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A TEXT-BOOK 
 
 OF 
 
 DENTAL HISTOLOGY 
 
 AND 
 
 EMBRYOLOGY 
 
 IXCLUDIXG 
 
 LABORATORY DIRECTIONS 
 
 BY 
 
 FREDERICK BOGUE NOYES, B.A., D.D.S. 
 
 PROFESSOR OF HISTOLOGY, NORTHWESTERN UNIVERSITY DENTAL SCHOOL 
 
 WITH 350 ILLUSTRATIONS A^ D 19 PLATES 
 
 LEA & FEBIGER 
 
 PHILADELPHIA AND NEW YORK 
 1912 
 
 C^/VcvwjMtU Vcx^^c^cJ?c/>8c^,?) 
 
Entered according to Act of Congress, in the year 1912, by 
 
 LEA & FEBIGER 
 
 in the Office of the Librarian of Congress. All rights reserved. 
 
 
 ^ 
 
 2_ 
 
TLo ms ffatbet 
 
 2)r« I£^num& IRoijee 
 
 Mboee long aiiD active professional career bas been Devoted, 
 
 witbout personal ambition or selfisb advancement, to 
 
 tbe 0OOD ot tbe Dental profession, anO wbose 
 
 unselfisbness anD sacrifice bave maDe 
 
 possible all tbat "IT bave C>one 
 
 or mas accomplisb 
 
PREFACE 
 
 It is indispensable for the successful treatment of disease 
 in the dental tissues that the dentist should acquire as 
 intimate a knowledge of structure as is essential to the 
 physician, and consequently that a parallel study of his- 
 tology should be followed. The development of biology has 
 placed histolog}' at the basis of all the medical sciences, for 
 as, in the last analysis, all physiology is cell physiology, and 
 all pathology is cell pathology, a knowledge of structure 
 and function is necessary for an intelligent conception of 
 the workings of the animal body in health and for the 
 restoration of normal function when impaired by disease. 
 
 Yet as recently as fifteen years ago, when the author 
 began teaching Dental Histology in the Northwestern 
 University, the subject was comparatively new in the cur- 
 riculum, and was considered rather unimportant and as 
 having little practical value. It would be impossible to 
 give adequate acknowledgment to the help received from 
 Dr. G. V. Black in developing the course. Every detail 
 was worked out in the closest cooperation with him, and 
 for years he guided and directed the work. 
 
 The object of a course in general and special histology 
 suited to the needs of dental students is to convey a definite 
 knowledge of the activities of these parts of the human bod}' 
 in terms of tissues and cells. This is the basis of every prac- 
 tical procedure. The structure of the enamel and dentine is 
 obviously the starting point in handling these tissues and 
 in the preparation of cavities, and the structure and function 
 of the pulp, the bone, the periosteum, and the peridental 
 membrane are similarly the basis for an understanding of 
 
vi PREFACE 
 
 their pathology and treatment. The study of the enamel in 
 relation to cavity preparation has proved to be of the greatest 
 value not only in forming better cavity walls, but also in 
 facilitating operation. For this purpose it is necessary to 
 understand both the structure of the enamel in itself, and 
 the arrangement of the structural elements in relation to the 
 tooth crown. To accomplish this, sections cut through the 
 crown in various planes must be studied and their relation 
 to it kept in mind. The modern dentist while looking at 
 the surface of a tooth must think of the enamel in terms of 
 its structural elements, and use this knowledge in handling 
 the tissue. In the following pages the enamel is studied 
 primarily in relation to operative dentistry. 
 
 The chapters on the pulp, peridental membrane, and 
 periosteum are likewise intended to emphasize the relation 
 of structure to function, and to impress the idea that the 
 treatment of disease in these tissues is in every instance a 
 biological problem. In forming true conceptions of caries 
 and necrosis, a knowledge of the intercellular substances and 
 their relation to the cells in the structure and function of 
 tissues is necessary. A chapter has, therefore, been devoted 
 to this subject. The study of the structure and development 
 of bone has very greatly modified treatment in orthodontia, 
 as it is now recognized that in all movements of the teeth 
 the results are accomplished by tissue changes under the 
 influence of mechanical stimuli. 
 
 Though there are many good books on general histology, 
 they are not fully adapted to the special needs of the dental 
 curriculum. The author has accordingly felt that teachers 
 and students of dentistry might find some advantage in a 
 work covering the subject from their own standpoint, and 
 embodying the results of his experience in teaching as well 
 as in research. In a word, this volume has been planned 
 primarily as a text-book for use in dental schools, and it 
 aims to provide students with a didactic text and teachers 
 with a course to follow. It contains directions for twenty- 
 two days of laboratory work, and an appendix giving tech- 
 nical methods for preparation of material for the classes. 
 
PREFACE vil 
 
 It suggests many fields for ot-iginal investigation, and tech- 
 nical directions which would enable any man to begin such 
 work. Let us hope that the benefits certain to result from 
 discoveries still to be made will lead some students and 
 practitioners to interest themselves in this inviting field. 
 
 Most of the illustrations are from the author's own nega- 
 tives. Those on the relation of enamel structure to cavity 
 walls are new as well as original in plan. The drawings 
 illustrating the periosteum and pathological conditions of 
 the pulp were made by Dr. G. V. Black, and he has writ- 
 ten the chapter on his machine for making ground sections 
 and the technique of its use. Some illustrations are taken 
 from other works, and these are duly credited in every 
 instance. Thanks are also due to Dr. Louis Schmidt, of the 
 Rockefeller Institute, who made the colored plates from the 
 author's specimens, and to A. B. Streetdain, of the University 
 of Chicago, for his work on some difficult diagrams. 
 
 Finally, the author wishes to thank his publishers for their 
 pains and patience in carrying out his wishes. 
 
 F. B. N. 
 
 Chicago, 1912. 
 
CONTENTS 
 
 INTRODUCTION 17 
 
 CHAPTER I 
 Homologies 19 
 
 CHAPTER II 
 The Dextal Tissues 28 
 
 CHAPTER HI 
 The Examel 38 
 
 CHAPTER IV 
 The Structural Elemexts of the Examel 43 
 
 CHAPTER V 
 Characteristics of the Examel Tissue 52 
 
 CHAPTER VI 
 
 The Directiox of the Examel Rods ix^ the Tooth Crowx . 65 
 
 CHAPTER VII 
 
 The Relatiox of the Structure to the Cuttixg of the 
 
 Examel , 73 
 
 CHAPTER VIII 
 
 The Structural Requiremexts for Stroxg ExNTamel Walls . 80 
 
 CHAPTER IX 
 The Preparatiox of Typical Examel Walls 89 
 
 CHAPTER X 
 Structural Defects ix the Enamel .,.,.... 107 
 
X CONTENTS 
 
 CHAPTER XI 
 Special Areas of Weakness for Enamel Margins . . . 124 
 
 CHAPTER XII 
 The Effect of Caries on the Structure of the Enamel . . 143 
 
 CHAPTER XIII 
 The Dentine 167 
 
 CHAPTER XIV 
 The Cementum 188 
 
 CHAPTER XV 
 Dental Pulp 201 
 
 CHAPTER XVI 
 Structural Changes in the Pathology of the Pulp . . . 219 
 
 CHAPTER XVII 
 Intercellular Substances 236 
 
 CHAPTER XVIII 
 Bone 247 
 
 CHAPTER XIX 
 Bone Formation and Growth 255 
 
 CHAPTER XX 
 Periosteum 262 
 
 CHAPTER XXI 
 The Attachment of the Teeth 271 
 
 CHAPTER XXII 
 Peridental Membrane 279 
 
 CHAPTER XXIII 
 
 The Cellular Elements of the Peridental Membrane. . 294 
 
 CHAPTER XXIV 
 The Mouth Cavity 323 
 
CONTENTS XI 
 
 CHAPTER XXV 
 
 Biological Considerations Fundamental to Embryology . 335 
 
 CHAPTER XXVI 
 Early Stages of Embryology 340 
 
 CHAPTER XXVII 
 
 The Development of the Tooth Germ 362 
 
 CHAPTER XXVIII 
 
 Th:e Relation of the Teeth to the Development of the 
 
 Face 374 
 
 PART II 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 (Twenty-four Periods in the Laboratory) 
 
 Period I 429 
 
 Period II 429 
 
 Period III 432 
 
 Period IV 435 
 
 Period V 435 
 
 Period VI 437 
 
 Period VII 439 
 
 Period VIII 440 
 
 Period IX 440 
 
 Period X 441 
 
 Period XI 442 
 
 Period XII 442 
 
 Period XIII 443 
 
 Period XIV 444 
 
 Period XV 445 
 
 Period XVI 445 
 
 Period XVII 446 
 
 Period XVIII 446 
 
 Period XIX 447 
 
 Period XX 448 
 
 Period XXI 449 
 
 Period XXII 450 
 
 Period XXIII 451 
 
 Period XXIV > 442 
 
xii CONTENTS 
 
 APPENDIX 
 
 CHAPTER I 
 
 The Grinding of Microscopic Specimens, using the Grind- 
 ing Machine 453 
 
 CHAPTER II 
 The Theory of Histological Technique 478 
 
 CHAPTER III 
 
 General Histological Methods 485 
 
 CHAPTER IV 
 Fixing Agents and Staining Solutions 496 
 
DENTAL HISTOLOGY 
 
 INTRODUCTION 
 
 The development in knowledge of the cell has had a most 
 profound effect upon the entire practice of medicine; in 
 fact, the progress of modern medicine has dated from the 
 studies of cell biology, the germ theory of disease being 
 only one of the phases of this development. In terms of the 
 cell theory the functions of the body are but the manifest 
 expression of the -activities of thousands or millions of more 
 or less independent but correlated centres of activity. If 
 these centres or cells perform their functions correctly, the 
 functions of the body are normal, but if they fail to perform 
 their office or work abnormally, the functions of the body 
 are perverted. In the last analysis, then, all physiology is 
 cell physiology, all pathology cell pathology. To modern 
 medicine, histology, or the cell structure of the organs and 
 tissues of the body, together with cell physiology, is the 
 rational foundation of all practice. This is as true for the 
 dentist as for the physician in regard to the soft tissues of 
 the mouth and teeth that he is called upon to handle. With 
 caries of the teeth, the disease which most demands the 
 attention of the dentist, the case is somewhat different. 
 Caries of the teeth is an active destruction, by outside 
 agencies, of a formed material which is the result of cell 
 activity, the teeth themselves being passive. The cellular 
 activities of organs and tissues of the body may have an 
 influence, but this is only in producing those conditions of 
 environment which render the activities of the destructive 
 
18 DENTAL HISTOLOGY 
 
 agent efficient in their action upon the tooth tissues. Though 
 the dental tissues are passive, the phenomena of caries can 
 only be understood when the structure of the tissues is 
 understood, and not only must the treatment be based upon 
 knowledge of the structure of the tissues, but the mechanical 
 execution of the treatment is facilitated by that knowledge 
 of structure. 
 
 In the preparation of cavities, the arrangement of the 
 enamel wall is determined by the knowledge of the direction 
 of the enamel prisms in that locality, and to a certain extent 
 the position of cavity margins must be governed by the 
 knowledge of the structure of the enamel. In the execution 
 of the work a minute knowledge of the direction of enamel 
 rods becomes the most important element in rapidity and 
 success of operation. The longer the author studies and 
 teaches the structure of the enamel in its relation to the 
 structure and preparation of enamel walls, the more he finds 
 himself using this knowledge at the chair in daily operations. 
 He believes that nothing will do more to increase facility, 
 rapidity, and success of operation than a close study of the 
 enamel structure. 
 
 All tissues are made up of two structural elements — cells 
 and intercellular substances. The cells give the vital char- 
 acteristics, the intercellular substances the physical character. 
 The cells are the active living elements, the intercellular 
 substances are formed materials produced by the activity 
 of the cells, and more or less dependent upon them to main- 
 tain their quality, but they possess no vital properties. They 
 surround and support the cells, and the physical character- 
 istics are given by them. An understanding of the relation 
 of cells and intercellular substances in the structure and 
 function of tissues is absolutely fundamental to a study of 
 dental histology, and should be acquired in a thorough study 
 of general histology before the subject is undertaken. 
 
CHAPTER I 
 
 HOMOLOGIES 
 
 Exoskeleton. — In studying the organization of animal forms 
 they are found, very early in the evolutionary stages, to de- 
 velop some sort of a framework, or skeleton, to support and 
 protect the creature. In the lower and earlier forms this 
 framework is formed entirely of some sort of shell upon the 
 outside of the creature, and consequently is called an exo- 
 skeleton. This may be either horny or chitinous in nature, 
 as in the insects, crabs, etc., or it may be calcified, as in the 
 shell-fish, or it may be both. The exoskeleton serves not 
 only as a supporting framework, but also as a protection. 
 
 Endoskeleton. — In the higher forms an internal framework, 
 or endoskeleton, is developed, which forms the scaffolding to 
 support the creature, but does not act as a protection. In the 
 first place, this is of cartilage, but may be changed into bone. 
 In lower forms of animals it remains always cartilage. In 
 man the cartilage is partly converted into bone, all of the 
 bones of the endoskeleton being preceded by cartilage. 
 
 The first trace of the endoskeleton is found in the lowest 
 form of vertebrate, the Amphioxus or Lancit, the lowest form 
 of fish, and appears as a rod or notochord in the dorsal 
 region. There is also an important difference in the nervous 
 organization (Figs. 1 and 2). In the invertebrate the nervous 
 system is represented by a larger or smaller ganglion in the 
 anterior or head end, corresponding to the brain; this is 
 dorsal to the alimentary canal. From this a ring passes 
 around the anterior end of the alimentary canal and unites 
 with a chain of ganglia ventral to it. The nervous system 
 of the invertebrate then is, with the exception of the brain 
 ganglia, ventral to the alimentary canal, and corresponds to 
 

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22 HOMOLOGIES 
 
 the sympathetic system in higher animals. It will be noted 
 that this arrangement puts the nervous system, which controls 
 the activity of the individual, in the most protected position. 
 The invertebrate crawling upon the ground is subject to 
 attack or injury from above, but it may be cut almost in 
 two before the nervous system is reached. 
 
 In the vertebrate the central nervous system appears as a 
 chain of ganglia dorsal to the alimentary canal and notochord 
 (Fig. 2). This difference is significant, and may be expressed 
 roughly in this way : The invertebrate framework is an out- 
 side protecting shell, upon which the creature depends for 
 protection. The vertebrate framework is an internal struc- 
 ture to facilitate motion and give support, and is accompanied 
 by a development of the nervous organization, so that the 
 creature protects itself by more rapid motion. In the inverte- 
 brate the digestive system is above or dorsal to the nervous 
 system, in the vertebrate the nervous system is in the upper 
 position, both structurally and functionally. 
 
 In ascending in the scale of organization the endoskeleton 
 increases in importance and development, while the exo- 
 skeleton decreases in importance and development. 
 
 From the standpoint of comparative anatomy the teeth 
 are not a part of the osseous system, but appendages of the 
 skin, and are to be compared with such structures in the body 
 as the hair and the nails. The teeth aie a part of the exo- 
 skeleton, and their relation to the bones of the endoskeleton 
 is entirely secondary for the purpose of strength, the bone 
 growing up around the tooth to support it. When the skin 
 of such an animal as the shark is examined, the entire sur- 
 face is found covered with small calcified bodies, which are 
 really small, simple, cone-shaped teeth. From the stand- 
 point of development the mouth cavity is to be regarded as 
 a part of the outside surface of the body which has been 
 enclosed by the development of neighboring parts, and the 
 dermal scales, or rudimentary teeth, which are found in the 
 skin covering the arches forming the jaws, have undergone 
 special development for the purpose of seizing and masti- 
 cating the animal's food. In the simplest forms there is only 
 
HOMOLOGY AND ANALOGY 
 
 23 
 
 a development in size and shape of these scales, and they 
 are supported only by the connective tissue which underlies 
 the skin. These teeth are easily torn off in the attempt to 
 hold a resisting prey, and in the shark (Fig. 3) they are 
 continually being replaced by new ones. In the more 
 highly developed forms, the bone forming the jaw grows 
 upward around the bases of these scale-like teeth to support 
 them more firmlv and render them more useful. 
 
 Fig. 3 
 
 Shark's skull (Lamna coinubica), showing succession of teeth. 
 
 Homology and Analogy. — In biology structures that are 
 similar in formation and origin are called homologous. 
 Structures that are similar in function are called analogous. 
 A structure or organ may be both homologous and analo- 
 gous to another, but not necessarily so. For instance, the 
 wing of a fly is analogous to the wing of a bird, because they 
 are used for the same purpose, but they are not homologous. 
 The wing of a bat and the wing of a bird are both analogous 
 and homologous, being used for the same purpose, and 
 
24 
 
 HOMOLOGIES 
 
 having similar structure and origin. The arm of man is 
 homologous to the wing of a bird but not analogous to it. 
 The jaws of a crab or beetle are analogous to the jaws of man, 
 but they are not homologous structures, as the jaws of the 
 crabs and insects are modified legs. The teeth are said to 
 
 Fig. 4 
 
 ,-^5^3^???; 
 
 Development of the hair: Sc, stratum corneum; SM. stratum malpighii; C, derma; 
 F, follicle; Dr, sebaceous gland; CZ, central, PZ, peripheral zone of hair germ; HK, 
 hair knob; P, beginning the formation ol the hair papilla; P', same in a later stage of 
 development when it has become vascular. (Wiedersheim, Comparative Anatomy of 
 Vertebrates.) 
 
 be homologous to the dermal scales of certain fishes, and to 
 the appendages of the skin, such as the hair and nails, because 
 they are similar in structure and origin (Plate I). 
 
 Comparison of Structure. — If the tooth is compared with the 
 hair in this way this will be better understood. The hair 
 
PLATE I 
 
 Comparison of Structure of Tooth and Hair. 
 
COMPARISON OF ORIGIN 
 
 25 
 
 may be considered as a horny 
 structure composed of epithelial 
 cells resting upon a papilla of 
 connective tissue. The tooth 
 may be considered a calcified 
 structure, formed by epithelial 
 cells, resting upon a papilla of 
 connective tissue, which is also 
 partially calcified. 
 
 Comparison of Origin. — From a 
 study of the development of the 
 tooth and the hair, the similarity 
 of their origin and structure be- 
 comes more apparent. 
 
 The first step in the develop- 
 ment of the hair is a thickening 
 of the epithelium at a point, the 
 epithelial cells multiplying and 
 growing down into the connective 
 tissue below, so as to make a 
 two-layered bag or cap, the con- 
 nective tissue groAving up in the 
 form of a cone-shaped papilla 
 into the cavity of the cap (Fig. 
 4). The epithelial cells of the 
 inner layer, next to the connec- 
 tive tissue, multiply rapidly and 
 develop horny material and are 
 pushed out from the surface of 
 the skin as the shaft of the hair. 
 
 Diugram to illustrate development of a 
 tooth: A. inner layer of enamel germ; B, 
 outer layer; C, remains of intermediate cells; 
 D, dentine; Z)./, dental lamina; ^.epithelium; 
 E.G, enamel germ; En, enamel; F, dental 
 furrow; L.D, labiodental furrow; M, con- 
 nective-tissue cells; O, odontoblasts: P, den- 
 tine papillaf R.G, reserve germ; V, blood- 
 vessel. (Cunningham's Anatomy.) 
 
 Fig. 5 
 
 III f'> 
 
 V \ ■ 
 
26 
 
 HOMOLOGIES 
 
 In the development of the tooth there is at first a thicken- 
 ing of the epithehum, and a mass of epithehal cells like that 
 forming the hair, hut larger, grows down into the connective 
 tissue (Fig. 5). This becomes bulbous, then invaginated, 
 forming a two-layered cap. The two layers are at first per- 
 fect and are farther from the surface than the epithelial 
 
 Fig. 6 
 
 Changes in the mandible with 
 
 buccal and lingual view. 
 
 structure which develops the hair. A cone-shaped papilla 
 of connective tissue, the dental papilla, grows up into the 
 cavity of the epithehal organ corresponding to the bulb of 
 the hair. 
 
 The inner layer of epithelial cells produce the enamel, the 
 outer layer of connective-tissue cells, covering the connective- 
 
RELATION TO THE BONE 27 
 
 tissue papilla, develop the dentine, leaving the pulp inside as 
 the remains of the dental papilla. 
 
 Relation to the Bone. — The relation of the bones of the jaws 
 to the teeth is entirely secondary and transient. They grow 
 up around the roots of the teeth to support them, and are 
 destroyed and removed with the loss of the teeth or the 
 cessation of their function. In this way the development of 
 the alveolar process appears around the roots of the tem- 
 porary teeth. All this bone surrounding their roots is ab- 
 sorbed and removed with the loss of the temporary denti- 
 tion, and a new alveolar process grows up around the roots 
 of the permanent teeth as they are formed. This develop- 
 ment of bone around the roots of the teeth leads to the 
 changes in the shape of the body of the lower jaw, increasing 
 the thickness from the mental foramen and the inferior dental 
 canal upward (Fig. 6) . When the teeth are finally lost this 
 bone is again removed and the body of the jaw is reduced in 
 thickness from above downward. These phenomena have 
 an important bearing upon the causes and treatment of 
 diseased conditions of the teeth, particularly those which 
 involve the supporting tissues. 
 
CHAPTER II 
 
 THE DENTAL TISSUES 
 
 Study of the structure of the teeth shows that all teeth, 
 from the simplest to the most complex, are composed of but 
 four tissues — enamel, dentine, cementum, and the pulp, or 
 formative tissue of the dentine. 
 
 Even the simplest placoid scales, as found in the skin of 
 the shark and dog-fish, contain these four tissues. In many 
 of the specialized forms of teeth some of these tissues may 
 be absent. For instance, in the bony fishes the teeth are 
 fastened to the bone by an interlocking of bone and dentine, 
 forming an ankylosed attachment, and the cementum is 
 absent; but in some of these there is also a slight formation 
 of cementum. In the tusks of elephants during the func- 
 tional period the dentine is not covered by enamel, but 
 when the tusk first erupted there was a slight enamel cap, 
 which was at once broken or worn off. In many instances 
 the enamel seems to be entirely absent, and for that reason 
 it has sometimes been called the most inconstant of the 
 dental tissues, but in every case in which the development of 
 the tooth has been studied an enamel organ has been found. 
 It is probably much more nearly correct to consider that in 
 all cases enamel is formed, but that it may be so thin and 
 transparent as to be very difficult to recognize, and very soon 
 may be entirely lost. 
 
 • 
 
 FUNCTIONS OF THE DENTAL TISSUES 
 
 The Enamel. — The enamel forms a hard protecting surface 
 or cap especially adapted to resist abrasion. It is the hardest 
 animal tissue, but brittle and inelastic, and dependent upon 
 
FUNCTIONS OF THE DENTAL TISSUES 29 
 
 the support of the elastic dentine for strength. Its func- 
 tion is to resist the abrasion of friction. Its arrangement in 
 many instances is found specially modified for this purpose. 
 The Dentine. — The dentine is the strong elastic tissue form- 
 ing the great mass of the tooth, and gives to it its strength. 
 Teeth that are subjected to stress and force are often made 
 up of dentine without enamel. If, for instance, the tusks 
 of the elephant used for such purposes as tearing down 
 branches, spading up the ground, and so on, were made 
 up entirely of enamel, they would break off the first time 
 they were locked in the branches or driven into the ground, 
 but the elastic dentine gives and bends and will stand great 
 stress. The teeth of many animals which use their tusks in 
 fighting are constructed on the same plan. 
 
 The Cementum. — The cementum furnishes attachment for 
 the connective-tissue fibers which fasten the tooth to the 
 bone or surrounding tissues. It is formed on the enamel and 
 dentine both before and after the eruption of the teeth. 
 The formation of the cementum on the surface of the root 
 fastens the surrounding connective-tissue fibers to the 
 tooth. The fibers are calcified along with the matrix of the 
 cementum which is built up around them. These fibres in 
 man and the higher animals extend to the bone and the 
 surrounding tissues and support the teeth against the forces 
 of occlusion, and hold the surrounding tissues in proper 
 relation to the teeth. The function of the cementum is, 
 therefore, to attach the connective tissue fibers to the surface 
 of the root. 
 
 The Pulp. — The pulp is the remains of the formative organ of 
 the dentine. In teeth of continuous growth it remains actively 
 functional throughout the life of the tooth, but in teeth of 
 limited grow^th, after the typical development of dentine, 
 it becomes functional again only in response to irritations, 
 which, however, may be local or reflex. The pulp performs 
 two functions — a vital function, the formation of dentine, and 
 a sensory function, the response to thermal change. 
 
 Summaxy. — The dental tissues, i. e., enamel, dentine, 
 cementum, and pulp, are so called not simply because they 
 
30 THE DENTAL TISSUES 
 
 are found in the human teeth, but because all teeth are 
 composed of these four tissues. 
 
 It is true that in comparative dental histology consider- 
 able difference exists in the microscopic structure of these 
 tissues from the teeth of different animals, but certain 
 characteristics are very persistent and quite characteristic 
 of each. 
 
 DISTRIBUTION OF THE DENTAL TISSUES 
 
 The arrangement and distribution of the dental tissues 
 in the structure of the human teeth is best studied in ground 
 sections cut longitudinally through the entire tooth (Plate II), 
 and series of transverse sections cut through the roots. For 
 this purpose the sections should not be too thin (from 10 
 to 20 microns). For the study of the arrangement of the 
 cementum and dentine in the roots at least three transverse 
 sections should be ground from each root, one from the 
 gingival, one from the middle, and one from the apical third. 
 
 The Enamel. — The enamel forms a cap over the exposed por- 
 tion of the tooth. Its function is to resist the abrasions of mas- 
 tication. It gives the detail of crown form to the tooth. It 
 extends to the gingival line, and, except in old age, is covered 
 in the gingival portions by the epithelium of the gingivus, 
 which lies in contact with it but is not attached to it. It is 
 thin in the gingival portion and is normally overlapped slightly 
 by the cementum at the gingival line. It extends farther 
 apically on the labial and lingual, and buccal and lingual, 
 than upon the proximal surfaces, especially on the incisors, 
 cuspids, and bicuspids. It is thickest in the occlusal third 
 of the axial surfaces, and on the occlusal surfaces of the 
 molars and bicuspids, especially over the cusps. In the 
 incisors and cuspids it is thickest in the occlusal third on 
 the labial and over the marginal ridges on the lingual and the 
 dento-enamel junction, which, though not parallel with the 
 surface of the enamel, is usually curved in the same direction. 
 
 In the molars and bicuspids the dento-enamel junction in 
 

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DISTRIBUTION OF THE DENTAL TISSUES 31 
 
 the occlusal thirds on the buccal and lingual is usually 
 curved in the opposite direction. That is, while the surface 
 
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32 THE DENTAL TISSUES 
 
 of the enamel is convex, the surface of the dentine is concave. 
 It will be seen that this not only gives a greater thickness 
 to the enamel in the region which will resist abrasion, 
 but also gives it a firmer seat upon the dentine. (Study 
 illustrations in Chapter X.) The dento-enamel junction is 
 seldom a smooth, even surface, but will appear scalloped 
 in sections, projections of dentine extending between 
 projections of enamel (Fig. 7). In three dimensions this 
 means that rounded projections of the enamel rest in 
 rounded depressions of the dentine surface, and pointed 
 projections of the dentine extend between the rounded pro- 
 jections of the enamel. This is similar but much less marked 
 than the interlocking of the papilla of connective tissue with 
 the projections of the Malpighian layer of stratified squamous 
 epithelium of the skin and mucous membrane. In some 
 cases these projections of dentine into the enamel may be 
 quite marked. This scalloping of the dento-enamel junc- 
 tion gives a stronger attachment of the enamel to the den- 
 tine, and accounts, partially at least, for the difference that 
 is observed in the ease with which enamel can be removed 
 from the dentine in the preparation of roots for crowns. 
 Where the two tissues join with smooth surfaces the enamel 
 can be comparatively easily cleaved away; Avhere the 
 scalloping is marked it is removed with much greater 
 difficulty. 
 
 The Dentine. — The dentine gives the strength to the tooth. 
 This should never be lost sight of in operations, and sound 
 dentine should always be conserved to the greatest possible 
 extent in the preparation of cavities. That the function of the 
 dentine is to give strength will be seen more clearly from a 
 comparative study of teeth modified for special functions. 
 The dentine forms the greatest mass of the tooth, the type 
 form being determined by it. The cusps and ridges, although 
 different in form, are still represented in the dentine as well 
 as the number and shape of the roots, while the detail of 
 the form of the roots is modified by the addition of the 
 cementum on the surface. 
 
 The dentine forms a layer of comparatively even thickness 
 
DISTRIBUTION OF THE DENTAL TISSUES 33 
 
 surrounding the central cavity or pulp chamber, which is 
 occupied by the formative organ. From this cavity a great 
 number of small tubules extend through the calcified dentine 
 matrix to the surface under the enamel and cementum. In 
 the crown portion the course of these tubules is characteris- 
 tically curved like the letter S or /, so that the tubules tend 
 to enter the pulp chamber at right angles to the surface and 
 to end under the enamel at right angles to the dento-enamel 
 junction (Plate II). On closer study these tubule directions 
 will be found to be more complicated, but in studying the 
 distribution of dentine they should be noted. In the root 
 portion the tubules are usually comparatively straight, 
 that is, without the double curve, and are at about right 
 angles to the axis of the canal. 
 
 The outer layer of dentine under low magnification pre- 
 sents a peculiar granular appearance, which is specially 
 apparent under the cementum. This is known as the 
 granular layer of Tomes, and is caused by irregular spaces 
 in the dentine matrix which communicate with the dentinal 
 tubules. 
 
 The Cementum. — The cementum covers the dentine in the 
 root portion, and in most cases slightly overlaps the enamel at 
 the gingival line. This is not always true, for in some cases it 
 just meets the enamel, and in others there is a space where 
 the dentine is uncovered between the enamel and the 
 cementum (Fig. 8). It has not been positively determined 
 whether this can ever be considered a normal condition, 
 and the author has some reason to suppose that the sec- 
 tions showing this condition were from teeth from which the 
 gums had receded and the cementum was destroyed. The 
 sensitiveness which is so marked in some cases, where the 
 gums have receded beyond the gingival line, is probably due 
 to the loss of cementum and the uncovering of the granular 
 layer of Tomes. 
 
 The cementum is thin and structureless in appearance in 
 
 the gingival portion when viewed with low powers, but 
 
 becomes thicker in the apical third. In the thicker portions 
 
 irregular spaces (lacunae) with radiating canals (canaliculi) 
 
 3 
 
34 
 
 THE DENTAL TISSUES 
 
 are seen. In life these spaces contain living cells (the cement 
 corpuscles), which correspond to the bone corpuscles found 
 in the lacuna of bone. Upon the convex surfaces of the root 
 the cementum is thin; upon the concave surface it is thicker. 
 This increases with age, and so the continuous formation of 
 cementum tends to round the outlines of the roots and to 
 
 Fig. 8 
 
 Gingival line, showing the relation of enamel and cementum. 
 
 unite them where they approach each other. The fibers 
 which are built in the cementum are often imperfectly 
 calcified, especially where the layers are thick, so that in 
 the ground sections they may often be easily mistaken for 
 canals, because the imperfectly calcified fiber has shrunken 
 in the preparation. 
 
ADAPT A TION IN DISTRIBUTION OF DENTAL TISSUES 35 
 
 ADAPTATION IN THE DISTRIBUTION OF DENTAL 
 TISSUES 
 
 If the teeth of mammals are studied in a comparative way 
 many modifications will be found in the relative amount and 
 distribution of the dental tissues, adapting the tooth to per- 
 form special functions. A study of these modified or special- 
 ized teeth will give a better understanding of the functions 
 of the tissues in the tooth. The human tooth may be taken 
 as a type of omnivorous tooth, and the arrangement and 
 distribution of its tissues has already been described. 
 
 Teeth of Continuous Growth. — In many animals the teeth 
 or some special teeth are developed as weapons for use in 
 fighting, or as implements to aid in securing food. It is 
 usually the cuspid teeth that show this modification, as in 
 the tusks of the boar and many species of the carnivora, the 
 tusks of the walrus, and other examples. In the case of the 
 elephant the incisors have been developed in the same way. 
 Whenever the teeth have been developed in size for uses 
 which require strength and the ability to withstand stress 
 and strain, the increase in size is by development of the mass 
 of dentine, the enamel often being entirely lost during the 
 functional period. If these teeth were composed chiefly of 
 enamel they would be too brittle. These tusks, which, as in 
 the case of the elephant, sometimes reach a weight of many 
 hundreds of pounds, are usually deeply embedded in the 
 bone, and the concealed portion is covered with a layer of 
 cementum which attaches the fibers, holding them to the 
 bone, but they retain a conical pulp in a cone-shaped pulp 
 chamber at the base of the tooth, which continues to form 
 dentine. The tooth is pushed out of the socket, as the 
 shaft of the hair is pushed out, by the multiplication of 
 cells covering the bulb. In this way the size of the tooth is 
 maintained as the exposed and functional part is worn off. 
 Strength and elasticity are required, therefore the dentine 
 is developed. The cementum which is formed on the 
 
36 THE DENTAL TISSUES 
 
 embedded portion for attachment of fibers is worn off as soon 
 as it is exposed to friction. 
 
 Chisel Teeth. — The incisors of the rodents, as rats, mice, 
 squirrels, and beavers, present an interesting modification for 
 a special function. These teeth are used as chisels for cutting 
 hard substances, as wood, shells of nuts, etc. Here strength 
 and hardness are required. The dentine is increased by 
 the continual function of a conical persistent pulp which 
 continues to form dentine, and the enamel organ is carried 
 down into the socket, to the base of the dental papilla, on 
 the labial, instead of stopping at the gingival line, as in the 
 human incisors. In this position it continues to build 
 enamel on the labial side of the dentine. The enamel 
 rods, instead of being straight, are twisted about each 
 other in a complicated fashion, giving the maximum of 
 hardness. As the incisors work against each other by the 
 movements of the jaw, the dentine is worn off on the lingual 
 side and the enamel kept in the form of a chisel edge. There 
 is also a modification of the temporomandibular articulation, 
 allowing the lower jaw to move forward and back as well as 
 up and down, but not laterally, so that the lower incisors 
 can be closed either lingually or labrally to the upper, and 
 in this way both the upper and the lower incisors are made 
 to sharpen each other in use. In this case there is need for 
 both strength and hardness, and both dentine and enamel 
 are continuously being formed at the base of the tooth 
 embedded in the socket, and the cementum is formed over 
 the embedded portions as the medium of attachment. 
 
 Grinding Teeth. — In a grinding tooth, as in the molar of 
 the horse and cow, and in a much more complicated form 
 in the elephant, the three tissues — enamel, cementum, and 
 dentine — are arranged so as to form, by the different rapidity 
 of abrasion, corrugated grinding surfaces like millstones. 
 The conditions can be understood if it is remembered that 
 the cusps in the dentine are very high, and are covered by 
 a comparatively thin layer of enamel. After the enamel is 
 formed, and while the tooth is embedded in its crypt in the 
 bone, cementum is formed, covering the surface and filling 
 
ADAPTA TION IN DISTRIBUTION OF DENTAL TISSUES 37 
 
 up the hollows between the cusps, so that the crown when 
 it first erupts is rounded, with only enamel showing at the 
 tips of the cusps. As soon as the tooth wears, the tip of 
 the enamel is worn through, so that the circumference of the 
 crown shows first cementum, then enamel, then dentine, 
 then enamel, then cementum, then enamel, and so on. The 
 foldings of the enamel often become very complicated, but 
 the most complicated forms can be understood in this way. 
 
 In describing the structure of the teeth and the arrange- 
 ment of the structural elements of the tissues, directions are 
 described with reference to three planes: The mesio-disto- 
 axial plane passing through the centre of the crown from 
 mesial to distal and parallel with the long axis of the tooth. 
 
 The bucco-linguo-axial plane, a plane passing through the 
 centre of the crown from buccal to lingual and parallel with 
 the long axis of the tooth. 
 
 The horizontal plane at right angles to the axial planes. 
 
CHAPTER III 
 
 THE ENAMEL 
 
 The enamel differs from all other calcified tissues: 
 
 1. In origin. 
 
 2. In degree of calcification. 
 
 3. In relation to its formative organ. 
 
 4. In the form of the structural elements of the tissue. 
 It is well to emphasize these points of difference, for 
 
 throughout dental and medical writing, reasoning by 
 analogy from bone conditions to tooth conditions, and 
 especially to changes in the enamel, is often found. For 
 instance, the argument has been made that because there 
 may be changes in the bones in pregnancy, "softening" of 
 the teeth would be expected. Many similar though less 
 crude arguments would not be made if it were remembered 
 that histologically, histogenetically, physiologically, and 
 morphologically the enamel stands alone. 
 
 Origin. — The enamel is the onl}^ calcified tissue derived 
 from the epithelium. All other calcified tissues are con- 
 nective tissues. Histogenetically, then, the enamel is 
 ultimately derived from the epiblastic germ layer, while all 
 other calcified tissues arise from the mesoblast. Thus, even 
 at the first step in the differentiation of cells, the enamel is 
 different and independent from bone, cementum, or dentine. 
 It is natural, therefore, to find the enamel differing from 
 bone in every other respect. On the other hand, the relation 
 of the enamel to the epithelium becomes more and more 
 apparent. For instance, imperfections in the structure of 
 the enamel during its formation are most likely to be pro- 
 duced by systemic conditions which affect the epithelium. 
 The eruptive fevers occurring during enamel formation often 
 
• DEGREE OF CALCIFICATION 39 
 
 produce imperfections of structure. Scarlet fever is most 
 pronounced in its epithelial effect, causing loss of skin, loss 
 of living epithelium of the alimentary tract, and often loss 
 of hair, and is likewise most likely to produce pitted and 
 atrophied teeth. In other words, the same poison which is 
 produced by the germ of scarlet fever causes the death of 
 epithelial cells, of the skin, of the hair bulb, of the mucous 
 membrane, and of the enamel organ. 
 
 The most recent work of Dr. Black shows the brown and 
 mottled enamel of certain localities to be found associated 
 with greatly freckled skin. Enamel, therefore, must be 
 considered as epithelial in origin and ultimately from the 
 epiblast, while all other calcified tissues are connective 
 tissue and ultimately of mesoblastic origin. 
 
 Degree of Calcification. — The enamel is by far the 
 hardest animal tissue. Chemically it is composed of water, 
 calcium phosphate, carbonate, and a small amount of 
 fluoride, magnesium phosphate, and a trace of other salts. 
 Normally it should contain no organic matter. Von Bibra 
 gives the following analysis : 
 
 Calcium phosphate and fluoride 89 . 82 
 
 Calcium carbonate 4.37 
 
 ^Magnesium phosphate 1.34 
 
 Other salts 0.88 
 
 Cartilage 3.39 
 
 Fat 0.20 
 
 It is very difficult to obtain enamel for chemical analysis 
 entirely free from dentine, and small portions of dentine 
 clinging to it are probably responsible for some of the 
 organic matter given in the above analysis. 
 
 In all the older analyses the enamel was said to contain 
 95 to 97 per cent, of inorganic matter, and 3 to 5 per cent, 
 of organic matter, while the percentage in dentine was given 
 as 72 per cent, of inorganic and 28 per cent, of organic, and 
 in bone as 68 per cent, inorganic and 32 per cent, organic 
 (dry compact bone). This in itself shows an enormous 
 difference in the degree of calcification between enamel and 
 the other hard tissues, but the results of more recent work are 
 
40 THE ENAMEL 
 
 still more remarkable. In most of the original studies of 
 the chemical composition, the enamel was broken into 
 small pieces and dried for some time at a temperature 
 above the boiling point of water, to drive off all the moisture. 
 The dry enamel was weighed and then ignited, and the loss 
 in weight taken as the amount of organic matter. In 1896 
 Dr. Charles Tomes,^ of London, published the results of his 
 chemical analysis of enamel, in which he showed that a large 
 part of the loss of weight in ignition was due to the loss of 
 water. He carried out ignition in tubes to collect the products 
 of combustion, and found that between red and white heat 
 from 2 to 3 per cent, of water was given off. This occurred 
 suddenly and with almost explosive violence, blowing large 
 pieces to fragments. While this did not account entirely 
 for all of the matter previously considered organic, the 
 character of the product of combustion and the observation 
 of the material during ignition led him to conclude that the 
 remaining portion was due to the dentine adhering to the 
 enamel, and that the enamel contained not more than a trace 
 of organic matter. 
 
 Dr. Leon Williams attacked the problem from the micro- 
 scopic and microchemical side, and was forced to the con- 
 clusion that normal enamel contains no organic matter. 
 No trace of organic matter can be found in sections of 
 enamel by staining. And if the enamel is dissolved by acid 
 and the progress observed, not a trace of organic matrix 
 can be found. The conclusion is therefore imperative that 
 enamel is composed entirely of inorganic matter, which has 
 been deposited and calcified in the form of the tissue by the 
 formative cells. In other words, enamel is formed material 
 produced by cells and laid down in a definite structure, but 
 it contains no organic matrix, while all other calcified tissues 
 are composed of an organic matrix of ultimate fibrous and 
 gelatin-yielding character, in which inorganic salts are 
 deposited in a weak chemical combination, and living cells 
 are retained in spaces of the formed material. 
 
 1 Journal of Physiology. 
 
RELATION TO THE FORMATIVE TISSUE 41 
 
 If bone or dentine is subjected to the action of acid, 
 the combination between the organic and inorganic matter 
 is broken up and the inorganic matter dissolved, leaving 
 the organic portion, which yields gelatin when boiled in 
 water, in the form of the original tissue. If enamel is 
 treated with acid the cementing substance between the rods 
 is first attacked and is dissolved more rapidly, then the rods 
 are attacked from their sides, and finally the tissue is entirely 
 destroyed, leaving no trace of structure. Apparently the 
 greater the dilution of the acid the greater will be the extent 
 of the solution of the cementing substance before the rods 
 are destroyed. 
 
 If bone or dentine is burned or ignited, the organic 
 matter will be driven off and the inorganic portion will be 
 left in the form of the tissue, still showing its structure. If 
 enamel is ignited, water of combination and whatever 
 foreign matter has clung to the pieces is given off, but the 
 form of the tissue is unchanged. To illustrate the difl^er- 
 ence by a crude comparison: Bone matrix may be likened 
 to a piece of cloth into which organic salts have been 
 deposited until it has become stifle and rigid, but the web 
 of the cloth is still seen. The salts may be dissolved out 
 and the cloth left, or the cloth may be burned out and the 
 salts left. The enamel may be compared to a fossil in which, 
 by molecular change, the organic matter has been removed 
 and inorganic matter substituted, so that no organic matter 
 remains, but the structure is preserved. If the inorganic 
 salts were dissolved, no trace of structure would remain. 
 On the other hand, by ignition, nothing but water can be 
 driven oft'. 
 
 Relation to the Formative Tissue. — The enamel is produced 
 by epithelial cells, which are lost and destroyed after the 
 tissue is completed. Any such thing, therefore, as a vital 
 change in the tissue is biologically unthinkable. After the 
 enamel is formed it can be changed only by chemical and 
 physical action of its environment. 
 
 All other calcified tissues are formed by connective tissue, 
 and remain in vital relation with connective tissue of undifter- 
 
42 THE ENAMEL 
 
 entiated character. Bone and dentine matrix are, therefore, 
 simply calcified intercellular substances containing living 
 cells in the spaces of the matrix, which maintain its chemical 
 quality. A change in the character or amount of the matrix 
 might possibl}^, therefore, be brought about by the vital 
 activit}^ of these cells. Moreover, the formed matrix is 
 always in vital relation with undifferentiated connective 
 tissue, which may at any time destroy or rebuild it. There 
 is, therefore, no basis for comparison between pathologic 
 conditions of bone and enamel. 
 
 The Form of the Structural Elements. — The enamel is made 
 up of prismatic rods of inorganic matter, held together 
 by an inorganic cementing substance. All other calcified 
 tissues are made up of fibrous intercellular substance, con- 
 taining inorganic salts and usually arranged in layers. The 
 structure of the enamel differs so greatly from all other 
 calcified tissues that it is difficult to compare them briefly. 
 
CHAPTER IV 
 
 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 The enamel is composed of two structural elements: 
 
 1. The enamel rods or prisms, sometimes called enamel 
 
 fibers. 
 
 2. The interprismatic, or cementing substance. 
 
 Fig. 9 
 
 £na 
 
 JO X) 
 
 Enamel Rods.— The enamel rods are long slender prismatic 
 rods irregularly five or sLx sided and alternately expanded and 
 constricted throughout their length (Figs. 9 and 10). They 
 are from three and four-tenths to four and five-tenths microns 
 
44 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 in diameter, and many of them extend from the dento-enamel 
 junction to the surface of the enamel. They are of the same 
 diameter at their outer and inner ends. This last statement 
 is emphasized, as the direct opposite is stated in some stand- 
 ard text-books of histology. In the formation of the tissue 
 they are arranged so that the expansions in adjoining rods 
 come opposite to each other, and do not interlock with the 
 constrictions, so that there is alternately a greater and a less 
 amount of cementing substance between them. 
 
 Fig. 10 
 
 Enamel rods isolated by scraping. (About SOO X) 
 
 It is evident that the outer surface of the enamel is much 
 greater than the surface of the dentine at the dento-enamel 
 junction. This greater area is obtained in two ways: 
 
 1. The rods are at right angles to the dentine at the 
 dento-enamel junction, but are seldom at right angles to the 
 outer surface. This may be illustrated by bending the leaves 
 
ENAMEL RODS 
 
 45 
 
 of a book, or cutting a stack of paper obliquely. The sheets 
 of paper are of the same thickness, but when cut at right 
 angles to the sheets the area of the cut surface is not so 
 great as when the leaves are cut diagonally. 
 
 2. Many of the enamel rods undoubtedly extend from the 
 dento-enamel junction to the surface of the enamel, though 
 it is difficult to follow individual rods through this distance, 
 
 Fig. 11 
 
 Enamel rods in thin etched section. (About 800 X) 
 
 but there are also short rods which extend from the surface 
 part way to the dentine. These short rods end in tapering 
 points between converging rods that extend the entire dis- 
 tance. The short rods are specially numerous in the most 
 convex portion of the surface, as over the tips of the cusps, 
 occlusal edges, and marginal ridges. These areas, therefore, 
 become of special importance in connection with the forma- 
 
46 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 tion of enamel walls, as will be considered in detail later on 
 (Fig. 11). 
 
 Differences between Enamel Rods and Cementing Substance. 
 ■ — AYliile the cementing substance and the substance of the 
 rods are both entirely inorganic, or, more correctly, are com- 
 posed entirely of mineral salts, they differ in physical and 
 chemical properties as follows : 
 
 1. The cementing substance is not as strong as the pris- 
 matic substance. 
 
 2. The cementing substance is more readily soluble in 
 dilute acids than the rod substance. 
 
 3. The cementing substance is of slightly different 
 (greater) refracting index than the substance of the rod. The 
 author wishes to emphasize these statements, as the exact 
 opposite is found in some of the standard texts, at least 
 concerning the first and second statements. The facts are, 
 however, so easily demonstrable that anyone may satisfy 
 himself without difficulty. 
 
 Relative Strength of the Enamel Rods and the Cementing 
 Substance. — The cementing substance is not as strong as 
 the substance of the rods. The most striking characteristics 
 of the enamel, and the first to attract the attention of the 
 student and the operator, are its hardness and its tendency 
 to split or cleave in certain directions. On examination it is 
 found that this is determined by the direction of the rods, 
 and is caused by the difference in strength between the two 
 substances. Sections ground at right angles to the rod 
 direction are very difficult to prepare because of the tendency 
 of the section to break to pieces. 
 
 If a section that is beginning to crack (Fig. 12) is studied, 
 the crack is found to follow the line of the cementing sub- 
 stance running around the rods. In some places a rod may 
 be split through its centre, but most of the rods remain 
 perfect, and the cementing substance breaks. In the same 
 way a section cut in the direction of the rods shows the crack 
 following the lines of the cementing substance (Fig. 13), 
 here and there breaking across a few rods, and then fol- 
 lowing the direction again; but the rods separate on the 
 
RELATIVE STRENGTH OF THE ENAMEL RODS 47 
 
 line of union, not at the centres of the rods. This fact 
 becomes fundamental in the cutting of enamel and in the 
 preparation of strong enamel walls. 
 
 Fig. 12 
 
 Transverse section of enamel rods. (.About bO X) 
 Fig. 13 
 
 Enamel showing direction of cleavage. (About 70 X) 
 
48 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 Relative Solubility of Enamel Rods and Cementing Sub- 
 stance. — If a thin section of enamel cut parallel witli the 
 direction of the enamel rods is mounted in water and hydro- 
 chloric acid (2 per cent.) is allowed to run under the cover- 
 glass and the action observed, it will be seen to attack the 
 cementing substance more rapidly, dissolving it out from 
 between the enamel rods and attacking their sides. If the 
 action is stopped the ends of the rods will be seen pro- 
 jecting like the pickets of a fence, as shown in the photograph 
 (Fig. 14). The more dilute the acid the greater will be the 
 distance to which the cementing substance is removed before 
 the rods are destroyed. 
 
 Fig. 14 
 
 The effect of acid on a section of enamel. 
 
 Etching. — If a section of enamel is ground at right angles 
 to the direction of the rods, mounted in glycerin and photo- 
 graphed, the outline of the rods will be seen with difficulty 
 (Fig. 15). The refracting index of the two substances is so 
 nearly the same that the section seems of almost uniform 
 transparency. The thinner the section, the greater will be 
 the difficulty of recognizing the rods. Oblique illumina- 
 tion and the use of a small diaphragm will, however, resolve 
 them. If the section is washed and treated with 2 per cent, 
 hydrochloric acid for a few seconds, washed, and remounted 
 in glycerin, the rods are distinctly outlined (Fig. 16). The 
 acid attacks the cementing substance and the surface of 
 the section is etched as if an engraving tool had been run 
 
RELATIVE SOLUBILITY OF THE EX AM EL RODS 49 
 
 around the rods. The fine grooves on the surface refract 
 the fight and outfine the rods. The difference in appear- 
 ance in longitudinal sections, that is, sections parallel with the 
 direction of enamel rods, is quite as striking. For the study 
 of enamel rod directions this etching is of the greatest impor- 
 tance. Only one side of the section should be acted upon 
 
 Enamel ground at right angles to the rods. Not treated with acid. (About 500 X) 
 
 by the acid, and the section should be mounted etched side 
 up. If etched upon both surfaces, the grooves in the lower 
 surface cannot be in focus at the same time as those of the 
 upper surface and will blur the definition. 
 
 The difference in the solubility of the rods and cementing 
 substance is beautifully illustrated in the effect of caries 
 4 
 
50 THE STRUCTURAL ELEMENTS OF THE ENAMEL 
 
 on the structure of the enamel (see ilhistrations in Chai)ter 
 XII), and caries of the enamel cannot be understood unless 
 these fundamental facts are remembered. The question, 
 "AVhat causes the difference in solubility between the enamel 
 rods and the cementing substance?" cannot be satisfactorily 
 answered at the i)rcsent time. While both the rods and the 
 
 Fig. 10 
 
 The same section as Fig. 15 after treatment with acid. (About 500 X) 
 
 cementing substance are normally composed entirely of 
 inorganic salts, there may be different salts in the two sub- 
 stances, or the salts may be in different physical condition. 
 There is great need for careful work in this field. Recent 
 work has strongly emphasized the distinctness of the two 
 structural elements of the enamel. 
 
DIFFERENCE IN REFRACTING INDEX 51 
 
 First, the study of the beginnings of caries of the enamel, 
 and the effect of caries upon the structure of the enamel, 
 brought out the difference in solubility in acids and showed 
 the extent of tissue injury before a cavity is formed. Later, 
 the study of atrophy developed the fact that certain patho- 
 logic or abnormal conditions may hinder or entirely pre- 
 vent the formation of the rods while the cementing sub- 
 stance is formed, and still more recently the investigation of 
 dystrophies of the enamel occurring in certain prescribed 
 localities, showed perfect rod formation and entire absence 
 of the cementing substance. These facts suggest the hypoth- 
 esis that the enamel rods and the cementing substance have 
 a different origin, or are formed by different cells, and that 
 pathological conditions may prevent the formation of one 
 and not the other. In view of these factors it is very neces- 
 sary that a new investigation of the process of enamel for- 
 mation be undertaken, as present knowledge of the process 
 does not explain such conditions. 
 
 Difference in Refracting Index between the Rods and the 
 Cementing Substance. — The cementing substance is of slightly 
 greater refracting index than the substance of the rods. If 
 it were not for this it would be impossible to see the rods in 
 unetched sections, either longitudinal or transverse. The 
 appearance of striation seen in longitudinal sections is also 
 dependent upon this difference in action on transmitted 
 light. 
 
CHAPTER V 
 
 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 From what has been said of the structural elements of 
 the tissue, their physical and chemical properties, and their 
 arrangement in the tissue, it is apparent that the striking 
 characteristics of the enamel are the result of these factors; 
 and that it can be intelligently dealt with only by thinking of 
 it always in these terms. 
 
 Enamel showing cleavage. 
 
 The enamel may be crudely compared to a pavement made 
 up of tall columns closely cemented together by an inorganic 
 cement. The wear comes on the ends of the columns, and 
 they furnish great resistance to the abrasion of friction. 
 
STRAIGHT ENAMEL 
 
 53 
 
 When supported upon a good and elastic foundation it is very 
 difficult to break it down, but when an opening has been 
 made in it, and the foundation removed from underneath, 
 the columns are comparatively easily split off and tumbled 
 into the opening (Fig. 17). This figure is crude, but it is a 
 very helpful one in learning to think of the enamel in terms 
 of its structural elements. 
 
 Fig. 18 
 
 Straight enamel rods. (About 80 X) 
 
 Straight Enamel. — Upon the axial surfaces of the teeth the 
 rods are usually straight and parallel with each other, and 
 most of them extend from the dentine to the surface. Such 
 enamel will split or cleave in the direction of the rods with 
 
54 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 comparative ease, and breaks down very readily when the 
 dentine is removed from under it. It will usually cleave 
 through its entire thickness and break away from sound 
 dentine when properly attacked with sharp hand instru- 
 ments. Such enamel is called straight enamel, as con- 
 trasted with gnarled enamel. It is best illustrated by 
 cutting sections lal)iolingualIy through the incisors, though 
 there is consi(leral)le variation in different teeth (Figs. 13 
 and 18). 
 
 Fig. 19 
 
 Gnarled enamel. (About 80 >0 
 
 Gnarled Enamel. — Upon the occlusal surfaces of the 
 molars and bicuspids, and especially over the tips of cusps 
 and marginal ridges, the rods are seldom straight and 
 
GNARLED ENAMEL 
 
 55 
 
 parallel through the thickness of the enamel, but are wound 
 and twisted about each other, especially in the deeper half 
 toward the dento-enamel junction. This is known as 
 gnarled enamel, and its appearance is in marked contrast 
 with straight enamel. 
 
 Fig. 20 
 
 Gnarled enamel. (About 50 X) 
 
 Toward the surface the rods are usually straight and 
 parallel for a longer or shorter distance, but as the dento- 
 enamel junction is approached they become twisted. This 
 is true of most of the occlusal surfaces of molars and bicus- 
 pids, but the gnarled condition extends farther toward the 
 surface-over the tips of the cusps, or the point at which the 
 rods were first completed in the growth of the crown. As 
 
5G CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 cleavage is caused by the difference in the strength of the 
 rods and cementing substance, it is easy to see that gnarled 
 enamel will not split or cleave easily when resting upon 
 sound dentine. This is often encountered in extending 
 occlusal cavities. The straight portion will split, but where 
 the rods begin to twist they break off, leaving a portion 
 resting on the dentine w-hich will resist the attack of any 
 cutting instrument from the surface (Figs. 19, 20, and 21). 
 
 Fig, 21 
 
 Gnarled enamel from etched section. (About 100 X) 
 
 The Effect of Structure on the Cutting of Enamel. — The two 
 
 kinds of enamel may be compared to straight-grained pine 
 wood and a pine knot. The first will split easily in the 
 direction of the fibers, the latter will split only in an irregu- 
 lar way and with the greatest difficulty. This difference in 
 the arrangements of the structural elements leads to the 
 difference in the feeling of various teeth to cutting instru- 
 ments, and is the basis for the clinical experience of hard 
 and soft teeth. It is not a matter of degree of calcification, 
 
APPEARANCES CHARACTERISTIC OF ENAMEL 57 
 
 but the arrangement of the structural elements, and gnarled 
 enamel will break down as rapidly under the effect of caries 
 as straight enamel would. 
 
 From a study of the positions in which the rods are 
 usually twisted about each other, and those in which they 
 are usually straight, it seems probable that the twisting is 
 due to movements in the dental papilla and the enamel 
 organ during the formation of the tissue. These movements 
 may be produced by variations in the blood pressure which 
 cause oscillations, or shifting of the tissues on each other. 
 These differences in the arrangement of the structural 
 elements of the enamel must be constantly kept in mind, 
 and will be referred to many times in connection with the 
 use of cutting instruments on the enamel and the preparation 
 of cavity walls. 
 
 APPEARANCES CHARACTERISTIC OF ENAMEL 
 
 Striation. — Striation is the appearance of fine light and 
 dark markings occurring alternately in the length of the 
 enamel rods. This is not unlike the striation of voluntary 
 muscle fibers, and has a similar cause. It is seen both in 
 thin sections cut in the direction of the rods, and in isolated 
 enamel rods. It is caused by the alternate expansions and 
 constrictions of the rods and the difference in the refracting 
 index between the rods and the cementing substance. 
 
 If isolated rods (Fig. 22) are observed with a -g^ or yV objec- 
 tive, they will be seen to be marked by alternate light and 
 dark areas across the rods; on changing the focus up and 
 down, the light and dark areas will change places, just as in 
 looking at a red blood corpuscle the centre may appear dark 
 and the rim light, or the centre light and the rim dark, 
 depending upon the exactness of focus. This is caused by the 
 refraction of the light as it passes through the convex and 
 concave portions of the rod. If the cementing substance were 
 of exactly the same refracting index as the rods, when the 
 rods were fastened together in the tissue there would be no 
 appearajice of striation, but as it is not, refraction of light 
 
58 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 occurs in ])assiiig from rod substance to cementing sub- 
 stance, and the striation is apparent in sections. There is 
 considerable difference in the distinctness of striation in 
 different sections of enamel. This is probably due to the fact 
 that the cementing substance has more nearly the same 
 refracting index as the rods in some specimens. When the 
 formation of enamel has been studied it will be found that 
 the enamel rods have been formed by globules which are 
 
 Fig 22 
 
 ^ 3^\ 
 
 Isolated enaniL'i r')d^<. iAiMun 1000 X.) 
 
 deposited one on top of the other to form the rods, and the 
 cementing substance fills up the space. The globules in 
 the adjacent rods come opposite each other, so that there 
 is alternately a greater and a less amount of cementing 
 substance between the rods. Each cross-mark, therefore, 
 represents a globule deposited in the formation of the rod, 
 and striation may be said to be a record of the growth of the 
 individual rods (Figs. 23 and 24). 
 
APPEARANCES CHARACTERISTIC OF ENAMEL 59 
 
 Fig. 23 
 
 
 
 1 
 
 1 
 
 ■ 
 
 ■ 
 
 ■ 
 
 i 
 
 
 
 B 
 
 ? 
 
 ^B 
 
 ^M 
 
 H 
 
 ■ 
 
 
 
 "-■■ 
 
 
 JH 
 
 ^^H 
 
 w^ 
 
 ^^ 
 
 
 
 
 
 '^H 
 
 m^Hi 
 
 ^K^ 
 
 
 
 m - 
 
 
 
 ^ 
 
 H^l 
 
 iv 
 
 
 . ;.' ' 
 
 flrv' r 
 
 
 
 
 m 
 
 B 
 
 
 
 U ';--> 
 
 
 
 
 rJH 
 
 ^Ktt_ 
 
 -: .v^ 
 
 
 
 E&nT'V 
 
 
 
 i 
 
 hm 
 
 ■ 
 
 
 
 H^ 
 
 
 
 
 «■ 
 
 ■ 
 
 
 ^^B 
 
 H| 
 
 1 
 
 — ^^ 
 
 ^•,-" .' '"- •. •-; 
 
 / .^ 
 
 ■ 
 
 ,-. ,. 
 
 ^^^i 
 
 MB 
 
 ■ 
 
 IKHI 
 
 IMH 
 
 IHIH 
 
 
 Knamei showing both striation and stratification. (About SO X) 
 Fig. 24 
 
 Enamel showing striation. (About 1000 X) 
 
60 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 Imperfections in the cementing substance render the 
 striation more apparent because they increase the difference 
 in refraction between the two substances. The action of 
 acid either upon isolated rods or upon sections renders 
 striation more apparent because it attacks the cementing 
 substance faster than the globules forming the rods, and 
 therefore increases the refraction. Von Beber has claimed 
 that the appearance of striation was caused by the action of 
 acid on the section, and that even in mounting in balsam 
 the acidity of the balsam affected the tissue. It is true that 
 any action of acid increases the distinctness of the cross- 
 striation, but it is not the cause of it. 
 
 Stratification, or the Bands of Retzius. — If longitudinal 
 sections of moderate thickness are observed with the low 
 power, brownish bands are seen running through the enamel, 
 which suggests the appearance of stratification in rocks. 
 These were first described by Retzius and were named after 
 him — the brown bands or striae of Retzius. A better name 
 would be incremental lines. 
 
 The bands of Retzius, or incremental lines, are caused by 
 actual coloring matter which is deposited with the inorganic 
 salts in the formation of the tissue. They are, therefore, 
 best seen with low powers and in sections that are not too 
 thin. In sections that are thinner than the diameter of a 
 single rod, or less than four microns, they become almost 
 invisible. For the study of the bands of Retzius sections 
 should be ground labiolingually through the incisors, bucco- 
 lingually through the bicuspids and molars, striking the 
 centre at the cusps. They may be studied also in mesiodistal 
 sections, but the sections should be in such a direction as to 
 be at right angles to the zones. Fig. 25 shows the tip of an 
 incisor in which the bands are very well marked. They are 
 seen to begin at the dento-enamel junction on the incisal 
 edge, and sweep in larger and larger zones around this 
 point. Each band represents what was at one time the sur- 
 face of the enamel already formed, and the line upon which 
 formation was progressing. They are, therefore, truly 
 incremental lines. The zones reach the surface of the 
 
APPEARANCES CHARACTERISTIC OF ENAMEL 61 
 
 enamel first at the point over the centre of beginning calcifi- 
 cation, and the succeeding bands extend from the surface 
 
 Fig. 25 
 
 Tip of an incisor. (About 50 X) 
 
 of the enamel, near the occlusal, to the dento-enamel junction 
 much farther apically, and corresponding lines are seen on 
 opposite sides of the section. In Fig. 26 the band which 
 
C)2 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 is at the surface at A and /I' reaches the dento-enamel junc- 
 tion at B and B\ This means that when the enamel rods 
 which form the surface at A were completed, the rods at 
 
 Fig. 26 
 
 Incisor tip showing stratification or incremental lines. Rods at A were fully formed 
 at the time the rods at B were Vjeginning to form. (About 50 X) 
 
 B were just beginning to be formed at the dento-enamel 
 junction. A layer of functioning ameloblasts occupied 
 this position. The bands of Retzius are always curved 
 
APPEARANCES CHARACTERISTIC OF ENAMEL 63 
 
 and usually pass obliquely across the enamel rods, but 
 are parallel neither with the dento-enamel junction or the 
 surface of the enamel. As they pass toward the gingival 
 
 Fig. 27 
 
 Stratification of enamel: the cusp of a bicuspid: De. dento-enamel junction: Ed, 
 enamel defecj showing in the heavj' stratification band; Ig, interglobular spaces in 
 the dentine. (About 40 X) 
 
64 CHARACTERISTICS OF THE ENAMEL TISSUE 
 
 the angle which they form with the axis of the tooth becomes 
 greater. Any disturbance of nutrition which affects the 
 formation of enamel is always shown in the increased distinct- 
 ness of the bands (Fig. 27). 
 
 The bands of Retzius, therefore, form a record of the 
 formation of the tissue, and by their study the points of 
 beginning calcification and the manner of the development 
 of the tooth crown may be followed. This will be con- 
 sidered again in connection with the grooves, pits, and 
 natural defects of the enamel. 
 
 Fig. 28 
 
 ^ 
 
 Lines of Schreger. (About 5 X) 
 
 Lines of Schreger. — These are lines appearing in the enamel 
 extending from the dento-enamel junction to or toward the 
 surface. They are caused by the direction in which the 
 enamel rods are cut. They may be seen in sections, but are 
 best shown by photographing the cut surface of the enamel 
 by reflected light and with very low magnification. The 
 rods are twisting about each other, and in one streak they 
 are cut longitudinally, in the next obliquely, and the 
 alternations of these directions cause the appearance of 
 the lines (Fig. 28). 
 
CHAPTER VI 
 
 THE DIRECTION OF THE ENAMEL RODS IN THE 
 TOOTH CROWN 
 
 In describing the direction of the enamel rods and their 
 arrangement in what may be called the architecture of the 
 tooth crown, they are always considered as extending from 
 the dento-enamel junction outward. This is not only con- 
 venient, but logical, as they are formed in that way, beginning 
 at the dento-enamel junction and being completed at the 
 surface. Enamel is formed from within outward, the cells 
 which produce it lying outside of the tissue already formed, 
 and there are many things about the arrangement of the rods 
 and their relation to each other that are understood only 
 when this is borne in mind. 
 
 The direction of the enamel rods is described by referring 
 them to the horizontal and axial planes, which have been 
 previously defined (page 37). The centigrade scale, that 
 is the division of the circle into one hundred equal arcs, is 
 used because those familiar with instrument nomenclature 
 are already familiar with these angles, and readily picture 
 them.i When a rod is said to be inclined 12 centigrades 
 
 1 In the centigrade division 
 the circle is divided into one 
 hundred parts, each called a 
 centigrade. One centigrade is 
 equal to 3.6 degrees of the 
 astronomical circle. 25 centi- 
 grades to 90 degrees, 12f cen- 
 tigrades to 45 degrees. The 
 cut gives a comparison of the 
 two systems of measuring 
 angles 
 
 2 70 
 
 180'' 
 Centigrade di\'ision. 
 
GO DIRECTION OF ENAMEL RODS IN TOOTH CROWN 
 
 occlusally from tlie horizontal plane, it means that if a 
 plane at right angles to the long axis of the tooth is passed 
 through the end of the rod at the dento-enamel junction, the 
 rod will lie to the occlusal of it and form an angle of 12 cen- 
 tigrades with it. In the same way, if a rod is said to be 
 inclined 12 centigrades buccally from the mesiodistal plane, 
 it means that if a plane parallel with the axis of the tooth, 
 and extending from mesio to distal, is passed through the 
 end of a rod at the dento-enamel junction, the rod will lie to 
 the buccal of it, and form an angle of 12 centigrades with it. 
 By a little practice with these terms the direction of the 
 enamel rods can be very easily and clearly pictured to the 
 mind. 
 
 The General Direction of Enamel Rods. — The general direc- 
 tion of the enamel rods has been variously described by 
 different authors, but all of these general statements are 
 very imperfect and often misleading. For instance, they 
 are sometimes said to radiate from the centre of the crown 
 or the pulp chamber, but it will be seen that this does not 
 apply to the rods which form the lingual slopes of the buccal 
 cusps, or the buccal slopes of the lingual cusps of bicuspids 
 and molars. 
 
 Again, they have been said to be, in general, perpendicular 
 to the surface, but it will be found from the study of sections 
 that there are very few places upon the surface where this is 
 true, and that in many places they are far from perpendicu- 
 lar to the surface. From a stud}^ of sections it wdll be seen 
 that the general arrangement of enamel rods, in the archi- 
 tecture of the tooth crown is such as to give the greatest 
 strength to the perfect tissue, and to furnish the greatest resist- 
 ance to abrasion in the use of the teeth for mastication. In a 
 buccolingual section through a bicuspid (Fig. 29), beginning 
 at the gingival line, the enamel is normally slightly overlapped 
 by the cementum, and in the gingival third the rods are 
 inclined more or less apically from the horizontal plane. 
 The degree of inclination varies considerably. It may be 
 as much as 12 centigrades, but is usually not more than 6. 
 In general, the more convex the surface the greater will 
 
THE GENERAL DIRECTION OF ENAMEL RODS 67 
 
 be the inclination. At some point between the junction of 
 the gingival and middle thirds and the middle of the middle 
 third of the surface they are in the horizontal plane and at 
 right angles to the axis of the tooth, and at this point they 
 are usually very nearly perpendicular to the surface. Passing 
 
 Fig. 29 
 
 Diagram of enamel rod directions, from a photograph of a buccolingual section of 
 an upper bicuspid. 
 
 occlusally from this point, they incline more and more 
 occlusally until in the occlusal third they reach an inclina- 
 tion of 18 to 20 centigrades occlusalh' from the horizontal. 
 
 The rods which form the tip of the buccal cusps do not 
 reach the tip of the dentine cusp, but the buccal slope of 
 
68 DIRECTION OF ENAMEL RODS IN TOOTH CROWN 
 
 the dentine. This becomes important, as will be seen later. 
 Over the tip of the dentine cusp the rods are in the axial 
 plane, but in this position they are usually very much 
 twisted. Passing down the lingual slope, they become more 
 and more inclined lingually from the mesiodistal axial plane. 
 
 Fig. 30 
 
 Diagram of enamel rod directions, drawn from a mesiodistal section of a bicuspid. 
 
 and the degree of inclination is related to the height of 
 the cusp — the taller the cusp the greater the inclination. At 
 the developmental groove or pit they meet the rods of the 
 lingual cusp, which are inclined in the opposite direction. 
 
 In a mesiodistal section (Fig. 30) the plan of arrangement 
 will be seen to be the same, the tip of the marginal ridge 
 corresponding to the tip of the cusp. In an incisor the 
 
THE GENERAL DIRECTION OF ENAMEL RODS 69 
 
 I 
 Fig. 31 1 
 
 Disturbance of enamel rod directions on labial surface of a cuspid. (About SO X 
 
70 DIRECTION OF ENAMEL RODS IN TOOTH CROWN 
 
 Fia. 32 
 
 Disturbance of enamel rod directions on lingual surface of same tooth as Fig. 33. 
 (About 80 X) 
 
SPECIAL AREAS 71 
 
 arrangement is similar, the lingual marginal ridge corre- 
 sponding to a rudimentary cusp. This general plan should 
 be studied in several sections of the various classes of teeth 
 before the rod direction is studied more minutely. 
 
 Effect of Atrophy. — Whenever an atrophy groove appears 
 upon the surface, the rod directions will be found to be more 
 or less disturbed. Fig. 31 shows a position on the labial 
 surface of a cuspid. In this position the disturbance of the 
 enamel rod direction is very marked. The rods tend to be 
 in whirls and the structure is more or less deficient. On the 
 lingual side of the same section (Fig. 32) the disturbance in 
 structure is so great that it is difficult to make out the rod 
 direction. Many such areas will be found in sections. Some 
 condition which has affected the nutrition of the enamel- 
 forming cells results in a local disturbance of the structural 
 elements. 
 
 SPECIAL AREAS 
 
 The Gingival Third. — There is much variation in enamel 
 rod direction in difierent teeth as the gingival line is 
 approached. The inclination apically from the horizontal 
 may be very great, as much as 12 to 15 centigrades in 
 some instances, as in Fig. 33, but this is exceptional. 
 It may be very slight, or the rods may be almost in the 
 horizontal plane. The direction of the rods in these areas 
 become very important in the preparation of the gingival 
 wall of proximal cavities, and cavities in the gingival third 
 of buccal and labial surfaces. 
 
 The Tips of the Cusps. — In studying the rod directions 
 in the region of the cusps and marginal ridges, it must be 
 borne in mind that the formation of enamel begins at the 
 dento-enamel junction, at separate points, and that the 
 growth is recorded in the tissue by the bands of Retzius, 
 each band having been at one time the surface of the enamel 
 cap then formed. In a buccolingual section the formation 
 of the buccal and lingual cusps will be shown (Chapter X). 
 While the little caps are growing they are being carried 
 apart by the growth of the dental papilla and enamel 
 organ, until the calcifications unite at the dento-enamel 
 
72 DIRECTION OF ENAMEL RODS IN TOOTH CROWN 
 
 junction. When this occurs the dental papilla has reached 
 its maximum mesio-distal diameter. The enamel organ, 
 however, will continue to grow, and as the rods are com- 
 
 FiG. 33 
 
 Direction of enamel rods in the gingival third. 
 
 pleted first just over the tip of the dentine cusp, the con- 
 tinued growth causes an increase in the inclination of the 
 rods in their outer portion. This often leads to a curving 
 of the rods at their outer portion. 
 
CHAPTER VII 
 
 THE RELATION OF THE STRUCTURE TO THE CUTTING 
 OF THE ENAMEL 
 
 • There are two methods of cutting enamel — to chop or 
 cleave it, or to shave or plane it. 
 
 Cleaving or Chopping Enamel. — In the cleavage of the 
 enamel the action of the instrument more nearly resembles 
 that of splitting ice than that of splitting wood. The ax for 
 splitting wood is strongly wedge-shaped, and the wedge 
 pries the fibers apart. In splitting ice a small nick is made 
 on the surface and then a sharp blow cracks the ice in the 
 direction of the cleavage. In a similar way the chisel 
 applied to the surface of the enamel makes a slight scratch 
 or bearing on the surface, and the force applied at a slight 
 angle to the direction of the rods cracks the tissue through 
 in the rod direction. The bevel of the instrument is 
 designed to give strength and keenness of edge, not to act 
 as a wedge. In order to cleave the enamel it is always 
 necessary that there be a break or opening in the tissue. 
 Only a small portion can be split off at a time. The edge 
 of the chisel should be placed on the enamel a quarter or 
 half a millimeter from the opening, rarely more, and so 
 piece after piece is split into the cavity. Fig. 34 shows a 
 section of enamel. The edge of the chisel is placed at 1, with 
 the shaft in the relation to enamel rod direction indicated; 
 a tap of a steel mallet will split oft' a piece, and the chisel is 
 moved back to position 2 and a second piece is split off. 
 Undermined enamel will split easily in this way. As soon as 
 a point is reached where the enamel rests on sound dentine, 
 it is recognized by the resistance. Straight enamel can be 
 split off from sound dentine without difficulty if attacked 
 in the proper way, but if the inner portion is gnarled and 
 
74 RE LA TION OF S TR UC T URE TO CUT TING OF EN A MEL 
 
 twisted, it can only be cleaved by removing the dentine 
 from under it. Such enamel, if resting on dentine, will split 
 
 Position of chisel in clea\ 
 
 as far as the rods are straight; but where they begin to twist 
 they will break off, leaving a portion which is very difficult 
 to remove by attacking it from the surface. If the dentine 
 
CLEAVING OR CHOPPING ENAMEL 75 
 
 is removed from under gnarled enamel, it will crack through 
 in an irregular way, following the general direction of the 
 rods. 
 
 In preparing teeth for crowns it is often necessary to 
 remove a large amount of enamel. This is always more 
 efficiently accomplished by the intelligent use of sharp 
 instruments than by force. The enamel on axial surfaces, 
 especially in the gingival half of the crown, is usually 
 straight, and if a cleavage line can once be established, 
 the enamel can be more easily and rapidly removed by 
 splitting it off piece after piece than in any other way. 
 In doing this a straight or contra-angled chisel is often the 
 most efficient instrument, and it must be remembered that 
 the "root trimmers" are more properly called "enamel 
 cleavers," and that they are used to cleave the enamel, 
 not to scrape or hoe it off, their form being adapted to give 
 a strong palm grasp of the instrument. 
 
 Fig. 35 illustrates the use of the enamel cleaver for the 
 removal of gingival enamel from an axial surface. The line 
 of cleavage being established, the edge of the instrument is 
 placed on the enamel half a millimeter from the broken edge, 
 and the force, which should be strong, quick, and sharp, is 
 applied in the direction indicated, and piece after piece is 
 split off, progressing from the occlusal toward the gingival. 
 In preparing the wall of a cavity the outline form should 
 be attained by cleavage, and this is the first step in the 
 preparation of the cavity. 
 
 After the enamel has been removed by cleavage to the 
 point where the margin is to be laid, the wall must be 
 completed by cutting the enamel in an entirely different 
 way. 
 
 Planing or Shaving Enamel. — In this manner of cutting 
 enamel the tissue is removed without reference to the rod 
 direction, and without injury to its structure (Figs. 36, 37, 
 and 38). The chisel is used like the blade of a plane. The 
 cutting edge is placed against the surface with the shaft of 
 the instrument almost parallel to it, and the tissue is shaved 
 away. In this way the rods that have been cracked apart 
 
76 RELATION OF STRUCTURE TO CUTTING OF ENAMEL 
 
 by the cleavage are removed, and the walls arranged in 
 terms of its structural elements so as to gain the required 
 strength of margin. 
 
 Fig. 35 
 
 ^— 
 
 The use of enamel cleaver in removing enamel. 
 
 Sharp Instruments. — Chisels and hatchets for use in cleaving 
 or planing enamel must be keenly sharp. If a dull edge is 
 
SHARP INSTRUMENTS 77 
 
 Fig. 36 Fig. 37 Fig. 38 
 
 The use of the chisel in planing or shaving enamel. (Black.) 
 Fig. 39 
 
 The relation of the edge of a sharp and a dull chisel. 
 
7S RELA TIOX OF STRUCTURE TO CUTTING OF ENAMEL 
 
 ])lacc(l on the surface of the enamel it will rest across the 
 ends of many rods, and force applied will only crumble them, 
 but will not split the tissue. The edge must be keen (Fig. 
 .39), so as to engage between the rods and so start 
 the cleavage. Cutting instruments as furnished by dental 
 supply houses are not tempered hard enough to hold an 
 edge. There is no fault to be found with the supply houses 
 for this, for they make them as the dentist wants them, and 
 any dealer will furnish hard-tempered instruments if they 
 are ordered. To use hand instruments successfully in cutting 
 enamel, the stock instruments must either be retempered or 
 the}' must be ordered hard tempered. The cutting edge of 
 
 Fig. 40 
 
 Fig. 41 
 
 Fig. 42 
 
 The use of the chisel in cleaving enamel. Opening an occlusal cavity. (Black.) 
 
 the blade of an enamel instrument should be straw-colored 
 when tempered. The chisel and hatchets are the instruments 
 for removing enamel. The burr is the instrument for 
 removing hard dentine. When the burr is used on enamel 
 it should be remembered that it is used as a revolving 
 chisel. It is by the thoughtful use of hand instruments 
 that knowledge of enamel rod direction is gained, and 
 only by the use of them can the enamel walls be pre- 
 pared in terms of their structural elements. In cleaving 
 undermined enamel the edge may be used either with a 
 pulling or a pushing motion. For instance, in opening up 
 a cavity in the occlusal surface of a bicuspid, the buccal 
 
SHARP INSTRUMENTS 79 
 
 portion of undermined enamel is split off by placing the 
 instrument as shown in Figs. 40 and 41 . The bevel of the blade 
 is held toward the cavity and the shaft of the instrument at 
 a slight angle to the rod direction, and the force is applied 
 in the direction of the shaft. The lingual portion may 
 be removed by placing the instrument as indicated in Fig. 
 42, the bevel of the blade away from the cavit}^ and the 
 force applied in the direction of the bevel by a pulling force 
 in the direction of the shaft. This is the way in which force 
 is applied on enamel cleavers. The pitch of the bevel in an 
 enamel cleaver and its relation to the shaft of the instrument 
 is extremely important, and the efficiency of an instrument 
 may easily be ruined by careless honing. Every time a 
 cutting instrument is applied to the enamel it must be 
 done with a knowledge of the relation of the cutting edge 
 and the force to the direction of the enamel rods, until it 
 becomes entirely automatic. The author emphatically 
 believes that the acquirement of this knowledge and skill 
 will do more to increase facility and success in the prepara- 
 tion of cavity walls than any other manipulative factor. 
 The preparation of enamel walls requires the continual 
 application of the knowledge of enamel structure. Enamel is 
 a very hard tissue, but it is composed of structural elements, 
 and walls prepared without reference to them will prove 
 their own weakness. 
 
CHAPTER VIII 
 
 THE STRUCTURAL REQUIREMENTS FOR STRONG 
 ENAMEL WALLS 
 
 From the consideration of the physical character of the 
 enamel, its structural elements and their properties, it is 
 evident that the strength of any enamel wall is dependent 
 upon the arrangement of the rods in the tissue which makes 
 up the walls and their relation to the dentine. Certain 
 requirements for strength can be clearly stated, and these 
 are applicable to all enamel walls. They cannot always be 
 secured with equal facility or perfection, but in proportion 
 as these principles are observed and attained the wall will 
 be strong; as they are imperfectly attained or ignored the 
 wall will be weak and unreliable. When these conditions are 
 understood very many failures can be clearly seen to have 
 been the result of their neglect. 
 
 Structural Requirements.^ — 1. The enamel must rest upon 
 sound dentine. 
 
 2. The rods which form the cavosurface angle must have 
 their inner ends resting upon sound dentine. 
 
 3. The rods which form the cavosurface angle must be 
 supported by a portion of enamel in which the inner ends of 
 the rods rest on sound dentine and the outer ends are 
 covered by the filling material. 
 
 4. The cavosurface angle^ must be so trimmed or bevelled 
 so that the margin will not be exposed to injury in con- 
 densing the filling material against it (Fig. 43). 
 
 These requirements should be considered one by one. 
 The Enamel Must Rest upon Sound Dentine. — That is, the 
 enamel plate must have the support of sound dentine, and 
 
 1 The cavosurface angle is defined as the angle formed by the surface of the tooth 
 and the wall of the cavity. 
 
ENAMEL MUST REST UPON SOUND DENTINE 81 
 
 all portions which are undermined by the removal of dentine 
 must be cut away. When the inner ends of the rods which 
 form the enamel plate rest upon sound dentine, the elasticity 
 of the dentine gives to the enamel a certain degree of elas- 
 ticity^, but the enamel itself without this support is extremely 
 brittle. Fig. 44 illustrates these requirements. The enamel 
 plate ABCD rests upon sound dentine. The rods which form 
 
 FiQ. 43 
 
 The structural requirements for a strong enamel wall. 
 
 the cavosurface angle at B run uninterruptedly to the den- 
 tine, and their inner ends rest on it at E. The rods B, E are 
 also supported by a portion of enamel, ABE, made up of 
 rods whose inner ends rest upon the dentine and whose 
 outer ends are covered in by the filling material, altogether 
 supporting the marginal rods like a buttress. And the cavo- 
 surface angle is bevelled, including from J to y of the enamel 
 6 
 
82 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 wall, so as to remove the sharp corner which would be in 
 danger of crumbling in under an instrument. A force that 
 causes it to give way will crack it through its entire thickness. 
 No filling material or substitute for the lost dentine can 
 restore the original condition. An enamel wall should be 
 
 Fig. 44 
 
 The structural requirements for a strong enamel wall: AB, the bevel of the 
 cavosurface angle. The rods forming the margin of the cavity at B reach the 
 dentine at E, and are supported by the portion ABE. 
 
 considered no stronger after the filling is inserted than it 
 was before. Moreover, when the dentine has been decalcified 
 or destroyed by the action of caries, the acid which has 
 decalcified the dentine has also acted upon the enamel, dis- 
 solving the cementing substance from between the rods, from 
 
EX AM EL MUST REST UPON SOUND DENTINE 83 
 
 within outward, often to a great extent, and the structure 
 is very imperfect. Enamel that has been so weakened will 
 not withstand the force of mastication, and sooner or later 
 will crack or break away from the filling material. It 
 should be removed and the wall formed in tissue whose 
 structure is perfect. Occasionally cases arise where an 
 
 Fig. 45 
 
 Improperly prepared enamel wall. The portion ABC has the inner ends of the 
 rods cut off and they do not reach the dentine. 
 
 operator decides to leave some unsupported enamel, but its 
 weakness and the possibility of restoring it if it breaks away 
 without destroying the original operation must always be 
 considered. It is sometimes supposed that it is only neces- 
 sary to have sound enamel resting on sound dentine, but by 
 looking at Figs. 45 and 46 it will be seen that the first require- 
 
84 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 ment may be present, but not the second. In these illustra- 
 tions the enamel plate is resting on sound dentine, but the 
 tissue has been cut in such a way that the inner ends of the 
 rods have been cut off. The rods that form the cavosurface 
 angle do not extend to the dentine, but run out on the 
 cavity wall at D, and the portion ABC is held together only 
 by the cementing substance. This is not strong enough to 
 
 B 
 
 \ 
 
 Fig. 46 
 
 c 
 
 ^ 
 
 Improperly prepared enamel -wall. The portion ABC is not supported by dentine. 
 
 sustain the force necessary to condense the filling material 
 or the forces received upon the surface of the tooth after 
 the filling is completed. It will crack on the line of the 
 cementing substance and chip out. The inclination of the 
 entire wall must be increased to a little more than to reach 
 the rod direction. Such a wall as this may easily be made, 
 in preparing a cavity wall, with a stone or a burr, but would 
 
THE RODS FORMING THE CAVOSURFACE ANGLE 85 
 
 be unlikely ever to be formed with hand instruments. Such 
 walls as this account for the chipping of many margins and 
 the failure of fillings along the gingival wall. The tissue 
 is cracked to pieces in inserting the filling material, and the 
 pieces fall out later. This occurs often in the gingival walls 
 of compound cavities. 
 
 Fig. 47 
 
 Enamel wall cut in the direction of the rods, 
 supported. It should be trimmed i 
 
 The marginal rods are not 
 the line indicated. 
 
 The Rods Forming the Cavosurface Angle Must be Sup- 
 ported. — This is the key to strong enamel walls. The more 
 perfect the support the stronger the wall. If an enamel wall 
 is cut exactly in the direction of the rods, as in Fig. 47, the 
 rods forming the margin are held together only by cementing 
 substance, and a comparatively slight force on the surface 
 in the direction toward the cavitv will break them off. If the 
 
86 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 same wall is trimmed, as indicated by the line, the same force 
 would do no dama^^e, as the rods which receive it are supported 
 by the portion which is covered by the filling material. 
 
 Fig. 48 
 
 The tip of a worn incisor. The rods forming the angle at A reach the dentine at C, 
 and are supported by the piece ABC. 
 
 It is interesting to note that in the wearing down of the 
 enamel by use, nature provides the same support for the 
 rods which form the angle of the worn and tooth surfaces. 
 Fig. 48 shows the tip of a worn incisor. The rods at A reach 
 
CLASSES OF CAVITIES 
 
 87 
 
 the dentine at B and are supported by the portion ABC. 
 When caries occurs on an abraided surface it starts by the 
 rods at the dento-enamel junction, chipping out and forming 
 a, protected niche for the lodgement of a colony. 
 
 Bevel the Cavosurface Angle. — It is not always necessarj' 
 to bevel the cavosurface angle where the rods are inclined 
 toward the cavity. In such places the rods forming the 
 margin are well supported and the angle need not be bevelled 
 unless it is so sharp that it would be in danger of being 
 injured. 
 
 Fig. 49 
 
 
 The two classes of cavities. Those with the rods inclined toward the cavity, ana 
 those with the rods inclined away from the cavity. 
 
 There are two reasons for bevelling the cavosurface angle: 
 (1) To protect a sharp angle from injury; (2) to gain support 
 for the marginal rods. The first occurs where the enamel 
 rods are inclined toward the cavity, the second where they 
 are inclined away from the cavity. 
 
 Classes of Cavities.— From a consideration of the direction 
 of the enamel rods in the tooth crown, and the positions where 
 caries begins on the enamel, enamel walls may be divided, 
 according to their structural type, into two classes (Fig. 49) : 
 
88 STRUCTURAL REQUIREMENTS FOR ENAMEL WALLS 
 
 1. Those in which the enamel rods are inchned toward 
 the cavity, characteristic of cavities on occlusal surfaces and 
 cavities beginning in fissures and pits. 
 
 2. Those in which the enamel rods are inclined away from 
 the cavity, characteristic of cavities on smooth surfaces. 
 
 In the firsi class it is comparatively easy to obtain a 
 strong margin, and this is fortunate, for when the filling is 
 completed the margin will be subjected to the full force of 
 mastication. In the second it is comparatively difficult to 
 obtain a strong margin, but only sufficient strength is 
 required to withstand the force of condensing the filling 
 material, a6 after the filling is completed it will be obliged to 
 withstand little force from mastication. 
 
 From a careful observation of the failures of filhngs (his 
 own and those of other operators), the author believes a 
 very large number are due to structurally imperfect enamel 
 walls. A study of enamel structure as related to cavity 
 preparation will do more to improve the quality of the 
 operation and to increase the facility of its execution than 
 any one factor. This study is a clinical study guided by 
 examination of the microscopic structure of the tissue. In 
 operating at the chair the detail of enamel rod direction as 
 it is applied to cavity preparation is learned, but to do so 
 hand instruments must be used and a sufficient knowledge 
 of the tissue must have been acquired to think of it always 
 in their use in terms of its structural elements. 
 
CHAPTER IX 
 THE PREPARATION OF TYPICAL ENAMEL WALLS 
 
 The steps in the preparation of an enamel wall are: 
 
 1. The cleavage of the enamel until the outline form of 
 the cavity is reached. 
 
 2. The trimming of the enamel walls. 
 
 3. The preparation of the margins. 
 
 Every enamel wall should be prepared according to these 
 steps. The first not only removes the tissue more or less 
 disintegrated and weakened by caries, but also places the 
 margin of the filling in a position where it is not likely to 
 be covered by the growth of a colony of bacteria. It also 
 determines the direction of the enamel rods so that the 
 walls can be completed in terms of its structural elements. 
 
 The second step is accomplished by the shaving or planing 
 process, and should always increase the inclination of the 
 entire enamel wall slightly, so as to extend a little beyond 
 the rod directions, and remove the portions that have been 
 cracked or splintered by the cleavage. After cleavage the 
 enamel wall will usually have a more or less whitish look. 
 This is caused by the cracking of the cementing substance 
 between the rods. The light is refracted by the air in these 
 microscopic spaces and imparts this whitish or snowy look 
 to the tissue. These portions are removed by planing or 
 shaving, and the tissue obtains its bluish translucent 
 appearance. 
 
 The third step is also accomplished by the planing process, 
 and should be carried out with two objects in mind: (1) To 
 so form the cavosurface angle that the tissue will not be 
 liable to injury in the condensation of the filling material 
 against it, and (2) to leave rods whose outer ends will be 
 
90 PREPARATION OF TYPICAL ENAMEL WALLS 
 
 covered by the filling material to support those which form 
 the actual margin of the cavity. 
 
 The steps in the })re])aration of enamel walls may be made 
 more clear by photomicrographs. Plate III shows a portion 
 of enamel close to a carious cavity which is to be extended 
 to the left. The chisel is placed close to the margin and the 
 portion is split off. The wall then appears whitish, for, as is 
 seen, the cementing substance has cracked in several places, 
 disturbing the structure, and in several places rods have 
 been broken across. The wall must now be planed so as to 
 increase the inclination of the entire wall slightly, and 
 finally the cavosurface angle must be bevelled, involving 
 from ^ to ^ of the thickness of the enamel wall to give 
 support to the rods forming the margins. In this case the 
 rods are straight and parallel, but in Plate IV they are twisted. 
 If the dentine is removed from under this enamel and the 
 chisel placed as indicated, the portion will be split out, but 
 not only has the tissue been splintered, but a considerable 
 portion is left in which the rods have been broken across. 
 By feeling of the margin with the chisel this can easily be 
 determined, and the angle of the wall must be increased by 
 planing so as to leave the wall in the position shown in Plate 
 IV, 3, and finally the cavosurface angle must be bevelled. 
 
 Preparation of Simple Occlusal Cavities. — Caries often 
 begins in the mesial and distal pits of the upper bicus- 
 pids, and in preparing the cavities for filling they must 
 be united. Fig. 50 is a buccolingual section through a 
 first superior bicuspid. Suppose caries has reached the 
 dento-enamel junction in both the mesial and distal pits, 
 and the}' are to be united along the groove. A small 
 spear drill is carried into the mesial pit until the dento- 
 enamel junction is reached, then a small inverted cone burr 
 is carried into the dentine just under the enamel and drawn 
 from the dentine to the surface of the enamel. When a 
 narrow cut has been made from the mesial to the distal 
 pit, a chisel placed at the edge of the opening wdll split out 
 the enamel as indicated in Fig. 51. Xow the walls must be 
 planed so as to bring the buccal and lingual walls into the 
 
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PLATE IV 
 
 T 'iF^'-^l l ^ 
 
 Preparation of Enamel Wall in Gnarled Enamel. 
 
 1, enaniel wall as cleaved, showing breaking across rods and slivering 
 at a. 2, wall as sniooth, but not extended to remove short rods whose 
 inner ends are cut off at b. 3, wall extended and trinimed to a position 
 of strength. D, dentine; De, dento-enamel junction; c, cavosurfaee angle; 
 6, point where inner ends of rods are cut off; a, slivering of the tissue. 
 (About 80 X) 
 
PREPARATIOX OF SIMPLE OCCLUSAL CAVITIES 91 
 
 axial plane, and the structural requirements will have been 
 completed (P'ig. 52). Fig. 53 shows the relation of the 
 cavity to the crown. 
 
 Fig. 50 
 
 Occlusal Assure in an upper bicuspid, showing direction of rods. (About 80 X) 
 
92 PREPARATION OF TYPICAL ENAMEL WALLS 
 
 It has often been advised to allow the filling to extend on 
 to the natural slopes of the cusps, as indicated in Fig. 54. 
 
 Fig. 51 
 
 / 
 
 # 
 
 The same section as Fig. 51. sbowing the position of the chisel in cleaving the 
 enamel to open the cavity. 
 
PREPARATION OF SIMPLE OCCLUSAL CAVITIES 93 
 
 Fig. 52 
 
 Preparation of enamel walls in occlusal fissure cavities (the same as Figs. 50 and 51). 
 
94 PREPARATION OF TYPICAL ENAMEL WALLS 
 
 Fig. 53 
 
 The relation of the cavity to the crown (the same as Figs. 50 and 51). 
 
 The trimming of the walls instead of lapping the filling material on the slope of the cusps. 
 
PREPARATIOX OF SIMPLE OCCLUSAL CAVITIES 95 
 
 It will be seen, however, that a stronger enamel wall and a 
 stronger edge of filling material will be obtained if the 
 
 Fig. 55 
 
 Caries beginning in an occlusal defect of a molar. (About SO X) 
 
 enamel wall is bevelled to the point where the margin of the 
 filling is desired and the filling finished to this position. 
 
96 PREPARATION OF TYPICAL ENAMEL WALLS 
 
 Fig. 55 shows a buccolingual section through a molar 
 with a small cavity in a mesial pit. Caries has undermined 
 
 Fig. 56 
 
 The preparation of the enamel walls of the cavity shown in Fig. 55 
 
 the enamel slightly toward the buccal, but has attacked the 
 enamel on the surface, extending toward the lingual farther 
 
PREPARATION OF SIMPLE OCCLUSAL CAVITIES 97 
 
 than the enamel has been undermined at the dento-enamel 
 junction. Applying the chisel to the surface, the undermined 
 enamel is split away, as is indicated in Fig. 56. The buccal 
 wall is planed until it is in the axial plane, and the cavo- 
 surface angle bevelled. It is not necessary to extend the 
 cavity to the lingual beyond the point where sound dentine 
 is reached, but the disintegrated enamel on the surface must 
 be removed. The enamel wall is, therefore, inclined about 
 6 centigrades lingually from the axial plane, and it is not 
 
 Fig. 57 
 
 ^^\ 
 A 
 
 The relation nf tl,e cavity to the crown (the same section as shown in Figs. 55 and 56). 
 
 necessary to bevel the cavosurface angle. The rods are 
 inclined toward the cavity and the rods forming the margins 
 are well supported, and the cavosurface angle is not so 
 sharp as to be endangered in condensing filling material. 
 Fig. 57 shows the relation of the cavity to the crown. 
 
 All occlusal defects should be filled as soon as the decay 
 has reached the dento-enamel junction, as all progress of 
 the disease beyond that point requires sacrifice of tissue 
 which otherwise would be saved, and the enamel wall becomes 
 less and less strong. Fig. 58 shows a much more exten- 
 7 
 
98 PREPARATION OF TYPICAL ENAMEL WALLS 
 
 Fig. 58 
 
 A larger cavity in (he occlusal surface of a molar. The position of the chisel in 
 opening the cavity 
 
 FiQ. 59 
 
 The preparation of the cavity shown in Fig .58. 
 
PREPARATION OF SIMPLE OCCLUSAL CAVITIES 99 
 
 Fig. 60 
 
 A gingival third caviti' in a bicuspid, showing the cleavage of the occlusal and i 
 
 gingival walls as cleaved. _ 
 
100 PREPARATION OF TYPICAL ENAMEL WALLS 
 
 Fig. 61 
 
 
 The preparatiun of the cavity shown in Fig. 60. 
 
GINGIVAL THIRD CAVITIES 
 
 101 
 
 sive occlusal cavity, one that has been neglected until 
 the enamel has been broken in, and as a result there was 
 much unnecessary loss of tooth structure. The chisel is 
 applied to the surface as indicated, and the undermined 
 enamel broken down until the sound dentine is reached. On 
 the buccal, the enamel wall is cut to the axial plane, and the 
 cavosurface angle bevelled. If the decay in the dentine had 
 reached the tip of the dentine cusp, it would be necessary 
 to remove the tip of the enamel cusp and incline the wall 
 
 A gingival third cavity in a molar. 
 
 about 8 centigrades buccally from the axial plane, in order 
 to obtain a strong wall, and then the cusp would be 
 replaced by filling material. On the lingual the undermined 
 enamel is removed, and the wall inclined slightly lingually 
 from the axial plane and the cavosurface angle bevelled a 
 little. Fig. 59 shows the relation of the cavity to the crown. 
 Gingival Third Cavities. — Fig. 60 is a buccolingual section 
 of a superior bicuspid, showing a break in the enamel in 
 
Fig. G3 
 
 '1. Wall as_cleaved. 
 Fig. 64 
 
 2. Wall as trimmed. 
 Preparation of occlusal wall of Fig. 62. (About 70 X) 
 
GINGIVAL THIRD CAVITIES 
 
 103 
 
 the position of a gingival third cavity. The occlusal wall is 
 cleaved to find the enamel rod direction, then planed to 
 
 Fig. 65 
 
 A ca-Wty in the lingual pit of a lateral incisor. The position of the chisel in 
 opening the cavity. 
 
104 PREPARATION OF TYPICAL ENAMEL WALLS 
 
 increase the inclination slightly, leaving it about 8 centigrades 
 occlusally from the horizontal plane, and the cavosurface 
 angle bevelled to obtain support for the marginal rods. 
 The gingival wall is prepared in the same way, inclined 
 gingivally about G centigrades from the horizontal plane, 
 and the cavosurface angle bevelled. Fig. Gl shows the walls 
 prepared. 
 
 Fig. 66 
 
 The preparation of the gingival wall of the cavity shown in Fig. 65. 
 
 Fig. 62 is a similar section from a molar. After chopping 
 away the occlusal wall until the cavity has been extended 
 to the point of greatest convexity of the surface, the wall is 
 seen to be in the condition shown in Fig. G3. Near the sur- 
 face some rods have broken across, and near the dento-enamel 
 junction the same thing has happened, but in the rest of the 
 distance the cleavage has followed the enamel rod direction. 
 The inchnation of the wall is increased by planing until 
 this roughness has been removed, and then the cavosurface 
 angle is bevelled to support the marginal rods, and prepara- 
 tion is complete, as shown in Fig. G4. 
 
GINGIVAL THIRD CAVITIES 
 
 105 
 
 Fig, 65 shows a cavity in the Ungual pit of a superior 
 lateral incisor. Caries has undermined the enamel to a 
 
 Fig. 67 
 
 The preparation of the cavity ishown in Fig. 65. 
 
lOG PREPARATION OF TYPICAL ENAMEL WALLS \ 
 
 considerable extent, and the cavity will have to be larger ! 
 
 than would otherwise have been necessary. Placing the • 
 
 chisel close to the occlusal margin, as indicated, the enamel i 
 
 is chij^ped away in that direction and around the circum- | 
 ference. On the lingual wall the chisel may be reversed 
 
 and used with a pulling motion, like a hoe. In this way i 
 
 the undermined enamel is chipped away and the tip of 1 
 
 the marginal ridge removed. The wall is then planed into 1 
 
 the horizontal plane and the cavosurface angle })evelled. \ 
 Fig. (36 shows the structure of the gingival wall, and Fig. 67 
 the relation to the crown. 
 
CHAPTER X 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 The formation of enamel begins at the dento-enamel 
 junction, and the tissue is laid down from within outward, 
 so that the enamel in contact with the dentine is formed 
 first and the surface of the crown last. Enamel formation 
 begins at several points, for each crown, the exact number 
 and position of which has been the subject of much investi- 
 gation. When enamel formation begins, these points are 
 close together, but they are carried farther apart by the 
 growth of the dental papilla, and are not united for some 
 time. The separate enamel caplets unite first at the dento- 
 enamel junction, and as the formation of the thickness of 
 the enamel progresses at these lines of union, there is always 
 more or less disturbance in structure. Even where the union 
 seems perfect, sections will show more or less disturbance of 
 enamel rod direction, arrangement of the rods, and relation 
 to the cementing substance. 
 
 Every operator and student of dental anatomy is familiar 
 with the developmental lines. On the occlusal surfaces they 
 are usually marked by well-defined grooves, but upon the 
 axial surfaces the grooves may be very slight, scarcely more 
 than slight depressions of the surface, and consequently 
 they are not thought of. It will be found, however, that on 
 these lines there is less perfect enamel structure, and con- 
 sequently the tissue is not as strong, and these lines must be 
 avoided in the preparation of enamel walls. The cause of 
 disturbance of structure will be better understood after study 
 of the development of the tooth germ and the formation 
 of enamel in the chapter on Dental Embryology, but some 
 
108 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 details of the cause should be touched upon here. The study 
 of the diagrams of the growth of the tooth crown will illustrate 
 the conditions (see Chapter XXVII), and shows a bucco- 
 lingual section through the tooth germ of a bicuspid just 
 before the formation of the dentine and the enamel begins. 
 The odontoblasts (dentine forming cells) and the ameloblasts 
 (enamel forming cells) are in contact at what will be the 
 dento-enamel junction. The odontoblasts form dentine 
 on their outer surface, beginning at the tip of the dentine 
 cusp, and progress from without inward and extend down 
 the slopes of the cusps. The ameloblasts form enamel on 
 their inner surface and progress from within outward and 
 down the slopes of the cusps. In this way little caplets of 
 dentine covered by enamel are formed over the horns of the 
 dental papilla; the caps are, of course, thickest where forma- 
 tion has been going on longest. While these caps are forming, 
 the dental papilla is increasing in size, and so they are carried 
 farther and farther apart (Figs. 68 to 73). As soon as the 
 calcifications reach each other at the dento-enamel junction 
 and unite, the increase in the diameter of the dental papilla 
 ceases. The layer of ameloblasts, which are tall col- 
 umnar cells, now cover the surface of the enamel and 
 receive their nourishment and the materials for the forma- 
 tion of enamel from the blood supply through the stratum 
 intermedium. As the blood supply comes from above, it is 
 evident that the cells high up along the slopes of the cusps 
 will receive most, while those at the bottom of the groove 
 get what is left. The formation is, therefore, more rapid 
 along the slopes and less rapid at the point of union. As 
 growth continues, this difference in supply increases, and 
 accordingly formation at the bottom of the groove is first 
 slowed and finally stopped, and the result is a defect. The 
 taller the cusps the greater will be the interference and the 
 deeper the defective groove. In studying sections (Figs. 74 
 to 78) it is very noticeable that teeth with long pointed 
 cusps have more open grooves, and the defect often extends 
 almost to the dento-enamel junction. 
 
STRUCTURAL DEFECTS IN THE ENAMEL 109 
 Fig. 68 
 
 *5''' « 
 
 Fig, 69 
 
 w 
 
 Diagram showing the growth of the crown of a bicuspid. 
 
110 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 Fig. 70 
 
 Fig. 71 
 
 Diagram showing the growth of the crown of a bicuspid. 
 
STRUCTURAL DEFECTS IN THE ENAMEL IJl 
 Fig. 72 
 
 Fig. 73 
 
 Diagram showing the growth of the crown of a bicuspid. 
 
112 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 Fig. 74 
 
 The sectioii from which Figs. 68 to 73 were drawn: A, tip of dentine cusp, B, 
 lines showing little caps of enamel formed before calcifications from separate centres 
 united; C, lines showing amount of enamel formed when calcifications united. 
 
 Fig. 75 
 
 Occlusal defect from an old tooth. 
 
Fig. 76 
 
 A deep open groove. 
 Fig. 77 
 
 A shallow groove. 
 
114 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 The bands of Retzius, which are the incremental Hnes of 
 the enamel, should be studied about these grooves. It will 
 be seen that they always dip down around the groove, and 
 that more enamel has been formed between one band (Figs. 
 84 and 85) and the next on the slope of the cusps than at 
 the bottom of the groove. In teeth with very flat, low cusps 
 the closure of the grooves may be very perfect, leaving only 
 a slight depression (Fig. 77). 
 
 Fig. 78 
 
 A very deep groove, showing the effect of caries at the bottom. 
 
 The importance of these defects as positions of beginning 
 caries cannot be overestimated, as they furnish ideal con- 
 ditions in areas that would otherwise be immune, and they 
 are the positions in which the attacks of caries are first 
 manifested. These occlusal grooves appear in great variety. 
 
STRUCTURAL DEFECTS IN THE ENAMEL 115 
 
 Fig. 79 
 
 The pit in a lateral incisor filled with coronal cementum. Interglobular spaces are 
 
 Fig. 80.— Occlusal surface of the lower third molar, showing the grooves. 
 Fig. 81.— The same tooth sliced for sectioning: 1, the piece from which the sec- 
 tion shown in Figs. 82 and 83 was ground. 
 
116 
 
 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 Some are simply shallow open grooves, in which the surface 
 of the enamel is perfect (Fig. 74); some are very deep and 
 entirely empty (Figs. 75, 7G, and 78) ; others are apparently 
 filled with a granular, more or less structureless calcified 
 material which appears to have been deposited in the groove 
 after the enamel was completed (Figs. 79, 84, and 85). This 
 is probably of the nature of cementum. It was formed after 
 the enamel was completed, but while the tooth was enclosed 
 
 Fig. 82 
 
 The section ground from 1, Fig. 81, showing the depth of the fissure. 
 
 in its follicle in the crypt in the bone. It is to be compared 
 with the coronal cementum that is characteristic of the 
 complex grinding teeth of the ungulates and other herbivor- 
 ous animals. A study of these defects furnishes the basis 
 for the operative rule that "all grooves must be cut out to 
 the point where the margin will be on a smooth surface." 
 For if they are not, a defect will be left at the margin of the 
 cavity which offers ideal conditions for the beginning of a 
 new decay. When caries begins in such a defect at the 
 
STRUCTURAL DEFECTS IN THE ENAMEL 
 
 117 
 
 margin of a filling, it progresses at the bottom of the defect 
 until the dento-enamel junction is reached, and then extends 
 in the dentine and may destroy the entire cro\yn without 
 
 Fig. S3 
 
 Higher magnification of the fissure shown in Fig. 82. (About 60 X) 
 
lis STRUCTURAL DEFECTS IN THE ENAMEL 
 
 showing upon the surface (see Chapter XII). The extent 
 of these defects is much greater than would be supposed 
 from the observation of the teeth in the mouth. Fig. 80 
 shows the occlusal view of a lower third molar, extracted 
 because of disease of the peridental membrane, from a man 
 aged about forty years. Examining these grooves with a 
 fine-pointed explorer, it would not stick any place. No 
 operator would think of cutting them out and filling them. 
 
 Fig. 84 
 
 An occlusal defect in a worn tooth. The fissure is filled with coronal cementum. 
 
 The crown w^as sawed through from buccal to lingual, as 
 shown in Fig. 81, and the piece marked 1 is shown in 
 Figs. 82 and 83. The grooves are open two-thirds of the 
 distance to the dento-enamel junction, and show slight 
 action of caries. Suppose caries had started in the central 
 pit, and a small round filUng had been made, open defects 
 would be left at the margin where every groove radi- 
 
STRUCTURAL DEFECTS IX THE ENAMEL 119 
 
 ated from the central cavity, and these would be just as 
 liable to recurrent decay as it was originally, and, if caries 
 occurred, it would progress at the depth of the groove, 
 reach the dento-enamel junction, and progress in the dentine, 
 
 Fig. 85 
 
 Higher magnification of Fig. 84. The fissure filled with granular calcified material. 
 Notice the direction of the bands of Retzius around the fissure. 
 
 until the occlusal enamel was so undermined that it would 
 break in under the force of mastication. On the other hand, 
 if the grooves are cut out to a point where the cavity margin 
 
120 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 will be on a smooth surface, there is no possibility of recur- 
 rent caries if the filling material is properly inserted. This 
 one illustration, which might be duplicated a thousand times, 
 
 Structural defects in developmental grooves on axial surfaces. 
 
 
 
 ] 
 
 Fig. 87 
 
 
 
 
 
 B 
 
 .-;: 
 
 A 
 
 A-C 
 
 "i; 
 
 V"5-^ 
 
 
 .!_ 
 
 
 -A 
 
 Structural defects in developmental grooves on axial surfaces. 
 
STRUCTURAL DEFECTS IN THE ENAMEL 
 
 121 
 
 therefore is the rational basis for the rule, "All grooves 
 must be cut out to their end." 
 
 Caries does not occur in all open grooves. Fig. 76 shows 
 an open groove in a section from a tooth in which the wear 
 indicates that it was not from a young person, but most of 
 the grooves that escape are not open, but more or less entirely 
 filled with structureless calcified matter or coronal cementum. 
 Figs. SO, S4, and So are very good illustrations of this class 
 of grooves. 
 
 Fig. 88 
 
 Defects on the axial surface in the enamel. 
 
 The condition in pits from which grooves extend, as the 
 lingual pits of incisors and the buccal pits of molars, show 
 the same condition as the grooves, except that the defect 
 is both broader and deeper. But pits that are sometimes 
 found on the tips of cusps and on smooth surfaces show an 
 entirely different structural condition, and will be considered 
 under atrophy in Chapter XII. 
 
122 STRUCTURAL DEFECTS IN THE ENAMEL 
 
 In places where the union of the enamel plates seems per- 
 fect, as, for instance, on the labial surface of the incisors or 
 the buccal surface of the bicuspids, and the line of union 
 is marked only by a slight depression of the surface, the 
 section will show disturbance of structure. Fig. 86, a 
 
 Fig. 89 
 
 A section through such a defect as that shown in Fig. 88. (About 80 X ) 
 
 drawing made by Dr. Black a good many years ago, shows 
 such a position. At the surface the rods and their arrange- 
 ment seem very perfect, but from a point about one-third 
 the distance to the dento-enamel junction there are no 
 
STRUCTURAL DEFECTS IN THE ENAMEL 123 
 
 rods at all, but apparently a number of calcospherites in 
 a granular calcific substance. In Fig. 87, another of Dr. 
 Black's illustrations, the rods are very irregular, and are 
 separated by large areas of structureless calcified material. 
 Grooves are often found in unusual or atypical positions. 
 Fig. 88 shows a groove running over the mesial marginal 
 ridge and down on the mesial surface. Fig. 89 shows a 
 section through such a defect. Notice the folding of the 
 enamel into the dentine and the disturbance of the rods 
 about the groove and between its base and the dentine. 
 
CHAPTER XI 
 
 SPECIAL AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 There are certain positions which in the perfect crown 
 are areas of great strength, but which, because of the pecu- 
 liar structure of the tissue in these places, become areas 
 of weakness when cavity margins are made in them. The 
 treatment of beginning caries would lead to no failures in 
 these positions, for cavity margins would never be extended 
 into them, except in the treatment of burrowing caries and 
 neglected cases. The extension of caries at the dento-enamel 
 junction often requires the extension of the margin into the 
 area of danger. In considering these areas and in the prepa- 
 ration of cavities, as well as the areas of imperfect structure 
 considered in Chapter X, it is important to place as much 
 emphasis on the necessity of 7iot extendirig cavity margins 
 into the areas of weakness, as on cutting away the dangerous 
 area and leaving the margin in a safe position, when the 
 area cannot be avoided. 
 
 In considering the relation of the enamel and dentine, and 
 in studying the arrangement of the enamel rod direction in 
 the "architecture" of the tooth crown, it has been pointed 
 out that the dentine cusps and the dentinal marginal ridges 
 are not directly under the corresponding points on the sur- 
 face of the enamel, but are nearer to the axis of the tooth. 
 The areas on the surface of the enamel, from the point 
 directly over the tip of the dentine cusp or ridge to the tip 
 of the enamel cusps or ridges, become areas of weakness 
 when a cavity is extended into them. 
 
 Fig. 90 is a photomicrograph of a buccolingual section of 
 a superior bicuspid, and Fig. 91 is a higher magnification 
 of the same, made to illustrate the condition. It will be 
 
AREAS OF WEAKXESS FOR ENAMEL MARGINS 125 
 
 seen that if decay has extended at the dento-enamel junction 
 to the tip of the dentine cusp, and the enamel walls were 
 left in the axial plane, the rods which form the surface of 
 
 Fig. 90 
 
 B A 
 
 Buccolingual section of upper bicuspid. Enamel is broken from grinding. A to B, 
 area of weakness for enamel margin. (About 20 X) 
 
 the enamel from the margin of the cavity to the tip of the 
 cusp "are not supported by dentine/' and would be likely 
 to be broken and fall aw^ay, leaving a defect at the margin 
 
Fig. 91 
 
 Enamel over tip of dentine cusp: D, dentine cusp. (About 80 X) From same 
 section as Fig. 90. 
 
AREAS OF WEAKNESS FOR ENAMEL MARGINS 127 
 
 of the filling. If decay beginning in the groove or pit has 
 extended only to point C, Fig. 90, the wall may be trimmed 
 in the axial plane and an ideal wall produced; but if it has 
 reached point D, Fig. 90, it must be inclined bucally, so as 
 to remove the tip of the cusp, as indicated in the dotted 
 line, and the cusp restored by the filling material. The 
 region of the surface indicated by A-B, while an- area of 
 
 Fig. 92 
 
 A bicuspid cut for sectioning. Sections were ground from the positions marked by 
 the lines 1, 2, 3, 4 in B, and are sho-svn in Figs. 93, 94, 95, and 96. 
 
 strength in the perfect tissue, becomes a position of weak- 
 ness when cavity margins are extended into them. A 
 careful observer wiU find many failures that are the 
 result of bad enamel wall preparation in these areas. The 
 same conditions exist in the region of the marginal 
 ridges. Figs. 96 and 97 show the mesial marginal ridge 
 of a superior bicuspid. If this is filled before the destruc- 
 tion of dentine has extended beyond the point A, the mesial 
 wall ma^ be cut in the axial plane as indicated; but if it 
 has reached the tip of the dentine ridge at point J5, it must be 
 inclined mesial ly, so as to reach the tip of the enamel ridge. 
 
128 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 FiQ. 93 
 
 Section ground from 1, Fig. 92, through the mesial oblique ridge. (About 30 X) 
 
AREAS OF WEAKNESS FOR ENAMEL MARGINS 129 
 
 Fig. 94 
 
 Section ground from 2. Fig, 92 (About 30 X), 
 
130 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 Fig. 95 
 
 Section ground from 3, Fig 92, through distal marginal ridge. (About 20 X) 
 
AREAS OF WEAKNESS FOR ENAMEL MARGINS 131 
 
 Fig. 96 
 
 Section ground from Fig. 92, through mesial part and marginal ridge. If caries 
 has extended at the dento-enamel junction to A, the -wall may be in the axial plane; 
 if it has reached B, the wall must be inclined as indicated by the dotted line. 
 (About 30 X) 
 
132 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 Figs. 98, 99, and 100 show the distal marginal ridge in a sec- 
 ond molar. Notice the inclination of the rods from the tip of 
 the dentine ridge. If decay has reached this point the wall 
 
 Fig. 97 
 
 A higher magnification of Fig. 96, showing enamel rod directions in the region 
 of the marginal ridge. 
 
 must be inclined distally, so as to reach the rod direction, 
 or a frail margin will be left and one which will not sustain 
 the force of mastication. Neglected caries in the lingual 
 
AREAS OF WEAKXESS FOR EXAMEL MARGIXS 133 
 
 pits of incisors often present the same conditions. Fig. 65 
 shows a section through such a pit in a lateral incisor, and 
 Fig. 06 shows the gingival wall. If this were prepared by 
 inclining the gingival wall only slightly, a very frail wall 
 
 Fig. 9S 
 
 An upper molar, showing the position of the section shown in Figs. 99 and 100. 
 
 Fig. 99 
 
 The section ground from Pig. 9S. 
 
 would be left. It should be cut down to the horizontal plane, 
 as indicated, and the marginal ridge restored by the filling 
 material r The same conditions are often encountered in 
 the preparation of simple cavities in the mesial or distal 
 surfaces of incisors, when caries has followed the dento- 
 
134 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 Fig. 100 
 
 The distal marginal ridge, showing the enanael rod direction. (About 30 X) 
 
AREAS OF WEAKXESS FOR ENAMEL MARGINS 135 
 
 Fig. 101 
 
 A B 
 
 An upper bicuspid, showing the position of the section shown in Fig, 102. 
 
 Fig. lO: 
 
 Section from the central piece shown in Fig. 101, showing the dinection of cleavage 
 on the mesial surface and the effect of caries on the tissue. 
 
136 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 enamel junction toward the lingual. Fig. 1 03 shows a superior 
 central incisor from which sections were cut as indicated. 
 Suppose caries to have begun in the region of the contact 
 point and to have extended to the point a. If the lingual 
 enamel wall were prepared at the line A, Fig. 105, a verj^ frail 
 
 Fig. 103 
 
 A superior central incisor, showing the position of sections in Figs. 104, 105, and 106. 
 
 Fig. 104 
 
 Section 1, Fig. 103, showing the enamel worn from the marginal ridges. 
 
 wall would result. Force coming upon this wall from the 
 Hngual, by the occlusion of the lower incisors, would be likely 
 to break out or crack a triangular piece of enamel, and the 
 filling would fail along the lingual wall. If, however, the wall 
 be laid in the line at B, a strong wall is produced, against 
 
AREAS OF WEAKNESS FOR ENAMEL MARGINS 137 
 
 which gold can be properly condensed without danger, and 
 which will withstand the force of occlusion. 
 
 Dentists are often tempted to prepare simple cavities in 
 the mesial surfaces of first and second bicuspids and occa- 
 sionally in the molars. If this is ever done, it must be with 
 the full knowledge both of the liability of recurrence of caries 
 and the structure of the enamel, for experience shows that 
 
 Fig. 105 
 
 Section 2, Fig, 103, showing position of weak and strong lingual walls. 
 
 such operations usually fail, either by recurrence of caries 
 at the buccogingival or linguogingival angles, or by the 
 breaking out of the enamel of the marginal ridge. Fig. 107 
 shows the mesial surface of a superior bicuspid. There was 
 a white spot on the contact point, but no actual cavity, as 
 the enamel rods had not fallen out. A section was ground 
 through this point, and Fig. 108 shows a photomicrograph 
 
Fig. 100 
 
 • "^ '^'' i^^B 
 
 m^^ 
 
 d \ 
 
 
 ^^^^^^HK'' 
 
 
 ^^^^^^^^^ 
 
 //^^ 
 
 ^^^^^^^^^^^^^^^B^M^^^^^Ika 
 
 
 A higher magnification of the mesial marginal ridge, shown in Fig. 105. 
 (About 60 X) 
 
 Fig. 107 
 
 Occlusal and mesial views of a superior bicuspid, showing position of section. 
 A beginning caries could be seen on the surface, but it does not show well in the 
 picture. The section from the buccal piece is shown in the following illustrations. 
 
AREAS OF WEAKNESS FOR ENAMEL MARGINS 139 
 
 of it. The enamel rods have fallen out of the disintegrated 
 area, and the decalcification in the dentine is shown (Fig. 109). 
 If this had been treated as a simple cavity the occlusal wall 
 would have required an inclination of 1 8 centigrades occlusally 
 from the horizontal plane to reach the enamel rod direction. 
 
 Fig. 108 
 
 The section ground froia the buccal piece, 1 
 
 There is very little support offered by the dentine for the 
 enamel of the marginal ridge, and the portion over to 
 the occlusal groove would be likely to be broken off by the 
 force of mastication. The conditions of the occlusal wall 
 are better shown in Fig. 110. 
 
140 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 Any number of illustrations of these conditions might 
 be made, but the subject may be summed up by saying: 
 
 Fig. 109 
 
 The region of the carious spot shown in Fig. 107, showing the disintegrated area 
 of the enamel and the action of acid on the dentine. (About 30 x) 
 
AREAS OF WEAKNESS FOR ENAMEL MARGINS 141 
 
 Fig. no 
 
 The enamel over the mesial marscinal ridge to the oblique groove, showing a region 
 of weakness for the occlusal wall of a simple proximal cavity. 
 
142 AREAS OF WEAKNESS FOR ENAMEL MARGINS 
 
 The surface of the enamel from the point directly over the 
 dentine cusp or ridge to the tip of the enamel cusp or ridge, 
 which is an area of great strength in the perfect crown, is a 
 region of weakness for an enamel wall. It is fully as impor- 
 tant not to extend into this area unnecessarily as to form 
 the wall properly when caries has extended so as to involve 
 it. And when caries of a smooth surface approaches a mar- 
 ginal ridge which receives the force of occlusion, the wall 
 must be extended so that the enamel receives full support 
 from sound dentine. 
 
CHAPTER XII 
 
 THE EFFECT OF CARIES ON THE STRUCTURE OF THE 
 
 ENAMEL 
 
 The action of acid upon the enamel has been fully con- 
 sidered in Chapter VIII, and it should be carefully studied 
 before considering the effect of caries on the structure of 
 the enamel, for this cannot be understood unless the relative 
 solubility of the rods and cementing substance and the 
 relationship of the two structural elements are clearly in 
 mind. 
 
 During the last ten years there has been a great increase 
 in knowledge of the beginning of caries of the enamel and 
 the extent of tissue injury before an actual cavity is pro- 
 duced. This has placed a tremendous emphasis upon the 
 value, for the preservation of the teeth, of the treatment 
 of caries in its early rather than in its later stages. It is 
 safe to say that if caries progresses until a patient is aware 
 of a cavity, the tooth has been injured more than is necessary 
 in the most radical treatment of the same cavity in its 
 beginning stages. One who has not studied carefully the 
 effect of caries on the structure of the enamel, so as to 
 recognize the extent of injury to the structure of the tissue 
 by its appearance to the naked eye, can never be considered 
 fit to prepare cavities as a treatment for the disease. The 
 beginnings of caries must be divided into two classes: (1) 
 Those occurring in natural defects of structure; (2) those 
 beginning upon smooth surfaces. 
 
 Caries Beginning in Natural Defects of Structure. — These are 
 the positions in which caries first appears and in which 
 it presents the greatest intensity, because they offer ideal 
 conditions. Such open grooves and imperfectly closed pits 
 in the enamel as have been illustrated in Chapter XI become 
 
144 THE EFFECT OF CARIES ON THE ENAMEL 
 
 filled with food debris, which furnish ideal culture media for 
 acid-forming bacteria. At the opening of the defect the 
 acid is washed away by the sahva as fast as it is formed, but 
 at the bottom of the groove it is confined and acts upon the 
 enamel, dissolving out the cementing substance from between 
 the rods and following the rod direction toward the dento- 
 enamel junction. The form of the disintegrated tissue in 
 such positions is always that of a cone or wedge, with the 
 apex at the opening of the pit or groove and the base toward 
 the dento-enamel junction. The formation of acid in these 
 positions is often so rapid and the confinement so perfect 
 that the carious process here manifests its greatest intensity, 
 the action often dissolving the rods as well as the cementing 
 substance and progressing across the rods. But even when 
 the action follows the rod direction, the form will be broader 
 toward the dentine, as the rods are inclined toward the 
 defect. Figs. Ill and 112 show split teeth illustrating the 
 disintegration of the enamel around occlusal defects. The 
 disintegration area appears white by reflected light because 
 the cementing substance has been removed from between 
 the rods and the resulting air spaces refract the light. As 
 soon as this disintegration reaches the dento-enamel junc- 
 tion, the acid formed passes through the now porous enamel 
 and acts much more rapidly upon the dentine. Because of 
 the branching of the dentinal tubules at the dento-enamel 
 junction, the action upon the dentine spreads rapidl}^ along 
 this line. Soon some of the loosened rods be ween the bottom 
 of the defect and the dentine are either entirely dissolved 
 or displaced or dislodged, and the microorganisms are 
 admitted to the dentine. The decalcified dentine matrix 
 becomes food material for the bacteria, and the space pro- 
 duced by the destruction of tissue furnishes greater space 
 for decomposing foodstufi^s. The acids formed attack the 
 enamel from within outward, producing what has been 
 called backward or secondary decay of enamel. At the mouth 
 of the defect the acid is still washed away, and there is 
 little action upon the tissue. The condition progresses, 
 therefore, until, as in Fig. 113, the entire occlusal enamel 
 
THE EFFECT OF CARIES ON THE ENAMEL 145 
 
 has been undermined, and all of the undermined area has 
 been greatly weakened by the solution of the cementing sub- 
 
 Fia. Ill 
 
 A split tooth, showing caries beginning in an occlusal groove. 
 
 Fig. 112 
 
 A split tooth, showing caries progressing in an occlusal groove. 
 
 10 
 
146 THE EFFECT OF CARIES ON THE ENAMEL 
 
 stance from between the rods. In general sections of such 
 areas as shown in Fig. 116 the disintegrated area appears 
 dark by transmitted Hght. Fig. 114 shows the progress of 
 secondary decay from an occhisal cavity. In this way it 
 often happens that the entire occlusal enamel is destroyed 
 before the original defect is noticeably enlarged. 
 
 The general form of the disintegrated area in caries 
 beginning in natural defects may be described diagram- 
 maticall}', as in the enamel a cone or wedge with the apex 
 toward the mouth of the defect and the base toward the 
 dento-enamel junction, and in the dentine a cone or wedge 
 with the base at the dento-enamel junction and the apex 
 toward the pulp. 
 
 Fig. 113 
 
 A split tooth, showing the undermining of the occlusal enamel by caries spreading 
 at the dento-enamel junction. 
 
 Caries Beginning on Smooth Surfaces. — Caries upon smooth 
 surfaces of the enamel is always due to the growth of a 
 colony of bacteria which becomes attached to the surface 
 by the formation of material, causing them to adhere to the 
 surface and at the same time confining their acid products 
 in contact with the enamel preventing its dissipation in 
 the saliva and allowing it to combine with the inorganic 
 
CARIES BEGINNING ON SMOOTH SURFACES 147 
 
 salts of the tissue elements. This is not the place to con- 
 sider the bacteriology of caries, but the effect upon the 
 structure of the enamel cannot be understood without a 
 
 Fig. 114 
 
 A section shoTving the undermining of the enamel and secondary or backward 
 decay at 1. 
 
 clear conception of the microbic plaques. A growth of 
 masses of microorganisms upon the surface of a tooth does 
 not constitute a plaque. ]Many very filthy mouths are 
 found where most of the surfaces of the teeth are covered 
 
148 THE EFFECT OF CARIES ON THE ENAMEL 
 
 by thick, furry masses, and where there is little or no attack 
 of the enamel. Either acid is not formed or it is at once lost 
 by solution in the saliva. Caries shows the greatest intensity 
 in comparatively clean mouths, in which something in the 
 nature of the saliva causes the bacteria to produce a tough 
 zooglea, which attaches them to the tooth surface and con- 
 fines the products of their activity. This zooglea presents 
 some of the phenomena of a dialyzing membrane. Through 
 it the microorganisms receive their food materials, and their 
 products are neutralized by chemical action on the surface 
 upon which the colony is growing. Colonies lodge in the 
 most favorable spots and extend from these points into 
 areas that are less liable to maintain their attachment. The 
 more perfect the confinement of the acid, and the more 
 rapid the rate of its formation, the greater w^ill be the intensity 
 of the destructive process. The more easily the colony is 
 able to maintain itself in its position and extend upon the 
 surface, the greater is the liability. As the colony becomes 
 thickest at the point of beginning, it is evident that the most 
 acid is formed here, and it is therefore the point of greatest 
 intensity. It is also the point at which the growth began, 
 and therefore the spot where the action on the tissue has 
 been longest in operation. It is also apparent that there may 
 be great intensity with limited liability, and great liability 
 with very low intensity, and the effect upon the tissue will 
 be different in the two cases. 
 
 The appearance of the tissue becomes an index for esti- 
 mating the intensity and liability in a given case. The char- 
 acter of the effect of the disease on the appearance of the 
 enamel, as well as the direction of the extension upon the 
 surface of the tooth, become most important factors in 
 the diagnosis of any case, and the diagnosis is the basis for 
 the treatment required. The increased appreciation of the 
 extent of disintegration of the enamel before an actual 
 cavity is apparent in a tooth has been one of the most 
 important results of Dr. Black's study of caries of the enamel 
 in the last ten years. The author has been intimately 
 associated with this work, and has been amazed at the extent 
 
PROGRESS OF CARIES 
 
 149 
 
 and character of the effect of caries upon the structure of the 
 enamel in what may be called the early stages of the disease. 
 Progress of Caries. — A colony of bacteria becomes attached 
 to the proximal surface of an incisor just to the gingival of 
 the contact point, and remains there some time. If the 
 surface of the tooth can then be examined, a white spot will 
 be seen at Fig. 115; the area appears white because the 
 cementing substance has been removed from between the 
 enamel rods, as will be seen later, and the air that occupies 
 the spaces diffuses the light. If a tooth is split through 
 such a spot and viewed from the surface, the appearance 
 
 Fig. 115 
 
 Fig. 116 
 
 A superior central incisor, showing a 
 white spot just to the gingival of the 
 contact point. 
 
 A split tooth cut through such a white 
 spot as is shown in Fig. 115. 
 
 will be as shown in Fig. 116. If a section were ground through 
 the spot and the tissue preserved, the ends of the enamel 
 rods would be seen pointed and projecting like the pickets 
 of a fence, giving the same appearance as that produced by 
 the action of acid upon a ground section, as illustrated in 
 Fig. 16, Chapter IV. The surface of the enamel is therefore 
 longer smooth, but roughened. The roughness may 
 
 no 
 
 often be felt by passing a very fine pointed steel explorer 
 over the surface. If the colony be dislodged at this stage it 
 is evident that it is much easier for a new one to become 
 attached. These whitened areas are often invisible unless 
 
150 THE EFFECT OF CARIES ON THE ENAMEL 
 
 the tissue is dried, because the saliva fills the spaces. If the 
 surface is dried the refraction of the light by the air whitens 
 the affected area. 
 
 A good comparison is furnished in a very familiar phenom- 
 enon. Snow is white because the air and the microscopic ice 
 crystals are of different refracting index, and the light is 
 diffused by passing from air and ice crystals. If a snowball 
 is saturated with water it loses its whiteness and becomes 
 translucent, because the water, which is nearly of the same 
 refracting index as ice, fills the spaces between the ice crystals, 
 and the light is not diffused. If the white area of such a 
 tooth is split through the centre with an aluminum disk 
 charged with emery powder, the enamel rods will be found 
 entirely separated by the solution of the cementing sub- 
 stance, and the cross-striation will be much more apparent 
 because the unevenness in the diameter of the rods has been 
 increased by the action of the acid. 
 
 Formerly it was impossible to grind a section through such 
 a spot and preserve the tissue. In 1902 the author ground, 
 by the old hand methods, a large number of sections for the 
 study of enamel rod directions. Fig. 107, Chapter XI, shows 
 the mesial surface of a bicuspid split for sectioning. There 
 was a white spot in the region of the contact point that can 
 scarcely be seen in the photograph. When the central 
 section was ground and mounted (Fig. 108, Chapter XI), 
 it was seen that the enamel was disintegrated through its 
 entire thickness and the acid had affected the dentine, and 
 all the enamel rods were lost from the disintegrated area. 
 Until methods were devised by Dr. Black, it was impossible 
 to preserve the tissue and examine its condition. These 
 methods demonstrate definitely that in the disintegrated 
 area the cementing substance is dissolved in large areas 
 before any of the rods are dissolved or destroyed. The 
 first sections of such areas were obtained by polishing the 
 surfaces and cementing the split tooth to the cover-glass with 
 balsam, completing the grinding and mounting without 
 loosening the section. In this way the spaces between the 
 rods were filled with balsam and so were held in place. 
 
PROGRESS OF CARIES 
 
 151 
 
 Fig. 117 shows a photograph of a section made in this way, 
 and the spaces between the rods and the distinct cross-stria- 
 tion are seen. Later it was fonnd that by dehydrating and 
 
 Fig. 11^ 
 
 A thin section of carious enamel ground on the cover-glass with balsam: E, sound 
 enamel; X, carious enamel in wliich the cementing substance had been dissolved 
 from between the rods. 
 
 immersing in a solution of brown shellac, the shellac could 
 be made to take the place of the lost cementing substance, 
 then the polished surface of the sawed-out section could be 
 fastened to the cover-glass with shellac, and the specimen 
 
152 THE EFFECT OF CARIES ON THE ENAMEL 
 
 handled more easily. Fig. 118 shows a photograph of 
 carious enamel made in this way. The rods are preserved 
 
 Fig. 118 
 
 Carious enamel ground on the cover-glass by the shellac method. In the region 
 X the cementing substance dissolved from between the rods has been replaced by- 
 shellac. 
 
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STAGES IN THE PROGRESS OF CARIES 153 
 
 in place and the dark shellac marks the disintegrated area 
 very clearly. 
 
 Stages in the Progress of Caries. — The progress of caries 
 on smooth surfaces of the enamel may be divided in three 
 periods, according to its effect upon the structure of the 
 tissue. 
 
 1. From the lodgement of the colony until the action 
 reaches the dento-enamel junction. 
 
 2. From the reaching of the dento-enamel junction until 
 the rods begin to fall out. 
 
 3. After a cavity is produced. 
 
 First Period. — The form of the disintegrated tissue in the 
 first period is always that of an irregular cone. Its base is 
 on the surface of the enamel, its outline is the boundary 
 of the colony, and the apex is toward the dentine in the 
 direction of the enamel rods from the starting point of the 
 colony. The inner boundary of the area is never even, but 
 shows flame-like extensions toward the dentine in the 
 direction of the rods. This is more marked in some cases 
 than in others, and sometimes suggests that the presence 
 of a colonv on the surface has been intermittent (Plates 
 V, YI, VII). 
 
 The boundary between the perfect and the disintegrated 
 area is usually marked by a darker area, the significance 
 of which is not now understood. If the disease progresses 
 continuously the affected tissue always appears white by 
 reflected light, but if the progress has been intermittent, 
 especially if there have been considerable periods in which 
 no colony has been attached to the surface, the area darkens, 
 becoming brownish or almost black. This is produced by 
 organic materials filling the space between the enamel rods 
 and decomposing, with the probable formation of sulphides 
 of dark color in the spaces. If immunity to caries is attained 
 before the effect upon the tissue has penetrated to the dento- 
 enamel junction, this will occur, and the spot changes from 
 a white to a brownish or black color. Such spots will be 
 found in some places on most teeth extracted from immune 
 persons. Work of Dr. Miller has indicated that such spots 
 
FiQ. 119 
 
 A section through a white spot in the first period of attack: X, disintegrated 
 enamel; E, sound enamel; D, dentine. 
 
STAGES IN THE PROGRESS OF CARIES 
 
 155 
 
 are more resistant to the progress of caries than perfect 
 enamel surfaces. At any time during the first period, there- 
 fore, the destruction may be arrested by the coming of 
 
 Fig. 120 
 
 A section through a carious spot in the first period. The attack has apparently been 
 slow and intermittent: X, disintegrated enamel; E, sound enamel; D, dentine. 
 
Fig. 121 
 
 X i. 
 
 \ 
 
 A section through a carious spot in the first period, showing the flame-like projections j 
 
 toward the dentine: X, disintegrated enamel; E, soimd enamel; D, dentine. | 
 
STAGES IN THE PROGRESS OF CARIES 157 
 
 immunity, which prevents the attachment of colonies to 
 the tooth surface by the formation of plaques. 
 
 ' Second Period — This period extends from the time when 
 the action of the acid reaches the dento-enamel junction 
 until the rods are destroyed or fall out. As soon as the 
 solution of the cementing substance reaches the dento- 
 enamel junction at the point of the advancing cone, the 
 solution of the inorganic salts from the dentine matrix 
 begins. It must be remembered that the acid is formed 
 by the microorganisms on the surface of the enamel, and 
 filters through the spaces between the enamel rods. The 
 decalcification of the dentine may be considerable, while 
 the surface of the enamel is still preserved. In this period 
 the swelhng of the surface is always noticeable. This results 
 in increasing the area of the contact and therefore allowing 
 the colony to extend its limits, increasing the extent of the 
 surface attack. This is especially noticeable toward the 
 gingival, and is shown in Plate V, which is, however, 
 shown in the first period of caries. In the disintegrated 
 area in this stage, as well as in the first stage, the diameter of 
 the enamel rods is always considerably reduced and the 
 striation rendered more apparent. In caries of great inten- 
 sity but low liability the reduction in the diameter of the 
 enamel rods is rapid, and they are soon destroyed, while 
 the area of the surface attacked is small (Fig. 122). 
 
 In caries of low intensity but great liability the diameter 
 of the rods is slowly reduced, while the area of surface 
 attacked, and consequently the area of disintegration, is 
 large (Fig. 123). These conditions should be studied in the 
 macroscopic appearance of caries at the chair. 
 
 The decalcified dentine matrix shrinks and more or less 
 of a space is formed under the enamel. 
 
 The action of the acid follows the tubules of the dentine 
 toward the pulp, and spreads through their branches later- 
 ally near the dento-enamel junction so that the form of the 
 disintegrated dentine is always that of a cone, with the base 
 at the dento-enamel junction and the apex toward the pulp 
 chamber. It is important, however, to remember that in 
 
158 THE EFFECT OF CARIES ON THE ENAMEL 
 
 Fig. 122 
 
 A tooth split through a spot, showing great intensity but low liability 
 
 Fig. 123 
 
 A tooth split through spots, showing low intensity but great liability. 
 
STAGES IX THE PROGRESS OF CARIES 
 
 159 
 
 this stage no microorganisms have entered the tissue, and 
 the effect upon it is the result of the action of substances 
 formed upon the surface. The extent of enamel disintegra- 
 tion and decalcification of dentine, in this stage, is much 
 greater than anyone supposed before such specimens as the 
 present illustrations were made. 
 
 Fig. 124 
 
 A drawing showing the microorganisms of caries growing through the dentins 
 tubules. (G. V. Black.) 
 
 Third Period. — This embraces the period after the enamel 
 rods have begun to fall out and an actual cavity is apparent. 
 As soon as this occurs the surface of the tooth at the point 
 where the formation of the colony began is destroyed and 
 the protected point is lost, and the extension of surface 
 attack ceases. The microorganisms are admitted to the den- 
 tine, where they grow through the dentinal tubules, spreading 
 rapidly at the dento-enamel junction (Fig. 124). The dentine 
 is always decalcified in advance of the penetration of the 
 microorganisms. The acid formed within the cavity attacks 
 
160 THE EFFECT OF CARIES ON THE ENAMEL 
 
 the cementing substance between the enamel rods, and 
 proceeding from the dento-enamel junction outward. This 
 is called secondary or backward decay of the enamel, and as 
 a result of it, large areas are disintegrated until they are 
 sufficiently weakened to break into the cavity. This con- 
 dition is shown in Fig. 1 14, in which the area indicated by A 
 has had the cementing substance entirely removed from 
 between the rods, and is in the same structural condition as 
 the disintegrated areas in the first and second stage. It is 
 safe to say that in the past few cavities have been filled 
 until the enamel has caved in. It is equally certain that in a 
 large proportion of cases, by the time this has happened, 
 the removal of all disintegrated tissue will require a greater 
 loss of tooth substance than would be required for the 
 prevention of a new surface attack, at the margin of the 
 filling, if the case had been treated as a beginning instead 
 of a burrowing decay. 
 
 ATROPHY 
 
 Atrophy is a disturbance in the structure of the enamel 
 caused by an arrest or perversion of the function of the 
 enamel-forming tissue during development. It may be 
 caused by any diseased condition which is serious enough to 
 produce marked disturbance of nutrition, but it is especially 
 liable to follow infectious diseases that affect the epithelium, 
 such as scarlet fever, measles, etc. There are all grades of 
 manifestation of this condition, from a slight disturbance 
 in the perfection of structure to complete loss of a portion 
 of the tissue. In all cases the portion of the tissue which was 
 being formed at the time of the disturbance shows a modifi- 
 cation of structure. The imperfect tissue, therefore, corre- 
 sponds to the bands of Retzius and follows their direction, 
 not the direction of the enamel rods. If, for instance, an 
 atrophied groove shows upon the surface of the enamel at 
 A in Fig. 26, the disturbance in structure will not follow the 
 enamel rods to the dentine, but the band of Retzius, and will 
 reach the dento-enamel junction at B. 
 
ATROPHY 161 
 
 Character of the Effect on the Tissue. — In atrophy the for- 
 mation of the rods is first affected, the cementing substance 
 becoming greater in amount. Conditions which produce 
 sHght disturbances are marked simply by a prominent band 
 of Retzius. These are iUustrated in Figs. 125 and 126. In 
 such sections the globules of which the rods are composed 
 are more imperfectly fused and the difference in the refracting 
 index between the rod substance and the cementing substance 
 is greater. The cementing substance also seems to contain 
 actual pigment. The zone of dentine which was being formed 
 at the same time usually shows a zone of interglobular spaces, 
 the character of which will vary with the character of the 
 defect in the enamel. 
 
 Fig. 125 
 
 Labial surface of a central incisor, showing atrophy grooves. The stain in the groove 
 makes it appear deeper than it is. Fig. 126 is a part of a section from this tooth. 
 
 In strongly marked cases it appears that no enamel at all 
 is formed during a longer or shorter period, and when forma- 
 tion begins again the portion that should have been formed 
 is left out and the new formation is telescoped on to the old. 
 When the pathological condition begins to affect the enamel- 
 forming organ, the rods are more and more imperfectly 
 formed and finally disappear, cementing substance contin- 
 uing to be deposited. The entire crown, therefore, is short- 
 ened and has the characteristic stunted appearance. This 
 will be understood by a study of Figs. 127, 128, and 129. 
 On the surface of the tooth, where the new formation joins 
 11 
 
162 THE EFFECT OF CARIES ON THE ENAMEL 
 
 the old, there is a groove the depth of which is determined 
 partly by the duration of the disturbance and partly by its 
 severity. 
 
 The grooves of atrophy are usually accompanied to a 
 greater or less extent by pits in which the formation of rods 
 seems to have failed entirely and the tissue is composed of 
 an imperfect granular material representing defective inter- 
 prismatic substance. 
 
 Fig. 126 
 
 injurj' from atrophy. The growth of enamel -was interrupted, but not stopped. 
 The zone of interglobular spaces in the dentine separates the dentine formed before 
 the interruption from that formed after. (Black.) 
 
 White Spots. — White spots are often seen (Fig. 130) in 
 the enamel of one or two teeth. The surface in these cases 
 is usually smooth and vitreous like the rest of the tooth. 
 Sections ground through such spots show that in the area of 
 the spot the rods have been perfectly formed, but no cement- 
 
ATROPHY 
 
 163 
 
 ing substance. When such an area is entirely within the 
 substance of the enamel, as they usually are, and the surface 
 of the tooth is covered with normal tissue, the spot remains 
 white, or with the same appearance it had when the tooth 
 erupted. If the defective structure reaches the surface they 
 may become more and more stained. 
 
 Fig. 127 
 
 Section of incisors, showing two zones of atrophy, appearing as two grooves on the 
 surface. (About 8 X) (Black.) 
 
 The spaces between the rods are occupied by air and the 
 refraction diffuses the light, causing the white appearance. 
 There is no way to remove these spots except by polishing 
 away the tissue, and this is never advisable. When the white 
 
164 THE EFFECT OF CARIES ON THE ENAMEL 
 
 area reaches the surface of the enamel it is not smooth and 
 poHshed, as in the normal condition, but rough and chalky. 
 One or two cases have been reported in which all of the 
 
 Fig. 128 
 
 Atrophy: A portion showing the incisal groove of Fig. 127 more highly magnified. 
 The dark Une separates the enamel of the first formation from that of the second. 
 Notice that the second is lapped on the first. The narrow line of interglobular 
 spaces is seen in the dentine. (Black.) 
 
 Fig. 129 
 
 The second, or gingival, groove shown in Fig. 127. The overlapping of the third 
 formation on the second is not as great, but the discoloration is greater. The band 
 of interglobular spaces in the dentine is much wider, (Black.) 
 
ATROPHY 
 
 165 
 
 enamel of all the teeth was of this character.^ Instead 
 of the normal color the tissue was originally white like a 
 sheet of paper, but it became very much stained and dis- 
 colored. It was soft and chalky and could be picked to 
 pieces with an instrument. When ground sections were 
 examined it was found that the interprismatic substance 
 was entirely absent and that the rods were standing unsup- 
 ported with spaces between them (Fig. 131). 
 
 Fig. 130 
 
 A section through a white spot in the enamel. In the white area the enamel rods 
 are without cementing substance between them. (Black.) 
 
 Mottled Enamel. — In certain restricted geographical areas 
 there seems to be a tendency to imperfections in the forma- 
 tion of the enamel. In these places the teeth of many 
 children present when they erupt white or mottled areas. 
 There may be only a few spots on a few teeth, or it may 
 
 1 Black's Operative Dentistry, vol. i, p. 35. 
 
166 THE EFFECT OF CARIES ON THE ENAMEL 
 
 involve all of the enamel of all the teeth. In some places 
 a large proportion of all the children born and brought up in 
 the district have teeth more or less disfigured. These mottled 
 teeth seem to be often accompanied by greatly freckled 
 skin. 
 
 Fig. 131 
 
 ;^/ 
 
 Enamel from a h^ection ground by Dr. Black trom near the cusps of one of the 
 teeth in Dr. Prunty's case, showing enamel rods breaking into bundles, which end 
 in spiculse. This enamel has no cementing substance between the rods. Its color 
 was white like paper. (Black.) 
 
 Nothing is now known as to the cause of this condition or 
 how the enamel organ is affected to produce this result. The 
 study of this enamel has shown that the enamel rods are 
 perfectly formed, but that the cementing substance is entirely 
 absent in the mottled area. When the spots are white the 
 spaces between the rods are entirely empty; when they are 
 brown or a dark color, they are filled more or less with some 
 sort of coloring matter. In many cases there is more or less 
 pigment in these spaces before the teeth erupt, and in some 
 cases they grow darker with age. 
 
CHAPTER XIII 
 
 THE DENTINE 
 
 The dentine may be defined as a connective tissue whose 
 intercellular substance is calcified. It is apparently homo- 
 geneous in structure, but penetrated by minute canals, 
 which contain protoplasmic projections from cells lying 
 within a cavity enclosed by the tissue. 
 
 The Function of the Dentine. — The dentine makes up the 
 mass of the tooth, giving to it its general form, each cusp 
 and root being indicated in it. It gives to the tooth its 
 elastic strength, and the enamel, being hard and very 
 resistant to abrasion, but extremely brittle, is dependent 
 upon the elastic support of the dentine. This has been 
 elaborated to a considerable extent in the chapter on the 
 Dental Tissues. The fact that the dentine gives the strength 
 to the tooth should never be lost sight of in operating, and 
 sound dentine should never be sacrificed unnecessarily in 
 the preparation of cavities. 
 
 Structural Elements of the Dentine.— The structural ele- 
 ments of the dentine may be stated as: 
 
 1. The dentine matrix. 
 
 2. The sheaths of Newman and the dentinal tubules. 
 
 3. The contents of the dentinal tubules or the dentinal 
 fibrils. 
 
 While these are the elements of which the tissue is com- 
 posed, there are other characteristic appearances found in 
 the dentine, caused by special conditions or arrangement of 
 these elements which must be studied. These are the granu- 
 lar layer of Tomes, the interglobular spaces and the lines 
 of Schreger, and secondary dentine. 
 
 Origin of the Tissue (Histogenesis). — The dentine, like 
 all of the other calcified tissues except the enamel, is a 
 
168 THE DENTINE 
 
 connective tissue, and is formed by the dental papilla, which 
 is a conical papilla of connective tissue rich in bloodvessels 
 and covered on its surface by the layer of dentine forming 
 cells, the odontoblasts. The dentine is formed from without 
 inward, leaving the remains of the dental papilla in the 
 cavity of the formed dentine as the dental pulp. Before the 
 tooth is erupted, and up to the time that the full length of 
 the root is formed, a characteristic thickness of dentine is 
 formed, which is called the primary dentirie. After this time 
 dentine is formed by the pulp only intermittently, in response 
 to irritations and trophic impulses, producing secondary 
 dentine. Secondary dentine is always more irregular in the 
 arrangement of the tubules, and more imperfect in structure 
 than the primary dentine. The boundary line between two 
 periods of dentine formation can always be picked out by 
 changes in the direction or character of the dentinal tubules. 
 
 The Dentine Matrix. — The dentine matrix is a solid, appar- 
 ently homogeneous, and very elastic substance, through 
 which the dentinal tubules extend. It is translucent in 
 appearance and slightly yellowish in color. In broken or 
 split sections to the unaided eye it has a yellowish color 
 by reflected light, and a characteristic luster due to the 
 refraction of light by the tubules. In ground sections, by 
 transmitted light, under the microscope, it is very translucent 
 and shows no indication of structure. 
 
 The matrix consists of an organic basis of ultimately 
 fibrous character, yielding gelatin on boiling, with which the 
 inorganic salts are chemically combined. The relation of 
 organic and inorganic matter in the dentine matrix is similar 
 to the condition in the bone matrix and that of all calcified 
 connective tissues. Apparently the organic basis is first 
 formed, and then the inorganic salts are combined with it in 
 a weak chemical union. If the dentine is treated with dilute 
 acid, the inorganic matter is dissolved and the organic basis 
 is left retaining the form of the tissue. If the organic matter 
 is burned out, it leaves the inorganic matter in the character- 
 istic form. 
 
THE SHEATHS OF NEWMAN 169 
 
 Von Bibra gives the following analysis of perfectly dry 
 dentine : 
 
 Organic matter 37.61 
 
 Fat 0.40 
 
 Calcium phosphate and fluoride 66 . 72 
 
 Calcium carbonate 3.36 
 
 Magnesium phosphate 1.08 
 
 Other salts 0.83 
 
 Dr. Charles Tomes pointed out that such analysis as 
 this failed to take account of about 8 per cent, of water 
 which is held as water of combination, and which is driven 
 off at about red heat. 
 
 It is evident that the organic matter in the dentine is of 
 two kinds — the organic basis of the matrix, which is of 
 gelatin yielding character, and the protoplasmic contents 
 of the dentinal tubules. Variations, therefore, in the pro- 
 portion of organic and inorganic matter in the dentine might 
 be caused by differences in the proportions of organic and 
 inorganic constituents of the matrix, or by variations in the 
 size of the tubules and the amount of material contained in 
 them. 
 
 If dentine changes in its degree of calcification with age, 
 this might be brought about by the reduction in the size 
 of the tubules, or by the adding of inorganic constituents 
 to the matrix. 
 
 The ultimately fibrous character of the dentine matrix 
 can be made out only in various stages of decalcification 
 and decomposition. In the original condition no trace of 
 the fibrous character can be seen. By maceration with 
 acids and alkalies the intertubular material assumes a 
 fibrous appearance, as if bundles of white connective-tissue 
 fibers had been fused together. There is apparently no 
 definite arrangement of these fibers and there is no indica- 
 tion of the arrangement of the substance in layers. 
 
 The Sheaths of Newman. — There has been much discussion 
 as to i:he character of these structures, which were first 
 discovered in 1863 by Newman. Some investigators have 
 denied their existence entirely, explaining the appearance 
 
170 
 
 THE DENTINE 
 
 in some other way. These structures are in no sense a 
 sheath surrounding tlie dentinal fibril and lying in the 
 dentinal tubule, but are that portion of the matrix which 
 forms the immediate wall of the tubule. That this material 
 differs from that which occupies the rest of the space between 
 the tubules is certain, and is shown by the examination of 
 ground sections, the action of stains upon ground sections, 
 and the action of the matrix when boiled with strong acids 
 
 Fig. 132 
 
 ""-•-- . --^ 
 
 ^■^Jtrmi., M 
 
 ' ^fsp >* ^-~---— ^ ™ 
 
 
 ■j 
 
 
 
 
 1 
 
 ^i 
 
 f c' 
 
 
 
 
 
 Dt 
 D 
 
 Dentine showing tubules in cross-section: Dt, dentinal tubules; D, dentine matrix; 
 S, shadow of sheaths of Neumann. (About 1150 x) 
 
 and alkalies. In Fig. 132, a photograph of a ground section, 
 there is evidently a difference in the refracting index of the 
 portion of the matrix immediately surrounding the tubules. 
 Apparently the sheaths of Newman are composed of a 
 material similar to that forming elastic connective-tissue 
 fibers, and known as elastin. This substance is very resistant 
 to the action of acids and alkalies. After the remainder 
 of the intertubular material has been destroyed by boiling 
 
DIRECTION OF TUBULES IN CROWN PORTION 171 
 
 with strong acid, the sheaths remain Uke hollow elastic 
 fibers, having the appearance of pipestems, which resist 
 long continued action of the boiling acid. Some authors 
 have suggested that the great elasticity of the dentine was 
 largely due to the presence of this substance. 
 
 The Dentinal Tubules. — The dentine matrix is penetrated 
 everywhere by minute branching tubules, which radiate 
 from the central cavity or pulp chamber and extend to the 
 outer surface of the dentine at the dento-enamel junction or 
 the dentocemental junction, where they end blindly or in 
 irregular enlargements. These tubules are from 1.1 to 3 
 microns in diameter. One hundred measurements^ made at 
 random from ground sections gave the extreme measure- 
 ment: 3, largest; 1.5, smallest; and average, 2.95. Fifty 
 measurements from one longitudinal section of tubules at 
 their pulpal extremity gave an average of 2.6; largest, 3; 
 smallest, 1.5; and 50 measurements at the dento-enamel 
 junction of the same section gave the following: Average, 
 1.2; largest, 1.5; smallest, 0.75. These measurements were 
 made with an eye-piece micrometer, using yV oil immersion 
 objective and No. 3 ocular. 
 
 At the present time there is a fertile field for investigation 
 offered in regard to the size of dentinal tubules. ]\Iany 
 statements have been made that have not been supported 
 by tabulated measurements, and no definite statement can 
 be made as to the variations and size of the dentinal tubules 
 in different teeth, the teeth of different animals, or in the 
 human teeth at different ages. 
 
 Direction of Tubules in Crown Portion. — In the crown 
 portion and the gingival portion of the dentine the tubules 
 pass from the pulp chamber to the dento-enamel junction, 
 or the dentocemental junction, in sweeping curves, which 
 were called by Tomes the primary curvatures. These 
 have been described as /- or *S-shaped (Fig. 133). The 
 tubule tends to enter the pulp chamber at right angles to 
 the surface, and to end at the dento-enamel junction at 
 
 ' KoUiker gives 5 microns, also Schafer; Owen, 2.5 microns. 
 
172 
 
 THE DENTINE 
 
 right angles to that surface. In the dentine forming the 
 axial walls of the pulp chamber the tubules make two bends 
 in passing from the pulp chamber to the surface of the 
 dentine. In the first the convexity is directed apically, in 
 the second it is directed occlusally. The outer extremity of 
 the tubule is, therefore, considerably farther to the occlusal 
 than the point at which it opens into the pulp chamber 
 
 Fig. 133 
 
 A section showing tlie primarj- curvatures of the dentinal tubules in the crown 
 portion. (About 20 X) 
 
 (Fig. 134). The outer part of this double curve is often 
 complex instead of simple (Fig. 135). The course of the 
 dentinal tubules is not a direct one, but that of an open 
 spiral. This may easily be demionstrated by changing 
 the focus up and down in examining sections cut at right 
 angles to the direction of the tubules. When examined in 
 longitudinal sections this spiral course gives to the tubule 
 
DIRECTION OF TUBULES IN CROWN PORTION 173 
 
 the appearance of having httle wavy curves throughout its 
 length. These have often been called the secondary curva- 
 tures. Each wave represents a turn in the spiral. As many 
 as two hundred have been counted in the length of a single 
 tubule, or about one hundred in a millimeter of length. 
 
 Fig. 134 
 
 A section showing the primary curvature of the dentinal tubules in the gingival 
 portion. (About 20 X ) 
 
 The dentinal tubules give off minute lateral branches, 
 which extend from one tubule to another. These are very 
 minute, and in the crown portion of the dentine are not at 
 all conspicuous, but in the region of the dento-enamel 
 
174 
 
 THE DENTINE 
 
 junction the tubules branch dichotomously, each fork having 
 about the same diameter as the original tubule (Fig. 136.) 
 These forkings of the tubules resemble the appearance of 
 the delta of a river on the map. The branches anastomose 
 with each other very freely. This anastomosis of the 
 tubules at the dento-enamel junction is very important in 
 determining the spreading of caries in this area. It 
 probably also explains the sensitiveness of this area noticed 
 in the preparation of cavities, which will be noted again in 
 considering the sensitiveness of the dentine. 
 
 Fig. 135 
 
 A section showing compound curves near the dento-enamel junction. (About 80 X ) 
 
 ' The Dentinal Tubules in the Root Portion.^In the root 
 portion of the dentine the tubules ordinarily show only the 
 secondary curves, their general direction being at right angles 
 to the axis of the pulp canal. Throughout their course they 
 give off an enormous number of very fine branches extending 
 from tubule to tubule. These are so numerous that in 
 suitably prepared sections they may be said to look like the 
 interlacing twigs of a thicket or the rootlets of plants in the 
 soil. Fig. 137 gives a very good idea of the appearance. 
 At the dentocemental junction the tubules end in irregular 
 
DENTINAL TUBULES IN THE ROOT PORTION 175 
 
 anastomosing spaces, which cause the appearance of the 
 granular layer of Tomes (Fig. 138). 
 
 From a consideration of the preceding it will be seen that 
 it is usually not difficult to determine whether a field of 
 
 Fig. 136 
 
 Dentine at dento-enamel junction, showing tubules cut longitudinally: Dt, dentinal 
 tubules; D, dentine matrix. (About 760 X) 
 
 dentine seen under the microscope was taken from the crown 
 or the root of a tooth. The structural characteristics of the 
 two regions may be summarized as follows: In the croicn, 
 the tubules show both the primary and the secondary curves. 
 
176 
 
 THE DENTINE 
 
 In the root, the tubules show only the secondary curves. 
 In the croum, the lateral branches are few and inconspicuous 
 and the tubules branch in a characteristic way at the dento- 
 enamel junction. In the root, the lateral branches are very 
 
 Fig. 137 
 
 Dentine from the root, showing tubules cut longitudinally. (About 700 X ) 
 
 numerous throughout the length of the tubule, and they 
 end in the characteristic spaces of the granular layer of 
 Tomes. 
 
 The Dentinal Fibrils. — In life the dentinal tubules are 
 occupied by protoplasmic projections of the odontoblasts 
 
THE DENTINAL FIBniLS 
 
 177 
 
 known as the dentinal fibrils, or fibers of Tomes. x\s the 
 dentine matrix is formed and calcified under the influence 
 of the odontoblasts, a portion of their protoplasm is left in 
 the tubules of the matrix as the dentinal fibril. These 
 
 Fig. 138 
 
 Granular layer of Tomes: L, lacunae of cementum; Gt, granular layer of Tomes; 
 Ig, interglobular spaces. (About 200 X) 
 
 structures were first described by Jolm Tomes, who recog- 
 nized tKeir true character. They may be demonstrated 
 in decalcified sections, and they will be seen projecting from 
 the odontoblasts, when the pulp is removed from a freshly 
 12 
 
178 THE DENTINE 
 
 extracted tootli, by cracking it and picking the pulp out. 
 In tliis way a portion of the fibril is pulled out of the tubules. 
 The fibrils will be considered more especially in connection 
 with the pulp, to which they properly belong. 
 
 The Granular Layer of Tomes. — The granular laj^er of 
 Tomes is the outer layer of the dentine next to the cementum. 
 The granular appearance is caused by irregular spaces in 
 the dentine matrix which connect with the ends of the 
 dentinal tubules, and wdiich are filled with protoplasm 
 continuous with that of the fibrils. Tomes first called 
 attention to this layer, and for this reason it bears his name. 
 
 With magnifications of from 50 to 100 diameter it is easily 
 seen in ground sections either longitudinal or transverse, and 
 appears as a layer filled with little dark spots or granules, 
 the spaces which have been filled with the debris of grinding. 
 It is separated from the cementum by a thin clear layer, 
 apparently of structureless dentine matrix, which is more 
 apparent in higher magnifications. The granular layer is 
 sometimes seen in the crown portion just under the enamel, 
 but it is never as well marked in this position. 
 
 The layer is seen in sections ground from freshly extracted 
 teeth as w^ell as from old dry teeth, showing that these are 
 true spaces and are not produced by the shrinkage of par- 
 tially calcified dentine matrix. Tomes called the spaces in 
 the granular layer "interglobular spaces," but this term 
 should not be used, as the structures generally known as 
 the interglobular spaces are different in location and char- 
 acter, and will be considered later. 
 
 The granular layer is not seen in decalcified sections. So 
 far as the author is aware, no one has called attention to this 
 fact before. In decalcified sections stained with hematoxylin 
 and eosin the position of the granular layer is always occu- 
 pied by a clear layer which takes the stain in an entirely 
 different way from the rest of the dentine matrix, and in 
 which no indication of spaces can be seen. While the 
 fibrils in the tubules through most of the dentine take the 
 hematoxylin stain and can be easily seen, they cannot be 
 
THE INTERGLOBULAR SPACES 179 
 
 followed into this clear layer, and no indication of proto- 
 plasmic contents of irregular spaces can be seen.^ 
 
 Most authorities state that the spaces of the granular 
 layer communicate with the canaliculi of the cementum, 
 as well as with the tubules of the dentine. This the author 
 has been unable to confirm. On the other hand, the granular 
 layer seems to be separated from the cementum by a thin 
 layer of dentine which is clear and apparently structureless. 
 This is separated from the cementum by a dark line, and 
 the first layer of cementum usually does not contain any 
 lacunae or canaliculi. This is supported by some of the 
 experiments that have been made with extracted teeth. In 
 experimenting on the diffusion of drugs through dentine, 
 it was found that liquids sealed in the pulp chambers of 
 extracted teeth could not be detected in the liquids in which 
 the teeth were placed unless the cementum was removed from 
 them. In the recent experiments of Dr. Southwell, of 
 Milwaukee, in which air was forced through the dentine 
 from the pulp chamber, to test the sealing of cavities by 
 filling materials, the air did not escape from the cementum, 
 which would be the case if dentinal tubules connected with 
 the canaliculi of the cementum. 
 
 If the spaces of the granular layer are filled with the pro- 
 toplasmic enlargements of the ends of the dentinal fibrils, 
 this would give a very reasonable explanation of the sensi- 
 tiveness of slight caries and erosion at the gingival line, as 
 the anastomosis through the granular layer would affect 
 the fibrils of the entire root. 
 
 The Interglobular Spaces. — There has been considerable 
 misunderstanding in dental histology in regard to these 
 spaces, owing to the confusing of two entirely different 
 things. Tomes called the spaces of the granular layer, 
 which have already been described, interglobular spaces. 
 As has been seen, they are true spaces in the dentine matrix 
 
 1 The appearance of the tissue in decalcified sections has led to some doubt in the 
 writer's mind as to the interpretation of the character of the laj-er by authors who 
 have described it. 
 
ISO 
 
 THE DENTINE 
 
 which connect with the dentinal tubules and are filled with 
 protoplasm. 
 
 In 1850 J. Czermak^ described areas of imperfectly cal- 
 cified dentine matrix, which appear as spaces in dried dentine, 
 and called them interglobular spaces. These have been so 
 called by most writers since. It seems important to the 
 author that the term be restricted to these and some others 
 used to indicate the spaces of the granular layer, which are 
 of entirely different character. 
 
 Fig. 139 
 
 A drawing showing a zone of interglobular spaces in the dentine. (Black.) 
 
 The interglobular spaces of Czermak are caused by some 
 disturbance in the calcification of the organic matrix of the 
 dentine. They occur in zones (Fig. 1.39) which correspond to 
 the dentine matrix, being calcified at a given time, and there 
 is usually seen a corresponding disturbance in the calcifica- 
 
 Beitrag zur Mikro-Anatomie der Menschlichen-Zahne. 
 
THE INTERGLOBULAR SPACES 
 
 181 
 
 tion of the enamel, which was being formed at the same 
 time and manifested as a more or less strongly marked 
 atrophy band. 
 
 Fig. 140 
 
 ^im^'^m^- 
 
 
 ^^ 
 
 ^-^'^^^^ 
 
 r^^r^^ 
 
 Interglobular spaces in dentine. (About 60 X ) 
 
 Fig. 141 
 
 Interglobular spaces in dentine. Some emptj', some filled wiiL JlLtis. Clbout SO >, ' 
 
 In the calcification of the dentine matrix the inorganic 
 salts are combined with the organic matrix in spherical 
 areas which become united. The boundaries of these areas 
 of uncalcified matrix are therefore very irregular, and made 
 
182 
 
 THE DENTINE 
 
 up of concave facets where they join the spherical surfaces 
 of the fully calcified matrix (Figs. 140 and 141). A study of 
 the illustrations and the appearance of the layer of forming 
 dentine next to the dental papilla of a developing tooth will 
 make this intelligible. 
 
 Fig. W2 
 
 Interglobular spaces in dentine: Ig, first line of interglobular spaces; Ig', second 
 line of interglobular spaces. (About 30 X ) 
 
 If the dentine is dried the organic matrix in these areas 
 gives up water and shrinks, and the interglobular spaces 
 become true spaces, partially filled with the shrunken matrix. 
 In this condition they can be filled with colored collodion 
 
THE INTERGLOBULAR SPACES 
 
 183 
 
 or any other material. If, however, they are studied in 
 sections of teeth which have never been allowed to dry, no 
 space appears, and the dentinal tubules continue without 
 change of course or diameter through the area. While they 
 are, therefore, not empty spaces, they are areas of the organic 
 matrix of the dentine which are bounded by globular sur- 
 faces of the fully calcified matrix, and their name is properly 
 significant. 
 
 Fig. 143 
 
 A root broken on a line of interglobular spaces. This tooth was extracted by 
 Dr. G. v. Black, and was pulled apart in extraction, a shows the form of the root 
 and a b the separation on the line of growth. (Black.) 
 
 Zones of interglobular spaces may occur at any portion 
 of the dentine, either in the crown or root, but they are more 
 common in the crown and near the enamel. Often more than 
 one zone can be seen, as in Fig. 142, which shows two dis- 
 turbances in calcification, and disturbances in the structure 
 of the enamel will be seen at corresponding positions. 
 
 The zones of interglobular spaces appear in all grades, 
 from a complete band of uncalcified matrix to widely 
 scattered patches. Fig. 143 shows a tooth in Dr. Black's 
 collection which was broken in extraction, because of the 
 presence of such a zone in the root. 
 
184 THE DENTINE 
 
 The interglobular spaces are of great importance in modi- 
 fying the direction of the progress of caries in the dentine. 
 
 The Lines of Schreger. — As in the case of the interglobular 
 spaces, there seems to be considerable misunderstanding in 
 the literature, and certain structures which have very 
 different meanings have been called the "lines of Schreger." 
 
 An arrest in the formation of dentine often occurs before 
 the crown is completed. When the activity has begun again 
 the dentinal tubules follow a slightly different direction. In 
 a longitudinal section this change in the direction of the 
 tubules produces a line. Several such lines may be seen 
 in a single section, though they are by no means to be found 
 in all longitudinal sections. 
 
 Schreger's lines have been most often confused with zones 
 of interglobular spaces, and they seem to be identical with 
 the incremental lines in the dentine described by Owen. 
 It is unfortunate that these names should have been used, 
 for a thoughtful study of the tissue makes their interpreta- 
 tion perfectly evident, and they are of no great significance. 
 
 Secondary Dentine.— It is by no means easy to define 
 secondary dentine or to pick out any particular piece of 
 dentine in a section and to say whether it is primary or 
 secondary. In general, the tubules are smaller, fewer, and 
 less regularly arranged in secondary than in primary dentine. 
 In general, it seems that the smaller the remainder of the 
 dental papillae becomes, the more imperfect dentine it 
 forms, until finally it simply throAvs down granular calcified 
 material. 
 
 The formation of dentine begins at the dento-enamel 
 junction, at a number of points in each tooth, and progresses 
 from without inward (strange to say, exactly the opposite 
 statement has been made several times in papers by very 
 prominent men). This matter will be taken up more in 
 detail in Chapters on Dental Embryology and Dentition. 
 It is enough to say here that in studying all sections of 
 dentine, whether cut longitudinally or transversely, the 
 formation of dentine began at the dento-enamel junction 
 and the dentocemental junction, and progressed toward 
 the pulp chamber. 
 
SECONDARY DENTINE 
 
 185 
 
 From the study of longitudinal and transverse sections 
 it is apparent that a certain typical amount of dentine is 
 formed before the tooth is erupted and while it is coming 
 into full occlusion. This is primary dentine. In it the 
 tubules are very regular in size and arrangement. From 
 
 Fig. 144 
 
 P 
 
 
 o^-*^«i^ 
 
 M^AVl 
 
 '•X-^it\»\uMiyiM: 
 
 Secondary dentine: A, margin of pnnuay LkiiUuc .-Lowiiiy u few ul thu tubules 
 continuing into secondary dentine; P, pulp chamber. (About 80 X) 
 
 this time on the formation of dentine is intermittent, and 
 apparenth' is the response to some outside condition. These 
 conditions may arise in the tooth in which the formation 
 occurs, or the irritation of one tooth may cause tissue forma- 
 tion in all or part of the others. It has not been determined 
 
186 
 
 THE DENTINE 
 
 whether such reflex trophic stimuH are confined to the same 
 lateral half or the same nerve distribution. Apparently the 
 formation of dentine proceeds again, after a pause, in all 
 teeth. It will seem, therefore, that the mere exposure of 
 the entire crown to conditions of thermo-change produces 
 sufficient stimulus to the pulp tissue to cause a renewal of 
 dentine formation. After the first period of rest the dentine 
 
 Fig. 145 
 
 A transverse section of a root, showing the reduction in the size of the pulp 
 and formation of secondary dentine. 
 
 formed in the second period is so nearly identical, and the 
 direction of the tubules so nearly the same, that it is usually 
 impossible to recognize the junction except at a few points 
 in the circumference of a transverse section. After each 
 period of rest, however, the difference in structure between 
 the succeeding portions becomes more marked. Fig. 144 
 
SECONDARY DENTINE 
 
 187 
 
 shows an area from a longitudinal section when the line A 
 was the pulpal wall of the dentine. There was probably 
 a considerable period of rest, when for some reason a new 
 formation of dentine was begun. But apparently only some 
 of the odontoblasts took part in the new formation of 
 
 A transverse section of a root, showing changes in the form of the pulp canal 
 by the formation of secondary dentine. 
 
 dentine matrix, for not nearly all of the tubules are con- 
 tinued, and those that do continue show a sharp change in 
 their direction and a difference in diameter and character 
 (Figs. 145 and 146). 
 
 These characteristic changes in the structure of the 
 dentine that is formed as the pulp becomes smaller seem 
 to the author of great practical importance. 
 
CHAPTER XIV 
 
 THE CEMENTUM 
 
 The cementum may be defined as a connective tissue whose 
 intercellular substance is calcified and arranged in layers 
 (lamellse) around the circumference of a tooth root, the 
 cells being found in spaces (lacunae) irregularly placed in 
 or between the layers. 
 
 Structurally the cementum is more closely related to the 
 subperiosteal bone than any other tissue, the only differences 
 being that in general the lacunse in bone are much more 
 uniform in size, shape, arrangement of the canaliculi, and 
 their position with reference to the lamellse than those in 
 cementum. In bone the lacunae are usually found between 
 the lamellae. In cementum the lacunae may be between the 
 lamellae, but they are more often enclosed within their sub- 
 stance and they are found most often where the lamellae 
 are thick. 
 
 Some writers have described Haversian canals in the 
 cementum, but the author has never seen anything that 
 could properly be called an Haversian canal in the cementum 
 from human teeth. Canals containing bloodvessels are not 
 uncommon, but in these the lamellae are never arranged 
 concentrically around the canal, as they are in Haversian 
 systems. For the last fifteen years the author has had under 
 personal observation, in the course of class work, not less 
 than 200 longitudinal sections, and 300 transverse sections 
 of the root, ground from human teeth, and in that time he 
 has never seen what could be called an Haversian canal. 
 In the same time he has examined many hundreds of 
 sections cut through the decalcified jaws of various mam- 
 mals, including the sheep, pig, cat, and dog, with the same 
 negative result. 
 
HISTOGENESIS 189 
 
 Function. — The function of the cementum is to attach to 
 the tooth the connective-tissue fibers which hold it in posi- 
 tion and support the surrounding tissues. 
 
 The formation of cementum begins as soon as the tooth 
 begins to erupt, and continues, at least intermittently, as 
 long as the tooth remains in place, whether it contains a live 
 pulp or not. 
 
 The function of the cementum cannot be too strongly 
 emphasized, and must be continually borne in mind. If, for 
 any reason, the tissues are detached from the surface of the 
 root, they can only be reattached by the formation of a new 
 layer of cementum on the surface of the root, which will 
 embed the surrounding connective-tissue fibers. In order 
 to accomplish this the tissues must lie in physiologic con- 
 tact with the surface of the root, and the connective-tissue 
 cells must be actively functional. 
 
 That the tissues may be reattached to the surface of a 
 root is both theoretically possible and clinically demonstrable, 
 but for it to occur, biological laws must be observed and the 
 conditions are very difficult to control, especially with the 
 old methods involving the excessive use of strong antiseptics. 
 It is well to remember ''that a dentist can never cure a 
 suppurating pocket along the side of a tooth root," but if 
 the conditions can be controlled the cells of the tissue may 
 form a new layer of cementum, reattaching the tissues and 
 so close the pocket. It is a biological problem, not a matter 
 of drugs, except as they are a means of producing cellular 
 reaction. 
 
 In view of its function, therefore, the cementum becomes 
 not the least but the most important of the dental tissues, for 
 no matter how perfect the crown may be, without firm 
 attachment the tooth becomes useless and is soon lost. 
 
 Histogenesis. — The cementum is formed by connective- 
 tissue cells lying between the fibers of the tissue which clothes 
 the surface of the root and which becomes specialized for 
 this function. Their origin is undoubtedly similar to that 
 of the osteoblasts, but they are not osteoblasts, either mor- 
 phologically or functionally, as will be seen later in the 
 
190 THE CEMENTUM 
 
 study of the peridental membrane, where the cementoblasts 
 and the formation of cementum will be considered. 
 
 Structural Elements. — The structural elements of the 
 cementum are: 
 
 1. The lamellae. 
 
 2. The lacunse and canaHculi. 
 
 3. The cement corpuscles. 
 
 4. The embedded fibers of the peridental membrane. 
 The Lamellae of the Cementum and Their Arrangement. — The 
 
 lamellae of the cementum resemble those of bone, but they 
 are very much more irregular both in thickness and appear- 
 ance. They may be extremely thin and almost transparent, 
 or they may be quite thick and coarsely granular. They 
 are not nearly as easily observed as those of bone, for in bone 
 the lamellae are marked off by the lacunae which lie between 
 them, while in cementum the lacunae may be entirely absent, 
 and when present are irregularly placed. 
 
 In the gingival portion of the root the lamellae are always 
 thin and very transparent, and lacunae are seldom seen. The 
 entire thickness of the tissue is transparent, and the appear- 
 ance of the lamellae can be seen only by using a very small 
 diaphragm or oblique illumination. In this position the 
 tissue is largely made up of embedded connective-tissue 
 fibers, which are, however, so perfectly calcified that they 
 cannot be demonstrated in ground sections. In decalci- 
 fied sections they are very easily seen. 
 
 The cementum becomes gradually thicker in the middle 
 third of the root, and is thickest in the apical third. It will 
 be seen that this increase in thickness is caused chiefly by 
 the greater thickness of each individual lamella. In longi- 
 tudinal sections the cementum is often found becoming 
 suddenly thicker at a certain point, and if examined closely, 
 it will be seen that each layer is continued apically, but with 
 greater thickness. Fig. 149 illustrates this condition near the 
 apex of the root. From a study of the lamellae, therefore, it 
 is apparent that the entire root is clothed with successive 
 layers, and that these layers are formed intermittently, but 
 continue to be formed as long as the tooth is in position. 
 
THE LAMELLA OF THE CEMENTUM 
 
 191 
 
 In a general way the number of layers is an index to the age 
 of the person at the time the tooth was extracted (Figs. 
 150 and 151). The rate of formation is not uniform; for 
 
 Fig. 147 
 
 
 ''■:■''> .^ 
 
 ^^ 
 
 / 
 
 kv\iiivv\\\\\vM\ \\\\n\Vu\\\\\VV\\wa\ 
 
 ^^UiV 
 
 Hjpertrophy of the cementum on the side of the root of a lower molar near the 
 neck of the tooth. From a lengthwise section, man: a, dentine; h, cementum; c, 
 fibers of peridental membrane. From 6 to c the cementum is normal and the incre- 
 mental lines fairly regular, but at d one of the lamellae is greatly thickened. At e 
 this lamella is seen to be about equal in thickness with the others. The next two 
 lamellae are thin over the greatest prominence, but one is much thickened at g, and 
 both at h. These latter seem to partially fill the valleys which were occasioned by 
 the first irregular growth. (1 in. obj.) 
 
 Fig. 148 
 
 WMfMMI&^^^MMiM^ 
 
 Hypertrophy from root of cuspid, man, in which the irregularity is confined to 
 the first lamella: o, dentine: 6, thickened first lamella: c, subsequent lamellae, which 
 are seen to be fairly regular. (1 in. obj.) 
 
192 
 
 THE CEMENTUM 
 
 instance, a number of layers may be formed within a short 
 time, and again, a considerable time may elapse between the 
 formation of one layer and the next. The time, however, 
 does not seem to determine the thickness of the layer. 
 
 Fig. 149 
 
 Apex of root of an upper bicuspid tooth with irregularly developed cementum: 
 a, a, dentine; b, b, pulp canals. The lamellse of cementum are marked 1, 2, 3, 4, 5, 
 6, 7, 8, 9; d, d, d, absorption areas that have been refilled with cementum. It will 
 be seen that the apices of the roots were originally separate, but became fused with 
 the deposit of the second lamella of cementum, and that in this the irregular growth 
 began and was most pronounced. It has continued through the subsequent 
 lamellse, but in less degree. It will also be noticed that the absorption areas, d, d, d, 
 have proceeded from certain lamellse. That between the roots has broken through 
 the first lamella and penetrated the dentine, and has been filled with the deposit 
 of a second lamella. Other of the absorptions have proceeded from lamellse 
 which can be readily made out. The small points, e, seem to have been filled with 
 the deposit of the last layer of the cementum, while others have one, two, or more 
 layers covering them. (2 in. obj.) 
 
 If a considerable number of teeth of persons of twenty 
 years of age were sectioned, the lamellse counted, and this 
 number compared with the number found in teeth extracted 
 
THE LAMELLA OF THE CEMEXTUM 
 
 193 
 
 from persons of forty, a fairly regular increase in the number 
 of layers will be noticed, and so on, for fifty, sixty, seventy, 
 or eighty years. 
 
 It is important to remember in connection with this 
 formation of cementum that the teeth move, more or less, 
 under the influence of natural forces throughout life, and 
 that every slight change in position must be accomplished 
 
 Fig. 150 
 
 A transverse section of a root extracted from a young person. The cementum is 
 thin, but is thicker in the grooves on the proximal sides. 
 
 by the formation of a new layer of cementum, to reattach 
 connective-tissue fibers in new positions or adjust them to 
 new directions of strain. 
 
 The first layer of cementum is formed while the tooth is 
 
 still in its crypt, but apparently no connective-tissue fibers 
 
 are calcified into it. This forms the first apparently clear 
 
 and structureless layer which lies next to the granular 
 
 13 
 
194 
 
 THE CEMENTUM 
 
 layer of Tomes (Fig. 138). Even in the teeth the entire 
 length of whose roots are formed before they begin to erupt, 
 there is no attachment until some stress comes upon the 
 crown. The tooth is lying loose in its crypt and can be 
 picked out with very little force. Bicuspids are often acci- 
 dentally extracted in the extraction of temporary molars. 
 x\s soon as the tooth comes through the gum a new layer 
 
 Fig. 151 
 
 A transverse section of a root from an old person. This root had carried 
 for many years. The section was cracked and one edge broken. 
 
 of cementum is formed over the entire root, attaching the 
 fibers to its surface, and as the tooth moves occlusally, layer 
 after layer is formed. This will be considered again in 
 connection wdth the peridental membrane. 
 
 The Lacunse and Canaliculi. — The lacunse of the cementum 
 correspond with the lacunse of bone. They difter from those 
 of bone, however, in that they are more irregular in shape, 
 size, position, and relation to the lamellae, and in the number 
 
THE CEMENT CORPUSCLES 195 
 
 and direction of the canaliculi radiating from them. In 
 bone the lacunae are fairly regular in shape, the long diameter 
 exceeding the short diameter by about one-third. Sections 
 cut through their long axis give an oval outline, the length of 
 which is about three times as great as the width. Sections 
 cut through their short axis give an oval outline, the long 
 diameter being about twice that of the short. The spaces 
 are, therefore, flattened between the lamellae. In cementum 
 there is no regularity whatever, either in size or in shape. 
 Some are a little larger than the lacunse in bone, some are 
 very much smaller. They may be almost exactl}^ the shape 
 of typical bone lacunae or they may be distorted into almost 
 any form, sometimes being almost stellate, often pear- 
 shaped, sometimes round, and occasionally pyramidal. The 
 lacunae of bone are fairly uniformly placed, and lie between 
 one lamella and the next.^ There is no regularity in the 
 relation of the lacunae of the cementum to the lamellae. 
 They sometimes lie between one lamella and the next, but 
 they are more often entirely in the substance of one. They 
 occur only where the lamellae are thick, and there may be 
 large areas with considerable aggregate thickness of cementum 
 in which there are no lacunae at all. 
 
 The number and direction of the canaliculi which radiate 
 from the lacunae of cementum is extremely irregular, but in 
 general there are more extending from the lacunae toward 
 the surface than toward the dentine. 
 
 The Cement Corpuscles. — The cement corpuscles corre- 
 spond exactly to bone corpuscles. They are the cells found 
 in the lacunae. These are simply embedded cementoblasts 
 and are typical connective-tissue cells. They are made 
 up of granular protoplasm and contain a faintly staining 
 nucleus. Extensions of the protoplasm undoubtedly extend 
 into the canaliculi. These cells bear the same relation to 
 the matrix of the cementum that bone corpuscles do to that 
 
 ' This is not absolutely correct, there being much more irregularity in the arrange- 
 ment of the lacunae in thick subperiosteal bone than in either cancellous or Haversian 
 system bone. To be strictly accurate, the above statement must be limited to Haver- 
 sian system bone (Plate X). 
 
196 THE CEMENTUM 
 
 of bone. What this is is not known in any definite way, but 
 it is known that when bone corpuscles are killed or die, the 
 matrix becomes a foreign body, and is either absorbed or 
 cut off from the portion in which the corpuscles are living, 
 to be absorbed or cast out as a sequestrum. The same 
 conditions are true of cementum. P'or instance, there are 
 many cement corpuscles in the lacuna in the region of the 
 apex of the root. If this portion be bathed in pus for a long 
 time, the cement corpuscles are killed, and the tissue becomes 
 saturated with poisonous materials, so that tissue cells 
 cannot lie in contact with it and live. In order to restore a 
 healthy condition, the necrosed cementum must be removed 
 mechanically until tissue is reached with which cells may 
 lie in physiological contact without injury. Conditions 
 which can only be understood through a knowledge of the 
 structure of the tissue often arise in connection with the 
 treatment of alveolar abscess. It should always be remem- 
 bered that the treatment of an abscess is a biological problem. 
 The Embedded Fibers of the Peridental Membrane. — The 
 embedded fibers of the peridental membrane are in the 
 strictest sense comparable with the fibers of Sharpe in bone. 
 They are, however, in many places much more perfectly 
 calcified. To appreciate the relation of the embedded fibers 
 to the matrix, the tissue must be studied both in ground 
 and decalcified sections. For instance, in the gingival 
 portion, from the study of ground sections, the presence 
 of embedded fibers would never be suspected, but if decal- 
 cified sections are studied it will be found to be almost 
 entirely composed of calcified fibers. In the middle and 
 apical thirds of the root, w^here the lamellse are thicker, the 
 calcification of these fibers is often not as perfect as that 
 of the rest of the matrix. In the preparation of ground sec- 
 tions, therefore, the imperfectly calcified fibers shrink and 
 consequently appear as canals in the cementum. In fact, 
 they have often been mistaken for canals. They are usually 
 not seen unless the section happens to cut in their direction. 
 These will be seen in many of the illustrations of cementum. 
 In Fig. 152 several layers are seen next to the dentine, in 
 
EMBEDDED FIBERS OF PERIDENTAL MEMBRANE 197 
 
 which no fibers appear, then in several layers the fibers are 
 plainly seen, and finally, the surface layers show no fibers. 
 This probably means that before and after these layers were 
 formed there was a change in the position of the tooth and 
 the fibers were all cut off in this area and reattached in a new 
 
 Fig. 152 
 
 Cementum near the apex of the root: Gt, granular layer of Tomes; L, lacunae; 
 b, point at which fibers were cut off and reattached. (About 54 X) 
 
 direction, adapting them to the new directions of strain. 
 It is often necessary to study ground sections very closely 
 to determine whether certain appearances are embedded 
 fibers or canaliculi radiating from the lacunae. The appear- 
 ance of these fibers should be studied in Fig. 153. It should 
 be noted that wherever special stress is exerted upon a 
 
198 
 
 THE CEMENTUM 
 
 bundle of fibers the cementum is thick around them. This 
 may be seen in decalcified sections in Figs. 220, 248 and Plate 
 
 Fig. 153 
 
 Two fields of cementum showing penetrating fibers: Gt. granular layer of Tomes; 
 C, cementum not showing fibers; F, penetrating fibers. (About 54 X) 
 
 XIV and in ground sections in Figs. 152 and 153. When the 
 next layer is formed, if the fibers are cut off, the additional 
 thickness of the last layer is removed. The unequal thick- 
 
EMBEDDED FIBERS OF PERIDENTAL MEMBRANE 199 
 
 ness of the last formed layer is not seen in the layers beneath 
 it to as great an extent. 
 
 Fig. 154 
 
 Record in the calcified tissue of an absorption repaired: D, dentine; Cm, cementum 
 filling absorption cavity. (About 40 X) 
 
 Fig. 155 
 
 Thick lamellae of cementum with many lacunae, filling an absorption in dentine: L, 
 lacunae; H, Howship's lacunae filled; D, dentine. (About 250 X) 
 
200 THE CEMENTUM 
 
 Absorption and Repair of the Cementum. — From what has 
 ah-eady been said about the cementum, it will be understood 
 that this tissue is continually undergoing changes, that new 
 layers are being added, and that often before an addition is 
 made there is absorption enough of it at least to cut off the 
 fibers. When an absorption occurs on the side of a root 
 which cuts into the dentine, the excavation in the dentine may 
 be filled by the dentine subsequently formed (Figs. 154 and 
 155). From a study of ground sections in class work such 
 absorptions are not uncommon. They probably occur when 
 the cusps first come into occlusion in eruption. 
 
CHAPTER XV 
 
 DENTAL PULP 
 
 Definition. — The dental pulp may be defined as the connec- 
 tive tissue occupying the central cavity of the dentine. 
 
 It is composed of embr^^onal connective tissue which is 
 more closely related to the tissue occupying the spaces of 
 cancellous bone than to any other. 
 
 Functions. — The functions of the dental pulp are: 
 
 1. A vital function, the formation of dentine. 
 
 2. A sensory function responding to thermal change and 
 chemical and traumatic irritation. 
 
 J^ital Function. — The vital function is the formation of 
 dentine and is performed by the layer of odontoblasts. 
 These cells also, by means of their dentinal fibrils, maintain 
 the same relation to the dentine matrix that the bone and 
 cement corpuscles bear to the matrix of bone and cementum. 
 When the pulp is removed from a tooth its dentine becomes 
 dead dentine in the same sense that bone in which the bone 
 corpuscles have been killed is necrosed bone. That there 
 is a constant reaction between the protoplasm of the odonto- 
 blasts and the substance of the dentine matrix, or that the 
 presence of the living protoplasm is necessary to prevent 
 degeneration of the matrix, is evidenced by the changes in 
 the physical properties of the dentine after the pulp has 
 been lost. That the tooth remains functional after the loss 
 of the pulp is due to the fact that, except at the minute 
 foramina, the dentine is not in physiologic contact with 
 any tissue excepting enamel and cementum, and that the 
 cementum attaches the tooth to the surrounding tissues, 
 receiving its nourishment from the surface and not from the 
 dentine. 
 
202 DENTAL PULP 
 
 When the pulp is removed and its place filled by a non- 
 irritating material, the dentine becomes entirely encased 
 in cementum, the foramina probably being covered over as 
 the subsequent lamellae are formed. The author wishes to 
 emphasize, however, the vital relations of the pulp to the 
 dentine matrix. Dead dentine is never as good as living 
 dentine, consequently a tooth from which the pulp has been 
 removed can never be considered just as good as one with 
 the living pulp. 
 
 The production of the dentine matrix is, of course, the 
 principal part of the vital function of the pulp. It is begun 
 in the development of the tooth before the dental papilla 
 is converted into the dental pulp, by being enclosed in the 
 dentine formed. After the tooth is fully formed the pulp 
 retains its ability to build dentine matrix as long as it 
 retains vitality, but this function is exercised only in response 
 to conditions of environment which are probably excited 
 through the intervention of its sensory function responding 
 to thermal change and chemical irritation. The sensory 
 function causes a trophic impulse which is manifested by the 
 production of another portion of dentine matrix reducing the 
 size of the pulp chamber. That this is a reflex and not purely 
 a local matter is indicated by the fact that formations of 
 dentine occur in one tooth when the irritation is in another, 
 and apparently the irritation of one tooth will excite dentine 
 formation in all of the teeth on that side, at least in some 
 instances. On the other hand, purely local responses are 
 found where a few odontoblasts respond to the irritation 
 of their fibrils by the formation of dentine. 
 
 This matter has been referred to under the heading of 
 secondary dentine, and it is best studied by the record it 
 leaves in the formed tissue. 
 
 The Sensory Function. — In regard to sensation, the pulp 
 resembles an internal organ, as in its normal condition it is 
 always enclosed in the cavity of the dentine. It has no sense 
 of touch or localization, and responds to stimuli only by 
 sensations of pain. The pain is usually located correctly 
 with reference to the median line, but apart from that it is 
 
STRUCTURAL ELEMENTS 203 
 
 located only as it is referred to some known lesion. If 
 several pulps were exposed on the same side of the mouth, 
 and in teeth of both the upper and lower arches, so that they 
 could be irritated without impressions reaching the peri- 
 dental membrane, if the patient were blindfolded it would 
 be impossible for him to tell which of the pulps was touched. 
 This characteristic becomes extremely important in diag- 
 nosis. 
 
 The pain originating from a tooth pulp may be referred 
 to the wrong tooth or to almost any point on the same side 
 supplied by the fifth cranial nerve. 
 
 The dental pulp is especially sensitive to changes in 
 temperature, amounting almost to a temperature sense, 
 having no exact parallel in any other tissue of the body. 
 This does not amount to a recognition of heat or cold as 
 such, but a special resentment to sudden changes. For 
 instance, if a tooth is isolated and so protected by non- 
 conductors that the soft tissues cannot be stimulated, and a 
 jet of hot and then cold water be thrown upon its crown, 
 it will respond to each with a sharp sensation of pain, but 
 the patient cannot tell which is hot and which is cold. It 
 is the sudden change that produces the reaction. This is 
 the basis of very important differential diagnoses, for, us is 
 true with most organs, in pathologic conditions its sensory 
 function is exaggerated. 
 
 Histogenesis. — The dental pulp is the remains of the dental 
 papilla. The dental papillae for the temporary teeth appear- 
 ing in the mesodermal tissue of the jaw arches very early in 
 fetal life. The cellular elements are at first very closely 
 placed and large, but they grow smaller and take on the 
 typical form of pulp cells as the intercellular substance is 
 increased. By the sixteenth week the dental papillae for 
 the temporary teeth are covered by a layer of tall columnar 
 cells, which will begin the formation of dentine about that 
 time. After the beginning of dentine formation the trans- 
 ition from the dental papillae to the dental pulp is very 
 gradual, and it would be impossible to draw any sharp line 
 of demarcation between them. 
 
204 
 
 DENTAL PULP 
 
 Structural Elements. — The structural elements of the den- 
 tal pulp are: 
 
 1. Odontoblasts. 
 
 2. Connective-tissue cells. 
 
 3. Intercellular substance. 
 
 4. Bloodvessels. 
 
 5. Nerves. 
 
 The Odontoblasts. — The odontoblasts are tall columnar 
 cells which form the outer layer of the pulp adjacent to the 
 dentine, and from which cytoplasmic fibrils extend into the 
 dentinal tubules. 
 
 Fig. 156 
 
 Odontoblasts and forming dentine: E, forming enamel; D, forming dentine; O, 
 odontoblasts; Dp, body of dental papilla. (From photomicrograph by Rose.) 
 
 The character of the odontoblasts changes very greatly 
 with the age of the tissue, and the activity of dentine forma- 
 tion. While the primary dentine is being formed they are 
 tall columnar cells, each containing a large oval nucleus, 
 
THE ODONTOBLASTS 205 
 
 rich in chromatin and located in the pulpal third of the 
 cell. From the dentinal end of the cell cytoplasm is con- 
 tinued, without any line of demarcation, into the dentinal 
 tubule as the dentinal fibril. In some instances two fibrils 
 may be sent from a single odontoblast. The character of 
 the odontoblast is beautifully seen in Fig. 156, a photograph 
 by Professor Rose. 
 
 Fig. 157 
 
 r t*:* T 
 
 
 
 % „«l 
 
 ^ts> g^ j^'i^ ..^^M 
 
 Odontoblasts. The section cuts obliquely through the odontoblasts: F, fibrils; 
 A^, nuclei of odontoblasts; A'', nuclei of connective-tissue cells; W, lajer of Weil, 
 not well shown. (About SO X) 
 
 After the tooth is erupted, but while the formation of 
 dentine is actively going on, the odontoblasts, while some- 
 what smaller, retain the same typical appearance. They may 
 be easily demonstrated either in decalcified sections or by 
 removing pulps from the pulp chamber of freshly extracted 
 teeth. Professor Salter has described two sets of processes 
 besides (Fig. 157) the dentinal fibril process. As a result of 
 teasing the fresh pulps, he considered that fine projections of 
 
 W 
 
206 
 
 DENTAL PULP 
 
 Fig. 158 
 
 Diagram of odonto- 
 blasts and dentinal 
 fibrils. (C.H.Stowell.) 
 
 the protoplasm extended from the sides 
 of the cells, uniting them to the adjoining 
 odontoblasts (Fig. 158). These he called 
 the lateral processes. He also described 
 cytoplasmic projections from the pulpal 
 end of the odontoblasts into the laj'er of 
 Weil. It is probable that these appear- 
 ances were the result of teasing, and are 
 not true structural characteristics, as the 
 work of other investigators has not con- 
 firmed their presence. It is easy to under- 
 stand how teasing the cells apart might 
 produce appearances which might be inter- 
 preted as processes, but careful work upon 
 sections does not show their presence. 
 
 In old pulps where the formation of 
 dentine has been intermittent and very 
 infrequent for a long time, the odonto- 
 blasts are smaller, lose their columnar form 
 more or less, and become pear-shaped or 
 globular. 
 
 As dentine is one of the most highly 
 specialized connective tissues, the odonto- 
 blasts are among the most highly differen- 
 tiated connective-tissue cells. They are 
 the only connective-tissue cells of colum- 
 nar form. Morphologically, they are very 
 similar to columnar epithelium, but epi- 
 thelial cells never have such processes as 
 the dentinal fibrils. Occasionally, in young 
 and actively growing bone, osteoblasts are 
 found which are distinctly columnar in 
 form, but they are never as tall as the 
 odontoblasts, and the nucleus is more 
 nearly in the centre of the cell. In the 
 case of the osteoblast the cytoplasmic 
 processes which extend into the canaliculi 
 correspond to the dentinal fibril process 
 of the odontoblast. ' The homologies be- 
 tween the osteoblasts and the odonto- 
 
THE MEMBRANA EBORIS 207 
 
 blasts have often been lost sight of in the discussions over 
 the character of the latter and their relation to the forma- 
 tion and sensitiveness of the dentine. 
 
 The Membrana Eboris. — The odontoblasts form a single 
 layer of cells on the surface of the pulp in contact with the 
 dentine. This layer was very early recognized to be related 
 to the formation of the dentine, and was called the memhrana 
 eboris, or the membrane of the ivory. The name has no 
 importance now except as it is found in the literature. 
 
 Size of the Odontoblasts. — From what has been said it will 
 be recognized that the size and shape of the odontoblasts 
 vary greatly in different sections. This is true not only of 
 pulps from different animals, and pulps at different periods 
 of development, but of different parts of the same pulp. In 
 the coronal portion of a pulp from a fully developed tooth, 
 but one in which the formation of dentine is still going on, 
 the average measurements would be about 5/^ in diameter 
 and 25 to 30/^- in height. During early stages of dentine 
 formation, before the crown is fully formed, they are con- 
 siderably larger and taller, and in the pulps of a calf they are 
 much larger than in smaller animals and man. In a con- 
 stricted pulp, as, for instance, in the mesial root of a lower 
 first molar, the odontoblasts on the constricted sides will 
 be shorter and relatively thicker than on the buccal and 
 lingual, where the long axis of the cell is in the direction of 
 the long diameter of the pulp, but this simply means that 
 the formation of dentine on the constricted side is relatively 
 farther advanced than on the buccal and lingual, and the 
 cells show older phases. It is evident that the supply of 
 nourishment to the cells in the constricted portions is more 
 imperfect, and that the ones farthest from the main vessels 
 are most affected, so that dentine formation is slowed and 
 made more imperfect here, while it still continues in full vigor 
 around the expanded portions of the pulp. This has been 
 spoken of in connection with the study of the dentine (see 
 Figs. 145 and 146). 
 
 Origin of the Odontoblasts. — The odontoblasts are special- 
 ized connective-tissue cells. It is therefore to be expected 
 
208 DENTAL PULP 
 
 that they should be formed from undifferentiated connective- 
 tissue cells as osteoblasts are formed from similar cells of the 
 inner laj^er of the periosteum. The odontoblasts are there- 
 fore developed from embryonal cells deeper in the pulp which 
 take their place in the odontoblastic layer. This probably 
 explains the appearance of some sections, and also, the author 
 believes, the views of some men in regard to the odontoblasts 
 and the dentinal fibrils. In some sections from old pulps the 
 odontoblasts seem to be in an incomplete layer, and their 
 form is more like that of typical connective-tissue cells. 
 
 Connective-tissue Cells. — The cells in the dental pulp, 
 aside from the odontoblasts, are typical connective-tissue 
 cells such as are found in embryonal tissue. They are of 
 three forms — round, spindle-shaped, and stellate. In the 
 crown or bulbous portion the cells are mostly stellate, while 
 in the root portion they are largely spindle-shaped, with the 
 axis of the spindle parallel with the canal. It seems difficult 
 for students to get an idea of their arrangement, and the 
 nucleus is often mistaken for the entire cell. The cells do not 
 lie in contact in a compact tissue, but are widely scattered 
 in the intercellular substance. There is a small ovoid 
 nucleus, which takes the stain deeply, surrounded by a mass 
 of granular protoplasm stretching away into very fine threads. 
 In the spindle-shaped cells the protoplasm is stretched out 
 in only two directions. In the stellate cells there may be 
 three, four, or more, stretching away in any direction. Plate 
 VIII was very carefully drawn with the camera lucida so as 
 to represent accurately the number, size, and position of the 
 cells in that field as seen with the yV oil immersion. It is 
 very difficult in a drawing to represent the third dimension of 
 space, and to show that some of the processes are extending 
 in a plane at right angles to the paper. An idea of this can 
 only be obtained by the very careful use of the fine adjust- 
 ment while studying the cells with the high power. 
 
 The round cells are probably white blood corpuscles or 
 undifferentiated connective-tissue cells which may develop 
 either into stellate or spindle-shaped. 
 
PLATE VII 
 
 . ft* 
 
 
 ^ * 
 
 v^ 
 
 
 A Field from the Coronal Portion of the Pulp froni 
 a Hunian Molar. 
 
 In the corner the stage micrometer shows y^o of ^ niilHmeter drawn 
 vvith the same^lens. The field sho^A^s the branching of a bloodvessel and 
 the connective-tissue cells of the pialp. Drawn from. J^ oil-ininiersion 
 lens with camera lucida. (About 12,000 X) 
 
THE BLOODVESSELS 209 
 
 The Arrangement of the Cells. — Immediately beneath the 
 layer of odontoblasts, for a space about one-half or two-thirds 
 as wide as the odontoblastic layer, the cells are very scarce, 
 making a clear line in many sections. This is known as the 
 layer of Weil, and contains many fine nerve fibers which 
 are not stained by ordinary methods. Beyond the layer of 
 Weil for a space perhaps twice as wide as the height of the 
 odontoblasts, the cells are very closely placed. Through the 
 remainder of the pulp they are much more widely but 
 comparatively evenly scattered. 
 
 The Intercellular Substance. — Very little is really known 
 about the character of the intercellular substance of the 
 pulp. It contains few fibers, and these in no way resemble 
 bundles of white or elastic connective tissue. The appear- 
 ance in the section is more as if a structureless gelatinous 
 material had been coagulated by the reagents. 
 
 There are, of course, connective-tissue fibers in connection 
 with the walls of the larger bloodvessels and nerves, and to a 
 certain extent in the gelatinous material. In studying the 
 intercellular substance in the sections it is necessary to 
 remember that it is filled with the protoplasmic projections 
 from the cells, and these are stained, appearing like fibers 
 in the matrix. There is need for further investigation of 
 the character of the intercellular substance. 
 
 The Bloodvessles. — The dental pulp is an extremely vas- 
 cular tissue, and the arrangement of the vessels, the structure 
 of their walls, and the nature of the intercellular substance 
 through which they run render the tissue especially sus- 
 ceptible to the pathological conditions which are associated 
 with alterations in the circulation. 
 
 Usually several arterial vessels enter the pulp through 
 foramina in the region of the apex. These vessels have their 
 origin in the rich vascular network of the cancellous bone 
 (Chapter on Peridental Membrane). The arteries follow 
 the central portion of the pulp, giving off many branches 
 as they pass occlusally, and finally form a very rich plexus 
 of capillaries near the surface of the pulp. From these capil- 
 laries the blood is collected into the veins, which follow 
 14 
 
210 
 
 DENTAL PULP 
 
 Fig. 159 
 
 *h 
 
 I 
 
 A section through the apex of a root showing three foraminse, A, B, and C. 
 
THE BLOODVESSELS 
 
 211 
 
 courses parallel to the arteries, leaving the pulp through the 
 same foramina in the region of the apex. It is important to 
 notice that an artery is entering and a vein leaving the tissue 
 
 Fig. 160 
 
 ..• \ 
 
 Diagram of the bloodvessels of the pulp. (Stowell.) 
 
 through very minute canals in the calcified dentine (Fig. 
 159). Dr. Stowell has made a very beautiful diagram of 
 the arrangement of the bloodvessels in a single-rooted tooth, 
 which is shown in Fig. 160. Preparations such as would 
 
212 DENTAL PULP 
 
 reproduce this diagram can be made by injecting the blood- 
 vessels with an inert material and destroying the soft tissues 
 by artificial digestion. 
 
 Structure. — The delicacy of the walls of the bloodvessels 
 is one of the most striking histologic characteristics of the 
 dental pulp. The largest arteries show only a few muscle 
 fibers in the media and a very slight condensation of fibrous 
 tissue for an adventitia. There is no distinct boundary 
 between the capillaries and the veins, and the vessels con- 
 tinue to have only a wall of endothelial cells after they have 
 reached a size much greater than that of capillaries. Because 
 of this peculiarity of structure the statement is to be found 
 in many text-books of histology that the largest capillaries 
 in the body are found in the dental pulp. These vessels 
 should probably not be considered as capillaries, but as 
 veins whose walls have the structure of capillaries. Even 
 in the largest veins the media is very imperfect, and there is 
 only a slight condensation of fibrous tissue to represent the 
 adventitia. This peculiarity of the bloodvessel walls in the 
 pulp renders the tissue peculiarly susceptible to hyperemia 
 and inflammation. 
 
 Fig. 161 is a photograph of a bloodvessel whose size can 
 be estimated from the number of red corpuscles seen in it, 
 and the wall is made up of a single layer of endothelial 
 cells. There is no indication of either media or adventitia. 
 The intercellular substance of the pulp being of gelatinous, 
 semifluid character, gives no support to these delicate walls. 
 
 In Plate VIII the author has drawn very carefully, with 
 the camera lucida, using a yV immersion lens, a field showing 
 the branching of a small bloodvessel. The size of the endo- 
 thelial cells, position of their nuclei in the wall of the vessel, 
 and the size, position, and shape of the connective-tissue cells, 
 are represented as accurately as possible. The field is from 
 the coronal portion of the pulp of a human molar. The caliber 
 of such a vessel as this would depend almost entirely upon 
 the blood pressure. The endothelial cells will stretch to a 
 very considerable extent under increased pressure, becoming 
 very thin at all points except around the nucleus. When 
 
THE BLOODVESSELS 
 
 213 
 
 the pressure is decreased the contractiHty of the protoplasm 
 pulls the cells together, making it thicker and less in diameter. 
 It is very important to remember these facts in connection 
 with hyperemia of the dental pulp. It is difficult in such 
 
 Fig. 161 
 
 A pulp bloodvessel, showing the thin wall: C, blood corpuscles in the vessel; 
 Bl, bloodvessel wall showing nuclei of endothelial cells; N, nuclei of connective- 
 tissue cells in the bodj- of the pulp; I, intercellular substance, showing a few 
 fibers. (About 200 X) 
 
 an illustration to give any representation of the third dimen- 
 sion of space, which is essential to a real understanding of 
 the connective-tissue cells of the pulp. These are bits of 
 cytoplasm with a nucleus forming a small irregular central 
 
214 DENTAL PULP 
 
 mass, from which the cytoplasm is stretched away in all 
 directions through the intercellular substance, ending in 
 very fine threads. 
 
 Plate IX is drawn in the same way from a transverse section 
 of the pulp of an unerupted tooth of a sheep. The vessels 
 are all cut transversely and are seen crowded with red blood 
 corpuscles. They are not distended, and some show slight 
 condensation of fibrous tissue around them. 
 
 In a normal pulp there are many capillaries so small that 
 a single corpuscle passes them with difficulty, but in patho- 
 logic conditions they become distended to many times 
 their normal diameter. All investigators have agreed in 
 finding no lymphatic vessels in the pulp. This is also an 
 important fact in connection with pathologic conditions. 
 
 The Nerves of the Dental Pulp. — Few subjects in connection 
 with dental histology have received more attention than 
 the distribution of the nerves of the dental pulp, especially 
 in relation to the sensitiveness of the dentine. Support 
 for almost any idea can be found in the literature, but many 
 of the conditions described have been shown to be errors in 
 microscopic interpretation, and many others have failed to 
 receive support by reinvestigation. The most recent work 
 upon this subject was done ten or twelve years ago by 
 Prof. Carl Huber, of Ann Arbor. The author has repeated 
 some of his work, and has never seen any specimen that 
 was contradictory to his statements. Usually three or four 
 nerve trunks enter the dental pulp through the foramina. 
 These contain from eight or ten to thirty or forty medullated 
 nerve fibers. They pass occlusally through the central 
 portion of the pulp, but almost immediately begin to give 
 off branches, which pass toward the periphery, branching 
 and anastomosing in their course. Most of the fibers lose 
 their medullary sheath very soon after leaving the nerve 
 trunk, proceeding as beaded fibers, made up of an axis 
 cylinder with nuclei scattered along it. A bundle of such 
 fibers, breaking up to be distributed to one horn of the 
 pulp, is shown in Fig. 162. Other fibers retain their medullary 
 sheath, following an independent course through the pulp 
 
PLATE IX 
 
 U. 
 
 \. 
 
 
 ^ % 
 
 '•r 
 
 r\* 
 
 .'J 
 
 /•••' 
 
 ''.^. 
 
 • ♦ 
 
 • • 
 
 ^ « 
 
 
 • 
 
 ^'i 
 
 3.-<^ 
 
 W 
 
 w ... 
 
 
 # ,^ 
 
 V > 
 
 A Field froni the Pulp of an Unerupted Tooth of a Sheep. 
 
 The bloodvessels are cut transversely. (About lOOO X) 
 
THE NERVES OF THE DENTAL PULP 
 
 215 
 
 tissue, until they reach the layer of Weil, where the sheath is 
 lost and they join the plexus of beaded fibers Wmg in this posi- 
 tion (Fig. 1G3). From the plexus in the layer of Weil beaded 
 fibers are given off, passing between and around the odonto- 
 blasts, forming a network around each cell, and even passing 
 over on to the end of the cell between it and the dentine, 
 but they have never been followed into the dentinal tubules. 
 In no instance and by no method that he has employed, has 
 Dr. Huber been able to demonstrate nerve fibers in the 
 dentinal tubules. 
 
 Fig. 162 
 
 
 x^^^^ 
 
 Nerve fibers in pulp from a human mol 
 
 (About 500 X) 
 
 The sensitiveness of the dentine, in view of these obser- 
 vations, is due to the presence of living fibrils, connected 
 with living odontoblasts which are in physiologic connec- 
 tion with nerve fibers. It is interesting to note that this is 
 the only instance in which a connective-tissue cell is inter- 
 mediate between the outside world and the nerve fiber. In 
 
216 
 
 DENTAL PULP 
 
 all other instances an epithelial cell is intermediate between 
 the environment and the nervous system. The sensitiveness 
 of the dentine is therefore due to the irritability of the cyto- 
 plasm of the fibril, transmitted through the continuity of 
 cytoplasm to the odontoblasts and their reaction upon the 
 surrounding nerve fibers. The irritation to the fibril may 
 be either traumatic, chemical, or thermal. For instance, salt 
 is sprinkled on exposed living dentine, and a sharp sensa- 
 tion of pain is the result. It may be supposed that chemical 
 
 
 Fig. 163 
 
 
 A 
 
 
 h •■ 
 
 B 
 
 
 u ■ 
 
 C 
 
 
 H 
 
 D 
 
 A ,| 1 j \ I:]:-: I 
 
 
 Rose's diagram of nerves and bloodvessels of the pulp. 
 
 changes are set up in the cytoplasm of the fibril which excite 
 changes in the cytoplasm of the odontoblasts. These react 
 upon the cytoplasm of the nerve fiber, and so are transmitted 
 to the nerve centre, being recognized, in consciousness, as a 
 sensation of pain. In the same way traumatic irritation 
 caused, for instance, by the cutting of dentine with a steel 
 instrument sets up changes in the fibril in the same fashion. 
 It is impossible to conceive of any vital activity of cyto- 
 plasm otherwise than as a form of chemical action or molec- 
 ular or atomic movement of its substance. 
 
THE NERVES OF THE DENTAL PULP 217 
 
 Certain clinical facts are well explained by these structural 
 facts. It is often noted in the preparation of cavities that 
 the dentine is most sensitive at the dento-enamel junction. 
 This would be expected when it is recalled that at the dento- 
 enamel junction the dentinal tubules fork and the fibrils 
 anastomose, so that an irritation to a few fibrils is not 
 simply transmitted to their odontoblasts and the nerve 
 endings in contact with them, but to all the fibrils, and so to 
 the nerves in contact "^ith all of the odontoblasts. The 
 presence of dilute acids render the cytoplasm of the fibrils 
 much more irritable. The dentine in a carious condition 
 is, therefore, much more sensitive than that in a sound or 
 normal area. The sensitiveness of extremely hypersensitive 
 dentine can often be greatly reduced, if not entirely over- 
 come, by cleansing the cavity thoroughly, washing with 
 tepid water followed by a dilute alkali, drying and sealing 
 for a few days, when it will be usually found that excavation 
 can be carried out without excessive pain. The sealing must 
 be perfect. If it is leaky the cavity will be more sensitive 
 than ever at the end of the delay. 
 
 Teeth in which the size of the pulp chamber has been 
 reduced by the formation of secondary dentine are usually 
 much less sensitive. By this formation, as has been seen 
 in the chapter on dentine, many of the tubules are cut off 
 and man}' of the fibrils reach the pulp only by anastomosing 
 with a few in the later formed dentine. The transmission 
 to the nerves of the pulp is thus made more difiicult and 
 imperfect. In all considerations of the sensitiveness of 
 dentine, the purely subjective and hysterical symptoms 
 must be carefully watched for. In many cases slight sensa- 
 tions are so magnified by fear and expectation as to be 
 considered intolerable. In such cases the diversion of 
 attention and the skilful use of suggestions are of more 
 value when coupled "\\'ith delicacy of manipulation and opera- 
 tive skill than any means of obtunding. In such cases, 
 although the operator is positive that the sensations are 
 slight, it ^^^ll never do any good to tell the patient so, or to 
 argue that what is being done cannot hurt. They must be 
 
218 DENTAL PULP 
 
 made to believe fully that something has been done to 
 destroy the sensitiveness, and then the attention must be 
 concentrated upon something, while the excavation is 
 lightly and skilfully performed. It makes very little 
 difference what is done, but it must attract the attention in 
 order to plant the belief that the sensitiveness has been 
 removed, and then the attention must be diverted until 
 the manipulation is completed. 
 
 The nerves of the pulp not only respond with sensations 
 of pain from the irritation of the fibrils in the dentinal 
 tubules, but because of their confinement in a calcified 
 chamber and the semifluid nature of the tissue, they are 
 very sensitive to pressure, either increased or decreased. 
 The normal response to changes of temperature, as well as 
 most of the pain in pathologic conditions of the pulp, 
 are probably caused by changes of pressure, through dis- 
 turbance of the blood circulation of the tissue. The nerves 
 of the pulp control the walls of the arteries through the 
 vasomotor reflexes, and also by trophic fibers control the 
 functional activity of the odontoblasts in the formation of 
 the dentine. 
 
 In a single tooth the irritation resulting from a carious 
 cavity is found to cause the formation of dentine not simply 
 in the region reached by the irritated fibrils, but upon the 
 entire wall of the pulp chamber and apparently also in other 
 teeth. It has seemed possible to the author that in some 
 instances osmotic conditions might be a factor in the pro- 
 duction of pain in the pulp, especially in the early stages of 
 caries. 
 
CHAPTER XVI 
 
 STRUCTURAL CHANGES IX THE PATHOLOGY OF THE 
 
 PULP 
 
 Because of its structural peculiarities, as well as the fact 
 that it is a tissue of embryonal character whose function has 
 been chiefl}' performed, the dental pulp is specially sus- 
 ceptible to certain pathologic conditions which produce 
 structural changes. These conditions are hyperemia, inflam- 
 mation, suppuration, and various forms of tissue degener- 
 ation. The dental pulp ofters specially good opportunities 
 for a study of the tissue changes that are characteristic of 
 these conditions, because in the normal state the tissue ele- 
 ments are comparatively widely scattered in an almost struc- 
 tureless intercellular substance. The changes in the blood- 
 vessels, the passage of cellular elements of the blood through 
 the bloodvessel walls and what becomes of them after they 
 enter the tissue can therefore be followed more easily than 
 in tissues which are crowded with cells. 
 
 HYPEREMIA 
 
 Hyperemia is defined as an increased amount of blood 
 in a part. It is usually divided into active and passive. 
 In active hyperemia the increase in the amount of blood 
 is due to the enlargement of the arteries supplying the part, 
 or the increase of blood pressure, or both. In passive 
 hyperemia it is due to an obstruction of the veins, so that 
 the blood is not allowed to escape as freely from the part. 
 In case of the dental pulp the conditions which cause an 
 active hyperemia produce, at the same time, a passive one. 
 H^^eremias are also classified as acute and chronic. 
 
220 STRUCTURAL CHANGES IN PATHOLOGY OF PULP 
 
 Acute Hyperemia. — It is one of the most important of the 
 pathologic conditions of the pulp, because it is one of the 
 most common, and is often the first of a series which result 
 in the final loss of the organ. It is this condition of the pulp 
 which most commonly calls the patient's attention to the 
 presence of a carious cavity. The destruction of the tooth 
 tissue as well as the irritation of the dentinal fibrils by the 
 acid produced, increase the irritability of the cytoplasm of the 
 odontoblasts, and the normal sensory function of the pulp is 
 greatly exaggerated. The response to sudden changes of 
 temperature which constitutes the normal sensory function of 
 the pulp is, in fact, a momentary acute hyperemia, which is 
 immediately recovered from by the return of the normal 
 caliber of the arteries. The reaction is brought about by the 
 vasomotor nerves which control the arteries. As soon as 
 the artery dilates a greatly increased amount of blood is 
 poured into the tissue, and all of the minute capillaries are 
 distended to three or four times their size (Figs. 164 and 
 165). Because of the semifluid character of the intercellular 
 substance, the pressure is transmitted to the nerves, and a 
 sharp lancinating pain is the result, which lasts until the dis- 
 tention of the bloodvessel subsides, which occurs in a few 
 seconds in normal conditions. 
 
 When a tooth is exposed continuously to sudden changes 
 of temperature, or to excessive heat, the function becomes 
 greatly exaggerated, and changes which would ordinarily 
 produce no effect will produce acute hyperemia. When the 
 irritability of the fibrils and the odontoblasts has been greatly 
 increased by the action of irritating agents, as in the progress 
 of caries, or when the thickness of the protecting dentine 
 has been greatly reduced, as in abrasion, or when consider- 
 able masses of gold are separated from the pulp only by a 
 thin layer of dentine, the same conditions result. 
 
 In this stage of hyperemia the only change in the tissue 
 that can be observed under the microscope is the distention 
 of the capillaries and veins, and as soon as the pain has 
 passed the tissue returns to a normal condition. 
 
 It is apparently, therefore, a functional disturbance, due 
 
ACUTE HYPEREMIA 
 
 221 
 
 to the increased irritability of the cytoplasm of the fibrils, 
 odontoblasts, and probably also of the nerve endings. The 
 rational treatment for such conditions is the removal of the 
 
 Fig. 
 
 164 
 
 
 .'.'l^B 
 
 m 
 
 r. 
 
 
 T\ 
 
 I 
 
 ■''V ' 
 
 
 ^m 
 
 %^' 
 
 Acute hyperemia. 
 Fig. 165 
 
 Acute hyperemia, higher power. 
 
222 STRUCTURAL CHANGES IN PATHOLOGY OF PULP 
 
 irritation which has caused the irritabiUty, and the complete 
 protection of the tooth from thermal change, until the rest 
 restores the normal function. 
 
 In order to observe the structural changes, the tooth must 
 be extracted during the paroxysm of pain, and should be 
 cracked and dropped at once into a fixing fluid, allowed to 
 remain there for about twenty-four hours, when the pulp 
 can be removed from the pulp chamber and embedded and 
 sectioned. In this w^ay the injection of the bloodvessels will 
 be preserved, and all of the capillaries and veins w^ill be 
 found crow^ded with corpuscles, and their distention will be 
 proportionate to the severity of pain at the time of the 
 extraction. 
 
 Acute hyperemia has two possible terminations aside 
 from recovery: (1) If often repeated, it may pass over into 
 chronic hyperemia; (2) if severe enough, it may end in 
 infarction. 
 
 Chronic Hyperemia. — When the paroxysms of acute 
 hyperemia are often repeated the endothelial cells of the 
 bloodvessel walls lose their power of contractility, and the 
 vessels remain permanently dilated. In this condition 
 the bloodvessels are dilated, even between the paroxysms of 
 pain, and thin w^alls often shoW' pouches and varicosed 
 enlargements. (See Black, American System of Dentistry, 
 vol. i, p. 846.) During the paroxysms of pain, both red 
 and w^hite blood corpuscles are forced through the blood- 
 vessel walls, and areas of breaking down red blood corpus- 
 cles may be found in the tissue. The number of cellular 
 elements in the pulp becomes greatly increased. Whether 
 this increase is due to the multiplication of the connec- 
 tive-tissue cells of the pulp or to the development into 
 tissue elements of w^hite blood corpuscles is a matter upon 
 which opinions differ. In the study of such specimens 
 the author has been unable to escape the feeling that many 
 of the white blood corpuscles develop into tissue elements, 
 w^hich, however, have not the form of the typical connective- 
 tissue cells of the pulp. 
 
 Fig. 166 is a photograph of a section of such a pulp, and 
 
INFARCTION 
 
 223 
 
 by comparison with Fig. 167 the increase in the number of 
 cellular elements is very apparent. 
 
 Chronic hyperemias are usually followed after exposure 
 of the pulp by inflammation and suppuration, but if this 
 
 Fig. 166 
 
 Chronic hyperemia, showing increase of cellular elements 
 
 is prevented by the treatment of the cavity, the tissue is 
 likely to undergo fibrous or other degeneration. 
 
 Infarction. — Complete infarction of the dental pulp is 
 rare, but partial conditions are not uncommon. The con- 
 dition is comparable to the conditions that occur in the brain 
 
224 STRUCTURAL CHANGES IN PATHOLOGY OF PULP 
 
 and other places supplied by end arteries, without anasto- 
 mosis, when the vessels carrying the blood from the part are 
 completely occluded. 
 
 The following clinical picture will occasionally be encoun- 
 tered with such a history. A tooth beginning to ache 
 suddenly, perhaps, because of a sudden exposure to change 
 of temperature, continues to ache violently several hours, 
 the pain being described as acute and lancinating, and so 
 severe as to be almost intolerable. Nothing done relieves 
 the symptoms in the least. Finally, the pain stopped almost 
 as suddenly as it began. The next morning the tooth is 
 more or less red in color, and by the time it reaches the 
 operator it begins to turn dark. What has happened is 
 represented in the following tissue changes. An extremely 
 acute hyperemia has occurred, the dilatation of the arteries 
 entering the apical foramina have compressed the more 
 delicate walls of the veins so as to occlude them completely, 
 and greatly increased blood pressure has distended all of 
 the capillaries and veins, forcing the red and white blood 
 corpuscles, as well as the serum, through their walls, filling 
 the tissue. Complete stasis has resulted after a few hours 
 in the death of all the tissue elements, at which time the 
 pain stopped. The serum has dissolved the hemoglobin 
 from the red blood corpuscles, filtered through the dentinal 
 tubules, discoloring the dentine and showing through the 
 enamel. Small areas of partial infarction are found in many 
 specimens after severe paroxysms of acute hyperemia^ which 
 may be recovered from entirely. 
 
 The severe pain which occasionally results from the 
 application of arsenic for the devitalization of pulps is due 
 to the acute hyperemia which is induced. The removal of 
 the arsenic application will not alleviate the pain, which 
 can be subdued only by the immediate extirpation of the 
 pulp. 
 
 Inflammation. — Inflammation of the pulp occurs ordin- 
 arily only after exposure, and follows a chronic suppurating 
 course, progressing along the veins; the line of demarcation 
 between the normal and inflammatory areas often being 
 
PULP NODULES 225 
 
 quite sharply marked. In the first stages the white cor- 
 puscles are seen along the walls of the vessels, passing through 
 the walls into the tissue in increasing numbers until the 
 tissue becomes a solid mass of cells and serum breaking 
 down into pus. This progresses until the entire tissue is 
 destroyed. There is the greatest difference in the rapidity 
 with which the stages follow each other and the extent to 
 which the inflammation spreads through the tissues before 
 the breaking down begins. This is probably due both to 
 the character of the invading microorganisms and the 
 resistance of the individual. These conditions are illus- 
 trated in Figs. 167, 168, 169, 170, 171, and 172. The for- 
 mation of pulp nodules is often noted in the deeper part of 
 the tissue in which inflammation is progressing (Fig. 169). 
 Occasionally centres of inflammation, progressing to abscess 
 formation, are found within the substance of the pulp (Fig. 
 174). These are apparently true intrapulpal abscesses and 
 present the characteristics of miliary abscesses in any other 
 tissue. 
 
 Degeneration. — The embryonal character of the pulp 
 tissue renders it specially susceptible to degenerative changes, 
 but the degenerative changes of the dental pulp have never 
 been adequately studied. It is extremely difficult to obtain 
 material. Teeth without histories are practically useless, 
 and large numbers of specimens are necessary. 
 
 Pulp Nodules. — In the cutting of large nuqabers of pulps 
 for the preparation of class work the author has been im- 
 pressed by the frequency with which hard nodules occur 
 in the tissue. These are apparently of several varieties, 
 some of which are calcified and others are not. They usually 
 occur in the coronal portion of the pulp near the opening 
 of the canals. They often occur in specimens in which the 
 tissue is otherwise normal. Fig. 176 shows a section with a 
 small, almost spherical nodule in the centre of the lower 
 part of the coronal portion. For a number of sections the 
 nodule was cut through as if it had a soft periphery, then it 
 took a nick out of the razor and was pulled out of the tissue, 
 the subsequent section showing the hole it had left (Fig. 177). 
 15 
 
226 STRUCTURAL CHANGES IN PATHOLOGY OF PULP 
 
 Fig. 167 
 
 Beginning inflanunation. 
 
 ^^"^ - - ^— ^-->-o<^/<r^^^;^^^ 
 
 ■V^'''--. 
 
 
 
 Minute inflammatory focus within the tissues of the pulp: a, a, arterial twigs; 
 b, a nerve bundle; c, collection of leukocytes. (Black.) 
 
PULP NODULES 
 
 227 
 
 Fig. 169 
 
 Section of dental pulp, showing the invasion of the inflammatory process along 
 the veins and the diapedesis of white blood corpuscles. 
 
 Fig. 17C 
 
 Inflammation of the pulp, showing pulp nodules. 
 
228 STRUCTURAL CHANGES IN PATHOLOGY OF PULP 
 
 Fia. 171 
 
 Inflammation of the pulp. 
 Fig. 172 
 
 y^ 
 
 '^^y^'- 
 -^'^4^, 
 
 '-i^-^^'r^; 
 
 Development of inflammatory tissue elements in the pulp: a, normal cells; 
 h, inflammatory elements; c, cells in process of division. (xVobj.) 
 
PULP NODULES 
 
 229 
 
 Fig. 173 
 
 
 
 
 m^ 
 
 
 
 
 Progressive suppuration of the pulp of an incisor: a, healthy tissue; 6, odonto- 
 blast laj'er, or membrana eboris; c, inflamed tissue, in which the veins are seen to 
 be dilated; d, line of demarcation of the suppurative process; e, pus. A part of the 
 crown portion of the pulp had been destroyed by suppuration, and in the remaining 
 portion it will be noted how the pulp is hollowed out, the process pursuing the 
 course of the veins and converging to the centre. (100 X, reduced.) (Black.) 
 
Fia. 174 
 
 Tntiapulpal abscess. 
 Fig 175 
 
 
 
 Abscess within the tissue of the pulp; the field includes about one-half of the little 
 pocket of pus. (About 250 X) (Black.) 
 
PULP NODULES 
 
 231 
 
 These conditions have been repeated many times in the 
 cutting of sections. In connection with inflammatory con- 
 ditions, nodules are often found in the deeper portion. 
 
 Fig. 176 
 
 Pulp nodules. 
 
 These are apparently calcoglobulin, and are to be compared 
 with the formation of phleboliths in the varicosed veins. 
 
 The nodules in the coronal portion of the pulp are usually 
 irregular in form and more or less nodulated. They present 
 
232 STRUCTURAL CHANGES IN PATHOLOGY OF PULP 
 
 Fig. 178 
 
 A small pulp nodule, as seen with a low power, showing its nodulation : 
 a represents the natural size. (15 X) (Black.) 
 
 Fig. 179 
 
 Section of a pulp nodule, showing many calcospherites, as pointed out by o, a 
 
 (Black.) 
 
PULP NODULES 
 
 233 
 
 an infinite variety of size, shape, and number. They often 
 contain calcospherites embedded in a granular, structureless, 
 calcified mass (Figs. 178 and 179). The calcospherites have 
 
 Fig. 180 
 
 Pulp nodules in the canal portion of the pulp. (15 X) (Black.) 
 Fig. 181 
 
 Nodules in root. A photomicrograph of the section from -which Fig. ISO was drawn. 
 
234 \STRUCTURAL CHANGES IN PATHOLOGY OF PULP 
 
 a small point at the centre and concentric rings around it, 
 but they usually make up a smaller portion of the nodule. 
 The nodules in the root portion are usually rounded in out- 
 line and completely calcified Figs. 180 and 181). 
 
 Fia. 182 
 
 Dental tumor within the pulp chamber: A, diagram of the tooth, with dotted line 
 showing the position of the section B. In B the pulp chamber is shown in section, 
 nearly natural size, showing the tumor within. C is an illustration of the tissue of 
 the tumor: a, a, the primary dentine; b, irregular tubules connecting the newgrowth 
 with the primary dentine — most of these are very dark and irregular; c, a calco- 
 spherite included in the mass; d, apparently a bloodvessel calcified; e, calcified tissue; 
 /, a finely granular mass; g, a spur of very transparent dentine. Dentinal tubules 
 appear at h, h. (Black.) 
 
 With the exception of the formation of calcoglobulin in 
 connection with inflammation, the author has never seen any 
 indication that pulp nodules were associated with patho- 
 logic conditions. They are apparently more common in 
 
PULP NODULES 235 
 
 the pulps of old and middle-aged people and are continually 
 found in the pulps from teeth that give no history of trouble. 
 They are apt to be found in mouths where there has been 
 considerable abrasion, or where dentine has been exposed by 
 caries; but they are just as apt to occur in the teeth that have 
 for some reason escaped, the irritation of one tooth causing 
 the deposits in the pulps of others as well as the one affected. 
 There seems to be a relationship between the irritation of 
 dentinal fibrils and these formations in the pulp. 
 
 Dr. Black has classified the hard formations occurring 
 within the pulp chamber under the following six heads : 
 
 1. Secondary dentine, a new growth of dentine, more or 
 less regular in formation, excited by abrasion, decay, or 
 other injury, by which the dentinal fibrils are subjected to 
 irritation at their distal ends. This has already been con- 
 sidered under the headings both of the dentine and the 
 pulp. 
 
 2. Dental tumor within the pulp chamber; an erratic 
 growth of dentine into the pulp chamber, united to the wall 
 by a pedicle. The structure is usually very irregular. These 
 are comparatively uncommon (Fig. 182). 
 
 3. Nodular calcifications among but not of the pulp tissue; 
 these are the irregular nodulated masses so frequently seen 
 either as large or small pulp stones. They contain many 
 calcospherites. 
 
 4. Interstitial calcifications of the pulp tissue; this is the 
 counterpart of calcifications elsewhere in the body, as in the 
 artery walls. 
 
 5. Cylindrical calcifications of the pulp, the tissues of 
 which are probably in a state of fibrous degeneration, usually 
 seen in the pulp canals (the so-called lead wire pulp). 
 
 6. Osteodentine; erratic formations showing both the 
 lacunse of bone and dentinal tubules. 
 
CHAPTER XVII 
 
 INTERCELLULAR SUBSTANCES 
 
 During the last hundred years, knowledge of living things 
 and all thought of their structure and function has entirely 
 changed. The cell theory has abundantly established that 
 the cell is the structural and functional unit of all living 
 objects, both plant and animal, and that all manifestations 
 of life are accomplished by the chemical activity of the 
 substance of the cell, which Huxley long ago designated as 
 "The physical basis of life." From a consideration of the 
 physical properties of cytoplasm, nothing is more apparent 
 than that the production of a highly organized body out 
 of it alone would be impossible. If the human body were 
 composed entirely of cytoplasm it would be a shapeless lump 
 of jelly. It is only by the production of material which has 
 physical properties of strength and rigidity through the 
 activity of the cytoplasm that the shape and function of a 
 highly organized creature is possible. This is accomplished 
 through the metabolism of the cytoplasm more or less 
 analogous to the building up of a secretion by the cells of a 
 gland, though there is no intention to suggest any direct 
 comparison between the two. In other words, all tissues are 
 made up of cells and intercellular substance, and the vital 
 characteristics are given to the tissue by the cells, the 
 physical characteristics by the intercellular substance. These 
 intercellular or extracellular materials possess none of the 
 vital manifestations, and are entirely dependent upon the 
 cells for their formation and maintenance. There is appar- 
 ently a constant reaction between the cell and the formed 
 material which constitutes the intercellular substance, for 
 even the most highly specialized of intercellular substances 
 represented by the dentine matrix changes in its properties 
 
INTERCELLULAR SUBSTANCES 237 
 
 if the cells are removed. If the cells in the bone matrix are 
 killed, that portion of the tissue becomes necrosed bone and 
 is as much a piece of foreign matter as if a piece of bone 
 toothbrush handle had been shot into the body. The fibers 
 of fibrous tissue have no ability to grow, to attach themselves 
 to any surface, or even to maintain their present form with- 
 out the presence of living cells or fibroblasts. There has 
 been a great deal of discussion as to the method of forma- 
 tion of intercellular substances by the cells, and the nature 
 of the reaction occurring between the cell and the formed 
 material after it has been produced. In several intercellular 
 substances the material passes through changes both of 
 physical and of chemical character, but these are carried out 
 by reaction with materials formed by the metabolism of the 
 cell, for if the cells are removed the formed material will not 
 go through any such changes. The intercellular substances, 
 therefore, while they are chemically extremely complex, 
 belong to the simplest classes of protein molecules, and have 
 no such complexity of atomic movement producing conditions 
 of recurrent unsatisfied afiinity, without which no idea of 
 the metabolism of living c}i:oplasm can be obtained. Chem- 
 ically, living cytoplasm may be roughly viewed as con- 
 stantly undergoing chemical changes which are almost 
 infinitely complex, and by means of which simpler substances 
 are acted upon and built into its own molecule. Complex 
 combinations are thrown off as products of its metabolism, 
 and simpler substances are formed as decomposition products, 
 or waste materials. Dr. Brooks often used to say in his 
 lectures that the most striking characteristic of living 
 things was their ability to react upon their environment in 
 such a way as to become better and better suited to it. 
 When living cytoplasm which is soft and without the physical 
 properties of strength and rigiditv requires protection from 
 physical influences, substances possessing these qualities 
 are produced by it. Intercellular substances, therefore, were 
 apparently formed by the cytoplasm in response to physical 
 conditions of its environment, and are one of the phases of 
 adaptation. 
 
238 INTERCELLULAR SUBSTANCES 
 
 In the higher forms of animal life the class of tissues 
 which have produced these formed materials, for the purpose 
 of support, rigidity, and connection, are called the connective 
 or supporting tissue. The formed materials are of two 
 classes — those which are to connect associated and dependent 
 parts, and those which give rigidity and protection. The 
 fibrous tissues are of the first class, and are made up of 
 materials possessing strength and elasticity. The bone and 
 cartilage belong to the second class, and give strength and 
 rigidity. The first sustain pulling stress, the latter shearing 
 or bending stress, though both possess a certain amount 
 of each. 
 
 Adaptability and the greatest range of variation are most 
 striking characteristics of connective tissue which develop 
 and change to meet all kinds of requirements of both mechan- 
 ical and physical environment to which they are subjected. 
 These variations are produced by the production of increased 
 amount of the intercellular material, its destruction, or the 
 change of its character, under the influence of the cells of 
 the tissue. No tissue responds more quickly to the demands 
 made upon it by development. When the muscles grow 
 larger and stronger by development, the tendons and the 
 bones to which they are attached change as quickly and in 
 proportion. From the appearance of the skeleton the experi- 
 enced anatomist can picture very accurately the muscular 
 development of the individual to whom it belonged. 
 
 The cell wall of plants may be used as one of the simplest 
 examples of supporting tissue. In this case each cell, in 
 addition to its other functions, produces its own supporting 
 substance. These may be observed in the cells of a growing 
 root tip. Plant an onion, by selecting one larger than a 
 small glass, fill the glass with water, and place the bulb on it. 
 If this is placed in a sunny window, in a few hours little 
 rootlets will be seen stretching down into the water. The 
 rootlets of a sprouting chestnut also make very good material 
 (Fig. 183). If these are embedded in paraffin, the develop- 
 ment of the cells and the formation of their supporting walls 
 can be observed. The young cells near the tip will be found 
 
INTERCELLULAR SUBSTANCES 
 
 239 
 
 to be a mass of granular protoplasm, with a large nucleus 
 in the centre, and a thin wall of cellulose which is the cell 
 organ of support. As the cell increases in size, vacuoles 
 appear in the cytoplasm which become larger and larger. 
 These vacuoles are filled with watery fluid which is not a 
 part of the cytoplasm. If the cell remained a solid mass of 
 cytoplasm, an enormous amount of food material would be 
 required, which would be out of all proportion to the work 
 which the cell is to perform. The vacuoles increase in size 
 
 Fig. 183 
 
 
 fp^rr 
 
 
 K, 
 
 1 
 
 K 
 
 1 
 
 
 J 
 
 ;> 
 
 'W 
 
 
 
 ^. 
 
 ^^$ 
 
 
 t§M 
 
 
 4 
 
 Yf 
 
 
 l^^ 
 
 jtaWia^^ 
 
 Cells from the growing tip of a chestnut seedling. 
 
 with the growth of the cell until there is a rim of cytoplasm 
 in contact with the cell wall, and a central mass of cytoplasm 
 surrounding the nucleus and connected with that at the 
 periphery by fine threads. In still further growth these 
 threads are broken, the nucleus is pushed to one side, and 
 the whole central portion becomes one huge vacuole. There 
 is now a cell wall, with a layer of cytoplasm covering its 
 inner surface, which is kept in reaction with the nucleus by 
 streaming around and around. This flowing of the cytoplasm 
 in plant cells may be easily observed in the dehcate stamen 
 
240 INTERCELLULAR SUBSTANCES 
 
 hairs of the ordinary Spiderwort, or in the cells of the 
 water plants Chara or Nitella, which are easily found in 
 most ponds. In this example it is seen that the cytoplasm 
 remains in contact with the formed material which it produces 
 for support, and that it is only sufficient in amount to form 
 and maintain this material. 
 
 In general histology it has already been noted that the 
 cells of connective tissue are very similar, and that the 
 tissues differ chiefly in the character and arrangement of 
 the intercellular substances. It has also been emphasized 
 that the connective tissues all originate from a common 
 form of embryonal connective tissue, or mesenchyme, and 
 change from one form to another in development. These 
 mutations of the connective tissues are its most striking 
 characteristic, and must be clearly grasped if the bone, as 
 an organ of support, is to be understood. For instance," 
 embryonal connective tissue is transformed into fibrous 
 tissue; fibrous tissue becomes arranged in a definite mem- 
 brane, and is transformed into cartilage, which is again 
 removed and transformed into bone. All these changes take 
 place to meet the requirement of mechanical conditions and 
 influences. 
 
 If the subcutaneous tissue of an embryo be examined in sec- 
 tions (Figs. 184 to 199) the cells will be found to be irregular 
 masses of cytoplasm with a nucleus in the central portion, 
 and fine projections stretching out in all directions through 
 an almost structureless intercellular substance. The fine pro- 
 jections of the cytoplasm meet those of the adjoining cells 
 and form a network holding everything together. Because of 
 the nature of cytoplasm, however, these possess very little 
 strength, and very soon fine thread-like fibers are found 
 appearing in the intercellular substance in contact with 
 cells. These unite with each other, forming continuous 
 fibers, and very soon a strong netw^ork is produced which 
 is entirely dependent upon the cytoplasm of the cell which 
 has formed and maintains it. If this tissue is now subjected 
 to pressure and strain, the cells become flattened out and 
 squeezed between the bundles of fibers, which take on 
 
INTERCELLULAR SUBSTANCES 
 
 241 
 
 parallel directions, and so a tendon is formed. A tendon 
 must be considered as a highly specialized form of connective 
 
 Fig. 184 
 
 Fig. 185 
 
 --KK<', 
 
 ^^:^- 
 
 -^ 
 
 
 ^^i-T-S 
 
 Embn,'onal connective tissue in an early The same, a little more developed, 
 
 stage of development, showing the cellular showing the cellular elements length- 
 elements embedded in the ground sub- ening in a common direction, 
 stance. 
 
 Fig. 186 
 
 ^--^"-ir^^^^--- - 
 
 The cells developed in spindle forms, fibroblasts with long filaments extending 
 from either end. 
 
 Fig. 187 
 
 The developed white fibrous tissue. 
 
 tissue, arranged to supply tensile strength. The degree of 
 specialization of the tissue is judged by the extent to which 
 16 
 
242 
 
 INTERCELLULAR SUBSTANCES 
 
 its characteristic features are developed, either in quantity 
 or quality. In the tendon the fine strong fibers have l)een 
 
 Fig. 188 
 
 ^ 
 
 Older white fibrous tissue, in which the cells are no longer seen, and showing 
 the wave like course of the fibers. 
 
 Fig 189 
 
 Coarse white fibers, made up of bundles of the fine fibers, and showing the mode 
 of division by splitting off of a portion of the fibers of the bundle. 
 
 Fig. 190 
 
 Coarse fiber breaking up into fine fibers. 
 
 gathered into bundles; a round nucleus would occupy too 
 much space. It has, therefore, become elongated and more 
 
INTERCELLULAR SUBSTANCES 
 
 243 
 
 or less rod-shaped, and the cytoplasm has been squeezed 
 out into thin leaf-like projections between the bundles. 
 
 Fig. 191 
 
 
 Cross-sections of coarse fibers, showing some of their various forms. 
 
 Fig. 192 
 
 Fig. 193 
 
 & 
 
 Reticular or elastic fibers, showing the mode of division 
 and the multipolar, or irregular, star forms of the cells 
 at the divisions. 
 
 Fig. 194 
 
 ^•/ 
 
 ^ o 
 
 Cross-sections of 
 the reticular fibers, 
 showing some of 
 their forms. 
 
 Connective-tissue cells from which reticular fibers are developed. 
 
 Each cell is in contact with several fibers, and each fiber in 
 contact with the cytoplasm of cells which have produced 
 them. 
 
244 
 
 INTERCELLULAR SUBSTANCES 
 
 Fig 195 
 
 Fig. 196 
 
 Network of elastic fibers from the point Network of elastic fibers teased out 
 
 of reflection of the mucous membrane of from elastic tendon, and showing the 
 the lip from the gums. usual mode of division. 
 
 Fig. 197 
 
 
 / 
 
 ^s-^ 
 
 ^yf^S' 
 
 Elastic fibers, showing their disposition to curl up when cut or broken. 
 
INTERCELLULAR SUBSTANCES 245 
 
 It must be supposed that there is a constant reaction 
 between the substance of the formed material and materials 
 produced by the metabolism of the c^i:oplasm. In patho- 
 logic conditions the metabolism of the cytoplasm is dis- 
 turbed, and there is a consequent change in the quality of 
 the fibers. So in some pathologic conditions a relaxation 
 and loss of tone is found in tendons and ligaments. In 
 inflammations of the gingivae the fibers become relaxed and 
 stretched, so that the gingivse are everted, but return to 
 their normal condition when the pathologic condition has 
 subsided, and the cells regain their normal metabolism. 
 
 Fig 198 Fig. 199 
 
 ■:-:>^'i-~/ir<'' . - r. ,, '^ ' ■',■■ 
 
 fc?:.-; .- :: .\.:::-:^- :-, --^^ '.:'.::-.. r-^.---..f^^^-& 
 Cross-sections of elastic fibers, 
 showing their forms as seen in Tissue of the dental pulp, in which the de- 
 
 a group passing between coarse velopment of the cells is not followed by any 
 white fibers. considerable formation of fibers. 
 
 To sum up w^hat has been said, it is apparent that both 
 phylogenetically and ontogenetically, intercellular substances 
 have been produced and are maintained by cells in response 
 to mechanical influences and to meet mechanical conditions. 
 In all higher animals certain tissues, the connective tissues, 
 have been set apart for this purpose, and the cells have been 
 specialized to respond to mechanical stimuli and develop 
 an intercellular substance adapted to the condition. This 
 makes the supposition necessary that an embryonal connec- 
 tive-tissue cell may develop into any specialized form and 
 that the kind of cell into which it develops will be determined 
 by the character of mechanical stimuli which it receives. 
 Just as the epithelial cells have been specialized to respond 
 to the environments of light stimuli, vibration of the air, 
 pressure, and chemical action which connect the organism 
 with its environment, connective-tissue cells have been 
 
246 INTERCELLULAR SUBSTANCES 
 
 specialized to respond to mechanical stimuli, by the pro- 
 duction of formed materials adapted to the mechanical 
 conditions. These conceptions are fundamental to an 
 understanding of bone structure and growth, and the muta- 
 tions of connective tissue in general. 
 
 In no branch of histology is a clear conception of inter- 
 iellular substances and the relation of cells to them as 
 mportant as in the study of the teeth and their associated 
 structures. Caries cannot be understood unless these 
 fundamental ideas have been appreciated, and many state- 
 ments in dental literature would never have appeared if 
 the nature of intercellular substance and the relation of 
 cytoplasm to it had been understood. 
 
CHAPTER XVIII 
 
 BONE 
 
 Defimtion. — Bone may be defined as a connective tissue 
 whose intercellular substance is calcified and arranged in 
 layers around nutrient canals or spaces. The cells are 
 placed in cavities, lacunae, between the layers, and receive 
 their nourishment through very minute channels, canal- 
 iculi, which radiate from them and penetrate the layers. 
 
 STRUCTURAL ELEMENTS 
 
 The structural elements of bone are : 
 
 1. Bone matrix, or intercellular substance, which is always 
 arranged in layers or lamellte. 
 
 2. The bone cells or bone corpuscles which are embedded 
 in the matrix between its layers. 
 
 3. Lacunae, or the spaces in which the cells are found. 
 
 4. CanalicuH, or the channels through the matrix by 
 which the embedded cells receive nourishment. 
 
 Bone Matrix. — The bone matrix is composed of a dense 
 organic basis of ultimately fibrous character which yields 
 gelatin upon boiling with water. With this inorganic salts 
 are combined in a weak chemical union, forming the hard 
 substance of bone. By treatment with acids the inorganic 
 salts can be removed, leaving the organic basis which retains 
 the form of the tissue. In this condition the rigidity of the 
 bone is destroyed. On the other hand, by calcining at red 
 heat the organic basis can be removed, leaving the inorganic 
 substances which retain the form of the tissue. In forma- 
 tion the organic basis is apparently formed first, and then 
 the salts of lime are combined with it, through the agency 
 of the formative cells, or osteoblasts. 
 
248 
 
 BONE 
 
 Bone Corpuscles. — Bone corpuscles are the cells lying in 
 the laciuiie. Each cell contains a single well-defined nucleus, 
 lying in the centre of a granular cytoplasm. The cell appar- 
 ently completely occupies the lacunae, and from the central 
 mass fine projections of cytoplasm extend through the 
 
 Fig. 200 
 
 From a section through the bone of a roebuck. The lacunae are seen from 
 above, and are filled with coloring matter. In places small dots are visible, which 
 represent the cross-sections of bone canaliculi. (850 X) (Szymonowicz.) 
 
 canaliculi, which brings the bone corpuscles in intimate 
 relation with certain area of bone matrix. The processes of 
 one cell anastomose with those of its neighbors through the 
 canaliculi, so that there is a continuous network of living 
 cytoplasm throughout the matrix. 
 
CANALICULI 
 
 249 
 
 Lacunae. — The lacunae are flat oval spaces about 20 
 microns long, 10 microns wide, and 5 or 6 microns thick. 
 Their shape, therefore, in sections depends upon the way in 
 which they are cut as illustrated in Figs. 200 and 201. When 
 
 Fig. 201 
 
 From a section through the bone of a roebuck. The lacume are seen from the side. 
 (850 X) (Szymonowicz.) 
 
 cut lengthwise they would appear as about 20 microns long 
 and 6 wide in profile, or as about 20 microns long and 10 
 wide when seen from above. 
 
 Canaliculi. — These radiate from the lacunae in all direc- 
 tions, opening into them by larger channels which branch 
 
250 BONE 
 
 and divide, becoming smaller as they pass farther into the 
 matrix. They anastomose freely with those from adjoining 
 lacunae. 
 
 THE VARIETIES OF BONE 
 
 There are three varieties of bone differing in the arrange- 
 ment of these structural elements. These are subperiosteal, 
 Haversian system, and cancellous bone. 
 
 Subperiosteal Bone. — This form of bone must be regarded 
 as primarily a formative arrangement and more or less 
 transitory, in which the layers are arranged parallel with the 
 surface, and under a formative membrane. It contains 
 canals (Volkmann's canals) with bloodvessels (Fig. 202), con- 
 nective tissue, etc. These penetrate the layers which are 
 never arranged concentrically around them. It is always 
 thin, that is, composed of comparatively few layers, and 
 when a considerable thickness is formed it is cut out from 
 within by absorptions beginning in the canals, and bone is 
 rebuilt with layers arranged concentrically around the chan- 
 nels formed. In this way subperiosteal bone is converted 
 into the second form. 
 
 Haversian System Bone.^In this variety the lamellae are 
 arranged concentrically around canals which contain blood- 
 vessels, nerves, and embryonal connective tissue, and from 
 which the cells in the lacunae are nourished (Fig. 203). These 
 canals are, in general, parallel with the surface or the long 
 axis of the bone and anastomose with each other. A canal 
 with the layers arranged around it constitute an Haversian 
 system. Between the Haversian systems are remains of 
 the subperiosteal layers (insterstitial lamellae) that were left 
 by the absorption, and for that reason have been called 
 fundamental lamellae. They have also been called ground 
 lamellae. Haversian system bone is often called compact 
 bone, and makes up the greater part of the shafts of the 
 long bone, and the plates of the flat ones. It is never allowed 
 to become greater in thickness than is necessary for strength, 
 and when sufficient thickness has been formed, the deeper 
 
CANCELLOUS BONE 
 
 251 
 
 part is cut out by absorptions in the Haversian canals, con- 
 verting them into large irregular spaces. The formation of 
 a few layers around these spaces transforms the second type 
 into the third or cancellous bone. 
 
 Ftg. 202 
 
 Fig. 203 
 
 Subperiosteal bone, showing 
 Volkmann's canals. 
 
 Haversian system bone; 
 a, Haversian canals. 
 
 Cancellous Bone. — In this variety the lamellae are arranged 
 in delicate plates surrounding large, irregular nutrient or 
 marrow spaces. These are filled by embryonal connective 
 tissue and contain bloodvessels and nerves. The plates of 
 cancellous bone are not arranged at haphazard, as might be 
 supposed from a casual observation of sections, but are 
 disposed in definite arrangement, which is determined by 
 the directions of stress on the compact bone which they 
 
252 BONE 
 
 support. (See illustrations in Chapter XVIII.) They are 
 not permanent and unchanging, but are continually being 
 rebuilt in new directions, in response to the mechanical con- 
 ditions to which the bone as a supporting organ is subjected. 
 
 THE ARRANGEMENT OF BONE 
 
 Compact Bone. — A knowledge of the structural elements 
 of bone can best be obtained by the study of sections 
 ground from the shaft of a long bone. An old dry bone 
 should be sawed across, near the middle of the shaft, in 
 two places, so as to cut out a ring about a quarter of 
 an inch thick. Then saw the ring through in two places 
 with an arc of about a quarter of an inch on the outer 
 surface. From this two slices should be sawed out, one 
 transverse to the long axis of the bone, the other parallel 
 with it. These are ground to not more than 8 or 10 microns 
 in thickness and mounted in hard balsam. From a study of 
 these two the arrangement of the lamellae, and the shape 
 and character of the lacunse can be made out. Upon the 
 outer surface of the transverse section will be found a larger 
 or a smaller number of layers of subperiosteal bone which 
 encircle the shaft, and consequently are called the circum- 
 ferential lamellae. The number of these layers will depend 
 upon the position from which the section is taken, and the 
 age of the bone. If the bone is increasing in circumference 
 at the point from which the section is cut, there will be a 
 considerable number of layers, and they will be easily seen. 
 If the bone has been growing smaller in circumference at the 
 point, there will be very little of subperiosteal bone, and it will 
 be comparatively hard to recognize. The greatest part of 
 the section will be made up of Haversian systems, in which 
 from two to three to five or six layers are arranged around 
 an Haversian canal. The lacunae appear as irregularly oval 
 spaces about 5 or 6 microns across and 15 to 20 microns in 
 length. From them a great many minute canals radiate 
 through the matrix both toward the Haversian canal and 
 
PLATE X 
 
 K -^ 
 
 
 f: 
 
 
 
 
 ^ if. V- 
 
 
 X W 
 
 ■ "- ^ V "^ "^ *^» - ' ^ 
 
 
 
 * 1 \ 
 
 <^ 
 
 M 
 
 
 -4-rf 
 
 ^ X - 
 
 
 
 i 
 
 i 
 
 s 
 
 
 ^ ^ , f 
 
 ^ < 
 
 V 
 
 .' .A. 
 
 
 < 
 
 
 Froni a Ground Cross-section of the Diaphysis of the 
 Human Metatarsus. (Szynionowicz. ) 
 
 a, outer ground lamellae; ;>, inner ground lan:iellae; c, Haversian laniellae; 
 d, interstitial laniellae. All canals and bone cavities are filled with coloring 
 nnatter and appear black. (90 X) 
 
COMPACT BONE 253 
 
 away from it. The character of these canaliculi can only be 
 appreciated by seeing them. They are filled in life by pro- 
 jections of the protoplasm of the bone corpuscles. They are 
 suggestive of the rootlets of plants running through soil, and 
 as in that case the rootlets are absorbing material from the 
 soil and reacting with it, in this case the protoplasmic con- 
 tents of the canaliculi is reacting with the matrix, maintain- 
 ing its quality. The portion of matrix through which the 
 canaliculi from one lacunae extend belongs to the bone 
 corpuscles which occupies the lacunse, as will be seen later. 
 These cells have been enclosed in the matrix which they 
 have formed. Between the Haversian systems will be found 
 a few layers of interstitial or fundamental lamellae. They 
 are the remains of layers which were formed under the 
 periosteum and were not entirely destroyed when it was 
 replaced by Haversian systems (Plate X). The amount of 
 interstitial lamellae varies greatly in different specimens, as 
 will be seen by comparing figures. 
 
 The Haversian canals anastomose with each other; this 
 will be seen in many specimens. Many Haversian sj^stems 
 will be found imperfect in form, as, for instance, those shown 
 in Plate X. This means that after these systems were com- 
 pleted, absorptions occurred in a neighboring canal which 
 attacked the layers of the system, and later a new system 
 was formed in this space by the deposit of concentric lamellae. 
 While bone is thought of as a hard and fixed tissue, it is con- 
 tinually being built and rebuilt in this way. It is onty by 
 the understanding of these possibilities that we get the ideas 
 that bone, while hard and rigid, is a plastic tissue and is con- 
 tinually being moulded by mechanical conditions to which 
 it is subjected. 
 
 It will be seen also that the arrangement of the lamellae 
 becomes a record of the changes that have occurred in the 
 formation of the tissue. The inner boundary of the section 
 next to the marrow cavity will show a few layers parallel 
 with the surface. These are known as the inner circum- 
 ferential lamellae. It is a mistake, however, to think of 
 them as surrounding the marrow cavity in the same sense as 
 
254 BONE 
 
 the outer circumferential lamellse surround the bone. If 
 the section has been cut at a little distance from the centre 
 of the shaft, it will have been noted that the marrow cavity 
 is penetrated by very delicate spicules, and that in fact the 
 marrow cavity is produced by the spaces of cancellous bone, 
 becoming larger and larger until they become one continuous 
 space. The inner circumferential lamellae are therefore the 
 layers which have been formed around an enlarged nutrient 
 or marrow space. 
 
 Cancellous Bone. — The cancellous bone can best be studied 
 in decalcified sections. A field from the central portion of a 
 flat bone will show its typical arrangement. It is made up 
 of delicate flattened spicules surrounding larger or smaller 
 irregular spaces which connect with each other very freely. 
 Each spicule is composed of a few lamellae which are arranged 
 around the space. The structure of the spicules often 
 becomes complicated by absorptions and rebuildings which 
 have occurred to change their direction. The tissue which 
 fills the spaces is a delicate, embrj^onal connective tissue 
 in which osteoblasts and osteoclasts appear in response to 
 mechanical conditions. It is richly supplied with blood- 
 vessels, nerves, and lymphatics. The lacunae and canal iculi 
 are in no respect different from those of the Haversian 
 system and subperiosteal bone. 
 
CHAPTER XIX 
 BONE FORMATION AND GROWTH 
 
 Bone is one of the latest tissues to be formed, and is 
 always developed from an antecedent connective tissue of 
 less specialized character. According to the character of 
 the antecedent tissue, bone formation is of two varieties — 
 the formation from cartilage, or endochondral bone forma- 
 tion, and that from fibrous connective tissue, without the 
 intervention of cartilage, or endomemhranoiis bone formation. 
 
 Endochondrial Bone Formation. — ^All of the bones of the endo- 
 skeleton are preformed in cartilage. The transformation of 
 cartilage into bone is rather a substitution than a transfor- 
 mation, for the original tissue is destroyed in the process, 
 aiKl a new and more highly specialized one substituted for it. 
 
 Before ossification begins the cartilage has taken on the 
 general form of the bone and is covered by a definite peri- 
 chondrium. Ossification begins at separate centres and 
 progresses through the cartilage, but the separate centres 
 do not unite until the bone is about fully formed. In the 
 long bone there are usually three centres — one near the 
 centre of the shaft, forming the hypophysis, and one near 
 either end, forming the epiphysis. These remain separated 
 by a layer of cartilage until the length of the bone has been 
 fully formed. 
 
 The first indication of the transformation of cartilage 
 into bone is an increase in the size of the lacunae and in 
 the amount of cartilage matrix, which also shows changes 
 in character, having lime salts deposited in it. The car- 
 tilage c^lls enlarge and show signs of degeneration, the 
 lacunse become arranged in rows, and as they increase in 
 size, more in the direction parallel with the axis of the 
 cartilage, the amount of matrix separating them is reduced. 
 
256 
 
 BONE FORMATION AND GROWTH 
 
 By this time the perichondrium, on the surface of the car- 
 tilage opposite to the centre, has developed osteoblasts which 
 begin the formation of subperiosteal lamellse upon the sur- 
 
 FiG. 204 
 
 Hyaline 
 cartilage 
 
 Area of 
 calcification 
 
 ).',.} Osteogeneiic 
 tissue 
 
 Perichondral 
 "*""" bone 
 
 Capsules containing 
 •many cartilage cells 
 
 From a longitudinal section of a finger of a three-and-a-half-months human 
 embryo. Two-thirds of the second phalanx is represented. At X a periosteal 
 bud is to be seen. (85 X) (Szymonowicz ) 
 
 face of the cartilage, and the perichondrium is transformed 
 into periosteum. Opposite the centre osteoclasts appear, 
 cutting into the cartilage, followed by buds of embryonal 
 tissue. The osteoclasts dissolve away the remains of the 
 
EXDOCHOXDRIAL BOXE FORMATION 
 
 257 
 
 cartilage matrix, opening up the spaces between the lacunae 
 and converting the rows of lacunae into irregular channels 
 or primary marrow spaces. Upon the spicules of calcified 
 cartilage matrix, osteoblasts arrange themselves and begin 
 to lay down lamelhie of bone. These changes progress from 
 
 Fig. 205 
 
 Cartilage cell 
 
 The place marked X in the preceding figure with stronger magnification. 
 (185 X) (Szymonowicz). 
 
 the centre in both directions, and all stages, from the typical 
 hyaline cartilage to the formation of bone, may be seen in 
 one section. These stages are illustrated by Figs. 204, 205, 
 and 206. 
 
 From now on the bone grows by progressive transforma- 
 tion of cartilage and by the growth of bone under the peri- 
 osteum, which will be considered under bone groT\i:h. 
 17 
 
258 
 
 BONE FORMATION AND GROWTH 
 
 Endomembranous Bone Formation. — The bones which are 
 not preformed in cartilage are formed directly from fibrous 
 
 Fig. 206 
 
 Periostenm^^^] 
 
 Periosteal bntl 
 
 Blood-i'essris 
 
 filled witJi^: 
 blood^ 
 
 Calcified |/'/^ij'r* ' /JJ 
 cartilage'*^' '*' " 
 
 Enlarged 
 cartilage 
 cells 
 
 From longitudinal section of a finger of a four months embryo. Only the diaphysis 
 of the second phalanx is represented. (85 X) (Szymonowicz.) 
 
 tissue. This is well illustrated in the mandible. In the 
 region of Meckel's cartilage and between it and the develop- 
 ing tooth germs the mesenchyme begins to show signs of 
 
ENDOMEMBRANOUS BONE FORMATION 
 
 259 
 
 specialization. Delicate fibers appear in the intercellular 
 substance. Along these the connective-tissue cells arrange 
 themselves, and, taking on the form of osteoblasts, begin to 
 
 Fig. 207 
 
 i3 
 
 C <9\0, 
 
 ^'.^>'^, N. ^ 
 
 ^'^-v.- 
 
 -^ -^( 
 
 
 _Primar\' 
 marrow 
 space. 
 
 
 
 
 
 Osseous 
 
 tissue. 
 
 Section through the lower jaw of an embr>-o sheep (decalcified with picric acid). 
 At a and immediately below are seen the fibers of a primitive marrow cavity Ij^ing 
 close together and engaged in the formation of the ground substance of the bone, 
 while the cells of the marrow cavity, with their processes, arrange themselves on 
 either side of the newly formed lamella and functionate as osteoblasts. (Bohm, 
 Davidoff, Huber.) (300 X) 
 
 lay down bone lamellae (Fig. 207). These stretch out through 
 the mesenchyme, forming a network of delicate spicules, 
 until they surround Meckel's cartilage, and grow up to the 
 
260 BONE FORMATION AND GROWTH 
 
 buccal and the lingual of the tooth germ. As soon as this 
 network of bone lamellee containing embryonal connective 
 tissue, in its primary marrow spaces, begins to take on 
 definite form, there is a specialization of the mesenchyme 
 surrounding it, developing into fibrous tissue which becomes 
 a periosteum. From this time onward the formation of bone 
 progresses, as will be described under the growth of bone. 
 
 Bone Growth. — If sections are cut transversely through 
 the shaft of a long bone from a fetus, the surface will be 
 found to be covered by a well-formed periosteum, which is 
 actively laying down layers of subperiosteal bone. The 
 central portion of the bone is made up of a network of 
 spicules surrounding primary marrow spaces, there being 
 no true marrow cavity. The formation of the subperiosteal 
 laj^ers does not progress at a uniform rate at all points* on 
 the circumference, but they are piled up at certain points 
 forming longitudinal ridges with grooves between them. 
 These grooves become arched across, enclosing part of the 
 connective tissue of the inner layer of the periosteum, and 
 contain bloodvessels and nerves. Soon after these spaces 
 are enclosed absorptions begin in their walls, destroying a 
 large part of the subperiosteal lamellae and forming primary 
 marrow spaces. As soon as these spaces have reached a 
 certain size the absorptions stop, and osteoblasts appear 
 upon the wall of the space and begin to lay down lamellae 
 upon its circumference, until an Haversian system has been 
 produced with an Haversian canal at its centre. In this way 
 the bone increases in diameter, and this process continues 
 until a considerable thickness of Haversian system bone is 
 formed. In all bone growth there is the alternation of 
 formation, destruction, and rebuilding, and it must be 
 remembered that this continues as long as the bone functions 
 as an organ of support. As the shaft becomes larger the 
 primary marrow spaces at the centre are enlarged by the 
 absorption, and a few lamellae are laid down again upon 
 their walls, until finally in the central portion of the shaft 
 the true marrow cavity is formed. As -the thickness of 
 Haversian system bone becomes greater, absorptions occur 
 
GROWTH OF MEMBRANE BONES 261 
 
 in the Haversian canals, cutting out large, irregular channels, 
 around which a few lamellse are laid down, and so the Haver- 
 sian system bone becomes converted into cancellous bone and 
 is opened into the marrow cavity as it grows larger. 
 
 Growth of Membrane Bones. — The growth of the membrane 
 bone progresses in a very similar way. As soon as the 
 periosteum is formed subperiosteal bone is laid down and 
 converted into Haversian system bone, forming the compact 
 plate of the surface, leaving the cancellous portion first 
 formed at the centre. When a certain thickness of compact 
 bone has been formed, absorptions occur in the Haversian 
 canals, converting the deeper portions into cancellous bone. 
 This process may be reversed. Absorptions may occur under 
 the periosteum, cutting deeply into the Haversian system 
 bone, and then a few subperiosteal layers be laid down 
 upon it. When this occurs lamellae are laid down around 
 the marrow spaces, converting the cancellous bone into 
 Haversian system bone to maintain the required strength. 
 In this way the bones are moulded into shape, adapting 
 them to the mechanical conditions to which they are sub- 
 jected. There is an oscillation between formation and 
 destruction, by which the balance adapted to the mechanical 
 conditions is maintained. It has often been noted that 
 bones are never allowed to become more bulky than is 
 necessary to perform their function. 
 
CHAPTER XX 
 
 PERIOSTEUM^ 
 
 Definition. — The periosteum is the formative and pro- 
 tective membrane which covers the outer surface of the bone. 
 All periosteum has certain structural characteristics in 
 common, but because of structural differences two classes 
 are recognized — attached and unattached — each of w^hich 
 may be simple or complex. Periosteum may thus be classi- 
 fied as follows: 
 
 1. Unattached simple. 
 
 2. Unattached complex. 
 
 3. iVttached simple. 
 
 4. Attached complex. 
 
 Function of Periosteum. — The importance to the dentist of 
 a knowledge of the structure and function of the periosteum 
 can scarcely be exaggerated. It has been the knowledge of 
 this tissue and its function that has led to all the advance- 
 ment in bone surgery of modern time. Repair and regenera- 
 tion of bone is largely accomplished through its agency. 
 
 The periosteum forms the immediate covering of all the 
 bones and is continuous over their entire surface except the 
 portion covered by cartilage. Each bone, therefore, has a 
 periosteum of its own which does not continue around the 
 articulation to the bones with which it joins. Bones that 
 are united by suture are, however, covered by a common 
 periosteum. If the flesh and overlying tissues are carefully 
 
 1 In the presentation of this chapter it is impossible adequately to express my 
 indebtedness to Dr. G. V. Black. Almost all of the illustrations are taken from The 
 Periosteum and Peridental Membrane, published by him in 1887. I have always felt 
 that this book had never received the attention it deserves. Only one thousand copies 
 of it were printed, and they were not sold until the orthodontists exhausted the edition. 
 The book is now entirely out of print and is very difficult to obtain. I have studied 
 this book for years and have repeated almost all of the work described in it, but I have 
 felt that it was impossible for text-book purposes to improve upon the illustrations. 
 
FUNCTION OF PERIOSTEUM 263 
 
 removed from a long bone, the periosteum will be seen 
 as. a smooth white, lustrous membrane, having much the 
 appearance of a tendon on most of its surface. But at some 
 places which correspond to the positions where muscles or 
 fascia were attached it appears ragged and dull, for the 
 tissues had to be cut to separate them from the outer layer 
 of the periosteum, to which they were firmly adherent. In 
 all other places the tissues separate easily in dissection; in 
 fact, are not attached at ah, except by the lightest of areolar 
 tissue, which is very easily broken, and the tissues may be 
 separated from the surface of the membrane with the finger 
 or the handle of a scalpel. Now, if the periosteum is slit 
 along a smooth surface with the scalpel and the handle 
 inserted between the bone and the membrane, it will be 
 found to separate readily from the bone over most of its 
 surface. If the process is watched closely, little strings will 
 be seen apparently running from the periosteum to the 
 bone, and being broken as they are separated. These are 
 mostly small bloodvessels which are running into canals in 
 the bone. In this process the periosteum seems like a closely 
 adapted sac or elastic glove, clothing the surface of the 
 bone, as if surrounding it in a fibrous bag. If the separation 
 of the periosteum from the bone is continued, it will be found 
 that it does not separate as easily in all places. As the 
 articular ends are approached it becomes suddenly fastened 
 to the underlying bone, and the blade of the knife must be 
 used. The periosteum now appears as a very thin, tough, 
 and inelastic membrane, that is torn with difficulty, but it is 
 so thin that it is difficult now to separate it from the bone 
 without cutting it through. When this point of attachment 
 is reached it seems that the periosteum is sinking into the 
 substance of the bone, and from the examination of its 
 structure it is found that this is practically what has 
 happened. 
 
 Comparing the periosteum to a sac surrounding the 
 bone, it is found sewed firmly down at the margin of the 
 cartilage around the articular ends. Besides the attach- 
 ment around the cartilage, the periosteum will be found 
 
264 PERIOSTEUM 
 
 adherent in the following positions: Where muscles or 
 fascia are attached to the outer layer of the periosteum; 
 where it approaches the insertion of tendons or ligaments; 
 and where the skin or mucous membrane seem attached 
 to the underlying bone, as around the auditory meatus, 
 the gums, mucous membrane of the nose, etc. In all such 
 positions the periosteum is firmly attached to the bone — in 
 fact, becomes a part of it — and through this medium the 
 connections between muscles, fascia, etc., and the framework 
 of the skeleton is accomplished. 
 
 This feature of the anatomy of the periosteum has never 
 been studied in the detail it deserves, especially by the 
 dentist. It is of the greatest importance in the manage- 
 ment of the diseases of bone, especially those involving 
 the formation of pus, for these lines of attachment deter- 
 mine the direction in which the pus will proceed along the 
 surface of the bone. When pus generated within the bone 
 reaches the surface, it will lift an unattached periosteum 
 and run along the surface until it reaches a line of attach- 
 ment. Here it can penetrate the periosteum more easily 
 than it can separate it from the bone. W^hen a line of attach- 
 ment is reached, therefore, the direction of the burrowing is de- 
 termined by the attached areas. The pus penetrates the peri- 
 osteum more easily than it separates its attachments from 
 the bone, but it lifts the unattached periosteum so easily that 
 it will often run along a line of attachment for a long distance. 
 
 These factors often become of great importance in deter- 
 mining the position in which alveolar abscesses will point. 
 For instance, if an abscess from a bicuspid root, or the 
 mesial root of a molar, reaches the surface of the bone 
 above the attachment of the buccinator, it cannot pene- 
 trate its attachment and pass downward to open on the 
 gum, but may run out over the surface of the muscle and 
 open on the cheek, producing the crow's foot scar so often 
 seen. An abscess from an upper cuspid may reach the 
 surface of the bone in the canine fossa between the attach- 
 ments of the nasalis and canius, and lift the periosteum 
 extending upward, and open at the inner canthus of the eye 
 
SIMPLE UNATTACHED PERIOSTEUM 265 
 
 between the orbicularis and the angular head of the quadratus 
 labii superioris. If these abscesses had been reached with a 
 lance, through the mucous membrane, at the proper time, 
 a disfiguring scar would have been avoided. Accurate knowl- 
 edge of the attached layers of the periosteum would have 
 made it certain that they could never point in the mouth 
 cavity without assistance. 
 
 Layers of the Periosteum.— Periosteum is always composed 
 of two distinct layers: 
 
 1. An outer or fibrous layer, which is essentially protective 
 and to which muscles and fascite are attached. This may be 
 either simple or complex. 
 
 2. An inner or osteogenetic layer which is essentially the 
 vital functioning layer, and is, as its name indicates, con- 
 cerned with the formation of bone. This may be either 
 simple or complex. 
 
 The Structural Elements. — The periosteum is composed 
 of the following structural elements: 
 
 1. White fibers in coarse bundles (in the outer layer). 
 
 2. White fibers in very fine bundles (in the inner layer). 
 
 3. Elastic fibers. 
 
 4. The penetrating fibers, or white fibers of the periosteum, 
 that in the growth of bone are included in its substance. 
 
 5. Embryonal connective-tissue cells. 
 
 6. Osteoblasts or bone forming cells. 
 
 7. Osteoclasts or bone absorbing cells. 
 
 Unattached Periosteum. — In the unattached periosteum the 
 inner layer is always simple, and the outer layer may be 
 either simple or complex, depending apparently upon the 
 requirements of protection. In general, the more exposed 
 the position the thicker is the layer, and the larger and 
 stronger the bundles of fibers of which it is composed. 
 
 Simple Unattached Periosteum. — Where the periosteum is 
 covered by a thick layer of muscles which are not attached 
 to it, as in the thigh, the thinnest and simplest form of 
 periosteum is found. An illustration, drawn by Dr. Black, 
 of the periosteum from the femur of a kitten will illustrate its 
 structure (Fig. 208). The outer layer is composed chiefly of 
 
266 
 
 PERIOSTEUM 
 
 bundles of white fibers, most of which run in a direction 
 parallel with the long axis of the bone. The bundles are com- 
 paratively small and much flattened, so as to be quite rib- 
 bon-like. The inner layer contains a much greater number 
 of cells lying among extremely delicate fibers. In its outer 
 portion many of the cells are embryonal in character. In 
 contact with the surface of the bone is a continuous layer of 
 
 xl 
 
 Fig. 208 
 
 :K 
 
 Non-attached periosteum from the shaft of the femur of the kitten: B, bone; 0, 
 layer of osteoblasts. In the central portion of the figure they have been pulled slightly 
 away from the bone, displa>-ing the processes to advantage. It will be observed that 
 the fibers of the periosteum do not enter the bone, a, inner layer of fine white fibrous 
 tissue (osteogenetic layer) showing the nuclei of the fibroblasts and a number of 
 developing connective-tissue cells, which probably become osteoblasts; c, outer layer, 
 or coarse fibrous layer, in which fusiform fibroblasts are also rendered apparent 
 by double staining with hematoxylin and carmine; d, some remains of the reticular 
 tissue connecting the superimposed tissue -with the periosteum. (y_r immersion.) 
 (Black.) 
 
 osteoblasts which are building subperiosteal bone in the 
 young animal, processes of their cytoplasm extending into 
 the canaliculi of the matrix which they have formed. At 
 one point in the illustration the osteoblasts are pulled off 
 from the surface of the bone and show these processes 
 stretched out of the canaliculi. 
 
ATTACHED PERIOSTEUM 267 
 
 Complex Unattached Periosteum. — In some places, espe- 
 cially where muscles or tendons perform sliding movements 
 over an unattached periosteum, the outer layer, instead of 
 being simple, may be very complex. This is illustrated in 
 Dr. Black's drawing (Fig. 209), from the periosteum of the 
 tibia of a young pig. In this instance the outer layer is 
 
 ^: 
 
 Fig. 209 
 
 
 
 Periosteum from the shaft of the tibia of the pig. lengthwise section, showing the 
 complex arrangement of fibers in the coarse or outer fibrous layer that sometimes 
 occurs under muscles that perform sliding movements upon it: B, bone; O, layer of 
 osteoblasts. The tissue has been pulled shghtly away from the bone in mounting the 
 section, and part of the osteoblasts have clung to the bone, some have clung to the 
 tissues, while others are suspended midway, their processes clinging to each, a layer 
 of fine fibers; inner or osteogenetic layer of the periosteum; 6, first lamina of the coarse 
 or outer fibrous layer, the fibers of which are, in this case, circumferential, exposing 
 the cut ends. It will be observed that there are ten lamina in the make up of the outer 
 layer, the lengthwise and circumferential fibers alternating. The ones marked / 
 and i are very delicate ribbon-like forms, which have shifted from their normal posi- 
 tion in the mounting of the section, so as to present their sides to view instead of 
 their ends, thus displaj-ing their structure to advantage. The illustration shows how 
 readily separable these lamina are. I, reticular tissue. (jV immersion.) (Black.) 
 
 composed of very much flattened bundles of white fibers, 
 arranged alternately longitudinally and circularly. Ten 
 layers may be counted in the section. The inner layer is of 
 the same character as in a simple specimen. 
 
 Attached Periosteum. — The attached periosteum differs 
 from the unattached by having the fibers of the inner layer 
 
268 
 
 PERIOSTEUM 
 
 arrano:ed in bundles, around which the bone matrix is 
 deposited hy the osteobhists, embedding them in the sub- 
 stance of the matrix and calcifying them with it. These 
 fibers constitute the penetrating fibers. They were first 
 described by Sharpey, and have been called Sharpey's 
 fibers. He, however, apparently did not understand their 
 importance or manner of formation. The fibers of the 
 inner layer are built into the substance of the bone in this 
 way wherever tissues are attached to the outer layer of the 
 periosteum. 
 
 ^ '^■^ - ' N ,*• 
 
 Simple attached periosteum: a, bone; h, osteoblasts; c, fibers of the inner layer; 
 D, bloodvessels of the inner layer; E, outer layer; F, muscle fibers attached to 
 outer layer. (Black.) 
 
 Simple Attached Periosteum.— Where the pull of tissues 
 attached to the outer layer of the periosteum is in one 
 direction, the fibers of the inner layer are inclined in the 
 same direction (Figs. 210 and 211). As the surface of the bone 
 
Fig. 211 
 
 A ptiotomicrograph of an attached periosteum similar to Fig. 210. From the alveolai 
 process of a sheep. (About 80 X) 
 
 Fig. 212 
 
 r' a 
 
 Attached periosteum from beneath the attachment of the muscles of the lower 
 lip of the sheep: .4., bone; S, osteoblasts, with the fibers emerging from the bone 
 between them; C, inner layer with fibers decussating and joining the inner side of the 
 coarse fibrous layer in opposite directions (this is rather an unusual form of this 
 layer of the periosteum); Z), coarse, fibrous layer; E, attachment of muscular fibers. 
 (Black.) 
 
270 PERIOSTEUM 
 
 is approached the fibers are gathered into strong bundles to 
 be inserted in the bone, the osteoblasts covering the surface 
 of the bone everywhere between the fibers. The outer and 
 inner layers are united by the interlacing of their fibers. 
 At the junction of the outer and the inner layers many 
 bloodvessels are seen. 
 
 Complex Attached Periosteum. — Where the pull upon the 
 outer layer is in many directions, the fibers of the inner layer, 
 after emerging from the bone, break up into smaller bundles 
 and anastomose in all directions, arching around to interlace 
 with the fibers of the outer layer, and in this way they sustain 
 force in all directions (Fig. 212). This is illustrated in Dr. 
 Black's drawing of a section of attached periosteum from 
 beneath the attachment of the muscles of the lower lip of a 
 sheep. 
 
CHAPTER XXI 
 
 THE ATTACHMENT OF THE TEETH 
 
 That the teeth are not a part of the osseous system, but 
 are appendages of the skin, supported in man by a special 
 development of bone forming the alveolar ridges of the 
 maxillary bones, is as well established as any fact concerning 
 human dentition. The work of Oscar Hertwig, published 
 in 1874, established very clearly the homology existing be- 
 tween the teeth and the dermal or placoid scales of the ganoid, 
 silurioid, and dipnoan fishes, both as to similarity of structure 
 and development. 
 
 Much has been written descriptive of the teeth of various 
 animals, their modifications of form, and attachment to 
 adapt them to modifications of function, and various classi- 
 fications of the means of attachment have been made. Of 
 these, perhaps the best and most logical is given by Charles 
 Tomes in his Dental Anatomy, describing four forms of 
 attachment: (1) By fibrous membrane; (2) by hinge-joint; 
 (3) by ankylosis; (4) by insertion in a socket. 
 
 These various forms of attachment will be taken up, and, 
 if possible, the comparison between them and the evolution 
 of the more complicated forms from the simpler will be 
 shown. The study must begin with an examination of the 
 structure and attachment of the placoid scales and the 
 simplest form of tooth, as illustrated in the shark. 
 
 Structure of Dermal Scales. — The dermal scales are composed 
 of a conical cap of calcified tissue developed from within out- 
 ward, by an epithelial organ, and corresponding in structure 
 to the enamel. This cap is supported upon a conical papilla 
 of calcified tissue formed from without inward, and corre- 
 sponding to dentine. In the outer layer the arrangement of 
 the fine tubules through the calcified matrix correspond very 
 
272 
 
 THE ATTACHMENT OF THE TEETH 
 
 closely to human dentine, but in the inner portions it is to be 
 understood only by considering the formation of the dentine 
 as progressing irregularly over the surface of the pulp and so 
 dividing the pulp tissue into portions enclosed in large canals, 
 from which the fine tubules radiate. The base of this partially 
 calcified papilla has a calcified connective tissue built on to it 
 by the derma or connective-tissue layer of the skin, which cor- 
 responds to cementum forming the basal plate, spreading out 
 
 Fig. 213 
 
 /M 
 
 
 m 
 
 Showing additions of bone of attachment to the bone of the jaw. (Tomes) 
 
 more or less in the connective-tissue layer of the skin, and into 
 which the fibers of this layer are built, so attaching the den- 
 ticle or dermal scale to the deep layer of the coreum. This 
 tissue very exactly resembles cementum. It is formed on 
 the dentine as the cementum of a human tooth is, and shows 
 the connective-tissue fibers embedded in it. In the ganoids 
 the basal plates of adjoining scales unite, forming the armor 
 plates of such fish as the sturgeon and gar-pike, and the 
 dentical remains projecting from the surface of the plates. 
 
 Attachment by Fibrous Membrane. — In the simplest teeth, 
 as of the shark (Lamna cornubica. Fig. 3), which are typical 
 
ATT AC HM EXT BY HINGE JOINT 
 
 273 
 
 dermal scales, there is an exactly similar method of attach- 
 ment, which may be taken as the simplest and most rudimen- 
 tary, or attachment in a fibrous membrane. That is, there is 
 no development or modification of the arch of the jaw, and 
 the teeth have no direct attachment to the bone; in fact 
 (Fig. 213), the jaws themselves are chiefly cartilage. 
 
 Fig. 2U 
 
 Attachment by hinge joint. Tooth of a hake: a, vasodentine ; h, pulp; c, 
 elastic hinge; d, buttress to receive/, formed out of bone of attachment; e, bone 
 of jaw; /, thickened base of tooth; g, enamel tip. (Tomes.) 
 
 Attachment by Hinge Joint. — The formation of the hinge 
 attachment as illustrated in many of the fishes (Fig. 214), 
 may be understood as a modification of the attachment in a 
 18 
 
274 THE ATTACHMENT OF THE TEETH 
 
 fibrous membrane in a more highly specialized creature. 
 These hinged teeth are found in many fishes and in the poison 
 fangs of snakes. The jaws are calcified, and the basal plate 
 or cementum may be considered as confined to, or specially 
 developed on, one side of the dentine papilla, which is also 
 more highly developed, especially in snakes. This cementum 
 is built and calcified around the fibers of the fibrous tissue 
 which pass directly to the bone of the jaw at that point. 
 This bone is to be regarded as an addition to the jaw specially 
 developed for each tooth. Thus, there is not only a modifica- 
 tion in the arrangement of the cementum, but a development 
 of bone for attachment of the tooth. The bloodvessels pass 
 through the fibers of the hinge to the pulp, and are not 
 affected by the motion of the tooth on the hinge; in fact, 
 the pulp seems to be attached to the hinge. There are many 
 complications of this method of attachment, but this may 
 be taken as the type and the manner of its modification from 
 the rudimentary conditions. The distinction, in this form 
 of attachment, from the dermal scale consists in a modifica- 
 tion of the arrangement of the cementum of the basal plate 
 and a development of bone from the jaw to attach fibers 
 which pass directly from cementum to bone. It should 
 also be said that there are developments in the hinge teeth 
 related to the third form of attachment, namely, ankylosis, 
 which cannot be understood until this form is studied. 
 
 Attachment by Ankylosis. — The third form of attachment, 
 ankylosis (Fig. 215), or direct calcified union with the bone of 
 the jaw, cannot be understood \\dthout a careful study of the 
 nature and formation of the dentine in these rudimentary 
 teeth. It is evident, from a study of the dentine of the 
 dermal scales, that compared with human dentine, the tissue 
 is rudimentary and not differentiated from other similar 
 connective tissues. The tubules are comparatively very 
 irregular, and resemble strikingly the tubules found in the 
 secondary dentine formed by a degenerating pulp. The 
 odontoblasts, or dentine-forming cells, are not like the highly 
 specialized cells which form the primary human dentine, 
 but resemble very closely simple spindle-shaped connective- 
 
ATTACHMENT BY ANKYLOSIS 
 
 275 
 
 tissue cells. The nucleus is larger and oval in form, and the 
 protoplasm stretches off from it in one direction intQ a fibril 
 
 Fig. 21; 
 
 Tooth of scarus, showing attachment by ankylosis. (Owen.) 
 
276 THE ATTACHMENT OF THE TEETH 
 
 instead of in two directions into a spindle. The cells are 
 much smaller than human odontoblasts and nearer the size 
 of ordinary spindle cells of the human pulp. In fact, they 
 look more like specially developed spindle cells than odonto- 
 blasts. The formation of dentine begins on the surface, at 
 the apex of a cone-shaped papilla of connective tissue, and 
 proceeds inward. If the formation continues uniformly 
 over the surface of the papilla, a solid layer of fine tubuled 
 dentine results; but it often proceeds irregularly, apparently 
 having special reference to the neighborhood of bloodvessels, 
 so that irregular projections of dentine are found on its 
 inner surface, dividing the pulp more or less into portions 
 enclosed in larger channels or tubes. These may be very 
 regular in arrangement and form around bloodvessels loops 
 embedding the bloodvessel in the calcified tissue, producing 
 what has been called vaso or vascular dentine; but the 
 formation is still from the surface of the pulp until it is 
 obliterated, except for what remains in the larger canals. 
 As distinguished from this formation of dentine we find in 
 the body of the dental papilla of many fishes the formation 
 of spicules of calcified tissue, which resemble neither den- 
 tine nor typical bone, shooting down through the substance 
 of the pulp. They are more to be compared with the first 
 formation of bone in membranes, or in the embryonal 
 connective tissue of the body of the human jaw, which is 
 afterward removed by absorption and replaced by true 
 Haversian system bone. These calcifications contain lacunae, 
 and have tubules or canaliculi running through them, and so, 
 as Tomes says, are intermediate between dentine and bone. 
 They divide the pulp into irregular spaces, and interdigitate, 
 or perhaps actually join, the formation of dentine which has 
 been progressing from the surface of the pulp. These 
 spicules run down into the bone of the jaw, forming an 
 actual calcified attachment for the tooth with the jaw; but 
 in this view of it it is to be regarded as a calcification or 
 rather a formation of bone in the pulp papilla interlocking 
 with the dentine. In some of the fishes, as in Scarus, there 
 is at the same time the remains of the cementum of the basal 
 
ATTACHMENT BY IMPLAXTATIOX IX SOCKET 277 
 
 plate formed on the outside of the dentine around the base of 
 the cone. Ankylosis is confined to the teeth of many fishes, 
 and may be stated as a modification from the dermal scale, 
 resulting in the reduction or loss of the basal plate and an 
 ossification of the pulp continuing through the connective 
 tissue at the base of the pulp to the body of the jaw. 
 
 Attachment by Implantation in Socket. — The development 
 of the fourth form of attachment, by implantation in a 
 socket, seems to be an evolution starting from the same 
 point but proceeding in a different direction (Fig. 216). It 
 
 Fig. 216 
 
 .-i, diagrams of transverse sections through the jaws of reptiles showing pleurc- 
 dont (a), acrodont (6), and theodont (c) dentitions. B. a, lower jaw of Zootoca 
 vivipara; b, of anguis fragilis. (After Leydig ) (Weidersheim, Comparative 
 Anatomy of Vertebrates.) 
 
 is associated with the very great increase in the size of the 
 teeth and consequent necessity for a stronger attachment. 
 The evolution of this is illustrated in the teeth of reptiles. 
 Weidersheim classifies the teeth of reptiles as (1) resting 
 upon a ledge on the lingual side of the jaw — pleurodont 
 dentition; (2) resting on a slight ridge around them — acro- 
 dont dentition; (3) lodged in permanent alveoli, as in the 
 crocodile — theodont dentiton. These three classes illus- 
 trate three stages in the development of the socket method 
 of attachment. 
 
 In the simplest form there is a cone-shaped tooth, attached 
 
27S THE ATTACHMENT OF THE TEETH 
 
 to the bone around its base, by the fibers being built into 
 the cementum and bone. There is little modification of the 
 rudimentary form, and little development of bone for its 
 attachment. In a higher form the tooth has become long or 
 peg-shaped, and the bone has grown up around a portion of 
 it to support it; but it is attached to the bone by connective- 
 tissue fibers, being built into the cementum on the surface 
 of the tooth and into the bone of attachment on the jaw. 
 The development of the form of the tooth to the peg from 
 the cone may be understood as a continuing of the develop- 
 ment of odontoblasts, and the formation of dentine (which 
 always begins at the apex of the cone) farther and farther 
 down the sides of the dental papillae. Then the formation 
 of the cementum, which begins around the base of the cone 
 and continues down on the outside of the calcified dentine, 
 covering its outer surface, and building the connective-tissue 
 fibers into the tooth. The development of bone accom- 
 panies, or rather follows that of the tooth, building the 
 other ends of these fibers into the bone which is developed 
 to support the tooth. 
 
 Summary. — To review the subject matter of this chapter, 
 all teeth have been evolved from the simple placoid scale. 
 In the simplest forms, as in the teeth of the shark, there is 
 no relation to the bone whatever, but the fibers of the sub- 
 cutaneous tissue are built into the basil plate of cementum. 
 As the tooth becomes larger and demands more support, 
 there is added to the bone of the jaw that which Tomes has 
 called bone of attachment. The osteoblasts build up addi- 
 tions to the jaw which surround and embed the fibers, so that 
 the fibers which were originally in the subcutaneous tissue 
 are fastened to the bone at one end and to the cementum 
 at the other. The evolutions of attachment by hinge joint 
 and by gomphosis are, therefore, direct evolutions from the 
 simple attachment in membrane. The form of ankylosis is 
 also evolved from the simplest type, but in this case the bone 
 of attachment is associated with the pulp, and the formation 
 of bone and dentine become interlocked and united. 
 
CHAPTER XXII 
 
 THE PERIDENTAL MEMBRANE 
 
 Ix one sense the peridental membrane may be consid- 
 ered as the most important of the dental tissues, for upon 
 it the usefulness of the teeth and their comfort to the indi- 
 vidual is dependent. It makes no difference how perfect a 
 crown may be, or how perfectly any damage which may 
 have occurred to it may have been restored, unless the 
 peridental membrane is in a healthy and fairly normal con- 
 dition, the tooth will be useless, and the individual would 
 be much more comfortable without it. 
 
 Definition. — The peridental membrane may be defined as 
 that tissue which fills the space between the surface of the 
 root and the bony wall of its alveolus, surrounds the root 
 occlusally from the border of the alveolus, and supports the 
 gingivus. It is necessary to emphasize the three parts of 
 the definition. The peridental membrane does not stop 
 at the border of the bone, but continues to surround the root 
 as far as the tissues are attached to it. In general, the dental 
 profession has thought of the peridental membrane as only 
 that tissue which occupies the space between the root and the 
 wall of its alveolus. As will be seen from a study of a section 
 later (Figs. 219 and 220), the structure of the tissue surround- 
 ing the root between the gingival line and the border of the 
 process is essentially the same as that in the alveolus, and 
 quite difterent from the much coarser fibrous mat forming 
 the submucous layer of the gum tissue. The peridental 
 membrane also extends into the free margin of the gum and 
 is the means of its support, holding the gingivae close to the 
 surface of the tooth and supporting them in the interproximal 
 spaces. The importance of this portion of the peridental 
 
280 THE PERIDENTAL MEMBRANE 
 
 membrane and the functions which it performs have been 
 strongly emphasized in the last few years, in their relation 
 to the extensions of caries and the beginnings of pyorrhea. 
 Most of the diseases of the peridental membrane which 
 result in the final loss of the teeth have their beginnings in 
 this portion. 
 
 Nomenclature.^ — The peridental membrane belongs to 
 the class of fibrous membranes which form the covering 
 of organs, the capsules of glands, and especially those 
 membranes which cover the organs of support. Its closest 
 relative is the periosteum in the attached portions, with 
 which it has many points of structure in common, but 
 it differs from the periosteum in any position in important 
 respects. It has often been called the alveodental peri- 
 osteum, but this name implies that the periosteum is folded 
 down into the alveolus and back upon the surface of the 
 root, which is an entirely erroneous conception of the mem- 
 brane. This idea would imply that it was a double mem- 
 brane having one layer covering the bone and another 
 covering the root, the two uniting in the middle portions. 
 But instead, the periosteum must be considered as stopping 
 at the border of the alveolus,^ and being united with the 
 peridental membrane around its circumference. ]\Iany 
 writers use the word pericementum in place of peridental 
 membrane. The author prefers and in this book will use 
 the term peridental membrane, though the two are synon}'- 
 mous. 
 
 Divisions. — Purely for convenience in description, the 
 peridental membrane is divided into three portions: The 
 gingival portion, that portion of the membrane which sur- 
 rounds the root occlu sally from the border of the alveolar 
 process and supports the gingivae; the alveolar ])ortion, the 
 portion of the membrane from the border of the process to 
 the region of the apex of the root; and the apical portion, 
 
 ' The student must be reminded that the word alveolus means a hole, and the 
 alveolar process, the portion of the bone which contains the holes. In dental writing 
 the word alveolus has often been incorrectly used in place of process or alveolar 
 process. 
 
PLATE XI 
 
 
 % 
 
 V, 
 
 V. 
 
 
 
 4' 
 
 
 "^s 
 
 -■^.■. 
 
 ■ 'f> , 
 
 i 
 
 4. 
 
 
 
 i' ■■ 
 
 
 ^i?**l*^iwirff 
 
 Longitudinal Section of Peridental Membrane. 
 
 Stained with heniatoxylin and eosin. Showing border of alveolar process. 
 
PLATE XI [ 
 
 n:,i 
 
 ^ '^ 
 
 f • 
 
 -ui 
 
 \ 
 
 i 
 
 f J ,: t ^." 
 
 
 \ '-S 
 
 ii'^-'li^tfi.wia. -v.. 
 
 te v§m 
 
 Longitudinal Section of Peridental Membrane. 
 
 Stained with hematoxylin and eosin. Showing part of the Ungual gingivus 
 and border of the alveolar process. 
 
PLATE XIII 
 
 % 
 
 J 7 
 -J 
 
 m 
 
 S- 4'. - 
 
 / 
 
 
 .'-*'* 
 
 
 ii4 
 
 
 H 
 
 V 
 
 
 / 
 
 Transverse Section of Peridental Membrane. 
 
 Stained with liematoxylin and eosin. Alveolar portion. 
 
FUNCTIONS 
 
 281 
 
 which surrounds the apex of the root and fills the apical 
 space. These are illustrated in the diagram (Figs. 217 and 
 218). 
 
 The Structural Elements. — These are: (1) White connective- 
 tissue fibers; (2) fibroblasts; (3) cementoblasts; (4) osteo- 
 blasts; (5) osteoclasts; (6) epithelial structures which have 
 sometimes been called the glands of the peridental membrane; 
 (7) bloodvessels; and (8) nerves. 
 
 Fig. 217 
 
 Drawing to show the arrangement of the fibers in a labiolingiial section through 
 an incisor of a kitten. (Black ) 
 
 Functions. — ^The peridental membrane performs three 
 functions: (1) A ph^-sical function — it maintains the tooth 
 in relation to the adjacent hard and soft tissues. (2) A 
 vital function — the formation of bone on the alveolar wall 
 and of cementum on the surface of the root. (3) A sensory 
 function — the sensation of touch for the tooth being exclu- 
 sively in this membrane. 
 
 It is necessary to emphasize the two parts of the physical 
 
282 
 
 THE PERIDENTAL MEMBRANE 
 
 Fig. 218 
 
 G -! 
 
 Al 
 
 Ap •! 
 
 Diagram of the fibers of the peridental membrane: G, gingival portion; Al, 
 alveolar portion; Ap, apical portion. (From a photograph of a section from incisor 
 of sheep.) 
 
ARRANGEMENT 283 
 
 function; the peridental membrane not only supports the 
 teeth in their relation to the bones which carry them, and 
 sustains them against the forces of occlusion and masti- 
 cation, but it also sustains the soft tissues in their proper 
 relation to the teeth. The second part of the physical 
 function is fully as important as the first, and the study of 
 the structure of the tissue related to it and the adaptation of 
 the form of the gingivse to the anatomic form of the teeth 
 and alveolar process, are important considerations which 
 should never be lost sight of in the making of artificial 
 crowns. 
 
 Classes of Fibrous Tissue. — The fibrous tissue of the 
 peridental membrane is entirely' of the white variety, but 
 may be divided into two classes. The principal fibers and 
 the indifferent or interstitial tissue. The former perform the 
 physical function of the membrane, the latter simply fill in 
 spaces between the bundles of fibers and surround and 
 accompany the bloodvessels and the nerves. 
 
 The Principal Fibers of the Peridental Membrane. — These 
 may be defined as the fibers which, springing from the 
 cementum, are attached at their other extremities to the 
 connective tissue supporting the epithelium, the fibrous mat 
 of the gum tissue, the cementum of the approximating tooth, 
 the outer layer of the periosteum at the border of the alveolar 
 process, or the bone of the alveolar wall. 
 
 Arrangement.— The principal fibers literally spring from 
 the cementum, the cementoblasts building up the matrix 
 around them and then calcifying both the matrix and the 
 fibers, in this way attaching them to the surface of the root. 
 In most places the fibers as they spring from the cementum 
 appear as good-sized bundles. A short distance from the 
 surface of the root they may break up into smaller bundles 
 which anastomose and interlace, passing around bloodvessels 
 and other fibers in their course and being again united into 
 large bundles for attachment at their other extremity. 
 
 To arrive at an understanding of the arrangement of the 
 fibers of the peridental membrane, sections must be cut 
 longitudinally, both from buccal to lingual and from mesial 
 
2S4 TIIE PKRJJJKXTAL MEMBRANE 
 
 to distal, and transversely through all portions of the mem- 
 brane. It therefore re(iuires the study of many sections to 
 work out a complete conception. After studying them out 
 completely in this way one is impressed with the beautiful 
 adaptation of their arrangement to sustain the tooth against 
 all the forces to which it is subjected, and to support the free 
 margin of the gum, so that it will lie closely against the 
 gingival portion of the enamel. It is necessary, however, 
 to remind the student that connective tissues are formed in 
 response to mechanical conditions and stimuli, and therefore 
 this arrangement must be considered, not as having been 
 designed to sustain the forces, but as being the result of the 
 forces to be sustained, and therefore beautifully adapted to 
 them. 
 
 Beginning at the gingival line, the fibers springing from 
 the cementum pass out at a short distance at right angles 
 to its surface and then bend sharply to the occlusal, 
 passing up into the gingivus and uniting with the fibrous 
 mat which supports the epithelium. These are much more 
 strongly marked on the Ungual than on the labial gingivus, 
 because in mastication the lingual gingivus receives more 
 pressure of food, which would tend to crush it down. A 
 little deeper the fibers springing from the cementum on the 
 labial and lingual pass out at right angles to the cementum 
 and are lost in the coarser fibrous-mat of the gum tissue. 
 The distance which they extend before lost in the coarser 
 fibers is always greater on the lingual than on the labial. 
 On the proximal sides the fibers springing from the cementum 
 at the same level, branch and interlace, passing across the 
 interproximal space, to be attached to the cementum of 
 the approximating tooth. These fibers are of the greatest 
 importance, as they produce the basket work which forms 
 the supporting framework for the interproximal gingivus. 
 A little farther occlusally the fibers as they come from the 
 cementum are inclined apically. A short distance from the 
 cementum they unite into very large and strong bundles 
 which join with the fibers of the outer layer of the perios- 
 teum, extending over the labial and lingual border of the 
 
ARRAXGEMEXT 285 
 
 alveolar process. On the proximal sides the fibers at the 
 same level are attached to the cementum of the adjoining 
 tooth, or are inclined apically, to be inserted in the bone of 
 the septum. These large bundles form a distinct la^'er, 
 which has been called the dental ligament, and bind them 
 together across the septum. They are the only fibers 
 which hold the teeth down in its socket. At the border 
 of the alveolar process, and in the occlusal third of the 
 alveolar portion, the fibers pass directly from the cementum 
 to the bone at right angles to the axis of the tooth. In 
 this position the fibers are larger and stronger, and show 
 less tendency to break up into smaller bundles in their 
 course than in any other portion of the membrane. In the 
 middle and apical thirds of the alveolar portion the fibers 
 are inclined occlusally as they pass from the cementum 
 to the bone. They spring from the cementum in compact 
 bundles, and show a strong tendency to break up into 
 fan-shaped fasciculi, spreading out as they approach the 
 bone, to be attached over a larger area of the alveolar wall. 
 These fibers literally swing the tooth in its socket and 
 support it against the forces of mastication. In the apical 
 region fibers springing from the cementum pass out in all 
 directions, spreading out in the same way, to be inserted 
 into the bone forming the wall of the apical space. 
 
 If force is exer^d against the lingual surface of an incisor, 
 the fibers on the lingual side of the root in the occlusal third 
 will sustain part of the strain, preventing the cro\NTi from 
 moving labially, and at the same time the fibers on the labial 
 side of the root in the apical space will also be under strain, 
 preventing the apex of the root from moving lingually. 
 The general plan of arrangemenit which has been described 
 is illustrated in Dr. Black's diagram made from a labio- 
 lingual section of an incisor of a young kitten (Fig. 217). 
 
 With this general plane of arrangement in mind individual 
 sections may be studied, examining the arrangement and 
 appearance of the fibers in detail. Figs. 219 and 220 show 
 the labial and lingual gingivae from an incisor of a sheep. 
 Notice that the labial gingivus is taller and thinner, and the 
 
286 
 
 THE PERIDENTAL MEMBRANE 
 
 fibers passing up into it are not as strongly marked. Notice 
 also the distance to which the final fibers of the peridental 
 membrane can be followed before they are lost in the coarser 
 mat of gum tissue. The lingual gingivus is broader and 
 flatter, and the fibers passing up into it form a strong 
 
 Fia. 219 
 
 •\^^ 
 
 Longitudinal section of the peridental membrane in the gingival portion, 
 from a lamb (the labial gingivus). 
 
 and well-defined band. Under higher magnification, fibers 
 would be seen cut transversely in the gingivus, which 
 pass around the tooth, helping to hold it closely against 
 the enamel. In Fig. 221 the fibers uniting with the outer 
 
ARRAXGEMENT 
 
 287 
 
 layers of the periosteum are very well shown. Taking 
 transverse sections in the gingival portion and remem- 
 bering that they are cut at right angles to these through 
 the same area, the distribution of the tissues will be bet- 
 ter understood. Fig. 222 shows a section cut close to the 
 
 Fig. 220 
 
 Longitudinal section of the peridental membrane in the gingival portion (the 
 lingual gingivus): D, dentine; N, Nasmyth's membrane; C, cementum; F, fibers sup- 
 porting the gingivus; Fi, fibers attached to the outer layer of the periosteum over 
 the alveolar process; F-, fibers attached to the bone at the rim of the alveolus; 
 B, bone. (About 30 X) 
 
 gingival line. At A the epithelium on the labial surface of 
 the gingivus is seen, and at B the epithelium lining the 
 gingival space. On the proximal sides of the roots the fibers 
 will be seen passing from the cementum of one tooth to that 
 of the next. Fig. 223 is a httle deeper and shows the fibers 
 
Fig. 221 
 
 Longitudinal section of peridental membrane of young sheep, showing fibers 
 penetrating the cementum: D, dentine; C, cementum, showing embedded fibers; 
 F, fibers running to the outer layer of the periosteum, covering the aveolaf process; 
 F', fibers running to the bone at the border of the process; B, bone. (About 80 X) 
 
ARRANGEMENT 
 
 289 
 
 attached around the entire circumference of the root. Begin- 
 ning at the middle of the labial surface, the fibers will be 
 found springing from the cementum and passing out at 
 right angles to it, to be lost in the fibrous mat supporting 
 the epithelium. The fine fibers of the peridental membrane 
 can be followed for about half the distance to the epithelium 
 before they are lost in the coarser mat of gum tissue, and 
 a fairlv definite boundary will be seen between what should 
 
 Transverse section of the peridental membrane in the gingival portion, from 
 young sheep. The roots of two temporarj' incisors are cut across. The epitheUum 
 lining the gingival space is shown part way around one. A, epitheUum on labial 
 surface of gingivae; B, epithelium lining the gingival space. (About 60 X) 
 
 be considered peridental membrane and the gum tissue. As 
 the distolabial angle of the root is approached, the fibers 
 passing from the cementum ten.d to swing around distally, 
 and pass to the mesiolabial angle of the adjoining tooth. 
 Along the proximal surface the network which supports the 
 interproximal gingivus is well shown. The fibers springing 
 from the cementum interlace and pass around bloodvessels 
 and fibers which are passing up into the gingivus, and finally 
 19 
 
290 
 
 THE PERIDENTAL MEMBRANE 
 
 are inserted into the cementiim of the next tooth. In this 
 way it will l^e seen that the teeth in the entire arch are firmly 
 bound together by the fibers in the gingival portion. This 
 
 Fig. 223 
 
 Transverse section of the peridental membrane in the gingival portion (from 
 sheep): E, epithelium; F, fibrous tissue of gum; B, point where peridental mem- 
 brane fibers are lost in fibrous mat of the gum; P, pulp; F' , fibers extending from 
 tooth to tooth. (About 30 X ) 
 
 explains the way in which the positions of all the teeth are 
 affected by the loss of a single one in the arch, and the way 
 in which the movement of one tooth will draw its neighbors 
 after it. It also explains the separation of the central 
 
PLATE XiV 
 
 Transverse Section of the Peridental Membrane in the 
 Occlusal Third of the Alveolar Portion (from Sheep). 
 
 .V, muscle fibers; Per, periosteum; .4/, bone of the alveolar process; Pd, peri- 
 dei-ital nien-ibrane fibers; P, pulp; D. dentine; Cm, eementum. 
 
ARRANGEMENT 
 
 291 
 
 incisors when the freniim labium passes through between the 
 teeth, and is inserted on the lingual surface of the alveolar 
 process. If these incisors are to be held together perma- 
 nently, normal attachment of fibers extending from the 
 cementum of one tooth to that of the other must be secured. 
 The fibers in this area are also well shown in Fig. 224, and 
 
 Fia. 224 
 
 
 ■'/-'. 
 ** s 
 
 A portion of the peridental membrane between two incisors of a young sheep, 
 showing the fibers extending from tooth to tooth. 
 
 it can be understood how they form foundation upon which 
 the interproximal gingivus rests. The first step in the 
 sagging of the interproximal gum tissue is the cutting off 
 of the fibers from the cementum, where it bends occlusally, 
 following the curve of the gingival line on the proximal 
 surface. 
 
 Plate XIV shows a transverse section in the occlusal third 
 
292 
 
 THE PERIDENTAL MEMBRANE 
 
 FiQ. 225 
 
 
 ■«^^- y.-'.j i -uv 
 
 W^ 
 
 ^^^!^fA m^^ . 
 
 0% 
 
 ^^4^^MP^^^K^^^9|H^^^H^^^jN^ t 
 
 % '-CV" 
 
 »""* 
 
 
 3 >« 
 
 < 
 
 
 Diagram of peridental membrane from section similar to Fig. 224. (From Maloc- 
 clusion of the Teeth, Dr. E. H. Angle.) 
 
 Fig. 22G 
 
 Fibers at the border of the alveolar process (from sheep): D, dentine; C, 
 cementum; F, fibers extending from cementum to bone; Bl, bloodvessel; B, bone. 
 (About 80 X) 
 
A RRA N GEM EN T 293 
 
 of the alveolar portion from the incisor of a sheep. Upon 
 the labial a few muscle fibers are seen and the periosteum 
 covering the labial surface of the process. Notice the medul- 
 lary spaces in the bone and the canals opening into the 
 peridental membrane and periosteum. The light line forming 
 the outer boundary of the dentine is characteristic. Two 
 layers of cement um are seen, and notice the thickening of the 
 layer where strong bundles are attached. At the middle of 
 the labial surface the fibers pass at right angles to the cemen- 
 tum and are attached to the bone, but as the distolabial 
 angle of the root is approached the bundles swing distally to 
 be attached in the bone. In Fig. 225, which was drawn very 
 carefully from this section, the arrangement of the fibers is 
 shown diagrammatically. Notice the way in which they 
 pass over and under each other and around the bloodvessels 
 which wind through them. This relation to the bloodvessels 
 is important, and will be considered again later in connection 
 with the blood supply of the membrane. The tangential 
 fibers at the angle of the root hold the tooth against the 
 forces which tend to rotate it in its socket. They are impor- 
 tant in connection with all rotating movements in ortho- 
 dontia. It has long been noted that rotations were the 
 hardest movements to retain, especially if the tooth were 
 moved in no other direction. In this case, if the tooth were 
 turned mesially the fibers at the distolabial angle would 
 spring the thin plate of the alveolar process as a bow is 
 bent, leaving a condition of stress in the tissue which will 
 tend to spring back into its old position and drag the tooth 
 with it. Notice the greater thickness of the membrane 
 on the lingual as compared with the labial. Figs. 221 and 
 226 show longitudinal sections at the border of the alveolar 
 process. Notice that the fibers can be seen running through 
 the entire thickness of the cementum. They are large, 
 strong fibers and branch \ery little in their course. Note the 
 bloodvessel that is shown in several of these sections, and 
 the way in which it gives oft' branches passing over the border 
 of the processes and toward the cementum. 
 
CHAPTER XXIII 
 
 THE CELLULAR ELEMENTS OF THE PERIDENTAL 
 MEMBRANE 
 
 Fibroblasts. — The fibroblasts are found everywhere between 
 the fibers which they have formed and to which they belong. 
 They are spindle-shaped or stellate connective-tissue cells, 
 having a more or less flattened nucleus and a body of granu- 
 
 Fig. 227 
 
 Fibers and fibroblasts from transverse section of membrane: F, fibers cut trans- 
 versely; F^, fibers cut longitudinally, showing fibroblasts. (About 80 X) 
 
 lar cytoplasm, which is squeezed out into thin projections 
 between the fibers. In sections stained with hematoxylin 
 the cells take the stain strongly and the fibers remain clear 
 (Fig. 227). In this way the fibers are marked out by the 
 cells which lie between them. The number of the fibroblasts 
 
CEMEXTOBLASTS 295 
 
 in the membrane decreases with age. They are large and 
 numerous in the membrane of a newly erupted tooth and 
 are comparatively small and few in the membrane around 
 an old tooth. This is, however, characteristic of fibroblasts 
 in connective tissue generally. Fig. 227 shows a small field 
 taken from the gingival portion of the membrane between 
 the teeth. The magnification is low, the photograph being 
 made with a f objective. The cells are seen as little dark 
 dots lying between the fibers, which are clear. Where the 
 fibers are cut longitudinally they appear spindle-shaped, 
 but where the fibers are cut across they appear star-shaped. 
 They will be seen better in photographs made with higher 
 magnification, but an adequate idea of their form can only 
 be obtained by studying sections very carefully with a J or -jV 
 objective and using the fine adjustment to gain an idea of 
 the third dimension of space. They are shown in many of 
 the illustrations of the epithelial structures. 
 
 Cementoblasts. — The cementoblasts are the cells which 
 form cementum. They cover the surface of the root every- 
 where between the fibers which are embedded in the tissue. 
 While these cells perform the same function for the cementum 
 as the osteoblasts do for bone, they are quite dift'erent in 
 form. They are always flattened cells, sometimes almost 
 scale-like, and when seen from above very irregular in out- 
 line. This irregularity in outline is due to the projections of 
 the cytoplasm around the fibers as they spring from the 
 cementum, the edges of the cell being notched and scalloped 
 to fit about them. There is a central mass of granular 
 cytoplasm which contains an oval and more or less flattened 
 nucleus, from which the cytoplasm extends in projections 
 passing partly around the fibers. Isolated cementoblasts 
 are shown in Fig. 228, drawn by Dr. Black. In order to 
 obtain an idea of the form of the cementoblasts, sections must 
 be cut at a tangent to the surface of the root, and just miss- 
 ing the surface of the cementum. In this way the fibers are 
 cut across and the cementoblasts are shown covering the 
 entire surface between the fibers. These are shown in Fig. 
 229, in which the fiUers are left perfectly clear in order to 
 
296 
 
 THE PERIDENTAL MEMBRANE 
 
 outline the cells more distinctly. In sections cut at right 
 angles to the surface of the roots (Figs. 240, 241, and 242) 
 the cementoblasts are shown as more or less flattened, but 
 no idea of the way in which they fit about the fibers can be 
 obtained. 
 
 Cytoplasmic processes extend from the body of the cemen- 
 toblasts into the matrix of the cementum. These correspond 
 to the processes of the osteoblasts which occupy the canal- 
 iculi of bone. . They, however, are not nearly as numerous 
 or as regular in their arrangement as the osteoblasts. Pro- 
 cesses extending from these cells in a direction from the 
 cementum out into the tissue of the membrane have not 
 been demonstrated. 
 
 Fig. 22S 
 
 4 ^ 
 
 t^^\^ 
 
 Isolated cemeotoblasts. showing the 
 form of the cell as it fits around the 
 fibers springing from the cementum. 
 
 >■■■'■ 
 
 Fig. 229 
 
 Cementoblasts as seen in a section 
 at a tangent to the root and just miss- 
 ing the cementum. The fibers are left 
 white, the ceils are shaded. 
 
 Cement Corpuscles. — Occasionally a cementoblast becomes 
 fastened down to the surface and enclosed in the matrix 
 that is formed. The}^ then lie in a lacuna and show processes 
 radiating from them into the canaliculi. These correspond 
 to bone corpuscles, but there is no such regularity of their 
 disposition or arrangement with reference to the lamellae, as 
 is shown in the case of bone. In man the cementum in the 
 gingi\'al half of the root is usually without cement corpuscles. 
 They often lie entirely within a single lamella instead of 
 
OSTEOBLASTS 297 
 
 between two, as is the case in bone. In general they are 
 found where the layers are thick and the embedded fibers 
 are not specially numerous. They are very often seen where 
 absorptions have been refilled by the formation of subse- 
 quent layers (Figs. 15-1 and 155). 
 
 It is by the activity of the cementoblasts producing a 
 new layer of cementum that the fibers are attached to the 
 surface of the root. In studying many sections, places are 
 found where the fibers, though lying in contact with the 
 surface are not attached to the cementum. In some places 
 it can be seen that they have been cut off by absorptions. 
 From a study of these layers it is evident that there is a 
 constant readjustment in the attachment of the fibers to 
 the root during the function of the tooth, which probably 
 adapt it to slight changes of position resulting from wear 
 and other conditions. It is important to remember that 
 whenever the fibers have been stripped from the surface 
 of the cementum, they can be reattached to it only by the 
 formation of a new layer of cementum, building the fibers 
 into it. This is certainly possible if the conditions are 
 properly controlled, but the cells of the tissue must be in a 
 normal and vitally active condition, and the surface of the 
 root must be such that they can lie in physiological contact 
 with it. The cure of a pyorrhea case, therefore, becomes a 
 biological problem. In this connection it is important to 
 remember that a surface of cementum which has long been 
 bathed in pus may be so filled ^\'ith poison that no cell can 
 lie in contact with it and perform its functions. 
 
 Osteoblasts. — The osteoblasts of the peridental membrane 
 are exactly like osteoblasts in other positions. They cover 
 the surface of the bone of the alveolar wall lying between 
 the fibers which are embedded in it. Even in the young 
 subject they are not found in every position, while in an 
 adjoining area the surface of the bone may be covered with 
 them. In the old subject they are generally absent or have 
 been reduced to flattened scales, which are very difficult to 
 demonstrate; but even in these cases areas will be found in 
 which osteoblasts are present. These are areas of active 
 
298 
 
 THE PERIDENTAL MEMBRANE 
 
 bone formation. The osteoblasts lay down bone exactly 
 as occurs in attached portions of the periosteum, but after 
 
 Fig. 230 
 
 PdM 
 
 PdB 
 
 Penetrating fibers in hone. A field from jjlute X\': Pil M, peridental membrane; 
 0¥, osteoblasts of peridental membrane; Ob^, osteoblasts of medullary space; Pd.B, 
 solid subperidental and subperiosteal bone with embedded fibers; Ms, medullary 
 space formed by absorption of the solid subperidental bone with embedded fibers; 
 H.B, Haversian system bone without fibers built around the medullary space, 
 (About 200 X) 
 
 HB 
 
PLATE XV 
 
 HB 
 
 PdB 
 
 Border of Growing Process. 
 
 Cm, eementum; Pd, peridental mennbrane; Pd. B, solid subperidental and 
 subperiosteal bone with enmbedded fibers; Ms, medullary space Ibrnned by 
 absorption of the solid bone; H. B, Haversian systeni bone without fibers; 
 Per, periosteuni. (About SO X) 
 
OSTEOCLASTS 299 
 
 a little thickness of this solid peridental bone has been 
 formed it is perforated by penetrating canals, on the walls 
 of. which absorptions occur, forming spaces about which new 
 Haversian system bone is formed. This is illustrated in 
 Plate XV. In this way only sufficient subperidental bone is 
 left to furnish an attachment for the fibers. 
 
 Fig. 230 shows a higher magnification of a small area. 
 The osteoblasts are seen between the fibers on the surface 
 of the alveolus, and the fibers can be followed through the 
 subperidental bone. A large absorption area has been formed 
 which has been partly rebuilt, and the new-formed bone 
 without embedded fibers is lighter in color. An under- 
 standing of this building and rebuilding of bone through 
 the agency of the peridental membrane is necessary to 
 understand the development of the face and everything in 
 connection with tooth movement, whether physiological 
 or artificial. 
 
 Osteoclasts. — The osteoclasts of the peridental membrane 
 are not constant elements. They appear and disappear in 
 response to the same conditions which lead to their appear- 
 ance and disappearance in bone. They are always large, 
 multinuclear cells, having from three or four to thirty or 
 forty nuclei (Fig. 231). They may appear upon the surface 
 of the cementum, upon the surface of the alveolar wall, or 
 within the medullary spaces of the bone. They are formed 
 from embryonal cells in the tissue in response to mechanical 
 stimuli. ^Morphologically they are in no respect difterent 
 from the osteoclasts in bone. 
 
 The osteoclasts are tissue destroyers and are the active 
 agents in the removal of any hard tissue. There is no difi'er- 
 ence in them, whether they are destroying the fibrous tissue, 
 bone, cementum, or dentine (Fig. 232). In order for them 
 to act, their cytoplasm must lie in actual contact with the 
 surface to be attacked. They do not first decalcify and then 
 remove, but apparently by applying their cytoplasm to its 
 surfacejthe cells destroy the intercellular substance, forming 
 hollows in the surface, into which the cells sink. These 
 hollows have been called Howship's lacunae. The cells 
 
300 
 
 THE PERIDENTAL MEMBRANE 
 
 usually appear in groups and spread out over the bone or 
 cementum to be attacked, but sometimes only two or three 
 will be found at a point on the surface of the bone, and these 
 will burrow into the substance, forming a penetrating canal 
 
 Fig. 231 
 
 Osteoclast absorption of bone over permanent tooth; Oc, osteoclasts; B, bone of 
 crypt wall; F, fibrous tissue of follicle wall; -4, ameloblasts. (About 62 X) 
 
 running through the bone (Figs. 233 and 234). In these 
 positions the osteoclasts are usually comparatively small. As 
 fast as the canal is formed the embryonal cells of the mem- 
 brane multiply and grow into the space and at any point 
 
OSTEOCLASTS 
 
 301 
 
 where absorption is going on the portion destroyed is immedi 
 ately replaced by embryonal connective tissue. 
 
 Fig. 232 
 
 Osteoclasts in cancellous bone near the peridental membrane; in some portions of 
 the field osteoblasts are seen. As bone is removed note how embryonal connective 
 tissue replaces it. 
 
 This will be noted in all the illustrations showing absorp- 
 tions. Whenever absorption is going on formation is also 
 
302 THE PERIDENTAL MEMBRANE 
 
 going on in an adjoining area. In this way the function 
 of the tissue is maintained until the last remnants of it are 
 destroyed. The general statement may be made that bone 
 formation is always accompanied by bone destruction, and 
 bone destruction by rebuilding. The result depends upon 
 which side the balance swings. The alternation of formation 
 and absorption in the removal of hard tissues is well illus- 
 trated in the absorption of the roots of the temporary teeth. 
 
 Fig. 233 
 
 
 Osteoclast absorption forming penetrators of canal: a, bone matrix; h, bloodves- 
 sel; c, embryonal connective tissue; d, new bone formation; e, osteoblasts, /, osteo- 
 clasts. (Black.) 
 
 The absorption does not begin at one point and spread 
 continuously over the entire surface of the root. If it did 
 so, all of the fibers would be cut out and the tooth would 
 drop off with at least a considerable portion of the root. 
 The process progresses in something of this fashion. At a 
 point on the side of the root near the apex, where the growth 
 of the erupting tooth produces pressure, osteoclasts appear 
 in the membrane, cutting off the fibers, displacing the cemen- 
 toblasts, and arranging themselves in groups on the surface 
 
OSTEOCLASTS 
 
 303 
 
 of the root. These dissolve away the cementum and sink 
 into the tissue, perhaps cutting into the dentine for a short 
 distance. By this excavation the pressure is relieved, the 
 osteoclasts disappear, cementoblasts are formed in the 
 
 Fig. 234 
 
 A longitudinal section through the remains of the alveolar process around the 
 root of a temporary tooth about to be shed (sheep): C, the cementum on the 
 remains of the tooth; B, penetrating canals cut through the labial plate of bone. 
 
 embryonal connective tissue, and the deposit of cementum 
 begins in the excavation, reattaching the fibers in this area. 
 As the rebuilding progresses, at a point a little farther occlu- 
 
Fig. 235 
 
 Root of a temporary incisor, sho-wing absorption and rebuilding of cementum 
 (from sheep): G, gingivus; D, dentine; Cm, cementum; Ab, absorption cavity, 
 showing Howship's lacunae; Cm'^, new-formed cementum. (About 50 X) 
 
OSTEOCLASTS 
 
 305 
 
 sally, osteoclasts appear and begin a new excavation. In 
 this manner the process continues. When the absorption 
 stops in the second point, it begins again at the first, cutting 
 
 Fig. 236 
 
 A transverse section through an incisor from the same jaw as Fig. 235, and at 
 the level of Cm^, showing the refilling of the absorption cavity by new layers of 
 cementum. 
 
 much deeper into the dentine, and, oscillating back and forth, 
 it progresses until all of the dentine may be destroyed, leav- 
 ing the hollow cap of enamel, and even then new-forming 
 20 
 
306 THE PERIDENTAL MEMBRANE 
 
 cementum to maintain the attachment will be fomid around 
 the circumference. In this way it will be seen that the 
 function of the tooth is maintained until its successor is 
 ready to take its place in a very short time. The importance 
 of this arrangement will be more fully appreciated after a 
 study of the relation of the teeth to the development of the 
 face. Fig. 235 shows a longitudinal section through a 
 temporary incisor of a sheep. At Ah an absorption has 
 just been completed, for the osteoclasts have disappeared. 
 The excavation is seen filled with embryonal tissue and 
 rebuilding is about to begin. At Cm an older and much 
 larger absorption space is seen which has been partially 
 replaced by a formation of new cementum reattaching 
 fibers. In Fig. 23G a transverse section of the root is seen 
 which is from the same jaw cut at the level of Cm, and shows 
 the absorption refilled. This patchwork performance goes 
 on in the same way in the bone of the alveolar process, and 
 its study is one of the most interesting phases of the rela- 
 tion of the teeth to the development of the face. Without a 
 clear idea of this it is impossible to understand how the 
 teeth, after their roots are fully formed, can move through 
 three dimensions of space and retain their function all the 
 time. 
 
 Epithelial Structures.— The epithelial structures of the 
 peridental membrane were first described by Dr. Black in 
 his volume Periosteum and Peridental Memhrane, pub- 
 lished in 1887. At this time Dr. Black considered them to 
 be of lymphatic character and named them endolymphatics. 
 His conception of them was that they were lymphatic 
 channels crowded with adenoid cells. Since then the form 
 and appearance of the cells and the character of their 
 reaction with staining agents has shown the cells to be of 
 epithelial character. In the same year that Dr. Black's 
 book was published, von Brunn^ described the same struc- 
 tures. He considered them as epithelial remains of the 
 outer layer of the enamel organ, growing down around the 
 
 'ArcLiv f. Anatomie, 1887. 
 
DISTRIBUTION 307 
 
 root beyond the gingival line where the formation of enamel 
 stops. It has seemed probable to the writer that this was 
 correct, but their histogenesis has not been sufficiently well 
 followed, and it presents an attractive field for research. 
 Although this is the origin of these structures, it has never 
 seemed proper to regard them as embryonal remains, for 
 while, like all the cellular elements, they are more numerous 
 in young people than in old, they are persistent throughout 
 life. They have been shown in the membrane from a man 
 aged seventy years, and it does not seem logical to suppose 
 that embryonal debris that was useless to the organism 
 would persist through life. Up to the present time, however, 
 nothing has been discovered about these structures to throw 
 any light upon their function. Specimens have strongly 
 indicated that they were important in some pathologic 
 conditions. Their cells have been found dead and degener- 
 ating in pathologic material beyond the point showing any 
 pathologic condition in other cells. These structures 
 have been observed in sections from man, sheep, cat, dog, 
 and monkey. The best material for their study is a young 
 sheep or pig. 
 
 Distribution. — These structures are composed of cords or 
 rows of epithelial cells, surrounded by an extremely delicate 
 basement membrane (Fig. 2.37). In some cases there is a 
 slight indication of a circular arrangement of connective 
 tissue around them. The cords lie very close to the surface 
 of the cementum, winding in and out anjong the fibers 
 (Fig. 238). They anastomose and join with each other, 
 forming a network the meshes of which are comparatively 
 close in the gingival portion (Fig. 239), and comparatively 
 wide in the apical portion, the cords becoming scarcer as the 
 apex of the root is approached, but the author has seen them 
 in sections from the apical third. 
 
 A binocular microscope was used to obtain a true concep- 
 tion of the way in which these cords wind in and out among 
 the bundles of fibers. The cords show a marked tendency 
 to run out into the membrane and loop back (Fig. 240), 
 coming very close to the surface of the cementum. 
 
308 
 
 THE PERIDENTAL MEMBRANE 
 
 The ends of the loops toward the cementum often show 
 enlargements which in some cases apparently lie directly 
 in contact with the cementum (Figs. 241 and 242). These 
 enlargements next to the cementum are shown in Fig. 240. 
 
 The Arrangement of the Cells. — There is no definite arrange- 
 ment of the cells in these cords. In some places there will be 
 a ring of irregular polyhedral or rounded cells which almost 
 exactly resemble a simple tubular gland. In other places 
 there is a pretty definite outer ring of cells and a central 
 
 Fig. 237 
 
 ^s^ 
 
 M 
 
 Diagram of glands of peridental membrane. (Black.) 
 
 mass enclosed by them. The cells are made up of granular 
 cytoplasm, each containing an oxoid nucleus that is rich in 
 chromatin. The author has spent much time attempting 
 to work out the relation of these cords to the epithelium 
 lining the gingival space, thinking that possibly they open 
 into it. In a few places structures appearing very much 
 like a duct have been seen, as shown in Fig. 244, but they 
 are apparently only unusually large cords. There is no 
 regularity in places where they are found, and no con- 
 
A sectioEL cutting diagonally through the root, showing the network of epithelial 
 cords, A; dentine, D; cementum, Cm. 
 
310 
 
 THE PERIDENTAL MEMBRANE 
 
 nection with the gingival space has ever been discovered. 
 Toward the gingival, as the gingival line is approached, the 
 cords seem to swing out away from the cementum, espe- 
 cially on the proximal side, and to pass up into the gingivus, 
 where they are lost among the projections of the epithelium. 
 
 Fig. 239 
 
 Transverse section of the perideni 
 the position of the epithelial cords. 
 in Fig. 241 is seen. 
 
 il membrane in the gingival portion, showing 
 At 1 the loop shown in higher magnification 
 
 Gland of Serres. — Salter, in his Dental Pathology and Sur- 
 gery, quotes Serres, who assigns the function of a gland to the 
 epithelium lining the gingival space. This, the writer believes, 
 is the first reference to an appearance in the tissues that has 
 been called the gland of Serres. It has long been noted that 
 the epithelium lining the gingival space was lighter in struc- 
 ture, composed of larger cells, and had no horny layer on its 
 
GLAND OF SERRES 
 
 311 
 
 surface, as is true of the epithelium on the outer surface of 
 the gingivus. Upon the proximal surfaces the projections 
 
 Fig. 240 
 
 Epithelial structures of the peridental membrane (from sheep): Fb, fibroblasts; 
 Ec, epithelial structures; Cb, cementoblasts; Cm, cementum; D, dentine. (About 
 468 X) 
 
 of the epithelium which extend down between the papilla^ of 
 connective tissue, which constitute the stratum papillaris, 
 are specially long, and in the connective tissue between 
 
312 
 
 THE PERIDENTAL MEMBRANE 
 
 them collections of small round cells are often found. It 
 is between these projections of epithelium that the cords of 
 epithelial cells which have been described are lost, and to 
 this portion of the tissue Dr. Black has again called attention, 
 
 Fig. 241 
 
 Epithelial structures: Ec, epithelial cord, apparently showing a lumen; Ch, cemento- 
 blasts; Cm, cementum; D, dentine. This loop is seen in Fig. 226. 
 
 as the gland of Serres. Sufficient work has not xo^t been 
 done upon this subject to know whether this is a constant 
 arrangement, or whether it is found only in certain animals, 
 or even whether it may not possib'y be pathologic. The 
 appearance is shown in Plate XVI and Figs. 245 and 246. 
 
BLOODVESSELS 
 
 313 
 
 Bloodvessels. — The peridental membrane possesses a very 
 rich blood supply. A number of vessels enter the membrane 
 in the apical portion from the medullary spaces in the bone. 
 
 Fig. 242 
 
 Transverse section, showing the cellular elements: Fb, fibroblasts; Ec, epithelial 
 structures: Cb, cementoblasts; Cm, cementum; D, dentine, (About 900 X) 
 
 Some of these, passing through canals in the apex of the root, 
 supply the dental pulp, others pass up through the mem- 
 brane. As they extend occlusally they give off and receive 
 branches which enter the membrane from the bone of the 
 
:U4 
 
 THE PERIDENTAL MEMBRANE 
 
 alveolar wall. In this way the caliber of the principal vessels 
 is maintained throughout their course in the membrane. As 
 they reach the border of the alveolar process they give oflF 
 
 Fig. 243 
 
 Epithelial structures (from sheep): /'7), fibrohhists; /w, ei)itheHal structures; Cb, 
 cementoblasts; Cm, cementuni; D, dentine. (About 700 X) 
 
 branches which anastomose with the vessels of the perios- 
 teum and gum tissue. These are shown in Plate XVII. 
 In the young membrane these vessels occupy a position 
 
A 
 
 m: 
 
 Wp^^^ 
 
 \ I 
 
 r 
 > 
 
 X 
 
 < 
 
 "^ ^^ ' 
 
BLOODVESSELS 315 
 
 closer to the bone than the cementiim, and as the membrane 
 becomes thinner they often come to He in grooves in the bone. 
 Vessels of any size are rarely seen close to the cementum, and 
 the capillaries in the membrane are rather scarce, though 
 they are more numerous than in most connective tissues of 
 as compact a character. The anastomosis of the vessels in 
 the membrane is quite rich. It is important to remember 
 
 Fin. 244 
 
 A very large cord which was at first mistaken for a duct. 
 
 that the cancellous bone of the process is richly supplied with 
 bloodvessels, and the anastomosis with the vessels of the 
 membrane, from the alveolar wall and over the border of 
 the process, is important in the consideration of pathologic 
 conditions. In alveolar abscess the vessels entering through 
 the apical space ma}' be entirely cut ofl', but this does not 
 disturb the blood supply of the rest of the membrane. The 
 removal of the pulp has often been advocated in the treatment 
 
316 
 
 THE PERIDENTAL MEMBRANE 
 
 Fia. 245 
 
 Longitudinal section, cut mesiodistally, similar to Plate XVI: D, dentine; Cm, 
 cementum which has separated from the dentine; Gs, gingival space; Ep, epithelial 
 projection from the lining of the gingival space, Ec, epithelial cords; Rs, small 
 round cells in the connective tissue. 
 
BLOODVESSELS 
 
 317 
 
 Fig. 24G 
 
 A longitudinal section cut mesiodistally: E, epithelium of the gingivus; Gs, gingivs 
 space; Cm, cementum -which has separated from the dentine; Ec, epithelial cords. 
 
318 THE PERIDENTAL MEMBRANE 
 
 of pathologic conditions of the membrane, on the ground 
 that the vessels entering the pulp rob the membrane of 
 blood supply, and that their removal made recovery more 
 certain. No one having a knowledge of the blood supply of 
 the membrane could advise this for this reason. 
 
 In their course through the membrane the vessels wind 
 between the principal fibers in a way that can only be appreci- 
 ated by studying sections with a binocular instrument, and 
 when this condition is realized it can be understood how some 
 inflammations in the membrane are set up. For instance, when 
 force is applied to a tooth, the principal fibers are stretched. 
 This causes them to close some spaces and open others. 
 The vessels in the closed spaces are constricted and the flow 
 of blood through them partly shut off. The vessels in the 
 enlarged spaces dilate to compensate. If the force is removed, 
 the dilated vessels are again constricted, and the constricted 
 ones enlarged, and the result is a literal sawing upon the 
 walls of the bloodvessels which in a very short time will set 
 up an acute inflammation. This is extremely important in 
 the application of force in orthodontia, and often also in the 
 use of the mallet in condensing gold, especially for young 
 patients. 
 
 Nerves. — The nerves of the peridental membrane enter 
 the peridental membrane in company with the bloodvessels. 
 Their source is the same as that of the bloodvessels. The 
 trunks entering in the apical space contain from eight or 
 ten to fifteen or twenty medullated fibers. Some of these 
 enter the dental pulp, others extend up through the mem- 
 brane, winding in and out among the fibers, generally 
 following the course of the bloodvessels. Many trunks 
 containing eight or ten fibers enter through the alveolar wall. 
 In this way a fairly rich plexus is formed, from which fibers 
 are continually given off to be lost in the tissue. They 
 probably terminate in beaded free endings. No special 
 nerve endings have been demonstrated. A few Pacinian 
 corpuscles have been seen near the gingival border. These 
 are not generally found, however. The nerves of the mem- 
 brane give to it the sense of touch, which is the only sensory 
 
CHANGES IX PERIDENTAL MEMBRANE WITH AGE 319 
 
 function of the membrane. As has been noted in connec- 
 tion with the dental pulp, the hard tissues and the pulp have 
 no sense of touch. The contact of any substance with the 
 surface of the tooth is reported to consciousness through 
 the medium of the peridental membrane. For instance, the 
 slightest touch of a delicate instrument produces a slight 
 movement of the tooth which affects the nerves between 
 the- fibers. The delicacy of this mechanism can be demon- 
 strated by the following experiment. Lightly touch the 
 surface of the enamel and the patient will tell at once not 
 only which tooth is touched, but whether a steel instrument 
 or a wooden point or some soft material was used. If, 
 however, the finger is placed upon a surface of the tooth and 
 firm pressure made in one direction, the contact of the point 
 will not be recognized. 
 
 The Changes in the Peridental Membrane with Age. — The 
 teeth are formed in crypts in the bone, and when they begin 
 to erupt the roof of the crypt is removed by absorption, 
 making an opening large enough for the crown to pass. As 
 the root is formed, the tooth moves occlusally and the 
 alveolus grows up around it, beginning at the margins of the 
 crypt. When the tooth first erupts, therefore, the alveolus 
 is much larger than the root, and the fibers of the peridental 
 membrane are very long. The size of the alveolus is reduced 
 by the formation of bone, by the osteoblasts on its wall, and 
 the size of the root is increased by the formation, layer after 
 layer, on its surface. In this way the thickness of the mem- 
 brane is reduced. Figs. 247 and 248 were made to illustrate 
 this change. They were photographed with as nearly the 
 same magnifications as possible, so as to compare the thick- 
 ness of the layers. In the first, there are but two layers of 
 cementum; notice the thickening of the last-formed layer, 
 to attach the strong bundles of fibers at the angle of the root. 
 The second is from a temporary tooth which has been in 
 position and function for a long time; notice the thickness 
 of the cementum and that the formation of bone and cemen- 
 tum has reduced the thickness of the membrane to not more 
 than one-third of its original amount. Notice also that the 
 
320 
 
 THE PERIDENTAL MEMBRANE 
 
 surface of both bone and cementum are not even, but 
 scalloped, and that where the cementum projects toward 
 the alveolar wall there is a depression in the bone, and where 
 the bone projects toward the cementum there is a depression 
 in the cementum. There is, therefore, a distinct tendency 
 
 Young membranes (from sheep): D, dentine; Cm, cementum; C'mi, thickening of 
 cementum to attach fibers at the corner; Pd, peridental membrane; B, bone forming 
 the wall of the alveolus, (About 80 X ) 
 
 for the two tissues to interlock but remain separated by a 
 layer of fibrous tissue. The author has never seen a specimen 
 showing a union between calcified substances of bone and 
 cementum. Two surfaces of cementum may become united 
 by direct calcification and the teeth fused together. This 
 
PRACTICAL CONSIDERATION 
 
 321 
 
 is illustrated in many freak specimens to be found in any 
 dental museum. It is often stated that a tooth had become 
 ankylosed to the bone, but to the author's knowledge no 
 specimen has ever been shown in which the separating 
 layer of fibrous tissue was not present. 
 
 Fig. 248 
 
 Old membranes (from sheepi: D, dentiue: Cm, cemeuiuiu; Pd, pendeutal mem- 
 brane; 5, bone forming the wall of the alveolus; P, pulp. (About 80X) 
 
 Practical Consideration. — These structural facts are of the 
 greatest practical importance, especially in the making of 
 gold fillings for young persons. Every operator has noticed 
 the greatest difference in the feeling of the instrument under 
 the mallet upon difterent teeth. In one instance it will ring 
 under the steel mallet as if the tooth were resting on an anvil; 
 21 
 
322 THE PERIDENTAL MEMBRANE 
 
 ill {uiotlier case it feels as if the tooth were resting on a 
 cushion. In the first case all of the force of the blow is 
 expended in the condensation of the gold. In the second, 
 a large proportion is lost in the movement of the tooth. 
 If the membrane is thin and the cementum and bone are 
 interlocked, the tooth is firmly supported. If the membrane 
 is thick and the fibers long, as in the first illustration, the 
 blow is dissipated in the sag of the fibers. The tooth is 
 jumping up and down in its socket. The force used is 
 dissipated, the gold is not condensed, and in a very short time 
 an acute inflammation is set up and the tooth becomes very 
 sore to the blows. This the author believes is the explanation 
 of the idea that gold will not preserve teeth for young 
 children. It has often been said that children's teeth are 
 too soft for gold fillings. The difficulty is not with the 
 enamel and dentine, but because of the thickness of their 
 membranes. The gold is not sufficiently condensed to 
 exclude moisture, and the fillings fail. Serious damage also 
 may be done to the membrane. The Museum of the North- 
 western University Dental School contains an object lesson 
 on this point. It consists in a bicuspid with a beautifully 
 condensed and finished gold filling, in a mesial occlusal cavity. 
 The history accompanying it is somewhat as follows: The 
 operation was undertaken for a patient aged about fourteen 
 years. The tooth became exceedingly sore under the mallet; 
 the filling was, however, completed and polished, but a few 
 days later the tooth was picked out with the fingers. The 
 peridental membrane had been literally hammered to death. 
 Stated in scientific terms, the fibers had sawed upon the 
 bloodvessels, exciting an acute inflammation, resulting in 
 complete stasis and the death of the tissue. In all opera- 
 tions where gold is to be condensed in teeth with thick 
 membranes they must be firmly supported so as to be held 
 rigidly against the blow. 
 
CHAPTER XXIV 
 
 THE IMOUTH CAVITY 
 
 Mucous Membrane. — The mucous membrane lining the 
 mxouth cavity is composed of a layer of stratified squamous 
 epithelium supported upon a tunica propria, which is usually 
 described as composed of two parts^ — the papillary layer 
 and the reticular layer. The epithelium and the tunica 
 propria make up the mucous membrane proper, which is sup- 
 ported upon a submucous layer composed of a coarse network 
 of white and elastic fibers, containing the larger bloodvessels. 
 
 The Epithelium. — The stratified squamous epithelium is 
 provided with a horny or corneous layer only in the por- 
 tions covering the alveolar process and the hard palate, or, 
 in other words, where the submucosa is firmly attached to 
 the periosteum (Fig. 249). In these positions the horny 
 layer consists of dead cells which have lost their nuclei 
 and whose cytoplasm has been converted into keratin or 
 horny material. 
 
 These scale-like cell remains are closely packed into a 
 protective layer. There is no distinct stratum lucidum 
 separating the dead from the living cells, as there is in the 
 skin. In the deeper portions the cells possess oval or rounded 
 nuclei and become larger and more polyhedral as the base- 
 ment membrane is approached. The cells of the deepest 
 layer next to the basement membrane are tall and approach 
 the columnar form, but are never much greater in height 
 than width. The deep layer is often called stratum ]\Ial- 
 pighii. The epithelium lining the gingival space and that 
 coverings unattached portions is without the horny laj'er, 
 and the cells are larger and more loosely placed. The poly- 
 hedral cells in the middle portion of the layer show distinct 
 
324 
 
 THE MOUTH CAVITY 
 
 intercellular spaces across which the cytoplasm extends 
 in intercellular bridges. 
 
 Isolated cells from this region show the broken bridges 
 projecting from their surface, and for this reason have been 
 called "pickle or prickle cells." In these positions the thick- 
 ness of the epithelial layer is usually greater than in the 
 attached portions of the membrane (Fig. 250). 
 
 Fig. 249 
 
 Stratified squamous epithelium covering the alveolar process: C, corneous layer; 
 P, papilla of connective tissue. (About 400 X) 
 
 Tunica Propria. — The connective-tissue layer of the mucous 
 membrane interlocks with the epithelial layer by means of 
 the tunica papillaris, which is composed of very delicate 
 white and elastic connective-tissue fibers. They are usually 
 about half as tall as the thickness of the epithelium, and 
 about one-third as wide as they are tall. The height and 
 character of the papillae varies greatly, however, in different 
 positions. In the red border of the lip and in the epithelium 
 
GLANDS OF THE SUB MUCOSA 
 
 325 
 
 lining the gingival space they are very tall and narrow, and 
 approach very close to the surface of the epithelium. Over 
 the gums and the palate they are much shorter and wider and 
 do not extend more than half-way through the epithelium. 
 These papilla? contain loops of capillary bloodvessels, and 
 in some special nerve endings are found. 
 
 Fig. 250 
 
 Stratified squamous epithelium from unattached mucous membrane of the mouth. 
 The corneous layer is absent. (About 200 X) 
 
 Reticular Layer. — The reticular layer joins the papillary 
 layer without any line of demarcation, and is composed of 
 the same kind of tissue, the fibers being arranged in a deli- 
 cate network. Everywhere in the tunica propria are found 
 ducts from mucous glands which lie in the deeper layers. 
 
 Submucosa. — The submucosa is composed of firm connec- 
 tive tissue in which the white fibers are in large, strong 
 bundles, and elastic fibers are scarce. It contains two 
 plexuses of bloodvessels, both more or less parallel with 
 the surface. The outer is composed of small vessels forming 
 a smalk meshed network, the deeper of large vessels more 
 \\idely separated. Lymphatic vessels everywhere follow 
 the course of the bloodvessels. 
 
326 
 
 THE MOUTH CAVITY 
 
 Glands of the Submucosa. — The siibmiicosa contains a great 
 many small tubular glands. These are distributed widely 
 over the tongue and membrane of the cheek and lip (Fig. 
 251). They are branched tubular glands, sometimes simple 
 
 Fig. 251 
 
 k. 
 
 Mucous 
 (jkmd' 
 
 Epitlielinm [ 
 of mucous. -.L 
 memhrane 
 
 Blood- 
 
 vessplfi 
 
 Epithelium 
 of mucous 
 membrane \/ 
 
 Cross sections^ 
 of muscle'-~-\{-~. 
 
 P- 
 
 
 '^O. Ffnir 
 
 ■ ]\ ' foJIicles 
 
 -yr—Fnt 
 
 -f-'Epidermis 
 
 f ^,(ll (1)1(1 s 
 
 _ _ - - - ^ of muscle 
 ibres 
 
 Mucous 
 membrane 
 tvith high'' 
 
 papillse " 
 
 Longitudinal 
 sections 
 
 Place irhere stratum 
 corneum begins 
 
 Section through the upper lip of a two-and-a-half-j^ear-old child. (14 X) 
 
THE TONGUE 
 
 327 
 
 and sometimes compound. The body of the gland is always 
 in the submucosa, though it may extend into the underlying 
 muscle. Some are serous and others mucous, while many of 
 the larger ones contain cells of both types. The secretion 
 of these glands is probably much more important than has 
 been supposed. 
 
 Nerve Endings in the Mucosa. — Sensory nerve endings of 
 two kinds are found in the mucous membrane. Krause's 
 end bulbs are found in many of the papillae, and other nerves 
 terminate in free endings lying between the epithelial cells. 
 
 Fig. 252 
 
 A section from the side of the tongue: E, epithelium; Sin, submucosa; Bv, blood- 
 vessels; Af, muscle fibers; G, mucous glands. 
 
 The Tongue. — The tongue is composed of a mass of volun- 
 tary muscle fibers arranged in complicated interlacing bun- 
 dles, covered by the mucous membrane. The most striking 
 characteristics of the mucous membrane of the tongue (Fig. 
 252) are: (1) The thinness of the submucosa, which holds 
 
328 THE MOUTH CAVITY 
 
 it closely to the mass of muscle and allows very little move- 
 ment of it; (2) the submiicosa in the dorsal surface contains 
 no glands, though there are glands among the muscle fibers 
 whose ducts pass through the submucosal (3) the presence 
 of the epithelial papilla upon its dorsal surface. The tongue 
 is imperfectly divided vertically on the median line by a band 
 of connective tissue forming the median raphe or septum, 
 which causes the depression at the central line of the dorsal 
 surface. 
 
 The Muscles. — The muscles of the tongue include two 
 groups — the extrinsic and the intrinsic. The extrinsic mus- 
 cles comprise the genioglossus, the hyoglossus, the styloglos- 
 sus, and the palatoglossus. These are all paired and extend 
 from the skull or the hyoid bone into the tongue. The 
 intrinsic muscles comprise the principal muscles of the 
 tongue, the lingualis. A transverse section through the 
 body of the tongue in the central portion shows a compli- 
 cated network of muscle fibers running in three directions — 
 longitudinally, transversely, and vertically. The longitu- 
 dinal fibers are arranged around the outer portion, forming 
 a cortical layer about 5 mm. thick. These constitute the 
 chief bulk of the lingualis, supplemented by fibers from the 
 styloglossus. The vertical fibers are mostly deeply placed 
 in the central portion on either side of the raphe. They are 
 chiefly derived from the genioglossus and radiate toward the 
 dorsal surface. The transverse fibers are entirely from the 
 lingualis except for a few from the palatoglossus. They arise 
 from the septum and interlace with the longitudinal and 
 vertical fibers. They break up into strands running between 
 the longitudinal fibers of the cortical portion, and spread 
 out to a submucous insertion. 
 
 The complicated movements of the tongue are accom- 
 plished by the contractions of these sets of muscles. When 
 the longitudinal fibers are relaxed and the transverse fibers 
 contracted the tongue is rolled and extended. When the 
 transverse fibers are relaxed and the vertical fibers contracted 
 the tongue is flattened. The division of the tongue on the 
 median line by the septum allows each half to work inde- 
 
THE PAPILLA 
 
 329 
 
 pendently, so that when the longitudinal fibers are con- 
 tracted on one side and relaxed on the other the tip of the 
 tongue is moved sideways. 
 
 The Papillae. — The roughness of the dorsal surface of the 
 tongue is caused by projections of the epithelium resting 
 upon the tunica propria, forming the papillae of the tongue. 
 These projections are not to be confused with the connective- 
 
 FiG. 253 
 
 Mucous membrane from the dorsal surface of the tongue of a kitten, showing 
 filiform and fungiform papillse. 
 
 tissue papillae in the tunica propria of the mucous membrane. 
 They are of three kinds — the filiform and fungiform papillae, 
 which are found over the entire dorsal surface, and the 
 circumvallate papillae, which are limited in number and 
 confined to the posterior portion. The filiform are much 
 the more numerous, especially near the tip of the tongue. 
 They are from 0.5 to 2.5 mm. in height, and often end in 
 brush-like strands of epithelial cells. 
 
3:^0 
 
 THE MOUTH CAVITY 
 
 The fungiform papillse form the red points on the surface 
 of the tongue, especially near the edges, because of the 
 
 Fig 254 
 
 Mucous membrane from the tongue of a rabbit, showing circum vallate papills 
 with taste buds on their sides. 
 
 Fig. 255 
 
 P h 
 
 s 
 
 A section of a taste bud: p, pore; g, gustatory cells; ep, epithelial cells; 
 s, sustentacular cells; h, bristles of the gustatory cells. (Schaefer.) 
 
THE TONSIL 331 
 
 thinness of their epithelium. They are low and rounded in 
 form, from 0.5 to 1.5 mm. in height, and are named from 
 their mushroom-like appearance. Fig. 253, a section from 
 the tongue of a kitten, shows the form of both of these 
 papillae. The circumvallate papillae usuall}^ number nine 
 or ten, and are arranged in a V-shaped form near the base 
 of the tongue, with the apex extending backward. They are 
 from 1 to 1.5 mm. in height and from 2 to 3.5 mm. in width. 
 They are surrounded by a depression, so that the upper 
 surface of the papillae is not much above the general level 
 of the membrane. 
 
 The Taste Buds.— These are found chiefly on the sides of the 
 circumvallate papillae (Fig. 254), though they are occasionally 
 found in the epithelium of the fungiform papillae and the soft 
 palate, and on the posterior surface of the epiglottis. They 
 are always entirely embedded in the epithelium and extend 
 through its entire thickness. The structures are ovoid in form, 
 with the rounded end toward the connective tissue and the 
 pointed end at the surface, where a small opening, the taste 
 pore, communicates with the mouth cavity (Fig. 255). INIost 
 of the cells are elongated and spindle-shaped, and arranged 
 like the leaves of an onion. Four varieties may be recog- 
 nized. The outer sustentacular cells form the outer layer 
 and are in contact with the epithelial cells. They are 
 elongated, with an oval nucleus near the centre. The inner 
 sustentacular are rod-shaped cells, more slender in form, 
 with a nucleus at the base. The neuro-epithelial cells are 
 elongated, spindle-shaped cells at the centre of the taste bud. 
 The nucleus is at the base of the cell, and from the opposite 
 end a stiff bristle-like process extends through the taste pore. 
 
 The basal cells are irregular in form with large oval nuclei; 
 they communicate with each other and the sustentacular 
 cells by cytoplasmic bridges. They form the base of the 
 taste bud. The function of the taste buds is probably 
 related to the function of deglutition rather than the sensa- 
 tion of taste. 
 
 The Tonsil. — In the posterior part of the tongue and the 
 wall of the pharynx is found adenoid tissue in the form of 
 
332 
 
 THE MOUTH CAVITY 
 
 solitary follicles lying in the tunica propria and invading 
 the epithelium. This adenoid tissue forms an organ which 
 Waldeyer has called the lymphatic pharyngeal ring. This 
 tissue is divided into three main masses — that lying in the 
 base of the tongue forming the lingual tonsil, that associated 
 with the palate and lying between the pillars of the pharynx 
 and forming the palatine tonsil, and that situated in the 
 pharynx or pharyngeal tonsil. 
 
 .^- 
 
 .^, 
 
 Epithe- 
 lium 
 
 TanicK 
 
 propria 
 
 Lympli 
 
 nodule 
 
 Oblique 
 
 section "j"'".- .^c;i^. 
 of duct — p--^ - -' 
 of raucous 
 gland 
 
 Muscle 
 
 fibres 
 cut — ii^. 
 
 trans- 
 versely 
 
 \^ar<rXZ- 
 
 Adenoid 
 tissue 
 
 Section through a Ungual foUicle in man: x, crypt. (50 X) (Szymonowicz.) 
 
 The Lingual Tonsils. — These are situated in the base of 
 the tongue between the circum vallate papillae and the 
 epiglottis. They are rounded masses of adenoid tissue 
 composed of solitary follicles lying mostly in the tunica 
 propria, and causing projections of the surface that are 
 easily seen. In the centre of each mass is a deep depression 
 forming a blind pouch, known as the crypt (Fig. 256). This 
 is lined with stratified squamous epithelium like that of the 
 adjoining mucous membrane except that at various places 
 
THE TONSIL 
 
 333 
 
 the lymphocytes have pushed their way through the 
 epitheUal cells, and escape on the surface. 
 
 Epithelium 
 of pharynx— -p- 
 
 Fig. 257 
 Epithelium of crypt 
 
 \ Fnllidc I 
 
 BJood vessel 
 
 X 
 
 Mucous glands<' 
 
 fe#f 
 
 
 '¥ 
 
 V 
 
 Connective-tissue 
 capsule 
 
 Section through a dog's tonsil. At x x there are seen leukocytes which have 
 wandered out from the follicles. (15 X) (Szymonowicz.) 
 
334 THE MOUTH CAVITY 
 
 The Palatine Tonsils.— These lie at the base of the tongue 
 between the anterior and the posterior pillars of the pharynx. 
 They are much larger than the lingual tonsils and are com- 
 posed of from ten to twenty follicles and a number of crypts. 
 The epithelium covering them is pierced in many places by 
 encroachments of the adenoid tissue. The crypts always 
 contain many lymphocytes (Fig. 257). These are what are 
 ordinarily called the tonsils, the infection of which produces 
 tonsillitis. 
 
 The Pharyngeal Tonsils. — These lie on the posterior wall 
 of the nasal phar^^nx above the level of the palate. Their 
 structure is similar to that of the palatine tonsil. The 
 crypts are five to six in number and are often clothed with 
 ciliated epithelium. Into them open the ducts of mixed 
 glands which form a distinct layer under the follicle. Here 
 also there is a migration of lymphocytes through the epi- 
 thelium. It is the hypertrophy of these which form the 
 adenoids so often found in children. 
 
CHAPTER XXV 
 
 BIOLOGICAL CONSIDERATIONS FUNDAMENTAL TO 
 EMBRYOLOGY 
 
 History.— Before beginning the study of embryology 
 some topics in general histology must be reviewed, and some 
 general biologic ideas considered. No real conception of 
 the complicated processes of individual development can be 
 obtained without laying a foundation in the study of the 
 cell as the units of life and the mechanism through which 
 the phenomena of life are manifested. 
 
 In embryology it is found that the individual in his physical 
 development passes through stages which correspond to 
 the development of the race or species to which he belongs, 
 and a like comparison might be drawn in mental develop- 
 ment and the acquirement of knowledge. This is specially 
 true of the subject of embryology. 
 
 Apparently the first ideas to occupy the speculative 
 thought of man when he became conscious of himself as an 
 independent being were the questions of his origin and the 
 relation to his environment and destiny. These have become 
 the basis for the development of all religious thought. 
 
 Up to the beginning of the nineteenth century all con- 
 siderations of these subjects were purely speculative. The 
 old question of "What is life?" received endless discussion. 
 In the nineteenth century this question has been dropped into 
 the background, and the question, "What is the mechanism 
 of life?" has been substituted for it. The consideration 
 of the latter question has resulted not only in the mar- 
 vellous advancement of medical knowledge and suigical 
 skill, but in the great development of deeper fundamental 
 thoughts. It must not be forgotten, however, that the 
 development of knowledge resulting from the considera- 
 
?)?)C) BIOLOGICAL CONSIDERATIONS OF EMBRYOLOGY 
 
 tion of the latter question has not and does not promise to 
 answer the old question, ''What is life?" any more than the 
 laws of electricity and their application to its use answer 
 the question, "What is electricity?" 
 
 The discovery of the cell hypothesis and the propounding 
 of the theory of organic evolution have been the greatest 
 factors in the unification of knowledge and the stimulation 
 of thought in these fields. It is interesting to notice that 
 these two theories, closely related as they have become, had 
 entirely independent origins and were long followed out with- 
 out any immediate connection. The theory of evolution w^as 
 based upon consideration of the forms of living things, their 
 distribution and adaptation to environment. 
 
 The Cell Theory. — The cell theory had its origin in the study 
 of miiuite forms. Its beginnings were made possible by the 
 development of the compound microscope, which revealed 
 their structure and showed them to be small bodies made up 
 of apparently a structureless, granular material which was 
 called protoplasm, or the ultimate substance of life. This 
 material, as its name indicates, was originally supposed to 
 be simple in structure and composition and to be the life 
 substance. Huxley's characterization of it as the "physical 
 basis of life" w^as the beginning of the study w^hich has 
 revealed it to be very far from a simple substance, but rather 
 extremely complex both in structural arrangement and 
 chemical composition. In more recent biology, therefore, 
 the word protoplasm is being dropped and the word cyto- 
 plasm or cell substance substituted for it. 
 
 The early history of the cell theory was obstructed in 
 its development by the remains of the old Greek idea that 
 living things could originate from non-living matter, that the 
 swamp breeds disease, and the decomposing body of an 
 animal bred maggots. It required fifty years of work on 
 the cell theory for Virchow, in 1850, to propound his thesis 
 that all living cells are derived from a preexisting cell, and 
 so establish the continuity of life, which has flowed on from 
 the beginning in an uninterrupted stream, each individual 
 being only a period. 
 
CELL DIVISION 337 
 
 When Schwank and Schleiden showed that the bodies of 
 both plants and animals, instead of being made up of homo- 
 geneous tissue, were composed of millions of structural 
 elements which they called cells, the consideration of both 
 plants and animals were for the first time put upon a common 
 basis. Naturally enough the first thing to attract attention 
 was the study of the form and arrangement of these struc- 
 tural elements in the tissues of animals and plants 
 
 In following out this study it became more and more 
 evident that, while infinitely varied in the detail of their 
 form and structure, all cells had a common plan of organi- 
 zation and possessed structural characteristics common to 
 all, at least in some stages of their history. 
 
 Relation of the Nucleus to the Protoplasm. — The first point 
 to be discovered in the internal organization of the cell was 
 the nucleus, the meaning of which and its relation to the 
 cytoplasm at once attracted attention. As the result of a 
 vast amount of work, it was gradually established that the 
 nucleus "exerts a controlling and directing influence over 
 the activity of the cytoplasm;" that a cell deprived of its 
 nucleus would continue to live for a longer or shorter time, 
 but that it would not grow and would not reproduce another 
 cell; that the phenomena of life manifested by destructive 
 metabolism would continue until the identity of the cyto- 
 plasm was destroyed, but there would be no constructive 
 metabolism. The work of the cytoplasm is, therefore, 
 dependent upon the character of the nuclear material. 
 
 Cell Division. — As first observed, cell division was supposed 
 to be an irregular cutting of the cytoplasm and the nucleus 
 in two, forming two individual cells. The cytoplasm by its 
 constructive changes does not continue to increase indefi- 
 nitely, but as soon as a certain size is reached it divides, a 
 portion of the nucleus going to each of the parts, which 
 immediately begin to increase in size. It was soon found 
 that cell division was not always so simple, and that in some 
 cases changes in the nucleus preceded the division of the 
 cytoplasm. Two forms of cell division are therefore de- 
 scribed, the simple or direct, and indirect or karyokinetic 
 
338 BIOLOGICAL CONSIDERATIONS OF EMBRYOLOGY 
 
 t'oll di\'ision. The simple is now known to l)e comparatively 
 rare. 
 
 Indirect Cell Division. — Indirect cell division must be con- 
 sidered as a means by which the chromatic material of the 
 nucleus is equally and systematically distributed to the 
 resulting cells. The nucleus, in cell division, contains a 
 beautiful structural mechanism, by which the material 
 which is to control the development of the resulting cells 
 and their activity is definitely distributed to them. In this 
 process there is no irregularity in the kind or amount of 
 material given to the two cells. 
 
 In this process the chromatin of the original nucleus is 
 divided into a definite number of pieces which are split in 
 two, and half of each sent to each new nucleus, where they 
 form its chromatin network. 
 
 The Vehicle of Transmission. — It was discovered that the 
 number of chromosomes was constant in every cell division 
 for all the cells of all the tissues of the given species, and was, 
 therefore, a characteristic of the species; and that in all the 
 cells of the body it was always an even number, and that 
 in the germ cells of the species the number of chromosomes 
 was exactly half that in the cells of the body. This led to 
 the immediate recognition of the chromatic material as 
 the vehicle of transmission. When in the study of fertiliza- 
 tion it was found that fertilization consists in the union 
 of two cells, each contributing both cytoplasm and nucleus, 
 and that the amount of chromatic material was equal from 
 each, and exactly half that found in the cells of the parent 
 body, the equality of the sexes in transmission was firmly 
 established upon a cytologic basis. It is interesting to note 
 that this equality had previously been claimed by the dis- 
 ciples of the evolutionary theory, and it was in this field 
 that the evolutionary theory and the cell theory first met 
 on common grounds (about 1875). 
 
 All the advancement in modern thought concerning hered- 
 ity and transmission has resulted from these discoveries. 
 The practical results are perhaps still more important in 
 the artificial breeding of plants and animals, adapting them 
 
CHEMICAL IDEAS 339 
 
 to their environment. The work of such men as Burbank 
 may be said to be the apphcation of the knowledge of the 
 mechanism of cell division and inheritance to horticulture 
 and agriculture. 
 
 Chemical Ideas. — At the present time the structural 
 mechanism of life, while inviting many fields for research, 
 may be said to have nearly reached the limit of possibilities 
 of observation, and at the present time the chemical phase 
 is attracting the greatest attention. Such questions as, 
 "How does the nucleus influence the activity of the cyto- 
 plasm?" are being eagerly investigated. Cytoplasm, while 
 enormously complex in chemical composition, must, never- 
 theless, always be thought of as performing its vital func- 
 tions by chemical activity. It is constantly building simpler 
 molecules into its own, and so increasing in amount. For 
 this its surface must be bathed in materials with which it 
 can react. It is evident that if the mass increased indefi- 
 nitely the volume would increase much more rapidly than the 
 surface, and this puts a limit upon the growth. 
 
 The constructive metabolism of the cytoplasm is depend- 
 ent upon the presence of the chromatin in the nucleus. In 
 the process of metabolism, therefore, there must be inter- 
 action between the chemical substances of the chromatin, 
 cytoplasm, and food material. The development of physio- 
 logic chemistry is rapidly affecting the ideas of the cause 
 and treatment of disease, and especially the production of 
 immunity and susceptibility. 
 
 If the dental profession is to keep pace with the develop- 
 ment in these fields and apply the results of investigation to 
 the treatment of diseases of the mouth, the study of the 
 fundamental sciences must be more thorough. 
 
CHAPTER XXVI 
 EARLY STAGES OF EMBRYOLOGY 
 
 Since fertilization consists essentially in the union of the 
 chromatin from two cells, and as the result of the union 
 restores the normal amount of chromatin for the cells of 
 that species, it is evident that in some way the germ cells 
 must be prepared for fertihzation by the loss of half their 
 chromatin. This process was first observed in case of the 
 ovum. 
 
 Maturation. — In observing fertilization of eggs of the star- 
 fish and various threadworms, it was noticed that before 
 fertilization occurred the nucleus of the ovum divided with 
 karyokinetic figures, forming three small bodies known as 
 polar bodies. This process is diagrammed in Fig. 258. In 
 reality, the ovum first divides, forming one polar body; the 
 polar body and the ovum both then divide again, so that the 
 result of the two series of division is the formation of four 
 cells, one of which is functional, three disappearing. This 
 process is practically universal in the formation of ova of 
 both plants and animals. The cells in the ovary which form 
 the ova are called oogonia. The cells formed from these 
 are the primary oocyte. The division of this cell produces 
 two secondary oocytes, of which one disappears later. The 
 division of the secondary oocyte results in the ovum and 
 three polar bodies. The number of chromosomes in the 
 primary oocyte is half the number characteristic of the 
 somatic cells, but they are made up of four pieces. In the 
 secondary oocytes they are the same number but double. In 
 the ovum and polar bodies they are the same in number and 
 single. 
 
 Spermatogenesis. — Exactly the same series of changes 
 occur in the formation of the spermatozoa. They are 
 
SPERM A TOGENESIS 
 
 341 
 
 illustrated in Fig. 259. On the outer wall of the seminiferous 
 tubules are two forms of cells, the spermatogonia and the 
 cells of Sertoli (Fig. 260) . The cell of Sertoli increases in size 
 
 Fig. 258 
 
 Diagram illustrating the reduction of the chromosomes during the maturation ot 
 the ovum: o, ovum; oc^, oocyte of the first generation; oc-, oocyte of the second 
 generation; P p, polar bodies. (McMurrich.) 
 
 and spreads out against the basement membrane, pushing 
 the spermatogonia away from it. They now divide, forming 
 two cells, one of which returns to the basement membrane 
 
342 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 and remains as the spermatogonia, the other becomes a 
 primary spermatocyte. The primary spermatocytes divide, 
 forming a secondary; the secondary divide, forming sperma- 
 tids, which develop directly into spermatozoa. By com- 
 
 FiG 259 
 
 jr^ 
 
 Diagram illustrating the reduction of the chromosomes during spermatogenesis: 
 sci, spermatocyte of the first order; sc^, spermatocyte of the second order; sp, 
 spermatid. (McMurrich.) 
 
 paring the diagrams they will be seen to correspond exactly 
 with the formation of the ova, except that all of the cells 
 are small and motile. The nuclear changes also correspond 
 to those of the ova, the primary spermatocyte having 
 half the number of tetrad chromosomes, the secondary half 
 
FERTILIZATIOX 
 
 343 
 
 the number of diad, and the spermatids half the number of 
 monad chromosomes. 
 
 Fertilization. — Fertilization is essentially the same in the 
 sexual reproduction of all plants and animals. It may be 
 easily observed in the transparent cells of such animals as 
 the starfish and the threadworm. The spermatozoon enters 
 the cytoplasm of the ova, where it immediately loses its 
 characteristic form and develops into a typical nucleus (Fig. 
 261). The ovum now has two nuclei, one of which is called 
 
 Fig. 260 
 
 Diagram showing stages of spermatogenesis as seen in diflferent sections of a 
 seminiferous tubule of a rat: s, Sertoli cell; sc^, spermatocyte of the first order; 
 sc^, spermatocyte of the second order; sg, spermatogone; sp, spermatid; sz, sperma- 
 tozoon. (Von Lenhossek's diagram, from McMurrich.) 
 
 the male pronucleus, the other the female pronucleus. These 
 both form chromosomes, the number from each being half 
 the number typical of the species. These are arranged as 
 usual between the centrosomes. They divide longitudinally, 
 each forming two, one of which passes to either centrosome, 
 where a new nucleus is formed, and in the meantime the 
 cytoplasm has divided so that two cells are formed. The 
 nuclear material of these two cells has, therefore, been 
 equally derived from the two parents, and it is to control 
 all of the future development of the individual. 
 
Fig. 261 
 
IIOLOBLASTIC SEGMENTATION 
 
 345 
 
 SEGMENTATION 
 
 Holoblastic Segmentation. — An idea of the development of 
 the embryo can perhaps best be obtained by following the 
 development of the frog. The frog's eggs are large and 
 easily observed, and they contain only a small amount of 
 yolk or food material, which does not obstruct the observa- 
 tion. The spherical ovum first divides into hemispheres ; these 
 
 Holoblastic segmentation. Segmentation of frog diagrammatically represented. 
 
 two cells are divided into four in a plane at right angles, 
 and the four are divided into eight by a plane at right angles 
 to the previous plane. This is best understood by examining 
 the illustration (Fig. 262). 
 
 Legend for Fig. 261 
 
 Fertilization of the egg of Ascaris megalocephala, var. bivale?is. (Boveri.) A, the 
 spermatozoon has entered the egg; its nucleus is shown at (f ; beside it lies the 
 granular mass of "archoplasm" (attraction sphere); above are the closing phases in 
 the formation of the second polar body (two chromosomes in each nucleus). B, germ 
 nuclei ($, J) in the reticular stage; the attraction sphere (a) contains the dividing 
 centrosome. C, chromosomes forming in the germ nuclei; the centrosome divided. 
 D, each germ nucleus resolved into chromosomes; attraction sphere (o) double. 
 E; mitotic figure forming for the first cleavage; the chromosomes (c) already split. 
 F, first cleavage in progress, showing divergence of the daughter chromosomes 
 toward the spindle poles (only three chromosomes shown) (Wilson.) 
 
346 
 
 EARLY STAGES OF EMBRYOLOCY 
 
 The lines of cell division proceed in a regular way, the 
 planes passing in such direction as to multiply the number 
 of cells by two in each set of divisions. Very soon the cells 
 
 Fig. 263 
 
 Four stages in the development of amphioxus, illustrating the formation of the 
 gastrula. I. The blastula, a hollow sphere of ceils; those at the lower pole larger 
 than those at the upper and filled with yolk granules. II Invagination of the 
 lower pole, because of more rapid growth of cells at the upper pole. III. The 
 gastrula, complete invagination; the creature is now a two-layered bag. A space 
 should be shown between the layers: bl, the mouth of the bag, or blastopore; 
 hy, inner layer of cells — -hypoblast; ep, outer layer of cells — epiblast. IV. The 
 gastrula will now e'ongate; the cavity becomes the alimentary canal; the blastopore 
 the orifice at one end. 
 
HYPOBLAST 347 
 
 around the black pole show a tendency to divide more 
 rapidly than those at the white pole. At this stage the 
 individual is made up of a hollow sphere of cells with a 
 space at the centre, the cells at the upper surface being 
 small and rapidly dividing, those at the lower surface large 
 and slowly dividing (Fig. 263). As this continues the sphere 
 becomes flattened on the bottom, and finall}' the lower surface 
 is turned inward until the sphere is converted into a hollow 
 bag or sac made up of two layers of cells, the outer of which 
 are small, the inner large, the two joining around the mouth 
 of the sac. This hollow bag stage is known as the gastrula. 
 The cavity of the sac is really a part of the outside world 
 around which the cells have grown, and will form the cavity 
 of the alimentary canal. The opening of the sac is known 
 as the blastopore, and will form the anterior opening into 
 the aHmentary tract from the mouth cavity. At this stage 
 the individual is made up of two kinds of cells, and is to be 
 compared in structure with the celenterates or such animals 
 as the fresh water hydra and the coral polyp. 
 
 Formation of the Germ Layers. — The cells which form the 
 outer layer of the gastrula are called the epiblast, the cells 
 which line it the hypoblast or entoblast. Where these two 
 layers join around the opening of the blastopore, a ring of 
 cells is formed which differs from both in form and arrange- 
 ment, and will form the mesoblast. In the process of cell 
 division from the ovum, therefore, three kinds of cells have 
 resulted which represent the first stage of specialization. 
 
 Epiblast. — From the cells of the epiblast will be formed: 
 (1) The epithelium of the surface of the body and all glands 
 that connect with it, the hair, the nails, and the enamel of 
 the teeth; (2) the epithelium lining the mouth and the nose 
 cavities and the lower part of the rectum; (3) the nervous 
 system and all of the organs of special sense. 
 
 Hjrpoblast. — From the hypoblast will be formed: (1) The 
 epithelium lining the alimentary canal and the glands 
 that open from it; (2) the epithelium lining the larynx, 
 trachea, and the lungs; (3) the epithelium of the bladder 
 and ureter. 
 
348 EARLY STAGES OF EMBHYOLOGY 
 
 Mesoblast. — From the mesoblast will be formed: (1) The 
 various connective tissues, including bone, dentine, and 
 cementum; (2) the muscles, both striated and unstriated; 
 (3) the circulatory system, including the blood itself and 
 the lymphatics; (4) the lining membrane of the serous 
 cavities of the body; (5) the kidney; (6) the internal organs 
 of reproduction. 
 
 Looking at these germ layers in another way, it may be 
 said that through the mechanism of cell division all of the 
 chromatin which is to control nerve cytoplasm has been 
 distributed to the epiblast; all that which is to contribute 
 the muscular activit}^ to the mesoblast, and so on. 
 
 Meroblastic Segmentation. — If the development of the 
 chick is compared to that of the frog they at first seem 
 to be very different. The ova of birds and reptiles are 
 provided with a vast amount of food material or yolk, 
 which is provided by the parent for the nourishment of 
 the embryo. It has been seen that the frog's egg con- 
 tains a certain amount of yolk, and that the presence of 
 yolk granules retarded the cell division. In the case of the 
 birds and reptiles the yolk granules have increased until 
 the active cytoplasm is left as a small disk floating on top 
 of a sphere of yolk enclosed in the yolk membrane. The 
 white spot seen floating on the top of the yolk of a hen's 
 egg is called the germinal spot. Before fertilization this is 
 a mass of protoplasm with a nucleus in the centre. When 
 segmentation begins it divides first into right and left halves, 
 then divides again by a line at right angles to the first one, 
 then the four cells are converted into eight cells, as if by a 
 circle, and the process continues in this way (Fig. 264). It 
 is best understood from the diagram. This type of segmen- 
 tation is known as meroblastic, while that of the frog is 
 holoblastic. 
 
 Mammalian Segmentation. — The mammalian ova contain 
 very little yolk, as the nourishment of the embryo is pro- 
 vided for in an entirely different way. The segmentation 
 is holoblastic (Fig. 265), but shows marked differences from 
 that of the frog, and characteristics similar to those of the 
 
MAMMALIAN SEGMENTATION 
 
 349 
 
 birds and reptiles, and this has been an added Hnk to the 
 evidence of the evokitionists, that the mammaha have been 
 derived in evolution from the reptiles. 
 
 Fig. 264 
 
 Segmentation of hen's egg IMeroblastic segmentation. 
 
 First five stages of segmentation (rabbit's ovum), o, b, c, d, and e In a, b, and c 
 the epiblast cells are larger than the hj'poblastic ones. In e the epiblast cells have 
 become smaller and more numerous than the hypoblasts, and the epiblastic spheres 
 are beginning to surround and close in the hypoblast cells: z.p, zona pellucida; 
 p.gl, polar globules; u, first epiblast cell; I, first hypoblast cell. 
 
350 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 After the first few divisions the cells of the upper pole 
 divide much more rapidly than those of the lower, and grow 
 
 Fig. 2GG 
 
 Sections of the ovum of a rabbit during the later stages of segmentation, showing 
 the formation of the blastodermic vesicle: a, gastrula stages; ent, hypoblast, en- 
 closed by ep, epiblast; b, fluid is beginning to collect and separate the epiblast 
 and hypoblast; c, the fluid has greatly increased in amount, the hypoblastic cells 
 adhering to the upper surface; d, the blastodermic vesicle; ect, the outer layer, epi- 
 blast; ent, hypoblast, the inner layer adhering to the inner surface of the epiblast 
 at the upper surface, forming the opaque area. 
 
 down over the others, enclosing them. When the large cells 
 have been entirely covered in by the small ones, the small 
 
Pig, 267 
 
 
 
 ^/.};-v@:5 
 
 ^W'..: 
 
 ~y, 
 
 5^^:|ifrr;r 
 
 
 -<'o:^. 
 
 m^^M^ 
 
 hz/. 
 
 m.^?T 
 
 'k^^iiM& 
 
 iz.cJuc. I . . 
 
 A series of sections through the neurenteric and notochordal canai of a mole 
 embryo: p.gr, tlie primitive groove: ep epiblast; me, mesoblast: hy, hypoblast; 
 m yr, meiluUary groove (Heap J 
 
352 EARLY STAGES OF EMBRYOLOGY 
 
 ones contiinie to multiply more rapidly and fluid collects 
 inside the sphere, leaving the large cells adhering to the inner 
 surface of the small cell layer at one pole of the sphere (Fig. 
 266) . At the upper pole where the sphere is made up of two 
 layers of cells there is an opaque spot, or the "area pellu- 
 cida," from only part of which the embryo is developed, the 
 rest forming organs to provide it with nourishment during 
 the embryonal condition. 
 
 Starting from the centre of the opaque area on the upper 
 surface of the sphere or blastula, there appears a streak 
 known as the primitive streak, caused by the appearance 
 of a rod of cells lying between the two layers, and from the 
 side of this rod or notochord a third kind of cell, different 
 from either the large or small cell layer, is formed. These 
 three kinds of cells make up the three layers of the blasto- 
 derm and represent the first step in differentiation; or, to 
 state it in a different way, all of the chromatin which (Fig. 
 267) directs nerve cell activity has been sent to the outer 
 small cell layer, or epiderm, all of the chromatin which 
 directs muscle cell activity, etc., has been sent to the new 
 cells of the third layer, or mesoderm, while the large cells 
 of the inner layer or hypoderm contain chromatin to direct 
 most of the secretory activities and the formation of the 
 epithelium of the elementary canal. 
 
 NERVOUS SYSTEM 
 
 Formation of Neural Canal. — The epidermal cells of either 
 side of the primitive streak grow rapidly, forming two 
 ridges with a groove between them, which grows deeper 
 and deeper until the ridges bend over and join, enclosing a 
 tube which is to be the canal of the spinal cord (Fig. 268). 
 The anterior end of this tube enlarges into three bulbs 
 which correspond to the ventricles of the brain, and as 
 they increase in sjze they fold over ventrally or toward the 
 centre of the sphere until the first and second are at right 
 angles to the original tubular part. 
 
FORMATION OF NEURAL CANAL 
 
 353 
 
 Fig. 268 
 
 Stages in the conversion of the medullary groove into the neural canal. From 
 tail end of embrj'O of the cat. jn.g, medullary groove: n.c, neural canal; ch, noto- 
 chord; ep, epiblast; hy. hypoblast; me, mesoblast; cce, celom; am, amnion. (After 
 Quain.) 
 
 23 
 
354 EARLY STAGES OF EMBRYOLOGY 
 
 As the outer layer forms the tul)e of the central nervous 
 system, the inner layer folds oft' a blind pouch from the 
 general cavity of the sphere which is to form the anterior 
 part of the alimentary canal (Plate XVIII). B}^ this time 
 development is complicated by the formation of the embry- 
 onal membranes, the amnion and allantois, but we may omit 
 these entirely for our purposes. 
 
 The diagram from Quain's Anatomy (Figs. 2G9 and 270) 
 illustrates the condition just described, showing the embryo 
 in longitudinal section, the bending over of the anterior 
 end of the neural canal to form the mid- and forebrain and 
 the foregut, or esophagus, a blind pouch ending anteriorly 
 under the mid-brain and posteriorly opening into the cavity 
 of the sphere now called the yolk sac. This pouch is lined 
 
 Legend for Plate XVIII. 
 
 Figs. 1 to 5. — Diagrammatic representations of longitudinal and cross-sections or 
 hen's egg in various stages of incubation. They illustrate how the embryo is de- 
 veloped out of the area pellucida, and the yolk sac, the serosa, and the allantois out 
 of the extra-embryonal area of the germ layers. The embryo is represented much 
 too large in relation to the yolk sac. The yolk is represented in yellow and the ento- 
 derm in green, ectoderm in blue, mesoderm in red, and the black dotted lines indicate 
 the limit to which the inner and outer germ layers have extended over the yolk. 
 The red dots mark the limit of the mesoderm: ak, outer germ layer (blue); mw, 
 medullary ridges or folds; A", neural tube; am, amniotic fold; vof, hof, saf, anterior, 
 posterior, and lateral amniotic folds; A. amnion, ah, amniotic cavity; S, serous mem- 
 brane; hu, dermal umbilicus; s/, lateral folds; kf 1, kf 2, head fold; afb, ifb, outer 
 and inner limb fold; iA;, inner germ layer (green); rr, its margin of overgrowth; 
 dr, intestinal groove; dg, vitelline duct; al, allantois; ds, interstitial sac; du, intes- 
 tinal umbilicus; mk, middle germ layer (red); mk, parietal layer of mesoderm; 
 mk, visceral layer of mesoderm; st, lateral limits of the same; dm, vm, dorsal and 
 ventral mesenteries; th\ bodj' cavity; th^, th^, embryonic extra-embryonic parts of 
 the same. 
 
 Fig. 1. — Cross-section through hen's egg on second day of incubation. 
 
 Fig. 2. — Cross-section through hen's egg on third day of incubation. 
 
 Fig. 3. — Longitudinal section through hen's egg on third day of incubation. 
 
 Fig. 4. — Longitudinal section through hen's egg beginning of fourth day of 
 incubation. 
 
 Fig. 5. — Longitudinal section through hen's egg on seventh day of incubation. 
 
 Fig. 6. — Cross-section through embryo, first day. 
 
 Fig. 7. — Diagrammatic longitudinal section through a selachian embryo. 
 
 Fig. 8 (Kolhkie). — Half of a cross-section through embrj'o chick (two daj-s). 
 
 Fig 9 (Kollikie). — Cross-section through embryo chick, beginning of third day. 
 
 Fig. 10. — Cross-section of chick (five days) in the region of the umbilicus. 
 
 Fig. 11. — Diagrammatic longitudinal section of embryo chick. 
 
PLATE XVIII 
 
 
 alf 
 
BRANCHIAL ARCHES 
 
 355 
 
 by hypoblast and covered by mesoblast and epiblast. The 
 heart has already begun its development in the mesoblast 
 on the ventral side of the foregut. 
 
 Branchial Arches. — There now appear what are called the 
 gill slits, openings from the foregut through its walls to the 
 
 Fig. 209 
 
 -Allantoic 
 Hi ad qut 
 
 Diagram of a longitudinal section of a mammalian embryo. Very earlj-, showing 
 the folding off of the embryo. (After Quain.) 
 
 surface of the embryo, which are separated by thickenings 
 of the wall forming arches around the gut known as the 
 visceral or branchial arches, at the centre of each of which 
 is found a. bloodvessel. These structures are to be compared 
 to the gills of a fish, which are slits through the wall of the 
 
356 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 esophagus to the outside, so that water taken into the 
 mouth nia}' pass out through the slits. At this time, too. 
 
 Fig. 270 
 
 Median sections through the head of embryo rabbits five (.4.) and six (B) milli- 
 rueters long: A, the opening from the foregut has not yet been made; B, the faucial 
 opening is shown at /; c, first brain vesicle; mc, midbrain vesicle; mo, medulla 
 oblongata; m, medullary epiblast; i/, infundibulum; spe, sphenothenoidal, be, sphe- 
 noidal, and sp.o, sphenooccipital parts of the basal cranii; i, foregut; ch, notochord; 
 py, buccal pituitary involution; a7n, amnion; h, heart. 
 
 Embryo showing branchial arches and stomodeum. 
 
PLATE XIX 
 
 Second aortic arch. 
 TJiird aortic arch. 
 
 Auditory vesicae. 
 
 Primitive 
 jugidar reni.N 
 
 Fourth aortic arch 
 
 ■ Fifth aortic, 
 arch. 
 
 Dorsal aorta. 
 
 Cardinal rein.- 
 
 Mid-gut. 
 
 Hind-gut. 
 
 Umbilical vein 
 
 First aortic arch. 
 
 , Olfactory pit. 
 
 ^_' 
 
 Maxilla ry p rocess. 
 
 Hyomandihular cleft. 
 
 Mandibular arch. 
 
 Aortic bidb. 
 
 Auricle. 
 
 Duct of Cnrier. 
 
 Ventricle. 
 
 Vitelline rein 
 Yolk sac. 
 
 Allantois. 
 
 Umbilical [allantoic artery). 
 
 Profile View of a Human Embryo Estimated at Twenty-one 
 Days Old. (After His.) 
 
 Showing branchial arelies and relation to bloodvessels. 
 
FRONTONASAL PROCESS 357 
 
 the arrangement of the bloodvessels exactly resembles that 
 of a fish, and tl^e individual may be said to be in the fish 
 stage of development. 
 
 Stomodeum. — Plate XIX, from Quain's Anatomy, and Fig. 
 271, from Hertwig's Text-Book of Embryology, shows the em- 
 bryo at this stage and the arrangement of the bloodvessels. 
 As the forebrain grows ventrally, the first visceral arch, or 
 mandibular arch, also grows in the same direction, and the 
 space between the inferior surface of the forebrain and the 
 upper surface of the first arch is the beginning of the mouth 
 and nose cavities, now called the stomodeum. From the 
 base of the mandibular arch is seen also the rounded bud, 
 which is beginning to grow forward along the base of the 
 forebrain to form part of the maxillary arch, and finally the 
 upper jaw. At this time also the area which is to develop 
 the sense of smell appears on each side at the outer and 
 lower portion of the forebrain. The olfactory areas grow out 
 of the base of the forebrain, at first being on the outside of 
 the head and in the later development being enclosed, 
 leaving an opening to the surface — the nostril. 
 
 If we have gained a correct idea of the conditions just 
 described by means of the pictures, it will be understood 
 that by the growing forward of the mandibular arch there is 
 left an almost cubical space between the lower surface of 
 the fore- and midbrain and the upper surface of the mandib- 
 ular arch (Fig. 271). This is a part of the outside world, 
 and is enclosed to form the mouth and nose cavities. This 
 process is best understood if we think of the development 
 from the anterior end of the forebrain of a process which 
 may be described as a curtain dropping down, making a 
 central piece, and the bud from the mandibular arch on each 
 side growing forward to unite with it, leaving a slit between 
 them and the mandibular arch which will be the mouth. In 
 order to get a correct idea of this process it must be followed 
 somewhat more minutely. 
 
 Frontonasal Process. — As the frontonasal process develops 
 it is made up of four rather bulk-hke portions (Figs. 272 and 
 273), two occupying the centre and which develop into the 
 
358 
 
 Olfactorj 
 pit 
 
 Lens. 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 Fig. 272 
 
 Maxillanj process. • 
 
 Mandibular arch. 
 
 Ilyo-mandihular cleft. 
 
 iuditory vesicle. 
 
 V 
 
 llyoid arch. 
 '^Sj^ Thyro-hyoid arch. 
 
 1 
 
 Simis 
 
 ..^ jjrsecervicalis 
 
 w 
 
 y 
 
 The beginning of the mandibular arch and the maxillary buds 
 Fig. 273 
 Cerebral hemisphere. 
 
 Fronto-nasal 
 process. 
 
 Stomodaenm. 
 
 Lateral nasal process. 
 
 1 -Eye. 
 ^ "- Processus (jlobularis. 
 ^ Maxillary process. 
 
 x^ Mandibular arch. 
 
 ^, 11 /o-mandibular cleft. 
 
 An embryo a little older than Fig. 272. Viewed from in front. Showing develop- 
 ment of maxillary bud.s and frontonasal process. 
 
FRONTONASAL PROCESS 
 
 359 
 
 intermaxillary bone containing the incisor teeth and the 
 centre of the lip; and two side or lateral processes which 
 grow out around the olfactory area and form the alee of the 
 
 Fig. 274 
 
 Embryo, a little older than Fig. 273. A. front view, frontonasal process, and 
 maxillary buds about to unite: 1, lateral nasal part of frontonasal process; 2, 
 maxillary bud; 3, mandibular arch; 4, hyoid arch. B, the same embryo with the 
 mandibular arch removed: 1, horizontal growth of the maxillary bud; 2, lateral 
 nasal process; 3, mesial nasal process; 4, globular processes which form the 
 horizontal part of the intermaxillary bone. 
 
 Fig 275 
 
 Head of an embryo of about seven weeks. (His.) The external nasal pro- 
 cesses have united with the maxillary and globular processes to shut off the 
 olfactory pit from the orifice of the mouth. 
 
360 
 
 EARLY STAGES OF EMBRYOLOGY 
 
 nose surrounding the nostril. These do not unite again with 
 the central parts, but the end stops over the point where the 
 maxillary bud unites with the central process (P'igs. 274 and 
 275). A failure of union causes the deformity of harelip, 
 the opening in the lip extending to one, or, if double, to both 
 nostrils. 
 
 When the central part of the frontonasal process has 
 united with the maxillary bud on each side the arch of the 
 upper jaw is complete and the original cubical space or 
 
 Fig. 27G 
 
 Mouth of olfactory 
 
 uit. (ir vosfril. 
 
 Palatid process of pro 
 cessm globulnris 
 
 Palatal part of maxil 
 lary process. 
 
 Maxillary 
 process 
 
 The head of an embryo with the mandibular arch removed. Looking up from 
 the mouth into the nose cavity. The union of the globular processes forming the 
 anterior part of the palate, and the horizontal ingrowths from the maxillary buds, 
 showing the way in which they unite from before backward, separating the nose 
 from the mouth cavity. 
 
 stomodeum is enclosed, leaving only the slit between the 
 maxillary and mandibulary arches which is to form the 
 mouth; but the enclosed space is in one chamber, there 
 being no separation between the mouth and nose cavities. 
 The time of this development in the human embryo may 
 be placed at about the fourth week. 
 
 Separation of Mouth and Nose Cavity. — The separation of 
 the mouth and nose cavities occurs by the development of 
 horizontal ingrowths from the three parts making up the 
 
SEPARATION OF MOUTH AXD NOSE CAVITY 361 
 
 maxilla and beginning at the centre and progressing back- 
 ward. First, a small triangular piece from the central part 
 of globular processes of the frontonasal process, this 
 uniting with the horizontal or palatal process of the maxil- 
 lary buds on each side until these reach the apex of the 
 triangle, which will be the intermaxillary bone, just a little 
 way back in the palate, and from here backward they 
 unite with their fellow of the opposite side. This is best 
 seen by removing the mandibular arch and viewing the 
 parts from below (Fig. 276, from Hertwig's Embryology). 
 
 The deformity of cleft palate is then a later development 
 than that of harelip, and either may occur without the other, 
 though they are usually found together. The cleft of the 
 palate usually turns to one side at the front, running out 
 between the cuspid and lateral unless it is double, when a 
 detached piece is found in the centre in front, containing the 
 incisors. As soon as the mouth and nose cavities are sepa- 
 rated and as fast as bone is formed in the jaws most of the 
 space is occupied by the tooth germs. 
 
CHAPTER XXVII 
 
 THE DEVELOPMENT OF THE TOOTH GERM 
 
 The Dental Ridge. — By the middle of the second month of 
 development the arches of both upper and lower jaws are 
 completed, and the palate has separated the nose and mouth 
 cavities. The first indication of the development of the 
 teeth is the multiplication of the cells of the epiblast in a 
 curved line on the crest of each arch in the area which is 
 to be occupied by the teeth. By this multiplication of cells 
 the epiderm is piled up in a ridge, projecting above the 
 surface, and at the same time the deep layer of the epiblast 
 is forced down into the underlying mesoderm (Fig. 277). 
 This structure is known as the dental ridge. In sections the 
 cells piled up against the surface are usually washed off more 
 or less by the reagents, but the depression into the mesoderm 
 is shown. On the hngual surface of this ridge, in the part 
 embedded in the mesoderm, the cells of the Malpighian 
 layer grow out lingually at right angles to the ridge, forming 
 a continuous shelf known as the dental lamina (Fig. 278). It 
 is important to remember that the lamina is continuous 
 along the entire extent of the ridge. 
 
 The Enamel Organ. — From ten points on the surface of the 
 lamina little buds of epiblast start and grow down into the 
 mesoderm, increasing in size and becoming bulbous at the 
 deep end. The bulbous portion gradually becomes flattened. 
 At this stage the bulb is composed of an outer layer of colum- 
 nar cells, continuous with the Malpighian layer of the ridge 
 and a central mass of large polyhedral cells (Fig. 179). 
 As the bud continues to grow into the mesoderm, the meso- 
 dermic tissue below it begins to condense and the cells of the 
 upper portion of the bulb, growing more rapidly, convert 
 the bulb into a two-layered bag. 
 
THE DENTAL PAPILLA 
 
 363 
 
 The Dental Papillae. — The cells in the condensed mesoderm 
 multiply and grow up into the cavity of this cap, forming the 
 beginning of the dental papillae. This stage is represented 
 in Figs. 280 and 2S1, in which the enamel organ is seen con- 
 nected with the lamina by a cord of epithelial cells, and 
 
 Fig. 277 
 
 The dental ridge. A section through the mandible of a pig embryo :)T r h^ : 
 
 two spicules of bone beginning to form, to the right ^leckel's cartilage. 
 
 made up of an outer layer of columnar cells known as the 
 outer tunic, and an inner layer of columnar cells lying next 
 to the dental papillae, known as the inner tunic. The poly- 
 hedral cells between the two layers fill the central part of 
 the enamel organ and have taken on peculiar appearance, 
 which has given to them the name of the stellate reticulum. 
 
364 THE DEVELOPMENT OF THE TOOTH GERM 
 
 The development of the tooth germ now progresses until 
 the dental papilla has taken on the typical form of the 
 tooth. The fully formed enamel organ for an incisor of a 
 sheep is shown in Fig. 282. The cord which connects the 
 outer tunic with the surface epithelium is not shown in this 
 section. 
 
 Fig. 278 
 
 Thedental ridge and denal lainina. 
 
 The Tooth Germ. — The tooth germ is composed of the 
 enamel organ, made up of the outer tunic, the inner tunic, 
 and the stellate reticulum, covering the dental papillae. 
 From the base of the papillae fibrous tissue develops, growing 
 upward around the entire tooth germ and enclosing it in a 
 definite wall or sac of fibrous tissue. This is known as the 
 dental follicle, or the folhcle wall. 
 
 The Dental Follicle.— This term has been used to indicate 
 not simply the connective-tissue wall, but all of the structure 
 
TOOTH GERMS OF THE PERMANENT TOOTH 365 
 
 enclosed in it. This use of the term, however, is confusing, 
 and the term should be confined to the fibrous sac. By the 
 end of the twelfth week the follicle wall has grown up so as 
 to enclose the enamel organ, and the epithelial cord which 
 has connected it with the surface is broken. 
 
 Tooth Germs of the Permanent Tooth.^ — Before the epithelial 
 cord is broken, from some point on the lingual surface of the 
 
 Fig. 279 
 
 WM 
 
 
 '.MW^W^ 
 
 -^ams^^ 
 
 A section through the ni:anliliuh;r arch: E, enamel organ; D, beginning of the 
 dental papilla; B, bone; F, fold from the side of the mandible to the base of the 
 tongue covering the beginning of the sublingual gland; T, tongue. 
 
 outer tunic or along the cord a bud of epithelial cells grows 
 out and turns down into the mesoderm, passing over the 
 follicle wall (Fig. 283). This continues to grow downward 
 until it has reached the position below and to the lingual 
 of the tooth germ for the temporary tooth, where it develops 
 into the enamel organ for the corresponding permanent 
 tooth. It goes through the same changes of form as has 
 been seen in the temporary teeth. 
 
.366 THE DEVELOPMENT OF THE TOOTH GEJiM 
 
 Beginning of Calcification. — About the sixteenth week 
 the tooth germs of all the temporary teeth have been com- 
 pletely enclosed in their follicles and the enamel organ for 
 
 Fig. 280 
 
 The enamel organ. The outer tunic connected to the lamina by the cord; the dental 
 papilla growing up into the cap. The spaces are skrinkage spaces. 
 
 the corresponding permanent teeth have begun their devel- 
 opment (Fig. 284). This illustration shows a section through 
 the lower jaw of a pig, and exhibits the tooth germs for two 
 incisors at about the stage of the closing of the follicle walls. 
 
BEGINNING OF CALCIFICATION 
 
 367 
 
 The buds for the permanent teeth are seen on the Ungual, 
 and the formation of enamel and dentine is just beginning 
 in the temporary teeth. Notice the remains of ^Meckel's 
 cartilage, and the extension of endomembranous bone 
 formation which is just beginning to form a periosteum on 
 
 Fig. 2S1 
 
 The enamel organ, a little older than Fig. 280. It shows the outer tunic, the 
 inner tunic, and the stellate reticulum. The dental papilla in the hollow of the 
 cap. The spaces are caused by shrinkage. 
 
 its surface. The bone has grown around ^Meckel's cartilage 
 and around the tooth gems on the buccal and lingual, 
 enclosing them in an open groove, which will later be com- 
 pleted and divided into separate crypts for each tooth. 
 Fig. 2S5 is from a similar specimen in the region of a tem- 
 
368 THE DEVELOPMENT OF THE TOOTH GERM 
 
 porary molar. The dental papilla is taking on the form of a 
 crown and the formation of enamel and dentine is ready to 
 begin. The cells on the outer layer of the dental papilla 
 
 Fig. 282 
 
 The tooth germ, from the manchlih ut a sheep The enamel organ shows the 
 outer tunic, inner tunic, and stellate reticulum. The dental papilla projects into 
 the enamel organ. The follicle is attached to the base of the dental papilla and 
 surrounds the enamel organ. The spicules of bone form the crypt wall. 
 
 have developed into odontoblasts, forming a single layer of 
 columnar cells lying in contact with the inner tunica of the 
 enamel organ. Here the formation of enamel and dentine 
 
BEGINNING OF CALCIFICATION 
 
 369 
 
 begins, the dentine slightly preceding the enamel. The 
 odontoblasts form and calcify dentine matrix from without 
 inward. The cells of the inner tunic or ameloblasts form 
 
 Fig. 283 
 
 The tooth germ showing the bud for the permanent tooth at P. Calcification is 
 just beginning: F, follicle wall; D, dental papilla; T, inner tunic; T', outer tunic, 
 ^', stellate reticulum; O, odontoblasts; A, ameloblasts, B, bone. 
 
 24 
 
370 THE DEVELOPMENT OF THE TOOTH GERM 
 
 and calcify the enamel rods and cementing substance, pro- 
 gressing from within outward. The line upon which the 
 odontoblasts and ameloblasts lie in contact, therefore, will 
 become the dento-enamel junction. The formation of 
 dentine and enamel begin at separate points, which are at 
 first very close together, but are carried farther apart by the 
 growth of the dental papilla, until they have progressed 
 along the dento-enamel junction and unite, when the increase 
 in the diameter of the dental papilla is stopped. This, per- 
 haps, will be better understood by studying Figs. 68 to 73. 
 
 Fig. 284 
 
 A section through the lower jaw of a pig embryo, showing germs of two incisors. 
 
 First Permanent Molar. — The origin and development of 
 the first permanent molar differs from that of all the other 
 permanent teeth in important respects. It is the only 
 permanent tooth whose enamel organ springs directly 
 from the dental lamina in the same way as those for the 
 temporary teeth. It is the only permanent tooth whose 
 crown is calcified before the individual is thrown upon its 
 
ORIGIN OF THE SECOND AND THIRD MOLARS 371 
 
 own resources for the obtaining of nourishment. Nature 
 seems to have taken special precautions in the formation 
 of this most important tooth. 
 
 About the seventeenth week, at a point on the dental 
 lamina, posterior to the enamel organs of the temporary 
 teeth, a bud starts to grow down into the mesoderm, which 
 
 Fto. 285 
 
 Germ of a premolar from an embryo pig. 
 
 develops into the enamel organ for the first molar, and by 
 the ninth month the follicle is complete and calcification has 
 begun. 
 
 The Origin of the Second and Third Molars — The enamel 
 organ for the second molar is formed from a bud given off 
 from the outer tunic of the enamel organ of the first molar. 
 
372 THE DEVELOPMENT OF THE TOOTH GERM 
 
 Tlie enamel organ for the third molar is formed from a bud 
 given ofl' from the outer tunic of the enamel organ of the 
 second, at about the third year. 
 
 Chronology.— The development of the teeth was first 
 investigated by Lagros and Magitot (about 1865). Since 
 that time their work has been repeated and verified by 
 several investigators. About ISSO Dr. Black repeated the 
 entire work of ^Magitot, and some of his illustrations were 
 used by Dr. Dean in his Translation of Magitot Memoir. 
 Magitot's table, showing the chronology of tooth develop- 
 ment, is given on page 378. 
 
•3b 
 
 
 
 
 <N if.:\-^. 
 
 CO 
 
 
 o 
 
 O 
 
 eriods at ^ 
 
 the teeth 
 
 eruptcc 
 
 6th moi 
 10th moi 
 16th moi 
 20th moi 
 Dth to the 
 inont h 
 24th moi 
 26th moi 
 28th moi 
 30th moi 
 
 1 
 
 00 
 
 2 5 
 
 2 o 
 
 c3 
 
 o 
 
 From 5 t 
 
 years 
 From 12 
 years 
 1 From 18 
 [ years. 
 
 O. CO 
 
 
 Periods at 
 
 which the 
 
 dentine cap 
 
 first appears. 
 
 1 
 
 • § 
 
 : & 
 
 : S 
 
 
 
 
 
 
 
 
 
 J3 • 
 
 o . ■ 
 
 
 
 ( 6th month 
 ( of fetal life 
 
 3d year 
 12th j'car 
 
 Closing of 
 the follicle 
 and rupture 
 of the cord. 
 
 
 M -5 
 .Sort 
 
 B 
 
 
 
 
 
 
 
 
 05 
 
 
 20th week 
 
 1st year 
 
 After the ) 
 Gth year j 
 
 Period at Time of the 
 which the appearance 
 dental bulb of the fol- 
 
 appears. licular wall 
 
 
 - 
 
 o 
 
 
 
 
 
 
 
 
 ?5 
 
 
 
 18th week 
 
 1st year 
 
 After the 
 6th year 
 
 
 
 
 
 
 
 
 
 
 -a 
 ■ S 
 
 
 
 17th week 
 
 1st year 
 
 f After the 
 1 5th year 
 
 Period at which 
 
 the enamel 
 
 organ first 
 
 appears. 
 
 
 
 i 
 
 •fa 
 
 
 
 
 
 
 
 
 
 
 
 
 15th week 
 3d mo. after ) 
 
 birth ; 
 
 3d year 
 
 
 
 
 
 'S. ^ 
 
 
 
 
 C o 
 
 013 
 
 >-iCC>-iCQi-iCC!.-i'H<M(M 
 
 uopi^uap AaBJoduiax 
 
 % c.2'S _ 50 2 = ~ = ::.^ -^ ^^ 
 
 C ~ C ~ C 3 (D ccTST! tn oo'3'O'O'a 
 
 noi-ji^uap ^nauBUijajj 
 
CHAPTER XXVIII 
 
 THE RELATION OF THE TEETH TO THE DEVELOPMENT 
 OF THE FACE 
 
 At birth the jaws contain all of the temporary teeth and 
 the first molars in a partially formed condition, and the 
 follicles for all of the permanent teeth except the second and 
 third molars. These very nearly fill the substance of the 
 bone. In the growth of the bones of the face and the changes 
 that occur in the transformation of the child to the adult 
 face, the teeth play a most important role. 
 
 Before considering this subject in detail it is necessary to 
 recall in this connection some things that have already 
 been emphasized. 
 
 RELATION OF THE TEETH TO THE BONE 
 
 In evolution the teeth originally had no connection with 
 the bone, it being formed later for their support. In embry- 
 ology the tooth is formed first, and the bone formed around 
 it. In this way the development of the individual repeats 
 evolution. In the study of the bone it has been emphasized 
 that the connective tissues have been specialized to meet 
 mechanical conditions, and that both ontogenetically and 
 phylogenetically they are formed in response to mechanical 
 stimuli. The mutations of connective tissue have been 
 dwelt upon, and especially the fact that a bone as an organ 
 of support always contains fibrous tissue, and that there is 
 a continual oscillation between formation and destruction, 
 by means of which it is perfectly adapted to its mechanical 
 environment. The transformations of bone in bone growth 
 have been pointed out, and these will be still more carefully 
 
RELATION OF THE TEETH TO THE BONE 375 
 
 studied in connection with the growth of the bones of the 
 face. 
 
 Some years ago the author undertook a study of the 
 structure and growth of the jaws and alveolar process, which 
 resulted in very important modifications of the conceptions 
 of the matter as given by standard texts. Tomes describes 
 
 Fig. 286 
 
 Tomes' diagram of development of mandible from infant to adult. 
 
 the process of development as essentially an addition at 
 the posterior portions of the jaws to make room for the 
 successively developed permanent molars, and illustrates 
 the process in diagrams (Fig. 286).^ The following quotation 
 states his view: 
 
 "But the main increase in the size of the jaw has been in 
 the direction of backward elongation; in this, as Kolliker 
 first pointed out, the thick articular cartilage plays an 
 
 1 Tomes' Dental Anatomj-, p. 208. 
 
376 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 important part. The manner in which the jaw is formed 
 might also be described as wasteful; a very large amount of 
 bone is formed which is subsequently, at no distant date, 
 removed again by absorption; or we might compare it to a 
 modelling process, in which thick, comparatively shapeless 
 masses are dabbed on to be trimmed and pared down into 
 form. 
 
 "To bring it more clearly home to the student's mind, if all 
 the bone ever formed were to remain, the coronoid process 
 would extend from the condyle to the region of the first 
 bicuspid, and all the teeth behind that would be buried in 
 its base; there would be no neck beneath the condyle, but 
 the internal oblique line would be a thick bar corresponding 
 in width with the condyle. It is necessary to fully realize 
 that the articular surface with its cartilage has successively 
 occupied every spot along this line; and as it progresses 
 backward by the deposition of fresh bone in its cartilage, it 
 had been followed up by the process of absorption, removing 
 all that was redundant." 
 
 In a similar way in any maxilla, the temporary dentition 
 is shown to occupy about the same space as the permanent 
 teeth, as far as the second bicuspid, and the adult is supposed 
 to be formed from the child by the building on of the bone 
 at the back as the molars are formed. 
 
 This conception is fundamentally misleading, for if the 
 infant mandible were to be shown in the relation to that of 
 the adult in three dimensions of space, it would be found 
 to be above and entirely within the adult mandible, and no 
 part of the bone which constituted the infant jaw is present 
 in the adult. In the upper, if the temporary teeth at two 
 years were figured in relation to those of the adult they 
 would be placed somewhere up in the nasal cavity. 
 
 The conditions are more correctly stated by saying that 
 forces exerted at the posterior portions of the jaw through 
 the development of the successive molars cause the bone 
 to grow downward, forward, and outward in the upper arch, 
 upward, forward, and outward in the lower, carrying the 
 bone into an entirely new position in space. 
 
RELATION OF THE TEETH TO THE BONE 377 
 
 In this process the peridental membrane, periosteum, 
 and articular cartilage all play their part, but all the bone 
 posterior to the second bicuspid cannot be thought of as 
 having been formed by the articular cartilage and modelled 
 into form by the periosteum, as might be inferred from 
 Tomes' statement. 
 
 Structure of Maxillae and Mandible. — Before attempting 
 to follow the growth of the bone in the development of the 
 face, the arrangement and distribution of the varieties of 
 bone in the structure of the mandible and maxillse should 
 be carefully studied. 
 
 Cortical Plate. — The outer surface of these bones is formed 
 of a compact layer composed partly of subperiosteal and 
 partly of Haversian system bone. This varies greatly in 
 thickness, depending upon the stress to be sustained. It is 
 called the cortical plate. 
 
 Cancellous Bone. — The centre of the bone is cancellous 
 in character and made up of thin plates of lamellae arranged 
 around large medullary spaces. The direction and arrange- 
 ment of these plates is determined by the forces received on 
 the cortical plates and the directions of stress to which they 
 are subjected. This was pointed out some years ago by 
 Walkoff in an elaborate study of the bones by the use of the 
 .r-rays. By this means he showed that the plates of cancel- 
 lous bone in certain areas had a definite arrangement which 
 was related to the attachments of certain muscles. From 
 the examination of sections of the mandible it will be found 
 that not only is the general form of the bone determined by 
 the forces to which it has been subjected, but also that its 
 minute inner structure is definitely arranged with reference 
 to these forces. The direction and arrangement of the 
 plates of cancellous bone are continually changed and rebuilt 
 to readjust them to the support of new conditions (Fig. 2.32). 
 
 Cribriform Plates. — The alveoli or sockets into which the 
 roots of the teeth fit are bounded by a thin, definite wall, 
 which is pierced by a great many openings. These have 
 been called the cribriform plates, or sieve-Hke plates. They 
 unite the cortical plates of the bone at the border of the 
 
378 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 alveolar process, and are fused with it, on their labial and 
 lingual sides. The cribriform plates forming the walls of the 
 alveoli are really made up of a thin layer of subperidental 
 bone, which has been built on to the plates of cancellous 
 bone, to attach the fibers of the peridental membrane (see 
 p. 299). Within the substance of the bone and surround- 
 
 Fia. 287 
 
 The distribution of bone in the alveolar process. 
 
 ing the course of the inferior dental artery and nerve is 
 found what Cryer has called the cribriform tube. This 
 extends from the point where the arteries and vein enter 
 the substance of the bone on the lingual surface of the 
 ramus, posterior to the alveolar process and below the 
 oblique line, and extends through the cancellous portion of 
 
RELATION OF THE TEETH TO THE BONE 379 
 
 the body of the bone, emerging at the mental foramina. 
 It is really a rather definite arrangement of the plates of 
 cancellous bone around the vessels and the nerves. 
 
 Alveolar Process. — If the adult alveolar process as seen in 
 the skull is examined, it is apparent that the bone is arranged 
 so as to give the greatest support T\dth the least possible bulk, 
 and where there is an increase in bulk it is to meet some 
 special force (Fig. 287). The incisors and cuspids are used 
 chiefly to bite off pieces of food, and when the food cannot be 
 
 Fig. 2S8 
 
 Skull of orang-outang. 
 
 bitten it is torn and wrenched away. This puts a heavy 
 strain in all directions on the roots of the teeth, which must 
 be suppoited by the bone. For this reason the roots of the 
 incisors are usually well covered "^ith bone through their 
 entire length. The cuspid root is long and the upper portion 
 of it so well supported in the bone at the side of the nose 
 and toward the orbit that the most convex portion of it is 
 sometimes uncovered. In animals that use the incisors 
 largely for tearing, wrenching, and fighting, the bone is 
 
380 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 greatly thickened over the incisor roots, as is shown in the 
 skull of the orang (Fig. 288). 
 
 In the upper molars the spreading of the three roots gives 
 abundant support against the direct forces of occlusion. 
 The grinding motions bring lateral pressure against the 
 inclined planes of the cusps, which is met by a thickening 
 of the process in its occlusal third (Fig. 287), forming a 
 heavier ring of bone, while the buccal roots are often exposed 
 in their middle third. In the molars the buccal incline of the 
 lingual cusps of the upper occlude with the lingual incline 
 of the buccal cusps of the lower when the jaws are brought 
 squarely together, and in the giinding motions the outward 
 pressure on the lower molars is supported by the great mass 
 of the bod}^ of the bone, while the inward pressure is sup- 
 ported by a thickening of the occlusal third, as the entire 
 alveolar process projects lingually from the body of the bone. 
 In the examination of any collection of skulls, the amount 
 and arrangement of the bone of the alveolar process will be 
 found to be an indication of the masticatory habits of the 
 individual. 
 
 In examining the sections through the bone of the alveolar 
 process, the adaptation of the arrangement of bone to the 
 force to be sustained should be constantly kept in mind. 
 
 Influence of Mechanical Conditions in Evolution. — Professor 
 E. D. Cope,^ in a long treatise on "The Mechanical Causes 
 of the Development of the Hard Parts in Mammals," has 
 elaborated the fact that the bones of the skeletons of all 
 mammals have been influenced in their development by 
 mechanical conditions, and that their present forms are 
 adaptations to physical environment. In this he states, 
 as a general* principle of structure, that the bone is most 
 dense, but least in amount, on the side in the direction toward 
 which forces have been exerted in development, and less 
 dense, but greater in amount, on the sides from which the 
 forces have been exerted. These statements should be 
 applied in the study of all the sections shown. 
 
 1 Journal of Morphology, 1888. 
 
RELATIOX OF THE TEETH TO THE BONE 381 
 
 An old dry mandible was sawed through in the positions 
 indicated in\he illustration (Figs. 289, 290, and 291). 
 
 FiQ. 289 
 
 Fig. 290 
 
 Human mandible, showing form of the bone and the positions from which 
 sections were cut. 
 
 The portion containing the bicuspid and molar on the 
 left side was ground through the molar to obtain a section 
 
382 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 Fio 291 
 
 Human mandible, showing form of the bone and the positions from which 
 sections v/ere cut. 
 
 Fig. 292 
 
 Ground section through the mandible -where the bicuspid had been extracted. 
 
RELATION OF THE TEETH TO THE BONE 383 
 
 Fig. 293 
 
 n-y. 
 
 ' / - f 4; 
 
 Transverse sections through the roots of two bicuspids and the first molar, 
 showing distribution of bone. 
 
384 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 l)arallel with the axis of the tooth. The portion between 
 the alveohis and second l)iciispid on the left side was ground 
 vertically through the area where the first bicuspid had 
 been (Fig. 292). The portion on the right side containing the 
 two bicuspids and molar was ground to give three sections 
 at right angles to the roots — one in the gingival third, one 
 about the middle of the root, and one just at their apices 
 (Pig. 293). The distal portions of the bone were decalcified 
 and sections cut through the alveoli of the second and third 
 molars (Figs. 294 and 295). 
 
 The Distribution of Bone in the Mandible. — In Chapter 
 XVIII, on Bone, it was stated that the arrangement of the 
 layers in the tissue could be read as a record of the manner of 
 formation. In the examination of these sections the arrange- 
 ment of the lamellae is to be studied in this way, as well as 
 the distribution of the varieties of bone. Where the bicus- 
 pid had been extracted the alveolus has been filled with 
 fairly compact bone, rounding over the border of the process. 
 The section ground through this position shows the buccal 
 and lingual cortical plates in U shape. The two plates are 
 braced together across the central portion by spicules of 
 cancellous bone. At the occlusal border the outline of 
 the old alveolus can still be seen by studying the section 
 carefully with the microscope. After the extraction of the 
 tooth the socket was first filled with connective tissue, which 
 was later transformed into bone, joining that of the alveolar 
 wall. At A, near the lower border, the subperiosteal bone is 
 found to be very thick, the bone evidently growing in that 
 direction. At B, near the occlusal border on the lingual 
 side, there have evidently been absorptions of the surface, 
 removing the Haversian system bone, and then a few layers 
 of subperiosteal bone have been reformed on the surface. 
 
 Fig. 296 shows a ground section through the molar. The 
 cribriform plates lining the alveoli join the cortical plates at 
 the border of the process. On the lingual side the wall of the 
 process is very thin, but is thickened in the occlusal third 
 to support the tooth against force exerted lingually. On 
 the buccal side the cribriform plate of the alveolar wall is 
 
RELATION OF THE TEETH TO THE BONE 385 
 
 Fig. 294 
 
3c% THE TEETH AND DEVELOPMENT OF THE FACE 
 
 Fig. 295 
 
RELATION OF THE TEETH TO THE BONE 387 
 
 connected with the cortical plate b}^ spicules of cancellous 
 bone. Below the apex of the root the cortical plates are con- 
 nected by cancellous bone in which the medullary spaces are 
 
 Fig. 296 
 
 A section ground through the first molar. 
 
 much larger. The same arrangement of the cortical plate and 
 its bracing is shown in Fig. 294, which cuts between the alveoli 
 of the second and third molar. Fig. 329 and Plate XV should 
 
388 THE TEETH AXD DEVELOPMENT OF THE FACE 
 
 Fig. 297 
 
 The buccal plate from Fig. 293. 
 Fig. 29S 
 
 •-•*.*A« !.-«.. TT. *JV ■*♦■ 
 
 /- •>.. : vr-f^T^^ .J ^ ■- */..:■ <^:.rr<«>t-_s»»n:*>*-^;r«K!-?»j»!«i»«»E«WJ8r«cra^ 
 
 Ift. 
 
 .. ^1- 
 
 
 
 
 -^?.^* 
 
 The lingual plate from Fig. 293. 
 
RELATIOX OF THE TEETH TO THE BOXE 
 
 189 
 
 be studied in this connection, remembering that the bone 
 has been formed and shaped by formation of subperiosteal 
 bone on its surface and subperidental bone at the border of 
 the process and their transformation into Haversian system 
 and cancellous bone. 
 
 Fig. 293 is cut transversely. Notice that the gingival 
 section has been turned over in mounting. Observe the 
 cribriform plates forming the walls of the alveoli, and the 
 way these are braced against each other and the cortical 
 
 Fig. 299 
 
 The bone between the alveoli cf the mesial and distal roots of the 6rst molar, 
 from Fig. 293. 
 
 plates by bands of cancellous bone. In accordance with the 
 principles noted, the buccal plate is thin and very compact, 
 while the hngual plate is much thicker, but more open in 
 structure, and the direction of growth has been toward the 
 buccal as the arch of the jaw increased in size. Fig. 297 
 shows the buccal plate with higher magnifications. Fig. 298 
 the lingual plate, and Fig. 299 the bone separating the alveoli 
 for the mesial and distal roots of the molar. The third figure 
 of this series shows only the tip of the distal root of the 
 
390 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 molar, but the arrangement of plates of cancellous bone 
 between the cortical plates is nicely shown. 
 
 The Maxilla. — In the maxilla the arrangement is exactly 
 on the same plan, the details being different because of the 
 difference in the shape of the bone. 
 
 THE GROWTH OF THE JAWS 
 
 It has long been noted that at birth the mandible is 
 straight, and with the eruption of the teeth the ramus 
 develops and the body increases in size. In this process the 
 thickness of the bone is increased from the mental foramina 
 to the alveolar border, and the body of the bone approaches 
 a right angle with the ramus. When the teeth are lost or 
 lose their function the alveolar process is destroyed and the 
 bone reduced in thickness from above downward until 
 the mental foramen comes to lie on the upper surface of the 
 bone. The mandible performs two functions, a respirator}^ 
 and a masticatory function, and it should be remembered 
 that these are influential in its development. The object 
 of this section is to give some conception of the direction of 
 growth in the development of the bones of the face and 
 the way in which the changes are brought about. 
 
 This can best be done by studying the series of skulls 
 from childhood to old age, in which the outer cortical plate 
 has been removed so as to show the developing teeth in their 
 crypts and the relation of the forming teeth to those already 
 in occlusion (Figs. 300 to 314). At birth all of the teeth 
 except the second and third molars have begun to develop, 
 and their tooth germs are lying embedded in the cancellous 
 substance of the maxilla. In the upper jaw they occupy 
 almost all of the space to the floor of the nose and orbit, 
 and there is little if any indication of the maxillary sinus 
 (Fig. 300). Each tooth germ is enclosed in a separate 
 crypt, the wall of which is formed by a cribriform plate. 
 The walls of the crypts are braced against each other and 
 the cortical plates of the maxillae by spicules of cancellous 
 
THE GROWTH OF THE JAWS 
 
 Fig. 300 
 
 391 
 
 MaxilliB at about eight inonlhs after birth, showing the unerupted tooth. 
 
392 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 hone surrounding medullary spaces. As the tooth develops 
 within its crypt, pressure is exerted and the crypt wall is 
 pushed backward through the cancellous bone. 
 
 Growth Force.^ — The force exerted by the growing tooth is 
 the result of the multiplication of cells in the tooth germ, 
 and is exactly comparable to the forces exerted by multi- 
 plication of cells in any position. For instance, the force 
 exerted by the multiplication of the cells in the rootlet of a 
 
 Fig. 302 
 
 Maxillae at about one year. 
 
 plant is sufficient to force pebbles aside and make an 
 opening through hard packed earth. Some attempts have 
 been made to measure the amount of force, but we can only 
 say that it appears to be considerable, acting through short 
 range. How this force is generated has been a matter of 
 much speculation and investigation It shows some points 
 of similarity with the swelling of wood fibers when water 
 is added. It apparently is related to osmosis, and has some 
 
THE GROWTH OF THE JAWS 
 
 393 
 
 direct relations to blood pressure. It is certainly a very 
 complicated matter, with chemical affinities at the bottom 
 of it. 
 
 Forces Influencing Bone Growth. — While the growing tooth 
 germs are producing force which causes conditions of stress 
 of the cortical plates, the growth of the tissues within the 
 mouth — the tongue and the associated organs — is exerting 
 
 Fig. .-303 
 
 Maxillae at one and one-half years. 
 
 pressure upon the lingual surfaces of the bone. The muscles 
 attached to their surfaces transmit force to the bone through 
 the periosteum, and the functions of mastication, deglu- 
 tition, and respiration are acting upon them. All of these 
 are mechanical stimuli, to which the connective-tissue cells 
 respond. In all the process of development the growth is 
 the result of all the forces to which the bones are subjected, 
 perfectly distributed through the substance of the bone bv 
 
394 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 the agency of normal occlusion. Any lack of harmony in 
 the proportion of these forces may allow the teeth to meet, 
 when they erupt, outside of the normal influence of their 
 cusps, causing the beginning of malocclusion. Any mal- 
 occlusion disturbs the balance in the distribution of forces, 
 
 Fia. 304 
 
 Maxillae in the second year, showing the relation of the erupting teeth. Note the 
 relation of the crypt of the second molar to the inferior dental canal. 
 
 and results in a disturbance of the development of bone, 
 which progresses during the entire period of development. 
 This must result in the lack of balance in the proportions 
 of the features which will be proportionate to the mal- 
 occlusion. 
 
THE GROWTH OF THE JAWS 
 
 395 
 
 It has been natural and almost inevitable, because of 
 their hardness, to think of bones as sohd and unchanging. 
 In the study of these skulls the bones of the face must be 
 viewed not as solid and rigid, but as containing millions 
 of active cells which are continually building and rebuilding 
 their substance. 
 
 Fig. 305 
 
 The complete temporary dentition (about three years), showing the relation 
 of the developing permanent teeth. 
 
 Usually somewhere between the seventh and ninth months 
 after birth the growth of the central incisors causes the absorp- 
 tion of the roof of their crypts, and the tooth moves occlusally, 
 cutting through the soft tissues (Fig. 301). The formation of 
 cementum on the surface of the root and of bone on the wall 
 of the crypt attach the connective-tissue fibers and form the 
 beginnhig of the peridental membrane. As the tooth moves 
 occlusally the bone grows up around it from the circumference 
 
396 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 of the crypt wall, converting it into the wall of the alveolus. 
 The root is not fully formed and the conical pulp filling 
 the funnel-like end exerts force by the multiplication of 
 cells and the blood pressure, which cause the tooth to move 
 occlusally and the bone to grow in that direction. At the 
 same time the pressure of tongue and lips exerts pressure on 
 
 Fig. 306 
 
 The complete temporary denlition and the first permanent molar. Note the 
 relation of the bicuspids to the temporary molars. (In the seventh year.) 
 
 the surfaces of the tooth and bone, influencing the direction 
 of bone growth. The jaw increases in thickness in the occlusal 
 direction and grows forward and outward. At the same 
 time the growth of each successively distal tooth is exerting 
 pressure upon those already erupted, causing them to move 
 farther in the occlusal direction. In Figs. 303 and 304 notice 
 the way in which the crypt walls are pushed downward by the 
 
THE GROWTH OF THE JA^yS 
 
 397 
 
 development of the tooth root until the inferior dental nerve 
 lies between the floor of the crypt and the cortical plate of the 
 lower border. In this way enough pressure may be produced 
 
 Fig. 307 
 
 Front view of the skull shown in Fig. 303. Note the relation of the permanent 
 incisors and cuspids to each other and the roots of the temporary teeth. 
 
398 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 to cause reflex nervous symptoms, which commonly precede 
 the eruption of the temporary molars, and so development 
 continues until all of the temporary teeth are in place. About 
 
 Fig. 308 
 
 Dentition in the eighth year. Note the position of the cuspids and compare 
 with Fig. 310. 
 
THE GROWTH OF THE JAWS 
 
 399 
 
 the sixth year the first permanent molars take their place at 
 the distal of the temporary teeth and their cusps interlock 
 (Fig. 305). The importance of these teeth can scarcely be 
 
 Fig. 309 
 
 The left side of the skull, shown io Fig. 30S 
 
400 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 overstated. They are not only to be the chief means of 
 mastication during the period in which the temporary teeth 
 are lost and replaced by their successors, but they are to 
 
 Fig. 310 
 
 Dentition in the eleventh year. Note the growth of the cuspids and bicuspids. 
 The second molar is about to erupt. 
 
THE GROWTH OF THE JAWS 
 
 401 
 
 maintain the relation of the jaws to each other. The way 
 in which these teeth lock determines the balance between the 
 forces exerted by the action of the muscles attached in the 
 region of the ramus, and those in the region of the svmphvsis 
 (Fig. 306). 
 
 Fig. 311 
 
 Dentition in the thirteenth year. Note the relation of the bicuspid crown to the 
 roots of the lower temporary molar. 
 
 A deviation from the normal relation of these teeth will 
 entirely change the direction of the forces, and will be mani- 
 fested by a modification in the development in the bone. 
 In the skull at this period the bicuspids are seen lying below 
 the temporary molars, and the second molar developing at 
 26 
 
402 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 the distal of the first. Their growth is transmitted through 
 the teeth to the alveolar process, and the addition of bone 
 results. The same skull viewed from in front (Fig. 307) shows 
 
 Fig. 312 
 
 The dentition of a young adult. The third molars have not erupted. 
 (About fifteen years.) 
 
THE GROWTH OF THE JAWS 
 
 403 
 
 the relation of the permanent incisors and cuspids to the tem- 
 porary ones. In the lower jaw the temporary centrals have 
 been lost and the permanent ones are forcing their way 
 between the temporary laterals. The crowns of the centrals 
 
 Fig. 313 
 
 Adult dfentition. Note the distance from the apices of the incisors to the lower 
 border of the mandible and the floor of the nose. 
 
 are wider than those of the teeth that were lost, and they 
 consequently exert pressure upon the mesial surfaces of the 
 laterals, pushing them apart and carrying them upward and 
 forward. 
 
 Studv the relation of the lower centrals, laterals, and 
 
404 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 cuspids in the development of the arches at from six to ten 
 years. Notice that the roots of the central are not fully 
 formed, that the lateral lies to the lingual of the temporary 
 lateral root, and with its mesio-occlusal angle below the 
 distal surface of the central. The development of the cuspid 
 has pushed the crypt floor through the cancellous bone 
 until it has reached the solid cortical plate, and still the for- 
 mation of the crown is not quite completed. The six teeth 
 
 Fig. 314 
 
 Edentulous jaws, showing loss of alveolar process. 
 
 form a triangle of which the centrals are the apex, and the 
 cortical plates from cuspid to cuspid the base. The com- 
 pletion of the roots of these teeth will carry the temporary 
 teeth, alveolar process and all, upward, forward, and out- 
 ward, thus increasing the distance from the mental foramen 
 to the symphysis and enlarging the arc of the law from 
 cuspid to cuspid. 
 
 In the same skull notice the relation of the upper incisors 
 and cuspids to the corresponding temporary teeth. They lie 
 
THE GROWTH OF THE JAWS 405 
 
 to the lingual of the roots of the temporary teeth, the lateral a 
 little to the lingual of the central and cuspid. The cuspid has 
 pushed back the floor of its crypt until it is braced against 
 the solid bone at the base of the malar process. The growth 
 of these teeth will first cause the temporary teeth to move 
 occlusally, the bone growing from the border of the process 
 to follow them. In this growth the distance from cuspid 
 to cuspid is increased and spaces appear between the tempo- 
 rary incisors some time before they are lost. 
 
 If such spaces do not appear, the development is not 
 progressing normally, and artificial force should be applied 
 to stimulate bone growth. If this is not done the permanent 
 teeth are sure to come in more or less rotated and out of 
 position. 
 
 In Figs, 308 and 309 the incisors have been pushed ofi' 
 and the permanent ones are beginning to move occlusally. 
 Notice the relation of the floor of the crypt to the floor of 
 the nose, and the root has scarcely begun to develop. In the 
 adult skull (Fig. 302) there is almost as much space from the 
 apex of the root to the floor of the nose as there is now from 
 the border of the alveolar process to the floor of the nose. 
 The result of the growth of the cuspids' roots is shown by 
 comparing Figs. 307 and 308 with Fig. 310. 
 
 The Importance of Proximal Contact. — The proper contact 
 of the teeth upon their proximal surfaces is necessary for 
 this development. If, for instance, the mesial angle of the 
 lower lateral fails to engage with the distal surface of the 
 central, but slips by to the lingual, the growth of the cuspid 
 will push it farther and farther past the central instead of 
 enlarging the arch. One of the cogs in the mechanism has 
 slipped, and the growth of bone cannot later be expected to 
 make room for the crowded teeth. 
 
 In the next stage of groT\i:h the increase in size is from 
 the mental foramen to the ramus, and is largely influenced 
 by the development of the roots of the bicuspids and the 
 second molars. Figs. 309 and 310 show the relation of 
 the second molar to the distal surface of the flrst, and 
 it will be seen that its growth exerts force upon the first 
 
406 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 Fig. 315 
 
 Fig. 316- 
 
 Figs. 315 and 316 were photogiaphed in the same relative size, to show the amount 
 and direction of growth, with the development of the full permanent dentition. 
 
THE GROWTH OF THE JAWS 
 
 FiQ. 317 
 
 407 
 
 Fig. 318 
 
 Figs. 317 and 31S -were pliotographet: in tlio same relative size, to shew the amount 
 and direction of growth, with the deveiupmeut of the full permanent dentition. 
 
I'la. 319 
 
 Fig. 320 
 
 Figs. 319 and 320 were photographed in the same relative size, to show the amount 
 and direction of growth, with the development of the full permanent dentition. 
 
Fig. 321 
 
 Fig. 322 
 
 Figs 321 and 322 were photographed in the same relative size, to show the amount 
 and direction of growth, with the development of the full permanent dentition. 
 
410 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 molar, and this is transmitted through the arch by means 
 of proximal contact. Notice the inclination of the bicuspid 
 roots, which help to carry the growth in the same direction. 
 After the second molar is in place the growth of the third 
 should exert the same force and room be provided for it (Fig. 
 
 Fig 323.— Two years. 
 
 Fig. 324. — Three years. 
 
 Fig. 325.— Six years. Fig. 326.— Ten years. 
 
 Maxillae photographed from the median line in the same relative size, to show the 
 
 amount and direction of growth. 
 
THE GROWTH OF THE JAWS 
 
 411 
 
 Fig. 327.— Twelve years. 
 
 Fig. 328.— Adult. 
 
 Maxillae photographed from the median line in the same relative size, to show the 
 amount and direction of growth. 
 
 311). The muscular action of the lips and tongue are specially 
 important in these last stages of growth, and particularly the 
 forces that are generated by the action of the muscles in 
 respiration and deglutition. The activity of the connective- 
 tissue cells in the bone require mechanical stimuli for their 
 maintenance, and as the muscular action is vigorous or 
 deficient, the growth of bone mil be full and normal or 
 imperfect and unbalanced. It appears often that the bone 
 activity becomes so sluggish that the growth of the third 
 molar cannot produce the effect it should, and it remains 
 impacted. A comparison of figures will show that while 
 room has been made for the third molar, all of the upper 
 teeth have moved downward, forward, and outward, and 
 the lower ones upward, forward, and outward. Compare 
 the distance from the apex of the incisor roots to the floor 
 of the nose and the lower border of the mandible in Figs. 
 312 and 313. 
 
 This process may be more fully realized by comparing 
 
412 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 the front views of the skulls (Figs. 315 to 322). They were 
 all photographed with the same lens and bellows length, 
 so as to make the pictures of the same relative size as the 
 skulls. Notice the increase in distance from the floor of the 
 nose and the floor of the orbit to the edges of the upper 
 
 Fig. 329 
 
 Bone from the buccal plate of the mandible of a young sheep, showing transforma- 
 tions of bone: 1, subperiosteal bone; 2, Haversian system bone; 3, Haversian system 
 bone, becoming cancellous. 
 
 incisors, and from the lower border of the mandible to the 
 edge of the lower incisors. It will be seen that if the infant 
 mandible were placed in relation to the adult mandible it 
 would lie entirely within the arch and in the mouth cavity, 
 while in the upper the temporary incisors in Fig. 322 would 
 be some place in the nasal cavity. In all of this growth the 
 
THE GROWTH OF THE JAWS 
 
 413 
 
 size of the air spaces increases with the movements of the 
 teeth, the floor of the nose and palate growing downward 
 and developing. This may be shown in Figs. 323 to 328, in 
 
 Fig. 330 
 
 The record in the arrangement of the lamellae of the growth of the mandibles. A 
 decalcified section from near the lower border of a human mandible. 
 
 which the right half of the maxilla has been removed from 
 dissected skulls and photographed from the median line. 
 
 Tissue Changes in the Physiologic Movements of the Teeth. 
 — All that has been said in regard to bone growth must be 
 
414 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 recalled in order to obtain a conception of the manner in 
 which these movements of the teeth and the development 
 of the bone are accomplished. Bone laid down under 
 
 FiQ. 331 
 
 A decalcified section from the lingual vertical plate of a human mandible, showing 
 the arrangement of lamella as a record of growth. 
 
 the periosteum and the peridental membrane has been 
 transformed into Haversian system bone and then made 
 cancellous, as illustrated in Fig. 329, which is taken 
 from the buccal plate of the mandible of a young sheep. 
 
THE GROWTH OF THE JAWS 
 
 415 
 
 Reversed changes have also been going on, the periosteum 
 cutting into the Haversian bone by absorption and the 
 cancellous bone being condensed into Haversian system 
 bone. These changes leave a record in the arrangement of 
 
 Fro. 332 
 
 Cancellous bone from a decalcified section of a human mandible, showing 
 reconstructions to change the direction of the spicules. 
 
 the lamellse, and may be studied in decalcified sections 
 (Figs. 330 to 333). Even the direction of the spicules of 
 cancellous bone are being constantly changed by absorptions 
 and rebuilding to adjust them to changes of stress. 
 
416 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 While the temporary teeth are moving occlusally, bone is 
 laid down under the peridental membrane at the border of the 
 alveolar process, which is at once cut out by absorptions and 
 replaced by Haversian system bone (Fig. 217). The alveolar 
 process becomes a veritable patchwork, as shown in Figs. 
 234 and 335. The permanent tooth developing in its crypt 
 produces conditions of pressure, and osteoclasts appear in 
 
 Fici. 3;33 
 
 Decalcified section of cancellous bone from a human mandible, showing absorptions 
 and rebuildings, changing the direction of the spicules. 
 
 all the medullary spaces, around and above the crypt, and 
 through the alveolar process, as well as on the crypt wall. 
 They are more active in the medullary spaces, cutting away 
 the spicules of bone, thinning and cutting apart the crypt 
 wall, and allowing it to be bent and pushed back. 
 
 Fig. 336 shows the alveolar process on the lingual side 
 of the temporary incisor, and illustrates the enlargement 
 
THE GROWTH OF THE JAWS 
 
 417 
 
 of the medullary spaces preparatory to the eruption of 
 the permanent tooth. Fig. 337 shows the labial plate of the 
 process, and notice that the bone is being formed under the 
 
 Fig. 334 
 
 A longitudinal section through the tip of the alveolar process of a temporary tooth 
 about ready to be lost: D, dentine; Cm, cementum, showing absorption and rebuild- 
 ing; Pd, peridental membrane; B, bone growing occlusally at the border of the 
 process; Hb, rebuilt Haversian svstem bone. 
 
 27 
 
FiQ 335 
 
 A longitudinal section through the temporary alveolar process, which is growing 
 occlusally to follow the temporary tooth. It is from the same series as Fig. 334, but 
 shows more of the bone. Study the absorptions and rebuildings, as shown in the arrange- 
 ment and character of the lamellae. Pd, peridental membrane: Po, periosteum. 
 
THE GROWTH OF THE JAWS 
 
 419 
 
420 THE TEETH AND DEVELOPMENT OF THE FACE 
 
THE GROWTH OF THE JAWS 
 
 421 
 
422 THE TEETH AND DEVELOPMENT OF THE FACE 
 
 periosteum and at spots under the dental membrane, while 
 the substance of the bone is being destroyed. 
 
 When the tooth is finally pushed off from the gum all 
 but a few bits of the alveolar process have been destroyed, 
 and as the permanent tooth comes through, bone formation 
 begins at the border, patching on to the remains of the old 
 process (Fig. 338). 
 
 In studying the absorption of bone around the crypt 
 walls, it has been noted that the osteoclasts appear first 
 in the cancellous bone (Figs. 218 and 219), surrounding the 
 crypts and outside of it. Absorptions here remove the spicules 
 which brace the crypt wall, and cut through the wall in such 
 a way as to allow it to be pushed back through the weakened 
 substance. In the same way in the movements of the teeth, 
 absorptions appear first in the spaces outside of the cribri- 
 form plates of the alveoli, until the remaining bone is weak- 
 ened sufficiently to spring under the pressure. All of the 
 sections of the mandible should be studied as a record 
 of these bone transformations, and especially in orthodontia 
 it should be remembered that appliances are used not to 
 push the teeth through the bone as a post would be pushed 
 through the mud, but to supply mechanical stimuli to 
 living cells whose activity will result in bone growth, carrying 
 the teeth into their proper positions, and finally that teeth 
 will remain only in the position in which all of the forces 
 to which it is subjected are balanced. 
 
PAET II 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 (TWENTY-FIVE PERIODS IN THE LABORATORY) 
 
 INTRODUCTION 
 
 It is assumed in this work that the student has had a 
 course in general histology, including laboratory work, that 
 he is familiar with the technique of handhng the microscope, 
 the technique of staining and mounting sections, and that 
 he is able to recognize at once the elementary tissues. The 
 same outfit is required as for general histology, including 
 slides and blank labels for them; cover-glasses; teasing 
 needles; forceps; section lifter; a tube of balsam; a funnel; 
 pipette; filter paper and lens paper; 6 one-ounce reagent 
 bottles containing xylol, absolute, 95, and 70 per cent, alco- 
 hols, hematoxylin, and eosin; at least two chip butter 
 dishes that can be used for staining; a box for the slides; 
 a notebook; a hard and a soft drawing pencil; a good eraser; 
 and a piece of clean, soft linen for wiping slides and cover- 
 glasses. 
 
 Teeth for Grinding. — It is difficult to obtain satisfactory 
 teeth for the grinding of microscopic sections, and the 
 student should bring to the laboratory a number of suitable 
 teeth from which selection can be made. Old, dry teeth are 
 absolutely useless for the purpose, however perfect their 
 structure may have been. When a tooth has been extracted 
 for some time the tissues dry out, giving up a considerable 
 amount of water, and consequently shrink. The shrinkage 
 of dentine and enamel is unequal, and the result is a cracking 
 
424 DIRECTIONS FOR LABORATORY WORK 
 
 of the tissue. The observation of the teeth in any skull will 
 reveal cracks in the enamel that may be seen with the naked 
 eye, the tooth often splitting lengthwise. Besides the cracks 
 that can be seen, the tissue is full of microscopic cracks. 
 When the grinding of sections from such teeth is attempted, 
 before the section is reduced to sufficient thinness for micro- 
 scopic observation, the enamel will break to pieces and be 
 lost. A tooth that is to be used for grinding must be placed 
 in solution as soon as it is extracted, and never at any stage 
 of the process be allowed to dry, until ready for mounting. 
 Any solution that will prevent decomposition will do for 
 this purpose. The best that I have found is a 4 per cent, 
 formaldehyde in 50 per cent, alcohol. This may be roughly 
 prepared by diluting 95 per cent, alcohol with an equal volume 
 of water and adding one part of formalin to nine parts of 
 the diluted alcohol : 
 
 Alcohol 45 c.c. 
 
 Water 45 c.c. 
 
 Formalin 9 c.c. 
 
 This solution not only prevents the drying, but has a 
 hardening action on the organic matter, which facilitates the 
 grinding. Teeth may be preserved in this indefinitely. 
 
 Teeth Required. — From his collection the student should 
 select for grinding an incisor or cuspid, a bicuspid, and a 
 molar. The teeth should be free from caries and their 
 crowns as perfect as possible. 
 
 The Relation of the Section to the Crown. — The practical 
 value of the study of ground sections depends upon obtaining 
 from them a knowledge of enamel rod directions in relation 
 to the tooth crown as well as the section. In operating the 
 teeth are looked at from their outside surface, but the 
 operator needs to see in the enamel not simply a hard and 
 extremely dense tissue, but a tissue made up of minute rods 
 whose general direction he knows beforehand. If a tooth 
 is selected and a section cut from it in a known position, and 
 the relation of the section to the crown remembered, the 
 direction of enamel rods can be placed in relation to the 
 
PREPARATION OF GROUND SECTIONS OF TEETH 425 
 
 entire crown as well as to the section. This is one of the 
 objects to be sought in the making of the outline drawings. 
 
 Location of the Section. — Having selected the teeth for 
 grinding, the next step is to locate the position and direction 
 of the section. This must be so placed as to cut the enamel 
 rods in their length. The section from the incisor or cuspid 
 should be ground labiolingaally, but the section from the 
 molar and bicuspid may be ground either buccolingually, 
 mesiodistally, or diagonally. The surface of the tooth 
 should be considered, and the section placed in an area in 
 which the student desires to discover the enamel rod direc- 
 tions and the structure of the tissue. The line of the section 
 should now be marked on the tooth with India ink and a 
 fine pen. 
 
 The Drawings of the Teeth. — After marking the position of 
 the section the tooth should be carefully and accurately 
 drawn, showing the position of the section as seen from the 
 axial and occlusal surfaces. 
 
 Grinding of the Section. — Every institution should have a 
 machine for the preparation of ground sections, but such a 
 machine is too delicate an instrument to be handled by 
 students. In the appendix will be found a chapter written 
 by Dr. Black describing the grinding machine and the 
 technique of its use. If one is available, the student may have 
 his sections ground for him and returned ready to mount, or 
 he may grind them himself, using the following technique: 
 
 Preparation of Ground Sections of the Teeth.— For this work 
 the student should have two large corundum stones not less 
 than four inches in diameter, one of "C" and one of "E" 
 grit. Corundum is very much better than carborundum for 
 this purpose. In grinding the stone should be kept revolving 
 slowly and moistened with a stream of water. Holding the 
 tooth against the flat side of the coarse stone with the 
 fingers, the tissues should be rapidly ground away until 
 the position marked for the section is reached, when the fine 
 stone should be substituted and the grinding continued just 
 enough to remove the scratches. The surface should now 
 be polished on the Arkansas stone until a very perfect surface 
 
426 DIRECTIONS FOR LABORATORY WORK 
 
 has been obtained. Wash the specimen clean and immerse 
 in several changes of 95 per cent, alcohol, and leave in absolute 
 alcohol in a closed bottle for several hours or over night. 
 Harden a drop of balsam on the centre of a clean slide by 
 warming it over a Bunsen burner to evaporate the xylol. 
 When the slide is cool the balsam should be neither sticky 
 nor brittle. Now remove the tooth from the alcohol, wipe 
 it dry, and, placing it on the balsam with the polished surface 
 next to the glass, gently warm the slide until the balsam is 
 thoroughly softened, and press the tooth down against the 
 glass and clamp it firmly in position, using a spring clip. 
 Set it away to harden thoroughly, when the grinding may be 
 continued. 
 
 Holding the slide parallel with the surface of the coarse 
 stone, the tissues may be rapidly removed until the section 
 is about as thin as a calling card, when the fine stone should 
 be substituted and the section reduced to the required thin- 
 ness. It should not be more than twenty microns in thick- 
 ness. In the final stages progress of the grinding may be 
 followed with a hand magnifying glass. Finally the surface 
 should be polished on an Arkansas stone. The specimen 
 should now be washed with alcohol, the balsam removed 
 with xylol, and brought to the laboratory in 95 per cent, 
 alcohol, where it is to be etched and mounted according to 
 the directions. 
 
 Every step in the above technique is important and must 
 be followed with minute care and accuracy. Not least 
 important is the cleaning of the slide. It sometimes happens 
 that the section will be loosened from the glass before the 
 grinding is completed. This is usually due to some fault 
 in the technique. When it happens it is best to finish the 
 grinding without attempting to refasten the section to the 
 slide. To do this the section should be held against the flat 
 side of the stone, using a fine-grained cork, a piece of box- 
 wood, or some similar material. The danger of breaking 
 the section, however, is much greater. 
 
 The Preparation of Transverse Sections of the Root. — For 
 this purpose one of the flattened roots furnishes the best 
 
DRAWINGS 427 
 
 material, as, for instance, the mesial root of a lower molar, 
 the root of a lower bicuspid, or of an upper second bicuspid. 
 Holding the root in a vice by the remains of the crown, 
 with a metal saw, saw off the tip of the root, removing an 
 eighth of an inch or less. Then saw off as thin a slice as 
 possible. In the same way saw^ out at least two other 
 sections, one from the gingival and one from the middle 
 third of the root. These should be dropped into a bottle 
 of formalin-alcohol until the grinding is completed. The 
 grinding is easily accomplished on the flat side of the corun- 
 dum stone, holding the section on the finger or under a 
 cork. The last grinding should be done on the fine Arkansas 
 stone. 
 
 Transverse sections of the root are easily ground and can 
 be made very thin. 
 
 Manner of Working in the Laboratory. — In no place in the 
 world can time be wasted more easily than in the histological 
 laboratory. The student should take the attitude of an 
 original investigator and study out the material for himself 
 as far as possible, remembering that he has a far better 
 opportunity than the man who worked out the details of 
 these structures. He must constantly try to picture the 
 structure, and imagine how it would appear if sectioned in 
 another direction. 
 
 Drawings. — Drawings from the microscope are made not 
 simply to occupy the student's time, nor as a record of what 
 he has done, but to make observation more accurate and 
 detailed, and to fix the impressions of structure more per- 
 fectly in mind. Many students excuse themselves for 
 careless and slovenly work by saying that they are not 
 artists. Anyone without any knowledge of the principles 
 of art can in a very short time acquire the ability to make 
 excellent microscopic drawings. A few principles of pro- 
 cedure will help greatly. The first of all is that a light 
 line can always be made darker, therefore the drawing should 
 always be kept light until the later stages. 
 
 After selecting a field, draw lightly the outline of the 
 principal masses and then the outKnes of the smaller ones. 
 
428 DIRECTIONS FOR LABORATORY WORK 
 
 In this way the proportion of objects in the field and their 
 relation to each other can be maintained. Never draw any 
 detail such as individual cells, nuclei, etc., until all of the 
 outlines are completed. Then work in the details in the 
 darker colored areas. The making of the outlines is by far 
 the most important stage in the drawings. 
 
 Each outfit should contain a 6 H and an H-B pencil and a 
 good eraser, which must be kept clean. The pencils should 
 be kept sharp and always used with a light touch upon the 
 paper. The beginner always tends to start his drawing by 
 making a circle. This should be avoided, for it is objects 
 that are being studied, not fields, and in many cases the 
 object cannot be bounded by a circle. There is also a ten- 
 dency to represent the object smaller on the paper than it 
 appears in the field. 
 
 The prime qualities in a microscopic drawing are accuracy 
 and correctness of detail. The drawings are made to show 
 all the detail of structure that can be observed. It often 
 happens that a drawing that looks very well shows very 
 little knowledge of the structure of the tissue which it 
 represents. 
 
 Stencilled Laboratory Notes. — In fifteen years of teaching 
 the author has found stencilled notes on the daily work in 
 the laboratory of very great assistance. There are always 
 variations in the appearance of the material which cannot 
 be anticipated before the sections are cut. Very often some- 
 thing will be seen unusually well that would not be men- 
 tioned in the text-book. Difterent stains may have been 
 used which would change the appearance of the tissues, and 
 for all of these things and many others daily notes are very 
 convenient. 
 
 USE OF DIRECTIONS FOR LABORATORY WORK 
 
 At the beginning of the laboratory period the first thing 
 to be done is to read through the directions for the day's work. 
 The amount of work for the dav is then clearlv in mind, and 
 
ETCHING AND MOUNTING OF GROUND SECTIONS 429 
 
 all the steps in any procedure that is to be undertaken are 
 understood at the beginning. It is necessary to divide the 
 time available, so as to accomplish the work indicated for the 
 day. 
 
 PERIOD I 
 
 Drawings of Tooth Surfaces Showing the Position of Sections. 
 — The object of these drawings is to show the relation of the 
 section to the crown from which it is ground, so that in 
 studying the enamel rod directions as seen in the sections, 
 they may be referred to the entire crown. The drawings 
 should be made from five to ten times natural size, and must 
 be made accurately to scale (Fig. 339). Measure the length 
 and the breadth of the tooth and lay out a rectangle, say 
 eight times these dimensions, to serve as a guide in drawing. 
 If the tooth is marked for a buccolingual section, stick the 
 apex of the root on a bit of wax and place the tooth on the 
 table with the buccal surface toward you. Do not change its 
 IDOsition until the drawing is completed, for to do so would 
 change lights and shadows. After getting the outline accur- 
 ately, work in the shadows so as to give the drawing round- 
 ness. Remember in doing this that you can always make it 
 darker, but you cannot erase without injuring the neatness 
 of the drawing. When the drawings are completed the sec- 
 tion is ready for grinding, which must be done outside of 
 the laboratorv, following the directions in Introduction to 
 Part II. 
 
 PERIOD n 
 
 Etching and Mounting of Ground Sections. ^ — At the desk will 
 be found 1 per cent, hydrochloric acid, dilute ammonia, and 
 vaseline, which are the only reagents not included in the 
 outfit and required for this work. The sections are brought 
 to the laboratory ground and ready to mount. Fill one of 
 the dishes with water and carefully wash the specimen free 
 from all debris of grinding. Dry the section between filter 
 
430 DIRECTIONS FOR LABORATORY WORK 
 
 Fig. 339 
 
 D/ST/IL SURFACE 
 
 BUCCAL 
 
 OCCLUSAL SURFACE 
 BUCCAL MARGIN 
 
 DISTAL M. 
 
 MESIAL M. 
 
 LING 
 
 LINGUAL M. 
 
 8 DIAMETERS 
 
 Drawings of occlusal and axial surfaces of a tooth, to show the relation of the section 
 to the tooth. (Drawn by W .A. Offil, 1910.) 
 
ETCHING AND MOUNTING OF GROUND SECTIONS 431 
 
 papers, so as to remove all moisture from the surface. Fill 
 one dish with 1 per cent, hydrochloric acid and the other 
 wdth dilute ammonia. Put a very little vaseHne upon the 
 tip of the finger, and holding the section by the root portion, 
 cover one surface of the crown portion with a very thin layer 
 of it. In doing this the vaseline should be wiped from the 
 centre toward the edges of the section, so as to prevent it 
 from running over on to the other surface. The vaseline is 
 to confine the action of the acid to one surface of the enamel. 
 Holding the section by the root portion, immerse the crown 
 in the dilute acid for thirty seconds, or until minute bubbles 
 can be seen forming upon the surface. Remove and immerse 
 at once in the dilute ammonia for a minute. Remove the 
 vaseline by carefully wiping the section vriih absolute alco- 
 hol or ether, and immerse in 95 per cent, alcohol. In this 
 it should remain while the slide and cover-glass are being 
 prepared. Obtain from the desk a cover-glass long enough 
 to cover the entire section and carefully clean both slide 
 and cover-glass. On the centre of the slide place a drop of 
 balsam that is as long as the section. Holding the slide over 
 a Bunsen burner or alcohol flame, warm it gently so as to 
 evaporate the xylol. In this process the drop will spread 
 out over the slide and the direction of spreading may be 
 guided by the heat. Allow the slide to cool and test the 
 hardness of the balsam with a teasing needle or the finger 
 nail. When cold the balsam should be just soft enough 
 to take the imprint of the needle or nail, but not be sticky. 
 If it is sticky it must be reheated; if, on the other hand, it is 
 brittle enough to chip, it must be scraped oft' from the 
 sUde and the process tried again. In the same way prepare 
 a film of balsam on the cover-glass. Remove the section 
 from the 95 per cent, alcohol and dry it for a few minutes 
 in the air (after wiping with filter paper). Place the section, 
 etched side up, upon the balsam on the slide, and place the 
 cover-glass on it balsam side doicn. Warm the slide gently 
 over the flame, while pressing the cover-glass down with the 
 handle of a teasing needle. As the balsam is warmed, the 
 slide and cover-glass are brought together, forcing the balsam 
 
432 DIRECTIONS FOR LABORATORY WORK 
 
 out to the edge of the cover-glass in all directions. All 
 excessive balsam should be squeezed out at the edges. 
 Place on the cover-glass a small piece of blotting paper or a 
 layer of cork, adjust some sort of a spring clip and put the 
 section away until the balsam is entirely hard. When the 
 balsam is entirely hard the excess may be removed by 
 gently scraping with a knife blade and wiping with xylol. 
 The section should now be labelled with the name of the tooth, 
 the direction and position of the section, the student's 
 name and number, and the date. 
 
 The mounting in hard balsam greatly improves the value 
 of the section, for the dentinal tubules and the lacunae of 
 the cementum are left filled with air and can be more 
 easily studied. Sections may, however, be mounted in the 
 ordinary way, in soft balsam. If the section is broken or 
 extremely thin, soft balsam should be used. 
 
 PERIOD m 
 
 Outline Drawings from Ground Sections. — The object of 
 the outline drawing is the study of the dental tissues, their 
 distribution, portion of the tooth formed by each, their 
 relation to each other, and the coarser points of their struc- 
 ture. To get the value from this work the drawings must be 
 made very accurately to scale and as large as the note book 
 page will allow. With the bole gauge or a millimeter rule 
 measure accurately the length of the section, multiply this 
 by eight or ten, and mark the length on a page of the draw- 
 ing book. Measure the width of the section at the point 
 of the greatest diameter and multiply this by the same 
 factor. Using this for the width and the previous measure- 
 ment for the length, lightly draw a rectangle, which is to be 
 used as a guide in the construction of the dra^\^ng. The 
 success now of the drawing depends on the accuracy and 
 number of the measurements. 
 
 First measure the vertical distance from the incisal edge 
 to the gingival line on one side of the section, and then on 
 the other, and mark these on the sides of the rectangle. 
 
OUTLINE DRAWINGS FROM GROUND SECTIONS 433 
 
 This will give the relative length of root and crown and the 
 difference, if any, in the position of the gingival line on the 
 two sides. Measure the vertical distance from the most 
 prominent point on the axial surface to the incisal edge or 
 the tips of the cusps. And so on, making every measurement 
 that can help in the formation of the drawing. In this way 
 the outline of the section should first be traced inside the 
 rectangle, then the dento-enamel junction, then the pulp 
 chamber is shown, and finally the cementum. Before 
 drawing the outline of the cementum, the section should be 
 placed under the microscope, using the low power, and the 
 cementum should be observed studying it from the gingival 
 line on one side of the section to the gingival line on the other. 
 
 It would be a waste of time to attempt to fill in the 
 structure of the tissue of the entire outline, and ofily certain 
 things are to be shown in these drawings. For that reason 
 fill in three portions of enamel and dentine and three portions 
 of cementum and dentine, using the low power objective. 
 Study first the bands of Retzius (page 60), and lightly 
 indicate their direction. Study the enamel rod direction, 
 beginning at the gingival line at one side and following it 
 around the crown to the other side. In a portion at the 
 incisal edge, or on the occlusal surface, indicate the rod 
 directions, and in the same way show them in a portion 
 near the centre of the axial surface on one side and near 
 the gingival line. Follow the dentinal tubules which end 
 next to the portions of enamel which have been filled in to the 
 point where they open into the pulp chamber, and indicate 
 their direction (page 171). In the same way fill in three 
 portions of the cementum and the dentine under them — 
 one in the gingival line, one near the middle of the root, and 
 one in the region of the apex (Fig. 340). 
 
 If any portion of the section has been lost in grinding, 
 that portion should be indicated by dotted lines, and in the 
 same way, if a portion of the crown has been lost by wear, 
 the original form may be added in dotted fines. 
 
 OutUne drawings should be made from each of the three 
 classes of teeth — one from the incisor or cuspid, one from a 
 28 
 
434 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 bicuspid, and one from a molar, and a laboratory period 
 should be devoted to each drawing. 
 
 Fig. 340 
 
 ENAMEL 
 ---ENAMEL RODS 
 
 'M~ DENTINE {TUBULES) 
 
 -PULP CHAMBER 
 
 CEMENTUM 
 
 Outline drawing of longitudinal section, made as a study of the dental tissues. 
 (Drawn by E. J. Schnaidt.) 
 
MINUTE STUDY OF ENAMEL AND DENTINE 435 
 
 PERIOD IV 
 
 Isolated Enamel Rods. — Obtain from the desk a fragment 
 of enamel which has been broken in the direction of the rods. 
 Place a drop of distilled water or glycerin on the centre of a 
 clean slide. jNIoisten the broken surface with a drop of 
 water and lightly scrape it with the blade of a broad, sharp, 
 chisel, holding the edge parallel with the surface and the 
 shaft at right angles to it. Dip the edge of the chisel in the 
 drop of liquid on the slide, and the scrapings will be left. 
 Cover with a cover-glass and study with the high power, using 
 a small diaphragm. Fragments of enamel will be found 
 made up of broken rods, some single and others in groups. 
 Note the diameter of the rods and the appearance of the 
 cross-markings, which will be seen if the light is properly 
 adjusted. Draw as seen with the high power. 
 
 Repeat this operation, using enamel that has been immersed 
 in 1 per cent, hydrochloric acid foi a number of hours. Com- 
 pare the appearance of the rods with those of the former 
 specimen and make a drawing as seen with the high power. 
 
 Find an old tooth with a large carious cavity, remove the 
 softened dentine without touching the enamel if possible. 
 Lightly scrape the whitened inner surface of the enamel next 
 to the cavity and mount the scrapings as before. Compare 
 the appearance of these rods isolated by the action of caries 
 with those of the previous specimen. Notice that the cross- 
 markings are more distinct and the expansions and con- 
 stiictions of the rods more prominent. Draw a few of the 
 rods as seen ^\'ith the high power, using the small diaphragm . 
 
 PERIOD V 
 
 Minute Study of the Enamel and Dentine. — Select a field 
 from one of the ground sections where the specimen is very 
 thin, and, if possible, where the entire thickness of the 
 enamel plate can be seen in one field with the f objective. 
 
436 
 
 DIRECTIONS FOR LABORATORY WORK 
 
 To select this field all of the enamel in the three sections 
 should be carefully studied with the low power, and the one 
 
 Fia. 341 
 
 DENTO-ENAMEL 
 
 ..^ni\?\ 
 
 Sr«P>GHT ROI 
 
 
 '•;t' 
 
 pEI'lNPu TL'SULI 
 
 High-power drawing of the enamel. (Drawn by A. B. Hopper. 1902-03.) 
 
MINUTE STUDY OF CEMENTUM AND DENTINE 437 
 
 chosen in which the rods can be seen best and can be most 
 easily drawn. Having selected the field, study the enamel 
 with the high power, beginning at the dento-enamel junction. 
 Note the form of the dento-enamel junction and the relation 
 of the two tissues at this point. Note the diameter of the 
 enamel rods and estimate it, using a red blood corpuscle as a 
 standard of measurement. Note the striation of the enamel 
 (page 57). Using both the low and the high power, draw 
 as accurately as possible the enamel from the surface to the 
 dento-enamel junction, showing all the details of structure 
 that can be made out. 
 
 The drawing should be made as long as the page will 
 allow, and need not be more than an inch wide, and should 
 include just enough of the dentine to show the dento-enamel 
 junction and the character of the dentine at that point (Fig. 
 341). Notice the diameter of the dentinal tubules, comparing 
 them with the red blood corpuscles and the enamel rods. 
 Note the amount of matrix that separates the tubules. 
 Observe the forking and the anastomosis of the tubules as 
 they approach the enamel, and follow them as far as possible. 
 
 PERIOD VI 
 
 Minute Study of the Cementum and Dentine. — With the low 
 power study the cementum in the three specimens, looking 
 for all the details of structure that can be made out (see page 
 ISl). In the gingival portions and often well toward the 
 apex, especially if the tooth is from a young person, the 
 cementum will be very thin and almost structureless in 
 appearance. ^Yith the high power, fine lines parallel with 
 the surface may be seen, which indicate the lamellae. In 
 the apical portion the cementum becomes much thicker, and 
 it will be seen that each layer is thicker and consequently 
 more easily seen. Little black spots looking like spiders 
 will be found in larger or smaller numbers. These are the 
 lacunae \\dth the canaliculi radiating from them. They were 
 filled in life by cement corpuscles. Look for embedded 
 
438 DIRECTIONS FOR LABORATORY WORK 
 
 fibers of the peridental membrane. In all of this work each 
 field should be studied with both the low and the high 
 power. 
 
 <*M- 
 
 A.e Hc\s 
 
 yr 
 
 Fig. 342 
 
 
 — CEMENTUM 
 
 4 
 
 \ ^^<^y^^ 't^ - GRANULAR LAYER 
 
 
 OF TOMES 
 
 DENTINE 
 
 PULP CHAMBER 
 
 Cementum and dentine. (Drawn by H. J. Lund and A. E. Hopper.) 
 
DRAWINGS OF TYPICAL CAVITY WALLS 439 
 
 The inner layer of the cementum next to the dentine is 
 clear and structureless, and the dentine adjoining it appears 
 with the low power as a granular layer known as "the granu- 
 lar layer of Tomes." Studied with the high power, the 
 appearance will be seen to be caused by irregular spaces in 
 the dentine matrix communicating with the dentinal tubules 
 and filled in life with protoplasm of the fibrils. Compare the 
 dentine in the root with that in the crown (page 171). 
 
 After studying all the cementum in the three sections, 
 select three fields, one from the gingival, one from the 
 middle, and one from the apical portion of the root, and 
 draw the tissues from the surface of the root to the pulp 
 chamber. Show all the details of structure that can be 
 made out with both low and high powers (Fig. 342). With 
 the high power search the cementum for the record of 
 absorptions which have been refilled by cementum. 
 
 PERIOD VII 
 
 Drawings of Typical Cavity Walls. — From the molar or 
 bicuspid section select a field in the region of a groove or 
 pit. Imagine a cavity to be prepared in this position. To 
 help in this, an ink line may be made on the cover-glass by 
 using a fine pen and Indian ink, or ordinary ink to which a 
 little sugar has been added. Xow, using both the high and 
 the low power, study the direction of the enamel rods as 
 they appear in the line of the cavity wall, and make a drawing 
 showing the structural requirements for a good wall in this 
 position. From any one of the three sections select a field 
 in the gingival third of the labial or buccal surface and 
 indicate the line of a cavity wall in the same way. Study 
 with the low and the high powers the direction of the enamel 
 rods as they appear in the line of the walls of the cavity, and 
 make a drawing showing the structural requirements for 
 good walls in these positions (page 80). 
 
440 DIRECTIONS FOR LABORATORY WORK 
 
 PERIOD vm 
 
 Outline Drawings from Transverse Sections of the Root. — 
 
 The ground sections of the root have been prepared and should 
 be brought to the laboratory in solution, ready for mounting. 
 The three sections should be mounted together under one 
 cover-glass, using balsam about the consistence of molasses. 
 The sections may be studied at once, but after the day's 
 work upon them they should have a spring clip adjusted 
 to the cover-glass and be put away until the balsam is thor- 
 oughly hard, otherwise they may work out to the edge of 
 the cover-glass. With the millimeter gauge measure the 
 length and breadth of each section, multiply the measure- 
 ments by twenty, and lay off a rectangle as in making the 
 longitudinal drawings. Draw the outline of the section and 
 the pulp chamber as accurately as possible before studying 
 the section with the inicroscope. With the low power 
 follow the dentocemental junction around each section and 
 draw it into the outline. Fill in half of each section, showing 
 the direction of the dentinal tubules, the position and char- 
 acter of the granular layer of Tomes, the number and posi- 
 tions of the lacunae, and the other structural characteristics 
 of the cementum. In this study the record of the reduction 
 of size of the pulp chamber which may be noted by changes 
 in the direction and the character of the dentinal tubules 
 (page 185). Label the section with the name of the root 
 from which it was ground, your name, and the date. 
 
 PERIOD IX 
 
 Study of Secondary Dentine and Cementum. — With the 
 low power find a field where there is a distinct demarcation 
 between dentine of earlier and later formation, and draw it 
 accurately with the high power. Compare the size of the 
 tubules, their number, their direction, and their diameter in 
 the earlier with the later formed dentine; is there any con- 
 
GROUND SECTIONS OF BONE 441 
 
 nection between the tubules of the two portions? Find a 
 similar field from a longitudinal section and study in the 
 same way, making an accurate drawing. 
 
 Search all of the ground sections with the low power until 
 a field is found where the dentinal tubules are cut trans- 
 versely. Adjust the high power objective and study the 
 field. Notice that by focussing up and down with the fine 
 adjustment the tubules seem to move in a circle, showing 
 the spiral course through the matrix. Using a red blood 
 corpuscle as a standard, note the size of the tubules, their 
 distribution in the matrix, and the amount of matrix sepa- 
 rating them. Look for the appearance of Newman's sheath, 
 which is that portion of the matrix forming the immediate 
 wall of the tubule. Draw accurately one field as seen with 
 the high powder. Study the cementum from all the ground 
 sections for an area showing absorption and rebuilding, and 
 if found, draw one field with the high power. Draw five or 
 six lacunae with their canahculi as seen with the high power, 
 selecting as great a variety of forms as possible. 
 
 PERIOD X 
 
 Ground Sections of Bone.— From a shaft of a femur or 
 humerus saw a disk about one-quarter of an inch thick. In 
 doing this notice the appearance of the marrow cavity 
 especially as you look into it toward the articular ends. 
 Saw the disk into sectors with an arc of about a quarter of 
 an inch on the outer surface. From this piece saw two thin 
 sHces — one at right angles to the axis of the bone, the 
 other parallel with it. These should be ground as directed 
 in the introduction for the grinding of transverse sections 
 of the root, and be brought to the laboratory ready to mount. 
 They should be mounted in hard balsam as described in 
 the mounting of longitudinal sections of the teeth. Label 
 the slide with the name of the bone from which the section 
 is taken and the direction in which it is cut. Study the 
 transverse section with the low power, working out the 
 
442 DIRECTIONS FOR LABORATORY WORK 
 
 arrangement of the lamellae and the distribution of the 
 subperiosteal and Haversian system bone (p. 252). Draw 
 the tissue from the surface of the bone to the marrow cavity. 
 This drawing should be not more than an inch wide and the 
 full length of the page. With the high power objective and 
 low power eyepiece, draw one or two Haversian systems. 
 
 Study the arrangement of the Haversian canals as seen 
 in the longitudinal sections. With the high power draw 
 at least three lacunae, showing one cut lengthwise, one trans- 
 versely, and one as seen from above. 
 
 PERIOD XI 
 
 Decalcified Bone. — One of the bones from a small animal 
 has been decalcified, embedded, sectioned, and stained with 
 hematoxylin and eosin. Receive from the desk two sections, 
 one of which is cut longitudinally, the other transversely. 
 Mount in balsam in the usual way. Label the slide with 
 the name of the animal, the bone from which it is cut, and 
 the direction of the section. Study the transverse section 
 with the low power, noting the bone corpuscles in the lacunae, 
 the tissue in the Haversian canals, and the marrow. With 
 the high power draw one field showing two or three Haversian 
 systems, one of which has been partially destroyed in the 
 building of another. Draw with the high power one field 
 from the marrow cavity. From the longitudinal section 
 draw, with the high power, one field showing osteoblasts in 
 a medullary space. 
 
 PERIOD XII 
 
 Comparative Study of Subperiosteal Bone and Cementum. — 
 For this day's work the previously mounted sections must 
 be used, the longitudinal sections of the teeth, the transverse 
 sections of the root, the ground and decalcified sections of 
 bone. Study the cementum and the subperiosteal bone as 
 shown in these sections and make one drawing of cementum 
 
DENTAL PULP FROM UNERUPTED TOOTH OF SHEEP 443 
 
 and one drawing of subperiosteal bone to show the com- 
 parison in structure. Compare the regularity in form and 
 arrangement of the lacunae in the bone with the irregularity 
 in form and position of the lacunae in cementum. Note that 
 in the bone the lacunae lie between the layers ; in the cementum 
 they may be between the layers or entirely within a single 
 layer. Compare the regularity in the arrangement and thick- 
 ness of layers with the corresponding irregularity in cementum. 
 Note the size, number, and arrangement of the canaliculi 
 radiating from the lacunae in bone, and compare them with 
 the canaliculi of the cementum. 
 
 PERIOD xm 
 
 Dental Pulp from the Unerupted Tooth of a Sheep. — An 
 
 unerupted molar or premolar of a yearling lamb was removed 
 from the lower jaw by splitting the bone. The pulp was 
 pulled out of the partially formed dentine embedded in 
 paraffin, sectioned, stained with hematoxylin and eosin. 
 Bring to the desk a clean slide with a drop of balsam upon 
 the centre of it and receive a section. Label the slide: " Pulp 
 from unerupted tooth of sheep, stained with hematoxylin 
 and eosin." Study first with the low power. Upon the 
 circumference of the section the layer of odontoblasts may 
 or may not be shown, depending upon whether in the 
 removal of the pulp the fibrils have pulled away from the 
 dentine, or the odontoblasts have been pulled oft' from the 
 surface of the pulp. They are usually present, at least in 
 spots. Note the number and arrangement of the blood- 
 vessels and the distribution of the connective-tissue cells. 
 With the low power draw a portion from the surface to the 
 centre, showing the layer of odontoblasts if present. With 
 the high power draw one field showing a bloodvessel and 
 the connective-tissue cells, taking particular pains to repre- 
 sent their forms correctly. If there are any odontoblasts 
 present draw one field showing them and the layer of Weil 
 (see page 209). 
 
444 DIRECTIONS FOR LABORATORY WORK 
 
 PERIOD XIV 
 
 Dental Pulp, Normal Human. — A number of human teeth 
 were cracked immediately after extraction and the pulps 
 removed from the pulp chambers. They were embedded in 
 one block of paraffin, sectioned, stained with hematoxylin 
 and eosin, and are ready to be given out. Bring to the desk 
 a clean slide with a drop of balsam on the centre and receive 
 a section. Label the slide: "Transverse section of pulp from 
 human teeth." There will be several sections in this specimen, 
 each from a separate pulp. With the low power follow the 
 circumference of each section, looking for places where 
 odontoblasts are present. Find the best field in the speci- 
 men and draw the layer of odontoblasts as seen with the 
 high power. Notice the fibrils which have been pulled out 
 of the dentinal tubules projecting from the ends of the 
 odontoblasts. If the section is parallel with the long axis 
 of the cells, they will appear as tall columnar cells with a 
 nucleus in the deeper end. If it is oblique to their axis the 
 layer may appear as two or three layers of oval cells. Just 
 beyond the odontoblasts the layer of Weil Avill be seen, 
 usually appearing as a clearer layer containing few cells and 
 about half as wide as the odontoblasts. Beyond this the 
 connective-tissue cells are thickly placed for a short distance, 
 and still deeper they are more widely scattered and about 
 evenly distributed in the rest of the pulp. 
 
 With the high power draw one field to show the form of 
 the connective-tissue cells of the pulp. With the low power 
 study the distribution of the bloodvessels in all of the sec- 
 tions. Select the best section and draw the entire section, 
 to show the size, number, and arrangement of the large 
 bloodvessels. With the high power draw a single field, to 
 show accurately the structure of a bloodvessel wall. 
 
ENDOCHONDRIAL BONE FORMATION 445 
 
 PERIOD XV 
 
 Dental Pulp, Pathologic Human. — By the cooperation of 
 the man in charge of the extracting room, or an extracting 
 speciahst, teeth with hving but inflamed or h\7)eremic 
 pulps were dropped as soon as extracted into a fixing fluid. 
 The teeth were afterward cracked and the pulps removed, 
 embedded, and sectioned as before. Bring to the desk a 
 clean slide with a drop of balsam on its centre and receive a 
 section. Label the sHde: "Pathologic pulp from human 
 tooth stained T^'ith hematoxylin and eosin." Follow the 
 same routine in studying these specimens as in the case of 
 the normal pulp. It is impossible to tell just what condi- 
 tions will be present. Compare the size and number of the 
 bloodvessels with those in the normal tissue, and the char- 
 acter and distribution of the cellular elements. Look for 
 nodules of calcoglobuU, especially in the inflammatory speci- 
 mens, and make a diagnosis of the condition, as shown in 
 the specimen. See the chapter on the Structural Changes in 
 the Pulp and Pathological Conditions for fuither assistance 
 on the work in this material. 
 
 PERIOD XVI 
 
 Endochondrial Bone Formation. — A forming bone from a 
 human fetus has been embedded, sectioned, and stained with 
 hematoxylin and eosin. Receive a section from the desk and 
 mount as usual. Study the specimen -^dth the low power, 
 identifying first the general arrangement of the tissues, 
 following from the unchanged cartilage to the development 
 of bone. Notice the subperiosteal layers on the surface. 
 Make a sketch of a sufficient part of the section to show the 
 changes from the typical hyaline cartilage to the young bone. 
 With the high power draw one field from a primary marrow 
 cavity, showdng osteoblasts laying down lamellse on one of the 
 spicules, and one field showing osteoclasts. 
 
446 DIRECTIONS FOR LABORATORY WORK 
 
 PERIOD xvn 
 
 Bone Growth. — A piece of a long bone from a very young 
 animal has been embedded and sectioned transversely to the 
 shaft. Sections have been stained in hematoxylin and 
 eosin, to be mounted as usual. Label the slide: "Growing 
 bone cut transversely, stained with hematoxylin and eosin." 
 Study first with the low power. On the surface of the section 
 will be seen the periosteum, in which the fibrous and osteo- 
 genetic layers can be easily recognized. Bone formation is 
 actively going on, laying down lamella^ under the periosteum 
 which are being transformed into Haversian system bone. 
 With the low power draw a portion of the section from the 
 periosteum to the centre of the bone. With the high 
 power draw a field showing the osteoblasts of the peri- 
 osteum, a field showing the absorption of subperiosteal bone 
 to form a medullary space, and a field showing osteoblasts in 
 a medullary space. 
 
 PERIOD XVIII 
 
 Periosteum from Attached Portion. — From a young kitten 
 a portion of a bone in a region to which muscles are attached 
 to the periosteum was carefully dissected out, removing the 
 attached muscle, and the tissue embedded in celloidin, the 
 sections cut parallel to the axis of the bone and perpendicular 
 to its surface. They have been stained in hematoxylin and 
 eosin, and are ready to be given out. Receive a section and 
 mount as usual. Label : " Periosteum from attached portion, 
 stained in hematoxylin and eosin." Study the specimen first 
 with the low power. The outer fibrous layer of the peri- 
 osteum will be seen with the muscle fibers attached to it 
 and the osteogenetic layer with the greater number of cells 
 taking the stain more deeply. Draw with the low power, 
 showing the tissues from the surface of the periosteum well 
 into the substance of the bone. With the high power study 
 the attachment of the muscle fibers to the outer layer of the 
 periosteum, the character and arrangement of the fibers of 
 
GINGIVUS AND GUM TISSUE 447 
 
 the outer layer, the interlacing of the fibers of the outer 
 and inner layer, the cells, and especially the osteoblasts of 
 the inner layer and the penetrating fibers that are built into 
 the bone. Draw the thickness of the periosteum as seen 
 with the high power, showing the details of structure as 
 accurately as possible. 
 
 PERIOD XJX 
 
 Gingivus and Gum Tissue. — The gingivus and gum tissue 
 covering the alveolar process down to the point of reflection 
 on to the cheek was dissected away from the teeth and jaw 
 of a sheep. The tissue was embedded in paraffin and sec- 
 tioned parallel \^'ith the long axis of the tooth. The sections 
 have been stained with hematoxylin and von Gieson, and are 
 ready to mount. Bring to the desk a clean slide ^^'ith a drop 
 of balsam on the centre and receive a specimen. Label the 
 section: "Gingivus from a sheep, stained with hematoxylin 
 and von Gieson." By this staining the cellular elements 
 will have a brownish color, the nuclei dark, the protoplasm 
 lighter, the white fibers should be bright red, and the elastic 
 fibers yellowish. It is a specially good stain for connective 
 tissue. Study with the low power. The epithelial will be 
 stained a brownish yellow or purple. It is a stratified 
 squamous epithelium made up of many layers of cells and 
 with a distinct horny or corneous layer on the surface from 
 the crest of the gingivus to the point where the mucous mem- 
 brane is reflected on to the cheek, or where it ceases to be 
 attached to the gum. This layer is yello\\Tsh in color, and is 
 made up of closely packed scales having no nuclei. They are 
 the remains of epithehal cells from which the protoplasm is 
 gone, leaving only the horny material which it had produced. 
 The portion of the epithelial lining the gingival space has no 
 corneous layer, nuclei being seen in the cells at the surface. 
 The cells are larger and more loosely placed. The connective- 
 tissue papillffi and the projections of epithelium which are 
 between them are extremely long. In the epithelium 
 covering the alveolar process the connective-tissue papilla? 
 
448 DIRECTIONS FOR LABORATORY WORK 
 
 are broader and not so deep, and the cells are much more 
 compactly arranged. At the point of reflection on the cheek 
 the epithelium changes its character abruptly, the corneous 
 layer disappears, the surface cells showing nuclei, the epi- 
 thelial layer is thicker and made up of larger and more 
 loosely placed cells. This change in the structure explains 
 why the epithelium is easily broken where a movable portion 
 of the membrane passes over the edge of an artificial denture. 
 When an infection reaches the connective tissue a sore is 
 produced that requires some time to heal. 
 
 Study the connective tissue, which is made up of coarse, 
 wavy bundles of white fibers taking the red stain. In the gum 
 tissue, that is, the portion of the section covering the alveolar 
 process, the bundles are very large and form a very coarse 
 network. Beyond the point of reflection the bundles are 
 finer and more delicate in their arrangement. Elastic fibers 
 take the yellowish stain. Notice the bloodvessels in the 
 connective tissue and the capillaries in the papillae. With 
 the low power draw the entire section so as to show the 
 character of the epithelium and the fibrous tissue in the three 
 parts. 
 
 With the high power draw the thickness of the epithelium 
 lining the gingival space and at the point where the mem- 
 brane is reflected to the cheek. 
 
 PERIOD XX 
 
 Peridental Membrane, Transverse Gingival. — The lower jaw 
 of a young sheep was sawed through between the teeth, 
 cutting the jaw into blocks each containing two teeth. The 
 crowns were broken off or opened so as to admit the fluids 
 to the pulp tissue. The tissues were decalcified, embedded, 
 and sectioned at right angles to the axis of the tooth. They 
 are cut from the gingival portion, and have been stained with 
 hematoxylin and eosin. Receive a section and mount as 
 usual. Label the slide : " Peridental membrane, transverse 
 gingival, stained with hematoxylin and eosin.'' A similar 
 block of tissue preserved in alcohol will be found at the desk. 
 
PERIDENTAL MEMBRANE 449 
 
 This should be observed so as to study out the relation 
 of the section to the gross appearance of the tissue. 
 
 Holding the section to the light, observe the distribution 
 of the tissue. Two roots '^'ill be seen cut across. Observe the 
 epithelium on the labial and the lingual, and possibly also that 
 lining the gingival space lying next to the root of one of the 
 teeth. By the aid of the low power sketch the outline of the 
 entire section to show the distribution of the tissues. Note 
 the demarcation where the finer fibers of the peridental 
 membrane unite with the coarser mat of gum tissue. Begin- 
 ning at the centre of the labial surface, follow the fibers 
 springing from the cementum to where they are lost in the 
 gum tissue or attached to the approximating tooth. Draw 
 the portion of the membrane between the two roots, accu- 
 rately representing the arrangement of the fibers. The epi- 
 thelial structures wall be seen lying between the fibers close to 
 the cementum, and should be showm in the drawing (p. 308). 
 
 With the high power study the cementoblasts and the 
 epithelial structures. Make a dra-^ing of one field, showing 
 all the details of structure as accurately as possible. 
 
 With the high power draw one field showing the fibrous 
 tissue between the roots and the relation of the fibroblasts 
 to them. This field should include a bloodvessel. 
 
 PERIOD XXI 
 
 Peridental Membrane, Alveolar Portion, Transverse. — The 
 sections for this work have been cut from the same block as 
 the preceding, but are in the occlusal third of the alveolar 
 portion and as close to the border of the alveolar process 
 as possible. Receive a section. Mount as usual and label 
 the shde : " Peridental membrane, alveolar portion, transverse, 
 stained with hematoxylin and eosin." 
 
 Study the general arrangement of the tissues and make a 
 sketch as in the case of the previous specimen. Note the 
 muscle fibers from the muscles of the lip attached to the 
 periosteum on the labial surface of the process, the bone of 
 the labial plate, the septum separating the alveoli, the peri- 
 20 
 
450 DIRECTIONS FOR LABORATORY WORK 
 
 dental membrane filling the space between the bone and the 
 surface of the root, the layers of the cementum, the dentine 
 and the pulp. 
 
 After studying the specimen with the low power as care- 
 fully as possible, draw the peridental membrane surrounding 
 one root, including the thickness of the labial plate of bone 
 with its periosteum and a part of the lingual plate. In this 
 drawing represent accurately the fibers of the peridental 
 membrane, their arrangement in the bundles, and the relation 
 of the bundles to each other and the bloodvessels. To do 
 this the fine adjustment must be used to obtain ideas of the 
 third dimension of space. With the high power draw one 
 field from the wall of the alveolus, showing the attachment of 
 the fibers to the bone, the osteoblasts on the surface of the 
 bone, and the other cellular elements. This field should 
 include a bloodvessel. With the high power draw the thick- 
 ness of the cementum at some point where a specially strong 
 bundle of fibers is attached. This should show the fibers 
 embedded in the cementum, cementoblasts on the surface, 
 and the branching and interlacing of the bundles. 
 
 PERIOD xxn 
 
 Longitudinal Section of the Peridental Membrane. — The 
 
 lower incisor of a young sheep was removed from the jaw 
 by sawing through between the teeth, leaving two teeth in 
 each block. The crowns of the teeth were broken off near 
 the level of the gum so as to admit the reagents to the pulp 
 chamber. The tissues decalcified, embedded in celloidin, 
 and sectioned. They were cut through from labial to 
 lingual, and only the ones from the central portion used. 
 They have been stained in hematoxylin and eosin and are 
 ready to mount. Mount the section as usual and label the 
 slide : " Longitudinal section through the peridental membrane 
 of a sheep, labiolingual, stained in hematoxylin and eosin." 
 First hold the section up to the light and note the relation 
 of the tooth to the bone and the soft tissues. Study the 
 section with the low power and make a sketch showing the 
 
TOOTH GERM 451 
 
 general distribution of the tissues. Show the pulp chamber, 
 dentine and cementum, bone, periosteum, gum tissue, and 
 epithelium. Do not attempt to fill in the drawing more than 
 diagrammatically, for it would require too much time. The 
 object of the drawing is to get the general relation of the 
 tissue before studying parts of it in detail. Compare the 
 form of the labial and the lingual gingivus and make a draw- 
 ing of the lingual, showing the details of structure as far as the 
 border of the process and as accuratel}^ as possible. With 
 the high power draw the thickness of the epithelium lining 
 the gingival space. Study the fibers in the occlusal third 
 of the alveolar process and make a drawing to represent them 
 accurately, showing the cementum at one side and the bone 
 at the other. The entire length of the root can seldom be 
 got in one section on account of the curve of the tooth, 
 so that the fibers can probably be studied to advantage in 
 the occlusal third of the alveolar process only. Draw one 
 field with the high power showing the bloodvessels. 
 
 PERIOD xxm 
 
 Tooth Germ. — The head of an embryo pig was embedded 
 in paraffin and sectioned at right angles to the snout. The 
 sections begin in the region of the incisors and far enough 
 back to cut through the nose cavity. They have been 
 stained in hematoxylin and eosin. Bring to the desk a clean 
 sHde and receive a section. Label the shde: "Tooth germ, 
 stained with hematoxylin and eosin." 
 
 The general form of the section will depend on the position 
 of the section through the head. At the desk is the head 
 of a similar embrj'o preserved in alcohol. This should be 
 observed so as to determine from the section its relation to 
 the head. By holding the section to the light and the use 
 of the low power, make a sketch of the entire section. Note 
 the epiblast covering the outer surface and lining the nose 
 and mouth cavity. The mass which is to form the tongue 
 lying between the roof of the mouth and the mandibular 
 arch. If the section is in front of the angle of the mouth 
 
452 DIRECTIONS FOR LABORATORY WORK 
 
 there will be no connection between the upper and lower 
 parts of the section. Notice the separation of the nose 
 cavity into right and left by a septum containing cartilage, 
 and the projections of cartilage from the side walls which 
 will form the turbinate bones. On either side of the septum 
 where it joins the palate will be seen little structures known 
 as Jacobson's organ, which later disappear. Notice Meckel's 
 cartilage in the mesodermic mass of the mandible. In the 
 epiderm of the outer surface the beginning of the formation 
 of hairs are to be seen. 
 
 With the low power follow the epiderm lining the mouth 
 cavity and look for the tooth germ. In each section there 
 are four chances for tooth germs, one on either side in the 
 upper and lower arches. Select the best one and draw it 
 as seen with the low power. The appearance will depend 
 entirely upon the stage of development. 
 
 With the high power draw enough of the enamel organ 
 to show the arrangement of the cells in the outer and inner 
 tunics and the stellate reticulum. 
 
 PERIOD XXIV 
 
 Tooth Germ. — Sections have been prepared in the same way 
 as in the preceding, but from the head of an older embryo, 
 in which the tooth germs are completely formed and calci- 
 fication is ready to begin. 
 
 Receive a section, mount, and label as before, and draw the 
 outline of the entire section. Note the changes in form and 
 in the tissue elements from the previous section. Bone 
 formation has begun both in the mandible and the maxilla. 
 The amount and distribution of this should be carefully 
 studied. 
 
 With the low power draw the entire tooth germ, selecting 
 the most typical one in the section. With the high power 
 draw one field showing ameloblasts, odontoblasts, and a 
 portion of the papillae. Find a field in which bone formation 
 is going on and draw it accurately with the high power. 
 
APPENDIX CHAPTEPv I 
 
 THE GRINDING OF MICROSCOPIC SPECIMENS, USING 
 THE GRINDING MACHINE 
 
 By G. V. BLACK, M.D., D.D.S., Sc.D., LL.D. 
 
 The Machine. — The basis of this machine is the larger 
 watchmaker's lathe known as No. 2. It must swing 4 
 inches, the length of the bed must be 12 inches, and be good 
 and solid. A test should be made of the alignment of the 
 lathe head to see that this is exact. If there is any inac- 
 curacy, another lathe should be selected. The power should 
 consist of one of the largest and strongest electric lathes, 
 or motors, made for the use of dentists. This power should 
 be transmitted to the lathe through an overhead shaft of 
 a length that will give good room to operate the lathe with- 
 out the motor being in the way. A pulley may be placed on 
 the left end of the shaft of the motor on one of the brass 
 carriers for grinding wheels. This pulley should carry a 
 good quarter-inch round leather belt. Its diameter should 
 be 2| inches. The pulley on the right hand end of the shaft 
 above should be 5 inches. This will reduce the speed one- 
 half and double the power. On the left end of the shaft 
 should be placed a copy — reversed — of the pulley on the 
 lathe-head, which has 4 grooves. This gives good varieties of 
 speed with each speed of the motor. Another small pulley 
 will be placed near the centre of the length of the overhead 
 shaft, the purpose of which will be explained later (Figs. 
 343 and 344). 
 
 The grinding apparatus is built upon a base fitted to the 
 lathe bed in the same wav as the lathe head, or tailpiece. 
 
454 
 
 APPENDIX CHAPTER I 
 
 Fig. 343 
 
 Figs. 343 and 344. — A general view of the grinding machine, showing particularly 
 the arrangement for transmitting the power from the electric motor to the machine that 
 does the work. All of this may be made out by reference to the picture while following 
 the text. The bed of the little lathe on the left hand is \2\ inches long, which gives 
 a good idea of the general dimensions. 
 
 The water is delivered to the grinding stone from a rubber bag or bucket hung on 
 the frame above through a rubber tube to the metal tube on a movable stand, which 
 may be so placed as to bring the brush at its end against the stone. This stand and 
 brush are better seen in Fig. 344. 
 
THE GRINDING OF MICROSCOPIC SPECIMENS 455 
 
 Fig 344 
 
456 APPENDIX CHAPTER I 
 
 It has one main shaft parallel with the lathe bed, in good 
 and sufficient bearings to maintain accuracy of alignment 
 and perfect steadiness for long continued usage (see Figs. 343 
 and 344). This shaft moves freely lengthwise, or back and 
 forward, while turning slowly in its bearings. On the end 
 of this shaft next to the lathe head — the forward end — 
 there is a larger portion,, or ring, and this end terminates 
 in a threaded nipple, upon which the removable grinding 
 disks are screwed firmly against the face of this larger ring, 
 to secure accuracy of adjustment. The use of these disks 
 will be more fully explained later. 
 
 On the rear end of this shaft, just back of its rear bearing 
 and abutting against it, a large movable nut is placed. This 
 is provided with a thumb screw by which it is made fast 
 at any point desired. Turning this forward pulls the shaft 
 back from the grinding stone. Turning it backward allows 
 the shaft to move forward against the stone. It has also 
 a finger reaching back over a graduated disk just to its 
 rear. This disk is made fast on the shaft, and the two 
 together constitute the micrometer, by which the thickness 
 to which specimens are ground is measured. The movable 
 nut has 40 threads to the inch. The graduation of the disk 
 is on the same principle as that on the screw calipers used 
 by machinists for fine measurements — one-thousandth of an 
 inch — but as this disk is li inches in diameter, the gradu- 
 ations of thousandths are so wide that one-quarter of one- 
 thousandth may readily be used. It differs in plan, in 
 that both the graduation and the parallel lines are placed 
 upon this disk. On the machinist's micrometer the lines 
 are placed on the shaft and the graduations on the nut. 
 The graduation is read from the side of the finger on the 
 movable nut, and the lines are read from its end. It is a 
 very perfect micrometer (Figs. 345 and 346). 
 
 The forw^ard movement of the shaft when grinding, and 
 also the pressure exerted upon the stone, are furnished by a 
 tailpiece placed behind it and attached to the lathe bed. 
 This has a plunger actuated by a spiral spring, which pushes 
 the shaft forward against the stone. The amount of pressure 
 
THE GRINDIXG OF MICROSCOPIC SPECIMENS 457 
 
 exerted in the grinding is controlled by the amount of com- 
 pression of this spring in fixing the piece to the lathe bed. 
 It may be much or little, as desired. Usually very little 
 pressure is used. When the movable nut has come against 
 the frame in which this shaft turns, the machine may con- 
 tinue to run, but the forward movement of the shaft stops 
 and the grinding ceases in consequence. Therefore there is 
 no danger of grinding a specimen thinner than the measure- 
 ment fixed upon. The further arrangement for finding this 
 measurement will be described later. 
 
 On the rear portion of the graduated disk, or wheel, a 
 portion or space is toothed, and connected with a worm 
 pinion or threaded shaft by which the main shaft is turned 
 in its bearings. A belt is attached over a wheel on the end 
 of this worm shaft, and extends to the third wheel, previously 
 mentioned, on the overhead shaft. When this belt is ad- 
 justed and the motor started, it causes the main shaft in 
 the grinding machine proper to turn slowly on its axis, while 
 being pressed against the stone by the tailpiece. By this 
 arrangement every part of the specimen fixed on the grind- 
 ing disk is brought successively against every part of the 
 rapidly revolving stone, and is cut perfectly level in all of 
 its parts. 
 
 The Grinding Disks. — The grinding disks are of brass, 
 accurately turned | inch thick, and If inches in diameter. 
 They have a threaded hole i inch deep in the back to fix 
 them to the nipple on the forward end of the shaft of the 
 grinding machine. A machine should have a half-dozen 
 or more of these, lettered or numbered on the edge, so that 
 records of each may be made when measuring preparatory 
 to mounting specimens for grinding. As the mounting of 
 specimens on others of these may proceed while the grinding 
 on one is going on (for the machine, being automatic, needs 
 little attention), this number at the least is necessar}^ for 
 rapid work. 
 
 The machine may be stopped and the disk removed from 
 the shaft by a few backward turns, the progress of the 
 grinding examined, the disk returned for further grinding, 
 
458 
 
 APPENDIX CHAPTER I 
 
 Fig. 345 
 
 Figs. 345 and 346. — The lathe with the grinding machine mounted upon it in position for work. On the left 
 next to the lathe head is the grinding stone surrounded by the spatter guard, which gathers all of the water 
 from the wheel and dehvers it through its hollow post into a rubber tube below the lathe bed, which conveys it 
 to a conveniently placed receptacle. The water comes from a rubber bag or bucket hung on the overhead 
 frame (see Fig. 343) through a rubber tube to the metal tube mounted on a movable stand so that the 
 brush through which it passes may be placed against the stone. The grinding machine proper is secured 
 to the lathe bed by the larger thumbscrew seen below. The point-finder is seen at the foot of the spatter 
 guard, and is secured by the middle thumbscrew seen below the lathe bed. 
 
 The shaft of the grinding machine (6 inches long) runs through its whole length, but is completely 
 covered in by its housings to protect its bearings from grit, except at its forward end (next to the grinding 
 stone). This part is protected by a swaddle held by a ring, which keeps the working bearing clean. On 
 this end the grinding disk is seen almost touching the stone. The micrometer is on the other end of the 
 shaft next back of the frame of the grinding machine. Next back of this is a toothed wheel made fast to 
 the shaft. This is actuated by the middle one of the belts descending from overhead (Fig. 343, the left 
 hand belt in Fig. 344). This belt passes over a wheel hidden from view and through a small worm shaft turns 
 the main shaft. Pressure for the grinding is supplied by a plunger actuated by a spiral spring seen at the 
 extreme right hand end. 
 
THE GRINDING OF MICROSCOPIC SPECIMENS 459 
 
460 APPENDIX CHAPTER I 
 
 etc., at any time during the progress of the work. The face 
 of the disk, which should be perfectly flat and parallel with 
 the face of the stone, should always be perfectly bright, 
 so as to reflect light through the specimen when it becomes 
 thin. This enables one to judge very closely of the thickness 
 by the eye (after sufficient practice), that sometimes proves 
 a valuable check on the setting of the measurement in the 
 beginning. 
 
 The Point Finder. — This is a piece of steel one-eighth of 
 an inch thick, fitted to the lathe bed and set against the face 
 of the lathe head, and made fast by a thumb screw passing 
 through the lathe bed from below. It has a strong arm which 
 passes around other fixtures between the lathe head and the 
 forward end of the base of the grinding machine. It is pro- 
 vided with a set screw, by which a range of variation can be 
 made in the distance of the forward end of the frame of 
 the grinding machine from the lathe head. When this is 
 in place and the measurement of a disk has been made and 
 recorded for the grinding of a specimen to a specified thick- 
 ness, the machine may be taken to pieces and set up again 
 and the grinding proceed without fear of disturbing the 
 measurement, so long as the set screw in the point finder is 
 not moved. It is often necessary during grinding to loosen 
 the grinding machine from the lathe bed, slide it back to 
 adjust something, to remove disks for examination of the 
 progress of the work, etc. This point finder, by preserving 
 the distance between the lathe head and the grinding 
 machine, enables one to do this at will, and again find his 
 exact point of measurement simply by sliding the frame of the 
 grinding machine forward against the set screw of the point 
 finder. This little device seems absolutely necessary to the 
 highest usefulness of the machine. 
 
 Lap Wheels and Grinding Stones. — I began my work of 
 grinding specimens by the use of lap wheels, but soon dis- 
 carded them because they are dirty. They cut much quicker 
 than stones, however, and may be used for the bulk of the 
 work when much grinding of very hard material is to be 
 done. They are not necessary in grinding teeth, bone, etc.. 
 
THE GRINDING OF MICROSCOPIC SPECIMENS 461 
 
 but in grinding the harder fossils, especially those impreg- 
 nated with the silica, and in some geological work they 
 become necessary. 
 
 The best lap wheel I have used is an aluminum wheel. 
 Brass or iron will do the work, but aluminum holds the grit 
 better, cuts with lighter pressure, and does the work quicker. 
 In using these I have fed them continuoush' by hand with 
 carborundum powder in soapy water, using a brush. 
 
 The Stones. — Anyone who is doing much grinding should 
 have a good supply of stones. I have a pair of carborundum 
 wheels, a pair of emery wheels, a pair of India oil stones, 
 and a pair of Arkansas stones. In each of these pairs one 
 is fine and the other coarser grit. Every stone is dressed 
 to a perfect face on the lathe head where it is to do its work, 
 with a black diamond held in the slide rest. 
 
 These stones, when put in good shape, seem capable of 
 doing an unlimited amount of work. The conditions of the 
 grinding prevents them from getting out of true. All that 
 seems necessary is to roughen them a bit with a picking 
 wheel when they become too smooth to cut well. For this 
 purpose a much smaller picking tool than the smallest 
 sold for the general mechanical uses seems desirable. This 
 picking wheel has sharp teeth of the hardest steel possible 
 on its periphery. It is held in a handle in such form that 
 the wheel is free to turn. In use it is held against the rapidly 
 rotating stone and slowly passed over its entire surface. 
 It may be held in the hand aided by a tool rest, or may be 
 arranged for use in the slide rest, which is the better form 
 for this work. 
 
 Watering the Stones. — In grinding, the stones are kept 
 wet in running ice water. A balsam that is too soft to hold 
 a specimen for grinding in water at room temperature will 
 hold it perfectly in ice water, because it is much harder when 
 cold. For this purpose, a receptacle for ice is hung on the 
 frame that holds the overhead shaft, and filled with bits of 
 ice and then filled with water. Both the ice and the water 
 must be clean, for the opening in the tube where it passes 
 the valve which regulates the flow is very small, and a 
 
462 APPENDIX CHAPTER I 
 
 little bit of dirt or trash might stop the flow. In this case 
 the specimen being gronnd would be burned instantly. A 
 bucket, or a large rubber bag, will answer for this purpose. 
 Then an ordinary rubber tube answers to conduct the water. 
 It is best to have this rubber tube to connect with a metal 
 tube mounted on a stand that may be placed in any position 
 wanted to deliver the water to the stone. This metallic 
 tube is provided with a valve for the regulation of the flow. 
 In its final end it should be provided with a brush of rather 
 long bristles, into which the w^ater is delivered and spread 
 upon the stone. This brush is made upon a short tube 
 fitted into the end of the metal tube. To make this brush, 
 first cover the plain part of the small brass tube with thick 
 shellac dissolved in absolute alcohol. Place a layer of the 
 bristles around it and wrap them tightly wath a fine, strong 
 thread. Then place more shellac over this and another 
 layer of bristles. Continue this until the brush is large 
 enough. Then wrap thoroughly with a cord in shellac, 
 let it dry, and then trim it up. Two of these have served 
 for four years of fairly hard usage. 
 
 Waste Water. — A spatter guard is made by bending a 
 |-inch round brass tube into a circle, the inner diameter 
 of which is the size of the stones used, and brazing the ends 
 solidly together. Then this is fixed in the lathe and one- 
 fourth of its inner circular diameter is turned away. The 
 grinding stones will then go inside this. Then this piece is 
 provided with a foot and hollow post and fitted to the 
 lathe bed with a washer and nut, the same as other pieces 
 are attached. This catches all w^aste water and through a 
 rubber tube attached to the end of its hollow post under the 
 lathe bed delivers it into a receptacle so placed by the table 
 as to receive it. This prevents all of the spattering of water 
 which would be thrown from a rapidly revolving wheel 
 without it. If it should be inclined to run over when a 
 very full stream is wanted, a piece of rubber dam may be 
 stretched over the foot and pulled to its upper end. This 
 may be caught under the guard in fastening it to the lathe 
 bed, and will deliver any overflow into a receptacle placed 
 
THE GRIXDIXG OF MICROSCOPIC SPECIMENS 463 
 
 to receive it. In this way nothing is wet or spattered with 
 water. 
 
 Preparation of Material. — In the preparation of material, 
 such as teeth, bone, etc., in histological work of ordinary 
 delicacy, the specimen is first ground fiat on one side by 
 hand on a rough stone 4 inches in diameter, on the motor, 
 and finished perfectly flat on one of the finer stones on the 
 lathe head. The piece is then washed clean and placed in 
 absolute alcohol for a sufficient time to remove all traces 
 of water, or, when cracking or injury from shrinkage is not 
 feared, it may be dried in the warming box. Then when 
 dried and warmed to about 120° F., it is ready to mount 
 with balsam on the grinding disk for grinding. 
 
 Management of Balsam. — I suppose the management of 
 balsam will always be a difficult problem with many per- 
 sons. ^Nlany, hovrever, learn it quickly. One may take 
 the dry balsam and dissolve it in xylol, and filter it at a 
 high temperature, say 110° or 120° F. Or one may use the 
 prepared balsam for microscopic mountings. In either case 
 it must be evaporated until stiff enough so that it will move 
 rather sluggishlv at 110° F., but will be fluid at 120° or 
 130° F. 
 
 Spiders and Dogs. — For using this another bit of apparatus 
 is necessary. A circular piece of steel made flat on the upper 
 surface is mounted on three legs 1| to 2 inches high. The 
 steel disk should have two rows of holes around its periphery, 
 the one row f inch inside the other. A hard rolled tool 
 steel wire, or rod -fo inch in diameter, should exactly fit 
 these holes. These rods should now be bent at right angles 
 with a short nib on the end, bent again at right angles, so 
 that it wdll point downward when the free end of the rod 
 is set into one of the holes. The length between these two 
 angles should vary from f to IJ inches in three dozen or 
 more pieces which should be prepared. The end which 
 goes in the holes should be cut so that it will not quite 
 reach the surface of the table when dropped into the holes 
 with the end of the nib on the surface of the circular plate. 
 These rods are called "dogs" (Fig. 347.) 
 
464 
 
 APPENDIX CHAPTER I 
 
 With this arrangement a warming box arranged with a 
 thermostat to maintain an even temperature, sufficiently 
 
 Fig. 347 
 
 The spider with a grinding disk upon it and a specimen laid on and secured by bent 
 rods called dogs. When these dogs are placed and pressed down through the holes in 
 the disk of the spider, they hold fast. With a little pressure of the finger outward 
 on the end of the rod below the disk of the spider, the dog slips up and is loose. The 
 disk of the spider is three inches in diameter. 
 
 high to soften the stiff balsam, is used. The specimen, the 
 balsam, the grinding disk, and the "spider" are placed inside, 
 and allowed to rest until they have reached the temperature 
 
THE GRINDING OF MICROSCOPIC SPECIMENS 4G5 
 
 desired. Then working quickly, a sufficient amount of 
 balsam is placed on the grinding disk, and the specimen 
 laid on it. This should be pressed down until it is seen 
 that all space under it is JSlled with balsam, but no con- 
 siderable excess should be used. It is well if this rest 
 so in the warming box for ten or fifteen minutes for the 
 balsam to soak well into the specimen. Then the grinding 
 disk, with the specimens, should be laid on the spider and 
 one of the dogs dropped into one of the holes in the steel 
 plate, that will bring its nib on to a part of the specimen 
 chosen. Then another, and still another, should be placed, 
 each with its nib on a different part of the specimen, so 
 that every part of it may be pressed flat on the disk. More 
 dogs should be added if necessary. Now each in turn is 
 pressed down a little, one after another, until all are exerting 
 about all the force the spring of the rods will exert without 
 permanently bending them. In this condition the whole 
 thing is again enclosed in the warming box. 
 
 At this time any number of specimens of teeth or bits 
 of teeth, bone, etc., that the face of the disk will hold may 
 be placed on the disk, and all may be ground together. 
 Four to six lengthwise sections of incisor or cuspid teeth 
 may be placed at once, or eight to twelve cross-sections. 
 It seems to be best practice, however, not to load the disk 
 too heavily. Four lengthwise sections will grind better 
 than six, as a rule. 
 
 Now, after the loaded disk had remained in the warming 
 box until all balsam that will come has been squeezed out 
 from under the specimens, all excess of balsam should be 
 very carefully removed, or wiped away, close up against 
 the specimens. Nothing clogs a stone and stops its cutting 
 more effectually than balsam smeared over it, and every 
 excess that may come against the stone should be got out 
 of the way. 
 
 When this is done the whole thing should be returned 
 
 to the warming box for from, one to four hours, so that it 
 
 may dry some about the margins at least. Then it may be 
 
 removed from the warming box and allowed to cool, and 
 
 30 
 
4G6 APPENDIX CHAPTER I 
 
 await convenience in grinding. It should, however, remain 
 secured on the si)i(ler by the dogs if it is to wait more than 
 a few hours, for the (Usi)()yition of dentine to warp in drying 
 may pull some i)art of the specimen from the disk. Under 
 these conditions, two or three days, or a week, will do no 
 harm. 
 
 When the grinding is completed, the disk is removed from 
 the machine and the specimens flushed with clean water, 
 and dried by the pressure of a soft napkin folded to several 
 thicknesses, or clean pieces of waste cotton fabric may be 
 used. Then the disk with its specimens should be laid in 
 a dish and sufficient xylol added to cover it, and allowed 
 to rest until the balsam has been dissolved and the specimens 
 released. This will usually require from twenty to thirty 
 minutes, or sometimes as much as an hour. When the 
 specimens are very thin they loosen much quicker than 
 when thick. Any material not penetrated by xylol, as 
 silicified petrifactions and stones, require much more time. 
 
 When the specimens have loosened, they are ready for 
 permanent mounting for microscopic study. 
 
 Rapidity of Grinding. — In order to make rapid progress 
 in grinding specimens, one should have six to ten grinding 
 disks, nearly as many spiders, and a large supply of dogs. 
 The machine is so nearly automatic in its action that it 
 needs but little w-atching, so that the preparation may be 
 going on while the grinding is in progress. One of the 
 principal points that needs attention is the flow of water. 
 But if the w^ater and ice placed in the receptacle are clean 
 and free from dirt or trash that may stop the flow^ of w^ater, 
 the only care is that the quantity of water is kept up. The 
 vessel should be large enough to hold a supply for several 
 hours. If the stone should run dry, the specimen would be 
 destroyed in a few seconds. 
 
 Setting the Measurement of Grinding Disks. — When begin- 
 ning any considerable series of grindings, the first thing of 
 importance is to try out and obtain a record of the measure- 
 ments of each grinding disk for the particular stone that 
 may be selected for finishing. I find that most persons, 
 
THE GRIXDTXG OF MICROSCOPIC SPECIMENS 467 
 
 after some practice, prefer to use a fine stone for the entire 
 grind. In grinding teeth, after roughing down the surface 
 that is to form the specimen, the back is also ground away 
 to a flat surface that will better accommodate the placing of 
 dogs in mounting on the grinding disks. These may be 
 made quite thin and reduce the grinding with the fine stone. 
 Then the stone selected is placed in the lathe head, seeing 
 to.it carefully that the face of the stone is clean. Then the 
 grinding machine is brought up in contact with the set screw 
 of the point finder. The tailpiece is placed in position and 
 pushed up so as to make some pressure on the shaft. Then, 
 with the large nut the shaft is so adjusted that the grinding 
 disk being tried comes close to the stone but does not touch 
 it. Now start the machine and note the running carefully, 
 and while doing so catch the adjusting nut of the micrometer 
 and move it one-thousandth at a time, and listen for the 
 first touch of the disk to the stone. The moment this is 
 heard, quickly reverse the movement of the adjusting nut, 
 and separate the disk from the stone. Try this again and 
 again, until you feel very certain of having detected the 
 first touch of the stone on the disk by moving the adjusting 
 nut half or a quarter of toV"o inch. At last, while it is touch- 
 ing, stop the machine in a position to see the finger on the 
 adjusting nut, and read the measurement and enter it on 
 your record for that disk. In setting for a grind with this 
 disk, turn the adjusting nut so as to draw the grinding disk 
 back from the stone 1-0^7 o" inch. When the specimens to 
 be ground are mounted on this disk, place it back on the 
 machine, start it, seeing that the iced water is running first, 
 and let it run until it ceases to cut, which it will do when 
 the forward movement of the shaft is stopped by the con- 
 tact of the adjusting nut of the micrometer with the rear 
 bearing of the shaft. 
 
 Then remove the disk and examine the specimens care- 
 fully. If the placement has been accurate, the specimens 
 will be too thick. Replace the disk carefully and turn the 
 nut forward so as to grind one-thousandth of an inch thinner, 
 or one mav do onlv a half of one-thousandth at a time. 
 
468 APPENDIX CHAPTER I 
 
 Repeat this until the section seems to be thin enough. 
 Then remove and mount the sections and judge them with 
 the microscope. By this time one will have arrived at an 
 accurate measurement of this disk, and the record will be 
 trustworthy for other grinds, and will not have to be repeated 
 until the wearing of the stone begins to leave the specimens 
 a bit thick. Then a half-thousandth of an inch will bring 
 it right. And so on, and on. Each disk will be treated in 
 the same way for each stone used, and if one is doing much 
 grinding all will be running on their records, and all go 
 smoothly. Recently a man who was grinding sections of 
 teeth for me made all of the preparations, preparatory 
 grindings, and disk mounts, ground and removed from the 
 disks ready for mounting forty full-length sections of central 
 incisors in six hours, and had his lunch during the time. 
 Every section was complete, was even in thickness in every 
 part, and all practically the same thickness — a thickness 
 chosen for the special studies in hand. 
 
 Grinding Frail Material. — While the machine facilitates 
 the production of the more ordinary sections to such a 
 degree as to be indispensable to one having many grindings 
 to do, it is in the production of sections of very frail material 
 that the grinding machine stands out as vastly superior 
 to other methods of grinding. In the study of caries of 
 enamel in which disintegration has rendered the remaining 
 tissue very frail and likely to fall to pieces before it is suffi- 
 ciently thin, we may obtain the required thinness and yet 
 retain all of the tissue. I have also produced exceedingly 
 fine sections of salivary calculus, and equally good sections 
 from small crumbs of serumal calculus. The production 
 of these is slow, but fairly certain of good results. 
 
 Also in grinding sections of fossil teeth, fossil woods, and 
 the like, in which very fine sections are too brittle to be 
 handled in any way except as stuck to glass, the machine 
 gives excellent results. In geological work it practically 
 removes the difficulties. Good sections of the very brittle 
 stones can be made with fair safety by grinding on the cover- 
 glass. 
 
THE GRINDING OF MICROSCOPIC SPECIMENS 469 
 
 Plans for Grinding Frail Material. — Much very desirable 
 material for microscopic investigation will be found that is 
 so frail, or at least so brittle, when reduced to sections thin 
 euough for microscopic investigation, that it will crumble 
 to pieces, either in the grinding or in the mounting, by the 
 ordinary processes. For grinding and mounting such 
 material the following processes have been slowly evolved. 
 These may be divided into the balsam process and the 
 shellac process. Such material that, when made fast to 
 a cover-glass and ground in hard balsam, is not liable to 
 go to pieces when this hard balsam is softened by sticking 
 the specimen and glass cover to a glass slide may be ground 
 in hard balsam. If, howveer, the different parts are liable 
 to separate and change position when the balsam softens, 
 shellac should be used for the grinding. I have had some 
 very sorrowful failures in grinding rare specimens of enamel 
 that had no cementing substance between the enamel 
 rods in hardened balsam. For when the softer balsam was 
 added to mount the specimen on the glass slide, the hard 
 balsam was softened and the enamel rods floated out of 
 position. All such material as will not hold together strongly 
 enough to prevent this should be ground in shellac. 
 
 To grind in hard balsam, the one side of the specimen may 
 be ground flat on the rough stone and then dried out in 
 absolute alcohol. Then the ground side should be saturated 
 to sufficient depth with soft balsam, and laid aside until 
 the balsam has become hard enough to grind smoothl}^ 
 Then the grinding and polishing of this first side should be 
 completed by grinding away all balsam from the immediate 
 surface, and sufficiently into the substance of the specimen 
 to produce a clean, smooth surface of the material. When 
 this has been done, and the surface dried, it should be 
 mounted on an ordinary cover-glass, the thickness of which 
 should have been measured and recorded. In this mounting 
 the cover-glass should be laid on a spider and weight enough 
 placed upon it to insure a perfect fit of the surface of the 
 glass. This should be subjected to about 120° F. heat for 
 from one to five or six hours, for the purpose of expressing 
 
470 APPENDIX CHAPTER I 
 
 the last bit of balsam possible from between the specimen 
 and the cover-glass. Then it may rest, awaiting the con- 
 venience of the operator, for several days, but the balsam 
 must not be allowed to become "brittle hard," because in 
 that case it loses toughness. All excess of balsam about 
 the margins of the specimen should be carefully removed 
 to facilitate the hardening of that which remains, and espe- 
 cially so that it may not come in contact with the grinding 
 stone, stick to its surface, and interfere with the cutting. 
 
 Good judgment must be acquired by practice as to the 
 hardening of balsam and shellac in these grinding processes. 
 The best idea of it that can be given in words is this. The 
 balsam or the shellac imist have become firm enough so that 
 it will not drag or allow the specimen to move while grinding 
 in iced water. Neither must it become hard enough to becoine 
 brittle, for then it becomes liable to break. 
 
 When ready, the specimen is mounted on the grinding 
 disk. This is done by first cleansing the disk, finishing with 
 x;y'lol, and then sealing the cover-glass to this with soft 
 balsam. This should be placed on the spider and well 
 weighted down with dogs. All excess of balsam should be 
 carefully wiped away from the margins of the cover-glass. 
 This may be quickly dried at 120° F., or more slowly at 
 room temperature. It should, however, be warmed for 
 a half hour or more, for the purpose of expressing as much 
 balsam as possible. This cover-glass will be well held for 
 grinding in iced water with only a little drying about the 
 margins, if all excess of balsam is cleaned away closely. 
 The balsam should not become very hard. 
 
 If the specimen is of considerable bulk and of a quality 
 of material that can be cut with a steel saw, the disk may 
 be caught in a vice "with leather-cushioned jaws to avoid 
 bruising," and the bulk of the material removed with a 
 jeweller's saw, leaving only a moderately thin section for 
 grinding. Or if the material is very hard, as stones, silicified 
 fossils, etc., the disks may be mounted upon the slide rest 
 and cut with the slicing disks, to be described later. 
 
 The specimen is now ready for the final grinding. The 
 
THE GRIXDIXG OF MICROSCOPIC SPECIMEXS 471 
 
 record for measurement with the particular stone to be 
 used in finishing has been made, tried out on unimportant 
 material, and the cover-glass has been measured and its 
 record made. With this data, the disk is screwed to its 
 place, the micrometer turned to the proper measurement 
 for the finish, the iced water arranged, the machine set in 
 motion, and it will do the rest. When coarser stones are 
 used for cutting away considerable material, I find those 
 with just a little experience prefer to gauge the amount 
 of the cutting by the eye for the coarse stone. 
 
 Removal of the Cover-glass from the Disk. — I remove the 
 cover-glass with the specimen from the grinding disk in 
 two different ways, as seems at the time best. 
 
 First, the grinding disk is placed on a heated piece of 
 metal that will warm the grinding disk quickly. Have a 
 stick of rather soft wood ready, the end of which is cut to 
 a rather sharp angle and thinned down almost in the form 
 of a blade. When the grinding disk begins to warm, catch 
 the margin of the cover-glass with the end of the stick and 
 begin to make steady pressure. As the disk warms, so as to 
 soften the balsam, the cover-glass will begin to move under 
 the steady pressure, slowly at first, but more rapidly later, 
 and will slide oft' the grinding disk before the specimen 
 is loosened. For this plan the cover-glass should be pretty 
 strong, one and one-half to two thousandths of an inch 
 thick. Otherwise there will be great danger of breaking 
 it. It is well in some cases to run just a little xylol around 
 the margins of the cover-glass and partially dissolve the 
 balsam that has become driest before the heating. Great 
 care must be taken not to allow the xylol to spread on to the 
 specimen, for it would loosen it very quickly. 
 
 The specimen is then turned downward and placed on a 
 tiny drop of balsam on a glass slide, and quickly pressed 
 down close and level. As the new balsam will soften the 
 old, it should not be moved further than quickly to apply 
 a light spring clip to hold it steady. The parts of the speci- 
 men are less likely to move if this is laid on ice for an hour 
 or more. 
 
472 APPENDIX CHAPTER I 
 
 The Use of Shellac. — In the second plan shellac is used 
 instead of balsam for hardening the specimen and holding 
 its parts together in the first grinding. This part of the 
 work is other\vise done in the same way. The drying of the 
 shellac requires more time usually than the balsam. 
 
 The attachment of the cover-glass to the grinding disk 
 is done in the same way as when balsam is used to hold the 
 specimen on the cover-glass — that is, with balsam. The 
 grinding proceeds similarly in every respect. 
 
 In the removal of the cover-glass from the grinding disk, 
 and mounting the specimen, comes the important differences 
 in the two processes. Xylol dissolves balsam very quickly. 
 But xylol does not dissolve shellac at all. Therefore, in- 
 stead of pushing the cover-glass of the grinding disk, the 
 disk is laid in xylol and the balsam dissolved out. In this 
 there is no danger of detaching or moving the specimen if 
 the handling is careful. When cleaned, it is inverted upon 
 a glass slide on a drop of balsam without fear of movement 
 of parts of the specimen, no matter how frail. 
 
 The Preparation of Shellac. — To keep shellac in condition 
 for this work has some difficulties. The dry scales should 
 be dissolved in absolute alcohol so as to make a moderately 
 thick varnish. It should then be filtered at a temperature 
 of 110° to 120° F., or be made thinner and filtered at 
 room temperature. Great care should be exercised to keep 
 the filtrate from exposure to a damp atmosphere, for it 
 absorbs water readily and then will throw down fine crys- 
 tals, which destroy its value for microscopic purposes. 
 
 After being filtered it should be evaporated in a close 
 warming box at about 110° to 120° F., to the consistence of 
 syrup. In doing this it is well to divide the supply into two 
 or three grades — a thinner, medium, and a thicker solution. 
 The thinner solution will be used for saturating frail speci- 
 mens before any cutting is done. The thicker solutions for 
 attaching specimens to the cover-glass for grinding. The 
 medium solution for either purpose, as the material may 
 seem to require. 
 
THE GRINDING OF MICROSCOPIC SPECIMENS 473 
 
 The Grinding from Crumbled Material. — There is often 
 important material for investigation that can be had only 
 in very small crumbs, or broken pieces, such as serumal cal- 
 culus, sands, crumbled bits of strange stones, or mixtures of 
 such material as is found in some of the coarser sands. These, 
 on microscopic investigation, ma^' tell important stories as 
 to their origin and throw important light upon geological 
 questions. In addition to the ordinary microscopic observa- 
 tion, the polariscope may be turned on these, and reveal 
 important facts as to their origin and structure. Also many 
 things will be found in botanical work, such as obtaining 
 sections of small seeds, and the like, which will give important 
 information. 
 
 Having done a few of these grindings, especially of the 
 very frail dental material, such as serumal calculus, extremely 
 frail fossil teeth, etc., plans of work more or less well adapted 
 have been developed. 
 
 For instance, I have obtained excellent sections of serumal 
 calculus, which can be had only in small crumbs or flakes, 
 in this wise. A small collection of these bits are first im- 
 mersed for a time in absolute alcohol, or until all air has 
 been removed if they are dry, or if the}' are freshly gathered, 
 until all water has been removed. Then a cover-glass is 
 prepared by covering its central part with the thicker 
 solution of shellac, and these crumbs are placed in this, 
 in what seems to be the best position for obtaining sections. 
 These are allowed to soak full of the shellac, under a close 
 cover, and then uncovered to dry up. Then, if some of the 
 pieces seem to need it, more shellac is added from time to 
 time, until the embedding seems sufficient. This may be 
 dried at room temperature, or in the warming oven at 110° 
 to 120° F. Shellac should not be subjected to much higher 
 temperatures for a considerable time, because continued 
 high temperature for many days together seems to injure 
 the strength. 
 
 When this is sufficiently hard for smooth grinding, and 
 before it has become too brittle (determining this point re- 
 quires some experience), the preparation is cemented to the 
 
474 ArrENDiX C If AFTER I 
 
 grinding disk with halsam and ground to such a point as 
 seems most favorable for obtaining sections. This point is 
 to be determined by frequent removal of the disk from the 
 machine and examination of the exposed surfaces of the 
 several pieces. 
 
 When this })art is done, the cover-glass is dissolved off 
 of the grinding disk by xylol. Then another cover-glass 
 is attached to the surface tvith the least possible amount of 
 shellac. This in turn is dried to the right consistence. Then 
 the last cover-glass placed — that is, the one on the side that 
 has been ground — is secured to the grinding disk with balsam. 
 When this has set it is placed on the machine and the first 
 cover-glass is ground away and the section ground to the 
 required thinness. They are again dissolved off of the grind- 
 ing disk, and may be at once mounted in balsam on the 
 microscopic slide. 
 
 Difficulties in Grinding. — In the grinding of material 
 enveloped in shellac, or in balsam, either of these materials 
 are apt to gum up the stone and stop the cutting, or render 
 the grinding very slow. When this is from balsam, it may 
 be quickly removed after drying the stone by washing w4th 
 xylol on a brush, or a bit of cloth, while the stone is slowly 
 revolved. 
 
 When clogged with shellac, the washing is done with 
 absolute alcohol. This requires much more time, and some 
 advantage may be obtained b}^ using pumice stone wuth the 
 cloth or with cork. After rubbing with pumice stone, a 
 very thorough washing with alcohol should be made to 
 remove the last particles of pumice, before re-beginning the 
 grinding. Even with this, the ground surface is apt to be 
 rough or scratched for a time by particles of the pumice 
 lodged on the stone. These will soon disappear, how'ever. 
 Yet the pumice should not be used in the last portion of 
 the grinding. 
 
 With much grinding of hard substances, the surfaces of 
 the stones become worn so smooth that the}' do not cut 
 well. Then the picking tool should be run over the surface 
 until it is perceptibly roughened. This will cause the stone 
 
THE GRIXDIXG OF MICROSCOPIC SPECIMENS 475 
 
 to cut briskly for a considerable time, and at first — following 
 such sharpening — the ground surface of the specimen is 
 likely to be full of scratches. In that case a smooth stone 
 should be used for the finishing. 
 
 Much care should be taken in keeping the stones in good 
 condition. Except in the ways mentioned, no dirt or grit 
 should be allowed to come in contact with their surfaces. 
 A single particle of grit lodged in the surface of the stone 
 will fill the whole surface of the ground section with scratches. 
 Although I shut up my stones in a close fitting drawer, I 
 find it necessary to cover each with a close fitting cloth that 
 is so closely woven as to exclude all dust. 
 
 In taking care of the machine itself, one cannot be too 
 careful. All of the bearings of the lathe head and of the 
 grinding machine should be swaddled with candle wick 
 saturated with oil to prevent the ingress of gritty particles. 
 This is especially needful when using the aluminum saws 
 and feeding them with carborundum powder. Then every 
 bearing about the whole machine should be especially pro- 
 tected to prevent the possibility of getting grit in the bear- 
 ings. Carelessness in such a matter will quickly ruin a 
 fine bit of mechanism. But with this care, such a machine 
 should continue to do its work well for a lifetime (Figs. 348 
 and 349). 
 
 The Slicing Mechanism. — This is an arrangement for slic- 
 ing very hard substances which cannot be cut with the 
 ordinary steel saw — such as the enamel of teeth, silicified 
 fossils, rocks, etc. This consists of an aluminum disk 
 fitted to the lathe head, and surrounded by a special form 
 of spatter guard that admits of the use of the periphery 
 for cutting, and an object holder fixed upon the slide rest 
 of the lathe. The object holder consists of a clamp that 
 grasps a brass tube slotted at the free end in which teeth, 
 or other objects may be made fast with plaster of Paris or 
 sealing wax for slicing. Or in place of this a brass man- 
 dril, upon the end of which there is a threaded nipple 
 by whidi any of the grinding disks may be attached. 
 These are fixed in the position of the ordinary tool post, 
 
476 
 
 APPENDIX CHAPTER I 
 
 and may be swung horizontally to any possible position 
 in relation to the aluminum disk. An object can there- 
 fore be so placed on the disk as to be cut in any direction 
 desired. Usually these are fixed upon the disk with sealing 
 wax. In using the aluminum disk it is fed with carborundum 
 powder suspended in soapy water to give it some stickiness. 
 
 Fia. 348 
 
 Figs. 348 and 3 19. — Arrangement for slicing very hard material. Fig. 348 is the more 
 ordinary view of the machine with the slide rest and object holder in position. In 
 Fig. 349 the lathe is turned about to give a better view of the slide rest, object holder, 
 spatter guard, and aluminum disk. In these illustrations the slotted tube is used 
 (see text) to hold the object being cut. Notice that the disk used for cutting is sur- 
 rounded by a spatter guard which is open for a space at one side so that the periphery 
 of the disk may be used in cutting. This guard gathers all water and grit used in 
 cutting, and delivers it into the pan below through its hollow post. When doing 
 this kind of work all of the bearings of the machine should be carefuUy wrapped 
 (swaddled) to keep them safe from intrusion of grit. 
 
 This is applied with a brush by hand, and is kept going so 
 constantly as to prevent the disk from running dry. The 
 ordinary aluminum plate, of twenty-four to thirty gauge, 
 may be used for making these. They are first cut in circles 
 
THE GRINDING OF MICROSCOPIC SPECIMENS 477 
 
 by hand, as large as the lathe will swing (4 inches), and then 
 are cut down to 3 J inches with a tool in the slide rest. These 
 are quickly made when wanted. They wear out rapidly, 
 and yet one of them will do much cutting of very hard 
 substances, and do it accurately and delicately. Rings may 
 readilv be cut from the ordinary test-tubes without special 
 
 
 Fig. 349 
 
 
 
 
 i 
 I 
 
 
 ■eg|L\ 
 
 \f)h^ 
 
 ^an -^\-.^^ 
 
 <^^^^S^ \^Hr 
 
 LJ* 
 
 ■^>'> 
 
 
 ml- 
 
 ^_aB»i 
 
 ■^^j0^^ 
 
 T 
 
 in. 
 
 000^'n^2 
 
 1 
 
 
 ' 
 
 ^ 
 
 danger of breaking. The crown of a molar tooth may be cut 
 into many slices; fossil teeth, silicified fossil woods, stones, 
 etc., may readily be sliced as thin as they can be handled 
 in the after-work of preparation. 
 
APPENDIX CHAPTER II 
 
 THE THEORY OF HISTOLOGICAL TECHNIQUE 
 
 The first requirement of histological technique is to 
 obtain a general view of the theory of procedure. Many 
 beginners make the mistake of supposing that directions 
 for histological technique can be followed like the receipts 
 of a cook book, or the directions for an experiment in chem- 
 istry. This is very seldom the case, and while it is always 
 necessary to follow directions accurately, it is still more 
 necessary to follow them intelligently. All histological 
 methods require judgment. For instance the length of 
 time required for xylol to replace absolute alcohol in a block 
 of tissue which is to be embedded depends upon the size 
 of the piece, the character of the tissue, the temperature, 
 and possibly some other factors. It is therefore impossible 
 to say exactly what time would be required, and the experi- 
 menter must use the judgment which has been acquired 
 as the result of experiment. In the same way no experi- 
 menter can make up a stain and be sure that it will work 
 exactly like the last lot made by the same formula until 
 he has tried it. Even with the same stain the length of 
 time required for staining a section depends upon the thick- 
 ness of the section, the character of the tissue, and the pre- 
 liminary technique it has been through. So that all time 
 directions must be considered as approximate, and to be 
 successful the experimenter must study, first, the object 
 to be obtained by the use of each reagent, and the peculiar 
 action of the reagent upon the tissue. 
 
 For observation with the compound microscope trans- 
 
THE THEORY OF HISTOLOGICAL TECHNIQUE 479 
 
 mitted light is ordinarily used. The object must therefore 
 be thin and transparent enough to allow the light to pass 
 through it. The higher the magnification the smaller the 
 field, that is, the smaller the portion of the tissue that can 
 be seen at one time, and the less depth of focus, and con- 
 sequently the thinner the sections must be. A section that 
 would be excellent for study with the f objective may be 
 almost valueless under a yV> ^nd sections that are splendid 
 under the yV might be of little value under the f . In other 
 words, the thickness of the section should be related to the 
 magnification with which it is to be studied, and to the size 
 of the structural elements which make up the tissue. For 
 the study of the organs and tissues of multicellular organisms 
 there are three general methods — (1) teasing, (2) macera- 
 ting, and (3) sectioning. 
 
 Teasing. — In this method a small portion of the living 
 tissue is torn apart with two needles in a drop of normal 
 salt solution or some indifferent medium which will not 
 aifect the tissue. In this way it is spread into a thin film 
 and squeezed a little between a slide and cover-glass so 
 as to separate the structural elements when they may be 
 directly observed. Of course, in stud^'ing such a preparation 
 it must be remembered that the tissue has been forcibly 
 torn apart and effects of violence must be looked for. These 
 often bring out facts of structure which would not otherwise 
 be as easily seen. After teasing the living tissue, staining 
 agents may be used to facilitate the study of structure. 
 The fresh tissues are often so transparent and made up of 
 substances of so near the same refracting index that very 
 little structure can be made out without the use of staining 
 agents. It must be borne in mind that staining agents 
 are of two classes, diffuse and selective. A diffuse stain 
 gives an even color to all of the tissue and facilitates the 
 study chiefly by rendering it less transparent. A selective 
 stain combines more readily with one portion of the tissue 
 than another, rendering it more conspicuous. Selective 
 stains therefore must be thought of as chemical agents which 
 combine with parts of the cell or tissue and demonstrate 
 
480 APPENDIX CHAPTER II 
 
 chemical differences in the structural elements. For in- 
 stance, basic anilines react with the chromatin of the nucleus, 
 producing a colored compound. The stain may then be 
 washed out of the section, leaving only the nuclei colored. 
 Acid anilines in general are diffusive stains giving a general 
 color to the cytoplasm. In a similar way certain stains 
 will react only or chiefly with intercellular substances, 
 rendering them more conspicuous. For staining freshly 
 teased specimens methyl green, the formula for which will 
 be found under the paragraph on stains, is an excellent 
 agent. Teased specimens are never very permanent, though 
 they may be preserved for a considerable length of time 
 by mounting in glycerin or glycerin jelly and putting a ring 
 of varnish or white lead around the edge of the cover-glass 
 so as to prevent evaporation. 
 
 Maceration. — When an organ is composed of more than 
 one tissue the structural elements may be separated by 
 selecting an agent which will act upon one and not upon 
 the others; for instance, the muscle fibers of a voluntary 
 muscle may be separated by treating a piece of tissue with 
 dilute alkali, which w^ill soften and dissolve the connective 
 tissue, allowing the muscle fibers to separate. In a similar 
 way dilute alcohol will soften the cementing substance be- 
 tween the epithelial cells. By first treating a piece of tissue 
 with the proper agent and then teasing, the form of the 
 structural elements of the tissue can be made out. By 
 treating a portion of connective tissue containing both 
 white and elastic fibers with dilute hydrochloric or acetic 
 acid, which dissolves the white fibers, elastic fibers which 
 could otherwise not be seen may be made out. Macer- 
 ating and teasing methods are of great assistance to the 
 study of tissues in sections, and it would be often very 
 difficult to obtain true ideas of structure from sections 
 without their assistance. 
 
 Sectioning. — For the study of the structural elements in 
 their relation to each other in the tissue sectioning is the one 
 method. As they exist in the body, however, some of the 
 tissues are too soft and others too hard to allow the cutting 
 
THE THEORY OF HISTOLOGICAL TECH XI QUE 481 
 
 of a thin enough shoe without disturbing the relation of the 
 structural elements. They must therefore be put through 
 rather an elaborate process in which the object of every 
 step must be understood. 
 
 Dissecting. — First of all, the material for histological work 
 must be absolutely fresh, that is, living. It must be remem- 
 bered that living cytoplasm is chemically different from 
 dead cytoplasm, and as soon as death occurs postmortem 
 changes begin which gradually destroy the structure. The 
 period from death to the beginning of histological methods of 
 preparation should be measured in minutes, not in hours. 
 Tissues that have been dead for a few hours will not react 
 with the staining agents so as to produce the brilliant 
 specimens that can be obtained from fresh material, and 
 often a few days will render material entirely useless except 
 for the grosser anatomical relations. The specimens to be 
 studied should be dissected while the cells of the tissue 
 are still alive, and in doing so the greatest care should be 
 used not to disturb the relation of the tissues. 
 
 Fixing. — Histologically this word means killing. After 
 dissecting out the tissue to be studied, and while the cells are 
 still alive, it must be immersed in some liquid that will 
 kill the cells and fix their structure as w^hen alive. The 
 pieces should be made small enough for the fixing agent 
 to penetrate them rapidly, and the size of the piece that can 
 be used depends upon the density of the tissue, its character, 
 and the nature of the reagent. Some fixing agents are very 
 much more penetrating than others. All fixing agents 
 coagulate or set the cytoplasm and tend to prevent shrink- 
 age. The success of all the following steps and the value 
 of the specimen for the study of detail of structure depend 
 upon the perfection of fixation. 
 
 The fixing agents most commonly used are bichloride of 
 mercury, potassium chromate or chromic acid, osmic acid, 
 alcohol, and formalin. The formulas for the same will be 
 found on pages 496 and 499. 
 
 Hardening. — Since all the fixing agents coagulate living 
 cytoplasm, they are also to a greater or less extent hardening 
 31 
 
482 APPENDIX CHAPTER II 
 
 agents., and after fixiii^r tissues may be handled with less 
 danger of disturbing the relation of the struetural elements. 
 Some fixing agents, espeeially chromic fluids, may be con- 
 tinued in their action as hardening agents until the tissue has 
 attained the proper consistency for sectioning, but, as a rule, 
 it is necessary to use other agents for this purpose. In all 
 cases the fixhig agent must he thoroughly trashed out of the 
 tissue before the process is continued. Alcohol is the uni- 
 versal hardening agent, and at the same time it removes the 
 water from the tissue. In carrying tissues from water to 
 alcohol several grades must always be used, and the more 
 delicate the tissue the more gradual must be the changes. 
 If a piece of tissue is taken from water and placed in 95 
 per cent, alcohol, the diffusing currents will be so strong 
 as to disturb structure and at the same time the hardening 
 action is so energetic as to produce shrinkage. From water a 
 tissue should never be placed in alcohol stronger than 70 
 per cent., where it should be allowed to remain for twenty- 
 four hours. From 70 per cent, it may be taken to 95 per 
 cent, for the same length of time, and from 95 per cent, to 
 absolute, which will entirely remove the water and prepare 
 the tissue for embedding. If the tissue is very delicate, 
 it should be placed in water, then in 50 per cent, alcohol, and 
 carried through in grades of 10 per cent, to 95 per cent. 
 
 Embedding. — In order to cut thin sections of tissue the 
 piece must be surrounded and infiltrated with some firm 
 substance which will not only support the entire piece, but 
 will soak through the tissue, filling all intercellular spaces 
 and supporting the individual structural elements. At the 
 same time the embedding material is used to fasten the tissue 
 firmly to a block of fiber or wood which can be grasped in 
 the clamp of the sectioning machine. Two kinds of material 
 are used for this purpose. Substances that are fluid when 
 warm, and solid when cold, as paraffin, or substances which 
 may be dissolved in volatile liquid and are solidified by evapo- 
 ration, as celloidin. In both of these methods the substances, 
 as a rule, are either oily or insoluble in water, and therefore 
 the tissue must be thoroughly dehydrated — that is, have all 
 
THE THEORY OF HISTOLOGICAL TECHNIQUE 483 
 
 the water removed from it before it is placed in the embed- 
 ding material. To accomplish this there should be at least 
 one change of absolute alcohol. From the absolute alcohol 
 the tissue should be placed in a fluid which is a solvent for 
 the embedding material, so that it will penetrate the tissue 
 more perfectly and rapidly. Heat is always injurious to the 
 tissue, and in embedding in paraffin, therefore, the tissue 
 should be kept in the melted paraffin for the shortest possi- 
 ble time and paraffin of as low a melting point as is consist- 
 ent with sufficient hardness for cutting should be used. In 
 embedding by evaporation the evaporation should not be too 
 rapid or the shrinkage will be increased. Tissues may be 
 kept blocked and ready to cut for a long time, but as a 
 general principle the shorter the time the more perfect will 
 be the specimen. 
 
 Sectioning. — For sectioning some sort of machine is 
 necessary, and many kinds have been designed, the general 
 principles of which are all the same. They consist of a 
 clamp which holds the knife and a clamp which holds the 
 specimen, and can be adjusted in such a way as to bring the 
 specimen in proper relation to the knife. The position of 
 the specimen is advanced by a micrometer screw so that 
 sections of any desired thickness may be sliced. The delicate 
 part of this machine is the micrometer screw. The essential 
 to the success of its working is the sharpness of the razor, 
 and for such specimens as decalcified bone the razor must 
 be heavy and strong, so that the edge will not spring in 
 cutting the hard tissue. 
 
 Staining. — The detail of staining process will be described 
 in the next chapter, but it must be remembered that stains, 
 as a rule, are water solutions and the sections must be carried 
 through the grades of alcohol to water before they are ready 
 for the stain. After staining they must be carried back 
 through the grades of alcohol, so as to remove the water 
 entirely before they can be mounted in balsam, which is 
 not soluble in water. 
 
 Mounting. — Except in serial work, but one specimen should 
 be placed on a slide, and this should be in the centre, leaving 
 
484 APPENDIX CHAPTER II 
 
 room at either end for a label. In serial work the sections 
 may be placed at one end of the slide, preferably the left 
 hand, leaving room at the right for one label. 
 
 Labelling. — Nothing in histological technique is more impor- 
 tant than labelling, especially in all research work. Through 
 every step of the process the specimen must be kept track 
 of, and a mixing of labels may spoil months of work. A 
 laboratory notebook containing a record of all material 
 and work should always be on the tables. I have found a 
 system of date and number convenient. For instance, on 
 June 4 a number of specimens are dissected out; in the note- 
 book the record of the source of the tissue is made; the 
 first piece is placed in a bottle of fixing fluid and the bottle 
 labelled 6-4-1911, No. 1; the second, 6-4-1911, No. 2, and 
 so on. In the notebook the description of each block and 
 the date and the hour when it was placed in the fluid is 
 recorded. In this way the tissue may be carried clear 
 through recording each step in the process, and when it is 
 sectioned and mounted we can follow its history in the 
 notebook. Every slide should be labelled first with the 
 date and the block number so as to follow its technique; 
 second, the name of the tissue, and third, the kind of stain- 
 ing. This should be placed on the right hand label, leaving 
 the left hand label for index and file number if the section 
 is preserved. 
 
 Indexing and Filing. — Many beginners make the mistake 
 of not indexing and filing their slides. They think because 
 they have only a few, that they can easily find anything 
 they want, and that they will wait until they have a larger 
 number before they begin a system, but when a large 
 number have piled up they can never find time to go back 
 and arrange them as they should be. And only one who 
 has failed in this way knows the annoyance of looking 
 through hundreds of slides to find one that he knows he has 
 someplace. 
 
APPENDIX CHAPTER III 
 
 GENERAL HISTOLOGICAL METHODS 
 
 Fixing. — As has been seen from the preceding chapter, 
 fixing is the first and one of the most important steps in all 
 histological methods. No degree of care in the latter steps 
 can make up for any imperfection in it. As a general 
 statement all fixing agents have advantages and disadvan- 
 tages so that in research work several should be tried and 
 their results compared. For class-room work, however, 
 minute details are not so important. Certain general 
 principles may be stated. Bichloride of mercury is espe- 
 cially adapted to the fixing of epithelium of the mucous 
 membrane. It, however, does not penetrate rapidly, and 
 small pieces must be used. Crystals are liable to form in 
 the tissue, and special precautions must be taken for their 
 removal. Flemming's and Zenker's fluids and the fluids 
 containing osmic acid are used chiefly in research. For 
 classwork the author uses Miiller's fluid and Miiller's fluid 
 and formalin almost entirely. Stains are apt to work better 
 after chromic flxing fluids. The formulas for several of the 
 best fixing agents with directions for their use are found in 
 the last chapter. 
 
 Washing. — Except for special purposes, fixing fluids are 
 washed out of the tissues in running water, and they should 
 be thoroughly removed. For this purpose the author has 
 made a galvanized iron tank in which a gauze tray divided 
 into small gauze compartments is suspended. The water 
 is brought into the tank through a rubber tube with the 
 mouth resting on the bottom, and leaves through a spout 
 at the top to which another tube can be attached. In this 
 
APPENDIX ('II AFTER III 
 
 way a lar^e number of specimens can be washed at once 
 and their identity followed. 
 
 Preserving Tissues. — After washing, the tissues should be 
 carried through the grades of alcohol, and may be preserved 
 for a considerable time in SO per cent, alcohol, but it should 
 be changed occasionally. 
 
 Choice of Sectioning Methods. — The choice between paraffin 
 and celloidin for embedding depends upon the character 
 of the section desired and the nature of the tissue. Small 
 objects and those of delicate structure, such as embryos, 
 dental pulps, etc., are best sectioned in paraffin. Large 
 pieces and blocks containing tissues of different densities 
 are more easily cut in celloidin. Paraffin can be cut much 
 thinner than celloidin, and is therefore preferable for the 
 minute study of cell structures with the high power. Cel- 
 loidin sections are more easily stained and are easier han- 
 dled and therefore preferable for the study of the arrange- 
 ment of tissues with low powers. The author prefers 
 celloidin sections for classwork whenever possible. 
 
 Embedding in Paraffin. — Tissues fixed and washed are taken 
 from 80 per cent, alcohol and placed in 95 per cent, for twenty- 
 four hours, then in absolute alcohol for the same length of 
 time, and the absolute alcohol should be changed once duiing 
 this period, from absolute alcohol to xylol, in which the tissue 
 should remain until it is clear and translucent. The time 
 in xylol should be as short as possible, as it has a harden- 
 ing action. From xylol it is placed in a solution of paraffin 
 in xylol, and from this to soft paraffin in the paraffin oven, 
 at a temperature of not over 52° or 53° C. In this it should 
 remain from one-half to six hours, when it is transferred to 
 hard paraffin in the oven for the same length of time. The 
 time in the oven should always be as short as is consistent 
 with a perfect infiltration. After sufficient time in hard 
 paraffin the tissue is blocked in the following way: A mould 
 is made by placing L-shaped pieces of metal together on 
 a flat slab. These are manufactured for the purpose. 
 Melted paraffin is poured in the mould and the tissue 
 arranged in it, placing it so that the sections will cut in the 
 
GENERAL HISTOLOGICAL METHODS 487 
 
 direction desired. A film of paraffin will harden at once on 
 the slab and the tissue can be placed very nicely with the 
 needles. As soon as a film has formed over the surface the 
 slab with the mould should be immersed in cold water, so as 
 to harden the paraffin as quickly as possible. When cold, 
 sections may be cut at once or the block may be preserved 
 in a pasteboard carton properly labelled. As a rule, paraffin 
 sections should be cut as soon as possible. 
 
 Paraffin. — The paraffin for embedding sections must be 
 of the best quality. That prepared for this purpose by 
 Griibler is preferable. It should be of two grades, that melt- 
 ing at 45° C, and that melting at 54° C. The hard paraffin 
 is mixed with the softer, so as to give a melting point at 
 about 52°. In winter softer paraffin should be used than in 
 summer, as the cutting quahty depends upon the adjust- 
 ment of the paraffin to the temperature of the room. If the 
 paraffin is too hard the sections are liable to tear and curl; 
 if it is too soft, the structure of the tissue will be disturbed 
 in cutting. Perfect infiltration is always necessary for good 
 sections. Chloroform or oil of cedar may be substituted for 
 xylol in this process. Xylol is most rapid, but has some 
 disadvantages in its action on the tissues, especially if left 
 too long. 
 
 Cutting Paraffin Sections. — If the specimen has been placed 
 at one end of the block, the other end of the paraffin may 
 be clamped in the microtome. If the piece is too small, 
 it should be fastened to a block of vulcanized fiber with 
 melted paraffin and the fiber block clamped in the specimen 
 holder. With a sharp scalpel the excess of paraffin around the 
 specimen should be trimmed off, leaving the block in a 
 rectangular form. The microtome knife is placed at right 
 angles to the microtome bed, and the side of the block 
 should be parallel with the blade. The specimen should 
 be brought up just to the edge and the first section cut. 
 The knife should be moved with a quick, sharp motion, as par- 
 affin sections are chopped when the knife is in this position. 
 The knife is pushed back, the block lifted with the micro- 
 meter screw so as to give a section of the proper thickness, 
 
488 APPENDIX CHAPTER III 
 
 and the second section cut. If the paraffin is of the proper 
 consistency and the block has been properly trimmed, the 
 edge of the second section will stick to the first and the 
 sections stretch out over the knife in a ribbon. The ribbons 
 may be transferred to a piece of clean white paper and com- 
 plete series of sections cut. When series are not required 
 larger specimens are often cut better by placing the blade 
 of the knife obliquely and drawing it with a slow, even 
 motion through the block. If the sections show a tendency 
 to roll up when the corner of the section begins to curl 
 over the edge of the knife, it may be caught with the tip of 
 a camel's-hair brush and so section after section transferred 
 to the paper. Paraffin sections should cut at a thickness of 
 from seven to ten microns, but sections as thin as one micron 
 may be cut from small blocks under ideal conditions. 
 
 Handling of Paraffin Sections. — For staining, paraffin sec- 
 tions must be fastened to the slide or cover-glass. If a few 
 sections are to be cut the slide is preferable; if man}^ sections, 
 as in the preparation of class work, square cover-glasses 
 should be used. In either case the glass must be absolutely 
 clean. A stock of perfectly clean slides and cover-glasses 
 should always be kept on hand (see p. 496). A thin film 
 of albumin fixative is spread upon the glass; this film must 
 be as thin as possible. The best way to spread it is to put 
 a drop of fixative on a glass slab or an ordinary slide, touch 
 the edge of the drop with the end of the little finger and 
 spread it over the cover-glass, wiping off all that can be 
 removed with the finger. Lay the cover-glasses film side up 
 on a piece of paper until the required number have been pre- 
 pared. As each section is cut it is laid on a cover-glass, 
 straightened, and pressed dow^n with a camel's-hair brush. 
 If the sections curl or wrinkle they should be floated on 
 water warmed just enough to soften the paraffin but not 
 melt it. As each section is cut it should be dropped on the 
 top of the water, where it will straighten out. When a 
 number have been placed on the surface of the water they 
 may be picked up by holding the cover-glass in the point 
 of the pliers and slipping it underneath the section and 
 
GENERAL HISTOLOGICAL METHODS 489 
 
 lifting it as on a section lifter. The water is drained off and 
 the cover-glass placed in the groove of the tray of a IMoore's 
 staining dish/ shown in Fig. 350. Each tray will hold about 
 thirty cover-glasses. They must now be thoroughly dried 
 by leaving them over night at room temperature or for a 
 shorter time in a warm oven, which should not be hot enough 
 to melt the paraffin. When dry, each cover-glass should 
 be picked up in the pliers and passed quickly through the 
 middle of a Bunsen flame, so as to coagulate the albumin, 
 or they may all be fixed at once in an oven. Heat that will 
 just melt the paraffin will coagulate 
 the albumin and hold the section on Fig Soo 
 
 the glass. By means of a little wire , — ^ --.^ 
 
 basket the tray with the thirty cover- (1^^^^ 
 glasses may now be carried from X^^J^F^ 
 one dish to another through the fol- ^^^--^ *- ^ 
 lowing necessary reagents. First, a ^ ^==^^^ 
 
 minute or two in xylol to remove the Morris staining dish. 
 
 paraffin; then absolute alcohol, then 
 
 70 per cent.; then water; Delafield's hematoxylin for five 
 minutes; distilled water to wash off the stain; acid alcohol 
 (70 per cent, alcohol to which 2 or 3 drops of hydrochloric 
 acid has been added to every 100 c.c. of alcohol); again 
 washed in tap water to remove and neutralize the acid 
 (some prefer alcohol to which a few drops of ammonia have 
 been added); 70 per cent, alcohol; eosin for thirty seconds; 
 70 per cent, alcohol, then 95 per cent., then absolute, and 
 finally xylol. From the xylol the sections may be mounted 
 or given out to the class. For class work a student brings 
 to the desk a clean slide with a drop of balsam on the 
 centre and receives a section. 
 Summary of Paraffin Method. — 
 
 Tissues in 80 per cent, alcohol. 
 
 95 per cent, alcohol, twenty-four hours. 
 
 Absolute alcohol (changed once), twenty-four hours. 
 
 Xylol, one-half to six hours. 
 
 1 These are manufactured by Bausch &, Lomb. 
 
490 APPENDIX CHAPTER III 
 
 X3I0I and paraffin, one-half honr. 
 
 Soft paraffin, one-half to six hours. 
 
 Hard paraffin, one to six hours. 
 
 Block. 
 
 Section. 
 
 Fix on glass. 
 
 Heat. 
 
 Xylol, one minute. 
 
 Absolute alcohol, one minute. 
 
 95 per cent, alcohol, same. 
 
 70 per cent, alcohol, same. 
 
 Distilled water. 
 
 Hematoxylin, five to ten minutes. 
 
 Tap water. 
 
 Acid alcohol. 
 
 Tap water or ammonia alcohol. 
 
 70 per cent, alcohol. 
 
 Eosin, thirty seconds. 
 
 70 per cent, alcohol. 
 
 95 per cent, alcohol. 
 
 Absolute alcohol. 
 
 Xylol. 
 
 Mount in balsam. 
 
 Label. 
 Celloidin Method. — Tissues fixed and washed are taken from 
 80 per cent, alcohol and placed in 95 per cent, for twenty-four 
 hours; then in absolute alcohol for the same length of time, 
 changing the alcohol once. Then into a mixture of absolute 
 alcohol and ether for twenty-four hours, from this into a 
 thin solution of celloidin, in which they should remain for 
 from two days to a week. From the thin solution they should 
 be placed in a thick celloidin solution, about the consistency 
 of syrup, for the same length of time. The tissues may be 
 kept in the celloidin solution indefinitely without injury, and 
 if the tissue is difficult to infiltrate it may be of advantage 
 to leave them in these solutions for weeks or months. In 
 this case the bottles must of course be perfectly corked to 
 prevent evaporation. 
 
GEXERAL HISTOLOGICAL METHODS 491 
 
 Blocking of Celloidin Material. — There are several methods 
 for blocking celloidin materials, of which the author prefers 
 the following: Thick celloidin is poured into a stender dish 
 or a small Petrie dish until there is enough to abundantly 
 cover the specimens, which are arranged on the bottom of 
 the dish. A match or bit of cork is placed under the edge 
 of the cover so as to allow slow evaporation. In a day or 
 two the celloidin will attain the consistence of a thick jelly. 
 A knife is now passed around each tissue and the celloidin 
 containing the specimen lifted out, and the excess of celloidin 
 is trimmed away. A vulcanized fiber block has one surface 
 dipped into the thick celloidin and the specimen arranged 
 upon it. Thick celloidin is now added to surround and cover 
 the tissue with its adherent celloidin. As soon as this is 
 hardened so as to form a film it is dropped into 80 per cent, 
 alcohol to harden the entire mass. In this it must remain 
 at least twenty-four hours before it can be sectioned. Tissues 
 embedded in celloidin may be kept for years in 80 per cent, 
 alcohol blocked and ready to cut without great injury to 
 the tissue. 
 
 Celloidin solutions for embedding should be kept in two 
 grades and labelled "thick" and "thin" celloidin. The 
 latter should be quite fluid, the former about a syrup 
 consistence. Scherring's celloidin is furnished in two 
 forms, in shreds and granules. The former will dissolve 
 more rapidly. About half an ounce is placed in a large- 
 mouthed bottle, and a mixture of equal parts of absolute 
 alcohol and ether added. It dissolves slowly and should 
 be shaken frequently. When this solution is sufficiently 
 thick, part may be poured into another bottle and diluted 
 with sufficient absolute alcohol and ether for the thin solution, 
 while the thicker portion is poured into a bottle for the thick 
 solution, and absolute alcohol and ether may be added to 
 the stock bottle to dissolve the residue. When blocking 
 tissues as described above the trimmings are dropped back 
 into the stock bottle. 
 
 Cutting Celloidin Sections. — The fiber block is clamped in 
 the specimen holder and adjusted. The knife is set diago- 
 
492 APPENDIX CHAPTER III 
 
 nally so as to cut with a drawing motion, and both the knife 
 and the block are kept flooded with 80 per cent, alcohol. 
 The sections may be allowed to pile up on the knife, and after 
 eight or ten are cut they are slid off with a camel's-hair 
 brush on to a section lifter and transferred to 80 per cent, 
 alcohol, in which they may be kept for some time. 
 
 Staining Celloidin Sections. — For transferring celloidin sec- 
 tions the most convenient thing is a small tea-strainer with a 
 handle. These may be got for a few cents at any hardware 
 store. By means of this the sections are transferred to 70 per 
 cent, alcohol, from this to distilled water, and are stained 
 from five to ten minutes in Delafield's hematoxylin. The 
 stain is then washed off with tap water, destained with acid 
 alcohol, w^ashed in tap water or ammonia alcohol, stained 
 thirty seconds in eosin, washed with 70 per cent, alcohol, 
 from this to 95 per cent., in which they should be given two 
 or three changes. From this they are transferred to beech- 
 wood creosote or some other clearing agent (see p. 503), 
 and in this they may be kept until they are ready to mount 
 or to be given out to the class. For class w^ork the student 
 brings to the desk a clean slide, and a section is placed upon 
 the centre of it. After blotting off the excess of oil he adds 
 a drop of balsam, covers with a cover-glass, and labels the 
 specimen. 
 
 Summary of Celloidin Method. — 
 
 Tissues in 80 per cent, alcohol. 
 
 95 per cent, alcohol, twenty-four hours. 
 
 Absolute alcohol, changed twice, twenty-four hours. 
 
 Absolute alcohol and ether, twenty-four hours. 
 
 Thin "celloidin, two days to a week. 
 
 Thick celloidin, the same. 
 
 Evaporate. 
 
 Block. 
 
 80 per cent, alcohol to harden or store. 
 
 Sections cut in 80 per cent, alcohol. 
 
 70 per cent, alcohol, one minute. 
 
 Distilled water. 
 
 Hematoxylin, five to ten minutes. 
 
GENERAL HISTOLOGICAL METHODS 493 
 
 Tap water. 
 
 Acid alcohol. 
 
 Tap water or ammonia alcohol. 
 
 70 per cent, alcohol. 
 
 Eosin one minute. 
 
 70 per cent, alcohol to wash. 
 
 95 per cent, alcohol, changed twice. 
 
 Creosote. 
 
 Mount in balsam. 
 
 Label. 
 Serial Sections with Celloidin. — It is difficult to cut series 
 of sections with the celloidin method. The simplest process 
 and one used with success is to carry the sections in order 
 from the knife to the slide, arranging three or four at one 
 end of it and leaving room for a label. Strips of porous 
 tissue paper are cut the proper size and one laid over the 
 sections to hold them in place. A thread is then lightly 
 wrapped around the slide and paper, when they may be 
 carried through the necessary agents for staining, in Naples 
 jars. After they are cleared the paper is removed, the excess 
 of the oil blotted off, the balsam put upon the section and 
 covered with a long cover-glass. 
 
 SPECIAL METHODS 
 
 Dental Pulp. — The unerupted premolars from a young 
 sheep furnish excellent material for the study of the dental 
 pulp. The jaws of sheep slaughtered for spring lamb can 
 be easily obtained from the stockyards, and while still warm 
 are placed in JNIiiller's fluid and formalin, in which they are 
 taken to the laboratory. The temporary incisors are still 
 in place and may be used for peridental membrane material. 
 
 With the bone forceps the cortical plate is removed and the 
 unerupted teeth dissected from their crypts. By grasping 
 the base of the dental papillae with the pliers the pulp may 
 be pulled out of the dentin. They should then be replaced 
 in Muller's fluid and formalin for twenty-four hours when 
 
494 APPENDIX CHAPTER III 
 
 tlu'v may be carried through the usual process, embedded 
 ill ])araffin, and sectioned. 
 
 Human Pulps. — By the cooperation of the extracting room 
 human })ulps for liistoh)gical work may be obtained. As 
 soon as extracted the tooth should be wrapped in a gauze 
 napkin, placed in the jaws of a heavy vise, which is carefully 
 tightened until the tooth cracks. The same thing may be 
 accomplished by a heavy hammer on an anvil. A few trials 
 of this will enable one to crack the tooth so that the pulps 
 may be easily remo\'ed without injury. The cracked tooth 
 is put in Miiller's fluid and formalin for twenty-four hours, 
 when the pieces of dentine are removed and the pulp care- 
 fully lifted out of the pulp chamber. It is then carried 
 through the regular process, embedded in paraffin, and 
 sectioned. If the teeth are not perfect clinical history should 
 be noted. 
 
 Periosteum. — Young kittens that have not attained their 
 full growth may be used for this purpose. The bone should 
 be very carefully dissected so as not to injure the periosteum 
 and then sawed in pieces, using a fine metal saw. It is usually 
 best simply to saw it in two at the middle of the shaft and 
 to fix it in Miiller's fluid and formalin. After fixing and 
 washing, it should be cut in small pieces and decalcified 
 in 2 to 5 per cent, nitric acid. A comparatively large volume 
 of acid should be used and a pad of cotton placed in the 
 lower half of the bottle, or the tissue suspended by a thread. 
 It is best to change the acid once a day. Decalcification 
 may require from two days to a week, and should be tested 
 by passing sharp needles through the tissues. As soon as 
 decalcified the tissue should be washed for twenty-four 
 hours in running w^ater, carried through the grades of alcohol, 
 and embedded in celloidin. The sections should be cut at 
 right angles to the shaft. 
 
 Peridental Membrane. — For class work the peridental mem- 
 branes of sheep are the best for study, as their fibers are large 
 and their direction easily observed. They are much better 
 than those of either cat or dog, in which the fibers are much 
 finer and the bone more dense. The jaws are brought from 
 
GENERAL HISTOLOGICAL METHODS 495 
 
 the stockyards in ]\Iiiller's fluid and formalin, the crowns 
 broken ofl' at the level of the gum so as to expose the pulp 
 chamber, and the jaws sawed through so as to leave two 
 teeth in each block, after which they are replaced in Miiller's 
 fluid and formalin for two days, decalcified in nitric acid, 
 and thoroughly washed. They may now be cut into small 
 blocks for transverse sections and embedded in celloidin. 
 Embryological Material. — For the study of the tooth germ 
 in class work embryo pigs of all ages are easily obtained. 
 The entire embryo should be at once placed in Miiller's 
 fluid or a saturated solution of picric acid and water. In 
 ^Miiller's fluid they should remain a week; in picric acid, 
 forty-eight hours. After fixing, the heads are cut ofi", thor- 
 oughly washed, carried through the grades of alcohol, and 
 embedded in paraffin. 
 
APPENDIX CHAPTER IV 
 
 FIXING AGENTS AND STAINING SOLUTIONS 
 
 Cleaning of Slides and Cover-glasses. — Slides or cover- 
 glasses on which paraffin sections are to be mounted must 
 be absolutely clean. They should be dropped in strong 
 sulphuric acid and allowed to remain a few minutes. The 
 acid should then be poured off and thoroughly removed with 
 water, and strong acetic acid poured on. After remaining 
 a few minutes wash the acid off thoroughly and wipe from 
 alcohol. Keep ready for use in a clean box. 
 
 Meyer's Fixative. — The white of an egg is chopped with 
 a pair of scissors and filtered through muslin, diluted with 
 an equal volume of glycerin, and a little sodium oxalate 
 added to prevent decomposition. 
 
 FIXING AGENTS 
 
 Flemming's Solution. — A good solution for fixing nuclear 
 structures is the chromic-acid solution of Flemming: 
 
 Parts. 
 
 Osmic acid, 1 per cent, aqueous solution 10 
 
 Chromic acid, 1 per cent, aqueous solution 25 
 
 Glacial acetic acid, 1 per cent, aqueous solution 10 
 
 Distilled water 55 
 
 Small pieces are fixed in a small quantity of the fluid for 
 at least twenty-four hours. They are then washed for the 
 same number of hours in running water and passed through 
 50, 75, and 80 per cent, each twenty-four hours into 90 
 per cent, alcohol. 
 
FIXIXG AGEXTS 497 
 
 A stronger solution is made as follows: 
 
 Part3. 
 
 Osmic acid, 2 per cent, aqueous solution 4 
 
 Chromic acid, 1 per cent, aqueous solution 15 
 
 Glacial acetic acid 1 
 
 Fol's Solution. — A modification of Flemming's solution: 
 
 Parts. 
 
 Osmic acid, 1 per cent aqueous solution 2 
 
 Chromic acid, 1 per cent, aqueous solution 25 
 
 Glacial acetic acid, 2 per cent, aqueous solutioa 5 
 
 Distilled water 68 
 
 Corrosive Sublimate. — An excellent fixing fluid is made by 
 saturating distilled water with corrosive sublimate. Small 
 pieces about 0.5 cm. in diameter are immersed in this fluid 
 for from three to twenty-four hours, then washed in running 
 water for twenty-four hours, and then transferred into 70 
 per cent, alcohol. After twenty-four hours the tissues are 
 placed in SO per cent, for the same length of time and then 
 preserved in 90 per cent. It often occurs that after changes in 
 temperature crystals of sublimate are formed on the surface 
 or in the interior of the object. For their removal a few 
 drops of iodine and potassium iodide are added to the alcohol 
 (P. Mayer). It is a matter of indifference whether the 70 
 per cent., SO per cent., or 90 per cent, alcohol is thus iodized. 
 In future treatment of the object, as well as in sectioning, 
 any such crystals of sublimate will not be found to be a 
 hindrance. In the case of delicate objects it is better to 
 undertake their removal after sectioning by adding iodine to 
 the absolute alcohol then used. 
 
 Acetic Sublimate Solution. — An excellent solution specially 
 used for embryonic tissues and for organs containing only 
 a small quantity of connective tissue. To a saturated 
 aqueous solution of sublimate, 5 to 10 per cent, of glacial 
 acetic acid is added. After remaining two to three hours 
 or more in this solution, the objects are transferred to 35 
 per cent, alcohol and then passed through the higher grades 
 of alcohol. 
 32 
 
498 APPENDIX CHAPTER IV 
 
 Picric Acid. — Small and medium-sized objects (up to 
 1 c.c.) are fixed in twenty-four hours in a saturated aqueous 
 solution of picric acid (about 0.75 per cent.). Objects of 
 considerable size may be left in this solution for weeks with- 
 out detriment. The tissues are then transferred to 70 or 
 80 per cent, alcohol, in which they remain until the alcohol 
 is not colored by the picric acid. Instead of a pure solution 
 of picric acid, the picrosulphuric acid of Kleinenberg, or 
 the picric acid of P. INIayer may be used. Picrosulphuric 
 acid is made as follows: 1 c.c. of concentrated sulphuric 
 acid is added to 100 c.c. of a saturated aqueous picric acid 
 solution. Allow this to stand for twenty-four hours and 
 dilute with double its volume of distilled w^ater. The picric 
 acid solution is made by adding 2 c.c. of pure nitric acid 
 to 100 c.c. of saturated picric acid solution. Filter after 
 standing for twenty-four hours. 
 
 Chromic Acid. — Chromic acid is used in a | to 1 per cent, 
 aqueous solution. Small pieces are fixed for twenty-four 
 hours, larger ones for a longer time. The quantity of the 
 fixing fluid should equal at least more than fifty times the 
 volume of the tissues to be fixed. After fixing, objects must 
 be w^ashed for at least tw^enty-four hours in running water,' 
 then through the grades of alcohols, and preserved in 80 
 per cent. Two to 3 drops of formic acid to every 100 c.c. 
 of chromic acid solution improve their fixing properties. 
 
 MuUer's Fluid. — 
 
 Potassium bichromate 2 to 2.5 grams 
 
 Sodium sulphate 1 gram 
 
 Water 100 c.c. 
 
 This solution requires a long time for fixing, at least 
 several weeks, and for large pieces several months. During 
 the first few weeks the solution should be changed every 
 three or four days and later once a week, until it remains 
 clear. Tissues should be thoroughly washed in running 
 water at least tw^enty-four hours. For some special purposes 
 it is better to wash in alcohol. Tissues should be carried 
 through the grades and preserved in 80 per cent, alcohol. 
 
FIXING AGENTS 499 
 
 Wliile tissues are in ^Nliiller's fluid they should be kept in 
 the dark. 
 
 Miiller's Fluid and Formalin. — 
 
 Muller's fluid 100 c.c. 
 
 Formalin 10 c.c. 
 
 The addition of formahn to Miiller's fluid greatly hastens 
 fixation. It is an excellent agent of great penetrating power, 
 and tissues stain very well after it. Twenty-four hours will 
 fix tissues of ordinary size, though they may be left longer 
 without damage. Bone fixed too long in formalin is liable 
 to be hard to cut. 
 
 Zenker's Fluid. — 
 
 Grams. 
 
 Potassium bichromate 2.5 
 
 Sodium sulphate . 1.0 
 
 Corrosive sublimate . 5.0 
 
 Glacial acetic acid . . .5.0 
 Water 100.0 
 
 Add the glacial acid in proper proportion to the quantity 
 of the solution to be used, and not to the stock solution. 
 x'Vllow the tissues to remain in this solution for from six to 
 twenty-four hours. Then wash in running water for from 
 twelve to twenty-four hours and transfer to gradually 
 concentrated alcohol. Crystals of sublimate which may be 
 present are removed with iodized alcohol. Zenker's fluid 
 penetrates easily and fixes nuclear and protoplasmic struc- 
 tures equally well without decreasing the staining qualities 
 of the elements. 
 
 Formalin. — Of recent years formalin, which is a 4 per cent, 
 solution of the gas formaldehyde in water, has been much 
 used as a fixing fluid. ^Nlake a solution by adding 10 parts 
 of formalin to 90 parts of water or normal saline solution. 
 Small pieces of tissue should remain in this for from twelve 
 to twenty-four hours, larger pieces a number of days or 
 weeks, and then transfer to 90 per cent, alcohol. 
 
500 APPENDIX CHAPTER IV 
 
 STAINING AGENTS 
 
 Delafield's Hematoxylin. — 
 
 Hematoxylin crj'stals 4 grams 
 
 Absolute alcohol 25 c.c. 
 
 Ammonia alum, aqueous solution 400 c.c. 
 
 Methyl alcohol 100 c.c. 
 
 Glycerin 100 c.c. 
 
 Dissolve hematoxylin crystals in absolute alcohol and add 
 to the alum solution, place in an open vessel for four days, 
 then filter and add the methyl alcohol and glycerin. 
 
 Hemalum (Mayer, 91). — One gram of hematin is dissolved 
 by heating in 50 c.c. of absolute alcohol. This is poured into 
 a solution of 50 grams of alum in 1 liter of distilled water 
 and the whole well stirred. A thymol crystal is added to 
 prevent the growth of fungus. The advantages of hemalum 
 is as follows: The stain may be used immediately after its 
 preparation, it stains quickly, never overstaining, especially 
 when diluted with water, and penetrates deeply, making it 
 useful for staining in bulk. After staining sections or tissues 
 are washed in distilled water. 
 
 Safranin. — 
 
 Safranin 1 gram 
 
 Absolute alcohol 10 c c. 
 
 Aniline water 90 c.c. 
 
 Aniline water is prepared by shaking up 5 c.c. to 8 c.c. of 
 aniline oil in 100 c.c. of distilled water and filtered through a 
 wet filter. Dissolve the safranin in the aniline water and add 
 the alcohol. Filter before using. 
 
 Stain sections fixed in Flemming's solution for twenty- 
 four hours and decolorize with a weak solution of hydro- 
 chloric acid in absolute alcohol (1 to 1000). After a varying 
 period of time, usually only a few minutes, all the tissue 
 elements will be found to have become bleached, only the 
 chromatin of the nucleus retaining the color. 
 
STAINING AGENTS 501 
 
 Methyl Green.— Stains very quickly. One gram is dis- 
 solved in 100 c.c. of distilled water to which 25 c.c. of absolute 
 alcohol is added. Rinse the sections in water, then place in 
 70 per cent, alcohol for a few minutes, transfer to absolute 
 alcohol for a minute, etc. 
 
 Hematoxylin. — Van Gieson's Acid Fuchsin-Picric Acid Solu- 
 tion. — Stain in any of the hematoxylin solutions, and after 
 rinsing sections in water counterstain in the following: 
 
 Acid fuchsin, 1 per cent, aqueous solution 5 c.c. 
 
 Picric acid, saturated aqueous solution 100 c.c. 
 
 Dilute with an equal quantity of water before using. The 
 hematoxylin stained sections remain in the solution from one 
 to two minutes, are then rinsed in water, dehydrated, and 
 cleared. 
 
 Hematoxylin-Eosin.— Sections already stained in hema- 
 toxylin are placed for two to five minutes in a 1 to 2 per 
 cent, aqueous solution of eosin or in a 1 per cent, solution 
 of eosin in a 60 per cent, solution of alcohol. They are then 
 washed in water until free from the stain, after which they 
 remain for a short time in absolute alcohol. In place of the 
 eosin solution a 1 per cent, aqueous solution of benzopurpurin 
 may be used for the following solution of erythrosin (Held). 
 
 Erythrosin 1 gram 
 
 Distilled water 150 c.c. 
 
 Glacial acetic acid 3 drops 
 
 Silver Nitrate Method. — Especially useful for staining inter- 
 cellular substances of epithelium, endothelium, and meso- 
 thelium, and the ground substance of connective tissues. 
 It may be used on either fresh or fixed tissues, fresh tissue, 
 however, being more satisfactory. Spread the tissues to be 
 stained in thin layers; immerse in a 0.5 to 1 per cent, solution 
 of silver nitrate from ten to fifteen minutes; rinse in distilled 
 water and place in fresh distilled water or 70 per cent, 
 alcohol or a 4 per cent, solution of formalin and expose to 
 direct sunlight until they assume a brown color. The sun- 
 light reduces the silver in the form of fine particles which 
 
502 APPENDIX CHAPTER IV 
 
 appear black on being examined with transmitted light. 
 The preparations thus obtained may be examined in glycerin 
 or dehydrated and mounted in balsam. 
 
 Glycerin. — To mount in glycerin transfer the sections from 
 water to the slide, cover with a drop of gl^xerin, and apply 
 the coverslip. Sections colored with a stain that would be 
 injured by contact with alcohol and where clearing is not 
 especially necessary are mounted this way. 
 
 Farrant's Gum Glycerin. — In place of pure glycerin the 
 following mixture may be used: 
 
 Gljcerin 50 c.c. 
 
 Water .50 c.c. 
 
 Gum arabic (powder) . . .50 grams 
 Arsenous acid 1 gram 
 
 Dissolve the arsenous acid in water. Place the gum- 
 arabic in a glass mortar and mix it with the water, then add 
 the gl^'cerin. Filter through a wet filter paper or through 
 fine muslin. To preserve such preparations for any length 
 of time the cover-glasses must be so fixed as to shut off the 
 glycerin from the air. For this purpose cements or varnishes 
 are used, by painting over the edges of the cover-glass. 
 These masses adhere to the glass, harden, and fasten the 
 cover-glass firmly to the slide, hermetically sealing the object. 
 Kronig's is one of the best formulas for varnish, and is made 
 as follows: Melt 2 parts of wax and stir in 7 to 9 parts of 
 colophonium and filter the mass hot. Before employing 
 an oil immersion lens it is best to paint the edges with an 
 alcoholic solution of shellac. 
 
 Silver Nitrate. — In thin membranes and sections the vessel 
 walls can be rendered distinct by silver impregnation, which 
 brings out the outlines of their endothelial cells. This may 
 be done either by injecting the vessel with a 1 per cent, 
 solution of silver nitrate, or with a 0.25 per cent, solution of 
 silver nitrate in gelatin. This method is of advantage, since 
 after hardening the capillaries of the injected tissues appear 
 slightly distended. Organs thus treated can be sectioned, 
 but the endothelial mosaic of the vessels does not appear 
 definitely until the sections have been exposed to sunlight. 
 
STAINING AGENTS 503 
 
 The injections of lymph channels, lymph vessels, and lymph 
 spaces is usually done by puncture. A pointed cannula is 
 thrust into the tissue and the syringe empties by a slight 
 but constant pressure. The injected fluid spreads by means 
 of the channels offering the least resistance. For this pur- 
 pose it is best to use aqueous solution of Berlin blue or silver 
 nitrate, as the thicker gelatin solutions cause tearing of the 
 tissues. 
 
 ■ Clearing Agents. — Clearing agents are substances of high 
 refracting index, mostly oils, which are used to displace 
 alcohol and prepare tissues for embedding and sections for 
 mounting in balsam. 
 
 Clearing agents for embedding in paraffin must be miscible 
 with alcohol and solvents for paraffin. They are called 
 clearing agents because the tissues become translucent and 
 clear in them. Xylol is the most rapid and probably most 
 used agent. It has, however, a hardening action on the 
 tissues, especially if they remain too long in it. Pure oil 
 of cedar wood when free from turpentine is an excellent 
 agent. Chloroform has been largely used for the same 
 purpose. 
 
 Before celloidin sections are mounted in balsam they 
 must be cleared. For this purpose an oil that will mix with 
 95 per cent, alcohol is desirable, as absolute alcohol softens 
 the celloidin. The oil used must not dissolve the celloidin, 
 and should not dissolve the stain. Beechwood creosote is 
 an excellent agent, and has been largely used. It clears 
 sections rapidly from 95 per cent, alcohol. Oil of bergamot 
 is an excellent agent, also oil of origanum; but in the latter 
 the oleum origani cretici and not the oleum origani gallici 
 must be used. A mixture of equal parts of oil of bergamot 
 and beechwood creosote has been used satisfactorily, and 
 is an excellent agent. A cheaper mixture is made of equal 
 parts of phenol, oil of origanum, and oil of cedarwood. 
 
INDEX 
 
 Absorptiox of roots of temporar}- 
 teeth, 302 
 
 Acetic acid and sublimate for 
 fixing, 497 
 
 Alveolar process, 379 
 
 Analog}', 22 
 
 Attachment of teeth, 271 
 by ankylosis, 274 
 in fibrous membrane, 272 
 by hinged joint, 273 
 by insertion in a socket, 277 
 
 B 
 
 Balsam, 463 
 
 management of, for grinding sec- 
 tions, 469 
 Bichloride of mercury for fixing, 
 
 497 
 Blocking celloidin material, 491 
 Bone, 247 
 
 arrangement of lamellae of, 252 
 canaliculi of, 249 
 cancellous, 251, 254 
 compact, 252 
 corpuscles of, 248 
 decalcified, 442 
 definition of, 247 
 formation of, 255 
 
 endochondrial, 255, 445 
 endomembranous, 258 
 growth of, 260, 446 
 Haversian canals of, 253 
 
 system of, 250 
 influence of mechanical condi- 
 tions on, 380 
 lacunae of, 249 
 
 Bone, matrix of, 247 
 
 structural elements of, 247 
 
 subperiosteal, 250 
 
 varieties of, 250 
 Branchial arches, 355 
 
 Calcoglobulix, 231 
 
 Calculus, grinding of sections of, 
 
 473 
 Cell division, 337 
 indirect, 338 
 
 theorv, 336 
 
 walls of plants, 238 
 Celloidin, blocking of, 491 
 
 cutting of, 491 
 
 method, 490 
 
 summary of, 492 
 
 sections of, serial, 493 
 staining of, 492 
 
 stock solutions of, 491 
 Cement corpuscles, 296 
 Cementoblasts, 295 
 Cementum, 188 
 
 absorption of, 200 
 
 canaliculi of, 194 
 
 cement corpuscles of, 195 
 
 distribution of, 33 
 
 embedded fibers of, 196 
 
 function of, 29, 189 
 
 Haversian canals in, 188 
 
 histogenesis of, 189 
 
 lacunae of, 194 
 
 lamella? of, 190 
 
 structural elements of, 190 
 Chromic acid for fixing, 498 
 Cleaning slides and cover-glasses, 
 
 496 
 
506 
 
 INDEX 
 
 Cleaning agents, 503 
 Cleft palate, 361 
 Connective tissues, 240 
 
 chemical relations of formed 
 
 material to cytoplasm, 245 
 mutations of, 240 
 relation of, to mechanical 
 conditions, 245 
 Corrosive subUmate, 497 
 Creosote, 503 
 
 Cutting celloidin sections, 491 
 paraffin sections, 487 
 
 Decalcified bone, 442 
 Delafield's hematoxylin, 500 
 Dental foUicles, 364 
 ligament, 285 
 papilla, 363 
 
 pulp, bloodvessels of, 209 
 cells of, arrangement of, 209 
 
 connective tissue of, 207 
 definition of, 201 
 degeneration of, 229 
 from unerupted tooth of a 
 
 sheep, 443 
 function of, 29 
 sensory, 202 
 vital, 201 
 hard formations in, 235 
 histogenesis of, 203 
 human, normal, 444 
 pathological, 445 
 hyperemia of, 219 
 infarction of, 224 
 intercellular substance of, 209 
 nerves of, 214 
 nodules in, 229 
 odontoblasts, 204 
 pathology of, 219 
 preparation of, method of, 493 
 structural elements of, 203 
 ridge, 362 
 tissues, 28 
 
 distribution of, 30 
 in adaptation, 35 
 Dentine, caries of, 157 
 chemical composition of, 169 
 dentinal fibrils, 176 
 distribution of, 32 
 
 Dentine, function of, 29, 167 
 granular layer of, 178 
 histogenesis of, 167 
 interglobular spaces in, 179 
 lines of Schreger in, 184 
 matrix of, 168 
 secondary, 184 
 sheath of Newman, 169 
 tubules of, 171 
 diameter of, 171 
 direction of, in crown, 171 
 in root, 174 
 Dermal scales, 22, 271 
 Development, beginning of calci- 
 fication, 366 
 chronology of, 372 
 of dental follicle, 364 
 papilla, 363 
 ridge, 362 
 of enamel organ, 362 
 of permanent molars, first, 370 
 second, 371 
 third, 371 
 of tooth germ, 362, 364 
 
 for permanent teeth, 365 
 Dissecting, 481 
 Drawings, 427 
 of teeth, 425 
 
 surfaces, 429 
 of typical cavity walls, 439 
 
 Embedding, 483 
 in paraffin, 486 
 Embryology, 335 
 
 biological considerations fund^ 
 
 mental to, 335 
 branchial arches, 355 
 chemical ideas related to, 339 
 early stages of, 340 
 fertilization, 343 
 formation of germ layers, 347 
 frontonasal process, 357 
 maturation, 340 
 neural canal, 352 
 preparation of material, 495 
 relation of cell theory to, 336 
 segmentation, holoblastic, 345 
 
 mammalian, 348 
 
 meroblastic, 348 
 
INDEX 
 
 507 
 
 Embryology, separation of nose 
 and mouth cavities, 360 
 spermatogenesis, 340 
 stomodium, 3.57 
 , transmission, 338 
 Enamel, action of acid in caries of, 
 150 
 stages in, 153 
 areas of weakness for cavity 
 margins, incisors, 136 
 marginal ridges, 127 
 simple proximal cavi- 
 ties in bicuspids and 
 molars, 137 
 tips of cusps, 124 
 atrophy of, structural effects of, 
 
 160 
 bands of Retzius, 60, 115 
 chemical composition of, 39 
 cleavage of, 73 
 
 cutting of, instruments for, 76 
 developmental lines in, 122 
 differences between rods and 
 cementing substance, 46 
 from other calcified tissues, 
 38 
 distribution of, 30 
 effect of caries beginning in 
 natural defect, 143 
 on smooth surfaces, 145 
 intensity and liability, 148 
 secondary or backward de- 
 cay, 146 
 on structure of, 143 
 of structure on cutting of, 56 
 etching of, 48 
 function of, 28 
 gnarled, 54 
 growth of cap of, 108 
 lines of Schreger in, 64 
 mottled, 165 
 occlusal grooves in, 115 
 origin of, 38, 362 
 planing of, 76 
 refracting index of rods and 
 
 cementing substance, 51 
 relation of, to formation tissue, 
 
 41 
 relative solubility of rods and 
 
 cementing substance, 46 
 rods of, 43 
 short, 45 
 
 Enamel, straight, 53 
 stratification of, 60 
 striation of, 57 
 structural elements of, 43 
 
 form of, 42 
 walls, structural requirements 
 of, 80 
 bevel of cavosurf ace angle, 
 
 87 
 classes of cavities, 87 
 gingival third cavities, 
 
 101 
 incisor pits, 105 
 in simple occlusal cavities, 
 
 90 
 steps in preparation of, 
 
 89 
 support of marginal rods, 
 85 
 of worn surfaces, 86 
 supported on sound den- 
 tine, 80 
 white spots in, 162 
 Endoskeleton, 19 
 
 relation of nervous system to, 22 
 Epiblast, 347 
 
 Epithelial structure in peridental 
 membrane, 307 
 arrangement of cells in, 
 
 308 
 distribution of, 307 
 Etching and mounting ground sec- 
 tions, 429 
 Exoskeleton, 19 
 
 relation of, to nervous system, 22 
 
 Farrant's gum glycerin, 502 
 ' Fastening teeth to grinding disks, 
 j 465 
 
 ; Fertihzation, 343 
 ' First permanent molars, origin of, 
 
 370 
 Fixative for paraffin sections, 496 
 Fixing, 481 
 
 agents, 496 
 Flemming's solution, 496, 497 
 Forces influencing bone growth, 
 
 393 
 I Formalin for fixing, 499 
 
508 
 
 INDEX 
 
 Formalin for 
 
 teeth, 424 
 Frontonasal process 
 
 preserving fluid for Growth of jaws, relation of first 
 molars, 398 
 357 tissue changes in, 413 
 
 of mandible, 375 
 of membrane bones, 261 
 Gum glycerin, 502 
 
 GiNGivus, gum tissue and, 447 
 
 support of, 291 
 Gland of Serres, 310 
 Glycerin for mounting, 502 
 Granular layer of Tomes, 178 
 Grinding of crumbled material, 473 
 difficulties in, 474 
 disks, 457 
 
 of frail material, 468 
 in hard balsam, 469 
 machine, 453 
 
 grinding of sections on, 463 
 of microscopic sections, 453 
 
 description of machine, 453- 
 
 457 
 fastening teeth to grinding 
 
 disks, 465 
 frail material, 468 
 grinding disks, 457 
 lap wheels and stones, 460 
 management of balsam, 463 
 measurement of sections, 
 
 467 
 point finder, 460 
 process of grinding, 462 
 rapidity of grinding, 466 
 removal of cover-glass from 
 
 disk, 471 
 spatter guard, 462 
 spiders and dogs, 463 
 waste water, 462 
 watering stones, 461 
 stones, 460 
 
 clogging of, 474 
 of tooth sections, 425 
 Ground sections of bone, 441 
 Growth force, 392 
 of jaws, 390 
 
 eruption of temporary teeth, 
 
 394 
 growth of air space in nose, 412 
 importance of proximal con- 
 tact, 405 
 influence of permanent incisors 
 and cuspids, 403 
 
 Hair, teeth and, comparison of 
 origin of, 25 
 of structure of, 24 
 Hardening, 481 
 Hemalum, 500 
 
 Hematoxylin, Delafield's, 500 
 eosin and, 501 
 Van Gieson's, 501 
 Histological technique, theory of, 
 
 478 
 Holoblastic segmentation, 345 
 Homology, 22 
 
 Hyperemia of dental pulp, 219 
 acute, 220 
 chronic, 222 
 Hypoblast, 347 
 
 Indexing and filing, 485 
 Infarction of dental pulp, 223 
 Inflammation of dental pulp, 224 
 Intercellular substances, 236 
 
 kinds of, 238 
 
 relation of cells to, 237 
 Isolated enamel rods, 435 
 
 Jaws, growth of, 390 
 
 Labelling, 485 
 
 Laboratory, manner of working in, 
 
 427 
 Lap wheels, 460 
 
INDEX 
 
 509 
 
 M 
 
 Maceratiox, 480 
 JMandible, growth of, 375 
 Maturation, 3-40 
 Membrana eboris, 207 
 Merkel's cartilage, 367 
 Methyl green, 501 
 Meyer's fixative, 496 
 Morris' staining dish, 489 
 Mounting, 483 
 Mouth eavitv, 323 
 
 epithelium of, 323 
 
 mucous membrane of, 323 
 
 nerve endings in, 327 
 
 submucosa of, 325 
 
 taste buds, 331 
 
 tongue, 327 
 muscles of, 328 
 papillae of, 329 
 Mucous membrane of mouth, 323 
 Mailer's fluid, 498 
 
 formahn and, 499 
 
 Odontoblasts, 204 
 Oil of bergamot, 503 
 
 of origanum, 503 
 Osteoblasts, 297 
 Osteoclasts, 299 
 absorptions by, 300 
 in burrowing canals, 302 
 Outline drawings of ground sec- 
 tions, 432 
 from transverse sections of 
 root, 440 
 
 Par AFFIX, cutting of, 487 
 embedding in, 486 
 kinds of, 487 
 
 method, summary of, 489 
 sections, staining of, 488 
 Pathology of dental pulp, 219 
 Peridental membrane, 279 
 absorption by, 300 
 arrangement of fibers of, 283 
 bloodvessels of, 313 
 
 Peridental membrane, cellular ele- 
 ments of, 294 
 cement corpuscles, 295 
 cementoblasts in, 295 
 changes in, with age, 319 
 definition of, 279 
 divisions of, 280 
 epithelial structure in, 306 
 fibroblasts in, 294 
 fibrous tissue of, 283 
 functions of, 281 
 longitudinal sections of, 450 
 nerves of, 318 
 nomenclature of, 280 
 osteoblasts of, 297 
 osteoclasts of, 299 
 practical considerations of, 
 
 321 
 preparation of material, 494 
 principal fibers of, 283 
 relation of cementoblasts to 
 
 cure of pockets, 297 
 structural elements of, 281 
 transverse alveolar, 449 
 gingival, 447 
 Periosteum, 262 
 
 attached, 264, 267, 446 
 complex, 270 
 simple, 268 
 classification of, 262 
 definition of, 262 
 functions of, 262 
 layers of, 265 
 
 macroscopic appearances of, 263 
 preparation of material, 494 
 relation of attachment of, to 
 
 burrowing pus, 264 
 unattached, complex, 265 
 simple, 265 
 Picric acid, 498 
 Placoid scabs, 22, 28, 271 
 Point finder, 460 
 
 Preparation of dental pulp mate- 
 rial, 493 
 of embryological material, 495 
 of grinding material, 463 
 of peridental membrane mate- 
 rial, 494 
 of periosteum material, 494 
 of shellac for grinding sections, 
 472 
 Preserving tissues, 486 
 
510 
 
 INDEX 
 
 Rapidity of grinding, 466 
 
 Reattachment of tissues to surface 
 of root, 297 
 
 Relation of nucleus to cytoplasm, 
 337 
 of section to crown, 424 
 of teeth to bone, 374 
 
 to development of face, 374 
 
 Removal of cover-glass from grind- 
 ing disk, 471 
 
 S 
 
 Safranin, 500 
 
 Schreger's lines in dentine, 184 
 
 Secondary dentine and cementum, 
 
 study of, 440 
 Sectioning, 480, 483 
 
 methods, choice of, 486 
 Segmentation, 345 
 holoblastic, 345 
 mammalian, 348 
 meroblastic, 348 
 Serial sections with celloidin, 493 
 Sheaths of Newman, 169 
 Silver nitrate, 501 
 injection, 502 
 Slicing mechanism, 475 
 Spatter guard, 462 
 Spermatogenesis, 340 
 Staining, 483 
 agents, 500 
 celloidin sections, 492 
 of fresh tissues, 479 
 of paraffin sections, 488 
 Stomodium, 357 
 
 Structure of mandible and maxilla, 
 377 
 distribution of bone in 
 alveolar process, 379 
 in mandible, 384 
 in maxilla, 390 
 
 Structure of mandible and maxilla, 
 influence of mechanical condition 
 in evolution of, 380 
 Subperiosteal bone, 250 
 
 and cementum, comparative 
 study of, 442 
 
 Taste buds, 331 
 
 Teasing, 479 
 
 Teeth, attachment of, 271 
 
 chisel, 36 
 
 for grinding sections, 423 
 
 grinding, 36 
 
 relation of, to bone, 27 
 to exoskeleton, 22 
 
 temporary, absorption of roots 
 of, 302 
 Tissue changes in the physiolog- 
 ical movements of teeth, 413 
 Tongue, 327 
 
 muscles of, 328 
 
 papilla of, 329 
 
 taste buds of, 331 
 Tonsils, 331 
 
 lingual, 332 
 
 palatine, 334 
 
 pharyngeal, 334 
 Tooth germ, 364, 451, 452 
 
 for permanent teeth, 365 
 Transmission, vehicle of, 338 
 Transverse sections of roots of 
 
 teeth, 426 
 
 W 
 
 Washing, 485 
 Watering the stones, 461 
 
 Zenker's fluid, 499 
 
■- -^^ 
 
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