircular canals of the ear. They are present in the same relative situations in all but the lowest vertebrates. One end of each tube dilates where it opens into the vestibule, forming an ampulla. Another process of the vestibule, passing forwards and downwards in the ear- mas^, is developed into the simple conical cochlea of birds, and the spirally- twisted cochlea of mammals. 24. In almost all vertebrates the mesoblastic invest- ment of the otic capsule becomes at least cartilaginous, very often bony : but the cartilage is scarcely ever com- plete on all aspects. It may be incomplete internally on the cranial side; or fenestra3 may be formed in it 10 MORPHOLOGY OF THE SKULL. [CHAP. externally, where the labyrinth abuts on certain secondary structures found in higher vertebrates. 25. Another organ of special sense, that of taste, is formed by epiblastic involution in the head. It does not_ present a manifest paired aspect, its involution being part of that of the mouth-cavity or alimentary vestibule, which discharges many functions besides that of taste. The oral involution early takes place in the angle between the downbent cranial vesicle and the fore part of the aliment- ary tube. The mesoblast outside the latter grows for- ward ventrally and laterally, surrounding a hollow ou the inferior surface of the head, into which the copious in- growing epiblast is received. The hinder limit of this involution is somewhere about the level of the fore end of the hindbrain. It is at a comparatively late period that a slit places the alimentary canal in communication with the oral cavity. 26. At about the point of junction of these two cavities, a diverticulum of epiblast is given off, passing upwards into the floor of the cranium : this forms the pituitary body already spoken of (§ 14, p. 6). The stalk of connection with the cavity beneath is obliterated, and the pituitary body always occupies a definite position in the cranial floor in front of the end of the notochord. 27. The mouth, throat, and neck are parts little differentiated in the embryo when already the neural vesicles have attained a high development. Subsequently they increase very greatly in size, growing both forwards and backwards. The mesoblast does not split at its ex- treme anterior end to form a visceral wall within the body-wall. A transverse section beneath the fore end of the notochord show.s a simple tubular cavity, which is the commencement of the digestive canal. In this region, as it grows more extensive, a series of thickenings arises on each side in a vertical plane, constituting what are called visceral folds. In general they are curved somewhat for- wards towards the middle line below, and the foremost of 1.] PRELIMINARY EMBRYOLOGY. 11 them grows especially forwards to envelope the oral invo- lution of epiblast. 28. The greatest number of these folds which occurs in vertebrates is nine ; frequently no more than six appear. The first pair of folds receives the name of mandibular ; the second, that of hyoid ; the remainder are named branchial. The valleys between the folds usually become perforated in more or less of their extent; thus there is established a series of visceral clefts, which may remain open throughout life, affording a channel of com- munication between the cavity of the throat and the exterior ; or some or all of the clefts may become closed at a later stage of development, and so remain during the life of the animal. 29. The boundaries of the mouth are primarily con- stituted, (1) anteriorly, by the nasofrontal jirocess which is the termination of the investment of the head be- neath the fore brain, and between the olfactory sacs ; (2) superiorly, by the floor of the cranial vesicles, forming the primary ^atoe ; (3) laterally and somewhat superiorly, by the maxillopalatine processes which grow forwards on each side along the line of the primary palate from the upper end of the mandibular folds ; and (4) inferiorly by these mandibular folds. 30. The visceral folds, and especially those named branchial, become the basis for the development of gills in varied forms, which may persist throughout life, or only during a portion of embryonic existence. In the higher forms, where no gills are developed, the appearance of the branchial folds is to a large extent evanescent, though not less certain and definite than in the other cases. 31. A brief reference to the vascular system will close this general account, and enable us to proceed to the history of the skeletal elements. The heart is formed in the mesoblast at the anterior region of the pleuro- peritoneal cavity. A tube proceeds from its fore end in the mid-ventral mesoblast: this is the bulbus aortce. It 12 MORPHOLOGY OF THE SKULL. [CHAP. passes along the ventral line into tlie neighbourhood^ of the visceral folds, and gives off a series of branches, which surround the fore end of the alimentary canal, one passing outwards and upwards along each visceral fold to gain the. dorsal side of the throat. These channels or aortic arches unite to form a pair of tubes corresponding to that from which they originated ventrally ; and these dorsal tubes pass backwards right and left of the notochord at some distance from it, uniting sooner or later to constitute the primitive dorsal aorta. 32. The aortic arches never quite maintain their primitive arrangement in adult life. They most nearly re- tain it in the permanently branchiate forms. But one or two of the anterior arches always become disconnected with the dorsal tube, and by means of secondary ramifications supply the head and brain with blood. One pair of these vessels, the internal carotid arteries, will be mentioned in succeeding pages, as passing into the cranial cavity in a definite position. Other portions of the aortic arches are differeiitiated in abranchiate conditions or forms of higher vertebrates, and furnish blood-vessels to internal respiratory organs : and finally but two pairs, or one pair, or one aortic arch, may be left in the adult in continuity with the dorsal aorta. 33. The skeletal elements with which we have to do arise entirely as developments of the me.soblast. Very early the mesoblast at the sides of the neural tube behind the head is divided by transverse partitions into a longi- tudinal series of more or less definite segments, the proto- vertehrw. These extend backwards as far as the end of the neural canal, but stop short anteriorly at the level of the posterior end of the third cerebral vesicle. The slightly differentiated mesoblast grows upwards so as to surround the neural canal, and also invests the notochord. 34. AVhen chondrification takes place, a continuous cartilaginous investment surrounds the notochord. This cartilage is ultimately segmented into vertebral pieces, so I.] PRELIMINARY EMBRYOLOGY. 13 that the lines between adjacent vertebrae are interme- diate between the dividing lines of the protovertebral segments ; thus each vertebral body corresponds with the contiguous parts of two protovertebrae. Lateral vertebral cartilages become gradually extended round the spinal canal on each side so as to form neural arches, which at a later period coalesce on the dorsal aspect of the canal. 35. By the time that the vertebra is complete in cartilage the notochord usually begins to degenerate, especially where it is encircled by the vertebral laodies or centra. In these situations it may become perfectly ob- literated. Very frequently remnants of the notochord persist in the intervertebral spaces. 36. Ossification commences in the vertebrae in the region of the centrum immediately surrounding the noto- chord. Other centres of ossification arise, one on either side of the neural canal. Where ribs are developed they grow outwards from the region of the vertebral centra. Some of the ribs may extend completely round the body- wall so that the corresponding pairs meet ventrally ; and a junction is elfected between several succeeding medio- veutral elements, to form a cartilaginous sternum. The Costal arches ossify by one or two centres on each side ; the sternum often by several paired centres, 37. No protovertebrae are formed by the side of the notochord in the cranial region. Much of the mesoblast investing the brain and constituting the visceral folds and other processes belonging to the head, undergoes a gradual but early transformation into cartilage, forming definite skeletal tracts before intercellular substance has appeared. It is the object of the following pages to trace in several types of vertebrates the rise and history of these carti- laginous tracts and of the bones developed within or in proximity to them. CHAPTER II. THE SKULLS OF THE DOGFISH AND THE SKATE. 38. The eggs of Elasraob ranch fishes, of which the principal are known as Sharks and Rays, when deposited, are enclosed in a strong horny capsule or "purse," secreted from the oviduct. This capsule is pillow-shaped, the corners being pointed in the Rays, and produced into long tendril-like processes in the Shark and Dogfish. The embryo remains enclosed in the purse until about six months after oviposition, and during this period the most important metamorphoses take place. First Stage. 39. In embryos of the lesser Spotted Dogfish {ScylKum canicida), from eight lines to an inch in length', the head and the branchial region are proportionately large and conspicuous, and external gills are present; the body is slender, tapering to a long thread-like tail. The embryo is extremely active, and has attached to it a yelk-sac of about three-quarters of an inch in diameter. 40. The neural tube in the head is completely closed, but the covering of the third or posterior cerebral vesicle is very thin. The latter vesicle (Figs. 1 and 2, C 8) lies directly above the anterior end of the notochord : the second vesicle (0 2) is immediately in front of the noto- chord, forming the foremost rounded part of the head ; while the first vesicle (C 1) is beneath the second, and is 1 These embryos correspond in most respects with stages M and N, described and figured by Mr Balfour (Journ. Anat. Vol. x. p. 563, and Plate xxv.). CHAP. ir.J THE DOGFISH: FIRST STAGE. 15 totally below the level of the notochord as well as ante- rior to it. Thus the mesocephalic or cranial flexure is already fully established. 41. Each sense capsule is seen in a very rudimentary condition as an infolding of the epiblast of the side of the head. These infoldings are at present subequal in size. The nasal sacs (Fig. 1, Na) are situated upon the infero- lateral surface of the head, external to the first cerebral vesicle. The young eyeballs (E) are almost vertically over the nasal sacs : above each eyeball is a noteworthy elevation of the skin, the supraorbital band. The ear-sacs (Au) are on a higher level and more posterior, flanking the hinder part of the third cerebral vesicle. Fig. 1." Head of emtryo Dogfish, 11 lines long. Tr. trabecula ; PI. Pt, palatopterygoid ; M.Pt, metapterygoid region; il/n, mandibular cartilage; Hy, hyoid arch ; Br. 1, first hraucbial arch ; Sp, muudibulo-liyoid cleft ; CU, first branchial clef t ; Lcli, so-called lachrymal cleft ; Na, olfactory rudiment; E, eyeball; Au, auditory mass; 01,2,3, cerebral vesicles; Hvi, hemispheres ; f.n.p, nasofrontal process. 42. On the under surface of the head is a large square mouth, bounded in front by the median antero- inferior projection of the head, called the nasofrontal jirocess (/. n. p.), lying between the nasal sacs ; behind, by the rudimentary lower jaw (Mn) ; and laterally by a pro- cess passing forward from the upper end of each half of the mandible {PI. Pt). Behind the mouth on each side are seen six clefts {Sp., CI.) curving downwards from the neural towards the haemal region of the neck roughly 16 MOllPHOLOGY OF THE SKULL. [CHAP. parallel to one another. The clefts on each side corre- spond, but they do not meet below. 43. Thick and somewhat prominent ridges intervene between the visceral clefts; and from the surface of each ridge or arch there arises a series of long filamentous external gills. The heart is plainly to be seen through the transparent skin in the middle line beneath, between and below the posterior arches just described. Behind the heart is the umbilicus, and on either side of the um- bilical region the rudiment of a pectoral fin projects. 44. The notochord in the cephalic region (Fig. 2, n.c.) lies beneath the neural axis, and is curved somewhat down- ■wards\ at the same time slightly tapering. It does not extend so far forwards as the third cerebral vesicle does, but ends above the middle of the mouth. In embryos of the Dogfish at this stage and also in Pristiurus embryos the anterior end of the notochord has been seen submo- niliform in outline, presenting from five to seven head- ings in the distance between its anterior extremity and the middle of the auditory region. 45. The pituitary body (p?/), lying behind the first or inferior cerebral vesicle, and connected with it by the infundibular process, is closely in front of and a little below the notochord ; it is the lower boundary of a space formed by the curvature of the neural axis, re- sembling the concavity of the hook of a crozier. This space is filled with delicate gelatinous tissue; it is the transitory "middle trabecula" of Kathke. 46. Flat bars of nascent cartilage, the paracJiordaW , are found in the hinder part of the cranial floor on either side of the notochord, extending backwards into the neck lor three or four times the length of their intracranial portion, without showing any vertebral segmentation. The inner edge of each parachordal is grooved to embrace 1 This downward cni-vature is considerably greater at an earlier stage ; see Balfour, Journ. Anat. Vol. x. PI. xxiv. G, H, and /. '' Fig- 5, p. 24, which shows these structures in the next stage, may be consulted ior many points in this description. 11.] THE DOGFISH : FIRST STAGE. 17 the notochord partially ; but they are not identified with its sheath, which is already cartilaginous. The para- chordals extend further forwards internally, where they reach the middle of the beaded region of the notochord ; externally, they pass nearly to the fore end of the audi- tory masses. In front they abut unconformably, at an angle of about 120°, upon the hinder part of the trabeculse. The parachordals are crescentically scooped and bevelled where they lie between the auditory capsules, and the latter rest upon the bevelled edge : they are wider behind this region, having a straight external edge. Fig. 2. Head of embryo Dogfish, 11 lines long, median longitudinal section. CI, 2, 3, cerebral vesicles; nc. notochord; pj/. pituitary body; m.tr. middle trabecula; i.n.p. internasal plate; m.sp. medulla spinaUs ; m. moixth; pit. pharynx; sp. spiracle; cl. visceral clefts; mn. mandibular, hy. hyoid, Or. 1, first branchial, arches; e.br. external branchiae. 47. Before the parachordals can be plainly made out, a pair of broad fiat bars is manifest in the hinder part of the down-turned cranial fioor, behind and below the first two brain-vesicles, and partly below the third. TLeir plane is directed forwards and downwards from the middle of the base of the third vesicle to the middle of the first. These traheculce (Fig. 1, tr) embrace the pituitary body and infundibulum by an arcuate inner margin, so as to leave between the two bars an oval space equal in width to themselves. In front they very nearly meet behind the nasal sacs, where they project bluntly towards one another. From this inner point their boundary sweeps B. M. ^ 18 MORPHOLOGY OF THE SKULL. [CHAP. crescentically outwards, and then again bends inwards ia front of the auditory capsules. The trabeculis are ex- panded where they abut against these capsules, wedging in for a little distance between them and the para- chordals. Posteriorly the trabeculse lie on the tore end of the parachordals, forming a slight elevation directed backwards, behind the pituitary region : they closely em- brace the fore part of the beaded portion of the notochord. 48. In front of the trabeculse and between the nasal sacs there is a median styloid tract of condensed tissue, broad behind, filling up the angle where the trabecular almost meet, and then growing forwards as a pointed rod lietvveen the nasal sacs and beneath the fore-brain. Be- fore the end of this stage the ear-sacs begin to acquire a cartilaginous covering : but excepting in the regions which have been described, the boundaries of the cranial cavity are entirely membranous and very diaphanous. 41). Each visceral arch has a cartilaginous axis, form- ing a more or less arcuate rod (Fig. 1, mn, hy, hr) ; and these cartilages will in future be referred to as the mandi- bular, hyoid, and branchial bars or arches. The upper ends of the branchial bars point directly inwards : but each of the two anterior arches sends forwards from its upper extremity a considerable flat process. The process from the mandibular arch is so large as to be really a continuation of it ; and it occupies the thick ridge which forms the lateral boundary of the mouth on either side : it is the palato-quadrdte plate. 50. There is very little upward expansion of the mandibular arch where it bends forwards; a little later, liganientous fibres connect the region of the bend with the side wall of the cranium immediately in front of the auditory capsule and below the exit of the trigeminal nerve. The palato-quadrate plate of either side grows to- wards its fellow, and ultimately meets it below the level of the eyes and behind the nasal sacs: the two become connected by ligament, and a deep groove is left between the bars and the nasal sacs. Tlie anterior process of the II.] THE SKATE : FIRST STAGE. ] 9 hyoid arch applies itself along the lower side of the auditory capsule posteriorly. The branchial arches remaia simple throughout this period. 51. During the growth of the processes of the first two arches, the upper part of the cleft between them becomes much wider, and is finally almost triangular, with the base above. The succeeding clefts also become wider above than below. The posterior edge of each arch developes a valance-like ridge or fold tending to cover the cleft behind it. The subsequent growth of these folds closes in the clefts very mucli, leaving only the adult branchial slits. Such folds entirely close most of the visceral clefts in abranchiate verte- brates. The fold on the hyoid arch is homologous with the opercular fold in osseous fishes. 52. In the youngest embryos at this stage a series of rounded papillse, the rudiments of external gills, are found on all the visceral arches except the last. Later on, these papillae develope into long filaments, each containing a single capillary loop. About ten of these spring from the hyoid and each of the branchial arches ; the mandibular has four, much shorter than the others, arising from its upper posterior edge in front of the first cleft. More in- ternally on the pjsterior edge of each arch bearing ex- ternal gills, is a series of cog-like projections, the rudiments of the internal gills : they form double series on the first four branchial arches. 53. In embryos of a Skate {Raia maculata), about an inch and a third in length, (Fig, 3), taken from the purse seven weeks after oviposition, development has not pro- ceeded farther than in the Dogfish already described. The extra length is due to the tail: in other respects it does not differ markedly from the larval Dogfish. In the whole cranial region there is no difference which re- quires special notice. 54.. The mandibulo-hyoid cleft (sp) early fills up below, and is pear-shaped above. The first branchial 2—2 20 MORPHOLOGY OF THE SKULL. [chap. cleft (between the hyoid and the first branchial arch) is similar to the preceding in its upper region, but retains the lower slit-like part. All the arches except the last branchial bear long spatulate external branchial fila- ments ; there are six or seven to each arch except the mandibular, which has only four. In an earlier stage the rudiments of the branchiae are seen as very numerous minute buds arranged in series. Certain of these shoot out to become the external transitory gill-filaments ; the others afterwards develope into the permanent gills. Fig. 3. C.3 -^^ M. Pi TOI Head of embryo Skate, \\va.. long. Tr. trabecula ; •pi. pt.,mn. palato- mandibular arch ; m. pt. metapterygoid cartilage ; h. m. hyomandibular ; Ay. remainder of hyoid arch; br.l. first branchial arch; pu. pineal gland; s^j. mandibulo-hyoid cleft or spiracle. 55. The mandibular and hyoid arches are more markedly bifurcate above than in the Dogfish, the anterior forks, however, being much the larger. The palato- quadrate region (pi. pt.) has the same relations as in the Dogfish : a separate cartilage (m. p4.) arises in the hinder fork of the arch, in front of the first cleft. It is placed in front of and below the auditory capsule, and is the rudiment of the spiracular cartilage. 56. Behind the first cleft, the anterior process of the hyoid arch is developed as a separate cartilage, lying below the auditory mass : it is the hi/omandibular element (/). m.). The hinder fork of the arch is loosely connected with the ear-sac behind, and it grows inwards towards II.] THE SKATE: FIRST STAGE. 21 the axial parts in a pointed manner ; it diverges upwards and backwards from the ventral part of the arch [hy), almost as much as the palato-quadrate diverges forwards from the mandible proper. 57. The branchial arches (br) in the greater portion of their extent, laterally and below, are simple unseg- mented arcs ; but a separate little curved pharyngo- hranchial element early developes in each, above and mesiad of the main bar, in the pharyngeal roof 58. The primary basal elements of the skull are already, clearly manifest in these young embryos, the parachordals forming the base of the hinder cranial region, and the trabecule of its anterior portion. The latter relation is somewhat masked by the mesocephalic flexure; the trabeculse appearing to be behind rather than below some parts of the brain. But they lie truly in the cranial floor, as is very evident after the flexure has passed away. The mandibular arch is seen in these types more distinct from other parts than in many ver- tebrate forms : its shape is already specialised almost to the adult condition. The origin of the hyomandibular element separately in the Skate is noteworthy as com- pared with its segmentation from the main hyoid bar elsewhere. In other respects these fishes show to great perfection the separate elements of which the skull is composed. Second Stage. 59. In embryos of the Dogfish from 12 to 15 lines in length the external gills are two or three times as long as in the preceding stage ; the four filaments on the mandibular arch are only one-third the length of those on the other arches. The mesocephalic flexure remains, although rapidly diminishing, and the " middle trabecula " is not absorbed ; the brain has become much more complex. 22 MORPHOLOGY OF THE SKULL. [chap. 60. The trabeculae (Figs. 4 and 5, tr.) are now fully chondrified, but they are still distinct from one another. They are relatively longer, and the intertrabecular space is Fig. 4. C.Hy Head of embryo Dogfish, l-J-in. long. Tr. trabecula ; pl.pt,qu. palato- quadrate bar; mck. meekelian cartilage; h.vi. hyomandibular ; c.hy. ceratohyal; (3,4,6, labial cartilages ; the branchial arches and external gills are indicated behind the hyoid arch. oblong, with rounded angles. Externallj'the trabecular edge is straightened, and so also is the anterior margin, which extends furthest forwards in the middle line. Postero- externally a crest rises from the hinder half of the trabeculae in the cranial wall, nearly reaching to the auditory mass, and arching back over the exit of the trigeminal nerve. The hinder part of each trabecula has coalesced with the corresponding parachordal {pa. ch.) ; the trabecula at the junction forms a low posterior clinoid wall, which is not continuous across the notochord, but which reaches to the auditory capsule and fuses with it by its thick outer extremity. The angle where the trabeculse and parachordals unite is fast lessening, and they will soon lie in one plane as a continuous basicranial bar on each side. 61. The parachordals (Fig. 5, pa. ch) are well chon- drified, as also is the distinct tubular tract surrounding the notochord ; the parachordals have not united. The apex of the notochord is Hat, the beaded appearance being II.] THE DOGFISH: SECOND STAGE. 2:3 lost; it is wedged in between the two halves of the clinoid wall just described, aad the hinder face of the pituitary body. The auditory capsules (au.) are chondrified ; they are elongated oval in shape, showing enlargements over the course of the semicircular canals. Most parts of the roof and walls of the cranium are still membranous. 62. The intevnasal element is not fully chondrified ; it is in the same condition as the nasal roofs, but distinct from them. Where it lies between the nasal sacs its relations are substantially as before ; but ante- riorly it has sent forward three prenasal lobes, a median rounded piece projecting forwards as the prenasal ele- ment [pn.), and a lateral pair of curved bifoliate lobes growing round and applying themselves to the antero- internai face of the nasal sacs below : these are the cornua trabeculce (c. tr.). 63. The palato-mandibular arch has undergone trans- verse segmentation just at its greatest bend : so that the arch now consists of an upper and anterior portion answer- ing to the folatopterygoid and quadrate regions of other vertebrates (Fig. 4, pi. pt., qu.), and of a lower and hinder portion, the meckelian or mandibular cartilage. The metapterygoid ligament is further developed, but retains its primitive relations ; while a stronger ligamentous union arises, below the remains of the first cleft (the spiracle;, between the posterior edges of the quadrate and articular regions of the mandible, and the front edge of the hyo- mandibular and ceratobyal (see following paragraphs) ; the upper portion of this ligament may be called symplectic, the lower mandibulo-hyoid. In this way by the weakness of the direct attachment of the jaws to the cranium, and their suspension from the hyomandibulars, their great mobility is provided for. 64. The cartilages of the upper and lower jaw just mentioned are simple finger-shaped cartilages, those of each pair becoming rounded towards the middle Ime, approaching one another, and being united by ligament. The palato-quadrate is the larger ; it is at first directed 24 MORPHOLOGY OF THE SKULL. [CHAP. forwards, then somewhat inwards, and finally almost abrupt}}' inwards so as to become nearly transverse where Head of embryo Dogfish, second stage ; basal view of cranium from above, the contents having been removed. CI, forebrain ; ol. olfactory sacs; «». auditory capsule; nc. notochord ; py. pituitary body; pa.ch. parachordal cartilage; tr. trabecula; inf. infundibulum ; c.tr. cornua trabeoulffi ; pn. prenasal element ; sp. spiracular cleft ; br. external branchiae; cl. 2, 4, visceral clefts. it lies behind the nasal sac and exactly beneath the anterior termination of the trabecula. The hinder or quadrate region is somewhat compressed, and lies in the cheek above its convex articular condyle. The meckelian cartilage is thick and solid where it receives the condyle in a concave surface, and curves but slightly in proceed- ing forwards and inwards to its rounded termination in the chin. 6.5. The upper part of the hyoid arch, including its forward process, has been segmented off in a position parallel with the articulation of the jaws. This hyoman- dihulur element [li. m. Fig. 4) developes an oblong articu- II-] THE DOGFISH: THIRD STAGE. 25 lating surface in relation to the infero-lateral region of the auditory mass at the middle of its length. Below this facet the cartilage bends forwards so as to be concave on its hinder margin, while the projecting angle in front is attached to the quadrate region of the upper jaw by the symplectic ligament. Below this again the hyomaa- dibular turns a little backwards, and bears a notch and a posterior angular process for articulation with the simple arcuate lower piece, the ceratohyal (c. hy.). Between the ventral ends of the ceratohyals there intervenes a flat shield-shaped hasihyal cartilage. 66. The branchial arches are divided into upper and lower pieces in a manner generally similar to the arches in front, forming einhranchial and ceratobranchial seg- ments ; and above each epibranchial a small separate cartilage is developed, turning inwards and backwards in the roof of the pharynx : this is the pharyngobranchial. 67. Morphological advance has been signalized in this stage by opposite processes in the cranial and the facial regions. In the former the elements which were primitively distinct, the trabecular and the parachordal, have coalesced : but the palato-mandibular and the hyoid bars have underarone seg^mentation, and in them the essential relations of the adult condition have already been established. The nasal region gives as yet only a mere indication of its future cartilaginous skeleton : while the cranium itself remains membranous for the most part. Third Stage. 68. Embryos of the Dogfish, from an inch and a half to two inches long, have undergone great metamorphosis. The external gills are still long, but the internal gills are functional. The mesocephalic flexure is lost ; so that the buccal cavity is not now behind the first cerebral vesicle, but lies below it. (See Fig. 6.) 26 MORPHOLOGY OF THE SKULL. [cHAP. 69. The parachordal cartilages {pa. ch.) have_ grown round the notochord, above and below, enclosing it com- pletely, and have coalesced with its cartilaginous sheath ; thus they unite with each other to form the basilar plate. The cranial part of the notochord is ^soon reduced to a mere posterior cone of gelatinous tissue in the hinder part of this plate. Behind, it bears a rounded knob on each side for articulation with the first vertebra ; in front, although it is fused with the trabecular plate, an emargi- nation in the middle line is very plain below, the cartilage becoming very much thinner. 70. Seen from below, the basilar plate is broad and subquadrate, generally convex, with a groove in the middle line. It extends widely outwards so as to support the auditory capsules entirely, and come into relation with the hyomandibular articulation. 71. The lozenge-shaped sub-pituitary space is now thinly floored by cartilage which is confluent with the trabecular plate all round, and rises into a low posterior clinoid ridge behind, directed a little forwards. The subpituitary cartilage is pierced in its widest part by the internal carotid arteries (i. car.), which, however, lie close together. The so-called "middle trabecula" is a mere fissure between the mid-brain and the hind-brain. 72. The trabecule have coalesced and now form a broad flat trabecular plate. This extends most widely out- wards behind, where a short curved process is sent out beyond the prominent antero-external angles of the basilar plate, being not yet perfectly confluent with the latter \ More anteriorly the trabecular plate not only forms the floor of the cranial cavity, but also partially supports the eyeballs. Each antero-external angle is curved outwards and somewhat backwards, being loosely connected with the ' The imperfect confluence of these regions, combined with an ex- aggerated growth external to the auditory capsules, produces a large fenestra in certain forms, as Carchurias ylaucus, and Lamna cornubica, bounded by an e.\tensive curved baud of cartilage. IT.J THE DOGFISH: THIRD STAGE. 27 palatine cartilage of the same side. This angle may be termed the antorbital or ethmopalatine process. 73. In front of the ethmopalatine region, the tra- becular plate, which has been gradually rising towards the level of the nasal roof, is suddenly narrowed, so that it has a nearly transverse anterior margin except in the middle line, where it is confluent with the internasal element. This internasal plate, which is not very wide, sends for- wards in the middle line from its transverse anterior margin a short linear prenasal rostrum. 74. The internasal plate gives off on its upper aspect a double ascending lamina, the moieties of which soon diverge outwardly as broad wings, and coalesce with the inner wall of each nasal sac. Each of these nasoseptal lamincB bears a small sigmoid cornu (c. tr.) in front, pass- ing inwards towards the prenasal cartilage. The roof and outer walls of the nasal sacs (na.) have very largely chon- drified, forming dome-shaped capsules; and in addition to the coalescence of each olfactory cartilage with the corre- sponding nasoseptal lamina, it is confluent with a cartila- ginous tract developed in the supraorbital band (§ 41, p. 15). 75. No cartilaginous laminae have arisen in the mul- tiplicate nasal membranes. The opening through which each olfactory crus reaches the corresponding nasal cavity is large, and is situated in the inner side wall of the dome behind. Owing to the outspreading of the nasoseptal wings, the opening is nearly horizontal at this stage ; sub- sequently it becomes slanted upwards and forwards. 76. Very much of the cranial roof and walls is chon- drified, the elements being more or less distinct, and more or less fused with the cartilages already mentioned. There is a flat cartilaginous cranial roof, or tegmen cranii (Fig. 6), extending from the supraoccipital or posterior cranial region (s. o.) to the level of the fore part of the eye- balls, and on either side is the curved supraorbital tract, continued anteriorly to a junction with the olfactory cap- sules. The front edge of the cranial roof is crescentic and 28 MORPHOLOGY OF THE SKULL. [CHAP. incomplete, exposing the olfactory crura as they diverge to their destinations. Head of embryo Dogfish, third stage ; median longitudinal section. CI, forehrain, 6'2, midbrain, my. myelon; na. olfactory sac; pa.ch. parachordal cartilage ; s.o. supraoccipital cartilage; pty. pituitary body; i.car. internal carotid artery ; c.tr. cornua trabecule ; i>}.pt. palato- pterygoid cartilage ; vick. mandibular cartilage ; b. hy. basiliyal ; c.hy. oeratohyal; h.Or.l, first hypobranchial ; c.br.l, first ceratobranchial ; ex.hr. extrabranchial cartilage; h. heart; e.cr. epicoracoid cartilage; behind pa.ch. vertebral centra. 77. Between each trabecula below and the supra- orbital band above, there is a cartilaginous sphenoidal wall continuous with both, extending from the auditory to the nasal regions. The supraorbital ridge forms a partial roof for the orbit, while the outer edge of the trabecula fur- nishes a partial floor. The optic nerve pierces the sphe- noidal wall at its anterior third. The trigeminal nerve passes out in the primary fissure between the sphenoidal wall and the auditory sac ; when they have coalesced, this fissure becomes crescentic, and may be called the tri- geminal foramen. The nasal branch of the ophthalmic nerve '(from the trigeminal) passes forwards in the angle between the sphenoidal wall and the preorbital part of the supraorbital band where it goes to join the olfactory capsule. The facial nerve emerges at the junction between the auditory mass and the trabecular plate, and curves upwards and backwards round the former. 78. The cartilaginous capsules of the auditory sacs are complete on both the inner and outer sides except where nerves and vessels pass. The capsules have coalesced, with n.J THE DOGFISH: THIRD STAGE. 29 the supraorbital band in front and above, with the supra- occipital cartilage and the tegmen cranii on the inner side above, and with the basilar plate below. The semi- circular canals are prominent and large, and the sur- rounding cartilage is correspondingly bulged outwards. Over the junction of the anterior and posterior semicircular canals an unchondrified tract is seen on the upper surface of the skull. The foramina for the glossopharyngeal and the vagus nerves are one behind the other, behind the auditory capsule, near the postero-extemal angle of the basilar plate. 79. The palato-quadrate bar has become much larger ; and it is very like the articulo-meckelian in form and size. The moieties of each jaw meet in the middle line, where a strong ligament unites them. At the symphysial ends the rods are terete and incurved ; proximally they are flat and sinuous. The articular region is gently scooped to work on the convex quadrate condyle. 80. The enlarged hyoid arch has undergone no im- portant change : the hyomandibular is articulated with the auditory capsule and the edge of the basilar plate below the horizontal semicircular canal. Between the ventral ends of the right and left hyoid bars there is a broad, flat, hasihyal plate of cartilage, (6. hy.), which is like a keystone to the arch. One more flat keystone occurs behind, between the two last branchial arches; but the hypobranchial pieces of the second and third arches turn backwards to reach the front edge of this basibranchial. The first hypobranchials are but small lobes (h. br. 1), and do not extend backwards. 81. The branchial arches are very similar, each con- sisting of four pieces. Their hypobranchials partially floor the throat cavity; their cerato- and epibranchials form its lateral boundaries which are very convex outwardly. The upper segments or pharyngobranchials are turned backwards in the roof of the throat. The fourth pharyngo- branchial is forked externally, the hinder fork being applied to the fifth arch so as to be contiuuous with its epibran- 30 MORPHOLOGY OF THE SKULL. [CHAP. chial ; there is a fifth ceratobranchial, from which no hypo- branchial is cut off. 82. Two other series of cartilages, called labials and extrahranchials, have now to be described. Three labial cartilages are related to the anterior and outer edges of each nasal capsule. The first is pyriform and is already furrowed above by two or three slime-glands ; the second and third are found in valvular folds of the nasal opening : the second is crescentic, and its lower horn partially floors the nasal aperture ; the third is ear-shaped, and lies on the outer edge of the capsule. A fourth labial, lanceolate in shape, underlies each palato-quadrate bar on the upper edge of the angle of the mouth; whilst a fifth labial, converging towards the fourth at the angle, is upon the external surface of the mandible. Similar cartilages, the extrahranchials {ex. br.), appear outside each of the first four branchial arches. They are pointed above and broader below; the last is very smalL 83. In an embryo of Raia macidata four inches long, taken from the egg-pouch three months after oviposition, the tail is twice as long as the body proper, and the yolk-sac is still of the size of a small walnut. Yet the skull has assumed the adult form and structure even more completely than in the Dogfish. On either side the flabelliform fin has grown forwards and united with the cheek, and consequently all the branchial clefts appear entirely on the ventral surface, while the mandibulo-hyoid cleft is seen dorsally as the spiracular opening. From the gill-slits the external branchite still project as long filaments. The nasal apertures are on the ventral surface, and the head is produced in front of them as a flattened rostrum, very distinct from the cranial convexity. The eyeballs lie in their sockets on the upper surface, bounded in front, internally, and behind, by the prominent supra- orbital ridges. 84. The general character of the metamorphosis is very similar to that of the Dogfish. The prenasal rostrum II.J THE SKATE: THIRD STAGE. 31 is much larger, and the first labials lie on either side of it in front, parallel with it. The internasal plate, con- tinuing the gradual elevation of the anterior part of the cranial floor, is much broader than in the Dogfish, being as wide as the cranial floor itself; and instead of giving rise to distinct nasoseptal wings, it simply rises and becomes thickened on each side, and is continuous with the nasal roofs. No fenestra arises in these cartilages nor is there any development of cornua in front. The olfactory crura pass into the back part of the nasal roofs through a large fenestra on each side in front of the fore-brain. A small antorbital plate is carried on the outside of the base of each nasal capsule, where it forms the antorbital boundary. 8-5. A large part of the cranial roof is unchondrified in front of the supraoccipital region. This median fbn- tanelle is elliptic in shape, and limited anteriorly by a transverse bar of cartilage, and laterally by the supra- orbital tracts. In front, the cranial cavity is quite un- closed by cartilage, as in the Dogfish. Instead of the former continuous parachordal mass in the basicrariial and cervical regions, there is an occipital segmentation, dividing aa occipital cranial tract from a cervical cartila- ginous tube, which is perforated at the sides by a series of fissures for the cervical nerves. 86. There is now a complete lunule of cartilage in the anterior wall of the spiracular opening. It is attached to the side wall of the cranium, below the antero-extemal or sphenotic process of the auditory capsule; and it bears a comb-like pseudobranchia. Below, the spiracular car- tilage is attached by ligament to the hindmost part of the palato-quadrate bar. 87. The upper jaw is directed quite transversely, instead of being concave behind, as in the Dogfish; the two palato-quadrate cartilages are closely in apposition in the middle line. The hinder jaw is similarly transverse, the mandibular cartilages being parallel to the palato-quadrate. The emarginate nasofrontal process of integument leaves 32 MOEPHOLOGY OF THE SKULL. [CHAP. the anterior jaw largely uncovered in the middle line. The small valvular nasal openings with the labial cartilages enclosed in the valves are situated on either side of the nasofrontal process. 88. The hyomandibular cartilage articulates with the auditory cajjsule beneath its supero-external [pterotic) ridge ; below, it is entirely detached from the rest of the hyoid arch, turning forwards behind the spiracle, which it guards, as the spiracular cartilage does in front. The hinder end of the palato-quadrate is strongly fastened by ligament to the lower end of the hyomandibular. 89. The growth of opercular skin-folds between the branchial arches has left only small sigmoid external open- ings to the five branchial sacs. They are portions of the lower parts of the original clefts, and they converge to- wards the middle line from before backwards. A special ear-shaped opercular flap covers each of the openings. At this perio'd the hyoid and branchial arches carry two sets of branchise in full function ; the external are at their highest development ; and the internal branchial laminae are perfect, though small. 90. The epihyal segment of the hyoid arch is attached by an irderhyal ligament behind the hyomandibular to the postero-external angle of the skull (pterotic ridge). The whole hyoid arch is smaller than the succeeding branchial arches — a great contrast to the Dogfish. The hypohyal is styliform, and widely separated from its fellow ; indeed it is attached by ligament to the first hyjDobranchial. (Com- pare Dogfish, § 80, p. 29.) 91. The branchial arches are almost bent double, in accordance with the flattened shape of the animal. The pharyngo- and epibranchials are above, the cerato- branchials external and inferior. As in the Dogfish, there is no separate fifth pharyngobranchial. A single series of cartilaginous rays proceeds externally from each arch ; these are pedate at their outer ends. The first hypo- branchial is elongated and slender, meeting and coalescino- with its fellow in the middle line. The second, third, and II- ] THE ADTTLT DOGFISH. 33 fourth hypobranchials are small. The fifth arch has its hypobranchials fused into a broad flat plate which lies between the last three branchial arches, having two di- verging cornua in front. 92. There are no extrabranchial cartilages, nor are there any oral labials. One rostral pair, and three nasal pairs in the valves of the nasal openings, are the only representatives of labials found in this Skate ; the internal labial related to the nostril is the largest, and projects backwards over the upper jaw. 93. It is of high interest to note the perfection of the cranial structures here described, when the embryos are still so small in proportion to their adult size, and also the sim- plicity of the changes by which they are developed from the primary elements. The chondrification of mesoblastic tissue, in continuity with the parts originating earliest, comprises most of the history of the proper cranial and capsular skeleton. The facial arches have attained the special arrangements found in this group by a method of segmentation, essentially resembling that which will be seen to occur in every type examined. The prenasal and ' the antorbital pieces are to be carefully borne in mind, for they throw light upon many facts to be afterwards noticed. The labial and extrabranchial cartilages and the branchial rays are stfuctures which are more specially characteristic of the Elasmobranchs, but they are charged with lessons for the student who accurately notices their relations. Skull of the adult Dogfish. 94. The changes which have to be described are not morphologically very important. The olfactory capsules have become relatively larger, and more closely ap- proximated. The incurved cornua projecting from the nasoseptal laminse have disappeared. The tirst labials, instead of being wholly anterior to the nasal region, extend backwards over the nasal roofs, and are pitted by numerous slime-glands. The second labials have partly B. M. 3 S-t MORPHOLOGY OF THE SKULL. [CHaP. coalesced with the anterior edges of the nasal domes, and with the adjacent antero-external angles of the intemasal cartilage. There is a considerable space between the superficial extrabranchials and the deeper branchial arches. Between the corresponding bars of each series, numerous cartilaginous rods are developed, situated in the septa between the successive gill-pouches. 95. The adult skull may now be described in detail. It is entirely fibrous and cartilaginous, excepting that there are closely-set superficial calcifications in tesseroe on most of the cartilages. There are no investing or " splint " bones ; but as a cartilaginous skull this is the most perfect and complete of its kind. 96. The chondrocranium is very flat both above and below; perforated behind by the foramen magnum, and open in front between the hinder boundaries of the nasal capsules. The latter are large domes attached to the antero-external angles of the cranium. Thus the olfactory region of the skull is much wider than the interorbital. A strong concave supraorbital ridge partly roofs each of the large eyeballs ; it forms a distinct antorbital plate, running down towards the antorbital process of the cranial floor behind the olfactory capsule, and at its opposite extremity coalesces with the front region of the auditory cartilage, where a blunt sphenotic process projects. Internal to each supraorbital ridge is a considerable groove, in the front part of which the supraorbital foramen opens. Mesially the cranial roof or tegmen cranii is convex. 97. The surface of the otic mass exhibits three well- marked prominences for the semicircular canals, anterior, posterior, and external ; and mesiad of the elevation for the anterior canal is a small opening, which is the remnant of the primitive involution of the ear-sac. There is also a strong external pterotic ridge, horizontally placed (pt. o.). 98. Viewing the skull from behind, the foramen magnum is seen, somewhat triangular in shape, bounded 11.] THE ADULT DOGFISH. 3o at the basal angles by the well-developed occipital con- dyles. Between these is a slight elevation, marking the original situation of the notochord. The lateral regions of the cranial cartilage extend widely outwards, the upper edge, as seen from behind, being longer than the lower, which is constituted by the basilar plate. The greater portion of each lateral mass of cartilage is periotic; but the foramen magnum is completely encircled by an occi- pital ring not due to the ear-capsule. Laterally, the basilar plate furnishes the lower part of the facet for the hyomandibular. 99. The inferior surface of the cranium is an oblong tract, of which the hinder half is the basilar plate, extend- ing under the ear-capsules, and the fore part is due to the trabecule. A notch under the front of the auditory region marks the separation between the basilar plate and the hinder angle of the trabecula. Farther forwards the trabecular plate is narrower ; but behind the nasal region it sends out an antorbital process on either side, to which the ethmopalatine ligament is attached. Between the olfactory sacs the trabecular cartilage is much narrowed, but it grows upwards and forwards to coalesce with the adjacent part of each nasal roof. On each side of this ethmoidal region the olfactory fibres pass through a large membranous fenestra down into the nasal sacs. These have their folds of membrane pinnately arranged, and con- taining no cartilage. The prenasal rostrum [p. n.) projects forwards and upwards for a short distance beyond the intemasal region. The inner wall of each olfactory cap- sule presents a large unchondrified space or fenestra ; it is in the region originally formed by the nasoseptal wing. 100. The long first labials (I. 1) meet in the middle line anteriorly to any other skeletal element, and curve backwards over the outer part of the olfactory capsules. The second labials {I 2) are partly confluent with the front of the capsules, and with the anterior angles of the intemasal plate at the base of the rostrum. A third labial 3—2 SG MORPHOLOGY OF THE SKULL. [chap. (l. 3) lies in the skin-fold protecting the outer edge of each nasal aperture. 101. Laterally, the skull presents continuous cartilage, embodying olfactory capsule, supraorbital bar, antorbital and postorbital (sphenotic) processes, auditory invest- ment, and basilar plate. The hinder part of the skull is very little higher than the front region. The rounded olfactory boss and the smaller antorbital prominence are succeeded by the concave orbital wall ; while, behind, the ear-cartilage with its sphenotic process {sp. o.) and pter- otic ridge (pt. o.) forms the outstanding buttress of the cranial edifice. 102. In this skull, having no bones or distinct car- tilages, the nerve foramina must be carefully studied as Fig. 7. Skull of adult Dogfish, side view. O.c. occipital condyle ; au. peri- otic capsule; pt. u. pterotic ridge; sp.o. sphenotic process; s.or. supra- orbital ridge; na. nasal capsule; p.n. prenasal cartilage; II. optic foramen; V, trigeminal foramen; pl.pt.,gu. palato-quadrate arcade; m. pt. metapterygoid ligament (including a small cartilage) ; pi. tr. ethmo-palatine or palato-trabecular ligament; mck. lower jaw; sp. spi- racle; h.m. hyomandibular ; c./ij/. ceratohyal; m. 7i. L mandibulo-hyoid ligament; ph.br. pharyngobranchial ; e.br. epibranchial ; c.br. cerato- branoliial; h.br. hypobranchial ; b.br. basibranchial ; ex.br. extra- branchial; Zl, 2, 3,4, 5, labial cartilages; the dotted lines within mc/c indicate the basihval. 1I-] . THE ADULT DOGFISH. 37 regional landmarks. On the upper surface there is a soiall foramen for the supraorbital nerve in a groove be- tween the fore part of the supraorbital ridge {s. or) and the frontal part of the cranial roof. The olfactory fenestra has already been described. The foramen for the optic nerve (Fig. 7. //.) appears low down in the middle region of the lateral (sphenoidal) wall of the cranium. The tri- geminal foramen {v.) is similarly placed in the hinder part of the sphenoidal wall, and the orifice for the facial is a little behind and below this. The external foramina for the glossopharyngeal and vagus nerves are on the pos- terior aspect of the skull, the former low down and exter- nal, marking the original separation between auditory mass and basilar plate ; the latter higher up, and at the side of the foramen magnum in the line of junction of the occipital and auditory regions. 103. Examining the inner surface of the skull, the pitui- tary body is seen to be guarded in front and behind by small cartilaginous ridges in which some bony deposit takes place. These markedly resemble the anterior and posterior clinoid vidges bounding the " sella turcica " of Mammalia ; and the re- semblance is strengthened by the entrance of the internal caro- tid arteries on either side of the pituitary body. With such facts may be coupled the likeness of the occipital articulation with the atlas to that of Mammalia. 104. The palato-mandibular cartilages, constituting the main part of the oral apparatus, are loosely swung from the sides of the basis cranii by two short ligaments. The upper jaw, or palato-quadrate arcade, is attached by the ethmopalatine ligament to the antorbital region ; by the metapterygoid ligament (m. pt) to the postorbital trabecu- lar angle behind the trigeminal foramen, and in front of the spiracle (sp.) ; and by the symplectic ligament to the hyomandibular behind. The metapterygoid and the sym- plectic ligaments are very closely connected, and some fibres of the former are attached to the hyomandibular cartilage. The hinder or quadrate end of the upper jaw {qu) forms a condylar surface for articulation with the lower jaw or articulo-meckelian element. 3S MORPHOLOGY OF THE SKULL. [OHAP. 105. The upper jaw is narrow behind, broad and cDnvex outwardly in the middle, and bends inwards to be united with its fellow by a strong ligament. The lower i;iw is very similar to the upper ; it is deep and outwardly convex behind, for attachment of the oral muscles. Teeth ai-e borne on the mucous membrane of the fore and inner ])art of both jaws. The fourth and fifth labials (/. 4 and ")), set on the upper and lower jaws obliquely, make an acute angle by the approach of their pointed hinder ends, considerably within the angle of the jaw. When the mouth is opened the angle between them becomes obtuse. There is a small cartilage in the anterior wall of the spiracle. 106. The large phalangiform hyomandibular {h. m.), the upper segment of the hyoid arch, is articulated with the side wall of the cranium beneath the pterotic ridge. The spiracle is in front of it, while distally it is connected with the quadrate region by the symplectic ligament. The stout and elongated ceratohyal (c. hy.) is articulated with the distal facet of the hyomandibular, and is locked within the everted postero-inferior edge of the mandible, being strongly bound to it by the mandibulo-hyoid ligament (m. h. I.). At its ventral end the ceratohyal is bilobate, and articu- lates by its anterior lobe with the broad basihyal plate, which it also slightly underlies behind. 107. The succeeding four arches have supero- trans- verse pharyngobranchials (^/i. br.), which are in contact with their fellows in the middle line beneath the vertebral column ; lateral epibranchials (e. hr.) and ceratobranchials (c. hr.), and infero-median hypobranchials [h. hr.) di- lected backwards. Each epibranchial has a small forward spur at its upper end. Normally the pharyngobranchials are turned somewhat backwards ; but when the pharynx is distended they may be directed transversely or even a little forwards. Each pharyngobranchial has at its lower end a backward spur. The fifth branchial arch is much smaller than the rest, having its pharyngobranchial con- fluent with the fourth, and no hypobranchial. The median II-] THE ADULT SKATE. 39 basibranchial {h. br.) lies between the last three arches, in contact anteriorly with the hinder hypobranchials, and with the fifth pair of ceratobranchials behind. 108. There is a vascular plexus (pseudobranchia) on the anterior side of the spiracular opening. The five gill- clefts are the external openings of as many pouches, the anterior and posterior walls of which bear transversely plaited branchial laminae. Internally each pouch opens into the pharynx. In the partition between each pouch is a series of cartilaginous rods, or branchial rays, most of them simple, extending from the hyoid and first four branchial arches towards the skin. Those in relation with the hyoid are pectinately branched ; three of them are in the hyomandibular region. 109. Four extrabranchial cartilages (ex. ir.), parallel to the ceratobranchials, are superficial to the correspond- ing sets of branchial rays. They are pedate below, and very greatly resemble the scapulo-coracoid cartilages situated immediately behind them (see Fig. 6, e. cr.). The fifth branchial arch has no gill-pouch behind it, no rays, and no extrabranchial. Skull of the adult Skate. 110. The most notable change in the appearance of the skull is produced by the great elongation of the pre- nasal rostrum (Fig. 8, p. «.), which has the first labials {I. 1) on either side of its fore extremity. The lateral portions of the intemasal plate where they adjoin the nasal capsules are continued into the rostrum, so that the cranial tioor is continuous with a long concavity on the hinder part of the rostrum. All traces of the external branchiae have disappeared; and cartilaginous branchial rays, about a dozen in the anterior wall of each gill- sac, extend from the arches outwards and backwards. The antorbital plate has become a thick backwardly and out- wardly curved rib-like bar, pointed behind, lying outside and above the upper jaw. 40 MORPHOLOGY OF THE SKULL. [chap. 111. The following are, briefly, the chief points in which the Skate's skull differs from that of the Dogfish :— Fig. 8. Skull of Skate, nearly adult. An. periotic capsule ; na. olfactory capsule ; pn. prenasal cartilage ; II, V. in the side wall of the cranium, optic and trigeminal foramina ; p/.j)t.,5it.palato-quadratebar ; mck. man- dibular cartilage ; m.pt. metapterygoid ; 7i.m. hyomandibular; i. 7i. Z. inter- hyal ligament; e.liy. epihyal; c.hy. ceratohyal ; h.hy. hypohyal; 7i.tr. 1,2,5, hypobranchials ; above them are seen the cerato, epi, and Ijharyngobranchials ; Zl,4, labials. the presence of a supero-median cranial fontanelle, and of a large prenasal rostrum ; the second labials not coalescing with the olfactory capsules ; the fourth labials becoming involved in the inner fold of the nasal openings, so that three pairs of labials protect these orifices ; the distinct- ness and size of the antorbital cartilage ; the large size of the caitilage («i. pt.) in the anterior wall of the spiracle'; the foim and forward direction of the hyomandibular, and its dissociation from the rest of the hyoid arch, except by a posteiior and superior interhyal ligament attached to a distinct epihyal ; the smaller size of the hyoid arch, and its close connection with the first branchial ; the absence ' The presence of one or more spiracular oartOages is constant in Sharks ; a small one is found in the Dogfish. II] SUMMAKY. 41 of a basihyal, and the junction of the first hypobranchials across the middle line ; the coalescence of the fifth hypo- branchials (A. br. 5) to form a broad median basibranchial plate ; the absence of the fifth pair of labials, and of the extrabranchial cartilages. 112. Summary. The history of these skulls is singu- larly uncomplicated ; but this does not necessarily indicate that the type is a low one. Paired basal elements arise at first, the trabeculee in the anterior portion of the cranial floor, and the parachordals behind. By their union, and by the chondrification of the greater part of the cranial wall in continuity with them, a simple cartilaginous cra- nium is formed, deficient in one or more regions above. The ear-cartilages are impacted in the cranial mass behind, and the nasal capsules have their own skeletal defence in front, coalescing with the cranial cartilage as well as with an internasal element ■which is at first distinct from the trabeculse, but afterwards fused with the cranial floor. The small prenasal spur found in the Dogfish is repre- sented by the great rostrum of the Skate ; while the antorbital region is characterised in the latter by the development of an antorbital rod extending backwards. 113. The skeleton of the jaws and gills possesses from its origin substantially its adult relations; and no part of it is at any time fused with the cranium proper. The upper and lower jaws are primarily continuous, although the bars are sharply bent ; but they are speedily cut in two. In the Skate an upper and posterior region of the mandibular arch chondrifies early to form the spiracular cartilage. ■ 114. The suspension of the jaws is effected by the intermediation of an element of the hyoid arch; but in the Dogfish the hyomandibular is the segmented upper extremity of the bar, while in the Skate it arises separately in the antero-superior region of the arch. The stout ceratohyal is in the Dogfish directed forwards and closely related to the lower jaw and the distal end of the hyoman- 42 MORPHOLOGY OF THE SKULL. [CHAP. II. dibular ; while in the Skate the hyoid arch is feebler, bflt is connected directly with the skull and the proximal end of the hyomandibular. It has an epihyal as well as a ceratohyal piece, and is in relation with the first branchial arch, 115. The branchial arches commence as simple rods, afterwards segmented into a series of pieces, the arches of opposite sides being connected by basal cartilages ; the Skate is distinguished by the separate origin of its upper branchial elements. Branchial rays are found in the partitions between the gill-pouches. 116. Two series of cartilages occurring less genei-ally in other types than in those which have been described are named labial and extrabranchial. These are developed between the primary skeletal elements and the skin, and enter into relation with the rostrum, the nostrils, the jaws, and the gill-clefts. CHAPTER III. THE SKULL OF THE SALMON. First Stage : Unhatched embryos, with simple facial arches. 117. The embryos here described have their neural tube open both anteriorly and posteriorly. The long tape- like . body lies on a yelk-sac, with the umbilicus very little constricted, so that its rim reaches to within a short dis- tance behind the mouth. The involutions to form the sense-organs are widely open ; yet the primary elements of the skull can be distinguished, though merely by being composed of more consistent tissue than the surrounding parts. The rods or bars to be spoken of are formed of the very small mother-cells of hyaline cartilage. 118. The enlargements of the neural tube to form the cerebral vesicles are very slight, not constituting any obvious bulging ; but the three vesicles are distinct, the first being largest, and having the landmark of the pituitary body (Fig. 10, py.) projecting backwards from its postero-inferior surface. There is no mesocephalic flexure of the neural tube. A little behind the pituitary body is the pointed anterior end of the notochord [n. c), lying just below the middle of the second cerebral vesicle. There is no clear distinction between the outer layer of epiblast and that which limits the neural tube. 119. Almost at the anterior extremity of the embryo, on its under surface, the olfactory sacs appear as small pits surrounded by a circular ridge of epiblast; the extreme open end of the neural tube is immediately above them. 44 MOKPHOLOGY OF THE SKULL. [chap. The involutions for the eyeballs are much larger, and un- closed ; they are behind the olfactory sacs, and lie quite Fw. 9. Embryo Salmon, about | inch long ; side view of head within chorion. ch. chorion; ol. olfactory sac; c. eyeball; au. auditory mass; s. 0.6. supraorbital baud; u.v. attachment of umbilical vesicle; cl. la- chrymal cleft ; V. 1, 2, 3, 4, 5, visceral arches ; mn. first or mandibular arch ; cl. 1, 2, 3, 4, 5, visceral clefts. on tbe inferior surface. The rudiments of the ear-sacs (au.) are by the sides of the third cerebral vesicle, and are consequently more dorsal than and posterior to the eye- balls ; their line of involution is open, and longitudinally placed. So large are the eye- and ear-sacs relatively to the skull, that they somewhat overlap one another. 1 20. The side wall of the head arching over the eye- ball is conspicuously thickened, forming the supraorbital band (Fig. 9, s. o. b.). Below and behind the eyeball, is a thickened suborbital arch, which extends forwards to the olfactory sacs of the same side. Between the anterior extremities of these arches is a space lined with thickened epiblast, the future mouth-cavity (Fig. 11, m.). This is bounded in the middle line and behind by the meeting of another pair of arches, the foremost of the visceral series. III.] THE SALMON: FIEST STAGE. 45 The latter curve forwards to the middle line below the auditory region, the successive arches having between them slits or clefts (cl.), which penetrate into the primitive ali- mentary canal. It is only at a later period that the moutli has a communication with the same cavity. The umbilical sac is attached in front to the head, in the space between the hindermost visceral arches ; the heart (Fig. 11, h.) is situated on the wall of the fore part of the alimentary canal, above or within the same region. Fig. 10. Embrj-o Salmon, about | inch long ; upper view of head, dissected,^ the neural tissue having been removed. tr. trabecula; pa.ch. parachordal cartilage; n.c. notochord ; oi(. auditory capsule, showing dilatations for semicircular canals ; py, pituitary body; e. eyeball; mn. mandibular, hy. hyoid, br. branchial, arches. The anterior aperture remaining in the neural tube is shown in the skin in front of the trabeculas. 121. The notochord ends pointedly at about the level of the front end of the ear-sacs, behind the pituitary body. On either side of it are parachordal bars (pa. ch.) cut off squarely in front, a little behind the apex of the noto- chord (Fig. 10). Laterally, the parachordals are flanked by the auditory involutions, and by the mesoblastic in- vestment which is forming a partial girdle of nascent car- tilao-e round them. The inner part of the ear-cavity shows the°several dilatations for the semicircular canals (Fig. 10). The nascent cartilage does not appear in the floor or on •iC MORPHOLOGY OF THE SKULL. [(."HAP. the lower outer side of the sac below the line of invo- lution. A little later, the auditory masses unite with the parachordals at two points on either side, one anterior and the other posterior ; thus a fenestra is formed in the base of the skull (see Fig. 14,/. s. o). ] 22. The axial prechordal region of the cranial floor is occupied by the trabeculoB (tr.), bowed outwartls on either side of the pituitary body. They are small and pointed behind, where they curve towards the notochord, without touching it ; and more expanded in front, where they approach one another, and lie under the fore part of the cranial cavity, and in the roof of the mouth. The intertrabecular space is not merely coextensive with the pituitary body, for the trabeculte lie in the floor of the fore- brain, and support it on either side. 123. A palatine bar is distinctly formed in the sub- ocular band ; it is simple and arcuate, reaching in front to the nasal sac, and behind nearly to the auditory capsule, where it overlaps the posterior extremit}' of the supraorbital ridge. The palatine rod is from the first clubbed at the fore end, and there solidifies earliest; it passes back into a fine point behind, much later in solidifying. The mandibular arch is at first curved but slightly forwards below, and may be nearly transverse, although embryos of the same age vary greatly in this respect. Its upper extremity curves inwards beneath the fore part of the ear-sac, towards the apex of the noto- chord. It soon becomes distinguishable into three re- gions ; (1) the metapterygoid apex, (2) a median swelling directed forwards, the orbital process, and (3) the lower or meckelian, elongated forwards as a spatulate expansion, but not yet meeting its fellow (Fig. 13, mt.pg., or. p.,mk.). 124. The hyoid arch (hy) has an enlarged and in- curved apex, which lies underneath the middle of the ear- sac. Its lower end becomes curved forwards behind the mandibular region. The two hyoids do not meet, but are before long separated only by a small median segment. I'l-] THE SALMON: FIRST STAGE. 47 the hasihyal (Fig. l.S, g. L). There are five pairs ol branchial rods {br.) behind the hyoid, not meeting in the fU*i Embiyo Salmon, about f inch long ; lower view of head, with the arches shining through. Na. nasal sacs; e. eyeball; au. ear-mass; tr. trabecula; pl.pt. palatine or subooular bar; mn. mandibular, hy. hyoid, br. branchial, arches ; pty. pituitary body ; m. mouth ; h. heart. middle line. They become progressively smaller from the front to the hinder one, and are all sigmoid in shape except the last, with their apices incurved over the cavity of the throat. 12.5. Most of the primary elements of the skull are present in this early embryo, which has not yet under- gone the mesocephalic flexure. The parachordal and tra- becular regions are definitely marked out, and the periotic tissue is commencing to chondrify and to unite with the parachordals. The palatine, mandibular, hyoid, and five branchial bars, are manifest in a simple unsegmented condition ; but the mandibular piece is already becoming differentiated into regions. 126. In order to simplify the description, no reference has been made to the remarkable asymmetry of Salmon embryos, and the great variations in the degree and kind of asymmetry. The head is usually twisted about its axis so tliat only one eyeball, which may be either right or left, is visible is M,ORPHOLOGY OF THE SKULL. [CHAP. ill an upper view. The rudimentary postoral arches (which share the general asymmetry at first) are distinct before the clefts between them are complete. The arches differ consider- ably in their degree of development ; the earliest distinct are the mandibular and hyoid bars, then the branchials, next the trabeculse, and most imperfect, the palatines. The chondri- fioation of the latter is very tardy. They enclose a soft pith e^-eu in the third stage. With this may be compared their long suppi'ession in the Frog. Second Stage : The metamorphosis of the hyoid arch. 127. The series of changes now to be noticed takes place in embryos while still within the egg, or only just hatched. The neural tube is completely closed, the cere- bral vesicles are greatly enlarged and very prominent, and their growth has produced a mesocephalic flexure (see Fig. 12). Thus the second cerebral vesicle is a large boss on the upper surface of the head, while the first vesicle is below and in front. Behind this is the eyeball, then the visceral arches ; but the mandibular arch is produced below (Fig. 13, mn.), forming the prominent hinder boundary of the mouth. The nasofrontal process (n. f. p.) has grown wide between the nasal sacs, separating the suborbital arches, so that the oral cavity is broad and quadrate. An opercular skin-fold is passing backwards from the upper part of the hyoid region. Dermal papilla, the rudiarents of the gills, are noticeable on all the branchial arches except the last. 128. The notochord remains relatively in the same position as at first ; kpt the parachordals are thicker in front, and unite with me trabecule. Behind, they extend farther, and are pointed; they also tend to invest the notochord more closely. Laterally, their union with the ear-cartilages becomes more and more marked, diminish- ing the fenestra in the line of union with the periotic mass (Fig. 14). The ear-capsules get much more exten- sively chondrified ; and at the line of junction between them and the parachordals there is a thickening and Ill-] THE SALMON: SECOND STAGE. 49 upward growth, which tends to form side walls and a roof to the cranial cavity. The semicircular canals produce It. 4 Embryo Salmon, partly hatched ; median longitudinal section of head. C 1, 2, 3, fore-, mid-, and hindbrain ; pij. pituitary body ; n.c. noto- chord ; tr. trabecula ; m. mouth ; mn. mandibular, hy. hyoid, br. bran- chial arches. bulgings in the ear-cartilage, and a small space remains unchondrified over the posterior canal {epiotic fenestra). 129. The mesocephalic flexure causes the trabeculae to run downwards as well as forwards ; anteriorly, how- ever, they curve upwards again, below the fore end of the cranial cavity (Fig. 12, tr.). Their hinder-hooked extremities bend inwards more and more towards the parachordals, and finally are completely fused with them. The time of this union is very variable ; sometimes it occurs before hatching, in other cases it is not perfect in the second week after that event. 130. In front of the pituitary region the trabecular become approximated along their whole length, and then gradually fused ; and at the same time a median longitu- dinal ridge arises. This ridge increases in prominence anteriorly, where the trabeculae lie in the nasofrontal process, and are laterally expanded, growing towards the subocular or palatine arches ; thus the ethmopalatine connection is being formed (Fig. 14). The fore extremi- B. M. 4 50 MORPHOLOGY OF THE SKULL. [chap. ties of the trabecnls^ develope a transverse ridge beneath, in the roof of the mouth ; above, they diverge from the median crest, and form a flat spatulate lamina on each side, partially overarching the olfactory sac. The remainder of the cranial investment is membranous; para- chordals, trabecule, and auditory cartilages constitute the whole of the chondrocranium. Fig. 13. or.p. Embryo Salmon, not long before hatching ; under view of head, with arclies 6een through. N.f.p, nasofrontal process ; ol. olfactory sac ; e. eyeball ; m. mouth ; tr. trabecula ; p.pg. palatine cartilage ; m. mandibular arch ; mk. mecke- lian region ; q. quadrate region ; mt.pg. metapterygoid region ; or.p. orbital process ; g.h. glosso- or basihyal ; h.h. hypohyal ; c.h. ceratohyal ; li.iit. hyomandibular. 131. The subocular or palatine bar remains little developed in this stage, forming the lateral wall of the mouth ; anteriorly it is in contact with the ethmopalatine process of the trabecula. Behind, there is a cleft between it and the eyeball, passing down into the palate. The mandibular arch undergoes more modification ; the inferior nieckelian part becomes bent at a large angle with the proximal portion, and meets its fellow in tVie middle line (Fig. 13). III.] THE SALMON: SECOND STAGE. 51 This flexure generally tates place forwards, but it may be directed backwards, so that very diverse appearances may be presented by the facial arches of diff'erent embryos, owing to this cause and also to the asymmetry of the creatures j but the morphological result is the same. 132. The proximal region of the mandibular arch beconies broadened, and an upper segment is cut off in a peculiar manner so as to form a ball-and-socket articu- lation. The ball is on the upper, the socket in the lower piece; and the junction is defended by an angular process of the latter, running up behind. Furthermore, the whole arch travels considerably downwards from the axial parts and becomes less closely attached to the cranial, wall. 133. Before the segmentation of the mandibular car- tilage, the mo.st striking division takes place in the hyoid arch. In addition to the median basihyal mentioned in § 124 (p. 46) a small globular piece, the hypohyal, is cut off from the inferior end of each hyoid bar ; while the rest of the arch is split into two along its entire length, so that one bar lies directly behind the other and in contact with it. The anterior of these is the hyomandihular, the poste- rior the ceratohyal (Fig. 13, hvi., c.h.). 134. At a later period, concurrently with the tra- velling downwards of the mandibular arch from the cranial mass, the hyomandihular diverges from the ceratohyal below, remaining in contact with it above. The hyoman- dihular being thus turned forwards, reaches the mandibular arch, and becomes applied to its upper segment and the proximal part of its lower segment (Fig. 14). 135. A further change of relation is subsequently effected. As the throat increases in size, the hyoman- dihular remains closely apposed to the auditory cartilage, while the ceratohyal is found lower and lower in position, as if it had slid down half way along the hinder edge of the hyomandihular (see next stage. Fig. 17). At this point the upper end of the ceratohyal becomes fixed by an 4—2 52 MORPHOLOGY OF THE SKULL. [chap. interhyal ligament, in which cartilage is subsequently de- veloped. The hyomandibular broadens above and developes Fig. 14. Embryo Salmon, not long before hatching ; lower view of skull dis- sected, the branchial arches having been removed. Tr. trabecula, with transverse ridge in roof of palate ; p.jig. palatine bar, near ethmopalatine conjunction; py. pituitary space ; h.c. notochord; ya.ch. parachordal cartilage which has coalesced with au. auditory ciipsule, except where a fenestra /.s.o. is left; mh. meckelian cartilage; q. quadrate region ; mt.pg. metapterygoid region ; the line should be pro- longed to the tract inside the meckelian bar ; h.m. hyomandibular turned forwards at its ventral end, and applied by its symplectio tract sy, to the quadrate region of the mandibular arch; c.liy. ceratohyal ; h.h. hypo- hyal ; g.h. basi- or glossohyal. a backward spur; below, it remains styliform. The mandibulo-hyoid cleft becomes gradually obliterated. 136. The branchial arches are not as yet segmented like the arches in front. They fuse in the middle line below, all but the last pair ; and then four small median azygous ventral pieces, the basibranchials, are cut out. The upper end^of these arches curve inwards most elegantly over the sides and roof of the throat. The hinder ^fifth) branchial pair remains much smaller than the rest. HI.] THE SALMON: THIRD STAGE, 53 137. The important period of development included in this second stage witnesses many significant changes. The mesocephalic flexure being fully established, the trabeculse unite with the parachordals, and the latter, in close union with the otic cartilages, form lateral and superior occipital growths. The anterior portions of the trabeculse unite and send up a median crest, the rudiment of the interorbital and nasal septa ; while lateral and anterior trabecular growths establish the ethmopalatine and the cornual regions. But the most intense interest attaches to the modifications by which the mandibular and hyoid arches are segmented, and their segments take up the relative positions essentially characteristic of Osseous Fishes. The preponderance of growth in particular regions effects all that to outward observation looks like the shifting of parts. It is, we hope, made plain how the mandibular elements, removed to a distance from the axial parts, derive their suspensorium from the arch behind, which is remarkably segmented for that purpose. The formation of definite hypo- and basibranchial pieces is to be noted at this period : the principal branchials do not yet present any segmenta- tion. Third Stage: Salmon-fry of the second week after hatching. 138. The head has become much larger, and the mesocephalic flexure is lost ; a longitudinal section shows the parts of the brain lying approximately in a straight line on the flattened cranial floor. The well-developed brain fills the proportionally large cranial cavity; the auditory protuberances are very marked at the sides. The articulation of the meckelian rod with the proximal man- dibular element is still farther removed beneath the level of the cranial floor, and the mandible has grown forwards underneath the fore part of the head. No ossifications in cartilage appear as yet; but parostoses' have arisen in 1 By parostosis is meant an ossiflcntion in subcutaneous fibrous tissue ; it may reach to and involve the perichondrium of a cartilaginous tract ; 54 MORPHOLOGY OF THE SKULL. [CHAP. mors tlian one region, t"he basicranial splint, or -para- tiplicnvid, being especially noticeable. In many point.s there are agreements between the cartilaginous cranium of the Salmon at this stage and the cartilaginous and osseous cranium of the adult Polyptenis ; see Traquair On the Cranial Osteology of Polypterus (Journ. Anat.\o\.v. p. 1G6). 139. The occipital region of the chondrocranium has progressed, but is yet far from perfect. The notochord Fig. 15. Salmon fry, second week after hatching ; upper view of sknll dissected, the brain heing removed, and the inferior parts somewhat displayed. Pa.ch. marks the anterior limit of the parachordal cartilage ; nc. noto- chord, lying in the posterior basicranial fontanelle produced by the recession of the parachordals ; o.m. the orbital muscles diverging from beneath the cranial floor in the notochordal region ; so. supraocci.pital tract; a.xc. anterior, h.s.c. horizontal or external, p.x.c. posterior semicircular canals ; tr. trabeculse ; t.cr. tegmen cranii, or cartilaginous roof of the cranium; fo. superior fontanelle, heart-shaped; s.n. nasal septum ; s.n.l. subnasal lamina ; mn. mesonasal cavity ; u.l. upper labial i.artilage ; p.pr]. palatopterygoid bar; 5. quadrate condyle; mt.pg. metapterygoid region of mandibular arch. fcuoh a bone may be denominated a membrane bone. Ectostosis sig- nifies ossification arising between perichondrium and the superficial cells of cartilage, which gradually becomes absorbed and replaced by bone. Endostosis is the direct calcification of cartilage. Ill] THE SALMON: THIRD STAGE. 55 is retracted so as to occupj' only the hinder half of the postpituitary (basilar) region, and the parachordal masses have receded from one another, leaving a considerable oval space (posterior hasicranial fontanelle) between them anteriorly ; while posteriorly they do not yet unite with each other in any portion of their length, although they invest the ndtochord closely behind (Fig. 15). 140. Laterally, the parachordals do not become broad, as in the Dogfish and Skate ; they form a floor for the hindbrain, and externally are fused with the periotic cartilages. Nevertheless, there is a distinct notch in front between the otic and parachordal elements ; and the latter also project backwards behind the ear-cartilages. Each of these hinder processes is divided into two lips ; these are the primordial articulating surfaces of the cranium with the first vertebra. Lateral occipital carti- lage now exists, largely fused with the ear-capsules, but rising free on the backward projections of the parachordals. Above, they have passed inwards on each side, and coa- lesced to form the supraoccipital plate {s.o. Fig. 15). 141. The ear-cartilages are not greatly modified ; the semicircular canals are large proportionately, and may be distinctly seen through the cartilage. On the under surface the fenestra formerly mentioned (§ 121 p. 46) in the line of junction between the periotic and the parachordal masses is much smaller relatively, being carried outwards by the latter growing underneath the former. The cartilage beneath the bulging vestibule is very thin, and a large otolith within it is well seen from outside. The upper and outer eminence of the ear-capsule (the pterutic ridge) is prominent, overhanging the articulation of the hyomandibular. 142. The hinder division of the trigeminal nerve escapes from the cranium by a foramen where the fore part of the parachordal is fused with the auditory carti- lao-e; its anterior (ophthalmic) division passes over the notch between these two elements. The foramen for the 56 MORPHOLOGY OF THE SKULL. [chap. facial nerve (Fig. 18, 7 a.) is behind that for the trigeminal (.5) on the inferior aspect of the skull ; while the glosso- pharyngeal and vagus nerves (S) emerge in the same line near the posterior part of the basis cranii. The orbital muscles {o.in.) pass forwards from their basicranial attachment to the orbit on each side of the pituitary body. 143. The hinder portion of the trabeculse, in the region of the pituitary body, does not show much change : slender arcuate rods enclose a somewhat triangular inter- trabecular space, much larger than the pituitary body ; and posteriorly they lie above and unite with the fore extremity of the parachordals. Anteriorly there is great change ; the brain-case has become elevated, and 1iie forebrain no longer lies directly on the trabecule, but on a membranous floor which forms the upper and inner boundary of each orbit, while a median vertical inter- TWS.pff- Salmon fry, second week after hatching; transverse section of head, througli forebrain and eyeballs. T.ev. tegmen cranii; m.cr. membranous cranial investment; CI 6. forebrain ; i.o.s^. interorbital septum ; 1, olfactory eras ; tr. coalesced trabeculse, with superior ridge ; ya. palatine cartilage ; mk. meckelian cartilage ; m. mouth ; e. eyeball. Bones: /. frontal; jia.s. paragphenoid ; ms.prj. mesopterygoid ; mx. maxillary ; d. dentary ; ar. articular. III.] THE SALMON: THIRD STAGE. 57 orbital septum connects this floor with the trabeculae (Fig. 16, i.o.s.). The latter are not horizontally placed, but arched upwards so as to unite at rather more than a right angle, and with a sharp edge upwards (Fig. 16, tr.) The trabeculae become broader where they coalesce, and at the front of the orbit they curve suddenly outwards on each side, forming the ethmopalatine processes, to which the fore ends of the palatine rods are connected (Fig. 15, p. pg.). The trabecular plate in its most anterior portion is broad and quadrate below, flooring the nasal sacs and ending witli a broad edge, truncated at the external angles, and showing very little distinction of free trabecular cornua. 144. The anterior part of the trabecular crest (the nasal septum) has increased in height; its moieties diverge in front and behind, so as to form crescents. The anterior fork becomes shallow towards its termination ; the posterior is more elevated, and unites with the ethmo- palatine process and with the cranial roof, to be presently described. In the diverging angle between the posterior forks of the internasal cartilage there is a median blind membranous pouch (Fig. 15, m.n.c), beneath which is the trabecular plate. The olfactory crura pass out of the brain- ctoe under the hinder part of the forebrain, and run in the orbit horizontally on either side of the interorbital septum, finally gaining a level a little above the coalesced trabeculse in the ethmoidal region, where they perforate the postnasal wall. 143. The fore pai't of the cranial roof has become chondrified. This cranial roof, or tegrnen cranii {t. cr ), is broad and quadrate, and convex above, covering the fore- brain and the anterior part of the midbrain. It is con- tinuous with the posterior forks of the nasal cartilages in front, and shelves down laterally to the ethmopalatine region, forming lateral ethmoidal or antorhital plates. There is no cartilage in the side wall of the cranium in the region of the tegmen; but each of its postero- external angles is continuous with a slender rod of carti- 58 MORPHOLOGY OF THE SKULL. [CHAP. lage developed in the supraorbital band, which curves outwards and backwards to join the fore part of the auditory cartilage (Fig. 17. s. or.). Thus there is a large heart-shaped membranous fontauelle in the roof of the chondrocranium, (/o. Fig. 15), bounded in front by the tegmen (which being produced somewhat backwards in the middle line causes an emargination in the fontanelle), laterally by the supraorbital bars and the ear-masses, and posteriorly by the occipital cartilage. 146. No part of the chondrocranium is ossified in this stage : but several parostoses have appeared. The chief of these is the azygous parasphenoid (Fig. 16, pa. s.), a flat bone lying underneath the cranial floor, extending from the nasal nearly to the hinder part of the auditory region. It is slightly forked both in front and behind. A delicate supraethmoid bone overlies the internasal septum, and there is a styloid frontal ossification over each lateral edge of the tegmen. Just beneath the front extremity of the trabecular plate is a rudiment of the premawlllary on each side. 147. The first important modification to be noticed in the facial parts is the coalescence of the primitively distinct elements, the palatine and the proximal mandibu- lar segment. The palatine or suborbital cartilage joins a forward growth from the mandibular, so as to become expanded behind as well as in front, remaining slender in the middle. The much enlarged upper mandibular seg- ment forms the fore part of the wall of the pharynx below the level of the cranial cavity, and is no longer directly connected with the skull. Its chief relation to the chondro- cranium is derived from the arch behind, the upper and forward portion of which is closely applied to it. Thus there is now but one continuous cartilage on each side, extending from the ethmopalatine trabecular process in front to the hyomandibular behind (Fig. 17). Two pairs of small nodular cartilages, the upper labials (l^, l.^ are found in front of the palatine cartilages, underneath the fore part of the trabeculse. The elongated articulo-meckelian Ill] THE SALMON: THIRD STAGE. 59 rod, or cartilage of the lower jaw, articulates with the condyle on the proximal mandibular piece, and is produced into a thick angle behind the condylar hollow. Fig. 17. S.Or KM no^-' G.% lUh M -E* ,S.O StsU ^•^'•^ jL^C^x ^^ Adult Salmon: side view of skull with all bones attached ; cartilaginous parts dotted. Cartilage bones: S.O. supraoocipital ; jEp. 0. epiotic ; PJ.O. pterotic ; Ft. palatine; Ms.Pt. mesopterygoid ; M.Pt. metapterygoid ; Qu. quad- rate ; Art. articular ; H.M. hyomandibular ; Sy. symplectic ; E.Hy. epi- ceratohyal, partly covered by P.Op. and I.Op. ; G.Hy. ceratohyal, hinder part ; H.Hy. hypohyal ; G.Iiy. glossohyal. Membrane bones : Pa. parietal ; Fr. frontal ; S.Eth. supraethmoid ; Na. nasal; Lch. lachrymal; Sb.Or. suborbital; S.Or. supraorbital; Pmx. premaxillary ; Mx. maxillary; Ju. jugal; P.Op. preoperoular ; Op. oper- cular; I. Op. interopercular ; S.Op. subopercular ; D. dentary ; Ang, angular; B.Brs. ba&ibranchiostegal ; ^rs.iJ. branohiostegal rays. 76 MORPHOLOGY OF THE SKULL. [CHAP. much broader, and considerably extended vertically, so as to descend to a level much lower thaa that of the cranial floor. At the outermost inferior angle of this palato- quadrate tract is a condyle for the mandible. The ex- tended hinder edge of the bar has a peg from the arch behind embedded in it in its lower part, while it overlaps the same arch above. The portion of the palatine car- tilage in front of the ethmopalatine connection is con- siderable, and bears a rounded boss looking upwards to the edge and base of the skull, and articulating with the overlying maxillary bone. 187. The dentigerous palatine {pi.), a rod-like bone, extends to the forward end of the bar : it has ossified most of the cartilage in front, but merely invests it behind ; only one-third of the length of the bone is be- hind the ethmopalatine conjunction. The mesopter3'goid bone (ins. pt.) invests the cartilage behind the palatine, overlapping the latter by a pointed end for half its length on the inner side of the bar, and passing back and broadening to overlap the quadrate and metapterygoid. It also overlaps the proper pterygoid below, and above clings round the upper edge of the cartilage and covei-s it externally for some extent. The pterj^goid embraces the lower edge of the cartilage from the jDalatine to the quadrate, and does not occujjy much space on either the inner or the outer surface, but most on the inner. 188. The metapterygoid (m. pt.) is a squarish bone occupying the postero-superior angle of the palatoquad- rate bar, and reaching to about half the height of the hyomandibular [h. m.), which it overlaps externally.. There is a tract of cartilasre between the lower edge of tills bone and the upper edge of the rather small trian- gular quadrate (qu.), whose inferior angle bears the carti- laginous mandibular condyle, and whose posterior edge is grooved for the peg from the next arch. 189. The investing bones related to these regions belong. to the upper jaw. Each premaxillary [pmx.) is a in.] THE ADULT SALMON. 77 splint of the corresponding trabecular cornu, passing backwards outside the prepalatine cartilage. It has an elevated prenasal plate, and a considerable dentary region. Its line is continued backwards by the smaller maxillary {mx.), which is overlapped by the premaxillary in front, and by the jugal (ju.) behind ; it also abuts upon the quadrate. The facial plate of the maxillary is small and little elevated; the oval margin is dentigerous in its whole extent. The jugal is a lanceolate bone not enter- ing into the edge of the jaw, but 1 ing above, on the maxillary and the quadrate. 190. The connective tissue encircling the orbit be- hind the ectethmoid, above the palato-quadrate bar, below the frontals, and in front of the hyomandibular, has given rise to about seven membrane-bones, which form a nearly complete ring. These orbital bones have some extension within the rim of the orbit and an ex- pansion on the face, which is more marked in the case of the hinder ones. The frontals partly interrupt the ring above, and enter into its circumference. 191. The lower jaw is an elongated bar, very ma.'isive behind, rising high in the coronoid precondylar region, having a projecting postcondylar or angular process, and a strong hook on each dentary bone at the symphysis. The proximal part of the cartilage has been ossified by the articular (art), but anteriorly this bone lies as an outside splint on the meckelian cartilage, which extends beyond it almost to the symphysis." The dentary (d.) embraces the fore end of the cartilage both externally and internally, but farther backwards it appears principally on the out- side, overlapping two-thirds of the articular, and terminat- ing by an upper and a lower fork. It is dentigerous on the anterior three-fourths of its upper edge, but not behind, where its upper fork mounts on the coronoid process of the articular. A small angular (ang.) lies on the angle of the jaw beneath the condyle, but the postcondylar proce.ss is formed by the articular. The symphysis of the lower jaw 78 MORPHOLOGY OF THE SKULL. [CHAP. is of small extent; the moieties are united by fibrous tissue. 192. The hyomandibular extends downwards from its elongated horizontal pterotic articulation behind the postorbitals and the metapterygoid. It is narrower as it descends : on its posterior edge in the upper third is a bony process covered by a knob of cartilage, the opercular condyle. The cartilage below the hyomandibular bone is a thick prominent elbow, which turns forwards at a little more than a right angle into the small peg-like symplectic bone {sy), fitting into the groove on the postero-internal face of the quadrate. 193. The inferior division of the hyoid arch is attached to the upper and anterior between its two ossifications, at the elbow just described. A small partially ossified interhyal cartilage intervenes, and the lower bar is directed forwards into the base of the tongue in the inter- mandibular space, being mainly compnsed of massive cartilage, flat proximally and thick distaliy. Its proximal third is ossified by the epiceratohyal (e. hy), its distal two-thirds by the ceratohyal (c. hy.) ; the fore extremity of the latter, covered by cartilage, fits into the posterior end of the smaller hypohyal mass {h. hy.), which has two distinct ossifications. An azygous basi- or glossohyal {g. hy) passes forwards into the tongue, having the hypo- hyals on either side behind, and being joined in the middle line posteriorly to the first basibranchial. The long glossohyal is ossified superiorly, and bears a row of teeth which project in the middle of the floor of the mouth. 194. The opercular membrane is a great semi-oval skin-flap extending backwards from the hinder edge of the hyomandibular and quadrate regions, and covering the upper part of the gill-arches. Four large membrane-bones are developed in this flap. The anterior, the preopercular {p. op.), occupies nearly its whole vertical extent, and is crescentic in shape, with two horns and a broad median III.] THE ADULT SALMON. 79 region ; the lower horn almost reaches the postcondylar process of the mandible. The preopercular is burrowed in a radiating manner by mucous glands ; and to its upper horn is attached a small supratemporal bone, one of the series of mucous bones which is continued by the scales of the lateral line. The bones of the orbital ring and the nasals also belong to this category. 195. The principal and largest opercular (op.) is sub- quadrate with very rounded angles ; it is overlapped in front by the upper horn of the preopercular, and has a cup on its upper anterior angle, for tlie opercular knob of the hyomandibular. There is a postero-inferior suboper- cular (s. op.), lying within the opercular, which overlaps its upper edge, and also within the next bone to be described, which is outside its forward region. The interopercular [i.op.) overlaps the lower anterior angle of the opercular and the anterior portion of the subopercular. It is ■ itself overlapped largely by the preopercular, and tied to the angle of the lower jaw bj' a strong ligament. Both sub- and iuteroperculars are very thin bones, of elongated oval shape. 196. The lower division of the hyoid arch has attach- ed to its under edge on the outside a series of twelve flat subfalcate bones, the br;inchiostegal rays {br. s. r.}. 1 he posterior is the outermost and largest, and overlaps the next, and so on to the front and smallest. A continuous elastic membrane encloses the whole, passing from the hinder edge of one ray to the fore edge of the one behind it. Where the folds of opposite sides meet in front there is an azygous membrane bone, or basibranchiostegal,- (6. br. s.), small anteriorly, with a high median crest and a , broad hinder basal plate. 197. The five branchial arches form a series in the walls of the throat. The first four bear gills, and are bent backwards at a right angle, the upper portions being about two-thh-ds the length of the lower and anterior divisions. The upper parts of the arches are also bent 80 MOKPHOLDGY OF THE SKULL. [CHAP. inwards under the hinder region of the cranial floor and the anterior vertebrae: and they present two divisions, a superior and smaller pharyngobranchial and a larger epibranchial, with a fibrous joint between. The fourth pharyngobranchial is attached to the lower hinder surface of the third. All four pharyngobranchials are more or less ossified and project almost directly inwards; they bear no teeth in this type. The forward epibranchials are the larger, and are shaft-like bones tipped with cartilage at both ends ; they bear an upper external pointed process. The fourth epibranchial is short and flattened. 19rS. The lower divisions of the branchial arches are attached to the upper by very perfect hinge-joints. From this joint the long shaft-like osseous ceratobranchial is directed forwards ; and the three first arches have a smaller hypobranchial piece seginented off and considerably ossified. The fifth branchial arch has only the lower division developed, and ossified as ceratobranchial ; it bears teeth on its inner edge, and is the inferior pharyngeal bono of Cuvier. The bones of the arches are solid in the middle and usually surrouud a cone of cartilage at either end. 199. The first and fourth branchial arches have pro- jecting from their antero-external margin about a score of small denticular bones, curving inwards ; the second and third arches have two series of these, one directed forwards and the other backwards. These little bones are so arranged as to form a colander through which the water is strained as it pours through the branchial clefts. The postero-external face of each arch is grooved from the top of the epibranchial to the bottom of the hypobranchial. In this groove lie the branchial vessels ; from it the gill laminse project. 200. The gill arches of opposite sides axe bound together mid-ventrally by a contmuous series of bones and cartilages. Each basibranchial, like the basihyal, runs in.] THE SALMON: SUMMARY. 81 forwards in frout of its arch. There is no segmentation of the cartilage except behind the third basibranchial : and the cartilage behind this point is unossitied, and bears both the fourth and the fifth branchial arches. Each of the three solid basibranchial bones is continuous by a considerable tract of cartilage with the next. 201. Summary. Considering first the history of tlie cartilaginous elements, we find parachordals and trabe- cula? in the floor of the cranium very early, and the union of trabecula with parachordal is complete before the para- chordals have united with one another. The latter quickly fuse with the adjacent otic cartilages, and an occipital ring is completed in continuity with these masses. The para^ chordals during growth become separated by a consider- able distance in their postpituitary region, and the trabe- cule bend outwards very definitely where they bound the pituitary space. In front they are speedily confluent, and become developed beyond the brain-case into lateral eth- moidal and prenasal or cornual regions, between which the nasal capsules lie. 202. The formation of a median interorbital septum above the coalesced trabeculse elevates the fore part of the brain. The septum is at first membranous, but afterwards chondrifies continuously with the trabeculse. The pre- cranial cartilage grows to a height corresponding with that of the septum, and differentiates into massive lateral eth- moidal, subnasal, internasal, and prenasal regions. Upper labial cartilages become connected with its cornual termi- nations. 203. The supraorbital tract is early chondrified, and is confluent in front with the lateral ethmoidal region, and behind with the auditory capsule. A strong cartilaginous cranial roof is developed from before backwards until it joins the supraoccipital growth, and only leaves a pair of small membranous foatanelles in the parietal region. The side walls of the cranial cavity anteriorly to the otic masses are not extensively chondrified. B. M. 6 «i MORPHOLOGY OF THE SKULL. [CHAP. 204. The mandibular, liyoid, and branchial arches arise as simple rods on each side ; and a palatine or sub- ocular bar is at the same time developed, having a definite attachment to the lateral ethmoidal region. The postoral arches undergo a segmentation by which they are divided into upper and lower portions. The inferior piece of the mandibular arch is elongated to constitute the axis of the lower jaw; the upper division grows forwards to join the palatine bar, forming a continuous palato- quad rate tract. It gradually descends from its proximity to the cranial wall, and is finally supported at a much lower level by the upper division of the next arch. 205. The segmentation of the hyoid arch occurs throughout its entire length, the anterior tract becoming superior, and the posterior inferior. The upper part is the suspeusorium of the lower jaw : it is articulated by an elongated surface with the otic cartilage, and is applied in front to the hinder edge of the upper mandibular seg- ment. The remainder of the hyoid arch, and also the branchial arches, become more or less segmented, but not remarkably modified. Median basal pieces connect the bars of opposite sides. 20G. The proportion of the chondrocranium which is replaced by osseous deposit is but small. The occi- pital ring has four bones, but they are separated by car- tilage. The feeble basi- and alisphenoids, with a peculiar massive orhitosphenoid produced by the junction of a pair of bones, are the other chief cranial ossifications in car- tilage, besides those related to the ear-capsules. Each of these acquires five bony centres, of which the pterotic is important as overhanging the hyomandibular articulation, while the prootic encroaches largely on the proper cranial rioor. The intrinsic ossifications of the facial bars are very distinct, their history is simple, and recapitulation is un- necessary. 2(;7. The subdermal or membraae bones are very nu- merous, the principal appearing sooner than the cartilage- btnes. Very important elements occur in relation to the III.] THE SKULLS OF FISHES. 83 base and roof of the cranium ; they are azygous beneath, and mostly paired above. The extent and relations of the parasphenoid demand especial attention. The precranial, palatine, and mandibular cartilages have series of bones related to them as splints ; while the large orbits are partly protected by the outward extension of bones related to the cranial ro(jf, and partly by special circumorbital bones. The great opejfcular and branchiostegal membranes have their own osseous skeleton, vi^hile the branchial arches possess many small tooth-like parostoses. APPENDIX ON THE SKULLS OF FISHES. 208. The skulls of most osseous finhes are constructed sub- stantially like that of the Salmon ; cartilage may persist to a greater or less extent, and anchylosis of bones may take place. In Siluroid and Cyprinoid fishes, however, an interorbital septum is not formed. The substance of the ethmoidal cartilage may become partially ossified by median (Cod) or paired (Pike) bones. In the Murmnoids the trabeculse become merely narrow bands of cartilage in the orbital region, arched forwards and upwards upon the parasphenoid. Anteriorly there is a median vertically-crested ethmoidal bone, deeply grooved beneath to receive a high crest from the dentigerous vomer. On either side of this crest lies a distinct simple trabecular rod. The olfactory nerves as they pass to the nasal sacs are en- closed by a pair of thick separate eotoethmoidal cartilages ; abovj the membranous olfactory capsule is a small nasal bone. There are no premaxillaries ; the vomer, dentigerous all the way, runs to the an- terior extremity of the beak ; the maxillaries are large and dentigerous. The hyomandibular is very large, witli two distinct heads widely separated : its distal part is directed downwards and forwards ; the symplectio is distinct. Between these two bones the rest of the hyoid arch is attached by the interhyal ligament. The epi- and ceratohyals are about equal in size, and there is no liypohyal. The apex of the suspensorial part of the mandib ilar arch is unossifled, and is let down to a position opposite the interhyal ligament, being embraced by sphnt-like processes of bone from the hyoman- dibular. There is a rod-like quadrate, and in front of it a minute tri- 6—2 Hi MORPHOLOGY OF THE SKULL. [CHAP. angular process of cartilage — all the pterygoid cartilage that exists : above it is a styloid palato- pterygoid bone. The lower jaw possesses, besides dentary and articular, a small thick coronoid on the inner face, wedged between the two larger bones. 209. The Silufoids resemble in important respects the holos- tean Ganoids ; many of them exhibit a remarkable combination of ganoid dermal plates with strongly ossified cranial cartilage. They agree with certain Ganoids, and differ from other Teleosteans in having no subopercular. In the Siluroid Clarias capensis the two dentigerous premaxillaries occupy a large part of the margin of the fore face, and behind them is a crescentic tooth-liearing plate, the anterior part of the single vomer. The maxillaries are minute, and bear a long cirriform filament ; above them each nasal capsule has an ossification comparable to a septo-maxillary (chap. v.). The nasal sacs are very small ; so are the orbits, which are placed very far forwards. The anterior part of the face is covered by a large broad dermo-eth- moid. Even in the adult, small unclosed spaces persist between the frontals and in the supraoccipital region. The palatine is a distinct rod-like bone attached to the lateral ethmoidal region, carrying the maxillary, and lying partly above and within the pterygoid and the mesopterygoid. The quadrate comes far forwards, and there is no metaptcrygoid. By comparing this skull with a Salamandriue form many instructive points of agreement are discovered. The Shall in Ganoids. 210. The holostean Ganoids have their chendrocranium ossified essentially like the Teleosteans, certain bones being sometimes missing. The single jugular plate of bone developed between the mandibular rami of Amia, and the two plates found in Polypterus in the same region, are structures not plainly comparable with anything in the Salmon. In other respects the student will find little diffi- culty in interpreting these skulls. In Polypterus^, for example, large basal and superior fontanelles persist in the chondroci-anium, which is ossified by an occipital ring of bones, by otic masses representing opisthotic and epiotic bones con- joined, by sphenoidal walls united in the cranial floor behind the 1 See Traquair, "On the Cranial Osteology of Polypterus," Joum. Anat. vol. x. p. 166. III.] THE SKULLS OF GANOID FISHES. 85 pituitary body, and also sending bony plates inwards in the anterior part of the cranial floor, which meet but do not anchylose; by sphenotics or " postfrontals," lateral ethmoids (" prefrontals ") ; and a median eth- moid in the extreme anterior part of the cranial cartilage, sending back an upper spur which is parosteal. There is no interorbital septum. The basioccipital ossification includes at least one vertebral centrum, the first neural arch being articulated with it above. The posterior margin of the sphenoid is slightly notched by the foramina for the hinder divisions of the trigeminal nerve ; near the middle of its lower margin is the optic foramen, above and behind which the motor nerves of the eyeball and the first branch of the trigeminal emerge. The principal roofing bones of the skull are paired parietais, frontals, and nasals, which become more or less ancliylosed in old specimens. The septo-maxillaries and median ethmoid appear slightly on the surface. The parasphenoid extends underneath almost the whole occipital region, and reaches forwards nearly to the extremity of the palate : it has remarkable basitemporal processes, and a minutely denticulated anterior region. The premaxillaries and maxillaries have considerable palatal plates. The palato-quadrate cartilages are ossified by small metapterygoids and quadrates, large pterygoids and mesopterygoids, and very small palatines ; the vomers continue the fine of the pterygoids, within and parallel to the palatal plates of the premaxillaries and maxillaries. The hyomandibular is rod-like, bent almost at a right angle in the region of the opercular condyle, and it has a small additional ossification at its upper end. There are large preoperculars or squamosals, considerable operculars, and small suboperculars. The superficial interspaces between the bones named are filled by many small dermal and membrane bones, which are more or less modified representatives of the lateral line series of scales. In Amia^ there is a chondrocranium perfectly complete, covered by a shield of suturally united dermal bones. The occipital cartilage bones are all present, two neural arches being articulated with the hinder part of the basioccipital. The basisphenoid is represented by a pair of ossifications on either side of the pituitary fossa; there are large alisphenoids, having a descending plate: the orbitosphi noid is simple. The ear-capsule has a complete set of bones, with the exception of the pterotic. The parasphenoid has 1 See Mr Bridge's valuable account in Jour. Anat., Vol. xi. HC) MOEPHOLOGY OF THE SKULL. [CHAP. basipterygoid wings extending to and propping up the splienotic. The supracrauial dermal shield consists of a median dermo-occipital, flanked by a pair of sapra-epiotics; of large paired frontals; a pair of nasals; and an anterior supraethmoid which partly underlies the nasals. The circumorbital bones are highly developed; there are tn-o large postorbitals which extend backwards by the side of the supra-epiotics. The palatine series of bones is normal. Each preniaxillary has two ascending processes surrounding the nostril and uniting above it, and beneath the nasal bone. The maxillary has a jugal behind it. The suspensorium and the branchial arches need not be remarked upon ; but the lower jaw is singularly inte- resting. The meckelian cartilage is largely persistent, ossified proxinially by four very distinct centres, three of which enter into the condylar surface, and distally by a mentu-meckelian. The splenial is in five distinct pieces, one being larger, but all den- tigerous. The other bones are dentary, angular, and surangular. The opercular set of bones is complete, with the addition of a supratemporal. 2U. In the Sturffeon there is a very solid cartilaginous skull, overlaid by dermal bones more or less representing the superficial bones in Teleosteans, and having a large parasphenoid at its base. The precranial region is of great extent, forming a massive beak, surrounded by very generalised dermal bones. The mouth is swung from a large hyomandibular, which has a small sheath of bone near the top, and swells out below into a broad solid plate of cartilage. To this is loosely jointed a symplectic piece, largely ossified, and turned forwards, bearing the upper and lower jaws. The broad meta- ptorygoid passes towards the middle line in the roof of the mouth, above and behind the continuous palato-quadrate curtilage which is partially ossified as pterygoid and palatine: sometimes a small quad- rate bone is found near the condyle. The margin of this palatine tract is flanked by the edentulous maxillary, which readies to the quadrate articulation, and bears a styloid jugal. In old specimens there is a separate mesopterygoid. In the lower jaw the meckelian carti- lage persists, surrounded by the dentary, and sometimes ossitied anteriorly by a mento-meckelian : there is also a small angular. The lower division of the hyoid arch starts from the hinder end of the symplectic, and has interhyal, ceratohyal, and hyiiohyal segments. III.] THE SKULL OF CERATODUS. p. opercular opening. r!4'2. The mandibular arch has become highly com- plex. The buspea.'-orium has increased in length nearly as much as the trabeculse, and is roughly parallel with the axis of the skull. But its hinder extremity is curved inwards and upv?ards, giving off two forks. The anterior and longer, the pedicle (Fig. 35, p.), is confluent with the cranial base and side wall just in front of the auditory cap- sule ; the posterior process (o) is closely applied to the antero- external and upper face of the same capsule, arching over the facial nerve. Between the capside and the two forks of the suspensorium there is a hourglass-shaped mem- branous space. 343. The paracranial part of the suspensorium is more than half its length, extending from the pedicle to v.] THE FROG: THIRD STAGE. 147 the palatine connective. The suspeusorium is at a lower level than the cranial floor, and consequently the membra- nous subocular fenestra {s.o.f.) is rather obliquely placed. The eyeball lies above it, and is well seen on the upper sur- face of the head: the temporal muscle is attached to much of the surrounding cartilage, and covered antero-externally by the orbitar process {or. p.), which has become large, elevated, and incurved over the side of the face. The palatine bar remains a small narrow pedicle, but it also developes on its posterior edge a small backwardly-growing lamina of cartilage, somewhat like the orbitar process. 344. In front of the palatine region the quadrate end of the suspensorium is short and stout, articulating with the meckelian rod, which is still short, and directed almost vertically upwards and inwards, so as to come very close to the trabecular cornu when the mouth is closed. Its proximal end is deeply and roundly notched to hinge upon the quadrate ; and it is joined to its fellow by fibrous tissue. 345. These meckelian rods do not appear to be of much functional importance as yet : their direction alone indicates this. The fore part of both jaws is occupied by the upper and lower labials. The upper pair are almost vertical falcate flaps of solid hyaline cartilage, thick and heart-shaped in section, the upper thick edge being grooved to embrace the decurved end of the trabecular horn. The inferior labials are drumstick-shaped rods, with the lower end stoutest, and the upper end attached to the anterior face of the meckelian bar near the symphysis. They nearly meet below. 346. The oral opening is very small, nearly vertical, and suctorial : but a horny dentigerous plate is developed in the front of each jaw between the labial cartilages and the skin. The upper plate is behind the corre- sponding labials ; the lower is below rather than behind them. The creases of the lips are also covered with small hooked teeth, in addition to the two principal plates. 10—2 J 48 MORPHOLOGY OF THE SKULL. [CHAP. Somewhat behind the mouth the oral cavity widens, and has lateral angles, as well as a deep angular fossa between the lower labials. Further backwards, the section of the cavity is simplified, becoming wider and less deep, with its lateral angles downwardly produced between the meckelian and the quadrate cartilages. 347. The hyoid arch (Fig. 35, hy.) remains completely below the level of the suspensorium : each moiety is a broad and massive plate, roughly four-sided, scooped within by an antero-inferior and a postero-snperior fossa. At its most elevated (median) part it articulates by a shallow cup- and-ball joint with the under surface of the suspensorium beneath the hinder edge of the orbitar process. Its blunt antero-inferior end articulates with the small pisiform basihyal. 348. The four branchial arches are at their full development, and are greatly modified since the last stage. They have all coalesced together both above and below, by the formation of a continuous thin bar of cartilage. From the epibranchial bar (opposite the second branchial) there project two or three small processes. The ventral connective articulates on either side with the rounded first and the rudimentary second basibranchial. This hypobranchial tract, behind the basi- branchial, overlies the pericardium ; and the arches are so much curved and thrown outwards in the side wall of the throat, that the hypobranchial and epibranchial connec- tives lie on nearly the same level ; and the arches, which were once immediately beneath the skin and parallel with the sides of the face, now hang like hammocks obliquely across the throat. The first and fourth arches are baggy and crumpled, and extremely thin. All are covered with papillae transversely placed, bearing the very abundant branchial tufts'. 349. We can now review this surpassingly interest- ing stage. If the student will make a comparison between ■■ For a full desoription of the branchial tufts in Bana pipiens, see P}dl. Trans. 1871, p. 156. V.J THE FROG : FOURTH STAGE. ] 4'J the form here described and the skull of the adult Frog, he cannot but be astonished at the contrast ; and he may go where he will in the broad field of the morphology of the skull, and returning with added knowledge and developed thought, he will find himself capable of deeper insight attended with intensified marvel in studying this minute skull. The figures on Plates V. and VI. in the Philosophical Transactions for 1871 will be found fruitful if looked at again and again. A shallow cartilaginous boat supports the brain, and united with it behind is a pair of ball-like masses containing the organs of hearing. Diverging horns in front of the brain-case support large labial cartilages. The mouth proper is entirely in front of the cranial cavity : there are rudiments of the future upper and lower jaws, but placed most dissimilarly to their ultimate directions : while large transitory labials predominate among the oral tissues. The mouth is suctorial, but also has a pair of transitory horny dentary plates, with other denticles. The side of the face is traversed by a large subocular bar, confluent with cranial cartilage in front and behind, and from its forward edge a lamina arises to roof over the temporal muscle. The hyoid arch has a massive and strange form, appearing to play a very subordinate part in the structure : and the gill-arches, framed into a complex basket-work, bound together above and below, bent almost double in the sides of the throat, are at the summit of their ascending development, soon to retrograde and become obsolete. Fourth Stage: Tadpoles with tails disappearing. 3.50. In this stage we consider the changes taking place in the skull during the gradual disappearance of the tail. Before the anterior limbs are seen externally, _ the lower jaw becomes much elongated, and the suspensorium, not increasing in size so rapidly as other structures, appears to be carried backwards and outwards, the palatine bar being greatly lengthened. By the time that the fore limbs are exposed, and the tail is reduced to half its 130 MORPHOLOGY OF THE SKULL. [chap. original size, the braiii-case is complete in cartilage. When the tail has disappeared, most of the cranial bones are manifest, the sphenethraoid or "girdle-bone" being a remarkable exception : the creature has become far removed from the ichthyic type, and in many respects S]3ecially amphibian. 351. The cranial floor is composed of almost con- tinuous cartilage, nearly uniform in level. The inter- trabecular region is fully chondrified, but behind the pituitary body a triangular median fenestra remains, with the base directed forwards ; it appears to be partly a remnant of the intertrabecular space, partly of the fissure between the parachordals which the notochord no longer occupies. In the basioccipital region the gelatinous re- mainder of the notochord lies in a deep median groove in the cartilage. FiK. 36. Tadpole with tail begiuniiig to shrink ; side view of skull without branchial arches. 77. c. notochord; an. auditory capsule; between it and cfA. the low cranial side wall is seen ; eth. ethmoidal region ; st. stapes ; 5, trigeminal foramen; 2, optic foramen; ol. olfactory capsules, both seen owing to slight tilting of the skull; c.tr. cornu trabeculaE) : u.l. upper labial, in outUne ; su. snspensorium ; pd. its pedicle ; ot.pr. its otic process ; or.p. its orbitar process; t.m. temporal muscle, indicated by dotted Unes, passing beneath the orbitar process; papg. palatopterygoid bar; mlc. meckelian cartilage; l.l. lower labial, in outline; c.h. ceratohyal; b.h. basihyal. The upper outline of the head is shown by dotted lines. ^'■] THE FROG: FOURTH STAGE. l.",! 852. The side walls of the cranium are complete, and confluent with th,e floor and with the auditory capsules ; above they form a slight inturued selvedge in the roof. The fore end of the skull is gently concave, pierced on either side of the middle line by the olfactory nerves. ' The optic nerve passes out in the side wall not far in advance of the auditory capsules. The supraoccipital cartilaginous roof extends as far as to the fore end of the ear-capsules, but is deficient on either side in its anterior half, leaving two oval parietal fontanelles. The cranium and the peri- otic cartilages constitute one broad continuous mass. The cranial roof is membranous in its anterior two-thirds, form- ing an oblong fontanelle. 353. In the earliest period of this stage the trabecular cornua remain distinct, although the hinder commissural (ethmoidal) region is more extensive than it was. The cornua become broader and more flattened, approaching the middle line ; the nasal sacs gradually come to lie upon them. Anteriorly the sacs are separated by soft tissue, but behind there is a cartilaginous vertical plate, the rudimentary nasal septum, growing from the thickening ethmoidal region in front of the brain-case. The ethmoidal cartilage passes directly into the palatine connective in the antorbital region (Fig. 36). 354. Later, the septum, tbe cornua, and the eth- moidal region become perfectly confluent, and the original trabecular moieties can only be distinguished antero- externally where the extremities of the cornua project inwards as small rhinal processes from the front and lower wall of the nasal capsule. The latter is almost completely invested by cartilage : the septum has grown forwards to the front of the face ; the floor is formed by the flattened cornua ; and a continuous aliseptal roof grows outwards from the upper edge of the septum and the ethmoidal region, curving' downwards into the external nasal wall. 355. The external nostrils are on the upper surface of the face, very near its fore end. They are bounded in 1"2 MORPHOLOGY OF THE SKULL. [c'HAP. front by a distinct but small prorhmal cartilage, derived from the upper labial, the lower part of which is sepa- rated, and covered by the premaxillary ; while the aliseptal lamina curves round behind, and bounds them infero- externally. In this external boundary an ectosteal rhinal bone arises, which extends in the outer wall down to the nasal floor. The roof of the cavity is mostly behind the external nostril, forming an alinasal pouch directed back- wards and outwards, distinct from the ethmoidal cranial wall and the palatine bar. The cartilage curves down- wards infero-externally as a falcate process to bouud the inferior or internal nostril (which is more posterior than the external), becoming continuous with the main part of the nasal floor. The internal nostrils are separated by a comparatively wide basinasal plate. 35f>. In previous stages we have seen the mandibular suspensorium as a subocular bar parallel with the cranial wall, and connected with it in front by a short palatine pedicle. The latter gets gradually elongated backwards and outwards, so that the original subocular bar is at length situated entirely behind the eyeball : first passing through a stage in which it forms a V with the palatine, the angle being subocular (Fig. 36). Finally, the palatine (or palatopterygoid as it may now be called) itself becomes a subocular bar, nearly parallel with the cranial wall, but descending to a lower level behind. The suspensorium, considerably shorter than the palatopterygoid, is directed forwards, outwards, and downwards, at an angle of about 50° with the cranial axis (Fig. 37). 357. The anterior or palatine pai-t of the subocular bar remains connected with the lateral ethmoidal region of the skull by a short transverse process beneath the alinasal cartilage; it also gives off antero-externally a short prepalatine spur. The palatine region soon becomes marked off from the pterygoid by the appearance of con- striction at the anterior third of the bar. Posteriorly the pterygoid is continuous with the suspensorium at the junction of its middle and lower thirds. v.] THE FROG: FOUllTH STAGE. 153 358. The orbitar process, formerly so large, does not advance in size, as the suspeasoriura increases. The latter is lengthened in front of the orbitar process, which con- , sequently, instead of being anterior to the eye, comes to be placed quite behind it, covering part of the hinder surface of the temporal muscle (Fig. 36, or. p.). It finally becomes an inconspicuous thin lamina on the fore edge of the suspensorium. 359. The upper or metapterygoid region of the sus- pensory cartilage is more and more differentiated into regions, though its size diminishes relatively to the qua- drate or lower tract. The attachment to the cranial floor remains, but the primary posterior process is first enlarged as a triradiate lamina, and then segmented off, to be trans- formed into the tympanic annulus in a later stage [a. t. Fig. 38). The upper posterior angle from which this cartilage has been cut off forms a rounded otic process which is apposed to the outer half of the front part of the auditory capsule. The hinder edge of the main part of the suspensorium becomes thin and shell-like, convex internally and concave externally. 360. The distal extremity of the suspensiorum (sit.) is concavo-convex, fitting accurately upon the condyle of the greatly elongated sigmoid mandible (nik.). The latter is thicker behind, and pointed in front; its course is forwards and inwards, subparallel with the cranial floor. The lower labial becomes identified with the symphysial end of the meckelian cartilage. The facial nerve passes out of the cranium in front of the periotic cartilage, and runs downwards and forwards on the inside of the posterior margin of the suspensorium, where it divides into a chorda tympani or mandibular branch and two lesser hyoid branches. 361. The hyoid arch loses its lozenge shape, and elon- gates, articulating with the middle and upper part of the hinder edge of the suspensorium instead of near its distal extremity. It is expanded above and below, and constricted in the middle. Furthermore, it coalesces early with the l.")-l MORPHOLOGY OF THE SKULL. [CHAP. soft azygous basihyal ; so that the arches of opposite sides are continuous below. Confluence also takes place between the basihyal and the basi branchial, thus fusing the hyoid and the branchials into one hyobranchial system. The hypobranchial regions have also lost their distinction in this broad medio-ventral plate of cartilage. The first and fourth branchial arches lose their fiattened expanded form, and all become veryslender (Fig.37). But each epibranchial connective sends upwards a series of six tooth-like lobes. Later, these same spurs degenerate and pass into fibrous tissue; while a process developed from the hypobranchial region of the first and second arches elongates, divergmg from the middle line. 362. Ossification is just commencing in the fibrous tissues of the cranial roof when the tail begins to dimi- nish in size : it advances very far before the end of this stage. Bones affecting the cartilaginous cranium arise some weeks later than the earliest membrane bnnes, but they may be mentioned first. There are two pairs, the prootics and exuccipitals (Fig. 37, pro., e.o.). The prootic embraces the trigeminal nerve at its exit from the cranium, and extends beneath the fore part of the auditor)^ capsule, and upwards in the side wall of the cranium in front of it, so as to be sickle-shaped. The exoccipital is vei-y similar, surrounding the foramen for the vagus nerve, ossifying the base of the condyle behind and the hinder part of the periotic mass in front. These bones commence in the perichondrium, and in the superficial cells of the cartilage {superficial endostosis) : they proceed to ossify the cartilage throughout. 363. Two pairs of parostoses arise on the cranial roof: the anterior {frontals) cover the part between the ear-capsules and the front end of the cranial cavity. They are much longer than the pmietals {pa.) which lie close together between the fore part of the auditory cap- sules. Later, the parietal and the frontal of each side begin to coalesce. The frontals do not quite roof over the great fontauelle in front ; the parietals seem to lie entirely upon ■^'■J THE FUOG: FOURTH STAGE. ]5.5 cartilage, but the cranial roof is imperfect beneath the centre of each bone. The nasals (na.) arise as small patches over the middle of each nasal capsule, and become crescentic bones with the concavity forwards and outwards. Fis. 37. Prt J'r A^ Jfr.O I Tmx Young Frog, with tail just assorted ; side riew ot skull. All. auditory capsule ; in front of it, cranial side wall; a.n. anterior nostril; st. stapes; vick. meckelian cartilage; b.hy. basihyobranchial plate; st.ffy. stylohyal or ceratohyal; br. 1, first branchial arch. Bones: e.o. exoccipital; pr.o. prootio; pa. parietal; fr. frontal; na. nasal; pvix. premaxiHary ; mx. maxillary; pt. pterygoid; sq. squamosal; qu.ju. quadratojugal ; art. artiordar ; d, dentary. 864. The parasphenoid underlies the cranial floor from the ethmoidal nearly to the end of the basioccipital region, being convex and subcarinate below, and scooped on its upper surface. It is almost as broad as the carti- laginous basis cranii, to which it is closely applied, tongue- shaped in its anterior two-thirds, and pointed behind. In the middle of the periotic region it acquires a basi- temporal wing on either side, ending sharply near the foramen ovale. When the parasphenoid has assumed this complete form, the vomers are only small irregular patches of bone on the surface of the palate, a little internal to each inferior nostril. 365. The premaodllaries (pmx.) and maxillaries (mx.) originate before the vomers. The former appear as short thick bars laid in front of the rhinal processes, with a nasal process on the upper surface of the face. The premaxil- 136 MORPHOLOGY OF THE SKULL. [cHAP. laries meet in the middle line, but their nasal processes are separated by a circular membranous space. The maxillaries are styloid bones continuing the line of the premaxillaries in "the upper lip, so as to form with them part of a hyperbolic curve. They have an ascending lamina on the face throughout most of their length, and end pointedly behind, outside the prepalatine spur of the palatine cartilage. They appear rather as splints to the nasal capsule than to the palatine bar. There is a liga- mentous union in the upper margin of the gape between the end of the maxillary bone and the quadrate condylar region. 36G. Delicate spicules of bone are found in the fibrous stroma lining the inner face of the suspensorium and of the pterygoid cartilage. The pteryguid lamina (pt.) becomes thicker close to the suspensorium, extending upwards as a needle (which expands to a spatula-form above) on its inner and anterior edge. There is a distinct ectosteal [metapterygoid) plate embracing the inner convex face of the hinder part of the suspensorium. Near the quadrate condyle a quadrato-jvgal {//u. ju.) has appeared, extending as a small style towards the maxillary. 367. On the outer face of the suspensorium a squa- mosal parostosis early arises, and soon acquires a sigmoid shape, being broader above and pointed below. Its upper or supratemporal region becomes transversely extended, and moulded on the remnant of the cartilaginous orbitar process. The dentary and articular bones {d., art.) arise similarly at first, as thin fenestrate plates lying in contact with the meckelian cartilage, or only slightly separated from it by fibrous stroma. The dentary is at first on the outer surface of the fore half of the mandible, the articular on the inner and lower surface of the hinder two-thirds, not however reaching to the angle. 368. The cranial cavity is now very largely bounded by cartilage, replaced to a slight extent by bone. The nasal region has taken definite form in continuity with the V.J . THE FROG: FIFTH STAGE. 157 primary trabecular cornua, forming tbe considerable olfac- tory capsules, and affording a basis for the anterior jaw- bones. Instead of a suspensorial subocular bar, the palato- pterygoid cartilage is subocular : the suspensorium is still directed a little forwards, but also passes very largely out- wards. It has become differentiated proximally, giving off the lamina for the tympanic ring, and forming a new simple otic process, as well as a rounded condyle on its inner cranial process or pedicle; the orbitar lamina is now insignificant. The transformation of the hyoid and bran- chial apparatus is very notable, tending to the formation of a single expanded basal plate, and a rod-like hyoid style, and to the disappearance of the branchial arches. 369. The bones existing at the close of this stage are the following: exoccipitals and prootics ; parasphenoid, parietals and frontals ; nasals, premaxillaries, maxillaries, vomers, and rhinals ; pterygoids, nietapterygoids, qna- drato-jugals, and squamosals ; dentaries and articulars. It is remarkable how indefinite are the boundaries between parostoses and ectostoses in the early stages of ossification in the Frog : the " membrane "-bones arise for the most part closely in contact with the face of the cartilage. Fifth Stage; Frogs of the first summer. 370. The young Frog during the first summer in- creases in bulk threefold, and at the same time its skull is greatly developed, especially with regard to ossification. The cranial cavity becomes less oblong, and broader be- hind ; the periotic masses are relatively wider and shorter. The direction of the suspensorium is altered by the gradual retraction of its lower extremity (Fig. 38). 371. The prootics and exoccipitals extend consider- ably into the surrounding cartilage, completely encircling the trigeminal and the vagus nerves respectively. A new bone, of great interest from its later development, makes its appearance as a narrow transverse splinter in the ethmoidal region immediately constituting the front wall loS MORPHOLOGY OF THE SKULL. CHAP. of the crtmium. This bone, arising as a true ethmoid, has to be subsetjuently d(!nominated sphenetlnnoid {eth.} ; it possesses both ectosteal and endosteal portions, formed synchronously ; at present no trace ot it can be seen from the sides or from below. Fig. 38. nt.m/c Young Frog, near end of first summer ; upper Tiew of skull, witli left niaudible removed, and the right extended outwards. h.o. basioccipital tract ; .5.0. supraocoipital tract ; /o. frontal fontanelle; e.n. external nostril; internal to it, internasal plate; a.t. tympanic ammlus. Bones : n.o. exoocipital ; jtr.o. prootic, partly overlapped by p. parietal ; j. frontal ; eth. rudiment of spheuethmoid ; na. nasal ; "prnx. pre- maxillary ; mx. maxillary ; p^. pterygoid, partly ensheathing the reduced cartilage; q.j. quadratojugal ; siy. sq^uamosal ; m: articular; d. dentary ; m.mk. mento-meckelian. have their peripheral cells directly calcified : but this bony matter is not deposited in tesserse as in the Elasmo- branchs, nor throughout distinct morphological regions ; the osseous granules do tend, however, to be aggregated where m another type a proper bone-territory would be established. The epiotic, supraocoipital, pterotic, basi- sphenoidal, and upper and lower nasal regions are those in which this superficial endostosis can be traced. The v.] -THE FllOU: FIFTH STAGE. 159 bony deposit is in very small grains, in semilunes ex- tending half round a cell, and in separate and connected rings. 373. The frontal and parietal of the same side still retain a partial suture between them: the parietal ove.- laps the ascending part of the prootic ; the frontal extends to the fore end of the great cranial fontanelle (Jo.), just behind the commencing sphenethmoid ; the parietofrontal of opposite sides do not yet meet by a considerable space. The nasals (?ia.) are much altered in shape ; the upper part is now a broad subquadrate plate, with a snjall spur running downwards and outwards on the ethmoidal cartilage : the two nasals are widely separated from one another. The rhinal bone is seen as a notched and grooved plate lying on the floor of each external nostril. 374. The premaxillaries and maxillaries {pmx., mx.) are much advanced, and dentigerous. Their nasal processes are considerably enlarged : those of the premaxillaries are completely apposed, and extend pretty evenly to the level of each external nostril ; those of the maxillaries rise to the outer boundary of the nostrils, and are higher behind, towards the ethmoidal spur of the nasals. Behind this region the nasal plate rapidly sinks, and the maxil- lary is a mere style passing backwards to within a short distance of the quadratojugal. The palatal laminae of the premaxillaries and maxillaries are of less extent than their nasal processes ; they are thin shelves which die away behind. The parasphenoid is broader in front and much more dense than formerly ; it does not rel. The lower part of the hyoid arch is a styloid cartilage loosely connected to the skull and suspensorium by ligaments. Ventrally it sends forward a little hypo- liyal lobe, and is fused with the broad shield-shaped basi- branchial plate. Posteriorly this plate bears remains of the liypobranchial portions of two branchial arches : the anterior are slender and unossified; the posterior pair are ossified, find embrace the larynx. The other regions of the branchial arches have disappeared. 382. The principal points to be borne in mind with re.spect to this period, in addition to the growth of bones previously existing and the moulding of the skull into its adult form, are the lievelopment of the ethmoid and mento- meckelian bones, the retraction of the suspensorium and of the lower jaw, the completion of the columella and the tympanic aimulus, and the disappearance of the branchial V.J THE ADULT FROG. 163 arches. The facial nerve passes over the cohimella in the same position as it passes over the upper member of the hyoid arch where that is early and fully developed. The Skull of the Adult Frog. 383. The skull of the frog is a flat semielliptical structure, broadest in the hinder region, where it is formed of continuous cartilage and bone. In front of this expansion, which occupies about one-fifth of the length of the skull, the narrow axial parts enclosing oc.c f,m.. Adult Frog : upper view of skull with inyesting bones and lower jaw removed, /.m. foramen magnum ; oc.c. occipital condyle ; p./o. parietal fonta- nelle ; /./o. frontal fontanelle ; 'g.rh. prorhinal cartilage ; e.n. external nostril; al.n. alinasal cartilage; u.V . modified upper labial; p.pa. pre- palatine spur. Bones: e.o. exoccipital; 'pr.o. prootic ; sp.etft. sphenethmoid ; pa. palatine; -pg. pterygoid; g.j. q^uadrato-jugal. the brain are separated from the lateral boundaries for about half the length of the head, by a large ovoid tract, 11—2 164 MORPHOLOGY OF THE SKULL. [CHAP. occupied by the eyeballs and by muscle. In the anterior part of the skull the axis and the lateral bars again unite in a precranial expansion. The external bars we have mentioned are carried at a level below that of the cranial floor. 384. When the investing bones of the skull are stripped off, the underlying cartilage and its proper bones are seen to present a somewhat different appearance. The continuous elliptical outline is lost, and the nasal cartilages appear as a very distinct mass attached to the axial or proper cranial region. 385. The cranial cavity is enclosed by continuous cartilage and cartilage-bone, except where nervous struc- tures pass out of it, and where the roof presents mem- branous fontanelles. Behind, there is an occipital region mostly posterior to the periotic masses. The auditory region is broad, and there is no distinction between periotic and cranial cartilage. The preauditory part of the cranial cavity is three-fifths of its length. 386. The brain-case is nearly twice as deep behind as it is in front (Fig. 41), and considerably wider above than below. Consequently its side walls in front of the ear-masses slope outwards as they ascend. With these peculiarities, the cranial cavity of the frog is of a very smooth and symmetrical type, without marked prominences or irregularities. The base is very flat beneath, and emarginate in the middle line behind. The condyles, largely ossified by the exoccipital bones, stand out on either side above the base (Fig. 41, occ. c), having about half the height of the skull. The exoccipitals extend nearly to the middle line of the cranial floor, and approach it almost as closely above ; the supraoccipital region has a thin film of superficial endostosis. 387. The exoccipitals have a large extension into the auditory masses, which are flattened and broad, with a curved outline externally. They are very consider- ably unossified, possessing indefinite films of superficial v.] THE ADXTLT FHOG. 165 endostosis, and a pair of large anterior bones, the prootics (pr.o.). These occupy the antero-internal regions of the periotic masses, are deeply notched below where the trigeminal and facial nerves pass out in front of the auditory tract, and extend forwards above these nerves to ossify a considerable part of the alisphenoidal region. The anterior canal and the fore end of the horizontal are especially girt by the prootic ; while the exoccipital is related to the posterior canal, and the hinder end of the horizontal. There is a pterotic ridge or tegmen tympani, external to the horizontal canal, and overhanging the stapes and tympanum. The exoccipitals contain the foramina for the glossopharyngeal and vagus nerves, at the junction of their auditory with their occipital regions. 388. The continuous cranial floor in front of the exoccipitals has some slight endostosis in its substance, but not sufficient to be denominated a bone. The car- tilaginous side walls of the skull present nothing remark- Fig. 41. pjnx 7 Adult Frog ; median longitudmal section of skull, lower jaw removed. occ.c. occipital condyle ; s.n. nasal septum ; 1, olfactory, 2, optic, 5, trigeminal, 7, facial, 8, glossopharyngeal and vagus foramina. Bones: e.o. exoccipital; pr.o. prootic; ■p. parietal; /. frontal; sp.eth. sphenethmoid ; pmx. premaxiUary ; pa.s. parasphenoid. able, except the very posterior position of the foramina for the optic nerves: two-fifths of the length of the cranial cavity is in front of them. The fontanelles on its upper surface consist of a pair of small oval parietal openings (Fig. 40, p.fo.) in the region between the prootic bones; and of one frontal {f.fo.) much larger, oblong in outline, with a triangular spur of cartilage projecting forwards from the posterior boundary. The cartilaginous 166 MORPHOLOGY OF THE SKULL. [chap. roof between the froDtal and the parietal fontanelles is feebly ossified by endostosis, a rudiment of a supra- sphenoidal bone. 389. The floor, side walls, and roof of the cranial cavity are ossified anteriorly by one bone, the sphen- ethmoid (Figs. 40, 41, sp. eth.). This bone extends further backwards in the side walls than in either the floor or the roof of the cranium. Below, it has a nearly transverse posterior margin ; in the roof it is concave behind, embracing the fore end of the frontal fontanelle. The olfactory crura pass through its cranial concavity to gain the nasal cavities ; a slight vertical ridge projects between the crura into the cranium, like a rudimentary crista galli. The sphenethmoid also extends forwards above and below in the ethmoidal region, aborting the cartilage, and ending by a blunt median projection in each case ; which point corresponds to the fore end of a median vertical plate connecting the upper and lower laminae : this is a true perpendicular ethmoidal (mes- ethmoid) plate (Fig. 41). Fig. 42. Adult Frog ; upper view of skull, with lower jaw removed, e.o. exoocipital; p. parietal;/, frontal; e. sphenethmoid; n. pm. premaxillai-y ; mx. maxillary; 7. quadrato-jugal; pt. pterygoid; sus. Buspensorium ; p.o. prootic ; op. opisthotic. 390. The investing bones of the cranium proper are very simple. There is a pair of parieto-frontals, joined '^■] THE ADULT FROG. 1G7 by suture, above, and a parasphenoid below. The parieto- frontals extend from the spheriethmoid, whose lateral regions they partially cover, to the exoccipitals. Ihey diverge from each other behind, exposing the cartilaginous cranial roof, and also in front. They are thick and smooth, and_ dip towards their sagittal suture : their parietal portions are conterminous with the prootics, and in front of them the bones dip over the edges of the cranial roof, investing its upper side walls for a small depth. 391. The parasphenoid underlies the whole cranial floor (separated from it by a perichondria] membrane) except in the occipital region, even sending a small spur backward between the exoccipitals. Its oblong basi- temporal wings are very wide, undergirding most of the width of the auditory masses, and lying between the prootics and the exoccipitals. In front of these wings the bone is very regularly linear, and subcarinate. 392. Beyond the sphenethmoid the whole of the precranial cartilages are unossified. A rather low septum .(s. n. Fig. 41), thicker above, runs forwards to the end of the snout. The nasal floor formed by the coalescence of the hinder half of the trabecular cornua is a broad thin flap, separated bj^ a notch from the antorbital or ethmopalatiae bar, extending outwards. Anteriorly the cartilage is expanded outwards and curved round the inner nostril on its outer side, so as nearly to enclose it. The broad anterior end of the cornu has sent forward in the middle a slender prorhinal process, and at its outer angle is bilobate (Fig. 40. p. rh.). 393. The roof of the capsule is broadest at its posterior region, where it has completely coalesced with the ethmopalatine bar. Both above and below, the cartilage is partially hardened by endosteal bone : one more notable patch than the rest is in the ectethmoid or antorbital region. The external nostrils are almost directly above the internal, and are protected by a lunule of cartilage which lies over the opening. This cartilage 168 MORPHOLOGY OF THE SKULL. [chap. is the upper two-thirds of the original upper labial (Fig. 40, u. I'.). The lower third has become a rounded nucleus which lies between the premaxillary and the nasal capsule in front. Within the nasal opening the Fig. 43. Adult Frog : under view of skull, with lower jaw removed. e.o. exoooipital; p.o. prootic; par. parasphenoid ; e. sphenethmoid ; V. vomer ; pm. premaxillary ; mx. maxillary ; q. quadrato-jugal : sus. sus- pensorium ; pt. pterygoid; 1, optic foramen; 2, trigeminal foramen; 3, glossopharyngeal and vagus foramen. septum sends out from its upper part a broad thin lamina of cartilage, which runs outwards to the outer and ante- rior wall of the nose, and unites with the subnasal floor ; half way outwards it becomes subdivided into two laminae, so that at this region the nasal cavity has three horizontal floors. The nasal passage, from external to internal nostril, is completely lined by the little subcylin- drical septo-maxillary. 394. On either side of the cranium is a large oval suborbital membranous space. It is bounded in front by the flat ethmopalatine bar, passing directly outwards and sending forwards a prepalatine spur (Fig. 40, p. pa), while 1)Osteriorly it is continued into the slender pterygoid bar. Behind, this passes into the mandibular suspensorium, marked by three principal regions, (1) an antero-inferior, the pedicle, directed inwards, and articulating by a v.] THE ADULT FROG. 169 thickened condyle with the auditory capsule in front of the fenestra ovalis ; (2) a supero-posterior, the otic process, attached to the fore part of the tegmen tympani ; (3) the quadrate or main suspensorium, directed backwards, downwards, and outwards, so as to reach nearly as far back as do the occipital condyles. Quja E.O O.C Adult (edible) Frog : under view of skull ; investing bonea removed on right side. (Huxley. ) O.C. occipital condyle ; p.n. posterior (inferior) nostril ; qxi. quadrate tract ; st. liy. stylohyal, partially shown ; eu. eustachian opening ; II. op- tic foramen; V. trigeminal foramen; X. glossopharyngeal and vagus foramen. Bones : e.o. exoccipital ; pr.o. prootio ; pa.s. parasphenoid (left half) ; g. sphenethmoid ; vo. Yomei; pmx. premaxiUary ; ma;, maxillary ; pi. palatine; pt. pterygoid; qu.ju. quadrato-jugal. 395. The palatine or antorbital bone invests the hinder surface of the transverse ethmopalatine bar; it is slender and almost crescentic, and slightly underlies the sphenethmoid. The pterygoid is elongated and more massive; it has transformed much of the cartilage of the subocular bar, and just touches the palatine bone in front. Behind, it is forked, passing inwards by a square (metapterygoid) process to support the pedicle, and back- wards as a narrow parosteal spur to clamp the under surface of the suspensorium almost as far as the condyle. The 170 MORPHOLOGY OF THE SKULL. [CHAP. quadrate cartilaginous region is only very slightly ossified by the hinder end of the quadrato-jugal. 396. The T-shaped squamosal {sq. Fig. 45) lies on the suspensorium and the external auditory region. Its horizontal tract is outside the prootic behind, and sends forwards over the pterygoid a considerable postorbital spur. At more than a right angle with this is set the descending portion, lying on the suspensorium and over- lapping the quadrate j^art of the quadrato-jugal. It is separated from the cartilage by perichondrium, and from the quadrate by periosteum. It lies beneath the tym- panic annulus. 397. The investing bones of the nasal and maxillary tracts have now to be described. The nasals are consi- derable bones, transversely placed on the nasal roofs, broad within, narrow externally, not nearly meeting in the middle line. In the middle of their anterior margin th?y are notched for the external nostril, and then ex- tend backwards and outwards along the nasal lamina of the maxillary to end in a sharp preorbital spur. The premaxillaries are of considerable breadth, are apposed but urianchylosed in the middle line, and possess both dentary and nasal plates. The latter project upwards and backwards by a curved spur in their middle region, touching the nasals. Mesially a tract of cartilage is uncovered by bone between the sphenethmoid, the nasals, and the nasal plates of the premaxillaries (see Fig. 42). The palatal plates of the latter are thin and quadrate, with a somewhat irregular surface. 398. The maKillaries are elongated, with extensive nasal lamiase in their anterior third; the septo-maxillary {s.mx. Fig. 4-5) appears between the inner and anterior angle of this plate and the nasal process of the premaxil- lary; the external nostril notches it behind this point. The nasal plate of the maxillary dies away, at first by a sudden decrease in the antorbital region, and then gradu- ally, the bone becoming a fine pointed style, largely over- lapping the quadrato-jugal. It is adjacent to the anterior v.] THE ADULT FROG. 171 third of the pterygoid, but diverges outwards from it, while the pterygoid curves inwards (Fig. 44). The palatal plate of the maxillary is but slight and ranges with that of the premaxillary, soon becoming insignificant. 399. The vomers are elegant trifoliate plates, wide apart like the nasals. The middle leaf, which is emar- ginate, and the narrow falcate posterior leaf, together largely surround the internal nostril. The pointed ante- rior leaf nearly reaches the suture between the maxillary and the premaxillary. The rounded stalk converges to- wards its fellow, lies partially under the sphenethmoid, and almost touches the parasphenoid ; this tract of the vomer is dentigerous. The septo- maxillary can just be seen between the middle and posterior leaflets of the vomer. Fig. 45. ICBt. St.hi/ . Adult Frog : side -view of skull, dissected. ox. occipital condyle ; st. stapes ; ty. tympanic membrane ; a.t. tym- panic annulus ; a.n. anterior nostril; qu. quadrate condyle ; mck. mecke- liau cartUage; st.hy. stylohyal cartilage; b.hy., b.br.l. basi-hyobranchial plate ; II. optic foramen ; V. trigeminal foramen. Bones : e.o. exocoipital ; sq. squamosal, partly covered by tympanic annulus ; pr.o. prootio ; fr.pa. parietofrontal ; g. sphenethmoid ; na. na- sal; prnx. premaxOIary ; s.ma;. septomaxillary ; mas. maxillary ; pa. s. para- sphenoid; j)f. pterygoid; qu.ju. quadrato-jugal ; 7?!.mc/c. mento-meckelian; h.br. hypobranchial or thyrohyal. 400. The lower jaw (Fig. 45) is comparatively simple, and very elongated, with its rami widely separated behind, and approaching each other rapidly in front. The meck- elian cartilage persists except in the symphysial region, 172 MORPHOLOGY OF THE SKULL. [CHAP. wbere it is replaced by a small cylinder of bone, the mento-meckelian, which however is partially derived from the remains of the lower labial cartilage. The articular region has an elevated coronoid part ; the condyle is a smooth egg-like mass, with its long axis longitudinal; it plays very freely beneath the smoothly-scooped base of the quadrate. The dentary, bearing no teeth, and conti- nuous with the mento-meckelian, runs back two-fifths of the length of the jaw, surrounding the cartilage ; the arti- cular' ensheaths the inner side completely, but very much of the cartilage is bare of bone externally. 401. It appears unnecessary again to describe the columella and tympanic annulus : the essential arrange- ment is given in §§ 379, 380. The anterior boundary of the tympanic cavity is traversed by the main facial nerve, which lies above the pedicle. The stapes itself is attached Fig. 46. uust Lit AdvUt Frog; columella aurig. St. stapes; i.st. interstapedial ; m.si. mediostapedial ; e.s%. extra- stapedial ; s.st. suprastapedial. to the margin of the fenestral fossa by a delicate band of fibrous tissue ; but much of the inner face is in immediate contact with the cavity of the vestibule. This fenestral fossa is worthy of some note ; it is an egg-shaped fossa of considerable depth, into which the prootic extends slightly ; the exoccipital does not reach to its posterior rim. The fenestra ovalis merely occupies the postero- inferlor third of the bottom of this fossa ; it is uniform, with the concave edge looking upwards and forwards ; the main otoconial mass can be seen through it. At an earlier period the fenestra was as large as the stapes, but the ' Prof. Huxley says this bone represents the angular, coronary, and splenial elements ; but it is here named articiilar from a consideration of transitional cases. See Huxley, art. 'Amphibia,' Encyc, Brit. Vol. i. p. 755. v.] THE ADULT FROG. 173 cartilage has latter has grown, and the surrounding enlarged to embrace it. 402. The slender curved hyoid arch (st. hy.) is at- tached to the periotic capsule a little way below the rim of the stapedial fossa at its anterior third. It has changed little since the Frog was two or three months old; it has not become ossified. Ventrally it passes into the antero- external angle of the broad hyobranchial plate, which is a great cartilaginous escutcheon with irregular patches of superficial endostosis on its surface (Fig. 47). It has lost Fig. 47. Adult Frog : hyobranchial plate. st.h. hyoid arch or stylohyal ; c. 7i. oeratohyal tract; h.Tiy. b.br. basihyobranchial ; br. remains of third branchial arch; th. fourth branchial or thyrohyal. the symmetrical foramina it formerly had ; and its various outgrowths are unossified. Its upper surface is gently concave ; and it is rendered exquisitely mobile by reason of its suspension by the sigmoid hyoid bars. The antero- external angle is produced into a leafy hypohyal lobe in front of the attachment of the hyoid cornu ; there is another broad lamina, the remnant of the first two branchial arches, just behind this attachn)ent, and a third more elongated process {br.), representing the third branchial arch, extends almost from the postero-external extremity of the plate. The fourth branchials (th.) are ossified as shaft-bones, a core of cartilage remaining, and 171 MORPHOLOGY OF THE SKULL. [CHAP. attached near one another at the posterior extremity of the hyobranchial plate; they are slender in the middle and expanded behind. 403. Summary. The primordial skull of the Frog, arising very early in development, possesses the typical elements quite distinct from one another ; but there is nothing to represent the palatopterygoid arch, and the suctorial mouth is situated far forv/ards. The mesocephalic flexure is early overcome, and at a subsequent period considerable labial cartilages form the functional jaws. When the external gills are at their fullest development, the face is enlarged proportionally to the brain-case, and the head is comparatively flat, the parts of the brain being arranged almost in a straight line. 404. By the time the tadpoles are half an inch long, extensive coalescence of the primitively distinct elements has occurred. A cranial floor is formed by the union of parachordals and trabeculee ; the latter give rise both to an internasal plate and to nasal floors. The mandibular arch unites with the trabecular cartilage both in front of and behind the orbit. The meckelian bar is cut off, and the definite suspensorium has articulated to it the unusually massive hyoid bar, which possesses no dorsal region. Subsequent metamorphosis puts the tadpole of an inch long in possession of a boat-shaped cartilaginous cranium united behind with the ball-like otic mas.ses ; while the mouth remains entirely in front of the brain-case, the suspensorium of the mandible (still functionally in abeyance) is a large subocular bar, bearing a superior lamina roofing the temporal muscle. The branchial arches have coalesced into a most remarkable basket-work, which is folded almost double in the sides of the throat. 405. As the tail is disappearing the cranium is found much more extensively chondrified, and nasal capsules are well formed. The character of the mouth is changed ; the labial cartilages become subordinate, and the two jaws functional. The mouth is elongated by the gradual v.] THE ADULT FROG. 175 retraction of the suspensorium, concurrently with a growth of (palatopterygoid) cartilage from the anterior confluence of the suspensorium with the trabecular cartilage, and the meckelian bar is correspondingly lengthened. The hyoid and branchial arches gradually dwindle, and ulti- mately form a rod-like arch and a broad basal plate. 406. During this great transformation most of the membrane-bones of the skull appear, and also the ex- occipitals and prootics. Some most characteristic bones are particularly late in development, for instance the sphenethmoid and the mento-meckelian. The series of parts which is identified as the upper part of the byoid arch, forming the columella, does not arise till after the young Frogs have taken to the land ; while another structure, which is related to the organs of hearing, the tympanic annulus, becomes connected with them, having been originally a postero-superior leaf cut off from the mandibular suspensorium. 407. The contrasts between the Elasmobranch skull, destitute of bones, the Salmon's, with its multitude of osseous plates little compacted together, and the Frog's, so highly specialised, and with bones so accurately moulded on cartilage, are very obvious, and need not be detailed here. But it may be serviceable to emphasize some of the features in which the Frog is unlike the Axolotl, both having skulls presenting at first sight many resemblances. The Axolotl's suspensorium is not a subocular arch at first ; its palatopterygoid cartilage is comparatively scanty, and very slight in the adult. The Axolotl has no labials, no orbitar process, no columella. The Frog developes a more complex branchial framework, which is more com- pletely absorbed than in the Axolotl. The retraction of the suspensorium is a special feature in the Frog, as well as its giving rise to a tympanic annulus. The early history of the trabecule differs much in the two types, especially as regards the parachordal extension, so marked in the Aj:olotl, and the trabecular cornua, which are early formed and of notable size in the Frog. 176 MORPHOLOGY OF THE SKULL. [CHAP. 408. The skull of the Axolotl is ossified by more bones, appearing earlier, and has less of permanent cartilage than the Frog. Among bones found in the former and not in the latter are the pterotic, opisthotic, and ectethmoid. The mento-meckelian and columellar ossifications of the Frog however are not represented in the Axolotl. APPENDIX ON THE SKULLS OF AMPHIBIA'. 409. The skull of Rana pipiens (the Bull-frog) presents marked distinctions from that of the common Frog. It is both highly generalized, having more features looking back to the Fishes, and very specialised, for no creature turns its suspensorium so far back. The larvae are much more Petromyzine in aspect than those of the Frog. The cranial box is very narrow in proportion to the width of the skull. There are many additional bones, frequently minute, and having the character of ganoid scales. Multiplication of bones takes place in several of the best defined tracts ; the metapterygoid and mesopterygoid are distinct from the pterygoid ; the palatine tract is divided into two bones ; there is an anterior parasphenoid distinct from the vomers ; a series of small bones represents the septo-maxillary, and another series covers the sus- pensorium. The tympanic annulus is of great size, and forms a complete circle. 410. In the small Frog, Pseudis paradoxa, the skull is pro- portionate to the body, and its details are of marvellous interest. In one stage of development the whole of the occipital and auditory regions is ossified by one continuous bone on either side. Later, each occipito-otic is cut into two bones ; and subsequently the hinder pair of bones (mainly occipital) unite into one, while the anterior pair remain separate as prootics. The parasphenoid is split down the middle in front, and has no posterior handle (or projecting spur behind the basitemporal wings). The parieto-frontals arise as one bone on either side, and are subsequently segmented into parietal and frontal. The intemasal cartilage is of great extent, but 1 For much valuable information see Prof. Huxley, art^ 'Amphibia,' Encyc. Brit., ninth edition, Vol. i. v.] THE SKULL OF DACTYLETHRA. 177 the nasal cavities are small The prepalatine region is considerable ; the antorbital cartilage is almost severed from its ethmoidal attaclunent. There are large suctorial cartilages, or labials. 411. The skull of the Toad {Bufo vulgaris) differs from that of the common Frog in the following respects. The orbitar process in the Tadpole forms a perfect arch over the temporal muscle ; the second upper labial coalesces in the adult with the internasal plate in front, and with the antorbital bar behind ; the palatine cartilage is segmented off both from the antorbital and from the pterygoid ; the upper part of the suspensorium in the adult rises above the "condyle of the pedicle ;" there is no separate metapterygoid bone ; the columellar cartilage is not segmented, subdivision taking place by the formation of two separate bony shafts, whose meeting-point is distill, not near the stapes ; the extrastapedial is a large semiov.al leaf, and the suprastapedial coalesces with the tegmen tympani ; the lower hyoid bar coalesces with the auditory capsule ; the car- tilaginous tympanic annulus forms only two-thirds of a circle ; the continuous nasal roof is only one-fourth the size of the same tract in the Frog ; the prorhiuals are much smaller. The Skull of Ductylethra^. 412. The skull in this remarkable form, the aglossal Cape To.ad, is as notable in its histoi-y and adult structure as any among the Anura. The larvae are very divergent in character from those of ordinary Frogs and Toads ; they present much general resem- blance in outward form to the extinct Coccosteus and Pterichthys. The mouth is suctorial, and small, but very wide, like that of Siluroid F.shes and Lophius. The lower jaw protrudes beyond the upper; a very long tentacle proceeds on either side from the upper hp, and there is no trace of the horny plates possessed in early life by most Frogs and Toads. In conformity with these characters the whole head is extremely flat or depressed, instead of being high and thick. There are no claspers beneath the chin. The branchial orifice is not confined to the left side, but exists also on the right. The tail, like the skull, is very Chimaeroic', the whole caudal region being narrow and elongated, and terminaxing ' See Phil. Trans. 1876 for figures and detailed description. B. M. 12 178 MORPHOLOGY OF THE SKULL. [cHAP. in a long, thin, pointed lasli. The fore limbs are not hidden beueatli the opercular fold. 413. The youngest larva? examined were an inch and a quarter in length, three-fifths of this being caudal. The fore and liind limbs at this stage are nearly equal in size, although subsequent growth makes them very unequal. The grade of development of the skull corresponds to that described in the third stage of the Prog. The cranial structures are nmch more flattened even than an external view would suggest; for the outer skin is very loose, and the subcutaneous stroma copious and gelatinous. Unlike the larvse of the ordinary Frogs and Toads, the tissues are very transparent, although richly supplied with brown pigment. 414. The cartilaginous cranial floor has a very astonishing aspect; as in the Skate's skull, there is a long, common, cranio-nasal valley, formed behind of the parachordal cartilages and notochord, and in front, for twice the extent, of the coalesced traheculaj, to whose transversely extended anterior edge a transverse labial cartilage is united. The periotic masses are confluent with the parachordal and trabecular cartilage, but the cranial floor extends considerably beliind them, and gives rise to exoccipital walls. The trabeculae form blunt side walls to the cranium in front of the auditory masses, and these elevations converge, narrowing the cranial cavity. The brain proper ends at a point in front of the periotic cartilages only one-fourth of their length, and the diverging olfactory crura are as long as the brain. The precrauial valley is embanked by the continued trabecular elevations, which again recede from one another, forming no transverse ethmoidal wall. The trabecular laminae in the sides of the precranial valley are but thin, and arched out- wards over a considerable infero-lateral concavity on either side. The precranial valley is filled with a watery tissue through which tlie olfactory crura run. 415. In the auditory capsule the fenestra ovalis is opening, but is merely filled with indifferent tissue. The tegmen tympani projects as a considerable lobe from the hinder part of the capsule, is narrower laterally, and then extends broadly from the whole anterior and antero-lateral margin. It runs forwards and outwards as a broad ribbon, equal in length and breadth to the capside, and then is confluent with a footsliaped flap having a pointed end forwards, v] THE SKULL OF DACTYLETHRA. 179 nearly reaching the orbitar plate of the suspensorium. This lobe is to be transformed into the tympanic auuulus. The facial nerve runs beneath the tegmen. 416. The mandibular suspensorium is broadly confluent behind with the trabecuKir cartilage in the whole of that tract where it forms a wall for the brain, and then diverges gently from it so as to leave a narrow falcate membranous space, the subocular fenestra, rounded in front, pointed behind. The suspensorium broadens as it passes forwards, and has a good-sized orbitar process, in a region not far behind the nasal openings. The quadrate condyle is just at the same level as these apertures; an;i at that region the sus- pensorium is confluent with the axial cartilage by a broad postnarial or palatine lamina and by a narrower prenarial lamina, which takes the place of the ligament uniting the cornu trabeculse of the Toad to the quadrate. The eyeball rests on the suspensorium somewhat outside the subocular fenestra. 417. The meckelian cartilage is small as compared with the existing portion of the hyoid arch. The symphysial end is pointed, and between the opposite points of the two bars is found a pair of short inferior labial cartilages continuing their line (very unlike the lower labials of the Frog's tadpole). The upper labials, as has been mentioned, are very different; the moieties have coalesced with each other, and to some extent with the anterior trabecular edge ; but on either side there is a sharp fissure between the labial and the cornu trabeculse, and the labial runs, as a gradually at- tenuating thread of true cartilage, to the extremity of the long tentacle, so that at this stage it reaches to the end of the abdominal cavity. 418. The hyoid arch shows a ceratohyal piece, with no segmen- tation ; it is broad and massive, yet sinuous, and articulates definitely with the suspensorium beneath the orbitar process. A basihyo- branchial plate separates the two hyoid moieties. There is no upper or columellar region at present. There are four branchial arches, all confluent above and below, separated laterally by three narrow oblique slits. The first arch is a large wide bag of cartilage, with sinuous walls, through which can be seen radiating rows of internal tufted gills. The second and third arches are comparatively narrow; the fourth is much wider and somewhat like the first. 12—2 180 MORPHOLOGY OF THE SKULL. [CHAP. Through the clefts can be seen the interdigitatiug papillae from which the dendritic gills grow. 419. In Tadpoles of Dactylethra at their largest size the proportionate width of the hinder part of the skull has greatly increased ; the precranial valley has become filled with cartilage in which the olfactory crura run; and from the auditory to the nasal sacs the floor of the skull is one wide sheet of cartilage (as in Sharks and Rays), gently convex in the middle, concave near the margins. Far forwards there is a small nasal septum. The labial tentacles are one third shorter : but there are two new pairs of labials around the inferior and two around the superior nares. The quadrate condyle has not relatively receded. There are two apposed frontals covering the supracranial fontanelle and bearing on their hinder surface a pair of small parietals. The parasphenoid (the first bone to appear) is dagger-shaped ; the cranial cartilage is affected by one continuous bone on either side, occupying the exoccipital and prootic tracts. 420. In Dactylethras with large legs and diminishing tail one of the most striking changes is the retraction of the quadrate condyle for a distance equal to one-third the length of the skull, it being now opposite the fore margin of the frontal. The precranial ethmoidal cartilage is still of great width, as also is the suspensorium ; a much narrower palatopterygoid pedicle has arisen, and the arcade is disengaged from the trabecular cornu. The tympanic annulus is definitely segmented off, and has become semicircular. The stapes is well chondrified, but there is no columella. The parasphenoid has grown forwards to the subnasal region, and hears a small vomer. The frontals have coalesced. Nasals, maxillaries, and pre- maxillaries have appeared. The meckehan cartilage has grown very much, with a thick proximal end, an ectosteal articular within, and a small dentary in front. The mouth is no longer Siluroid, but Batrathian. The hyoid arch is still very large and strongly marked, but somewhat disengaged from its suspensorial articulation. 421. In the adult Dactylethra an enormous transformation has taken place, for we find a narrow elongated cranial cavity, quadrate condyles at the level of the periotic masses, and a very large subocular fenestra separating the cranium from the palatopterjgoid bar. The part of the cranial walls and floor between the nasal and "^•] THE SKULL OF PIPA. 181 the auditory regions is nearly all solid bone, but this sphenoidal bone scarcely extends into the ethmoidal reg-ion. The long superior fontaneUe is covered by the IdVge parieto-frontal. There is a triradiate supraethmoid upon the cartilage in front of the cranium, its median ray overlying the nasal septum. The nasal region, with its cornua, abbreviated first labials, and numerous pairs of valvular secondary labials, presents many features of resemblance to the Elasmobranchs. 422. The parasphenoid reaches nearly to the extremity of the snout, the single vomer lying beneath it behind its termination. There is no palatine and no jugal ; and the hinder end of the maxillary is far distant from the quadrate. The large pterygoid behind underlies the ^uspensorium, extending inwards to touch the short basitemporal wings of the parasphenoid. The suspensorium is not quite directly attached to the cranial wall, but is confluent with the auditory cartilage without dividing into very marked processes. The elongated mandible presents much cartilage, and has but two bones, the articular and dentary. There is a very large auditory columella, in which the cartilage is not segmented, but there are two shaft-bones of nearly equal length, the inter- and mediostapedial. The extrastapodial is a large cartilaginous plate apphed over the outer face of the squamosal, qiiadrato-jugal, and quadrate, and embedded in the tympanic membrane; around it is the attenuated tympanic annulus, whose upper deficiency is supplied by two small tympanic bones. The lower hyoid tract, from being massive, has become very slender ; it is not closely related to the auditory capsule, but is suspended froni it behind and below the attachment of the suspensorium by a ligament of some length- Medioventrally the two bars are fused with one another and with the basibranchial tract. The fore part of the original branchial pouch has become a long and broad plate on either side. Prom the basibranchial tract project backwards a pair of branchials ("thyro- hyals") ossified as shaft-boues, and supporting the larynx. Tim Skull of Pipa. 423. Embryos of the remarkable Surinam Toad, Pipa monstrosa, taken from the dorsal pouches when 9 lines long, have a skull as highly developed as the earliest described larvae of Dactylethra, liS2 MORPHOLOGY OF THE SKULL. [CHAP. while as yet they are coiled upon a little differentiated yelk-mass, the tail almost meeting the chin. The limbs are well deyeloped and free externally, and yet the cerebral vesicles are very bulbous, and the opercular flap is free and small. There are no homy jaws, no labial cartilages, no traces of branchiae ; although the branchial and aortic arches are formed. The ear-sacs with the hindbrain are very large, taking up half the head. In many respects, tadpoles of the common Frog and Toad only half the length of these embryos are more advanced; they are free and active at one third their length. But in Pipa the embryo is very unlike a tadpole : it appears never to have gills; and the metamorphosis is perfect before extrusion from the dorsal pouch. 424. The skull is essentially very comparable to that of Daoty- lethra, with considerable differences in relative size of parts. The hinder region is predominant, with a long cephalic notochord; the brain reaches nearly to the anterior extremity of the head. Tlie cartilage has not united around the notochord, buttheintertrabeoular space is already filled in with cartilage, forming a broad trabecular plate, continued outwards in the mandibular fuspensoria ; the sabocular fenestra is a mere chink. There is a well marked otic process of the suspensorium. The tegmen tympani is not so con- spicuous a feature as in Dactylethra. The fenestra ovalis is forming. The trabecular cornua are free. The hyoid and branchial cartilages are on the whole very similar to those of Bactylethra, although feebler. This is their highest state of development. The aortic arches show no secondary branches. 425. Metamorphosis is complete while the larva remains in the maternal pouch. In creatures still rolled up, and little more than half an inch long, as well as in others showing their faces at the mouths of the dorsal pouches, seven and a half lines long, the ossi- fication is already intense. The head and body are only one-fourth of an inch longer than when the yelk-mass was but little lessened ; the original ground-plan, so to speak, is filled in with but little modification in the size of the territories. 426. Two of the most remarkable features of the chondro- cranium are, the absorption of most of the cartilage in the floor of the skull, including some of the postpituitary tract, and the very great increase in the subocular fenestra. The palatopterygoid is V ] THE SKULL OF PIPA. 18:1 a slender rod of cartilage. Tiie columella is complete at this early jieriod, contrasting with its late development in the Frog. The ceratohyal has become entirely absorbed; the branchial arches are reduced to a form substantially agreeing with their condition in Dactylethra. 427. The notochord is but little diminished relatively : it liei mainly ^ipon the basilar plate behind, and over the parasphenoid in front, free from relation to cartilage. On either side behind, a fissure marks the division between the almost absorbed auditory cartilage and the basilar plate ; in front of that chink the auditory, b.isilar, trabecular, and suspensorial cartilages are united, the right and left tracts being widely separated. About the region of the optic nerve the trabecula is reduced to a normal size. Prom thence each bar rapidly widens, running inwards to unite with its fellow and form the ethmoidal floor, upon which lie the olfactory lobes, and the anterior fourth of the cerebral hemispheres The cartila- ginous cranial wall, continuous with the trabeoilar base, is a very thin vertical lamina extending from the auditory capsule to the ethmoidal region, united with the ethmoidal floor just described, and forming a narrow anterior ethmoidal wall to the cranium : all this is entirely unossifled. It is continued forwards into a com- paratively narrow nasal septum with recurrent cornua trabecula», and is laterally produced into the almost transverse antorbital and palatine tract. There is a small alinasal roof growing on either side from the top of the spptum ; and the nasal investment is completed by three pairs of upper labial cartilages, two of which extend into the upper lip. 428. Tliere is a strong prepalatine spur from the ethmopalatine bar, running forwards parallel with each trabecular cornu. Behind this the main bar is very elongated, joining the fore face of the suspensorium just within the condyle. The suspcnsorium is very difi'erent from its primary condition : the pedicle in front of the auditory capsule is narrow, but joins broadly with the base and side wall of the cranium ; its direction is forwards and inwards from the quadrate condyle ; formerly it was directed backwards. The otic process almost continues the lino of the pedicle backwards : it is not fused with the periotic mass ; it is directly behind the palatopterygoid bar. The quadrate tract is little extended: it ]S-t MORPHOLOGY OF THE SKULL. [CHAP. furnishes a bilobed condyle. Instead of being situated at the level of the anterior end of the suboculMr fenestra, it i.s now below its posterior angle. The columella extends forwards over tlie otic process to the palatopterygoid bar. The process of the suspensoriuni bearing the condyle for the ceratohynl has disappeared in correlation to the absorption of that cartilaghious plute. 429. The cranial bones present remarkable features. There is an occipito-otic mass, possessing small exoccipital tracts, nearly meeting below in the middle line a strong supraoccipital bridge, beneath which a cartilaginous plate remains, and very complete otic in- vestments, which have eaten away most of the cartilage behiw, leaving it more extensively above. The extreme external and an- terior regions of the capsule remain unaffected by bone. The very large parasphenoid greatly resembles that of an adult i)erennibran- chiate or a young caducibranchiate Urodele. It ends behind about tlie middle of the auditory region by a sharp spur; and in front beneath the nasal septum by another sharp spine ; it is broadest in the ethmoidal region ; there is no vomer. The cranial fontanelle is more than covered by a great single parieto-frontal bone, ex- tending backwards to the supraoccipital : an anterior median spur is due to a supraethmoid ectosteal plate. 430. The nasals are semicircular bones notched in front for the external nares : in front of each is a small preorhital and a smaller septo-niaxillary. The premaxillaries and maxillaries are flat, simple bones, lying entirely beneath the anterior cartilaginous ^tructures. The maxillaries are falcate, ending behind in sharp p lints, a little in front of the pterygoids; the palatopterygoid cartilage is bowed farther out than the maxillary bone. A delicate styloid palatine bone extends between the parasphenoid and the maxillary. The pterygoid lies underneath the con-esponding carti- lage and the suspensoriuni, almost touching the parasphenoid : its transverse tract is much the larger. There is no quadrate ossifi- cation ; the meckelian cartilage is largely invested by the ectosteal articular, reaching nearly to the symphysis ; the dentary is largest at the symphysis. The squamosal consists of a lamina on the sispensorium, hidden by the columella ; and a horizontal extension at the side of the auditory capsule. The columella is fastened to the simple unossified stapes ; the elongated cartilage is not v.] THE SKULL OF PIPA. 185 segmented, but it is invested by an iiiterstapedial and a mediostapedial shaft-bone, and is continued forwards into the enlarge 1 extrastapedial cartilage, surrounded by the U-shaped tympanic anuulus : there is no suprastapedial. 431. The skull of the adult Pi pa, although presenting no struc- turjs which cannot be interpreted by the last described form, is one of the most remarkable pieces of osseous architecture that can be imagined. Cartilage is almost entirely absorbed ; the bony tracts have become greatly extended and massive, and by the alteration of their relative size the skull wears an extremely different aspect. Its breadth has become half as gi-eat again as its length, the auditory capsules being thrust out on long stalks. The quadrate condyles are retracted to a position parallel with the occipital condyles, though at a lower level. The cranium is excessively flat- tened, and the bones of the roof and floor meet laterally. The maxillaries combine with the pterygoids to bound the upper jaw. 432. The posterior view of the skull presents a bony arc, convex upwards, bearing numerous bosses for muscular attachment, and consisting almost exclusively of the occipito-otic mass. In the basi- oecipital region a small median tract is unossified : the rest of the foramen magnum is encircled by bone, and no cartilage is left. The occipital mass extends outwards to the squamosal and quadrate bones, and nearly to the quadrate condyle. The occipital condyles are very outwardly directed. The prootic tract of the oecipito-otic mass reaches out to the tympanic apparatus, which is close to the quadrate condyle. The parasphenoid is a broad oblong bone, pos- teriorly filling up an angle between the right and left occipito-otic masses where they nearly touch below, anteriorly presenting a median spur beneath the nasal septum, and a pair of palatine wings reaching to the maxillaries. There is no vomer. The massive fronto-parietal is twice as broad in the ethmoidal region as it is behind, where it overlaps the supraoccipital tract It has a median supraethmoidal spur, and large lateral wings extending on either side down to the maxillary. There is no cartilage in the cranial walls: the deficiency is supplied by the bevelling of the parasphenoid upwards on either side to meet a long orbital plato of the parietofrontal. There is no sphenethmoid. ISO MORPHOLOGY OF THE SKULL. [cHAP. V. 433. Cartilage is persistent in the ethmoidal, nasal, and ethmo- palatine tracts. The prepalatine spur is retained, and also a slender part of tlie palatopterygoid bar, not nearly reaching the suspensoriuni or the antorbital region. The nasal septum remains, but is very low ; the alinasal cartilage is small ; the recurrent trabecular cornua are long, slender, and straight, diverging from the septum at about 45°; a small prenasal spur projects forwards from the septum; all but one pair of labial cartilages have vanished. Tlie large triangular nasals extend forwards to the extremity of the snout, where they meet; they are notched for the external nares. and overlap the parietofrontal. Below the narial opening are the preorbital and septo-niaxilhiry. 434. The preraaxillaries and maxillaries are edentulous palatal plates. The latter are much the larger, have a spur within the internal nostrils, and extend backwards under the pterygoid bones for half their length. The anterior part of the pterygoid is of considerable breadth, bounding the subooular space, which is wider than the cranium ; while the eyeball is quite small, and rests against the anterior external part of the pterygoid. Posteriorly, the pterygoid is transversely extended beneath the otic mass so as to reach the parasphenoid and come very near the middle line ; there is also a smaller process extending outwards and backwards beneath the quadrate. The suspensorium is undistinffuishable from the otic mass (although some cartilage remains) except by the fact that the quadrate is a distinct considerable bone. The squamosal clasps the outer edge of the otic mass and the upper and hinder face of the quadrate. The columella has its extrastapedial relatively dilated, and surrounded by an almost perfect ring of cartilage, the tympanic anuulus. The stapes and the fenestra ovalis are surrounded by an annular elevation of cartilage. Tlie facial nerve emerges from the upper surface of the otic mass about half way towards the quadrate condyle ; it passes over the columella. The moieties of the edentulous lower jaw have a fibrous symphysis, and are very rib-like ; the articular bone runs nearly to the chin, and is covered by the dentary, which is but two-tliirds its length. CHAPTER VI. THE SKULL OF THE COMMON SXAKE. ( Tropidonut us natrix. ) 435. The early phases of the development of the skull in this form were long ago observed by Rathke, and his account of them is not by any means superseded. The first two stages given below are abridged from Eathke's Entwickelungsgeschichte der Natter, 1839 \ First Stage. 436. The trabeculte at their first appearance form two narrow bands, consisting of the same gelatinous substance as that constituting the whole investment of the notochord, not sharply differentiated, but only thickened and more solid parts of that half of the cranial floor which lies under the anterior cerebral vesicle. Posteriorly they are separated by a small interval, and thence sweep in an arch to about the middle of their length, separating as they pass forwards ; afterwards they converge, so as to approach one another very closely, or even to come into contact. Altogether they form, as it were, two horns, into which the investing mass of the notochord (parachordal tract) is continued forwards. The elongated ' See Huxley, Proc. Boy. Soc, Nov. 1858, p. 56, and Elements of Comparative Anatomy, p. 237. 188 MORPHOLOGY OF THE SKULL. [CHAP. (intertrabecular) space between them, moderately wide in the middle, is occupied by softer formative substance, upon which rests the infundibulum and the forebrain. Anteriorly the trabeculiB reach to the front of the head, and bend slightly upwards into the frontal wall : but there is also a small lateral projection or cornu passing outwards on either side. 437. The middle trabecula' grows, with tlie brain, further and further into the cranial cavity, and as tlie dura mater begins to be now distinguishable, it becomes more readily obvious than before, that tlie middle trabecula raises up a transverse fold of it, which traverses the cranial cavity trans- versely. The fold itself passes laterally into the cranial wall ; it is highest in the middle, where it encloses the median trabecula, and becomes lower externally, where it forms, as it were, a sliort ala proceeding from the trabecula. With increasing elongation, the (middle) trabecula becomes broader and broader towards its free end, and, for a short time, its thickness increases. After this, however, it gradually becomes thinner, without any change in its tissue, till, at the end of its second period, it is only a thin lamella, and after a short time (in the third period) entirely disappears. 438. In Mammals, Birds, and Lizards, that is, in those anin a s in general, in which the middle cerebral vesicle is very strongly bent up and forms a protuberance, while the base of the brain exhibits a deep fold betweeu the infundibulum and the posterior cerebral vesicle, a similar part to this median trabecula of the skull is found. In these animals, also, at a certain very early period of embryonic life, it elevates a fold of the dura mater, which passes from one future petrous (prootic) bone to the other, and after a certain time jjrojects strongly into the cranial cavity. Somewhat later, however, it diminishes in height and thickness, until at last it disappears entirely, the two layers of the fold which it had raised up coming into contact. When this has happened, the fold diminishes in height and eventually vanishes almost completely. 1 This account of Rathke's " middle trabecula" is given at length in order that ita siguificauce may be prominently brought forward. VI.J THE SNAKE: SECOND STAGE. 1^9 Second Stage. 439. The trabeculte attain more solidity, acquire greater distinctness from the surrounding parts, and assume a more determinate form, becoming in fact fili- form, so that the further forward, the thinner they appear. They increase very little in thickness, but far more in length, during the growth of the head. Quite anteriorly, they coalesce with oue another, forming an internasal tract. As soon as the olfactory organs increase markedly in size, the septal region is moderately elongated and thickened, without becoming so dense as the hinder part of the trabeculse. The lateral prolongations which now proceed from the internasal tract become little denser, although considerably enlarged. 440. The lateral parts and upper wall of the cranium, with the exception of the auditory capsules, remain merely membranous, consisting in fact only of the cutaneous covering, the dura mater, and a little interposed blas- tema, which is hardly perceptible in the upper part, but increases in the lateral walls, towards the bdse of the skull. Third Stage : Emhryo Snakes about an inch and three- quarters in length. 441. These embryos have recovered from the meso- cephalic flexure ; but the visceral clefts remain open, three fairly distinct, the fourth obscure. The head (Fig. 48) has a very monstrous appearance, being covered with a series of bulbous protuberances. The nasal region is rounded and beaked : the olfactory cavities are formed, and the nasal meatus is pervious. The eyes (e.) are very large, being twice the diameter of the auditory and nasal sacs. Although the mesocephalic flexure is lost, there is a remarkable bending of the head upon the neck. 442. The angle of the mouth is directly below the anterior margin of the auditory sac. The inner nares 1!)0 MORPHOLOGY OF THE SKULL. [CHAP, are formed, opening together in one common depression on the mid line of the palate. The maxilloijalatine cheek -mass, from the nasal sacs to the angle of the mouth, is distinguishable into two tracts, which are better seen when the lower jaw is removed. There is an outer or maxillary, and an inner {jalatopterj'goid tract. The lower jaw is of equivalent length and thickness to the upper; and between its rami can be seen a pair of rod-like prominences containing the rudiment of the tongue and it.s sheath. ppg Embryo Snake If incli long ; side view of head with facial arches seen through. C 1, 2, 3, cerebral vesicles; ol. nasal sac; e. eyeball; an. auditory mass; m. month; ppff. palatopterygoid tract; mk, meckelian cartilage; (/. quadrate; tg. rudiment of tongue; liy. hyoid arch; c\. 1, 2, visceral clefts. 443. At this stage the chondrocranium is already well formed. The cranial notochord (cA. Fig. 49) is short, and twisted upon itself, and does not reach more than two-fifths of the distance between the occipital end of the skull and the pituitary body. The parachordal cartilages are united at two points behind the pituitary body and in front of the notochord. There is one broad bridge immediately in front of the notochord ; the other is narrower, and lies immediately behind the pituitary body. Between these bridges there is a large oval space vacant of cartilage (j3. h. c. f.). The anterior part of the VI.] THE snake: THIKD STAGE. 1»1 parachordal cartilage is turned outwards opposite the postpituitary bridge, to form a hooked process, with a hinder concave margin. Just outside the same bridge the carotid artery pierces the cartilage on either side : this point marks the junction of the parachordal with the trabecular cartilaoes. o Fig. 49. /t.l.o/ s.n Emlirjo Snake, about IJ inch long ; chondroeraumm seen from above, the brain and jaws having been removed. s.o. supraoccipital tract; au. auditory capsule; a.s.c, li.s.c, p.s.c, anterior, horizontal, and posterior semicircular canals; ch. notoehord; i.m. basilar plate; p.h.c.f. posterior basicranial fontanelle ; al.s. ali- ipbenoid cartilage ; ptij. pituitary body; tr. trabecula; o.s. orbitosphenoid cartilage ; II. optic nerve ; na. nasal capsular cartilage. 444. The trabecule {tr.) pass forwards as rounded rods, wide apart opposite the pituitary body, rapidly converging in front, but remaining separated by a distance almost equal to their own width as far as the nasal region. The tract in which the trabeculse remain distinct is about one-third the length of the skull. In front the trabecular coalesce, forming a considerable internasal plate ; and develope a low nasal septum which is continuous at its upper edge with the nasal capsules. There are lateral 192 MOEPHOLOGY OF THE SKULL. [CHAP. processes at the sides of the fore end of the septum — the trabecular horns ; and a median anterior downgrowth, the prenasal cartilage. 445. The nasal glands lie right and left of the septum, but on a lower plane, in the anterior part of the palate. The whole nasal wall has chondrified; seen from above each nasal roof («a.) appears as a reniform cartilage, the two being in contact back to back, and the external nasal opening occupying tlie hilus. The nasal cartilages are relatively large as compared with the trabeculae, and have coalesced with nearly the whole extent of the nasal septum. 446. The posterior region of the basicranial cartilage has two mammillary condyloid projections, one on either side of the notochord ; and a little in i'ront of these are the anterior and posterior (or rather external and internal) condyloid foramina. The chondrocranium is completed in the occipital region laterally and superiorly. The supraoccipital region (s. o.) forms a prominent boss in the middle line behind, and anteriorly is shaped like a rectangular wedge fitting between the auditory capsules ; but the union of the original supraoccipital moieties is not yet complete, and their lateral union with the auditory capsules is also imperfect. 447. The periotic masses have a remarkable tri- angular shape ; the long base of the triangle is external, while its apex is situated internally, at the fore part of the supraoccipital wedge : the two capsules nearly meet in the cranial roof. The front part of the capsule diverges much farther from the middle line than the hinder portion : it is also longer and reaches to the level of the middle of the posterior basicranial fontanelle. The semicircular canals are relatively very large and distinct; the anterior and posterior meet by less than a right angle at the inner (supraoccipital) part of the capsule ; and the ampullcE are all well seen from above. The great supracranial fontanelle is margined behind almost entirely by the somewhat concave inner and upper edges VI. J THE SNAKE: THIRD STAGE. 193 of the auditory capsules, along which margin the large anterior canals run. 448. Opposite the apex of the notochord in the lower edge of each capsule the membranous labyrinth is budding into a rudimentary cochlea lying just mesiad of the external canal. Behind and above this swelling, on the infero- lateral face of the capsule, is the fenestra ovalis, containing a membranous plate which is chondrifying continuously with the hyoid arch. A diverticulum can be seen growing backwards from the fenestra ovalis; this is the commence- ment of the fenestra rotunda, which afterwards becomes separated from the former by a bar of cartilage. Behind these and internal to them the glossopharyngeal and vagus nerves are seen passing out in the boundary between the otic mass and the occipital cartilage. 449. Between the out-turned anterior termination of the basilar cartilage and the side- wall of the otic mass is a large rounded notch, just opposite to the postpituitary fontanelle and of similar size. Part of this notch is occupied by a small independent ear-shaped cartilage, having a narrow end forwards, and its edge convex inwards; this is the alisphenoid cartilage {al. s.). The orbitonasal branch of the trigeminal nerve passes out between this cartilage and the auditory capsule, and then forwards into the orbit. The second and third divisions of the trigeminal also pass out behind this alisphenoid cartilage. 4-50. The only chondrified tracts in the visceral arches are found in the mandibular and upper part of the hyoid (Fig. 48). The quadrate (q.) is segmented from the meckelian rod, and lies loosely external to the upper and outer side of the auditory capsule, its posterior otic process, which is sub-bifid, nearly reaching to the posterior canal. The part answering to the pedicle in the frog is a short rounded lobe. The shaft of the quadrate is directed forwards and downwards, and the condylar articulation is on as high a level as the external semicircular canal. The mandible (mk) is a slender sigmoid rod, continuing the line of the quadrate shaft, and having a long angular B. M. 13 194 MORPHOLOGY OF THE SKULL. [CHAP. process ascending behind the condyle. The two mandibular bars nearly meet in the chin. 451. The whole of the hyoid arch (hy) is only about half the length of the meckelian cartilage. It is a small rib-Uke bar, with a bilobate proximal extremity similar to that of the quadrate ; and a curved distal arc turned backwards. The proximal part is closely applied to the infero-lateral surface of the auditory capsule, and the hinder and inner of the two lobes coalesces with the stape- dial plate filling the fenestra ovalis. The rounded outer tubercle is free from the otic mass and is directed forwards. The posterior region of the hinder part of the hyoid is ready to be cut off as a free piece homologous with the mandible ; it is separate a little later, and diverges further backwards. 452. At this period the chondrocraniura of the Snake is fairly complete, without bony deposit. The notochord occupies only the hinder part of its original territory ; the trabeculce and parachordals have united, and while union has taken place at two places behind the pituitary body, there is a considerable posterior basicranial fontanelle. An occipital arch and alisphenoid cartilages complete the list of proper cranial elements. Anteriorly the trabeculae are parallel for a considerable distance without uniting, but in the nasal region they coalesce with each other and with the cartilages of the nasal capsules : cornual and prenasal tracts are likewise present. The auditory capsules have a notable triangular form, and already possess a rudimentary cochlea. The mandibular arch is divided into upper and lower pieces; the tissue in the maxillo- palatine process has not chrondrified. The palatal surface possesses median, submedian, and lateral longitudinal thick- enings, which are nearly prepared for osseous deposit. The simple hyoid bar is continuous with the stapedial plate in the fenestra ovalis. "^'l] THE SNAKE: FOURTH STAGE. 195 Fourth Stage: Embryos about two and a half inches long. 453. The head is more elongated, and much less monstrous. The cerebral vesicles are still well marked from above, but not nearly so protuberant as before (Fig. 50). The jaws have lengthened correspondingly with the rest of the head. The visceral clefts are entirely obliterated. Ossification is considerably advanced in several regions. 454. The general form of the basis cranii has altered very little from that of the more advanced individuals of the last stage. The fontanelle behind the pituitary body is larger, and the space between it and the end of the notochord is less than it was. The two moieties of the occipital roof have completely coalesced. The cranial part of the notochord lies on the united parachordals or basilar plate, and is surrounded by a bony sheath which is spreading into the substance of the cartilage right and left, forming the hasioccipital {h. o.). In the cartilage on either side of this is a bone (e. o.) pierced by the foramen for the hypoglossal nerve and extending as far as that for the vagus, which it is beginning to enclose. These exoccipitals can be seen postero-laterally on the occipital roof. The auditory capsules have altered their external shape very little; they are still unossified. The diverticulum to form the cochlea is a rounded bud extending downwards and forwards below and in front of the fenestra ovalis. It has no definite fenestra (rotunda) in the auditory wall as yet. 455. The small alisphenoid cartilage {al. s.) is still earshaped, and is placed somewhat longitudinally. Its hinder dilated extremity is applied to the front of the auditoiy capsule below, and its anterior end lies along the edge of the basilar plate, behind the region of the pituitary body. A notch in the hinder part of the alisphenoid embraces the posterior division of the trigeminal nerve (5), which passes backwards ; and the remainder of the trigeminal emerges from the cranial cavity in the space between the concave notch of the front aspect of the 196 MORPHOLOGY OF THE SKULL. [chap. basilar plate and the concave edge of the alisphenoid look- ing inwards. 4.56. The postpituitary bridge of cartilage separating the pituitary space from the basicranial fontanelle remains as it was in the last stage ; the external carotid artery pierces the cartilage on either side, and on the outer edge of this region is a bony rudiment ossifying the cartilage, I'Sf Embryo Snake, about 2 J incbea long; side view of head, dissected; BOYeral membrane bones haying been removed. CI, 2, 3, fore-, mid-, and hind-brain; oc.c. occipital condyle; a.o. supraoccipital cartilage; a.sc, h.sc, p.sc. anterior, horizontal and posterior semicircular canals ; al.s. alisphenoid cartilage ; o.s. orbito- spheuoid cartilage; tr. trabecula ; s.n. nasal septum; pn. prenasal cartilage ; ol. upper part of nasal capsule ; 2, optic foramen ; 6, trigeminal foramina ; /.o. fenestra ovalis ; st.h. stylohyal cartilage; co. columella; mk. meckelian cartilage ; tg. tongue. Bones: b.o. basioccipital ; e.o. exoccipital ; b.s. basisphenoid ; p. parietal ; /. frontal ; iilc. maxillary ; p><^- palatine ; pg. pterygoid ; t.pi. transpalatine ; q. quadrate. the two ossifications coalescing later to form the sphenoid {b. s.). The long uncoalesced portions of the trabeculse {tr.} in front of the pituitary body remain closely apposed; and between them is appearing a deli- cate styloid bone, enlarged into a spatulate plate just in VI.] THE SNAKE: FOURTH STAGE. 197 front of the pituitary body, beneath the broader part of the intertrabecular space. This is the origin of the -para- sphenoid. The small infero-lateral granular tracts of the cranial (orbitosphenoidal) wall above and in front of the optic foramina have not yet chondrified. The nasal cartilages have not changed in form, but the septum (s. n) is higher. The trabecular cornua are now indistinguishable from the nasal capsules ; and the prenasal spur (p. n.) is curved downwards and backwards iu front of them. 4.57. On either side of the midbrain in its lower region a thin film of bony matter is giving rise to the parietal (p.); and the frontals (/.) are similar bony plates on the sides of the forebrain, nearly meeting beneath the cranial cavity, above the trabeculse. Also on the inner face of each olfactory roof there is a thin shell of bone, the nasal; and these two shells lie back to back above the low septum. In front of the nasal cavities a bilunate plate of bone has appeared, the azygous premascillary. 458. On the side and below the level of the nasal septum is a large reniform nasal gland with a duct which passes downwards and outwards. This gland is covered above by a delicate film of bone, which enlarges inwards into a vertical plate near the septum; this is the septo- maxillary. Mesiad of the duct, the lower surface of the gland on either side is invested by a bone, the vomer, having a convex surface downwards. Two small (labial) cartilages are attached to the duct of each nasal gland. 459. Along the anterior region of the palate is a pair of bony styles lying external to the trabecule, and partly underneath the nasal cavities. In its middle region each of these palatine bones (pa.) sends inwards a process (the ethmopalatine) towards the trabeculse, just opposite the pointed end of the parasphenoid: it curves over the posterior nasal passage, which opens in the middle of the palate. A pair of slender gently-curved bones begin immediately beneath the hinder end of the palatines, and pass outwards and backwards to the inner- face of the quadrate cartilage. These are the pterygoids [pg.). Neither 19S MORPHOLOGY OF THE SKULL. [CHAP. palatines nor pterygoids arise in cartilage. In the margin of the cheek and upper lip there is a splinter of bone reaching from the outer angle of the premaxillary to opposite the posterior end of the palatine — the maxillary (inx.). Between the posterior end of the maxillary and the middle of the pterygoid, lying very obliquely, is another slender bone with a broad end towards the maxillary: this is the trans^alatine (t. pa.). 4C0. The quadrate, instead of being a rib-shaped carti- lage in the same line as the meckelian, passing downwards and forwards, is broad and fan- shaped, and its narrow lower part is rapidly ossifying as the quadrate bone (y.). The curved upper edge is applied loosely to the side of the auditory capsule, and is separated from it already by a delicate sickle-shaped plate, the squamosal, whose pointed anterior end is attached to the fore part of the capsule. 4G1. The meckelian cartilage, now very elongated, is gently sigmoid, and prolonged into an angular knob behind the quadrate articulation. The rods of opposite sides are widely separated by connective tissue. Films of parosteal bone have appeared in relation to it ; the dentary in front and outside, forking behind, and exposing a considerable tract of cartilage; behind it, applied to the outer face of the jaw, is the suranqidar, reaching to the condyle. On the inner surface there are three splints, enlarging from before backwards — the splenial, the coronoid, the angular, the latter reaching nearly to the angle of the jaw. Ectostosis has arisen in relation to the cartilage near the articulation, and extending into the angular process; this is the articular. 462. The backward direction of the hyoid arch is now much increased, and the rod has become slender and less curved [co.). The joart resembling the tubercle of a rib, instead of turning forwards, is directed back- wards, and is applying itself to the posterior edge of the stapedial plate. A lamina has been separated from the hinder and upper edge of the bar, forming a distinct stylohyal cartilage (st.h.), which is somewhat heart-shaped with the apex directed backwards. '^"'■J THE SNAKE: FIFTH STAGE. l!)l) 463. We have here no record of great transformation in cartilaginous parts; and the principal changes of relation are those by which the upper mandibular segment and the hyoid arch are carried backwards, the former becoming at the same time more loosely connected with the brain-case. The bones which have appeared in cartilage are the basioccipital, exoccipital, basisphenoid, quadrate, and articular. The principal membrane-bones are also present, the low lateral position of the frontals and parietals, the azygoiis condition of the premaxillary, and the occurrence of the septo-maxillary and transpalatine being points of interest. The auditory capsule is as yet unossified. Fifth Stage : Embryo Snalces nedr and up to the time of Hatching. 404. The skull has made great progress towards its adult conformation ; the brain-case is very straight, and the original protuberances are scarcely distinguishable on the upper surface. Ossification has extended rapidly, but is still far from complete : many new centres are arising. 465. The basioccipital has become a large heart-shaped bone, the notochord with its bony sheath (cephalostyle) still lying above it, though united with it ; the bone is separated by considerable tracts of cartilage from the sur- rounding elements. The pointed end of the basioccipital lies backwards, and is flanked on either side by a tubercle of cartilage, forming the two somewhat projecting lobes of the median condyle. In front the bone is extending beyond the original cartilage into the posterior basicranial fontanelle. The paired basisphenoidal ossifications described in the last stage have coalesced into one bone, ossifying the pre- chordal (postpituitary) bridge of cartilage, and beginning to extend backwards into the fontanelle. It passes for- wards also on either side of the pituitary body, as far as the posterior end of the parasphenoid, Avith which it ultimately coalesces. The internal carotid arteries perforate 200 MORPHOLOGY OF THE SICULL. [CHAP. the bone. There is no ossification of the trabecular cartilage in front of the basispheuoid. 466. The exoccipitals have extended, so as to meet by suture over the foramen magnum. The remainder of the supraoccipital region is ossified by a distinct supra- occipital, which is five-sided, broadest transversely between the highest part of the auditory capsules. Its concave anterior margin is the hinder border of the great supra- cranial fontanelle : posteriorly it is conterminous with the exoccipitals, and laterally with the epiotics. The glossopharyngeal nerve pierces the hinder end of the ear- cartilage ; the vagus passes out between the capsule and the exoccipital ; the hypoglossal pierces the exoccipital. 467. The auditory capsule has acquired its three most constant ossifications. The epiotic lies at the side of the supraoccipital, and occupies the most elevated part of the capsule, where the anterior and posterior canals unite : it is the smallest of the periotic bones. The opisthotic is related to the posterior part of the horizontal canal and the lower part, with its ampulla, of the posterior canal. It is of considerable vertical extent, and passes downwards as a wedge between the fenestra ovalis and the fenestra rotunda (of the cochlea), also passing behind the latter so as almost to encircle it. Its anterior process nearly meets the prootic beneath the fenestra ovalis, over the neck of the cochlea, whose bulb is still unossified. 468. The rest of the capsule is enclosed by the prootic, which includes most of the anterior and horizontal canals with their ampulhe, and surrounds the meatus intemus. This opening lies in front of the lower ray of a triiadiate synchondrosis between the three periotic bones. Beneath the two amj^uUas the prootic sends down a wedge-like process into the floor of the cranium between the basioccipital and the basisphenoid. In this wedge there are three foramina, two iarge ones, anterior and inferior, giving passage to the anterior and the posterior divisions of the trigeminal nerve ; and a smaller above VI.] THE SNAKE: FIFTH STAGE. 201 between them and the meatus internus, for the facial nerve. 469. External to this lower part of the prootic, standing on the obliq^ue, hinder, and outer edge of the basisphenoid, and underpropping the anterior part of the prootic, is a small four-sided bone which has ossified the alisphenoid cartilage, as well as an additional band of membrane bridging over the notch which previously existed in the caitilage, (§ 455, p. 195). Through the foramen thus formed the anterior portion of the tri- geminal nerve passes after its exit from the prootic. The concave hinder part of the alisphenoid is in front of the posterior division of the trigeminal, which jDasses almost directly backwards : the facial nerve is immediately above this. There is one other cartilage bone in the cranial wall, the orbitosphenoid ; it is triradiate, wedged in between the frontal and parietal, above and in front of the optic foramen. 470. The parietals, very solid and thick, now occupy a large portion of the side-walls of the brain-case, and reach inferiorly to the lateral edges of the basisphenoid (anterior part) and the front of the alisphenoid and prootic ; they send backwards a horn along the upper edge of each prootic and epiotic, reaching the antero- lateral angle of the supraoccipital. In front of the anterior ampulla the parietals also send a vertical ridge inwards as a partial boundary between the midbrain and the hindbrain. At present they have not roofed over the brain-case, but simply lie in its sides. The frontals are more complete above, and below nearly meet, above the parasphenoid. Their side-wall is concave externally, the parietals being convex. 471. The nasals extend much further over the nasal capsules than they did; and the vomers and septo- maxillaries are becoming solid, and taking the shape which they possess in the adult. The small azygous (edentulous) premaxillary has developed two distinct palatal processes, one on either side of the median pre- 202 MOEPHOLOGY OF THE SKULL. [cHAP. nasal spur. The parasphenoid has grown large after the pattern described in the last stage. 472. The palatines are hatchet-shaped : the blade of the hatchet is very concave below where the bone arches liver the posterior nasal meatus. There is a small external and a larger anterior process ; all these parts are related to the ethmoidal region of the skull, (between the eye and the nose). The handle of the palatine passes directly backwards, and lies above the fore end of the pterygoid. This latter is very much enlarged ; it is a spindle-shapad bone extending somewhat outwards and far backwards to the inner side of the quadrate ; its middle region is broad, and supports the curious transpalatiue, which is forked anteriorly, receiving the posterior end of the maxillary. Tliis is now much elongated and dentigerous, bounding the anterior half of the gape, ami towards its hinder end having a horizontal expansion which supports the innei' fork of the transpalatine. 473. The squamosal is an oblong splint-like bone attached loosely by its fore end to the auditory capsule over the anterior ampulla; behind, it diverges backwards and outwards. On its inner face is a much smaller splint, the supratemporul. The handle of the fan-shaj)ed quadrate is much longer than it was, and its extended upper edge is still cartilaginous : it applies itself o\er and outside the posterior end of the squamosal Thus it has lost all direct connexion with the cranium, and is also directed definitely backwards. The quadrate condyle is now behind the level of the basloecipital region ; it Ibrms a hinge-joint, -with an angular process (like an olecranon) behind, and a raised rim in front. 474. The posterior p;irt of the meckelian cartilage is well ossified by the articular ; anteriorly the cartilage persists to the front of the jaw, covered on its outer side by the dcntary. In front of the middle of the jaw the broad ends of two bones meet in a vertical line : one of these, directed forwards, is the splenial, the other, lying behind, is the coronoid ; both end by a point. A VI.j THE ADULT SXAKE. 203 narrow angular invests the lower edge of much of the articular, extending back to the region of the joint. The surangular is on the outer side of the same region, but is twice as large. 475. The columella is now composed of a bony stapedial plate and a shaft continuous with it, all but the distal third of the original cartilage being ossified. The stapedial plate itself is nearly straight behind, and convex in front ; convex externally and scooped within. The ascending (tubercular) process of the shaft is above and behind, the capitulum below and in front. To the curved posterior half of the sigmoid shaft a thick subcrescentic (stylohyal) bone is attached. It has ossified all but the extremities and the free under edge of its cartilage : and it is very tubercular and rough. Its outer face is attached to the inner surface of the quadrate, and both it and the columella extend behind the posterior edge of that bone. 476. The condition of the skull above described requires careful attention to details for its full compre- hension ; the growth of the parietals and frontals is especially noteworthy. The anterior halves of the trabeculoe remain unossified ; but supraoccipital, pro-, epi-, and opisthotic, alisphenoid, and orbitosphenoid centres have appeared. The quadrate condyle is now carried behind the basioccipital region, and it is connected with the side of the cranium by the intervention of the squamosal, — a very novel feature. The columella and the stylohyal have become ossified. The Skull of the Adult Snake. 477. The cranial investment constitutes an exceed- ingly strong box, the bones being united by even sutures, sometimes slightly squamous. The bony substance is very dense, almost like ivory, with very little diploe; and a large number of the sutures that were present at the time of hatching have disappeared. The bones of the face are also dense, but instead of being welded together, they are e '204 MORPHOLOGY OF THE SKULL. [CHAP. for the most part loosely connected with one another, as well as with the brain-case, by elastic ligaments constituting the most mobile facial apparatus to be found among Vertebrates. The axial parts are condensed into a comparatively small space, while the jaws even in their most contracted state are widely extended on either side of and behind the rest of the skull. The brain rests upon the almost flat cranial floor, without being elevated upon an interorbital septum. The hyoid apparatus is as insignificant as in any vertebrate type, while branchial arches are non-existent. 478. The cranial box is of tolerably even breadth, for the ear-masses do not project laterally as in other types, but are rather elongated. The occipital ring is much flattened, and is continuous to a great extent with the auditory capsule. The basioccipital is a broadly heart-shaped plate [h.o. Fig. 52), its rounded posterior apex constituting the median transversely-ova'l occipital condyle. A considerable part of each exoccipital (e.o.) appears in the cranial floor, and an equal portion in the cranial roof; and the postero- superior edge of each over- laps the corresponding part of the atlas. The exoccipitals meet above (Fig. .53), and are separated by the condyle below : each has coalesced with the contiguous opisthotic. The supraoccipital is now apparently larger than before, for it has annexed to itself the two epiotics. 479. The largest of the periotic bones, the pi'ootic (Fig. 51, pr.o.), is apparently quite distinct; but it has coalesced by its outer face in front with the little four- sided perforate alisphenoid. Internally the prootic rests equally upon the antero-external edge of the basioccipital and the outer edge of the basisphenoid. Anteriorly it is in contact with the thick hinder edge of the lateral plate of the parietal, with its inturned thick vertical crest. Siiperiorly at its fore part it adjoins the postero-lateral edge of the parietal, and the epiotic behind. Here, as in Reptiles generally, a Y-shaped suture is persistent between the three periotic bones, which never unite with VI.J THE ADULT SNAKE. 205 each other, but always with some adjacent bone. (When there is no alisphenoid, the prootic remains distinct from any other element.) The nerve foramina do not need further description beyond that given in the last stage. 480. The large irregularly four-sided basisphenoid has coalesced with the parasphenoid, to form a very curiously-shaped bone (Fig. 52). There is a posterior clinoid wall arching over the hinder part of the pituitary body. The latter rests upon a bony floor formed by the backward growth of the parasphenoid and by a concomitant growth of bone (basisphenoidal) from the whole margin of the intertrabecular space. Thus a sort of subcranial hollow is formed, which besides being partially roofed by the posterior clinoid wall, is covered at either side by a shelf passing inwards from the base of the parietal, on which the brain very largely rests. The subcranial hollow contains the pituitary body, a quantity of fibrous tissue, and the internal carotid arteries, which pass into it laterally beneath the parietal shelf, having previously perforated the basisphenoid. 481. The basisphenoidal ossification proceeds forwards on either side of the intertrabecular space to a point just under the very large optic foramen. Anteriorly to this point the trabeculse are entirely unossified, and lie in a pair of almost closed channels, on either side of the parasphenoid. The latter has a rather broad base and a crested summit which is wedged in between the frontal bones. Anteriorly to the brain-case the parasphenoid becomes compressed and knife-like, wedging in between the hinder ends of the vomers. 482. The two parietals have completely anchylosed into one, with no crest along the line of union. The bone is somewhat pentagonal above, partly overlapping the supraoccipital behind, and the epiotics laterally. Just in front of its projecting postorbital angle the parietal is overlaid by a small oblong but slightly curved postorbital membrane bone. The front edge of the parietal recedes at the mid-line, and is slightly overlapped by the frontals. 206 MORPHOLOGY OF THE SKULL. [CHAP. 483. The side-wall of the cranial cavity is provided by three bones, prootic, parietal, and frontal, the larger m, A. Or, Fig. 51. Adult Snake ; blue view of skull, with jaws removed. tr. trabeeula ; a.n. anterior nostril ; II. optic foramen. Bones: bo. basioccipital ; e.o. exoccipital ; s.o. snpraoccipital ; ^I'.o. prootic; 6.s. basisphenoid; ai.8. alisphenoid ; pa. parietal; pa. lateral plate of parietal; />. frontal; fr'. lateral plate of frontal; o.s. orbito- sphenoid; 2>a.s. parasphenoid ; vo. TOmer ; na, nasal ; pmx. premaxillary ; ;. septo-maxillary ; a.or. antorbital or prefrontal; p. or. postorbital or postfrontal ; col. columella. third being afforded by the parietal (Fig. 51). The alisphenoid does not appear in the cranial wall at all, being thrust out by the parietal and prootic. The inner face of the parietal wall is deeply scooped for lodging the optic lobes ; and the external face is correspondingly convex. The hinder edge of the hollow is very thick and almost vertical, projecting inwards as far as the prootic does ; from the lower edge of the bone on either side the shelf over the subcranial cavity proceeds inwards. 4S4'. The optic foramen is much larger than the optic nerve which passes through its centre ; its boundaries are, behind, the antero-inferior part of the parietal wall; in front and below, the parasphenoid ; above, a fan-shaped fenestra between the anterior edge of the parietal and the posterior edge of the frontal wall. The anterior part or handle of this space is occupied by a very small trilobate bone, the orbitosphenoid. 485. The frontals are very solid bones, not anchylosed to one another, but coapplied so as to form a single small tubular cavity. The inferior meeting-place is in the cranial VI.] THE ADULT SNAKE. 207 floor above the parasphenoid ; supero-externally they expand to form a supraorbital ridge over each orbit. In the extreme anterior part of the cranium a median wall grows downwards from each frontal and separates the fore pa.rt of the cerebral hemispheres, which are somewhat divergent in front; but even these partitions, though in contact, are unanchylosed to one another. The frontal floor is here more expanded than behind; while at the same time the outer wall is deficient infero-laterally, the space being filled in by fibrous tissue, supported by an ingrowth of the prefrontal. This membrane-bone (a. or.) lies at the side of the fore end of the frontal, and bounds the orbit anteriorly; its upper angle wedging in between the frontal and the nasal. 486. The trabeculse have united underneath the fore part of the frontals, and become compressed into a vertical ethmoidal plate passing on into the nasal septum. The hinder part of the nasal capsules are also wedged in beneath the fore part of the frontals. The nasal septum is highest behind, but nowhere very high, for the nasal roofs con- tinuous with it diverge very gently at first and are highly arched. The nasal bones (na.) lie like shells upon the cartilages, dipping down between the two almost to the septum. Externally they narrow as they descend towards the maxillaries: in front of these they are concave for the passage of the external nostrils. Anteriorly to the nasals the walls of the capsules dip gradually and are con- fluent, the trabecular cornu fcirming a bulging front-wall, resting on the premaxillary, and scooped by the external nostril. A very small spur of cartilage (prenasal) grows backwards between and underneath the extreme fore end of the capsules. 487. In the fore part of the nasal floor, on either side of the vomers, is a small tongue of cartilage, which may be connected on one side with a trabecular cornu, and which passes more directly outwards behind towards the outer process of the vomer. At the angle where the outward divergence takes place is another small piece of cartilage. 208 MORPHOLOGY OF THE SKULL. [chap. ■which converges a little towards its fellow of the opposite side, and lies in a rounded notch of the vomer. These are two upper labial cartilages. Fig. 52. ISLPr.O ||Ai.g Art Adult Snake ; under view of skiUl. ox. occipital condyle ; n.g. nasal gland. Bones : b.o. basioccipital ; e.o. exoccipital ; pr.o. prootio ; b.s. basi- sphenoid ; pa.s. parasphenoid ; vo. vomer; pmx. premaxillary ; mx. maxillary ; pi. palatine ; pt. pterygoid ; tr. transpalatine ; qu. quadrate ; st.hy. small stylohyal on quadrate; art. articular; ang. angular: s.ang. surangular ; cor. coronoid ; spl. splenial ; d. dentary. 4)88. The greater part of each nasal floor is occupied by the bone called septo-maxillary, which is vertically extended along the low nasal septum, and after under- lying the cavity extends upwards for some distance in the outer nasal wall towards the nasal. The transverse plate of the septo-maxillary which is a floor to the nasal cavity is a roof to the large reniforni nasal gland, which itself is bedded in a hollow of the vomer. The latter bone (vo.) has a longitudinal plate applied to its fellow beneath the nasal , septum, and a transverse wing which is cupped to receive the nasal gland, and notched antero-laterally to give exit VI.] THE ADULT SNAKE. 209 to its duct. The inner (posterior) labial applies itself closely to this notch and extends inwards in the hilus of the gland, expanding upon its duct. 489. The azygous premaxillary (pmx.) is a small triradiate edentulous bone, with a short median nasal process above, and- a longer double palatine process below. On either side it reaches the maxillary, which is a rather long slightly-curved dentigerous rod, thicker in front and flatter behind. It is attached by fibrous tissue at its anterior end to the premaxillary, at its anterior third (inter- nally) to the descending edge of the prefrontal, and behind passes under and is supported by the broad transpalatine. It occupies less than half of the gape, the rest of which in the upper jaw is margined merely by fibrous tissue, there being neither jugal nor quadratojugal. 490. The palatine (pi) is a small dentigerous rod of bone not half as long or wide as the pterygoid: in front of the middle it sends inwards a small plate beneath the posterior nasal passage. Its anterior third lies beneath the prefrontal: behind this it diverges gently outwards, not lying at all under the brain-case, and is attached to the outer side of the fore end of the pterygoid. The latter (pt.) is a large somewhat falcate bar, thick m the middle, and nearly as long as the distance from occiput to pre- maxillary; i% reaches from the palatine behind the occiput to an extent nearly equivalent to half the length of the skull; it obliquely crosses the inner face of the suspensorium, and extends even behind the quadrate condyle to the most posterior part of the angle of the mandible. The trans- palatine (tr.) is a curious hatchet-shaped bone, passing obliquely from the posterior end of the maxillary to rest upon the broadest part of the pterygoid. Its antero- external process touches the palatine and is attached to it. 491. The squamosal (sq., Fig. 53), the uppermost bone of the suspensorium, is almost horizontally placed, passing also a little downwards ; it is oblong, rounded in front, and wedge-shaped behind, where its downward face is covered B. M. 14 210 MORPHOLOGY OF THE SKULL. [CHAP. -with articular cartilage. It lies on and is attached by ligament to the anterior part of the prootic, and is slightly in contact with the temporal region of the parietal. The quadrate {qu.) is obliquely extended above, with a carti- laginous face lying over the bevelled hinder and external Adult Snake : skull seen from above. Bones: s.o. supraocoipital ; e.o. exoccipital ; pr.o. prootic ; pa. parietal ; /)'. frontal ; va. nasal ; pmx. premaxillary ; a. or. antorbital or prefrontal ; p. or. ■postoihital or postfrontal; pi. palatine ; pt. pterygoid; tr. transpala- tiue ; »i.r. maxillary ; sq. squamosal ; 511. quadrate ; st.hy. minute stylohyal on quadrate; art. articular; s.ang. surangular ; ang. angular; d. deutary. edge of the squamosal: there is a synovial joint between the two bones. The quadrate becomes more slender and thicker below, and bears a rounded condyle for a perfect hinge joint. Its direction is outwards and backwards at an angle of about 45° with the middle line. 4f)2. The lower jaw reaches as far as to the front of the maxill; ry bone, and is gently arcuate : its dentigerous VI-] THE ADULT SNAKE, 211 dentary {d.) is just half the length of the ramus. The bones are on the whole unchanged from the last stage. The broad junction of the coronoid and splenial is at two- fifths the length of the jaw from its anterior extremity. The angular {ang) and surangular (s. ang) are of great length, and are in some degree consolidated with the articular {art.) ; the meckelian cartilage is almost entirely absorbed. 493. The columella with its stapedial plate lies under the horizontal canal, in a recess which leads to the fenestrse ovalis and rotunda ; it is relatively much diminished, is entirely bony, and the sigmoid mediostapedial part is now no longer than the long axis of the stylohyal. This latter is a little scale-like bone, partly anchylosed with the posterior edge and inner face of the quadrate a little above its middle. By the growth of the suspensorium it is carried away from the columella to a distance of four times its own length. The small supratemporal has com- pletely coalesced with the squamosal. A small pair of cartilages is sometimes found at the sides of the larynx, which may represent the distal part of an arch (Huxley). 494. Summary. In this type the non-coalescence of the trabeculae in their postnasal tract, combined with their late persistence, is a feature of striking interest. Further- more, we note a marked diminution in the relative pro- portion of the cartilaginous structures to the rest of the skull. The cranium is only posteriorly roofed by cartilage ; the anterior part of its lateral wall also acquires very little cartilage, and the orbito- and alisphenoids are very feeble, and contribute but slightly to the cranial investment. The occipital and auditory bones are very complete, showing much more likeness to Osseous Fishes than to Amphibians ; the basisphenoid is ossified by two centres, and then coalesces with the large parasphenoid which by its median upward growth separates the trabeculse. But the fore part of the cranium has yet more notable features, in the first appearance of the frontals and parietals being low 14—2 212 MORPHOLOGY OF THE SKULL. [CHAP. down laterally, in the perfect coalescence of the parietals superiorly, and the formation of inferior plates, above the trabeculse and supporting the brain. 49-5. The palate is of a distinctive " cleft'' type, and the palatal bones present an extraordinary elongation and high specialisation of parts. A new bone, the transpalatine, is added, extending between the posterior end of the maxillary and the middle of the pterygoid. The latter, as in forms previously described, is connected with the (juadrate, but it has a perfection of structure not before manifested. The premaxillary is azygous ; the septo- maxillaries are very perfect and interesting ; " labial " cartilages are again found in relation to the posterior nares. The suspensorium of the mandible is notable (1) as giving rise to but one bone, the quadrate, which ossifies it entirely ; (2) for altering its direction during development from forwards to backwards ; (3) in being quite dissociated from contact with the cranial wall, and loosely united with the long squamosal, both bones being backwardly directed. The lower jaw has a very full number of distinct ossific centres. The side wall of the skull possesses membranous antorbital (prefrontal) and postorbital (postfrontal) bones. The small representative of the hyoid arch is divided into two parts, of which one persists as the simple columella, separated by some distance from the minute stylohyal piece. 496. There are but few points of special community between this skull and the types examined in the Amphibia, while there are many features which remind us of the Osseous Fish and the Bird. While the specialisation of membrane-bones and diminution of cartilage and cartilage-bones is far beyond what we have seen in the Salmon, yet in the occipital and otic bones, the para- sphenoid, and the palato-mandibular structures there are many resemblances between these two forms. But there is lost almost entirely the conformation of parts necessi- tated by the function of breathing in water ; and the remnants of the postmandibular arches very early after VI-] THE SKULL OF THK TURTLE. 213 their appearance take up the relations to which they are adapted in the adult. In many respects we are on the high plane of vertebrate development; the membrane bones are co-applied and united with the cartilage-bones to form a strong defence for the brain; the bones of the palate have a form and relations which need but little modification to present the Avian type; the suspensorium is one definite bone, the quadrate. From the consideration of this skull alone, the association of Amphibians and Reptiles in a group distinguished on one hand from Fishes, and from Birds on the other, becomes a contradiction to anatomical truth. APPENDIX ON THE SKULLS OP REPTILES. 497. The skulls of venomous snakes vary in thnir palatal structures in a manner which finds its extreme expression in Crotalus (the Rattlesnake), Here the shortening and specialisation of the maxillary, bearing the poison-fang, combined with great mobility of the other palatal bones, are the distinctive characters. The premaxillaries are small; and the maxillaries are capable of being moved from their normal position so as to erect the poison- fangs, by the same muscular action which opens the mouths In Typhlops the palatines are slender, and transversely placed, behind the posterior nares. The pterygoids lie beneath the cranial floor, and do not extend to the quadrates. In Tortrix the quadrate is short, and directly connected with the cranium, the squamosal being insignificant. The Skull of the Turtle, 498. In the Turtle {Chelone) at the time of hatching^, the chondrocranium is much more developed than in the Snake. The occipital roof is extensive, reaching beneath the hinder fourth of ^ See Huxley, Anatomy of Vertebrates, p. 239. ^ This description will explain most of the structure of the adult skull. 214 MORPHOLOGY OF THE SKULL. [CHAP. the parietal bone; yet the supracranial fontanelle is large. The floor of the cavity presents cartilage almost perfectly continuous, the posterior clinoid ridge being prominent ; but there is a posterior basicranial fontanelle. The ossifications of the floor of the cranium are a large basioccipital, and a basisphenoid arising by three centres, two pajred and lateral, behind the pituitary body, as in the Snake, and one anterior and median, representing the rostrum of the same animal. In front of this the presphenoidal cartilage is low at first, and then becomes suddenly compressed vertically, forming an interorbital septum beneath the optic nerve. This cartilage attains its greatest height as mesethmoid, and then is gently lowered where it constitutes the nasal septum, finally terminating antero-inferiorly in the median preuasal cartilage. 499. From the whole of the upper edge of the interorbital septum arises on either side a large semilunar orbitosphenoid cartilage, which ends posteriorly just above the optic nerve. The space behind this in the cranial wall is largely ossified by a descend- ing plate of the parietal, there being no alisphenoid cartilage. The nasal cartilages form very perfect roofs and side-walls to their capsules, and, curving inwards laterally, give rise to rudimentary turbinal plates. Posteriorly the nasal wall constitutes a steep antorbital plate. In its hinder part the capsule is almost entirely invested by the large ethmo-nasal bone. Anteriorly the lateral wall is continuous in cartilage with the nasal fioor, so that the capsules form two perfect cylinders. 500. The exoccipitals are vertical and oblong, perforated by the vagus and hypoglossal nerves. The supraocoipital region is large, with a great crest or spine growing backwards ; it possesses one considerable ectostosis. There are three auditory centres, but much of the capsule remains cartilaginous. Anteriorly is the prootic; supero-posteriorly the epiotic, passing forwards and inwards, and uniting with the supraocoipital; and postero-inferiorly the opisthotic, remaining permanently distinct. Between these bones is a triradiate tract of cartilage, the remains of which persist through life as a suture. There are large parietals (with lateral cranial plates), and frontals. 501. The quadrate has a large swollen otic region closely applied to the auditory capsule, and an anterior (orbital) process growing VI.] THE SKULL OF LIZARDS. 215 forwards and inwards to the outer side of the descending plate of the parietal. This is not ossified in the adult. The pterygoids are long and broad, and unite with one another in the middle line beneath the cranial floor. Each pterygoid has an angular ascending process applied to the inner face of the quadrate, and also abutting against the outer face of the parietal beneath the orbital process of the quadrate ; it is ossified from membrane. In a groove on this osseous process is found a tract of cartilage, whose apex nearly reaches the apex of the orbital process, forming a right angle with it. It becomes invested with a bony shaft, and constitutes a small epipterygoid ; it is subsequently developed into an oblong plate of bone wedged in between the parietal and the ascending process of the pterygoid, and has been mistaken for an alisphenoid. 502. The palatines are united with the pterygoids behind, and with the median vomer in front and above. The latter bone is expanded beneath, on a level with the small inferior palatal plates of the palatines, which are here seen for the first time, tending to carry backwards the orifice of the posterior nares in the palate. There is no transpalatine, and no septo-maxillary. The premaxillaries are small; the maxillaries large, and jugals and quadrato-jugals con- tinue the series to the quadrate. There is a postfrontal (membrane bone) behind and above the orbit ; and further back, a squamosal at the sides of the auditory capsule and above the quadrate. The temporal fossa is roofed over by the junction of the postfrontals and squamosals internally with an external plate of the parietals, and inferiorly with the jugals and quadrato-jugals. The dentaries are anchylosed in the adult into one bone. The postoral arches present much more development than in the Snake. There is a broad basihyal with anterior (larger) ceratohyal cornua and posterior smaller (branchial or " thyro-hyal ") processes. These cornua are all ossified. They are not directly connected with the axial parts; but there is a long columella auris, ending above in the stapes. The Skull of Lizards. 503. In a typical Lacertilian (as Varanus, Iguana) the hinder region of the skull is very completely ossified. Basi- and exoccipitals enter into the composition of the single condyle. The basioccipital is broad, the exoccipitals ascend to two-thirds of the height of the 21G MORPHOLOGY OF THE SKULL. [CHAP. foramen wiagnum, which is completed by the large supraoccipital. The auditory capsules, together with the exoocipitals, are extended widely outwards; the opisthotic unites with the exoccipital; the epiotic with the supraoccipital ; the prootic remains distinct. 504. The basisphenoid is ossified from a pair of ectosteal centres behind the pituitary body ; and sends out on either side a large basi- pterygoid process to articulate with the inner aspect of the ptery- goid. The basisphenoid extends forwards for some little distance into the trabecular cartilage on either side of the pituitary space, but in front of this the cartilage is unossified and passes into the extensive interorbital septum, which again is continued into the na:sal septum. The membranous floor of the pituitary space is supported by the posterior broad end of the delicate styliform parasphenoid, which underlies the posterior two-thirds of the interorbital septum. The latter is largely fenestrate, and but slightly ossified. There is no alisphenoid, but a membrane occupies the cranial wall from a little behind the optic foramen to the front margin of the prootic (which is notched for the trigeminal nerve). The parietal de- scends slightly into this tract. The orbitosphenoid cartilage is uxteusive, but very largely fenestrate ; the cartilage extends for some distance behind the optic foramen. There is an orbito- sphenoid ossification below the fenestra. The nasal septum is unossified : the nasal passage is floored on either side by a large septo-maxillary, which ascends as a small vertical plate against the septum. 505. The premaxillary as a rule is azygous, having a long nasal process. The nasals may be large as in Iguana, or very small ;is in Monitors. There are two membrane-bones in the antorbital region, both of which may be perforated ; the upper or prefrontal is the larger, and has an orbital plate; it narrows into an arcuate spur between the orbits, uniting by suture with the frontal. There is one supraorbital bone in Monitor, Iguana has two or three, the Sciucoids a considerable series. The frontals are simple superior bones, united with the parietals by a straight coronal suture, whose ends abut on the postorbitals (postfrontals). These membrane-bones (peculiar to reptiles) are always large, and rest on the squamosal behind, and sometimes on the jugal in front. As in Snakes, the parietals are always densely anchylosed, except for the small parietal foramen, so characteristic of these forms, which may be behind the VI.] THE SKULL OF CROCODILES. 217 coronal suture, in its course, or slightly in front of it. Posteriorly ii large horn is sent out on each side from the parietal, passing bacliwards to the ear-capsule and the parotic process of the ex- occipital. The jugal is sometimes large, reaching to the postfrontat and parietal. The quadrato-jugal is absent except in Hatteria. The squamosal is connected with the posterior spur of the parietal, and may be also united with the postfrontal. There is a supra- temporal behind the squamosal. The fossse which lie between the external bones of a Lizard's skull may be distinguished as supratemporal, between the parietal, postfrontal, and squamosal ; post-temporal, between the parietal, supraoccipital, and auditory prolongation ; and lateral temporal, between the squamosal and postfrontal above, the jugal and quadrate in front and behind, and the quadrato-jugal ligament below. 506. In the Geckos the external lateral bones of the skull are small or absent, so that the upper and lower lateral bars are not constituted. In Sphencdon both arcades are complete, and the quadrate is anchylosed to the squamosal, quadrato-jugal and ptery- goid, and the front of the ear-capsule. There are two premaxilla- ries. In Mosasaurus the pterygoids articulate with one another in the middle line behind the posterior nares. In Chamwleon the skull has an interorbital septum, but no epipterygoid . The quadrate bones in Ainphishcena are directed downwards and forwards, and there is no interorbital septum. In Chamseleon there is a backwardly produced median crest from the occipital and parietal bones, joined by a pair of lateral prolongations from the squamosals. The orbit is closed by a process of the jugal, but the lovyer arcade is not completed by a quadrato-jugal. The quadrate is articulated with the squamosal and the auditory capsule, but the pterygoid does not reach it. The Skull of Crocodiles. 507. The hinder region of the skull is very complete, and there are large alisphenoids. There is an interorbital septum ; and the orbitosphenoids are little developed. There is no parietal foramen. The bones anterior to the cranial cavity are greatly elongated, and the palatal plates of the maxillaries and palatines are very large, meeting in the middle line, and forming a secondary floor to the nasal passages ; this palate is carried far backwards by the develop- ment of a palatal plate on the pterygoids. The latter also extend 218 MORPHOLOGY OF THE SKULL. [CHAP. VL upwards to the base of the cranium, being suturally united, and are united externally with the upper and inner surface of the quadrates. The Tomers are usually concealed by the palatal plates. The large quadrate is firmly fixed in the side of the skull, though not anchy- losed. The tympanic cavity is bounded by almost all the bones of the hinder part of the skull, the epiotic and supraoccipital excepted. The opisthotic is confluent with the exoccipital. The lateral bony arches of the skull are very complete and strong, forming consider- able fossae resembling those of the Lacertilia. The Eustachian tubes are very complex. ( 219 ) CHAPTER VII. THE SKULL OF THE COMMON FOWL. First Stage : Embryo at the end of the fifth day of incubation. 508. The head of the emhiyo chick has at this period become well-defined, with notable bulbous protuberances of the cranial vesicles, and very large eyeballs. The mesocephalic flexure is at its greatest intensity ; the head is bent over so as nearly to touch the neck ; the mouth faces the chest. The cerebral hemispheres are already well-developed on the inferior aspect of the head (Fig. 54, c.h.) ; the midbrain is prominent in front of the cranial bend ; while the hindbrain is elongated and comparatively small. The olfactory pits (n.) are little depressions below and behind the forebrain, and just internal to the fore part of the eyeballs. The auditory vesicles are some way above and a little behind the eyeballs, at the sides of the posterior region of the hindbrain. 509. The mouth (?w.), beneath and behind the fore- brain, has an oblong aperture, bounded in front by the nasofrontal process (n.f), which separates the two olfactory- depressions ; behind by the first visceral arch, and laterally by the maxillopalatine processes (s.m.) growing forwards from the sides of that arch, and lying underneath the eyeballs. The nasal pits, as they deepen, acquire a break in their rim, giving rise to a groove directed obliquely downwards towards the cavity of the mouth. The naso- 220 MORPHOLOGY OF THE SKULL. [chap. frontal and maxillopalatine processes abutting ou this groove on either side, tend to deepen it : at a later period they form a bridge over it, and thus enclose a channel from the nasal pits to the mouth, opening on its roof. Emljryo Chick of the fourth day of iucnhation; head viewed from helow as an opaque object (Foster and Balfour). The ueok has beeu cut across between the third and fourth visceral folds. c.Ji. cerebral hemispheres; f.b. first cerebral vesicle; op. eyeball «./. nasofrontal process; vi. mouth cavity; s.vi. superior maxillary o maxillopalatine process from /. 1, the first visceral (mandibular) fold f. 2, /. 3, second and third visceral folds ; v. nasal pit. lu the section of the neck, al. alimentary canal ; m.c. neural canal cJi. uotochord ; ao. below, dorsal aorta; v. vertebral vein; ao. above, bulbus arteriosus. •510. Three visceral folds (Fig. 54, /. 1, 2, 3) exist in the side-walls of the throat, the hinder being the smaller; and they are most prominent ventrally. Behind each fold there is a cleft on either side, opening into the throat. .511. The primary parts of the cranio-facial skeleton, from being constituted of stellate cells, are mostly passing into hyaline cartilage. The cranial part of the notochord (Fig. 55, m.c), slightly constricted in two places, extends forwards nearly to the pituitary region (pt.s.). It is flanked on either side by the parachordals {pa.ch.) : there vn.] THE FOWL: FIRST STAGE. 221 is a prechordal bridge, and from this region the trahecnhr {tr.) bend downwards and diverge to surround the pituitary space. A discontinuity of these primary elements has not been discovered in any earlier stage. No separate carti- laginous auditory capsule is formed in the chick ; but FiR. Hn. Embryo Cliick, fifth day of incubation; view of cranial structures from above, the upper part of the head having been sliced away horizon- tally ; part of the first vesicle and the lower part of each eveball are left. The cartilaginous parts are those indicated by dark horizontal shading. c.v. 1, forebrain ; e. eyeball ; tr. trabecula (the line stops short of the part indicated) ; pt.s. pituitary or intertrabecular space ; 5, notch for trigeminal nerve ; l.g. process which becomes the lingula sphenoidalis ; q. quadrate cartilage ; h.s.c. horizontal semicircular canal ; cl. cochlea ; pa.cJi, parachordal cartilage ; n.c. notochord; 9, foramen for hypoglossal each parachordal is from the first continuous in the middle of its outer aspect with the chondrified wall of the ear-sac, in which the cochlea (cl.) and semicircular canals already begin to be evident. A little way in front of the ear-mass the parachordals are suddenly narrowed, forming a notch for the passage of the trigeminal nerve (5). In front 222 MORPHOLOGY OF THE SKULL. [CHAP. of this there is a slight angular expansion, on each^ side of the front end of the notochord ; and at this point a small transverse crest exists, marking out exactly the line about which the mesocephalic flexure has taken place. 512. The space enclosed by the descending trabecular is longitudinally oval : below this region the paired rods coalesce to form a broad internasal plate, lying behind the Embryo Chick, fifth clay of inoubation ; head viewed from below, witli skeletal parts seen through. c.tJ. 1, forebrain ; e. eyeball; n. nasal pit; f.n. nasofrontal process; ()'. points to one of the trabecular horns, recurrent in the nasofrontal process beneath the level of the main part of the trabeculse, which are seen on either side of pt.s. the pituitary space ; mr. subocular bar, in which pa. palatine and pg. pterygoid tracts are indicated ; q. quadrate ; mk. meokeiian cartilage ; b.h. basihyal ; c.h. oeratohyal ; h.hr. basibranchial ; c.br. ceratobranchial ; e.hr. epibranchial ; 1, 2, 3, visceral clefts. forebrain. It is emarginate in the middle line, and extended into a horn-shaped process on each side. These cornua partially arch round the external nasal openings. The inferior part of the internasal plate is curved some- what backwards upon the remainder, so that the cornua look directly backwards (see Fig. 56). VII.J THE FOWL : FIRST STAGE. 223 513. In the inner (palatine) region of the maxillo- palatine process, the curved palatopterygoid bar (sub- ocular in position) is beginning to be established, but it is considerably later in solidifying than the mandibular arch (Fig. 5Q,pa.,pg.). The latter is already segmented into tvvo pieces ; (1) a smaller tuberous quadrate cartilage {q.) lying behind the eyeball, and closely apposed to the antero- external angle of the parachordal and otic cartilage behind the exit of the trigeminal nerve ; and (2) a meckelian rod (nik.) lying in the first visceral fold, having a sinuous articular surface for the quadrate and a slight angular outgrowth behind. The meckelian bars nearly meet in the middle line. The first visceral cleft (1), behind the quadrate, is commencing its development into the complex tympanic cavity. 514. The hyoid arch contains ventrally a pair of small ill-differentiated rods (c. h.) and an azygous piece or hasihyal (h. h.). The second visceral cleft is fast closing, and behind it the first branchial arch has a pair of well- defined ceratohranchial rods (c. hr.) nearly meeting in the middle line, surmounted by very small distinct epi- hranchial pieces. A hasibranchial ih. hr.) is forming in the middle line behind. At this early stage the hyoid arch has begun to lie within the mandibular, just as the first brancliial comes to lie within the massive hyoid arch in the Osseous Fish. Already the third visceral arch of the chick has outgrown the second, — a state of things universal hi the class of Birds. 515. We here see that a grade of structure is reached in this skull as soon as its rudiments can be made out, which is only attained by a series of changes in the skulls of inferior types. The basicranial elements are continuous notwithstanding the mesocephalic flexure ; the internasal plate and comua are equally complete and continuous. The otic masses are also coalesced from the first with the special cranial structures. The separate development of the upper piece of the mandibular arch is a feature of high specialisation; the future palatopterygoid, so important 224 MORPHOLOGY OF THE SKULL. [CHAP. in the adult, is already distinguishable. The slightness of the skeletal parts in the hinder arches is related to the absence of any necessity for breathing in water. Second Stage : Sixth and Seventh Days of Incuhation. 516. The head, still disproportionately large, is now five lines long, and the rudimentary cranium is well chon- drified. The mesocephalic flexure has greatly diminished, and the head is more squared and less bulbous (Fig. .57). The forebrain occupies nearly the front half of the cranial cavity ; the midbrain lies behind ; while the hindbrain, considerably smaller, is entirely below the hinder part i)i the midbrain. The mouth, instead of being totally inferior, is now a space, the axis of which points more forwards than downwards. Fig. 57. /" 1 f i, Embryo Cliick, sixth day of incubation ; head seen from below. (After Huxley.) 111. cerebral hemispheres; u,. eyeball; g. nasal pit; fc. nasofrontal process ; ;. maxillopalatine process ; 1, mandibular arch ; 2, hyoid arch ; .r. first visceral cleft. The cavity of the mouth is seen between k and 1 ; the darkest part indicates the opening into the throat. 517. The trabecular region of the cranium has be- come longer than the parachordal. At the angle formed by the bending of the cranial floor, marked by a slight crest in the first stage, there has arisen the high ridge. VII.J THE FOWL: SECOND STAGE. 225 directed upwards and somewhat backwards, known as the posterior clinoid wall, protecting the pituitary body beliind. The internal carotid arteries enter, the cranial cavity in front of this wall. The parachordal cartilages are well consolidated, although the notochord is not obliterated. Posteriorly the cartilage grows outwards and upwards around the neural tube, forming exoccipital regions, and tending to complete the occipital ring. An external growth of the exoccipital behind the ear-mass [tympanic wing) comes to overarch the tympanic cavity (e.o. Fig. 58). 518. The trabecular rods at the sides of the pituitary body appear slightly bulged out and more distinct from the parachordals than in the first stage, though perfectly continuous with them. In older embryos there is an alisphenoid (lateral) tract of cartilage {a.s.) partially com- pletiug the base of the cranium, separated from the otic mass by the trigeminal nerve below, but united with it out- side the nerve. Externally this alisphenoid cartilage has an oblique projecting ridge, partially bounding the orbit behind, and overarching the quadrate. The trabecular cartilage remains low for a little distance, as far as the exit of the optic nerves. Underneath the fore part of the basilar region the fibrous stroma is thickening to recei\e the basitemporal ossification. 519. In front of the optic foramen and rising consi- derably above its level, the brain has become elevated upon a vertical cartilaginous internasal and interorbital plate, oblong in side view, thinner above, but thick below where it is directly formed by the coalesced trabecula?. It extends continuously to the fore extremity of the head, and its front margin is almost vertical ; it is continued below into a rounded knob, directed a little backwards; and its upper edge is united on each side with the small cartilaginous nasal roof. 520. Within a very short time in incubation the an- terior part of this vertical plate grows forwards below, so as to form the axis of a beak projecting downwards and B. M. * 15 226 MORPHOLOGY OF THE SKULL. [CHAP. forwards : the beaked part (prenasal cartilage) curving below the level of the basicranial axis. Thus we have an oblong interorhital plate (Fig. 58, p.s.), with a little Fig. 58. a-ln bhr Embryo Chick, seventh day of incnhation ; side view of skull. o.c. occipital condyle; nc. notoohord ; e.o. exoccipital tract, with its tympauic wing; s.o. supraoccipital tract; h.is.c. horizontal semicircular canal, ^.sx. posterior semicircular canal, both seen through the cartilage ; a.s. alisphenoid cartilage, with its external ridge ; 2, optic foramen ; f.s. prespheuoidal region ; below it, interorhital septum ; j).|;. antorbital plate ; etli. ethmoidal region ; al.e. aliethmoidal cartilage ; fl/ji. alinasal cartilage; p.n. prenasal cartilage; pa. palatine, ^,9. pterygoid tract; 5. quadrate; %t. stapes; /.r. fenestra rotunda; mk. meckelian bar; c./t. ceratohyal ; hM. basihyal ; 6. Jr. basibrauchial ; c.hr. ceratobranchial ; e.hr. epibranchial. spine projecting upwards in the front wall of the cranial cavity; and a triangular internasal plate, with its apex directed downwards and forwards. This internasal plate rapidly becomes differentiated into three regions, distin- guished by tlie nature of the nasal outgrowths continuous with its upper and anterior edge. In front there is the alinasal valvular structure (al.nyi, protecting the external nostril; behind this the aliseptal roof in the proper septal region, with soft coiling growths already projecting down- wards and inwards from it; and finally the aliethmoidal roof {ale.) growing from the highest part of the vertical plate, coiling to form an upper turbinal, and also develop- ing a transverse vertical partition (p.p.) between the nasal CLivities and the orbits {antorbital plate). VII.] THE FOWL: SECOND STAGE. 227 521. The arcuate rod in tbe maxillopalatine process (pa.,pg.) is elongated to reach from the front of the quadrate cartilage to the middle of the nasal septum. In section these rods and the thickenings in which they lie appear as the walls of the cleft palate. The quadrate (Fig. 58, q.) is now a very definite triradiate cartilage ; its antero-internal process passes under the trigeminal nerve, but does not reach the cranial wall. The postero- external or otic process forms a large hook extending along and applied to the side of the ear-mass, and reaching to and partially overarched by the exoccipital (tympanic) wing. This otic process overarches the tympanic cavity, and the fenestrse ovalis and rotunda (/. r.) are on the wall of the capsule beneath and within it. The quadrate condyle for the mandible, looking forwards and downwards, is grooved so as to be double. The meckelian rods (mk.) are long sinuous cartilages approximated anteriorly, extending as far forwards as the beak ; thickest behind, where they fork into two processes, the inner and the posterior angu- lar processes. 522. A small piece of cartilage (stapedial or colu- mellar), at first continuous with the ear-capsule, has become detached from it below the otic process of the quadrate, and consequently its thick base fills up an oval fenestra (ovalis), behind and below which is another mem- branous fenestra (rotunda), in the wall of the rudimentary cochlea (Fig. 58, st.,f. r.). 523. The history of this part has been observed more completely in the House-Martin (Ghelidon urhica) where the upper end has been observed at first continuous with the auditory cartilage, but dilated at its free- end into a fan-shaped extrastapedial plate which is pointed above and below, the upper process being the rudimentary supra- stapedial, and the lower being continuous with a delicate ligamentous tract, which chondrifying afterwards becomes the infrastapedial (see Fig. 74). From the lower end of this ligament there passes backwards at a right angle a small fiat oblong cartilage : this exactly corresponds to. 1.5—2 228 MORPHOLOGY OF THE SKULL. [CHAP. the stylohyal which Prof. Huxley described in the young Crocodile (see his paper on the Representatives of the Incus and Malleus, P.Z.S.). The dilated extrastapedial part has the same shape as described by the same author iu Hatteria. .524. The basal hyoid and branchial cartilages are the same in number as in the last stage, but are more solid, and the lateral moieties are directed backwards. 52.5. Comparing the skull at this stage with its pri- mary condition, we see that a marvellous advance has taken place, elevating it from an elementary grade to one characterized by the notable interorbital septum. Already the skull is definitely Sauropsidan, and in several points Struthious. The hinder part of the cranial investment is proceeding to form its occipital ring ; the anterior has developed the main outline, not only of the interorbital framework, but also of the beak which distinguishes the Bird. The prenasal cartilage is a very fit model on which the premaxillaries of any type of bird might be formed ; in no bird whatever, let the shape of its face be what it may, is the type of the first model wholly lost, although in every case the primordial structure very early undergoes wasting. The palate is cleft as in Struthious Birds ; the present articulation of the quadrate is like that which is persistent in them. Likewise the temporary condition of the branchial arches in this stage agrees with their form in adult Struthionids. The angular processes of the jaw already indicate the tyjie of the animal, for they have a peculiar development in the whole Gallo-Anserine series, attaining their most extraordinary foim in the Tetraonine group. Third Stage : Enibryos in the middle of the second week of incubation, with heads from eight to nine lines in length. .526. The skull has made great progress towards a bird-like form. In this stage it is in some respects parallel VII.] THE FOWL: THIRD STAGE. 229 with the adult skull in certain Teleostean Fishes, where there is a free development of the cartilaginous frame- work, which is comparatively little ossified ; whilst the parostoses are delicate, although numerous. The Lump- fish (Cyclopterus) and the Salmon possess such skulls. 527. When the axial parts of the skull are dissected out, they are seen to be very easily divisible into two main regions : a large posterior rounded expansion, more than hemispherical,and an elongated vertically-compressed plate, lying between the orbits and between the olfactory organs, and flanked anteriorly by moderate-sized appendicular cartilages belonging to the nasal sacs. The posterior expansion is remarkable for the curious way in which the periotic masses are compressed and curved so as to be moulded into the rounded form of the cranium (see Fig. 60). Thus a considerable portion (the cochlear) of the ear-mass is in the cranial floor, very median in position and far forwards : but the region containing the canals is much compressed and tilted backwards, rising high in the side wall, where it becomes postero-external, to accommo- date the large brain ; so that all the semicircular canals are visible in a posterior view. 528. The notochord has relatively retired, and is separated from the pituitary body by a distinct bridge of cartilage ; this forms the somewhat elevated posterior clinoid wall, and the cartilage shelves downwards on either side of it. The cranial notochord is slightly constricted in two places, thus presenting three subequal regions, the anterior cylindrical, the other two fusiform; the hinder of these is enclosed in a bony sheath, and at the posterior extremity of the skull by a cartilaginous bridge above and below. This is the only point where the parachordal cartilages have as yet united ; but here they completely invest the notochord, and form the single rounded occi- pital condyle, the two mammillee which first appear becoming one. Anteriorly indeed the parachordals have somewhat -withdrawn from the middle line, leaving a 230 MORPHOLOGY OF THE SKULL. [CHAP. lanceolate chink in which the notoehord lies : this is the posterior basicranial fontanelle [p.h.f. Fig. 62). 529. The occipital ring of cartilage is complete ; the foramen magnum is obliquely directed backwards (/. m. Fig. 59), and the supraoccipital region is almost vertical; the latter is emarginate in the middle line, both in front and behind. A pair of triangular exoccipitals (e.o.) are ossifying ; they extend from the lateral margin and base of the foramen magnum forwards to the foramen for the vagus nerve. The exoccipital region sends a lamina of cartilage (tympanic wing of the exoccipital) around the side and base of the hinder part of the ear-capsule. This wing is the posterior boundary of a scooped antero-inferior hollow in the periotic mass, which is to become the tym- panic cavity (Fig. 59, ty.) ; at present it is filled by a soft flocculent stroma, soon to be absorbed. There is but a slight roof to this cavity (tegmen tympani or pterotic ridge). 530. Mesiad of this tympanic hollow, and extending in front of it, is a thick rounded boss of cartilage, protruding both within the cranial cavity and on its inferior aspect, and containing the finger-like cochlear process of the ear- cavity {cl. Fig. 62). These cochlear protuberances are remarkably near to the median line of the cranial floor, at the sides of the middle region of the notoehord. The whole of the periotic capsule is unossified at present. 531. The circumpituitary or basisphenoidal region is perforated by a nearly circular pituitary hole, into which the pituitary body passes, and through which the internal carotid arteries enter the cranial cavity. The axial carti- lage is narrow in the posterior cliuoid region, then a little expanded outwards and much extended vertically; and projecting from the circumpituitary cartilage antero- laterally is a curved process or lingula of cartilage, extending downwards and backwards for a short distance. From the hinder half of the basisphenoidal region a considerable elongated aUsphenuidal lamina arises (a.s. Fig. 59), passing outwards and backwards on either side in the cranial floor, and ascending a little into the side VII.] THE FOWL: THIRD STAGE. 281 wall ; coalescing at its extremity with the periotic mass, but leaving an elongated space unfilled by cartilage between the two tracts. The cranial surface of the alisphenoid conforms closely with the concave curvature of the hinder part of the cranium (see Fig. 62). 532. In the cartilage of the antero-lateral boundary of the pituitary fossa is a pair of small bony centres ; they are continuous, however, with the parosteal ossifica- tion of the rostrum to be hereafter mentioned. Just in front of these basisphenoidal centres, where the compression of the basicranial cartilage to form the interorbital septum begins, is a pair of small distinct cartilaginous plates, affording articulating surfaces for similar cartilages deve- loped on the pterygoid bones (indicated in Fig. 59). The interorbital septum is now of much greater depth, carrying the fore part of the brain up to a high level. The pre- nasal continuation of the nasal septum is thickened above, and almost straight below, though slightly downbent at its extremity {p.n. Fig. 59). The trabecular cornua have become lost in the lower anterior part of the alinasal cartilage. 533. The frontal, parietal, and nasal bones have appeared; they need not be described in the present stage. The bones of the floor of the cranium and of the palate must be noticed more fully, because of the great interest of their history. The subcranial region which in the Frog is ossified by the basitemporal wings of the parasphenoid, is here supplied with a pair of distinct and large basi- temporal bones (b.t. Fig. 59), which extend from near the median line, beneath the cochlea, and so far outwards as to constitute a floor for the tympanic cavity : their anterior limit is near the fore margin of the alisphenoid cartilage. These ossifications arise in a thick web of fibrous tissue in the hinder part of the palate ; and the matrix is abundant in the middle line, extending forwards to the bone next to be described. The Eustachian tubes run forwards and inwards above the anterior edge of these bones, and meet in the middle line, beneath the pituitary fossa. 232 MORPHOLOGY OF THE SKULL. [chap. Embryo Chick, middle of second week of incubation ; under view of skull, "with arches removed. nc. notochord; f.m. foramen magnum; s.o. supraoccipital tract; !l, foramen for hypoglossal nerve ; p-S-i!- posterior semicircular canal ; 8, foramen for glossopharyngeal and vagus nerves; ty. tympanum; (.<.•. internal carotid artery passing above the basit?mporal bone; a.s. alisphenoid cartilage ; nc. notochord diagrammatically seen in the posterior basicranial fontanelle ; p.n. prenasal cartilage. Bones: e.o. exoccipital ; b.t. basitemporal ; r.b.s. rostrum; at its hinder end on either side the basisphenoid centres ; pa. palatine ; pc/. pterygoid; px. premaxillary ; mx. maxillary; pmx. maxillopalatine plate of maxillary ; j. jugal ; q.j. quadrato-jugal ; q. quadrate. 534. The interorbital septum rests upon a grooved fusiform rod of bone {r.b.s.), representing the anterior part of the parasphenoid of the Sahnon, the Axolotl, and VII.J THE FOWL: THIRD STAGE. 233 the Frog, and the whole of that of the Snake and the Lizard. This ossification in Birds receives the special name of rostrum ; its posterior extremity, at the anterior part of the pituitary space, has crept into the perichon- drium and the superjacent cartilage on either side. 53.3. The premaxillaries (px.) are formed as double laminae fnasal and palatal), united at the outer edge : above, they nearly meet by their nasal processes, although the prenasal cartilage protrudes between them in front : below, they lie outside it. The posterior extremity of each is forked, the outer being the dentary process, and the inner the palatal. From each premaxillary along the margin of the upper jaw to the quadrate condyle there are found three subequal styliform parostoses, maxillary (mx.), jygcil (J.), and quadrato-jugal (qj.). The maxillary at its anterior third sends inwards an earshaped process over the palatine bone; this is the maxillopalatine process {pmx.). 536. The palatine bones {pa. Fig. 59) are flat, pointed in front, where they pass between the palatal pro- cesses of the premaxillaries and the prenasal cartilage ; flattened behind and somewhat arcuate outwards, leaving a pair of long palatonasal" spaces between them and the nasal septum and the rostrum. Posteriorly they lie broadly against the rostrum, and then turn a little outwards, be- coming pointed where' they articulate with the pterygoids. The latter (pg.) are sigmoid clubs, flattened in front where they articulate with the cranial axis and the points of the palatines; and subcylindrical behind, articulating by a cupped face with a small knob on the quadrate cartilage, just above its principal condyle. The articulation of the pterygoid with the basis cranii is by the intervention of a distinct plate of cartilage playing against the basi- pterygoid cranial plate formerly mentioned (§ 532). The tissue in which the palatines and pterygoids are formed does not become true hyaline cartilage before being ossified, although fast tending towards that condition ; and more or less cartilage may appear in different parts of these tracts in various birds. 234 MORPHOLOGY OF THE SKULL. [CHAP. 537. The ascending portion of the quadrate cartilage is enveloped in an ectosteal sheath (g.): the mandible is becoming covered with its splints, the dentary, splenial, surangular, and angular pairs (Fig. 63, p. 242) : there is no coronoid. The stapes or columella is fully formed, but not ossified. The two small ceratohyals have united in the greater part of their extent, forming a small arrow- head-shaped lingual cartilage. Shaftbones are appearing in the ceratobranchials. 538. This stage is marked by the more perfect mould- ing of the occipital region, and the great development of the nasal parts. The constriction of the notochord and the ossification surrounding its posterior portion are of great interest. The tympanic cavity is plainly indicated, and the Eustachian tubes acquire a notable position above a pair of bones, the basitemporals. Attention should be especially directed to the origin of these bones and of the rostrum.' Few ectosteal centres have appeared ; exocci- pitals, basispbenoids, quadrates, and ceratobranchials. The principal membrane-bones of the roof of the skull, the palate, and the jaws are now found ; but the vomer is not yet in existence. Fourth Stage : Embryo at end of second and heginning of third week of incubation : head about an inch long. 539. The chondrocranium is rapidly becoming ossi- fied, its general form remaining substantially the same. The notochordal ossification has extended on each side into the parachordal cartilage, but the condyle is still unossified. The basioccipital bone thus originating is broadest in the middle, and is most prominent above along the notochordal line [b.o. Fig. 62). The posterior basicranial fontanelle (p.bf.) has become a wide space between the clinoid wall and the basioccipital. The exoccipitals (e.o.) have increased in size, and possess a pro- cess directed outwards in the tympanic wing ; and paired VII. J THE FOWL: FOURTH STAGE. 235- supraoccipitals (Fig. 60, s.o.) have appeared,, ossifying most of the cartilage above the foramen magnum. p'sc e.o n.c ^^ Embryo Chick, end of second week of incubation; posterior view of cranium. ox. occipital condyle ; nc. indicates the superior position of the notochordal remnant ; p.s.c. posterior semicircular canal ; h.s. horizontal semicircular canal ; t.eo. tympanic wing of exoccipital cartilage ; sp.o. sphenotic process ; fo. supracranial fontanelle. Bones : e.o. exoccipital ; s.o. supraoecipital ; sg. squamosal ; /. frontal ; p. parietal. 540. The anterior part of the otic cartilage, in the cochlear region and below the anterior canal, is ossified as prootic [pr.o. Fig. 61), and perforated by the auditory nerve. The outer and hinder view of the much tilted auditory mass shows nothing but cartilage ; a small opisthotic osseous wedge, however, has appeared in the edge of the capsule adjacent to the exoccipital {oj>.). Already the supraoecipital is ossifying the cartilage around the posterior part of the anterior canal : and the exoccipital is enclosing the posterior canal below. 541. The rostiiim, the basisphenoidal ossifications connected with it, and the basitemporals are gradually becoming one bone, together with another considerable pair of osseous tracts {pretemporal wings) developed in stroma laid down upon the lingulse formerly mentioned (§ 531), and extending outwards and backwards to the upper extremity of the quadrate along the anterior margin of the basitemporal {p.t.p. Fig. 65, p. 246). The 236 MORPHOLOGY OF THE SKULL. [CHAP. basitemporals, now uniting, include between them and the floor of the skull, a space in which the Eustachian tube and the carotid artery of either side run forwards Pig. 61. Embryo Cbiok, end of second week of incubation ; inner view of hinder part of cranium, with median section of basilar and supraocoipital regions. o.c. occipital condyle ; 6.0. basioccipital ; jir.o. prootio ; a.sc. anterior semicircular canal; s.o. supraoccipital ; 0^. opisthotic ; e.o. exoccipital ; 7, foramen for facial nerve ; 8, foramen for glossopharyngeal and vagus ; 9, foramen for hypoglossal. and inwards. The pretemporal wings and the basi- temporal plate meet at an angle, and enclose a considerable cavity anteriorly, continuous with the tympanic; these are the anterior tympanic recesses. .542. The proper basisphenoidal ossification has ex- tended in an annular form round the pituitary space (Fig. 62), and into the clinoid wall and the margin of the posterior fontanelle. The alisphenoid cartilage [a.s. Fig. G2) has become complicated in its relationships, being united with the fore part of the ear-mass above and below (the main part of the trigeminal nerve issuing between them) ; and also continuous by its base with the posterior clinoid wall and the side of the pituitary region. It has an external and posterior sphenotic process (sp.o.), and is also considerably curved forwards. A membranous fontanelle (a.s.f.) arises in its centre ; and both in front and behind this, distinct ossific centres appear in the cartilage : these afterwards unite to form one alisphenoid bone. 543. The remainder of the chondrocranium in front of the pituitary body is unossified. The anterior part of VII.] THE FOWL : FOURTH STAGE. 237 the brain is borne laterally upon membrane and covered by membrane and membrane-bones (see Fig. 63, p. 242) ; it is supported mesially by the elevated cartilaginous inter- orbital septum, which has a convex superior edge and a concave posterior margin, on either side of which the optic nerves diverge. The septum has a somewhat thickened lower edge where it rests upon the rostrum. The continuity of the interorbital with the nasal septum is interrupted to a great extent by the development of a vertical hourglass-shaped craniofacial fenestra (see Fig. 66, p. 250, in front of m.e.) in the cartilage just in front of the termination of the parasphenoidal rostrum. At the same spot the cartilage is expanded into an oval plate at the base, corresponding to the primary breadth of the coalesced trabeculse. 544. There is another fenestra of great interest in the posterior and upper region of the interorbital septum ; it is as large as the last-named, and may be termed inter- orbital fenestra; its backward termination comes very near to the limit of the cartilage {i-o.f Fig. 63). Yet one other feature of interest must be mentioned ; the median cartilage at the extreme anterior end of the brain- cavity sends backwards and upwards a sharp spike in its roof, the sole representative in the Fowl of the ichthyic tegmen : close beneath it the olfactory nerves emerge. 545. There is nothing especially to be noted about the proper nasal septum, except that it is grooved in front of the craniofacial fenestra for a branch of the orbito- nasal nerve, which passes from the upper wall of the orbit over the lateral ■ ethmoidal plate into the nasal roof, and then bends downwards and inwards in front of the fenestra, grooving the septum. "When it has reached nearly the bottom of the septum, the nerve passes under a small cartilaginous bridge, and turns forwards along the base of the septum to its peripheral distribution. The almost cylindrical prenasal cartilage, to which the septum slopes, is at its highest relative development {'p.n. Fig. 62). 546. The top of the ethmoidal and nasal septum is 238 MORPHOLOGY OF THE SKULL. • [CHAP. continuous with various outgrowths on either side, arching over and extending into the different regions of the laby- rinth. The nasal roof is complete on each side along its te.o Embryo Chick," end of second week of incubation ; upper view of skull, the braiu and parostoses being removed. o.c. occipital condyle; a.s.c. anterior semicircular canal; i.e.o. tympanic wing of exoooipital ; cl. cochlea ; j).6./. posterior basicranial fontanelle; ft.s. pituitary space; a.cl. anterior cliuoid ridge; j),cl. posterior clinoid ridge, outer part ; 5, trigeminal foramen ; 5a. foramen for its orbitonasal branch ; as.f. alisphenoid fenestra ; s-p.o. sphenotic process ; ji.s. presphenoidal region, or top of the interorbital septum, the great depth of which is not perceptible ; ctli. ethmoidal region ; aX.e. aliethmoidal cartilage; al.s. aliseptal cartilage; al.n. alinasal cartilage; ■p.n. prenasal cartilage. Bones: h.o. basioccipital ; e.o. exoccipital; s.o. snpraocoipital ; ^r.o. prootic ; a.s. alisphenoid ; around ^(.s. basisphenoid. whole length (Fig. 62), but the side-wall growing from it is deficient in certain regions, exposing externally the superior or proper upper turbinal coil and the inferior or lower turbinal of Anthropotomy. There is no " middle turbinal." The lateral ethmoidal region furnishes a large vertical subquadrate antorbital plate, separating the orbit from the nasal cavity. The distribution of the olfactory VII.] THE FOWL : FOURTH STAGE. 239 nerve is over a bag-like involution of the aliethmoidal roof, continuous behind with the antorbital wall. The inferior turbinal coil is united with the postero-inferior angle of the same wall, but is elevated anteriorly. Further forward, in the alinasal region (al.n.) there is a curiously sinuous wall, with an alinasal turbinal depending from the roof and curved in correspondence with the sinuosity of the wall. The nasal opening in the cartilage is a long notch passing backwards and upwards, and rounded behind, the alinasal cartilage being enfolded around it everywhere and passing into the alinasal turbinal (see Figs. 69—71, p. 252). 547. The cranial cavity is now partially invested by membrane-bones above, but they are principally lateral, leaving a large median fontanelle. The parietals (p. Fig. 60) are directly in front of the ear-capsules and the supraoccipital moieties ; they are expanded below, where they are overlapped by the squamosals, and narrower above. The squamosals (sq.) are broad squarish plates overlying the lower part of the parietals and frontals, and •reaching down to the postfrontal or sphenotie process of the alisphenoid and the top of the quadrate. The frontals (/.) are thin hollow shells of bone, covering a large part of the crown of the head ; they are bent suddenly inwards along the upper edge of the orbit, forming a considerable orbital plate in each case, but leaving a large membranous space between it and the interorbital septum (Fig. 64). They become sharp-pointed anteriorly over the ethmoidal region. 548. The nasals are flat narrow plates lying over the frontals behind, and on the aliseptal region of the nasal cartilage ; they fork anteriorly behind the alinasal regiou, sending an upper process along the nasal process of the premaxillary, and a lower process down to the maxillary [na. Fig. 64). Outside the hinder part of the nasals is a rather broad supraorbital ossification {Ich. Fig. 64), with a downward imperforate spur occupying the lachrymal region and applied to the outer edge of the antorbital plate. 240 MORPHOLOGY OF THE SKJLL. [CHAP. 549. The premaxiliaries and the rest of the bones of the upper jaw and palate, and also the quadrate and the lower jaw, are further advanced in development, becoming more solidified, and perfecting the various relations already described. A new bone has arisen in the palate, viz. the vomer (v. Fig. 6.5, p. 246), which is however but a very small style, lying under the nasal septum, behind the points of maxillopalatine processes of the maxillaries. 550. In several respects the skull at this period reminds the skilled observer of conditions found in the Tinamous. Cartilage-bones have increased in number; there are paired supraoccipitals, prootics, opisthotics, and alisphenoids. The remarkable manner in which the basis cranii is compounded will attract attention ; the fenestra- tion of the orbitonasal septum is of scarcely inferior interest. The cartilages of the nasal labyrinth are now modelled as to their main outlines. The supraorbital and the vomerine ossifications are the principal new formations in membrane. Fifth Stage : The Chirk about the second day after hatching. 551. The skull of the ripe chick is interesting in many respects ; it is profitable for comparison with the skulls of the Fish, the Reptile, and the immature Mammal. Looked at generally, it comes much closer to the nearest congeners of the typical Fowl than does that of the adult bird. The drawings of this form might, with very little modification, serve as diagrams to illustrate the structure of the skull in any Bird above the Struthionidse; compared with the Aves Prsecoces, the skull is like theirs at the same period ; but it corresponds to that of nestlings of the Aves Altrices at about the end of the first v/eek after hatching. 552. The period of a week has sufficed for very great changes in chondrous and fibrous tracts ; and now is the best time for catching the true form of many of the osseous VII.] THtSr.FOWL: FIFTH STAGE. 241 territories, although some have already lost their distinct- ness, whilst others have not yet appeared. The ectosteal tracts have set up endostosis in the cartilage lying between their inner and outer plates, so that they are being enlarged intrinsically as well as from the immediately overlying (perichondria!) fibrous tissue. Save in the instance of the early-grafted paraspheuoidal elements, the parosteal tracts or splint-bones are still altogether free from union 'vith the endoskeletal parts, whether bony or cartilaginous. The hyaline cartilage furnishing the inter- spaces and ^leadlands of the ectosteal plates has become very dense through the abundance of the cheese-like inter- cellular substance ; it is semitransparent when thick, and ■for the most part of a lilac colour. The splint-bones are still fibrous, but are beginning to become smooth through the continual ossification of aponeurotic layers. As a correlate of this exogenous growth, the first deposit of bony matter is being absorbed in many places, so as to form diploe ; this process is most advanced in the basi- temporals and squamosals. 553. The extensive occipital plane, swelling back- wards above, is largely ossified, although there are con- siderable chondrous tracts remaining. The basioccipital extends into the occipital condyle, and it is consideralily underfloored by the basitemporal plate in front (Fig. 63). The exoccipitals ossify the lovfer half of the sides of the foramen magnum. They are very irregular in shape, extending considerably into the ear-cartilage : they are perforated by the vagus and hypoglossal nerves. The supraoccipital centres have coalesced almost completely to form a large bone bounding the upper half of the foramen magnum, which is pointed above ; superiorly, the margin of the bone is curved like a fan, and abuts on the parietals. 554. Looking at the internal surface of the cranium, it will be seen that nearly all of the arch of the anterior semicircular canal (a.s.c.) lies in the supraoccipital {s.o.), being supero-posterior in position, owing to the backward- B. M. IG 242 MORPHOLOGY OF THE SKULL. [CHAP. tilting of the whole auditory capsule. Nearly the whole of the posterior canal and part of the horizontal lie in the exoccipital. .55.5. Below and in front of the supraoccipital, the prootic {pr.o.) ossifies most of the remainder of the auditory mass : but there is a small e.piotic lying attached to the prootic in the anterior part of the recess for the 2>ap ^^i;^,, ( liick two days old ; median longitudinal section of skull, the brain beiDR remoYed. a.s.c, anterior semicircular canal in supraoccipital Lone ; i.e. foramen for internal carotid artery; b.pg. basipterygoid process; p.s. presphenoiilal region; i.o.f. interorbital fenestra; i.t.h. interior turbinal ; n:n. na^al uen'e passing downwards and forwards ; .5.?!. nasal septum ; pn. prenasal cartilage; 2, optic foramen; 5, trigeminal foramen; 7, foramen for facial uerve ; 8, foramen for glossopharyngeal and vagus nerves ; inh. meckeliau cartilage, which is also indicated in outline beneath the splenial bone ; p.a.p. posterior angular process. Bones: b.o. basioccipital ; e.o. exoccipital; s.o. supraoccipital; o^. opisthotic ; pr.o. prootic ; ?j.f. basitemporal conjoined with basisphenoid ; r.h.s. rostrum; p. parietal; si;, squamosal; f. frontal; a.!<. alisphenod; rtlr. ethmoid; p.e. mesethmoid ; p.r. premaxillary ; n.px. its nasal jnoceas ; d.px. its palatal plate ; ar. articular ; i.a.p. its internal angular process; u. angular; su. surangular ; sp. splenial; d!. dentary. tlocculus. BetweerL the pi'ootic and the exoccipital is a wedge of bone, the opisthotic, visible internally (op.) : the vagus nerve (iSj enters its foramen between this and the exoccipital. On the outer aspect of the skull the opisthotic can be seen separating the fenestra ovalis from the fenestra rotunda, also arowina- round the latter. It does not reach the posterior occipital wall. The prootic has a scooped anterior crest, continued on to the ali- and basisphenoid, and helping to form the resting place for the midbrain : VII.] THE FOWL: FIFTH STAGE. 243 the crest is notched in front for the trigeminal nerve. The large backwardly-projecting plate of the prootic is pierced about its middle by the facial and auditory nerves (7). 556. Immediately in front of the basioccipital and as far as the rostrum the cranial floor is a thick mass of bone, for the coalesced basitemporals have united with' the outspread pretemporal wings, and a section shows that their diploe is continuous (Fig. 63, h.t.). The basi- temporal plate presents a pair of large convexities behind, and its anterior very contracted extremity is notched be- neath the junction of the Eustachian tubes, which run forwards in its substance, as before described (§ 541, p. 28.j). Immediately in front of this region are the two oval carti- laginous basipterygoid articular plates, looking outwards and downwards on the side of the basisphenoid, which is slightly pedunculate on either aspect to carry them. The alisphenoid {a.s.) which arose from two centres is now one' bone on each side, distinct from the basisphenoid, and retaining its fenestra; it forms a slightly bulging wall to the lower part of the back of the orbit, and is almost transverse in position: it is applied to the prootic behind, and its upper surface articulates with the large orbital plate of the frontal. 557. The rounded rostrum (r.h.s.), grooved above for the interorbital cartilage, is still perfectly distinct, reaching to the cranio-facial fenestra, where it curves up- wards in front of the base of the ethmoidal cartilage. The latter is now completely separated from the nasal septum below — the fenestra has become a large notch. The inter- orbital septum and fenestra remain the same as before, but the anterior (ethmoidal) portion of the perpendicular plate is ossified by a double ectosteal lamina (mesethmoid), not reaching to the bottom of the septum. This bone (p.e.) is thick and solid anteriorly where it bounds the cranio-facial notch, and at its upper angle; behind, it does not nearly reach the interorbital fenestra. 558. From the upper edge of the mesethmoid the cartilage curves down laterally and inwards to form the 16—2 ■Hi MORPHOLOGY OF THE SKULL. [chap. swollen upper turbinal; the antorbital wall is as before. The nasal septum has a posterior angulated margin sepa- rated by a considerable distance from the fore end of the niesethmoid ; but above, there is continuous cartilage, forming a jJermanent isthmus. The septum sinks antero- iuferiorly into the prenasal cartilage ( p.ii.), which within the last three days has become reduced to about a fifth of iis former bulk and half its length: this condition is retained till about the middle of the first winter. The nasal septum and turbinal coils are unossified. Fig. 64. Chick two days old ; external lateral view of skull. n.c. occipital condyle; p.s. prcsphenoid region; in front of it, inter- nrliit^il fenestra; p.p. antorbital plate, al.n. alinasal cartilage; a.n. external nostril; II. optic foramen; V. trigeminal foramen: art. articular .surface of lower jaw. Bones: e.o. exoccipital ; s.o. supraoccipital ; sq. squamosal; pa. parietal; />. frontal; al.s. alisphenoid ; b.s. basisphenoid ; m.eth. mes- etlmioid; Ich. lachrymal; na. nasal; pmx. ^lemaxiiiaxy ; ?hx. maxUlary ; pi. palatine; pt. pterygoid; ju. jugal; q.ju. quadrato-jugal ; St. stapes; qu. quadrate ; ang. angular ; s.ang. surangular ; d. dentary. r).19. The roof- and wall-bones of the cranium, the frontals and parietals, have grown rapidly, obliterating njuch of the great upper fontanelle. The parietals (Fig. 04, pa.) are narrow transversely- placed oblong plates, fitting accurately to the posterior margin of the frontals VII.] THE PaWL : FIFTH STAGE. 24."; (which they partially overlap), and the anterior margin uf the supraoccipital. The parietals are flanked below by a pair of bones larger than themselves, but more irregular, the squamosals (sq.) More than half the squamosal can be seen on the inner surface of the cranium, between the oblique lower edge of the parietal and the outer (binder) edge of the alisphenoid {sq. Fig. 63) ; externally it clamps the sphenotic cartilaginous process of the alisphenoid, and overarches the quadrate and the tympanic cavity. 560. The frontals (fr.) are the largest cranial bones; they are very convex above, and their extensive incurved orbital plate is bounded by a sharp semicircular supra- orbital ridge. The line formed by this ridge is continued farther down in front by the suture of the nasal with the lachrymal, and behind by the junction of the squamosal with the sphenotic process, thus bounding the very regu- lar socket for the large eyeball. A membranous region remains in the cranial floor between the edge of the orbi- tal plate of the frontal, the alisphenoid, and the inter- orbital septum, from the ethmoidal region to the common optic foramen. The frontals are approximated anteriorly to the coronal (parieto-frontal) suture, and then agaux diverge and become pointed in front, leaving the top of the mesethraoid uncovered, — a persistent condition. 561. The broad posterior edges of the nasals (im.) cover the pointed anterior extremities of the frontals : they are twisted upon themselves, and extend downwards and outwards and then forwards below the alinasal carti- lages {al. n.) ; they also send a sharp fork above the same, by the side of the nasal processes of the premaxillary. The supraorbital plate of the lachrymal [Ich.) is large and oval ; its anterior portion remains slender. 562. The premaxillaries (pmx.) have coalesced into a single strong triangular bone, which has a solid rostral portion in front of the nasal septum and equal to it in length. Its nasal processes are united where they pass between the nares, but permanently distinct behind and above, where they reach the mesethmoid. These ^46 MORPHOLOGY OF THE SKULL. [chap. processes are longer than the inferior sharp dentary margin (Fig. 65, d.px), which is one-fourth longer than the palatal processes {p.px.). Each palatal process passes under the apex of the palatine bone. The axial part of the premaxillary is grooved below, and the hinder half of the groove is filled by the small prenasal cartilage {p.n.Yig. 63). ■563. The slender maxillary (Figs. 64, 65) now rises in front into the angle between the descending crus of Fig. 65. }>tj> Chick two days old ; under view of skull, -with lower jaw removed ; the upper bones are shown in outline on either side of the interorbital septvim. oc.c. occipital condyle ; f.m. foramen magnum ; eii. anterior opening of Eustachian tubes after traversing basitemporal bone; pn. prenasal cartilage ; i)f. sphenotic process ; Uj. tympanic cavity ; 8, foramen for glossopharyngeal and vagus nerves ; 9, foramen for hypoglossal nerve. Bones: h.o. basiocoipital ; e.o. exoccipital ; s.o. supraocoipital ; b.t. basitemporal ; p.t.p. pretemporal wing of basisphenoid ; qu. quadrate ; sq. squamosal; u.s. alisphenoid ; r. vomer; pa. palatine; pq. pterygoid; d.px. dentary process of premaxUlary ; p.p.r. palatal process of pre- maxillary ; 7)1. maxillary ; 7?ia!.p. maxillopalatine plate of maxillary ; ,/. jugal ; q.j. quadiatojugal ; I. lachrymal ; /. frontal. the nasal bone and the hinder edge of the dentary process of the premaxillary. The maxillopalatine plates {mx.p. VII.] THE FOWL : FIFTH STAGE. 247 Fig. 65) are broader and reach nearly to the mid line, being separated partly by the nasal septum and partly by the small vomer, which is rounded in front, and split for a short distance behind. The forks of the vomer (v.) articu- late with the inner and anterior points of the inner plates of the palatine bones, which lie side by side mesially, nearly concealing the rostrum. Behind these plates the palatine bones {pa. Fig. 65) are thickish angular rods, both together having an elongated lyriform shape. The hinder extremities are carved slightly outwards for arti- culation with the pterygoids. The latter (pg. Fig. Q6) are short stout mallet-shaped bones, the handles being- turned outwards and a little backwards to articulate by a cup with a little ball on the quadrate (§ 536), and also sending up above this a small .epipterygoid plate. The head of the mallet is notched in front to receive the rounded posterior end of the palatine. The inner face of the head has a distinct oval plate of cartilage upon it, exactly corresponding to and articulating with the basi- pterygoid plate on the basisphenoid. 564. The quadrate (qu. Figs. 64, 65) has the apex of its orbital spur unossified; a similar condition is persistent in the Chelonia, The otic process has two cartilaginous surfaces close together; a rounded upper and outer head fitting in a cupped space on the squamosal, and a smaller One gliding on a cartilaginous facet of the prootic. Tliere is a slight projection on the front of the otic process, but there is no postero-internal head as in most Birds, the relation to the tympanic wing of the exoccipital being- lost (§ 521). The shaft of the quadrate is short, and bears a rounded condyle, almost transverse, and divided into a larger outer and a lesser inner portion. The quad- rate is connected with the maxillary by the intervention of slender jugal and quadrato-jngal bones (j., q.j.), the latter being bent inwards to clasp the side of the quad- rate above the condyle. The base and shaft of the stapes are ossified, and its outer end is a triradiate cartilage. The tympanic cavity will be described in the adult. 248 MORPHOLOGY OF THE SKULL. [CHAP. 565. A new bone has appeared in the mandible — a rudiment of the articular {art. Figs. 63, 64); it projects inwards from the condylar region as an internal angular process, tipped with cartilage (t.a.j;. Fig. 63). The posterior angular process (p.a.p.), and the shaft of the meckelian cartilage, which is three-fourths of the length of the jaw, are unossified. The dentaries of opposite sides have coa- lesced; they lie outside the cartilage and partly embrace it; their lengtTh is three-fourths of that of the jaw, and they are forked behind. (Jn the inner face of the cartilage is a long-oblong splenial (sp.), leaving both the point and the hinder part of the cartilage bare. The angular below and the surangular above the posterior part of the carti- lage leave its inner face considerably exposed, bnt they cover it externally, and meet. 566. Ossification has appeared in the two small cornua (minora) of the hyoid, and the- ossification is coo- tinuous across the lingual cartilage. The median basal cartilage has two styliform bones, one in front of and the other behind the " thyrohyals" (=lst branchials). The only ossification in the latter cartilages in the last stage was a lower one on either side ; there is now a second upper bone, but the curved tip is cartilaginous, as are the apices of all these osseous tracts. 567. The types suggested by a view of the chick's skull at this stage are the Hemipodiine, the Pterocline, and the Columbine outliers of the Gallinaceous group : subsequent development remove,-^ the Fowl greatly from its lower and more generalized relatives on one hand, and from its higher and more specialized congeners on the r)ther. Every detail which has just been given will be fonnd pregnant with meaning by the student who has an affinity for this subject. The coalescence of the various bones in the basisphenoidal region, the extension of the exoccipitals and the supraoccipitals into the ear-capsides, and the appearance of the epiotics and mesethmoid, are the chief events that have affected the brain-case. The VII.J THE FOWL : SIXTH STAGE. 249 prenasal cartilage is relatively much reduced. The pre- maxillaries and dentaries respectively now form but one bone ; and bony deposit has arisen in the stapes, the articular region of the mandible, and the small re- presentatives of the hyoid and branchial arches. Sixth Stage: Young Fowls up to nine months old. 568. In a chick three weeks old there are two features to be noted, (1) a distinct osseous centre in the sphenotic process (§ 542), subsequently uniting with the alisphenoid, and (2) the appearance of a small opisthotic plate on the outer occipital plane, in a notch of the supero- exterual part of the exoccipital, soon becoming united with it. 569. In a chicken of two months old the structures have increased in massiveness, and the roofing bones of the skull are connected by delicately-toothed sutures; only narrow synch ondrosial tracts are left in the cranium. An inner view of the occipito-otic region shows the three bones of the periotic capsule, as well as the alisphenoid, still remaining distinct; nor has the basisphenoid yet coalesced with the basioccipital or with the alisphenoid. 570. Above the optic foramen, wedged in between the alisphenoid and the interorbital septum, a four-sided bone has arisen in membrane on each side, and there is a smaller pair in front of and above the larger, helping to fill in the fenestrse left unoccupied by the orbital plates of the frontals. These are the anterior and posterior orbito- sphenoids (o.s. Fig. 66). The anterior half of the inter- orbital septum is ossified, the mesethmoid encroaching on the anterior margin of the interorbital fenestra. 571. In chickens from three to four months old all the splint bones of the skull, with the ex'ception of the early engrafted parasphenoidal elements, are separable by maceration ; only the symmetrical premaxillaries and dentaries have united at the mid line, the former very early, the latter soon after hatching. Looking at the 250 MOEPHOLOGY OF THE SKULL. [chap. skull below and behind, externally, the large supra- occipital still shows sutures with the exoccipitals, and they ■with the basioccipital — the latter being still distinct from the basisphenoid and its basitemporal plate. The posterior basicranial fontanelle is indicated by a deep median longi- tudinal chink in the posterior part of the basisphenoid. The complex basisphenoid is distinct from the alisphenoids, prootics, and basioccipital; but the small epiotic is uniting with the prootic and supraoccipital, and the opisthotic with the prootic and exoccipital. The sphenotic is con- fluent with the alisphenoid. The composition of the occi- pital condyle of three elements is still manifest. 572. In young fowls of the fir.st winter, from seven to nine months old, a large number of embryonic characters persist (Fig. 6(i). The skull has become very thick and solid, with copious coarse diploe. The cranium proper is Fig. 66. pay Fowl of first winter ; median longitudinal section of skull. ft, 0. basioccipital perfectly continuous with liasitemporal, basisphenoid, and rostrum; ji . prootic; i.o. supraoccipital; ii.j. piiuitary fossa; J), parietal; sq^. squamosal; a.s. alisphenoid; o.^. orbitosphenoid ; /.frontal; m.e. mesethmoid ; t'. vomer ; J3x. premaxillary ; A. dentary ; sp spleiiiul ; s.a, surangular; a. angular; ar. articular. Cartilage is seen in the interorbital septum behind the mesethmoid ; in the nasal septum in front of that bone, sending forward the small prenasal spur ; apd in the lower jaw slightly between the angular and surangular. not half the length of the elongated skull, and is extremely narrow where it ends above the interorbital septum. The bones of the occipital ring, the periotic mass, and the basi- VII.] THE FOWL: SIXTH STAGE. 251 sphenoid are entirely confluent, as also are the anterior parts of the premaxillaries and dentaries. All the other lines of suture are still traceable. 573. In the more advanced specimens a small patch of hone can be seen between the orbitosphenoids in the cartilage of the septum, between the interorbital fenestra and the optic foramen; this is the first appearance of the presphermid. Cartilage remains beneath this bone, in front of the interorbital fenestra, down to a vertical suture which divides the anterior part of the basisphenoid from the postero-inferior part of the mesethmoid; this connec- tion is above the original rostrum, and is produced by extension of ossification. The mesethmoid {m.e.) has ex- panded superiorly into the aliethmoidal cartilage on either side, forming a flat plate, which can be seen between the frontals and nasals and behind the nasal processes of the premaxillaries (see Fig. 70). The mesethmoid ossifies the posterior part of the aliethmoidal roof, and extends behind into the spike mentioned in § 544 : below this is a groove for the olfactory nerve, passing beneath the alar expansion on either side, in the angle between it and the vertical plate. 574. The rest of the nasal labyrinth is entirely un- ossified. The hinder part of the roof is flattened above, and its lateral edges grow inwards to form the upper tur- binals {u.ib. Figs. 69, 71), which near the middle line are again bent outwards, forming a bullate pouch opening outwardly; its edge is confluent behind with the ant- orbital wall. In this region the nasal septum is deficient by reason of the craniofacial notch, except for a slight carti- laginous ridge above, separating the nasal branches of the trigeminal nerve {n.n) as they pass forwards to reach the septum. 575. No middle turbinal coil is developed on the face of the antorbital plate below the upper turbinal. In front of and below the latter, the inferior turbinal {i. tb. Figs. 68, 71) forms a not very uniform scroll, coiled twice, the blind face of the coil being inwards, and its long axis 252 MORPHOLOGY OF THE SKULL. Pigs. 67—71. [chap. FJR. 71. eth p.mx Views of uasal structures of Fowl of first winter. Fig. 67. Transverse section through most expanded part of alinasal region; [n.th. Fig. 71). Fig. 68. Transverse section between aliethmoidal and aliseptal regions (through \.tb. Fig. 71). Fig. 69. Transverse section in aliethmoidal region, with inferior turbinal cut away, in region of cranio-facial notch of septum (through u.lh. Fig. 71). Fig. 70. Transverse section tlirough most solid part of ethmoid, showing antorbital plate [fji. Fig. 71). Fig. 71. View of nasal labyrintli from within, the septum being removed ; less magnified than the preceding figures. s.«. nasal septum; al.n. alinasal cartilage; n.th, alinasal turbinal; 71. to. nasal wall; i. (J. inferior turbinal; h.(6. upper turbinal; al.e. ah- ethmoid cartilage ; a.or. (Fig. 70), jip. (Fig. 71), antorbital plate; n.ij. nasal vii.J THE fowl: sixth stage. 258 gland; 1, olfactory nerve ; n.n. nasal nerve (from trigeminal), appearing as displaced from relation to the septum in Fig. 71 ; m.n. middle narial passage. Bones : n.px. nasal process of premaxillary ; d.px. dentary process of same ; n. nasal (in Fig. 68 the lower fork of the nasal is cut through and seen separate from the upper) ; /. frontal ; ;. lachrymal ; p.e. mesethmoid ; eih. ethmoid; v. vomer; pa. palatine; r. rostrum; vix. maxillary; p.>nx. palatal plate of maxillary. directed obliquely. It grows out of the nasal roof and outer wall in front of the aliethmoidal region. Behind and below this, the nasal wall becomes imperfect for a .space ; and the hinder termination of the inferior turbinal is confluent with the lower outer angle of the antorbital plate. 576. Another turbinal occupies the fore part of the nasal cavity ; it is the alinasal turbinal {n. th. I'igs. 67, 71), whose blind face is also inwards ; at most it only forms half a coil. It arises from the outer edge of the nasal roof, but outside it the alinasal cartilage {al. n. Fig. 67) forms a complete lateral wall, continuous with the nasal floor except where it is slit obliquely for the anterior nasal opening. Posteriorly the alinasal turbinal, where it lies below the anterior part of the inferior turbinal, grows inwards, and joins a remarkable wing of cartilage con- tinuous with the nasal septum, and pierced on either side by the nasal nerve Tthis is a development from the original bridge over this nerve, § 545). Anteriorly, the upper part of the alinasal cartilage terminates as a small ear-like process on either side, and between these the prenasal cartilage, which has not lessened since the time of hatch- ing, lies in a groove of the premaxillary. The whole alinasal tract lies in an ovoid sj)ace forD:ied between the processes of the premaxillary, and also partly in the notch between the processes of the nasals. In the Fowl the aliseptal region is very small in comparison with the aliethmoidal behind and the alinasal in front of it. 577. The bones of the palate have merely grown larger without altering their relations ; those of the lower jaw are beginning to coalesce, and the posterior as well as the internal angular process is ossified by the articular. 254 MORPHOLOGY OF THE SKULL. [chap. The Skull in Foiuls several years old. 578. By the time the fowl is a year old most of the cranial sutures are completely closed ; aud then, year by year, the skull becomes more and more dense, the peri- osteal layers filling in all spaces in the bony walls except those needed for the, transmission of vessels and nerves. Thus in an aged bird (Figs. 72, 73) the occipital, otic, pos- terior and anterior sphenoidal, and ethmoidal regions have become one continuous bone, with the old landmarks all removed ; and scarcely a sign is left of the once highly complex conditions of these parts. This state of things is quite normal for a Bird ; and although carried to a very high degree in the Fowl, j^et in some birds the obliteration of all signs of the once composite condition of the skull is still more j^erlect. Fig. 72. -^jp Fowl several years old ; side Tiew of Bkull. Bones : sq. squamosal ; p. parietal ; pf. sphenotic process ; f. frontal ; vie. mesethinoid ; }. lachrymal; px. premaxillary ; mx. maxillary; V. vomer ; j. jngal ; q.j. quadrato-jugal ; between ti. and j. palatine ; j){. pteyrgoid ; g. quadrate ; ar. articular; a. angular; d. dentary. ;")7!K In the side view of the skull of an old male (Fig. 72) it is seen that the face has gained considerably upon the cranium as compared with the condition found in the newly-hatched chick ; although the maxillary bones imx) are relatively smaller than ever, being minute styles with a very small maxillopalatine ingrowth. VII.] THE ADULT FOWL. 255 680. In an upper view the anterior part of the suture between the frontals is visible : the nasals have coalesced behind with the frontals, but are distinct from the forked nasal process of the premaxillary. Between these bones (frontals, nasals, and premaxillary) a considerable penta- gonal tract of the top of the ethmoid is apparent. The following bones are permanently separate : lachrymals, raaxillaries, jugals, quadrato-jugals, quadrates, pterygoids, palatines, vomer. 581. The whole interorbital plate (m. e.) has become one bone. In some old birds a trace of the interorbital fenestra can be seen ; in others its outline is distinguish- able, but it is filled up with periosteal bone. The ali- sphenoidal fenestra is also occupied by bone, and the alisphenoid has coalesced with the orbital plate of the frontal. Above this, the two pairs of orbitosphenoids, the presphenoid, and the anterior part of the orbital plates of the frontals, have coalesced together ; and the fenestra which formerly existed on either side, between the edge of the orbital plate and the interorbital septum, is oblite- rated excepting for the antero-superior chink by which the olfactory nerve runs forwards on either side of the top of the mesethmoid. 582. In the cartilaginous isthmus above the cranio- facial notch (§ 557) two small osseous centres have appeared, one behind the other; and there is a small nodule on either side between them. The rest of the nasal labyrinth is unossified save that in some cases a little bony matter passes from the mesethmoid to the antorbital plate, which otherwise hangs down from the ethmoidal roof free from the median parts. 583. An aponeurotic tract extending forwards from the middle of the outer surface of the squamosal, has become ossified, and meets and often coalesces with the sphenotic process, forming a temporal bridge. The occi- pital plane is much more vertical than in former stages, and the auditory capsule has recovered from its great obliquity, and is more erect than in the second stage. A 256 MORPHOLOGY OF THE SKULL. [chap. partial bony septum passes between the halves of the fore- brain from the line of coalescence of the frontals, and there is a transverse septum corresponding to the coronal or parieto-froutal suture, to some extent sepa.rating the fore from the midbrain. A sharp crest passes from the pro- jecting ridge of theprootic (in front of the meaf » intern us) inwards and forwards to the sides of the jioste ir clinoid wall of the basispheooid. The arch of the an ior semi- circular canal, which is now nearly vertical, is connected by a ridge of bone with the fronto-parietal sept" in. Kg. 73. Fowl several years old ; under view of skull with lower jaw removed. no. Bupraoccipital; b.o. basioecipital ; eii. orifice of Eustachian tubes ; 7)/. spheuotic procuKs; pt. pterygoid; p. palatine; f. vomer ; ^)x. pre- maxillary ; vix. maxillary ; j. jngal ; q.j. quadrato-jugal ; q. quadrate. 584. The basisphenoidal region, including the basi- temporal plate, is of great thickness, and the thickening of the cranial floor reaches back nearly to the occipital condyle. The pituitary fossa is a deep well, elegantly rounded, turning backwards below, beneath the posterior clinoid wall ; and it is perforated by the internal carotid arteries. One of these arteries enters the basis cranii outside VII.J THE ADULT FOWL. 25^ and in front of the exit of each vagus nerve, in a notch between the temporal wing of the exoccipital and the outer and posterior angle of the basitemporal, and these two bones coalesce together to form a bridge outside the artery. When the outer table of the bone, together with a portion of '■he diploe, is removed in the auditory region, the .vnok nembranous labyrinth is seen to be enclosed in a bonj ^ise exactly corresponding to the membranous structure?^ ivithin, the cochleae appearing in the floor of the sku'i'i at a moderate distance from each other, like little boD 585. Besides the anterior tympanic recess which reaches forwards above the narrow Eustachian tubes (§ 541), there is at the summit of the tympanic cavity au upper ti^mpanic recess', running into the diploe of the occiput on each side. The base of the stapes and the two fenestrap (ovalis and rotunda) lie at the bottom of a , 'I recess in the tympanic cavity, ->o. The tympanic membrane is attached to a delicate f bone some distance within the margin of the bony tyrL.;j^nic hollow produced by the tympanic wing of the exoccipital : this margin is extended outwards by a thick fold of skin to form the proper meatus auditorius externus. The fine rim of attachment for the membrane can be traced just outside the three principal recesses of the tj^mpanum, the anterior tympanic, the posterior tympanic, and the fenestral recess. The bony regions which furnish the attachment are the occipito-otic and basitemporal. The membrane is fixed in front to a strong aponeurosis passing down from the squamosal to the extremity of the pretemporal wing of the basisphenoid (§ 541), behind the head of the quadrate. This aponeurosis becomes ossified in old birds, thus excluding the quadrate from direct relation with the tympanic cavity in this type. 1 In' the Crocodile these recesses meet above the occipital arch hetween the outer and inner tables of bone. B. M. 17 258 MORPHOLOGY OF THE SKULL. [CHAP. In birda wliicli have a second head on the otic process of the quadrate, the aponeurotic band here spoken of is attached to the quadrate and the hinder head is included in the cavity ; the band itself frequently becomes the seat of tympanic ossifi- cations, of which the number varies from one to seven in different types. 587. The columella (Fig. 7-1) is a vory delicate styloid bone, having an ovoid stapedial plate in the ^nestra ovalis. Externally it is cartilaginous, where it ab'is against the middle of the tympanic membrane and distends it. The stapedial plate [st.) and the shaft of the columella or mediostapedial {m.st.) form a continuous bone. Distally there are three cartilaginous rays. Two outer ones, the Fig. 74. Adu'.t FoTv', side view of columella auris. St. stapedial (bony) plate ; m.st. mediostapedial (bone) ; the remaining parts ai-e cartilaginous; «.sf . extrastapedial ; s.st. suprastapedial ;/. fe- nestra between these bars ; i.st. iuterstapedial ; st.h. stylohyal. supra- and infrastapedial {s.st, i.st), pass at right angles from the shaft. The upper is bifurcate, and is attached by a membranous band to the po.sterior part of the tym- panic roof. The third or median process (extrastapedial, e.st) is broader than the rest, spatulate, and decurved. Extending from its outer edge to the extremity of the suprastapedial is a bar of cartilage, a fenestra (/.) being left m the angle between the extra- and suprastapedial This bar lies against the middle and anterior part of the tympanic membrane towards the quadrate. The small infrastapedial is directed downwards and a little forwards, VII.J THE ADULT FOWL. 259 and is sometimes expanded telow. This lower region was distinct in an early stage (§ 523), but has become confluent with the infrastapedial. It represents the top of the stylohyal {st.h.). 588. The mandible varies considerably in different individuals ; in some specimens there is scarcely a trace of the great fenestra seen on either side in the Tetraonidap between the forks of the dentary and the hinder elements. There is usually some remnant of the suture between the posterior wedge of the dentary and the angular and sur- angular ; but all the other sutures close. 589. The cornua (minora) of the hyoid have coalesced to form a glossohyal : but the bone is tipped with cartilage anteriorly, and a chink -like foramen remains in the median part of the bone. This elegant sagittiform tongue-piece Kg. 75. Fowl several years old; liyobranohial apparatus from above. G.h. ceratohyal ; 6r. 1, first basibranchial ; b.br. second basibranohial ; c.h: ceratobranohial ; e.br. epibranehial. (Fig. 75, c. h.) articulates by a synovial joint with the first basibranchial (br. 1), which itself has a similar articulation with the long slender proximally-ossified second basi- branchial (6. br.). The branchial arch articulates on either side by a synovial joint with the posterior end of the first basibranchial, but the proximal (ventral) piece or cerato- branchial (c.br.) is united to the distal or epibranehial (e. br.) by a mass of fibrous tissue : the ceratobranchial is cartilaginous above, and the epibranehial at both ends. 590. Summary. A striking feature in this history is the acceleration of the determining events ; the charac- ters which according to the theory of evolution are due 17—2 260 MORPHOLOGY OF THE SKULL. [CHAP. to very far-reaching inheritance, are assumed with great rapidity, the skull being definitely Sauropsidan at the end of the first week of incubation. Those details of structure, however, by which the Fowl is differentiated from other birds, are much later and slower in appearing, several of them being only gradually acquired after the first year of life. There is no fact in the early development of the Fowl's skull which does not illustrate or receive illus- tration from the previous histories. This book might be indefinitely enlarged if these correspondences were all pointed out: but the student who patiently meditates will perceive them for himself in due time. 591. Commencing with the simplest stage, we find the axial parts at the base of the skull developed con- tinuously, as if the disjunction found in lower types were overpassed and their later condition of union at once attained. Conversely, distinct elements formed out of simple bars in other forms are here separate at first, without the necessity of any process of segmentation. That which was an event of development before has now become a- primary condition. Again, the first appearance of the liyoid and branchial arches in the Fowl corresponds with the condition attained after metamorphosis in the Frog. 592. It is to be remarked bow small a proportion of the brain-case itself is preformed in cartilage. The same elements are present as in previous cases, but their pro- portionate size in the structure is much less, and their total share in the completed skull is really small. Yet unity of plan is manifest in the occurrence of the same bony centres in cartilage, even if they form but insig- nificant nodules. Together with these facts in relation to the chondrified parts, we notice the increased size of the membrane-bones, the perfection of their adaptation in the adult organism, and their intricate interlocking with cartilage-bones. If the parasphenoidal elements in the Fowl are compared with the parasphenoid as previously VII.] THE FOWL: SUMMARY. 261 described, their essential likeness as well as their special diversities will be better understood. 593. In the first stage we see an elementary skull, already advanced to a condition reached in lower forms by coalescence and segmentation. In the second stage the principal outlines of the skull are visible: the occipital ring and the orbitonasal septum are in existence. There is a cleft palate like that of Struthious birds, and the angular processes of the mandibular cartilage indicate the type to which the Fowl belongs. In the next period numerous membrane-bones have appeared, and a few centres in cartilage ; the nasal region is highly developed. In the fourth stage, still before hatching, the skull has attained many of its bones, the vomer being a notable addition ; and the bony cranial floor has advanced in its remarkable process of composition. It is not till after hatching that some notable osseous centres appear, and the coalescence of bones becomes very marked : this goes on gradually until adult age, when most of the sutures are obliterated. The proportionate length of the precranial part of the skull increases, the bones become thick and solid, and mostly smooth externally. Much of the nasal labyrinth remains cartilaginous throughout life ; some cartilage remains in the columella, the mandible, the palatopterygoid tracts, and the hyobranchial apparatus. Although bony deposit appears in the palatine and ptery- goid tissues before true cartilage is formed in them, yet wherever in these tracts the ossification does not extend, cartilage is subsequently found. 262 MORPHOLOGY OF THE SKULL. [CHAP. APPENDIX ON THE SKULLS OP BIRDS\ 594. Notwithstanding the great varieties of outward form to be seen in the skulls of Birds, there is extremely little modification in the general morphological relations of parts. The palatal structures present the largest amount of change, yet some of the features of importance in that respect are only to be discerned after very careful study. In Struthious birds there is a much greater per- sistence of sutures than in higher forms; the parietals are notably large. The rostrum is of greater size proportionally than in any other group of Birds. The cranio-facial notch is absent. The body of the basisphenoid intervenes (below) between the openings of the Eustachian tubes ; in nearly all other birds the basitemporals meet below these tubes so that they open together close to the middle line. Only in some of the Struthionidae are the Eustachian tubes at all enclosed in bone, and this occurs by the projection of a ridge of the basitempnral to meet another from the " pretympanic wing.'' The mesethmoid ossifies the whole interorbital and nasal septum. The otic process of the quadrate is not divided into two heads ; its anterior fork remains tipped with cartilage in the adult. In this last particular, as well as in the structure of the palate, the genus Tinamus agrees with all Struthious birds, and this compound group may be spoken of as dromceognathous {Droniwus, the Emeu). The vomer is very broad, and unites in front with the maxillopalatine plates, while behind it receives the posterior extremities of the l^alatines and the anterior ends of the pterygoids, which are thus prevented from entering into any extensive articulation with the rostrum. There are strong basipterygoid (bony) processes from the basisphenoid, and these articulate with the pterygoids near their DUter or hinder ends. The basitemporals are but feebly developed. 595. All other types have the posterior ends of the palatines and the anterior ends of the pterygoids articulating directly with the rostrum. The Fowl belongs to the schizognathous type (having a cleft palate). The vomer tapers to a point anteriorly; while posteriorly it embraces the rostrum. The maxillopalatine plates 1 See Huxley " On the Classification of Birds," Proc. Zool. Soc. 1867, and "On the Alectoromorphse," ih. 1868; also Parker, Art. "Birds," Encycl. Brit., Vol. iii. VII.] THE SKULLS OF BIRDS. 263 leave a broader or narrower fissure in the palate between them- selves and the vomer. The restricted Gallinaceous group (or Alecto- romorphse) excludes the Pigeons and Tinanioua. The other principal schizognathous types are the Plovers, the Cranes and Rails, the Gulls, Pigeons, and Auks, and the Penguins. In Pigeons and Sand- grouse the vomer is absent. Some, as the Albatross and the Gull, possess an os uncinatum or antorbital bone ; others, as the Hum- ming-Birds and Kagu, have septo-maxillaries. All except the Alec- toromorphse have mesopterygoids. 596. A group named desmngnathous is distinguished by the following characters : vomer mostly small or absent ; maxillopalatine plates united across the middle line, either directly or by the inter- mediation of ossifications*)n the nasal septum. Falcons and Geese are examples of what may be termed direct desmognathisni ; the maxillopalatine plates meet below at the middle line as in the mam- mal. In the Falcon the nasal septum is anchylosed to this hard palate; in the Goose it remains free. Indii-ect desmognathism is exemplified in Eagles, Vultures, and Owls ; the maxillopalatine plates are anchylosed to the nasal septum, but are separated from one another by a chink. In Dicholophus cristatus the maxillopalatine plates are united by harmony suture and not by coalescence. In Megalcema asiatica they are closely articulated with, and separated by, a median ossification beneath the nasal septum, the "median septo-maxillary." The most advanced condition may be called double desmognathism, where the palatines as well as the maxillo- palatine plates coalesce to form a continuous palate ; this is seen in Podargiis, and to a less extent in ihe large Hornbills. In Ducks and Swans a pair of ossicles is found between the palatines, stretching towards tlie maxillopalatine plate. In several Ardeicke an additional bone, the postmaxillary, is formed behind the angle of the maxillary (this occurs also in the Emeu). In various desmo- gnathous forms the palatines unite for a considerable distance behind the posterior nares, and send down a vertical crest :tt their junction. This is largest in the Pelican, where all the parts in front of the very mobile cranio-facial hinge are anchylosed into one mass, and the nasal labyrinth is in its most aborted state. In Pheeuicopteni-i the median palatine keel is exquisitely thin in front, but where the palatines extend beneath the rostrum, a separate lamella is sent down from each bone and the two are bound together by fibrous 264 MORPHOLOGY OF THE SKULL. [CHAP. tissue, and between them the vomer is wedged : the ethmopalatine spurs are enormously long. In Sula alba the basitemporals are relatively as small as in Dromrvus; the small Eustachian tubes, un- covered by bone, open at a little distance apart, in a wide shallow fossa where the three parasphenoidal elements meet. There is a postraaxillary ; the cranio-facial hinge almost rivals that of the Parrot and Toucan; the columella auris is very long and bent, with a long attenuated cartilaginous infrastapedial process, terminated by a bony fusiform stylohyal. The hinge for the mandible is very far back, as also in the Cormorant. 597. In the (desmognathous) Aetomorphce, in addition to the hooking of the beak, there is another character; the maxillopalatine plates are generally united with an ossiftcatiou beneath the nasal septum, or " median septo-maxillary." The vomer is azygous ; the palatines often have a mediopalatine where they unite, or a pair of niesopterygoids. In some forms the supra-orhital process of the lachrymal is very large; in others (Hawks, &c.) there is a distinct supra-orbital at its extremity. In the Sparrow-Hawk distinct pterotic and sphenotic centres are developed ; and the orbitosphenoids are preceded by cartilage. 598. The desmognathous Parrots are very imiform, having the most complete cranio-facinl cleft, with a perfect hinge-joint between the frontal and nasal regions. There is no vomer; the palatines are vertically elongated posteriorly, while anteriorly they are hori- zontally flattened, and movably united with the rostrum. The lachrymal and postorbital (or sphenotic) bend towards one another, and unite by the intervention of a large os uiicinatum or antorbital. In some also the temporal fossa is bridged over by the union of the zygomatic process of the squamosal with the os uncinatum. The nasal septum is a thick wall of I one ; an annular ossicle is found in the ahnasal CAxiWugeoi Melopsitlacus undiilatiis ; in Palxornis torquata, this part is largely ossified and anchylosed to the upper jaw, and the alinasal turbiual is partly calcified. The Hornbills and Toucans have a median ; epto-maxillary in front of the azygous vomer. In Poclargus, when the lower palatine floor is cut away, there are to be seen three small. ossicles, the vomer (anterior), and two medio-pala- tines. In Kingfishers and Hoopoes there is no vomer. 693. The cegithognathons type (including the Passerines) is VII.] THE SKULLS OF BIRDS. 265 distinguished by the union of the vomers with the alinasal wall and turbinal ; and the embryo at least has a pair of " upper labial " car- tilages, which the vomers ossify either partially or entirely. In most cases there are septo-maxillaries, attached to the angles of the vomers. In the fledgling Rook the basitemporal plate is an almost transverse band of bone ; the rostrum bears no basipterygoid pro- cesses ; the cranio-facial notch is nearly complete, but the nasals and nasal processes of the premaxillaries are set into the frontals as thin spurs. The palatines develope cartilage at their hinder (outer) angle, and this part ossifies separately and late, like a transpalatine. The mesopterygoid spur of the pterygoid coalesces with the palatine. The maxillopalatine processes are hooked and flattened. There is a chain of seven small tympanic bones, the principal of which be- comes a perfect ring in the Crow, and surrounds the siphonium. In many iEgithognathse the lachrymal cannot be seen at any stage ; in many that have it, it soon anchyloses either with the nasal in front or with the ethmoid behind. 600. There is incomplete oegithognathism in the Turnix group ; the large labials are imperfectly ossified by the vomers, and these bones are only strongly attached to the nasal labyrinth by fibrous tissue. The following forms of complete segithognathism may be distinguished : (1) in Pachyrhamphus, Pipra, &c., the labials are often only imperfectly ossified by the vomers, and these centres are distinct from those ossifying the alinasal cartilages ; but the union of the two tracts is complete ; (2) in a great number of the Passerines, and in the Swifts, the labials are small and completely ossified by the vomers, but the bony deposit extends largely into the alinasal wall and turbinals ; (3) a compound condition, namely, desmognathism in a perfectly segithognathous type, the maxillopalatiues uniting with a highly ossified alinasal wall and nasal septum {Gymnorhina, Para- discea, Artamus); the transpalatine process is a long spike. > 601. The Saurognathous group (Woodpeckers and "Wrynecks) have their palatal structures arrested at a most simple and La- certian stage, whilst in other respects they are metamorphosed and specialised beyond any other kind of birds. The principal characteristics are as follows : the retention and ossification of the trabecular comua ; the great number and distinctness of the vo- merine series of bones (vomers, septo-maxillaries, median septo- 2G6 MORPHOLOGY OF THE SKULL. [CHAP. VII. maxillary); absence of a distinct mesopterygoid ; presence of a mediopalatine ; no distinct transpalatine ; abortive maxillopalatine plates ; a distinct palatomaxillary on one side only. The nasal labyrinth is unusually simple; the inferior turbinal, which has three coils in Rhea and Tinamus, and two in most birds, in Gecinus is two-winged, and in Yunx makes less than a single turn. In Gecinus, which is yery specialised, the columella has two suprastapedial spurs and two infrastapedial bands, which have united with the stylohyal. The small ceratohyals in the Woodpeckers early coalesce into one arrowhead-shaped bone, and behind it is a very long highly-ossified and elastic basihyal. Joined to this behind is a pair of cerato- branchials, half its length ; the epibranchials are four times the length of the ceratobranchials, and, passing first down the sides of the upper part of the neck, they curve gently upwards and forwards, lying in a furrow on the top of the skull ; bending slightly from the middle line, they end on the nasal roof. CHAPTER VIII. THE SKULL OF THE PIG. First Stage: Embryos 7^ to 8 lines long. 602. The earliest embryos of the Pig in which the foundations of the skull are perceptible present strong resemblances in the cephalic region to those of lower vertebrates; nevertheless a few special features of mor- phological elevation are manifest. In a front view the second cerebral vesicle is seen as the uppermost of the neural bulbs, although it does not advance quite to the front ; thus the forehead is sloping. The distinct cerebral hemispheres occupy the middle of the lateral aspect of the head, and are together rather broader than the second vesicle. Immediately beneath the hemispheres are the nasal sacs, reniform in shape ; they are bounded inferiorly by the lateral angles of the nasofrontal process in which the trabecular cornua are contained. The eyeballs, not very prominent, are seen on either side of the front view, above and behind the nasal sacs. Between the latter is a nasofrontal tract of modera.te width, forming with its horns the anterior and upper margin of the mouth. This opening is at present quite inferior and transverse, bounded laterally by maxillopalatine processes which grow forwards and downwards from the continuous super- ficial tissues of the sides of the head, and posteriorly by the mandibular arch, which constitutes a transverse bar below, and has its moieties convex outwardly at the sides. Behind the mouth the skin of the broadish throat is con- tinuous and unmarked ventrally. 268 MORPHOLOGY OF THE SKULL. [chap. 603. In a side view (Fig. 76), the great proportionate size of the second cerebral ve.sicle (C2), which occupies the whole crown of the head, is particularly marked. The third vesicle (C3) is much smaller, and on a lower level. C/..9 Embryo Pig, two-thirds of an incli long ; side view of liead and neck. G 1, 2, 3, cerebral vesicles; e.n. external nasal opening; e, eyeball; an. auditory mass ; tg. tongue-rudiment; mn. mandibular arcb; pa.mx. maxillopalatine process; cl. 1, 2, 3, 4, visceral clefts; hy. byoid arch, covered by the end of the fore limb. There is a considerable depression between this vesicle and the spinal region, the cervical part of which is bent at a right angle with the dorsal, giving the embryo a strange humpbaclied appearance. The olfactory sacs are seen at the antero-inferior extremity of the head ; the eyeballs (e) are above and behind them in the angle between the first and second cerebral vesicles; and the pyriform ear-sacs {au), of approximately the same size as the other sense capsules, are below the hinder half of the third vesicle. The skin over them, as well as over the adjacent cerebral vesicle, is very thin as yet. 604. The side wall of the mouth and throat presents a continuous skin, interrupted below by four successive vertical clefts. The maxillopalatine process (pa.mx.) is perfectly continuous with the wall of the mouth and throat above, and projects downwards and forwards in the subocular "^"I"-] THK PIG : FIRST STAGE. 2G9 region, a cleft remaining between it and the eyeball. The hinder visceral clefts are overlapped by the palmate rudi- ment of the forwardly-directed fore limb. 605. In a median longitudinal and vertical section the brain vesicles are seen to be hollow, having only a film of soft neural substance lining the membranous cra- nium. The mesocephalic flexure is even more intense than in other types. The notochord, underlying the hinder vesicle, bends suddenly upwards so as to be situated in front of it. It does not extend so far as the summit of the third vesicle or the base of the second, but ends in a free blunted point, just opposite to that superior and posterior infolding of the membranous cranium which partially severs the second and third vesicles. Above the apex of the notochord is a mass of gelatinous stroma (the "middle trabecula") forming the hilus of the long kidney- shaped second vesicle. In front of the notochord at its upward bend is the pituitary body, and immediately an- terior to that, the small first cerebral vesicle giving off its large hemispheres. The internal carotid artery is distinct on either side of the pituitary body. 606. Parachordal hyaline cartilages (Fig. 77, pa. ch) already exist, being rather below the level of the noto- chord (nc.) on either side ; they follow its curve, stopping somewhat short of its apex. The ear-capsules lie external to the parachordal cartilages, with walls commencing to chondrify. They are tuberous bodies, having a nearly straight inner margin, and a sublobular outer edge, and are broadest behind. The aqueductus vestibuli is left as an opening in the cartilage on the upper and inner edge : the facial nerve enters the capsule a little behind this, its passage being the aqueductus Fallopii. The semi- circular canals are but just beginning to bud out from the postero-superior region of the ear-cavity, and the cochlea is indicated by a styliform anterior projection of the same cavity. There is an external deficiency in the capsule, filled with a plug of gelatinous stroma. The glossopharyn- !70 MORPHOLOGY OF THE SKULL.' [chap. geal (8«), vagus (85), and hypoglossal (9) nerves pass through soft stroma in the posterior angle between the auditory sac and the parachordal cartilage. The trigeminal is distinguishable in front of the ear-sac. Fig. 77. -py ^a.ck 1LC Embryo Pig, two-tldrds of an inch long; elements of the skull seen somewhat diagrammatioally from below. pa.ch. parachordal cartilage; nc. notoohord; au. auditory capsule; 2>y. pituitary body; tr. trabeculffi ; c.tr. trabecular cornu; ^jji. prenasal cartilage; e.n. external nasal opening; ol. nasal capsule; p.jjg. palato- pterygoid tract enclosed in the maxillopalatine process ; vm. mandibular arch; hy. hyoid arch; th.h. first branchial arch; 7i(, facial nerve; 8fl, glossopharyngeal; 8&, vagus; 9, hypoglossal. 607. The trabeculse (tr.) are well marked out in the fore part of the cranial floor, lying beneath the olfactory sacs and the first cerebral vesicle, and extending upwards and backwards to embrace the pittiitary body. They are shaped like callipers, and their pointed (notochordal) apices are some distance apart. They reach nearly as far back as the fore end of the parachordals, but are con- siderably below them in position. The trabecular curve is not moulded on the sides of the pituitary body, but is very distinct from it ; and the intertrabecular space is much more extensive than that body. In their fore part they very early coalesce and thicken : the commissure viu.j THE PIG: FIRST STAGE. 271 gradually extends backwards, until the uncoalesced part merely corresponds to the pituitary body. The fore end of each trabecula does not take part in the fusion, but becomes first clubbed and then bent somewhat outwards and inferiorly, forming the trabecular cornua [c.tr.); and these cause the skin to project on either side in the palate, mesiad of the internal nostrils. Between these promi- nences of the cornua a median backwardly directed process soon arises, which includes an azygous growth from the trabecular commissure, the-prenasal or rostral process. (Figs. 77, 7S,i9n.) 608. The trabeculse, as well as the other skeletal ele- ments, are not yet truly cartilaginous, although unmistakeable; they are embedded in a gelatinous tissue rich with young cells, whose protoplasmic substance takes up carmine very freely. The differentiation of the rods is at present a matter of degree, that part of the blastema which will become hyaline cartilage being the most compact and crowded with young cells ; next to this the nascent perichondrium ; and the most gelatinous parD outside is the rudimentary condition of the areolar connective tissue. 609. The nasal sacs are intimately related to the front part of the trabeculse ; and are already complicated. There is a squarish median internasal I'egion, of consider- able thickness, beneath the first vesicle, and in front of the at present down-turned trabecular extremity. The inner walls of the nasal cavities are simple, but the outer walls exhibit various processes projecting inwards. There is a lower bilobate process, the anterior lobe of which be- comes the ahnasal turbinal (see Fig. 79, p. 281), and the hinder lobe the inferior turbinal. Another swelling below the fore part of the nasal roof is the rudiment of the nasal turbinal; while a mass in the hinder angle beneath the olfactory crus gives rise to the upper and middle turbinals and the true olfactory region. By reason of these processes the nasal meatus is already tortuous, first ascending, pro- ceeding backwards, and then descending; but the external and internal nares are on the same level, partitioned from 'Ut MORPHOLOGY OF THE SKULL. [chap. oDe another by the forward part of the maxillopalatine process and the tissue containing the trabecular cornu. 610. The internasal mass contains a pair of cartilagi- nous septal laminae, each being continuous below with the corresponding trabecular moiety at its inner and upper edge, very near the middle line, and forming a concave inner wall to each nasal sac. The arch extends considerably into the upper nasal boundary ; it may be called aliseptal cartilage. 611. It has already been stated (§ 602) that the nasal sacs are beneath the cerebral hemispheres. A transverse vertical section through the head at the region of the internal nares cuts through the olfactory crura partially Fig. 78. Embryo Pig, two-thirds of an inch long; palatal view, the lower jaw and lower side of throat having been removed. p. prominence enclosing prenasal cartilage; c.tr. that covering the trabecular cornu; ol. nasal sac; e.n. external nostril; i.n. primary in- ternal nostril; pn.mx. maxillopalatine process; mt. section of meckehan cartilage; hy. section of hyoid arch; tli.h. section of branchial arch, ap- plied to lower extremity of hyoid ; py. tissue beneath the pituitary fossa. separated by a median ethmoidal partition of mesoblastic tissue ; the trabeculas lying as rounded rods somewhat below this partition ; and the irregularly shaped nasal VIII.] THE PIG: FIRST STAGE. 27^) sacs beneath the cranial cavity and external to the trabe- culae. The thick maxillopalatine process forms the outer and lower margin of the section (see Fig. 78), enclosing, in that part which is close to the internal nostril, the small rudiment of the palatopterygoid bar. In the upper and outer tract of the maxillopalatine process the cleft between it and the upper facial region (§ 604) is cut through ; it passes downwards into the posterior portion of the nasal passage, and upwards to the base of the fore part of the orbital region (the future "inner canthus"); it be- comes the lachrymal passage. 612. The palatopterygoid bars {p-pg., Fig. 77) are less definitely developed at present than the other skeletal elements ; they are small sigmoid granular rods, in the inner edge of the maxillopalatine processes, which extend inwards to form a considerable part of the palatal roof. They are continued forwards beneath the trabecular region, approximating as they advance ; but they do not meet, so that there is a primary cleft palate or median palatal hollow. The posterior ends of the palatopterygoid bars diverge outwards, instead of turning inwards like the postoral arches. 613. The mandibular bars {mn. Fig. 77) are much stouter and more perfect than the palatopterygoids ■, they occupy most of the lower jaw, but do not meet in the middle line. Each bar is sigmoid, and strongly inhooked proximally, where it extends towards the auditory sac. It comes into close relation with the first visceral or mandi- bulo-hyoid cleft, which at its upper part is of considerable extent owing to the thickness of the throat-wall. Tlie cleft becomes constricted at the point where the mandi- bular bar is in contact with its anterior wall ; this is the position of the future tympanic membrane. 614. The hyoid arch (Fig. 77, %.) is much like the mandibular ; but it is flatter, and the right and left bars approach one another more closely below, at an obtuse angle ; its apex also is more sharply inturned, making the B, M. 18 274 MORPHOLOGY OF THE SKULL. [CHAP. shoulder stand out like the tubercle of a rib. The apex of the bar grows into relation with the mandibulo-hyoid cleft behind, in a similar manner to the bar in front. The main part of the facial nerve (7a) curves backwards round the fore part of the auditory capsule, and behind the angle of the hyoid bar. At some distance behind the hyoid, in the lower part of the third visceral arch (th.h), is a much smaller cartilage on each side, attached to the fore part of the sides of the larynx. This is homologous with the branchial rods of lower vertebrates. 615. The primordial skeleton of this most highly- specialised Mammal is as simple as that of the lowest Fish we have examined; the head, with its intense mesocephalic flexure, compares very well in shape with that of the Skate or the Frog. The elements are more distinct from one another than in the Fowl. There is everything to show the persistency of morphological impress; the vis- ceral clefts which appear, all of them transitory except one, are more in number than the arches which contribute anything to the adult skeleton. The embryo of the air- breathing Pig gives evidence of its relationship with forms breathing through water. The prenasal nodule and the little cornua are further manifestations of unity with other tvpes, cunningly adapted and hidden in subsequent his- tory. 616. But the Pig even at this stage shows evidence of its high grade. The notable posterior clinoid ridge is prepared for by the high upward curving of the para- chordals, above the trabecular level. The early internasal fusion of the trabecule and the development of the rudi- ments of turbinal folds while as yet the sense capsules are sub-equal in size, are indications of the future predomi- nance of the olfactory organs. A distinctive palatopterygoid tract exists in the maxillopalatine process of the mandibular arch, but the palatopterygoid itself shows no appearance of being a process from the mandibular cartilage. The principal arches of the skull at their proximal extremity Vm.] THE PIG : SECOND STAGE. 273 already begin to be specialised ; the first visceral cleft is becoming tortuous, and the apex of the mandibular arch (future manubrium of the malleus), -which is plainly com- parable to the apex of the otic process of the quadrate iti Birds, grows iuwards into the cleft in a way not occurring in the Fowl, so as to become involved in the tympanic membrane. 617. Here then in the last of our types we find the same parachordal and trabecular elements in the cranial floor and the same relations of the notochord as in pre- vious cases; with two principal arches, mandibular and hyoid, and smaller rudiments, the palatopterygoid and first branchial. All these parts are quite distinct at first. But there are already two openings to each nasal cavity, and these are approximately on the same level ; the "posterior nares " are in the anterior part of the oral cavity, as in Dipnoi and Amphibia. The roof of the mouth, or primary palate, is the floor of the cranium. A free growth of car- tilage, binding the various bars together, would produce a very good parallel to the cartilaginous skull of Lepidoiiren. Second Stage : Embryos one inch long. 618. In this stage chondrification has fairly set in, although the cells of the hyaline cartilage are still close to- gether. Ossification has also commenced in fibrous tracts related to the month. Differentiation of parts has gone on very rapidly in the olfactory and auditory regions, whilst the embryo has merely become longer by one half. The mesocephalic flexure is partially lost, but the apex of the notochord and the direction of the parachordals still make a considerable angle with the trabeculaj. 619. The parachordals have grown round the uoto- chord to a large extent, but do not unite in front of it. They are narrow in the interauditory region, and much broader behind where they coalesce with the inner and hinder edges of the periotic masses; they extend back- 18—2 276 MOEPHOLOGY OF THE SKTTLL. [CHAP. ■wards considerably behind the latter, ending pointedly on either side. The occipital ring of cartilage is developing. 620. The trabeculse have coalesced in their whole extent except around the pituitary space, which is itself becoming floored with cartilage; and this cartilage unites with the parachordal below the level of the clinoid ridge. The cranial floor is being extended laterally by trabecular outgrowths, after the manner in which the occipital ring is being formed; thus alisphenoidal and orbitosphenoidal laminae are arising. 621. The alinasal cartilage has become completelv chondrified up to the fore end of the nasal region, and the primary moieties are more or less confluent with the trabeculae and their cornua into a single cartilaginous septum. This is thick and low in front, but much thinner and more elevated behind, in the mesethmoid region. The outer nasal wall is fully chondrifled, and sends cartilaginous processes into the various turbinal folds. There is no trace of separate trabecule in the base of the mesethmoid ; that lamina merely gradually thickens towards its base, and just outside the mucous membrane covering it below is the internal nostril on either side. The base of the mesethmoid tract is underlaid by an elongated granular mass which is undergoing endostosis to form the azygous vomer. A dense premaxillary stroma is found under the trabecular cornua in front, ready to ossify; while far on either side of the ethmoidal region there is a faint beginning of the maxillary, above and quite distinct from a rudi- mentary tooth-pulp. The compressed granular palato- pterygoid rod is considerably mesiad of this tooth-pulp, bounding the median cleft of the palate. 622. In the ear-sac the semicircular canals and the coils of the cochlea have now become differentiated. The median lobular protuberance of the outer wall of the capsule seen in the first stage is separate to a great extent from the rest of the auditory wall, but remains as a plu^, VIII.] THE PIG : SECOND STAGE. 277 thus forming the stapes, lying in the fenestra ovalis. It has two papular elevations on its external surface, and is covered externally by tissue commencing to chondrify. 623. By the thickening of the side-wall of the throat the first visceral cleft is so much lengthened that it may now be called tympano-eustachian canal ; its Eustachian part is directed forwards into the mouth ; its tympanic portion is becoming closed by the formation of the tympanic membrane as the upper end of the mandibular arch grows inwards towards the ear-capsule. An external tube out- side the tympanic chamber is being formed by the growth of an opercular skin-fold round the primary orifice of the cleft ; cartilage soon arises in this fold. The external edge of the periotic mass is produced as a pterotic ridge or tegmen tympani, covering and partially surrounding the upper ends of both the mandibular and the hyoid arches. 624. The mandibular bar becomes more and more incurved at its proximal extremity, so that it is very hook-shaped (see Figs. 80, 81, representing a later stage). It has a small bulbous end, and a thickened portion at the bend or shoulder, -which articulates with the upper part of the next bar. The incurved apex becomes the manubrium, or handle of the malleus, and the remainder is the meckelian cartilage. It is the ingrowth of the manubrium into and over the tympanic canal that, carry- ing the skin with it, gives rise to the tympanic membrane. The head of the malleus further grows out into a boss at its junction with the main bar, for the attachment of the tensor tympani muscle. 625. The proximal region of the hyoid bar is more differentiated than the preceding arch, and also undergoes distinct segmentation. The incurved and backwardly- directed hook has been cut off from the shaft, and becomes shaped into the incus with its processes; while the remaining bar shifts its position backwards along the incus ; its proximal end becomes somewhat two-headed, to articulate externally with the tegmen tympani, and to 278 MORPHOLOGY OF THE SKULL. [CHAP. unite internally with the ear-sac close in front of the exit of the facial nerve. 626. It was the tubercular part or shoulder of the un- segmented arch which came into contact with the man- dibular arch growing backwards and forming the tympanic membrane : while the apex or head was bent inwards and backwards towards the ear-sac. Consequently, when seg- mentation takes place, the tubercular part of the hyoid is left as the front and outer end of the incus, articulating with the head of the malleus (Fig. 81). A small boss arises on the outer edge of the incus, to form the short crus; the part of the incus growing towards the ear-sac remains as the loner crus, and its small rounded head is the orbicular process, coming into close contact with the stapes. 627. The distal part of the mandibular arch extends forwards in the lower jaw ; and half encircling it on the outer side is a mass of granular tissue or nascent cartilage, rcjuiform in section, in the axis of which the dentary bone is being formed. The remainder of the hyoid arch (which niay be called stylo- and ceratohyal) extends downwards to the fore part of the larynx, and is fastened to a median cartilage, which is the first basibranchial (basihyal of human anatomy) ; this lies between a pair of small carti- lages which represent the first branchial arch. 628. lu this minute embryo some of the features most characteristic of the adult as belonging to the mam- malian group are already distinguishable. Chondrification has advanced greatlj', and is rapidly completing its work. The notochord is still visible, but the basilar plate is well f)rmed, though incompletely united as yet with the auditory capsules; and the exoccipital region is arising. The trabeculse have fused together almost completely, but are not yet united to the basilar plate ; lateral carti- lag(.'s, however, are growing in the ali- and orbitosphe- noidal regions. The cochlea and semicircular canals are well formed. The nasal labyrinth has increased in size and completeness ; and a median septum is found instead VIII.] THE PIG: THIRD STAGE. 279 of two distinct inner nasal walls. The trabecular cornua appear as recurrent cartilages. 629. Thepalate is still distinctively cleft; there are rudi- ments of the vomer and the rnaxillaries. The palatoptery- goid bar does not completely chondrify, but has already begun to be ossified. As a contrast to this, the stroma of the dentary (very distinct from the meckelian cartilage) chondrifies very largely before it ossifies. The outer ear is also becoming cartilaginous, while the proximal parts of the two principal arches are rapidly assuming their adult relations. The mandibular apex becomes shaped into the parts of the malleus, without any segmentation taking place ; v>'hile the hyoid, becoming bent so as to ai'ticulate with the malleus, gives rise to the incus, which is also in relation to the commencing stapes formed out of the tissue of the ear-capsule. The incus being segmented from it, the main hyoid arch is withdrawn backwards, approaching the stapes and becoming connected with it and the ear-capsule by an interhyal ligament. Third Stage : Embryos an inch and a third long. 630. There is now continuous cartilage occupying the whole median basicranial and facial regions ; the lateral cranial walls are partially chondrified, the occipital ring being complete. The parachordals have united in a basilar plate, entirely enclosing the diminished notochord (Fig. 80). The anterior end of the basilar plate forms the hinder boundary of the pituitary space (posterior clinoid wall, p. cl. Fig. 79) ; but does not rise so_ high relatively, nor form so great an upward curve as did the parachordals in the first stage. No remnant of the noto- chord is found in this posterior clinoid wall — a sharp contrast to the first stage, when the notochord projected beyond the parachordals. 631. The basilar plate (b. o.) is narrow between the ear-capsules/ which are continuous with it; internally 280 MORPHOLOGY OF THE SKULL. [CHAP. and especially posteriorly they are on the same level with it, but externally they are elevated so as to con- stitute the lower half of the lateral cranial wall. Behind the auditory masses the basilar plate widens, and curves gradually upwards on both sides to form the occipital arch. The supraoccipital region (s.o.) rests on and is continuous with the hinder half of the ear-capsules. The foramen magnum {/. m.) is large and transversely ellip- tical, with a supero-mediau notch : the crescentic condyles are external to the foramen, extending towards the middle line below. G82. The foramen for the vagus and glossopharyngeal nerves (Fig. 79, 8) is immediately behind the ear-mass internally; the hypoglossal foramen is somewhat behind this. The auditory cartilage, protruding into the cranial cavity, presents several depressions and openings. There is a depression beneath the arch of the anterior canal to receive the cerebellar process (flocculus) ; antero- inferiorly the facial and auditory nerves perforate the cartilage, close together, but separated by a distinct bridge. The condition of the auditory cavity will be described later. 633. In front of the basilar plate, the floor of the cranial cavity presents a deep pituitary cup (jo.y.), floored with cartilage; a slightly elevated anterior clinoid wall, with a depression in front of it on which the optic chiasma lies (presphenoidal region, p.s.): and a more elevated ethmoidal region, the level of which is approxi- mately the same as that of the basilar plate. The (_4hmoidal cartilage projects slightly upwards at the extreme fore part of the brain case, forming a rudimentary anterior wall. 634. The lateral portions of the cranial floor and the lowest regions of its side walls contain cartilaginous orbitosphenoidal and alisphenoidal growths, continuous Avith the median floor. The alisphenoidal cartilage {al. s.) is short and thick, and is attached to the anterior and Vin.] THE PIG: THIRD STAGE. 281 posterior clinoid tracts. Its most noteworthy part ex- tends downwards to the palatopterygoid bar, to form the external pterygoid cartilage. Between the alisphenoid and the ear-sac is a large shallow fossa for the Gasserian ganglion, and the largest division of the trigeminal nerve Fig. 79. Embryo Pig, an inch and a third long; median longitudinal section of head, with nasal septum remoTed. al.tb. alinasal turbinal; i.tb. inferior turbinal; n.ti. nasal turbinal ; m.tb. middle turbinal; u.tb. Upper turbinal ; cr.p. cribriform (ethmoidal) plate (the line should be prolonged to a point over the turbinal folds) ; p.s. presphenoid region; o.s. orbitosphenoid ; al.s. alisphenoid; pi/. pituitary fossa; p.cl. posterior clinoid ridge; au. ear-capsule; a.s.c. an- terior semicircular canal; p.s.c. posterior semicircular canal; s.o. supra- occipital cartilage; l.s. position of the lateral sinus in dura mater; f.m. foramen magnum ; o.c. occipital condyle; 6.0. basilar plate ; 2, optic foramen ; 5, foramen ovale ; 8, foramen for glossopharyngeal and vagus nerves; 9, foramen for hypoglossal; mk. meckehan cartilage; above it, the tongue; c.hy. ceratohyal. (the inferior maxillary) passes out between these two cartilaginous regions (5). The superior maxillary division of the trigeminal passes out between the orbito- and ali- sphenoids. The orbitosphenoids (o.s.) are far the larger, spreading from their presphenoidal base into a sickle- shaped lamina extending from the fore part of the cranium almost to the auditory masses. The optic nerve (2) passes out by a foramen in the base of the orbitosphenoid. The thin frontals have arisen in the stroma outside and above the outer edge of the orbitosphenoids. 282 MORPHOLOGY OF THE SKXTLL. [CHAP. 635. The nasal cavities extend backwards beneath the cranial cavitj' in its ethmoidal portion, and on each side of the median ethmoidal plate the nasal roof is soft, and the olfactory lobes of the brain lie upon it. The nerve filaments perforate the roof to reach the turbinal growths of the nasal sacs ; and very soon cartilage appears between the groups of nerve fibres, and gradually becomes perfected into a cribriform plate {cr. jj.). 636. The nasal cavities already occupy more than half the length of the head. Rather more than half their extent belongs to the snout, which is a rostrum-like pro- jection from the bulbous head. The nasal septum is a complete vertical lamina of cartilage, of considerable and almost uniform height, being most elevated at the junc- tion of the snout with the cranial box. The septum bears no conspicuous projections on its lateral surfaces ; but superiorly it is continuous with the upjoer and outer carti- laginous nasal walls, which are distinguishable from before backwards into alinasal, aliseptal, and aliethmoidal re 'ions. Anteriorly the septum is broader at the base, owing to its continuity with the primary trabecular cornua ; but a little posteriorly these cornua have become recurrent car- tilages distinct from the septum ; and they lie for some distance close to its base, diminishing to small styliform rods. The greater portion of the septum is floored by condensed tissue very like nascent cartilage, separating it from the roof of the mouth, or from the palatal plates where they are uniting to form the true palate. This tissue early ossifies as the vomer. 637. The outer nasal wall becomes more complete and more complex in passing from before backwards; each of the main turbinal outgrowths from the upper and outer walls contains a small cartilaginous blade continuous with the capsular wall. The nasal sacs are widest where they underlie the fore part of the cranial cavity. They diminish in size as they approach the presphenoidal region, under which they die out: this is the part of the nasal cavity known as the sphenoidal sinus. The preinaxillaries VIII.J THE PIG: THIRD STAGE. 283 have appeared in the roof of the mouth, beneath each nasal mass, sliort of its extreme anterior extremity. 638. The trihedral mass of stroma forming the outer part of the palate roof and the upper lip, sends inwards in its middle region a thin tongue of tissue to meet its fellow and form the secondary palatal roof This meeting takes place at present only for a short distance ; elsewhere the palate is still cleft. The palatopterygoid rod lies just at the junction of this inner palatal region with the outer stroma; it is kidney-shaped in transverse section. Beneath the hinder part of the nasal capsule this imperfect carti- lage is ossifying to form the palatine ; further back a small rudiment of the pterygoid appears (the future interna] pterygoid plate), adjacent to the downgrowth from the alisphenoid which becomes the external pterygoid plate. The maxillary bones first arise beneath the nasal sacs under the anterior cranial region, in the palatal roof It is evident that it is the palatal part of the maxillary which is the first formed ; outside and behind it, a row of tooth- pulps is developing, close to the edge of the lip, corre- sponding to a similar set in the lower jaw. 639. The floor of the mouth is occupied by the pro- minent tongue, and below its level on either side is the stroma in which the meckelian cartilage is embedded. This primary rod is flanked externally and partly em- braced by a dentary stroma, which ossifies rapidly, be- coming however distinctly cartilaginous in the interim. The dentary bone is vertically elevated behind ; a row of tooth-pulps lies above it in the greater part of its extent. The meckelian cartilages have fused together anteriorly. 640. The arrangement of the parts of the ear-capsule is as follows. The coils of the cochlea (cl. Fig. 80) are fully developed, and are entirely anterior, and near the middle line and base of the skull. The facial nerve passes into the capsule above and externally to the cochlea, and curves backwards and outwards to reach the roof of the. tympanic cavity. The anterior semicircular canal 284 MORPHOLOGY OF THE SKULL. [CHAP. arches high up on the antero-external part of the auditory mass : the horizontal canal is almost directly below it, and Fig. 80. Embryo Pig, an inch and a third Ions; posterior view of a section through the basal region of the skull. The section being not per- fectly transverse shows a different condition on each side. The left-hand view shows parts in front of those displayed on the right ; the manubrium of the malleus mb. is cut through at its thickest (posterior) part, and the main part or shoulder ml. is seen, at the part which articulates with the incus ; within mb. is the tympanic cavity t.c. On the right side the incus i. is seen, covering the malleus, and having its short cms cut away; its orbicular head o.ob. is applied to the stapes st. The stylohyal cartilage st.h. is seen in oblique section as it passes down- wards and forwards. The supraoccipital cartilage s.o. is seen as if discon- tinuous with the auditory tract on the left side, in consequence of the section passing through the notch in the front end of the cartilage. h.o. basilar cartilage, narrow between the ear-capsules; a little of its upper surface is seen, as well as the section enclosing the notochord nc. ; ha. a. basilar artery; j.i'. jugular vein, with a coiled branch; l.c.i. long crus of incus ; aq.v. aqueductus vestibuli ; cl. cochlea. Under the basilar cartilage the throat is partially represented. the main part of the labyrinth is internal, behind the cochlea. The posterior canal, convex backwards, touches the horizontal canal externally, and ends internally in the labyrinth. The middle and hinder part of the external edge of the ear-mass, containing part of the posterior canal, projects as a roof or tegmen tympani (pterotic ridge) over the tympanic cavity ; so that the fenestra in which the stapes lies places the labyrinth in direct contiguity VIII.] THE pig: third stage. 285 ■with the tympanic cavity. The short or external crus of the incus articulates with the tegmen. 641. The tympano-eustachian canal has increased in length and become more definite; it is completely oc- cluded by the tympanic membrane, in which the handle of the malleus (Figs. 80, 81, mb.) is embedded. The latter piece has become flattened, but is not segmented from the remainder of the mandibular arch. The incus [i.) has Fig. 81. Embryo Pig, an incli and a third long ; side view of mandibular and hyoid arehes. The main hyoid arch is seen as displaced backwards after segmentation from the incus. tg. tongue ; mlc. meckelian cartilage ; ml. body of malleus ; mb. manu- brium or handle of the malleus; i.«2/. tegmen tympani; i. incus; st. stapes; i.hy. interhyal ligament; st.h. stylohyal cartilage; h.h. hypohyal ; h.h. basibranchial ; th.h. rudiment of iirst branchial arch ; 7a, facial nerve. similarly progressed, applying the orbicular head (o.oh.) of its long crus to the stapes (Figs. 80, 81, st.) The latter has begun to assume its distinctive shape, the two tubercles which it bore at first being now connected by a bridge of cartilage in the fenestra ovalis. The main hyoid rod (st.h) is attached by ligament to the orbicular end of the incus, and is in contact with the auditory capsule behind the stapes. In this interhyal ligament (i.hy.) a small cartilage has arisen. The facial nerve (7a.) passes (above the tympanic membrane) to get behind the hyoid arch, parallel with which it proceeds to its destination, 642. The main hyoid bar passes directly downwards from the auditory region for some .distance, and then 286 MORPHOLOGY OF THE SKULL. [CHAP. curves forwai'ds and inwards, being connected with the small hypohyal segment (k.h.). The latter is articulated to the median azygous rudiment of the next arch (basi- branchial), and to its lateral or branchial cartilages (so- called basi- and thyrohyal). "The proper territories of each investing bone in the Pig evidently ouly want thiie that they might all become true cartilage ; ossification sets in too soon for the formation of the intercelhilar substance, but each tract, before ossification, is a true morphological element or organ." {Fhil. Trans. 1874, p. 306.) 643. We have uow considered the structure of the skull in an embryo Pig only an inch and a third long, and have found it completely constituted as to cartilage, with all the normal elements continuous, viz. basilar plate, trabeculse, occipital ring, auditory capsules, ali- and orbito- sphenoids, nasal septum, walls, and turbinals. The princi- pal arches have almost perfected their permanent rela- tions. The predominance of the orbitosphenoid cartilages, the large size of the nasal capsules, their extension back- wards beneath the brain-case, and the formation of the cribriform plate are to be noticed. The true cartilage- bones have not yet appeared ; but there are present the vomer, frontals, premaxillaries, maxillaries, palatines, ptery- goids, and dentaries. Fourth Stage: Embryo two and a half inches long. 644. Development has proceeded very rapidly ; the snout is proportionately longer, and the forehead has become sloping instead of vertical. The well-marked granular territories that at first invested the primordial skull are now largely ossiiied, and these ossifications are massive in relation to so small a skull. The cartilaginous parts also are undergoing endostosis at many points. 645. The cartilaginous cranium has not changed much in its main outlines : the floor has thickened considerably. "^^"■J THE PIG: FOURTH STAGE. 287 The cribriform ethmoidal plate has now four slender bars passing between the olfactory filaments, and a common outer band which does not extend backwards to the orbito- sphenoidal cartilage. The latter has grown forwards to unite with the aliethmoid cartilage, and backwards to overlap the antero-external angle of the auditory cartilage, quite overshadowing the alisphenoid. But the external pterygoid plate (Fig. 82, e.-pg.) arising from both the basisphenoidal and the alisphenoidal regions, is well de- veloped, and articulates with the palatopterygoid bar. Fig. 82. Embryo Pig, 2 J inches long ; under view of skull with lower jaw removed. /.m. foramen magnum ; oc.c. occipital condyle ; au. auditory capsule ; m.t. tympanic membrane ; e.pg. external pterygoid process ;j9.s. presphenoid region ; c. tr. trabecular cornua. Bones : 6.o. basioccipital ; e.o. exoccipital; s.o. supraocoipital ; ty. tym- panic ; b.s. basisphenoid ; sq. squamosal; pg. pterygoid; pa. palatine; V. vomer ; d.px. dentary plate of premaxillary ; p.px. palatal process of same ; mx. maxillaiy ; j. jugal. 646. The cartilage bones are principally endosteal at present, although they rapidly gain the surface and affect 288 MOEPHOLOGT OF THE SKULL. [CHAP. the perichondrium. There is a thick hasioccipital (h.o ) in the basilar plate, seen both above and below ; it is shaped like a spear-head, and is fast obliterating the notochord. In the exoccijyital region there is a large growth of bone (e.o.) extending inwards towards the upper edge of the foramen magnum, and outwards and downwards in the external or paroccipital process. Besides this, a distinct ossification arises in the massive condyle, which coalesces with the exoccipital before long. The supraoccipital (s.o.) is double at first, but the two patches run into one another in a day or two. 647. The cartilage in the pituitary floor is ossifyi'ifr to form the basisphenoid {b.s.) ; a little later, a distil t centre arises on either side for the alisphenoid. There is no presphenoid, but a small orbitosphenoid ossification is found outside and behind each optic foramen. The auditory capsule is unossified at present. 648. The hinder nasal region has become much broader than the anterior. The upper and middle turbinal folds with their included cartilaginous laminae occupy the back part of the nasal cavity ; and the inferior turbinal is further forward. Anteriorly the recurrent trabecular cornua {c.Lr.) still persist, on either side of tlie base of the septum. 649. The vomer (Figs. 82, 83, v.) embraces the base of the septum along the hinder two-thirds of the nasal region. The premaxillaries are found on the under surface of the fore part of the snout, not reaching to its extremity. Each has an expanded dentary portion (d.px.) grooved by tooth-sacs, and a small medic-palatal spur (p.px.) ex- tending towards the vomer. The end of the snout around and above the external nostrils is unclothed by bone : but behind this region the premaxillary of each side sends up a facial lamina to meet the outer edge of the nasal, also comintr in contact behind with the facial plate of the maxillary. The nasals, frontuls, and paridals foi-ni a. ■Viii.j THK I'lu: iuuiiiii STAGE. 289 regular double series of thick bones. The parietals do not extend so far back as to the supraoccipital ; they occupy the main part of the vertex of the skull. The frontals are the largest pair. They stretch considerably over the face, touching the lachrymal and maxillary bones ; they arch over the orbit, and send inwards from the eave an orbital plate which is in the side wall of the skull, and reaches the orbitosphenoid cartilage. The anterior and posterior median fontanelles (fronto-parietal and parieto- occipital) are still widely open. 650. The maxillary [mx.) has its facial and palatal plqtes largely developed. The facial lamina extends from tl , preraaxillary along the lower part of the face to under- lap the jugal. Above, the maxillary is in contact with the frontal and lachrymal, and does not form any part of the margin of the orbit. The lachrymal is a small bone en- closing the upper end of the orbitonasal (lachrymal) canal, and occupying some space on the face between the frontal and jugal bones. The latter {j.) bounds the lower margin of the orbit, and passes backwards underneath the zygo- matic process of the squamosal, nearly to the ear-sac. 651. The squamosal (sq.) is a large triradiate mem- brane bone, with a squamous part lying on the infero- lateral wall of the cranium, between and below the frontal and parietal ; it has also a long sickle-shaped process directed backwardly by the side of the ear-capsule above and outside the tympanum ; and a zygomatic process growing outwards and forwards to the back part and external edge of the orbit, resting on the jugal. At the point of junction of these three parts the squamosal has an orbicular concave surface, external to the tympanic membrane; with this surface the osseous mandible ar- ticulates. 652. Returning to the palate, the greater portion of it is occupied by the large oblong palatal plates of the maxillaries, which do not meet in the middle line but leave the vomer exposed between them. The maxillaries, B. M. 19 290 MORPHOLOGY OF THE SKULL. [chap. are grooved by vessels in the middle of their palatal plates, whilst the dentary portions are hollow and shell-like, containing large growing tooth-gemns. The palatines {pa.), pointed in front, rod- like behind, extend outwards and backwards on the palate from the rnaxillaries to the external pterygoid cartilaginous plate. Their ascending lamina i-eaches to the hind part of the nasal septum. In- ternally to the hinder portion of each palatine, is a small osseous nodule, the pterygoid (pff-). Embryo Pig, 2^ inches long; vertical section of head, showing structures between and beneath the orbits. p.e. mesthmoid plate; vi.th. middle turbinal coil; e. eyeball; m.n. nasal passage above secondary palate; ink. meckeliau cartilage; b.hy. basibrancliial (body of hyoidj ; c.hy. ceratohyal; th.h. first branchial arcii; tg. tongue. Bones : i'. vomer ; pa, palatine ; cr. coronoid process ; ar, articular tract ; d. dentary ; j. jugal. 6o:3. The proper mandible is now an extensive ossifi- cation of considerable vertical extent, ensheathing its own cartilage, sending up a backwardly curved coronoid process behind (Fig. 83, cr.), and having a cartilaginous condyle beneath the curve of the coronoid, articulating with the squamosal. There is a protuberant angular reo-ion of cartilage below the condyle. The osseous symphysis is not formed at present : but the symphysis of the meckelian rod is very extensive. This cartilage lies within the bony mandible unchanged ; the malleus and its manubrium have become ossified, but are still perfectly continuous VIII.] THE PIG: FIFTH STAGE. 291 with the rest of the primary arch, just within the articular condyle. 654. The tympanum is becoming rapidly perfected ; the Eustachian tube opens into the back of the mouth beneath the basisphenoid. The tympanic membrane is partially enringed by a tympanic parostosis (Fig. 82, ty.) deficient behind. The incus is ossifying : behmd it the stylohyal is continuous with the ear-capsule below the horizontal semicircular canal. At its lower end the stylo- hyal region passes into the hypohyal. The hypohyals, basibranchial (6. Ay.), and the pair of branchials {th.h.) are articulated together; and they support the root of the tongue. 655. This stage is an excellent one for comparison with the adult Fish, Amphibian, or Reptile, or the ripe chick of the common Fowl. It also corresponds very closely with an early stage of the skull of Balwna japonica, figured by Eschricht. The non-segmeutation of the car- tilage of the mandibular arch, and the articulation of the dentary with the squamosal, at once distinguish the mam- malian type ; yet if the palate be compared with that of the young Ostrich {Phil. Trans. 1866, Plate Vii. Fig. 4), the conformity is more remarkable than the ditference. The new bones which have appeared are the four occipi- tals ; the basi-, ali-, and orbitosphenoids ; the nasals and parietals ; the lachrymals, jugals, squamosals, and tym- panies ; the incus and the malleus. Fifth Stage: Embryo six inches long from. snout to ischium. 656. The general form of the head has not altered much ; the snout is longer and its upper surface slopes more gradually from the frontal region forwards. The bones have become very dense ; on the roof of the skull they are applied to each other edge to edge by sutures, in 19—2 292 MORPHOLOGY OF THE SKULL. [CHAP. certain places even overlapping. The anterior fontanelle (tronto-parietal) is still open, but is much lessened ; the parietal and occipital bones form a good lambdoidal suture. The sickle-shaped process of the squamosal which lies alongside of the ear-capsule, has an increased surface, projecting backwards and upwards to the supraoccipital (Fig. 84, S.O.), and downwards in apposition with the paroccipital process (p.oc.) of the exoccipital. The end of the snout, around the small external nostrils, is unclothed by bone. The nasals project forwards mesially. The orbital plate of the frontal has developed very largely, so as to form the whole bony roof of the orbit. The upper and lateral bones of the skull require but little relative change, with increase of size, to bring them to their adult condition. 657. The previously existing cartilage-bones have in- creased in size. The bones forming tlie occipital ring are not yet in contact with one another. The exoccipitals (Fig. 84, e.o.) ossify the condyles and the paroccipital jDrocesses. The supraoccipital [s.o.) is a large concave shell ot bone, separated by a considerable tract of cartilage from the exoccipitals. 658. The basisphenoid is a thick bone, extending partially into the posterior clinoid ridge, and continuous ^vith the alisphenoids, which are of considerable size, and extend downwards into the external pterygoid plates. The prespheuoid cartilage is well ossifieci, and continuous with the orbitosphenoids, which have ossified the inner half of the corresponding cartilages. The latter have not grown with the growth of the skull, and consequently appear relatively contracted ; they are separated from the aliethnioid cartilage anteriorly, and are at some distance from the auditory capsule behind. The remainder of the facial axis and nasal septum is one sheet of solid cartilage, l>erfectly continuous on either side with the nasal laby- rinth and its now highly complex turbinal growths aud cribriform plate. VIII.] THE PIG: FIFTH STAGE. 293 659. The proper periotic bones are now distinct. Tlie prootic (pr.o.) is an endosteal patcli surrounding the meatus internus, lying under the fore part of the cochlea and extending supero-posteriorly to the junction of the anterior and posterior canals : externally it is seen above and in front of the fenestra ovalis (f.ov.). The opisthotic Fig. 84. Embryo Pig, six inches long ; outer view of occipital and auditory regions. t.ty. tegmeu tynipani;/.ot'. fenestra ovalis; a.s.c. anterior, h.s.c. hori- zontal, ^.s.c. posterior semicircular canals; f.r. fenestra rotunda, the line should be carried upwards to the angle behind f.ov.; la. facial nerve ; p.oc. paroccipital process ; o.c. occipital condyle ; st.h. stylohyal cartilage. Bones, in some cases not well indicated ; e.o. exocoipital; s.o. supra- oooipital ; pr.o. prootic ; op. opisthotic ; ep. epiotic. (op.) is on the under surface of the capsule behind, and covers the most bulbous part of the cochlea below: one of its processes lies between the fenestra ovalis and the fenestra rotunda {f.r) close in front of the head of the stylohyal cartilage. The epiotic centre {ep.) is at present but a small scute ahove the head of the latter cartilage and below the hinder end of the tegmen tympani or pterotic ridge {t.ty.). (J60. The structures of the middle ear have acquired almost their full development. The stapes is not yet ossified. The tympanic bone is still an imperfect ring,' but is thicker and broader, although no meatus externus 2.94 MORPHOLOGY OF THE SKULL. [dHAP. is as yet formed. There is an additional small ossicle clinging to the inner edge of the tympanic : it arises m the connective tissue forming the floor of the tympanum ; it is a small os bidlce. 661. The palate has not become much broader : it is now regularly oblong. The maxillaries meet thepre- maxillaries, leaving the incisive foramen near the middle line on each side. The palatal plates of the maxillaries meet all along the middle line, hiding the vomer. The palatal plates of the palatine bones continue the general surface of the palate, being triangular with the bases ad- jacent and the apices directed outwards and backwards. The ascending laminae of the palatines curve round the posterior nares and ascend to the base of the nasal laby- rinth in its hinder region. The pterygoids are quite small. The ossification of the lower jaw is almost complete ; the coronoid process is narrow and curved backwards ; the condyle is still cartilaginous. Sixth Stage : Keiu-born Pigs. 662. Since the last stage the head has almost doubled in length, and the process of ossification has gone on very rapidly : the form of the skull has become much more specialized. The whole of the occipital and sphenoidal cartilages are ossified. The spheno-occipital synchondrosis is of small extent, and a scarcely thicker tract of cartilage remains between the basi- and presphenoid. 663. The perpendicular ethmoid and nasal septum are still unossified ; but the inferior turbinals are almost completely, and the middle and upper turbinals (so-called lateral masses of the ethmoid) are partially converted into endosteal bone. The cribriform plate is soft, and so is the snout ; but this latter is everywhere burrowed with vessels prior to ossification. The external nostrils are here in- ferior, the alinasal cartilage being continued forwards and VIII.] THE PIG: SIXTH STAGE. 295 downwards, and having coalesced with the recurved tra- becular horns and their continuation backwards along tlie lower edge of the septum. The external nostril is bounded anteriorly and partially externally by the alinasal cartilage, externally by a cartilage of the " labial" category, distin- guished as appendix alas nasi, and within and behind by the trabecular cornu. The prenasal cartilage is now und istiuguishable. 664. The bony palate is now most compact, and its postero-external angles are continued by the pterygoids and the external pterygoid plates. The pterygoid is fixed posteriorly to the solid nut-like tympanic, indenting it — a temporary state of things only. The lower jaw is well ossified, while the adjacent meckelian bar has degenerated into a band of fibrous tissue. 665. The three periotic centres have completely ossified the capsular cartilage, and coalesced with one another. The exoccipital (Fig. 85, e.o.) and its parocci- pita] process {p.oc.) are apposed to the periotic bone behind, the process being separated from the stylohyal bar (st.h.) by the facial nerve (7a), which lies in the styloid canal, and gives off at its upper part its anterior branch (the chorda tympani, 7a) running upwards and forwards to curve over the tympanic membrane and enter the Glaserian fissure. 666. The processus gracilis of the malleus (the con- tinuation of the primary mandibular arch) is reduced to a style ending in fibrous tissue (Fig. 86, p.gr.) ; the manu- brium (m6.) is flat and slightly arcuate ; the head (ml.) articulates with the incus by a synovial joint, the minia- ture of the tibio-astragalar joint in the same animal. The head of the malleus sends inwards a rounded process {{.p.m.), and the manubrium at its base has a posteiior process to which is attached the tensor tympani muscle. 667. The hollow of the tegmen tympani (t.ty.) has the head of the incus in its hinder recess, which pos- 296 MORPHOLOGY OP THE SKULL. [CHAP. teriorly has a round cup-like facet for the short eras of the incus (s.c.i.); the head is also partially roofed by the ingrowing squamosal. The hooked long crus of the mcus [Lc.i.) is tipped by a distinct orbicular ossification {o.o.) Fig. 85. Fig. 85. The Pig at birth ; outer view of auditory capsule, &a., the squa- mosal and tympanic having been removed. 0.6. cs bulte; pr. promontory; f.ov. fenestra ovalis ; st. stapes; oh. OS orbiculare ; pr.o. prootio ; t.ty. tegmen tympani ; l.c.i. long orus of incus; a.xc. h.sc. p.sc. semicircular canals; s.c.i. short crus of incus; sq. a small piece of the squamosal in the hinder part of the tympanum ; i.o. exoccipital; oc.c. occipital condyle; ep. epiotio ; s.m.f. stylo-mastoid foamen; 7a, facial nerve; la', chorda tympani branch; f.r. fenestra rotunda; poc. end of paroccipital process; st.h. stylohyal; ch. cerato- liyul; b.h. basihyal ; tJi.h. "thyrohyal." Fig. 86. Auditory chain of bones. ml. body of malleus ; mb. manubrium ; p.gr. processus gracilis; i.p.m. internal process ; i. incus ; s.c.i shortcrus; /.c.i. long crus ; i.hy. iniei- hyal; o.o. os orbiculare; st.m. stapedius muscle; st. stapes. applied to the cup on the summit of the arch of the stapes. The base of the latter fits into the fenestra ovalis. VIII.] THE PIG: SEVENTH STAGE. 297 The^ little nucleus of cartilage which was early foi-med in the interhyal ligament {i.hy.) is now attached by its base below the head of the stapes, while its pointed distal end is buried in the fibres of the stapedial muscle {st.m). 668. In the head of the stylohyal cartilage, confluent below and behind the preceding parts with the auditory capsule, a bony centre has appeared, forming the tympano- hyal. The middle of the bar, outside the skull, ossifies as the stylohyal. The distal end of the bar is tied by ligament to the small hyoid cornu (c./i.) ; the larger cornua {th.h.), belonging to the first branchial arch, are ossified. Seventh Stage : The Skull of a Pig six months old. 669. In this stage the greater number of the sutural landmarks, largely obliterated in the adult, are still in existence. The long angular skull is an irregular pyramid, with two equal and two unequal sides and an oblique base. A complete contrast in outward form to the human skull, that of the Pig is the straightest of all the types; it is very strongly built, but its bone-tissue is inferior in density to that of the Sheep, being intermediate in this respect between the bone of a Ruminant and that of a Cetacean. The flat top of the skull, with its orbits flush with the surface, indicates the semi-aquatic habits of its owner; and the depth and squareness of the base of the pyramid is correlative with the high neck and strong shoulders. 670. The long straight nasals overlap the snout in front, and are articulated by suture along their outer margin with the upper edge of the long premaxillaries, and for a less extent with the maxillaries ; they terminate in a transverse line of suture with the frontals. The latter together form a somewhat pentagonal plate, divided along the middle line by the sagittal suture. The anterior third is deeply grooved, the grooves issuing from the 298 Morphology of the skull. [chap. supraorbital foramina; the posterior half of the outer margin of the bones bears the thick and somewhat promi- nent supraorbital ridge. The frontals have a large orbital plate, bounded behind and above by the short_ postorbital process, and lower down and within by the orbitosphenoid. The upper surface of the parietals is narrow, and divided by the continuation of the sagittal suture ; laterally they are much compressed, forming the inner wall of the large temporal fossa; behind, the upper part of the high supra- occipital abuts against them. 671. The premaxillaries have a large facial and a lesser palatal region, the palatal spurs being slender and compressed, lying together in the middle line, with an anterior palatine foramen on either side. The huge maxillary forms most of the side of the face and the anterior root of the zygoma ; and the part containing the hindermost tooth-socket is bound to the external ptery- goid plate and the descending part of the palatine. The horizontal plates of the maxillaries form tbree-fourths of the grooved and ribbed hard palate. The palatine bones complete the palate, being much longer at the middle line than externally. Altogether, the median suture of the hard palate is two-thirds of the length of the skull. 672. The inner ascending plate of the palatine articu- lates with the vomer, and sends forward a long scoop-like process beneath the lateral ethmoidal masses. Externally and behind they form a thick boss, which articulates with the maxillary and the external pterygoid process on the outside, and with the proper pterygoid on the inside. The latter bone has a very thin ascending part, and an upper squamous plate underlying the presphenoid — a separate piece (mesopterygoid) in some examples. 673. The thin dentate posterior end of the vomer extends to the same transverse line as the mesopterygoid plate ; in front it reaches almost to the fore end of the premaxillaries. Its lower edge lies, behind, on the np- VIII.] THE pig: seventh stage, 299 turned edge of the palatal plate of the palatine bones, and anteriorly to this, on the long harmony-suture of the palatal plates of the maxillaries. Its upper edge is grooved by the nasal septum. The complex nasal labyrinth is well ossified, but the septum, between the inferior turbi- nals, is still soft. The common trabecular and alinasal cartilage projects forward beyond the bony structures of the skull, and in the adult becomes ossified endosteally as the median snout-bone. 674. The lachrymal is nearly equally developed with- in and without the orbit ; it articulates with the frontal above and behind, with the maxillary in front, and with the jugal below. It has both an upper and a lower canal, immediately in front of the rim of the orbit. The jugal or malar bounds the lower and anterior portion of the orbit : it is massive in front, while its hinder half shelves away -under the zygomatic process of the squamosal. About an inch of space intervenes between the highest part of the jugal and the postorbital spur of the frontal. 675. The squamosal by its squamous lamina forms the lower posterior boundary of the side of the skull, overlapping the hinder edge of the frontal and the lower part of the parietal. It is turned outwards at its middle region below, to form the transversely- extended glenoid cavity for the mandibular condyle. The zygomatic pro- cess passes forwards from the outer side of this articular surface to rest upon the jugal. The line of the zygoma is continued backwards and upwards by an acute ridge rising towards the upper part of the supraoccipital, bounding the deep temporal fossa. The squamosal also coalesces with the upturned mouth of the external auditory meatus, and thence gives two processes, the post-tympanic, fixed on the side of the tympanic bone, and in front of this, a post- glenoid process. In front of the glenoid facet the squa- mosal is strongly sutured to the alisphenoid and to its external pterygoid plate. 676. The main part of the tympanic bone is somewhat 300 MORPHOLOGY OF THE SKULL. [CHAP. like a large filbert, ridged below ; it principally consists of a mass of sqnare-chambered diploe. It has a small cavity, and across its mouth the tymjDanic membrane is stretched; from its outer angle behind, a tubular meatus has grown upwards and backwards. There is a large foramen lacerum for nerves and vessels along the inner and hinder edge of the tympanic bone, between it and the basisphenoid and basioccipital. Part of the periotic mass is visible through this foramen ; it i.s verj' largely unanchylosed to the sur- rounding parts. The tympanic abuts on the alisphenoid in front, the squamosal externally, and the exoccipital behind. The post-tympanic spur of the squamosal on the outside, the tympanic bone on the inside, and the paroc- cipital process behind, encircle a canal through which the styloid cartilage and the facial nerve descend, and in which the tympanohyal bone is impacted. 677. The gi-eat occipital plane is scooped above, beneath a strong transverse backwardly-turned ridcre. The supraoccipital sends a wedge forwards between the parietals, expands laterally above, and descends so as to occupy a small part of the margin of the foramen magnum. The exoccipitals spread widely outwards, meeting the hinder edge of the squamosals and their post-tympanic processes, and lying behind the epiotic or " mastoid " portions of the periotic bones. From this region the exoccipitals run downwards to form the paroccipital pro- cesses ; while their middle region projects and constitutes the diverging semioval occipital condyles. Within the paroccipital process, on the base of the skull, is a con- siderable foramen for the hypoglossal nerve. 678. The basioccipital is a pentagonal bone joinino- the exoccipitals by suture, and separated from the basi*^ sphenoid by a narrow synchondrosis. The basisphenoid, half the length of the basioccipital, is continuous with the ascending alisphenoids. A narrow cartilaginous tract separates the basi- from the presphenoid, which is hidden below by the vomer. The large orbitosphenoids appear yui.J THE lSJiUi.i.s Oi< MAMMALIA. SOI in the posterior part of the orbit around and ahove the optic foramen. No change of importance has taken place in the auditory ossicles : the preceding description (p. 295) is sufficient. 679. The mandibular rami are quite distinct from each other. They are large deep bones, with a small notch between the coronoid and articular regions, the coronoid process scarcely rising higher than the condyle. 680. In the three preceding stages the enlargement of the face as compared with the brain-case, and the develop- ment of the tracts which are specially strengthened lor muscular attachment, have given the skull approximately its adult conformation. We may note that the ossifications of the auditory capsule were found in the fifth stage, as well as the small os bullte ; the turbinals, the os orbiculare, the tympano- and stylohyals. and the hyoid cornua in the sixth ; while it is only in the latest stage that the mes- ethmoid and cribriform plates are ossified, and the tubular part forming the meatus is added to the tympanic. Further description of the adult skull of the Pig may be found in Prof. Mower's Osteology of the Ma-inmaliu. APPENDIX ON THE SKULLS OF MAMMALIA. 681. The Mammalian skull presents great uniformity in its broad niorihological features, and tlie extraordinary varieties of external contour which are found depend mainly upon the relative size and prominence of bones, their distinctness or anchylosis, or their over- lapping one another. Remarkable cases of asymmetrical skulls occur among the Getacea. The most important forms of the Manmialian skull are admirably elucidated in Prof. Flower's Os'eolopp of the Mammalia, in Prof. Huxley's Lectures on Comparative Anatomy, and his Anatomy of Vertebrated Animals. We are yet in great ignorance of the history of the skull in most Mammalian groups; conseouently only a few facts of importance in general moipholugy can be noticed here. 302 MORPHOLOGY OF THE SKULL. [CHAP. 682. Reptilian affinities in the skulls of Echidna and Ornitho- rhynchus are found in the absence of an ascending ramus in the mandible, in the slight bending of the cochlea, in the stapes being im- perforate and columelliforni, in the malleus being very large and the incus small. In the Marsupials many of the cranial sutures persist throughout life; the squamosal, the periotic mass, and the tympanic remain separate, not uniting to form a " temporal" bone. Very fre- quently the jiigal extends far backwards and furnishes part of the articular surface for the mandible. The internal carotid arteries pierce the basisphenoid and enter the bottom of the pituitary fossa, as in Birds, instead of coming in at the sides of the basisphenoid. There is an anterior tympanic recess comparable to that of Birds (called alisphenoid bulla). The palatal plates of the palatines have large deficiencies in Marsupials, and in the Hedgehog. Palatal plates of the pterygoid exist in Mijrmecophacja, in Alanis, and many Ce- tacea. The pterygoid is hollowed by cells in Cetacea. In Manis several bones of the skull have air-passages, and by this means the two tympanic cavities are in communication with one another. 683. In Ruminants the external pterygoid process comes dis- tinctly from the basisphenoid. The whole sphenoid mass is developed from a pair of alisphenoid and a pair of orbitosphenoid centres; the basi- and presphenoid centres do not appear. The tympanohyal has two centres in the Sheep. In Rhinoceros the squamosal sends down a large postglenoid process, which unites with the post-tympanic process of the same bone to form a sort of external auditory meatus, the tympanic bone not possessing a tubular part. The "mastoid" portion of the periotic bone is hidden by the combined post-tympanic process of the squamosal and paroccipital process of the exoc- cipital. The horns of Uafjulata are well known and need not be described here. In Sireuia the frontals are prolonged into broad supraorbital processes, and the nasals .are abortive. The anchylosed tympano-periotic mass can be readily separated from the rest of the skull. 684. In Cetacea the supraocoipital and interparietal separate the small parietal.s and unite with the frontals, each of which has a great supraorbital plate. The very large maxillary extends far backwards and outwards, often largely overlapping the frontal, as well as far forwards, bounding nearly the whole of the gape. The premaxillaries have very long nasal processes extending to the an- '-"'•J J.nii. oivuuLio \ji} MAMMALIA. 303 terior nostrils, which are superior in position. The nasal passages are nearly vertical, the nasals and turbiiials saiall or rudimentary. The periotic mass is but loosely connected with the squamosal and tympanic, and is readily isolated. The lower jaw has no ascending ramus, and the condyle is posterior. The hyoid is broad and well ossified, with two pairs of bony comua. In the Carnicora there is a remarkable tympanic bulla partly due to the periotic cartilage and partly to an extension of the tympanic bone. The periotic portion is traversed by the internal carotid artery. For the struc- ture and variations of the bulla see Prof. Flower's Osteology and his important paper in Proc. Zool. Soc. 1869. 685. In Rodcntia the presphenoid is mostly very distinct as a median bone; it is seldom so in the other Mammalian groups, the median tract being ossified by the orbitosphenoids. There is fre- quently a large distinct posterior ossification of the alisphenoid. In the Rabbit the presphenoid is high and much compressed, so as to form an interorbital septum; and the optic foramina run into one, as in some Seals. The tympanic and periotic bones are anchy- losed together but not with other bones. The pituitary fossa is permanently uufloored by bone, so that there is a perforation in the middle line in the adult skull. The premaxillary is very large, with great palatal plates, and is much perforated. In the Guinea-pig {Cavia aperea), the pterj'goids do not reach the base of the cranium, but are fixed on basipterygoid processes like those of Ostriches, which are really homologous with the external pterygoid plates of the Pig. From the sides of the basis cranii above and behind these basiptery- goid processes a long continuous bony lingula passes backwards on either side, extendii)g to the front of the auditory bulla ; at the end of each is a delicate sigmoid bone, separate at first, anchylosed afterwards. The first answers to the pretympanic wing of the Bird, the second to the basitemporal. In old specimens the vomer is still found in two separate moieties. These facts indicate the higli anti- quity of this type. The vomer appears on the palate in the Cat and the Pangolin. In the Capybara and the Guinea-pig the alveolar border of the maxillary is prolonged far backwards beneath the orbit so as to unite with the squamosal at a level with the anterior border of the glenoid fossa. In some Rodents, as Dipus, Chinchilla^ there is in addition to the tympanic bulla another expansion above the tympanic cavity, forming a rounded prominence on the postero- 304 MORPHOLOGY OF THE SKULL. [CHAP. external angle of the skulL In the Beaver it forms a definite angular Ijrocess. 686. The Hedgehog has large pretympanic wings whose poster or margin nearly reaches the foramen magnum. In Moles and Shrews much of the skull is primarily cartilaginous : a large periotio crest projects from the upper edge of the ear-capsule, distinct from the supraoccipital, and becomes ossified, occupying a large part of the cranial wall. The squamosal is partly outside it, also appearinf in front and below. The Human Skull. 687. The circumstances which contribute most to modify thj form of the human skull and the condition of its component hot ,is compared with that of animals, are — 1st, the proportionally larv . size of the brain and the corresponding expansion of the cranial bones which enclose it; 2nd, the smaller development of the face as a whole, and especially of the jaws, which brings the facial bones almost entirely under the fore part of the brain-case, insteadof in front of it, as occurs in all animals, with the partial exception of the anthropoid apes ; and 3rd, the adaptation of the human skeleton to the erect posture, which, as regards the head, is attended with the sudden bend of the basicranial axis at a considerable aiigle upon the line of the erect vertebral column; and along with this the advance of the occipito-vertebral articulation to such an extent as to make the head nearly balanced on the upper extremity of the spine. The downward opening of the nostrils, the forward aspect of the orbits and eyes, the nearly vertical forehead, and more or less oval-shaped face, are accompaniments of these human peculiarities in the form of the head, which, together with those already mentioned, strongly contrast with the smaller cranium and its strong crests of bone, the larger projecting face and jaws, and the other characteristic features of the skull in most animals^ 688. A general acquaintance with the topography of the human skull will be assumed, in order that the references to its structure 1 This paragraph is quoted from Quain and Sharpey's Anatcmj, 8th edition, Vol. i. p. 73 ; this work also contains much of the' iuiormation (riven below, but ditterently arranged, gee also Huxley and Flower in the works before mentioned. Vm.J THE HUMAN SKULL. 305 and development may be abbreviated. There is on the whole a move cor ilete consolidation or anchylosis of the osseous elements than in mc, I, other mammals. Features of contrast between man and many lower forms are the union of the frontals and the moieties of the mandible; of the premaxillary with the maxillary; the distinctness of the presphenoid; the fusion of the interparietal with the supra- oceipital; the large proportionate size of the squamosal and its uniQn with the periotic mass; the comparatively small size of the tympanic, and the absence of an auditory bulla; the occurrence of fewer and less extensive elements in the hyoid arch. 689. Briefly catalogued, the following are the elements distinctly diiSiSfloped in the human skull, in comparison vviih the bones as named Yitorks on human anatomy : Occipital = basioccipital, exoccipitals, supraoccipital, interparietal. Sphenoid = basisphenoid (including sella turcica), alisphenoids with external pterygoid plates, pterygoids, presphenoid, orbitosphe- noids. Ethmoid— mesethmoid, ectethmoids with proper superior and inferior turbinals. Inferior turbiuals or niaxillo-turbinals ; vomer; sphenoidal turbiual. Parietals, frontals, nasals, lachrymals; premaxillaries, maxillaries, palatines; jugals (malars). Temporal, composed of several principal parts : — squamosal (in- cluding zygomatic process), tympanic (including tympanic ring, ex- ternal auditory meatus, and postglenoid process), and petromastoid (including prootic, epiotic, opisthotic); tympanohyal, stylohyal. Malleus, incus, os orbiculare, stapes. Inferior maxillary = dentary, coronary, splenial, mento-meckelian. Hyoid = basibranchial (body), ceratohyals (lesser comua), first branchials (greater cornua). 690. The occipital bone is almost horizontally placed, and the condyles are not far from the middle of the floor of the cranium. The supraoccipital originates by four centres, in pairs above and below : these .speedily unite, but four fissures long remain between them. The upper pair, developed in membrane, represent the interparietal. There are in early life anterior and posterior fontanelles in the middle line, at the anterior and posterior angles of the parietals ; there is also a fontauelle at the postero-inferior angle of each parietal. B. M. 20 306 MORPHOLOGY OF THE SKULL. [CHAP. The basioccipital and basispheiioid are separated by synchondroses up to the twentieth year. In the adult large air-cells are found in the conjoined basi- and presphenoid. A considerable portion of tlie nasal septum remains unossified, in front of the mesethmoid. There is a persistent aliseptal cartilage on either side, antero-inferior to the nasal bone, and united with the top of the septum ; also a pair of alinasal cartilages, each bent on itself, and the two co-appMed over the termination of the septum. Thus the alinasal cartilages bound the first part of the nasal cavities above, mesially, and externally, but do not reach to the base of the nostril in either position. 691. The alisphenoid nuclei appear about the eighth week of foetal life between the foramina rotunda and ovalia, and spread thence outwards, and also downwards into the external pterygoid processes. The basispheiioid arises from two granules, lying side by side in the sella turcica, and uniting about the fourth month. After their union two other centres appear, forming the lingulse ibasi- temporals), just outside the carotid grooves. The internal pterygoid plates (proper pterygoids) arise by separate nuclei in the fourth month. The alisphenoids are united to the basisphenoid in the first year after birth. The orbitosphenoids appear as a pair of nuclei, one outside each optic foramen; these extend by growth into the orbitosphenoid cartilages (lesser wings of sphenoid). Another pair of nuclei is found on the inner side of the optic foramina, and the presphenoid is formed by their union, or there may be an independent centre. At birth the alisphenoids are only suturally united with the lingulse, which are slill large in comparison with the basisphenoid. The orbitosphenoids and presphenoid are anchylosed together above the optic foramina, but the part of the orbitosphenoid beneath the fora- men abuts against the basisphenoid. Later still, the basisphenoid becomes larger in proportion to the lingute, and the posterior clinoid processes are ossified. The sphenoidal turbinal appears after birth, applied to the front of the body of the sphenoid (pre- and basi- .sphenoid). Each is in early life a hollow pyramid formed of three lamince; an inferior, constituting the adult turbinal; an external or orbital portion, situated in the adult between the orbital plates of the lateral ethmoid, alisphenoid, frontal, and palatine ; and a superior, forming the inner wall and roof of the original sphenoidal sinus, be- coming partially absorbed and partially united to the presplienoid, which by the enlargement of the nasal cavities and sphenoidal sinuses VIII.] THE HUMAN SKULL. 307 is ultimately reduced to the thin sphenoidal septum and rostrum. Tlie presphenoid is for a year or two broad, and rounded inferiorly ; it gradually becomes narrower and more prominent. 692. Ossification arises in the ectethmoids (orbital plates) in the fourth or fifth month, and gradually extends into the turbinal coils ; during the first year themesethmoid with the cribriform plate is ossified by a single nucleus, uniting with the lateral masses about the beginning of the second year. The ethmoidal cells are developed in the fourth or fifth year. It is only after birth, and with the gradual advance to adult years, that the spheno-occipital and spheno- ethmoidal synchondroses are obliterated ; the occipital mass is com- pletely united into one bone ; the vomer may become anchylosed with the mesethmoid. The epiotic fenestra existing for a long time beneath the arch of the superior semicircular canal is finally occupied by bone. 693. The maxillary, arising in the maxillopalatine process, begins to ossify immediately after the mandible and the clavicle, from several nuclei. Bfclard (Meckel's Archie, vi. 432) describes centres for the alveolar arch, the palatal plate, the orbito-malar tract, the nasal and facial, and the incisor. These are united at the end of the third month of foetal life. The antrum begins as a shallow depression seen before birth on the inner surface of the bone ; it deepens, extends outwards, and gradually separates the orbital and palatal portions, which at birth are close together. The premaxillary, occupying the nasofrontal process, is distinct, bat is covered on its facial aspect by a process of the maxillaiy, which unites with it completely before birth. In all young skulls an incisor fissure is traceable on the palate between the premaxillary and the maxillary, passing outwards from the incisor foramen to the front of the canine socket. The palatine arises by one centre ; the vomer by one, which developes two laminse embracing the septal cartilage. These two plates undergo increased union from behind forwards up to puberty, formmg a single median plate, and ultimately leaving only a groove on the anterior and superior surface, in which the mesethmoid fits. The nasal and lachrymal have one centre each; the jugal (malar) has one or two ; the inferior turbinal one. 694. The squamo-zygomatic portion of the temporal arises by one nucleus, the tympanic by another around the tympanic^mem- 308 MORPHOLOGY OF THE SKULL. [CHAP. brane, anT these centres soon unite. The periotic capsule ossifies late, by three centres ; (1) the prootic, forming the roof of the cochlea, the su] erinr and part of the posterior semicircular canals, the in- ternal auditory meatus, the tegmen tympani, and the upper part of the girdle of the fenestra ovalis ; thus the prootic gives rise to the greater part of the petrous portion of the temporal bone, as well as the upper portion of the mastoid ; (2) the opisthotic, constituting all of the petrous portion visible on the base of the skull, the floor of the cochlea, the investment of the fenestra rotunda, and half the girdle of the fenestra ovalis ; it also developes the lamina which completes tlie carotid canal, and furnishes the inner part of the floor of the tympanum ; (3) the epiotic, forming the lower and main part of the mastoid process. 695. At birth the petromastoid or periotic bone is separated from the squamosal by cirtilage ; bony union takes place in the first year. The "mastoid" tract is then flat, the glenoid fossa shallow, the articular eminence of the zygoma scarcely perceptible, the styloid process cartilaginous. The external auditory meatus is only de- veloped after birth, by the arching outwards of the united bones, and the growth of the special tubular part from the external surface of the tjmi)anic riu?. The mastoid process gets prominent in the second year, but its cells' only arise after puberty. A tympanohyal is found at birth and is distinct for a few years after, as a little cylin- drical plug, in a depression in the hinder border of the tympanic, just antero-internally to the stylomastoid foramen. It is soon anchy- losed with the periotic mass, and becomes ensheathed by the vaginal process of the tympanic. The styloid cartilaginous process is con- tinuous with the tympanohyal, begins to ossify before birth, and may remain unanchylosed with surrounding parts even to middle life. 696. The moieties of the mandible arise at first in fibrous tissue around the meckelian cartilage. Mr Callender^ has discovered a mento-meckelian ossification of the cartilage at the symphysis; and there appear to be splenial and coronary elements on the inner side, In addition to the deutary on the outside. The upper end of the meckelian cartilage becomes ossified as the malleus, and the carti- lage extends downwards and forwards between the tympanic bone and the periotic capsule ; the processus gracilis of the malleus, lying 1 See Phil. Tram. 1869, p. 163. VIII.] THE HUMAN SKULL. 809 in the Glaserian fissure between the squamosal and the tympanic, is the limit of ossification, the rest of the cartilage becoming gradually obliterated. The hyoid arch is represented above by the incus, the tympanohyal, and the styloid process or stylohyal. The auditory ossicles are at first altogether outside the tympanic cavity; and as the latter enlarges, its mucous membrane is reflected around the ossicles. The stapes is described as having three ossiflc centres. The hyoid bone is developed by five centres, for the two pairs of cornua and the body. 697. The dates of appearance of the diflferent osseous centres are as follows ; sixth or seventh week of gestation, dentary, premaxillary, maxillary, frontal ; seventh to eighth week, basioccipital, exoccipital, supraoccipital, basisphenoid, alisphenoid, presphenoid, squamosal, parietal, palatine, vomer, nasal, lachrymal, jugal or malar; fourth month, pterygoid, lateral ethmoid (middle and upper turbinal) ; fifth month, inferior turbinal ; fifth and sixth months, periotic bones ; eighth month, hyoid (great cornua, body, small cornua), tympanic, tympanohyal ; stylohyal j first year, sphenoidal turbinal, mesethmoid and cribriform plate. 698. The union of distinct bones takes place in the following order: the two basisphenoid centres unite in the fourth month of foetal life; the pterygoid and the external pterygoid plate in the sixth month ; the presphenoid and basisphenoid in the eighth month. During the first year after birth the periotic mass unites with the squamosal, the alisphenoid with the basisphenoid, and the moieties of the lower jaw coalesce at the symphysis. The second year witnesses tlie union of the frontals with one another, and of the mesethmoid with the lateral masses. The exoccipitals coalesce with the supra- occipital between the second and fourth years, witli the basioccipital from the fifth to the sixth year. The basioccipital and basisphenoid are anchylosed only after the twentieth year. The styloid ossification unites with the "temporal" in adult life; the osseous union of the hyoid cornua with the body of the bone does not occur till after middle age. CHAPTER IX. THE MORPHOLOGY OF THE SKULL. 699. Having now described in a condensed fashion the more important facts which have been ascertained respecting the history and structure of the skull in various forms, we proceed to summarise those facts, and to indicate some conclusions which appear reasonable. A larger measure of consideration is asked for these conclusions than if they merety arose from a verification of the facts which have been detailed; for they have become deve- loped in the course of many years of exploring work, in concomitance with a transition from the darkness of archetypal fancies to the clear light of actual verifiable history. In many cases the views expressed are based upon a much more extensive acquaintance with skulls in all stages of development than has been indicated ; a mass of detailed information remains for future publi- cation. The Cartilaginous Skull. 700. In all cases the primary elements of the cartila- ginous brain-case consist of two pairs, arising beneath the matrix from which the dura mater developes ; (1) the para- chordals posteriorly, under the greater part of the hinder division of the brain ; and (2) the trabeculse beneath the forebrain. Whatever cartilage appears subsequently in the brain-case grows nearly always in direct continuity with these elements, by gradual chondrification of mesoblast. In this way a more or less complete cartilaginous box is ultimately formed. CHAP. I X.J THE CARTILAGINOUS SKULL. 311 701. The notochord is related to the whole length of the parachordals and to the hinder extremity of the traheculse. The stratum in which it originates is imme- diately below the nervous axis, and somewhat above that in which the cartilaginous .elements arise. The ante- rior end of the notochord is often curved upwards towards the second brain vesicle ; but in some cases it is turned downwards again between the traheculse ; and it has moreover been detected in Elasmobranchs actually bent double, and running backwards some distance before terminating. In other forms the flexure of the noto- chord is very slight ; it simply bends downwards a little. In embryo Sharks and Skates from half an inch to an inch in length, the notochord is seen to present a number of headings in its extreme anterior portion ; this feature is evanescent. In the Fowl at an early stage the cranial notochord has two constrictions dividing it imperfectly into three spindle-shaped regions ; and this continues observable for a considerable time. So far as we can make out, there is nothing in the structure of the noto- chord itself which gives us very definite information about its segmental relations or about the segmentation of the skull. The point to be remembered is, that the notochord has a relation to the traheculse as well as to the para- chordals. 702. The parachordals become identified sooner or later with the tube of tissue surrounding the notochord, which in several cases (Bombinator, Dogfish, &c.) is defi- nitely chondrified previously to this confluence, and which is continuous with the cylinder of cartilage surrounding the rest of the notochord. Each parachordal becomes coalesced with the trabecula and the ear-capsule of its own side, and developes more or less of the lateral occi- pital wall, before uniting with its fellow. In fact, in the majority of cases the parachordals during development recede from one another in their anterior half or two- thirds; and concurrently with this, the notochord becomes, by the growth of surrounding parts, apparently retracted 312 MORPHOLOGY OF THE SKULL. [CHAP. in the cranial floor, so as to appear only between the hinder part of the parachordals. Thus for a time there is consti- tuted a posterior basicranial fontanelle, or membranous space in the hinder part of the floor of the chondro- cranium. 703. When the parachordals unite in the region where the notochord still persists, it is by the growth of car- tilaginous bridges both over and under it. The bridge beneath the notochord is very marked and becomes thick ; the cartilage is thinner above, and often non-existent for a long time, so that the notochord lies in a groove on the basilar plate constituted by the union of the parachordals. In many cases where a basicranial fontanelle exists, the ■cartilages do not approach one another again, and the fontanelle is only closed by bony growth; but in other types, especially in Birds, the space is nearly obliterated by the growth of cartilage, a later bony deposit completing the work. The whole of the cranial notochord is gradually aborted in most instances, and its place is occupied by car- tilage ; but in various forms a remnant is left as a slender string, embedded in the basioccipital bone or cartilage. 704. The length of the parachordal cartilages com- pared with other parts varies greatly. In some cases (Salmon, Fowl) the parachordals stretch anteriorly almost to the extreme notochordal apex, and posteriorly to a considerable distance behind the ear-capsules, being at first much more bulky than the trabeculae. It is note- worthy that the parachordals extend in the Elasmobranchs into a very considerable region of the neck. A seg- mentation takes place afterwards, separating the cranial i'nan the cervical cartilages at the usual place, and forming two basal condyles convex backwards. In all other known forms the hinder extremity of each parachordal primarily forms a condyle : and thus in every cartilaginous condition of the vertebrate skull where condyles are formed, there are two occipital condyles. 705. The parachordal is always bent outwards more IX.J THE CARTILAGINOUS SKULL. 313 or less, behind and in front of the ear-capsiUe, and partially undergirds it. In Elasmobranchs it grows quite underneath the otic mass, and is even carried outwards somewhat beyond it. The paroccipital process in higher vertebrates is an extension of the parachordal around, outside, and beyond the ear-capsule. The preauditory growth of the parachordal is more or less in confluence with the hinder end of the trabecula. 706. The trabeculse are at first simply solidifications of tissue, afterwards chondrified, in the sides of the floor of the first cerebral vesicle, which almost bulges down between them. The hinder termination of the trabeculse (Salmon, Frog) is often very slender at first. In others, as in Urodeles, they are largest behind, and early embrace the anterior half of the cranial notochord ; in Sharks they are about of equal breadth throughout. In cases where they are early in contact with the para- chordals, they lie over them at an angle. The two tra- becule have always a more or less lyriform appearance, being approximated in front, beneath the fore part of the brain-case, and also behind. In the posterior portion of the intertrabecular space the infundibulum and pituitary body are found ; but the primary interval between the trabeculse is related to the whole of the first cerebral vesicle, and not merely to these bodies. 707. The primitive trabeculse are flat or more or less rounded in section. The position they occupy in the head undergoes change in consequence of the mesocephalic flexure; at first, where early distinct, they are on the whole on the same level as the parachordals ; but the mesocephalic flexure may bend the former down to an angle of 120° or more with the latter, and in other cases they occupy this position when first distinguishable. As growth advances this flexure is entirely lost, and the level of the trabeculse and the parachordals may again become approximately identical. 708. It is on the whole in the lower types that the 314 MORPHOLOGY OF THE SKULL. [CHAP. trabeculoe are perceptible as primitively distinct from the parachordals ; and this condition would appear to be due to the mesocephalic flexure. In higher forms (Bird, Pig) they can at no time be found clearly separated, although a demarcation may appear to exist as the rudiment of the posterior clinoid ridge. Where the dimensions of the trabeculas are least bulky behind, they may barely embrace the apex of the notochord : in every case where they are distinct they do this. Where they are of greater size posteriorly, they are related to a varying length of the notochord ; in the extreme case (Axolotl), to more than half the cranial notochord, tapering backwards in close contact with it, and attaining their greatest width opposite the notochordal apex. 709. There is a most interesting variation in the relative size and date of appearance of parachordals and trabeculaB. In some types the parachordals are large and definite from the first, and the trabeculae disproportionately small and little solidified (Salmon); in others the trabeculae are clearly manifest and solid, and have taken up their definite relations considerably before the parachordals have appeared at all (Axolotl, Frog). In Elasmobranchs the two are very similar, but the trabecule chondrify first. 710. If not existing at the earliest time of solidifi- cation, a prechordal (postpituitary) bridge is formed as a meeting-place of trabeculs and parachordals, except where the parachordals do not extend so far forwards. In most cases this bridge appears to be wholly or in great part formed by the trabeculse. Furthermore the posterior clinoid ridge appears usually to be clearly a trabecular product, either arising as a transverse ridge on the united basicranial cartilage (Fowl, Pig), or being produced at first by the trabeculas overlapping the parachordals. 711. At the broadest region of the trabecula, in front of the notochordal apex, a small curved tongue or lingula may be given off, remaining free from other parts, and directed backwards (Fowl) ; or such a process may be IX. J THE CARTILAGINOUS SKULL. 315 large and become united with an antero-external process of the parachordal {Carcharias glaums). 712. The anterior extension of the trabeculse at their first appearance varies very greatly with the extent of the early development of the snout. They always from the beginning underlie the whole of the anterior part of the brain-case ; but in some cases (Frogs and certain Newts) they pass between the nasal sacs to the prenasal region very early, and throw out cornua. In others they very soon extend to the internasal region, where the septum subsequently forms. The trabecular modifications in the region of the cranial cavity will be first dealt with. 713. The trabeculse underneath the fore-brain, which are at first separated by an interval, but are closest together anteriorly, in many forms very early become approximated all along their length, except where the pituitary body and the carotid arteries lie between them. .They may be placed quite flat together (Elasmobranchs) or obliquely at an obtuse or right angle looking down- wards (Salmon). Coalescence quickly takes place in all the region of apposition, and a trabecular plate occupies the whole floor of the brain-case in front, adapting itself to the shape of the brain, being flat or gently curved, often rising more or less towards the anterior or ethmoidal extremity of the brain-case. The prepituitary part of the trabecular cartilage may develope a distinct anterior clinoid ridge, and thus a squarish or circular pituitary fossa may become limited. At a later period in many cases a thin floor of cartilage, continuous with the trabe- culse, arises in the bottom of the pituitary fossa, interrupted only by the perforations for the internal carotid arteries. The trabecular plate sometimes extends outwards beyond the cranial cavity, so as to partially support the eyeballs. 714. In Amphibia generally the primary trabeculse remain apart where they underlie the forebrain, but in the Anura a thin cartilaginous floor unites the bars and fills up the primordial fontanelle. In Snakes the trabeculse 316 MOEPHOLOGY OF THE SKULL. [CHAP. are permanently separate as far as the nasal region, and retain a simple form. In Eels they are separate anteriorly. 715. The most remarkable modification of the cranial trabecular region is that which gives rise to an interorbital s-aptum in Osseous Fishes, Lizards, and Birds. The roof of the mouth gradually becomes separated by a considerable distance from the floor of the cranium, and the deeper portion of the eyeballs comes to lie between the palate and the cranial floor. A median membrano-cartilaginous vertical wall (interorbital septum) separates the orbits from one another; and the cartilage which it acquires is usually due to the growth of a median trabecular crest. Thus the forebrain becomes supported mesially on a wall which rises high in front of the pituitary region; and lateral outgrowths from the top of this septum underlie the sides of the brain. The interorbital septum may be thick and solid, or thin with some thickening at the base ; and a greater or less extent of it may never chondrify, or ra.ny become membranous after having once been carti- laginous. 716. When the trabeculaj primarily extend as two distinct rods into the internasal region, they are early coalesced into an internasal plate, which forms the base of the nasal septum where that is definitely developed. The same term of internasal plate is also applied when the internasal chondrification, continuous with the trabeculse, is single from the first. This plate may be a simple bar at the base of the internasal region, very like that pro- duced b}' the early coalescence of the two trabeculse as above described, appearing as a continuation from the level of the cranial floor and sometimes permanently retaining this form; or it may first arise as a prolonga- tion of the vertical interorbital septum just described. In any case, anteriorly to the internasal tract, an azygous prenasal structure is very often developed; either directed straight forwards (Skate), or curved downwards as the axis of the beak (Bird). In the Skate it attains a very large development forwards as the axis of the rostrum. IX.] THE CARTILAGINOUS SKULL. 317 and has a grooved upper surface continuous with the internasal floor. In the Chick at an early stage, and in the Tortoise, it is bent \inderneath the axial parts and becomes recurrent. Where the adult has a very blunt snout, the prenasal element is short or almost absent (Salmon, Axolotl, Frog). 717. The main part of the nasal septum may at first arise double, as in the Pig, the two laminae very early coalescing. When the nasal organs are widely separated there are wings of cartilage (nasoseptal laminae), which arch widely outwards from the basal internasal region and unite with the olfactory capsules. They may become so outspread in adult life as to appear continuous with the internasal plate, only rising gently on each side. 718. Lateral outgrowths always proceed from the axial cartilages in the nasal region. The olfactory organs become supported by anterior and posterior growths, the trabecular cornua, and the antorbital or lateral ethmoidal cartilages. The cornua in some types are visible at almost the earliest stage of the growth of the skull, as outwartl curvings of the trabeculse, jutting forwards in front of the nasal sacs : and they may soon show traces of division into two lobes, of which one, external, broadens and partly underlies the nasal sac and defends it anteriorly, while the other bends inwards and downwards, becoming more or less recurrent underneath the internasal floor; and finally these recurrent cartilages, in some Birds, coalesce into one, quite independently of the prenasal part, which exists at the same time. 719. The antorbital cartilages in some respects agree with the cornua, in having a similar relation to the nasal capsule, in arising early from the trabeculae at their anterior junction : and they may be almost completely recurrent (Skate). In Urodeles they arise quite separately from the ethmoidal angles of the skull. I'hey frequently acquire a greater vertical depth than the cornua, in accordance with the height of the corresponding parts ; 318 MORPHOLOGY OF THE SKULL. [cHAP. and tliey may become the points of attachment or coa- lescence of a region of the mandibular arch, or cartilagi- nous parts developed from it. The antorbital frequently coalesces with the back of the nasal Avail, and in that circamstauce is sometimes carried far outwards by the growth of the capsule, beim^ disjoined from the axial part from which it at first arose. 720. We have now to trace the formation of a cartilaginous brain-case in the tissue surrounding the primary membranous cranium, in continuity more or less perfect with the basal structures which have been described. Separate cartilages are seldom developed in any part of the cranial boundary, but in many cases it is quite im- possible to determine whether the chondrification has been by a conversion of the cells at every point of the tissue, or by a proliferation of the cells in cartilage already existing : probably both processes' occur. The formation of lateral walls in cartilage is frequently not so complete as the furnishing of basal parts : and their extent and style seem to be very much influenced by the impaction of the three pairs of sense-capsules. The roof is still less perfectly chondrified in many types, and has its own special regions of fenestration. 721. In all cases a cartilaginous occipital ring, bound- ing the foramen magnum, is formed by the upward growth of the parachordals on either side, meeting above and coalescing. This ring is perfectly definite and continuous behind the periotic masses : but is complicated with them in their hinder region. The occipital ring in most cases is not limited to the region behind the ear-capsule, but the cartilage extends above and supero-laterally between the ear-capsules m the cranial roof, sometimes stretching for- wards for two-thirds of their length. The occipital invest- ment may be left somewhat imperfect below in the noto- chordal tract ; but in these cases there is a remnant of the notochord. 722. A cranial wall distinct from the ear-capsule cannot be clearly made out in the auditory region. IX- J THE CARTILAGINOUS SKULL. 319 Further forwards there is a cartilagiaous wall, interrupted by uerye foramina and by fenestras. This wall may be approximately vertical in lower types; but in ascending the scale, it becomes more and more tilted outwards and overhanging, so that ultimately (Pig) parts which were constituents of the wall in lower types enter into the composition of the floor of the large cranium, and the greater part of the wall and roof is unchondrified, and forms a vast fontanelle in the cartilaginous skull. It is especially to be noted that the types which pos- sess an interorbital cartilaginous septum are as a rule deficient in their cranial walls. The regions of lateral cartilage will be defined when we come to speak of nerves; but it must be mentioned here that in some cases (e.g. Snakes) a small cai-tilage called alisphenoid arises in the tract immediately in front of the ear-capsule, in the cranial wall or external part of the floor; and this may also appear as a process of basicranial cartilage, not cut off from it. In front of this a larger wing of cartilage, the orbitosphenoid, may be developed, also having definite relations. 723. The lateral cranial wall originates in the simpler types by the growth of a longitudinal crest upon the trabeculse ; this is most prominent behind, and often early confluent with the auditory capsule behind and the nasal in front. Where the cranial region is definitely marked off (in all but Elasmobranchs), this crest is continued in- wards in the ethmoidal region, the two crests meeting so as to form with the floor a trough or barge-like hollow in which the brain lies : and at the same time the ethmoidal tract may be confluent with the nasal sacs. 724. The lateral chondrification may become con- tinuous with a superior (supraorbital) ridge of cartilage connecting the ethmoidal and nasal regions with the auditory, and finally the chondrification may proceed in- wards towards the centre of the cranial roof. The tegmen cranii (Frogs, Sharks, Rays, &c.) is of varied size, usually leaving one large fontanelle in front, or two hinder 320 MORPHOLOGY OF THE SKULL. [CHAP. small ones in addition (Frog). The growth may take place chiefly from before backwards, as in the Salmon and Frog ; in the Salmon it almost entirely covers the brain-case with a thick roof or tegmen, excepting only two small postero-lateral or parietal fontanelles in which cartilage is permanently deficient. In Urodeles the growth of the cranial floor may be arrested early and the tra- beculse may remain comparatively widely separated, and finally appear as totally absorbed into the side walls, form- ing part of them ; so that the cranium has two complete side walls but no cartilaginous roof or floor in the trabecular region. The Sense Capsules. 725. The organs of special sense influence the archi- tecture of the skull in a very marked degree, the eyeballs being no less potent than the nasal and the auditory capsules in affecting the external form of the skull and the style of its intimate structure. The sense-capsules have in common a more or less complete cartilaginous investment, and an intimate association with and protec- tion by the cranial cartilage. They are situated in pairs at the sides of the skull, and are related to a more or less extensive fenestration of the cranial cartilage where they abut upon it. Each capsule receives a special nerve of sense, besides being supplied with other nerves. The cartilaginous investment is not perfectly complete in any case, or if it is complete at any period, does not remain so, orifices occurring through which the efficient agencies in exciting sensation may work, or in some cases as the relic of a primary involution. But the olfactory invest- ment differs from the other two in being usually like a cap or dome, more or less widely open below, while they are more complete, and rather resemble balls. 726. The auditory masses are on the whole of larger size relatively to the cranium in lower types, and in ascending we find them more and more subordinated to ^-^•J THE SENSE CAPSULES. S21 the enlarging brain-case, and inwrought into its general contour. In the lower groups the height of the ear- capsule is nearly equal to that of the cranium, and its breadth may be as great ; and the close impaction of the organ upon the brain-case does not allow of the formation of a distinct cranial wall of cartilage at that region, and leads to a coalescence of cranium and capsule at all the borders, with marks of distinctness at various points. Thus we may speak of the lateral tract where there is no distinct cranial cartilage as constituting a large auditory fenestra. In higher forms we are not able to perceive the formation of cranial and auditory cartilages separately from one another; the growth is continuous from the first: but the real fenestration may become obvious at a later stage, after ossification has taken place, by the lack of anchylosis with other parts throughout the whole or great part of the borders of the periotic mass. 727. In many instances a small postero-superior region of the capsule never chondrifies, or remains unchondrified till a late period of development, forming an epiotic fe- nestra. It marks the primary connection of the auditory mass with the epiblast. Another fenestra is formed in the Salmon infero-mesially, appearing at first as a space between the parachordal cartilage and the capsule; but the former travelling outwards and the capsule lying over it, and becoming identified with it, the fenestra travels also outwards, and beconies really an opening in the lateral aspect and floor of the vestibule, afterwards closed. Its ultimate position is similar to that in which the stapes arises in Amphibia. A deficiency occurs in the cranial surface of the ear-mass in some Teleostean fishes, so that there is no cartilage at that part between the membranous labyrinth and the cranial cavity. 728. A remarkable feature about the ear-capsule is the formation at an early period in nearly all types of an external more or less horizontal projection, called the pterotic ridge : it is related to and often contains part of B. M. 21 322 MORPHOLOGY OF THE SKULL. [CHAP. the horizontal semicircular canal. Where there is no tympanum this ridge overhangs the articulation of the appendicular elements of the skull. When a tympanic cavity is developed, this ridge is specialised into a tegmen tympani, or tympanic roof; and then there is a definite scooping of the ear-mass beneath the ridge. This cavity may be still more defined by a posterior boundary and ])artial floor derived from the exoccipital cartilage, and by a cartilaginous floor derived from the ear-cartilage itself (auditory bulla). 729. The surfaces of articulation and regions of coalescence of the periotic capsule with appendicular parts will be mentioned later; but one point must be referred to here : namely, that a fenestra (ovalis), which has an intimate relation to the function of hearing, arises in the part of the capsule adjacent to the tympanum, either by dehiscence of the cartilage, or by the cutting out of a small segment, the stapes, which remains in the hole out of which it had been cut. The stapes is formed in other cases by the chondrification of the tissue in the membranous fenestra. In the Urodeles the origin of the stapes may be described as the cutting off of an opercular fold over the fenestral slit; and here the stapes is only connected by ligament with parts at some distance from it. But in higher forms the stapes is always closely tied to, or coalesced with, or originates in continuity with ex- traneous elements. Another fenestra (rotunda) originates in the wall of the cochlear rudiment in many Reptiles, and occurs in a similar situation in all Birds and Mammals. 730. The mobile eyeballs are not amalgamated with the cranium like the auditory masses ; but the cavities in which they aie lodged show most adraii-ably the plia- bility of the axial parts, and the manifold shades of adap- tation which they can as.sume. Whether the eves occupy the sides of the whole fore part of the brain-case, or are embedded beneath it, or elevated above it (Chimsera), or carried outwards (Zyggena), the orbits are bounded by the same cartilaginous structures little modified. In the ^^•j THE SENSE CAPSULES. 823 simple^ lateral position of the eyes (Sharks), the outward extension of the trabecular base of the cranium provides a partial floor for the orbit, the projecting supraorbital tract protects it above, and the antorbital and postorbital or sphenotic terminations of that tract wall it in before and behind. The palatal structures and aponeurosis form the rest of the orbital floor. In the Amphibians, which have simple lateral orbits, there is little furnishing of protecting laminae ; but the history of the suborbital car- tilages in the Anura is of great interest, the subocular bar being first the mandibular suspensorium and after- wards the palatopterygoid cartilage. The eyeballs in au early stage rest almost entirely on cartilage ; but as the subocular fenestra is gradually widened with the growtli of the head, the eyes come to rest almost exclusively on membrane. In these types the optic foramen is always larger, often much larger, than the nerve which passes through it, the rest of the space being filled by membrane. 731. When the eyeball is more deeply embedded and more thoroughly protected, we have a modification of the cranial cartilage which is as if the lower regions of its side-walls were forced together and converted into a mere vertical septum. The fore part of the cranial cavity is elevated upon this septum, the portion of the brain therein contained becoming small proportionately, and lying be- tween the upper part of the eyes, or even totally above them. In this condition, as growth proceeds, a more or less extensive fenestration of the septum very frequently occurs, and there may also be (Lizard) fenestration in the slanting cranial roof of the orbit (orbitosphenoidal tract), as well as an optic foramen larger than the nerve. 732. In Birds the orbit is protected by the cranial wall slanting outwards above and behind, by the pro- minent supraorbital ridge above and behind, and by the antorbital or postnasal plate of the nasal capsule, which may be prolonged into an external and inferior process (os uncinatum of Finches and Parrots). No very definite floor is developed, the palatine tracts being little specialised 21 2 324 MOEPHOLOGY OF THE SKULL. [CHAP. in relation to the orbit. In the Mamixial an additional circumstance influences the position and relation of the orbits, namely, the extension backwards of the nasal cap- sules beneath the fore part of the cranium and between the eyes. In its cartilaginous condition the orbit is roofed by the orbitosphenoidal tract, and partly floored and walled by the lateral ethmoidal cartilage. It is subsequently comi^leted by bones, but may continue to be very largely supported by membrane beneath. A very considerable por- tion of the orbit may in these cases be below the level of the cranial floor. 733. The sclerotic investment of the eyeball is car- tilaginous in most types except the mammalian. In many Elasmobranchs and some Osseous Fishes the eyeball is sup- ported near the entrance of the optic nerve by a movable cartilaginous rod or pedicle, articulated with the ' sclerotic and with the cranial cartilage. 734. In its simple condition in the Elasmobranchs the nasal capsule is a dome of cartilage widely open below. Its sensory membrane is prodiiced into many folds, but these are not supported by cartilage. The capsular carti- lage does not grow out from the cranium, but early be- comes continiious with it behind. More or fewer labial cartilages become involved in the valvular inferior open- ing, and may coalesce with the proper nasal investment. A lateral union of the capsules with the internasal cartilage occurs by the intervention of nasoseptal lamimB. A fenes- tra may arise in the inner superior aspect of the capsule, in addition to the large nerve fenestra more posteriorly, through which the olfactory fibres pass downwards. 735. In the Salmon separate capsular cartilages can- not be said to exist : there is at an early period a median internasal tract of cartilage with anterior and posterior horns, and a floor formed by the conjoined trabeculse. In the adult the membranous nasal capsule lines a scooped hollow on either side of the massive snout cartilage, and is protected by bones. In most Urodeles there is the I^-] THE SENSE CAPSULES. 825 same massive internasal cartilage and anterior growth from the cornua; but there is more or less of distinct roofing cartilage belonging to the capsule ; no labial car- tilages here become related to the nasal openings. 736. In the Frog a special vertical nasal, septum is formed. The olfactory sacs rest at first on the anterior prolongations of the trabecule. Later, they lie between them and the distal end of each mandibular suspensorium — a remarkable condition. Finally, the nasal septum being formed, a supero-external proper nasal roof arises, and the cornua and labials are dovetailed into the bounda- ries of the capsules, so as to leave two openings, a superior and an inferior. The roof bends downwards behind so as to form a hollow conchoidal recess. In other Anura which have a nasal septum, the true capsular cartilage may be very imperfect or almost Bon-existent, together with a most extensive adaptation of true labials to the defence of the sacs (Pipa, Dactylethra). In the Frog we see the first appearance of cartilaginous laminae within the capsule at its fore part. Similar laminae occur in many Reptiles. 737. In Birds and Mammals the olfactory capsules rise to a very high grade of complication. They may increase in size so as to occupy far the greater part of the length of the skull; but some considerable part of the complication connected with them is due to bones which are not preformed in cartilage ; these need not enter into our consideration of the capsules proper. Essentially we may consider the capsules in these classes as consisting of the median vertical septum, and of outgrowths from it or arising continuously with it, forming roofs, side-walls, posterior and partial anterior walls, and more or less of a floor on the external aspect, by folding in of the side- wall. In a few forms this floor becomes complete (Fal- cons). From the inner surfaces of these capsular cartilages laminae of cartilage proceed inwards towards the septum and become coiled in very various styles. Three principal regions are distinguished, aliethmoidal (posterior), aliseptal (middle), and alinasal (anterior), the latter being related r)2C MORPHOLOGY OF THE SKULL. [CHAP. to the external or anterior nostril. Tnrbinal laminae may arise in all these regions, having special characters of their own: those of the aliseptal region (inferior turbinal, nasal turbinal) are often preponderant in size ; those of the ali- ethmoidal region (middle and upper turbinals) are espe- cially related to the sense of smell. In Mammals all these growths are seen at their highest perfection, and in many cases the nasal sacs burrow backwards beneath the cranial cavity into the sphenoidal region. Both in the Bird and in the Mammal there is an adaptation of primordial labial cartilages to the purposes of the anterior nostril : in the Pig this orifice is inferior. The posterior nostril is also provided with special labial cartilages, appearing in the vomerine region, in Snakes and many Birds. 738. In the Mammalia the space in which the ol- factory fibres pass down into the capsules is at first a large membranous fenestra comparable to that for the optic nerve of the Frog; the fibres pass in a scattered fashion through this membrane. During growth carti- lage appears in the interspaces between the fibres, and forms the cribriform plate continuous with the top of the hinder part of the nasal septum. Another fenestration, corre- sponding to that which occurs in the interorbital septum, arises in the nasal septum of many Birds, and this may become very large, aborting the previously existing car- tilage so much as to produce a large notch, opening below: this is principally in the aliseptal region. In some cases there ai'e several fenestraa' in the septum. The Arches. 739. The arches appended to the cranium have cer- tain resemblances to those of the trunk. They enclose a cavity, are liable to segmentation, and, have very similar nerve-relations. The non-attachment of the gill-arches to the cranium or to vertebra; contrasts with the adherence of ribs to the vertebral column. IX.] THE ARCHES. 827 740. All the postoral arches may bear gills in some vertebrates or at some period of development. The man- dibular and hyoid are highly specialised in regard to various functions; the last branchials also may be con- siderably modified. A simple idea of the series may first be considered, viz. the average branchial arch in most Fishes. A continuous rod of cartilage originates in each visceral fold at the sides of the throat, bending more or less backwards in the pharyngeal roof, free from the cranium and vertebrae, and passing forwards and inwards below. The inferior ends of each pair of arches usually become connected by the intervention of a median key- stone piece or basi branchial, very comparable with the azygous elements of the sternum. By a simple segmenta- tion these branchial arches are perfected. Four segments arise, and a more or less perfect jointing allows of mobility of the parts. The upper and the lower segments are usually more or less horizontal in position; the two others are the more important, and are chiefly lateral. The number of the branchial arches in Fishes varies from five to seven ; the hindermost of them is modified, attached to the one before it, and deficient in one or more of its segments. The basibrauchials become attached to oue anotlier in the median ventral line. 741. The hyoid arch in Fishes becomes divided by seg- mentation into the same number of parts as the branchials, and there is a basihyal : but the mandibular arch on each side may be only segmented into two main pieces, a sus- pensorial and a meckelian. The suspensorial cartilage in all oviparous vertebrates has a forward prolongation which enters into the maxillopalatine process of the cheek, and may be so large as to constitute the entire upper jaw and enter into no definite union with other parts (Shark) ; or again, it may unite with a retral antorbital or palatine growth to form the main palato-quadrate bar (Frog, Sal- mon). In this case we have a conjunction of evidently distinct parts. The meckelian cartilage does not undergo segmentation, nor does it form a basal piece. In some 328 MORPHOLOGY OF THE SKULL. [CHAP. cases the right and left cartilages fuse at the symphysis. The joint between the lower jaw and the suspensoriuni appears to correspond in position to those between the epibranchials and ceratobranchials. 742. The relations of the upper end of the mandi- bular arch to the cranium, and the methods of suspension of the lower jaw, offer numerous points for consideration. In certain forms the mandibular arch is directly and strongly fixed to the cranial floor or wall, or to the ear- t'apsule ; and concurrently with these conditions there is a complete distinction between the hyoid arch and the man- dibular; or again, the former is so tied to the latter as to take a considerable part in the suspension of the lower jaw. When the mandibular suspensorium entirely or principally supports the lower jaw, the skull has been called autostylic^; when it largely shares the work with the hj'oid, the term amj^hlstylic is applied. But in certain cases tBe mandibular arch does not retain its primary relation to the cranium, but becomes distant from it, and not directly fastened to it ; it is firmly tied to the upper portion of the hyoid arch, which thus becomes the prin- cipal suspensor of the jaw ; these skulls are hyostylic. 743. The primary relation of the mandibular arch to the cranium is to its side-wall and base. In the Axolotl it is at first applied to the trabecule far forwards, but soon becomes placed in the usual situation, namely, just in front of the ear-capsule and near the primary apex of the trabecula. An ascending process in some forms brings the arch into relation with the side-wall of the cranium (vnrious Urodeles) ; and either of these regions may coalesce with the cranial cartilage. As a result of transformation, or a.s a primary relation in higher types, the mandibular arch may be brought into connection with the capsular cartilage of the ear, may develope a surface for articulation with it (Frog), or may unite with it. A backward prolongation of the upper region of the arch (otic process) may lie 1 Huxley, Ceratodus,\.c. ^^•] THE ARCHES. 329 along the side of the capsule or coalesce with it. Further- niore, a hinder and superficial leaf of the suspensorium IS cut off in many Anura, and becomes adapted to the tympanic membrane as the annulus. 744. The suspension of the lower jaw in hyostylic skulls is effected by a smaller or greater modification of the primitive form of the hyoid arch. The upper part of the arch either arises separately, or is cut off from the rest of the arch ; being carried forwards at its lower end, it is firmly attached to the lower part of the suspensorium and to the jaw. In a primitive condition (Shark) the upper end of -this hyomandibular element is articulated both with the basis cranii and the auditory capsule, under- neath the pterotic ridge. 745. The segmentation of the hyoid arch to form this hyomandibular may either be transverse or oblique, in a position comparable to that of the joint between the epi- and ceratobranchials; or it may be longitudinal (Salmon), affecting neai-ly the whole length of the arch, the anterior (hyomandibular) piece becoming superior, and the posterior inferior. In am phi- and autostylic skulls the hyomandibular piece may be comparatively small, or may not exist. 746. In many Anura and Sauropsida, when the hyoid arch chondrifies, it appears to be only the lower portion of an arch. Sometimes at an early, sometimes at a comparatively late period of metamorphosis, a series of structures originates near the auditory capsule and behind the mandibular suspensorium, in precisely the same situa- tion as the hyomandibular, the nerve and cleft relations being identical. They failed formerly to be recognized as portions of the hyoid arch, because they did not originate continuously or contemporaneously with its lower segments, or because in the adult they may not be continuous, their directions being different. These structures form the columella auris, and always enter into connection, sometimes coalesce, with the stapes belonging 330 MORPHOLOGY OP THE SKULL. [cHAP. to the auditory capsule. One or more segmeuta may arise in this upper hyoid tract; and its outer end is usually in relation with the tympanic membrane, and helps to support it. 747. In Mammalia, while the primitive condition is more complete, the metamorphosis is in some respects more extraordinary. Both the mandibular and the hyoid arches are of full length and development at first; and their upper ends become related to each other and to the tympanic cavity. The mandibular arch is never seg- mented; but its meckelian region does not form part of the functional lower jaw. Its upper tract, remaining small, is specially moulded for purposes connected with hearing, lies in the tympanic cavity, and supports by one of its processes the tympanic membrane. The upper part of the hyoid arch is bent inwards to be fastened to the stapes, and then the hook, apposed by its elbow to the top of the mandibular arch, is cut off, becoming the incus; the main part of the arch is carried backwards and coalesces with the auditory capsule. The processes of the incus are exactly paralleled by the processes of the columella in Frogs. 748. Retrogression in the cartilaginous cranium of particular forms can hardly be said to exist, except in the Urodeles, where some of the axial basicranial cartilage is absorbed. But as we pass from branchiate to higher types the proportionate amount of cartilage that occurs in the cranial investment is diminished. 749. The mandibular arch can scarcely be said to undergo retrogression : at a varying period of development it ceases to grow, and becomes a less important factor in the lower jaw in each ascent of type. The hyoid and branchial arches may however be said to retrograde, but the change is related to new functional adaptations. These phenomena occur especially in the caducibranchiate Am- phibia. The main (lower) part of the hyoid arch, from being short and massive, and attached to the suspensorium, IX.] THE ARCHES. 331 becomes long and slender and attached to the ear-capsule; in some it is entirely absorbed. In Frogs the branchial arches are originally quite distinct from one another, but become united into a basket-work by transverse bars both above and below; and then gradually during the loss of the water-breathing function, the whole branchial struc- ture is almost completely absorbed, leaving rudiments which are related to the larynx, and which unite with a broad basihyobranchial plate very much resembling a sternum. lu other cases the branchial arches in becoming modified are variously tied to one another; and the rem- nants are of very different patterns. Frequently what looks like a process of transformation is merely a cessation of growth combined with adherence to a basal attachment. 750. In the abranchiate forms the early hyoid arch may elongate, remaining slender, and there may be con- tinuous cartilage between the basihyal region and the ear-capsule; or a portion of the arch becomes merely ligamentous, leaving cartilage proximally and distally. One pair of branchial cartilages is found, ceasing to grow at an early date; they mostly do not disappear, but become applied to, and support the larynx: a relation to the orifice of the breathing apparatus in air-breathers parallel with that of the branchial arches to the respi- ratory orifices in water-breathers. In the Frog and in Mammals paired branchial rudiments partially embrace the larynx. In Birds it is the posterior end of the basi- branchial bar that is applied to the larynx: in Urodeles it is the same element which is thus adapted, but it is com- pletely segmented off from the remaining parts. In Lizards there is no application of the branchial arches to the larynx; and there is a considerable first branchial arch. In Woodpeckers there is no basibranchial, but an immensely long first branchial arch is extended completely, in other birds partially, round the head, and lies over the nasal roof. 832 MORPHOLOGY OF THE SKULL. [cHAP. The Cranial Nerves. 751. The relations of the brain-case to the foramina of exit of the cranial nerves are of much importance in the study of the skull. The positions of these nerves assuredly serve as landmarks; but their precise significance cannot yet be definitely expressed. A certain order is al- ways observed in their arrangement; but the distances of the foramina from one another vary greatly, and two of the nerves (trigeminal aud vagus) often perforate the cranium as two or more trunks. The order in which the nerves pass out from before backwards is the following: olfactory, Optic, oculo-motor (several), trigeminal, facial, auditory, glosso- pharyngeal, vagus, hypoglossal. These nerves are certainly not of equal morphological value. How far they correspond, in relation to segments of the body, with the ordinary nerves, e.g. of the dorsal region, is not yet ascertained'. Consequently deductions from them with regard to the composition of the cranium are premature. Those who hold that the olfactory and optic nerves are of so special a nature as to be quite unconnected with the ordinary segmental arrangement of nerves, are certainly not en- titled, from the jjosition of these foramina, to make any dogmatic conclusions respecting the cranial segments and their homology with vertebral segments. We believe that these nerves will ultimately be found to have some definite connection with primordial segments and their nerves. 752. The position in which the cranial nerves emerge may be generally defined to be the lateral region of the basis cranii or the lower part of the side-walls. The emergence is never in the middle line, the nearest approach to it being in the case of the olfactory nerves. These mostly pass out of the primordial cranium through a large fenestra in the back part of the nasal walls, the fibres 1 See Mr Balfour's very able discussion of this question in his Deve- lopment of Elanmobranch Fishes, Jonrn. Aunt. Vol. xi.. Part 3, April 1877, and also Mr Marsball's valuable paper on the cranial nerves of the Fowl in the same journal. ^•^■J THE CRANIAL NERVES. 333 being not closely packed, but more or less widely dis- tributed in the fenestra. The olfactory fenestra is at or very near the anterior end of the cranium. 753. The optic nerves enter the hinder or middle region of the orbits by a foramen frequently larger than the nerve. The motor nerves of the eyeball may enter it by separate small foramina behind the optic foramen, or in common with the anterior division of the trigenainal. The latter is always situated near the junction of the cranial wall with the ear-capsule. Several branches pro- ceed from it, becoming distinct before emergence. They may all pass out of the brain-case by a common foramen, or by as many as three distinct foramina, corresponding to the three main divisions of the nerve. The facial nerve is more or less closely connected with the auditory, and in higher forms one foramen may be common to the two nerves In many Anura the facial emerges in front of the ear-capsule, in the same foramen with the trigeminal. In other forms it bores into the front part of the ear-ca]3sule and has a winding course within its anterior and lower region. In Sharks and Axolotls the facial foramen is in the line of junction between the true basilar cartilage and the ear-capsule. The auditory nerve enters its capsule directly. The glossopharyngeal and vagus foramina are always found in or near the line of junction of the oc- cipital and auditory cartilage; being more in the base or side-walls according to the position of this junction. They may emerge by a common foramen; or by two foramina not far from one another; or the vagus, separated into a number of strands, may have a foramen for each, as in Notidanus. The glossopharyngeal appears frequently as if included in the auditory cartilage : the vagus not so. The hypoglossal nerve (where it exists) comes out more internally than these nerves, just in front of or near the condyle. 754. The greater part of the tracts in front of and above the mouth are supplied by branches of the tri- geminal nerve. The main anterior branch (ophthalmic 334 MORPHOLOGY OF THE SKULL. [CHAP. or orbitonasal) has a general distribution throughout much of this territory, especially around and in the orbits and nasal cavities. In Birds its nasal branch becomes specially related to the prenasal cartilage: and it may pierce through the anterior facial tissue and appear on the ex- treme fore part of the face in them and in mammals. There are several indications that this nerve is a dorsal branch, whose distribution is extended far forwards'. The co- extension of the orbitonasal and the trabecular growths is very remarkable, whatever may be its interpretation. 75.5. Of the oculomotor nerves it may be said that they supply a series of muscles which are developed, at least in lower forms (up to the Frog), from the first muscular segment of the body, appearing primarily on either side of the anterior part of the notochord ; and permanently taking origin from the basicranial canal in the Salmon. The oculomotor nerves are more or less clearly divisible into an anterior and a posterior division. 756. The orbitonasal nerve runs along the side-wall of the skull in its orbital path; frequently it ascends high up under the supraorbital ridge. In every case where an ascending process of the mandibular suspensorium is developed, this lies above the orbitonasal at its divergence from the remaining trigeminal trunks. The two other divisions of the trigeminal are the superior and inferior maxillary. They diverge almost directly outwards in most cases over the suspensorium in autostylic skulls. The distribution of the superior maxillary corresponds tolerably closely with the extent of the maxillopalatine process: that of the inferior maxillary with the lower jaw. 757. The facial nerve in almost all cases sends for- wards a notable branch, soon after its exit from the cranial cavity, outside and at a little lower level than the orbito- nasal. This is the palatine or vidian nerve, and it is supplied to preoral parts like the last, but does not extend 1 Vr Balfour has brought forward this view prominently in his paper iihove cited. ^^•j THE CRANIAL NERVES. 335 SO far forwards morphologically: it has nothing to do with the prenasal region. 'I'he facial trunk further divides sooner or later into two brandies, one of which (known as the chorda tympani, in mammals), turns forwards and unites more or less closely with the inferior maxillary nerve. The forking of the facial is always above the first visceral cleft, and is often very closely related to it. Con- sequently also this branching always takes place in or in close proximity to the ear-capsule. The course of the facial in the periotic cartilage is notable for its great concavity looking backwards — which may be matched by the concavity looking forwards, frequently seen in the course of the glossopharyngeal. The posterior branch of the facial is primarily related to the fore edge of the hyoid arch. The distribution of the facial is very compli- cated and little explained in higher animals ; it spreads over a large territory on the face, and is the motor nerve connected with the development of facial expression. 758. The glossopharyngeal nerve is not so much implicated in the ear-capsule as the facial, but yet may be united by branches both with the facial and the vagus. It supplies the hinder part of the hyoid and the fore part of the first branchial arch, the branches being separated by the second visceral cleft where that exists. The vagus is distributed to the branchial arches, its branches (like so many distinct nerves) forking above the arches after the manner of the glossopharyngeal. Besides this the vagus is distributed to many tracts of the body, and its whole significance cannot be entered upon here. In abranchiate forms it supplies those parts which are in any way recognizable as corresponding to branchial arches : but this is but a very small fraction of the whole distribu- tion of the nerve. The hypoglossal is the nerve of several muscles behind the hyoid arch as well as in front of it. 759. It should be noticed how remarkably several of the cranial nerves pass forwards, and appear to go beyond any simple segmental distribution. The orbitonasal, the vidian, the glossopharyngeal, and the hypoglossal are all of 336 MOBPHOLOGY OF THE SKULL. [CHAP. this nature, while we see the vagus distributed far backwards, even in the simplest forms: thus, supposing evolution to have occurred, the lowest types referred to m this book have passed very far beyond any primordial ar- rangement of segments and nerves that can be conceived. Flan and Segmentation of the Cartilaginous Skull. 760. We are thus led naturally to consider what conception can be formed of the real structure of the cartilaginous skull. This must necessarily involve refer- ence to the cartilaginous vertebral column, which shares with the brain-case the function of surrounding and pro- tecting the axial nervous system and emitting nerves for distribution over the body. 761. We do not conceive of the skull as being com- posed of a number of coalesced vertebrae; not having perceived indications of any process of coalescence in the embryo, and being unaware of any evidence of the past occurrence of such a transformation in ancient times. A large portion of each vertebral centrum is due to chondri- fication of a tube of tissue around the notochord. Su. h chondrification takes place distinctly around the cranial part ot the notochord in two types examined; and this cartilage is no doubt homologous with the spinal cartilage just mentioned. As evidence relating to its segmental value in the chondrocranium we have merely the constric- tion of the cranial notochord in two places in the Fowl, giving the appearance of three notochordal segments; and oDe constriction in the Urodeles. The notochord and its enveloping cartilage are never found in the anterior part of the cranium, and consequently, judging by ordinary stand- ards of homology, the prepituitary structures contain nothing resembling the main part of the vertebral centra. 762. The brain becomes enveloped in lower verte- brates by almost continuous cartilage, except where nervous J-S-.J THE CARTILAGINOUS SKULL. 3S7 structures pass out. The origin of this investment from two paired bars, the trabeculse and parachordals, which have different histories, do not originate contemporaneously in some types, and lie in the base of the anterior and posterior regions of the cranium, is apparently unlike any- thing in the history of the vertebral column. "What then is the meaning of these bars? Are they similar to one another ? 763. Viewing embryos at the stage when the meso- cephalic flexure is strongest, gives rise to the idea that the trabeculse and parachordals are not of the same order. The early anterior coalescence of the trabeculse presents some resemblances to the coalescence of the ventral ex- tremities of a visceral arch. The curved outline of the trabeculse in various forms further gives an appearance of possibility to the view that the trabeculse might be forwardly turned visceral arches supporting the brain; but the same fact has made it conceivable by some that the trabecule are a pair of down-turned neural arches. 764. Yet- when we consider that the trabeculae, like the parachordals, arise in the mesoblast in the floor of the cranium; that the occurrence of the mesocephalic flexure does not change these relations, and would appear to be due to the great expansion of the brain at an early stage, producing a discontinuity of the basal bars at the region of the curvature ; that the flexure becomes oblite- rated, and the basal cartilages coalesce into one on either side, and then by junction of the right and left bars a continuous floor arises; that the side walls of the fore part of the cranium are formed by growth from the trabe- culse, just as, posteriorly, the walls are formed from the parachordals; that nerves are similarly emitted through the trabecular and the occipital walls; when it is seen, in short, that the trabeculse are neural in all their relations, as completely as and in similar fashion to the para- cliordals; it seems impossible to resist the conclusion that the trabecule and the parachordals must be placed in one and the same category. li. JI. " 22 338 MORPHOLOGY OF THE SKULL. [cHAP. 765. If we further consider in what way the structure of a wide neural tube will differ from that of a simple cylindrical one, it will appear that the formation of a broad floor is a feature of unlikeness, but only in degree. If then the trabeculas and parachordals are looked upon as representing in a continuous form the basal parts of a series of cartilages forming the neural arches of vertebra, flattened and widened, we think some advance is made in comprehending the skull; and we are then prepared to view the lateral parts of the cranium as continuous neural arches protecting the brain, extending into the roof and uniting more or less extensively with each other. And just as in certain forms of vertebrse the bases of the neural arches nearly or quite meet above the centrum surrounding the notochord ; similarly in the part of the cranium destitute of notochord, the bases of the neural investment meet and form a basal tract, but without any proper centrum beneath. 766. The most complete brain-case of this kind is found in forms which do not develope bones ; in other types the cranium is mostly never perfected in cartilage ; often the roof is very incomplete, and even the side walls are considerably deficient. The occipital region always however remains more or less perfect ; sometimes it is imperfect basally in the notochordal tract. 767. There is no definite evidence of segmentation in the history of the highly-perfected chondrocranium of the Elasmobranchs, unless the separate origin of the tra- beculee and parachordals is regarded as such, which we cannot think. It has been said that it is still very doubt- ful what is the real significance of the position of exit of nerves from the cranium as regards segmentation. Until the doubts as to the homology or equivalence of the cra- nial nerves are cleared up, it would be rash to frame a theory of skull structure upon their disposition. If we trusted to them, we should be led to consider that at fewest six segments are rcpre.sented in the simplest skull, ^^•J THE CARTILAGINOUS SKULL.- 339 Without reckoning more than one as related to the vagus'. 768. In Elasmobranchs there is a precranial floor perfectly continuous with the cranium, but it is destitute of a roof, and its side-walls unite with 'the nasal capsules. Where shall we mark off the termination of the cranium and the commencement of that which is not cranium ? We cannot tell. We see no logical ground for separating into two categories the cartilage between the nasal sacs o'f a Skate and the cranial floor : the scooping of the rostrum in the same skull as well as its continuity of origin from the trabecula appears to indicate that it is not to be, dissociated from the cranial floor. These parts then must have originally had some relation to body- segments. The long precranial valley found in an early stage of Dactylethra may be considered as throwing light on this matter. 769. There appears to be no reason to doubt that the Elasmobrauch skull is in many ways the lowest and simplest type we have examined ; and it leads us to specu- late on the former existence of a still simpler form of skull, related to a large number of segments of the body, in which the brain was less concentrated and little ex- panded, and extended along a lengthy brain-case. Ascent of type appears to correspond to expansion of the capacity of the cranium together with concentration of the brain, its retraction from between the nasal capsules, and in higher forms its very great diminution in the interorbital region, or almost total retraction behind the orbits. 770. Thus we view the chondro cranium as having had a long history; and it may be impossible to discover and 1 Thia mode of computation is adopted provisionally, in preference to adding the full number of branchial arches to the cranial segments. We do not see that it follows, because a nerve is supphed to several seg- ments, that the portion of the skull to wbioh it is related must be itself a condensation of several segments. The emergence of the vagus by several cords in Notidanus may be an indication that a series of cranial segments corresponding to the branchial arches has disappeared. S40 MORPHOLOGY OF THE SKULL. [CHAP. formulate accurately the number of segments of the pri- mitive vertebral body to which the present chondrocranium corresponds. It appears to have grown beyond the pri- mordial simple construction, and to have arisen into a higher and more complicated form, more perfected and specialised in relation to conditions of life, combining all kinds of potentiality, of adaptability to different uses. If these views are read in connection with those of Professor Huxley on Amphioxus (Proc. Roy. Soc. 1874), their great agreement will be perceived. 771. Then in those higher types in which bone occurs, we get a more or less distinct origination of alisphenoid and orbitosphenoid pieces in the cranial wall ; but we do not find any warrant for saying that each of these is homolo- gous with a neural arch. It does not appear to us that the cranium is a structure made up of such pieces soldered together; their distinctness is like that of disconnected por- tions of a continuous structure ; they belong to the higher type into which the brain-case grows as an osteocranium. 772. The intimate relation of the three sense-capsules to the cranium has already been noticed at length. Each pair is built into the cranium in its own style, being vari- ously and very perfectly protected or supported by it. A partial floor, inner wall, and roof may be provided for each organ by cranial cartilage, in addition to its own chon- drifications. In ascending from the lower to the higher types we are struck by the fact that each organ appears to press continually closer to the middle line, and either to compress or get beneath the cranial cavity or cartilage, la the Salmon we find a very simple state of the inter- and prenasal cartilage, which is expanded into a thick mass not directly comparable with the internasal region of a Dogfish. There is a simplification of this condition in the Urodeles, the inner walls of the proper nasal cap- sules and the median tissue coalescing. But in higher forms a process occurs which appears to be intelligible by imagining the lateral parts of the internasal plate of IX-] THE CARTILAGINOUS SKULL. 341 the Dogfish, and the inner nasal wall, to be elevated and constituted into vertical walls, which being approximated unite to form the nasal septum. In these types also it becomes increasingly difficult to distinguish what is inters nasal (trabecular) cartilage and what belongs to the proper capsular wall. The floor of the nasal sac in the Frog and other forms is made out of the primary cornua, and appears very comparable to the basilar cartilage underlying the ear-capsule in the Elasmobranchs and in the Axolotl, and the trabecular shelf supporting the eyeballs in the former group. 773. Approach of the orbits to the median line of the head occurs in many forms, concurrently with development of an interorbital septum : and the brain is either elevated above them or much retracted. The approach of the ear-capsules to the middle line in Birds and Mammals appears to be a phenomenon of the same order : but ifi them the organs are largely covered by the brain in its region of highest development. 774. The fenestration of the cranial cartilage by the sense-capsules has also been alluded to : the capsules appear to be related to three tracts of cranial cartilage, and at any rate in some forms cartilage is aborted where they abut. This is exemplified in the fenestration of internasal, interorbital, and orbitosphenoidal tracts, and the frequently greater size of the olfactory and optic foramina than the nerves which pass through them ; as well as by the non-formation of lateral cranial cartilage internal to the ear-capsules in Elasmobranchs and others, a fact less perceptible in higher forms in consequence of the con- tinuity of cranial and capsular cartilage in them from the first. 775. It does not appear that the sense-capsules bear any sure testimony to the segmental structure of the skull. They were probably originally situated in definite seo'ments, but their large size may have extended them over the tracts due to several primordial segments. See- 342 MORPHOLOGY OF THE SKULL. [cHAP. iug what is our uncertainty about the composition of the brain-case, we cannot tell what segments of it especially correspond to the capsules: only the regions can be roughly indicated. 776. Of what value are the visceral arches as indica- tions of the composition of the skull? The mandibular arch is constantly related to the basis cranii just in front of the trigeminal foramen; the hyoid to the basilar plate where it extends outwards beneath the ear-capsule, and in other cases to the median region of the same capsule; the branchial arches are behind these, and not directly ( onnected with the cranium. Do all the branchial arches belong to the segments which the cranium represents ? They are certainly all supplied with nerves from the cranium; but to relate all the branchial arches to cranial segments requires the assumption of the disappearance or condensation of a series of body segments so far as their axial and neural parts are concerned ; and our reckon- ing of the number of segments that have thus disappeared will vary according to the view taken of the perfect quota of branchial arches. If we view the first two branchial arches as specially related to the cranium (namely, those Mipplied by the glossopharyngeal and the first branch of the vagus), and the hinder branchial arches as belonging reall}'' to the neck, we have four arches related to the cranium at the side of and behind the pituitary body'. The bearing of this view on the structure of the osteo- cranium must be considered later. 777. We believe that it is justifiable to regard the cornua trabeculas of the Dogfish and Frog, (recurrent cartilages of Pig, Passerine Bird,j and the antorbital plate or bar, as preoral representatives of visceral arches; and the method of segmentation of certain palatine tracts in Urodeles, Lizards, and Birds gives some colour to the imagination that another segmental piece is there to be ' We have no wish to cast any discredit on the opposed view, of the disajipearance or condensation of segments in the hinder part of the skull. The question cannot yet be decided. ^^•] THE OSSEOUS SKULL. 843 traced; but the light is very uncertain at present. There IS no certainty that all the body segments concerned in tlie cranium are provided with representatives of arches. -But on this view, derived from arches or their representa- tives, not fewer than seven segments are concerned in the formation of the cartilaginous skull. 778. The meaning of the labials and other superficial cartilages is very obscure. No definite relations between the labials and the arches can be made out at present: yet their almost constant occurrence, their predominance in the Lampreys and Elasmobranchs, in the early stages of Frogs, and the importance of the changes in which they take part, lead to the idea that they must have some intelligible position in reference to the skull. The extra- branchials of the Dogfish are at any rate superficial cartilages related to the branchial arches, and they appear to be homologous with the scapulo-coracoid cartilages. The Osseous Skull. 779. Calcareous deposit occurs in vertebrates in the following tracts; epidermis or epithelium (enamel of teeth, outer layer of Ganoid scales): dermis (dentine of teeth, Ganoid and Teleostean scales) : subcutaneous fibrous mesh; immediately outside the perichondrium of a cartilaginous tract (parostosis) ; immediately within perichondrium, and eating into cartilage, (ectostosis) : a little way beneath the surface of cartilage (superficial endostosis): and deep in its substance (true endostosis, central or subcentral). In most of these tracts the calcification may be such as not to gain the title of bone; but in all except the first, true bones may result from the process. 780. In various embryos it has been noticed that the calcification of teeth occurs before the bony plate that bears them and unites a series has appeared. The prin- cipal membrane-bones arise previous to any ossification of cartilage. On the other hand, some types very deficient Mi MORPHOLOGY OF THE SKULL. [CHAP. or entirely wanting in ectostoses have superficial calcifica- tions of cartilage ; and in others, deep endosteal deposit takes place before ectostosis, but it is soon either absorbed or superseded by the latter. 781. Any of the calcified tracts mentioned above may unite with the next so as to be undistinguishable from it. Dermosteal and parosteal tracts are frequently conjomed: again and again do bones developed parosteally graft themselves on cartilage as ectostoses, or ectostoses_ extend into outer tracts of fibrous tissue: and ectostoses similarly unite with endostoses. 782. These facts, combined with our present know- ledge of fossil forms, seem to furnish a clue to the pro- cesses by which the bony skull has become what it is in various types. We conceive of primordial vertebrates which possessed a calcified exoskeleton and a cartilaginous brain-case; these calcifications becoming true bones, and continually ossifying deep tracts; the chondrocraniura gradually acquiring its proper bony centres; the parostoses extending more deeply and being applied to and moulded upon the cartilaginous parts, in many cases uniting with their proper bones, or aborting the cartilage. 783. Scales and dermal bones in vertebrates have probably a relation to the segmentation of the body. The bony armour of Callichthys, the lateral line series of scales each with its mucous duct, of Teleosteans, the bony scutes of the Sturgeon and the Crocodile, are but a few out of man}' facts suggestive of this idea. The dermal bones and parostoses of the head are representatives of the same thing, occurring serially and in rows which can be ti'aced more or less clearly. This relation has been proved with regard to the parostoses related to the cartilaginous parts of the sternum and shoulder -girdle'; and if a student desired to be prepared in the best way for understanding the difiiculties and complicated relations of tracts of bone and cartilage in the skull, he could not do better than 1 On the Shoulder-Girdle and Breast Bone. Bay Society, 18C6. ^^•] THE OSSEOUS SKULL. 345 master the simpler problem of the sternum and shoulder- girdle. 784. The body of Callichthys is enveloped in a right and left series of elongated supero-lateral and infero-lateral (ossified) dermal plates. The supero-lateral bones are directed downwards and forwards, the infero-lateral down- wards and backwards. The upper and lower series are very inuch alike; only the upper plates are pierced at their base in front and notched behind, for mucous glands. If this portion were cut off by a suture in each upper plate we should have a series comparable to the mucous scales of the lateral line of typical Teleostei. This does in fact take place in the first cincture behind the head, forming a post-temporal bone. The cinctures are not perfect above or below; space is left for small dermo-spinal plates and spines in the fins ; and for dermo- ventrals and ventral spines. 785. In the work referred to it is shown, in the case of Callichthys, that the two supraoccipital derm- bones, the single parietal, the symmetrical frontals, the single dermo-ethmoid are all serial homologues, whether azygous or symmetrical, of the upper three-fourths of the supero-lateral dermal plates clothing the body. The lower part of the first body-plate (containing the mucous duct) is cut off as a supratemporal. Then this series becomes double around the eye, and we have the dermal post- frontal, and postorbital, and supraorbital, the suborbital and lachrymal, and then the nasal. The infero-lateral plate is subdivided where it is in relation with the shoulder-girdle, into three ; supraclavicle, clavicle, and interclavicle. The opercular repeats the supraclavicle, the subopercular (not found in Callichthys) the clavicle; but the hyoid dermo- cincture is completed by the many branchiostegals (three in Callichthys); the basibranchiostegal is a lower spine- bone, and seems to correspond with the interclavicular region. The squamosal, in relation to the mandibular pier, carries on the series of supraclavicle and opercular; the clavicle and subopercular are represented by the inter- 346 MORPHOLOGY OF THE SKULL. [CHAP. opercular; the dermal bones of the lower hyoid and interclavicular regions are represented by the jugulars, and the splints of the mandible. The maxillary, jugal, and quadrato-jugal, homologous with part or the whole of the squamosal, belong to the infero-lateral series in the fore part of the head. 786. The ossifications which occur in the cartilaginous brain-case and its anterior continuations, independently of the proper capsular bones, are comparatively few, and on the whole simple in character so long as we have to do merely with cartilage. The most constant ectostoses in the cranium are the exoccipitals, appearing on either side of the foramen magnum, enclosing the vagus and often the glossopharyngeal and hypoglossal nerves, bearing the occipital condyles where there are two, or contributing the lateral portions when the condyle is single. In every skiill which is trulj^ ossified these bones occur ; and when neither basi- nor supraoccipital exists, the exoccipitals extend largely into the floor and roof of the cranium, very much after the manner of the neural arches of vertebrae in various animals. 787. The orbitosphenoid is, next to the exoccipital, the most constant bone of the same category. It is the ossification of the lateral cranial wall anteriorly, or of so much of it as developes cartilage ; it is perforated by, or is anterior to the exit of the optic nerve. Very fre- quently it is conjoined in development with the similar tract in front (ethmoidal), beyond the brain-case: and the lateral ossifications may become united by median bone, forming the complicated sphenethmoid. It is noteworthy that the orbitosphenoid may arise by two centres on each side. The alisphenoid is the only other bone which can be classed with these : it is in the lateral cranial wall, or so much of it as is cartilaginous, between the exit of the optic and the trigeminal nerve, or partially or entirely enclosing the latter. It also in some cases arises by two centres on each side. Both the orbito- and alisphenoids IX.J THE OSSEOUS SKULL. 347 may lie in the lateral part of the floor of the cranium in higher types with large brains. 788. The basioccipital, rarely found in Amphibia, but occurring in all higher types, is an expression of ossifica- tion arising immediately around the notochord, after the manner of the main part of a vertebral centrum, and extending on either side into the proper parachordal car- tilage. Developing at first in the Chick around the hinder portion of the cranial notpchord, it subsequently grows so as to enclose the whole of its three segments, when the notochord has relatively retired and the posterior basi- cranial fontanelle is established. In the Axolotl, where no basioccipital is formed, there is an apical ossification around the notochord (cephalostyle), which afterwards lo.ses its identity in the parasphenoid, or is absorbed. We thus see that the basioccipital corresponds very closely with a vertebral centrum, but its relation to the whole of the cranial notochord makes it doubtful whether it is not equivalent to two or three vertebral centra. 789. The next bone, the basisphenoid, corresponds to a certain extent with the basioccipital, in being more or less laid down in cartilage which is the result of the junction of lateral masses, the trabeculse, as the basi- occipital is largely due to the parachordals. But the basi- sphenoid has no relation to the notochord, or to anything like a vertebral centrum. A basisphenoid formed from cartilage occurs less frequently than a basioccipital ; in most cases it arises from a pair of centres on the antero- lateral margin of the pituitary body, which afterwards join. It is not till we reach the Birds and Mammals that a basisphenoid arising from a single median centre is found, but even in the highest form, in Man, we find a pair of centres. The basisphenoid may be increased in bulk very largely by ossification in adjacent membrane. 790. A rudiment of an anterior median centre, the presphenoid, is found in Lizards, and more markedly in Birds: it is in the upper part of the interorbital septum, 348 MORPHOLOGY OF THE SKULL. [CHAP. the expression of conjoined and compressed trabeculte. A distinct presphenoid is marked in various Mammals, most definitely in rodents ; but in many cases the region is ossified by the inward extension and union of the orbito- sphenoids. Anteriorly to this, the median cartilage is ossified by a vertical mesethmoid where a true nasal septum is found : it occupies the hinder part of this region and the fore part of the interorbital. The cribriform plate in Mammals is simply a lateral ossification continuous with the top of the back part of the mesethmoid, and extending through the interspaces between the olfactory fibres where they pass out of the cranium. The meseth- moid has a definite extent forwards, not passing beyond the cranio-facial fenestra (Birds), or into the aliseptal region as formerly defined : but in various birds two or three centres may ossify part of the anterior region of the septum. 791. Other forms of ossification of precranial cartilage are found in the Cod tribe, where there is an anterior median ethmoid, and in the Pike, where small paired ossi- fications invade the cartilage. One lateral ectostosis has to be mentioned, the ectethmoid (prefrontal) in the ant- orbital wall, occurring in the Salmon and again in Mam- mals. In the adult Pig a prenasal bone is found in the extreme anterior cartilage of the head (prenasal and ali- nasal). Ossifications in the turbinal ingrowths of the nasal capsules arise only in Birds and Mammals. They present no important phenomena beyond those of the cartilaginous structures in which they take their origin. 792. In the ear-capsule we have a series of bones very definitely related, and very persistent. The prootic is about the anterior semicircular canal, and is related to the antero-inferior region of the capsule; the epiotic is over the junction of the anterior and posterior canals ; the opisthotic is postero-inferior, and frequently furnishes the hinder part of the margin of the fenestra ovalis. These are the most constant ossific centres, remaining distinct from one another either throughout life or till an advanced IX-J THE OSSEOUS SKULL. 349 stage, in Fishes, Amphibians, and Reptiles. In Fishes there is a pterotic over the external canal, frequently- bearing the horizontal articular facet for the hyoman- dibular : this centre reappears in the Axolotl, but is not found in the higher types. There may be another centre in front of this, the sphenotic, occupying the antero- external region of the capsule ; but the cartilage in which it arises is always of a composite character, being due to a confluence of proper cranial cartilage with that of the capsule. 793. There is a strong tendency for the epiotic centre to iinite with the supraoccipital, and the opisthotic with the exoccipital ; and in some forms the periotic bones do not arise separately, but the supraoccipital and exoccipitals extend into the epiotic and opisthotic regions respectively. Where there is no supraoccipital, the exoccipital may ossify the whole posterior portion of the capsule. In Birds the epiotic and opisthotic are quite small, and the adjacent bones grow into the auditory cartilage and then annex these centres to themselves. The prootic on the other hand has much more independence and persistence ; it is the most constant periotic bone, and is never aborted by adjacent bones. It may grow far into the floor of the cranium, and may even occupy a considerable portion of its roof in Amphibians ; sometimes it further developes a process which lies in the side wall of the skull anteriorly to the capsule, and is comparable to an alisphenoid. 794. The cartilage-bones of the outworks of the skull have in their way, as curious a persistence as those of the cranium proper. But some regions comparatively early in the ascent of types become affected by mem- brane ossification, and cease to develope cartilage, or develope it only when the ossifying force is exhausted, or develope it in the embryo and abort it under the influ- ence of the growth of neighbouring parostoses. The parts which persist as cartilage-bones are selected, as we ascend the scale, for specially important functions, being usually 350 MORPHOLOGY OF THE SKULL. [CHAP. reduced in relative size and becoming more elegantly shaped. 795. It appears that the palatine of the osseous Fish represents the antorbital or palatine of Amphibia, and partially the os uncinatum of a Bird. It cannot be de- finitely stated whether the pterygoid and mesopterygoid of an osseous Fish belong to a proper snbocular arch or to the forward growth of the upper mandibular segment : probably the latter is the case. There is no doubt that the metapterygoid and quadrate of Fishes belong to the upper mandibular segment ; and they correspond regionally with the epi- and pharyngobranchials. In the classes above Fishes the palatine and pterygoid are very largely formed out of membrane, the cartilage being either ossified in addition, or more or less ahurted as in Amphibia; or only coming into existence at a late period, in various tracts, which remain permanently unossified, as in Birds. A specialised portion of the pterygoid tract may however exist in cartilage v/ithin and above the membrane-ptery- goid, and ossification of this dev.elopes the epipterygoid of Lizard and Turtles. 796. In Reptiles and Birds the whole upper mandi- bular segment has but one bone, the quadrate ; and this may be said to answer in a general way to both quadrate and metapterygoid of the lower forms; but it is more highly specialised, and itself becomes, like the palatine and pterygoid tracts, increasingly unified with the cranium. The mandible usually has but one centre, the articular, corresponding to a ceratobranchial ; in the Frog and in Man there is an inferior or anterior (mentomeckelian) centre, apparently like a hypobranchial. But the whole meckelian cartilage never becomes prop(jrtionally so much ossified as the lirancliial cartilages. In ascending the scale the articular is very persistent, but becomes small in the Birds (largest in Struthionida^) and specialised in firm. In Manimals no articular is formed, and the repre- sentative of tlie quadrate (the malleus) is very small, ^^■■•J liciiii