UNIVERSITY of GLASGOW James Ireland Memorial Library 30114011828152 Hosp/t/\L (^1 1 A - Glasgow University Library ALL ITEMS ARE ISSUED SUBJECT TO RECALL GUL 96.18 Digitized by the Internet Archive in 2014 https://archive.org/details/b21449594 A MANUAL OF DENTAL ANATOMY HUMAN AND COMPARATIVE BY CHAELES S. TOMES, LLD., F.E.S., P.R.C.S. SEVEN1H EBI1ION EDITED BY H. W. MAEETT TIMS, M.A. (Cantab.), M.D., M.Ch. (Edin.), F.L.S., F.Z.S. UNIVERSITY READER IN ZOOLOGY, BEDFORD COLLEGE, UNIVERSITY OB1 LONDON, AND PROFESSOR OF BIOLOGY, ROYAL VETERINARY COLLEGE, LONDON ; AND A. HOPEWELL-SMITH, L.R.C.R, M.R.C.S., L.D.S. (Eng. PROFESSOR OF DENTAL HISTOLOGY AND COMPARATIVE ODONTOLOGY, UNIVERSITY OF PENNSYLVANIA, U.S.A. ; JOHN TOMES PRIZEMAN OF THE ROYAL COLLEGE OF SURGEONS OF ENGLAND ; FORMERLY SURGEON AND LECTURER ON DENTAL ANATOMY AND PHYSIOLOGY AT THE ROYAL DENTAL HOSPITAL, LONDON. WITH 300 ILLUSTRATIONS LONDON J. k A. CHUECHILL 7 GREAT MARLBOROUGH STREET 1914 PREFACE TO THE SEVENTH EDITION. The present edition of Mr. C. S. Tomes' well-known Manual has been to some extent re-arranged. As the author has pointed out in the preface to the previous edition, the require- ments of the Student of General Zoology are not identical with those of the medical and dental student. The accumulation of new observations tends to accentuate this difference. It is on this account that the re-arrangement of the subject-matter has been made. In Part I. the facts dealt with are such as are of more immediate value to the dental or medical student ; while Part II. contains that which is of greater importance to the Student of Comparative Anatomy. An attempt has been made to incorporate in the volume the results of recent research, a very difficult task to accom- plish fully, owing to the number of and rapidity with which new facts in almost every branch of the subject are being constantly brought to light. The references to the literature have been considerably amplified in the hope of rendering the work of identification simpler. As this edition is brought out under joint editorship it is but right that the measure of responsibility should be some- what defined; while Part I. is the work of both Editors, the responsibility for Part II., as well as the re-arrangement of the subject-matter, rests solely upon one (H. W. M. T.). PREFACE TO THE SEVENTH EDITION. That part of the last chapter dealing with the teeth of pre- historic man, has been written by the author himself. To him we desire to offer our grateful thanks for so generously allowing us an entirely free hand to deal with the book as seemed best. We also wish to acknowledge our gratitude to Professor Fawcett of British University for kindly writing a resume of his recent researches upon the development of the human jaws. To various authors and publishers our thanks are due for permission to include numerous illustrations ; and lastly, we wish to express indebtedness to Mr. T. H. E. Winston, Assistant in the Library of the Royal Society of Medicine (now Librarian at Guy's Hospital) for his invaluable assistance in tracing and verifying all the references to the literature, and to Miss Mary L. Hett, Demonstrator of Zoology, Bedford College (University of London), for her kindly help in the preparation of the Index. H. W. MARETT TIMS. A. HOPEWELL-SMITH. London. PKEFACE TO THE SIXTH EDITION. The accumulation of a somewhat voluminous literature, much of which, while falling short of carrying complete conviction, yet tends to unsettle the ideas hitherto prevalent upon such points as the methods of evolution and the homologies of the teeth, renders the task of presenting in small compass any general view of the subject one of ever- increasing difficulty. In endeavouring to give an outline of the more important facts of Odontology and of the theories based thereon, I have hoped that this book may serve as an introduction to a more extended study of the subject, but a difficulty has arisen in the choice of the matter to be included, for that which will satisfy the general student of biology will be somewhat in excess of the requirements of the medical or dental student. References could not of course be entirely omitted from a W'Ork of the kind, though at the same time it is not desirable to burden a text-book with too many ; it has been my endeavour, not, I fear, always consistently carried out, to give references only to such works and papers as either form the authority for accepted views, or deal with subjects which are the ground of debate at the present time, and are distinctly unsettled. In places it has appeared to me to be better to err on the side of repetition than to be constantly referring the reader to another page, more especially as the book may be referred to by those who do not read it through. vi PREFACE TO THE SIXTH EDITION. And I take this opportunity of making a general apology to any of those of whose writings I may have made use without specific acknowledgment in the text. CHARLES S. TOMES. 9, Park Crescent, London, W., 1904. CONTENTS. 1 CHAPTER PART I. I. The Teeth of Man 7 n. The Dental Tissues — Enamel .... 30 in. Ortho-dentine and its Yarieties . . 61 IY. The Tooth Pulp — Cementum — The Gum — The Alveolo-dental Membrane .... 107 Y. The Development of the Teeth 132 YL Calcification of Enamel, Dentine and Cementum 162 YIL The Development of the Jaws — The Eruption of the Teeth ... 218 YIII. The Attachment of Teeth .... 219 PART II. 1A. General Introductory Remarks . . 262 A. The Teeth of Fishes ..... ir> n n 21 o 11. The Teeth of Amphibia, Reptiles and Birds . 305 VTT All. The Homologies of the Mammalian Teeth 337 ~\r TTT XIII. The Evolution of the Forms of Teeth . 365 XIY. The Teeth of Mammalia — Prototheria . 391 XY. The Teeth of Metatheria. — Marsupialia 399 XYI. The Teeth of Eutheria — Insectivora, Galeopi- thecid^e, Cheiroptera, Carnivora 426 XYII. The Teeth of Edentata, Rodentia and TlLLODONTIA . . . . . 470 Villi. The Teeth of Cetacea, Unculata, Amblypoda AND CONDYLARTHRA 490 XIX. The Teeth of Hyracoidea, Proboscidea and 537 XX. The Teeth of Primates 565 MANUAL OF DENTAL ANATOMY HUMAN AND COMPAEATIVE. INTKODUCTION. The range of the subject of Dental Anatomy turns upon the meaning which is attached to the word " Tooth ; " but although this chapter might most appropriately open with a definition of this word, it is very much easier to explain what is ordinarily understood by it, than to frame any single sentence which shall fulfil the requirements of logical definition. Most vertebrate and a great many invertebrate animals have certain hard masses in or near to the entrance of the alimentary canal, i.e., the mouth. By these hard masses, sometimes of a bony and sometimes of a horny nature, various offices in connection with the prehension or com- minution of food are performed, and to them the term " teeth " is applied. Whilst, however, in some animals these functions are performed by corneous structures throughout life, recent researches have shown that these may sometimes at first underlie and subsequently replace true calcified teeth, as in Ornithorhyncus, in other cases horny plates overlie and bury rudimentary calcified teeth. In many animals, teeth have come to be used for purposes other than those of nutrition, such as for sexual warfare; but it can hardly be doubted that true teeth primarily had to do with the nourishment of their possessor. The subject of the homologies of the teeth cannot be fully entered upon until the details of their development have been mastered ; still, a few words may even at the outset be devoted to the elucidation of their real nature. The mucous membrane which lines the alimentary canal D.A. 1 n 2 A MANUAL OF DENTAL ANATOMY. is continuous with the external skin, with which it blends at the lips. Now, if a young dog-fish just about to be hatched, be examined, it will be found that it has no distinct posterior lip, but that its skin turns in over the rounded jaw without interruption. The skin outside carries spines (dermal denticles, placoid scales),* and these spines are continued over that part of it which enters the mouth and bends over the jaws, being slightly larger in this latter position. It may occur to the reader that it is a remarkable thing that the skin and its associated structures should be found in the mouth at all, but a very easy explanation is afforded by the facts of embryology. At the stage immediately prior to the appearance of the embryo, the egg is enveloped by cells which are differentiated into two layers. In the neighbourhood where the embryo is about to appear, a third layer becomes separated off and placed between the other two. The origin of this third layer varies in different animals. In Amphioxus, for example, it is derived entirely from the inner layer. The most superficial of the three layers is called the epiblast, the internal one the hypoblast, and the middle one the mesoblast. From the epiblast are ultimately formed the enamel of the teeth, the epithelium of the mouth (with the exception of that which covers the tongue and the back part of the floor of the mouth), the lining of the anus, the skin and its appendages, hair and nails, etc., the epithelium of skin, glands, and a variety of other structures, such as the nervous system. From the mesoblast are derived the dentine and cementum, the skeleton, and all the connective tissues of the body, the muscular system, etc., etc. From the hypoblast are formed the epithelium of the whole alimentary canal, from the back of the mouth as far as the anus, and the other internal epithelia of various organs derived from the alimentary canal, such as the lungs, liver and pancreas. Now, as the embryo begins to take definite shape, that part which in the future will become the intestinal tract has for some time a widely open communication on its ventral surface into the general interior cavity of the egg or yolk sac, but the embryo grows forwards and backwards in such a way as to form a blind pouch at either end, the one in front and the other behind, which are thus still in communication with the cavity of the yolk. Were this state of things to be perpetuated while the alimentary canal becomes gradually pinched off and finally closed from the yolk sac, there would be neither mouth nor anus, and the intestine * " The placoid scale has the structure of dentine ; is covered by enamel, and is continued at its base into a plate formed of osseous tissue." Gegenbaur's " Comparative Anatomy," translated by F. Jeffery Bell, p. 424. (The term " Dermal denticle" is therefore preferable to that of " Placoid scale," since it better expresses both the structure and position of the spine. — Eds.) INTKODUCTION. 3 would terminate in blind extremities near either end of the embryo. At the hinder end this sometimes happens, and the result is that the animal is born with an imperforate anus. At this stage the parts are named the foregut (the blind anterior end), the midgut, and the hindgut. The mouth is formed later by an invagination of the epiblast from the surface, so as to make a pouch (stomodoeum) reaching down to the foregut, but it is still some time before the septum between them breaks down. The anus is formed in a similar way, the invagination (proctodeum) being much shallower and occurring at a later date. Thus we have in what is to be the mouth, a portion of the actual exterior or future skin of the embryo carried in, and so it ceases to be a matter for surprise that we should find in the mouth skin structures, viz., dermal denticles. It is, however, pointed out by Dr. Wortman (5), on the authority of Mr. Byder, that the teeth of those fish which have teeth upon the gill arches and pharyngeal bones, lie beyond the limits of the invaginated integument, and are therefore of hypoblastic derivation. He goes on to add that, if this be true (which is not certain), the generalisation that all teeth are modified dermal spines would be incorrect. It might be, however, an example in which identical structures have been produced in a similar manner from tissues of different origin, and in all probability be attributable to the same cause, viz., repeated stimulation of a particular point, which eventually gave rise to a calcified papilla. The more probable explanation of the origin of teeth in these positions is that they are derived from lateral invaginations of the epiblast, similar in character to the .anterior stomodoeum. These invaginations meet with lateral evaginations from the walls of the foregut. The bilaminar membrane separating the cavities of these epiblastic and hypoblastic pouches ultimately ruptures, thus giving rise to the pharyngeal gill slits, which are consequently formed in a manner comparable with that of the mouth. Likewise, the origin of the teeth upon the gill arches will be the same as of those situated in the mouth.* If the growth of the dog-fish be followed, those spines of the skin which cover the jaws become developed to a greater size than those outside. A groove, without spines, appears between the jaw and the lip, and the identity and continuity of the two become to some extent masked. No one can doubt, whether from the comparison of adult forms or from a study of the development of the parts, that the teeth of the shark correspond to the teeth of other fish, and these again to those of reptiles and mammals. It may be clearly demonstrated that the teeth * The arrangement of the pharyngeal teeth and the degree to which they are developed vary greatly. Many of these have been described by Colonel Shepherd in a series of papers which have recently appeared in the Zoologist. Eds. 1-2 . 4 A MANUAL OF DENTAL ANATOMY. of the shark are nothing more than highly developed spines of the skin, and therefore it is inferred that all teeth bear a similar relation to the skin. This is what is meant when teeth are called " dermal appendages," and are said to be perfectly distinct from the internal bony skeleton of the animal. The teeth of the shark and of many other creatures are not only developed, but always remain imbedded in tough mucous membrane, and never acquire any connection with the underlying bone. Indeed, all teeth alike are developed from a part of the mucous membrane, and any connection which they may ultimately get with the bone is a secondary matter. As has been well expressed by Dr. Harrison Allen " if the hairs of the scalp were to be inserted into the skull, or of the moustache into the upper jaw, we should express great astonishment, yet such an extreme proposition is no more remarkable than what is seen to take place in the jaws ; " again, "the feathers of certain birds making impressions on the radius, the whalebone pendent from the roof of the mouth, are examples of this same association of tegumentary appendages with the bones." To these examples may be added the horny plates of the Ornithorhyncus, which are pure hardenings of the stratum corneum of the oral epithelium, but nevertheless have definite beds provided for them on the bones. In their simpler forms, then, teeth are met with as very numerous spines, differing but little from the spines of the skin except in size, and still less from one another. Indeed, if we take the spines of the skin where, as happens in many cartilaginous fish, some are developed to aD especially large size for protective purposes, we shall often find them abso- lutely indistinguishable from teeth upon the jaws, so far as minute structure and general form go, though they may perhaps exceed the teeth in size. Nickerson (3) has minutely investigated the development of the scales of Lepidosteus, which, at all events at an early period, carry upon them small spines. He regards these spines alone as perhaps homologous, with the scales of the shark, the whole scale of Lepidosteus then repre- senting the fusion of a number of placoid scales and consisting, with the exception of these little spines, of their basal portions only. Some- what similar conclusions have been arrived at by 0. Hertwig (2) for the Ganoid scale ; and by Marett Tims (4) for the scales of the Gadidce, INTRODUCTION. 5 In many fish, the teeth in some cases more specialised than those of the shark, are scattered over almost every one of the numerous bones which form part of the walls of the mouth and pharynx. In .the amphibia the teeth are more restricted in position, being found only upon the jaws and in connection with the vomers. In reptiles they are even more restricted, whilst in mammals they are absolutely confined to the premaxillary, maxillary, and mandibular bones. In fish and reptiles it is the exception for the teeth in different parts of the mouth to differ markedly from each other ; in mammals it is the rule. Most teeth owe their hardness to an impregnation with salts of lime ; the organic matrix may, however, be of proteoid substance, keratin, in which case the tooth is of horny consistence, and is spoken of as " cornified ; " or the matrix may be, like that of bone, gelatigenous (that is to say, not gela- tine, but collagen, the anhydride of gelatine which passes into gelatine after prolonged boiling), in which case the tooth is more richly impregnated with salts, and is spoken of as " calcified." Horny teeth, so far as they have been investigated, consist of aggregations of cells of the stratum corneum of the oral epithelium, and they are penetrated on their under side by the papillse, whose enormously exaggerated epithelial coats have built them up. The great mass of a calcified tooth is usually made up of " dentine," which gives to it its characteristic form, and often practically constitutes the whole tooth. To this may or may not be added enamel and cementum. The simplest teeth are hollow cones, and have no roots ; they are calcifications of papillae, and, as has already been mentioned, are sometimes practically identical in structure and form with similar spines upon the skin. As these simple teeth are generally very numerous, it has occurred to some morphologists that multicuspid teeth have originated by the coalescence of several simple cones, each cusp thus being the representative of an original separate tootb. But there is a good deal to be said against the accept- ance of such a view ; the question can, however, be better discussed at a subsequent page. It will suffice to say here that it is much more probable that additional cusps arise by some process of evolution. 6 A MANUAL OF DENTAL ANATOMY. It is thus usual to speak of horny and calcified teeth ; but the development of the former is less fully known, and appears to vary in different cases. Hence it is impossible to determine the exact relation in which they really stand to calcified teeth. Before proceeding to consider the structure and comparative anatomy of teeth in detail, it may be well now to give a description of the human teeth, as being the more familiar. References. (*) Allen, Harrison. " Studies in the Facial Eegion." Philadelphia, 1875, (2) Hertwig, 0. "Ueber das Hautskelet der Fische." Morpholo- gische Jahr., 1876, Bd. ii., pp. 328—395 (parti.); 1879, Bd. v., pp. 1—21 (part ii.) ; Bd. vii., pp. 1 — 41 (part iii.). (3) Nickerson, W. S. "The Development of the Scales of Lepido- steus." Bull. Mus. Comp. Zool. (Harvard), 1893, vol. xxiv., pp. 115—138. (4) Tims, H. W. Marett. "The Development, Structure, and Morphology of the Scales in some Teleostean Fish." Quart. Journ. Micr. Sci. London, 1906, vol. xlix., N.S. pp. 39—69. (5) Wortman, J. L. " American System of Dental Surgery." Phila- delphia, 1887. PART I. CHAPTEE I, THE TEETH OF MAN. The Parts of a Tooth. — For the purpose of description the three external parts of a tooth are distinguished hy name, viz., the crown, neck, and root. This distinction is made in describing human teeth, and is applicable to the great majority of mammalian teeth, though there are some forms in which no such differentiation of parts can be seen. The crown is that portion which is exposed above the borders of the gum, and is in human teeth coated with enamel ; the neck is that portion which corresponds to the edge of the gum, and intervenes between the edges of the bony sockets and the edge of the enamel ; the root is that part which is enclosed within the bony socket, and is covered by cementum. Of these it is to be remarked that the " neck," although a convenient and necessary term for descriptive purposes, marks an arbitrary division of less importance than that expressed by crown and root ; also that, although this division into three parts can be made in the case of socketed teeth of limited growth, no such distinction of parts can be made in teeth of perpetual growth. Special names have been applied to the various surfaces of the crowns, as, owing to the curvature of the alveolar border, terms which have reference to back, front, or sides would in different parts of the mouth indicate different surfaces, and so lead to confusion. The lips, cheeks, tongue, and the median line of the mouth, however, are not open to this objection, so the surfaces which are directed outward towards the lips, or cheeks, are called "labial," or "external," and those inwards towards the tongue 8 A MANUAL OF DENTAL ANATOMY. " lingual," " palatal," " vestibular," or " internal." The inter- stitial surfaces are named " proximal " and " distal." * Incisors. — Of these there are four in each jaw : two first and two second incisors. f Their working surfaces form wedges, or obtuse and blunt-edged chisels, calculated to divide food of moderate consistency. The Upper Incisors. — The first incisors are very much larger than the second, and, viewed either from the back or front, taper with some regularity from the cutting edge to the apex of the root, the neck not being marked by any strong constriction. The crown of the tooth, as seen from the front, A , B FlG. 1. — Front and side view of a left upper first incisor. A. Labial surface. B. Distal surface, b. Neck. c. Root. is somewhat square or, more strictly oblong, its length being greater than its breadth. The distal (median) side, by which it is in contact with its fellow, is a little longer than the proximal, so that the distal angle of the crown is a little lower and, as a necessary conse- quence, a little more acute than the proximal angle of the cutting edge. Near to its base the crown narrows rather abruptly, so that at the neck a space is left between the contiguous teeth. * The terms "mesial" and "distal," which are in general use clinically, are used In relation to the middle line of the mouth, but the terms " proximal " and " distal " are here suggested in reference to the articular ends of the jaws, as being more in accordance with anatomical usage. — Eds. f The terms "first incisor" and " second incisor " are habitually employed to denote the human incisors from the clinical point of view, but this nomenclature is open to the objection that it is by no means certain that these teeth are the homologues of the first and second incisors of other mammals. It is also more correct to use the term "premolar" instead of "bicuspid" and "first, second and third molars " in place of " six-year old, twelve-year old, and wisdom tooth " respectively. — Eds. c THE TEETH OF MAN. 9 The labial surface is slightly convex in each direction, and often presents two extremely shallow longitudinal depressions, which end at the cutting edge in slight notches, though in well-formed typical .teeth these grooves are not distinguishable. In recently erupted teeth the thin cutting edge presents three slight tubercles, which soon wear down and disappear apparently by use. The edge of an incisor may be regarded as formed by the bevelling off of the dentine of the lingual surface, which is nearly flat from side to side, with a slight tendency to con- cavity, while from above downwards it is distinctly concave, and is often marked by longitudinal depressions similar to those on the labial surface. The lingual surface towards the gingival margin terminates in a distinct prominence, often- times amounting to a bounding ring of enamel, termed the basal ridge, or, in the language of comparative anatomy, the cingulum. It is variable in the extent of its development ; it rarely rises into a central prominence at the back, but in the angle where the ridges of the two sides meet, a deep pit is sometimes left in the enamel, though this must be regarded as a departure from the normal. It is a favourite site for caries. The crown terminates on the lingual and labial aspects of the tooth in a curved line, the convexity of the curve being directed towards the gum ; on both median and distal surfaces the curve is less regular, and its contour would be more correctly described as V-shaped, the apex of the V being towards the crown of the tooth and away from the gum. The transverse indentations of the enamel seen both on the lingual and labial surfaces, though more especially on the latter, are marks of arrest of development, and, common as they are, are to be regarded as abnormalities. The first incisors are larger than the second, though not in so great degree in man as is the case in the anthropoid apes. The pulp cavity bears a general resemblance to the external contour of the tooth. Towards the cutting edge it is very thin, and is prolonged at its two corners to a slight extent into cornua ; at the neck it is cylindrical, and it is also cylindrical in the root, tapering gradually till it approaches the apex, when it becomes suddenly constricted. Upper Second Incisors are in every dimension slightly smaller than those just described. They widen somewhat 10 A MANUAL OF DENTAL ANATOMY. abruptly near to the cutting edge, but below this they taper fairly regularly to the end of the root. The labial surface is convex in each direction, while the lingual surface is perhaps rather flatter than that of the first incisor. The outer angle of the crown is far more rounded or sloped away than in the first, and the proximal surface, turned towards the canine, is in a slight degree convex. The median surface may be slightly concave. The enamel terminates towards the gum in contours precisely similar to those which are found in the other incisors ; but the basal ridge, or cingulum, is often more strongly pronounced, and the presence of a central tubercle upon it is less infrequent. From this greater prominence of the cingulum, and consequent more marked depression in front of it, caries is more frequent FlG. 2. — Labial and distal view of a left upper second incisor. upon the lingual surfaces of the upper second than upon those of the upper first incisors. The pulp cavity is, relatively to the whole tooth, perhaps a little larger than that of the first incisor ; in other respects the same description will suffice. The Mandibular (Lower) First Incisors are very much narrower than those of the upper jaw, not more than Fig. 3. — Labial and distal view of a mandibular first incisor. half the width at their cutting edges, which again are much wider than the necks of the teeth. THE TEETH OF MAN. 11 From before backwards they are broad at the neck ; hence the roots are very much flattened from side to side, and rotation is inadmissible in the attempt to extract them. The enamel contour at the neck is similar to that of the upper incisors, but there is no well-marked cingulum. The Mandibular Second Incisors, unlike the upper teeth, are distinctly larger than the first in each of their dimen- sions, but more especially in the length of their roots, which are much flattened, and often present on their sides a median longitudinal depression, sometimes amounting to an actual groove. The proximal angle of the crown is rounded off like that of the upper lateral incisors, though not so markedly. The Canines are in all respects larger teeth than the incisors ; not only are their crowns thicker and stronger, but their roots are very much longer. The crown terminates in a blunt point, which lies in a straight line with the long axis of the root. A feebly-pro- nounced line or ridge runs down the outer surface of the tooth from this point to the neck. The crown slopes away both Fig. 4. — A maxillary canine, showing (A) labial, (B) lingual and (C) proximal surfaces, the basal cusp and the three ridges which con- verge towards it. before and behind the point or cusp, but as that side of the tooth which lies next to the premolar is convex, and, as it were, produced towards that tooth, the slope is longer on the proximal than on the distal half of the crown. The asymmetry of the shape of the crown makes it easy to determine at a glance to which side of the mouth the canine belongs. The internal or lingual surface is not concave like that of the incisors, but is in a slight degree convex, and a median A B C 12 A MANUAL OF DENTAL ANATOMY. ridge extends from the apex of the cusp ; this ridge where it meets with the ridge which borders the lingual surface and corresponds with the cingulum of the incisor teeth is often developed into a well-marked prominence or slight cusp. In transverse section the neck is somewhat triangular, with the angles rounded, the outer or labial being much wider than the lingual aspect. The Mandibular Canines are less pronounced in form than the corresponding upper teeth ; the point is more blunted, the root shorter, the perpendicular labial ridge not traceable, and the want of symmetry between the distal and proximal halves of the crown less marked. The lingual surface has perhaps a greater tendency to concavity. The Premolars (Bicuspids) are eight in number, two on each side of both upper and lower jaws, and they probably correspond to the third and fourth premolars of the typical mammalian dentition, the first and second premolars being generally regarded as unrepresented in man. The Maxillary Premolars. — The crown, when examined on its occluding surface, is roughly quadrilateral, its outer or labial border being, however, larger and thicker than its inner. The teeth are carried round the curve of the alveolar border mainly by means of this difference in size in the external and internal portions of the canines and the two premolars. The crown has two cusps, of which the outer is the larger, stouter and broader. The outer and inner surfaces (labial and lingual) are convex and smooth, with no basal ridges at the edge of the gums. It is, however, not uncommon, especially in the lower races of mankind, to see some indication of a median ridge upon the outer surface, denned by slight grooves upon either side. The inner and outer cusps are not joined by a ridge. Instead of this there is a deep fissure, the ends of which turn somewhat abruptly outwards towards the outer aspect of the tooth. Perhaps the cingulum has been elevated to form the inner cusp, forming slight elevations bordering the anterior and posterior (distal and proximal) edges of the grinding surface. The distal surface is concave. The root is single, and much compressed from side to side. Very frequently, however, it is double for the greater part of its length, and, if not so divided, is then marked by a groove THE TEETH OF MAN. 13 upon each side, indicating a tendency towards such division. The outer border of the root is often also marked by a longi- tudinal furrow, which may amount to complete division. The tendency to division is so constant, both in the lower and higher races, that it may be regarded as almost normal. In fact, a premolar may have three perfectly distinct roots, like a molar, and like the premolars of monkeys, in most of whom three is the normal number of roots. The first premolar is more variable in respect of its roots than the second. It is thought by Topinard that in the passage from a three- rooted to a single-rooted type the postero-external root is lost, the antero-external and internal roots becoming fused. The second upper premolar differs from the first in that the difference in size between its outer and inner cusps is less, the inner cusp being relatively considerably larger, FlG. 5. — Occluding surface of a maxillary premolar. and indeed often preponderating over the outer cusp in length, so that the outer and lingual surfaces are nearly equal. The pulp cavity in the crown is famished with distinct cornua. At the neck it is very much flattened from side to side, being often reduced to a mere fissure, which is, however, considerably larger at its two extremities than in its middle. Hence the pulp cavity of an upper premolar is difficult to fill, a difficulty increased by the impossibility, without the aid of a skiagram, of always discovering what number of roots it has, as their division sometimes takes place in varying situations. The Mandibular Premolars are smaller teeth than those of the upper jaw, and are quite distinct in shape. The outer cusp is bent inwards, and the corresponding surface of the crown is very convex. The inner cusp is very feebly developed, and is connected with the outer by a low ridge ; it is also narrow, so that the tooth is somewhat triangular as looked upon from above. The root is rounded, a little larger on its outer side than on its inner, and tapers regularly towards its point. The pulp 14 A MANUAL OF DENTAL ANATOMY. cavity is cylindrical at the neck, and also tapers regularly to the apex of the root. The cornu of the pulp which corresponds to the inner cusp is feebly developed. The second lower premolar differs a good deal from the first ; its crown is more nearly square in shape and larger in FlG. 6. — A mandibular first premolar, seen from the lingual side, showing the preponderance of its outer over its inner cusp. all its dimensions. The inner cusp reaches to a higher level and is larger, and the greater development of the ridges which bound the anterior and posterior borders of the occluding surface makes it attain to such shapes as to render the tendency towards a transition from the premolar type to that of a true molar very evident. The General Characters of the Series. — The differences between a well-developed incisor, canine, or premolar are so strongly pronounced that the resemblances which underlie them are apt to be overlooked, and it might be supposed that in shape they had little in common. Nevertheless a very distinct gradation may be traced, and it is usual to meet with teeth which possess in a marked degree transitional characters. If the external angle of a second incisor be sloped off more than usual, while at the same time its cingulum and basal prominence be well marked, it makes no bad imitation of a diminutive canine ; and such incisors are often to be met with by any who search for such deviations from the normal form. Thus the form characteristic of a second incisor, if it be a little exaggerated, very nearly approximates to the form of a canine ; indeed, in the teeth of the orang the second incisor is to all intents a diminutive canine ; and in the present discus- sion the great comparative size of the canine, which is traceable to readily intelligible causes, may be put aside, as it tends to obscure the point to be here insisted on. Between the canines and the premolars a similar relation- THE TEETH OF MAN. 15 ship in form exists, and it is more apparent in the lower than in the upper jaw. The fact that at the base of the inner or lingual aspect of the canine may be found an elevation of the cingulum, in many, instances amounting to a low cusp, has been already noted ; and it has been pointed out that the inner cusp of the first lower premolar is both smaller and lower than the other, indeed often can hardly be said to exist. A longitudinal section through the crowns of the two teeth will demonstrate without the necessity of further description that the basal cusp of the canine and the inner cusp of the premolar are the same thing, differing only in degree, while it is inte- resting to note that the pulp chamber in the latter has hardly any prolongation towards the small inner cusp, so that the Fig. 7. — Sections of a mandibular canine and first premolar, showing the characters common to the two. resemblance between the two teeth is thus made more complete. This close relationship of canines and premolars will be considered in the chapter on the Homologies of the Teeth ; for the present purpose it will suffice to merely point out its existence. The transition from the premolars to the molars is more abrupt ; at least, it is not so easy to point out exactly how the cusp pattern of the one would arrive at the form of the other. But it merely needs an exaggeration of the differences existing between a canine and a first premolar to make a good imitation of a second premolar. If any one will take the trouble to make mental note of the deviations in form which he meets with in human teeth, he will find that they almost invariably consist of approaches towards the form of the teeth on either side of them ; and will infallibly be led to the conclusion that incisors, canines, and premolars are not three patterns of teeth perfectly distinct, 16 A MANUAL OF DENTAL ANATOMY. and each sui generis, but that they are modifications of one and the same pattern. The Maxillary Molar teeth have quadrilateral crowns, the angles being much rounded off. It may be premised that the first molar is more constant in shape than the second (it was regular in 99 per cent, of skulls examined), and this latter than the third ; with this proviso the first and second may be described together. The masticating surface carries four subequal cusps, two labial or external and two lingual or internal ; the antero- internal cusp is distinctly the largest, and it is connected with the postero-external cusp by a thick oblique ridge of enamel, the remaining two cusps having no such connection. This oblique ridge on the upper molars is met with in man, the anthropoid apes, and certain new-world monkeys. Fig. 8.— Masticating surface of a first maxillary molar on the left side ; the oblique ridge connects the antero-internal with the postero- external cusp. The grooves which separate the cusps pass down on to the labial and lingual surfaces of the crown, but are lost before reaching the gum. At the place where they terminate, how- ever, there is often a pit, which is a very favourite situation for caries, especially on the outer aspect of the teeth. It is very rare to see the grooves passing down upon the proximal or distal surfaces of the crown, a raised border of enamel generally cutting them short in this direction. When one cusp is suppressed, it is always the postero- internal. The roots are three in number, two external or labial and one internal or palatal. The latter is the largest, and runs in a direction more strongly divergent from the axis of the crown than the other roots. It is directed obliquely inwards towards the roof of the palate, is subcylindrical, and often curved. The external roots are less cylindrical, being mutually com- pressed, so that their largest diameter is transverse to the dental arch ; the anterior is rather the larger of the two and is THE TEETH OF MAN. 17 more strongly pronounced on the side of the neck of the root. The anterior labial root is occasionally confluent with the palatine root, but still more frequently the posterior labial and palatine roots are confluent. Occasionally, also, four distinct roots may be met with. The third molars of the upper jaw resemble in a general way the first and second, that is, when they are well developed and placed in a roomy dental arch. But amongst most civilised races it may almost be said to be exceptional for these teeth to be regular either in form or position, so that extreme variability prevails among them. The two inner tubercles are often blended together, and the roots confluent, forming an abruptly tapering cone, the apex of which is often bent and crooked, so that but little vestige of the three roots can be traced, the pulp cavity even being quite single. The Mandibular Molars.— The first lower molar is the most constant in form, and is somewhat the largest. Its grind- ing surface presents five cusps. Fig. 9. — Occluding surface and side view of a first right mandibular molar, the five cusps of which are indicated by figures. Four cusps are placed regularly at the angles of a square, these being divided from one another by a crucial fissure ; the posterior arm of the crucial fissure bifurcates, and between its diverging arms is the fifth cusp, which may occupy the median line of the tooth, but more often is somewhat to the outer side. It is distinctly visible when viewed from the labial aspect. The transverse fissure passes over the limits of the occlud- ing surface, and on the outer surface of the tooth may end in a pit, which is a common site for caries ; although it occasionally passes over the lingual surface, it is here less pronounced. Lower molars are implanted by two roots, placed anteriorly and posteriorly ; the roots are much flattened from 18 A MANUAL OF DENTAL ANATOMY. before backwards, and they are very usually curved slightly backwards. In the median line of each root there is usually a groove, by the deepening of which four roots may be produced ; or this may happen with one root only, so that a three-rooted tooth is the result. The first lower molar had five cusps in 82 per cent, of all skulls examined. The second molar does not greatly differ from the first, save that the roots are more often confluent, and the fifth cusp less marked, even if it exists at all. It is present in about 24 per cent, of skulls. Fig. 10. — Second lower molar of right side, the four cusps being indicated by figures. The tooth figured is not so square as it should be, its posterior cusps as well as its posterior root being smaller than the normal. The third mandibular molar is seldom so small as the corresponding upper tooth, and its crown is often large even when its roots are very stunted. It has five cusps, as a rule, and bears a more or less close resemblance to the molars which precede it. It is either two-rooted, or if the roots be confluent, a groove usually marks a tendency to a division into %two fangs. Fig. 11. — The third left mandibular molar. It is stated by Professor Owen (3) that, although the third molar is the smallest of the series, the difference is less marked in the Melanesian than in the Caucasian races, adding also that the triple implantation of the upper and the double im- plantation of the lower is constant in the former races. More extended observations have overthrown this statement as a THE TEETH OF MAN. 19 positive dictum to be accepted without exceptions, but it may nevertheless be taken as expressing a general truth. In lower molars the inner, and in the upper the outer, cusps are the longer. The Deciduous "Teeth. — The milk teeth differ from their permanent successors by being smaller and having more bulbous crowns, so that the neck is more strongly constricted. The incisors and canines are somewhat similar to their successors, the canines, however, being relatively shorter and broader than their representatives in the permanent dentition. The first upper molars have three cusps, two external and one internal ; the second more nearly resemble the permanent molars. The second lower deciduous molar has four cusps and resembles a second lower permanent molar. The roots of the deciduous teeth diverge from the neck at greater angles than those of permanent teeth, in consequence of their more or less completely enclosing between them the crypts in which the latter are developing. The Dental Formula of Man. — In viewing the gradational characters which exist between the various human teeth, it must not be forgotten that some links in the chain have dropped out. The typical number of teeth in existing mammals may be regarded as 44, i.e., • 3 1 4 3 13Clpm4-m-3 =44' whereas man has only 2 1 2 3 i-ciPm-m-= 32. It has been usual to suppose that the teeth which are missing in man are the third incisor on each side and the first and second premolars, but reasons have been advanced which throw doubt upon this conclusion as regards the incisors (2, 6). From the study of cases of cleft palate it has been found that it is not at all certain that the cleft usually runs along the site of the suture between the premaxillary bone and the maxilla ; for it often appears to be well within the limits of the former, and so lends some support to the idea of Albrecht (*) 2—2 20 A MANUAL OF DENTAL ANATOMY. that there are two intermaxillary bones on each side,* and that the cleft runs along the division between them. And it is far from uncommon for a tooth of incisor type to lie beyond the cleft and close against the front of the canine. To this tooth Sir W. Turner (5), pending decision as to its homologies, gives the name of " precanine." The argument put briefly is this : a tooth outside the cleft and close to the canine is of so common occurrence that its position there must be due to something other than accident when the normal number of incisors is not exceeded. But there is a case on record in which this precanine existed although there were four incisors upon the intermaxillary portion, which was isolated by a double cleft ; in this specimen, therefore, the precanine was evidently the third incisor of the normal mammalian dentition, and it is therefore not unlikely that it is always so ; if this inference be correct, then the lost incisor in man is probably the second. The Arrangement of the Teeth in the Dental Arches. — The teeth of the upper jaw are ranged along a curve of larger dimensions than those of the lower, the incisors passing in front of the corresponding lower teeth, and the external cusps of the premolars and molars closing outside those of the lower teeth. There are, however, some points to be noted in the relation borne by the upper to the lower teeth, by which it is brought about that each tooth is antagonised by portions of two teeth in the other jaw, and has therefore not merely a single opponent. The upper incisors and canines when the mouth is closed, from the larger size of the arch in which they are arranged, shut over and in front of the lower teeth, concealing the upper thirds of their crowns ; while the external tubercles, or cusps, of the premolars and molars of the lower jaw are received into the depressions between the external and internal tubercles of the similar teeth in the upper jaw, thus allowing the external tubercles of the upper teeth to close externally to the outer tubercles of the lower row. Beyond the incisors, each lower tooth closes somewhat in front of its corresponding upper tooth. * Comparative anatomy, however, lends no support to this view. THE TEETH OF MAN. 21 In consequence of this arrangement of the tubercles, the whole surfaces of the crowns of the opposing teeth are used during mastication, this act being performed by bringing the external tubercles of the lower molars opposite to those of the upper row, whence, by the lateral motion of the mandible inwards, their external tubercles pass down the inclined sur- faces of the external and up those of the internal tubercles of the upper teeth, crushing in this action any interposed substance. It will also be observed that, from the difference of width in the incisors of-; the two jaws, the first incisors of the upper extend over the first and half of the seconds of the mandibular series, and that the superior seconds lie over the remaining half of the inferior seconds and the anterior half of the canines of the lower jaw. The upper canines close over the halves o£ the lower canines and first premolars, while the first premolars impinge on the half of the first and half of the second premolars of the lower row. The second upper pre- molars close upon the anterior third of the opposing lower first molars and the posterior halves of the second premolars. The upper first molars occlude with the posterior two-thirds of the first and one-third of the second molars of the lower jaw, while the second upper molars close upon the unoccupied posterior third of the second and the anterior third of the third molars. The maxillary third molar being smaller in size than that of the lower jaw, is perfectly opposed by that portion of the latter left unoccupied by the second mandibular molar. By this admirable arrangement no two teeth oppose each other only, but each tooth in closure of the jaw impinges upon two, so that should a tooth be lost, or even two alternate teeth, still the corresponding teeth of the opposite jaw are to some extent opposed, and thus remain useful. For when a tooth is wholly unopposed a process is apt to be set up in the jaw by which the useless organ is gradually ejected. The direction of the teeth in the upper jaw is vertically downwards and slightly forwards, while those of the mandible are placed vertically, the molars tending slightly inwards. It should be borne in mind that the description of the human teeth here given is that which may be regarded as typical for the European races of to-day. The teeth of other human races as well as of those of earlier and of prehistoric 22 A MANUAL OF DENTAL ANATOMY. times exhibit certain points of difference. These will, how- ever, be considered in a later chapter in which the dentitions of the Primates are dealt with from the comparative point of view. THE FACIAL AND MAXILLARY BONES. Without dwelling upon details which are to be found in every anatomical text-book, there are some considerations of especial interest to the odontologist which are often passed with little notice. Thus the close interdependence between the dentition and the general appearance of the face, and, indeed, even the shape of the skull as a secondary result, is apt to be forgotten. Thus, to take a lion as an example, the snout is broad, owing to the wide separation between the canines, which gives these teeth a good purchase in grasping a living prey. Its shortness enables them to be used at a greater mechanical advantage than would be the case were they further removed from their fulcrum at the joint, and the breadth of the face below the eyes is conferred by the widely spreading zygomatic arches, which are obliged to be wide to give passage to the very powerful temporal, and attachment to the masseter muscles. The size and strength of the muscles are correlated with a lateral compression of the cranial portion of the skull, together with the development of bony ridges to form additional surface for the origin and attachment of muscles acting upon the mandible, thus largely influencing the contour of the face. Such a skull is in marked contrast with the long thin snouts of the Insectivora, whose forceps-like front teeth as a rule merely pick up unresisting prey ; or with the long weak snouts of the horse and the Ungulata generally. The face of the boar, again, is largely determined by the great muscles which move the jaw, and by the bony processes which give attachment to them. The adult anthropoid ape has a shorter face, associated with a dentition with closely-packed teeth, not very divergent in shape from the human type, but he remains prognathous, and with a face that is disproportionately large as compared with his cranium, because he has teeth that are relatively THE FACIAL AND MAXILLARY BONES. 23 larger and more powerful than those of man, and he has a diastema between them to admit his great canines. In the young ape, however, with a less powerful dentition, the contrast with man is much less marked. Or, if the human face be taken as an example, and the jaws be extended forward a little, the teeth projected and the mouth widened, a coarse animal type of face is copied ; and, conversely, by a reduction of the maxillary region, perhaps even below the limits which will afford space for the regular disposition of the teeth, a refined oval type of face is formed, such as is represented in many of the ancient Greek sculptures. The jaws of a negro are large relatively to the cranium, as are also those of exceptionally big men, though this is not universally true. In rickets the jaws are disproportionately small. The motions executed in mastication, as well as the crown- patterns of the individual teeth, differ much according to the nature of the food. Hence it happens that in different animals the muscles of mastication and the type of tooth-pattern are very variously developed. Thus in the Herbivora there is great lateral movement of the jaw, as anyone may observe, while the pterygoid muscles, and especially the external pterygoid, attain to a very large relative size. On the other hand in the rodents, which move their jaws backward and forward in gnawing, the masseter muscle is enormously developed, and has a very marked general back- ward direction. Although it is not strictly true, the masseter and temporal muscles may be said to be developed in mammals in an inverse ratio to one another : when one is large the other is small. But though the former is at a maximum in the Carnivora, in which little lateral movement of the jaws is possible, the latter is highly developed in many of the class. In the great apes, the temporal muscle becomes enormously developed only at the period of second dentition ; this fact, conjoined with its size, which in Ungulates seems to have some relation to the presence or absence of canines, would incline one to suppose that it is useful in that rapid closure of the mouth appropriate to the seizure of prey or to fighting. It may be noticed that, whilst the powerful (masseter, 24 A MANUAL OF DENTAL ANATOMY. temporal, and internal pterygoid) muscles close the jaws, only slight and weak ones (digastric, mylohyoid, and geniohyoid, aided perhaps by the platysma) open them, and the weight of the mandible is enough per se to open the mouth. The position of repose is neither complete closure nor opening of the jaws ; in persons with nasal obstruction the habitual position is one with the mouth somewhat more widely open, owing to the difficulty of breathing through the nose, a fact which may cause an irregularity in the disposition of the teeth. Just as the muscles of mastication vary in their relative development in accordance with the food to be dealt with, so also do the salivary glands. As a rule herbivorous creatures have large parotid glands ; that is to say, those creatures which deal with the driest food and masticate it the most, have this gland largely developed. For instance, it is very large in the ox and other ruminants ; in herbivorous marsupials it is larger, and in the carnivorous section smaller, than the submaxillary gland. When an especially viscid fluid is required, as, for example, that which lubricates the tongue of an ant-eater, it is furnished by exceedingly large submaxillary glands. The form of the glenoid cavity also bears an intimate relation to the type of dentition of the animal and the nature and extent of the movement of its jaws. Thus in a child it is nearly flat, with no well-marked sur- rounding elevations ; its axis is transverse, and little rotary motion is made use of. In the adult it is deeply sunk ; the axis of the condyle is oblique, and rotary movements are largely made use of in triturating food. In the Felidce, it is strictly transverse ; their teeth, adapted for slicing but not grinding, would gain nothing by lateral motion, which is rendered quite impossible by the manner in which the long transverse condyles are locked into the glenoid cavity by strong processes in front and behind. Curiously enough, the interarticular cartilage is present, but as the condyle never moves forward, the cartilage is not attached to the external pterygoid muscle. In the Ungulata the condyle is rounded, the ascending ramus long, the pterygoid muscles large, and the glenoid cavity shallow. In the whale, which of course does not THE FACIAL AND MAXILLARY BONES. 25 masticate at all, there is no interarticular cartilage and no synovial membrane; the articulation is reduced to a mere ligamentous attachment. In rodents there is no postglenoid process, so that the condyles have a large play backwards, useful for employing the teeth in gnawing. The articulation of the human lower jaw is peculiar, and allows of a degree of play quite unusual in a joint. The ovoid condyles when the jaw is at rest are lodged in depressions, the glenoid fosses of the temporal bones, formed partly by the squamous and partly by the vaginal portions of the bone. In the anterior-posterior direction the fossa is much larger than the condyle which rests upon its anterior portion ; the posterior half of the cavity is rough, and lodges a portion of the parotid gland ; the anterior is smooth, and is bounded in front by the eminentia articularis, which >is the middle root of the zygoma, enters into the formation of the joint, and is coated over by cartilage even beyond its summit. Between the condyle of the lower jaw and the temporal bone lies a movable interarticular fibro -cartilage, which is an irregular bi-concave oval plate, the edges of which are united with the capsular ligament, so that the joint is divided into two cavities, furnished with separate synovial membranes, unless when, as sometimes is the case, the flbro- cartilage is perforated in its centre. It is thickened at its posterior border, which lies in the hollow of the glenoid fossa. Above it the bone is thin, the dura mater being close at hand, and it may be useful in deadening shocks. The startling effect of a " knock -rout " blow is due to shock transmitted through the condyle. The lower surface of the cartilage is concave, and, as it moves forward with the condyle in its excursions, it provides the latter with a concave seating in which to revolve, in all positions of the mandible, for it is more closely attached to the condyle than to the skull. An interarticular fibro-cartilage is constant in, and peculiar to the Mammalia. The exact nature of the movement of the mandible in opening and closing the mouth was studied by John Hunter, and more recently by other observers, especially with regard to the results produced by the laxity of the articulation and 26 A MANUAL OF DENTAL ANATOMY. the consequent latitude of movement allowed to the condyle. In the Felicia the glenoid cavity is so complete and fits with such accuracy the form of the condyle (an interarticular cartilage being, however, interposed), that only a pure hinge motion is possible, and consequently any part of the mandible Fig. 12. — Condyle of the lower jaw A and glenoid fossa B of a tiger, must, in the opening of the mouth, describe an arc of a single circle having for its centre the condyle. In any animal, however, in which the condyle is free to move forwards, this will only be true so long as the condyle remains stationary, in other words, so long as the centre round which the arc is described remains in the same place. It was formerly supposed that during the opening of the moutli to a small or moderate extent the condyle did, in man, remain in the same position, and that it was only when the opening was somewhat extreme that it passed forward and on to the eminentia articularis. It has, however, now been demon- strated, first by Dr. Luce and subsequently, by other methods, by Mr. Dolamore and the author (4), that this is not the case, but that the condyle commences to travel forwards from the first. Its motion of translation is 'not equal for equal increments in the opening of the mouth, being at first less, then increasing in the middle of its course, and finally, as the condyle has mounted the eminence, again diminishing. It becomes then obvious that the path described by a point in the mandible, say by the mandibular incisors, cannot be a portion of any one circle, but would be more nearly THE FACIAL AND MAXILLAKY BONES. 27 represented by successive short segments, each described from a more advanced centre. In man, however, the sliding motion of the condyle is not, as in some creatures, simply forwards : its track ^ is determined by the form of the surface of the glenoid cavity, so that, as it slides along, it travels along a curve the concavity of which looks forwards and upwards. The resultant then, the path described by the mandible, is compounded of two circular motions, just like the path of a geometric cutter or a geometric chuck; in other words, the rotation of the mandible upon its condyle as a centre is complicated in its ultimate results by this centre, the condyle itself, all the time travelling along a short curved path, namely the under-surface of the glenoid fossa and the eminentia articularis. But this last-mentioned path, though considerably curved, is a short one, and so the resultant motion of the mandible does not depart from the arc of a circle to any very wide extent. It is obvious that the shape of the glenoid fossa and of the eminence will affect the result, and without entering into details, for which the reader is referred to the papers themselves, it may be briefly stated that the effect of this combination of movements is to carry the mandible further forwards than it would other- wise go, and to approximate its motion more nearly to a straight line ; — that is to say, to make it travel almost upon the arc of a circle larger than that which would be traced by its rotation round its own condyle. It may be that this more forward path is an advantage to an animal which carries its head more or less at right angles to its neck ; an animal, such as a lion or a dog, which carries its head horizontally, has plenty of room to open the mouth widely without touching its neck, whereas in man or in a higher ape the wide opening of the mandible, if it were not at the same time carried forward, would make pressure on the anterior structures of the neck. Be that as it may, it will be obvious that no simple circular motion round any fixed centre can truly represent the path of the mandible ; nevertheless the actual path does not greatly diverge from the arc of a circle described round a point generally an inch or an inch and a half below the condyle, and somewhat behind it. Thus the old statement that the axis of rotation lay upon the plane 28 A MANUAL OF DENTAL ANATOMY. of the masticating surfaces of the teeth prolonged backwards is not accurate, but is not very far from the truth. A curious point which came out in the progress of the inquiry is that the track pursued in opening the mouth, as shown by tracings, is not identical with that pursued in closure, the latter generally lying in front of the former and only becoming coincident at the last moment, just before the teeth meet. When the mouth is forcibly closed the lower teeth only are moved ; the upper teeth have to be rigidly fixed, while the Fig. 13. — Superior maxillary bone of the right side. 1. Body. 2. Tuberosity. 3. Incisors. 4. Canines. 5. Premolars. 6. Molars. 7. Malar process. 8. Nasal process. 9. Alveolar process. teeth of the lower jaw are brought forcibly against them with more or less of shock. And whilst these blows have to be received, resisted, and ultimately borne by the cranium, it is obviously desirable that they should be distributed over a sufficiently wide area, so as not to be felt unpleasantly. The Maxillary Bone. — The ascending nasal process is very stout, and serves to connect the maxilla strongly with the frontal bone, which also in the region in question is powerfully developed ; the thick malar process gives rigidity and resistance to lateral movements of the jaws, and carries off the strains to the lateral walls of the cranium ; the maxilla is buttressed at the back by the pterygoid processes. Thus, though the maxilla is hollowed out, so as to be lightened by the antrum and the nasal fossa, it forms a firm and resistant base against which to exercise force, THE FACIAL AND MAXILLARY BONES. 29 The alveolus of each individual tooth consists of a shell of comparatively compact bone of minute thickness, the lamina dura, which is imbedded in a mass of loose cancellous bone. This dense shell comes into relation with the dense cortical bone of the jaw mainly at its free margin, near to the neck of the tooth. Over very prominent roots, such as those of maxillary canines, a portion of alveolus is at times wanting, so that in a macerated skull the root is exposed to view. This difference between the bone constituting the immediate alveoli and that constituting the shell of the jaw-bones is very strongly marked in some mammals, notably in the manatee, in which there appears to be a continual migration of teeth forward taking place during the animal's life (see p. 561). Bibliography. (x) Albrecht, P. "Die morphologische Bedentung der seitlichen Kieferspalte und die wahrscheinliche Existenz von vier zweichenkiefern bei den Saugethieren." Zoolog. Anzeiger, 1879, Bd, ii., pp. 207—213. (2) Edwards, H. H. ' 4 The Missing Incisors in Man. Which are they ? " Brit. Dental Assoc. Journ., Dec. 18, 1885, vol. vi., pp. 725 — 735. (3) Owen, B. Odontography. Text and Atlas. London. 1840-45. (4) Tomes, C. S. and Dolamore, W. H. " Some Observations on the Motions of the Mandible." Trans. Odontol. Soc. of Gt. Brit., April 22, 1901, vol. ,xxxiii., pp. 167—192. (5) Turner, Sir W. "The Belation of the Alveolar Form of the Cleft Palate to the Incisor Teeth and the Inter- maxillary Bones.'" Journ. Anat. and Phys., 1885, vol. xix., pp. 198—213. (6) Wilson, A. " The Missing Incisors in Man. Which are they ? " Brit. Dental Assoc. Journ., 1835, April 15, vol. vi., pp. 241—243. CHAPTEE II. THE DENTAL TISSUES. — ENAMEL. Generally speaking, calcined teeth are composed of one or more structures which are in great measure peculiar \to the teeth. Notwithstanding the existence of certain transitional forms, it is not possible to doubt the propriety of a general division of dental tissues into three, viz., Dentine, Enamel, and Cementum. The first named of these constitutes the greater part of all teeth, and so far predominates in mass over the other constituents that, in very many cases, the tooth would nearly retain its form and character after the removal of the enamel and cementum. This central body of dentine, enclosing the pulp, is very often covered by a cap of enamel, which forms the exposed surface of the tooth ; this may be very partial, as in the eel or the newt, in which animals only this enamel-capped tip of the tooth projects far above the surface of the mucous membrane; or it may cover a much larger proportion of the tooth, as in man. Perhaps the most usual condition is that the enamel invests the whole crown of the tooth, terminating at about the level to which the gum reaches, as in the human and most other mammalian teeth of limited growth. In teeth of persistent growth the enamel extends down into the socket as far as the base of the tooth. In such cases it may embrace the whole circumference of the dentine, as in the molar teeth of many rodents, or it may be confined to one side only, as in their incisor teeth, where by its great hardness, it serves to constantly preserve a sharp edge as the tooth is worn away. The enamel is believed to be quite absent from many teeth ; thus the order Edentata, comprising sloths, armadillos, and ant-eaters, have it not ; the narwal, certain cetaceans, some reptiles, and many fish also have none. But although it might appear an exceedingly simple matter to determine whether a tooth is or is not coated with enamel, as a matter of fact in practice it is not always easy to be certain upon this point. When the enamel is tolerably thick there is no difficulty in making sections which show it THE DENTAL TISSUES — ENAMEL. 31 satisfactorily, bat when it is very thin it is apt to break off in grinding down the section. And even when it does not, it is in such cases usually quite transparent and structureless, and the outermost layer m of the dentine being also clear and structureless, it is very hard to decide whether the appearance of a double boundary line is a mere optical effect due to the thickness of the section, or is indicative of a thin layer of a distinct tissue, which might be either enamel or cementum, for very thin layers of cementum look very much like enamel. The author's investigations upon the development of the teeth of fish and reptiles have led him to suspect that rudi- mentary layers of enamel may exist upon many teeth on which their presence has not been recognised, for he has found that the formative enamel-organs universally occur, at least they exist upon all tooth-germs which have been adequately examined. Upon the teeth of snakes, which were stated by Professor Owen to be composed only of dentine and cement, he has endeavoured to show that a thin layer of enamel exists, and that there is no cementum. The frog has an enamel-organ as distinct as that of the snake, but it is not certain that there is enamel upon its teeth, although there is an appearance of a thin coat of distinct tissue. The armadillo has an enamel-organ, but observations have failed to discover any enamel or anything like it upon the adult teeth, and Professor Sir Wm. Turner has made a similar observation upon the narwal. At all events it may be said that in these and many other creatures no functional development of enamel takes place : whether it does or does not exist in an extremely thin and rudimentary layer has become a question of much less significance, since the author has shown the presence of an enamel-organ to be universal at an early stage. Hence one may hesitate to fully endorse Sir Eichard Owen's generalisation that dentine is the most and enamel the least constant of dental tissues ; it is to be remembered that it may be so in completed teeth, but recent researches into dental development have very materially modified the concep- tions formed as to the relations of the tissues to one another, and must lead one to examine carefully into such deductive statements before accepting them. The remaining dental tissue is cementum, which clothes, in a layer of appreciable thickness, the roots of mammalian teeth, 32 A MANUAL OF DENTAL ANATOMY. and reaches up as far as the enamel, the edge of which it may overlap to a slight extent, or entirely cover, or even not extend up to, so that when the cementum is present upon the crown, it occupies a position external to that of the enamel. Cementum occurs universally upon the teeth of mammalia, but it is not always confined to the root of the tooth. In many organs of persistent growth it originally invested the whole crown, and after it has been worn from the exposed grinding surface, continues to invest the sides of the tooth. (See the description of the complex teeth of the elephant, ox, horse, etc.) It is probably entirely absent from the teeth of snakes, and indeed of very many reptiles ; in the reptilian class, at all events, it would appear to be confined to those in which the teeth are lodged either in sockets or in a deep bony groove, as the author is unacquainted with any tooth anchylosed to the jaw in which it exists, unless one is inclined to include under the term cementum the tissue which has been designated " bone of attachment." (See " Implantation of Teeth.") Chemical Constituents. — Upon the outer surface of the dentine the enamel forms a cap of a very much harder and denser material. In its most perfect form it is by far the hardest of all tissues met with in the animal body, and at the same time the poorest in organic matter. It has been usually stated that in the enamel of a human adult tooth there is from 3*5 to 5 per cent, of organic matter, but this is, as will presently appear, an error, there being probably only about 1 per cent. The lime salts consist of a large quantity of calcium phos- phate, some carbonate, and a trace of fluoride; in addition, there is a little magnesium phosphate. Von Bibra (x) gives two analyses of enamel : ADULT ADULT MAN. WOMAN. Calcium Phosphate and Fluoride Calcium Carbonate Magnesium Phosphate Other Salts 89-82 4-37 1-34 •88 3-39 •20 81.63 8*88 2*55 •97 5-97 a trace Cartilage . Fat Organic Inorganic 3-59 96-41 5*97 94-03 THE DENTAL TISSUES — ENAMEL. 33 Hoppe-Seyler (7), who believes that the salt mainly present in bone, dentine, and enamel is a compound salt, in which three molecules of calcium phosphate are combined with one of calcium carbonate, Cai0 C03 (P04)6 (just as in the mineral apatite two molecules of fluorine are combined with three of calcium phosphate), gives as his analysis : Ca10 C03 (P04)6 . . ... 95-35 Mg. HPO* 1-05 Organic substance . . . . . 3 '60 In 1899 F. Bertz published an Inaugural Dissertation (Wiirtzburg) on the chemical composition of the teeth. An abstract of this paper is given in the " Jahresbericht liber die Fortschritte der Thier-chemie," vol. xxx., 1900, p. 457. Bertz gives the following table for the composition of the teeth of man and of the calf : Menschenzahne. Kalberzahne. Dentin. Schmelz. Schmelz. Organic substance . 29-15 6-822 16-56 CaO . . 38'18 50-224 44-243 MgO . . 1-508 0-732 0-955 P205 . . 30-244 40-693 37-019 S03 . 0-378 0-296 0-316 Fl . 0-471 1-089 0-771 99-931 99-856 99-864 The most recent analysis of dried enamel has been under- taken by Dr. Lovatt Evans (Proc. Int. Med. Congress, 1913, Section XVII.), who finds in 3'659 grm. of human enamel 39*56 c.c. of gas consisting of carbon dioxide 30'21 c.c, and nitrogen 9*35 c.c, thus giving a percentage of 0'55 carbon and 0*4 nitrogen, and in elephant enamel a percentage of 0*97 carbon and 0'57 nitrogen. His results therefore show that the organic matter in dried human enamel probably amounts to between 1 and 2 per cent., and in elephant enamel over 2 per cent. A large number of other analyses have been from time to time published, in which the proportion of organic matter ranges from two to seven or eight per cent. The author undertook a series of experiments with the object of ascertaining what the nature of this organic matter d.a. 3 34 A MANUAL OF DENTAL ANATOMY. was (12), but failed to discover, in any notable quantity, any substance giving proteid or proteoid reactions. It is expressly stated in describing some of the published analyses that the organic matter was estimated simply by the loss on ignition, and this appears to have been the case in all, for in none of them is water set down as a constituent of dried enamel. Hoppe-Seyler's statement that infantile enamel contains more organic matter, and otherwise differs from adult enamel, has been cited as an argument that enamel is not outside the pale of nutrition. But this argument is inapplicable, for his observations do not relate to the complete enamel of erupted teeth. He used the teeth of a newly-born infant, whose teeth were not erupted and whose enamel was far from being com- plete. It is difficult to see how it would be possible to obtain such enamel in adequate amount for certain analysis, and to be at the same time sure that it was quite free from adherent dentine and enamel-organ. Hence his analysis of infantile enamel is omitted in detail, as it appears so open to fallacy. But the author's original analyses showed that, in the case of elephant's enamel, which was selected because, on account of its thickness, it was comparatively easy to get tolerably free from adherent fragments of dentine or cementum, there was as much as four per cent, of water left after the enamel had been long dried at a temperature of 300° F. This water, which was apparently held in somewhat close chemical combination, was, under ignition, given off suddenly, and with such violence as to cause the fragments of enamel to crepitate and fly to pieces, and it was possible to collect it in a chloride of calcium tube and to weigh it, so as to prove that the total loss on ignition was very nearly accounted for by the water given off. It would be out of place to repeat here in detail all the experiments, as they are to be found in the paper referred to (12) ; it will suffice to say that probably there is no organic matter at all (in any noteworthy amount) in enamel, and that that which has heretofore been set down as organic matter, because it was lost on ignition, is simply water combined with the lime salts. When fragments of enamel are ignited in a hard glass tube, it is seen that the surfaces which had lain in contact with dentine or cementum become blackened, but that fractured THE DENTAL TISSUES — ENAMEL. 35 surfaces in the midst of the enamel do not blacken at all but remain snowy white ; it is thus shown that the very minute traces of organic matter found are mainly due to the practical impossibility of getting enamel quite free from adherent dentine or cementum, a conclusion which is confirmed by the microscopic examination of the trifling residue left when such enamel is dissolved in acids. As a check experiment a solution of dissolved dentine matrix was prepared, and this was added to a solution of enamel salts so as to introduce one quarter per cent, of organic matter ; this gave abundant proteid reactions. It has long been a familiar fact that enamel treated with acid disappears, whereas the organic matter in the prismatic layer of the shell of the mollusc Pinna, which bears a superficial resemblance to enamel, retains its structure and is actually more bulky than before decalcification, although it only amounts to 1*3 per cent, of the weight of the shell employed. | Enamel is then to be regarded as an inorganic substance composed of lime salts which have been formed and deposited in particular patterns under the influence of organic tissues, which have themselves disappeared during its formation. A very minute trace of organic matter is sometimes to be extracted by maceration of powdered enamel in caustic potash or baryta water, the subsequent addition of acetic acid pro- ducing a cloudiness ; the quantity obtainable is far too small for identification, but the reaction above mentioned would indicate that it was either mucin or nucleo-albumin ; it might have been derived from the fluids of the mouth, but in any case is in too small amount to be of much significance. The rods of enamel, obtained by scraping or crushing, hardly undergo any alteration in form or appearance by ignition, except that they become a little more granular ; but this slight amount of alteration seems to go to confirm what has already been said as to the absence of organic matter, for the expulsion of the combined water might easily cause a slight difference in the appearance of the crystalline salts. The water is probably combined with the calcium phos- phate ; tribasic calcium phosphate when prepared by any wet method retains, in combination, one or more equivalents of water, which it will not part with below red heat, and the author's analyses show the presence of water in about (though 3—2 36 A MANUAL OF DENTAL ANATOMY. not absolutely exactly) the proportion of one equivalent. This slight discrepancy, there being a little too much water, may be due to enamel being from its physical structure very difficult to dry absolutely ; or perhaps some of the calcium phosphate retains two, and the rest only one equivalent of water. Distribution. — Enamel is of very common occurrence in the teeth of fish, reptiles and mammals, and is also to be found upon many dermal scales in fish ; this is only to be expected, as the occurrence of an enamel-organ in early development is absolutely universal. But it varies in amount, forming sometimes a complete coating, sometimes a very partial investment, sometimes a mere spear point to give sharpness to the tooth, and sometimes it appears to be wholly absent. In teeth of limited growth it terminates at the neck of the tooth, where it overlaps or is slightly overlapped by or fails to reach the cementum. Of these variations of termination it would appear that, as a rule, the two tissues actually meet one another. Choquet has found that in 27'5 per cent, of specimens he examined there was a breach of continuity, a minute surface of dentine being exposed. Where there is a complete coat of cementum over the crown this always lies outside the enamel, which is then between the cementum and the dentine. The cap of enamel on a human tooth is of varying thick- ness, being thicker in the neighbourhood of the cusps than elsewhere. Its outer surface is finely striated, the stria3 being transverse to the long axis of the crown. They are called " imbrication lines." In addition to this fine striation there may be a few deeper and more pronounced grooves, or pits, which are pathological and are marks of checks in development more or less complete. The enamel of some animals is almost structureless. Such is the nature of the little caps which, like spear points, surmount the teeth of some fishes, and which from their extreme brittleness are often lost in preparing [My friend Mr. Gowland Hopkins, F.B.S., tells me that the literature on the chemistry of the teeth is fairly fully given in the " Handbuch d. Biochemie/' by Hans Aron (1909), but I have not been able to obtain a copy of this work.— H. W. M. T.] THE DENTAL TISSUES — ENAMEL. 37 sections, so that their very existence has long been overlooked. Some of these enamel tips, however, present an appearance somewhat similar to the enamel of Sargus (p. 59). But the absence of structure is after all a mere question of degree : in the commonest form of enamel, such as that of the human teeth, there is a finely prismatic structure, very apparent in imperfect teeth but far less so in well-formed ones, so that even when constituent rods cannot be distinguished this is merely an indication that calcification has progressed a little farther than in human teeth : if calcification only goes far enough, all structure, if not destroyed, might at all events be masked from sight. Histology of normal Enamel. — The structure of human enamel has been called fibrous ; that is to say, it has a cleavage in a definite direction, and is capable of being broken up into columns or rods, which seem in trans- verse section to approximate more or less closely to pentagonal or hexagonal forms brought about by their mutual apposition. The rods run from the dentine towards the free surface ; this is, however, subject to many minor modifications. The curved and decussating course of the human enamel rods renders it difficult to trace them throughout their length, but the structure of the enamel of many lower animals (especially the rodents) is more easily intelligible. Enamel such as that of the manatee, in which all the rods pursue a perfectly straight course, is of comparatively rare occurrence, but among the rodents the courses pursued by them are simple, and pro- duce very regular patterns, which are constant for particular families. Thus in the Sciuridce, a section of this tissue, whether longitudinal or transverse, appears divided into an outer and inner portion, in which the individual rods, although con- tinuous from the dentine to the free surface, pursue different directions. As seen in longitudinal section, the enamel rods start from the dentine at right angles to its surface, and after passing through about two-thirds of the thickness of the enamel in this direction, abruptly bend upwards, forming an angle of 45° with their original course. In the inner two-thirds as seen in transverse section, they are found to be arranged in horizontal layers, each layer being a single rod in thickness ; in successive layers they pass to the right and to the left, 38 A MANUAL OF DENTAL ANATOMY. crossing those of the next layer at right angles, and thus making a pattern of squares in the inner two-thirds of the enamel. But in the outer third, where the rods bend abruptly upwards, those of superimposed layers no longer pass in opposite directions, but are all parallel ; in fact no longer Fig. 14. — Transverse section of dentine and enamel of a beaver ; in the inner half the rods of contiguous layers cross each other at right angles, in the outer they are parallel. Owing to the peculiarity of the direction in which they run in this animal, this figure has for clearness been rendered somewhat diagrammatic. It is obvious that a transverse section, in order to show the decussation of superimposed layers clearly, must be parallel with the layers, i.e., at an angle of 60° with the long axis of the tooth. But this obliquity will not be right for the outer portion in which the rods are yet more inclined upwards. Hence in the figure this has been corrected, and the simpler outer layer drawn as though the plane of the section were really parallel to the fibres in both layers. The reader is requested to ignore the parallel lines in the upper portion of the figure, which are due to an unsuccessful attempt to represent appearances actually seen in a single section. admit of distinction into alternate laminae. Thus each layer of enamel rods passes in a very definite direction, and, seen with those of other layers, forms a very characteristic pattern ; but the enamel rods are not in any part of their course curved. In the beaver the course of the rods is not quite so simple. Examining a longitudinal section first, instead of the rods starting off at right angles to the surface of the dentine, they THE DENTAL TISSUES — ENAMEL. 39 at first are inclined upwards towards the apex of the tooth at an angle of 60° with the dentinal surface ; then, after passing through about half the thickness of the enamel, they turn up abruptly again, so that they here make an angle a little less than 30° with the dentine. It follows from this that no one transverse section can show very plainly the direction of the rods in both parts of their course. The most instructive transverse section is one cut parallel with the layers near to the dentine ; this will plainly show the successive layers passing to the right and to the left as in the squirrel, but the yet more inclined fibres of the outer half of the enamel will be cut across obliquely ; for this reason the figure here given may be regarded as so far diagrammatic that the effect of this obliquity of cutting has been partly corrected, and the figure compounded from two sections, in each of which one set of rods was parallel with the plane of the section. It is still further complicated by the fact that the outer layers are not only inclined upwards but also sideways towards the middle line of the head : thus the middle line of the head would be to the right of the section as represented. As regards the decussation of the rods of alternate layers, it is similar to that of the Sciuridce, but it differs in the laminae being slightly flexuous instead of pursuing perfectly straight lines. In the porcupine family very much more complex patterns are met with, the enamel rods being individually flexuous, and their curves not confined to one plane ; the individual rods pursue a serpentine course, and cannot be followed far in any one section. Near to the surface, however, they all become parallel, the enamel thus conforming with that of other rodents in being divided into two portions (at least so far as the course pursued and the pattern traced by its rods in its inner and outer parts can be said to so divide it). The Leporidce form an exception ; their enamel has no such lamelliform arrange- ment, but is built up merely of slightly flexuous columns. By tracing the courses of enamel rods from the simple pattern found in the manatee through that of the squirrel, beaver, and porcupine, it is seen how a very definite arrange- ment, at first simple, becomes modified into something a little more complex, till at last it reaches a degree of complexity that looks like mere disorder. No one unfamiliar 40 A MANUAL OF DENTAL ANATOMY. with the enamel of other rodents, looking at the enamel of the porcupine, would be able to unravel the very indefinite-looking, structural chaos before him ; but had he studied forms in some degree transitional he could not doubt that the tortuous, curving course which he saw the rod to be pursuing was nevertheless perfectly definite and precise, and formed part of a regular pattern. It is very usual for the individual rods, like dentinal tubes, to pursue a spiral course. In well-formed human enamel the rods are not very strongly marked, it being only in imperfect enamel that they are pronounced, and it is stated by Otto Walkhoff (15) that in different groups of rodents there are gradations between sharply defined rods and an almost homogeneous structure. Speaking generally, the rods are well defined in primates, carnivora and cetacea, whilst in marsupials they are seen with difficulty prior to the application of acid. In human enamel on the whole the rods are parallel, and run from the surface of the dentine continuously to that of the enamel. Their paths are not, however, either perfectly straight or perfectly parallel, for alternate layers appear to be inclined in opposite directions, while they are also wavy, forming several curves in their length. The curvature of the enamel rods is most marked upon the masticating surface ; while on the sides, the layers, alternating in the direction of their inclination as just described, are in planes transverse to the long axis of the crown, and correspond to the fine imbrication lines on the surface of the enamel, which appear to be caused by their outcrop. The curvatures take place in more than one plane ; in other words, the course of the individual rod is more or less a spiral. Although most rods run through the whole thickness of the enamel, yet inasmuch as the area of the outer is much larger than that of the inner surface of the enamel, and the individual rods do not undergo much alteration in size as they pass outwards, many supplemental rods are present in the outer portions which do not penetrate far inwards. The actual structure of enamel, which might have been thought to be a sufficiently simple matter of observation, is even yet not settled. It is an example of the difficulties which beset any observation of structures which have a high refractive index, being difficult to reduce to extreme THE DENTAL TISSUES — ENAMEL. 41 thinness. Deceptive appearances may arise from the thickness of the section, from diffraction lines and other effects, from other than critical illumination, and from the smallest alteration of focus, so that with high powers there is always left a doubt as to the proper interpretation of that which appears to be seen. The rods of human enamel are about 5/x in diameter, though in places they may exceed this a little. The individual rods are, under a low power, to all appearances Fig. 15. — Striatum of human enamel (from a photograph by Mr. Leon Williams). The specimen was ground fresh, mounted in balsam, and not treated with acid. structureless in perfectly formed human enamel, but a faint transverse striation, fainter, but otherwise not unlike that of voluntary muscle, is so general that it cannot be regarded as pathological, although it is most strongly developed in imperfect brownish enamel. The striation in question may be seen even in a single isolated rod, and is not necessarily continuous over adjacent columns, though it generally is so ; it is rendered more apparent by the slight action of diluted acids upon the rod. The striae are from 3yut to 6ft apart. Very various interpretations of this appearance have been given 42 A MANUAL OF DENTAL ANATOMY. It has been attributed to "an intermittent calcification5' of the enamel rod (Hertz), but is with more probability referred to varicosities in the individual rods (Kolliker, Waldeyer). It is very marked in the enamel of the common rat, which shares with that of other Muridce the peculiarity of having the individual rods almost serrated, those of adjacent crossing layers being fitted to one another with great exactness. In human enamel the adjacent rods, if united without any intermediate cementing medium, and pursuing courses slightly different, must of necessity be of slightly irregular form, or else interspaces would be left, which is not found to be the Fig. 16. — Isolated rod from the shell of Pinna, showing transverse striation (photographed by Mr. Mummery). case. Thus the " decussation of the rods " seemed a plausible explanation of this appearance of striation ; indeed, individual rods do present an appearance of slight varicosities, repeated at regular intervals. That the striation of enamel rods is due to this cause is confirmed by Febiger, an American expert in the resolution of diatoms, to whom enamel sections were submitted for his opinion by Dr. Xavier Sudduth. That the striation of voluntary muscle is due to such varicosities has been strongly urged by Dr. Haycraft (6), and this is confirmed by Professor Ewald of Strasburg (5), who finds that by pressing muscular fibre upon a film of moist collodion that substance reproduces the striation, and that by overstaining with silver THE DENTAL TISSUES — ENAMEL. 43 nitrate, so that the fibres are rendered absolutely opaque, it may be shown by reflected light, the surface appearing to be made up of regularly recurring hills and valleys. Yon Ebner (4), however, holds that the cross striae are due to the action of weak acid, and that the reason that they are seen in dry sections is that the Canada balsam in which they are usually mounted has a feeble acid reaction, and says that a similar etched pattern may be produced by acids on some crystals. He does not, however, infer that the rods are therefore Fig. 17. — Decalcified matrix of pinna shell. necessarily crystalline, but only that there are physical differences in their different parts ; but this is controverted by Leon Williams and others, who have found the striae without acids or balsam being used. Transverse striation apparently due to varicosities is also to be seen in the rods of Pinna. The comparison with the shell of Pinna must not be strained too far. Nevertheless, the resemblance of this prismatic structure to an enamel with rods on a bigger scale, is sufficiently close to be in some measure instructive, although the tissue is composed of carbonate and not of phosphate of 44 A MANUAL OP DENTAL ANATOMY. lime, and though nothing is known of its development A transverse section of pinna shell shows the pattern produced by the approximation of the sharply defined rods, but it is not possible to make out the existence of a cementing substance. But if a section be rubbed down and then fastened to a glass slide and subsequently decalcified in situ, the rods wholly disappear, the pattern being, however, still maintained by the organic honeycomb in which they lay, and by which, presum- ably, their form was moulded. If a piece of human enamel be similarly treated, nothing is left upon the slide, so that it is certain that, whatever may have been the case during its formation, when it is finished the organic matrix is gone. And here again the resemblance fails in many respects, for it is probable that the organic mould is of a quite different nature ; indeed, as will presently be seen, no thoroughly distinct interstitial substance exists in enamel. The recent researches of Leon Williams (16) have thrown much light upon the structure of enamel, and illustrated as they are by very beautiful and untouched photomicrographs, must be accepted as, at all events, indisputable as respects that which can be seen, although exception might be taken to his statement that photography can show all that the human eye can see. In a sense this is true, though, as Professor Sir E. Schafer has said, it is with one's finger on the fine adjustment that, after all, the intimate structure of any tissue must be unravelled. Still photographs have the unquestionable value that, unlike a drawing, they cannot be strained to express the preconceived ideas of the observer ; on the other hand, high- power photographs represent truly only one plane, and appearances due to other planes out of focus in the thickness of the section occur, to say nothing of diffraction errors which may mislead. He completely demonstrates that enamel is a solid tissue, that is to say, that the minute canals said by von Ebner to be present in it have no real existence, and that this observer is also mistaken in supposing that the striation is an artificial appearance induced by reagents. Leon Williams believes that the completed enamel rod is a regular beaded calcified column, the beadings of which are the cause of the appearance of striation, and that these beaded rods are embedded in a matrix which fills up all the interstices, THE DENTAL TISSUES— ENAMEL. 45 is itself calcified, and does not differ much in refractive index from the rod itself. The varicosity of the rods and their consequent transverse striatum is of variable distinctness ; it is least seen, according to Leon Williams, nearest to the dentine. The difficulty of deciphering the intimate structure of a transparent calcified material is very considerable at best, and, to borrow an illustration from Leon Williams, in enamel r i jrIG.# is. — Human enamel washed with dilute hydrochloric acid (photo- graphed and prepared by Mr. Leon Williams). The beaded appearance of the enamel rods is very clearly seen. it is much as though we had made a complex pattern with beaded glass rods, then embedded them in melted glass of a lower melting point, and then set to work to unravel the pattern of the original rods. According to this observer, each rod is composed of more or less globular masses arranged at definite intervals and united in their lengths by smaller rods or bands. The rods thus built up touch, or almost touch, those around them, and the interstices, sometimes smaller, sometimes [larger, are filled in with the matrix material. The varicosities in) the rod 46 A MANUAL OF DENTAL ANATOMY. lie in its length, and may be more or less fused together ; those of one correspond with those of the next, and lie side by side, and do not interdigitate with them. On the subject of the ultimate constitution of enamel Leon Williams thus expresses himself, " The entire mass becomes so completely calcined that in normal mature enamel there is left no trace of organic matter. Only the original form remains duplicated in mineral substances like a fossil." Such, then, are the most recent views as to the nature of enamel, and they are confirmed by the action of orseille, a stain which will colour bone and dentine matrix without prior decalcification, but which fails to impart the smallest tinge to enamel. But although both alike appear to consist practically of salts Fig. 19. — From human enamel, softened in chromic acid, until it was friable. A. Longitudinal. B. Oblique section. alone, there seems to be a little difference between the rods and the interprismatic substance, which may be chemical or may be merely physical, for under the action of acids the rods are at first rendered more distinct. Leon Williams believes that the acid acts first upon the inter-prismatic material, whilst other observers have thought the contrary ; whichever be true, it is certain that the first effect is to render the rods more distinct in outline. A curious phenomenon, rather inexplicable in the light of recent research, is sometimes, but not constantly, seen when enamel is subjected to the action of weak chromic acid ('25 per cent.): the striation is rendered very plain, and the axes of the rods are dark and stained green by the reduced chromium sesquioxide. But this appearance cannot always be produced at will (Fig. 19), THE DENTAL TISSUES— ENAMEL. 47 It has been already mentioned that the action of weak acids is to render the rods more distinct, and the nature of their union with one another and of the interstitial substance, if there be one, has led to much discussion. Bodecker (3), basing conclusions upon the examination of thin sections stained with chloride of gold, held that enamel is built up of columns of calcined substance, between which minute spaces exist filled by a material which takes the stain deeply. He states that it gives off exceedingly fine thorns, which apparently pierce the rods at right angles to their length, so that it forms a close network very intimately mixed up with the calcined portion of the tissue. It is not of uniform thickness, but is beaded, and Bodecker attributes to it a role of far greater importance than that of a mere cementing substance, for he regards it as being an active, protoplasmic network, which renders the enamel much more " alive " than it had hitherto been considered to be. He con- siders it to become continuous with the soft contents of the dentinal tubes through the medium of large masses of proto- plasmic matter, which he believes he found at the margins of the enamel and dentine. On this Klein (8) remarks that, " the enamel cells, like all epithelial cells, being separated from one another by a homo- geneous interstitial substance, it is clear that the remains of this substance must occur also between the enamel rods ; in the enamel of a developing tooth the interstitial substance is larger in amount than in the fully formed organ. It is im- probable that nucleated protoplasmic masses are contained in the interstitial substance of the enamel of a fully formed tooth, as is maintained quite recently by Bodecker." Yon Ebner who has investigated the histology of enamel, holds that there is an uncalcified cement substance between the rods, which appears to be traceable into continuity with Nasix^th's membrane, which latter, however, acids cause to become detached, so that this continuity may be merely apparent. Bodecker's views have, however, never obtained much credence, and the result of Leon Williams's observations seems to completely dispose of them, as also of those of von Ebner, the appearances being explicable without the assump- tion of the presence of a noteworthy amount of organic matter. Many stains, and notably chloride of gold, will stain crevices 48 A MANUAL OF DENTAL ANATOMY. even in mineral specimens, so that ,too much reliance must never be placed on the results of the reactions of stains alone. And the writer's own analyses, proving that there is no organic matter or only the most minute trace of organic matter in completed enamel, render it difficult to accept the existence of a complete network (cf. p. 34). In a more recent address Leon Williams combats the explanation offered by WalkhofT, that the double contour seen in longitudinal sections is an optical effect, and reaffirms his belief in an interstitial substance between the rods, or " fibres " as he prefers to call them. In transverse sections he has photographed the rods as being more round than hexagonal or pentagonal, so that there must be interspaces of necessity, and his skill as a microscopist is such that no one is less likely to be deceived in observation. Yet such are the tricks that high powers may play in produc- ing deceptive appearances of things which do not exist that a shade of doubt must always remain. He holds, however, most strongly that whatever is between the rods is calcified, and does not contain organic matter in any quantity. In fractured enamel, the line of fracture is said to run through the centre of the rods, and not, as might have been expected, through their interspaces. There is also an appearance of striation upon a far larger scale, consisting of brownish lines, which are never, or very rarely, quite parallel with the outer surface of the enamel, but which nevertheless preserve some sort of relation with it and the surface of the dentine. These are known as the " brown striae of Ketzius," and, as they coincide with what was at one time the outer surface of the enamel cusp, are in some sense marks of its stratification in its original deposition. Von Ebner suggests that they are due to the entrance of air into spaces between rows of enamel rods, but this is strongly controverted by Leon Williams, who not only denies the existence of spaces into which air could enter, but has shown with some degree of probability that they are due to a real pigmentation, since they can be demonstrated on a section which has never been allowed to dry. It has been recently suggested by Zsigmondy (17) that they are due to actual imbrications of the ends of the enamel columns, being thus produced by the grinding down of a longitudinal section. THE DENTAL TISSUES— ENAMEL. 49 There is another class of markings seen upon the surface of a section of enamel, known as " Schreger's lines." These are dependent upon the different directions of contiguous groups of rods, and can be seen by reflected light, when they appear as light bands, and by transmitted light, when they assume a black aspect. Fig. 20. — " Enamel spindles," which communicate with the dentinal tubes. A few cavities, or at least what look like cavities, of irregular form often exist in the coronal enamel close to the surface of the dentine, and the dentinal tubes sometimes communicate with them. Hopewell- Smith calls them "enamel spindles." Per- haps these are to be regarded as abnormal, but they are not uncommon, and are disposed with some regularity about the margins of the apices of the underlying dentine cusps, as shown in longitudinal sections- Eomer says that they have D.A. 4 50 A MANUAL OF DENTAL ANATOMY. organic contents in the fresh state, and. suggests that they contain nerve endings. Bodecker, of course, regards them as filled up by protoplasm. Walkhoff considers that prior to enamel deposition the surface of the dentine is absorbed, and that they are dentine tubes left unabsorbed — an unlikety hypothesis. Irregular fissures and spaces also occur upon the outer surface of the enamel, which also have no special significance save, perhaps, as predisposing causes of dental caries. Tubular enamel. — In man, however, dentinal tubes may occasionally be seen to enter the enamel, passing across- the amelo-dentinal junction, that is, the boundary between the two tissues, and pursuing their course without being lost in irregular cavities. As was first pointed out by Sir John Tomes, the passage of tubes continuous with the dentinal tubes into and through a great part of the thickness of the enamel takes place in marsupials with such constancy as to be almost a class characteristic. The only exception to the rule amongst recent marsupials occurs in the teeth of the wombat, in which no dentinal tubes enter the enamel ; those extinct marsupials which have been examined present, as might have been expected, a structure in this respect similar to that of their nearest allies amongst the v'ecent genera. The enamel of the wombat is peculiar also in another respect, in the fact that it is covered by a strong and remark- ably uniform layer of cementum. The penetration of the enamel by dentinal tubes is not, however, a peculiarity quite confined to the marsupials, for it is to be found in some rodents (e.g., the jerboa), and in some insectivores (e.g., the Soricidce), and in many fish. Waldeyer and Hertz doubt the passage of the tubes of the dentine into the enamel ; as Kolliker observes, it is difficult to see how they can doubt it, even after mere observation of a single specimen ; moreover, it is also capable of experimental demonstration, for if an acid capable of removing the enamel be applied to one of these sections of marsupial teeth so as to dissolve away the enamel, the freed tubes are left hanging out from the edge of the dentine, thus putting the matter beyond all possibility of doubt. The penetration of the enamel by tubes continuous with the dentinal tubes seems rather common THE DENTAL TISSUES — ENAMEL. 51 in fish ; these tubes in the enamel grow smaller as they leave the dentine and do not reach the exterior of the enamel ; Fig-. 21. — Enamel and dentine of a kangaroo (Jfacropus major'). The dentinal tubes in the dentine (A) are furnished with numerous short branches at the line of juncture with the enamel ; they are dilated and a little bent out of their course, while beyond the dilatation they pass on through about two-thirds of the thickness of the enamel (B) in a straight course and without branches. Only a part of the whole thickness of the enamel is shown in the figure. C = an isolated tubule. examples may be found in Serrasalmo, in the barbel and in many extinct fish. The tubes, probably, run in the rods and not between them, though it has been stated by von Ebner that the tubes in 4—2 52 A MANUAL OF DENTAL ANATOMY. marsupial enamel do not lie in the rods themselves but in their interspaces, and he gives a figure of a transverse section in which all but one of the tubes appear to do so. Leon Williams has photographed some transverse sections of marsupial enamel, in which three-fourths of the tubes appear clearly to be in the substance of the rods. The remaining fourth appear as though they were between them. But it is a priori highly improbable that they occupy both positions, and a position within the rod is much more readily reconcilable with the appearances seen in the development of marsupial enamel (see p. 171). Although it has been usual to speak of marsupial enamel as penetrated by dentinal tubes, this is a somewhat improper use of terms. The enamel has a tube system, and its tubes are continuous with those of the dentine, but the tubes in the enamel are wholly and entirely an enamel formation, derived from the epiblast cells (see p. 175), which at the beginning of enamel development join up with the ends of the dentinal fibrils in the first-formed layer of dentine, or perhaps even earlier with the odontoblasts, before the formation of any dentine. The penetration of enamel by tubes is met with in many fish, and some instructive examples may be met with in the sharks. In some fossil sharks and Ganoid fish, e.g., Lepidotas, there is a rich crop of tubes in the enamel which are continuous with those of the dentine, becoming curiously curved as they course through the inner half of the enamel, and lost before they reach its surface. Doubt has been expressed by Dr. Paul whether the author was correct in describing the outer layer of the tooth of Lamna as fine tubed dentine, it being, in his opinion, more like enamel. Paul's criticism (9) led to an examination of the teeth of Selachia which had unexpected results. (14) It had not escaped the notice of older observers that the outer layer of the teeth of some Ganoid and Elasmobranch fishes differed from the more familiar dental tissues, and special names were applied to it, such as Ganoin, Vitrodentine, and Placoinschmelz, When it is thin, as in many rays, it looks like an ordinary enamel, but when it is thick it diverges both from the ordinary types of enamel and of dentine. It rasembles enamel in that it almost completely disappears THE DENTAL TISSUES— ENAMEL. 53 under the action of an acid, no coherent matrix being left ; it is highly refractive, is very hard, and will not stain, remaining white even when, under the influences of fossilisation, the dentine matrix is stained dark brown by iron. It is not known whether any analyses of it are available, but it is noticeable, if it is watched while dissolving in an acid, that while copious bubbles are given off from the dentine, very few come off from the enamel, thereby indicating that the latter is poorer in carbonates. On the other hand, it is penetrated for some distance (in certain forms) by thin tracts of dentine matrix as well as by tubes : a lamination parallel with the surface can often be seen, a banded structure is visible in transverse section, and it often contains interglobular or lacunar spaces unlike any other known enamel. It thus participates in the characters both of dentine and enamel, though inclining more towards the latter, and its true position cannot be determined without reference to its development, which it would be premature to 54 A MANUAL OF DENTAL ANATOMY. describe here. It may, however, be mentioned that whilst it is formed in a mesoblastic tissue, viz., the exterior of the dentine papilla, the cells of the enamel-organ appear to take an active part in its calcification, so that in its development also it is intermediate between the two tissues. In Gale us (one of the Lam niche), the surface of the dentine is somewhat festooned towards the enamel ; the dentine is of the osteodentine type (p. 97), and therefore not very richly tubular, but some tubes do enter the enamel. Not only do tubes enter the enamel, but by staining after slight washing with acid, it may be shown that tracts of dentine matrix enter with them ; this is common to all, but the depth to which r — " 1 / Fig. 23.— Tooth of Lamna. The outer Layer, dark at its inner and trans- parent at its outer portion, is enamel ; the centre core is dentine. these tracts penetrate, and their size, vary in different genera and families. This at first sight would seem to lend probability to Dr. Paul's suggestion that the penetration of enamel by tubes continuous with the dentinal tubes in marsupials and others could be explained by the dentine matrix growing out into the enamel, and so carrying tubes in between the enamel rods. But as has been shown elsewhere (13), this explanation is not true THE DENTAL TISSUES— ENAMEL. 55 as regards marsupials, seeing that the tubes in enamel appear developmentally to belong wholly to the enamel (cf. p. 175). Besides the tubes brought in from the dentine, the enamel of Lamna has a rich tube system of its own, these tubes being much coarser than the dentinal tubes, and about one-third of the way through the enamel they communicate with irregular lacunal spaces, a condition which has not been noticed in other enamel. These are both numerous and large, giving a confused appearance to this region ; beyond it the tubes are continued on to the outer surface, near which there is a more or less differentiated layer. In Galeus this outer layer is much more distinct, and has every appearance of parallel tubes running in from the surface like those of Sargas (Fig. 28). The varieties in the disposition of the tubes and of the Fig. 21. — Enamel of Galeus (one of the Lanmidce), showing penetration by tubes running in from the dentine. lacunal spaces are almost infinite, and it is only possible here to instance a few ; Jaeckel states that characters in the dentine, and especially in the enamel, are constant for genera or groups of genera. A fossil shark from the London clay, not identified, is yet more interesting. In this specimen (Fig. 25), the enamel is quite white, but the dentine has taken a strong brown stain, and shows clearly that the festooning of the dentine surface is carried further than in Lamna, so that tracts of dentine, their tubes going with them, enter the enamel for a greater distance, somewhat less than one-third of thickness of this tissue, which is three millimetres thick. [Rose (10) argues against these tissues being regarded as enamel, and claims that an exceedingly thin layer of real enamel does exist over them as a glaze. My paper had not then been published, so that Rose had not before him the considerations which, it contains, and the two 56 A MANUAL OF DENTAL ANATOMY. papers were so nearly contemporaneous that I had not seen hie when mine was first written, though it was published in time for me to refer to and consider it in my paper before its actual publication in the " Philos. Trans." (18) As I can only claim the tissue as an enamel on the balance of considerations, and have presented facts which tell both ways, it is possible that I may not have made a convert of Rose. But, on his hypothesis, calcification ought to commence on the exterior and to advance inwards, the calcification of the core of osteo- or trabeculo- dentine only commencing when calcification has reached in to it. But that is not the case (cf. p. 183): the first calcification is the borderline beneath this enamel layer, in fact, where enamel calcification ought to commence if this line be the outside of the real dentine, as I hold it to Fig. 25. — Enamel of a fossil shark, not identified, from the London clay. At the left-hand bottom corner is a small amount of dentine ; at the right upper side of the enamel is seen the distinct outer layer in which there is an appearance of tubes entering from the surface. be. And I do not attach much importance to double refraction as a test, though when it is pronounced it is useful. In common with Rose himself, I have often seen it in undoubted dentine, so that it is an adjunct, but not an infallible test. Of the disputed structure I wrote {Joe. cit.) : " It is seen to shine out brightly under dark ground illumination, its refractive index being high ; and it is doubly refractive, though this docs not stand for much, as the osteodentine in Lamna is also doubly refractive, and with polarised light shows a well-marked black cross intersecting the concentric laminne round each Haversian system. C.S.T.] THE DENTAL TISSUES— ENAMEL. 57 Whilst the tubes appear to run through the whole thick- ness of the enamel they are larger near to the dentine and again near to the surface. The differentiation of an outer layer, already noted as traceable in Lamna and Galeus, is much more marked in this London clay fossil ; the figure, however, exaggerates it, as it is from a photograph and the layer was yellower than the rest of the enamel. Eine lacunar spaces exist along the Fig. 26. — Enamel of Heterodontus (Cestracion) ; the lower part of the figure consists of vascular dentine. inner margin of this belt, which shows a fine striation perpendicular to the surface. The thickness and distinctness of this outer layer vary in different Plagiostomes, it being quite absent in some of them. Occasionally the striation of this outer layer is of such character that it has the appearance of tubes entering from the free surface. The outer surface of the tooth of the archaic existing shark Heterodontus is very peculiar. The pattern looks like fine tubes running in from the surface, upon which they open with large mouths, and they are gathered into bundles, so as to make, as they radiate inwards, a well-defined pattern. 58 A MANUAL OF DENTAL ANATOMY. Analogy, and its development so far as it has been studied in somewhat imperfectly preserved material, lead to the inference that this corresponds to the enamel of other Selachia, and, possibly, only to the specialised outer layer just described. Longitudinal sections of all these enamels show with varying distinctness a lamination parallel with the surface. Transverse sections show, in such as have been examined, a peculiar structure of bands which leave between them oat-shaped interstices ; in these interstices are found to be the prolongations of dentine matrix already described. This Fig. 27. — Transverse ground section of the enamel of Galeus. structure is not found in any enamels or dentines hitherto described. In Sargus, or the sheep's-head fish, and in various other fish, the enamel is penetrated by a system of formations which are not continued out of or derived from the dentine, but belong to the enamel itself. These markings, as seen in the figure (Fig. 28), run at right angles to the external surface of the enamel, proceed inwards without branching for some little distance, and then, at about the same point, bend abruptly at an angle, and give off numerous branches. The mesh work produced by the crossing of the tubes at all sorts of angles in the inner part of the enamel is so complicated as to render it impracticable to reproduce it in a drawing. That portion of enamel next to the dentine is without any appearance of canals. Von Boas (2), describing similarly constructed enamel of scaroid fishes, says that the canals open upon the outer surface of the enamel. THE DENTAL TISSUES — ENAMEL. 59 Though they have every appearance of being tubes, it is difficult to make coloured fluids enter them. If a thin section be treated with dilute hydrochloric acid and watched during its action, short rods appear projecting from the surface, but they soon dissolve away. All these forms of tubular enamel, especially those in which the tubes open on and enter from the surface, were very unintelligible, as it was hard alike to see what advantage they could confer upon their possessors, or how they were developed. But if the author's conclusions as to the development of enamel (cf. description of development of marsupial enamel) be accepted FlG. 28. — Enamel of the sheep's-head fish (Sargus oris). The enamel is penetrated by a system of formations, probably tabes, which enter from its free exposed surface, pass in for a certain distance in straight lines, and then, abruptly bending at an angle, cross one another, and produce a complicated pattern in the inner third of the enamel. the difficulty of interpretation disappears. If all enamel in its development passes through a tubular stage, then these are merely arrests of complete development and perpetuations of a stage which is transitory in placental mammals. Befeeences. (*) vox Bibea. " Chemische Untersuchungen iiber die Knocken und Zahne des Menschen und der Wirbelthiezen, etc." 1884. Schweinfurt. ('2) VON Boas. "Die Zahne der Scaroiden." Zeitschr. f. wissensch. Zoolog., 1879, Bd. xxxii., pp. 189 — 21G. (3) Bodeckee, C. F. W. "Anatomy and Pathology of the Teeth/*' New York, 1894. (4) vox Ebxee, Y. " Strittige Frangen iiber den Bau des Zahn- schnieltzes." Sitzungsb. d. k. Akad. d. Wissensch. Math-naturw. CI. Wien, 1890, Bd. xcix., pp. 57 — 104. (5) Ewald, B. "The Cross-striping of Muscle." Nature, Nov., 1892, p. 92. 60 A MANUAL OF DENTAL ANATOMY. (6) Haycraft, J. B. " Upon the Cause of the Striation of Voluntary Muscular Tissue." Proc. Roy. Soc. Lond., 1881, vol. xxxi., pp. 360 — 379. (7) Hoppe-Seyler, E. " Handbuch der Physiologisch und Patho- logisch-chemischen Analyse, etc." Berlin, 1893 (new ed.). (8) Klein, E., and Smith, E. Noble. " Atlas of Histology." London, 1879-80. (9) Paul, E. "Some points of interest in Dental Histology; The Enamel Organ." Dental Record, 1896, vol. xvi., pp. 493—517. (10) Rose, C. " Ueber die verschiedenen Abanderungen der Hart- gewebe bei Niederen Wirbeltieren." Anat. Anzeiger, 1898, Bd. xiv., pp. 21—31 and 33—69. (n) Stjddtjth, Xavier. "American System of Dental Surgery." 1887. (12) Tomes, C. S. " On the Chemical Composition of Enamel." Journ. Physiol., Cambridge, 1896, vol. xix., pp. 217—223. (ls) " On the Development of Marsupial and other Tubular Enamels, with Notes upon the Develop- ment of Enamel in General." Phil. Trans. Lond., 1898, vol. clxxxix., pp. 107—122. (14) " On the Structure and Development of the Enamel of Elasmobranch Fishes." Phil. Trans. Lond., 1898, vol. cxc. B., pp. 443-461. (15) Walkhoff. " Die normal e Histologic Menschlichen Zahne einschliesolich der mikroskopischen Tecknik." Leipzig, 1901. (16) Williams, Leon. Various Papers in Dental Cosmos. 1896 — 1902. (17) Zsigmondy, Otto. "Uber die Retziusschen Parallelstreifen im menschlichen Schmelze." Osterreich. Zeit. fur Stomatol. Vienna, 1913. CHAPTER III. ORTHO-DENTINE AND ITS VARIETIES. The dentine constitutes the greater part of every tooth, which thus, even after the removal of the other tissues, pre- serves somewhat of its characteristic shape. Several varieties exist in which those peculiarities of structure which differentiate it from hone are less marked, so that a point is sometimes reached at which it is hard to say whether a par- ticular structure should more rightly he described as dentine or as bone. It will be most convenient to commence with a description of that variety of dentine which differs most markedly from bone, or, in other words, which has the most typical " dentinal " structure ; and for that purpose the tissue met with in the teeth of man and the majority of mammals (though it is by no means confined to that class), and known under the name of " hard " or " unvascular " dentine, may be selected. This may also be called " ortho-dentine." Dentine is a hard, highl}7 elastic substance, in colour white with a slight tinge of yellow ; it is to some extent translucent, its transparency being often made more striking by contrast with the opacity which marks the first advent of dental caries. When broken, a silky lustre is seen upon the fractured sur- faces, which, being in the main due to the presence of air in its tubes, is more apparent in dry than in fresh dentine ; its fracture is sometimes described as finely fibrous. The mass of dentine consists of an organic matrix richly impregnated with calcareous salts ; this matrix is everywhere permeated by parallel tubes, which radiate, with some minor exceptions, from the pulp cavity towards the surface of the tooth. Chemical Composition. — The organic matrix of dentine, after the removal of the lime salts by an acid, is a yellow material of cartilaginous consistence, which when dried is not unlike horn ; it is insoluble, and does not swell up in water, but dissolves when submitted to prolonged boiling, and is in fact converted into gelatine. It is similar to the substance 62 A MANUAL OF DENTAL ANATOMY. obtained in like manner from bone, known as collagen, a substance with a highly complex chemical composition. Collagen. — The white fibres of connective tissue, and the greater part of the organic matrix of bone, dentine and cementuin, consist of this substance. Chemically it is nearly related to gelatine, of which it is the anhydride. By boiling in water for some little time it takes up an equivalent of water and becomes converted into gelatine ; conversely, gelatine, by being heated to 130° C, may be dehydrated and converted into collagen, thus, C102 Hir,l N31 029 — H2 0 = C102 H149 N31 O2R. (Gelatine) (Collagen) Elastin is another albuminoid substance which is abundant in ordinary elastic tissue. It is very insoluble, but may be dissolved by concen- trated sulphuric acid, or concentrated nitric acid, or hot caustic potash, which, however, all decompose it. The linings of Haversian canals, of the lacunee and canaliculi, and the sheaths of Neumann, to be presently described, are probably composed of this substance. It contains no sulphur, and thereby differs from keratine, which it in some respects resembles. Though insoluble in any fluid which does not decompose it, it is slowly digested by pepsin or by trypsin. When the decalcified matrix of dentine is boiled so as to form gelatine a small insoluble residue is left, and the same result may be attained by treating dentine with an acid sufficiently strong to bring the collagen into solution. A like effect is obtained with bone. This residue, which consists of a material singularly resistant to the action of acids and alkalies, appears to consist of elastin. In ivory, which is a highly elastic form of dentine, the elastin amounts to about 2*7 per cent, of the dried decal- cified matrix, and it is, without doubt, as can be seen by a microscopic examination of the residue above alluded to, derived from the immediate walls of the dentinal canals or sheaths of Neumann, of which this residue wholly consists. As has been already pointed out in the case of enamel, an important error pervades the current text-book analyses of dentine. The usual method of estimating the total salts is by ignition, that is to say, after fully drying the tissue at 212° F., by burning away the organic matter, weighing the residual ash which contains all the salts, and computing the loss as organic matter. In this way from 69 to 72 per cent, of salts is generally found, and dentine is usually said to contain from 31 to 28 per cent, of organic matrix, the amount of the loss. But if, instead of doing this, the lime salts are dissolved out ORTHO-DENTINE AND ITS VARIETIES. 03 and then the collagen dried and weighed, it will be found that it falls far short of this, only amounting to about 19 per cent, of the dried dentine. The cause of this discrepancy, which has been long overlooked, is that dentine fully dried at 212° F still contains something like 8 per cent, of water, which was held in chemical combination with the lime salts, and can only be driven out at red heat ; in fact, the calcium phosphate, like any calcium phosphate prepared by wet methods, holds in combination one or more equivalents of water. Thus fresh or wret dentine loses on drying at 212° F. about 10 or 11 per cent, of free water ; the dried dentine, when ignited at bright red heat, loses by decomposition of the hydrated salts 8 per cent, more water. The composition of dentine is therefore : Fresh human dentine,- — Free water (which can be dried out at 212° F.) 10-0 Dry dentine 90*0 100-0 . And of dried human dentine, — Organic matter . . . . 19*6 X Combined wTater [ .... 8*4 Salts J . .-. . 72-0 - ioo-o The proportion of salts to organic matter varies widely in different dentines ; thus elephant ivory (of the tusks) consists of— Organic matter 34*0 Combined water j 8*5 Salts J . . . 57*5 100-0 Whilst the dentine of an elephant's molar contains a pro- portion of salts nearer to that of man — Organic matter . . . .21*0 Combined water | .... 9'0 Salts )..... 70-0 100-0 64 A MANUAL OF DENTAL ANATOMY. Subject to the above correction the following may be accepted as giving the constitution of dentine. Von Bibra (2) gives the following analysis of perfectly dried dentine : Organic matter (tooth cartilage) . 27*61 Fat 0*40 Calcium phosphate, and fluoride . 66*72 Calcium carbonate . . . . 3 '36 Magnesium phosphate . . .1*08 Other salts 0'83 Von Bibra gives another analysis : Cartilage 20*42 Fat 0*58 Salts 1-00 Magnesium phosphate . . . 2*49 Calcium phosphate, and fluoride . 67*54 Calcium carbonate . . . . 7*97 And Berzelius (x) gives : Gelatine and water . . . 28*00 Sodium salts 1*50 Magnesium phosphate . . . 1*00 Calcium phosphate . . . . 62*00 Calcium fluoride .... 2*00 Calcium carbonate . . . . 5*50 Gallippe (6) gives : Water and organic matter . . 25*29 Mineral matter, consisting of — Soluble ash (alkaline chlorides, and phosphates) . . . 0*54 Calcium carbonate . . . . 0*35 Magnesium carbonate . . 1*13 Calcium 45*11 Magnesium . . . .1-67 Phosphoric acid . . . . 23*70 Silicates 0*41 Undetermined . . . . 1*8 The dentine of many mammals is very much richer in magnesium phosphate than human dentine. Even the latter, OKTHO -DEN TINE AND ITS VARIETIES. 65 it would seem, from the discrepancies existing between the various analyses, is variable in composition ; but, on the whole, it may be said that, amongst inorganic constituents of dentine, calcium phosphate largely preponderates ; from 3*5 to 8 per cent, consists of calcium carbonate ; a much smaller proportion consists of magnesium phosphate, while calcium fluoride exists in traces only. Great discrepancies exist in the amount of carbonates estimated, and this arises from the great practical difficulty in making a quantitative analysis of small proportions of carbonic acid in a substance of which only very small quantities are obtainable, sources of error, which cannot be eliminated, thus creeping in. It was the opinion of Hoppe-Seyler that the salt which chiefly hardens bone, dentine and enamel is a double salt, a com- pound of three equivalents of calcium phosphate with one of calcium carbonate, comparable to the mineral apatite. Bone salt, Ca9 (P04)6, Ca C03 or Ca10 C03 (P04)6. Apatite, Cai0 Fl2 (PO^e- A variety of apatite, Cai0 Cl2 (PO^. For a discussion of the probabilities of this being the case the reader is referred to Haliburton's " Physiological Chemistry," and to papers by the author in " Journal of Physiology," 1896 (19), and "Trans. Odontol. Soc," 1896 (20). The researches of Black (3), which have been in the main con- firmed by independent analyses and of Gallippe (6), indicate that the differences in the percentages of lime salts in various human teeth have been over-estimated, and that there is but a very small range between the most perfect and the most frail teeth. But it is remarkable that the dentine of molars contains a slightly higher percentage of salts than that of incisors, the former containing 2 per cent, more: this was indicated by the specific gravity experiments of Gallippe and subsequently confirmed by the author's analyses (19). Of the chemical constitution of the dentinal fibrils themselves little is known, except that they have been supposed not to consist of albumen, for, on treating pulverised dentine with salt solution rendered alkaline with sodic carbonate, only *1 to *75 per cent, of albumen was extracted (24) by two independent investigators. Still, this is not conclusive, as the comparative b.a. 5 66 A MANUAL OF DENTAL ANATOMY. areas of the dentinal tubes and the solid matrix, as well as their respective specific gravities are not known. In the matrix of human dentine there is, under ordinary circumstances, little, if any, structure to be made out ; but the investigations of Howard Mummery have shown that in caries of the teeth, which are then to some extent decalcified, an appearance like connective tissue fibres can be seen in the matrix, and indications of the same thing can also be detected in some normal dentine. This observation, which accords with the descriptions by von Ebner of a similar connective tissue in the matrix of bone and of dentine, is strongly confirmed by what obtains in some vaso-dentines. In this tissue the absence of a fine tube system renders it much more easy to see the nature of the matrix, which, under the influence of weak decalcifying agents, sometimes splits up into parallel fibres corresponding exactly in size and direction to the bundles of connective tissue fibrils which occur with complete regularity on the surface of the pulp. Of the matrix of hard tubular dentine, then, it may be said that there is normally no visible structure, but that it some- times breaks up in such a manner as to indicate that there really is some trace left of connective tissue fibres which were once present. The action of acids in decalcification tends to soften and make the connective tissues generally swell up and become less visible, hence the difficulty of establishing their presence. The Dentinal Tubes. — As has been already mentioned, the matrix is everywhere permeated by tubes, the precise direction of which varies in different parts of the tooth, so that the following description of their course must be taken as merely in a general way descriptive, and not as of universal or precise application. Each tube starts by an open circular mouth upon the surface of the pulp cavity ; thence it runs outwards, in a direction generally perpendicular to the surface, towards the periphery of the dentine, which, however, it does not reach, as it becomes smaller, and divides into branches at a little distance beneath the surface of the dentine. Near to the pulp they are so closely packed that there is little ORTHO-DENTINE AND ITS VARIETIES. 67 room betweeen them for the matrix, while near to the outside of the tooth they are more widely separated : their diameter is also greater near to the pulp cavity. The dentinal tubes do not pursue a perfectly straight course, bat describe curves both on a larger and a smaller scale. The longer curves are less abrupt than the others, and are termed the " primary curvatures." They are often compared to the letter /^ to which they bear a certain amount of resemblance, and are more pronounced in the crown than in the root. The secondary curvatures are very much more numerous and are smaller. The actual course of the dentinal tube is, in c Fig. 29. — Dentine (d) and cementum (c) of the narwhal, showing contour lines due to rows of interglobular spaces. most places at all events, an elongated spiral, as may be very well seen in thick sections transverse to the tubes ; by altera- tions in the focus of the high-power objective of the microscope the appearance of the tube making a spiral turn is very striking. The effect of an elongated spiral viewed on its side will of course be only slight undulations, such as are the secondary curvatures of the tubes. The spiral course of the dentinal tubes is most strongly marked in the roots of teeth. When a transverse section of dentine is viewed, bands or rings, concentric with the pulp cavity, are seen, and the same bands may be seen in longitudinal section. Such a striated or laminated appearance in the dentine may be due to two causes ; and some little confusion has arisen in the nomen- 5—2 68 A MANUAL OF DENTAL ANATOMY. clature, owing to its double origin not having always been kept in view. Such striae may be due to the presence of rows of interglobular spaces, or to the coincidence of the primary curvatures of neighbouring dentinal tubes ; that is to say, each tube bends at the same distance from the surface, and the bend makes a difference in the optical properties of the dentine at that point. Schreger described these latter: "the lines of Schreger," therefore, are markings, ranged parallel with the exterior of the dentine, and are due to the curvatures of the dentinal tubes. The " contour lines " of Owen, even in his own works, include markings of both classes : i.e., those due to the curvature of Fig. 30. — Termination of a dentinal tube in the midst of human dentine. the dentinal tubes, and those due to laminae of interglobular spaces, such as are met with in the teeth of Cetacea. Ketzius had seen and described contour markings due to inter- globular spaces, though his name is not usually associated with them. Passing outwards the tubes often divide into two equally large branches ; they also give off fine branches, which anas- tomose with those of neighbouring tubes. In the crown of a human tooth these fine branches are comparatively few, until the tube has reached nearly to the enamel, but in the root they are so numerous as to afford a ready means of dis- tinguishing whence the section has been taken. The small branches above alluded to are given off at right angles to the course of the main tube, which, however, itself frequently divides and subdivides, its divisions pursuing a nearly parallel course. The tubes are subject to slight varicosities, and their course is sometimes apparently interrupted by a small interglobular space, as is to be seen in an extreme degree in the dentine of the Cetacea. OKTHO-DENTINE AND ITS VARIETIES. 69 Owing to their breaking up into minute branches, some of the tubes become lost as they approach the surface of the dentine, and apparently end in fine pointed extremities. Some terminate by anastomosing with terminal branches of others, forming loops near to the surface of the dentine ; others terminate far beneath the surface in a similar way. Some tubes pass into the small interglobular spaces which constitute the " granular layer " described by Sir John Tomes, while others again pass out altogether beyond the boundary of the dentine and anastomose with the canaliculi of the lacunae in the cementum, when they exist in that tissue. The enamel also may be penetrated by the dentinal tubes, though this, when occurring in the human subject, must be regarded as exceptional and almost pathological in its nature (see Fig. 20). As has, however, been mentioned in speaking of the enamel, in most of the Marsupials, in a good many fish, e.g., Spherodus, Serrasalmo, Barbel, and in certain other animals, it is a perfectly normal and indeed characteristic occurrence, difficult though it be to see how such a relation of parts is brought about in the course of development of the two tissues. Dentinal Sheaths. — If dentine be exposed to the action of strong acid for some days, a sort of fibrous felt, or if the action of the acid has gone further, a transparent slime alone remains. Examined with the microscope, this proves to be a collection of tubes ; it seems to be, in fact, made up of the immediate walls of the dentinal tubes, the intervening matrix having been wholly destroyed. Two facts are thus demonstrated : the one that the tubes have definite walls, and are not simple channels in the v matrix ; the other, that these walls are composed of some- thing singularly indestructible. Indeed, the walls of the dentinal tubes are so indestructible that they may be demon- strated in fossil teeth, in teeth boiled in caustic alkalies, or in teeth which have been allowed to putrefy. Although Kolliker was, perhaps, the first to describe and figure these isolated tubes, they are generally known as the "dentinal sheaths of Neumann," the latter writer having more fully investigated and described them. The precise chemical nature of these sheaths will be more conveniently considered under the head of calcification : similarly inde- 70 A MANUAL OF DENTAL ANATOMY. structible tissues are, however, to be met with surrounding the Haversian canals and the lacunae of bone. It was the opinion of Neumann, as it was also of Henle, that the dentinal sheaths are calcified ; but the proof of this is very difficult, as they cannot be demonstrated, or isolated, to any extent in dentine, unless it has been decalcified. Their existence as distinct from the fibrils has been denied by Magitot, Sudduth, and recently by Komer. Transverse sections of dentine present fallacious appear- Fig. 31. — Ground section of human dentine, which has been submitted to the action of strong acids so as to isolate Neumann's sheaths by the destruction of the intervening matrix. ances, owing to the thickness of the section giving to the tube a double contour which may be easily mistaken for a special wall. Immediately round the opening of the canal, or "lumen," as it is called, there is, however, generally a thin yellowish border, which may perhaps be the sheath of Neumann. In the earlier stages of caries, before the dentine is much softened, the walls of the canals become strikingly apparent. But it must be remembered that the dentinal sheaths can only be fully demonstrated by processes which amount to a partial destruction of the dentine, and that they are therefore in some degree at all events artificial ; it may OKTHO-DENTINE AND ITS VARIETIES. 71 be that they have no real existence until they are displayed more clearly by the action of these agents. If that is so, the immediate surroundings of the soft fibril differ somewhat in chemical constitution from the parts of the matrix which are more remote, so that under the action of destructive agents the matrix is split up into the sheathing layers round the fibrils and the more soluble residuum of the matrix. Rose has succeeded, by Golgi's rapid method of nitrate of silver staining, in rendering Neumann's sheaths perfectly black, a transverse section of dentine thus prepared reminding one of the familiar tobacco-pipe appearance of dental caries, only that the sections of tobacco pipe are black. As the intervening matrix remains unstained, it is clear that Fig. 32. — Transverse section of dentine. The appearance of a double contour is so much exaggerated as to make the figure diagrammatic, but it fairly represents the contour shown by nigrosin staining. Neumann's sheaths differ from it in the degree of calcifica- tion, an interpretation which is borne out by the fact that in a longitudinal section the youngest dentine layer so stained is also intensely black. If, prior to treatment by Golgi's stain, the tooth be pre- pared by the Koch-Weil method, the soft parts, including the dentinal fibres, do not take the stain, so that in the transverse section the fibril is seen as a bright point in the middle of the black Neumann's sheath. In longitudinal section the black sheath is fringed all along the course of the tubes by processes more or less bridging across to the next ones, but the dentinal fibrils have not been demonstrated in these branches. Rose concurs in describing Neumann's sheaths as especially resistant to acids and also to caries ; hence the distinctness of the " tobacco-pipe appearance." The author has succeeded in satisfactorily staining them 72 A MANUAL OF DENTAL ANATOMY. by means of nigrosin ; a preparation so treated would resemble Fig. 32, save that the opening of the tube would be light, and be surrounded by an intensely black border in place of the light one of the figure. Dentine would thus be considered as a tissue made up of a calcified matrix, resembling that of bone, permeated by the soft fibrils, just as bone is permeated by canaliculi with soft contents, but having this peculiarity, that the latest formed portions of matrix, namely, those immediately embracing the fibrils, differ sufficiently from the bulk of the matrix in chemical constitution to enable them to be isolated as sheathing tubes, or to be stained in situ. Dentinal Fibrils. — The canals which everywhere per- meate the dentine are not empty, a fact which might be inferred from the difference in translucency and general aspect of dry and fresh dentine, whether seen in mass or in FlG. 33. — Transverse section through the tooth of a hedgehog stained with gold chloride. Semi-diagrammatic (Marett Tims). thin section ; neither are they, as was at one time supposed, tenanted merely by fluid. This is well seen in the accompanying- figure (Fig. 33). Each canal is occupied by a soft fibril, which is continuous with a cell upon the surface of the pulp (odontoblast); the existence of these soft fibrils was first demonstrated by John ORTHO -DENTINE AND ITS VARIETIES. 73 Tomes, who thus, to use the words of Waldeyer, " opened the way to a correct interpretation of the nature of the dentine." Henle, in his " Allgemeine Anatomie" (1841), a translation of a portion of which is to He found in the " Archives of Dentistry " (1865), figured and described projections from dentinal tubes. These he described as calcified and rigid, adding that by the use of acids they may be made flexible ; he speaks of the tube as empty, save when blocked by granular calcareous matter, and alludes to fluids entering it by capillarity ; and lastly, he says nothing whatever of the connections of the pulp with the tubes. Miiller (as translated in Nasmyth on the " Structure of the Teeth," 1839) says, "In breaking fine sections of the teeth perpendicularly to the fibres, I have frequently seen the latter projecting a little at the fractured edge. In such cases they are quite straight and not curved, and seem to be not at all flexible. Hence it follows that the tubes have an organised basis, a membrane, and that this is stiff and brittle, and probably saturated with calcareous salts, but weak and soft in a decalcified tooth." The whole importance of Sir John Tomes' discovery lay in the fact that dentine is permeated by soft, uncalcified structures ; and what is yet more significant, that these soft fibrils, permeating the hard dentine, proceed from the pulp. In no sense, therefore, did Henle anticipate this discovery. In 1854 Lent figured processes from the odontoblasts which he con- ceived to be concerned in the formation of dentine ; but in the earlier editions of the " Histology"- of his friend and teacher, Professor Kolliker, although Lent's discoveries are described and adopted without reservation, no mention of the real structure of dentine occurs. In a later edition, however, Professor Kolliker says, "After Tomes had described a soft fibre in each tube, I fell into the mistake of supposing that these fibres and the tubes were one and the same." The circumstances under which the dentinal fibrils can or cannot be discovered are said to be as follows, and may be taken as indications of a distinction between the dentinal fibrils and the dentinal sheaths : — If a tooth section be submitted to the action of a caustic alkali and boiled in it, or be allowed to completely putrefy, so that the soft parts are entirely destroyed, the dentinal sheaths can still be demonstrated, but the fibres can in no way be brought into view (Kolliker). The dentinal sheaths may be demonstrated also in fossil teeth, as has been shown by Hoppe (10) and others. In fresh dentine every odontoblast sends a process into the dentinal tubes (Tomes, Kolliker, Lent, Waldeyer, Neumann), 74 A MANUAL OF DENTAL ANATOMY. and it has been found possible to demonstrate both the sheaths and the fibres in the same sections (Neumann, Boll). In transverse and even in longitudinal sections of decalcified dentine the fibrils may be recognised in situ (Kolliker). The contrast between the dentinal sheaths and the fibrils is this : — The sheaths are very indestructible, and can be demon- strated in teeth which have undergone all sorts of change ; the soft fibril is no longer demonstrable when the tooth has been placed in circumstances which would lead to its soft parts perishing. In dentine, then, there exist (i.) a matrix permeated by canals ; (ii.) soft fibrils contained in these canals, or " dentinal fibres " ; and probably also (iii.) special walls to these canals, or " dentinal sheaths " ; and it now remains to consider these in further detail. In sections of small fragments of dentine taken from the Fig. 34. — Section of dentine from the edge of which hang out the dentinal sheaths, and beyond these again the fibrils (after Boll). edges of the pulp cavity and including the surface of the pulp, the dentinal fibrils may be seen stretching from the cells of the superficial layer of the pulp (odontoblasts) into the dentinal tubes, as owing to their being extensile they may be stretched or drawn out from the tubes for some little distance without being broken across. In the same way they may be seen stretching across like harpstrings between two pieces of dentine, when this is torn by needles, and they can be thus shown in fresh fragments just as well as in those of decalcified dentine. When stretched to a considerable extent their diameter becomes diminished and they finally break, a sort of bead sometimes appearing at the broken end (J. Tomes). This would seem to indicate that the substance of the fibril is of colloid consistency, and that its external portions are in some degree firmer than its axial portion. The dentinal fibrils are well seen in Figs. 33 and 35 in which many are attached to the odontoblast cells. Modern methods of histological research have made the ORTHO-DENTINE AND ITS VARIETIES. 75 demonstration of the fibrils, which are continued from the odontoblast cells and enter the dentinal tubes, an easy matter ; they are of some size, and, judging from appearances, are pretty firm in consistence, as they look somewhat stiff where they are seen projecting from the surface of a torn-away pulp. They stain very readily with haematoxylene, gold chloride, &c;, and many other stains are slightly taken by them. Another easy method of staining the fibrils is to treat the pulp with a salt of iron followed by tannin, of course after a thorough washing. The dentinal fibril is capable of being stained with carmine ; in young dentine it is more easily stained, especially near the Fig. 35. — Surface of the pulp with the odontoblast layer in situ. The dentine fibrils pulled out of the dentinal tubules hang like a fringe from the odontoblast layer ; dentine fibrils are also seen hanging out from the edge of the dentine, to which, to the right of the figure, a few odontoblasts remain attached. pulp cavity, and the following figure (Fig. 36) is taken from such a section of dentine from a partially-developed human incisor. The matrix is slightly stained with the carmine, indicating that it has not yet become fully impregnated with salts, and in the centres of the clear areas, dark spots deeply stained with carmine are to be seen, the latter being transverse sections of the dentinal fibrils in situ. The author has observed precisely similar appearances in the thin young dentine of calves' and pigs' teeth ; Kolliker also mentions that the dentinal fibril may be recognised in situ in transverse sections of fresh dentine. Bo'decker finds that the dentinal fibrils stained darkly with chloride of gold, when viewed in transverse sections under a magnifying power of 2,000 diameters, do not appear round 76 A MANUAL OF DENTAL ANATOMY. but somewhat angular, and give off tiny lateral offshoots which seem to penetrate the dentine (See Fig. 36). In the matrix itself there is an appearance of a faint network when it has been stained with gold, and from this Bodecker infers, though probably erroneously, that the dentine is penetrated everywhere by a network of living plasm, derived from, though far finer than, the dentinal fibrils. (Cf. results of Golgi's stain, p. 71.) Probably the angularity of the fibril, which, as figured by him, is much smaller than the canal, is due to its having shrunk under the action of chromic acid or some such reagent. According to Neumann, in old age the fibrils atrophy or become calcified ; some observers have failed to detect them near to the periphery of the dentine, far away from the pulp FlG. 36. — Transverse section of dentine ; in four of the dentinal tubes, the dentinal fibrils, deeply stained with carmine, in the preparation from which this figure was drawn, are seen. The fibrils arc somewhat shrunken, owing to the action of the glycerine in which the section is mounted. cavity. But here they would naturally be more minute, and it is more probable that the manipulations had failed to demonstrate them than that they were absent, for Bodecker has traced them to the very periphery of the dentine. Dr. Beale has seen prolongations of the nucleus of the cell towards the base of the fibril, though in the example which he figures it does not enter it. Dentinal fibrils were demonstrated in the Rcj)tili<( and Amphibia by Santi Sirena and by the author; and also by the latter in the few fish examined with that purpose. Of their real nature some doubts are entertained ; they are certainly processes of the odontoblasts, and their substance seems identical with that of the protoplasm of the cell, for they are of tolerably large size, and are larger at their origin than at their terminations. According to Hopewell- Smith (8) the odontoblasts vary considerably in size, those in ORTHO-DENTINE AND ITS VARIETIES. 77 the coronal part of the pulp of a fully grown tooth measuring 25ju to 30/x in length, and about 5/x in breadth. Nerve end organs in the ordinary sense of the word they are not ; but there are many examples of cellular structures which are connected with the termination of sensory nerve fibres, such as the goblet cells in the olfactory membrane of the frog, and it is quite possible that the odontoblast cells may stand in some such relations to the nerves of the pulp. Coleman once suggested that it was possible that the odontoblasts might have some tactile function, and Hopewell-Smith (9) points out their resemblance to multipolar ganglion cells of the spinal cord. Fig. 37.— a. Dentinal fibrils, b. Amorphous matrix, c. Odontoblasts. d. Nuclei of odontoblasts, e. Stellate cells. /. Nerve extremities which are continuous with the branched cells. After Magitot. According to Magitot the nerves of the pulp become continuous with a layer of reticulate cells which lie beneath the odontoblasts ; these communicate freely with the processes of the odontoblasts, so that there is a very direct chain of communication between the dentinal fibril and the nerves of the pulp. Magitot speaks very positively as to the accuracy of his views, which as yet, however, have not been confirmed by other investigators (Fig. 37).* Yet another view of the nature of the dentinal fibril is advocated by Klein (u, p. 183), who holds that the odontoblasts are concerned only in * An objection has been urged against the view that the prolongations from the odontoblasts into the dentinal tubules (the dentinal fibrils) have any sensory, i.e.. nervous, function, on the ground that the odontoblasts are of mesoblastic origin, whereas the nervous system as a whole is derived from epiblast. There is, however, a tendency at the present time to lay less stress than formerly on the three germ-layer theory. It is therefore possible that in future this objection may lose some of its force.- — Eds. 78 A MANUAL OF DENTAL ANATOMY. the formation of the dentine matrix, and that the dentinal fibrils are long processes of the deeper cells in the above figure, which run up between the odontoblasts and enter the dentinal canals. In a later paper (n) Magitot also impugns the accuracy of the views ordinarily accepted as to the structure of dentine, denying the existence of any special walls to the tubes, and further arguing that it is undesirable to think or speak of the channels in dried dentine as tubes at all. For, he argues, they are not tubes in the fresh state, seeing that the fibrils are adherent to the matrix and form a part of it, and that they were originally precisely the same tissue. He would prefer to speak of dentine as being a fibrillar tissue included in a hard and homogeneous matrix. Fig. 38. — Section showing non-medullated nerve fibres passing from the pulp between the odontoblasts and entering the dentinal tubules. From Mummery's paper from British Dental Journal, October 1, 1913. In this opinion Magitot does not stand alone : Xavier Sudduth does not believe in the existence of a specialised sheath, and Eomer (15) also holds strongly that the processes of the odontoblasts constitute the whole contents of the dentinal tubes, i.e., that they lie in canals in the dentine matrix, which have no special walls. According to him, therefore, the residue left after the destruction of the matrix by strong acids (Fig. 31) consists of the processes of the odontoblasts and of these alone. These views, however, do not differ substantially from those in the text, save that Magitot does not recognise the existence of that transitional tissue which others believe to be there, and call the sheaths of Neumann. The balance of evidence is against this view of Magitot and Sudduth. ORTHO-DENTINE AND ITS VARIETIES. 79 If they were right one would have the anomaly of sensation being conducted by a material so indestructible by acids as to have departed far from the protoplasmic condition which is associated with functional activity, unless the penetration of actual nerve fibres, be admitted. Both Morgernstern -and Romer believe that non-medullated nerve fibres pass from the pulp into the dentinal tubes, and end in the " enamel spindles," and Howard Mummery (13) is of the opinion that, as they approach the plexus of Raschkow, the nerves of the pulp, passing between and around the odontoblasts, form a narrow marginal plexus from which they extend, as non-medullated fibres, into the dentinal tubes, and accompany the dentinal fibrils to their terminations beneath the enamel or cementum (Fig. 38). The observations of Mummery appear to receive confirmation in a recent paper by Deppendorf (5). Mummery's iron preparations clearly show that fine fibres traceable into connection with indisputable nerve bundles deeper in the pulp do run out straight towards the dentine and right up to it as though they entered it, which, perhaps, they do. In this connexion the very remarkable specimens of Dr. Dentz, of Utrecht, may | be mentioned. These, which were cut from embryos, when about half the thickness of the dentine was formed, show pear-shaped bodies arranged with regularity a short distance within the border of the dentine, the smaller ends being directed towards the pulp, and their large ends connected with one or more dentinal tubes. These pear- shaped bodies have two or three large nuclei, and present a close resemblance to some forms of nerve-end organs. Amongst many difficulties as to their interpretation is the fact that the examination of a large number of sections of dry deciduous teeth has quite failed to reveal the spaces which might have been expected to have contained them, and their relatively large size (^Jo °^ an inch) would render the spaces conspicuous. The observation of Boll is also very suggestive. He found that by treating a perfectly fresh pulp with ^ per cent, solution of chromic acid an immense number of fine fibres could be demonstrated, a great many of which projected from above the surface, as though they had been pulled out of the dentinal tubes, but although they pass up from a plexus of 80 A MANUAL OF DENTAL ANATOMY. dark-bordered nerve fibres beneath the membrana eboris between the cells of that layer, their passage into the dentine remains a mere matter of inference. There can be no question that the sensitiveness of the dentine is due to the presence of soft organised tissue in the tubes, and is not a mere transmission of vibrations to the pulp through a fluid or other inert conductor. The peripheral sensitiveness of a tooth can be allayed by local applications which it would be absurd to suppose were themselves con- ducted to the pulp ; moreover, it is within the experience of every operator that after the removal of a very sensitive layer of caries, an area of dentine is reached, which, though nearer to the pulp, is far less sensitive, a condition quite inexplicable, Fig. 39. — Dentinal tubes terminating in the spaces of the granular layer. except upon the supposition of a different local condition of the contents of the tubes. With reference to the probabilities of actual nerve fibres entering the dentinal tubes, it must be remembered that, in those tissues which are naturally so thin as to present great facilities for examination, nerves of a degree of fineness unknown elsewhere have been demonstrated ; in other words, the easier the tissue is to investigate, the finer the nerves which have been seen in it, while dentine is among the most difficult substances conceivable for the demonstration of fine nerve fibrils. Interglobular Spaces. — In the dentine which underlies the cementum an immense number of these spaces exist, giving to the tissue, as seen under a low power, an appearance of granularity and constituting the "granular layer " of Sir John Tomes. On account of the greater abundance of these spaces in that situation, they are more evident beneath the cement than beneath the enamel, and many of the dentinal tubes appear to terminate in them. ORTHO-DENTINE AND ITS VARIETIES. 81 Although the name " interglobular spaces " is strictly applicable to the structures constituting the granular layer of dentine, it was not to these that it was first applied. When a dried section of dentine is examined, dark irregular spaces clustered together and usually most abundant at a little distance below the enamel, are often to be seen, particularly if the section has been made from a brownish, imperfectly developed tooth. The spaces have a rounded outline, furnished with short pointed processes, and in favourably prepared sections it Fig. 40. — Interglobular spaces in dentine. may be seen that their outlines are formed by portions of the surfaces of closely apposed spheres, and globular contours may often be detected in the solid dentine near to them, as is shown in Fig. 40, taken from a section boiled in wax in order to render it very transparent. Although these large spaces are very common, they are not universally present, or at any rate they cannot always be demonstrated. They are perhaps not to be regarded as perfectly normal, but are rather indications of an arrested development at that spot. The occurrence of globular forms during the early stages of calcification will again be alluded to in connection with the development of teeth ; but although the term " interglobular " is thus strictly applicable, the use of the word " spaces " is not so correct. In dry dentine it is d.a. 6 82 A MANUAL OF DENTAL ANATOMY. true that they are, as Czermak described them, spaces filled with air ; but that they are so is only due to the fact that their contents are soft, and shrivel up in drying. In the fresh condition the interglobular " space " is perfectly full, its contents often having the structural arrangement of the rest of the matrix, or else consisting of soft plasm ; in the former case, the dentinal tubes pass across and through it without any interruption or alteration in their course. This fact, as well as the softer nature of the contents as compared with the rest of the dentine, is well illustrated hy a section in the author's possession which was taken from a carious tooth, near to the affected surface. In this the bacteria had effected an entrance into some of the tubes, giving to them a varicose or beaded Fig. 41. — Section of carious dentine, in which some of the tubes are enlarged by the ingress of organisms, which have developed with greater freedom in one or two of the tubes where they cross the inter- globular spaces. appearance, and causing their enlargement. But when the micro-organisms reached the interglobular space, the less amount of resistance, or possibly the more favourable pabulum accessible, led to their more rapid development, so that the tubes within the confines of the space are many times more enlarged than those outside ; nevertheless the continuity of the tubes across the space is well demonstrated by the multiplication of micro-organisms having followed them with exactitude. It sometimes happens that indications of spherical forms with faintly discernible contours resembling those of the interglobular spaces may be seen in dried sections, in which no actual spaces occur. The appearances are perhaps produced by the formation of an interglobular space, the contents of which have subsequently become more or less perfectly calcified. The term " areolar dentine " formerly applied to this is falling into disuse, although it can very well ORTHO-DENTINE AND ITS VARIETIES. 83 be applied to one form of adventitious dentine, the outcome of dental caries. Globular forms, showing concentric lamination like an onion, are very distinct in the outer part of the dentine in some fossil reptiles. (Credner, 4.) The exact nature of the contents of the interglobular spaces is not very certain ; they may easily be tinted by carmine, and it is said that they may, like the dentinal sheaths, be isolated by the destruction of the rest of the matrix in acids. Like Neumann's sheaths, they take a deep black stain with Golgi's process. Bodecker says that there is soft living plasm abundantly distributed in the smaller interglobular spaces which con- stitute the granular layer, and that this is in very free com- munication with the soft fibrils in the tubes on the one side and with the soft contents of the lacunae and canaliculi of the cementum on the other. Varieties, — In the dentine so far described, which is that variety known as hard or unvascular dentine (ortho-dentine), some degree of nutrition is perhaps provided for by the penetration of the whole thickness of the tissue by protoplasmic fibres, the dentinal fibrils ; but this nutrition may be effected in a different way, and there are other varieties of dentine known in which dentinal fibrils have never been shown to exist. For descriptive purposes the author would classify dentines as : (i.) Hard, unvascular, or ortho-dentine, (ii.) Plici-dentine. (iii.) Vaso-dentine. (iv.) Osteo-dentine. Another classification has been proposed by Rose, to which fuller reference will be made at a future page. He does not recognise plici-dentine as a variety, and proposes the term trabecular dentine instead of osteo-dentine (see p. 102). Of these ordinary hard dentine has been sufficiently described. Plici-dentine. — This is a variety of hard dentine, not very distinct in its essential nature, though at first sight widely dissimilar. In ordinary dentine the dentinal tubes radiate out from a pulp and pulp chamber of simple form. If that form is rendered complex by foldings of its wralls, the dentinal tubes still running off at right angles to that portion of the pulp to 6—2 84 A MANUAL OF DENTAL ANATOMY. which they immediately belong, a " plici-dentine " results. It is merely an ordinary dentine with its pulp folded up and wrinkled into a greater or less degree of complexity. Fig. 42. — Section of plici-dentine with the pulp in situ (^Lepidosteus). a. Prolongation of the pulp. p. Connective tissue framework of pulp. d. Dentine. In the teeth of Varanus (monitor lizard), or of L&pidosteus, for example, the process of calcification of the pulp takes place in such manner that in the upper half of the tooth a cap of ordinary unvascular dentine, in which the tubes radiate from Fig. 43. — Transverse section across the crown of the tootli of Vanum*. near to its base. The central pulp cavity is produced outwards into processes, and it might be said that the dentine is arranged in plates with some little regularity round its periphery. a single central pulp cavity, is formed. But in the lower part of the tooth slight longitudinal furrows appear on the surface, which, on transverse section, are seen to correspond to involu- tions of the dentine ; the dentine is, as it were, in folds. The ORTIIO-PEXTIXE AND ITS VARIETIES. 85 pulp on section might be compared to a paddle-wheel, the floats on which correspond to the thin flat radiating processes of the pulp ; but as yet the central axial pulp chamber is unaltered. In Lepidostevs (the North American Gar-pike), a little lower down, as represented in Fig. 44, there is no longer a central simple pulp chamber ; the inflections round the periphery have become relatively much deeper, and the centre Fig. a. — Transverse section across the base of a tooth of Lepulosteus spatula. At the exterior are regularly disposed radiating plates of dentine, each with its own pulp cavity, while the central area is com- posed of more or less cylindrical pulp chambers, each of which forms the starting point for its own sj-stem of dentinal tubes. The pulp chambers are made dark in the figure for the sake of greater distinct- ness. of the tooth is occupied by a tissue irregular, but not otherwise unlike the dentine of Myliobates (cf. p. 88) ; that is to say, there are a number of columns of pulp, each of which forms an axis whence a system of dental tubes radiate. The outrunning plates of dental pulp, which on section radiate out like the spokes of a wheel, do not always remain single ; they may divide simply into two branches, as may be seen in the section across the base of the tooth of Lejndosteus ; or sometimes there are several branches. A MANUAL OF DENTAL ANATOMY In Lepidostevs oxyurus there are simple Inflections, and a central pulp cavity ; in L. spatula the inflections are branched, and the central pulp cavity all filled up. In the foregoing figure of the base of a tooth of Lepidosteus some few of the outrunning pulp chambers are seen to be bifurcated, while the central mass of the tooth is composed of dentine permeated by pulp canals which pursue a longitudinal course. A slight further modification leads up to the com- Fig. 45.- — Transverse section of a tooth of Labyrinthodon. (After Owen,) The letter a is placed in the centre pulp chamber ; the letter b marks the lines of separation between the systems of dentinal tubes which belong to each lamina of pulp; these lines of demarcation were formerly supposed to be occupied by cementum. plicated structure of the dentine of Labyrinthodon, in which a maximum of complexity is attained, although the clue to its intimate structure is afforded by the teeth of Varanus or of Lepidosteus. The lamina? of pulp, with their several systems of dentinal tubes, instead of passing out in straight lines like the spokes of a wheel, pursue a tortuous course as they run from the small central pulp chamber towards the surface. Not only do they undulate, but they also give off lateral processes ; and at their terminations near to the surface of the tooth, the thin lamina? of pulp (so thin that the radiating pulp chambers are mere fissures) become dilated ; so that on section circular ORTHO-DENTINE AND ITS VARIETIES. 87 canals are seen at these points, as is also the case at the points where subsidiary processes branch olY. The wavy course pursued by fche radiating plates of dentine, and the disposition of the tubes round the dilated portions of the pulp chamber, render the general aspect of the dentine structure very complicated; the several "systems"* are united to one another by an inosculation of the terminal branches of the tubes in some few places, but more generally Fig. 46. — From tooth of Labyrinthodon, showing the nature of the connec- tion between the contiguous dentinal systems, a and b as in previous figure. (After a drawing by Sir John Tomes.) by a clear layer containing radiate spaces, something like the lacunae of cementum. Hence Professor Owen has described the tooth as consisting of radiating plates of dentine, between which equally convoluted plates of cementum penetrate. But, as was pointed out by Sir John Tomes (18J, the mere presence of lacuna- like spaces is not sufficient to prove the presence of cementum, inasmuch as they occur on a small scale in the granular layer of dentine. Moreover, when cementum and enamel are both * The term <; dentinal system" is applied to the portion of dentine in which all the tubes radiate from a single section of pulp chamber ; thus the tooth of Labt/ri/itJtudon is made up of many dentinal systems ; the same may be said of Myliobates. 88 A MANUAL OF DENTAL ANATOMY. present, the cementum is always outside the enamel, whereas at the upper part of the tooth of the Labyrinthodon the characteristic inflections take place within a common invest- ment of enamel which is not involuted. Thus the whole of the tissue constituting the very complex pattern of the Labyrin- thodon tooth is dentine, and the cementum does not, as was usually supposed, enter into its composition at all. So far the foldings have been lateral, and in many cases the tip of the tooth was a simple cone without any foldings. There is another kind of dentine which has much in common with these plici-dentines, but which is a little more difficult to Fig. 47. — Transverse section of the dentine of Myliobates. classify. This is found in the teeth of the Cape ant-eater (Orycterojms) ; in the teeth of Myliobates, a large ray ; or in the teeth of the rostrum of the saw-fish (Pristis). In the Myliobates (Fig. 47) the flat pavement-like tooth is permeated by a series of equidistant, parallel, straight canals, running up at right angles towards the surface, from the upper end and sides of which, systems of dentinal tubes radiate, just as the tubes radiate from the single pulp chamber of a human tooth, save that they run for a comparatively short distance. In transverse sections the tubes are seen radiating from these canals, and at their terminations sometimes inosculating with the terminal branches of the tubes of another system. The channels contain prolongations of the vascular pulp, which are distinct in the upper part of the tooth, but become intimately united together at its base, where the disposition of the ORTHO-DENTINE AND ITS VARIETIES. 89 channels ceases to be regular, and, as a consequence, the systems of dentinal tubes pass from them in various directions without producing the symmetrical patterns which characterise the upper part of the -crown. When the tooth comes into use and its immediate surface gets worn off, the ends of the perpendicular pulp canals would be laid open, were it not that they become blocked by the deposition of a transparent homogeneous tissue within them? Fig. 48. — Longitudinal section of lower incisor of Galeop'dlieeus. resembling the similar tissue which closes the Haversian canals of an antler about to be shed. Instead of being regarded as a plici-dentine, such a tooth might be said to be built up of a series of small, parallel, fused denticles, or exceedingly broadened and fused cusps. Without attaching too much weight to the comparison, if we imagine the comb-like tooth of Galeopitkecus, a lemurine animal, to be devoid of enamel, and the cusps to be squeezed together into lateral contact, we should have a structure closely resembling a single row of the denticles of Myliobates. The point which it is wished to emphasise is that the dentinal systems in the latter are of the ordinary type of a fine-tubed or ortho-dentine, and it is only in the manner of their aggregation that they come to present a very different- looking structure. Hence any classification which removed 90 A MANUAL OF DENTAL ANATOMY. such a dentine far from an ortho- dentine would appear to be misleading. The arrangement of the tubes in Pristis is, how- ever, a little different : they are fewer and are more branched, and do not fill up the periphery of each system so completely. Vaso-dentine. — In dentine, as so far described, the existence of a system of fine tubes — dentinal tubes — has been the most Fig. 49. — Section of dentine from a freshly caught hake. d. Dentine matrix, cp. Capillary channels with blood vessels hanging out from the edge, containing here and there abundant blood corpuscles. conspicuous feature, but there are tissues equally entitled to be called dentine in which no such tube system exists at all. Fig. 50. — Tooth of Ostracion. a. Enamel, h. Capillary channels in the dentine, c. Tulp chamber. But, as it would seem necessary that some living material should permeate any tissue which is for any length of time to ORTHO-DENTINE AND ITS VARIETIES. 91 be retained in close connection with the rest of the organism, these dentines are not solid, but are permeated by a much larger tube system, the canals being in this case channels in which blood circulates through capillaries, so that during the life of the animal the teeth look quite pink, and in a section cut from the fresh tooth the coats of the capillary Fig. 51. — Hinged [tooth of hake. a. Vaso-dentine. h. Pulp. e. Elastic hinge, d. Buttress of bone to receive /, formed out of bone of attach- ment, e. Bone of jaw. /. Thickened and rounded free base of tooth. g. Enamel tip. may be seen to hang out from the edge, and the vessels to be full of blood corpuscles (Fig. 49). The arrangement of the vascular canals is regular and striking, reminding one of the appearance of the vessels in an intestinal villus ; in fact, an intestinal villus petrified, whilst its capillary network remained pervious, and red blood continued to circulate through it, would form no bad representation of a typical vaso-dentine tooth. 92 A MANUAL OF DENTAL ANATOMY. In all the vaso-dentine teeth with which the author is acquainted, the pulp chamber is of simple form, so that the conical pulp can he pulled out, and no tissue, or next to none save the hlood vessels, enters the canals, which are therefore of almost uniform diameter, and each capillary almost or quite fills and fits its canal. The matrix does not present a very definite structure, though there is generally some appearance of lamination parallel with the surface, and what has just been said with reference to the canals requires modification in so far as there are, in dry specimens, slight narrow spaces, thorn-like in Fio. 52. — From the tooth of the hake. The thorn-like processes are seen, as also a faint lamination of the matrix. section, which run from the canals between the lamina* (Fig. 52). These are not present in all vaso-dentines, but may be found in a good many, as, for example, in the hake, from which the figure has been drawn. Hose describes the outer part of the hake's tooth, beyond the vascular loops, as vitro-dentine. But the author does not lind that the dentine here differs either in the manner of its development or in its ultimate structure from the dentine which intervenes between the vascular loops, and hence to give it a distinctive name is unnecessary and misleading. The outer portion of a cod's tooth is more distinct in structure, but its character is not such as to lead one to apply the term vitro-dentine to it. As a rule the vascular canals do not extend quite out to the surface, but terminate sometimes in a bounding canal, and sometimes without one, at a very definite distance within fche surface (Fig* 58) ; this perhaps represents the original limit of the capillaries in the formative pulp. But however that OliTIIO-DKNTIXTC AND ITS VARIETIES. 98 may be, the structure of the matrix outside them is, in some examples, very interest ing in vi -w of recent invest igal ions into the development of dentine. This outer layer looks exactly like a calcified connective tissue which has preserved all its form, and it is not obscured by the occurrence in it of any tube system, interglobular spaces, or other structures. Such in brief are the characters of a typical vaso-dentine, but there are vast numbers of forms of this tissue which serve to bridge over the gap which would seem to exist between ii and hard dentine. Thus in the conical teeth of the flounder, Fig. 53. — Tooth of flounder, a. Hard dentine. b. Vascular canals. c. Enamel tip. For other examples of vaso-dentine, see the figures of the teeth of ChatodoR (F.ig. 149, p. 290), of Odontostomus (Fig. 131, p. 251). etc. and indeed of many flat fish (Pleuronectidai), the tips of the teeth are composed of ordinary fine-tubed dentine, surmounted by enamel tips. In this part of the tooth the tubes radiate out- wards in the ordinary way from the pulp chamber, and are both numerous and regular, but lower down in the tooth the dentinal tubes become less numerous, and paripassu with their diminution, vascular canals begin to make their appearance till, as the base of the tooth is reached, tolerably typical tubeless vaso-dentine is found. It may be supposed, therefore, that the two sorts of tubes are capable of taking the place of one another, and that the nutrition of the dentine may be provided for either by proto- plasmic material running for a long distance from the pulp 94 A MANUAL OF DENTAL ANATOMY. through dentinal tubes, or by blood brought nearer to hand by the vessel-carrying canals, but that both are not required, and so do not co-exist in full development. But from the flounder's tooth it may he learnt that hard dentine and vaso-dentine are not fundamentally dissimilar, and that they may pass into one another by imperceptible gradations, so that it cannot he said exactly at what point the name of vaso-dentine is to be given to it. And within the limits of a single family of fish — the Gadidce or cod-lish family — there are to be found all grades of vascularity, from the complete and regular system of vascular channel existing in a h FIG. 54.— Dentine and cementum of Megatherium ; the latter lies to the right, a. Vascular dentine. I>. Fine-tubed dentine, o. Cementum. d. Dentine. the hake, to the sparse canals of the haddock and the pollock ; in Lotelld there are neither dentinal tubes nor vascular canals, but a simple laminated structure. Kose states that the very small teeth of young haddocks are without vascular canals, and calls them vitro-dentine, and also states that the small teeth of very young pike consist of normal dentine without an osteo-dentine (trabeculo-dentine) core. But size of teeth alone does not determine a departure from the normal type. Thus the tiny teeth in the gill arches (" gillrakers ") of the hake consist of a beautiful vaso-dentine, whilst among the Gadidce are fish with teeth of large- size but only simple structure. (Of. 0. S. Tomes, -'.) The great extinct Megatherium possessed a dentine which was very rich in vascular canals, but their arrangement was a little different from that just described, as may be seen in the figure (54) to the left of which lies the inner portion of the oirnio-PExnxK and its varieties. 95 dentine, very rich in vascular canals ; this merges into a fine- tubed dentine (6), the vascular canals all ending at the same distance from the surface, while outside this again conies the very vascular cementum (c). The typical vasb-dentine teeth appear to be a good deal softer than those which are composed of line-tubed dentine, and accordingly they are generally either completely coated, or at all events tipped with enamel. It occasionally occurs that a vascular canal persists in the hard dentine of a human tooth. In such cases there is no relation between these canals and the course of the dentinal Fig. 55. — Vascular canal in human dentine. tubes, the latter being (as is seen in Fig. 55, drawn from a specimen shown to the author by Dr. Andrews at Cambridge, Massachusetts) displaced from their course by it. These canals, which occasionally occur both in human and other mammalian teeth, are to be regarded as abnormalities. There is yet another modification of the vascular canal occurring as an invariable character in certain teeth, which is of great interest from the point of view of evolution, since it exemplifies vascular canals losing their function and in process of disappearance. Towards the base of the teeth of Sargus, for instance, loops of uniform calibre, with their concavities directed towards the pulp, may be seen. No one familiar wuth the canals of vaso-dentine can for a moment doubt that they were formed around a vessel, but though such was their origin, the vessels soon ceased to be permeable, and their 96 A MANUAL OF DENTAL ANATOMY. open ends stop far short of the pulp-chamber, in the midst of the dentine. It is interesting to note that their ends are not abruptly cut off and sealed, but that they taper down and are continuous with one, two, or three dentinal canals (Fig. 56). Nothing is known of the contents of these loops in the fresh tooth, but it hardly admits of doubt that this partial persistence of the vascular channel, speedily abandoned in the further building of the dentine, indicates descent from a form in which these canals were functional, and the type of dentine a real vaso-dentine. The walls of these canals are smooth, just as in a typical vaso-dentine, but there is yet Fig. 56. — Vascular loops from tooth of Sargus, another form of canal which, in this respect, has departed a little more from its pristine type. In the outer part of the dentine of the manatee a series of regular loops are to be seen (Fig. 57), which at first sight look like, and in the earlier editions of this book were described as, vascular canals. Just as in Sargus, their general arrange- ment and fairly uniform diameter, &c, lead one to the conclusion that they mark the site of former vessels. But in place of their interior being smooth, a higher power discloses that it has numerous irregular sharp prolongations running out from it, between which rounded projections encroach upon the lumen of the canal. In point of fact the walls bounding the canal strikingly recall the walls of an interglobular space, and the whole canal might appropriately be compared to an enormously elongated inter- globular space. As in Sargus, nothing is known of the OKTHO-DENTINE AND ITS VAKIETIES. 97 contents in the fresh state ; but as in that fish, it is hardly possible to doubt that here also there is evidence that in some ancestral form there was a circulation through these channels. FlG. 57. — Dentine of manatee, showing partial persistence of vascular canals. The method of their disappearance is interesting; the dentinal tubes run into or past them, and had calcification gone a little further, to the obliteration of the above so-termed " long " interglobular spaces, the last trace of the ancestral vaso-dentine would have disappeared. Osteo-dentine. — This is a tissue which in its most typical form is marked off from hard dentine, plici-dentine and vaso- dentine alike by certain structural peculiarities and by the method of its development. It sometimes approaches very nearly to bone, from which indeed it has few essential points of difference. But there are many transitional forms, so that it is often hard to draw a sharp line of distinction between them. The difference can hardly be fully appreciated until the development of dentine has been described ; but it may be at once mentioned that osteo-dentine is not wholly formed upon the surface of the pulp, and it results from this, that whereas the pulp-chamber has a definite outline in those forms thus far described, and the pulp can be bodily pulled out, in osteo- dentine calcified trabecule shoot right through the pulp, there is ultimately no sharply defined pulp-chamber, and the pulp cannot be pulled out, the hard and soft tissues being inextricably mixed up. And though there are numerous large channels, often much larger than those of vaso-dentine, they are less regular, do not D.A. 7 98 A MANUAL OF DENTAL ANATOMY. in their arrangement so closely follow the form of capillary loops and in a fresh tooth contain masses of pulp-structure as well as blood-vessels. The pike's tooth affords a good example of osteo-dentine. Its surface is formed of a layer of fine-tubed tissue, almost like ordinary dentine, but this soon gives place to a coarsely channelled tissue, containing elongated spaces filled with pulp, Fig. 58.— Tooth of common pike. a. Outer layer of fine-tubed dentine. b. Inner mass of osteo-dentine. from which canaliculi, like those of a bone lacuna, branch off in all directions, but do not extend tar. The fine-tubed layer which forms the exterior differs from ordinary dentine in the respect that the tubes, although parallel in their outer portion, in the absence of a general pulp cavity, as they pass inwards become gathered together into fewer and larger tubes, which again originate from the somewhat irregularly shaped channels in the interior of the tooth. ORTHO-DENTINE AND ITS VARIETIES. 99 Their relationship to the fine tubes which occur in less abundance in the deeper parts of the tooth is thus shown, and the close resemblance of these dentinal tubes to the canaliculi of ordinary bone becomes apparent. It is stated by Sternfeld (17) that flat cells line all the surfaces, and that processes from these enter the tubes. The structure of the teeth of sharks show a large amount of variability; thus the teeth of the great white shark (Carcharias), which is so formidable in the tropics, are com- posed of ordinary fine-tubed dentine, with the exception of their bases, which are of osteo-dentine. On the other hand, the teeth of the Porbeagle shark (Lamna) Fig. 59. — Outer layer of the dentine of the tooth of a pike. The fine canals are seen to be gathered into bundles, which spring from the large canals of the axial portion of the tooth. Rose would distinguish this outer layer as normal dentine, postulating a change in the formative cells, which he holds to have been at first odontoblasts, afterwards osteoblasts. But this is an assumption not proven. consist of a central core of osteo-dentine surrounded (?) with enamel. (Fig. 60.) In an osteo-dentine, medullary canals of varying size run, with a direction, roughly speaking, parallel to the long axis of the tooth, anastomosing with one another, and from their sides wavy bundles of fine tubes radiate, but do not run far ; that is to say, its dentinal tubes do not radiate from any one central pulp chamber, but from an indefinitely large number of canals. When, as happens in some sharks, these medullary canals are very regular in size, are placed at nearly uniform distances from each other and run parallel with one another towards the surface, a structure not widely different in places from that found in Myliobates obtains. But as in Myliobates there are few teeth, whereas in most rays there are a great many, 7—2 100 A MANUAL OF DENTAL ANATOMY. it is possible that its structure may be the result of fusion of many denticles, a process actually observed in Ceratodus, and in the Cochliodontidce, a family of Palaeozoic sharks. Its development is not sufficiently well known to settle the question.* At the other extremity of the series canals much more branched and irregular in their course are found, the tubes radiating from them reduced in amount and extent, some Fig. 60. — Tooth of a species of Lamna, consisting of a central mass of osteo-dentine with a fine-tubed tissue which may probably be regarded as enamel, upon its surface, and which is not homologous with that on the surface of the pike's tooth. * degree of lamination of the matrix, and, in short, a tissue which is practically like ordinary bone. The similarity of the channels of pulp in osteo-dentine to Haversian canals in bone is in some respects close ; so similar that when teeth consisting of osteo-dentine become, as in many fish they do, anchylosed to the subjacent bone, it becomes impossible to say at what point the dentine ends and the bone commences ; and this difficulty is intensified by the fact that the bone of many fishes lacks lacunae, and is almost exactly like dentine. In many fish the outer compact layer of the jaw- * Gegenbaur admits the reality of a concrescence in Ceratodus, but denies, on the authority of Treuenfels, its occurrence in Myliobates. ORTHO-DENTINE AND ITS VARIETIES. 101 bone has fine parallel canaliculi, and no other structure is visible, so that it bears a close resemblance to the external layer of these osteo-dentine teeth. Osteo-dentine was defined by Owen as dentine in which the matrix is arranged in concentric rings around the vascular canals, having lacunae similar to those of bone. But neither of these characters are to be found in many teeth which, if the manner of their development is to be taken into account, are unquestionably composed of osteo-dentine ; and so they cannot be made use of for purposes of definition, although lacunas and lamination of the matrix are far more often present in osteo-dentine than in the other varieties of dentinal structure. It will be seen from what has already been stated that the varieties of dentine blend with one another, and it becomes a matter of difficulty to frame satisfactory definitions, especially in the case of osteo-dentine, without taking into account its development. The varieties of dentine may be grouped thus : (A.) Dentines developed upon the surface of a pulp, by calcification of its surface layers. Dentinal tubes when present, radiating from a single axial pulp chamber or from its folds. (i.) Hard, unvascular dentine, thoroughly per- meated with dentinal tubes, which radiate from a simple central pulp chamber. Example — Human dentine, (ii.) Plici-dentine, permeated with dentinal tubes, which radiate from a pulp chamber rendered complex in form by in-foldings of its walls. Examples — Lepidosteus Labyrinthodon. (iii.) Vaso-dentine, with dentinal tubes few or absent, but abundant capillary channels with blood circulating through them. Example — Hake, (iv.) Dentines which might be regarded as aggregations of fused parallel denticles, as forms of folded dentine, or as transi- tional towards a very regular osteo-dentine. Examples — Myliobates, Pristis. 102 A MANUAL OF DENTAL ANATOMY. (B.) Dentines developed by calcification extending through the interior of a pulp, not by calcification of a surface layer. Dentinal tubes have relation to the medullary tracts left, and not to an axial pulp chamber. (v.) Osteo-dentine, or Trabeculo-dentine; with dentinal tubes which radiate from numerous medullary channels, and which may be only short and few in number, or may be fully developed. Lamination of the matrix and spaces like bone lacunae occur in some osteo-dentines. Examples — Pike, Lamna. Eose, in a valuable paper (16), points out that, with the exception of true vaso-dentine, which is first known to occur in some cretaceous fish (Empo- and Pachy-rhizodus), all of the modifications of dentine are found to exist in the earliest known Silurian fish, and he thinks that normal dentine was probably the oldest calcified tissue. In the same paper Eose proposes a different classification of dentines, viz. : Ortho-dentine (under which heading would come Vaso-dentine and Vitro-dentine as subdivisions). Trabecular dentine. Osteoid dentine. Bone dentine. The criterion which he selects for his definition of normal dentine is that it shall have been developed in an even line from without inwards immediately under an epithelial sheath, and therefore will have a single undivided pulp cavity. I am not in the least concerned to defend my classification because I happen to have proposed it myself, and I have repeatedly pointed out that it is not in all respects satisfactory. But I do not find Eose's classi- fication in practice to be any better, and, in fact, I think that the applica- tion of his criterion leads one into difficulties, necessitating the grouping together of forms rather divergent, and separating others which are, practically identical, into different classes. Thus Carcharias has dis- tinctly normal dentine, whilst other Selachia have, equally distinctly, osteo-dentine. Yet in their relation to the epithelial sheath these two forms are absolutely similar. In like manner the incipient in-foldings of such teeth as Varanus (near its base) by Eose's criterion remain normal dentine, but precisely the same sort of folding carried further is called by him trabecular dentine. To the term trabecular dentine, in itself, I have no objection : indeed, I think it a better descriptive term than osteo-dentine : it is to Eose's application of the term in leading to a grouping together of forms which ORTHO-DENTINE AND ITS VARIETIES. 103 appear to me essentially different, that exception is taken. My reasons for differing from him are given at greater length in a paper (23). Eose adopts fully my definition of vaso- dentine, but does not retain plici-dentine as a separate distinct variety, although he speaks of " real folds of pure dentine," in Rhizodus, Megalicthys, etc., my plici-dentines coming under his trabeculo-dentines, where I do not think they belong. And he distinguishes as vitro-dentine some structures which I do not think are distinct or require a distinctive name, e.g., the outer portion of a hake's tooth, which differs in no respect from that which lies deeper, save that the vascular canals stop short of it (cf. p. 92), and brings under this heading others which I consider preferably as enamels, e.g., the hard outer layer of Selachian teeth (cf. p. 53). (C. S. T.) It remains to be added that the same pulp may undergo a change in the manner of its calcification ; that is to say, that after having gone on with surface calcification for a certain length of time, this may give place to a more irregular internal calcification into an osteo-dentine. This is especially prone to happen after injury, and is often exemplified upon a large scale in elephants' tusks, the pulps of which, normally engaged in calcifying the odontoblast layers into ivory, may after an injury calcify irregularly and solidify into a coarse osteo-dentine. It will then be easy to understand that so-called secondary dentine, produced in a pulp which ordinarily forms hard dentine, may partake of the character of hard, of vaso-, or of osteo-dentine. Thus the pulp of a sperm whale's tooth becomes obliterated by a development of secondary dentine, which sometimes forms irregular masses loose in the pulp chamber, and some- times is adherent to and continuous with the dentine previously formed. The structure of these masses is very confused. Tubes, of about the same diameter as dentinal tubes, abound ; but they are often arranged in tufts or in bundles, and without any apparent reference to any common points of radiation. Irregular spaces, partaking of the character of interglobular spaces or of bone lacunae, abound ; and vascular canals are also common. In the human tooth secondary dentine occurs in the teeth of aged persons, in which the pulp cavity is probably contracted in size, and is also very frequently formed pathologically, as a protection to the pulp when threatened by the approach of dental caries (called by Hopewell- Smith " adventitious den- 104 A MANUAL OF DENTAL ANATOMY. tine ") or by the thinning of the walls of the pulp cavity through excessive wear. The accompanying figure, represent- ing one of the cornua of the pulp chamber from a molar tooth affected by caries, is a good example of pathological adventitious dentine. It occasionally happens that the pulp resumes its formative activity, and new dentine is developed, which with the exception of a slight break or bend in the continuity of the tubes, is almost exactly like normal dentine. More often, A Fig. 61. — Areolar adventitious dentine (X 250): A = areolar dentine; D = primary dentine ; P = pulp tissue. Secondary (adventitious) dentine filling up one of the cornua of the pulp cavity. From a human molar affected by caries. however, the boundary line between the old and the new is marked by an abundance of irregular spaces and globular contours, whilst further in the mass of new secondary dentine, the tubular structure again asserts itself more strongly. The axial portions of many teeth of persistent growth, for some little distance below the working surfaces, become filled by a calcification which has much in common with the secondary dentines ; that is to say, it is of coarser and less definite structure than the peripheral parts of the same tooth. OKTHO-DENTINE AND ITS VABIETIES. 105 For example, in the armadillo the axial portion of the tooth is made up of a tubular tissue, but the tubes are crowded together into irregular bundles, and instead of running from the interior towards the surface, they run more or less along the axis of the tooth. In another member of the Edentata (the sloth) the axial portion differs much more from the out- side, being very much softer and more porous, containing, in fact, a large number of vascular channels, and being very apt to break away in the process of rubbing down the section. In some rodents the final closure of the axial tract takes place almost by a continuance of the formation of normal fine- tubed dentine with very little secondary dentine of different structure, while in others there is a large area of dentine with vascular tracts in it. In the partly persistently growing incisor of the kangaroo there is a mass of secondary dentine which is sometimes almost glassy looking and structureless. In some specimens of secondary dentine from an elephant's tusk Howard Mummery found an abundant connective tissue stroma after decalcification. References. (x) Berzelius. Vide Retzius. (2) von Bibra. Vide Chapter II. (references). (3) Black, GL V. " An Investigation of the Physical Characters of the Human Teeth in Relation to their Diseases, etc. Dental Cosmos, May, 1895, vol. xxxvii., pp. 353 et seq. (4) Credner, H. T. Abhandl. Math.-phys. Leipzig. 1893. (5) Deppendorf, T. " Beitrage zur Kenntnis der Innervierung der Menschlichen Zahnpulpa und des Dentino- Deutsche Monatssch. f. Zahnheilkunde," Sept., 1913. Heft 9, 31 Jahrgang, pp. 689—719. (6) GtAlippe, V. " Recherches sur la Constitution Physique et Chimique des Dents a l'Etat de Sante et de Maladie." Comp. Rend, des Seances et Memoires de la Societe de Biologie, 1884, t. i., 8th ser., Paris, pp. 289—292. (7) Henle. Allegemeine Anatomie. 1841. (8) Hopewell-Smith, A. "On Dentogeny.'1 Dental Record, 1889, vol. ix., pp. 337 —343. (9) "The Histology and Pathology of the Teeth and Associated Parts." London. 1903. (10) Hoppe, F. " Untersuchungen iiber die Constitution des Zahn- schmeltze." Virchow Arch. f. Path. Anat. Berlin, 1862, bd. xxiv., pp. 13—32. 106 A MANUAL OF DENTAL ANATOMY. (u) Klein, E. Vide Chapter II. (references). (12) Magitot, E. "De la Structure et du Developpement du Tissue Denticulaire dans la Serie Animale." Comp. Eend. Acad. d. Sci., Paris, 1888, t. xc, pp. 1298— 1301. (13) Mummery, J. H. "On the Distribution of the Nerves of the Dental Pulp." Phil. Trans. Eoy. Soc, Lond., Feb., 1912, vol. ccii. B., pp. 337—349. (14) Eetzius, von A. " Bemerkungen iiber den innern Bau der Zahne, mit besonderer Eiicksicht auf den im Zahnkunchen verkommenden Eiihrenhau. " Miiller's Arch. f. Anat. u. Phys., 1837, pp. 487—565. (15) Homer, O. "Zahnhistologische Studie." Freiburg-in-Baden. 1900. (16) Eose, C. " Ueber die verschiedenen Abanderungen der Hautge- webe bei Neideren Wirbelthieren." Anat. Anz., 1898, bd. xiv., pp. 21—31 u. 33—69. (17) Sternfeld, A. " Ueber die Struktur des Hechtzahnes, Inbeson- dere die des Vasoden tines." Archiv. f. Mikr. Anat., bd. xx., 1881—2, pp. 382—412. (18) Tomes, Sir J. "On the Structure of the Dental Tissues of the Order Eodentia." Phil. Trans. Eoy. Soc, Lond., 1850, pt. ii., pp. 529—569. (19) Tomes, C. S. Journ. of Phys. (vide Chapter II., references). (20) " Notes upon Dentine and Enamel." Trans. Odontol. Soc. of Gt. Brit., 1896, vol. xxviii., N.S., pp. 114—132. (21) " On Differences in the Histological Structure of Teeth occurring within a Single Family — the Gadidse." Jan., 1899. Quart. Journ. Micr. Sci., vol. xli., N.S., pp. 459—471. (22) " On the Development of Marsupial and other Tubular Enamels, with Notes upon the Develop- ment of Enamel in General." Phil. Trans. Eoy. Soc, Lond., 1898, vol. clxxxix., pp. 107 — 122. (23) ■ " Upon Eose's Proposed Classification of the Forms of Dentine." Anat. Anzeiger, 1898, bd. xiv., pp. 343—348. (24) Joint Authors. Trans. Illinois State Dental Soc. 1898. CHAPTEK IV. THE TOOTH PULP CEMENTUM THE GUM THE ALVEOLO -DENTAL MEMBKANE. The pulp occupying the central chamber or pulp cavity was the formative organ of the dentine, and consequently varies in its anatomical characters according to its age. As well as being the remains of the dentine-papilla of the tooth-germ, it is the source of vascular and nervous supply to the dentine. The pulp may be described as being composed of a very delicate connective tissue with spaces filled with a jelly-like material, the fibres of which are probably principally the processes of the cells. The finely fibrous element of the pulp is not, however, brought into view unless special stains are employed which colour it ; if this be done, it is found that there is far less unoccupied space than was formerly supposed. The cells are fairly abundant. Some ordinary fibrous con- nective tissue is discoverable in it, and nerves and blood- vessels also ramify abundantly in its substance. The intercellular matrix, remaining in material proportion whilst the cells are not greatly developed and the fibrous element is very delicate, gives to the pulp a character common to many embryonic tissues. At an early period the cells of the dentine papilla are some- what rounded embryonic cells, a surface layer of specialised cells, the future odontoblasts, making their appearance only about the time at which calcification commences. The deeper cells grow out into processes, but these remain very fine, and at no period are large bundles of connec- tive tissue fibres to be found unless the pulp has undergone degeneration in advanced age. In a specimen cut from a loose tooth in an aged person, which was given to the author by Hopewell- Smith, the cellular, vascular and nervous elements have all completely disappeared, and there remains nothing but connective tissue, the bundles of which run into the dentine. It is possible that the con- spicuousness of this connective tissue may partly be due to 108 A MANUAL OF DENTAL ANATOMY. the fact that it is in no degree concealed by the abundant cells of an ordinary pulp, but it seems probable that the connective tissue itself is also coarser and more abundant. The entrance of connective tissue fibres into the dentine is of importance in connection with the question of the entrance of nerve fibres into the dentine, as some observers regard the structures described as nerves as possibly being connective tissue fibres. The cellular elements of the pulp are arranged, as seen in transverse sections, in a direction radiating outwards from the centre ; this is most marked in the highly specialised layer of cells which form the surface of the pulp, and are termed odontoblasts. The odontoblast layer, sometimes called the mcmbrana Fig. 62. — Surface of human pulp, showing odontoblasts with their pro- eboris, because it usually adheres more strongly to the dentine than to the rest of the pulp, and is therefore often left on the surface of the dentine when the pulp is torn away, consists of a single row of large elongated cells, of a dark granular appearance, with large and conspicuous nuclei near to the ends furthest from the dentine. The sharp contours possessed by the odontoblasts in pulps which have been acted on by chromic acid, alcohol, or even water, are absent in the perfectly fresh and unaltered condi- tion, and it is believed that they have no special investing membrane. They have two sets of processes, one peripheral, and one central. Of these the dentinal process (equivalent to the dentinal fibre) enters the canal in the dentine, and the individual odontoblasts may be furnished with several dentinal processes. A cesses. A. Dentinal process. B. Nucleus. THE TOOTH PULP. 109 Dr. Aitchison Kobertson (12) describes the central or pulp process and the odontoblasts in the ox as being very long and fine, but says that -there is no lateral union: his specimens, however, appear to have been mostly teased out, and the cells figured hardly look like odontoblasts. Lateral processes, by which neighbouring odontoblasts communicate with one another, have been described, but there is some doubt whether they have any real existence. When dentine formation has commenced, the odontoblasts are united at their extremities nearest to the dentine by a sort of collar, Fig-. 63. — Odontoblast cells with their (A) dentinal processes and (B) collars. Beneath and to some extent between them are seen the fusiform cells and connective tissue fibres of the pulp. but this is of the nature of imperfectly formed material, and is not properly a process of the odontoblast cell. The process which enters the dentinal tubes is of some size, tapers slightly, and extends from the cell in a line coincident with its longitudinal axis. The membrana eboris covers the surface of the pulp like an epithelium. The odontoblasts vary much in form at different periods; in the youngest pulps, prior to the formation of dentine, they are rounded, or rather pyriform. During the period of their greatest functional activity the end directed towards the dentine is more or less square, though tapering to a slight extent into the dentinal process; while in old age they become comparatively inconspicuous, and assume a more rounded or ovoid shape. They are altogether larger in the coronal than in the radicular regions of the pulp, and, in transverse sections of the tissue, are seen to be shorter and A 110 A MANUAL OF DENTAL ANATOMY. rounder in its constricted portion than in its longer diameter. In cylindrical pulps their lengths are the same. The odonto- blasts are slightly separated from one another, an effect which is rarely seen in the enamel cells — a fact which, if due to shrinkage through the use of histological reagents, would indicate either that they have no definite limiting membranes, or else that they are of a softer material. But it is very possible, especially in view of recent researches into the development of dentine, that they are really slightly separated from one another. They usually, however, adhere together firmly at their ends nearest to the dentine, so as to form here a continuous sheet. The general matrix of the pulp, as has been before noted, is of firm, gelatinous consistency ; it is a little more dense upon the surface, whence has perhaps arisen the erroneous idea that the pulp is bounded by a definite membrane. The vessels of the pulp are very numerous ; three or more arteries enter at the apical foramen, and, breaking up into branches which are at first parallel with the long axis of the tooth, finally form a capillary plexus immediately beneath the cells of the membrana eboris. The nerves enter the apical foramen of the root by one large trunk and three or four minute ones. After pursuing a parallel course, and running in close association with the vessels, mean- while giving off some branches which anastomose but little in the expanded portion of the pulp, they form a rich plexus beneath the membrana eboris, as has been described by Kaschkow and many subsequent writers. But here our exact knowledge almost ends, for the nature of the terminations of the nerve fibres in the pulp is not with certainty known ; the nerve fibrils, which are extraordinarily abundant near to the surface of the pulp, often form meshes, but this does not appear to be their real termination. Boll, as has already been mentioned, investigated this point, and found that if a pulp be treated for an hour with very dilute chromic acid solution, an immense number of fine non-medul- lated nerve fibres, which he succeeded in tracing into continuity with the larger medullated fibres, may be discerned near to the surface of the pulp. The ultimate destination of these nerve fibres is uncertain ; but he has seen them passing through the membrana eboris, and taking a direction parallel to that of the THE TOOTH PULP. Ill dentinal fibrils in such numbers that he infers that they have been pulled out from the canals of the dentine. Still, what- ever may be the .probabilities of the case, he has not seen a nerve fibre definitely pass into a dentinal canal, and the observations of Morgenstern (8), Komer (13) and others are not convincing, as Golgi's staining method may lead to false appearances. Howard Mummery, using as a stain an iron salt followed by tannin, has succeeded in showing that from the nerve bundles which are distributed to the surface of the pulp very fine fibres are given off, which run towards the dentine, and the author has examined these preparations with him. Whilst there are appearances suggestive of their entering it, no certain conclusion could be reached, such is the difficulty of an investigation where very delicate structures are in relation with a calcified material. Nevertheless, recently, by staining according to the method of Eamon y Cajal, Mummery is of the opinion that nerve fibres do actually penetrate the dentinal tubes. As has been stated in the previous chapter, Mummery's observations seem to have been confirmed by Deppendorf (p. 79). Eetzius (u) investigated the nerves of the teeth in some fish, batrachians and reptiles, using Golgi's stain and finding practically similar conditions in all. Fine non-medullated fibres pass upwards in the pulp, subdividing as they go. They are beaded, and arrive at the region of the odontoblasts where they end close to the dentine, but they were never observed to enter it. Their actual terminations were fine short ramifications, and now and then the ends are swollen into little knobs, the apices of the pulps being without nerves. In fish, the nerves break up into a plexus in the soft tissues between the teeth, and only fine non-medullated fibres enter the pulps at all. In young mice the ultimate fibrils, beaded as in fish and reptiles, pass between the odontoblasts and there terminate in free ends, and do not penetrate into the dentine. Carl Huber (4) employed methylene blue, injected into the carotids of rabbits and cats immediately after death. He states that nerves course up the pulp in bundles of eight or ten medullated fibres, which towards the surface anastomose by exchange of fibres or by actual branching at the nodes of 112 A MANUAL OF DENTAL ANATOMY. Kanvier. Below the odontoblast layer they break up into numerous beaded non-medullated fibres and form a plexus by crossing and recrossing. They terminate by fine ends or granules near to the free ends of the odontoblasts, sometimes appearing to almost enclose the cells in a basket-work of fibres. He states that they never make any connection with the odontoblasts, nor could they be expected to, being them- selves epiblastic and the odontoblasts mesoblastic in origin.* A few medullated fibres reach as far as the odontoblasts. These observations are therefore in complete accord with those of Eetzius, and have been, to some extent, confirmed by Hopewell-Smith (3b). Boll's observations have not been fully confirmed by any subsequent worker in the field, nor had they been definitely controverted until Fig. 64. — Non-medullated nerves ending amongst the odontoblasts. (After C. Huber.) Legros and Magitot (5) stated that they had fully satisfied themselves that the nerves become continuous with the branched somewhat stellate cells which form a layer beneath the odontoblasts, and through the medium of these cells with the odontoblasts themselves. (See Fig. 37.) If this view of their relation to the nerves were correct the sensitive- ness of the dentine would be fully accounted for without the necessity for the supposition that actual nerve fibres enter it, for the dentinal fibrils would be in a measure themselves prolongations of the nerves. Dr. Eobertson (loc. cit.) describes the nerves of the tooth pulp of the ox, his specimens mostly having been made by teasing fresh pulp in 1 per cent, solution of osmic acid. He describes large bundles of medul- lated and non-medullated fibres, the former being the more numerous, and in the teased specimen the axis-cylinder of medullated fibres was often seen stretching far beyond the sheath. He believes that the axis- cylinders of medullated nerves lose their sheath, and after running for a greater or less distance, become continuous with the pulp process of the odontoblasts, which therefore, with their dentinal processes, become nerve-end organs, so that he so far agrees with Magitot. * See footnote on p. 77. THE TOOTH PULP. 113 Oscar Eomer (13), using the intra-vitam method of methylene blue staining, agrees with Eetzius and Huber up to a certain point, but further states that the fine fibres, after passing between the odontoblasts, actually enter the dentinal fibrils and so pass into the dentine, but his figures fail to exclude the presumption of error. Morgenstern (p) goes yet further and says that two axons enter each canal and reach the outer boundary of the dentine, or even pass into the enamel. It will hence be seen that some uncertainty still attaches to the odontoblasts and their functions, but to this it will be necessary to recur in treating of the development of the dentine. This difference of opinion as to the function of the odonto- blast cells never can be finally settled until the nerve- endings of the pulp are satisfactorily demonstrated. The idea that the odontoblasts themselves are nerve-end organs was advo- cated by Coleman, and more recently in a modified form by Hopewell- Smith (3) ; but there are some objections to this view, amongst others being the occurrence of well-marked odontoblasts in places where they seem not only useless but decidedly misplaced as nerve-end organs, and the observation of Eetzius that there are parts of teeth where there are odontoblasts, but which are not reached by any nerve fibres at all. Towards the outer surface of the pulp, but beneath the odontoblasts, Weil has described a layer about which there has been, and indeed still is, some controversy. It goes by the name of the " basal layer of Weil," and consists of a comparatively pale and transparent zone lying between the inner ends of the odontoblasts and the pulp. He describes it as consisting of fine connective tissue fibres which communicate with the processes of the odontoblasts. Howard Mummery (9) points out that though visible in many of his preparations made by the Koch-Weil process, it is absent near to the growing base of young teeth, and it is in the crown that it is most pronounced. He disagrees with Weil as to the connection of the fibres of this layer with the odontoblasts, saying that he has often traced these fibres through the cell layer into the dentine matrix. Yon Ebner altogether doubts its existence, attributing it to the pulp shrinking while the odontoblasts are held up to the dentine by their processes. Rose shares Yon Ebner' s view. Weil compares it to a basement membrane, urging that a fibrillar D.A. 8 114 A MANUAL OF DENTAL ANATOMY. structure has been detected in many undoubted basement membranes, but whether it be an actual or an artefact structure, there seems little reason to regard it as anything more than connective tissue pulp stroma. Walkhoff regards it as a result of atrophy of the odontoblast layer, and says it does not exist in young active pulps. But this is not in conformity with recent research. It has already been mentioned that the pulp undergoes fibroid alterations in advanced age, its diminution in size", at times, by its progressive calcification and the addition thus made to the walls of the pulp cavity being the most conspicuous changes which occur. In pulps which have undergone a little further degeneration, the odontoblast layer becomes atrophied, and the fibrillar connective tissue more abundant, coinci- dently with the diminution in the quantity of the cellular elements. Finally, the capillary system becomes obliterated by the occurrence of thrombosis in the larger vessels, the nerves undergo fatty degeneration, and the pulp becomes reduced to a shrivelled, unvascular, insensitive mass. These changes may go on without leading to actual putrefactive decomposition of the pulp, and are hence not attended by alveolar abscess ; but a tooth in which the pulp has undergone senile atrophy is seldom securely attached to its socket. The pulps of the teeth of some animals become eventually entirely converted into secondary dentine, but it would seem to be very generally the case that those teeth which exercise very active functions and last throughout the life of the creature retain their pulps in a more or less active condition. No lymphatic vessels are known to exist in the tooth pulp. Halle's experiment of injecting Prussian blue into the tissue showed that there are neither lymph spaces nor lymphatic vessels in it. But further experiment by painting Prussian blue upon an exposed pulp in a dog, and sealing it in, proved that somehow the particles passed through the whole pulp and were found in the lymphatic glands, possibly passing by way of the liquid of the tissue, or possibly carried by wander- ing cells (Morgan,7). The lymphatic glands which receive the products from the teeth and gums are, according to Morgan : (i.) The supra-hyoid glands, which lie between the bellies of the digastric muscle, drain the anterior part of the THE TOOTH PULP. 115 lower gums and teeth, and pass the lymph on to the submaxillary group, which are numerous and small. These lie in the digastric triangle along the lower edge of the jaw. (ii.) The superficial cervical glands which lie along the course of the external jugular vein receive some lymph from the posterior part of the gum, while the deep cervical lie along the internal jugular; indirectly through the submaxillary group they receive lymph from the structures of the mouth. According to the text-books the lymph from the upper gums and teeth is received by the deep facial glands ; they are situated by the side of the pharynx behind the buccinator muscles, and it is suggested by Morgan that cases of suppura- tion in these glands may not be recognised, but be mistaken for quinsy, as they are close to and behind the tonsil. But for some unknown reason suppuration in the lower lymphatic glands is far more common than in these upper ones. CEMENTUM. This tissue forms a coating of variable thickness over the roots of the teeth, sometimes, when the several roots are very close to one another, or the cementum is thickened by disease, uniting the several roots into one. The cementum is ordinarily said to be absent from the crown of the teeth of man, the carnivora, &c, and to commence by a thin edge just at the neck of the tooth, meeting the enamel to a slight extent. It is, in normal circumstances, thickest in the interspaces between the roots of molar or premolar teeth ; it is, however, often thickened at the end of a root by a patho- logical hyperplasia. In compound teeth, the cementum forms the connecting substance between the denticles (see the figures of the tooth of the Capybara, the elephant, &c), and, before the tooth has been subject to wear, forms a complete invest- ment over the top of the crown. The cementum also covers the crowns of the complex-patterned crowns of the teeth of ruminants. Tho cementum here, is the most external of the dental tissues : a facii which necessarily follows from its being derived more or less directly from the tooth follicle. Both physically, chemically, and also in respect of the manner of its development, the cementum is closely allied to 116 A MANUAL OF DENTAL ANATOMY. bone. It consists of a laminated calcified matrix or basal sub- stance, and, if large in amount, lacunae. Vascular canals corresponding to the Haversian canals of bone may be rarely met with, but it is only in thick cementum that they exist and, in man, perhaps in hyperplasia more often than in the thick healthy tissue. The lamellae of the cementum are thinner towards the neck of the tooth, being thickest at the apex of the root, but the number of the lamellae is about the same in all parts. Soon after the completion of a tooth, there are but few lamellae, so that usually an adult has cementum far thicker Fig. 65. — Thick laminated cementum from the root of a human tooth. than a child, an aged person again perhaps having more than an adult. The matrix is a calcified substance, which, when boiled, yields gelatine, and if decalcified, retains its form and struc- ture : it is, in fact, practically identical with the matrix of bone. It is sometimes apparently structureless, at others finely granular, or interspersed with small globules. The lacunae of hyperplasic cementum share with those of bone the following characters : in dried sections they are irregular cavities, elongated in the direction of the lamellae of the matrix, and furnished with a large number of processes. The processes of the lacunae (known as canaliculi) are most abundantly given off at right angles to the lamellae (see Fig. 65), and, again, are more abundantly directed towards the exterior CEMENTUM. 117 of the root than towards the dentine. The lacunae differ from those of bone in being far more variable in size, in form, and in the excessive" number and length of their canaliculi. In this latter respect the lacunae of the cementum of cetacean teeth are very remarkable. Many of the lacunae are connected, by means of their canaliculi, with the terminations of the dentinal tubes (Fig. 66) ; they, by the same means, possibly intercommunicate with one another, while others of their processes are directed towards the surface, which, however, in most instances, they do not appear to actually reach. The lacunae assume all sorts of peculiar forms, especially in the thicker portion of the cement. Here and there lacunae are to found which are furnished FlG. G6. — Lacuna of cementum which communicates with the terminations of the dentinal tubes, a. Cementum. b. Dentine. with comparatively short processes, and are contained within well-confined contours. Sometimes such a line is to be seen surrounding a single lacuna, sometimes several lacunae are enclosed within it ; lacunae so circumscribed are called " encapsuled lacunae," and were first observed by Gerber in the cementum of the teeth of the horse (they are specially abundant in the teeth of the Peris sodactyla). By cautious disintegration of the cementum in acids these encapsuled lacunae may be isolated: their immediate walls, just as in bone, being composed of a material which has more power of resisting chemical reagents than the rest of the matrix. The encapsuled lacunae may perhaps be regarded as individual osteoblasts, or nests of osteoblasts, which have to some extent preserved their individuality during calcification. In the fresh condition it appears probable that the lacunae are filled up by soft matrix, which shrinks, and so leaves 118 A MANUAL OF DENTAL ANATOMY. them as cavities in dried sections. It can hardly as yet be said that the question of the contents of the lacunae has been finally settled, though the researches of Bodecker and Heitzmann have gone far towards doing so. According to these writers each lacuna contains a pro- toplasmic body, which they term the cement corpuscle, with a central nucleus. This nucleus may be large and surrounded by but little protoplasm, or it may be small ; or there may be many nuclei. Heitzmann, after staining with gold chloride, claims to have demonstrated protoplasmic matter in the thin layer of cemen- tum of the neck of the tooth, and while the author by no means agrees with him in regarding as protoplasm everything which will stain with chloride of gold, yet clinically there would seem to be very little doubt, that the cementum in this locality is sensitive, and that there is an a priori probability that there is some uncalcified tissue there. At the same time it must be remembered that semi-calcified tissue can be stained with silver nitrate, and sometimes also with gold, on the periphery of the pulp, in interglobular spaces, and in Neumann's sheaths, and one would quite expect to find, in the interstices of such globular forms as can be made out in this thin cementum layer, such semi- calcified matrix material. Heitzmann also throws doubt upon the perforating fibres that are to be described as being Sharpey's fibres ; he interprets them also as protoplasmic, in which view few histologists will share. The cementum corpuscles communicate freely with one another by offshoots, those of large size occupying the conspicuously visible canaliculi of the lacunae, whilst the finer offshoots are believed by Heitzmann to form a delicate network through the whole basis substance or matrix. The corpuscles of the cementum near to the external surface give off numerous offshoots which communicate with protoplasmic bodies in the alveolo-dental periosteum. By this means it would appear as if the cementum can remain alive even when the pulp of the tooth is. dead, and thus the tooth be in no way a mere foreign body, dead and inert. - Like bone, cementum contains Sharpey's fibres ; that is to say, minute rods running through it at right angles to its CEMENTUM. 119 own lamination, and, as it were, perforating it. These are probably calcined or uncalcified bundles of connective tissue. And it is by the medium of these that the alveolo-dental periosteum adheres to the cementum. Where the cementum is very thin, as, for instance, at the neck of a human tooth, it is generally to all appearances structureless, and does not contain any lacunae and therefore no protoplasmic bodies ; nevertheless, lacunae may be some- times found in thin cementum, as, for example, in that thin layer which invests the front of the enamel of the rodent-like tooth of a wombat. Hopewell- Smith is of the opinion, based on the examination of many sections, that there are no lacunae in perfectly healthy human cementum, and that their presence is a sign of subsequent pathological increase in the tissue. He believes that cementum per se is insensitive, but that tactile sensations may be conveyed through it to the underlying dentine (3c). The cementum at the neck is also devoid of lamellae ; it appears to be built up by direct ossification of osteoblasts, the characteristic shape of which may be traced in it. Bodecker describes it as permeated by a fine but abundant network of soft living matter. The larger dentinal tubes fall short of the boundary line at the neck, but according to him a fine protoplasmic network crosses it. He states that it has a covering of epithelial elements, like those of the gum. The outermost layer of thick cementum is a glassy film, denser apparently than the subjacent portions, and quite devoid of lacunae, being homologous with the peripheric lamellae of the long bones. On the surface it is slightly nodular, and may be described as built up of an infinite number of very minute and perfectly fused globules ; this is, in fact, the youngest layer of cement, and is closely similar to that globular formation which characterises dentine at an early stage of its development. The cementum is very closely, indeed inseparably, connected with the dentine, through the medium of the "granular" layer of the latter ; the fusion of the two tissues being so intimate that it is often difficult to say precisely at what point the one may be said to have merged into the other. And in this region there is an abundant passage of protoplasmic filaments across from the one to the other. 120 A MANUAL OF DENTAL ANATOMY. Nasmyth's membrane. — Under the names of " Nasmyth's membrane," " enamel cuticle," or " persistent dental capsule," a structure is described about which much difference of opinion has been, and indeed still is, expressed. Over the enamel of the crown of a human or other mammalian tooth, the crown of which is not coated by a thick layer of cementum, there is an exceedingly thin membrane, the existence of which can only be demonstrated by the use of acids, which cause it to become detached from the surface of the enamel. "When thus isolated it is found to form a continuous transparent sheet, upon which, it is said, by staining with nitrate of silver, a reticulated pattern may be brought out, as though it were made up of epithelial cells. It is exceedingly thin, Kolliker attributing to it a thickness of only one twenty-thousandth of an inch, though probably this is incorrect ; nevertheless, it is very indestructible, resisting the action of strong nitric or hydro- chloric acid, and only swelling slightly when boiled in caustic potash. Notwithstanding, however, that it resists the action of acids, it is not so hard as the enamel, and becomes worn off tolerably speedily, so that to see it well an unerupted tooth, still enclosed in its dental capsule, should be selected. The singular power of resistance to reagents which characterises it proves nothing more than that it is a tissue, imperfectly calcified, on the border-land of calcification, so to speak, since similarly-resistant structures are to be found lining the Haversian canals, the dentinal tubes, the surface of developing enamel, the lacunae, &c. When burnt it gives off a smell like that of burning horn. In Sir John Tomes' opinion (14) it was to be regarded as a thin covering of cementum, and the author gave additional evidence in support of this view in a paper referred to hereafter. The more recent investigations of Paul(10) have, however, thrown a fresh light upon the matter, and have shown that the points upon which the author chiefly relied are open to quite a different explanation. He employed unworn teeth, fixed in a chromate solution, and detached the membrane by a 5 per cent, solution of nitric acid ; he has since simplified the process by decalcifying the enamel at once by immersion in a phloro- glucin and nitric acid solution without preliminary fixation. The membrane can easily be stained in eosin or in Ehrlich's CEMENTUM. 121 acid hematoxylin, washed, and mounted in Farrant's medium. This at once shows the reticulated pattern ; but that this is not due to the impressions of the ends of the enamel prisms is proved by its being too large — ten times too large according to Paul — and, in addition, there are large nuclei which are so related to the pattern as to show that it is produced by the margins of the cells of which they are the nuclei. His specimens, from one of which Fig. 67 is taken, leave no Fig. 67. — Nasmyth's membrane, showing the stained nuclei. From a specimen prepared and photographed by Dr. Paul. doubt but that it is a membrane composed of a layer of flat epithelial cells. It is obvious that a layer of flat epithelial cells in this situation can have only one source, namely, some part of the enamel organ, and Paul found that in cutting sections of the dental capsule he was able to find upon its inner surface a layer of flat epithelial cells, which could be traced to be the external epithelium of the enamel-organ. Hence the present state of knowledge on the subject must lead to the conclusion that Waldeyer was right, and that Nasmyth's membrane is a product of the external layer of the enamel-organ. 122 A MANUAL OF DENTAL ANATOMY. In addition to the layer of epithelial cells, there is a thin structureless pellicle inside them, i.e., between them and the enamel, which is of such consistence as to make the isolated Nasmyth's membrane tend to curl up. Upon the inner surface of this membrane the pitted impressions of the ends of the enamel prisms may sometimes be seen, as described by the older observers. Like the epithelial layer, this membrane must, from its position, be a product of the enamel organ. Paul suggests that it may be of some service in protecting the enamel by its power of resistance to acid, especially in those depressions which are the most vulnerable points in a tooth, from which F IG. 68. — From a section of a premolar tooth, in which the cementum, c, is continued over the outside of the enamel, a. The dentine is indicated by the letter b. it would not get worn off as it does from the more exposed regions. It seems possible that this inner portion may be merely a sheet of imperfectly calcined enamel, the last products of the effete enamel cells, and so correspond with the thin sheet of tissue which may be raised from the surface of forming enamel at all stages of its growth. Several views have been held as to the nature of Nasmyth's membrane. It has been thought (1) to be a thin layer of cementum, too thin to show the characteristic structure of cementum, as supposed by Sir John Tomes and by the author ; (2) to be a final product of the enamel cells or ameloblasts (Kolliker) ; (3) to be derived from the external epithelium of the enamel organ. The grounds upon which the author believed it to be a thin layer of CEMENTUM. 123 cementuin were that in a large number of animals there is a thick and functional layer of cementum outside the enamel, and that it now and then happens that the m cementum upon a more or less abnormal human tooth, instead of terminating at the neck of the tooth, is continued up over the exterior of the enamel. This occurs less uncommonly than is generally imagined, and the foregoing Fig. 68 represents a portion of the crown of such a tooth. If sections be made of the grinding surfaces of such teeth as present rather deep fissures in these situations, well marked and unmistakable lacunal cells, or encapsuled lacunae, will be met with occasionally. Now and then an encapsuled lacuna may be found occupying a shallow depression in the enamel which it just fits, but more commonly a dozen or more are crowded together in a pit in the enamel, where they are usually stained of a brownish colour. The occurrence of lacunae in these situations is far from rare : Sir John Tomes' collection contains several good examples of them in these positions. Nasmyth's membrane, thin though it is over the exterior of the Fig. 69. — Encapsuled lacuna occupying a pit in the enamel. enamel, is thickened when it covers over a pit or fissure, and when isolated by an acid is seen to have entirely filled up such spots (Fig. 70). In these places, then, where the encapsuled lacunae are to be found, Nasmyth's membrane also exists, a fact which alone would lend some probability to the view that it is cementum. The general absence of lacunae in Nasmyth's membrane might be due to the fact that it is not thick enough to contain them ; just as the thinnest layers of unquestionable cementum also are without lacunae. In sections of an unworn premolar which was treated with acid subse- quently to its having been ground thin and placed upon the slide, the author has several times been fortunate enough to get a view of the membrane in situ ; it then appears to be continuous with an external layer of cementum, which becomes a little discoloured by the acid employed to detach Nasmyth's membrane from the enamel. Caush apparently had observed the same thing when he wrote (2), that Nasmyth's membrane seems to be continuous not with the cementum but with the membrane covering the cementum, that it will stain with fuchsin, and that there 124 A MANUAL OF DENTAL ANATOMY. is no apparent difference between that covering cementum and that covering enamel. It is pointed ont by Underwood that these observations are not wholly discordant with those of Paul, inasmuch as by the time Nasmyth's mem- brane is formed, the cells of the external epithelium of the enamel-organ are fused with the dental capsule from which cementum is developed. The author was formerly inclined to regard it as young and incomplete cementum, and to consider it as representing (upon the human tooth) the thick cementum which covers the crowns of the teeth of herbivora ; and Magitot, who made many researches upon this subject, entirely concurred in this view, which had also the support of Professor Wedl. FlG. 70. — Nasmyth's membrane, set free by the partial solution of the enamel, a. Nasmyth's membrane, b. Dentine, d. Mass occupying a pit in the enamel, e. Enamel, a'. Torn end of Nasmyth's membrane. /. Termination of the cementum. It has, however, been pointed out by Paul that these observations are not inconsistent with the interpretation of its nature which he advocates, for the dental capsule envelops the crown of the tooth in man as well as in certain herbivorous animals, in which it does form coronal cementum ; and it is quite possible that it might deposit encapsuled lacunae in pits where there was room for them, and that these would appear to be in Nasmyth's membrane, which also extends into the pits; nor is its apparent continuity with the cementum at the neck more difficult to account for. To render the matter certain, Nasmyth's membrane should be demon- strated between the enamel and cementum in the teeth of animals possessing thick coronal cementum, but on this point Paul is not, as yet, prepared to speak with certainty. CEMENTUM. 125 Nasmyth, who first called attention to its existence, regarded it as the " persistent dental capsule " ; a view of its nature not very materially differing from that formerly advocated in these pages. Professor Huxley described it as being identical with the membrana performatwa ; that is to say, with a membrane which covered the dentine papilla prior to the occurrence of calcification, and which afterwards came to intervene between the formed enamel and the enamel-organ. The objections to the acceptance of this view of its nature are so inextricably wrapped up with other objections to Professor Huxley's theory of the development of the teeth that they cannot profitably be detailed in this volume ; it will suffice to say that evidence and the weight of authority alike point to there being no such true membrane as this membrana performativa in the place in question. Waldeyer and Kose hold that it {i.e., Nasmyth' s membrane) is a product of a part of the enamel-organ. After the completion of the formation of the enamel they believe that the cells of the external epithelium of the enamel-organ become applied to the surface of the enamel and there cornified ; in this way they account for its resistance to reagents, and for its peculiar smell when it is burnt. According to the statement of Legros and Magitot, the layer of cells in question (external epithelium of the enamel-organ) is atrophied before the time of the completion of the enamel — a fact which, if confirmed, is fatal to Waldeyer's and Paul's explanation. This is a point which requires to be investigated afresh in the light of recent researches. Magitot", in a later paper (5) on the subject, gives his adherence to the view that it is cementum. Kolliker, who dissents strongly from the views of Waldeyer, and admits some uncertainty as to its nature, provisionally regards it as a continuous and structureless layer furnished by the enamel cells after their work is completed. THE GUM. The gum is continuous with the mucous membrane of the inside of the lips, of the floor of the mouth, and of the palate, and differs from it principally by its greater density. Its tenacity is in part due to the abundant tendinous fasciculi which it contains, and in part to its being closely bound down to the bone by the blending of the dense fibrous fasciculi of the periosteum with its own. The fasciculi springing from the periosteum spread out in fan-like shape as they approach the epithelial surface. There is thus no sharp line of demarcation between the gum and the periosteum when these are seen in section in situ. The gum is beset with rather large, broad-based papilla?, which are sometimes single, sometimes compound. The epithelium is composed of the stratum corneum, the stratum 126 A MANUAL OF DENTAL ANATOMY. luciclum, and the stratum, granulosum ; and cylindrical cells, the rete malpighi, form its deepest layer. Small round aggregations of pavement epithelium are met with at a little depth, or even imbedded in the surface. In or in the neighbourhood of developing tooth-sacs epithelial aggregations, the so-called "glands of Serres " may be seen, and are remains of the dental-lamina, or tooth-band, which has undergone fenestration subsequently to the completion of its original function. The gums are rich in vessels, but remarkably scantily supplied with nerves. Mucous racemose glands abound. Black considers that the " glands of Serres " are not glands, but he believes that their constituent cells resemble connective tissue cells, and hence does not apparently think that they are epithelial " nests." At the necks of the teeth the gum becomes continuous with the periosteum of the internal surface of the alveoli, into which it passes without any line of demarcation. THE ALVEOLO-DENTAL MEMBRANE. The Alveolo-dental periosteum, or Periodontal Membrane, is a connective tissue of moderate density, devoid of elastic fibres, and richly supplied with nerves and vessels. It is thicker near to the neck of the tooth, where it passes by imperceptible gradations into the gum and periosteum of the alveolar process, than in other parts. The general direction of the fibres is transverse ; that is to say, they run across from the alveolus to the cementum, without break of continuity, as do also many capillary vessels. A mere inspection of the connective tissue bundles, as seen in a transverse section of a decalcified tooth in its socket, will suffice to demonstrate that there is but a single " membrane," and that no such thing as a membrane proper to the root and another proper to the alveolus can be distinguished ; and the study of its develop- ment alike proves that the soft tissue investing the root, and that lining the socket, are one and the same thing : that there is but one " membrane," namely, the alveolo-dental periosteum. At that part which is nearest to the bone the fibres are grouped together into conspicuous bundles ; it is, in fact, much like any ordinary fibrous periosteum. On its inner aspect, THE ALVEOLO-DENTAL MEMBRANE. 127 where it becomes continuous with the cementum, it consists of a fine network of interlacing bands, many of which lose them- selves in the surface of the cementum. But although there is a marked difference in histological character between the extreme parts of the membrane, yet the markedly fibrous elements of the outer blend and pass insensibly into the bands of the fine network of the inner part, and there is no break of continuity whatever. The actual attachment, both to the cementum and to the bone, takes place by means of the connective tissue fibres, which pass right into the hard structures, which they traverse for some distance, and in this situation are known as Sharpey's fibres. They pass through all the lamellae of the cementum, and there are appearances of shrinkage in dry preparations which would lead to the inference that they were not very fully calcified ; in some portions of the cementum it seems to be almost composed of them, as at the neck of a tooth (Black). This writer states that they may be especially clearly seen in the pig, and that " they are the principal fibres of the peri- dental membrane included in the cementum in its growth, and furnish the means of taking firm hold of the peridental membrane upon the root of the tooth. They are white con- nective tissue fibres, the ends of which are included in the matrix of the cementum sufficiently to make them apparent when the lime salts are removed, but when both are calcified they cannot be demonstrated except in cases where there is imperfect calcification of the fibres, as has been mentioned above." The thickness of the membrane appears to undergo a diminution with age, by calcification encroaching upon it from both the side of the bone and of the cementum. Malassez (6) urges that an ordinary periosteum in this situation would be too tender for the purposes of mastication and that as it is not a mere enveloping membrane, but is composed of fibrous bundles, which serve to " sling the tooth in its place," it should be called the alveolo-dental ligament. He further compares it with the fibrous bands which in some fishes serve to tie the tooth down to the bone where no tooth sockets exist, and holds that it is strictly homologous with these. 128 A MANUAL OF DENTAL ANATOMY. At the surface of the cementum it is more richly cellular, and here occur abundantly large soft nucleated plasm masses, which are the osteoblasts concerned in making cementum. The fibres, whether in longitudinal or in transverse sections, rarely pass straight in the shortest possible line from the bone to the cementum, but they usually pursue an oblique course, which probably serves to allow for slight mobility of the tooth without the fibres being stretched or torn. Black (*) gives the following account of the direction of the fibres : Near the neck of the tooth some of the fibres arch over the summit of the alveolus and blend with the periosteum under the gum. A little lower down they pass across horizon- tally, though when seen in transverse sections they do not pass quite directly, but are more or less tangential to the sur- face of the cementum, while in the apical half of the tooth they have an upward direction as they pass from the cemen- tum to the bone. As age advances the alveolar dental mem- brane may become thinner, chiefly because it is encroached upon by the additional thickness of the cementum. The blood vessels are most abundant in the membrane mid- way between the bone and the cementum, or rather nearer to the latter, but close to it there is a rich capillary plexus with- out large vessels. A good many of the arteries enter the apical egion and divide to be distributed partly to the tooth pulp and partly to the periosteum, some of them passing from the apex to the gum ; they anastomose freely with vessels in the bone and with those of the gum, so that the blood supply is not easily interfered with. The vascular supply of the root membrane is, according to Wedl, derived from three sources : the gums, the vessels of the bone, and the vessels destined for the pulp of the tooth, the last being the most important. The nerve supply also is largely derived from the medullated nerves distributed to the dental pulps ; other filaments come from the inter-alveolar canals, viz., those spaces in the bone, containing nerves and vessels, which are situated in the septa separating the alveoli of contiguous teeth. It should be borne in mind that the tooth-pulp and the tissue which becomes the root membrane have sprung from the same source, and were once continuous over the whole base of the pulp. A recognition of this fact makes it easier to realise THE ALVEOLO-DENTAL MEMBRANE. 129 how it comes about that their vascular and nervous systems are so nearly identical. Several observers have laid stress upon the occurrence of cells upon the surface of the cementum, deep down in the tooth sockets, which are unlike osteoblasts but are obviously epithe- lial cells. It is claimed by von Brunn that the epithelial sheath of Hertwig from the enamel-organ extends far below the region where enamel is to be formed, and that it is, in fact, co-extensive with the dentine, thus necessarily intervening BV. between the dentine and the cement-forming tissue. He describes the connective tissue bundles as growing through it to attach themselves to the dentine, and thus cutting up the remains of the enamel-organ into small isolated groups of cells, which are to be found here and there in the adult alveolo- dental periosteum. Black described, in the sheep, another type of cells which he believed to be lymph cells lining lymph canals; these are always found close to cementum. He believed also, as corroborative of this view, that he had D.A. 9 130 A MANUAL OF DENTAL ANATOMY. been able to trace pus infiltration along these chains of cells. He { 1899) still adheres to the idea that the accumulations of cells of epithelial character found deep down in the socket are not the remains of the epithelial sheath of Hertwig, but are really glandular structures. He describes them as winding about and branching in every direction, and gives a diagram of their arrangement as seen in a perspective longitudinal view of the membrane, constructed from a number of sections. This diagram is eminently suggestive of their being the remains of the epithelial sheath which has become fenestrated like the spent tooth band (p. 142), and after careful study of his text and figures it appears to the author that the attribution to them of glandular nature rests upon the slenderest foundation. The human tooth is connected with the living organism very intimately, even though its special tissues are extra-vascular. For blood vessels and nerves enter the pulp in abundance ; and the dentine is organically connected with the pulp by the dentinal fibrils. A favourable place in which to study the nature of the alveolo- dental membrane is the rapidly changing tooth of the crocodile. In it the membrane is very thick, and its bundles of connective tissues very apparent. Keferences. (*) Black, G. Y. "The Fibres and Glands of the Peridental Mem- brane." Dental Cosmos, Philad., February, 1899, vol. xli., pp. 101—122. (2) Cause:, D. E. " Is Nasmyth's Membrane Modified Cementum ? " Journ. Brit. Dental Assoc., 1898, vol. xix., pp. 296—297. (3) Hopewell- Smith, A. " Some Observations on the Cellular Elements of the Dental Pulp." Trans. Odontol. Soc. Gt. Britain, 1894, vol. xxvi., N.C., pp. 9— 32. (8b) « < Histology and Patho -histology of the Teeth and Associated Parts." London, 1903. (3c) " An Introduction to Dental Anatomy and Physiology; Descriptive and Applied." London, 1913. (4) Huber, Carl. " The Innervation of the Tooth-pulp." Dental Cosmos, Philad., October, 1898, vol. xl., pp. 797 —811. THE ALVEOLO-DENTAL MEMBRANE. 131 (6) Legros, Ch., et Magitot, E. " Developpement de 1'Organe Dentaire chez les Mammiferes (Contributions a 'Etude du Developpement des Dents, 3me Menioire)." Journ. de l'Anat. et de la Physiol., 1881, t. 17, Paris, pp. 60- 95. (6) Malassez, L. " Sur le Eole des D6bris Epithelieux Paradent aires." Archiv. de Physiol. Normale et Pathologique, 1885, t. vi., 3",e series, pp. 309— 340, et 379— 419. (") Moegan, G. "Lymph Glands in Relation to the Teeth and Gums." Brit. Dental Journ., August 15, 1903, vol.xxiv., pp. 521—528. (8) Morgensteen", M. " Ueber die Innervation des Zahnbeines." Arch. f. Anat. und Entwicklungsgesch. Leipzig, 1896, pp. 378—394. (9) Mummery, J. H. "Basal Layer of Weil." Journ. Brit. Dental Assoc., vol. xiii., 1892, pp. 777—781. (10) Paul, F. T. " The Enamel Organ." Dental Eecord, 1896, vol. xvi., pp. 493—498. (n) Eetzius, G. " Biologische Untersuchungen, Neue Folge." IV. (1892). "Zur Kenntniss der Nervenendi- gungen in der Zahnen," pp. 65, 66. V. (1893). "Ueber die Nervenendigungen in der Zahnen bei Amphibien," pp. 40, 41. VI. (1894). ", Ueber die Endigungsweise der Nerven an den Haaren des Menschen," pp. 61, 62. (12) Eobertson, W. G. Aitchison. On the Eelation of Nerves to Odontoblasts, and on the Growth of Dentine." Trans. Eoy. Soc, Edin., 1890—91, vol. xxxvi., pt. ii., pp. 321—335. (ls) Eomer, O. " Zahnhistologische Studien," 1899. (14) Tomes, Sir J. " Dental Surgery." London, 1859 (later editions edited by C. S. Tomes). 9—2 CHAPTER V. THE DEVELOPMENT OF THE TEETH. The development of the teeth is a process which, while subject to modifications in the different groups of vertebrates, nevertheless retains in all of them certain essential characters, so that it becomes possible to embody its main features in a general account. Prior to the commencement of any calcification there is always a special disposition of the soft tissues at the spot where a tooth is destined to be formed ; and the name of " tooth-germ " is given to those portions of the soft tissue which are thus specially arranged. A part only of the soft structures making up a tooth-germ becomes converted into the dental tissues by a deposition of salts of lime within their own substance. The details of this conversion can be better dis- cussed at a later page ; for the present it will suffice to say that the three principal tissues, namely, dentine, enamel, and cementum, are formed from distinct parts of the tooth-germ, and that it is customary to speak of the enamel-germ and the dentine-germ. The existence of a special cement-germ is asserted by Magitot, but as yet his description awaits confirma- tion. In the older anatomical works which the student may have occasion to consult, the process of tooth development is described as being divided into periods, under the names of "papillary," "follicular," and "eruptive" stages. These stages were based upon false conceptions, upon theories now known to be incorrect, and have now been absolutely abandoned. The account of the development of the teeth given in the following pages (based, in the case of man and mammals, upon the researches of Kolliker, Thiersch, Waldeyer, Rose, Leche, and others ; in the case of reptiles and fishes, upon those of Huxley, Santi Sirena, Hertwig, Tomes, and others) will thus be found to differ from the older accounts published by that deservedly great authority, Sir Richard Owen. Modern THE DEVELOPMENT OF THE TEETH. 133 methods of research have made it easy to demonstrate facts heretofore not demonstrable : yet fifty years ago Professor Huxley demonstrated in a remarkable paper (9) the incorrect- ness of certain of the theories then promulgated. Of the general accuracy of the following description the author is, however, fully satisfied, and most of the facts may be easily verified by any one desirous of so doing. True tooth-germs are not formed quite upon the surface of the jaw, but are situated at a little distance beneath it, in many animals ultimately coming to lie at a considerable depth. A few exceptions to this statement may be cited. The dermal denticles of sharks, which are homologous with their teeth, are formed near to the surface, as are the first teeth of Triton (newt), according to Eose, who says that they sink in subsequently. Eose also appears to regard the tooth germs of some osseous fish as in the same category, but Leche dissents from this, and, indeed, considers Edse's view as merely a question of terms. e d a be Fig. 72. — Developing dermal denticle of a Selachian. (After Hertwig.) a. Enamel cells. b. Epithelium of surface, c. Dentine papilla. d. Formed dentine, e. Formed enamel. /. Cells of the mesoblast. Every known tooth-germ consists in the first instance of two portions, and two only, the enamel-germ and the dentine- germ ; and these are derived from distinct sources, the former being a special development from the epithelium of the mouth and consequently epiblastic, the latter from the more under- 134 A MANUAL OF DENTAL ANATOMY. lying parts of the sub-mucous tissue, and therefore mesoblastic. Other things, such as a tooth capsule, may be subsequently and secondarily formed, but in the first instance, every tooth- germ consists of an enamel-germ and a dentine-germ only, and the simplest tooth-germs may never develop any additional parts- The existence of an enamel-organ in an early stage is therefore perfectly independent of any subsequent formation of enamel. But even before this was known the author had shown that it may be found in the germs of teeth which have no enamel. It exists, in fact, in every known tooth-germ. That part of the tooth-germ destined to become dentine is often called the dentiae-papilla, having acquired this name from its papilliform shape ; and in a certain sense it is true that the enamel-organ is the epithelium of the dentine-papilla. Yet, although not absolutely untrue, such an expression might mislead by implying that the enamel-organ is a secondary development, whereas its appearance is antecedent to that of the dentine-germ. The most general account that one can give of the process is that in the future tooth-bearing part of the jaw there is an involution of the superficial epithelium, or rather of its deep layer. This downward growth consists of a double layer of cells, which in section has somewhat the appearance of a simple tubular gland. At certain points a further development of the epithelium takes place, forming enamel-organs, beneath each of which a specialisation of the mesoblastic tissue goes on, forming the dentine-papilla. The details of this process vary in different creatures. In Mammalia the earliest changes which will ultimately result in the formation of a tooth are traceable at a very early period. In man these occur before ossification has set in, while the lower jaw is represented solely by Meckel's car- tilage as yet not fully formed and imbedded in embryonic tissue and while the lateral processes, which will ultimately become the maxillary bones, have only just reached as far as the median process or pre-maxillary bone. That is to say, about the fortieth or forty-fifth day of intra-uterine life, in the situa- tion corresponding to the future alveolar border, there appears in transverse sections an ingrowth of epithelium extending along the whole length of the jaw, rounded on its deeper aspect. This rapidly extends more deeply into the subjacent mesoblastic THE DEVELOPMENT OP THE TEETH. 135 tissue, so that it comes to appear in section like a tubular gland, although it is in reality a continuous sheet or lamina of epithe- lium,* the primitive tooth-band or " Zahnleiste " of German authors. In some animals, e.g., in Artiodactyles, and in some marsupials, there is a considerable heaping up of epithelium over the tooth-band (see Fig. 75). Leche takes exception to these figures as not representing an occurrence typical of all mammals at this early stage. Nevertheless, it is a condition which is present to a greater or less degree in many mammalian embryos. To this heaping up of epithelium (Zahnwall) over the position of the ingrowth, though it occurs in man but to a Fig. 73. — Embryo at end of fifth week. 1, 2. First two visceral arches. a. Maxillary process, t. Tongue, b. Eye. c. Lateral naso-frontal process, nf. Median naso-frontal process. (After Carpenter.) very slight extent and in only a part of the jaw, Rose attaches importance, as he considers it to be a remnant of free tooth formation in the epithelium such as occurs in the larval teeth of fishes and amphibians. About the same time that the tooth-band becomes differentiated, an ingrowth of epithelium takes place all around the outer surfaces of the jaws in the situation which is to be occupied by the vestibulum oris, or groove between the lips and cheeks and the jaws ; the future history of this ingrowth * The epithelium having been removed by maceration or by keeping a specimen in dilute spirit, a groove would result, and this is probably what was seen and described by Groodsir as the " primitive dental groove : " but as the student will gather from the text, there is at no time any such thing as a deep open groove like that described by him, unless it results from maceration and consequent partial destruction of the specimen. 136 A MANUAL OF DENTAL ANATOMY. being that it widens out and the cells which fill its interior atrophy, so as to leave an open groove. To this ingrowth the Germans give the name of " Lippen- furche," a name for which no English equivalent has as yet obtained currency, and a certain amount of difference of opinion as to its exact origin exists. Kose considers that the tooth-band and the lippenfurche have a common origin, and that the primitive tooth-band splits longitudinally, its outer division forming the lippenfurche and its inner division being the true tooth-band, whilst Baume holds that the true tooth-band originates in the lippenfurche. On the other hand, Leche, an equally careful observer, considers that neither of these statements has a general applicability, and that the lippenfurche and the tooth-band have a simultaneous and independent origin, at all events in many animals. The question does not appear to be of any paramount importance, as, even taking Rose's own figures, the divarication of the two epithelial inflections takes place at a time when neither is advanced beyond the merest beginnings. The successive stages of tooth development have been made more intel- ligible by the researches and models of Rose, who has made use of Bora's method of modelling. A complete series of sections of uniform and known thickness are made and photographed or traced with a camera lucida to the same scale. The outlines of every fourth or fifth section are then copied on to tracing-paper affixed to sheets of wax, the thickness of the wax bearing a definite relation to the magnification of the sections and to the thickness and number of those represented by each sheet. The wax is then cut out round the line of the tracing, and the various sheets of wax welded together. It is obvious that if the wax sheets are of a thickness exactly propor- tionate to the thickness and number of sections to which each corresponds, and if the sheets so cut out are placed with their flat surfaces together, they will, after being united by running a hot spatula along their edges, exactly represent, on a magnified scale, the object in the solid. But it is of course impossible to represent thus more than one set of structures at a time, though, by making a tracing round any set that may be selected, a solid model of that structure is the result. The result of a study of models brings into prominence the following facts : the original inflection (tooth-band, epithelial lamina, or zahnleiste) at an early stage appears, according to Kose, to divide into two portions, one of which, the outer, runs nearly perpendicularly outwards from the jaw, and is ultimately connected with the formation of the lip THE DEVELOPMENT OF THE TEETH. 137 furrow, whilst the inner, with which the reader is more immediately concerned, passes into the tissues beneath, with a slight inclination towards the middle line. This stage is reached in the human embryo about the forty- eighth day ; then near to, but not quite at, the free edge of the Fig. 74. — Three stages in the development of the humau tooth-germ. The right-hand side of the figure is the lingual or that towards the middle line. (After Hertwig.) a. Primitive dental furrow (the existence of which is doubtful), b. Tooth-band in section, c. Enamel-organ of first tooth, d. Free end of tooth-band, to the inner side of which are formed the enamel-organs of successional teeth. tooth-band small club-shaped thickenings appear at each point where a tooth is about to be formed. Soon each cellular accumulation assumes a bell shape, into the concavity of which a papilla-like specialisation of the submucous tissue grows. It is to be particularly noted that this formation of bell- shaped future enamel-organs is not at the free edge of the tooth-band or dental lamina, but only very near to it, so that 138 A MANUAL OF DENTAL ANATOMY. the margin of the band is free to go on growing past them, deeper into the submucous tissue, as may be seen in the foregoing diagrams taken from Hertwig (7). It is from this further growing part of the tooth-band that the enamel-organs of successional teeth, whether one or more, will ultimately be derived. Thus the enamel-organs develop from the labial side of the tooth-band and not from its lower free extremity. When the bell-shaped cap which constitutes the enamel- organ begins to form, it is, of course, somewhat crescentic when viewed in section. The inner horn of the crescent is formed to some extent by the terminal end of the tooth-band, while the outer bends outwards from it a little way up, and the whole is not detached to any appreciable degree by constriction from the general body of the tooth-band ; so that for a little time it looks as if the end of the tooth-band were wholly used up in its formation. The band begins to grow rapidly beyond it (Fig. 7 4), and from this further prolongation any successional teeth-germs, if there are to be any, will be formed. The tooth-germs, as has been stated, are formed always upon the outer or labial aspect of the tooth-band, which is inclined inwards towards the middle line ; hence it follows that when a tooth-germ lies upon the lingual side of another, it necessarily belongs to a later generation of teeth, and vice versa. The position of the tooth-germs with relation to the tooth-band has been aptly compared to " swallows' nests built against a board." Some writers attach so much importance to the growth of the free edge of the tooth-band beyond and to the lingual side of the forming tooth-germ that they consider that wherever it exists there is a possibility, a potentiality, of a further generation of teeth being developed.* While the tooth-band is continuous round the whole jaw, its further development, resulting in the formation of tooth- germs, takes place at detached points only, where teeth are going to be formed. These appear about the ninth week of intra-uterine life in man. The further development of an individual enamel-organ may now be described. The first result of rapid pro- * For a full discussion on this point, vide Wilson and Hill, Quart. Journ. Micr. Sci., London, 1896. THE DEVELOPMENT OF THE TEETH. 139 liferation of cells is to bring it somewhat into the form of a Florence flask; then, as the future dentine-papilla becomes more defined and pushes "into it, the enamel-organ assumes the form / 2 Fig. 75. — Three stages in the development of a mammalian tooth-germ. (After Frey.) a. Oral epithelium heaped up over germ (Zahnwall). b. Younger epithelial cells, c. Deep layer of cells, or rete Malpighi. d. Inflection of epithelium for enamel-germ (Zahnleiste). e. Stellate reticulum. /. Dentine-germ. g. Inner portion of future tooth-sac. h. Outer portion of future tooth-sac. i. Vessels cut across, k. Bone of jaw. These figures are not quite complete, in that they do not show the continuation of the tooth-band beyond and to the lingual side of the tooth-germs, and in that the Zahnwall is but a doubtfully typical structure. of a bell with a handle, like a dinner-bell, as may be seen in the accompanying figures. At the same time the enamel-organ recedes from the tooth-band so that it comes to be connected with the latter only by a narrow double line of cells, as seen in section ; this goes on ultimately to the detachment of the enamel-organ from the tooth-band. 140 A MANUAL OF DENTAL ANATOMY. Leche distinguishes three stages in the development of an enamel-organ, which are of use in describing rudimentary organs which do not proceed to full development, by giving a definite indication of the stage to which they have gone on. 1. The bud-shaped germ. 2. The bell or cap-shaped germ, without differentiation of its constituent cells. 3. The bell-shaped germ, with differentiation of its outer and inner epithelia, or complete enamel-germ. According to Leche, the detachment of the formed enamel- organ from the tooth-band is the necessary accompaniment of the formation of the next new successional tooth-germ, and so long as the tooth -band retains its connection with the surface epithelium and has a down-growth beyond the last- formed tooth-germ, so long is there a possibility of the develop- ment of more generations of teeth. He holds, therefore, that in a region where these appearances obtain, it is a fair inference that tooth-building is not yet concluded. In the enlargement which constitutes the young enamel- organ (bud-shaped stage) the cells upon the periphery are columnar, polygonal cells occupying the central area of the enlargement. Very soon the terminal enlargement, as it grows more deeply into the jaw, alters in form ; its base becomes flattened, and the borders of the base grow down more rapidly than the centre, so that its deepest portion presents a concavity looking downwards ; it might be com- pared to a bell, suspended from above by the thin cords of epithelium which still connect it with the epithelium of the surface, or it might in section be described as crescentic, the horns of the crescent being long, and pointing downwards. Coincident with the assumption of this form by the enamel- germ, is the appearance of the dentine-germ ; but it will facilitate the description of the process to pursue the development of the enamel-organ to a further stage. A shallow furrow (the primitive dental furrow) has by this time appeared on the free surface of the epithelium in the mouth, which corresponds with the situation of the tooth-band (cf. Fig. 74). The cells on the periphery of the enamel-organ remain prismatic or columnar, but those in its centre become transformed into a stellate network, in which conspicuous nuclei occupy the centre of ramified cells, the processes from THE DEVELOPMENT OF THE TEETH. 141 which anastomose freely with those of neighbouring cells (see Fig. 82). This conversion of the cells into a stellate reticulum is most marked quite in the centre of the enamel- organ. Near to its surface the processes of the cells are short and inconspicuous, and the whole process strikingly recalls the phenomena of colloid degeneration as observed in certain tumours. The transformation of the cells occupying the centre and constituting the bulk of the enamel-organ into a stellate reticulum goes on progressing from the centre outwards, but it stops short of reaching the layer of columnar cells which constitute the surface of the enamel-organ, next to the dentine- papilla. A narrow layer of unaltered cells remains between the stellate cells and the columnar enamel cells and is known as the " stratum intermedium." Leon Williams, however, does not consider that the cells have changed into a stellate form, but that this stellate appearance is an intercellular substance, from which the cells have been washed out. He states that the spaces are not empty in preparations rapidly fixed in Fleming's or Hermann's solution and examined in glycerine, but that the contents are always lost in balsam preparations. Williams is, so far as is known, the first to have observed and described the contents of these spaces ; but the author does not think that his observation is at all inconsistent with the view that the cells have undergone a sort of colloid degeneration, for it is characteristic of colloid changes for the nucleus to be pressed out against the periphery, and as the change goes on in a myxoma, or in colloid degeneration of a cancer, a stellate-looking tissue comes to be formed by the squeezing outwards of the nucleus and approximation of the walls of neighbouring cells, whilst the colloid material is frequently lost from the interior in cutting the section. Eve (4) has also shown that in certain cystic tumours of the jaws the epithelium which enters into them undergoes a sort of colloid degeneration, resulting in appearances almost precisely resembling the stellate tissue of the enamel-organ. The cells become much enlarged, and the nuclei pressed against the cell- walls. The protoplasm of the individual cells then becomes merged into a general mass, but the nuclei with por- tions of the cell- walls attached to them remain, so that the resultant tissue has much the appearance of flattened cells with processes. In some places a drop of colloid substance forms within the individual cell, and the protoplasm is expanded out into a thin layer, so that the nucleus is pressed against the cell-wall, thus producing the characteristic signet-ring appearance. Similar forms are to be found in enamel-organs. Thus a cystic epithelium, probably identical in its origin with the enamel-organs, may undergo a degenerative change, which brings it into close similarity with this part of the enamel-organ. 142 A MANUAL OF DENTAL ANATOMY. Thus far all the cells constituting the periphery of the enamel-organ are alike : they are columnar or prismatic ; but from the time of the appearance of the dentine-papilla those which come into relation with it become much more elongated and greatly enlarged, while those round the outer surface con- stituting the " external epithelium " of the enamel-organ do not enlarge ; indeed according to some authors, they even com- mence to flatten and to atrophy at this early period. The cells which lie like a cap over the dentine-germ or " papilla " as they elongate and their nuclei recede towards their extremities, take on the character to be presently described as belonging to the " enamel cells " (enamel epithelium, internal epithelium of the enamel-organ, adamantoblasts, or ameloblasts). > The enamel-organ, then, consists (proceeding from without inwards) of an "external epithelium," a " stellate reticulum," a " stratum intermedium," and an "internal epithelium," the external and internal epithelia being continuous at the edges or base of the enamel-organ, while at its summit the external epithelium remains still, through the medium of the " neck of the enamel-organ," in continuity with the cells of the tooth- band, and through them with the cells of the " stratum Malpighi." Thus the enamel-organ is entirely derived from the oral epithelium, with which, by means of its " neck," it long retains a connection, so that it, and whatever products it may afterwards give rise to, are obviously to be regarded as " epithelial structures," and epiblastic in their origin. But it is the enamel-organ alone which is directly derived from the epithelium ; the origin of the dentine-germ is quite distinct. Degenerative changes begin to appear in the common tooth- band so soon as the tooth-germs are well on their way in their development, and fenestrations begin to form in it, so that, instead of being a flat and complete sheet, it becomes a cribri- form sheet ; hence a fallacious appearance of its being broken up and no longer continuous with the surface results when individual sections are viewed, it often happening that the plane of the section coincides with one of the perforations in the sheet. But, as has already been hinted, this change does not go on uniformly everywhere in the dental lamina, but only where its work is nearly accomplished ; thus at the back of the THE DEVELOPMENT OF THE TEETH. 143 mouth, where more teeth have to be developed, it is still a complete sheet, and to the lingual sides of the milk teeth where the successional teeth are to be formed, it also remains complete and continues to grow. The same sort of change takes place in the necks of the enamel-organs, and ultimately these, as well as the dental lamina itself, break up into isolated islands of epithelium, which sometimes seem to proliferate a little, and constitute the little masses of epithelium known as the "glands of Serres." It is possible that these little nests of epithelium may be the site of certain new growths which sometimes arise in the jaws.* So far the ingrowth of the epiblastic epithelium into the mesoblastic tissue beneath it has been described, but wher- ever this has gone on to the formation of an enamel-organ, the mesoblastic tissue seems to be stimulated to growth, leading to the formation of the dentine-germ. Leche thinks that the differentiation and crowding of the cells of the mesoblast may be at first merely the mechanical effect of the vigorous ingrowth of epithelium, as he has observed it near to the end of the tooth-band, i.e., at points where tooth-germs are not going to be formed. However that may be, in the embryonic tissue of the jaws, some little distance beneath the surface, and at a point corresponding to that ingrowth of cells and subsequent enlargement of the same which goes to form the enamel-organ, appears the first trace of a dentine-germ, t This is at first a mere increase in the opacity of the part, with- out any visible structural change, and it occupies the con- cavity of the enamel-organ. Thus the dentine-germ appears early, indeed almost simultaneously with the formation of the definite enamel-organ, but the latter is far in advance of the former in point of structural differentiation, and the earliest changes resulting in the formation of the enamel-organ are strikingly visible before a dentine-germ can be discovered. Hence it has been suggested that the enamel-organ governs * For a further consideration of epithelial nests in connection with developing teeth, vide p. 360. f The term " dental papilla,'1 although eminently convenient, is associated with an erroneous feature of the older views upon tooth development ; where it is employed in the following pages, the student must guard against the miscon- ception that free papillae at any time exist in any animal. 144 A MANUAL OF DENTAL ANATOMY. and determines the ultimate form of the tooth. According to Dursy (3), the dark halo which becomes the dentine-bulb is, like the inflection of epithelium which forms the enamel-germ, continuous all round the jaw, while eventually it develops into prominences at the points corresponding to the enamel-germs of future teeth, and atrophies in their interspaces. From the base of the dentine-bulb prolongations pass out- wards and slightly upwards, so that they in a measure embrace the free edge of the enamel-organ, and at a somewhat later period they grow upwards till they fairly embrace the whole enamel-organ. These prolongations are the rudiments of the dental sac or capsule. In their origin, therefore, the dental sac and the dentine-organ are identical, and spring from the submucous tissue : they contrast with the enamel-organ, which, as before said, is derived from the oral epithelium. To recapitulate briefly the facts which are now established beyond all question, the early mammalian tooth-germ consists of three parts, one of which, the enamel-organ, is derived from the epithelium of the surface ; the other two, the dentine-organ and the dental sac, originate in the midst of solid embryonic tissue at a distance from the surface ; the one is ecderonic or epiblastic, and the other enderonic or mesoblastic. The enamel- organ is formed by a rapid increase of cells near to the bottom of a process which dips in from the stratum Malpighi of the oral epithelium ; the dentine-germ and the dental sac are formed in close continuity to this enamel-organ from the sub- mucous tissue. If there were a " basement membrane " demonstrable in the mucous tissues at this early period (which there is not) the enamel-organ and the dentine-organ would lie upon the opposite sides of it. Malassez insists much upon the significance of remnants of epithelium left after the atrophy of the enamel-organ ; some of these he believes that he has found in the alveolo-dental membrane (Fig. 71). This is confirmed, as to developing teeth, by von Brunn (2), who states that in rodents the enamel-organ extends far down, in fact, the whole length of the roots, and figures the fibres of the alveolo-dental periosteum as growing through it to take hold of the cementum ; it seems possible that these cells are the same which Black and Xavier Sudduth consider to be portions of a lymphatic system. THE DEVELOPMENT OF THE TEETH. 145 Before leaving the subject of the enamel-organ, it may be mentioned that several writers, notably von Brunn and Ballowitz (1), hold that the primary function of the enamel- organ is that of determining the form of the future tooth ; even going so far as to suggest that its calcification to form enamel is in some animals a secondary function, taken on later. In support of this contention they urge that enamel-organs are universal, even where no enamel is found : and, that in teeth where there is a partial investment of enamel on the dentine the enamel-organ extends far beyond the enamel limits. Moreover, they trace extensions of the enamel-organ right down to the bottom of the roots in those teeth where the whole crown only is invested with enamel. Indeed, von Brunn says that it is to be traced between the roots of a three- rooted tooth, so that it may be compared to something envelop- ing a three-legged stool, creeping round its edges, its under- side, and finally growing down as an envelope to each leg. Its presence is confirmed by Rose, who says, that so soon as its increase downwards ceases, so does that of the dentine. Thus von Brunn holds that everywhere where dentine is to be formed there is an antecedent " form-building " investment of enamel- organ ; and to this, and not to any retrogression, he would attribute the enamel-organ of Edentata. But whatever may be thought as to its " form-building " functions, recent researches into aborted and rudimentary teeth of mammals render it quite certain that in that class at all events a complete dentition of enamelled teeth was the rule, and that those which have lost, or partly lost, their teeth have arrived at this state by degeneration, so that it seems a fair inference to suppose that where enamel is absent, that also is in the nature of a degeneration. The development of the teeth in lower vertebrates has, as already mentioned, very much that is common to them and to the higher, the principal difference lying in the arrangements for succession. Elasmobranch Fishes. — If a transverse section through the jaw of a dog-fish (Scyllium canicula) be examined, it will be found that the developing teeth lie upon the inside of the cartilaginous jaws, the youngest being at the bottom (Fig. 76). Progressing upwards, each tooth is more fully D.A, 10 146 A MANUAL OF DENTAL ANATOMY. calcified till, at the border of the jaw, are situated the functional organs and those teeth whose period of greatest usefulness is passed and which are about to be cast off in the course of that slow rotation of the whole tooth-bearing mucous membrane over the border of the jaw which is constantly going on. In this figure four teeth advanced in calcification are repre- sented, while beneath them are three tooth-germs in earlier stages of development. Of the former two only are fully b'iG. 76. — Transverse section of lower jaw of a dog-fish. a. Oral epithelium. b. Oral epithelium passing on to flap. c. Protecting flap of mucous membrane (thecal fold), d. Youngest dentine pulp. e. Youngest enamel-organ. /. Tooth about to be shed. g. Cartilage of jaw. Note the great development of enamel cells over the third papilla from the base. protruded through the epithelium, the third being in part covered in ; the remaining teeth are altogether beneath the surface of the epithelium, and therefore shut off from the cavity of the mouth, if the soft parts be all in situ. All the teeth not fully calcified are covered in and protected by a reflexion upwards of the mucous membrane (c in the figure), which serves to protect them during calcification. But although this may be termed a fold reflected upwards, it is not, as was supposed by Sir Eichard Owen, a free flap, THE DEVELOPMENT OF THE TEETH. 147 detached from the opposite surface on which the teeth are developing ; there is no deep open fissure or pouch running round inside the" jaw, as would in that case exist, and the epithelium does not pass down on the one side to the bottom of such fissure, and then ascend upon the other as a distinct layer. On the contrary, we have here that inward growing inflection of epithelium known as the " tooth-band " (zahn- leiste). Although the fold is very easily torn away from the tooth-germs which it covers in, yet in the natural condition it is attached, and there is no breach of surface : the epithelium of the jaw is seen in the figure, in which the epithelial layer is represented as broken just at the point between the third and fourth teeth, where the tooth band dips in. The conditions met with in the Elasmobranch fishes are peculiarly favourable for the determination of the homologies of the several parts of the tooth-germ and of the formed tooth.* At the base of the jaw, where the youngest tooth- germs are to be found, the tissue whence the dentine-papillae arise blends insensibly with that making up the substance of the thecal fold on the one hand, and, on the other, with that clothing the concavity of the jaw and giving attachment to the teeth. No sharp line of demarcation at any time marks off the base of the dentine-papilla from the tissue which surrounds it, and from which it springs up, as would be the case in mammalian or reptilian tooth-germs ; all that can be said is, that the dentine-germs are cellular, the cells being large and rounded, while in the rest of the mucous membrane the fibrillar elements preponderate, so that it passes by imperceptible gradations into the densely fibrous gum, found on the exposed border of the jaw. The dentine-germs, and consequently the dentine, are indisputably derived from the connective tissue of the mucous membrane immediately subjacent to the epithelium, nor can it be doubted that the enamel-organs are simply the modified epithelium of that same mucous membrane. Of course there is nothing new in this conclusion, which had been already arrived at by the study of other creatures, but the sharks happen to demonstrate it with more clearness than those other animals in whom the original nature of the process * Compare the description of the dermal spine (pp. 3 and 133). 10—2 148 A MANUAL OF DENTAL ANATOMY. is more or less masked by the introduction of further com- plexities. Hence it is worth while to study carefully the relations of the epithelium constituting the enamel-organs with that of the surface of the mouth. As has been already mentioned, in the normal condition of the part there is no deep fissure on the inner side of the jaw, but the epithelium passes across (from the interspace between the third and fourth teeth in the figure) on to the protecting fold of mucous membrane (c in Fig. 7 6). The epithelium in this situation does not, then, consist simply of one layer going down on the one side and covering the tooth-germs, and then reflected up at the bottom to coat the inner side of the thecal fold, but it is an inflexion of the deep layer of a tooth-band ; it is termed the " enamel-organ" because over the tooth-germs these epithelial cells assume a marked columnar character, and are very different in appearance from the epithelium else- where, especially at that stage during which enamel formation is most active. The terminal portion of this tooth-band runs beyond the youngest enamel-germ, which forms a bell-like cap over the eminence of connective tissue which constitutes the earliest dentine-germ, and in section is of the form shown in the figure. The surface next to the dentine-papilla consists of elongated columnar cells, with nuclei near to their middles, while the rest of its substance is made up of much smaller cells, some of which have inosculating processes, so that they constitute a sort of finely cellular connective tissue, very different in appearance from anything met with in mammalian enamel-organs. It is sufficiently consistent to keep up the continuity of all the enamel-organs, even when displaced in cutting sections, so that the whole might almost be described as forming one composite enamel-organ. The columnar cells already alluded to invest the whole surface which is directed towards the forming teeth, but they atrophy somewhat in the interspaces of the tooth-germs. Before proceeding further in the description of the develop- ment of the tooth-germs, it will be well to refer again to an earlier stage in the growth of the dog-fish, in which the relation subsisting between the teeth and the dermal spines is still well seen. On the lower jaw of the young dog-fish there is no lip : THE DEVELOPMENT OF THE TEETH. 149 hence, as is seen in the figure, the spines which clothe the skin come close to the dentigerous surface of the jaw. Although there are differences in form and size, a glance at the figure will demonstrate the homological identity of the teeth and the dermal spines. As the dog-fish increases in size, this continuity of the teeth with the dermal spines on the out- side of the head becomes interrupted by an extension of the skin to form a lip. This happens earlier in the upper jaw than in the lower, and at first the spines are continued over the edge and the inside of the newly-formed lip — from these situations, however, they soon disappear. In structure, the teeth and the dermal spines are, in many species, very closely similar ; the latter are, however, much less often shed and Fig. 77 — Section of lower jaw of young dog-fish, showing the continuity of the dermal spines of the skin under the jaw, with the teeth which lie above and over its end. reproduced, so that it is less easy to find them in all stages of their growth ; the author believes, however, that they follow a course essentially similar to that of the teeth. It is stated by Gegenbaur (5) that in Selachia the mucous membrane of the mouth is clothed with spines of a structure similar to that of the teeth, and that these spines are often limited to particular regions, extending back as far as the pharynx — these same regions in Ganoid and Osseous fishes being occupied by conspicuous teeth ; and Her twig (7) has shown that the dermal spines are developed in a manner precisely similar to that described in the teeth, save that the germs are even less specialised. There are certain peculiarities in the enamel-organ of Selachia, and also in the dentine-papillae, which may be more advantageously described in connection with calcification. 150 A MANUAL OF DENTAL ANATOMY. Teleostei or Osseous Fishes. — In passing from the consideration of the development of the tooth-germs of Elasmobranch to those of Osseous fishes, the first difference to be noted is this : whereas in the former each tooth-germ was, so far as the enamel-germ is concerned, easily seen to be derived from the general tooth-band, in the latter each enamel-germ apparently often rises independently, and, as it were, de novo. At all events, so far as the author's inves- tigations go, no obvious connection between the germs of teeth of different ages is visible ; but Heincke (6) says that in the pike new enamel-organs may be derived from older ones. This apparently independent origin of an indefinite number of teeth, having no relation to their predecessors, only occurs Fig. 78. — Tooth-gernf of an eel. d. Neck of enamel-organ, e. Enamel- cells, a. Cap of enamel, b. Cap of dentine, c. Rudimentary enamel cells opposite to that part of the dentine-germ where no enamel will be formed. in the osseous fish : in them it is likely that the germs so soon become detached that their origin from any common tooth- band is masked : of the development of the teeth of Ganoid fish next to nothing is known. The oral epithelium, which varies much in its thickness and in other characters in different fishes, sends down a process which goes to form an enamel-organ, whilst a dentine- papilla in rising up to meet it comes to be invested by it as with a cap. The after-history of the process depends much on the character of tooth which is to be formed. If no enamel, or but a rudimentary coat of enamel, is to be formed, the cells of the enamel-organ remain small and insignificant, as in the mackerel. If, on the other hand, a partial investment of enamel is found upon the perfected tooth, such, for instance, as the little enamel tips upon the teeth of the hake (see THE DEVELOPMENT OF THE TEETH. 151 Fig. 51), then the after-development of the enamel-organ is very instructive. Opposite to the apex of the dentine-papilla, where the enamel- cap is to be, the cells of the enamel-organ attain to a very considerable size, measuring about of an inch in length ; below this the investing cap of the enamel-organ does not cease, but it is considered to be in a sort of rudimentary condition. Thus, although the enamel-organ invests the whole length of the dentine-papilla, its cells only attain to any considerable size opposite to the point where the enamel is to be formed. The knowledge of this fact often enables an observer to say, from an inspection of the tooth-germ, whether it is probable that the perfected tooth will be coated with enamel or not. In any case an enamel-organ will be there, but if no enamel is to be formed, the individual cells do not attain to any considerable degree of differentiation from the epithelium elsewhere; in other words, the whole enamel-organ will partake of the character of the lower portion of that represented in the figure of the tooth-germ of the eel (Fig. 78). Although of course there are many differences of detail arising from the very various situations in which teeth are developed in fish, so great uniformity pervades all which have been examined, that the development of the teeth of reptiles may now be considered, merely adding that it is not altogether true to say that the teeth of fish in their development exemplify transitory stages in the development of mammalian teeth. Amphibia and Reptilia. — So far as the appearances presented by the individual germs go, there are few differences worthy of note to be found by which they are distinguishable from those of either fish or mammals. The enamel-organ is derived from the tooth-band, and the dentine-organ from the submucous tissue in a very similar manner ; nevertheless, there are points in the relation which the successional tooth-germs bear to one another, and to the teeth already in situ, which are of some little interest. The constant succession of new teeth met with amongst almost all reptiles renders it easy to obtain sections showing the teeth in all stages of growth. Upon the inner side of the jaw there will be found a region occupied by these forming teeth and by nothing else, which may be called the " area of tooth development " ; this is bounded on the one side by the bone and teeth which it carries, and 152 A MANUAL OF DENTAL ANATOMY. on the other by a more or less sharply defined wall of fibrous connective tissue. In the newt, for example (Fig. 79), to the left of the tooth in use are seen three tooth-sacs, in serial order, the youngest being nearest to the median line of the mouth. As the sacs increase in size they appear to undergo a sort of migration towards the edge of the jaw, while simultaneously new ones are constantly being developed beyond them. The ingrowth of the epithelium is obviously the first step apparent ; this ingrowth of a process of epithelium or tooth-band always extends beyond and inside the youngest dentine-papilla, and each new enamel-organ is formed from its side near its growing FlG. 79. — Section of upper jaw of Triton cristatus (newt). To the inner side of the tooth attached to the bone are three younger tooth germs. end. New enamel-organs are therefore not derived directly from the epithelium of the surface, but from the deep end of the tooth-band. In this amphibian, the developing teeth spread out for a con- siderable distance towards the palate, and thus, being free from crowding, the relations of the enamel-organs of three or four successional teeth of serial ages may be studied in a single section ; and the arrangement so disclosed may be advantage- ously compared with that seen in the dog-fish (see Fig. 76). The tooth-sac of the newt is a good example of the simplest form of tooth-germ, consisting solely of an enamel-organ and a dentine-germ, without any especial investment. The "germ " THE DEVELOPMENT OF THE TEETH. 153 is wholly cellular, and on pressure breaks up, leaving nothing but cells behind it. The cells of the enamel-organ are large, and resemble those- of the eel ; the teeth of newts have a partial enamel tip, like those of the fish referred to, but differ from them in being bifurcated, as is very early indicated by the configuration of the enamel-organ. In the frog there is a peculiarity in the manner in which the two jaws meet, the edentulous lower jaw, which has no lip, passing altogether inside the upper jaw and its supported teeth, and so confining the area of tooth development within very narrow limits. Consequently the author has been unable to satisfy himself whether the new tooth-germs, or rather their enamel-organs, are derived from the primitive tooth-band or spring up de novo ; analogy would indicate the former, but appearances tend towards the latter supposition. There is also a peculiarity in the development of the teeth of some reptiles in that the first formed tooth is developed, like the dermal spine of a shark, very close to the surface ; in fact, Kose affirms that the first tooth-germ of the crocodile is actually an upstanding papilla. This, however, is not confirmed by Leche, who says he has never seen an actual free papilla, but he describes and figures in the iguana a little first rudimentary tooth almost in the surface epithelium, and shows that the tooth -band sinks in deeper and deeper by the side of this, till at last the succes- sional teeth are formed as deep as possible on the surface of the jaw-bone, into which, however, they do not pass. In the lizards the new tooth-germs are formed a very long way beneath the surface, so that the neck of the enamel-organ becomes enormously elongated, for the dentine papilla is, just as in the newt, situated quite at the level of the floor of the area of tooth development. The teeth of the lizards have a more complete investment of enamel, hence the enamel cells are developed upon the side of the dentine-germ to a much lower point than in the newt. The germs also acquire an adventitious capsule, mainly derived from the condensation of the connective tissue around them, which is pushed out of the way as they grow larger. The further progress of the tooth- germ being identical with that of mammalia, its description may be for the present deferred. Some lizard-like animals, such as Sphenodon and the 154 A MANUAL OF DENTAL ANATOMY. chamseleon, present no evidence of successional teeth being formed, though in them, though acrodont, a " tooth-band " is to be seen to the inner side of the functional tooth (Kose). In ophidian reptiles (snakes) several peculiarities are met with which are very characteristic of the order. A snake's method of swallowing its food would seem to render the renewal of its teeth frequently necessary ; although no data are known by which the probable duration of the life of an individual tooth could be estimated, the large number of teeth which are Fig. 80. — Transverse section of the lower jaw of common English snake. e. Involution of epithelium. /. Oral epithelium. 1, 2, 3, &c. Tooth- germs of various ages. 8. Tooth in place, cut somewhat obliquely, so that its tip apparently falls short of its surface, and does not project above the mucous membrane. developiug in reserve, all destined to succeed to the same spot upon the jaws, would indicate that it is short. The author has seen as many as seven successional teeth in a single section, and their arrangement, particularly in the lower jaw, which undergoes great displacement while food is being swallowed, is very peculiar. The numerous successional tooth-sacs, instead of being spread out side by side as in the newt, are placed almost vertically, and in a direction parallel with the surface of the jaw-bone ; they are, moreover, contained in a sort of general investment of connective tissue — THE DEVELOPMENT OF THE TEETH. 155 a species of bag to keep them from displacement during the expansion of the mouth. The inward growing process of oral epithelium enters this collection of tooth-sacs at its top, and may be caught sight of here and there as its prolongations wind their way by the sides of the tooth-sacs to the bottom of the area. Here the familiar process of the formation of an enamel-organ and f Fig. 81.— Developing teeth of a snake. /. Oral epithelium, e. Neck of the enamel-organs, b. Dentine-pulp. c. Enamel cells, d. Dentine. 1, 2. Very young germs. 3, 4. Older germs. dentine-papilla may be observed, in no essential point differing from that which is to be seen in other animals. That the derivation of each enamel-organ is from a part of that of the tooth-band is very obvious ; the dentine-organs are formed in relation with the enamel-germs, but apparently independently of one another. As the tooth- sacs attain considerable dimensions, a curious alteration in position takes place ; instead of preserving a ver- tical position, they become recumbent, so that the developing tooth lies more or less parallel with the long axis of the jaw. The utility of such an arrangement is obvious : were the tooth to remain erect after it has attained to some little length, its 156 A MANUAL OF DENTAL ANATOMY. point would probably be forced through the mucous membrane when the mouth was put upon the stretch ; but while it lies nearly parallel with the jaw no such accident can occur. The tooth does not resume the upright position until it finally moves into its place upon the summit of the bone. As has already been mentioned, there is a well-developed enamel-organ with large enamel cells ; from these a thin layer of enamel is formed, and thus the thin external layer upon the teeth of snakes is probably true enamel, and not, as has been usually supposed, cementum. The essential feature in the development of reptilian teeth is the indefinitely repeated succession which is the typical condition, though, as already mentioned, in some instances it has become limited or entirely lost : this seems to have been the case in the most mammalian of reptiles, the Anomodont group. The question of succession has, however, of late years, become so important, and there is such a voluminous literature, that it will require special notice, and a section will be devoted to it. Mammalia. — The mammalian enamel-organ, as has already been stated, forms a cap-like investment to the dentine papilla, and it is itself thickest over the apex of the latter, becoming more attenuated as it approaches the base. It is entirely surrounded by an epithelial layer, which, upon the inner surface, applied to the dentine-bulb, consists of very elongated columnar cells, and takes the name of internal epithelium of the enamel-organ, and upon its outer surface the name of external epithelium of the enamel-organ. The great bulk of the early mammalian organ consists of a stellate tissue, which passes somewhat abruptly through the medium of a layer of rounded cells, the stratum intermedium, into the enamel cells (ameloblasts), or internal epithelium. The essential portion of the enamel-organ is this layer of " enamel cells," which give rise to the enamel. In lower animals, and most, if not all, reptiles, the whole enamel-organ is represented by little else than this layer of " enamel cells." The internal epithelium (enamel cells) consist of an exceed- ingly regular and perfect columnar epithelium, the individual cells becoming by result of their mutual apposition fairly symmetrical hexagons. THE DEVELOPMENT OF THE TEETH. 157 They are four or five times as long as they are broad, and the nucleus, which is large and oval, lies near to that end which is farthest from the dentine. According to Waldeyer (u) the sides of the cells only are invested by membrane, the protoplasm being without investment at its two ends. On the other hand Leon Williams describes structures at both ends of the cells, and calls them the " inner and outer ameloblastic membranes." Towards the base of the dentine-germ, where the internal epithelium merges into the external epithelium, the cells are not so much elongated, and they pass gradually into the cubical form of these latter cells. At their attached extremities the enamel cells are prolonged into processes which are con- tinuous with the cells of the stratum intermedium, so that it Fig. 82.— Cells of the stellate reticulum of the enamel organ. (From Frey's "Histology.") has sometimes been concluded that the enamel cells, as they are used up in the formation of enamel, are recruited from the cells of this layer. The stratum intermedium consists of cells intermediate in character between those of the bordering epithelium and the stellate reticulum ; they are branched, but less conspicuously so than the stellate cells. The stellate cells proper are characterised by the great length of their communicating processes, and the interspace of the meshes is occupied by a fluid rich in albumen, so that the consistence of the whole is little more than that of jelly. As the structure in question constitutes the major part in bulk of mammalian enamel-organs, these have been called the enamel jellies, or enamel pulps. The function and destination of this portion of the enamel- organ is not very clear : enamel can be very well formed with- 158 A MANUAL OF DENTAL ANATOMY. out it, as is seen amongst reptiles and fish, and even in mammalia it disappears prior to the completion of the tissue, so that a great deal of the enamel is formed after the external and internal epithelia have come into contact. It has been supposed to have no more important function than to fill up the space subsequently taken up by the growing tooth. It has been pointed out by Dr. Paul (loc. cit.) that this is after all not an unimportant office ; the tooth-sac is regular in outline, the apices of the cusps, &c, of the forming tooth very much the reverse ; and it is familiar to every histologist that the distribution of the stellate tissue is very unequal, even at the period of its greatest development, the forming tooth in some places coming close to the tooth-sac, in others having a great thickness of stellate reticulum over it. Its thickness bears no relation whatever to the amount of enamel that is to be formed : thus, as just mentioned, over the apices of the cusps it is often almost absent. The external epithelium of the enamel-organ is composed of cells cubical or rounded in form, and is of little interest save in that it is a matter of controversy what becomes of it. Waldeyer holds to his opinion that, after the disappearance of the enamel pulp and the stratum intermedium, it becomes applied to the enamel cells, and on the completion of the enamel becomes cornified and converted into Nasmyth's membrane. Kolliker and Legros and Magitot dissent from this opinion, the latter stating that the atrophy of these cells commences early, and that they actually disappear prior to the complete atrophy of the organ. The external epithelium was seen by Nasmyth, Huxley, and Guillot, but it was not very fully described until investigated by Kobin and Magitot. Kecently Dr. Paul has adduced strong evidence in favour of Waldeyer's view, which has already been quoted in relation to Nasmyth's membrane. So simple a matter as the vascularity or non-vascularity of the enamel-organ is not yet settled : Wedl asserts that it contains no vessels, Magitot and Legros sharing this opinion ; Xavier Sudduth has uniformly failed to detect vessels in it, but Dr. Lionel Beale, on the other hand, states that a vascular network lies in the stratum intermedium, and this is confirmed by Professors Howes and Poulton (10) in the rat ; and Hopewell- THE DEVELOPMENT OF THE TEETH. 159 Smith and Marett Tims in a joint paper (8) have recently described the presence of vessels, containing blood corpuscles, in the enamel-organ of the wallaby. See Fig. 83. Paul, however, has uniformly failed to discover vessels in it, ' and is inclined to believe that those observers who have thought that they have detected them have been deceived by the very close resemblance- to the stellate tissue of the enamel which is borne by the tissues external 'to the tooth-sac, which, after the disappearance of the stellate reticulum Fig. 83. — Showing the presence of blood-vessels within the stellate reticulum of an enamel-organ of Macropus Mllardieri (after Tims and Hopewell-Smith. Proc. Zoolog. Soc. Lond., 1911). and the coalescence of the external and internal epithelium, would come to lie close against the enamel cells, bringing with them their rich supply of vessels. Leon Williams believes that in some animals the stratum intermedium itself becomes transformed into papilliform elements which contain vessels, and claims that in this respect the enamel-organ is vascular, and that his untouched photographs prove this. On the other hand, he states that this does not occur at the outset of enamel formation, and, apart from the inherent improbability of a change of an important kind in the formative organs without any corresponding change in the resultant tissue, the author cannot see that the photographs exclude Dr. Paul's explanation, as the stellate tissue seems always to have disappeared prior to the existence of these papillae. For his own part, the author has had under observation a very large 160 A MANUAL OF DENTAL ANATOMY. number of enamel-organs, and he has never seen a blood-vessel which appeared to him to be unquestionably in the enamel-organ. At all events, the origin of blood-vessels in the tissue in the ordinary way of blood- vessel formation is not likely to happen in a purely epithelial mass : if they do not originate in it, then, if there at all, they must have grown into it from without, i.e., from the tooth-sac. But none of those who claim that they exist have either described or figured their origin in either of these methods. Hopewell-Smith, who believes in the vascularity of the enamel organ, has a photomicrograph in his "Histology and Patho -Histology of the Teeth and Associated Parts," which is apparently quite inconclusive. The inner surface of the enamel- organ, where it is applied to the dentine bulb, presents a perfectly smooth outline, but its outer surface is thus, at all events, in some animals indented by numerous papillary projections, into which enter blood-vessels of the dental sacculus. It is believed that they exercise an influence on the formation of the enamel, to which again reference will be made ; but their existence is denied by Sudduth during the period of activity of the organ. Beferences. (*) Ballowitz, E. " Das Schmeltzorgan des Edentaten, seine Aus- bildung im Embryo und die Persistenz seines Keimrandes bei dem erwachsener Thiere." Archiv. f. mikr. Anatomie. 1892. Bd. xl, pp. 133—157. (2) von Brunn", A. (a) " IJeber die Ausdehnung des Schmeltzorganes in seine Bedentung f. der Zahnbildung." Archiv. f. mikr. Anatomie. 1887. Bd. xxix., pp. 367—383. (6) " Beitrage zur Kentniss der Zahnentwick- lung." Archiv. f. mikr. Anatomie. 1891. Bd. xxxviii., pp. 142 — 156. (s) Dursy, Emil. ' ' Entwickelungsgeschichte des Kopfes des Men- schen und der hoheren Wirbelthiere." Tubingen. 1869. (4) Eve, E. S. " On Cystic and Encysted Solid Tumours of the Jaw, with Observations on the Structure of the Enamel-organ." Trans. Odontolog. Soc. Gt. Britain. 1885. Vol. xviii., pp. 39—61. (5) Gegenbaur, 0. " Manual of Comparative Anatomy" (Eng. trans.). London. 1903. (6) von Heincke, F. " Untersucheingen uber die Zahne niederer Wirbelthiere." Zeitschr. f. Wissenschaft. Zool. 1873. Bd. xxiii., pp.495— 591. (7) Hertwig, 0. (a)" Ueber der Bau und Entwickelung der Placoid- schuppen und Zahne." Jenaische Zeitschr. f. Naturw. 1874. Bd. viii., pp. 331—404. THE DEVELOPMENT OF THE TEETH. 161 (b) " Ueber das Zahnsystem der Amphibien und seine Bedeutung fur die Genese des Skelets der Mundhohle." Archiv. fiir mikr. Anatomie. 1874. Bd. xi., Suppl., pp. 1—208. (c) Lehrbucb der Entwickelungsgeschichte. Jena. 1888. (8) Hopewell-Smith, A., and Tims, H. W. Marett. " Tooth-germs in the Wallaby, Macropus billiardieri." Proc. Zoolog. Soc. Lond. 1911. Part iv., pp. 926 — 942. (9) Huxley, T. H. "Development of the Teeth and on the nature and import of Nasmyth's Persistent Capsule." Quart. Journ. Micr. Sci., Lond. 1853. Vol. i., pp. 149—164. (10) Poulton, E. B. "True Teeth and the Horny Plates of Ornitho- rhynchus." Quart. Journ. Micr. Sci., Lond. 1888. Vol. xxix., N.S., pp. 9—48. (u) Waldeyer, W. "Strieker's Histology." New Sydenham Soc, London. 1873. Vol. i. D.A. 11 CHAPTEK VI. CALCIFICATION OF ENAMEL, DENTINE AND CEMENTUM. Before proceeding to an account of the actual formation of the enamel it will be useful to discuss the question of calcifica- tion in general. A tissue is said to be "calcified" when the organic structures of which it is composed are hardened and stiffened by impregnation with salts of lime. The impregnation with lime salts may go on so far, that the residual organic matrix is reduced to a very small proportion, as is exemplified in the case of adult enamel, in which the organic constituents make a very minute percentage of the whole, so that practically the enamel wholly disappears under the influence of an acid ; or the organic matrix may persist in sufficient quantity to retain its structural characteristics after the removal by solution in an acid of its salts, as is the case with dentine, bone, and cementum. There are two ways in which a calcified structure may be built up : the one by the deposition of the salts in the very substance of a formative organ, which thus becomes actually converted into the calcified structure ; the other by a formative organ shedding out from its surface both the organic and inorganic constituents, and thus, so to speak, excreting the resultant tissue. An example of the latter method is to be found in the shells of many molluscs, in which the mantle secretes the shell, and is able to repair fractures in it, without itself undergoing any apparent alteration ; while the formation of dentine, bone, and enamel* are examples of calcification by conversion. The insoluble salts of lime are altered in their behaviour by association with organic compounds, a fact which was first pointed out by Eainey (9), and has been more recently worked out by Harting (4) and Ord. If a solution of a soluble salt of lime be slowly mixed with another solution capable of precipitating the lime, the resultant * All observers are not, however, agreed as to the formation of the enamel. (Cf. p. 167). CALCIFICATION OF ENAMEL. 163 lime salt will go down in the form of an amorphous powder, or, under some circumstances, in minute crystals. But in the presence of gelatine, albumen, and many other organic compounds, the form and physical character of the lime salts are materially altered, and in the place of an amorphous powder there are found various curious but definite forms, quite unlike the character of crystals produced without the intervention of the organic substance. Eainey found that if calcium carbonate be slowly formed in a thick solution of mucilage or albumen the resultant salt is in the form of globules, concentrically laminated in structure, so that they may be likened to tiny onions. These globules, when in contact, become agglomerated into a single laminated mass, appearing as if the laruinse in immediate apposition blended with one another. Globular masses, at one time of mulberry-like form, lose the individuality of their con- stituent smaller globules, and become smoothed down into a single mass. Eainey suggests as an explanation of the laminated structure that the smaller masses have accumu- lated in concentric layers which have subsequently coalesced ; and in the substitution of the globular for the amorphous Fig. 84. — Calcospherites. 11—2 164 A MANUAL OF DENTAL ANATOMY. or crystalline form in the salt of lime when in contact with various organic substances, he claimed to find the clue for the explanation of the development of shells, teeth, and bone. At this point Professor Harting took up the investiga- tion, and found that other salts of lime would behave in a similar manner, and that by modifying the condition of the experiment very various forms * might be produced. But the most important addition to our knowledge made by Professor Harting lay in the very peculiar constitution of the " calcospherites," by which name he designated the globular forms seen and described by Rainey. That these are built up of concentric laminae like an onion has already been mentioned, and Rainey was aware that albumen actually entered into the composition of the globule, since it retained its form even after the application of acid. But calcoglobulin was also found in a comparatively amorphous condition, formed without any precipitate round a crystal of calcium chloride plunged into albumen ; and it was further found that the presence of phosphates modified the result. A precipitate obtained in gelatine and consisting of car- bonate alone was at first membranous, but passed rapidly into the globular and crystalline forms : a precipitate of phosphate alone was at once crystalline without passing through any colloidal stage, but a precipitate of bicarbonates and phosphates retained for an indefinite time this intermediate form ; and it is noteworthy that in bone, dentine and enamel phosphates and carbonates are invariably associated. Harting failed to produce calcospherites when experimenting with coagulated albumen, with tendons, or with decalcified bone matrix, but he succeeded with cartilage. But Harting has shown that the albumen left behind after the treatment of a calcospherite with acid is no longer ordinary albumen: it is profoundly modified, and has become exceedingly resistant to the action of acids, alkalies, and boiling water, and, in fact, resembles chitin, the substance of which the hard integuments of insects consist. For this modified albumen he proposes the name of " calcoglobulin," as it appears that the lime is held in some sort of chemical combination, for its last traces are very * Thus he was successful in artificially producing " dumb-bell " crystals. CALCIFICATION OF ENAMEL. 165 obstinately retained when " calcoglobulin " is submitted to the action of acids. The " calcospherite," then, has a true matrix of calco- globulin, which is capable of retaining its form and structure after the removal of the great bulk of the lime, and this stains strongly with carmine and other stains. Now it is a very suggestive fact that in the investigation of calcification, structures remarkable for their indestructibility are constantly met with ; for example, if dentine is destroyed by the action of very strong acids, or by variously contrived processes of decalcification, putrefaction, &c, there remains behind a. tangled mass of tubes, the "dentinal sheaths" of Neumann, which are really the immediate walls of the dentinal tubes. Or if bone be disintegrated by certain methods there are separated large tubes, found to be the linings of the Haversian canals (Kolliker), and small rounded bodies recognisable as isolated lacunae. The translucent pellicle of Nasmyth's membrane affords also another excellent example of this peculiarly indestructible tissue. In point of fact, as will be better seen after the development of the dental tissues has been more fully described, there exists very constantly, on the borderland of calcification, between the completed fully calcified tissue and the formative matrix as yet not fully impregnated with lime, a stratum of tissue which in its physical and chemical properties very much resembles " calcoglobulin." It should also be noted that globular spherical forms are very frequently to be seen in the enamel cells, at the edges of the thin cap of forming dentine, and around the interglobular spaces (see Fig. 40) ; moreover, isolated spherules of lime salts have been described by Eobin and Magitot as occurring abundantly in the young pulps of human teeth, as well as those in the herbivora, where their presence was noted by Henle. Perhaps all deposit of lime salts commences in this way. In an instructive paper, Sims Woodhead (15) advocates the view that a calcification is in some measure a change in the direction of degeneration, for which there is needed (1) devita- lised tissue in which albuminoid matter is present ; (2) a layer of formed material such as fibrous tissue or some membrane 166 A MANUAL OF DENTAL ANATOMY. (in bone both of these are represented by the matrix, the fibrous tissue of which, in the centre of one of the trabecular, is practically devoid of cells, the layer near the surface being merely a membrane in which a few cells remain); (3) the layer of proliferating cells always found in the region of any foreign or dead mass, these cells being the osteoblasts of bony tissues, and amongst their functions, as regards calcification, perhaps the most important being the generation of carbonic acid. He writes : " The osteoblasts lay down a matrix of formed material ; the more active the cells within certain limits the greater the relative amount of the matrix. This matrix may be looked upon as inert or dead organic matter, which corresponds to a membrane through which dialysis may take place, or rather the layers near the two surfaces may be so considered, and as the molecular combinations of the phosphoric acid and lime take place around the osteoblasts (which, as before stated, during their active formative stages give off carbonic acid to render the lime for the time being insoluble), there is a con- tinuous process by dialysis of separation of these lime salts. We look upon the formed dialysing membrane that serves to separate the lime salts prepared in its immediate neighbour- hood by the carbonic acid forming cells ; this carbonic acid causes a throwing down of phosphates of lime, with a small proportion of lime in which the phosphoric acid is usually replaced by carbonic acid." If Hoppe Seyler is correct in supposing that the salt which hardens bone, dentine, and enamel is a double salt, in which one equivalent of lime carbonate is combined with three equivalents of lime phosphate, then the share taken by the carbonic acid set free under Sims Woodhead's theory becomes easily understood. (Cf. p. 35.) It must be remembered, however, that in all these experi- ments carbonates give the best results without phosphates, and the experiments must be regarded as a parallel to, rather than as actually reproducing, what goes on in actual calcification. Calcification of the Enamel. — Although the calcification of the dentine commences before that of the enamel, it will be convenient to describe that of the enamel first, as being perhaps a somewhat simpler and more easily intelligible process. CALCIFICATION OF ENAMEL. 167 Two views have been held as to the formation of this tissue, (i.) that it is formed by the actual conversion of the cells of the enamel-organ into enamel ; (ii.) that it is in some sense secreted or shed out by these cells. In support of this latter theory the names of no less authorities than Huxley, Kolliker, Magitot, and others may be adduced, but some of the grounds on which their decisions were based are appearances susceptible of a different interpretation. Kolliker considers that the cells do not undergo any direct conversion, but that the enamel is shed out from the ends of the enamel cells, the enamel fibre therefore corresponding in size and being continuous with the enamel cells whence they were shed out. The cells would thus elaborate, and, so to speak, plaster from their free ends the material in question upon the already-formed enamel, they themselves retaining their own integrity. Without laying too much stress upon an analogy which is only partial, there may be cited the resemblance in the resultant products in the case of enamel, and in the case of the shells of certain molluscs, say, for example, Pinna. In the latter the shell is formed intermittently by the margins of the mantle, which are closely applied to it, but are at any time separable, so that it is difficult to suppose that any con- version of its constituent cells can take place. It would rather seem certain that something must be shed out from their ends, which is hardened by the deposition in or with it of lime salts. The shell of Pinna contains so large an amount of organic matter that its structure is retained even after the removal of the lime by means of an acid ; and it is conceivable that the cells of the mantle are at work, elaborating their products like so many bees plastering wax upon a honey- comb. But whether the enamel cells manufacture something which they shed off their extremities, or are themselves converted into hard tissue by the deposition of lime within their own substance, no one doubts that they preside over the process and determine the form of the result. The cells of the internal epithelium of the enamel-organ or enamel cells have been already in some measure described. They are elongated cells, forming a very regular columnar epithelium, and are hence rendered more or less hexagonal by mutual apposition ; they vary in their length and diameter 168 A MANUAL OF DENTAL ANATOMY. in different animals, but always correspond in diameter with the complete enamel rods. To secure uniformity of nomenclature, the name ameloblast has been proposed for them, as being better comparable with the terms odontoblast and osteoblast. Although they are connected with the cells of the stratum intermedium by a process at their bases, they often adhere more strongly to the enamel, when once this has begun to be formed, than to the rest of the enamel-organ, so that when a dental capsule is opened the enamel cells are most easily obtained by scraping the surface of the enamel. The cells thus torn away often have tapering processes at the ends directed towards the enamel, and are called " Tomes' processes." The cells are also slightly enlarged at these extremities, especially if they have been immersed in glycerine or any such Fig. 85. — Enamel cells with Tomes' processes. fluid which causes their shrinkage, perhaps because this end of the cell having received a partial impregnation with lime salt at its periphery, and therefore being rigid, is unable to contract with the rest of the cell. These enlarged, everted ends often show a very sharp contour, their trumpet-like mouths tending to confirm the statement of Waldeyer, that the protoplasm of the cell is not covered by membrane at its ends. The nucleus is large and oval, often occupying the whole width of the cell, and it lies near to that end of the cell which is farthest from the forming enamel. It contains granules of chromatin, and is of delicate reticular structure ; there are also generally one or several nucleoli. Sometimes the nucleus is square or angular. It is not uncommon for it to appear crescentic on section, the horns of the crescent being directed towards the young enamel (Fig. 90), and there is also sometimes an appearance as though the horns were continued into filamentous processes. CALCIFICATION OF ENAMEL. 169 If such processes have any real existence, it is very possible that these shapes of the nucleus may be due to its being, so to speak, dragged upon during its recession from the forming enamel ; for there is practically no doubt that a single ameloblast forms the whole length of an enamel rod, and therefore is constantly receding as more enamel is formed. There is also a delicate reticular appearance in the body of the cell, and, in addition, there are to be seen little spherules or oval bodies, transparent, structureless and highly refrac- tive, which sometimes attain to considerable size. These were first described by von Spee, and afterwards by Andrews, who believed that they represent the formative material of enamel, and attached therefore much importance to them. They stain black with osmic acid, as does also the youngest layer of forming enamel (Eose), and it has been pointed out Fig. 86. — Enamel cells ; the two on the left have been shrunk by immer- sion in glycerine and present the open trumpet-shaped ends described in the text. by Leon Williams that, in double-stained sections, they often take the same colours as the contents of the blood-vessel, whence he concludes that they have some albuminous con- stituent. Ameloblasts vary much in length. They are longest when enamel formation is most active, and a material thickness of enamel is to be formed. Their distribution coincides always with the extent to which functional enamel is going to be developed upon the tooth, as is observed in the develop- ment of the teeth of fishes. As has been already mentioned, the ameloblasts in some parts of the enamel-organs of certain fish, such as the eel and perch, and certain Batrachia, e.g., the newt, have dimensions very greatly exceeding those of the cells in the remainder of the organ. These highly-developed bodies, three times as long as the corresponding ones lower down upon the dentine papilla, are in the position of the terminal cap of enamel which characterises the teeth (Fig. 78). More- 170 A MANUAL OF DENTAL ANATOMY. over, in the tooth sac of the poison fang of a viper, the distribution of the large cells coincides with that of the enamel on the finished tooth, (Fig. 88). Notwithstanding all that has been written upon the subject of enamel development, the problem is so difficult that some doubt still remains as to the exact modus operandi of the enamel cells, though no one now doubts that they are the active agents in the process, and their diameter corresponds exactly to that of the completed enamel rod in the particular animal. Tomes' processes are of universal occurrence in all mammals Fig. 87. — Enamel cells, showing the apparently reticular structure of the plasm of the cell, the globular bodies (in the left-hand cell), and the •] Tomes' processes. At the top of the figure is a portion of the stratum intermedium, x 1,000. which have been examined, and there is no doubt that they enter the youngest layer of enamel. And it has long been known that from the surface of growing enamel acids can detach a fenestrated membrane, as though, at this stage at all events, the cementing material were a little more resistant than the axes of the rods, and it has been inferred that Tomes' processes had been pulled out of the holes thus demonstrated. Many years ago it was pointed out that Tomes' processes were of unusual length in Marsupials, and that this might have some bearing upon the question of enamel development ; CALCIFICATION OF ENAMEL. 171 it is probable that in marsupial enamels lies the clue to enamel development in general (llb). Andrews (2), had pointed out that a fine fibrillar structure is to be seen near growing enamel between the ameloblasts Fig. 88. — Transverse section of the tooth-sac of a poison fang (Viper). The crescentic pulp (a) is surrounded by a layer of dentine (d~). Ex- ternal to this is a layer of columnar ameloblasts, which, upon the exterior of the tooth., in the situation where a thin layer of enamel is to be formed, are large conspicuous cells. As they pass in between the horns of the crescent, into that part which will ultimately be the poison canal, their character is lost, and their places taken by the stellate cells (/). No enamel is formed in this latter position. and the enamel, and these fibres have been seen and figured also by Leon Williams ; but neither of these observers have appreciated their full importance, owing to their having only investigated enamels in which their existence is very transitory. Fig. 89. — Fenestrated membrane raised from the surface of forming enamel by dilute hydrochloric acid. But in the tubular enamel of marsupials they persist and can be seen extending the whole distance between the ameloblasts and the layer of young dentine, and can hence be studied to far greater advantage. They are there clearly seen to be merely very long Tomes' processes (which are in fact fibrils 172 A MANUAL OF DENTAL ANATOMY. broken off short), and it is obvious that they are the prolonga- tions of the plasm of the ameloblasts, which, as rightly described by Waldeyer (12), have no membrane across their free ends ; their behaviour to stains is the same as that of the cell contents. Although it is convenient to speak of them as fibrils, they do not show any minute fibrillar structure. Where they leave the parent cells they are apparently not much, if at all, smaller than these, the small size of Tomes' processes in FlG. 90. — From a developing tooth of Macropus. At the right-hand top corner is the stratum intermedium, next the ameloblasts, with some- what angular nuclei ; and from these soft fibrils extend in places right across to the dentine, which is the dark mass at left bottom corner of the figure. A short distance from the ends of the ameloblasts the fibrils are seen to change their direction abruptly. human enamel being perhaps due to their having been attenuated by being pulled from the enamel. They are soft and extensile, so that they may be drawn down to a mere thread before they break (see Fig. 91), and their direction exactly coincides with that of completed enamel rods, running straight where these do so, and taking abrupt bends when these bend, as they often do, almost at right angles in certain parts of the kangaroo's enamel. In decalcified sections of developing marsupial enamel, if, CALCIFICATION OF ENAMEL. 173 as generally happens, they are in the least degree oblique to the axes of the rods, there is a most conspicuous appearance of a sort of honeycomb, the septa of which take stains strongly. The Tomes' processes, and their continuations the fibrils, enter the spaces of this honeycomb, and in favourable sections which happen to be transverse to the honeycomb are seen to fill the holes. This conspicuous honeycomb is exactly the same thing as the fenestrated membrane already alluded to (p. 171) in human and other mammalian enamels, but in Fig. 91. — Ameloblasts, Tomes' processes, and youngest part of enamel of Halmaturus, which is cut slightly obliquely, and. has the appearance of a honeycomb. The Tomes' processes are drawn out thin by the slight displacement of the ameloblasts, especially at the left of the figure, and their continuations may in places be seen in the openings of the honeycomb, x 1,000. enamels which are to have solid rods, it occupies only a very small vertical depth, while in tubular enamels it is of material thickness. In addition to marsupials it can be seen in the pig and the sheep quite recognisable as a honeycomb, and it is distinctly visible in some of Leon Williams' photographs. But although it is comparatively thick in marsupials, yet it is in them, like the fenestrated membrane of older observers, a transitional structure belonging to one — the earliest — stage of enamel calcification, and when a little farther advanced towards completion it loses its resistant power and becomes 174 A MANUAL OF DENTAL ANATOMY. dissolved by the decalcifying agent. Hence in decalcified sections the fibres are seen at first, close to the ameloblasts, free ; then attached together by the honeycomb ; while deeper in the forming enamel they are more or less free again. The exact relationship of the fibrils to the cells of the honey- comb was at first difficult to understand, for while in cross section they are seen to occupy the lumen of each cell, and could be in this position in longitudinal section also, yet, especially when drawn out so as to become attenuated, they FlG-. 92. — The ameloblasts of Halmaturus cut so obliquely as to be hardly- recognisable in the figure, lie to the left upper side of the figure ; next comes the honeycomb region ; while the lower part of the figure is occupied by more developed enamel, in which the fibrils may be seen, cut obliquely, as darker masses. Thus in the earliest honeycomb stage the septa stain more strongly than their contents, but at a later stage the fibrils stain more deeply than the septa, x 1,000. seem to be adherent to the septa. Moreover, the septa of the honeycomb are thicker than the amount of interstitial sub- stance found between the rods in complete enamel would account for. But it was found that although their external portions ran continuously into the substance of the septa, their internal portions were still soft, so that the fibril slightly pulled upon, in cutting the section, took on a sort of hour- glass shape, its ends attached to the ameloblasts and to the septa of the honeycomb remaining of their original diameter, while the intervening portion was drawn out into a thread. Hence it appears that the first changes in the fibril towards CALCIFICATION OF ENAMEL. 175 calcification take place in its exterior, and progress towards its interior, which in the case of marsupial tubular enamel they may never reach, many of the rods continuing to have a canal with soft contents down its axis. But in enamels with solid rods the full calcification of the whole thickness of the fibril follows very closely upon the heels of the first honeycomb change, and so the nature of the process is masked. It is probable, therefore, that the ameloblast itself does not calcify ; That each ameloblast furnishes a fibrillar process, con- FlG-. 93. — Araeloblasts of Hahnaturus showing their relation to Tomes' pro- cesses and to the honeycomb ; the latter is cut slightly obliquely, so that, below, a second cell is opened into. tinuous with its own plasma which serves for the entire length of an enamel rod ; That calcification and the changes preparatory to calcifica- tion take place in or round each fibril, quickly reaching its central axis in solid enamels, but never reaching it in some of the rods of tubular enamels. It will be seen that the appearances are consistent either with the calcification of the fibril, or with calcification going on round it, and, so to speak, reducing it down to smaller dimensions or obliterating it. The exceedingly small amount of organic matter present in completed solid enamel might seem to point to the latter being the preferable view, but were this so, one might have expected 176 A MANUAL OF DENTAL ANATOMY. the individuality of the rods to be lost, whereas there appears to be an appreciable amount of interstitial substance demonstrable between them. It has been pointed out by Leon Williams that the axis of the ameloblasts seldom coincides with that of the rods ; this is the case, the ameloblasts always standing nearly at right angles to the dentine surface. But though these fibrillar prolongations start off in the same direction as the axis of the cell, they, in many cases, change their direction quite abruptly a little way from the cell, and the directions which they then assume do correspond exactly with those of the rods in the completed enamel. The identity of the fibrils with the ultimate canals of marsupial enamel is further shown by the former, in the more advanced parts of the enamel, being spiral. If this theory of enamel formation be correct, it serves to explain much that was heretofore perplexing. In the first place in marsupials, the extent to which the tube system exists varies much in different members of the group, and even in those in which it is most rich, the tubes become smaller as they approach the outer surface and disappear well before they reach it ; so that it is quite obvious that the same enamel-organ is capable of forming both tubular and solid enamel, and there can hardly be any very fundamental difference in the processes. In examining a marsupial enamel-organ no dissimilarity can be seen between the different ameloblasts — every one has its long Tomes' process. Yet in the finished enamel the majority of the rods are solid, as may be seen in Leon Williams' beautiful photographs of transverse sections of kangaroo enamel. Whilst taken over a considerable area, the tubes seem somewhat evenly distributed, though sometimes two or even three contiguous rods contain tubes, similarly four or five contiguous solid rods may easily be found. On the whole a tube occurs in about one out of every four rods in the kangaroo. Again, the enamel-organ of a marsupial, prior to the formation of any enamel, is absolutely indistinguishable from that of another mammal ; and tubular enamel reappears sporadically amongst other very diverse mammals, for instance in the Hyrax, in certain Insectivora, and in the Jerboa among rodents CALCIFICATION OF ENAMEL. 177 But if tubularity is a stage through which every enamel rod passes, the difficulty of accounting for animals, whose allies have all lost it, re-acquiring or developing it, disappears. And although the examination of human or other solid developing enamels might never have led to this interpreta- tion, yet a re-examination of them, after having arrived at it, convinces the observer that it is equally true of all. Andrews unquestionably had the fibrils of the ameloblasts under observation, and was probably the first to describe them. But he described them as coming from the cells of the stratum intermedium, which they certainly do not ; and he speaks of them as a fibre network, which, when the enamel rod is fully calcified, is wholly obscured ; and neither he nor Leon Williams appreciated the importance of their role nor their relations to the rod; indeed, it is to be doubted if it were discoverable elsewhere than in a tubular enamel. The latter observer noticed the complexity of the course of the fibres which he saw, but apparently did not connect it with the complexity of the course of the enamel rods in the animals from which his sections came. But in a much earlier paper (*) he came very near to the above views when he wrote : "As the formation of the enamel progresses, these cells recede, leaving within the formed enamel what appears to be a fibre of living matter in the centre of the enamel prism." Rose's views nearly coincide with these, and he correctly figures the appearances so far as they are visible in human enamel. He, however, takes a somewhat different view of the process ; he describes the young enamel as like a honeycomb with Tomes' processes in the cells of the honeycomb. He considers that young enamel, like young dentine, has an organic matrix fairly rich in lime salts, but as yet not by any means fully calcified, and that the lime salts are first deposited fully in the Tomes' processes, so that at a certain stage young enamel consists of thin rods (calcification in Tomes' processes) with a large amount of interpris- matic substance ; the rods then thicken, so that finally there is but a mere trace of cement substance between them. Yon Spee and Andrews consider that the globular bodies described by them in the ameloblasts, and particularly near their free ends, coalesce and merge into the enamel substance, the fibres they describe perhaps serving as a sort of scaffolding to determine their arrangement. This again is not far from the true nature of the process, and this idea is adopted, with modifications, by Leon Williams (13), who, in his later paper, holds that these globular bodies pass out from the ameloblasts, and are seen to be connected with one another by plasm strings, several to each rod, which strings can be seen in the body of the ameloblast, where they constitute the reticulum above described. He holds that in the formed enamel rod these globular bodies are more or less compressed into disk-like shapes, and are sometimes nearly, or quite, melted into one another. This is in effect very much going back to the view of Harting, who writes : " Les prismes, qui composent l'email des D.A. 12 178 A MANUAL OF DENTAL ANATOMY. mammiferes, ne sont evidemment que des piles de calcospherites tres petites," etc. Leon Williams further says : " Around the skeleton thus formed, which constitutes the real structure of enamel, the albumen-like substance flows, supplying the cement substance, and probably the mineral matter for the whole." This view differs from that propounded in the text, in that it does not recognise the preponderant importance of the ameloblast processes and in that it puts first the formation of the rods, to be afterwards joined together by a cement substance, whereas it is probably beyond dispute that the first thing formed is the periphery of the rod and the uniting cement substance, for in the honeycomb the septa of contiguous cells are already united and appear homogeneous, which is a sequence of events quite different from that which he indicates. Kolliker, partly upon the ground that ameloblasts can be seen of the same size and form at all stages of enamel formation, held that enamel formation was one of secretion, the enamel being shed out, so to speak, from the free end of each enamel cell ; hence the rods of the enamel will correspond in size and number with the cells of the enamel epithelium. The processes of the ameloblasts he regards as being fragments of this hardened secretion which are still clinging to the parent cell. Legros and Magitot (5) revived this view, describing each cell as ter- minated, towards the forming enamel, by a little plate of dense material through which by some process of osmosis the constituents of enamel travel out. They note that these plates often cohere so as to form a sheet, but say nothing of their being perforated. No one, however, who had seen the enamel cell of a marsupial with the tapering process five or six times as long as itself which had been pulled out of the young enamel would be satisfied with the excretion theory as so expressed. Schwann believed that the enamel cell was constantly increasing at its free end (i.e., that next to the enamel), and that the new growth, or youngest part of the cell, is calcified as fast as it is formed. This view differs little from that of Kolliker, who prefers to express it by saying that this end of the cell is constantly shedding off or secreting a material which becomes external to itself. Sir John Tomes, Waldeyer, Hertz, and many others, believed that the cell growth takes place not at this free end, but at the attached nucleated end, and that it is the oldest portion of the cell itself which receives an impregnation with salts and forms the enamel. Upon the question of the nature of the globular appearances and smaller beads observed in the ameloblasts, any positive opinion must be still reserved. It may be that von Spee, Andrews, and Williams are right in believing it to be material elaborated for the purpose of calcification, and they may be right in supposing that these masses migrate bodily from the ends of the cells. The evidence does not seem, however, to be conclusive. CALCIFICATION OF ENAMEL. 179 Leon Williams describes appearances which lead him to believe that there is a membrane at both ends of the ameloblast, because in some double-stained preparations there is a sharply defined line of a different colour, both between the enamel and the dentine at the earliest period of calcification, and at all periods between the ameloblasts and the stratum intermedium. But differences in staining occur frequently in the very same tissue, and merely indicate a slight difference in state, and it is perfectly certain that there is no membrane over the free end of the ameloblast after the formation of its fibrillar prolongation; whilst in intervals between two fibrils there is often a very defined line, which, if the section does not happen to lie exactly along the central axis of the cell, looks to be continuous over many cells and their processes, so that the appearances are open to a different interpretation. He himself is aware that later on there is no membrane and can be no membrane ; and appearances of membranes- seen in section are open to so many fallacies that, where there is an a 'priori improbability in the existence of one in a particular position, it is safer to doubt its existence until it is demonstrated more extensively. With regard to the other constituents of the enamel-organ there is not very much to be said. Behind the ameloblasts come the large round cells of the stratum intermedium. It has been customary to attribute to these the function of recruiting the enamel cells, with which some observers have believed them to be connected by processes. After the dis- appearance of the stellate reticulum, the tooth sac, rich in blood-vessels, closely applies itself to the stratum intermedium ; and hence may have arisen the supposition that the enamel organ contains blood-vessels, the strong resemblance of the structures just outside the stellate reticulum lending itself to this mistake. Since,- then, this layer is made up of large cells and intervenes between the ameloblasts and their blood s.upply, it is highly probable that it takes some active share in the elaboration of the materials required for enamel formation, but what that share is is unknown. The inner surface of the tooth sac is frequently produced inwards into papilliform eminences, to which some observers have even attributed the function of planning the pattern of the enamel rod, and when it has come down upon the stratum inter- medium, these are said in some cases to become even more conspicuous. Leon Williams, seeing this large development of the cells of the stratum intermedium distributed over vascular papilli- form shapes, has attributed to the cells a glandular function, 12—2 180 A MANUAL OF DENTAL ANATOMY. and sets down to them a very large share in enamel formation in the way of elaborating products for calcification. But this is hardly a safe inference. In the first place this particular structure does not arise till after some enamel is already formed, and there is no known difference between the enamel formed before and after its development ; moreover, it is only in some animals that it is at all well marked, and their enamel is not different in any known way from those that do not possess it. The ultimate fate of the external epithelium was treated of when describing the minute anatomy of Nasmyth's membrane, so no more need be said here. Upon the whole, then, it must be admitted that although advance has been made in recent years towards a clearer con- ception of the process, there still remains a great deal to clear up with regard to precise knowledge of enamel formation. The Calcification of the Enamel in Fish— On this subject knowledge is somewhat fragmentary. The process has been investigated in a few osseous fish and in Selachians (lie and im)m It was hoped to find methods transitional between those peculiar to Selachians and those found in mammals when investigating osseous fish, but instead another quite peculiar process was found, the bearings of which on enamel develop- ment in general are a little difficult to decipher. In the Gadidce small lance-like or spear-shaped caps of enamel sur- mount the apices of the teeth (see Fig. 51), the rest of the organs being apparently devoid of enamel, or perhaps having only a rudimentary layer. In an early stage the tooth-germ is just like a mammalian tooth-germ. But very soon preparation is made for the enamel caps, which will ultimately remain in a groove at the apex of the dentine, so that they do not greatly increase the diameter of the tooth. At the period when the calcification of the dentine is about to commence, the ameloblasts, where the cap of enamel is to be, enlarge greatly, and there comes to be a con- siderable space in the tooth sac (which is formed by a condensed connective tissue) above the dentine papilla. Up to this point the ameloblasts, which are about 19 /jl in length, only differ from the mammalian type in that their nuclei are not CALCIFICATION OF ENAMEL. 181 quite so sharply marked, and the plasma stains somewhat more freely. When a thin shell of dentine has been calcined, the space above the dentine occupied by the ameloblasts has again greatly increased, and has attained to the full dimensions of the ultimate enamel cap; the ameloblasts have disappeared altogether, having become transformed into a tissue which in longitudinal section looks like a very delicate connective tissue with elongated meshes. Seen in transverse section it is found Fig. 94. — Tooth-germ of hake (Merlucius). b. Stained outer border of enamel stroma, c. Dentine papilla, d. Formed dentine, es. Enamel stroma. The formed dentine has disappeared in decalcification, leaving merely traces of its former structure. (From " Phil. Trans.," B., 1900.) to be perforated by holes arranged with the utmost regularity (Fig. 95). No nuclei are to be found in this stroma, but round its outer surface are closely grouped nuclei which are the same size as, and may possibly actually be, the nuclei of the ameloblasts. Later on in transverse section the holes already alluded to have become smaller and are surrounded by thick borders which take a stain, but although the outer portions of these areas are not sharply denned they have not actually coalesced with one another (Fig. 96). Without entering into full details, which may be found in 182 A MANUAL OF DENTAL ANATOMY. the paper referred to, the peculiarity of the process is that instead of the ameloblasts, as in mammalia, building enamel with a very small amount of transitional tissue between the cells and the fully formed enamel, these cells become bodily transformed into a stroma, which is of the full size of the enamel to be formed, disappearing themselves as individual sharply-defined cells. Blood-vessels are very abundant in the connective tissue which forms the capsule, but none enter the enamel-organ. Hence arises the anomaly of lime salts being separated out Fig. 95. — Transverse section of enamel stroma of hake, taken in the region indicated by the lowest dotted line marked b in the preceding figure, x 120. (" Phil. Trans.," B., 1900.) and deposited in a stroma of considerable thickness without there being any conspicuous cells to effect its segregation, unless indeed the nuclei of the ameloblasts have persisted. Apparently the enamel rods are at first tubular, for the centres of the bodies figured in Fig. 96 look empty : if they are solid columns the material of which they are composed will not take any stain. The enamel when finished is at first sight structureless, but in fortunate sections a striation running in from the free surface, something like that seen in Sargus (see Fig. 28), can easily be traced. The development of the enamel of Sargus, Labrus, etc., has CALCIFICATION OF ENAMEL. 183 not been fully worked out, it being next to impossible to get satisfactory preparations owing to the forming teeth being enclosed in crypts -of dense bone, which prevents fixing fluids from getting thorough access to them ; but preparations show that the calcification of their enamels is preceded by the formation of an extensive stroma such as has been described in the Gadidce. In the elasmobranch fishes the departure from the mam- malian methods is still more extreme, and the tissue formed is sufficiently different to have been variously described as enamel and as vitrodentine (see p. 52). An early tooth-germ does not differ much from an early mammalian tooth-germ : there is the cap of enamel-organ, and a dentine-papilla, the surface cells of which are not at all differentiated (cf. Fig. 99). But before any calcification has Fig. 96. — From the outer and darker peripheral portion of the section last figured. (" Phil. Trans.," B., 1900.) commenced, the peripheral cells of the dentine-papilla have become spindle-shaped, with immensely long processes which become parallel with the surface, and finally constitute a very definite layer of considerable thickness. When calcification commences it is not on the surface of the dentine-papilla, but a little way inside it, beneath the specialised layer just described. Hence this fibrillated layer is left between the cells of the enamel-organ and the first formed dentine, which latter speedily cuts it off more or less completely from the subjacent pulp, and it is noteworthy that this layer corresponds pretty closely in thickness with the future enamel. So far no mention has been made of the enamel-organs, the changes described having taken place entirely in the mesoblastic dentine-papilla. If a vertical section of the jaws of a selachian be examined, the nature of the succession places before the observer, at a glance, a series of tooth-germs of all ages, the 184 A MANUAL OF DENTAL ANATOMY. youngest being at the bottom (cf. Fig. 76). In this, e.g., in Lamna, the ameloblasts are about 18/x in length : over the next papilla they have increased slightly, while over the third in respect of age and position they suddenly extend to 65/u, 70ft, or even more, often being quite four times the length of the younger ones. Again, in the next tooth-germ above this, they have suddenly dropped to about 16/x, and at the same time have changed their appearance and their staining reactions completely. In the youngest ameloblasts the plasma stains pretty freely, even with nuclear dyes : in the largest cells it does not stain at all, and it is very transparent, while a Fig. 97. —Portion of young tooth germ of Lamna. x 240. a. Ameloblasts, not yet fully developed, b. Specialised surface layer of pulp. c. Den- tine pulp. their nuclei stain deeply. Over the next older tooth the glassy appearance is gone, the plasma stains and the nuclei do not, so that they look like holes in the cell. Now this enormous development of the ameloblast cells coincides in point of time with the calcification of the outer fibrillated layer of the dentine-papilla, which is very rapidly accomplished. As soon as it is done they are obviously spent and degenerated, as seen by their shrinkage and the change in their nuclei. It is hardly possible, therefore, to doubt that they effect its calcification, especially when it is remembered that this layer has already been cut off from the subjacent dentine-papilla by the rim of calcification of dentine which has CALCIFICATION OF ENAMEL. 185 already been formed. Moreover, there is no other function for them to fulfil, as no other tissues are formed, and one cannot suppose that 'their sudden and enormous development is for no purpose. The process, as described, is common to all of the elasmo- branch fishes, but it varies in degree according to the tissues to be ultimately formed. Thus while the fibrillar layer is thick in Lamna, Carcharias and numerous others which form thick enamel of comparatively complex structure, and the ameloblasts are enormous at the date of their functional activity, in Raia, in which the enamel is thin and not m i — cu Fig. 98. — Tooth-germ of Carcharias, at an older stage than Fig. 97. X 200. a. Enormous ameloblasts. b. Specialised outer layer of dentine pulp. c. Body of dentine pulp with calcification commencing between it and the specialised layer. unlike mammalian enamel in appearance, the fibrillar layer is also thin and the ameloblasts only attain to 48/jl in length. And it does not make the least difference so far as the facts here enumerated are concerned whether the tooth is to consist, as in Carcharias, of orthodentine, or, as in LamnCt, of osteo- dentin : in either case the calcification commences away from the surface and along the line between the fibrillar layer and the deeper part of the dentine-papilla. The facts above mentioned have been partly described by other previous observers, but they have not been put together, nor their 186 A MANUAL OF DENTAL ANATOMY. bearing appreciated. But it is interesting to note that Nickerson (8), in describing the development of the scales of a ganoid fish, had written: — "What is very curious in their development is that, although by the increase of the dermis in their neighbourhood, a sort of papilliform eminence projecting up into the epidermis is formed, the calcification of the scale takes place, not at the line of junction of the two, but wel in the substance of the dermis, of which a thick layer intervenes between the epithelium and the outer surface of the forming plate," though he goes on to misinterpret their ultimate product. It is obvious that he had under observation a precisely similar process, though in the then state of knowledge he misinterprets the nature of the ultimate product, denying the relationship of the outer hard layer to enamel on the ground that it is mesoblastic in origin. It is evident, therefore, in so-called ganoid or elasmobranch fish the enamel one has to deal with is a tissue which is in a sense hybrid and upsetting to one's previous conceptions. Both the mesoblastic dentine-papilla and the epiblastic enamel-organ have contributed to its formation, and it is open to argument what it shall be called, though from reasons already given (p. 54) on the balance of its characters it may be regarded as enamel. Every grade exists between it in its most aberrant forms and the enamel of Rata, which latter no one, apart from the knowledge of these facts, would for a moment have hesitated to describe as enamel. If an endeavour is made to reconcile these different methods of enamel formation and to place them in their proper relation to one another, some difficulty must arise. Provisionally they may be grouped thus : — (A.) Enamels which are not wholly epiblastic. These are laid down by the operation of epiblas- tic ameloblasts in a matrix which is derived from a modification of the surface of the meso- blastic dentine papilla. (B.) Enamels which are wholly epiblastic. (1.) The ameloblasts lose their integrity early and are transformed into a delicate stroma in which enamel calcification goes on e.g., Gadidce, Labrus, Sargus, and no doubt many others. CALCIFICATION OF ENAMEL. 187 (2.) The ameloblasts persist throughout the whole period of enamel formation and are not bodily transformed into a stroma. Their free ends are, however, continued into long processes which during enamel formation run through the whole thickness and some of them remain uncalcified. This has been worked out only in marsupials, though doubtless other tubular enamels such as that of Hyrax, Jerboa, shrews, etc., are similarly formed. (3.) The ameloblasts persist through the whole period, and the processes of their free ends pass only a short distance into the forming enamel, full calcification following very closely upon the heels of any alteration. This obtains in all placental enamels which are not con- spicuously tubular. It will be observed that the above grouping shows an increasing specialisation and importance of the ameloblasts, which at first only take a share in the result and after- wards effect the whole process unaided. But the gaps are wide, and it may be expected that further research may reveal the stages by which the dentine-papilla surrendered its share in the process. THE DENTINE-OEGAN. The dentine germ, or dentine-bulb, of which the origin has been already described, at first was nothing more than a part of the mesoblastic myxomatous tissue of the jaw which had become richer in vessels and cells than other neighbouring parts, but which did not present any structures essentially different from those found around it. It very speedily assumes the form of the apex of the future tooth, becoming, if it be a canine, simply conical (it has, however, been alleged that the canine papilla has at first a hint of two cusps, thereby showing- its relationship to the premolars) ; if a tooth with two cusps, bicuspid, etc. At an early stage the cells of the surface of the dentine, papilla in no way differ from those which lie deeper (Fig. 99). But as the time approaches for the commencement of 188 A MANUAL OF DENTAL ANATOMY. calcification a more highly specialised layer of cells, which will ultimately form the surface, begins to be differentiated (Fig. 100). These cells, probably largely concerned in the formation of the dentine, form a very distinct layer, which, after the com- mencement of calcification, adheres more strongly to the formed cap of dentine than to the rest of the pulp, and so is often pulled away with the former when the two are separated ; hence this layer of cells (odontoblasts) has obtained the name of membrana eboris, or membrane of the ivory ; but the student must Fig. 99. — Surface of pulp of young tooth-germ. A layer of ameloblasts not as yet fully developed covers the surface ; the surface cells of the pulp are not yet differentiated, so that no indication of the odontoblast layer is seen. (After Dr. Paul.) beware of falling into the mistake of supposing that it really is a " membrane " properly so called. The origin of these cells has been carefully described and figured by Paul, who points out that some dentine matrix begins to be formed before they are by any means completely differentiated. Calcification of the Dentine— The dentine is formed upon the surface of the dentine-bulb, or papilla, from without inwards, so that no portion of dentine once calcified can receive any increase in external dimensions ; all additions must take place upon the interior of the dentine cap. The nature of the dentine-bulb has already been to some extent THE DENTINE -ORGAN. 189 described; it remains to consider somewhat more minutely the histological features of its surface. The cells constituting the membrana eboris, to which Waldeyer has given the convenient name of " odontoblasts," form an exceedingly sharply defined layer upon the surface of the dentine, being arranged in a single row ; the cells immediately beneath them differ strongly from them, so that there is not so marked an appearance of transitional structure as may be seen in the stratum intermedium of the enamel-organ. Nothing whatever like the linear succes- sion of formative cells, which, by coalescence at their ends, X 250 Fig. 100. — From above downwards are seen the (S. K.) stellate reticulum, (S. I.) cells of the stratum intermedium, (A.) the ameloblasts, and (P.) the dentine pulp. Near to the surface of the latter the cells are assuming a direction perpendicular to the surface, and the odontoblast layer is beginning to be indicated. (After Dr. Paul.) went to form the dentinal tubes, as described by the older writers, is to be seen. The odontoblast cells vary in form according as the dentine formation is actively going on or not, but at the period of their greatest activity they are broad at the end directed towards the dentine cap, so as to look almost abruptly truncated. The several processes of the cells have already been described ; there are, however, sometimes several " dentinal processes " proceeding from a single cell, and Boll has counted no less than six. The cells are finely granular. The nucleus is oval, lies in that extremity of the cell which is farthest from the dentine, 190 A MANUAL OF DENTAL ANATOMY. and is sometimes prolonged towards the dentinal process so as to be ovoid or almost pointed. The peripheral process passes into the tubes of the dentine, and it frequently happens that, when the membrana eboris is only slightly separated from the dentine, these processes, which constitute the dentinal fibrils, may be seen stretching across the interval in great numbers. The odontoblasts, as may be seen from Fig. 63, are placed Fig-. 101. — General view of tissues at the time when a thin layer of dentine has been formed ; from a photograph given to the author by Leon Williams. At the top, to the left, is seen the stratum intermedium, next the ameloblasts, then young dentine, and, below, the odontoblasts, which may be seen sending processes into the young dentine. rather closely together, and there is not very much room for any other tissue between them, so long as the formation of dentine is actively going on. Prior to its commencement, however, the cells are not so square at their ends, and the appearance of the extreme edge of such a pulp suggests the idea that they are embedded in a transparent and structureless jelly, which projects a little beyond them. When the pulp has completed, for the time being at all events, the formation of the dentine, the odontoblast cells THE DENTINE-OBGAN. 191 become more elongated and more rounded in their outline and taper off towards and into the dentinal process, instead of having truncated ends. Several more or less divergent views as to the exact method of development of the dentine have from time to time held the field, the question turning mainly upon the part played by the odontoblasts. The close relation of these cells to the dentine, their change in form according as dentine building is or is not actively going on, and the absence of any other of the large cells which elsewhere are associated with the elaboration of special products, would naturally lead to the inference that they are the chief factors in the segregation of lime salts and A Fig. 102. — Odontoblasts in situ. (After Waldeyer.) This figure also shows at A. a layer of formed matrix not yet calcified. The pulp processes are not illustrated. their incorporation in the dentine, a view which has obtained many adherents. According to this view, which is supported by Waldeyer, Frey, Boll, Lionel Beale, and many other of the older writers, the dentinal fibrils, the dentinal sheaths, and the matrix between these latter, are alike derived from the metamorphosis of the odontoblast cell. In other words the three structures in question may be considered as being three stages in the conversion of one and the same substance : thus there is the dentinal fibril in its soft condition, little more than the unaltered protoplasm of the cell ; then the dentinal sheath, one of those peculiarly resistant sub- stances which lie on the borders of calcification ; and lastly, the matrix, the completed, wholly calcified tissue. Thus, it has been supposed that the most external portions of the odontoblasts undergo a metamorphosis into a gelatigenous matrix, which is the seat of calcification, while their most central portions remain soft and unaltered as the fibrils. Intermediate between the central per- manently soft fibril and the general calcified matrix is that portion which immediately surrounds the fibril, namely, the dentinal sheath. This view differs but little from that of von Ebner and Eose, which, 192 A MANUAL OF DENTAL ANATOMY. however, is to some extent intermediate between the older theory and that advocated by Mummery : they believe that the whole of the dentine is derived from the odontoblasts, but that while their axial portions persist as dentinal fibril, their peripheral portions become metamorphosed into a delicate fibrillar gelatigenous tissue, a connective tissue in fact, which, forming the matrix, receives an impregnation with lime salts. Just as in the case of enamel, there are writers who hold that the odontoblasts do not themselves become calcified, but that they preside over the secretion of a material which is converted into dentine. Thus Kolliker and Lent believe that while the canals and their contents are continuations of the odontoblasts, the matrix is a secretion either from these cells or from the rest of the pulp, and so is an "intercellular " substance. Their view is therefore intermediate between the excretion and conversion theories ; and Kolliker goes on to say, " since the dentinal cells are immediately drawn out at their outer ends into the dentinal fibres, and do not, as was formerly thought, grow out in such a manner that the dentinal fibre is to be regarded only as the inner part of the cell, so it is not possible to derive the dentine immediately from the cells." But is not Kolliker thinking and writing of those aged, spent cells which his pupil Lent figured ? No one could speak of a young, active odontoblast as "drawn out into the dentinal fibril." A properly prepared section of young developing dentine shows that the cells are square and abrupt towards the dentine ; they do not taper into the dentinal process in the smallest degree, and there is little room for any intercellular substance. Hertz coincides with Kolliker in regarding the matrix as a "secretion from all the dentinal cells in common, which stands in no definite histological relation to the individual cells," but his figure also probably is represen- tative of an adult inactive surface of pulp, in which dentine formation has almost ceased. Kolliker and Lent are of opinion that a single cell is sufficient to form the whole length of a dentinal fibril, not having seen evidence of active cell growth in the subjacent layer of the pulp, from which they could infer that the membrana eboris was supplemented by new cells from below. In a later edition of his work, however, Kolliker speaks with much more hesitation on this point. Magitot held that the whole of the dentine is " a product elaborated by the odontoblasts," but neither secreted by nor formed by the con- version of the odontoblasts, and he denied the existence of the sheaths of Neumann in toto. Klein believes that the odontoblast forms the matrix only, whilst the dentinal fibrils are processes continued up between the odontoblasts from a subjacent layer of stellate cells. Robin and Magitot formerly held that the dentine matrix was formed by the transformation of the odontoblast cells, but that the tubes were interspaces between these latter, not corresponding with the axes of the cells. Baume (3) holds that the odontoblasts secrete a material which calcifies, rather than that they are themselves converted. THE DENTINE-OKGAN. 193 And Hope well -Smith believes that the odontoblasts take no part in the formation of the matrix of dentine, which is furnished by smaller cells near the pulp surface. It will be remembered that von Ebner has shown the existeDce of a delicate fibrillar structure both in bone and in dentine ; and his investigations have been carried further by Mummery (7), whose view, briefly expressed, is that the dentine matrix is due to a calcification in a delicate connective tissue which has not been derived from the odontoblasts at all, but from the general connective tissue of the pulp, i.e., that the Fig. 103.— Longitudinal section at apex of radical portion of pulp in human premolar, showing odontogenic fibres in continuity with the dentogenetic zone, (x 350.) At A is seen the formed dentine; at B a layer of forming dentine, the " dentogenetic zone," to which a number of connective tissue bundles (C) adhere. (After a drawing by . Howard Mummery in the Phil. Trans. Koy. Soc.) dentine matrix is of an inter-cellular nature. He confirmed von Ebner's observation that in the completed matrix now and again traces of a much masked fibrillar structure were to be found, especially in teeth which had undergone the slow decalcifi- cation attendant upon the early stages of dental caries. But he further showed that by the use of Koch's method as modified by Weil a connective tissue was to be demonstrated in the periphery of the pulp, which connective tissue apparently ran continuously into the matrix, somewhat as Sharpey's fibres run into bone. According to him, dentine is developed very much like a membrane-bone, there being, however, this difference: that d.a. 13 194 A MANUAL OF DENTAL ANATOMY. in the case of dentine long protoplasmic processes from each odontoblast cell persist, though the cells themselves do not become enclosed in the forming tissue. In the case of bone, some of the osteoblasts do become invested and persist as the branched cells which occupy the lacunae. He has traced these connective tissue bands passing between the odontoblasts and running into the dentine, while on the other hand, their con- tinuity with the general connective tissue of the pulp is easily demonstrable. They are, however, not well seen in sections which run exactly in the plane of the length of the odonto- blasts, which in that case are apt to obscure these very delicate fibrils somewhat ; and there appear to be cells applied to these A Fig. 104. — Decalcified section of tooth of hake. A. Formed dentine. B. Connective tissue bundles running into it without break of con- tinuity. C. Connective tissue framework of the pulp. bundles of connective tissue which are distinct from the odontoblasts, and recall the cells upon the osteogenetic fibres of bone. For the details of the reasons which have led him to adopt this view, the reader must refer to his original paper ; but a very strong argument in its favour is derived from a study of the development of vaso-dentines. In the latter there can be found a very distinct layer of straight parallel fibres which abut upon the forming dentine. They really are a layer of specially arranged connective tissue fibres. For their nuclei are inconspicuous and they quite lack the fully developed appearance of true odontoblasts. Though they so far resemble them as to pull away in a con- tinuous sheet adherent to the dentine, and of perfectly uniform thickness, yet in suitable preparations they can be seen to be continuous with the connective tissue framework of THE DENTINE-ORGAN. i95 the pulp, which in vaso-dentine pulps is far more strongly developed than it is in the pulps of ortho-dentine. There being no dentinal tubes to mask it, the dentine can often be seen splitting up in decalcified specimens, into bundles of the same size as the fibres of the pulp layer. Moreover, it is much more easy to see the fibres alluded to running into the formed dentine matrix on the one hand, and on the other into the connective tissue deeper in the pulp in vaso-dentine than in hard tubular dentine. In the peripheral or first-formed part of the dentine of many vaso-dentine teeth there is a structure which might Fig. 105a. — Decalcified section from a bake. The connective tissue fibres are seen forming a layer as distinct as, though greatly thicker than, an odontoblast layer. In the formed dentine at A the vascular canals are seen, and at C the general connective tissue stroma of the pulp. Fig. 105b. — Outer layer of dentine of a cod. almost be termed a petrifaction of the connective tissue stroma unchanged. This is well seen in Fig. 105b, drawn from the tooth of a cod. It cannot be said that there is any funda- mental difference between the development of a vaso-dentine and that of ortho-dentine, for the very same tooth may be composed at one part of the one and at another of the other, as is well exemplified in the tooth of a flounder (see Fig. 53), so that it is quite safe to use the one tissue as illustrating the development of the other. If then it be true, as it appears to be, that the dentine matrix is formed in a connective tissue basis, it becomes a question what share the odontoblasts take in the process. It may be taken as certain that the odontoblast is concerned in Fig. 105a. Fig. 105b. C 13—2 196 A MANUAL OF DENTAL ANATOMY. the development of the dentinal tubes, keeping them open as it were by its process, the dentinal fibril ; and it seems clear that the same odontoblast forms the whole length of the fibril, itself receding backwards as the dentine grows ; it seems also possible that it is concerned in the development of the sheaths of Neumann. But inasmuch as Mummery believes that he has seen other cells applied to the connective tissue fibres, both in man and in fish, if he is right, it becomes doubtful whether the odontoblasts alone are concerned in elaborating the calcifying material. On the whole, however, their general resemblance to the layers of osteoblasts which clothe the surfaces of developing bone renders it probable that they are active agents in the development of the dentine, and it is certainly known that dentinal tubes are not formed in their absence (lla). It has been mentioned (p. 101) that the different varieties of dentine merge into one another, and that also dentine in some of its varieties is not distinguishable from bone. The con- nective tissue theory of its development therefore derives no little support from comparative anatomy, for it brings the development of dentine into line with that of bone, and, on the other hand, with that of structures very like dentine, which occur in the endo- and exo- skeletons of a great many fish. But other views are still entertained by some authors. There is another feature in the development of dentine which has not so far been alluded to. During the period of active growth of this tissue, but not afterwards, there is a layer between the odontoblasts and the completed dentine, which appears to be in a transitional stage. This, the dento- genetic zone, usually takes the stain much less deeply, or, in the case of double staining, takes a different colour from that of the rest of the dentine when this has been decalcified. In sections prepared by Weil's process it stains more deeply than the dentine. In this layer an appearance of striation, parallel to the surface of the pulp cavity, can sometimes be seen, and these striae can be faintly seen in the completed dentine near to it. In this connect on it is interesting to note that decalcified dentine sometimes splits up into laminae concentric with the pulp, and F. J. Bennett noted that in teeth long kept in glycerine, which has a slowly decalcifying action, a similar THE DENTINE-OKGAN. 197 splitting occurred. The layer of incomplete dentine is, like Neumann's sheaths and the contents of interglobular spaces, stained a deep black by Golgi's method of using nitrate of silver. The thinnest layer of dentine, such as may be found at the edges of the dentine cap, is soft and elastic, and so transparent as to appear structureless. Where it has attained a somewhat greater thickness, globules begin to appear in it, which are smaller in the thinner and larger in the thicker portion of the dentine cap. As they are actually in the substance of the cap, their growth and coalescence obviously go on without any very immediate relation to the cells of the pulp ; in point of fact, a process strictly analogous to that demonstrated by Eainey and Professor Harting (see p. 162) is going on. Thus in the formation of the first layer of dentine a stage of metamor- phosis preparatory to impregnation with calcareous salts dis- tinctly precedes that full impregnation which is marked by the occurrence of globules and their subsequent coalescence. The occurrence of these globular forms, and consequent large interglobular spaces, in the deeper parts of adult dentine, is therefore an evidence of arrest of development rather than of any otherwise abnormal condition. When the formation of the dentine and enamel has gone on to the extent of the crown of the tooth having attained its full length, the reproduction of new formative pulp (in teeth of limited growth) takes place only over a contracted area, so that a neck, and finally one or more roots, are the result of its conversion into tooth substance. In teeth of constant growth, however, no such narrowing of the formative pulp takes place, but the additions to the base of the tooth are of constant or ever-increasing dimensions, as in some tusks, which are thus conical in form. Little is known as to the rate of increase of teeth of persistent growth, nor as to whether this increase is constant or intermittent. But the tusk of a walrus which was in the possession of the late Professor Charles Stewart, seems to distinctly indicate that its growth was intermittent. This tusk has over a considerable part of its surface been severely weathered, a small portion having been protected. The protected surface is smooth and in no way unusual, but the weathered portion bears circumferential rings slightly more 198 A MANUAL OF DENTAL ANATOMY. prominent than their interspaces. Towards the apex or younger part of the tooth these are about an inch apart, but the interspaces diminish with regularity towards the base, where at last they attain a fairly constant separation of about a third of an inch. They can hardly be due to anything else than intermissions of growth, and each interspace probably represents the annual increment, large while the tooth was young and, as the creature grew older, short and subsequently smaller. It is said that the number of roots which would have been developed at the base of a particular dentine organ may be inferred from the vessels, i.e., that in a single-rooted tooth the vessels would, even at an early period, form a single fasciculus ; in a double-rooted one, similarly, they would be arranged in two bundles, so that the ultimate formative activity will be exercised around one, two, or three centres of nutrition. Aitchison Robertson found by feeding rabbits with madder that about 1 mm. a week was added to their incisors, but that their molars increased more slowly. The rate of growth of human dentine has not been accurately determined but the tables of Magitot and of Rose indicate a far slower rate of addition than this (cf. p. 214). There is some reason to think that dentinal tubes once formed are possibly capable of farther calcification, by which their calibre becomes sensibly diminished. Thus Sir John Tomes, speaking of the incisor teeth of rodents, states, " The tubes which proceed from the pulp cavity near the base of the tooth are, in most cases, perceptibly larger than those that are situated higher up ; hence it follows that, as the latter were once near the base of the tooth, the dentinal tubes undergo a diminution of calibre after their original formation. In the teeth of the Sciuridce I have found a difference of size amounting to a third or half between the tubes near the base and those near the surface in wear." Lionel Beale calls attention to the fact that the hollows of the canals are larger nearest to the pulp, and smallest at the periphery of the tooth, in other words, at the oldest part. Also that calcification is still slowly going on even in advanced life, so as often to lead to the obliteration of the peripheral tubes,. There is, too, the statement of THE DENTINE-OBGAN. 199 Eobin and Magitot, that the teeth become more rich in calcareous salts as age advances, so that analyses of human teeth show great discrepancies. It is difficult to see how a dentinal tube once formed can become contracted to a third or half of its diameter unless it is believed that that which was at first the soft tissue (dentinal fibril) occupying its canal may become at its periphery metamorphosed into " dentinal sheath," while that which was originally this latter has passed into the condition of matrix. But it has been urged (see p. 74) chat the dentinal sheath has no separate existence in the dentine until after its disintegration by a strong acid. The explanation has, however, been suggested that the odontoblasts and their processes differ in size during different periods in dentine formation (Hopewell- Smith) ; and probably in this observation lies the whole truth of the matter. Calcification of Vaso-dentine. — Like ortho-dentine, this is developed in a uniform layer upon the surface of a simple pulp, which can be pulled out of the cap of formed dentine at any time. During the calcification of ordinary dentine there is an abundant plexus of capillary vessels a very short distance beneath the odontoblast layer, but this recedes as dentine formation goes on, and does not, except as a matter of accident, get included in the forming dentine. Never- theless, a moment's reflection will show that (except in the earliest stages, before any dentine is formed) this plexus must at a prior time have occupied the place now taken possession of by the inward-marching odontoblasts and dentine. But in the calcification of a formative pulp into vaso-dentine this recession of its vessels before the advancing border of calcification does not take place ; the whole vascular network of the papilla remains and continues to carry blood circulating through it, even after calcification has crept up to and around it. Just as in a hard dentine, the calcification takes place on the surface of the pulp, but owing to the great development of the connective tissue elements, the odontoblasts are not so conspicuous, although, by search, round cells like osteoblasts are to be found there. Indeed, as has already been seen, the very same pulp with 200 A MANUAL OF DENTAL ANATOMY. an odontoblast layer is able, at the tip of the tooth, to form a finely tubular dentine, and lower down to gradually change its methods so as to leave vessels carrying red blood through special channels. (See Fig. 53.) The typical odontoblasts seem to be concerned in keeping open the tubes of dentine, in which their protoplasmic processes remain. The capability of a pulp to change its method of calcification is very prettily illustrated in the teeth of some fish, of which the Sargus may be selected. In this fish the dentine pulp for some time goes on forming Fig. 106. — Base of the dentine of the crown, and the implanted portion of the tooth of Sargus. The surrounding bone in which the latter is imbedded is indicated in outline only. A. Fine tubed dentine ; B. Vaso- dentine. a fine-tubed hard dentine in which no vascular canals exist, but at a certain point a sudden break occurs, which (Fig. 106) coincides with the commencement of the implanted portion of the tooth. This implanted portion differs from the true dentine tooth above it, in that it contains at first a few, and then no dentinal tubes at all, and it does contain vascular loops and the somewhat irregular spaces which are to be found in the bones of these fish. Moreover, at the base of the tooth above the break, towards the outer surface of the dentine, a few vascular loops begin to appear, foreshadowing the more complete change which is about to occur (Fig. 56). Tbe great peculiarity of the process is that there is no apparent change in the structure of the formative pulp, which is continued down with its odontoblast layer just the same, not the smallest THE DENTINE-OKGAN. 201 difference being distinguishable between the pulp above and below the break. Below it, the tooth pulp with its odonto- blast layer goes on at first forming an additional portion of Fig. 107. — Diagrammatic representation of tooth and tooth-sac of Sa?'gus, showing the relation of the implanted portion to the odontoblast layer and the tooth-sac. the same diameter as before, but it is thinned down pretty speedily to an edge. The after-history of this deeper portion of the tooth is that it is imbedded in and thoroughly fused with the surrounding bone of the jaw. Calcification of Osteo-dentine.— With the exception of Fig. 108. — Point of junction of the two portions, more highly magnified. the thin external layers (see Fig. 59), which are developed from a superficial layer of not very highly specialised cells, osteo-dentine is built up in a manner fundamentally different from that in which ortho-dentine, plici-dentine, and vaso- dentine are constructed. For it is not, like these, a surface formation ; it is not laid 202 A MANUAL OF DENTAL ANATOMY. down in a regular manner upon the exterior of a pulp, and it has no relation to an odontoblast layer, if, perhaps, its thin exterior shell be excepted. So soon as this has been formed, its inner surface becomes roughened by trabecule shooting inwards into the substance of the pulp, which speedily becomes traversed completely by them, as well as by the connective tissue bundles which are continuous with them. Thus the pulp, being pierced through in every direction by these ingrowths, cannot be withdrawn, like the pulp of a hard or of a vaso -dentine tooth, from the interior of the dentine cap. Osteoblasts clothe, like an epithe- lium, the trabecule and the connective tissue fibres attached to them, and by the calcification of these the osteo-dentine is formed. The process is exactly like the calcification of any membrane- bone, and the connective tissue bundles remind one of those which are believed to be the occasion of the formation of Sharpey's fibres in bone. In the case of teeth which are going to be anchylosed to the subjacent bone, these fibres run con- tinuously from the interior of the dentine cap down to the bone, and calcification in and around them binds the two inseparably together. It is interesting to note, especially in connection with the fact that some observers believe Sharpey's fibres to be elastic, that the hinged teeth of the pike (see Fig. 133) owe their power of resilience entirely to the elasticity of these connective tissue bundles, which do not become completely calcified ; although at an early stage it would be quite impossible to say whether a particular tooth was going to be anchylosed, or to be a hinged tooth with elastic attachments. Thus it will be seen, that if the connective tissue theory of dentine development be accepted, the development of the several varieties of dentine, which seem to run into one another structurally by almost imperceptible gradations, comes into line and so seems more intelligible. Still it may be possible that in each kind, as particularly clearly seen in osteo-dentines, osteoblastic cells, as apart from odontoblastic cells, may produce the dentine matrix. Dental Follicle. — By the time that calcification is com- mencing, there is a tooth-follicle or tissue forming a tough capsule-like investment around the dentine-germ and enamel- THE DENTINE-OBGAN. 203 organ. At an early period of development the tissue forming the dentine-papilla of a mammalian tooth is seen to be pro- longed outwards and upwards from its base (see h in Fig. 75) ; these processes appear to grow rapidly upwards, so as to embrace the enamel-organ ; but w7hether this is really so, or whether it is merely that the ill-defined tissue in which the dentine-forming organ has itself originated is in this region also becoming more pronounced, it is hard to say ; probably the latter is the more correct. This upgrowth from the base Fig. 109. — Tooth-sac of a calf (semi-diagrammatic). a. Tooth-sac. a\ a!1. Its outer and middle portions, b. Stellate cells of enamel- organ, c. External epithelium of enamel-organ, d. Internal epithe- lium of enamel-organ, e. Odontoblasts. /. Dentine pulp or papilla. g. Vessels in dentine pulp. i. Points where the sac becomes fused with the base of the dentine papilla. of the dentine-papilla is the first appearance of a special dental sacculus, which is thus derived from sources identical with that of the formative organ of the dentine. While these changes are going on, the tooth-sac is be- coming lodged in a widely open gutter of bone, which is being rapidly formed at its sides and under its base. If at this stage (see Figs. 75 and 110) the gum be stripped off the jaws, the developing tooth capsules are torn off with it, and are inseparable from it except by actual cutting, thus leaving the gutter of bone quite bare and empty. In fact, 204 A MANUAL OF DENTAL ANATOMY. 10 Fig. 110. — Transverse section of the lower jaw and developing back tooth of a lamb, copied from Waldeyer (Henle's " Zeitschrift f. Kat. Med." 1865). In its outlines the figure is faithful to nature ; it is so far diagrammatic that more of the structure than could be seen with the magnifying power employed is introduced. 1. Dentine germ, with its border of odontoblasts. 2. Formed dentine. 3. Formed enamel. 4. Points where the inner epithelium and the outer epithelium of the enamel become continuous. 5. Enamel cells or internal epithelium. 6. External epithelium of enamel-organ. 7. Stellate reti- culum of enamel-organ. 8. Papillary projections into the enamel-organ. 9. Connective tissue around the sac, becoming continuous above with that of the gum (9a) ; this constitutes what is called the tooth-sac. 10. Vessels and nerves of the jaw. 11. Bone of lower jaw. 12. Periosteum of the jaw. 13. Heap of epithelium over the young tooth. 14. External skin with its epidermis. 15. Muscular bundles from floor of mouth, THE DENTINE-ORGAN. 205 the capsule or sac consists of almost the whole of the con- nective tissue which intervenes between the special dentine and enamel-germs, and the bone — the latter originating deep in the tissues and independently of any periosteum, which is not yet differentiated. In the first instance the follicle wall is only distinguished from the connective tissue external to it by being somewhat richer in cells, vessels and fibrillar elements, being in fact more condensed or more compact. The sacs, when at their fullest development, are divisible into two layers, an outer thin firm wall, and an inner looser tissue, not very dense. At the base of the tooth-sac, the follicle wall is not separable nor distinguishable from the base of the dentine-papilla with which it blends. The follicle wall is richly vascular ; and over the surface of the enamel-organ it is prolonged inwards in the form of villous or papilliform eminences, pro- jecting into the external epithelium of the enamel-organ. To these prominences, which are analogous to the papillae on the free surface of the gum, some authors attach much importance, as having an influence upon the direction of the enamel rods, and so regulating the pattern formed ; but this view is by no means universally accepted. The internal or softer and looser portion of the follicle wall, which has a consistency but little firmer than that of the stellate reticulum of the enamel-organ, is much developed in ungulates, where there is to be a deposi- tion of coronal cement. This differentiation of a portion of the dental-sac is thought by Legros, Robin, and Magitot, to be sufficiently pronounced to justify its designation as a distinct " cement-organ," though many later writers altogether dispute this. THE CEMENT-OBGAN. Cementum is, according to these last mentioned authors, developed, just as bone is, in two distinct methods. Where it is not to be very thick, and is to clothe roots, the ossification takes place in membrane (the alveolo-dental periosteum), but where it is to form a thick layer over the crown, as in ruminants, a cartilaginous cement-organ is formed, and a calcification analogous to formation of bone in cartilage ensues. Thus the cement-organ is found in those animals only which 206 A MANUAL OF DENTAL ANATOMY. have coronal cement, such as in some of the Ungulata. In a calf embryo about the time that dentine calcification is com- mencing, there may be distinguished beneath the follicle wall and above the enamel-organ a greyish layer of tissue thick enough to be distinguishable with the naked eye, and of firmer consistence than the enamel-organ, from which it also differs in being richly vascular. But though it exists at this early period, it is not till later, when, after the completion of the dentine and enamel imme- diately beneath it, its own function is about to come into play, that it attains to its characteristic structure. This Legros Fig. 111. — Cement-organ of a calf. (After Magitot.) a. Fibroid matrix. b. Cartilage cells and capsules. and Magitot designate as fibro-cartilaginous, as there appear in it characteristic cartilage cells, containing one, two, or rarely three cells, which have spherical or ovoid nuclei. In those creatures which have cementum upon the roots of the teeth only, no special cement-organ exists, but osteoblasts which calcify the cementum are furnished by the tooth-sac. It is said that the inner layer of the tooth-sac is concerned in the formation of the cement ; that the outer layer, conjointly with the surrounding connective tissue, is converted into the aiveolo-dental periosteum. In human teeth the parts of the follicle wall or sac cease to be distinctly distinguishable at a comparatively early period, and their importance is not such as to call for any very detailed description. THE CEMENT-OEGAN. 207 Another structure, once thought important, and now known to be a mere bundle of dense fibrous tissue, is the " -gubemaculum" . Each permanent tooth-sac during its growth, has become invested by a bony shell, which is complete, save at a point near its apex, where there is a fora- men. Through this foramen, very conspicuous when the surrounding bone is broken away, passes a thin fibrous cord, which is called the "gubemaculum," from the notions enter- tained by the older anatomists that it was concerned in directing or effecting the eruption of the tooth. The guber- nacula of the front permanent tooth-sacs perforate the alveo- lar process of the jaws and blend with the gum behind the necks of the corresponding milk teeth ; those of the premolars unite with the periosteum of the alveoli of their deciduous predecessors. The Calcification of Cementum. — Just as is the case with bones elsewhere in the body, cementum may, perhaps, be formed in two distinct ways — by membranous ossification, and by ossification in a fibro-cartilage, the former method obtaining upon the roots of teeth, and the latter upon those crowns where the cement-organ described by Magitot exists. At the time when the crown of a tooth appears through the gum, it alone is complete, and the root has yet to be calcified. As each portion of dentine of the root is completed, it is coated with a closely-adherent vascular membrane which is, in fact, the follicle wall, and which is to become, when the cement is formed, the alveolo-dental periosteum. The inner or dentinal face of this membrane presents a layer of large cells, the osteoblasts of Gegenbaur, and it is by their agency that bone or cementum is formed. These osteoblasts are themselves a special development where bone is about to be manufactured, as was clearly explained in the following extract from a paper by Sir John Tomes and the late Mr. De Morgan, (10) who termed them osteal cells :■ — " Here (towards the bone) in the place of cells with elongated processes, or cells arranged in fibre-like lines, we find cells aggregated into a mass, and so closely packed as to leave little room for intermediate tissue. The cells appear to have increased in size at the cost of the processes which existed at an earlier stage, and formed a bond of union between them. Everywhere about growing bone a careful examination will reveal 208 A MANUAL OF DENTAL ANATOMY. cells attached to its surface, while the surface of the bone itself will present a series of similar bodies ossified. To these we propose to give the name of osteal cells." Behind the osteoblasts and very near to the perfected cementum lies a reticulum or network, which looks like con- nective tissue, and is confused from the interlacing of the processes. Many of these processes pass into, and are lost in the clear, structureless matrix of the already-formed cementum, and persist in it, as Sharpey's fibres. Where tendons or ligaments are inserted into a bone, the fibres or bundles of the ligament are continued into the bone as penetrating fibres (Sharpey's fibres). Von Ebner holds that they are composed of bundles of fibre-like connective tissue, and thinks that they are not calcified, but merely imbedded in a calcified matrix. It is more commonly thought that in the development of bone, to which cementum is closely similar, bundles of fibres like those of connective tissue (osteogenetic fibres) appear, and become stiffened and calcified by a globular deposition of lime salts in them, these globules coalescing. Thus a membrane-bone is at first a network of spiculae, beyond which extend the soft osteogenic fibres. Externally to the fine-meshed network which has been well figured and described by Lionel Beale, the soft tissue sur- rounding the root partakes more of the character of ordinary fibrous tissue, and may be teased out into fibrils. The fibrous bands run mainly in a direction from the alveolus towards the tooth. Many of them pass through the whole thickness of the soft structures, extending from the bone of the alveolus to the cementum of the tooth, becoming lost at each extremity in the one tissue or the other. Here and there osteoblasts become enclosed in the forming matrix, and remain as the bone corpuscle of the future bone, occupying its lacunae. In carmine-stained preparations from the teeth of calves a round nucleus may sometimes be seen lying in the stellate " lacuna " ; the nucleus soon disappears, and plays no active part in determining the form of the lacuna. The nucleus may also be seen in the developing bones of human foetuses, and, though this is difficult to understand, the traces of the nucleus seem to be beautifully preserved in the lacunae of a supposed THE CEMENT-OKGAN. 209 Pterodactyle bone from the Wealden, a section from which was figured by Sir John Tomes in the paper referred to. Exactly as calcification, advancing with irregularity in the interior of an individual cell, fails to render it homogeneous by pervading its whole substance, so it may fail so completely to unite con- tiguous cells as to obliterate their contours. A lacuna, surrounded by such a contour line, mapping the limits of the original cell, or cluster of cells is what is termed an " encapsuled lacuna." That which determines the formation of a lacuna, or an encapsuled lacuna, at any particular spot, is unknown. All that can certainly be said upon the subject is embodied in the following extract from the paper by Sir John Tomes and Dr. Morgan above alluded to : " We see the boundary of the original lacunal cells only in those cases where the lacunae have but few, or are entirely devoid of canaliculi. It would appear to be a law, to which there are few, if any, excep- tions, that when anastomosis is established between adjoining lacunae, the lacunal cells blend with the contiguous parts, and are no longer recognisable as distinct bodies." The connective tissue-like fibres are not thought by all histologists to be the primary sources of bone matrix, but some regard the nbrillated ground substance as derived from a part of the osteoblasts. Thus Klein considers that the osteoblasts form both matrix and bone corpuscles. " Each osteoblast by the peripheral portion of its cell substance gives origin to the osseous ground substance, while the central protoplasm round the nucleus persists with the latter as the nucleated bone-cell. The bone-cell and the space in which it lies become branched. For a row of osteoblasts a row of obloDg or round territories is found, each composed of matrix, and in it a nucleated branched cell. The outlines of individual territories are gradually lost, and a continuous osseous lamina, with its bone-cells then results. The ground substance is from the outset a network of fibrils ; it is at first soft, but soon becomes impregnated with inorganic salts, the process commencing at the ' point of ossification.' The bone-cells, with their processes, are situated in corresponding lacunpe and canaliculi, just as in the adult osseous substance." Schafer urges as an objection to this view that there is no indica- tion in the formed bone of the cell areas, nor of partly converted osteoblasts ; and if the periphery becomes converted into ground substance and the centre and nucleus persist as the contents of a lacuna, the osteoblast should be far larger than the lacuna, which is not the case. This objection, however, applies with less force to the very large " osteoblast" cell, which is as large as the encapsuled lacuna D.A. 14 210 A MANUAL OF DENTAL ANATOMY. According to Kolliker, the cementum is first deposited in isolated scales, which coalesce with one another, rather than in a continuous sheet. In the teeth of the Primates, the Carnivora, Imectivora, &c, the cementum, at least in any appreciable thickness, is confined to the roots of the teeth. M. Magitot states that the calcification of the cartilaginous cement- organ of ungulates differs in no respect from that of other cartilages, but in his description he merely states that patches of calcification appear here and there in the deepest portion of the organ, coalesce, and come to invade its entire thickness ; and, further, that the cement at the period of eruption is constituted of "osteoblasts" regularly grouped round vascular canals, and included in a ground substance finely striated. (6) But where intra- cartilaginous ossifications occur elsewhere in the body a temporary bone is formed by the calcification of the cartilage matrix, which is subsequently absorbed and swept away, as marrow-containing channels appear in it and bore their way through it, substituting for the calcified cartilage a bone developed from osteoblasts ; and ultimately all remains of the calcified cartilage or temporary bone disappear. Thus all bone, whether developed in cartilage or in membrane, is formed alike, the calcified cartilage merely forming a temporary framework or scaffolding, in and amongst which the bone is formed from osteoblasts. But M. Magitot does not describe in any such detail calcification of cartilage and subsequent removal to give place to an osteoblast-derived bone, though he speaks of the cartilaginous cement-organ as a transitory or temporary structure. Historical Survey of the Development of the Teeth. It may perhaps assist the student, who may be perplexed in endea- vouring to reconcile the statements of various authors, to give a succinct history of the views upon the development of the teeth from time to time set forth. Before the time of Goodsir (1838), the initial stages of development were described by Baschkow, somewhat vaguely as proceeding under- neath the mucous membrane ; he did not, however, trace out in what precise manner the several parts of the tooth-germ originated. The paper of Goodsir, giving, in the place of somewhat general notions a very definite and intelligible description of observations, was accepted without question by most anatomists, if not by all. Accordingly there may be found in all the text-books at and after that period, and in some even up to the present day, the description given by Goodsir reproduced almost without alteration, so that it will be worth while to briefly relate what his views were. He believed that at an early period in foetal life there appeared a con- tinuous open groove, running round the whole circumference of the jaws ; that from the bottom of this groove there arose isolated and uncovered papillae, corresponding in number to the milk teeth ; that these papillae became covered in by the deepening of the groove and the meeting of its two edges over them, whilst at the same time transverse septa were formed, so that the several papillae became enclosed in their own THE CEMENT-ORGAN. 211 separate follicles. With the details of the process as described by him the reader need not be concerned ; it will suffice to remember that he dis- tinguished the four stages : a primitive dental groove, a papillary stage, a follicular stage,- and an eruptive stage (the latter of course at a long subsequent period). Not only were these views accepted quite without question, but they were even extended to explain the development of the teeth in reptiles and fishes, and thus in the Odontographies of Professor Owen and Professor Giebel may be found accounts of development of the teeth in these animals which are perfectly in accord with Goodsir's theory, but which, in fact, are far more inaccurate than the same theories were as applied to mammalian teeth. Even so careful a writer as Professor Huxley, who was the first to point out that these stages really did not exist either in the frog, the mackerel, or certain other fish, accepted them without question as true of mammals. Markusen* (1849) gave upon the whole a correct account of the process, referring the enamel to the oral epithelium, and Professor Huxley (1854), whilst demonstrating that the stage of free papillae was not to be found in certain fish and reptiles (a fact also made out in the newt by Dr. Beale), clearly and strongly expressed the same view as to the origin of the enamel-organ, and hence of the enamel. It would be wrong, however, to set small value upon the observations of Groodsir. They were a great step in advance, and were as accurate as the methods of investigation then in use would allow of ; moreover the error in observation is very easy to account for. The epithelium having peeled off as a result of de- composition or the use of weak spirit, the state of things left does not widely differ from that described. The subject rested for many years without further advances, but in 1863 Kolliker demonstrated, beyond all cavil, the real origin of the enamel organ and its relations to the oral epithelium, the dentine-organ, and the dental sac. His views, substantially correct, have been elaborated by Waldeyer, Kollman, Hertz, Legros and Magitot, Wedl, Hertwig and others and only in minor particulars have they been modified. The development of the teeth of reptiles was found by a pupil of Kolliker, Santi Sirena, to have several features in accord with that of mammalian teeth; and the author's researches on the teeth of Batrachia, fish and reptiles, elsewhere detailed, have proved a striking general similarity in the process throughout the vertebrate kingdom. Membrana Preformativa. — To the student of dental development few things are more perplexing than the conflicting statements which he reads in various works as to the nature and position of the Membrana 'preformativa ; after having mastered with difficulty some one description of its character, he finds that many of the most recent authors altogether deny its existence. * In the resume given by Messrs. Legros and Magitot, before referred to, due credit is not given to the papers of Markusen and Huxley (1849, 1854) (although they are alluded to), and it appears that too much is given to that of Natalis Guillot (1858). 14—2 212 A MANUAL OF DENTAL ANATOMY. According to the older theories of tooth development, under the thrall of which earlier authors have written, the tooth-germ was in the first instance a free, uncovered papilla of the mucous membrane which sub- sequently sank in and became encapsulated, &c, &c. (see pp. 129 and 210). Moreover, it was taught by the older histologists that fine homogeneous Abasement membranes " were to be found in a great variety of situations, amongst others beneath the epithelium of the mucous membrane, and that these were physiologically of much importance, inasmuch as they formed defining limits, through which structures did not pass. As a necessary consequence of these views, it was assumed as a matter of course that the " dentine papilla " must be covered over by a " basement membrane," or membrana preformativa. Thus this membrane necessarily intervened between the enamel-organ and the dentine-papilla, and hence gave rise to difficulties in the under- standing of the calcifying process. Henle considered that evidences of its presence speedily became lost, but that ossification proceeded in oppo- site directions upon the two sides of this membrane ; from within out- wards in the case of the enamel, from without inwards in the case of the dentine. Huxley, starting on the same hypothesis as to its position, namely, that it was placed between the enamel organ and the dentine papilla, came to a different conclusion as to its after-fate. Eelying upon the fact that a continuous sheet of tissue or membrane can be raised from the surface of the developing enamel (see p. 172), he concluded that this was the original membrana 'preformativa, that it afterwards became Nas- myth's membrane, and that enamel was developed, without the direct participation of the enamel-organ, seeing that a membrane separated the two. Kolliker strongly affirmed the existence of the membrana preformativa, and in the older edition of his Histology held that it became converted into Nasmyth's membrane. There are thus three situations assigned to the membrane covering the dentine papilla, or membrana preformativa : — (i.) Between the dentine and the enamel (Henle) : (ii.) Between the enamel and the enamel-organ, i.e., outside, the enamel (Huxley) : (iii.) Between the dentine and the pulp (several writers of less authority). Many writers deny its existence altogether, explaining on other grounds the appearances observed. Markusen believed that it was nothing more than the part of the papilla first ossified ; and Lionel Beale definitely denies the existence of a membrane in any one of the three situations above detailed, as do also Hertz, Wenzel, and Waldeyer. Eobin and Magitot have offered a plausible explanation of the appear- ance of a limiting membrane over the pulp, which is briefly this : the formative pulp is rich in a clear substance of gelatinous consistency (which, in fact, forms its chief bulk), reminding the observer of the tissue THE CEMENT-ORGAN. 213 contained in the umbilical cord. This is somewhat more dense towards the surface, where it forms a matrix for the odontoblasts and projects beyond them, so as to appear, in section or at a thin edge, like a sort of varnish to the papilla. Prom its greater density near the surface, it may become corrugated, and so seem like a folded or torn membrane. It is difficult to imagine that such a membrane exists upon papillse formed at such a great distance from the surface as those of the snake or the lizard (Fig. 81) ; if there be such a membrane, it might be a secondary development upon the surface of the mass of cells which primarily con- stitute the rudiment of the dentine-papilla, and in that case is not a part of the general basement membrane of the oral mucous membrane ; or else it must have been carried in as a sort of cul de sac in front of the inward growing process of the epithelium, to which in that case it would belong rather than to the dentine-germ. Neither of these suppositions commends itself as probable ; and a still greater obstacle to the accept- ance of a membrane in this position is afforded by the structure of marsupial teeth (see Figs. 21 and 90), in which the membrane would be everywhere perforated by the soft contents of the dentine and enamel tubes. Huber considers that the membrana preformativa is a product of the ends of the odontoblasts, which by a coalescence form a continuous sheet, and that this by its calcification becomes the granular layer of Tomes, a view which seems most highly improbable. It must be remembered also that basement membranes have for the most part been found to consist really of flattened cells, and to be very often incomplete and fenestrated. The latest contribution to the subject is in Leon Williams's paper upon enamel. By staining (apparently with Ehrlich-Biondi stain) he has found an appearance of a distinct line of different colour at that end of the enamel cells where they approach the dentine-papilla, and also at their other ends, i.e., between them and the cells of the stratum intermedium. Now a difference in colour with a double stain means a difference of chemical or, perhaps, physical consti- tution or of reaction to a certain extent; but whether this region of difference can be rightly, on that evidence alone, described as a membrane may be doubted, and Leon Williams expresses himself with some caution as to the nature of the stained lines. And Ehrlich-Biondi is a very fickle stain, very slight causes apparently determining a difference. Thus the completely-formed and partly-formed dentine and enamel will often stain two different colours. In former editions of this book Magitot's table giving the date of appearance of the various tooth-germs and the size of their calcified caps was given ; but in this, as well as in the last edition, Eose's table has been adopted, the latter having shown that in all probability the age of some of Magitot's specimens was not correctly ascertained. Eose has followed His in his determinations of the age corresponding to certain lengths of foetus, and some of the principal dates thus ascertained are as follows : — (i.) First appearance of the dental lamina takes place from the thirty- fourth to the fortieth day of intra-uterine life, (ii.) About the tenth week the anterior eight papillae form and become 214 A MANUAL OF DENTAL ANATOMY. THE CEMENT-ORGAN. 215 216 A MANUAL OF DENTAL ANATOMY. imbedded in their respective enamel -organs, the remaining two following in a week or so. The separating of their enamel- germs from the tooth-band begins about the fourteenth week, (iii.) In the fourteenth week the tooth- band extends beyond the lowest milk molar, and about the seventeenth week the papilla of the first permanent molar becomes developed, (iv.) In the sixth month after birth the beginnings of the second molar appear. (v.) At about three and a quarter years the commencement of the third molar is seen. Eeeerences. (*) Andrews, E. Vide Williams, L. J. (2) " Calcification of Teeth." World's Columbian Dental Congress, Chicago, 1894, i., p. 288. (3) Batjme, E. " Odontologische Fortschungen." Theil I., Leipzig, 1882. (4) Harting, P. " On the Artificial Production of Some of the Prin- cipal Organic Calcareous Formations." Quart. Journ. Micr. Sci. Lond., 1872, vol. xx., N.S., pp. 118—123. (5) Legros, C. II., and Magitot, E. " Morphologie du follicule den- taire chez les mammiferes." Journ. de TAna- tomie, Paris, 1879, t. xv., pp. 248 — 293. ■ " Developpement de l'organe dentaire chez les mammiferes." Journ. de l'Anatomie, Paris, 1881, t. xvii., pp. 60 — 95. (7) Mummery, J. H. " Some Points in the Structure and Development of Dentine." Phil. Trans. Eoy. Soc. Lond., 1891, vol. clxxxii. b, pp. 527 — 545. (8) Nickerson, W. S. "The Development of the Scales of Lepi- dosteus." Bull. Mus. Comp. Zool., Harvard, 1893, vol. xxiv., pp. 115 — 138. (9) Eajney, G. " On the Formation of the Skeletons of Animals, and other Hard Structures formed in connection with Living Tissues." Brit, and Foreign Med.- Chirurg. Eeview, London, 1857, vol. xx., pp. 451—476. (10) Tomes, J., and de Morgan, 0. " On the Structure and Develop- ment of Bone." Proc. Eoy. Soc. Lond., 1852, vol. vi., pp. 203—208. (") Tomes, C. S. (a) "A Case of Calcification of a widely exposed Pulp."' Trans. Odontol. Soc. Gt. Brit., 1895—6, vol. xxviii., pp. 183 — 190. (b) " On the Development of Marsupial and other Tubular Enamels, with Notes upon the Develop- ment of Enamels in general." Phil. Trans. Eoy- Soc. Lond., 1897, vol. 189 b, pp. 107—122. THE CEMENT-OEGAN. 217 (c) " Upon the Structure and Development of the Enamel of Elasmobranch Fishes." Phil. Trans. Eoy. Soc. Lond., 1898, vol. 190 b, pp. 443— • 464. (d) ' ' Upon the Development of the Enamel in certain osseous Eish." Phil. Trans. Eoy. Soc. Lond., 1903, vol. 193 B, pp. 1—12. (12) Waldeyek, W. " Structure and Development of the Teeth." Strieker's Histology, London, pp. 463 — 493. i13) Williams, L. J. " On certain Disputed Points in the Develop- ment and Histology of the Teeth." Dental Cosmos. Phil., 1884, vol. xxvi., pp. 641—658. (14) Williams, Leon. "On the Formation and Structure of Dental Enamel." Dental Cosmos. Phil. 1896, vol. xxxviii., pp. 101, 265, 456. (15) Woodhead, Gr. Sims. " On Inflammation in Bone." Trans. Odontol. Soc. Gt. Brit., 1892—3, vol. xxv., pp. 30—53. CHAPTER VII. THE DEVELOPMENT OF THE JAWS — THE ERUPTION OF THE TEETH. At an early period in the development of the embryo there is a single primitive buccal cavity, which is subsequently divided into a nasal and an oral cavity by the palatine plates growing horizontally across it ; the pharynx behind the hinder end of the primitive buccal cavity remains undivided. About the third week in the human embryo the foregut (see p. 3) becomes enlarged in the head region to form the pharynx. The hypoblast which lines it grows transversely outwards on each side at four different levels, these outgrowths being met by corresponding ingrowths of the epiblast from the surface. The bilaminar membranes thus formed at each spot, soon become ruptured. Four clefts are thus formed, of which the first is the largest and the fourth the smallest ; these pass right through, so as to become actual slits. The tissues intervening between them become thickened and strengthened, forming vertical bars between the visceral clefts ; the same thing happens in front of the first and behind the last, so that there are five bars or visceral arches to the four clefts. In man these clefts are of course transitory structures and no true slits are formed, but they remain open in fish and some Amphibia, and are familiar to us as the gill-slits of fish. The intervening bars develop vascular filamentous processes which serve for respiration, forming the gills. The fish takes the aerated water into its mouth and expels it through the slits between its gills or visceral arches. On their inner aspect these arches in fish often carry teeth which in form and structure resemble those in the mouth.* The visceral arches have, however, a further importance to the student of the development of the jaws, for inside the first * For a more detailed account of the pharyngeal teeth, the reader is referred to a series of papers which have recently appeared in the Zoologist by Colonel Shepherd— H. W. M. T. THE DEVELOPMENT OF THE JAWS. 219 visceral arch, that is to say, in front of the first visceral cleft, a cartilaginous bar is developed, which runs round to meet its fellow of the opposite side. This is known as Meckel's cartilage ; it starts from the base of the cranium in the immediate neighbourhood of the future ear, and at this end of it two of the auditory ossicles, the incus and malleus, are ultimately formed.* The most recent work on the development of the maxilla and mandible in man, is that by Prof. Fawcett, of the University of Bristol, who has most kindly written the following resume of his investigations : — " The Human Maxilla is developed as a covering bone to the nasal capsule in close connection with the branches — but not the main trunk — of the superior maxillary nerve which lies at first much above the maxilla. This bone consists of two originally independent elements, the maxilla proper and the premaxilla, either of which may ossify before the other. But in any event incomplete fusion takes place between them at a very early period, so that their original independence is not manifest when viewed from the exterior. The maxilla commences to ossify in the neighbourhood of the canine tooth-germ and rapidly grows upwards and downwards, inwards and backwards. The upward growth produces a great part — the posterior — of the nasal or frontal process. The downward growth produces the outer alveolar walls ; the inward one, the palatine process, and the backward one, both outer alveolar and malar processes. These parts are visible at the 19 mm. stage. The anterior dental nerve runs forwards under the palatine process. At a later stage the palatine process increases greatly in height and at the same time grows backwards, leaving a gap between itself and the outer alveolo-malar process through which the, dental nerves run. Still later the palatine process gives off a dependent ridge on the medial side of the milk molar tooth-germs which forms the inner alveolar wall and about the same time— 37 mm. — metaplastic cartilage forms along the alveolo-molar process, but this rapidly becomes converted into bone. "* The development and morphology of the auditory ossicles is as yet not accurately determined (2, 3). 220 A MANUAL OF DENTAL ANATOMY. Up to the 30 mm. stage the main trunk of the superior maxillary nerve lies some distance above the very small tri- angular orbital surface of the maxilla, but at the 30 mm. stage a groove, the infraorbital groove, soon becomes evident and its walls subsequently close in to form the infraorbital foramen. During all but the very early stages the height of the maxilla is relatively small, and the orbital surface is only separated from the roof of the tooth sockets by a thin plate of bone. With the advent of the antrum of Highmore which is at first an outpouching of the lateral nasal cartilage and long retains its cartilaginous lining, the orbital floor is separated from the alveolar roof, and this separation increases with the age of the bone. At the 100 mm. stage an off-shoot of the lateral nasal capsule which partially encircles the lachrymal duct becomes invaded by ossification and incorporated within the maxilla. The maxilla is thus derived mainly from membrane, but in part from metaplastic cartilage and in part from the primordial cranial capsule ; and by one centre of ossification. The Premaxilla arises by two centres, one the palato-facial, which forms part of the hard palate, appears on the face and contributes to the formation of the frontal process of the complete maxilla. This centre contains the incisor tooth-germs. The other centre to avoid confusion had best be called (as originally by Rambard and Renault) tJie subvomeran. It forms that part of the premaxilla which lies mesial to Stenson's duct, and is only visible from below when the anterior palatine fossa is of large size. It appears normally at the 50 — 55 mm. stage and soon commences to fuse with the palato-facial centre. It may be regarded as a covering bone to the anterior paraseptal cartilage. The premaxilla and maxilla commence to fuse along the outer alveolar wall almost as soon as they are formed ; elsewhere, fusion is delayed as is well known, to a comparatively late period, that between the nasal processes preceding the remainder. The Human Mandible, the second bone in the body to ossify, being preceded only by the clavicle in that respect, is developed by two centres, one for each half, which commencing in ' membrane ' in the immediate neighbourhood of the man- dibular nerve gradually involves certain cartilages, one of THE DEVELOPMENT OF THE JAWS. 221 which is Meckel's cartilage, and the others are cartilages which arise quite independently of Meckel's cartilage, and are not to be considered as parts of the primordial chondro- cranium. Ossification in Membrane. — The mandible arises primarily in the interior of the mandibular arch of the embryo which is perforated from back to front by the mandibular branch of the fifth cranial nerve. This nerve on passing forwards divides into two terminal branches, one which runs forwards towards the middle line — the incisor branch, the other which takes a more lateral course — the mental nerve. On each side of the mandibular nerve and its forward continuation, the incisor nerve, the core of mesoblast occupying the incisor Fig. 112. — Horizontal section of mandibular arch with contained structures, at second stage of development. mandibular arch condenses to form two denser masses, viz. one median to the nerves in question, the forerunner of Meckel's cartilage, the other lateral to the nerves, the fore- runner of the membranous mandible. The mandibular nerve and its incisor continuation come thus to lie in the interval betweeen the more median Meckel anlage, and the more lateral membranous jaw anlage (Fig. 112). Ossification in membrane commences in the membranous anlage in the angle between the incisor and mental branches of the mandibular nerve at the 17 — 18 mm. stage ; it proceeds backwards underneath the mental nerve, which thus comes to lie in a groove on the upper aspect of the bone (Figs. 113, 114). At a little later stage, just before the 19 mm. stage, the anterior wall of the mental groove forms a spur which grows 222 A MANUAL OF DENTAL ANATOMY. backwards over the mental nerve, and will at the 19 mm. stage fuse with the hind wall of the groove and so complete the mental foramen (Fig. 115). During the same period the anterior part of the floor of the mental groove proliferates in an inward direction, insinuates itself between the incisor nerve and Meckel's cartilage, gradually overlapping the latter, by so doing forms the first commencement of the internal alveolar wall, and at a little later date the outer and inner alveolar walls become connected with one another by a bony bridge which arching over the incisor nerve forms the incisor canal, the first canal con- taining nerve to be formed in the mandible (the inferior dental canal containing the inferior dental or mandibular nerve not being formed till the late 60 mm. stage). By the 30 mm- stage there is much advance in growth of the membrane bone, especially in the backward direction, and the first sign of a coronoid process is now visible. The condyle region is still in a " membranous " or connec- tive tissue condition. In the region near the incisor nerve more active growth is taking place, for two inwardly directed shelves have now made their appearance, one which grows from the inner alveolar wall above Meckel's cartilage, the other from the main mass of bone below the inner alveolar wall which grows inwards below Meckel's cartilage. In course of' time these inwardly directed shelves will meet on the medial side of Meckel's cartilage, and enclose it from near the middle line to the region of the second milk molar tooth in the backward direction. incisor nerve Meckel's cartilage . Figs. 113, 114. — Side views of developing mandible. THE DEVELOPMENT OF THE JAWS. 223 In transverse (Coronal) section the appearance is as shown in the accompanying diagram (Fig. 115). At the 42 mm. m stage, Meckel's cartilage becomes invaded by ossification, usually opposite the first and second incisor tooth buds. Prior to doing so the perichondrium becomes Fig. 115. markedly thinner opposite these tooth buds, and osteoblasts seem to invade the cartilage opposite the tooth buds from the neighbouring membrane bone. The final result is that Meckel's cartilage — int. alveolar wall^ -mandib.nerve bone --/-outer alveolar wall Fig. 116. Meckel's cartilage undergoes ossification and incorporation in the mandible from near the symphysis to the second milk molar tooth-germ, but not by a separate centre of ossification. inner alveolar walh-~ bony shelve s.< incisor nerve of'^-outer alveolar wall ^--mental nerve Meckel's cart main sten 117. Ossification in other Cartilages.— At a comparatively late period cartilage becomes developed in the unossified con- 224 A MANUAL OF DENTAL ANATOMY. nective tissue from which the membranous jaw is formed, and at the following sites : At the 50 — 55 mm. stage, a mass of cartilage appears in the region of the condyle (Fig. 118), this mass is shaped like a carrot, the large end forming the condyle, the small end tapering away under the root of the coronoid process. This cartilage is capped at its free end by dense connective tissue which does not chondrify. It is not like ordinary hyaline cartilage, containing very little matrix, and it com- mences to ossify almost as soon as formed, but it is not completely ossified till quite late, traces being found at birth even. As in the case of Meckel's cartilage, there is no independent ossification, it is simply a case of invasion from the neighbouring membrane bone, and it takes place more rapidly on the inner than the outer side. At the 80 mm. stage a strip of cartilage of the same nature as the last appears along the anterior border of the coronoid process (Fig. 119). This becomes invaded by ossification from the neighbouring membrane bone. At the 100 mm. stage, a mass of cartilage appears along the top of both the inner and outer alveolar walls (Fig. 120) : this is continued downwards along the symphysial margin of the mandible and outwards towards the future digastric impression. It, like the others, is normally ossified by invasion from the neighbouring membrane bone. All these cartilages are usually classified as metaplastic cartilages, and have no connection whatsoever with Meckel's cartilage, nor are they normally ossified independently. It is possible however that two of them rarely may be, viz., the Coronoid and the Symphysial. Prof. Fawcett has a specimen of monkey's jaw (species unknown) in which there is an independent bony element in the tip and anterior border of the coronoid process, which —f- membrane bone ' 50S5m.m. fc$\--cart. Fig. 118. THE DEVELOPMENT OF THE JAWS. 225 corresponds very well with the coronoid strip of cartilage of man, and Kerckringius has described such a mass in man. The writer has never seen it. It is possible that the ossa mentalia which are so often found in the symphysial region are the result of independent ossification of the symphysial cartilage. Meckel's Cartilage. — This in early foetal life is a con- FlG. 119. densation of the mesoblast of the mandibular arch, placed on the medial side of the mandibular (inferior dental and incisor) nerve. Soon it segments off the incus, and developing tumefactions on its posterior end forms the malleus anlage, the tumefaction producing the head and handle (Fig. 121). The cartilage runs obliquely from behind forward in the mandibular arch, and ends at first some distance from the middle line in a blunt-pointed extremity which has not Fig. 120. chondrified when ossification commences in the membranous tissue lateral to it. At a later stage, 24 mm., the anterior extremity of the cartilage begins to swell out so that when it is cut coronally it has a wedge shape. The base of the wedge is upwards, and the inner basal angle forms a growing point, which, lying in the loose connective tissue in the symphysial tissue, bends backwards and comes into close contact with its fellow, in some cases even fusing with it as it' does in many lower animals, e.g., marsupials. Occasionally this fused point may be segmented off to form an azygos cartilage in the upper part of the symphysial connec- d.a. 15 226 A MANUAL OF DENTAL ANATOMY. tive tissue, and it is possible that the unpaired os mentale which sometimes exists may be derived from independent ossification of this cartilage. Except at the "growing point" just alluded to, Meckel's cartilage undergoes ossification from the middle line back- wards to the region of the second milk molar tooth, and is incorporated with the mandible within the same limits. Behind the tooth -germ in question the cartilage ultimately disappears, but does not become the long internal lateral ligament. That is formed from connective tissue quite independently of the cartilage. During its growth Meckel's cartilage tends to increase in length more rapidly than the Fig. 121. membranous jaw ; it consequently tends to buckle up behind the region of the second milk molar tooth, and in one case it has been found so bent as to be cut twice in the same coronal section. The perichondrium surrounding Meckel's cartilage is very thick save where it is in close contact with the membrane bone. In cleft palate and hare-lip the cleft occurs between the endo- and mesognathion ; a third dwarfed incisor is sometimes found on the inner side of the cleft, and suggests that the incisor missing from the typical mammalian dentition is I2. With the later history of the other visceral arches we have not much concern. It will suffice to note that the second cartilaginous bar, called the hyoid bar, arises very close to Meckel's cartilage, and, indeed, has been considered by some anatomists to possibly participate in the formation of the malleus and incus. The more usual view is, however, that it in great part dis- appears, but that it is in part converted into the styloid process, stylo-hyoid ligament, and lesser cornu of the hyoid bone. The bar of the third arch, called the thyro-hyoid, in THE DEVELOPMENT OF THE JAWS. 227 great part also disappears, but its lower end goes to form the greater cornu of the hyoid bone." The student who wishes to pursue this subject further is referred to the very excellent and clear account to be found in the latest edition of " Quain's Anatomy." The later changes which are undergone by the jaws during the development, eruption, and loss of the teeth, have long engaged the attention of anatomists, and amongst others of Hunter, who was the first to arrive at a tolerably correct appreciation of the process. In the first edition of Sir J. Tomes' " Dental Surgery," the results of a very extensive series of observations carried out upon maxillae collected by himself were detailed, confirming in the main Hunter's con- clusions, but adding many new points to our knowledge ; and from this work the author has borrowed largely in the present chapter. The late Sir G. Humphrey, who had overlooked these descriptions, which were never published in any other form than as an introduction to the "Dental Surgery," instituted a series of experiments upon growing animals, which tended towards the same conclusions. As a means of giving the student a guide in his reading of the following pages, and a clue to the results towards which he is being led, a preliminary statement, which does not pretend to scientific accuracy, may perhaps be useful ; while the description given will relate for the most part to the lower jaw, because its isolated position, bringing it into relation with fewer other bones, renders it more easy to study ; not that any difference of principle underlies the growth of the upper jaw. The different parts of the lower jaw answer for different purposes ; one division of its body having a very close and exclusive relation with the teeth, the other serving distinctly other purposes, as well as forming a base of support for the dental apparatus. The alveolar portion of the jaw, that which lies above the level of the inferior dental canal, is developed around the milk teeth : when they are lost, it disappears, to be re-formed again for the second set of teeth, and is finally wholly removed after the loss of the teeth in old age. The portion of jaw below this line, which is essential to deglutition and respiration, is late in acquiring any consider- able development. Once formed, it is never removed, save 15—2 228 A MANUAL OF DENTAL ANATOMY. that when in advanced old age the muscles of mastication are no longer in full use it becomes, to a slight extent only, wasted. In order to understand the drift of the following description, it is essential to keep in view the different life histories of those two parts of the jaw just alluded to. In an early foetus, long before the necessity for respiratory movement or deglutition has become imminent, a thin lamina of bone has begun to be developed beneath the tooth-germs, forming, as it were, a semicircular gutter running round the jaw, in which the developing tooth-sacs are lodged. The thin gutter of bone is thus formed above and outside Meckel's cartilage, and intervenes between the rudimentary inferior maxillary vessels and nerves, and the teeth. The sides of the bony furrow rise as high as the top of the tooth-germs, but they do not arch over and cover them in in such manner as the permanent tooth-germs are arched in, for the long furrow is widely open at the top. At the time of birth, the two halves of the mandibles are as yet not anchylosed, but are united only by fibrous tissue. "The alveolar margins are deeply indented with large open crypts, more or less perfectly formed. The depth of these bony cells is only sufficient to contain the developing teeth and tooth-pulps, the former rising to the level of the alveolar margins of the jaws. At this period the crypts or alveoli are not arranged in a perfectly uniform line, nor are they all equally complete. The septa, which divide into a series of cells that which at an earlier age was but a continuous groove, are less perfect at the back than at the front of the mouth. The alveoli of the first incisors of the upper and the lower jaws are a little larger within than at the orifice, and this difference is made still greater by a depression upon the lingual wall of each for the reception of the germ of the corresponding per- manent tooth. They are divided from the crypts of the lateral incisors by a septum which runs obliquely backwards and inwards towards the median line. The sockets for the second incisors occupy a position slightly posterior to those for the other incisors, and are divided from the canine alveoli by a septum which proceeds obliquely backwards, and in the lower jaw, as regards the median line of the mouth, outwards. By the arrangement of these divisions the alveoli of the first THE DEVELOPMENT OF THE JAWS. 229 incisors are rendered broader in front than behind, and these relative dimensions in the sockets of the second incisors are reversed, as shown in Fig. 122. The crypts of the canine teeth are placed a little anteriorly to those of the second and FlG. 122. — Upper and lower jaws of a nine months foetus, the teeth having been removed from the jaws on one side to show the extent to which they are calcified at this period. ( Two-thirds life size.) a. Alveoli of second incisors, b. Alveoli of canines, c. Alveoli of second temporary and first permanent molars. A bristle has been passed through the inferior dental canal. nearly in a line with those of the first incisors, giving to the jaws a somewhat flattened anterior aspect. While the main bulk of the lower jaw is thus made up by the alveoli of the teeth, in the upper jaw the alveoli descend but little below the level of the palatal plates, though the sockets are tolerably deep. The antrum as a special and 230 A MANUAL OF DENTAL ANATOMY. distinct cavity cannot be said to exist, being merely repre- sented by a depression upon the wall of the nasal cavity, the alveolar cavities therefore being separated only by a thin plate of bone from the orbits. The figure represents also the extent to which calcification has advanced in the various teeth. A full half of the length of the crowns of the first incisors, about half that of the second, and the tips only of the canines are calcified ; the first deciduous premolars are complete as to their masticating surfaces; the second have their cusps more or less irregularly united, in many specimens the four cusps being united into a ring of dentine, the dentine in the central depression of the crown being as yet unformed. During the formation of the permanent teeth, very similar relations exist between the amount of calcification in the incisors and canines ; thus when, as sometimes happens, the development of the teeth proceeds very imperfectly up to a certain date, and then changes for the better, it may be that the lower half of the crown of the first incisor, somewhat less of the second, and the extreme tip of the canine will be honeycombed, while the remainder of the tooth will be perfect, thus perpetuating an evidence of the stages to which each of these teeth had at that particular period attained. Having noted in some detail the characters of the jaws of a nine months foetus, we may pass on to the consideration of those changes which precede the cutting of the deciduous teeth. A general increase in size takes place, new bone being developed at all those points where the maxillae are connected by soft tissue with other bones, as well as from their own periosteum. But the increase in dimensions does not take place in all directions equally, so that material changes of form result. In correspondence with the elongation of the tooth-sacs, the alveoli become increased in depth, and their edges circle inwards over the tooth-sacs ; active development of bone takes place in the sutures uniting the two halves of the jaws to one another, which is compensated by the inclination inwards towards the middle line of the alveoli of the first incisors. In the lower jaw the articular process, at first hardly raised above the level of the alveolar border, grows rapidly up? the direction of the ramus at first remaining 232 A MANUAL OF DENTAL ANATOMY. estimated, and may as well at once be mentioned. As the jaw undergoes increase in size, large additions are made to its surface by deposition of bone from the periosteum, neces- sarily lengthening the canal. The additions to the canal do not, however, take place quite in the line of its original course, but in this added portion it is bent a little outwards and upwards. If we rasp off the bone of an adult jaw down to the level of this bend, a process which nature in great part performs for us in an aged jaw, or if instead we make due allowance for the alteration, the mental foramen becomes an available fixed point for measurement. The mental foramen, which undergoes most of its total change of position within a few months after birth, comes to correspond with the centre of the socket of the first deciduous premolar ; later on it corresponds with the root of the first premolar, which is thus shown to succeed, in exact vertical position, the first deciduous premolar. On the inner surface of the jaw the tubercles for the attachment of the genio-hyo-glossus and genio-hyoid muscles are in the foetus opposite to, and very little below the base of the alveoli of the first incisors, a position which they afterwards hold with regard to the permanent incisors. The upper of the two pairs of processes are about at the same general level as the mental foramen. The general result arrived at by measurements taken from these fixed points is that the alveolar arch occupied by the permanent teeth which have had deciduous predecessors, namely the incisors, canines, and premolars, corresponds very closely with the whole alveolar arch of the child in whom the temporary dentition is complete; and that the differences which do exist are referable, not to any fundamental alteration in form or to interstitial growth, but to mere addition to its exterior surface ; or, more briefly, that the front twenty of the permanent succeed vertically to the deciduous teeth, the increase in the size of the jaw in an adult being due to additions at the back in the situation of the true molars, and to the surface. Thus, if measurements be taken across between the inner plates of the alveoli on either side at the points where they are joined by the septa between the first and second deciduous premolars, and at about the level of the genio-hyo-glossus THE DEVELOPMENT OF THE JAWS. 283 tubercles, it will be found that the difference is slight, if any, notwithstanding that in other dimensions there is a very great difference between the jaws of a nine months foetus and of a nine months child. Again, if imaginary lines be stretched across between these two points, and from their centre lines be drawn forwards to the spince mentales in the same two jaws, these will be found to differ but little in length in the two specimens. But if, instead of measuring to the spiiue mentales, the Fig. 123. — Diagram representing a jaw of a nine months foetus, superimposed upon an adult jaw, to show in what directions increase has taken place. line had been carried further to the anterior alveolar plate, a great difference would have been observable ; in point of fact, contemporaneously with the development of the crypts of the permanent teeth inside them, the deciduous teeth and their outer alveolar plates are slowly pushed outwards, a process the results of which we see in the separation which comes about between each one of the deciduous teeth prior to their being shed, when the process of dentition is being carried on in a perfectly normal manner. Measurements taken for the sake of comparing adult jaws with those of an eight months child give closely similar 234 A MANUAL OF DENTAL ANATOMY. results, which the author has endeavoured to roughly embody in the accompanying figures. In these it is shown that the increase in the dimensions of the jaw has taken place in two directions : by prolongation backwards of its cornua concomitantly with the addition at the back of the series of teeth of the true molars, which follow one another at considerable intervals of time ; and by additions to its exterior surface by which it is thickened and strengthened. The study of the growth of the jaw in vertical depth is also very instructive. We find that, as has already been mentioned, the history of that part of the jaw which lies below the inferior dental canal is very different from that which lies above. From the time of birth to that at which the deciduous teeth Fig. 124.— Lower jaw of child. The horizontal line marks the level of the inferior dental canal. begin to be cut, the jaw below that line has been making steady but slow progress in vertical depth ; the alveoli, above that line, have been far more active but far more intermittent in their development. Again, passing from the nine months foetus to the seven years old child, in whom the deciduous dentition is complete, the framework of the jaw below the imaginary line has attained to a depth almost equal to that which it is seen to have in an adult ; in the adult again it corresponds pretty well with that in an aged jaw. The alveolar portion, however, is far deeper in the adult than in the child (this difference is not sufficiently well marked in the figure), and in fact constitutes almost the whole increase in vertical dimensions during the passage from fhe child's to the adult's form of the jaw. In the lower jaw we may then take it as proven that the basal THE DEVELOPMENT OF THE JAWS. 285 portion has little relation to the development of the teeth, but that the alveolar, or upper portion, is in entire and absolute dependence upon them, a point to which reference will again be made in speaking of the eruption of the teeth. Fig. 125. — Lower jaw of an adult. It remains to speak in some further detail of the precise means by which the enlargement of the jaw is effected. To a slight extent there is formation of bone going on at the symphysis prior to complete anchylosis taking place. The Fig. 126. — Lower jaw of an aged person, the dotted lines indicating the out- line of the parts removed by absorption, as the jaw assumes the form characteristic of advanced age. share taken by this in increasing the size of the jaw would, however, appear to be but small after the termination of the intra-uterine period. Additions to the surface at the edges of the alveoli and at the base of the jaw are continually going on, and bring about that addition to the exterior already noticed. 236 A MANUAL OF DENTAL ANATOMY. 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 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, again removed 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 premolar, 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 realise that the articular surface with its cartilage has successively occupied every spot along this line ; and as it progresses backwards by the deposition of fresh bone in its cartilage, it has been followed up by the process of absorption removing all that was redundant. On the outer surface of the jaw we can frequently discern a slight ridge, extending a short distance from the head of the bone ; but if the prominence were preserved on the inner surface, the inferior dental artery and nerve would be turned out of their course. We have thus a speedy removal of the newly-formed bone, so that a concavity lies immediately on the inner side of the condyle ; and microscopic examination of the bone at this point shows that the lacunae of Howship, those characteristic evidences of absorption, abundantly cover its surface, showing that here at least absorption is most actively going on. In the same way the coronoid process, beneath the base of which the first, second, and third molars have successively been formed, has moved backwards by absorption cutting away its anterior, and by deposition adding to its posterior surfaces. The periosteum covering the back of the jaw is also active in forming the angle and the parts thereabouts. It is worth while to add that the direction of growth in young jaws is marked by a series of minute ridges. In like THE DEVELOPMENT OF THE JAWS. 237 manner the characteristic marks of absorption are to be found about the neck of the condyle, and the front of the coronoid process, and those of active addition about the posterior border, so that the above statements rest upon a basis of observation, and are not merely theoretical. Two cases of arrested develop- ment of the jaw (4) lend a species of experimental proof to the theory of the formation and growth of the jaw above given. There are authors, however, who maintain that the growth of the jaws is not merely a backward elongation of the cornua, together with additions to the external surface, but that an " interstitial growth " takes place. Wedl inclines to this latter view, and the question cannot, in the opinion of the author, be held to be absolutely settled. Although it is difficult to form any definite conception of interstitial growth in a tissue so dense and unyielding as bone, the doctrines promulgated in the foregoing pages have the support of a priori probability, there are some rather para- doxical facts to be met with in comparative odontology. Never- theless, there can be no doubt that backward elongation, as teeth are successively added, is sufficiently near the truth in the case of human and most mammalian jaws for practical purposes. In the museum of the Royal College of Surgeons there is a series of models of the same mouth from the age of four up to the age of twenty-one which the author carefully examined (5), with the result of fully confirming the conclusions arrived at in the foregoing pages, and he further instituted a comparison with the jaws of certain of the anthropoid apes. In these latter the same conclusions as to the absence of growth in the region occupied by the deciduous teeth do not hold good, as during the change from the deciduous to the permanent dentitions there is a large antero-posterior elongation. But this elongation takes place almost entirely in the extent of the intermaxillary bones and is much greater in the orang and the gorilla than in the chimpanzee ; and this difference appears to correspond with, amongst other things, the date of the closure of the intermaxillary suture, which takes place much earlier in the chimpanzee than in the other two genera of anthropoid apes, and seems to bear a proportion to the extent of development of the canines. 238 A MANUAL OF DENTAL ANATOMY. Thus a large part of the prognathism of these apes is due to long-continued growth in the intermaxillge, and it would be very interesting to ascertain if the prognathism of the lower races of mankind is also due to the obliteration of the suture being deferred longer than in the European races ; but unfor- tunately, the material for this investigation does not appear to exist in this country. It remains to notice the changes in form which the ascending ramus and the angle of the jaw undergo. In the fcetus the ramus is but little out of the line of the body of the jaw, and the condyle little raised above the alveolar border. Gradually the line of development, as is indicated even in the adult jaw by the course of the inferior dental canal, takes a more upward direction ; copious additions of bone are made on the posterior border and about the angle, so that in an adult the ramus ascends nearly at right angles to the body of the jaw. In old age, concomitantly with the diminution of muscular energy, the bone about the angle wastes, so that once more the ramus appears to meet the body at an obtuse angle. But all the changes which mark an aged jaw are the simple results of a superficial and not of an interstitial absorption, corresponding with a wasting of the muscles, of the pterygoid plates of the sphenoid bone, etc. Eruption of the Teeth. — The mechanism by which teeth, at the date of eruption, are pushed upwards into place is far from being perfectly understood. The simplest theory would appear to be that they rise up in consequence of the addition of dentine to their base ; in fact, that their eruption is due to the elongation of their roots. Various very strong objections have been brought forward, clearly proving that this cause is quite inadequate to explain all that may be observed. In the first place, teeth with very stunted roots— which may be practically said to have no roots — are often erupted. Again, a tooth may have the whole length of its roots completed, and yet remain buried in the jaw through half a lifetime, and then be erupted. Moreover, when a healthy normal tooth is being erupted, the distance travelled by its crown often greatly exceeds the amount of addition to its length which has gone on during the same period. To tarn to comparative anatomy, the tooth of a crocodile THE DEVELOPMENT OF THE JAWS. 239 moves upwards, tooth-pulp and all, obviously impelled by something different from mere elongation. The author's researches upon the development and succession of reptilian teeth clearly show that a force quite independent of increase in their length alters the position and "erupts" successive teeth. It has been suggested by Constant Q) that the blood pressure may be the impelling force, the pulp and tissues beneath the tooth being more rich in their vascular supply than those above it. It seems very possible that the blood pressure keeping up a state of general tension may operate to push a solid body in any direction in which there is a diminished resistance, to take up, so to speak, any unoccupied place ; but it is difficult to see how it could be efficient without some such concomitant action of absorption. For the movement of an erupting tooth is not always in the direction of its long axis ; for instance, the developing tooth of the frog, the newt (Figs. 79 and 156), or of the crocodile takes an oblique course, by which it travels underneath the old tooth, before moving upwards into position. But if its path be prepared by absorption of the structures in its way, then it is very possible that the blood pressure, keeping up a certain general tension, may tend to move it along a track of diminishing resistance. Some teeth of persistent growth, e.g., the lower incisors of rodents, show a movement which cannot be accounted for in this manner. For, as is indicated by the nerve going to the open base of the pulp being bent back on itself, the base of the tooth has travelled backwards for a considerable distance, although its apex is constantly moving forwards. Underwood suggests that the cause may be the same as that which moves a shark's tooth, which apparently is the movement of the soft parts in which it is imbedded. This would seem tantamount to attributing its movements to changes in the alveolo-dental periosteum, and the direction of the fibres in an erupting crocodile tooth would lend some support to this idea, which, however, by no means explains everything. In man, towards the eighth month of childhood the bony crypts which contain the temporary teeth in the front of the mouth begin to be removed. At the back of the mouth the 240 A MANUAL OF DENTAL ANATOMY. crypts still retain their inverted edges ; indeed, further develop- ment of the crypts is still going on in this part of the mouth. When a tooth is about to be cut, very active absorption of its bony surroundings goes on, particularly on the anterior surface, the bone behind it being still required to form part of the crypt of the developing successional tooth. But no sooner has the crown passed up through the very wide and free orifice so formed than absorption gives place to deposition, Fig. 127. — Jaws of a male nine months old, in which the eruption of the teeth is just commencing. and the bone rapidly develops so as to loosely embrace the neck of the tooth. Additions to the margin of the alveoli keep pace with the gradual elongation of the roots of the teeth ; as this is a moderately rapid process, the alveolar portion of the jaw increases in depth almost abruptly. But it does not do so uniformly all over the mouth ; if it did the teeth could only be closed at the back of the mouth, unless the rami elongated by an equally sudden accession of new bone. The front teeth are erupted first, and the jaw deepens first in front ; later on the back teeth come up, and the jaw is deepened posteriorly ; meanwhile the elongation of the rami THE DEVELOPMENT OF THE JAWS. 241 has been going on slowly, but without interruption. Thus is brought about a condition of parts allowing the whole series of teeth to come into their proper mutual antagonism. It was pointed out by Trousseau that the eruption of the teeth is not a continuous process which, once commenced, is carried on without intermission to its completion, but that it is interrupted by periods of repose. The teeth are, according to his statement, cut in groups ; the eruption of the teeth of each group being rapid, and being succeeded by a complete cessation of the process. Individual variations are numerous ; but the following may be taken as an approximation to the truth : — The lower first incisors are erupted at an age ranging from six to nine months ; their eruption is rapid, and is completed in ten days or thereabouts ; then follows a rest of two or three months. Next come the four upper incisors ; a rest of a few months ; the lower second incisors and the four first molars ; then a rest of four or five months. The canines are peculiar in being the only teeth of the temporary set which come down between teeth already in position. To this, as well as to the greater length of their root (although it is not quite clear what this has to do with it), Trousseau ascribes the great length of time which their eruption occupies, it taking two or three months for its com- pletion. According to him, children suffer more severely from constitutional disturbance during the cutting of these teeth than of any other, but the late Dr. West thought that the eruption of the first molars causes the most suffering. It may also be noted that the canines during their development lie farther from the alveolar border than do the other teeth, so that they travel a greater distance ; obviously not merely from the elongation of the root, which is wholly inadequate to effect such a change in position. The dates of the eruption of the milk teeth vary much, no two authors giving them alike ; but the whole of the deciduous teeth are usually cut by the completion of the second year. Cases in which incisors have been erupted before birth are not very uncommon. At a time when the crowns of all the deciduous teeth have been fully erupted, their roots are still incomplete, and are widely open at their bases, so that it is d.a. 16 242 A MANUAL OF DENTAL ANATOMY. not until between the fourth and sixth years that the temporary set of teeth can be called absolutely complete. At the sixth year, preparatory to the appearance of any of the permanent teeth, the temporary teeth may be observed to be slightly separated from each other ; they have come to occupy a more anterior position, pushed forward, it may be, by the great increase in size of the crypt of the permanent teeth behind them. The general relation of these to the temporary teeth may be gathered from the accompanying figure, in which it will be noticed that the canines lie far above and altogether out of the line of the other teeth, and that a Via. 128. — Normal well-formed jaws, from which the alveolar plate has been in great part removed, so as to expose the developing permanent teeth in their crypts in the jaw. slight degree of overlapping of the edges of the permanent central and lateral incisors exists. The premolars lie in bony cells which are embraced pretty closely by the roots of the temporary molars, and it hence happens that extraction of these latter sometimes brings them away in their entirety. The first permanent molars are erupted in a manner closely similar to that described as occurring with the temporary teeth ; that is to say, their bony crypts become widely opened out by absorption, the crown passes out, and the new bone is rapidly formed which loosely embraces the neck, prior to any considerable length of root being formed. Last, then, follows the absorption of the root of the tern- THE DEVELOPMENT OF THE JAWS. 243 porary teeth, a matter first accurately investigated by Sir John Tomes. The root, at or near to its end, becomes excavated by shallow cup-shaped depressions ; these deepen, coalesce, and thus gradually the whole is removed. Although absorption usually commences on that side of the root which is nearest to the successional tooth, it by no means invariably does so ; it may be, and often is, attacked on the opposite side, and in many places at once. FlG. 129. — Jaws of a six-year old child. In the upper jaw complete sockets are seen where the temporary incisors have been shed. The cement um is usually attacked first, but eventually dentine and even enamel come to be scooped out and removed by an extension of the process. That part of the dentine, however, which immediately surrounds the pulp appears to have more power of resistance than any other part of the tooth, and thus often persists for a time as a sort of hollow column. The absorption of the temporary teeth is absolutely 16 — 2 244 A MANUAL OF DENTAL ANATOMY. independent of pressure. The varying position of the excava- tion has already been noticed, and it may be added that in many lower animals, for example, the frog or the crocodile, the growing tooth-sac passes bodily into the excavation made before it in the base of the preceding tooth, while if pressure had had any share in the matter the cells of its enamel-organs, etc., must have inevitably been crushed and destroyed. Again, when the absorption and shedding of the first teeth have taken place early, before their successors are ready to appear, perfect little sockets are formed behind the lost temporary teeth (Fig. 129), cutting them off from the per- manent teeth destined to follow them. Absorption, too, may attack the roots of permanent teeth, which is another reason for regarding the process as not necessarily dependent upon the approach of a displacing tooth. Closely applied to the excavation produced by absorption is a mass of very vascular soft tissue, the so-called " absorbent-organ." The surface of this is composed of very large, peculiar-looking cells, bearing some little resemblance to those known as " myeloid cells," or the " giant cells " of recent authors. Microscopic examination of the excavated surface shows it to be covered with small hemi- spherical indentations, the " lacunae of Howship," into each of which one of the giant cells was fitted, and in which they may sometimes be seen in situ. In what manner these giant cells, or " osteoclasts," effect their work is not known, but their presence where absorption of hard tissues is going on is universal. Some suppose that they put forth amcebiform processes, others that they secrete an acid fluid, but nothing very definite is known. A curious parallel is afforded by the manner by which a fungus can drill and tunnel through and through the dentine, as may be very constantly observed in teeth long buried. The process of absorption once commenced does not neces- sarily proceed without intermission, but may give place for a time to actual deposition of osseous tissue on the very surface eroded ; probably by the agency of the absorbent cells them- selves, which are capable of being calcified in the excavations they have individually made. These alternations of absorption and deposition, so common a result of inflammations of the pulp or of the alveolo-dental periosteum as to be diagnostic of the former occurrence of THE DEVELOPMENT OF THE JAWS. 245 these maladies, often occur during the normal process of the removal of the deciduous teeth, and result in the deposition of a tissue not unlike cementum in excavations made in the dentine, or even in the enamel. The eruption of the permanent teeth is a process closely analogous to that of the temporary set. Kapid absorption of the bone, especially on the exterior surface of the crypts, takes place, and an orifice very much larger than the crown of the tooth is quickly opened out. Hence it is that the slightest force will suffice to determine the direction assumed by the rising crown : a fragment of a root of a temporary tooth, the action of the lips and tongue, etc., are all potent agencies in modifying the arrangement of the teeth. The temporary teeth stand vertically, the permanent teeth in front of the mouth obliquely, thus opening a space between the first incisors and the first premolar for the canine, which during development was above or below the line altogether. And, inasmuch as the crowns of the teeth are on the whole much larger than their necks, it would be manifestly impossible for them all to come down simultaneously. The permanent teeth usually make their appearance in the following order : — First permanent molars, about the Seventh year ; a little later, the lower first incisors, upper first and second incisors, the first premolars, the canines, the second premolars, the second permanent molars the third permanent molars. The period of eruption is variable. From a comparison of several tables, the principal discrepancies are seen to relate to the date of the appearance of the canines and the second premolars. The canine would certainly appear to belong to the eleventh and twelfth years ; but some authors consider that the second premolar is usually cut earlier, others later than this date. We may now revert to the phenomena observed in the alveolar processes. They were first built up as crypts with overhanging edges enclosing the temporary teeth ; then they were swept away, in great part, to allow of the eruption of the temporary teeth ; and next they were rebuilt about their necks to form their sockets. 246 A MANUAL OF DENTAL ANATOMY. Once more, at the fall of the deciduous teeth, the alveoli are swept away, the crypts of the permanent teeth are widely opened, and the permanent come down through the gaping orifices. When they have done so, the bone is re-formed so as to closely embrace their necks, and this at a period when but little of the root has been completed. Take for example the first upper or lower molars : their short and widely-open roots occupy the whole depth of the sockets, and reach respectively nearly to the floor of the Fig. 130. — From a child aged fourteen. The specimen well exemplifies the fact that the height of the alveolar edge corresponds exactly to the posi- tion of the neck of each tooth, on which it is wholly dependent. A temporary tooth (the first right lower temporary molar) has been elevated, so that it has attained to the level of the surrounding permanent teeth, and the edge of the socket follows the level of the neck of the tooth. antrum and inferior dental canal. No growth, therefore can possibly take place in these directions ; the utmost available depth has already been reached, and as the roots lengthen the sockets must be deepened by additions to their free edges. It is impossible to insist too strongly upon this fact, that the sockets grow up with and are moulded around the teeth as the latter elongate. Teeth do not come down and take possession of sockets more or less ready made and pre-existent, but the socket is subservient to the position of the tooth ; wherever the tooth may chance to get there its socket will be built up round it. THE DEVELOPMENT OF THE JAWS. 247 Upon the proper appreciation of this fact depends our whole understanding of the mechanism of teething ; the position of the teeth determines that of the sockets, and the form of the pre- existent alveolar bone has little or nothing to do with the disposition of the teeth. Hence during the period of eruption of the permanent teeth the level of the alveolar margin is seen, in a dried skull, to be extremely irregular, the edges of the sockets corresponding to the necks of the teeth, whether they have attained to their ultimate level, or have been but just cut. And when temporary teeth have been retained for a longer period than is natural, they sometimes become elevated to the general level of the permanent teeth (which is considerably higher than that of the temporary teeth), so that they take their share of work in mastication. When this is the case the alveoli are developed round them, and come to occupy with their tooth a higher level than before. Enough has perhaps been said to illustrate the entire dependence of the alveoli upon the teeth, a relation of which dentists every day avail themselves in the treatment of regula- tion cases ; it remains to say a few words as to the forces which determine the position of the teeth. Inasmuch as when a tooth leaves its bony crypt, the bone does not at first closely embrace it, but its socket is much too large for it, a very small force is sufficient to deflect it. And, indeed, a very slight force, constantly operating, is sufficient to materially alter the position of the tooth, even when it has attained to its full length. Along the outside of the alveolar arch the muscular lips are exercising a very symmetrical and even pressure upon the crowns of the teeth ; so also the tongue, with equal symmetry, is pushing them outwards : between the two forces, the lips and the tongue, the teeth naturally become moulded into a symmetrical arch. When the crowns of the teeth have attained such a level as to come in contact with their opposing teeth, they very speedily, from readily intelligible mechanical causes are forced into a position of perfect correspondence and antagonism ; and even at a somewhat later period than that of eruption, if this antagonism be interfered with, the teeth will often rise up so as to readjust themselves in position, 248 A MANUAL OF DENTAL ANATOMY. Kefepences. t1) Constant, T. E. ''The Eruption of the Teeth." 3rd Oongres Dentaire Internat. Paris, 1900. Vol. ii., pp. 180—192. (2) Gaupp, E. "Die Eeichatsche Theorie." Archiv. Mr Anatomie nnd Entwickelungs. Suppl. Bd., 1913. Leipzig. (3) Jenkinson, J. W. "The Development of the Ear-bones in the Mouse.'' Journ. Anat. and Phys., July, 1911. Vol. xlv. (Third Ser. Vol. vi.), pp. 305—318. (4) Tomes, Sir J. " Dental Surgery." 1859. London. (5) Tomes, 0. S. " Studies upon the Growth of the Jaws." Trans. Odontol. Soc. Gt. Brit., 1891-2. Vol. xxiv., pp. 143—158. CHAPTER VIII. THE ATTACHMENT OF TEETH. Although the various methods by which teeth are fixed in their position upon the bones which carry them pass by grada- tional forms into one another, so that a simple and at the same time absolutely correct classification is impossible, yet for the purposes of description four principal methods may be enumerated, namely, attachment by means of fibrous membrane, by a hinge, by anchylosis, and by implantation in bony sockets. Attachment by means of Fibrous Membrane.— An excellent illustration of this method of attachment is afforded by the sharks and rays, in which the teeth have no direct connection with the cartilaginous, more or less calcified jaws, but are embedded solely in the tough fibrous membrane which covers them. This, carrying with it the teeth, makes a sort of sliding progress over the curved surface of the jaw, so that the teeth once situated at the inner and lower border of the jaw, where fresh ones are constantly being developed, rotate over it, and come to occupy the topmost position (cf. descrip- tion of the dentition of the sharks). That the whole fibrous gum, with its attached teeth, does really so slide over the sur- face of the jaw was accidentally demonstrated by the result of an injury which had been inflicted upon the jaws of a shark. The fibrous bands by which each individual tooth of the shark is bound down are merely portions of that same sheet of sub-mucous membrane which furnished the dentine-papillae. The gradual assumption of a tough fibrillated structure by that portion of the membrane which is contiguous to the base of the dentine-papilla may be traced (no such fibrous tissue being found at the base of young papillae), very dense bands being attached to the bases of the completely calcified teeth. A large number of fish have their teeth attached to short pedestals of bone by means of a sort of annular ligament, which allows of a slight degree of mobility. This would seem to be the case in many of those whose teeth have been 250 A MANUAL OF DENTAL ANATOMY. generally supposed to be anchylosed, such as the Sargus (Fig. 106) and perhaps the Eel (cf. Fig. 108 and Fig. 135), although in the completed tooth the joint is so close that mere inspection of the developing tooth gives a hint as to the true nature of the attachment. By the limitation of this ligament to one side, where it is greatly strengthened, we pass by easy transitions to those more specialised arrangements characteristic of hinged teeth. Attachment by an Elastic Hinge— The possession of movable teeth, able to yield to pressure, and subsequently to resume the upright position, was formerly supposed to be confined to Lophius (the Angler-fish) and its immediate allies. The author has, however, found hinges in the common Pike (Esox), and in some of the Gadiclce (Cod tribe), as well as in certain other predatory fish ; so that, as they occur in these fish so widely removed from one another in other respects, it is probable that further investigation will bring, and, indeed, is bringing, to light many other examples of this very peculiar method of attachment, eminently suited to, and hitherto only discovered in, fish of predatory habits. In the Angler, which obtains its food by lying in ambush on the bottom to which it is closely assimilated in colour, many of the largest teeth are so hinged that they readily allow an object to pass into the mouth, but, rebounding again, oppose its egress. These teeth are held in position by dense fibrous ligaments radiating from the inner side of their bases on to the subjacent bone, while the fronts of the bases of the teeth are free and, when the teeth are pressed towards the throat, rise away from the bone. The elasticity of the liga- ment is such that when it has been compressed by the tooth bending over towards it, it returns the tooth instantly into position with a snap. Many of the teeth of the Angler are, like most fishes' teeth, anchylosed firmly. Amongst the very peculiar predatory fish which were obtained by the Challenger from great depths, and which have been described by Dr. Gunther, through whose kindness the author has had the opportunity of examining them, are several which possess hinged teeth. Thus in Batliysaurus ferox, which has a crocodile-like snout, the teeth are of no very great size, but they are attached by ligamentous hinges which allow of their being bent backwards and inwards. The teeth are not THE ATTACHMENT OF TEETH. 251 perched upon any definite pedestals, so that the motion is not very exactly limited to one plane. But in Odontostomus hyalinus there are some more highly specialised hinged teeth, which are laterally compressed, and have a sort of barbed point which recalls the form of some primitive bone fish-hooks. The vomer carries two such teeth of great length, behind which come some smaller ones. They are perched upon little Fig. 131. — Hinged tooth of Odontostomus, a predatory fish from great depths, a. Enamel penetrated by dentinal tubes, b. Dentine. bony pedestals of such form that, the attachment being made by means of an elastic ligament on one side, the motion per- mitted to the tooth is strictly confined to one plane. The Hake (Merlucius, one of the Gadidce) possesses two rows of teeth, the outer and shorter of which are anchylosed, while the inner and longer are hinged. In some respects these hinged teeth are more highly special- ised than those of the Angler, which they resemble in being attached by an elastic hinge on their inner sides, the elasticity 252 A MANUAL OF DENTAL ANATOMY. of which is brought into play by its being compressed, or, at all events, bent over upon itself. The hinge in the Hake is peculiar in structure, and does not, as in the Angler, consist of fibrous tissue. On the inner side of the tooth the formation of what is apparently dentine matrix goes on so that a thin edge is continued right down to Fig. 132. — Hinged tooth of hake (^Merlucius). a. Vaso-dentine. b. Pulp. c. Elastic hinge, d. Buttress of bone to receive /. formed out of bone of attachment, e. Bone of jaw. /. Thickened base of tooth. g. Enamel tip. the bone, which it joins just as though the tooth were going to be anchylosed there. But though matrix is formed, it is not calcified, and so the tooth is united to the bone on the side by a thin piece of flexible matrix. When seen in section this is found to be divided in the middle by a space which stops short of reaching either the top or the bottom of the flexible portion, and in this space are seen sharply-bordered fibres which resemble elastic tissue in appearance. Possibly this space and THE ATTACHMENT OF TEETH. 258 its contents serve to increase the elasticity of the hinge. Of course in a decalcified section this portion looks and stains exactly like the rest of the matrix which was calcified, except that it does not contain vascular canals. These, however, are continued down to a lower level on the inside than on the outside of the space, and probably calcification also had advanced to a lower level here, as a dried tooth ends in a thin edge in this situation. The pulp is highly vascular, and its vessels are so arranged that, by entering the pulp through a hole in the flexible portion, which is near the axis of motion, they escape being stretched or torn during the movements of the tooth. The free edge of the base of the tooth itself is modified so as to be particularly fitted for resisting the jars to which a movable tooth must at times be exposed, and so also is the pedestal of bone upon which it strikes. As is seen in the figure, the base of the tooth on the side opposite to the hinge, is thickened and rounded. The advan- tages which such a form must possess over a thin edge when striking against the bone is sufficiently obvious. This thickened edge is received upon a little buttress of bone, and it occupies a much higher level than the opposite thin edge to which the hinge is attached, so that the tooth cannot possibly be bent outwards without actual rupture of the ligament. And what is not a little remarkable is that, whilst the Hake, the most predatory of all the Gadidce, is possessed of these very perfectly hinged teeth, other members of the family have teeth movable in a less degree, and others again have teeth rigidly fixed. Indeed, the Hake itself has its outer row of teeth anchylosed. So that within the limits of a single family we have several steps in a gradual progression towards a very highly specialised organ. In the hinged teeth already alluded to the purpose served by their mobility seems to be the catching of active fish, and the elasticity resides solely in the hinges ; but the common Pike possesses many hinged teeth which seem to be more concerned in the swallowing of the prey after it has been caught, and there is no elasticity in the hinges, the resilience of the teeth being provided for in another way. The teeth which surround the margins of the jaws are anchylosed, and they are more or less solidly filled up in their 254 A MANUAL OF DENTAL ANATOMY. interior with a development of osteo-dentine, which, by becom- ing continuous with the subjacent bone, cements them to it. The manner of development of this is by rods of calcifying material shooting down through the central pulp (see p. 201). In the hinged teeth also these trabecule shoot downwards and become continuous with the subjacent bone, but instead of Fig. 133. — Hinged tooth of Pike. a. Dentine, b. Elastic rods formed of uncalcified trabecule which might have become bone. c. Hinge, not itself elastic, d. Bone of attachment, e. Bone of body of jaw. rigidly ossifying they remain soft and elastic, so that the tooth is like an extinguisher fastened down by a large number of elastic strings attached to different points on its interior, and hinged at one side. The elasticity is very perfect. The teeth depressed and suddenly released return with an audible snap. The elasticity resides solely in these strings, for if they be divided by care- fully slipping a cataract needle under the tooth without injur- THE ATTACHMENT OF TEETH. 255 ing the hinge, the tooth will stay in any position into which it is placed. The method in winch the resilience of these teeth is utilised will be described subsequently, under the heading " The Teeth of Fishes." Thus we have examples of hinged teeth occurring in several distinct orders of fish : in Acanthopterygii (Lophius), in Anacanthini (Merlncius), in Physostomi (Esox, Batltysaurus, Odontostomus), whilst on the other hand they are not universal even within the limits of well denned families. Hence it is noteworthy (i.) that hinged teeth have arisen independently in families of fish widely removed from one another, and (ii.) that, whilst the general object of mobility and elastic resiliency is attained in all, it is by different mechanisms, and apparently by the least modification possible of the existing fixed teeth of the family. Attachment by Anchylosis. — In both the socketed and the membranous methods of attachment, an organised, more or less vascular, membrane intervenes between the tooth and the jaw-bone. In the mode of attachment now under considera- tion there is no such intervening membrane, but the calcified tooth substance and the bone are in actual continuity, so that it is often difficult to discern with the naked eye the line of junction. The teeth may be only slightly held, so that they break off under the application of but a moderate degree of force, or they may be so intimately bound to the bone that a portion of the latter will usually be torn away with the tooth. A very perfect example of attachment by anchylosis is afforded by the fixed teeth of the Pike, of which the central core is composed of osteo-dentine. The method by which the entire fusion of this tissue with the bone beneath it takes place has already been alluded to, the similarity of its method of calcification with that of bone rendering the fusion easy and complete. From the accounts which pass current in most books it would be supposed that the process of attachment by anchylosis is a very simple matter, the base of the dentine papilla or the dental capsule, by its calcification cementing the tooth on to a surface of the jaw-bone already formed. In a few animals (examined), however, the author has found that this con- 256 A MANUAL OF DENTAL ANATOMY. ception does not at all adequately represent what really takes place. It seldom, perhaps never, happens that a tooth is attached directly to a plane surface of the jaw which has been formed previously. The union takes place through the medium of a portion oE bone (which may be large or small in amount) specially developed to give attachment to that one particular tooth, and after the fall of that tooth is itself removed. For this bone the author proposed the name of " bone of attachment," and it is strictly analogous to the sockets of Fig. 134. — Section of tooth and. a portion of the jaw of a Python, showing the marked difference in character between the bone of attachment and the rest of the bone. those teeth which have sockets. It is well exemplified in the Ophidia, a description of the fixation of the teeth of which will serve to convey a good idea of its general character. If the base of one of the teeth with the subjacent jaw-bone, be submitted to microscopical examination it will be found that the layer of bone which closely embraces the tooth contrasts markedly with the rest of the bone. The latter is fine in texture, its lacunae with their very numerous fine canaliculi, very regular, and the lamination obviously referable to the general surface of the bone. But the " bone of attachment " is very coarse in texture, full of irregular spaces very different from the regular lacunae, and its lamination is roughly parallel with the base of the tooth. The dentine of the base of the tooth also bends inwards (Fig. 134), and its THE ATTACHMENT OF TEETH. 257 tubes are lost in the osseous tissue, a blending so intimate resulting, that in grinding down sections the tooth, and the bone of attachment often come away together, the tooth and this bone being more intimately united than this special bone is with that of the rest of the jaw. A study of its development also proves that it has an intimate relation with the tooth with which it is continuous, for it is wholly removed with the fall of the tooth, and is again formed for the next tooth which comes into position. The periosteum of the rest of the jaw-bone appears to take an important share in the formation of this special bone substance, and the tooth capsule, by its ossification, apparently contributes little. Hertwig has thrown out the suggestion that parts of the tooth sockets and of the basal plates met with in fish and reptiles represent the cementum, and that it may even be that the bones surrounding the mouth have originated from the coalescence of such plates ; but this view does not commend itself to the author. In the frog the teeth are commonly described as being attached by their bases and outer surfaces to a continuous groove, of which the external wall is the higher. Such is, however, an inadequate description of the condition, the tooth as seen in section being attached on its outer side by a new development of bone, which extends for a short distance up over its external surface. For the support of its inner wall there springs up from the subjacent bone an osseous pillar, which is entirely removed when that tooth falls, a new pillar being developed for the next tooth. When the teeth are, as in many fish, implanted upon what to the naked eye appears to be nothing more than a plane surface of bone, a microscopical examination generally, in fact in all specimens examined by the author, reveals either, that the individual teeth are implanted in depressions much larger than themselves, the excess of space being occupied by new and specially formed bone ; or else, that the teeth surmount pedicles which are closely set "together, the interspaces being occupied with a less regular calcified structure. A good example of the latter method is afforded by the Eel (Fig. 135), in which each tooth surmounts a short hollow cylinder of bone, the lamination, etc., of which differs markedly d.a. 17 258 A MANUAL OF DENTAL ANATOMY. from that of the body of the jaw-bone. When the tooth which it carries is shed, the bone of attachment, in this case a hollow cylinder, is removed right down to the level of the main bone of the jaw, as is well seen in the figure to the left of the teeth in position. Under a higher magnifying power the bone at this point would be found to be excavated by "Howship's lacunge." As an anchylosis, the implantation of the teeth is less perfect than that of the teeth of the snake, for the dentinal tubes at the base of the tooth are not deflected and do not in any sense blend with the bone beneath them. Accordingly, the teeth are far less firmly attached, and break off quite readily. Indeed, the actual attachment may perhaps Fig. 135. — From lower jaw of an eel. a. Bone of jaw. b. Bone of attach- ment, d. Dentine. /. Enamel, g. Space vacated by a shed tooth. be of a fibrous nature, although a bone of attachment pedestal is formed. Another instance of the formation of a kind of pedestal, which in this instance is sunk within the substance of the surrounding bone, is found in Sargus (Fig. 106). Here the attachment of the working tooth to the subjacent portion is by means of fibrous tissue (Fig. 108), though the submerged portion appears in this case to be made by the tooth pulp itself. A transition towards a socketed type of implantation is seen in some of the Cod family. Thus in the Haddock the teeth surmount a hollow cylindrical " bone of attachment," resembling in many particulars those of the Eel. The teeth THE ATTACHMENT OF TEETH. 259 do not, however, simply surmount the bony cylinders, but are continued for a short distance within them, definite shoulders being "formed which rest on the rims of the cylinder. The base of the tooth does not, however, contract or taper any more, and is widely open, so that it cannot be considered that any close approximation to a root is made. The pulp cavity of the tooth becomes continuous with the cavity of the osseous cylinder, into which the pulp is for a short distance continued. Not only is the pulp continued down, but its odontoblast layer is here traceable, and this relation of parts recalls that seen in Sargus. (Cf. Fig. 107, p. 201.) The bony supports of the teeth originate in many osseous Fig. 136. — From lower jaw of a haddock, a. Bone of jaw. b. Bone of attachment, d. Dentine of tooth. trabecule which spring up simultaneously from the bone of the jaw beneath the new tooth ; these coalesce to form a net-like skeleton, which rapidly becomes filled in by the progress of ossification. So far as the author's researches enable him to conclude, there is this much in common in all forms of attachment by anchylosis, that no matter how different the naked eye results of the process may be, the tooth as it comes into position, is secured by an exceedingly rapid development of bone, more or less directly an outgrowth from the jaw-bone itself, and which is in some unseen manner stimulated into activity by the proximity of the tooth. The teeth of the mackerel present an interesting variety of attachment by anchylosis. The margins of the jaws are very thin, and by no means fleshy, and in them there is a 17—2 260 A MANUAL OF DENTAL ANATOMY. deep groove between the outer and inner plates of the bone. In this groove the teeth are attached, their sharp points projecting beyond the edges of the bone, and they are held in their places by a network or scaffolding of " bone of attach- ment " which is developed between their sides and the inner surface of the bone. They are, so to speak, hung up in their place, and their open bases rest on nothing, or at least on nothing hard. Attachment by Implantation in a Socket. — In this, as in anchylosis, there is a special development of bone Fig. 137. — Tooth of mackerel, showing its peculiar mode of anchylosis. a. Tooth, h. Bone of jaw. c. Gums. d. Bone of attachment. which is modelled to the base of the tooth, but, instead of its being in actual close continuity with the dental tissues, there intervenes a vascular organised membrane in great part, at all events, derived from the tooth-sac. The manner in which the sockets are, so to speak, plastered around the roots of the teeth and are perfectly subservient to and dependent on them, has already been described. Little, therefore, need be added here, save that the soft tissue intervening between the bone and the tooth is not separable, either anatomically or developmentally, into any two layers, but is a single membrane termed the " alveolo -dental peri- osteum." That it is single when completed is a matter of absolute certainty ; there is no difficulty in demonstrating it in situ, with vessels and bundles of fibres traversing its whole thickness, from the tooth to the bone, or vice versa. THE ATTACHMENT OF TEETH. 261 The nature and development of the sockets in those few reptiles and fishes which have socketed teeth require further examination. From what has been seen in sections of the jaws of a young crocodile, the author is inclined to regard them as in all respects similar to the alveoli of mammalian teeth. At all events, they are not developed in that same subserviency to each individual tooth ; on the contrary, successive teeth come up and occupy a socket which is more or less already in existence. Although there are animals in which implantation in a spurious socket is supplemented by anchylosis to the wall or to the bottom of the socket, few examples of anchylosis occurring between the tooth and the bone of the socket have ever been met with in man, or indeed in any mammal exemplifying a typical socketed implantation of the teeth. Eeference. (!) Tomes, C. S. "Studies upon the Attachment of Teeth." Trans. Odontol. Soc, Gt. Brit., 1874-5. Yol. vii., pp. 41—56. PART II. CHAPTEK IX. GENERAL INTRODUCTORY REMARKS. Before proceeding to the description of the teeth of Fishes, Keptiles, Mammals, of which it is only possible to give the barest outline, it seems desirable, at the risk of some recapitulation, to devote a few pages to those general principles which underlie and render interesting and intelligible, the details with which we shall be concerned. Before the day of Darwin it was customary to explain those resemblances and points of identity which the study of comparative anatomy and of development revealed upon the supposition that there was some sort of type or standard organisation and that all others were arrived at by modifi- cations and departures from this type, such modifications being introduced with a direct purpose in view, in order to fit the creature to a special habit of life. This " type " theory suggested a clue to the explanation for the possession by an animal of some highly specialised organ which detailed comparative study reveals as but the homologue of some simpler structure present in allied animals ; and like- wise to the reduction or suppression of an organ in one animal, which is of ordinary size and functional activity in its congeners. This is as true of teeth as of any other organ ; indeed, they afford many admirable examples. Thus the tusks of the boar or of the Sus babirussa (figured on p. 513), large and peculiar though they be, are not new developments, but are merely the canine teeth which all pigs possess, and which in this species attain to unusual dimensions. In the same way the enormous straight tusk of the male Narwal is nothing more than an incisor tooth of one side, the fellow to which has been checked in its development ; but this latter is not missing, for it remains throughout the GENEEAL INTRODUCTORY REMARKS. 263 life of the animal buried within its socket. In the female Narwal both of the teeth, being rudimentary, are permanently enclosed within the sockets, and are, of course, not of the smallest service to the animal, directly or indirectly. Further- more, as has been shown by Professor Sir W. Turner, in young specimens, a second pair of vestigial incisors are to be found which in the adults have disappeared. The modern school of morphologists, rejecting the " arche- type " theory, refer these correspondences detected between dentitions, upon the whole dissimilar to one another, to a more intelligible cause, namely, inheritance. Assuming, as every evidence compels us to assume, that the many divergent forms which we observe have been derived by progressive modifications and differentiations from fewer and simpler ancestral forms, we shall have no difficulty in seeing how, by such processes as are well known to occur, namely, the dwindling of disused organs and the exaggerated development of those which are of considerable use, great differences may ultimately result. To illustrate what is meant by this so-called " adaptive modification," this suppression of things that are not needed, and increased development of those most used, we may allude to the dentitions of non-venomous and venomous snakes (p. 316). In them we see that the typical dentition in the snake, as exemplified by the Pythons, consisted of three complete rows of teeth, two in the upper jaw, one of which occupied the maxilla, and one in the lower. Then in the Colubrine Opisthoglypha some of the maxillary teeth become enlarged and grooved, while the snakes are only doubtfully poisonous ; then, further, in the Proteroglypha the front teeth of the maxilla become grooved and enlarged to form the poison fang, the complete row of maxillary teeth becoming reduced to a few small teeth behind it, the maxilla itself undergoing reduction in an antero-posterior direction. Finally, in the Vipers the poison fang is enormously developed, the rest of the maxillary teeth are lost, and the maxilla so shortened as to be almost square, and coincidently acquiring the power of movement of rotation through a wide angle, so as to lay the poison fang down flat when not in use. This mobility of the maxilla was foreshadowed in the Cobra, 264 A MANUAL OF DENTAL ANATOMY. in which the bone is but slightly movable. The same serial modification is to be observed in the poison fangs themselves (p. 320), which range from the very complete canal of the Vipers, traversing the whole length of the fang and terminating in an orifice at the apex through the closure of the laterally- placed groove in the Cobra, and the open grooves of some of the other Proteroglypha, to the grooveless teeth of the non- poisonous snakes. Thus there can be found amongst living snakes a most complete chain of transitional forms of poison apparatus. But in many poisonous colubrine snakes the three or four small and useless teeth lingering upon the maxillary bone, though their function has gone, serve to indicate to us in some measure the gradual process by which that singularly perfect adaptation of means to an end, the poison apparatus of the viper has been attained. Good examples of rudimentary teeth are to be found in the larval teeth of the edentulous sturgeon, or of the whale- bone whales, or in the teeth buried beneath the horny plates of the deflected portion of the Dugong's jaw, all of these being of absolutely no service to their possessors. Some teeth have disappeared utterly ; thus the upper incisors of Kuminants are gone, and no rudiments have been detected at any stage ; others still remain in a stunted and dwindled form, and do not persist throughout the lifetime of the animal, as for instance the first premolars of the horse, or two out of the four premolars of most bears. The teeth of Ornithorhynchus do work for their possessor till the animal approaches its adult size ; they are then curiously supplanted by a horny development of the gums. Darwin believed that any modification in the structure of a plant or an animal, which is of benefit to its possessor, is capable, nay, is sure of being transmitted and intensified in successive generations, until great and material differences have more or less masked the resemblance to the parent form. Just as man, by picking out and breeding from those modifications of form, etc., that please him best, has been able, in the course of a few years — in a length of time altogether infinitesimal, as compared with the time during which the surface of land and sea has been nearly in GENERAL INTRODUCTORY REMARKS. 265 its present form, to say nothing of the enormously longer earlier geological epochs — to profoundly modify the breeds of dogs, of horses, of. pigeons and of numbers of plants, all of which are known to have had a common origin ; so in nature forces are and ever have been in perpetual operation effecting the same result. A pigeon-fancier desires a pigeon of particular plumage, with some feathers differing slightly from any pigeon he has ever seen or heard of * ; he knows by experience that little variations are for ever arising, and that by watching a sufficient number of young ones, and rigorously picking out those which at all tend in the direction required, he will get what he requires and will even state with confidence that in so many years he will make a breed with the peculiarity desired. And exactly as the required plumage is obtained, so in nature the tooth that is " wanted," i.e., the dentition that is excellently well adapted to do its work, may have been, elaborated by the operation of that law known as " survival of the fittest." The number of animals born, or of seeds germinating is vastly in excess of the capabilities of the area in which they live, to support them, so that a weeding-out process, the death of a large number, is a necessity. And those are the most likely to die or to fail to breed which are, in any way, placed at a disadvantage. It is, therefore, quite enough that one of the small varia- tions for ever arising in animals shall be of advantage for us to see how the peculiarity is likely to be transmitted and intensified in successive generations without the intervention of man, which we know can effect such a result. The question has been well presented by Wallace, who points out that we must not think so much of variations in individuals as in groups of individuals ; for instance it is a familiar fact that people vary in height, so that any hundred persons may be divided into fifty taller and fifty shorter persons. Now if a little extra height were of advantage, many or most of the fifty would be placed under circumstances to experience it, though some might not. In the same way, if we grouped one hundred animals whose teeth varied a little in * An eminent pigeon-fancier, Sir J. Sebright, told Mr. Darwin that he could produce any given feather in three years. 266 A MANUAL OF DENTAL ANATOMY. respect of strength into the fifty weaker and fifty stronger, it is easy to see that the stronger fifty would get the better of the others in the struggle for existence on the whole, would be more certain to propagate their kind, and would repeat in a majority of their progeny those peculiarities which had helped themselves to survive. Thus the doctrine of natural selection, or survival of the fittest, is as fully applicable to the teeth of an animal as to any part of its organisation, and the operation of this natural law will be constantly tending to produce advantageous or " adaptive " differences. On the other hand, the strong power of inheritance is tending to preserve even that which, in the altering conditions of life, has become of little use, and thus we may understand vestigial teeth to be those which are in process of disappearance, having ceased to be useful to their possessors, but which have still for a long time lingered upon the scene. And it must be remembered that, just as natural selection brings an organ up to its fullest standard of efficiency, so the cessation of selection will tend to allow its variations in a downward direction to establish themselves. In other words, the continuance of that selection which elaborated an organ is required to maintain it at that standard. For the efficient action of natural selection, it will be seen that two conditions are essential : the one a competition amongst the organisms concerned, and the other a tendency to vary, the variations being capable of being inherited. Now, however produced, it is a familiar fact that variations occur, and that they are capable of perpetuation, as every florist or breeder of animals knows. It was formerly supposed that the variations seized upon by natural selection and intensified in successive generations were of necessity small, so small that it was difficult to see how their advantage would really be felt. But since attention has been drawn to this subject, and careful measurements and weighings have been taken of large numbers of wild specimens of the same species, it has been found that the variations are often far from being insignificant in extent. Then Wallace (6) points out that the variation in common species often reaches 20 per cent, of the size of the part impli- GENEKAL IN TRODUC TOBY REMAKES. 267 cated, and this without reference to the general size of the animal. The jaws of the wolf may be taken as an example ; in ten specimens variation to the extent of an inch and a half in length occurred. If there be an advantage in it, it is easy to see how in a few generations a very distinct advance in the direction of increased length or shortness of jaw might be established. The same species will be found to vary in size according to the locality whence it has come. Thus a species having a large range in latitude will generally be found larger in its northern than in its southern areas. Variability in the number of teeth is much more common than has generally been supposed. Thus Bateson (*) found Fig. 138. — Skull of a placental rodent (Cobpybara), showing general charac- ter of a rodent's dentition. supernumerary teeth in 8 per cent, of the jaws of Gorillas, Orangs, and Chimpanzees which he examined. In Phocidce supernumerary teeth were present in 7 per cent, of the specimens examined, in 4 per cent, of Otariidce, in 3 per cent, of wild and in 8 per cent, of domestic Canidce, in less than 2 per cent, of wild, but in 9 per cent, of domestic Felidce. Before leaving this section of the subject, an instructive illustration of the operation of these agencies may be given. It is easy to understand how a " rodent," type of dentition may be beneficial to its possessor by rendering accessible articles of food wholly unavailable for creatures which have no means of gnawing through a shell or other hard body. Now it happens that in three regions of the world, more or less completely cut off from one another, three animals, in 268 A MANUAL OF DENTAL ANATOMY. parentage widely dissimilar, have arrived at dentitions of " functionally rodent " type. Thus in Australia, a region practically wholly monopolised by Marsupials, a marsupial, the Wombat, has a dentition very much like an ordinary placental Rodent. In the island of Madagascar, one of the very few parts of the globe without indigenous rodents (except a few Muridce), a Lem urine Fig. 139. — A. Milk teeth of the Lemurine Cheiromys, with the permanent incisors just coming into place. It differs from any rodent by- haying many milk teeth, i. Permanent incisor. i 2. Deciduous incisor, c. Deciduous canine, d, d 2. Deciduous molars. I. Lower permanent incisor. I 2. Lower deciduous canine, d a, d b. Lower deciduous molars. B. Reduced outline figure of its permanent denti- tion, in which it closely mimics the true rodents. animal, the Cheiromys, has a dentition modified in a similar direction (though it is probably employed to obtain a different sort of food) ; and elsewhere, scattered all over the world, we have the ordinary Rodents. In fact, three sets of creatures, as widely different from each other in parentage as they well could be, have been modified by natural selection until they have dentitions by no means identical but for working purposes not unlike. Such instances are examples of what is now known as " convergent evolution." It cannot be supposed that these three creatures have arrived at their peculiarities by the way of common origin ; GENERAL INTRODUCTORY REMARKS. 269 their ancestry must have been widely different, the regions in which they live have been isolated from one another for countless years, and- yet they have each got to a " practically rodent " type of dentition. Of extinct Lemurs little is known and of the ancestry of Cheiromys nothing ; but in the compact group of Marsupials, still living in Australia, we are able to dimly see some of the progressive steps which seem to tend towards this rodent form of dentition. In Australia, roughly speaking, there are nothing but Marsupials ; in Madagascar more Lemurs than anything else ; and in each case, out of the material at hand, natural selection has manufactured a "rodent" dentition. At the same time the force of inheritance is seen in each of them retaining characteristics of the groups whence they have been derived, so that underlying the prima facie resemblance in the teeth there are points in their several dentitions whereby the wombat shows its marsupial affinities, and the Aye-aye its quadrumanous relationships. The approximation to a rodent type of dentition by a number of widely different animals has led to the sugges- tion being made, that it does not follow that all existing Rodents have had a common origin. Thus Oscar Schmidt (4) writes : — " A comparison of the very different shapes of the molars in the rodents among one another, and the approxima- tion of many genera — not as yet decided rodents — to the rodent type (for instance the wombat, the fingered animal — Aye-aye — and the rock coney), render it extremely probable that even our present rodents are not of one and the same origin. The fact remains, animals of different derivation having attained a similar exterior, succeed extremely well in the struggle for existence, or even better, in their endeavour to obtain food. Unlike as they may be in one point, they are incontestably alike, i.e., in the development of continuously growing incisors." An example of the same kind of thing is to be seen in the occurrence of hinged teeth in various families of fish (see p. 250), in which the requirements of a hinge and of a means for restoring the bent-down tooth to the upright position are attained by different mechanisms according to the differences in the raw material, if such an expression may be allowed. Examples of " convergence " are of course not confined to 270 A MANUAL OF DENTAL ANATOMY. the teeth, but occur throughout the animal kingdom, and in many different organs of the body. In addition to these modifications which are of direct use to the individual in the way of assisting in the procuring of food, etc., any character which would enable one male to get an advantage over other males, and so render him more certain to propagate his kind, will be sure to be transmitted and intensified. Thus we can understand how the males of some species have become ornamented ; how the males of many birds have come to sing ; and what is of more immediate concern to us, how the males of some animals have become possessed of weapons which the females have not. The possession of weapons by the male is strikingly exemplified in the teeth of some animals. The males of many frugivorous monkeys have canine teeth much larger than those of the females ; they are cut late, coincidentally with the attainment of sexual maturity, and are useful to their possessors as weapons in their combats with other males. The male narwal has its single elongated tusk ; the male dugong has tusk-like incisors ; in the respective females these same teeth are insignificant. But the most striking instance of the teeth being modified, so as to serve as weapons for sexual combat, is afforded by some members of the group of ruminants, amongst whom, as Cuvier long ago pointed out, those which are armed with horns have no canine teeth, and vice versa — a generalisation which, although subject to slight exceptions, remains upon the whole true. The male musk-deer (Moschus moschiferus) has canine teeth of enormous length, while it is quite without horns ; the female has canine teeth so reduced as to be functionless. The male muntjak, which has very short horns has canine teeth, but of much smaller size, than those of the musk-deer. Other examples of hornless deer furnished with canine teeth are to be found in Swinhoe's water-deer (Hydropotes inermis) and in the Elaphodus cephalophus (which has very small antlers), a Chinese deer more recently discovered, and in the Tragulidce. It is obvious that males furnished with weapons more powerful than their fellows will be more likely to prove victorious in their battles, to drive away the other males, to monopolise the herd of females, and so to transmit their own GENERAL INTRODUCTORY REMARKS. 271 peculiarities to offspring, which will again be favoured in the same way. Thus it is easy to see how, amongst grega- rious animals, the development of teeth serving as sexual weapons is likely to be favoured, generation after generation, until canines as highly specialised as those of the musk-deer, or the wild boar, are attained. It will suffice to indicate to the reader that he must be prepared to find that the teeth are profoundly susceptible of modification, but that, amid all their varied forms, the evi- dences of descent from ancestors whose teeth departed less from the typical mammalian dentition are clearly traceable Fig. 140. — Cranium of Moschus (male), showing the long canine tooth. in the existence of vestigial teeth and other such characters. And, although it is by no means probable that more than a part of the agencies which are at work have been recog- nised, natural selection and sexual selection appear to be competent to produce most of the phenomena of modifica- tion observed. There remains one other influence, much more obscure in its nature, to be touched upon, namely, " correlation of growth " or " concomitant variation." When we find that when horns are developed canine teeth are absent, or that, after a boar has been castrated, his tusks cease to grow, although we may be quite unable to conceive the precise manner in which the one thing influences the other, we can see that there is a consistency in the development of the sexual weapon ceasing coincidently with the destruction of the sexual apparatus, or in the fact that two kinds of weapon are not developed in the same animal. 272 A MANUAL OF DENTAL ANATOMY. But there are some correlations of growth of a still more recondite nature, in which the connection is less obvious. Of this kind is the relation which exists between peculiarities of the skin and of the teeth : the Edentata, abnormal in their skins, are different from most other Mammalia in their teeth ; whales, yet more aberrant in the nature of their skins, have only rudimentary teeth, in place of which, after birth, plates of whalebone are found. Darwin (8) has collected a number of curious instances of relations existing between hair and teeth. In general terms it may be said that any great abnormality in the hair goes hand in hand with an abnormality of the teeth. Thus there is a breed of dogs found in Turkey which is almost hair- less, and which has very few teeth, the dentition being reduced to a single molar on each side, together with a few imperfect incisors * ; in the human subject inherited baldness has been often found to be associated with inherited deficiency of the teeth ; on the other hand, redundance of hair has in several cases been accompanied by absence of teeth. In the case of the now famous hairy family of Burmah, the peculiarity of silky hair being developed over the face was transmitted to a third generation, and in each case the teeth were very deficient in number. A number of years ago a hairy man and his son, said to have come from the interior of Bussia, were exhibited in London, and they were almost toothless.! A good many years ago a hairy woman (Julia Pastrana) was exhibited in London, of whom it has commonly been reported that she had an excessive number of teeth. Certain it is that her mouth was very prominent, and that she was described as " dog-faced " and " pig-faced," but models have been presented to the Odontological Society by Mr. Hepburn which are indisputably known to be models of her mouth, and * The author has described an example of great reduction of the teeth in an individual specimen of the very hairy monkey Colobus caudatus (5). f The man's mouth exemplified the dependence of the growth of the jaw upon the presence of teeth. Ordinarily the increase in size between childhood and adult age takes place by a backward elongation, which allows for the successive development and eruption of the molars behind the spaces occupied by the temporary teeth. But this man never had any true molars, and no such backward elongation of the jaw had ever taken place, so that, though he was a full-sized man, his jaw was no larger than a child's. GENEKAL INTRODUCTORY REMARKS. 273 these do not show any excessive number of teeth. The teeth, at least such of them as can be seen, are enormously large, but the mouth is- affected with general hypertrophy of the gums and alveolar processes to such a degree that only a few of the teeth can be made out. But this does not make her case the less interesting to the odontologist, for in the huge teeth, the enormous papillae of the gum, and the redundant hairs on the face, we have evidence of a disposition to hypertrophies of the integument affecting in different places the different tegumentary append- ages which happen to be there. And that the teeth are dermal appendages has been shown at a previous page (see p. 4). But we must not go further than to say that great abnormality of hair goes Hand in hand with abnor- mality of teeth. On the other hand, redundance of hair has in several cases been accompanied by absence of teeth. He would indeed be a rash man who ventured to assert that he had recognised all the agencies which are at work in the modelling of animal and vegetable forms ; but it is safe to say that, at the present time, we are acquainted with " natural selection," or " survival of the fittest," an agency by which variations beneficial to their possessors will be preserved and intensified in successive generations ; with " sexual selection," which operates principally by enabling those possessed of certain characters to propagate their race, while others less favoured do not get the opportunity of so doing; and with " concomitant variation " between different parts of the body, an agency much more recondite in its operations, but by which agencies affecting one part may secondarily bring about alterations in some other part. And operating in the contrary direction, we have a certain fixity of organisation, so that the power of inheritance is constantly asserting itself by the retention, for a time at all events, of parts which have become useless, and by the occasional reappearance of characters which have been lost (atavism). With regard to the direct action of environment upon the teeth of the individual or upon the race within a compara- tively few generations, some rather contradictory statements have been made as to the effect of high feeding, improvement of breed, etc., upon the cutting of the teeth, early maturity being of course one of the aims of the improvers of farm stock. d.a. 18 274 A MANUAL OF DENTAL ANATOMY. With regard to horses, neither high feeding nor improved breed seems to have made any difference, e.g., a Highland pony at five years has as full a mouth as the most pampered horse. But in cows it seems to be different. Thus Highland cattle have all the permanent teeth at four and a half or five years of age, whereas some improved breeds have a full mouth at three years (Williams (7)). Prof. Brown (2) has been unable to trace any change in thirty years, though, if the older writers were accurate, a real acceleration has taken place over much longer periods. He, however, quite denies that high feeding, of a litter of pigs for example, will produce any immediate effect, and strongly lays down that their dentition is a reliable test of age. From what has been said, the reader will have gathered that, in order that evolution may go on, it is necessary, firstly, that animals shall vary ; and, secondly, that these variations shall be inherited ; granted these two propositions, the process is intelligible. That animals do vary we have already seen, but various views are current as to the causes of that variation. One school, supported by the very numerous measurements of prawns taken by the late Prof. Weldon (prawns were selected as being creatures of which it was easy to procure large numbers), holds that the variations are, so to speak, fortuitous ; that is to say that they take place round a mean, in the same pro- portions as would result were they determined by the tossing up of a coin, those which are advantageous getting preserved by selection, while those which are not die out. The other school hold that they are by no means fortuitous, but that they take place along certain defined lines towards which there is an ever-acting tendency, and that the force in operation is that of the environment and habits of the animal. This involves another consideration, namely, whether characters acquired by an animal in its lifetime can be and are transmitted to its progeny. And on this point also naturalists are divided into two camps : those who hold that the agencies enumerated are all-sufficient, and, with Weissman, hold that qualities acquired by the individual during its life- time under the influence of outside conditions can never be transmitted; and those, the so-called Neo-Lamarckians, who believe that they can. GENEEAL INTRODUCTORY REMARKS. 275 Amongst the strongest advocates of this latter view are several of the American naturalists who have given much attention to teeth", and base their arguments largely upon them, especially Profs. Cope, Ryder, and Osborn, to whose views we shall have from time to time to recur. But as their arguments relate chiefly to what has been observed with regard to mammalian teeth, it may be more convenient to defer their consideration for the present. Keferences. (x) Bateson, W. "On some cases of Variation in Secondary Sexual Characters statistically examined." Proc. Zool. Soc, London, 1892, pp. 585—594. (2) Brown, Gr. T. "Dentition as indicative of the Age of the Animals of the Farm." Journ. Roy. Agricult. Soc, 1882, vol. xviii., pp. 385—438. (3) Darwin, C. "Plants and Animals under Domestication." London, 1889. (4) Schmidt, O. "Mammalia." International Scientific Series, Lond., 1885. (6) Tomes, C. S. "Partial suppression of Teeth in a very Hairy Monkey" (CuJobus caudatus). Trans. Odon- tolog. Gt. Brit. 1897-8, vol. xxx., N. S., pp. 30—35. (6) Wallace, A. E. " Darwinism." London, 1889. (7) Williams. "Dentition of the Horse and other Animals," Brit. Journ. Dental Sci., 1882, vol. xxv., pp. 364—371. 18—2 CHAPTER X. THE TEETH OF FISHES. In the following pages the limits of space forbid more than a brief account of a few typical forms. In the class of fish the task of selection of forms for description is no easy one ; for the almost infinite diversity of dentition which exists in it makes it a matter of peculiar difficulty to frame any general account, or to do more than present before the reader a description of a few individual types from which he may gather, as best he can, a general idea of piscine dentition. The modern classification * of the class Pisces separates off a sub-class Agnatha, which is without paired fins and without a lower jaw. It comprises The Cyclostomata (vel. Marsipobranchii) which embrace the Lampreys and Myxinoid fishes, as well as some extinct forms. Pisces — Aquatic, cold-blooded animals breathing by means of internal gills. More or less covered with scales and with appendages in the form of fins. I. Elasmobranchii. — Mouth ventral. Body covered with dermal denticles. Operculum absent. Tail heterocercal. Skeleton cartilaginous. Heart with a contractile conus arteriosus. Spiral valve in the intestine. Optic nerves not decussating or only partly decussating. A. Plagiostomi (Sharks and Rays). B. Holocephali (Chimcera). II. Teleostomi. — Mouth terminal. Body usually covered with cycloid or ctenoid scales. Operculum present. Tail usually homocercal. Spiral valve usually absent. Skeleton contains bone corpuscles. Optic nerves decussating. * For further details as to classification, characters, etc., the reader is referred to the Cambridge Natural History, " Fishes," or to any standard text-hook of Zoology. THE TEETH OF FISHES. 277 A. ACTINOPTERYGII. 1. Chondrostei (Sturgeon). 2. Holostei (Garpike). 3. Teleosti (Bony fishes generally). B. Ceossopterygii (Polypterus). III. Dipnoi. — Have two pairs of nostrils. Breathe by gills or swim -bladder. (Ceratodus, Protopterus, Lepidosiren.) The Cyclostomata comprise the lampreys and the very peculiar parasitic fish, the Myxine and Bdellostoma, which bore their way into the bodies of other fish, the cod being often attacked by the Myxine. The lampreys are also pre- datory, attaching themselves by their sucking mouth to the bodies of other fish from which they scrape off the flesh. As the name implies they have a circular mouth, the margins of which are beset with rows of small conical teeth, there being two larger blade- shaped teeth in the centre, sometimes called from their relative position the mandibular and maxillary teeth. Until comparatively recently little was known of the struc- ture of horny teeth, but they have been investigated by Beard (1> 2), who finds that the horny cone rests upon a slight dermal papilla, and fits into special epidermal depressions at the base of the papilla (in Petromyzon fluviatilis) ; but in P. marinus there are superimposed cones " like a nest of Chinese boxes." Each of these layers arises from a separate epi- dermal depression, which goes on continually forming horn, so that the under cones are in no sense reserve teeth, for as each tooth is worn away at the apex fresh horny matter is formed below and pushed superficially. There is thus no resemblance to the teeth of higher vertebrates. In a young lamprey there are to be found what at first sight look like true tooth-sacs, but the dental-papilla never forms any odontoblasts, and the epithelium which corre- sponds to the enamel-organ produces horn. This is true of the marginal teeth, but further in towards the centre the teeth are formed simply in the basal layers of the epithelium, without the intervention of any sort of tooth-sac. But ill the Myxine and Bdellostoma, there is an in- teresting arrangement of structures. They have a large 278 A MANUAL OF DENTAL ANATOMY. sharply-pointed median tooth and two comb-like smaller teeth upon the tongue, and the working surface of the teeth is composed of horn similar both in structure and devolopment to that found in the lamprey. Beard (lj 2) had described the tooth of Bdellostoma as consisting of a cap of horn, thick and strong, and of a bright yellow colour. Beneath this comes a layer of epithelium, and next to this a hard calcined material, which he regarded as some form of dentine. Some doubt, however, has been thrown upon the nature of this calcified dentine by the late Prof. Howes (4), and by Warren (9). The horny cap is fitted into an epithelial groove at its base, so that it increases in length by the cells of this groove becoming cornified, and in thickness by a similar conversion of the epithelial layer beneath it. The hard cone which forms, so to speak, the body of the subjacent tooth, Beard supposed it to be an anomalous structure, not closely corresponding with any known form of dentine, but yet being the product of an odontoblast layer upon the pulp, which latter remains in the base of the cone of dentine in the usual way. The great hardness of the tooth and of the horny cap renders it a very difficult matter to obtain good sections, and hence the minute structure is difficult to determine. There is a layer of epithelium, in the proper situation, having the characters of an enamel-organ in so far as the presence of long columnar cells is concerned. But Warren, who investi- gated it later, strongly affirms that there is no real calcification, but merely a succession of horny caps superimposed on one another and developed from an epithelial groove after the usual manner of horny teeth. Were Beard's view correct, it would be difficult to conceive that the presence of the calcified cap could be of much, if any, service whilst buried beneath the horny cap, and we should regard these structures not as an early and simple form of tooth, but as being degenerated teeth. They would thus lend support to the idea, already held by some, that these fish had as ancestors fish which had jaws, and teeth carried upon them. Marett Tims (6) has suggested the possibility of the derivation of the horny teeth of the Cyclostomes from the scales of the Teleostomi. THE TEETH OF FISHES. 279 It is to be noted that the relation, described by Beard, which the horny cone bears to the dental-papilla and its dentine, is entirely different from that borne by the horny teeth of Ornithorhynchus, in which animal the horny plate, which replaces the teeth in the adult, lies beneath the teeth. It has been suggested by Beard that the fusion of the lingual teeth of Myxine into a serrated horny plate may indicate the manner in which the serrated horny jaws of chelonians may have originated, as a substitution of horny tissue for true teeth upon which they were once superimposed. Similar speculations have been indulged in as to the manner Fig. 141. — Tooth of Bdellostoma, semidiagrammatic, after Dr. Beard. d. Calcified dentine cap. e. (?) Enamel, h. Horny tooth, he. Epi- thelial groove in which the base is formed. in which the bird's bill may have originated from the substitu- tion of a number of coalescent horny teeth for true teeth, but the material for such generalisation is not yet to hand. But, as has already been mentioned, no little doubt rests on this determination of structures, and it is now thought that these teeth are entirely horny. Elasmobranchii. — As in the matter of teeth the Palaich- thyes, or Elasmobranchii, comprising the Sharks and Bays and the Ganoid fish, present somewhat simpler conditions than are met with in the osseous fish, it will be convenient to describe their teeth first. In Sharks the scales of other fish are represented by cal- Ite 280 A MANUAL OF DENTAL ANATOMY. cified papillae, which have much the same structure as their teeth : to these the " shagreen," as shark skin is termed, owes its roughness. The mouth is a transverse, more or less curved fissure, opening upon the under surface of the head at some little distance behind the end of the snout. Hence it is that a shark in seizing its prey turns over upon its back, or at all events upon its side. The jaws, which are made up of the representatives of the palato-quadrate arch and of Meckel's cartillage, neither true maxillae nor premaxillae being present, are cartilaginous in the main, and therefore shrink and become much distorted in drying. The shape of the jaws differs in the various Fig. 142.— Lower jaw of Lamna. a. Edge of flap of mucous membrane which covers the teeth not yet completed. groups of Plagiostomi, in some, each of the two jaws form a tolerably perfect semicircle, while in others they are nearly straight and parallel to one another (see Fig. 142, and Fig. 146) ; but in all the rounded working surface of the jaw is clothed or encased by teeth, arranged in many parallel concentric rows. The teeth which are situated upon the edge or exposed border of the jaw, are usually erect, whilst the rows which lie behind them, farther within the mouth, point backwards, and are more or less recumbent, not having yet come into full use. In this respect, however, marked difference exists among various genera of sharks ; for instance, in the great tropical THE TEETH OF FISHES. 281 white shark the teeth which lie on the border of the jaw are erect, and all the successive rows are quite recumbent ; whereas in many of the dog-fish the inner aspect of the jaws forms an even rounded surface along which the rows of teeth are disposed in every intermediate position between those f ally recumbent at the innermost part of the jaw and those fully erected upon its exposed borders. Only a few of the most forward rows of teeth are exposed, a fold or flap of mucous membrane covering in teeth which are not as yet fully calcined and firml attached to the gums. Fig. 143. — Transverse section of lower jaw of a dog-fish. a. Oral epithelium. b. Oral epithelium passing on to flap, from which it is torn in the figure, e. Protecting flap of mucous membrane (thecal fold). d. Youngest dentine pulp. e. Youngest enamel organ. /. Tooth about to be shed. g. Calcified crust of jaw. In Lamna, which may be taken as fairly illustrative, the teeth are arranged round the jaws in concentric rows with great regularity, those of the successive rows corresponding in position to the teeth of older rows, and not, as is the case in some other sharks, to their interspaces. They are attached by being embedded in a densely fibrous gum, which closely embraces their bifurcated bases ; and this dense gum, carrying the teeth with it, slides bodily upwards over the inner face of the jaw, and outwards over its border, beyond which, to borrow a phrase from geological science, it has an 282 A MANUAL OF DENTAL ANATOMY. "outcrop." The second and third rows of teeth are only partially erect, the rows behind these lying recumbent, and being in the fresh state covered in by the fold of mucous membrane, which, being dried and shrunk in the specimen figured, falls far short of its original level. Thus rows of teeth, originally developed at the base of the jaw, are carried upwards, come to occupy the foremost position on the border of the jaw and are cast off when they pass the point / in the figure. It is thus easy to understand why shark's teeth are so abundantly found in a fossil condition, although other indications of the existence of the fish are comparatively rare ; for every shark in the course of its life casts off great numbers of teeth, which fall to the bottom of the sea and become embedded in the deposit there forming. The teeth are never anchylosed to the jaw, nor have they any direct connection with it, but, as before mentioned, are retained by being embedded in a very tough fibrous membrane. The nature of their fixation has been more exactly described on another page (p. 249). It may be worth while to condense from Professor Owen the description of the manner in which it was proved that an actual sliding or rotation of the membrane does really take place, and that the whole jaw itself does not become slowly everted. The spine of a sting ray had been driven through the lower jaw of a shark (Galeus), passing between two (vertical) rows of teeth which had not yet been brought into use ; when the specimen came under observation the spine had remained in this situation, transfixing the jaw, for a long time, as was evidenced by all the teeth of these two rows both above and below it, being stunted and smaller than their neighbours. Hence the development of these teeth, which ultimately came to be at some little distance from the spine, had been profoundly modified by its presence, and it is difficult to under- stand in what manner this could have effected them had they not, at an earlier period of their growth, lain in more imme- diate proximity to it. But if the membrane with the teeth attached, did move slowly along the surface of the jaw, this difficulty at once disappears. The forms of the teeth in various sharks though different and characteristic, nevertheless vary somewhat with age in some species, and present differences in size and form in the THE TEETH OF FISHES. 283 upper and lower jaws, or in different parts of the mouth of the same individual. For instance, in Lamna, in the upper jaw, the teeth of the third vertical row, counting from the middle line, are very small, while in both jaws there is a gradual diminution in the size of the teeth towards the back of the mouth. More- over, many of the skates and rays exhibit sexual differences in their dentitions, in the male the teeth being simple and pointed whereas in the female they may present a plate-like character. Thus, although it is often possible to refer a particular tooth to its right genus or even species, much care is requisite in so doing. The teeth of the bloodthirsty white shark (Carcharias) are triangular flattened plates, rounded on their posterior aspect, with trenchant, slightly serrated edges. It has been pointed out by Owen that if the relation between the size of the teeth and that of the body were the same in extinct as in recent sharks, the dimensions of the teeth of the tertiary Carcharodon would indicate the existence of sharks as large as whales. The intimate relationship between the teeth and the dermal spines, which from the standpoint of development, has been illustrated at p. 3 and pp. 134 and 149, is apparent also in their histological structure. There are many dermal spines to be met with in the sharks and on the back or tail of rays, which seen alone could not possibly be distinguished from teeth, the resemblance both in outer form, in minute structure, and manner of development being most complete. The tooth figured on p. 100 is a fair example of a structure very common among the sharks, viz., a central body of osteo-dentine, the outer portion of which is covered by an enamel (?) of peculiar structure. Fine tubed dentine is, however, also frequently met with, as is the case of the genus Carcharias. Dental tissues occur in other parts of the mouths of Plagios- tomes than upon the jaws, not only in the embryonic stages, but in the adult. Thus Professor Sir W. Turner has described (8), very numerous comb-like appendages five inches in length upon the branchial arches of the Basking Shark (Selachc maxima), which apparently perform the same function as whalebone in straining the water. These combs are formed of a variety of dentine (? osteo-dentine), and closely resemble in structure the true teeth, which are, however, very small in this shark. 284 A MANUAL OF DENTAL ANATOMY. In the seas of Australia there exists a shark, the Cestracion philippi, with a very aberrant dentition, to which great interest attaches, inasmuch as it is the sole surviving representative of forms once spread all over the world. In the front of the mouth the teeth are small and very numerous ; they are flat plates fitted by their edges to one another, while from their centres spring up sharp points, soon worn off when the tooth reaches such a position upon the jaw that it comes into use. Proceeding backwards, the teeth cease to be pointed, increase in size, and become fewer in each row ; a reference to the figure will convey a better idea of their general form than any Fig. 144. — Lower jaw of Cestracion philippi (Heterodontus). a. Young teeth not yet in use. b. Large grinding back teeth, c. Small pointed front teeth. The new teeth are developed at the bottom of the series on the inner side, and, just as in other sharks, are covered in by a flap of mucous membrane. description. Those which have come into use are, towards the back of the mouth, always much worn ; their shedding and renewal takes place, as in other sharks, by a rotation of the mucous membrane over the surface of the jaw, so that, as might have been expected, large numbers of the isolated fossil teeth of Cestracionts are to be met with. The teeth of Cestracion are fitted for the trituration of hard substances, and for such they are used, its food consisting of shell-fish, etc. They consist of a body of dentine par- taking of the character of osteo- rather than of vaso-dentine, whilst the surface layer is made up of sharply-defined layers of fine parallel tubes, curiously gathered into bundles, which THE TEETH OF FISHES. 285 taper both towards the surface and towards the deeper part of the tooth. A comparison with other selachian teeth indicates that this, layer may be regarded as a curiously modified enamel (p. 57). The extinct Cestracionts extended far back in time, being met with in palaeozoic strata. They were equally widely Fig. 145. — Rostrum and under side of the head of a small Pristis. a. Mouth. b. Rostrum, c. One of the rostral teeth. The teeth, with which the margins of the jaw are covered, are so small that they cannot be represented in this figure. distributed in space. The size of many of the teeth indicates the existence of forms much larger than the recent timid and inoffensive Cestracion philippi. Many of the extinct forms 286 A MANUAL OF DENTAL ANATOMY. are known only by isolated teeth ; of others, portions of the jaw with teeth in situ have been discovered ; thus fragments of the jaw of Acrodus, the isolated fossil teeth of which have been compared to fossil leeches, wifh seven teeth arranged in series, have been met with. The Pristis, or Saw-fish, so far as the mouth is concerned, is in no way remarkable, its teeth being small and blunt, like those of many rays. The snout is, however, prolonged to an enormous length, and is shaped like a gigantic spatula, its thin edges being beset by dermal spines of large size, arranged at Fig. 146. — Upper and lower jaw of 3Iyliobat.es. At the mosaic pavement formed by the broad flattened plates which constitute its teeth is seen, these being the oldest teeth which are about to be shed off in conse- quence of the rotation of the whole sheet of mucous membrane over the surface of the jaws. The letter b indicates the under surface of one of the plates, which is seen to be finely fluted on its edges. regular intervals, and implanted in distinct sockets. These dermal spines, or rostral teeth, as they are sometimes termed, are not shed and replaced, but grow from persistent pulps. In structure they somewhat closely resemble the teeth of Myliobates (see p. 88), being made up of parallel denticles, in the centre of each of which is a pulp cavity or medullary canal. What use the Saw-fish makes of its armed snout is not very certainly known, but its rostral teeth are of interest to the odontologist for several reasons — one being that they are dermal spines, having a structure all but identical with that of THE TEETH OF FISHES. 287 the actual teeth of another ray, the Myliobates, while the scales of Pristis have a structure such that it is easy to imagine that these large spines are an aggregation of a number of the ordinary scales, enlarged and fused together. A second point of interest is that they are socketed, a manner of implanta- tion not at all common amongst the teeth of fishes ; and yet another is that they grow from persistent pulps, also unusual in fishes. Broadly speaking, the teeth of the Kays (skates) differ from those of typical sharks by being individually blunter, and being more closely set, so that they form something approach- ing to a continuous pavement over the jaws, with but little interspace left between the teeth. The dentigerous surface of the jaw is very much rounded, and in some is completely encased under a pavement of teeth. Thus, in Myliobates, the powerful jaws are straight from side to side, while their working surfaces from back to front are segments of a circle. The teeth form a thick and strong pavement over the jaws, the manner of their formation and renewal conforming with the teeth of other Plagiostomi. The severe use to which they are put is indicated by the extent to which the grinding surfaces of those teeth which have come into use are worn down. Several genera have the jaws thus covered, the number of the teeth differing ; thus Myliobates has a central series of very broad, oblong teeth, to the outer sides of which are three rows of small hexagonal teeth. In (Etobatis the large oblong central plates constitute the whole armature of the jaw. Amongst ganoid fish (Chondrostei) great diversity of dentition exists. Thus the sturgeons have no teeth, the mouth being at the lower surface of the snout, and being protrusible as a sort of suctorial tube. In the larval stages, however, the sturgeon possessed teeth. In the allied Spatularia there are numerous minute teeth, whilst there are many extinct ganoids with large blunt-pointed teeth upon the palate and mandible. The American Lepidosteus, the structure of whose teeth has been described on p. 85, has a long pointed snout furnished with large sharp conical teeth. Some of the extinct ganoids, e.g., Dendrodus and Rhizodus, had conical teeth of reptilian appearance. Others combined chisel-shaped teeth in front with large hemispherical or 288 A MANUAL OF DENTAL ANATOMY. flattened teeth on the palate and part of lower jaw, whilst others again were only furnished with crushing teeth. The great majority of ganoid fish are extinct, and it is now claimed that these extinct forms link the ganoids directly with the Teleostei, or osseous fishes. The Dipnoi or lung fishes which are able to breathe for periods out of the water, present some resemblance to the Amphibia. Lepidosiren has a peculiar dental arma- ture, the margins of the jaws being formed by serrated plates anchylosed to the bone. These plates have upon their edges five deep angular notches, the prominence of the upper Fig. 147. — Edge of jaw of Lepidosiren, in which are developed superimposed plates of enamel. Prepared and photographed by Mr. Leon Williams. plate corresponding to the notches of the lower ; and the edge is kept somewhat sharp by the disposition of the hard tissues. On the edge of the bone lie superimposed layers of a tissue which appears to be enamel, and a blunt edge is kept up by these resisting wear longer than the bone in which they are embedded. The attention of the author was called to this structure by Mr. Leon Williams, and of which there appears to be no published description. THE TEETH OF FISHES. 289 The cutting plates of the upper jaw are developed near the median line of the palate and on the palate bones, and in front of them there are conical piercing teeth upon that forward prolongation of the cartilage which takes the place of a distinct vomer ; these have sometimes been described as being upon the nasal bone. It would seem that the two conical piercing teeth serve as holdfasts, while the cutting Fig. 148. — Upper jaw of Ceratodus. From a recent specimen. A somewhat similar condition exists in Phyllodus, an extinct fish from the London clay, known only by its teeth, and referred to the Wrasse family. In this there are superimposed layers of dentine and enamel, not embedded in bone. The dentine is, however, in very small proportion , and a still further reduction in the dentine would bring it very much to the condition of the enamel plates of Lepidosiren, so that this explains the apparent anomaly of successive plates of enamel developed without dentine. This structure is unlike that found in recent Wrasses, and suggests doubt as to the affinities of Phyllodus. edges of the deeply-notched plates are brought into play to slice up the food. Both in structure and general disposition the dental plates in Lepidosiren are paralleled by the teeth of Ceratodus, for some time known only as a fossil, but of which recent examples have been captured in Queensland ; this resemblance was suspected some years ago by the late Professor Moseley, before the affinities of Ceratodus were known. D.A. 19 290 A MANUAL OF DENTAL ANATOMY. Semon (5) has shown that the vomerine and palatine teeth originate as distinct rounded tubercles, corresponding to the roots of the adult tooth. The dental plates are thus shown to be the result of concrescence, of the fusion of teeth at first separate. The Teleostei, or osseous fish, form the group which com- prises all the fish most familiarly known to us, and within its limits the variation in dentitions is so great that few, if any, general statements can be made about them. It is not uncommon to find teeth crowded upon every one of the Fig. 149. — Teeth of a species of Chcetodon. The recurved enamel tips are apparently structureless. bones which form a part of the bony framework of the mouth and pharynx, and the teeth are sometimes in countless numbers. Such is the variability that even within the limits of single families great differences in the teeth are to be found. The teeth of fish are of all degrees of size and of fineness. In some (Chsetodonts) the teeth are as fine as hairs, and are so soft as to be flexible ; they formerly were supposed to be horny, but the larger ones are composed of a vascular dentine surmounted at the tip by a curious little hook of enamel. In the finest teeth there is hardly room for vascular canals, THE TEETH OF FISHES. 291 but no one can doubt their derivation from the larger forms, so that the idea that these teeth are horny must be abandoned in toto. Teeth which are very fine and very closely set are termed "dents en velours," " ciliiform " or " setiform " ; when they are a little stouter, " dents en brosse," or " villiform," and when still stronger and sharper, " dents en cardes." Teeth that are conical, wedge-shaped, spheroidal, and lamelliform, are all to be met with. And there are some fish, e.g., some of the large Siluroids, which have very strong, large teeth, an inch and a half or more long, and very firmly anchylosed to the bone. The long and very strong snout of the Sword Fish, formed by a coalescence of the maxillary and intermaxillary bones, which is able to pierce a plank, is roughened on its lower surface by villiform teeth which can be of no use, and are therefore to be regarded as rudimentary survivals. In the common pike the mouth is crowded with sharply- pointed teeth, having a general inclination backwards, and being in some parts of the mouth of larger size than in others. The margin of the lower jaw is armed with teeth of formidable size and sharpness, the smallest teeth being at the front, where they are arranged in several rows, and the largest being about the middle of the side of the jaw. A pike, as is well known to anglers, when it has seized a fish, often holds it across its mouth, piercing and retaining it by means of these largest teeth ; then, after holding it thus for a time, and so having maimed it and lessened its power of escape, it swallows it, generally head foremost. The tenacity of the pike's hold is often illustrated when it takes a bait, and retains it so firmly that when the angler " strikes " the hooks do not get driven into the fish's mouth ; but after tugging at the bait for a time the pike releases it, and the angler finds that it has never been hooked at all. The margin of the upper jaw is not bordered by teeth, save at the front, where the intermaxillary bones carry a few teeth of insignificant dimensions ; indeed, it is rather exceptional for the true maxillary bones to carry teeth in osseous fish. The roof of the mouth presents three wide parallel bands of teeth, those in the median band (on the vomer) being directed backwards, those upon the lateral bands (on the palatine bones) 19—2 292 A MANUAL OF DENTAL ANATOMY. backwards and inwards. Some of the latter teeth are very large, but not quite so large as those at the sides of the lower jaw. The marginal teeth are firmly anchylosed, but the teeth upon the palate are all hinged, and in such a manner that Fig. 150. — Jaws of a pike, viewed from the front, with the mouth opened more widely than is natural, so as to bring the teeth into view. a. Group of teeth situated on the palatine bone. h. Group of teeth situated on the vomer, c. Group of teeth situated on the lingual bone. d. Specially large teeth, placed at intervals round the margin of the lower jaw. e. Group of teeth on the intermaxillary bones. The diagram represents the direction in which the hinged teeth of the vomerine and palatine bands can bend. they can only bend exactly in one direction. Those of the vomerine band which lie in the middle line, will bend back- wards only; those upon the outer margins of this band backwards, with an inclination outwards. Those of the lateral or palatine bands bend obliquely backwards and inwards, at an angle of about 45° with the median line of THE TEETH OF FISHES. 293 the mouth, or somewhat more directly backwards. To a body sliding over them in one direction they offer no resistance, bending down as 'it passes, and springing up as the pressure is removed from them, but to anything moving in any other direction they are rigidly-fixed sharp curved stakes, impeding its further progress. An elongated body of some size, such as a living fish, can only be swallowed by the pike when it is arranged lengthwise in the mouth ; crosswise it cannot possibly enter the throat. The hinged teeth on the palate seem admirably arranged for getting the fish into a longitudinal position and keeping it there ; for if we imagine the fish's body held up against these teeth, and consider the direction in which the hinging of the teeth allows them to yield, it will be seen that every motion tending to arrange the body lengthwise, either in the median line of the mouth or in either of the interspaces between the vomerine and palatine bands of teeth, will meet with no obstruction, but in every deviation from this position it will be caught on the points of the teeth and resisted. Thus with the pike's mouth shut, and the prey kept up against the palatine teeth, even its own struggles will be utilised by allowing every movement tending to place it aright, and every other being stopped by the bands of hinged teeth entangling it. The structure of these teeth, and the mechanism by which they are rendered elastic, have been already described (p. 254). The lingual bone, and the three median bones behind it, carry small teeth arranged in oblong patches ; the internal surfaces of the branchial bones (which support the gills) are armed with similar small teeth ; while the last or fifth branchial arch (which carries no gills, the bones forming it being called inferior pharyngeal bones) possesses larger teeth. The superior pharyngeal bones (which are median portions of the four anterior branchial arches) also carry recurved teeth larger than those which line the rest of the internal surfaces of each of the branchial arches. The pike's mouth and pharynx thus fairly bristle with teeth, all directed somewhat backwards ; and any one who has been unfortunate enough to have allowed his fingers to get entangled in the mouth of a living pike will realise how small a chance of escape its living prey has when once it has been seized, 294 A MANUAL OF DENTAL ANATOMY. The teeth of the pike are composed of a central body of osteo-dentine, on the outside of which is a layer in which the dentinal tubes are directed towards the surface, as in hard or unvascular dentine (see Fig. 58) ; while the outermost portion of all is a very dense and hard film, which is perhaps enamel. The teeth are anchylosed to the bone, and are very frequently renewed, their successors being developed at one side of their bases (cf. p. 301). Though the pike has rather more teeth than many other fish, it may be taken as a fair example of most osseous fishes in this respect. Space will only allow of a few of the more exceptional forms being here described. The angler (Lophius piscatorms), another predatory fish, found on our own coasts, with an enormous mouth and disproportionately small body and tail, lies hidden in the mud, or crouched upon the bottom, and makes a rush upon smaller fishes which approach sufficiently near to it. It is remarkable for the manner of attachment of the teeth, some of the largest of which upon the edges of its jaws do not become anchylosed, but are so attached, as has been described at p. 250, as to allow of their bending inwards towards the mouth, but not in the opposite or indeed any other direction. The teeth of the outer row are firmly anchylosed to the margins of the jaw, and the far larger hinged teeth form a sort of irregular second row. The benefit of such an arrangement to a fish of its habit is sufficiently obvious ; its teeth allow the utmost freedom of entry, but offer obstacles to anything getting out again. This arrangement of teeth, long supposed to be unique, is closely paralleled in a very different fish, the Hake (Merlucius, one of the Gadidce). This fish, the most active and predatory of the Cod family, follows shoals of pilchards and of herrings, themselves active fish, and feeds upon them. The margins of the jaws carry two distinct and regularly arranged rows of teeth, an outer smaller row which are anchylosed, and an inner longer row which are hinged. They are very sharp, being tipped with spear points of enamel, and are recurved. In the fresh state they look quite red, being composed of a richly vascular vaso-dentine. The family of Gadidce or Cod fishes, which the best authorities state to be a well-defined and natural group, present an interesting series of THE TEETH OF FISHES. 295 gradations between teeth immovably fixed, teeth admitting of slight motion, and highly specialised hinged teeth such as those of the Hake. The author has examined the teeth of a considerable number of Gadidce (7) and found that they presented wide variability in the structure of their teeth, at the same time these differences are linked together by every gradation. Thus are seen the rich and regular disposition of vascular canals in the Ling and Hake, the less abundant system of canals in the Cod, whilst in the Eock Cod (Motella) the dentine is laminated and con- tains few canals, and in Lotella, a fish found in Japan, and allied to the Hake, there are no vascular canals and no dentinal tubes, but a strongly marked lamination is seen. Whilst in members of the family which are very closely related there is a similarity of histological structure, the extent to which the vascular canals are present and the pattern in which they are arranged does not always accord in its variations with the generally accepted classification of the genera. Beduction in size does not imply simplification of structure, for the tiny teeth termed gill-rakers, which cover the inner surfaces of the gill arches of the Hake, are identical in structure with its larger teeth, and, on the other hand, the very large teeth of Uraleptus are found to be quite simple in structure. It is difficult to understand by what process of selection differences of minute structure have been brought about, for it would seem as if from the point of view of efficiency it would matter little what the minute structure be, so long as a tooth be strong enough and sharp enough. More detail on this subject will be found in the paper referred to (7). In several of the deep sea fish dredged by the Challenge!' from depths to which light does not penetrate, hinged teeth have been found (cf. p. 250). Many of these deep sea fish have very formidable dental armaments, and, curiously enough, one of these, which has exceptionally long upper teeth, has a downward projection from the lower jaw, which serves to protect them while closed, an arrangement elsewhere only met with in extinct mammalia, such as Dinoceras and Machairodus, figured later on in this work. All of them are carnivorous, the toothless forms feeding on animalcule which live at great depths or sink down from lesser depths, and all are akin to forms which inhabit lesser depths from which the very deep sea fauna has been gradually modified. The dentition of many might be called eccentric, the teeth in front of the mouth being so long that in closure of the mouth they pass outside the opposite lip for a con- siderable distance. Sometimes these very long teeth are hinged ; sometimes they are anchylosed. Another curious dentition is possessed by the Wolf-fish 296 A MANUAL OF DENTAL ANATOMY. (Anarrhichas lupus), also an inhabitant of British waters, and sometimes to be seen in London fishmongers' shops under the name of the sea-cat. The intermaxillary teeth are conical, bluntly pointed, and set forwards and outwards ; these are antagonised by somewhat similar teeth in the front of the lower jaw. The palatine bones carry short, bluntly conical, or round-topped crushing' teeth in a double Fig. 151. — Bones of the mouth of the wolf-fish (Anarrhichas lupus). The letter a indicates the divergent pointed teeth which occupy the inter- maxillary bone ; the letter d indicates the similar teeth which are attached to the front of the mandible, on the middle and back parts of which are round-topped crushing teeth («?). Strong crushing teeth are found also upon the palatine bones (7/), and upon the vomer (c). row ; the vomer is also armed with a double row of very much larger and shorter teeth. The lower jaw, with the exception of its anterior part, is occupied by teeth of similar character. All the teeth of the Wolf-fish are anchylosed slightly to the bone, a definite process from which forms a sort of short pedestal for each tooth. The jaws are worked by muscles of great power, and it seldom happens that a specimen is examined in which some of the teeth are not broken. It THE TEETH OF FISHES. 297 feeds upon shell-fish, the hard coverings of which are crushed by the blunter teeth, while the pointed front teeth apparently serve to tear the shell-fish from the rocks to which they are commonly attached. In the group of fish known as " Gymnodonts " (naked- toothed), the teeth and the margins of the dentigerous bones form a sort of beak, which is not covered by the lips. The example here figured consists of the upper and lower jaws of the Diodon, so called because it appears to casual observers to have but two teeth. A kindred fish, in which the division of each jaw in the middle line is conspicuous, is similarly Fig-. 152. — Jaws of the Diodon. a. Base of the dental plates, where new lamellae of dentine are being developed, b. Margin of jaw, formed mainly by the sides of the denticles, c. Compound tooth, made up of the superimposed lamella? of dentine anchylosed together. called Tetrodon. The jaw consists of teeth and bone very intimately fused together ; the broad rounded mass (c in the figure), which lies just inside the margin of the jaw, is made up of a number of horizontal plates of dentine, the edges of which crop out upon its posterior surface ; and these are united to one another by the calcification of the last remains of the pulp of each plate into a sort of osteo-dentine, the different hardness of the two tissues keeping the surface constantly rough, as the plates become worn away. The whole margin of the jaw is similarly built up of smaller horizontally-disposed denticles, or plates of dentine, which are, as they wear down, replaced by the development of fresh 298 A MANUAL OF DENTAL ANATOMY. plates, which are added from beneath, where they are developed in cavities situated low down in the substance of the bone. The new teeth or plates of dentine thus formed at the base of the hemispherical masses within the jaws (at the point a), or low down in the substance of the jaw, do not come into use by the ordinary process of displacing their predecessors and themselves being in turn replaced, but fresh plates only come into use by the actual wearing away of all that is above them, both dentine and bone, so that they come to form the topmost FlG. 153. — Edge of jaw of Pseudoscarus (?). a. Denticles, b. Bone of jaw. portion of the jaw. The margins of the jaw are, however, mainly built up of dental tissues, there being but little bone in their interspaces. Tetrodon has not the rounded triturating disc of the Diodon, or has it but feebly represented ; and the margins of the jaws are sharper. In the parrot-fishes (Scarus), which are very nearly allied to the Gymnodonts, somewhat similar beaks are found, the individual teeth being more conspicuous. The whole outer surface of the jaw near to its working edge is covered by a THE TEETH OF FISHES. 299 sort of tesselated pavement, formed by the several teeth which are pressed together into a mass, but they form only the outer surface and the immediate edge, so that the soft bone forms a part of the working surface, or would do so but that, by its more speedy wear, it leaves the edge, formed by dentine and enamel, always prominent and more or less sharp. The structure and succession of these teeth have been carefully described by J. von Boas (3), and the differences between the several genera pointed out. He describes cementum as binding the denticles together and forming a part of the working edge, but that which he describes as cementum appears to be that tissue which the author has termed " bone of attachment." See p. 256. In a section of a jaw in the possession of the author, which he believed to have belonged to a Gymnodont fish but which bears a remarkably close resemblance to that figured by von Boas as being a jaw of Pseudoscarus, a very beautiful arrange- ment serves to preserve the sharpness of the edge of the jaw. The denticles are conical, and form a series of hollow super- imposed cones with the points directed upwards. They consist of dentine and enamel, and the point of the subjacent cone fits into the hollow of that above it so closely that in von Boas' specimen the dentine of the older tooth is in great part absorbed to make way for the point of its successor ; the working denticle thus comes to be little more than a hollow cone of enamel. This is not the case in the specimen referred to, in which there is a quantity of dentine left in each denticle. This vertical series of superimposed sharp cones lies in the midst of the somewhat thin jawbone, fused together by bone of attachment (? cementum), and enclosed between the inner and outer plates of the jaw. The bone, being considerably softer than the dentine, wears down much faster, so that the edge is always formed by a pro- minent sharp tooth, which, as the wearing down of the bone progresses, falls off, and the next one beneath it comes into play. The arrangement recalls the way in which a scythe or a chisel is assisted in keeping its edge by being made of a plate of steel welded between two plates of softer iron. The pharyngeal bones are also remarkable ; the two lower are united into one, and the stout bone so formed is armed with teeth ; it is antagonised by two upper pharyngeal bones 300 A MANUAL OF DENTAL ANATOMY. similarly armed. It carries teeth which are anchylosed to it, and which are so disposed as to keep the surface constantly rough. When they are freshly formed the teeth have flattened thin edges, something like human incisors. They are coated with enamel, and thus, when calcification has proceeded so far as to obliterate their central pulp cavities, after the tooth is worn to a certain point (c in Fig. 154) it presents a ring of enamel, inside which is a ring of dentine, and inside this a core of secondary dentine, as seen in the figure. Owing to the different hardness of the three tissues, a constant roughness of surface is maintained. The upper pharyngeals are similarly armed ; and as the teeth and the supporting Fig. 154. — Lower pharyngeal bone of Pseudoscarus. a. Posterior border at which the teeth are unworn, c. Oval areas formed by teeth, the points of which are worn off. b. Anterior edge of bone, at which the teeth are almost completely worn away. bone wear away, fresh teeth are developed at the front, so that the whole bone undergoes a sort of gliding motion backwards, the armature of the lower pharyngeal being renewed in a similar manner, save that new teeth and bone are developed at its posterior instead of its anterior extremity. The teeth are developed in bony crypts, below the youngest functional teeth, and perforations in the roofs of the crypts give passage to the connecting band between the tooth-sac and the mucous membrane. The Sheep's-head fish (Sargus), to which attention has been drawn on account of the peculiarities of structure which it well exemplifies, has curiously human-like incisors in the front of the mouth, and round-topped crushing teeth inside. No more fitting place will occur for noticing the stout THE TEETH OF FISHES. 301 pharyngeal teeth which are met with in so many fish.* Some fish, which are edentulous so far as the mouth proper is concerned, have the pharyngeal bones armed with teeth ; in the carp and its allies, edentulous so far as the mouth proper is concerned, the two lower pharyngeal bones carry long pointed teeth, which partly oppose one another, and partly oppose a sort of horny tubercle, which is supported on a process of the base of the occipital bone. A few fish are quite without teeth ; the sturgeon, whose mouth forms a protrusible sucker, is edentulous, as are also the pipe fish and the little sea-horse {Hippocampus), now so common in aquaria. These are to be regarded as degenerations from ancestors which possessed teeth, and not as fish which have never from the earliest times acquired teeth. As a rule fish are remarkable for the great number of their teeth, which are being constantly shed off and replaced by successors an indefinite number of times. In all the fish hitherto mentioned in these pages, it happens that the teeth in different parts of the mouth differ in size and in the function which they have to perform, but this is only so because a few striking forms have been naturally selected for description. It is far commoner for all the teeth of fish, particularly of those fish which have countless numbers of teeth, to be very nearly alike in form and size in all parts of the mouth. Usually, fish do not comminute their food very fully, but make use of their teeth simply for the prehen- sion of prey, not submitting the food to any mastication whatever ; their teeth are hence often mere sharp cones, slightly recurved, or set pointing backwards. Thus, though the mouth of the common pike is beset with an immense number of sharp teeth, its food is swallowed whole, and very often is alive when it reaches the stomach, the sole purpose served by the teeth being the prevention of its escape when once it has been seized. Implantation of the teeth in sockets is not usual in the class of fish. Still many examples of it may be found ; for example, the Barracuda pike (Sphyrcena) has its lancet-shaped teeth im- planted in distinct sockets, to the wall of which they are said * For a more detailed account of the pharyngeal teeth of fishes the reader is referred to a series of papers by Colonel Shepherd in recent numbers of the Zoologist (H. W". M. T.^ 302 A MANUAL OF DENTAL ANATOMY. to become slightly anchylosed ; the file-fish and others might also be cited. And although the succession of teeth is usually from the side, in some cases the successional teeth are developed in alveolar cavities within the substance of the bone, and displace their predecessors in a vertical direction, as happens in the pharyngeal teeth of the Wrasses, or the incisors of the Sheep's-head fish (Sargus) ; Lepidosteus also has its teeth affixed in incomplete sockets, to the walls and bases of which they are anchylosed ; this is not a very un- common arrangement with the teeth of the fish when they are socketed at all. As has been already mentioned, sexual differences may be met with between the teeth of the male and female, as in some species of Skate, though such differences are by no means common. Although not strictly speaking a dental character, it may not be out of place to mention here the peculiar armature of the jaw of the male Salmon at the breeding season. The end of the lower jaw becomes prolonged, and turned upwards at its point ; the stout cartilaginous hook thus formed is of such dimensions that it has to be accommodated during closure of the mouth in a deep cavity formed for it between the intermaxillary bones. In some Canadian salmon this process is supposed to be constant in the older males, but in the British fish it disappears, and only exists at the breed- ing season. After spawning it is still strongly developed, and the salmon is a foul fish and is called a Kelt. It is used apparently as a battering ram, and salmon are constantly found killed, with their sides deeply gashed by the charges of their opponents. Not much can be said in general terms of the structure of the teeth of fish. The bulk of the teeth of most fishes is made up of one or other modification of vaso-dentine or osteo- dentine ; this is often glazed over upon its exterior by a thin film of enamel, so thin as often to appear structureless. Unvascular dentine also forms the teeth of many fish, and in some is remarkable for the fineness of its tubes; in fact, every form of dentine, from fine-tubed hard dentine to tissue indistinguishable from coarse bone, is to be found in this class. Dentins of very complex structure (labyrintho-dentine) is met with in some fish ; and an example from the Lepidosteus (American garpike, a ganoid fish) has been figured on p. 85. THE TEETH OF FISHES. 303 Enamel is often present in a very thin layer, glazing the exterior of the dentine ; sometimes it forms a mere tip, a sort of spear-point to the tooth, as in the Eel, the Hake and in Chaetodonts (see Figs. 135 and 149), and sometimes it is very thick, and itself permeated by systems of tubes (see Fig. 28). Cementum is of comparatively rare occurrence in fish. Professor Kolliker has shown that in a very large number of fishes the skeleton more nearly resembles dentine than true bone in its structure. This is especially the case upon its surface, the jaw of the Wrasse, for instance, presenting a finer- tubed layer with no lacunae or other irregular spaces ; whilst the dermal scales and the protective spines of fish are made up of a tissue closely resembling dentine (cf. Professor William- son, (10)). We may say, then, that just as in the external skin bony or dentinal plates are developed for the purpose of pro- tecting it from destruction by attrition, so for a similar purpose teeth are developed in that portion of the mucous membrane which covers the jaws. With regard to their development (cf. pp. 133, 145, 150), it may be added that in an embryo Pike the first teeth are formed in a primitive fashion, there being an upstanding dentine-papilla with so little specialisation of the surrounding epithelium that it resembles in its manner of formation a dermal placoid spine (Fig. 72), and so far invalidates the statement that the enamel-organ is always the first thing to appear. Though in most cases this simple condition after- wards gives way to the formation of tooth-germs of an ordinary type, situated more deeply in the submucous tissues, it to some extent persists in the case of «the very small and numerous teeth found in some parts of the mouth. A change in the methods of development after the appear- ance of the earliest teeth has been observed in other fish. Thus, the earliest tooth-germs of Myliobates are rounded and all alike, but in the second generation enlargement of the middle teeth occurs, without, it is said, any evidence of coalescence of several smaller ones, and the earliest teeth of Cestracion are all alike, and none resembles the flattened molariform teeth afterwards formed (Friedmann and Kose, as quoted by Burckhardt). 304 A MANUAL OF DENTAL ANATOMY. Beferences. (x) Beard, J. (a) "The Nature of the Teeth of the Marsipobranch Fishes." Morph. Jahrb., 1889, Bd. iii., pp. 727 —753. (2) „ (6) " The Teeth of Myxinoid Fishes." Anatom, Anzeiger, 1888, Bd. iii., pp. 169 — 172. (3) von Boas, T. E. V. "Die Lahne der Scaroiden." Zeitschr. Mr Wissensch. ZooL, 1879, Bd. xxxii., pp. 189—215. (4) Howes, G. B. "On the Affinities, Inter-relationships and Syste- matic Position of the Marsipobranchii." Proc. and Trans., Liverpool Biolog. Soc, 1892, vol. vi., pp. 122-148. (5) Semon, B. "Die Zahnentwickelung der Ceratodus Forsteri.' Denteschr. Med.-Naturw. Gesell., Jena, 1901, iv. (Forschungsreisen Australian, 1), Lief, iii., pp. 113—133. (6) Tims, H. W. Marett. "A Suggestion as to the Nature of the Horny Teeth of the Marsipobranchii." Proc. Camb. Philosoph. Soc, 1906, vol. xiii., pp. 383 —386. (7) Tomes, C. S. "On Differences in the Histological Structure of Teeth occurring in a Single Family — the Gadidae.' ' Quart. Journ. Mic. Sci., 1898-99, vol. xli., pp. 459—469. (8) Turner, Sir W. " The Structure of the Comb-like Branchial Appendages and of the Teeth of the Basking Shark (Selache maxima)" Proc. Eoy. Soc. Edin., 1878-80, vol. x., p. 457. (9) Warren, E. " On the Teeth of Petromyzon and Myxine." Quart. Journ. Mic. Sci., 1902, vol. xlv., pp. 631—637. (10) Williamson, W. C. " On the Microscopic Structure of the Scales and Dermal Teeth of some Ganoid and Placoid Fish." Phil. Trans. Eoy. Soc, Lond., 1849, vol. cxxxix., pp. 435 — 475. CHAPTEK XI. THE TEETH OF AMPHIBIA, REPTILES, AND BIRDS. In these classes the teeth are never so numerous nor so widely distributed upon the bones of the mouth as in fish. A double row of teeth arranged in concentric lines in the upper jaw, between which a single row of teeth upon the lower jaw passes when the mouth is closed, is an arrangement common amongst Amphibia. The outer of the two rows of teeth in the upper jaw is situated upon the premaxillary and maxillary bones, and usually extends further back than the vomerine or inner row. Almost all batrachians and reptiles have an endless succession of teeth ; but there are some lizards (e.g. the Chamaeleon and Hatteria, now generally known as Sphenodon) in which no replacement takes place. Only a few reptiles (e.g. Hatteria, Ophisaurus, a snake-like lizard akin to the blindworm, Chamseleon) have a row of vomerine teeth. Amphibia. — From the type of dentition just described are many deviations ; thus the toads are edentulous, and the frog has no teeth in the lower jaw. The teeth of the frog form a single row upon the margin of the upper jaw, their points projecting but little above the surface of the mucous membrane, and the vomerine teeth are few in number and cover only a small space. The edentulous lower jaw passes altogether inside the row of upper teeth, and having rounded surfaces and no lip, fits very closely against the inner sides of the teeth. Thus it leaves very little room for the young developing tooth-sacs, which are accommodated with the space required for the attainment of their full size, by the absorption of the older solid bone and of the tooth which has preceded them, in the following manner. The teeth are attached to the bone by anchylosis, each tooth being perched upon a little pedestal of bone specially formed for it. The successional teeth, the germs of which originally lay at the inner sides of the old teeth, commonly undermine the sides and bases of the pedestals of the latter, and move bodily beneath them, so that the new tooth D.A. 20 306 A MANUAL OF DENTAL ANATOMY. completes its development in what was once the pulp cavity of its predecessor. The teeth of the frog consist of a body of hard dentine, coated with an exceedingly thin layer of enamel, the existence of which has been doubted by some writers ; but a study of the tissues of the tooth-sac renders it probable that the transparent layer, which is undoubtedly present, is really enamel. The teeth of the newt and its ally the salamander are remarkable for having tips of enamel, somewhat like those of the eel (see Fig. 135), save that they are bifurcated, the one point being larger and longer than the other. The armature of the mouth of the tadpole is peculiar and interesting. Upon its cartilaginous jaws are tough, horny plates, something in general shape like a turtle's bill ; but in addition to these the inner margins of the lips are set with tiny horny spines or hooks seated upon expanded bases. The apices of these spines have frequently three minute points arranged longitudinally, the median point being the most pronounced. Each one of these spines is the product of the cornification of a single epithelial cell, which has become of very large size and shaped into an extinguisher-like form. Beneath the one in use are a number of others in serial stages, ready to succeed when the top one falls off. The beaks are formed in exactly the same way, save that the cells being closely placed side by side, a continuous sheet is formed instead of isolated spines. These horny cases are shed off prior to the commencement of true tooth formation. Some extinct batrachia were of large size ; the Laby- rinthodon, the structure of whose teeth has already been described (p. 86), was furnished with a marginal row of teeth in the upper jaw, of which some few were of larger size and greater length than the others. In the lower jaw, the teeth, which are similar to those of the upper, are disposed in some sense in an incomplete double row, the series of smaller teeth not being interrupted by the occurrence of the larger tusks, but passing in unbroken series outside them. The Laby- ririthodon was possessed also of palatine teeth. The teeth were anchylosed to slight depressions or sockets, and the successional teeth were probably developed, as in TEETH OF REPTILES. 307 the frog, at the inner side of the bases of the teeth already in position, as there are no indications of crypts within the bone. Reptilia. — In many reptiles teeth are developed for the merely temporary end of effecting an exit from the egg-shell. This purpose is sufficiently answered by the hard snout of the crocodiles and by a sort of snout developed in Chelonia, but snakes and lizards have sharp teeth, developed on the pre- maxillary bones, which are afterwards lost and these are true teeth, formed of dentine. (i.) The Chelonia, comprising the Tortoises and Turtles, have no teeth, but the margins of the jaws are sheathed in horny cases, variously shaped in accordance with the habits of the animal, being sharp and thin-edged in carnivorous and blunt and rugged in herbivorous species. It is stated by Eose that a tooth band exists, but that it does not go on to the formation of any definite tooth-germ rudiment. Some extinct forms of reptiles, the flying dragons (Ornitho- sauria), were also devoid of teeth, and had very bird-like beaks, presumably covered with horn. It is difficult to make any very general statements as to the teeth of reptiles, as so large a variety exists. Many, however, have simple conical teeth, either attached in grooves as in the great extinct fish-lizards (Ichthyosaurus), in distinct sockets as in the crocodiles, or anchylosed by their sides or by their bases to the bone, as in most lizards. Usually they are confined to the margins of the jaws, but as already mentioned, a second row of vomerine teeth sometimes exists. Less commonly, palatal teeth, flattened to form a sort of pavement to the roof of the mouth, are met with ; such are to be found in several extinct reptiles. In structure many of them consist of a fine- tubed dentine, with a coat of enamel, also of simple structure, but there are not wanting teeth of very complex structure, similar to those found in Labyrinthodon among Batrachia. The complexity of pattern arises, however, more from the existence of plici-dentines than of vascular dentines, although in some, for example the Iguanodon, there are medullary channels arranged with regularity. (ii.) In Crocodilia the teeth are confined to the margins of the jaws, where they are very formidable in size and sharpness. 20—2 308 A MANUAL OF DENTAL ANATOMY. The individual teeth are generally conical, sharply pointed, and often a little compressed from side to side, so as to possess sharp edges : but they vary much in form in different species. The teeth are lodged in distinct tubular alveolar cavities (thecodont), to the walls of which they do not become anchylosed. They are tolerably constant in number in the same species. In parts of the mouth certain teeth are developed to a greater length than those nearest to them; thus, in the Crocodile proper, the first and fourth lower and the third Fig. 155. — Jaws of the crocodile. The first, fourth, and eleventh teeth in the lower jaw, and the third and ninth in the upper, are seen to attain to a larger size than the others. and ninth upper teeth are specially large. This character is common to alligators and many true crocodiles, the former being distinguished by the first and fourth lower teeth being received into deep pits in the upper jaw.* The Garial, or slender snouted crocodile of the Ganges, has long and very narrow jaws, with slender recurved" teeth, its jaws recalling those of a dolphin, like which it feeds exclu- sively on fish and, although the creature may attain to the length of 20 feet, is not dangerous to mammals. Various crocodiles are intermediate in respect of the length and width of their jaws between the garial and the broad-nosed alligator ** It is doubtful whether these distinctions are characteristic. — Eds. TEETH OF REPTILES. 309 In structure the teeth of crocodiles consist of hard, fine- tubed dentine, with an investing cap of enamel, and in addition a coating of "cementum on their implanted portions. As already mentioned, they are implanted in tubular sockets ; new successional teeth being continually developed at the inner side of their bases, and as the latter attain to a certain size, absorption attacks the base of the older tooth, and its successor moves into the space thus gained, so that it comes to be situated vertically beneath the older tooth. In its further growth it causes yet more absorption of the older tooth, which it ulti- FiG. 156. — Transverse section of the lower jaw of a young alligator. a. Oral epithelium, b. Bone of socket, d. Dentine of old tooth. 2. Tooth next in order of succession, which is causing absorption of one side of the base of the older tooth. 3. Young tooth-germ. mately pushes out in front of it, sometimes carrying the remains of the old tooth like a cap upon its own apex when it first emerges. Each new tooth vertically succeeds its prede- cessor ; hence no additional teeth are added, but the young newly-hatched crocodile has as many teeth as a full-grown one- In the extinct Ichthyosaurus the teeth, while forming an armature not unlike that of some of the crocodiles, were not implanted in distinct sockets, but were lodged in a continuous shallow groove, with but slight indications of transverse divisions. (iii.) Lacertilia. — The scaly reptiles, comprising the lizards and the snakes, run somewhat into one another. 310 A MANUAL OF DENTAL ANATOMY. The food varies from fruit and the like to insects, eggs, and small animals. The lizards have more or less conical teeth, though in some the cone is much flattened down and rounded. Again, the cone may be complicated by serrations or vertical ridges and prominences. More rarely there are subsidiary cusps, generally set length- wise, and there are but seldom inner or outer cusps which would bring it at all into the semblance of a mammalian tooth. The teeth are generally composed of a central body of hard dentine, more or less completely invested by a cap of enamel, and they are attached to the bone by anchylosis. When the tooth is anchylosed by its outer side to an external parapet of bone, the creature is said to be FlG. 157. — Lower jaw of a lizard (Varanus gouldii). a. Foramina leading to cavities of reserve. " pleurodont," when by the end of its base it is attached to the summit of a parapet it is " acrodont." The succession of teeth in most lizards is constant, new teeth being developed at the inner side of the bases of the old teeth, which become undermined by absorption, and fall off when the successional tooth has attained to a certain stage in its development. The accompanying figure of the lower jaw of a Monitor lizard will give an idea of a dentition common in the group. The teeth are neither very large nor very numerous, there being about 30 in the jaw ; towards the front of the mouth they are a little more pointed than at the back, but the differences in this respect are not striking. At the inner side of the bases of the teeth are seen foramina which lead into the spaces in which new teeth are being developed. TEETH OF REPTILES. 311 Amongst the lizards considerable variety in the form of the teeth themselves exists, some having thin serrated edges, others being exceedingly blunt and rounded, but in the general disposition of the teeth there is considerable uniformity. The teeth of some lizards consist at their apices of ordinary osteo-dentine, with a simple central pulp cavity, but at their bases of plici-dentine with numerous subdivisions of the pulp cavity, as is seen in the Monitor lizards {Varanus, see p. 84). One Mexican lizard (Heloderma suspectwn) has the reputation of being poisonous, and has teeth which are grooved both back and front ; but it is doubtful whether its harmful powers have not been exaggerated. In Heloderma the salivary glands of the lower jaw, probably the submaxillary glands, lie close against the under side of the bone. In a dissection made by Professor Stewart upon a specimen in the College of Surgeons museum, there appear to be a number of ducts which seem to actually perforate the bone, and they emerge by a series of small holes which lie in the sulcus between the lip and the teeth, close to the necks of the teeth. In the Python the corresponding gland also has many ducts which open in a similar position, but they attain to it without any perforation of fche bone, and there is no reason to suppose that their secretion is at all poisonous. In Heloderma there is, however, no doubt that the secretion is poisonous ; indeed the bite of a specimen in the Zoological Gardens has been found to be fatal to small animals, and Dr. Weir Mitchell states that in one instance it has been known to be fatal to man. It is, however, of gentle disposition, and not disposed to bite. The teeth of the Iguana are somewhat slender, and the tops are flattened, lobed, and serrated. They are pleurodont, and the successional tooth moves sideways into the pulp cavity of its successor. Although most reptiles have the teeth subequal in size, in the Agamoid group of lizards it is common to find tusk-like teeth in the canine region, and molariform teeth at the back. In the common blind-worm, which is zoologically a lizard which has lost its legs, the teeth are small and conical, like those of the snakes. The Chamseleons, which differ markedly in many respects from other lizards, are peculiar in having no replacement of 312 A MANUAL OF DENTAL ANATOMY. their teeth, though, according to Kose (12), new teeth are constantly throughout life being added to the back of the series. The absence of any succession of functional teeth is shared by another very peculiar lizard found in New Zealand. Sphenodon, the sole survivor of a very ancient and primi- tive group of reptiles, and it and its allies are now separated from the lizards and elevated into a separate order. Its very unusual dental armature was described by Gunther(3), and has been subsequently re-investigated by Spencer Harrison (4) and by Maggs and Payne (7). The premaxillary bones and the corresponding regions of the mandible each carry three teeth, which become fixed at their bases and are anchylosed to the bone. By wear they become knife-edged with a single notch, so that it used to be said that there were only two teeth in this situation, and their form recalls that of the gnawing incisors of rodents. There were originally fourteen other teeth on each side which are acrodont, and are embedded for a short distance in the jaw. An inner row is carried upon the palatine bones, but the vomer is toothless, though Harrison describes vestigial teeth there also. There is no functional replacement of teeth, so that when the teeth are worn down, as happens in old specimens, the actual sharp margins of the bone come into play as masticating organs. It occurred to the author as possible that these hard polished surfaces might have a coating of dentine upon them, but a microscopic examination which, by the kindness of Dr. Gunther, he was enabled to make, proved that the dense ivory-like surface was true bone, and had no relation to dental structures. With the exception of some fish, and there in a far more limited degree (cf. p. 297), there are few instances of actual bone being exposed and used for masticating purposes, though Harrison states that this happens in the true lizard, Uromastyx. The tooth-band has been said to be continued beyond the germs of the functional teeth, although no successional teeth are formed, and Harrison has since described the existence of calcified embryonic teeth which never cut the gum. Thus at an early age its dentition conforms more closely ) TEETH OF REPTILES. 313 to that of its class, and it may be mentioned that its extinct allies do not all share its peculiarities, some having strongly developed teeth, as well as teeth on the vomer. This animal presents an antero-posterior elongation of the condyle of the lower jaw, a character unique among reptiles. (iv.) The Ophidia, or snakes, were formerly divided into two groups, the venomous and non-venomous snakes, but this division has been found to be unsatisfactory. They are, there- fore, now usually grouped, with the exception of a few less important families allied to the first, into the Pythons, the Colubrine snakes, and the Viperine snakes. Another method of grouping the snakes is based entirely upon their teeth : 1. Opoterodonta. Teeth in one jaw only. Blind snakes. 2. Aglypha. Teeth in both jaws. None grooved. 3. Opisthoglypha. Posterior maxillary teeth grooved. (4. Proteroglypha. Anterior maxillary teeth grooved. 15. Solenoglypha. Anterior maxillary tooth isolated and perforated by a canal. (These last two are often grouped together under the name Proteroglypha.) In the small family of blind snakes the eye is rudimentary. Teeth may be present in the upper or in the lower jaw but not in both, and the mouth is incapable of wide opening. They are subterranean in their habits, and feed on larvae, ants, &c. Of other snakes, the Pythons are the least specialised in their characters, and may be described first. They have one row of teeth in the lower jaw, and two rows in the upper jaw. In the latter the maxillary bones carry one row, while a parallel internal row is supported upon the palatine and pterygoid bones. The teeth are all strongly recurved, and firmly anchylosed to the bone. They consist of a central body of non -vascular dentine, coated by a very thin layer of enamel (there is not, as is generally supposed, any layer of cementum, the enamel having been erroneously supposed to be such). The two halves of the lower jaw are connected at the symphysis by an exceedingly elastic ligament. Their articu- lation with the temporal bone is through the medium of an elongated movable quadrate bone, and is such as to allow of their being widely separated from the skull and from one 314 A MANUAL OF DENTAL ANATOMY. another, which permits of the dilatation of the mouth rendered necessary by the large size of the creatures which a snake swallows. The teeth of the snake are simply available for seizing prey and retaining it, as snakes invariably swallow their prey whole, and in no sense masticate it. As the object to be swallowed is often so disproportionately large as to make the process of deglutition appear an impossi- bility, the mouth and pharynx have to undergo great dilatation. Fig. 158. — One half of the skull of a python (without the lower jaw) seen from below, a. Intermaxillary bone. b. Maxillary bone, carrying the outer row of teeth. 2. — Transverse section of tooih-sac of poison fang of viper, prior to the complete closure of the poison tube by the meeting together of the two cornua of the dentine, a. Dental pulp. d. Dentine, e. Ameloblasts of cellular tissue which ultimately disappear. previous movements reversed, and the parts return to their first position." The poison fang is a long, pointed, slightly recurved tooth, traversed by a canal which commences on its front surface, near to the bone, and terminates also on its front surface, a little distance short of its point ; in the figure a bristle has been passed through it, and shows the points where it com- mences and terminates (Fig. 161). This tube conveys the poison into the puncture, its upper orifice being in close rela- tion with the end of the duct of the poison gland. As may have been gathered from what has already been said in colubrine poisonous snakes the canal is more or less TEETH OF EEPTILES. 321 visible on the exterior of the tooth, where an apparent fissure marks the point where the two lips of the groove have met. Thus the poison fang- of Hydrophis, although through a part of its length the canal is quite closed in, has a very marked line along its anterior surface, and in section it looks much as would the dentine in Fig. 162, if the two cornua had their rounded extremities brought together into actual contact, without, however, their rounded outlines being altered. By imagining an anterior groove to be deepened, and finally converted into a canal by its edges growing up and meeting over it, we shall have a fair conception of the nature of the tube in a poison fang, which is thus really outside the tooth. It might thus, at least in its canaliculated part, be regarded as a thin flattened tooth bent round so as to form a tube. Just as there are gradations in the armature of the maxillary bone, which link together the extreme forms of the harmless Python and the venomous Kattlesnake, so there are gradations in the form of the grooving and the completeness of the canal between the colubrine and viperine poisonous snakes. But in the poison fang of a viperine snake the lips of the groove are flattened and fitted to one another, so that not a vestige of the join can be seen upon the smooth exterior of the tooth. In the following figure (Fig. 163) the pulp cavity is seen to be a thin flattened chamber partly surrounding the tube formed for the conveyance of the poison. The poison-fang is exceedingly sharp, its point being continued some little distance beyond the place where the poison canal opens on the front of the tooth. This disposition of parts has been copied in the points of syringes for making subcutaneous injections. The dentine is continued down to a very fine point, and it is cased by an exceedingly thin layer of enamel, not much more than g of an inch in thickness in our common English viper ; thus the utmost sharpness is secured, without loss of elasticity, which would have ensued had its point been made up of brittle enamel only. Enamel covers the whole exterior of the tooth, but does not extend into the poison canal in the viperine snakes, but in Hydrophis probably this is the case. As the point is simple, the tooth-germ of a poison-fang only becomes distinguishable from that of any other ophidian tooth d.a. 21 322 A MANUAL OF DENTAL ANATOMY. after the tip has been formed, when a groove appears on its side. (See 8 and 9 in Fig. 164.) The large size of the canal in the viperine snakes enables them to send a jet of poison through it, whereas only a drop can be forced through the small canal of a cobra. It being the habit of poisonous snakes to make use of these weapons to kill their prey, which they consequently do not swallow alive, it would obviously subject them to no little inconvenience to be without these weapons for any considerable length of time, while from their habit of striking living prey the long fangs must be very liable to be broken off by the Fig. 163. — Transverse section of the poison fang of a rattlesnake, a. Pulp cavity, d. Dentine. jumping away of the creature struck, to say nothing of the great force with which the blow is given. In the most typical (viperine) poisonous snakes the succes- sion of teeth is conducted upon a plan which is unique, and which is excellently adapted to save loss of time in the replace- ment of a lost poison fang. Upon the movable maxillary bones there is space enough for two poison fangs, side by side ; only one, however, is fully anchylosed to the bone at a time, and occupies a place to the extreme right or extreme left of the bone, as the case may be, leaving vacant space for another by its side. When the tooth in use falls, it will be succeeded by a tooth upon the vacant spot by its side, not upon the spot upon which TEETH OF REPTILES. 323 itself stood, so that the places on the right and the left of the bone are occupied alternately by the tooth in use. Thus, in Fig. 161, the poison fang of the snake's right side is seen occupying a position on the extreme outside of the maxillary bone, while its left poison fang is fixed on the inside of the maxillary bone. The upper boundary of Fig. 164 is formed by the flap of mucous membrane which covers in the poison fang when at rest. Nos. 1 and 2 lie in the pouch formed by it, the section Fig. 164. — Transverse sections of the reserve poison fangs of a viper. 1. Tooth at present in use, in its recumbent position ; were it erect it would be withdrawn from view, or else seen in longitudinal section. 2. Tooth which will next succeed to No. 1. 3, 4, 5, etc. Tooth-sacs numbered in the order in which they will succeed. happening to be taken from a specimen in which the tooth was about to be changed. In most specimens one tooth only, the tooth actually in use, is seen in this position. A flap hanging freely across this space serves apparently to keep teeth of the one series from getting over to the other side, and probably serves to hold in place the reserve tooth when the older tooth is erected for biting. The reserve poison fangs, as many as ten in number in the -rattlesnake, are likewise arranged in two parallel series, in which the teeth exist in pairs of almost equal age. The tooth in use is thus derived alternately from the one and the other 21—2 324 A MANUAL OF DENTAL ANATOMY. series, as is indicated by the consecutive numbers of the figure, a septum of connective tissue keeping the two series of teeth distinct from one another. The teeth being arranged in pairs of almost equal age, suggest that the succession is both rapid and regular. All the reserve teeth lie recumbent in and behind the sheath of mucous membrane which covers in the functional tooth. This arrangement of the successional teeth in a paired series does not exist in the Cobra, in which the successional teeth form but a single series. Perhaps this may serve to explain the preference of the snake charmers for the Cobra, which would probably take longer to replace a removed poison fang than would a viperine snake.* But in the colubrine venomous snakes (cf. p. 316) the successional poison fang sometimes makes its way to a spot to the side of its predecessor, so that there may possibly be no loss of time ; and notwithstanding that they are in a measure transitional forms between the harmless and the viperine snakes, some of them are most virulently poisonous and deadly in their bite.t This arrangement of two distinct chains of younger develop- ing organs, all destined to keep the creature always supplied with one organ in a state of efficiency, is, so far as I know, without parallel. A clue to the nature of the arrangement seems, however, to be afforded by some of the colubrine poisonous sea snakes (cf. p. 317), in which it was found by West that in some forms which retain more than one large tooth upon the maxilla the second tooth had moved forwards till it stood nearly or quite at the side of the first, and something of the same kind was noted in the Cobra. It would seem, then, that the viperine snake has really retained two of its maxillary teeth, although, their develop- * An inquiry inserted in an Indian newspaper elicited the following answer : — " I have frequently seen snake-charmers exhibit snakes of the family Viperidce, chiefly the Baboia russelli and Ecliis carinata. I have also been told by some snake-charmers that they considered the Baboia even more poisonous than the cobra ; and judging from the cautious way in which they handle these snakes — never lifting them off the ground without first putting a stick on their necks to hold them down — I feel pretty sure they all consider the vipers more dangerous than the cobras." f A more detailed account of the succession of poison fangs has been given in the " Philos. Trans.," 1876, Part i. TEETH OF EEPTILES. 325 ment alternating, it has only one in use at a time ; and that the second tooth and its chain of successors have come to be arranged in pairs with the first tooth and its successors, thus explaining an otherwise very anomalous arrangement. Like other ophidian teeth, the poison fangs become anchylosed to the bone which carries them, their secure fixation being aided by the base of the tooth being fluted, as well as by a sort of buttress work of new bone, being thrown out to secure each new poison fang as it comes into place. L. Kathariner (6) has investigated the development of poison fangs in the viper, and holds that the two series arise from but one tooth band, a view also held by Eose. If this be true, it renders less probable the suggestion made in the text (p. 324), that the functional teeth represent not one, but really two of the once numerous maxillary teeth. However, Voerckel (l3) holds that there are two separate tooth bands, a conclusion which seems d priori much more probable. Kathariner gives a valuable description of the means by which each new tooth comes into close relation with the poison ducts, and holds, amongst other points, that the originally solid epithelial rudiment of the poison ducts sends extensions down to each series of tooth germs, and that the successional teeth travel along this opened-up track to their places. He denies the existence of enamel on these teeth, affirms that of an enamel cuticle, and states that there is not only cementum (which term he prefers to the author's "bone of attachment"), but also a modified dentine, which he styles fibro -dentine intervening between the ordinary dentine and the cementum. Dinosauria. — Some of these huge extinct creatures, which flourished during the secondary period, when the mammals were but feebly represented, attained a length of sixty feet or thereabouts, though there were also some small forms. They present affinities with the crocodiles and were also in some respects bird-like, the great Iguanodons, of which some fine examples are to be seen in the Brussels Museum, sitting up on their hind legs and tails, and having but short front legs. These Dinosaurs had a distinct chin bone and a contour of the front of the jaws more or less sharp and beak-shaped, and devoid of teeth. The teeth were not implanted in distinct sockets, and in many of the group had expanded and flattened crowns like those of the Iguana, whence the name Iguanodon, but it must not be supposed that they therefore had any particular affinity with that lizard. In them the outer alveolar wall 326 A MANUAL OF DENTAL ANATOMY. was considerably higher than the inner, and the transverse septa not very complete. The roots of the teeth were more or less perfectly cylindrical, and the enamelled crowns com- pressed and expanded, with trenchant edges. The tooth of the Iguanodon will serve as a fair example of a Dinosaurian tooth. The crown is greatly expanded, and presents anterior and posterior sharp notched margins ; the enamel is laid over the outer surface of upper teeth, and the inner of lower teeth. The enamelled surface is ridged, so that as it wears down a notched edge is maintained. Moreover the maintenance of a sharp edge is further secured by the dentine on the enamelled side of the crown being of the hard unvascular variety, that on the inner being vaso-dentine and therefore softer. The remnant of the pulp ossifies, and comes into use, as these teeth remained at work until worn quite to a flat surface. The root portion was smooth, round, and curved. Professor Marsh (8) has described and figured a peculiar Dinosaurian dentition, in a reptile to which he gives the name of Stegosaurus. The teeth are slightly compressed transversely and are covered with a thin enamel ; the roots are long and slender, implanted weakly in separate sockets. But at the inner side of the root of the tooth in use were no less than five successional teeth, in graduated stages of development, ready to ultimately take its place ; so large a number of successional teeth has not hitherto been met with in a Dinosaur. FIG. 165.— Tooth of Iguanodon. (After Owen.) TEETH OF REPTILES. 327 Anomodont Reptiles. — This reptilian order, established many years ago by the late Sir Richard Owen, contains a considerable number of forms which, so far as their teeth are concerned, are somewhat divergent, the order being formed upon various characters of the skull and skeleton. The original fossils came from South Africa, whence a large number have since been obtained, but others have been found in Europe and America, also in deposits of the Triassic and Permian periods. They have been investigated by the late Professor Huxley, Professor Cope, and many others, but the most complete account is to be found in a series of elaborate papers by Professor Seeley, published in the " Philos. Trans.," 1888—1895.* By the last-named writer they are divided into two sub- orders, the Therochelonia and the Therosuchia, each of which is of the greatest interest to the odontologist, the former because they present such a highly aberrant dentition, and the latter because of the close general resemblance which the teeth in some members of the group present to those of mammals. Of the Therochelonia, so styled on account of resemblances in the palate to that of Chelonia, the genus Dicynodon of Owen is best known. In it the only teeth known are two exceedingly long and strong tusks which project down from the upper jaw in the position where canine teeth usually occur. The extent to which the tusks are developed varies considerably in different species, and even in the two sexes of the same species. One specimen at the British Museum of what seems to be Dicynodon microtrema has a number of small molars. It is not known how far some of the specimens with small molars, such as Prodicynodon, Dicelurodon or Pristerodon, may possibly be young Dicynodons (2). The margin of the toothless mandible was rather sharp, just as in turtles and tortoises, and was, perhaps, covered with a horny bill, as may also have been the maxilla. Seeley points out that the tusk is not truly a canine because it is implanted some distance behind the premaxillary suture, * Our knowledge of the fossil reptiles of South Africa has been greatly extended during the past few years by the admirable researches of Dr. R. Broom.— H. W. M. T. 328 A MANUAL OF DENTAL ANATOMY. but this is not of much importance since, for reasons given on a succeeding page, the canine can no longer be regarded as a morphological entity, and we may hence apply the term to any caniniform tooth ; and, as it is implanted in a socket considerably produced, it may easily have commenced its development close to the suture. The other group, the Therosuchia, present dentitions very varied in character, some (Procolophon) have conical, cylindrical teeth of sub-equal size, with expanded bases in close union with the jaw, and teeth upon the palate ; the teeth of the Fig. 166.— Dentition of Cynognathus (after Seeley), with single upper tooth on large scale ; quarter natural size. upper and lower jaws interdigitate, and all the usual characters of a reptilian dentition are present. But the main interest lies in the sub-group, the Theriodontia, where for the first time we meet with teeth which may be divided into incisors, canines, and molars, and in which the general appearance of the dentition is distinctly mammalian. Nevertheless the composite lower jaw remains, in the dis- tinctness of its component bones, thoroughly reptilian, and the molars show in different forms many gradations between the simple reptilian cones (Mlurosaurus) and the tuberculated many-cusped teeth {Diademodon) . TEETH OF REPTILES. 329 A general idea of these dentitions may be gained from the genus Cynognaihus here figured. The incisors stand in a rounded curve, and have slightly serrated margins ; there are four in each of the pre-maxillse, and in the lower jaw there are probably three on each side, the outermost being the largest, as in Carnivora. There is a diastema in front of the large upper caniniform tooth, while the corresponding tooth in the lower jaw passes up inside it, being received into a deep fossa. Behind this tooth follow nine others which, on account of the greater simplicity of the first five, may be divided into five pre-molars and four molars. The molars have a principal cusp and several subsidiary cusps. The enamel terminates in a distinct margin, and there is a slight hint of a division into two roots ; there are, how- Fig. 167.— M3 of Diademodon x 1. (After Seeley.) ever, no internal cusps, and the upper and lower molars work upon one another with a shearing action. A point of great interest is that there is an indication of a cingulum, which, in other forms, e.g., Nythosaurus, is well marked. The advocates of the tritubercular theory place the cingulum in a very unimportant position, explaining the genesis of tooth cusps in quite other ways, whereas those who do not accept this theory in its entirety attribute to the cingulum a much more important role as the source of additional cusps. If, then, the Theriodonts lie upon, or close to, the direct line of mammalian ancestry, as most zoologists consider that they do, the appearance of the cingulum at so early a period seems significant, for here, according to the one school, are additional cusps in posse. In number the teeth vary considerably ; some had as many as six incisors on each side of the upper jaw (Pristerognatlms), 330 A MANUAL OF DENTAL ANATOMY. while others (Deuterosaurus) had the molars reduced to one on each side. Two-rooted teeth, not otherwise known amongst Eeptilia, appear in Tritylodon. But the molars in some attain to a much higher degree of differentiation, so that Seeley writes of Diademodon that " if its teeth had occurred isolated it would have been legitimate to have referred them to mammals." The molar teeth are of quadrate form, with two or three cusps on the anterior border, one tubercle on the anterior and several on the posterior border ; there is also a tubercle in the middle of the crown. They are usually single-rooted, and often show much attrition. The pre-molars were small, the canine large and fluted ; the incisors are unknown. The pre-molars were generally small, circular, and tuberculate, but in some allied forms they were laterally compressed. Some of these reptiles attained to considerable size ; thus, Sir Kichard Owen (10) described, under the name of Cynodraco, one as large as a lion, which had eight incisors in the lower jaw, of which the first is the smallest, and a canine of moderate size. The upper incisors are not known, but there were a pair of upper canines of such size that they extended down along the outside of a flattened portion of the lower jaw, like the canine teeth of Machairodus and those of Dinoceras. This protection of an especially long upper tooth by a corresponding downgrowth of the lower jaw is by no means an unusual provision of nature. Besides being met with in the instances cited, it is seen iu Tinoceras, another of the Dinocerata, and in Chauliodus, a deep-sea fish dredged up by the " Challenger." It is, however, not a universal structure, as it is quite absent in the musk deer. The hinder margins of these canines were trenchant and finely serrated. The molar teeth are not known. In the Anomodontia there is no evidence of a replacement of the canine, but in genera with numerous small molars, e.g., Dioelurodon, Prodicynodon and others, there is evidence of an indefinite replacement of the molars. Quite recently (2b) Broom has discovered evidence of a mammal-like dental succession in the Cynodont reptiles, while in the Therocephalia dental replacement has long been known in the incisors and canines. A genus to which the name Anumodon has been given had TEETH OF KEPTILES. 331 nothing but two tusks in the upper jaw, the lower being wholly edentulous, and the Placodontia had incisor-like teeth, in front, with rounded, flattish teeth at the back, the palate being furnished with similar teeth, quite unlike those of any other reptile. It has been mentioned that the lower jaw consists of all the usual components of the reptilian jaw, but the dentary bone forms so large a proportion of the whole, that it is suggested that this is the chief element in the mammalian jaw. The Pterosauria, or flying reptiles, have, since the dis- covery of toothed birds, become of special interest to the odontologist. The wings were stretched membranes, like those of a bat, and the measurement across their tips in some of the largest must have been twenty-five feet ; but most of those known were much smaller, from ten to fifteen inches in total length of body. In the Pterodactyls the jaws are furnished with long, slender, sharp teeth in their whole length ; but in Bamphorhynchus the anterior extremities of the jaws are without teeth, and it has been conjectured that these portions were sheathed in horny beaks. And Professor Marsh (8) has discovered, in the same forma- tion in which he found the toothed birds, several species of Pterodactyls wholly without teeth, for which the generic name Pteranodon is proposed. Its jaws, which are more like those of birds than those of any known reptile, show no traces of teeth, and the pre- maxillaries seem to have been encased in a horny covering. THE TEETH OF BIRDS. Prior to the discovery by Marsh, in 1870, of the remains of birds with teeth in the cretaceous formations of Western Kansas, little was with certainty known about the existence of teeth in any bird, although one or two fossils, leading to the suspicion that birds might have possessed teeth, were known. The state of knowledge up to that time has been clearly summarised by Woodward (15), to this effect : that it had been long supposed that no examples of teeth were to be met with amongst the birds, although some, such as the Merganser, have the margins of the bill serrated, so that the 332 A MANUAL OF DENTAL ANATOMY. functions of teeth are discharged by this horny armature of the jaws. The point of the bill is hooked, and for a little way behind the point is smooth, the corresponding part of the lower bill being also smooth. The lower bill is furnished with one row of pointed horny teeth, which fit in between two rows on the upper bill, the latter raking backwards. They are apparently horny, near the apices there are some fine canals, and each is seated upon a vascular papilla ; their bases coalesce, so that there is a continuous horny sheath to the jaw. It' has been said that bony prominences correspond to these so-called teeth, but in the specimen examined by the author no trace of such a thing could be seen. In the fossil bird described by Professor Owen, from the London clay, under the name of Odontopteryx toliapicus, the form of the bill is not known, but the margins of the jaws are furnished with strong bony prominences. Geoffroy St. Hilaire had described a series of vascular pulps as existing on the margin of the jaw of parroquets just about to be hatched, which, though destined to form a horny bill, and not to be calcified into teeth, yet strikingly recall dental pulps. Then there is also the famous fossil Archceopteryx, an anomalous, oolitic bird, with a long and jointed tail, which possessed teeth. In the first specimen described there was a flaw in the evidence, inasmuch as the toothed jaw was not in situ, and so possibly might have belonged to another creature ; but in the specimen now at Berlin there can be no question. In successive expeditions, conducted under great difficulties owing to the extremes of heat and cold, and to the hostility of the Indians, the remains of no fewer than one hundred and fifty different individuals referable to the sub-class Odon- toenithes have been obtained by Professor Marsh ; they are classified under nine genera and twenty species. They are referable to two widely different types, one group consisting of comparatively small birds, with great power of flight, and having their teeth implanted in distinct sockets (Odontotomy, illustrated by the genus Ichthyornis as a type), the other group consisting of very large swimming birds without wings, and having teeth in grooves (Odontolcce, type genus Hesperornis). THE TEETH OF BIRDS. 333 In Ichthyomis the teeth were about twenty-one in number in each ramus, all sharp and pointed, and recurved ; the crowns were coated with enamel, and the front and back edges sharp but not serrated. They are implanted in distinct though shallow sockets, and the maxillary teeth are a little larger than those opposing Fig-. 168. — Mandible of Ichthyomis. (After Professor Marsh.) A. Side view, showing the teeth in situ. B. View of upper surface, showing the sockets in which the teeth were implanted. them ; the pre-maxillaries were probably edentulous, and perhaps covered with a horny bill. In the lower jaw the largest teeth occur about the middle of the ramus, those at its posterior end being materially smaller ; and the sockets are deeper and stronger than in the upper jaw. The succession takes place vertically, as in Crocodiles and Dinosaurs. The genus Hesperomis, probably diving birds, includes 1* M.S. A B 334 A MANUAL OF DENTAL ANATOMY. species six feet in length. As has already been mentioned, the teeth are not implanted in distinct sockets, but lie in a continuous groove like those of Ichthyosaurus. Slight pro- jections from the lateral walls indicate a partitioning off into sockets, but nothing more than this is attained, and after the perishing of the soft parts the teeth were easily displaced, and had often fallen out of the jaws. The premaxillary bone is edentulous, but the teeth extend quite to the anterior extremity of the lower jaw. In one specimen there are fourteen sockets in the maxillary bone, and thirty-three in the corresponding lower ramus. The successional tooth-germs were formed at the side of the base of the old ones, and, causing absorption of the old roots, Fig. 169.— (After Professor Marsh.) A. Hesperovnis regain, with succes- sional tooth in an excavation at its base ; enlarged eight diameters. B. Tooth of Mosasaurus princeps, half natural size, seen nearly in profile. migrated into the excavations so formed, grew large, and ultimately expelled their predecessors, as is seen in the accompanying figure. In structure these teeth consist of hard dentine, invested with a rather thin layer of enamel, and having a large axial pulp-cavity. The basal portion of the roots consists of osteo- dentine. The outer side of the crown is nearly flat, the inner strongly convex : the junction of these surfaces is marked by a sharp ridge, not serrated ; and in addition the teeth are curved backwards, so that the anterior surface is convex and the posterior concave. n form the teeth of Hesjjeromis present a close resem- blance to those of Mosasaurus, a great extinct lizard. A B THE TEETH OE BIRDS. 335 Indeed, as Marsh observes, " in all their main features the teeth of Hesperomis are essentially reptilian, and no anatomist would hesitate to refer them to that class, had they been found alone. Combined with the other reptilian characters of Hesperomis . . . they clearly indicate a genetic connection with that group." In the dentine, contour lines are abundant ; the enamel is so dense as to appear structureless, and there is no coronal cementum. The foregoing account is condensed from the magnificent volume published by the United States Government Geo- logical Exploration. (" Odontornithes : a Monograph," etc., by 0. C. Marsh, Professor of Palaeontology, Yale College.) With these notable exceptions, the jaws of all known birds are toothless, the horny cases forming their beaks taking the places and fulfilling the functions of teeth. It would therefore seem possible that the ancestral birds all had true teeth, and it is possible that a sufficiently extended search might reveal rudimentary teeth surviving beneath the functional horny bill, or even possibly above it, like those of Ornithorhynchus. Indeed Eose has found that in Sterna the tooth band exists, though no rudiments are to be found of more differentiated tooth-germs ; though, judging from his figures, some might doubt the propriety of identifying so small an inflection of epithelium as being with any certainty a rudimentary tooth-band. Eefeeences. (*) Alcock, A. and Eogers, L. "On the Toxic Properties of the Saliva of certain Non-poisonous Colubrines." Proc. Eoy. Soc, Lond., 1902, vol. lxx., pp. 446 — 454. (2) Broom, B. (a) " On Some New Genera and Species of Dicynodont Eeptiles, with Notes on a few others." Bull. Amer. Mus. Nat. Hist., 1913 (Sept. 13th), vol. xxxii., pp. 441 — 457. (b) " On Evidence of a Mammal-like Dental Succession in the Cynodont Eeptiles." Bull. Amer. Mus. Nat. Hist., 1913 (Sept. 23rd), vol. xxxii., pp. 465 —468. (3) Gunthee, A. "Contributions to the Anatomy of Hatter ia" (Bhynchocephalus, Owen). Phil. Trans. Eoy. Soc, Lond., 1867, vol. clvii., pp. 595—629. 336 A MANUAL OF DENTAL ANATOMY. (4) Harrison, H. Spencer, (a) " Hatteria punctata, its Dentitions and its Incubation Period." Anatom. Anzeiger, 1901-2, Bd. xx., pp. 145—158. (b) "The Development and Succession of Teeth in Hatteria punctata." Quart. Journ. Micr. Sci., Lond., 1901, vol. xliv., pp. 161—213. (5) Huxley, T. H. "The Anatomy of Vertebrated Animals." London, 1880. (6) Katheriner, L. " Ueber Bildung und Ersatz der Giftzahne bei Giftschlangen." Zoolog. Jahrb. Abth. fiir Anatomie, 1897, Bd. x., pp. 55 — 92. (7) Maggs, W. A. and Payne, J. L. "Notes on the Dentition of Sphenodon." Trans. Odontol. Soc, Gt. Brit., 1901-2, vol. xxxiv., pp. 9—19. (8) Marsh, 0. C. American Journ. of Science, 1876-1880. Numerous Papers. (9) Nicholson, E. "Indian Snakes." Madras, 1870, 2nd ed., 1874. (10) Owen, R. "Evidences of Theriodonts in Permian Deposits else- where than in South Africa." Quart. Journ. Geolog. Soc, Lond., 1876, vol. xxxii., pp. 352 — 363. (u) Rogers, L. "On the Physiological Action of the Poison of the ffydrophidw." Proc. Eoy. Soc, Lond., 1903, vol. lxxii., pp. 305—319. (12) Rose, 0. " Ueber die erste Anlage der Zahnleiste beim Menschen." Anatom. Anzeiger, 1893, Bd. viii., pp. 29 — 32. (13) Voerckel. " Untersuchungen der Giftzahne von Pelias" Leipzig, 1895. (u) West, G. S. (a) " On the Buccal Glands and Teeth of certain Poisonous Snakes." Proc. Zoolog. Soc, Lond., 1895, pp. 812—826. (&) " On the Histology of the Salivary, Buccal and Harderian Glands of the Colubridw, with notes on their Tooth-succession and the Relationships of the Poison -duct." Journ. (Zoology) Linnean Soc, Lond., 1898, vol. xxvi., pp. 517 — 526. (15) Woodward, H. " On Birds with Teeth." Popular Science Review, 1875, vol. xv., pp. 337 — 350. CHAPTER XII. THE HOMOLOGIES OF THE MAMMALIAN TEETH. Although it is generally believed that the ancestors of the mammals are to be sought amongst the reptiles, and some of these, as we have seen (p. 328), had attained to a differentia- tion between the teeth in different parts of the mouth, such differentiation is not universal amongst the mammals. There are some mammalian orders in which the teeth are alike in all parts of the mouth, and are of simple form, usually simple cones ; it also happens that this simplicity of form frequently goes hand in hand with the absence of any Fig. 170. — Jaws of Common Dolphin (Homodont). functional succession in the teeth. Hence Homodont and Monophyodont * are terms often applicable to the same creatures. Of Homodont mammals, i.e., those which have similar teeth in all parts of the jaws, certain of the Cetacea afford good examples. Thus the Dolphin (Fig. 170) has a great number of sharp conical teeth, those of the lower jaw alternating with and fitting in between those of the upper jaw, and it is by some believed that the teeth of higher mammals are derived from some such type of dentition, though not from these particular ones, for the Cetacea do not stand in the line of descent, but have retrograded so far as their teeth are concerned. * The term Monophyodont was originally introduced by Owen to signify animals with a single set of teeth. At that time dried skulls only had been examined. With the application of histological methods it was found that in most cases, hitherto regarded as truly Monophyodont, f imctionless representatives of other dentitions were to be found. Strictly speaking, therefore, there are but very few Monophyodont mammals. The term may, however, be conveniently retained in the more limited sense of animals with a single functional dentition, such, for example, as the toothed whales. — H. W. M. T. D.A. 22 338 A MANUAL OF DENTAL ANATOMY. But it is far more common for the teeth in different regions of the mouth to present material differences of form, and such dentitions are termed Heterodont. Those animals having a Heterodont dentition are possessed of two sets of teeth, a milk and a permanent, i.e., Diphyodont. A superficial survey of the teeth of those mammals which possess two sets of teeth (Heterodont diphyodonts) will indicate that, notwithstanding the apparent anomalies brought about by adaptive modifications, a correspondence exists between the several teeth of different animals. That is to say, we can generally identify incisors, premolars, and molars. Nay, more, when an animal has less than the full typical number of a particular class of teeth, we can ordinarily say with some degree of certainty which of them it is that are absent. Teeth are divided into incisors, canines, premolars, and molars, but these classes, though convenient, do not all admit of quite satisfactory definition. Incisors are defined as teeth implanted in the premaxillary bone, a definition which has the merit of being precise ; while on the whole there is a certain resemblance running through incisor teeth in most mammals. But the definition of lower incisors as being the corresponding teeth in the lower jaw is a good deal less satis- factory, because they are not situated upon any distinct bone. It has even been denied that there can be a true homology between a maxillary and a mandibular tooth. Incisors usually have but a single root, but in Galeopithecus (erroneously called a flying Lemur) the outermost of the upper incisors are implanted by two roots, as is also the canine, which does not differ in any marked degree from the incisors. Molars are teeth at the back of the mouth, which have no functional milk predecessors. They are generally subservient to grinding the food. Premolars are teeth situated, in the case of the upper jaw, in the maxillary bone and in front of the molars, usually differing from them by being smaller and more simple in form, and, in typical Diphyodonts, by having displaced deciduous predecessors.* But they are not always simpler in * The classical definition of premolars and molars as being cheek teeth with or without predecessors respectively entirely fails when non-erupted teeth are taken into account. Numerous investigators have found evidences of vestigial teeth to the outer side of the so-called molars. Though difficult to eradicate THE HOMOLOGIES OF MAMMALIAN TEETH. 339 form, nor smaller (e.g., in the horse), nor do they always dis- place deciduous predecessors (e.g., they do not all do so in the Marsupials, and -the first premolar but rarely displaces a deciduous predecessor in other Mammalia), so that this definition is not absolutely precise. Molars and premolars are often grouped together under the term " cheek teeth," and one palaeontologist, Ameghino (x), abandoned the distinction between premolars and molars, and has numbered the teeth behind the canine continuously from one to seven. Any objection that can be raised to the name of premolar on the score of a short strictly logical definition being impos- sible applies with tenfold force to the canines. (4) The nearest approach to a good definition is that which describes the canine as the next tooth behind the premaxillo- maxillary suture, provided it be not far behind it ; and the lower canine as the tooth which closes in front of the upper canine. A great deal of confusion has arisen out of the twofold sense in which the word " canine " is used. If it were always applied to the first tooth in the maxilla of the typical mammalian dentition, quite irrespective of its size, &c, and to the lower tooth closing in front of it, no objection to its employment could be made, inasmuch as it would designate truly homologous organs. But it so happens that the tooth in question is, in a very large number of familiar animals, developed to a large size and sharply pointed for use as a weapon, and so with the word canine there comes to be associated a teleological idea, and we come to be dissatisfied with calling the first maxillary tooth " canine," when there is some other tooth which is doing its special sort of work. On the other hand, if we are to leave out of court all con- siderations as to size, purpose to which it is to be applied, and so forth, there is nothing left to make it deserving of a name distinguishing it from the four premolar teeth behind it. So we must be content with some such statement as the following : — A very large number of animals, notably the Carnivora, the old nomenclature, the term " cheek teeth " is undoubtedly preferable. Though it is possible in most cases to distinguish the premolars from the molars by their crown-patterns, it is impossible in other cases to do so. — H. W. M. T. 22—2 340 A MANUAL OF DENTAL ANATOMY. have one tooth, situated a little way from the front of the mouth, developed to an unusual length and sharply pointed for use as a weapon. The tooth which has undergone this adaptive modification is usually the first which lies in the maxillary bone, in fact, the foremost of the premolar series ; but it occasionally happens that it is some other tooth which has undergone this modification. When the term canine is used it should generally mean a tooth so modified, and generally, but not always, should be alluding to the same tooth, i.e., to the tooth which in the typical mammalian dentition comes next behind the outermost incisor — the first of the premolars, if we count five premolars instead of four. It would obviously be very inconvenient to abolish the term canine, and some naturalists will continue to call the tooth Fig. 171. — Side view of the dentition of Anoplotherium. (After Owen.) v — premolars, m = molar teeth. which fulfils the function " canine," whilst others will apply the term to the homologous tooth, whatever be its shape. But it should be borne in mind that its significance is merely equivalent to " caniniform premolar," and thus that in describing the dog's dentition we should be accurate were we to say that it has five premolars, of which the first is canini- form. To those who accept the doctrine of evolution it is not needful to say more, as it is hardly possible to resist the conclusion that the teeth of the parent forms were, like those of the present monophyodonts, not much differentiated from one another, as is exemplified by the actual ungulate Anoplo- therium. Then, as animals diverged and became modified in accordance with their requirements, their teeth would become so far differentiated that they would admit of being classified. Thus the Carnivora would have attained to a stage of differen- tiation in which the canine is functionally certainly deserving THE HOMOLOGIES OF MAMMALIAN TEETH. 341 of a distinction, whereas along other lines of descent, differen- tiation having not proceeded so far, or having proceeded in a somewhat different direction, it would not merit a distinctive appellation. It may be desirable to point out a few instances of the difficulties to which those anatomists are committed who call some tooth a " canine' ' in every case where a tooth is situated in the maxillary bone, close behind the suture which connects it with the inter-maxillary bone, whether that or any other tooth be large and pointed, " caniniform " or not. In typical Ruminants, the upper jaw lacks both incisors and canines (with certain exceptions), but in front of the lower jaw there are grouped together eight teeth, closely fitted to one another, and of almost exactly similar size and shape. The outermost pair of these teeth are called canines, because (i.) in some allied species the tooth in this situation is Fig. 171a. — Oreodon culbertsonii. (After Leidy.) It will be observed that in the upper jaw the four premolars of the typical mammalian dentition are behind the "canine," but that in the lower jaw the tooth which would fulfil the functions of a canine is the first of these four, and therefore is not the corresponding tooth to the " canine " in the upper jaw. more pointed; (ii.) because this tooth shuts in advance of the upper canine when the mouth is closed in those allied creatures which have an undoubted upper canine ; (iii.) because it is cut later than the others (Owen) ; (iv.) because six is the typical number of mammalian incisors. The first three reasons are weak, because form (i.) is a very unsafe guide to homology, and as to the lateness of its development (iii.), it succeeds to the third incisor, by Professor Owen's own showing, after about the same lapse of time which separated the eruption of the second and third incisors. Oreodon, an extinct Euminant with caniniform teeth, has the eight incisors in the lower jaw in addition to a caniniform tooth, which is the fifth tooth counting from the front. With reference to the relative positions of the upper and lower teeth, determining which is and which is not "the canine" (ii.), no one, looking at the dentition of Oreodon, would be inclined to hesitate which teeth he shall call " canines" ; yet the lower caniniform tooth shuts behind the upper, and, therefore, according to this test, it is not a true canine. In the Lemurs there are similarly four or six procumbent teeth 342 A MANUAL OF DENTAL ANATOMY. occupying the front of the lower jaw, of which the outermost pair are called canines, although not in the smallest degree meriting that name for any other reason than that they close in front of the caniniform tooth of the upper jaw, for they are just like the incisors. But it is in the Insectivora, which in some respects represent an ancient and generalised mammalian type, that the greatest difficulties occur. To the mole no fewer than four dental formulae have been assigned, all turning upon the identification of the canine. The difficulty is this : The upper tooth, which looks like a canine, has two roots, and it together with its deciduous predecessor (Spence Bate) is implanted within the limits of the premaxillary bone. With regard to this, Leche and Marsh point out that the canine is two-rooted in all the oldest known mammals, and that the reduction to one root goes hand in hand with the specialisation of its crown. Moreover, Leche does not attach much importance to its relation to the premaxillary bone, as he states that he is prepared to show that teeth rooted in the premaxilla in one species, and FlG. 172. — Upper and lower teeth of the common mole. Tn it, just as in Oreodon, the teeth which fulfil the functions of canines are not corre- sponding teeth in the upper and lower jaws. therefore incisors, are to be found in allied animals rooted in the maxilla. And besides this, the lower tooth, which answers the purpose of, and looks like, a canine, closes behind instead of, as in other Insectivora, in front of the great upper teeth. Ericulus has a sharp, long, two-fanged tooth, in pattern of crown an enlarged premolar, in position of upper canine, and no caniniform tooth in lower jaw. Centetes has typical canines, like a Carnivore. Hemicentetes. The so -called canine, differs in no respect from the premolars behind it. Erinaceus. So-called upper canine, two-rooted, and like the premolars which follow behind it. Gymnura. Upper canine-like tooth has two roots ; a single- rooted lower pointed tooth closes in front of it. Macroscelis and Petrodromus. The third or outermost incisor is two- rooted, long, and sharp, and plays the part of a canine. . Potamogale. A small tooth, in no respect different from the other premolars, is called a " canine." f Ncib. Siz* THE HOMOLOGIES OF MAMMALIAN TEETH. 343 Thus in some of the groups no tooth has been lengthened and pointed, so as to serve as a canine ; in others it is the wrong tooth, i.e.,, not the same tooth as in the Carnivora, or as in other Insectivora. Consequently, in the Insectivora the elevation of a tooth into caniniform length and character is a mere adaptive modification, which may affect an incisor or a premolar, or no tooth at all. Wortman states that though a premolar is often so modified, no example is known of a lower incisor becoming caniniform to oppose an upper canine. As regards the other premolars, there is no difficulty in their determination in those dentitions in which all four milk molars are present, and are succeeded by permanent successors, but this by no means always happens. In a very large majority of cases, the first tooth behind the canine is repre- sented in one dentition only; hence it remains uncertain whether it is a milk tooth which persists, or a permanent tooth which has had no representative in the milk series. M. F. Woodward (8b) discusses the question, pointing out that where there is no succession we may be dealing with a retarded member of an early or an accelerated member of a later set. In the Mole, in which there is no succession as regards this tooth, the enamel-organ of pml does not appear till after those of the milk teeth, but contemporaneously with that of pc, which tooth, however, appears long before any of the other permanent teeth, so that the date of the appearance of its enamel- organ is not of much assistance. He further points out that in those cases amongst mammalia in which there is a succession of pml, the canine is either vestigial as in Hyrax, or is widely separated from the next premolar as in Tapirus indicus, and further suggests that the usual suppression may be due to the great development of the canines. The first premolar is wanting or greatly reduced in all forms with great canines, and he thinks that the somewhat enlarged deciduous canine slightly stunts dpml, while the enormous permanent canine crowds out and totally suppresses ppml, so that he concludes by regarding the solitary pml as really a persistent milk tooth (as dpml), a conclusion reached by Osborn on palseontological evidence. The first premolar is apt to be very small, and it is remark- able that although it is usually almost functionless, it is apt to be long retained. Moreover, its eruption is apt to be later than that of the unquestionable second, third and fourth deciduous premolars, so that it has some claim to be considered as a member of the second series in those cases in which no tooth change occurs. 344 A MANUAL OF DENTAL ANATOMY. But these difficulties which have been briefly pointed out do not prevent us from forming a fairly definite idea of what the usual complete mammalian dentition is, and for brevity it is usual to write this out in symbols, thus : — •3 1 4 3 AA 13crpm4m3 = 44- This is what is styled a typical mammalian dentition, and when teeth are absent from it, we can frequently say which are the absentees. But the occasional occurrence of a larger number of teeth amongst recent mammals, and the dentition observed amongst extinct mammals, have led Osborn (5) to consider that the typical dental formula of primitive mammals was i 4, c 1, pm 4, m 8. As it is not always the same tooth which is absent, a fuller dental formula may be written thus : — i1 i2 i3 1 pm1 pm2 pm3 pm4 m1 m2 m3 c 1 ii i2 i3 1 pmi pm2 pm3 pm4 mi m2 m3 .1.2.3 1 1.2.3.4 1.2.3 orirr^73cipmi.2.3.4mr72T3- The general, though not invariable, rule would appear to be that incisors were lost from the outer end, the third being absent when there are only two incisors ; premolars from the front of the series being the first to drop out of the formula, and molars from the hinder end, the third being the first to disappear. There are, however, many exceptions to this rule — e.g., the first incisor is the first to disappear in the otter, walrus, and some few others, and a question has been raised as to the homologies of man's two incisors. When premolars are missing, it is said that they are lost from the front of the series. Thus in the Baboon the first and second premolars are absent, so that its dental formula written in full is : — .1.2.0 1 0.0.3.4 1.2.3 1 1T2T0 C T pm 0.0.3.4 m 17273- This is generally true, but there are many exceptions, of which the following may be given. In many bears the second premolar is often lost, as is also the third, but the first and the fourth are very constant ; this is also true of some bats.* * This is ascertained by the examination of allied forms, in which the third premolar is found to be so small that it might almost be termed vestigial. THE HOMOLOGIES OF MAMMALIAN TEETH. 345 To make this more clear, Oldfield Thomas proposes to write out in full the dentition, thus : — Bear : — .1/2.3 1 1.0.0.41.2.0 1 1T2T3 c I pm 1.0.0.4 m iT273' Although modern marsupials have usually been considered to possess but three premolars, the original number was four, as in placental mammals. Thus in the extinct Triconodon* which was probably marsu- pial, Thomas has shown that all four premolars are present, and it is the last or fourth which alone shows vertical succession. From a study of the genus Phascologale, one of the Busy ur idee, in which pm2 and pm3 are very constant, but pm4 variable and even reduced to a minute and functionless tooth in one species, it has been suggested that in Dasyurus, with its two premolars, it is pm4 that has disappeared. A comparison of many specimens (in one of which (Phascologale) four premolars were present, pm2 being smaller than the others ; and a skull of Dasyurus in which a rudimentary tooth was present between its two premolars), indicates that the tooth which has disappeared was pm2. ™ ^ n 1.0.3.0 Thus Dasyurus has pm ^ — ^ — ^ — q> A rrru i 1.0.3.4 and Thylacmus pm - — ^ — ^ — -. [It is doubtful, however, whether the homologies of the teeth, when one or more of the series is absent, can be correctly interpreted from an examination of the skulls of adult animals, whether recent or fossil. In all cases where possible a microscopical investigation of the developing teeth should be made, in order to determine the position of any tooth- vestiges if such happen to be present. The homologies of the missing cheek-teeth in the marsupials will be further discussed in a subsequent chapter.] Another difficulty arises from teeth which are beyond question of the permanent or replacing series, not persisting through the life of the animal, but being shed off. The wart-hog is a conspicuous example of the early loss of teeth which clearly belong to the permanent series, all the teeth (premolar and molar) in front of the last great molar being cast off, and the dentition ulti- mately reduced to — .211 13cTmI- Amongst placental mammals the normal number of the molars is three. The Tenrec, however, according to Oldfield Thomas, cuts a small fourth true molar late in life ; this, coupled with its tri tubercular pattern of molar teeth, is indicative of great antiquity. Of living * The systematic position of 'Triconodon is doubtful. 346 A MANUAL OF DENTAL ANATOMY. mammals only the marsupials and Otocyon, a dog-like animal, have four true molars. It is very unusual for the full typical number 44 to be exceeded ; among recent placental mammals Otocyon exceeds it, but some fossil Ungulates have more than the full number, and so have some marsupials. Whether the immediate ancestral forms of mammalia were homodont like the Cetacea or the Armadillo, or merely pre- sented a much simpler and less differentiated series of teeth than recent mammals, may be a matter of doubt, but if we could place in series all the mammals that have ever existed it would be impossible to determine the point at which a differentiation towards some specialised type could properly be said to commence, so gradual are the transitions in all cases in which they are more or less fully known to us. THE MILK DENTITION, Some forty years ago Professor Owen called attention to the fact that those mammals in which the teeth, situated in different parts of the mouth, are alike in form (homodont), as a rule, develop only one set of teeth. Those animals which, on the contrary, have teeth of different size and form in various parts of the mouth (heterodonts), develop two sets of teeth, a " milk " set, which is displaced by a permanent set. But although this is true of a large number of animals if we take only functional teeth into account, even then exceptions are to be noted. Thus the nine-banded armadillo (Tatusia peba) is a true homodont ; its teeth are all very nearly alike, they are simple in form, and they grow from persistent pulps. Yet it has been shown by Kapp, Gervais, and Sir William Flower, to have a well-developed set of milk teeth, retained until the animal is of nearly full size. Thus it is a diphyodont, at the same time that it is a true homodont mammal. But no milk dentition has been definitely observed in the sloths, nor indeed at present has it been detected in other Armadillos, with the exception of a doubt- THE MILK DENTITION. 347 fully distinct species, so that as far as functional teeth are concerned many homodont animals are monophyodont. But recent researches have clearly shown that the term monophyodont is no longer of much use, for even in apparently monophyodont animals the tooth-germs of another set of teeth, sometimes slightly calcified, sometimes yet further reduced, have been found. Hence there hardly remains room for doubt that the primitive condition of mammalia was that of diphyodontism, and that where only one set remains in full evidence, the other has been lost by the suppression of that which is not needed. Upon the whole, our information respecting the "milk " or deciduous dentition remains still defective ; but much light has been thrown upon the subject by the recent investigations of Kiikenthal, Kose, Leche, Oldfield Thomas, M. F. Woodward, Marett Tims, Wilson and Hill, and others. The perpetual replacement of teeth lost or shed in regular course, which characterises the dentition of fish and reptiles (polyphyodont) , finds no distinct parallel in the case of mammals, none of whom develop more than two functional sets of teeth, though Leche and many others believe that there are undoubted evidences of additional series. [In this connection, attention may be drawn to an interest- ing example of polyphyodontism in a girl aged two years and nine months. Details of the case were communicated to the International Medical Congress held in London in 1913, by Dr. J. H. Gibbs, of Edinburgh. In the right half of the mandible, which was considerably swollen, there were three definite rows of teeth, the uppermost of which were fully erupted. Separated by a thin shelf of bone, occurred a second series consisting of a lateral incisor, a geminated canine, milk molars and a single permanent molar. The third row, again separated from the second by another thin plate of bone, contained also milk molars and a solitary permanent molar. The teeth of the third series, however, presented a remarkable feature ; though situated in the right side of the mandible, morphologically they were left- sided teeth. A crypt between the roots of the first erupted premolar indicated the existence of another dentition, though the developing tooth escaped observation. 348 A MANUAL OF DENTAL ANATOMY. Below the third series of teeth was a mass of soft connective tissue. From the fact that this mass contained columnar and cubical epithelium, Gibbs was of opinion that it afforded an indication of an attempt to develop a still further series. If this be the correct interpretation, five serial dentitions are represented.] The accompanying figure (Fig. 173) indicates the conditions present. The deciduous or milk set of teeth may be of any degree of completeness. The milk teeth in man answer the require- Fig. 173. — Drawing of the tumour made after the operation, showing the teeth in the relative positions (Reproduced from Dr. J. H. Gibbs1 article " Polyphyodontism in a child" in the Transactions of the Seventeenth International Congress of Medicine, 1918, by permission of the author). ments of the child up to the age of seven years, and in the Ungulata they commonly remain until the animal has assumed its adult proportions. On the other hand, in many " diphyodont " animals the milk teeth disappear very early indeed, as in the mole (see Fig. 172) ; whilst there are many instances of the milk teeth being absorbed in utero. So that, in the extent to which the milk teeth are developed, the greatest variability is found to exist. A perfectly typical milk dentition represents with a consider- able degree of accuracy, but upon a reduced scale, the adult THE MILK DENTITION. 349 dentition of the animal, any sexual differences which may exist in the latter being but very feebly marked in the former. There are, however, several exceptions ; such are found in the bats and in the edentates. Keference will be made to these subsequently. As a general rule, the hindmost of the milk teeth bear more resemblance to the true molars which come up behind them than they do to the premolars which come up from below to Fig. 174. — Permanent and milk dentitions of a dog : the latter is well developed. Nat. size. (After Blower.) Fig. 175. — Permanent and milk dentitions of a seal {Plioca greenlandica). Nat. size. displace them, the latter being generally of a somewhat simpler form. In what may be termed the normal arrangement, each tooth of the milk series is vertically displaced by a tooth of the permanent series ; but many examples may be cited of particular milk teeth which have no successors, and, on the contrary, of individual permanent teeth wnich have no deciduous predecessor. Although the views advocated by him have become in some respects obsolete, the papers of Sir William Fowler (2) remain 350 A MANUAL OF DENTAL ANATOMY. valuable in giving an instructive series of comparative develop- ment of milk dentitions, and from them the accompanying illustrations are borrowed : — Within the order Carnivora, the dog and many others have a thoroughly well-developed set of milk teeth, which do service for some time ; in the bear the milk teeth are relatively smaller, and are shed very early ; in the seal they are vestigial and functionless, and are absorbed before birth, so that -knob siz& Fig. 176. — Permanent and milk dentitions of an elephant seal (Cystophora proboscidea). ^Tiett size FlG. 177. — Teeth of the monophyodont grampus QOrca capensis). four figures are copied from Sir W. Flower's paper.) (These in the specimen figured the deciduous incisors had already disappeared. In the elephant seal the milk teeth are yet more feebly developed, and the difference between its dentition and that of the monophyodont homodont cetacean {Grampus) is not great ; an observation which is the more interesting, inasmuch as this seal in other characters than its teeth approaches towards the cetacean group. From these and other facts, Sir William Flower argued that the permanent set of teeth of diphyodonts correspond to the single set of monophyodonts, and THE MILK DENTITION. 351 that the milk dentition, where it exists at all, would be some- thing superadded. This was a question upon which there was at that time room for doubt, and although the facts adduced by Sir William Flower afforded some grounds for argument that this was the true interpretation, yet the known facts of development pointed the other way.* Subsequent investigations carried out by means of complete series of serial sections of marsupial jaws by Leche and Kukenthal appeared to have rendered Flower's view untenable. In recent years further evidence has led to a Fig. 178. — Permanent tooth germ and aborted milk germ of the canine of a hedgehog, x 100. (After M. F. Woodward.) resuscitation of that interpretation. As this evidence is mainly derived from a study of the tooth-genesis in the Mar- supials, the question will be more fully discussed in a later chapter. The question turns largely upon the interpretation attached * This discussion is now merely of historic interest. It was based on the view then current, that the monophyodont marsupials were on the direct line of descent of the Eutheria. In the marsupials but one tooth is replaced, pm4. Hence the conclusion that monophyodontism was primitive, the single pml representing a newly arising second dentition. The discussion centred around the question — Does the dentition of the marsupials represent the successional series of the eutheria, with a deciduous set coming into being ; or, is it a persistent milk dentition, with one tooth only of the permanent series as yet evolved?— H. W. M. T. 352 A MANUAL OF DENTAL ANATOMY. to outgrowths of the tooth-band, which may or may not really represent aborted tooth-germs. The accompanying figure represents a stage of suppression of a milk tooth, in which, although the enamel-organ is but little differentiated, it is impossible to doubt that we have a true tooth-germ, as an irregular calcification has taken place at its end. (Cf. Fig. 178.) But- in a great number of outgrowths of the tooth- band which have been regarded as representing tooth-germs there is no such calcification to settle the question. The epithelial lamina or tooth- band (zahnleiste) descends into the mesoblastic tissue somewhat obliquely, inclining towards the tongue, and the buds which go on to form tooth-germs appear upon its labial side at, or close to, its end for the time being. Consequently, as its free end FlG. 179. — I. Diagram of indefinite succession in fish or reptiles. II. Diagram (hypothetical) of functional milk and permanent germs of mammalia, showing premilk and post-permanent rudiments. III. Diagram of typical mammalian succession, all tooth-germs save the milk and per- manent having totally disappeared. In all the diagrams germs which proceed to functional calcification are marked by darker shading. goes on growing deeper into the tissues on the lingual side of , the already forming tooth-germ, we are justified in saying any additional specialisa- tion of the dental lamina which is situated on the lingual side of a formed germ belongs to a later generation of teeth, and conversely that any similar growth of the lamina which lies on the labial side of a formed tooth-germ belongs to an antecedent generation. Now, whilst in such a specimen as that figured on p. 146 we have in view at one time many generations of tooth-germs, we do not find in mammals that more than two attain to functional development, and the Fig. 179, III. represents what may be regarded as typical in the mammalia. Here we perceive that the germ of the successional tooth is at the end of the dental lamina — that it has, so to speak, used it all up, and that there is no prolongation beyond it. But this is by no means invariably the case. Sometimes we may find a little prolongation of the dental lamina beyond and to the lingual side of the developing succes- sional germs, and this is regarded by many writers as an indication of a post-permanent dentition in posse. Similarly we may find an outgrowth to the labial side of the milk tooth-germ, and d fortiori of the succes- THE MILK DENTITION. 353 sional germ, and this is nowadays sometimes interpreted as a vestigial premilk dentition. But no one has, so far as is known, seen in any one specimen vestiges of all four at once* ; had they done so, they would, of course, have to be interpreted as premilk (vestigial), milk (functional), permanent (func- tional) and post-permanent (vestigial). Eose believes that he has found evidences of a series antecedent to the premilk dentition in an early human foetus, and a similar statement has recently been made in regard to the Dugong (p. 560). On the one hand, if mammals be descended from reptiles, which have for the most part an indefinite succession of teeth, it might perhaps be expected that some traces of the suprjressed series of teeth might be found. On the other hand, it would indicate a retention of characters over a very Fig. 180. — Tooth-germs of a pig, from a specimen given to the author by Dr. Marett Tims. The right is the lingual side of the specimen. long period of time, and tooth-germs may disappear in toto, as for instance the upper incisors of ruminants, although they have unques- tionably been present at a comparatively recent period in the ancestors of existing ruminants. As an example of the appearances, which lead to diversities of interpre- tation, the accompanying figure (Fig. 180) may be given. In it the milk * It is scarcely to be expected that all four dentitions should be found in any one specimen at the same time, for presuming them all to be present in the same animal, the development of the milk tooth will have sufficiently advanced to obliterate all traces of the premilk dentition before the post-permanent down growth will have become separated off from the enamel-organ of the successional tooth. — H. W. M. T. 1>.A. 23 354 A MANUAL OF DENTAL ANATOMY. tooth-germ is fully developed, and to its lingual side is the commence- ment of the germ of the successional tooth, both of these ultimately attaining to full functional development. But above the milk germ and to its labial side is an outgrowth of the tooth-band, which, if it is to be regarded as an aborted tooth-germ at all, must be referable to a premilk series. It is, however, not accepted as certainly representing a tooth- germ by Marett Tims and others who have studied this specimen. Thus, it maybe objected that many outgrowths of a tooth-band are not much differentiated, and that they are hardly worth calling tooth-germs, even with the added qualification " vestigial." But it must be remembered that mammals are of great antiquity, and that we should not expect to find any remains of long- lost structures highly differentiated. In view of the hypotheses which are occupying so much of the atten- tion of naturalists upon the relations of the two (or more) sets of mam- malian teeth, it becomes important to decide how much differentiation of the epithelial lamina we shall be justified in regarding as adequate evidence of a tooth-germ, but unfortunately there is no agreement on this funda- mental point. Kiikenthal and Eose in particular, and following them a good many other writers, appear to be satisfied with remarkably little ; almost any proliferation, of the cells of the tooth-band leading to its being thickened or "swollen," so that it is bud-shaped on section, is for them a reduced tooth-germ ; and Leche, whilst distinctly pointing out that neither a bud- shaped (as seen in section) enlargement of the band, nor even a connec- tive tissue thickening about its free end, can be held to establish it as a tooth-germ, yet in practice seems to accept as tooth-germs a great many structures which show no more than this. It is true that such a swelling does take place where a tooth-germ is about to be formed. It is true, also, that in certain cases we are able to trace a downward series from a well- marked and calcified tooth-germ to ultimately nothing more than such a swelling ; but it is also true that such swellings of the tooth-band do take place beyond the second tooth-germs of many mammals, and, in fact in other positions where we have not the smallest ground for assuming that a tooth- germ has ever existed beyond the evidence of the swollen lamina itself. Hence, as is clearly pointed out by Messrs. Wilson and Hill (7), mere enlargement of the tooth-band, even accompanied by some condensation of the mesoblastic tissue round it, is not adequate evidence. Of course, if it goes on to the bell- shaped cap with full differentiation of its outer and inner epithelia, and still more if there be the smallest calcification, no one can feel any scepticism. But unfortunately a very large proportion of the supposed germs upon which speculation is rife, and upon which theories are freely built, are merely of this doubtful character. If we could find in any mammal all four sets of tooth- germs going on to calcification, or even to such an amount of differentiation as would show that the outgrowths were really aborting tooth-germs, all difficulty of interpretation would disappear ; but unfortunately this is not the case. We do not find all four at once ; neither are the sup- posed rudiments so far differentiated as to put their nature beyond all doubt (cf. p. 347). THE MILK DENTITION. 355 So when we have to deal with what are apparently three sets of rudiments successional to one another, the question always has to be decided for the particular case, to which of the supposed five dentitions do the functional tee'th of the individual belong, before we can assign a place to the extra rudiment, if, indeed, true tooth rudiment it be. In the whole group of marsupials (with the exception of the Wombat) no tooth change had been demonstrated except in the case of an individual tooth, the last premolar, but here the vertical succession is curiously constant, though in the case of some of the Dasyuridcv it is shown in process of reduction and disappearance. The question as to whether this is a true case of tooth succession will be fully discussed in the chapter dealing with the dentition of Marsupials. This succession of a single tooth had become established in very early geological times (e.g. Tricon od on), and has been retained with remarkable constancy. Then came the researches of Kiikenthal and Rose, who showed that the tooth-band, beyond and to the lingual side of the germs of the functional marsupial teeth, become swollen, and they gave to these enlargements the full significance of tooth-germs (which has since been denied them by Wilson and Hill) . Kiikenthal regarded the func- tional dentition of marsupials as being a persistent milk dentition, with remaining rudiments of a suppressed (or first appearing ?) second denti- tion, a view which obtained a large amount of acceptance. Leche subsequently discovered that actual calcification, though of an abortive kind, does take place in Myrmecobius in germs which obviously belong to an earlier series than the functional teeth. On the basis of Kiikenthal's discovery, this writer strongly held to the view that the functional teeth of all marsupials are persistent milk teeth ; he was hence compelled to interpret these rudiments as belonging to a "premilk" series, for they lay to the labial side of the functional germ. This interpretation rests entirely upon the correctness of the identification of the functional teeth of the whole marsupial group as persistent milk teeth. And this interpretation of the functional teeth of marsupials in turn rests upon the fact that supposed germs of a successional set — that is to say, germs upon the lingual side — of the developing functional teeth have been recognised in a considerable number of mar- supials by Kiikenthal, Leche , Woodward, Rose, Tims, and others, so that, if we acknowledge these as tooth-germs at all, the functional teeth are certainly not the latest set represented by vestiges, which would seem to be in favour of the view that the teeth of the marsupials really are per- sistent milk teeth. However, even granting this, there remains a shadow of doubt, for Marett Tims thinks that, inasmuch as in Diddphys Rose has seen what he regards as three sets of germs, and it was the middle one which developed, it is possible that after all the milk teeth are suppressed, the permanent teeth developed, and a post-permanent set of vestigial germs* are still to be found. The matter is still open to argument. * This opinion rests not merely upon the conditions found in the marsupials, but also on a comparison with the conditions existing in many Eutheria. — H. W. M. T. 23—2 356 A MANUAL OF DENTAL ANATOMY. But if tbe one set of functional teeth of marsupials are permanent teeth, then Leche's premilk teeth become ordinary aborted milk teeth, and Myrmecobius merely stands alone among marsupials in having some calcification occurring in both sets.* Woodward thus puts the state of ideas upon the subject of the premilk dentitions :— " In the anterior part of the jaw of the marsupials we find the following sets of teeth : (1) a small vestigial set, often calcified ; (2) a large functional set ; and (3) a series of lingually-placed swellings of the dental lamina (Kiikenthal, Eose, Leche, and Woodward) closely resembling in appearence the first indication of the successional germs in the Placentalia. It is generally believed that the second set repre- sents the milk dentition, and the third the reappearing permanent teeth of Fig. 181. — Myrmecobius. x 50. Tooth-germ of functional canine, to the left of which is seen a slightly calcified rudiment of a tooth of an earlier generation. (After Leche, according to whose interpretation they represent germs of a milk canine and of a premilk canine.) higher mammals, the first vestigial set being regarded as premilk dentition. If the structures described by Eose and Leche in man, the ox and the hedgehog, are the homologues of the calcified labial set of the marsupials, then there can be no doubt that this latter is a premilk denti- tion : but these structures in the Placentalia are slight, though said to be constant, and some may doubt the correctness of this interpretation, especially on comparing these labial teeth with the undoubted reduced milk dentition in many insectivores, and they may consequently be inclined to regard, as Tims has, the labial toothlets of the marsupials as reduced milk teeth, and thus revert to the discarded Flower and Thomas hypothesis. * Uncalcified representations of the so-called " premilk " dentition are to be found in other Marsupials. — H. W. M. T. THE MILK DENTITION. 357 It will no doubt be difficult to convince such believers that their interpre- tation is incorrect until we can find some premilk teeth in the Placentalia better developed than those of Leche and Eose, and we can only admit that ontogeny is not yet as conclusive on the point as one could wish." * And, finally, Messrs. Wilson and Hill (loc. c#.),f finding in Peramcles some vestiges (calcified) corresponding to those of Leche in Myrmecobius, and wholly denying the right of the Kiikenthal and Eose " swellings " to the lingual side of the functional germs to be regarded as tooth-germs at all, reopen the whole question, and would say of the marsupials that they all present a more or less completely aborted milk dentition, and afunctional permanent dentition homologous with that of placental mammals, and thus confirm the view previously suggested by Marett Tims. Though personally the author is inclined to accept the views of Messrs. Wilson and Hill, and of Marett Tims, they have not as yet found universal credence. Thus Carlsson holds that the functional dentition is lacteal, and that rudiments of permanent teeth do exist, of which only pm3 is fully developed. Adloff , Bild, and Deppendorf also accept Leche's prelacteals. Bild affirms that traces of prelacteal germs are found in pigs, but his figures are not very conclusive as to whether the outgrowths in this situation really deserve to be considered as tooth-germs, and Adloff says that prelacteal rudiments exist in Eodents also, but these too may be regarded as doubtful. As the reader will have gathered, the question of the exist- ence of traces of either prelacteal or post-permanent dentition is by no means yet settled. To roughly summarise recent views, it may be said that, according to Leche's latest dicta, in marsupials the functional teeth belong to the milk dentition. There are rudiments, sometimes calcined, of a premilk set ; of the " permanent " teeth one only is fully calcined, and often functional, the others are vestiges. In Placental mammals there are but small traces, probably never calcined, of a premilk series. The milk teeth (with the possible exception of the true molars : see 1, p. 358) never persist through the whole life of the animal, but a " perma- nent " set replaces them in the adult animal. A "post-perma- nent set is represented in some animals by bands beyond the permanent tooth-germs, but these never calcify. * This quotation is taken from a paper by M. F. Woodward published in the Anatomischer Anzexger in 1896. Since that date evidence has been gradually accumulating in favour of the view that the functional dentition of the Marsupials does correspond to the Eutherian successional series. — H. W. M. T. t This paper, though published in the same year as that by Woodward, did not appear until some months later. — H. VV. M. T. 358 A MANUAL OF DENTAL ANATOMY. The milk dentitions occur in various degrees of reduction, thus : a. One (for instance, dpnii in most mammals) or more (e.g., Erinaceidce) fail to attain to full calcification, without the rest of the milk teeth being aborted. b. The whole milk dentition (except, as above, and the true molars) * is more or less rudimentary (e.g. Pinnipedia) ; and in some forms is absorbed without even being cut, as in some Phocidce, Eodentia, and Chiroptera. c. The milk dentition (except as regards the true molars as referable to it) does not attain to any calcification at all (Shrews and many Kodents). d. The milk dentition is neither represented by rudimentary nor by erupting teeth (? Bradypus). This is a considerable modification of Leche's earlier views, but it does not meet with universal acceptance. Thus Kviken- thal (and Leche formerly) holds that the teeth of Cetacea are milk teeth, and that it is the permanent series in them which is suppressed, though Leche regards it as still somewhat an open question. As regards the true molars, several ideas have been propounded : — 1. That, as suggested by Owen and others, these teeth, having had no predecessors, and having arisen from the primitive tooth-band, must be regarded as members of the milk series (Kiikenthal, Kose, Leche, &c). 2. That they belong to the second or permanent series of dentitions (Woodward, Magitot, Marett Tims, &c). 3. That they are not homologous with either, but represent the two series by a fusion of germs or by using up the material whence two series might have been founded (Kukenthal's later view, Schwalbe). 4. That each true molar is the terminal member of a separate series of successive dentitions, the rest being all lost. With regard to 3, Marett Tims (6), whilst discussing the genesis of tooth forms, expresses a belief that each molar may represent an antero- posterior fusion of two teeth belonging to the same series, but he rejects Kukenthal's theory that a lateral fusion of two teeth belonging to * This exception is made on the assumption that the molar teeth belong to the milk series, which is doubtful. — H. W. M. T. THE MILK DENTITION. 359 successive series has taken place. Adloff, however, states that he has seen the fusion of a prelacteal germ in Rodents with a lacteal tooth, Bild affirms the same thing in pigs and Kukenthal in the Dugong, all therefore supporting the concrescence theory. Hopewell-Smith and Marett Tims have also recently described appearances in the developing molars of Macropus billardieri which may indicate a fusion of enamel-organs (Fig. 182). A fusion of cusps is known to occur in Ceratodus and in Sphenodon among lower vertebrates. The question is therefore still subjudice. Lydekker has, how- ever, been influenced by the evidence accumulated, to the extent of disclaiming the views which, in common with Flower, he formerly held (3), and now regards the dentition of mar- supials, disregarding for the moment the replacing tooth, as consisting of persistent milk teeth plus true molars. This leads him to further inferences, which are of considerable import- ance. Thus, he regards the original mammalian dentition as consisting of milk teeth plus true molars. These last he considers as homologous with the milk dentition rather than with the so-called permanent dentition, replacing teeth (or teeth of the second dentition), which may or may not occur, and which he regards as the thing superadded and not as being the constant typical set of teeth. In this connection he points out that the hindmost milk molar always closely resembles the first true molar, so that there is no abrupt tran- sition at this point where both are in place at the same time. He does not agree with Ameghino (p. 339) that it is desirable to abandon the term premolar, but on the contrary would prefer to designate the milk molars (as ordinarily understood) as milk premolars, teeth which may or may not be displaced by permanent premolars. Hence he would write down the milk dentition of man as di | dc j dpm |, and the permanent dentition as .21 23 12clpm2m31 For many years past it has been usual to consider that there was a marked difference in the dental formulae of mar- supials and of placental mammals, it being held that the former had but three premolars and four true molars, whilst the latter had four premolars and three true molars. Oldfield 360 A MANUAL OF DENTAL ANATOMY. Thomas has shown that this is not certain, and the advance of palaeontology has thrown additional difficulties in the way of accepting the distinction, so that Lydekker and many others now hold that the typical dental formula is the same for both, namely, four premolars and three true molars ; but to this subject it will be necessary to recur in dealing with the teeth of marsupials. There is a reason, or some show of a reason, for the succes- ' sion taking place as far back as the premolars, the molars being exempt from change. In all mammals the whole length of the jaw, at the time of birth and afterwards, is occupied by tooth-germs and afterwards by teeth. It is well ascertained that the manner of growth in the jaw is by backward elonga- tion, and that that portion which is occupied by the molars actually does not exist at the time of birth, so that the tooth- band, boring backwards in this region, dips down and has to lose its vertical connection with the epithelium of the surface. Tooth-change might therefore be expected to be limited, as in fact it is, to that portion of the jaws which exists early, while the animal is small. A succession of teeth could not exist in the molar region, because during the reign of the milk teeth in front the molar region itself does not exist. [There is reason for believing that predecessors to the true molars once existed in several animals, e.g. Guinea-pig; as " concentric epithelial bodies " occur to the labial side of the developing molars. These bodies are precisely similar to the those figured and described by Wilson and Hill in Omitho- rhynclius, and shown by them to be tooth-vestiges.] If we consider that the point of the primitive diphyodontism of the mammals is established, and that they are probably descended from polyphyodont ancestors, no further explanation of the existence of successional teeth is called for. But the view propounded by Baume and adopted by Oscar Schmidt should just be noticed. They suggest that the two dentitions were originally disposed in line, and that the origin of succes- sional teeth can be traced back to the shortening of the facial region, which gave no room for the full number of tooth-germs to lie side by side ; the result of this crowding being that they came to lie upon the top of one another, each alternate one becoming displaced and retarded; then that the teeth lying nearest to the surface, having to be used first, get developed THE MILK DENTITION. 361 first, and thus the milk teeth are placed at a disadvantage owing to the hostile position of their successors, which cause their absorption and ejection. The investigation of these questions is further complicated by the fact that there are quite numerous instances of " per- manent " teeth, that is teeth unquestionably belonging to the second set, which are shed off early, and do not remain in place through the lifetime of the animal. An example of this is to be found in the Wart-Hog (Phacochcerus), which loses successively all its premolars and the first and second true molars, the last true molar alone being truly persistent. It has already been mentioned that amongst homodonts no succession of teeth has been observed in the Cetacea ; but Kiikenthal states that the teeth of toothed whales (Beluga, Tarsiops, and Globiocephus) are the milk teeth, inasmuch as germs of successional teeth which never cut the gum are to be found to their inner sides, while Leche considers the evidence insufficient.* In the Shrews the milk series is very greatly reduced (p. 432), while in the Bats the milk teeth are homodont and their successors heterodont. In those Edentates in which a succession is known, the first teeth are heterodont, their successors homo- dont. No succession is known to take place in any of the Edentata, save the armadillo, and in the very aberrant form Orycteropus (see p. 475). Amongst heterodonts there are several Kodents which have no deciduous teeth, e.g., the rat. Probably the dugong has deciduous incisors, but no other milk teeth. The elephant has no vertical succession, save in the incisors, but instead has a succession of teeth from behind forwards in the molar region. As regards the " premilk " and post-permanent germs, it may be repeated that both alike stand or fall according to the import attached to the prolongations of the dental lamina beyond the permanent tooth-germs. If these swellings are not tooth-germs, then there is nothing to lead us to speak of a post-permanent dentition, as no one has seen a well-differ- entiated tooth-germ, much less any calcification in this region. Again, if they be not tooth-germs, then the functional teeth * The arguments as to the interpretation of the Cetacean dentition are similar to those given above in relation to the Marsupial dentition and to the homologies of the molar teeth. — H. W. M. T. 362 A MANUAL OF DENTAL ANATOMY. of marsupials are ordinary permanent teeth, and the " premilk " germs of Leche, Woodward and others, described also, but not Reptilia. Mammalia (hypothetjcal) Mammalia. (generalised) Dog, Seal, Man. Ornithorhynchus Cetacea Marsupialia Some Edentates Tatusia. Some Edentates Manid/e.? diagram showing the number of dentition 5 represented in the various groups of the mammalia, {maritt tims) so styled by Marett Tims and Wilson and Hill, which do go on sometimes to abortive calcifications, are merely the ordinary milk set in a certain stage of suppression. The author THE MILK DENTITION. 363 regards both "premilk" and "post-permanent " rudiments as at present hypothetical, and the evidence insufficient to establish their existence.*- But the literature upon this subject has become so profuse, and the data require such detailed descrip- tions, that it is quite impossible to enter into the discussion in the pages of a small text-book, so the reader must be referred to the original sources, of which he will rind an excellent summary in Messrs. Wilson and Hill's paper. (7) In the accompanying diagram an attempt has been made to summarise at a glance the present position of our knowledge with regard to the interpretation of the mammalian dentitions. References. (x) Ameghins, F. "On the Primitive Type of Plexodont Molar of Mammals." Proc. Zool. Soc, London, 1899, pp. 555—571. (2) Flower, W. (a) "On the Development and Succession of the Teeth in the Marsupialia." Phil. Trans. Roy. Soc, London, 1867, vol. clvii., pp. 631—641. (b) " The Homologies and Notation of the Teeth of Mammalia." Journ. Anat. and Phys., 1869, vol. iii., pp. 262—278. (c) ' 1 On the First or Milk Dentition of the Mam- malia." Trans. Odontol. Soc. Gt. Brit., 1870-71, vol. iii., pp. 211—233. (3) Lydekker, R. " The Dental Formula of the Marsupial and Placental Carnivora." Proc. Zool. Soc, London, 1899, pp. 922—928. (4) Moseley, H. N. and Lankester, E. Ray. " On the Nomenclature of Mammalian Teeth, and on the Dentition of the Mole (Taljm euro- poea) and the Badger [Males toxus)." Journ. Anat. and Phys., 1869, vol. iii., pp. 73 — 80. (5) Osborx, H. F. "On the Structure and Classification of the Mesozoic Mammalia." Journ. Acad. Nat. Sci., Philadelphia, vol. ix., p. 186. (6) Tims, H. W. Marett. " The Evolution of the Teeth in the Mam- malia." Journ. Anat. and Phys., 1903, vol. xxxvii., pp. 131 — 149. * There is now less reason for hesitation in accepting the evidence of the presence of premilk vestiges than was formerly the case. Doubt may still exist as to the value of the post-permanent down growths of the dental lamina. — H. W. M. T. 364 A MANUAL OF DENTAL ANATOMY. (7) Wilson, J. T. and Hill, J. P. " Observations upon the Develop- ment and Succession of the Teeth in Perameles, together with a Contribution to the Dis- cussion of the Homologies of the Teeth in Marsupial Animals." Quart. Journ. Micr. Sci. London, 1897, vol. xxxix., pp. 427—588. (8) Woodward, M. F. (a) "On the Teeth of the Marsupialia, with Special References to the Premilk Denti- tion." Anat. ; Anzeiger, 1896, Bd. xii., pp. 281—291. (b) " On the Dentition of certain Insectivora," Proc. Zool. Soc. Lond., 1896, pp. 557 — 594. CHAPTER XIII. THE EVOLUTION OF THE FORMS OF TEETH. Our knowledge of extinct mammalian forms being very largely based upon their teeth, which are often better preserved than other parts owing to the extreme hardness of their tissues, much attention has been devoted to the manner in which their patterns have been arrived at and to the forces which may be supposed to have acted in their modification. It has been suggested by J. A. Ryder (8) that the pattern of the molar teeth of herbivora is the result of the extent and direction of the excursions of the mandible when it is in use, and so depends upon the form of the glenoid cavity and of the condyle ; that hence the greatest modification is to be found nearest to the articulation where the greatest force is exerted. Thus " bunodont " animals, i.e., those that have rounded conical cusps upon their short-rooted teeth, have a cylindrical condyle ; selenodonts, or those with crescentic ridges on the crown surfaces of the molars, have a condyle which is expanded and plane ; while lophodonts, or those with transversely ridged teeth, have a globular condyle. It has been pointed out by Cope that in many instances the articular surfaces of bones assume forms very similar to those which would be modelled by two somewhat plastic masses working upon one another, and it is a familiar fact that if a dislocation be left unreduced, the surfaces of bone in contact will become so altered as to form a sort of articulation in the new place. But even if we grant the one much-disputed point, namely, that characters acquired by the individual in its own lifetime can be transmitted to its offspring, there is a special difficulty remaining in the case of teeth in adopting the simple mechanical explanation that the teeth are, so to speak, drawn or squeezed into their forms, whether in one or in ten thousand generations. For, unlike a bone, which is very much alive, and is constantly growing and being renewed after it has come 366 A MANUAL OF DENTAL ANATOMY. into use, that portion of the tooth which is subject to these direct influences is hard and rigid, and its form, whatever it be, in that individual is unalterable. In order to alter the form of a masticating surface by direct mechanical means, the influence would seem to need to be brought to bear upon the teeth while they are yet soft, when they are still buried within the jaw in their bony crypts. And we cannot safely assume that structures like enamel-covered dentine can be altered by pressure, the essential character of dentine being its elasticity, and that of enamel its rigidity. It therefore follows that the tooth is of a nature very little likely either to be deformed by pressure, or being deformed, to retain the deformity. Cope, quoting this objection, states that Kolliker and others have established the existence of osteoblasts on dentine as well as upon bone, but this in no way appears to the author to meet the objection, as the only place where osteoblasts can occur is on the inside of the dentine, i.e., on the walls of the pulp cavity, and it is not therefore easily understood how they can help the matter short of removing the whole thickness of the dentine. Moreover, it would appear that we have no right to assume that the lateral excursions of the jaw and the form of the condyle antedated the special form of the teeth. Thus, however tempting the doctrine of Cope's kineto- genesis, by which the very accurate articulation of the cusps of the teeth with one another, as well as their variety of forms, is sought to be accounted for, it hardly commends itself as possible, at all events in the form in which it was promul- gated. It has been pointed out by Osborn that in the palseonto- logical record it is found that the points of greatest wear in the molars of more ancient forms are just the places in which in succeeding periods of time new cusps appear. Granting the inheritance of acquired characters, this seems more easy to follow. It is a familiar fact that the stimulus of excessive wear, conveyed probably through the dentinal fibre, does stimulate the corresponding odontoblasts to increased action, and that new dentine is built on to the wall of the pulp cavity to thicken it at the threatened point. To the mind of the Neo-Lamarckian it would not be a long THE EVOLUTION OF THE FOEMS OF TEETH. 367 step from this to the earlier and stronger development of dentine at this point in a succeeding generation, i.e., to the development of a. new cusp. But if we accept this idea, then we are confronted with a difficulty in that Cope's view of the plastic yielding, if it may be so expressed, and Osborn's idea of a reaction leading to an increased resistance, seem contra- dictory, as they would both arise as a consequence of forces not very different acting upon a tooth. Moreover, Cope's idea seems to involve a greater degree of fixity in the attachment of the teeth than practical experience finds to obtain. For a tooth quickly yields to a constant or frequently recurrent pressure, especially in any lateral direc- tion, so that one would expect the teeth of the ruminant to move in their sockets rather than to be affected in form. Cope himself recognises this as a factor which has determined the outward leaning of the incisor teeth of the lemurs, which perhaps use their front teeth to comb out their fur. Another difficulty arises from the fact that in the mutual adaptation of the working surfaces the development of a cusp on the one tooth involves a depression on the other, yet action and reaction must have been equal on the two points. Wortman extends the idea of kinetogenesis yet further, thinking that the cingulum may be a wrinkle or fold produced by long-continued pressure applied vertically, and that division of single roots into two may have resulted from a backward and forward rocking of the tooth in mastication. Cope thus formulates the influences which he conceives to be the most important : — 1. Increase of size of a tooth, or of a part of a tooth, is due to increased use, within a certain maximum of capacity for increased nutrition. 2. The change of direction and use of a tooth takes place away from the direction of greatest, and in the direction of least resistance. 3. It follows from their greater flexibility that the crests or crowns of teeth yield to strains more readily than do the cusps. 4. The increase in the length of crests and cusps in all directions, and therefore the plications of the same, is directly as the irritation from use to which their apices and edges are subjected, up to the limit set by the destructive effects of such use, or by the recuperative energy of nutrition. 368 A MANUAL OF DENTAL ANATOMY. • 5. The direction of growth of the branches of a V, or of the horns of a crescent, will be in the direction of movement of the corresponding parts of the opposite jaw. Apart from the investigation of the actual forces at work, naturalists have sought to discover the probable ancestral form of mammalian teeth, and the steps by which they have arrived at their ultimate complexity. The earliest theory is that known as Concrescence, under which it is suggested by Kiikenthal and Kose (though the idea is much older than their time)* that the occurrence of multi- cuspid teeth may be explained by the shortening of the jaw and consequent crowding of the teeth leading to the coalescence of several simple conical teeth to form a single tooth with several cusps ; thus they would regard the five-cusped human molars as derived from the coalescence into one tooth of five originally distinct simple conical teeth. This view was sup- ported by Virchow, who thought that a case in which the place of a molar had been taken by three peg-shaped denticles with separate roots was an instance of atavism. This coalescence might take place in the length of the jaw, and result in a shortened jaw with a smaller number of more complex teeth, or it might take place laterally, in which case the teeth fused would be of successive series, i.e., of different dentitions. On this latter supposition Kiikenthal would regard a molar tooth with its outer and inner cusps as the product of the lateral fusion of a milk tooth or two milk teeth with one or two of the permanent series, t Although it is true that some observers have believed that they have seen evidence of the coalescence of more or less rudimentary tooth-germs with the germs of functionally- developed teeth in their immediate neighbourhood, there is but little evidence that such fusion, even if it does take place at all, has the effect of adding cusps, or otherwise altering the form of the absorbing germ. Hopewell- Smith and Marett Tims have figured in a recent paper what they suggest as * As far as I have been able to discover Gervais (1854) was the first writer to advance any theory in explanation of the evolution of the molar pattern. His theory was practically that now known as the Concrescence theory. — H. W. M. T. ■j- The most recent discussion of the Concrescence theory is that by Bolk (2), who advances certain modifications of his own. For a detailed account of which the reader is referred to his somewhat lengthy paper. — H. W. M. T. THE EVOLUTION OF THE FOKMS OF TEETH. 369 possible evidence of the fusion of tooth-germs in the molar teeth of a Wallaby (3). In the case of Ceratodus (p. 289), and of Sphenodon (p." 312), the coalescence of tooth-germs, originally distinct, to make up a continuous sheathing mass of dental tissue has been observed. Marett Tims whilst rejecting Kukenthal's idea that molars represent the fusion of teeth belonging to two successive series, has expressed the opinion that it is possible that they may be built up by the fusion of two teeth antero-posteriorly, of two teeth of the same series in fact (10b). Ameghino has pointed out that the numbers of individual teeth and of cusps upon each tooth stand in an inverse ratio to one another (*). FlG. 182. — Showing the sub-division of the enamel organ into two parts. ? a fusion of two distinct enamel organs (Hopewell Smith and Tims. Proc. Zool. Soc., London, 1911). Still, palaeontology does not afford much definite support to the concrescence theory. Of course the geological record is very imperfect, and there are but few cases in which we can watch the gradual evolution of a complex tooth form over a long series of ages, but where we can, concrescence does not appear to afford a good explanation of the facts observed. Another difficulty in accepting the concrescence theory is pointed out by Osborn. Long after a settled form has been arrived at and perpetuated for long periods of time, long after, so to speak, the component separate teeth have been forgotten, we have the phenomenon of new cusps arising. It is difficult to suppose in such a case that a long-lost tooth thus reappears to make a new cusp, and hence there must be some other way of genesis for added cusps. The Cingulum theory is another explanation which has been D.A. 24 370 A MANUAL OF DENTAL ANATOMY. offered, and is strongly urged by Marett Tims (loc. ext.). The cingulum is a ridge which surrounds the neck of a tooth, and is a very ancient structure, for all primitive teeth are com- pletely surrounded by it, and even the strongest advocates of the tritubercular theory to which reference will subsequently be made, recognise its importance. Thus Osborn says, " As is well known, the hypocone is an upgrowth from the cingulam. By its disappearance in some regions, and by its elevation into prominence in others, the form of a tooth may become profoundly modified, and it thus comes to be regarded as a sort of mother of cusps." Marett Tims leans to the cingulum hypothesis as explaining much of the evolution of tooth forms in complexity, and mention may be made of a very instructive Fig. 183.— Upper molar teeth of (A) Urutrichus ; (B) Mole (Talpa) ; (C) Chrysochloris (the Cape iridescent mole). The four principal cusps (Mivart) are lettered a, b, c, d, the others being elevations of the cingulum of secondary importance. Woodward suggests that their homologies are : a. Paracone. b. Protocone. c. Metacone. d. Hypocone. series of comparisons of the molar teeth of Insectivora by Mivart (5), pointing out that within the limits of this group a great variety of patterns is met with, the several modifications being connected by transitional forms. A V-shaped tooth of tritubercular aspect occurs in some of the Insectivora of Madagascar and Africa, and in many others there is a more complex or W-shaped pattern. Often upon the upper molar teeth of Insectivora there are four principal cusps (lettered a, b, c, d, in the figure) which are more or less connected by ridges ; such simple teeth are met with in the elephant mouse (Macroscelides) and hedgehog. The cingulum is well developed in most of the group, and the further complexity of the crowns, which often bristle with sharp points, is brought about by the elevation of the cingulum into long sharp points, equalling, or exceeding in length, the principal cusps of the tooth. THE EVOLUTION OF THE FOKMS OF TEETH. 371 Thus in Urotrichus, a Japanese creature having affinities with the mole, the external cingulum is elevated into three distinct pointed cusps, united by ridges with the two principal cusps, an arrangement which gives a sort of W pattern to the surface, while to the inner side the cingulum forms another cusp, so that there are in all eight cusps ; the common mole has the third cusp developed from the outer cingulum, but its two inner principal cusps are fused together and lose their distinctiveness. The suppression and fusion of cusps is carried to a much greater extent in the compressed teeth of the iridescent mole (Chrysochloris), but there are inter- mediate forms which render it easy to identify its reduced parts with those corresponding to them in the mole or in Urotrichus. Speaking generally, it may be said that new cusps are added to the number already existing by the cingulum becoming elevated into points ; it is not very unusual to see subsidiary cusps obviously originating in this way upon human molars. Kidges may variously connect the cusps. Mivart suggests that the coalescence of two or more cusps to form an exceed- ingly elevated point is illustrated by the carnassial tooth of Carnivora. To this transformation certain marsupial teeth form the clue, as they afford unquestionable evidence of such coalescence by a gradational series of small modifications in. this direction occurring in allied creatures. But this view is at variance with that of Cope and Osborn. Whilst Mivart regarded the simple tritubercular tooth as derived by com- pression and fusion of cusps from the others, they would advocate a course of modification the converse of this. The Tritubercular theory, which has a large literature of its own, owes its present form chiefly to the writings of Cope, Osborn, and others of the American school of naturalists. On the Continent it has some adherents, but in this country it has for the most part received only a guarded and partial recognition. Its essential feature is that in the evolution of teetli from the simple cone a stage was reached at an early period in which three cusps were arranged in a triangle. Not only was this tritubercular tooth reached early, but it was possessed by a considerable number of the earlier mammals, and is still retained in some existing forms. This triangle 24—2 372 A MANUAL OF DENTAL ANATOMY. of cusps, or Trigon, once attained is believed to have shown such fixity of type that not only has it come down comparatively unchanged in some animals to the present day, but that it can be traced, and its component cusps identified, even in teeth which have become more complex in form and have lost all superficial resemblance to the primitive tritubercular tooth. The early steps may be thus described : To the primitive cone (protocone) were added two lesser cones, one in front (paracone) and one behind (metacone). To such a tooth with its three cusps in an antero-posterior linear series, the name Triconodont is applied, and is to be seen in the teeth of the fossil Triconodon. The accessory anterior and posterior cusps enlarged and became displaced sideways, so that there came to be in the upper teeth two outer and one inner cusp, and on the lower two inner and one outer cusp. [Such a tooth has now assumed the Tritubercular pattern, i.e., having a triangular crown surface with a cusp at each angle. In the upper molars the internal apical cusp is the protocone, and is regarded as the representative of the primitive haplodont cone. The anterior basal cone is the paracone, the posterior the metacone. In order to distinguish the corresponding cusps in the lower molars, the suffix -id is added. Hence they are named protoconid, paraconid, and metaconid respectively. The triangular pattern of the upper molars is reversed in the lower teeth, so that the apical protoconid is placed externally, the paraconid and metaconid being at the anterior and posterior angles respectively, and are placed to the inner side of the crown surface.] It is found that in the very generalised form, Dromaihe- rium, the teeth are little more than simple cones ; in Amphilestes there is a triconodont stage ; while in Spalaco- therium the three cones, instead of being placed in a line, have become in the upper teeth two outer and one inner, and in the lower one outer and two inner cusps. At this stage we have the tritubercular tooth, and the teeth of the upper and lower jaws alternate with one another like the teeth of a dolphin (Fig. 170), an arrangement which with slight modification is exceedingly common in the mammals of the early geological periods. The next stage in complexity consisted in the addition to THE EVOLUTION OF THE FORMS OF TEETH. 373 the primitive triangles (trigons) of the posterior heel (talon), which carried a fourth cusp (hypocone). Such a tooth is termed trituberculo-sectorial, and was quite common amongst the Creodonts, and indeed other early mammals. The added heel in the lower teeth may carry another cusp besides the hypocone, to which the name entocone is given, whilst the Fig. 184. Fig. 184. — Diagram (after Cope) of upper (shaded) and lower (clear) teeth. (a) Stage reached in Triconodon. (b) Stage of ideal tri tubercular teeth approached by Menacodon. Fig. 185. — 1. Tooth of Dromatherium. 2. Tooth of Triconodon. 3. Tooth of Spalacotherium, inner aspect. 4. Working surface. 5. Outer aspect (after Osborn). Fig. 186. — Lower jaw of Am,philestrs, from the Stonesfield slate, x 2 (after Owen). upper teeth often have two additional cusps styled protoconule and metaconule. So far the hypothesis commends itself by its simplicity as a generalisation, and indeed the convenience of having 374 A MANUAL OF DENTAL ANATOMY distinctive names for corresponding cusps, when that corre- spondence can be determined, is so great that the terminology is very commonly employed, even by those who hesitate to adopt the theory in its entirety. The stages of trituberculism, a term first employed by Kutimeyer, are thus set forth by Osborn : — I. a. Haplodont Type. — A simple conical crown, with single root, not well marked off from the crown. Not as yet known among primitive mammalia. b. Protodont Type. — One main cone with lateral accessory cuspules ; root grooved. Ex. Dromatherium from American Trias. II. Triconodont Type. — One central cone with two lateral accessory cones ; root double. Ex. Triconodon. Fig. 187. — Upper molars of Primates, Anaptomorphus to Homo, showing evolu- tion of the hypocone from the cingulum. A. Anaptomorphus. B. and C. Other Primates. D. Man. (After Osborn.) III. Tritubercular Type. — Crown triangular with three main cusps, the central or principal cone internal in upper molars and external in lower molars. Ex. Spalacotherium and Asthenoclon. IY. Trituberculo -sectorial Type. — Ex. Some living carnivores, In these various sub-types the primitive triangles form the main body of the crown, to which other secondary cusps have been subsequently added. By this means the teeth no longer wholly alternate, but some portions of them meet by their working surfaces. Nevertheless the alternation of that part of the teeth which is constituted by the primitive triangles remains, so that each triangle is opposed not by another, but by the added portion of the tooth, the talon of one falling into the hollow of the opposing trigon ; the protoconule and metaconule always appearing at the spot where the talon abuts upon the trigon. When, however, it is sought to apply the theory to more complex teeth, difficulties arise, and it even appears that in THE EVOLUTION OE THE FOBMS OF TEETH. 375 the upper and lower teeth of the same animal the subsequent additions of cusps do not follow the same order, and their homologies become doubtful. The cusps of the primitive triangle may not all persist, even though other cusps are being added. Thus the formation of a quadritubercular tooth is often arrived at, not by the simple addition of one cusp to the primitive triangle, but by the addition of two and the more or less complete suppression of one of the three of the primitive triangle. Thus the following tables represent the names given : — Cusps of Lower Molars. g & | Antero-external = protoconid. rS J Antero -internal = paraconid. .§ .3 ] Intermediate (or after the loss of the paraconid antero- PhH I internal) = metaconid. Postero-external = hypoconid. Postero -internal = entaconid. Postero-internal (sixth cusp) = hypocone. Antero-intermediate = protoconule. Posterior intermediate between metacone and hypocone = metaconule. According to Osborn, the cusps of human molars are identified as follows : — Cusps of Upper Molars. U) (Antero -internal = Lower Molars. Antero-buccal = protoconid ] . Antero-lingual = metaconid ' ^ Postero-buccal = hypoconid | Postero-lingual = entaconid fHeel. Postero -mesial = hypoconulidi Upper Molars. Antero-palatal = protoconel Antero-buccal = paracone rTrigon. Postero-buccal = metacone J Postero-palatal = hypocone — Heel. 376 A MANUAL OF DENTAL ANATOMY. The evolution of the human upper molar from Anapto- morphus, an Eocene lemuroid, is illustrated in the foregoing figure after Osborn (" American Naturalist," 1897, where a particularly clear summary of trituberculy is given : — " The talon or talouid always appeared first in the lower molars, and as it increased in dimension other cusps came in, so that in lower Eocene mammals the lower teeth generally bore six cusps : the protoconid, paraconid, and metaconid in the trigon and the hypocone, hyperconulid, and entaconid on the talon." Thus, to borrow Osborn's words, "it was ready for transformation into the molar of a primate, an ungulate, or a carnivore, as the case might be." To further illustrate the application of this nomenclature, Fig. 188. — Diagram of two upper and two lower molars, adapted from Osborn in order to show relation of cusps. The shaded lower molars are supposed to be transparent, and to be looked at from below. The cusps of the primitive triangle are marked heavily, and are united by strong lines, those of the lower being solid, those of the upper ringed, a. Protoconid. b. Paraconid. c. Metaconid. d. Hypoconid. l.Protocone. 2. Paracone. 3. Metacone. 4. Hypocone. taking the lower teeth, the lower carnassial tooth of the carnivora has the primitive triangle, with connecting ridges and an additional low hypocone. The same type occurs in many marsupials and insectivores (see Fig. 183). Add the hypoconid and entaconid to the primitive triangle, and we have a common type of quinquetuberculate molar ; then suppress one of the cusps of the triangle (the metaconid in Carnivora or the paraconid in Ungulata), and we get the quadricuspid bunodont molar. Or, to express it in other words, the divergencies occur in the direction towards bunodont teeth, in which a quadritubercular form is assumed by the loss of one of the primary cusps and the addition of a heel which contains two secondary cusps, or otherwise towards a sectorial THE EVOLUTION OF THE FOBMS OF TEETH. 377 type in which the heel is low, and the three primary cusps are connected by cutting ridges. In the foregoing diagram an attempt is made to show the relations of the different cusps in upper and lower molars of a bunodont type. The alternation of the primitive triangles is well seen, as also the position of the hypocone and the other additional cusps. Such is in brief the doctrine of trituberculism, the termino- logy of which has gained currency in most recent literature. For the present the determination of the homologies of the cusps set forth by Osborn still retains some hold, though it will most probably come to be modified in the future ; for this reason, in the later part of this book little attempt has been made to give these homologies in describing the teeth of individual mammals.* It may save repetition and assist the reader not familiar with geology to give a table (p. 378) presenting the classification generally in use, leaving out the older formations in which mammals do not occur. The names of some of the actual forms alluded to are placed opposite their respective periods. The earliest mammalian dentitions known to us carry us back no further than Mesozoic times, and of the Mesozoic mammals we may say that they had mostly smooth cerebral hemispheres, no inflection of the angle of the lower jaw, and forty-four or more teeth. Of these the canines had two roots and the premolars and molars were little differentiated from one another. If we except the Multituberculata to be presently described, all the teeth of the Mesozoic mammals are in some stage of trituberculism ; indeed, this applies also to the early Eocene (Tertiary) mammals. Of the few mammals known to have existed in Mesozoic times, about a dozen genera have been obtained from a single locality : the Purbeck bone bed. This is a fresh water deposit found near to Swanage, and it is no larger than the floor of a very moderate-sized room, and only six inches deep. From this tiny patch comes most of our record of English Mesozoic * Though the terminology is still retained, the number of adherents to the implied doctrine is steadily diminishing ; indeed it seems doubtful if Prof. Osborn is as ardent a supporter as formerly. — H. W. M. T. 378 A MANUAL OF DENTAL ANATOMY. mammalia, and in it are forms so differentiated, as for example Plagiaulax, as to clearly indicate that at this date mammals SOZOIC. Triassic — Lower, upper First appearance of mammals, Dromatherium, Microlestes. s o Jurassic — Liassic, Oolitic Ctenacoda, Plagiaulax and other Multituberculates, Triconodonts and Trituber- culates. Second, Cretaceous — Lower, upper Multituberculates and Tritu- berculates, Marsupials. i Cainozoic. " Eocene — Lower Middle... ... Upper Polymastodon. Ancient types of Ungulata, Carnivora, Insectivora, Lemurs, Creo- donts, Condylarthra. Hyracotherium, Dinocerata, Phenacodus, Insectivora, Chiroptera, Hyracoidea, Artiodactyl and Perissodac- tyl ungulates, Eodents, Fish, Lemurs, and Mon- keys, Creodonts. Creodonts, Mesonyx, Oreo- dons, Tapirs. Extinction of Multitu- berculata. Condylar- thra, Tillo- donts, Dino- cerata, some Creodonts. o 1 Ph M H tf H H Miocene — Lower Middle Upper Mesohippus, Creodonts, Pure dogs and cats, monkeys, pigs and peccaries, opossums, Insectivora, Eodents, Creo- donts, Oreodon. Miohippus, Two - horned rhinoceros. Protohippus, Hipparion, Mas- todon, Edentates, Ehino- cerous, Deer, Llamas and Camels. Titan- otheres, Hyracodon. Creodonts, Hycenodon, Oreodon. O p3 Pliocene L r Equus, Mastodon, Sloth, Ilycenidoi, Mustelidce. HQ? 1 Pleistocene < Eecent Mammoth, Horse. Man. had long existed and were present in the country in great numbers, yet we know nothing at all of them. THE EVOLUTION OF THE FOKMS OF TEETH. 379 Ke/cently a considerable number have been found in Wyoming, in Jurassic formations, and the late Professor Marsh, pending evidence which would place them either amongst the Metatheria or Eutheria, proposed for them the provisional orders Panto- theria and Allotheria. He considered that his Pantotheria were probably placental, and his Allotheria, which had fewer teeth, greater specialisation of the premolars and molars and an inflected angle to the lower jaw, were probably Marsupial mammals, but he infers from what is known of the earliest mammals that the placentals are not descended from the Marsupials, but that both have arisen from a common stock, perhaps of Monotreme character. The Amercian Mesozoic mammals, known only by teeth and jaws, were very similar to the European forms. The dentition of Amphitherium of which the lower jaw only has yet been found, is one of the earliest known. It was obtained from the Jurassic formations ; sixteen teeth were present which, so far as in their forms can tell us, were 1 8 °I Pm 6 m 6- Of Mesozoic mammals in general, it may be said that the earliest are known by isolated jaws, or even by a few detached teeth only, but those coming a little later are better known, largely by the magnificent finds which have rewarded the American naturalists. A large series of early mammals have come from the Puerco beds of New Mexico, which are either Eocene Tertiary, or even a little earlier (Osborn and Earle (7). They are remarkable for the large number of orders represented, from the multi- tuberculate allies of Plagiaulax to representative of the Pri- mates, which appear already to have been both abundant and specialised. No bunodont forms occur in the Mesozoic period, but are first met with in the Cretaceous perio'd. The early and very general assumption of the tritubercular type indicates that the simpler forms are to be sought outside the mammalia, and it suggests that where we find them in mammals they may be due to retrogression. One prevalant type of Mesozoic mammals resembles some- what the polyprotodont marsupials ; there are three or more 380 A MANUAL OF DENTAL ANATOMY. lower incisors, the canines are pronounced, and the cheek teeth are cuspidate, reaching in some cases the number of twelve. But it is uncertain whether they were really marsupials. Another type of mammals, termed Multituberculata, on account of the many tubercles which their molar teeth present appeared in Mesozoic times and extended on into the Puerco beds. They may be provisionally divided into two groups, one of which includes Polymastodon and its allies, and the other comprises Plagiaulax and allied forms. It must not be inferred that they were necessarily marsupials ; indeed, some writers consider them to have possibly been Monotremes. Professor Marsh has described several Mesozoic forms like Fig. 189. — Lower jaw of Ctenacodon. (After Marsh.) Plagiaulax, but upper and lower jaws not having been found together in situ, it is somewhat problematical which upper belongs to which lower jaw. The lower jaw of one of these, to which the name Ctenacodon has been given, found in strata of Jurassic age, has a single long pointed incisor, four compressed cutting premolars, and two minute tubercular molars behind them. They are so strongly differentiated that they cannot be regarded as ancestral types, and the manner of succession reminds us of that seen in Marsupials. The Plagiaulacidce have compressed cutting premolars resembling somewhat those of Hypsiprymnus. A great controversy once raged as to the probable diet of these forms. Professor Owen, basing his arguments largely upon the presence of premolars which possessed elongated and sharp- edged blades, held that Plagiaulax was carnivorous, saying THE EVOLUTION OF THE FOEMS OF TEETH. 381 that it possessed an effective dental machinery for predatory life ; Dr. Falconer and Sir W. Flower have shown this view to be untenable, or at least unsupported by adequate evidence. Professor Marsh remarks that all the mammals older than Tertiary times may have been insectivorous, but that Plagiaulax shows an adaptation to some special food, whether animal or vegetable is as yet unknown. The true molars do not, however, resemble those of existing marsupials ; on the other hand, the angle of the lower jaw is inflected as in them. Polymastoclon (Cope), besides being possessed of multi- tuberculate molars (there are no less than twenty-eight oval Fig. 190. — Dentition of Poly mastodon. (After Osborn.) The grinding surfaces of the multituberculate upper molars are shown separately. cusps upon the first upper molar), had two long slender rodent-like incisors in both upper and lower jaws, with the enamel confined to the anterior surface ; in the upper jaw there were in addition two small incisors. In the lower molars there were normally only two rows of cusps, though occasionally signs of an additional row are present. Upon the existence of these teeth has been based yet another theory of the genesis of mammalian teeth, viz., the Multitubercular or Polybuny theory, upheld by Forsyth Major (4). It has been pointed out by him that amongst the earliest mammals known to us are some which were not tritubercular, but multitubercular, and that there is just as 382 A MANUAL OF DENTAL ANATOMY. much ground for supposing that the tritubercular tooth, when it does occur, has been arrived at by a process of reduction in cusps, as that it is derived from a triconodont type. And Ameghino cites Proteodidelphys, an ancient form (Lower Cretaceous), as possessing six cusps with indications of others. Hence he regards six cusps as a primitive form, and any smaller number as being a reduction. This brings us to a consideration of some of the difficulties which stand in the way of the tritubercular theory. Some authorities differ in the determination of the homo- logies of the cusps. There are those who strongly dissent from the proposition that the protocone is on the inside in the upper teeth, holding that it is on the outside in both upper and lower teeth. Again, Scott (9) considers that what is true of the molars is not true of premolars, claiming that even where the premolars attain to a close resemblance or identity with the molars, yet the resemblance is superficial, and that it has been arrived at by a different process, the protocone in them being on the outside in both jaws. He proposes for the premolars a slightly different nomen- clature, namely, protocone, deuterocone, tritocone, tetracone, in the order of the successive appearances of cusps in the palaeontological series. If this could always be ascertained, this nomenclature would seem an improvement, but the imperfection of the geological record renders this not always possible. He considers that the internal cusps of premolars have been derived from elevations of the cingulum, and not from any ancestral tritubercular cusp. To this view Cope lends some support, remarking that we "seem to have here an excellent illustration of the origin of two identical structures by different evolutionary routes," rather than regarding it as a difficulty in the acceptance of the theory generally. [It is still an open question whether the premolars have or have not followed the same order of cusp evolution as the molars. In the older Cope-Osborn (Tritubercular) theory it was held that the evolution was different in the two series, but now, in what Osborn terms the " newer theory," (5c), the opposite conclusion is maintained. This is termed the ''Premolar Analogy Theory," which holds that "the superior molars originally acquired trituberculy in a manner similar to that which can be traced in the premolar metamorphosis." This idea was first suggested by Huxley in 1880. It is supported by Schlosser, and by Wortman and THE EVOLUTION OF THE FORMS OF TEETH. 383 Gidley on palseontological grounds. It would appear that both embryology and palaeontology furnish evidence in support of the view that the antero- external cusps of the upper molars represent the primitive reptilian cone. Marett Tims' views are a slight modification of this (10b). He believes that the course of evolution of the molars and premolars was originally the same, but that in the case of the former, an antero -posterior con- crescence of the teeth has taken place, so that in reality both external cusps of the molars represent the reptilian cones of originally separate cheek teeth. The internal and subsidiary cusps he derives from that archaic structure the Cingulum. In this way one of the great objections to the Cope-Osborn theory, viz., the implied rotation of cusps in the evolution of the tritubercular from the triconodent pattern is avoided.] Another stumbling-block has been pointed out by Marett Tims (10a) in the acceptance of the Tritubercular theory. It would be in accordance with other facts in development if the primitive cusp, even though in the adult it was going to be reduced in size below the others, should be the first to appear in the course of development. But it happens that in some Primates, Marsupials, Ungulates, Insectivores, Rodents, and Canidse, this is not the case; the protoconid does, but the protocone does not develop first : and in some it is even not the second to appear, the paracone always and the metacone usually, appearing in the tooth-germ before it. With the exception of two Insectivores, Centetes and Ericulus (and some doubt is thrown even on these), in all animals which have been adequately examined the paracone and the protoconid are the first to appear ; the evidence of ontogeny therefore points to these being homologous cusps, they being in each instance the antero-external cusps. This may seem a small matter, but if the reversal of the primitive triangle (cf. p. 372) does not take place, this initial and, from the point of view of the theory, very important stage is a fallacy. Whilst Osborn points out that antagonism would lead naturally to the longest (protocone) cusp being on the inside in the one jaw and on the outside in the other, this functional adaptation does not help much to establish the homology of these longest cusps with one another. Marett Tims further points out that the majority of the fossil forms, on which the tritubercular theory is chiefly based, occur in strata not very different in age, and that to 384 A MANUAL OF DENTAL ANATOMY. him at least the evidence of palaeontology is as deficient as that of embryology is damaging. But whatever the ultimate fate of the theory may be, morphologists will always owe a debt of gratitude to its advocates in that they have studied, and induced others to study, the evolution and homologies of the cusps with an accuracy which would in all probability not have been attempted but for the theory, and have reduced to order the nomenclature * and the history of the evolution of the complex teeth of ungulates, which were previously in a chaotic condition. Besides the introduction of new cusps, the pattern of a tooth Fig. 191. Fig. 192. Fig. 191. — Horse incisor, longitudinal section. Fig. 192. — Horse incisor, showing the mark at various ages. may be complicated by infoldings of its surface, whether on its sides or on its apex. If we take a simple conical tooth with one cusp, such as a canine, and grind or wear down its apex till the terminal portion of enamel is removed, its blunted end will present a more or less circular area of dentine, surrounded by a rim of enamel. If we imagine a tooth with four long similar cusps, * The reduction to order here referred to is unfortunately now again reduced to disorder. The terms Protocone, etc., may still be used to denote certain cusps, but certainly not with an assured implication of morphological significance, such as was intended when the terms were first adopted. — H. W. M. T. THE EVOLUTION OF THE FOEMS OF TEETH. 385 we shall at a certain stage of wear have four such areas, while eventually, as the tooth gets worn down below the level of the bases of the cusps., there will come to be a single larger area of dentine surrounded by enamel. Thus in those teeth, the grinding surfaces of which are rendered complex in pattern by the presence of several cusps, the pattern changes from time to time as the tooth wears down; while the addition of thick cementum, filling up the interspaces of the cusps, adds a further element of complexity, as is seen in the teeth of most herbivorous creatures. The change of pattern induced by the wearing down of the surface to a lower level is well and simply illustrated by the " mark" of the incisor teeth of a horse. In an uncut, and therefore perfectly unworn, tooth, the condition of the apex may be compared to the finger of a glove the tip of which has been pushed in or invaginated. The depression so formed is, like the rest of the surface, coated with enamel, and with a thin layer of cementum. When the tooth is worn down to a considerable extent there is a field of dentine, in the centre of which is an oval ring of enamel, within which is a space filled with the debris of food, &c. This constitutes the " mark," and as the tooth becomes further worn down, below the level of the bottom of the pit, the " mark " disappears, and a plain area of dentine results. Somewhat similar inflections occur abundantly also upon the sides of teeth. The inflection of the surface, which in the incisors of the horse is of the simplest possible form, may be cruciform, or variously waved and broken up, thus leading to all sorts of complications of surface. As the tooth becomes worn, the longitudinal inflections, running in from the sides, may also be oblique or variously waved, or they may extend through the entire width of the tooth, thus cutting it into a series of plates of dentine and enamel, fused into one tooth by the cementum. (See Fig. 196.) A common pattern of tooth is formed by the junction of the two anterior and two posterior cusps by simple ridges ; and the cingulum may connect the outer ends of these two ridges ; such a tooth is seen in the Tapir, and in Palceotherium . By the varied obliquity of these ridges, and by the introduction of secondary inflections, patterns apparently dissimilar are arrived at. In the molar teeth of the horse, arrived at by a modification d.a. 25 386 A MANUAL OF DENTAL ANATOMY. of the Palseotherium type, we have a surface constantly kept rough by the varying hardness of its different constituents. In a worn tooth there are two islands of cementum upon a general field of dentine bounded by tortuous lines of enamel, and on the inner side a sort of promontory of dentine, bounded by enamel. The tortuous lines of enamel by virtue of their hardness will, at all stages of wear, be more prominent than the dentine or the cementum, and will hence maintain the efficiency of teeth as grinders. The patterns of grinding surface thus produced are very constant for allied species, so that an individual tooth of a herbivore may sometimes be correctly referred to its genus and always to its family. But as it will be necessary to recur to this subject from time to time, and especially in connection with the Ungulata, Fig. 193. — Molar tooth of horse, showing the characteristic pattern of its grinding surface. it will suffice for the present to point out that such corre- spondences do exist, and that all the complexities of pattern found may, in practice, be reduced to some few types. The development of additional cusps from upgrowths of the cingulum, and the suppression or fusion of pre-existing cusps, may be traced by a comparison of the teeth of allied animals, and thus connecting links are found between patterns at first sight very dissimilar. The order Proboscidea affords, however, so instructive an instance of the manner in which an exceed- ingly complex tooth has been derived from a simple one, that it may* be mentioned in this place as an example. The tooth of the elephant is so strikingly unlike other teeth, that it might at first sight be supposed that it is more essentially different than is really the case. The clue to its nature is afforded by the teeth of an extinct Proboscidian, the THE EVOLUTION OF THE FOKMS OF TEETH. 387 Mastodon. If the second true molar of one of the mastodons (Tetralophodon) be taken as a starting point its crown is found to be made up of four strongly pronounced transverse ridges, the summits of which are made up of rounded eminences (whence the name Mastodon, from fxao-ros, a nipple). The A R FiGr. 194. — Second upper molar of Mastodon (longirostris), from Falconer. A. Side view. B. View of the crown surface. About one-eighth natural size. The four transverse ridges, b, c, d, e, are seen to be, to some extent, divided into outer and inner cusps by a longitudinal cleft, much less deep than the transverse indentation. At the front there is a slight elevation of cingulum into a " talon " (a), and a similar one at the back of the teeth ; by its further elevation additional ridges or cusps might be formed. four transverse ridges coalesce at their bases, and the crown is supported upon a number of roots corresponding to the ridges. If we take the next tooth, or the third true molar, the general character remains the same, except that there are five ridges, and indications of as many roots ; still the general correspondence of the ridges with the cusps of less aberrant Fig. 195. — Molar tooth of an Asiatic elephant, showing the transverse plates of dentine bordered by enamel, c, cementum; d, dentine ; e, enamel. teeth is obvious. The crown is coated by enamel, over which there is a thin layer of cement, which does not fill, up the whole interval between the ridges. Thus the tooth, though very large, is not a very aberrant one ; it is obviously nothing more than a tooth in which the somewhat numerous cusps are connected by transverse ridges, and are very long and strongly pronounced. 25—2 388 A MANUAL OF DENTAL ANATOMY. To convert the tooth of a mastodon into that of an elephant, we should have to multiply the number of ridges,* to further increase their depth, to fill up solidly the interspaces between them with cementum and to stunt the roots. The completed tooth of an elephant is a square or rather oblong mass, from the base of which spring contracted and stunted roots. It consists of a common pulp cavity, small in proportion to the bulk of the tooth, and deep down in the mass; from it many thin laminse are sent up towards the surface, each forming the core of an area of dentine enclosed by enamel ; and the inter- spaces of these exaggerated cusps are solidly filled in by cementum. Between the Mastodon and the Indian Elephant are a number of transitional forms in which we are able to trace the gradual modification of the not excessively aberrant tooth of the Mastodon into the very peculiar huge molar of the FIG. 196. — Molar of Capybara, showing the transverse plates of dentine and enamel united to one another by cementum. Indian Elephant. And Andrews has recently found in Miocene formations in Egypt forms which illustrate the evolution of the Proboscidea from less aberrant mammalian types. The numerous transverse plates of the elephant's grinders are united by dentine at their bases, and a common pulp cavity and truncated roots are formed ; but in this last respect the molar teeth of the Capybara depart still further from the ordinary type, for, being molars of persistent growth, their numerous transverse plates of dentine and enamel do not become continuous, and there is no common pulp cavity. It is as though in an elephant's grinder the plates, which are for a long time distinct, never coalesced, but continued to grow on separately, being united with their fellows by cementum only. Further differences may be due to the partial or complete absence of one or another of the constituent tissues. * This multiplication of the ridges may be explained by an antero-posterior fusion of originally separate teeth (10b). — H. W. M. T. THE EVOLUTION OF THE FOBMS OF TEETH. 389 In what might be termed a typical tooth there should be a single axial pulp cavity surrounded by hard dentine, this again being capped over the crown by enamel and entirely invested by cementum. The layer of coronal cement may be absent, as in man or the Carnivora ; or the investment with enamel may be only partial, as upon the front of a Kodent incisor, or on the tusk of a Pig or Hippopotamus ; or a tooth may be composed solely of a mass of hard unvascular dentine, coated with cementum, as in the tusk of an Elephant. And just as endless varieties of teeth may be produced by the suppression, or partial suppression, of certain of the tissues, so differences may be brought about by the occurrence of other than the three usual tissues. Thus the remains of the central pulp cavity often become occupied by calcified pulp, forming " osteo-dentine " ; this, which occurs in man as an almost pathological condition, is perfectly normal in many animals, in the sperm whale for instance, or in the constantly growing teeth of the sloth, the central axes of which are occupied by dentine permeated by vascular canals. Kefeeences. (x) Ameghino, F. "On the primitive type of the Plexodont Molars of Mammals." Proc. Zool. Soc. Lond., 1899, pp. 555 — 571. (2) Bolk, L. " Die Ontogenie der Primatenzahne." 1913, Jena. (3) Hopewell-Smith, A., and Tims, H. W. Maeett. " Tooth-germs in the Wallaby, Macropus billardieri." Proc. Zool. Soc. Lond. 1911, pp. 926—942. (4) Majoe Poesyth, 0. L. " On some Miocene Squirrels, with remarks on the Dentition and Classification of the Scuirince" Proc. Zool. Soc. Lond., 1893, p. 179. (5) Mivaet, St. Gr. " Notes on the Osteology of the Insectivora." Journ. Anat. and Phys., 1868, vol. ii., pp. 117 — 154. (6) Osboen, H. P. (a) " The Evolution of Mammalian Molars to and from the Tritubercular type." Amer. Naturalist Philad., 1888, vol. xxii., pp. 1067—1079. (b) ' ' Nomenclature of Mammalian Molar Cusps." Ibid., 1892, vol. xxvi., p. 436. (c) " Trituberculy ; a review dedicated to the late Professor Cope." Ibid., 1897, vol. xxxi., pp. 993 —1016. (d) " Evolution of Mammalian Molar Teeth." London, 1907. 390 A MANUAL OF DENTAL ANATOMY. (7) Osborn, H. F. and Earle, C. " Fossil Mammals of the Puerco Beds." Bull. Amer. Mus. Nat. Hist., 1895, vol. vii., pp. 1 — -70. (8) Eyder, J. A. (a) " On the mechanical genesis of tooth-forms." Proc. Acad. Nat. Sci., Philad., 1878, pp. 45—80. (b) "Further notes on the mechanical genesis of tooth-forms." Ibid., 1879, pp. 47 — 51. (9) Scott, W. B. "The Evolution of the Premolar teeth in the Mammals." Proc. Acad. Nat. Sci., Philad., 1892, pp. 405—444. (10) Tims, H. W. Marett (a). " On the tooth genesis in the Canidce" Journ. (Zool.) Linnean Soc. Lond., 1896, vol- xxv., pp. 445 — 480. (6). "The Evolution of the Teeth in the Mam- malia." Journ. Anat. and Phys., 1903, vol. xvii., N.S., pp. 131—149. CHAPTER XIV. THE TEETH OF MAMMALIA. — PROTOTHERIA. The ancestry of the Mammalia remains still somewhat uncertain, and the Theriodont reptiles are possibly not in the direct line of descent, though they present certain resemblances to mammals, and are perhaps not far from the stem. The mammalia are usually now divided into three groups — Prototheria, Metatheria, and Eutheria. The Prototheria comprise numerically but few animals, as no extinct forms are known with any certainty, and of living animals only two families exist, namely, the Omithorhynchidce and the Echidnidce. They stand at the bottom of the mammalian class, and present many points of affinity with lower vertebrates, par- ticularly with Sauropsida and Amphibia. Metatheria, like the preceding group, have characters which place them low in the mammalian scale. They are numerously represented at the present day, comprising the animals known as Marsupials, which practically monopolise the Australian region, but exist also in the South American continent. In former times they existed over other portions of the globe, and some of the earliest mammalian fossils are perhaps referable to the group. Eutheria comprise all the mammals in whom the young are nourished by means of an allantoic placenta. It must not be supposed by the student that the Metatheria of the present day are descended directly from the Prototheria, or that the Eutheria are direct descendants of the marsupials. It is more probable that exceedingly early and unknown mammalian forms have given rise to each of these stems, and that the Prototheria have advanced but little, the marsupials a good deal, giving rise to a large number of forms, while the Eutheria have far outstripped the others, and have been and are supplanting them in all directions. 392 A MANUAL OF DENTAL ANATOMY. PROTOTHERIA. The Echidna, or Scaly Ant-eater, is, so far as is known, entirely edentulous at all stages of its growth. Its palate and tongue are, however, beset with hard spines. The Ornithorhynchus, which in its soft parts and skeleton alike differs from higher mammals in points which approach the characters of Sauropsida, is furnished with wide flattened Fig. 197. — Upper and lower jaws of Ornithorhynchus. From the lower jaw the skin, &c., has been removed, but it remains upon the upper. a. Anterior horny plate, b. Posterior horny plate, with remains of tooth sockets, 1, 2, 3. jaws, in which horny plates fulfil the function of true teeth ; from this peculiarity comes its name of duck-billed Platypus. Caldwell discovered that, though possessed of functional mammary glands, it lays eggs, and subsequently Professor Poulton in 1888 found that at an early stage tooth-germs were present, as he supposed, underneath the horny plates. Following upon this, Oldfield Thomas found that its true teeth came into actual use, and that they were not beneath but superficial to the horny plates. THE TEETH OF MAMMALIA. — PKOTOTHERIA. 393 To take first the dentition, if such it can be called, of the adult animals, the horny plates with which the jaws are ultimately furnished are four in number in each jaw, the anterior plate being a thin long band with a longitudinal ridge and a furrow in its inner side, and the posterior plate a flat, broad-topped mass, the surface of which is roughened by a series of ridges separated from one another by concavities, Fig. 198 Fig. 199 Fig. 198. — Horny plate, a, of a half-grown Ornithorhynchus, with empty pits, b, for the reception of the teeth. Fig. 199. — Horny plate with teeth in situ. c. Long cusp of tooth. After Professor Stewart, from a specimen in the Royal College of Surgeons Museum. the ridges of the opposing plates interdigitating with one another. These plates are simply pronounced thickenings and hardenings of the oral epithelium, and their under- surfaces are penetrated by long papillse, each of which sends up a pro- longation of soft, deeply-staining cells from its apex. The plates are at their sides quite continuous with the stratum corneum of the epithelium, of which their harder portions are composed ; there is no calcification, and no bony structure in them. Somewhat similar horny structures occur upon the tongue. 394 A MANUAL OF DENTAL ANATOMY. A full account of the structure and form of these plates will be found in a paper by Poulton (2), which is devoted to a full description of his most interesting and significant discovery of the existence of the true tooth-germs, which he then believed to underlie the forming horny plates. From want of material he was somewhat uncertain as to the extent to which the calcification of these teeth goes and what afterwards becomes of them, but he saw that there were the germs of four teeth in the upper on each side, and probably the same number in the lower jaw ; and they occupy a widely open furrow, which he thought was subsequently occupied by the horny plate. [In a more recent paper by Professors Wilson and Hill (6) the tooth development in Ornithorhynchus has been described. Fig. 200. — Schematic figure showing' the actual tooth-germs in Ornitho- rhynchus. (After Wilson and Hill, Quart. Journ. Micr. Sci. Lond., vol. li., N.S., 1907.) In a young specimen 80 mm. in length, " each jaw shows two definite papillated enamel-organs," and in front of these a " calcined vestigial toothlet." In an older specimen with a length of 250 mm. there are present in each jaw "two very large enamel-organs, each with a number of calcined cusps," representing the two large multicuspidate teeth of the young adult. Anteriorly to these in the upper jaw an enamel-organ with a single calcined cone, and still more anteriorly another rudimentary papillated enamel-organ. In the lower jaw, in front of the two large multicuspidate teeth, is a minute calcified tooth, while behind them is a well-developed uncal- cified enamel-organ. On comparison with the dentition of the young adult the THE TEETH OF MAMMALIA. — PROTOTHEKIA. 395 authors conclude that in the two specimens which they examined there are " representatives of five quasi-permanent teeth " in each jaw, together with " vestigial representatives of deciduous predecessors to the most anterior of these five tooth-elements." The accompanying figure (Fig. 200) taken from Wilson and Hill's paper show all these in a diagrammatic way, with the exception of the most posterior upper molar, fx iff Fig. 201. — 1. Left upper teeth of Ornithorhynclms. 2. Left lower teeth. 3. Molar teeth of Microlestes, much magnified. (After Oldfield Thomas, Proc. Roy. Soc. Lond., vol. xlvi., 1890.) which in these stages is only represented by a special thickening of the dental lamina. These observations are of very considerable interest as they not only demonstrate the presence of two dentitions in the Monotremes, as had previously been surmised by Tims (5), but they also afford evidence of concrescence in the molar teeth.] In this connection it is interesting to note that Professor Huxley, in writing of the Monotremes, had years ago expressed the conviction that there is good reason to suppose that edentulous forms are modified descendants of toothed forms. 396 A MANUAL OF DENTAL ANATOMY. Oldfield Thomas (4) and subsequently Professor Stewart (3) found that the true teeth of Ornithorhynchus are cut, and for a time are in actual use. They are twelve in number, two on each side being of some size, and the third very small. The upper teeth have broad-topped crowns, with two long cusps on the inner edge, and a crenated border along the outer edge with many small cusps ; in the lower this is reversed. They have low broad crowns with short stunted roots, by which, however, they are for a time fairly firmly held. The crown pattern of these teeth are strikingly like those of the fossil Microlestes, as is shown in Fig. 201. Instead of being beneath the horny plates, they are on the top of them, and their implantation is peculiar ; the expanded crowns narrow rapidly at the neck, and they are surrounded by a very dense and thick epithelium, almost horny, which rises into a ring round them and dips underneath the expanded portion, so that the tooth lies in a sort of cup of horny con- sistency. This cup is not complete at the bottom, but the roots pass through it and fit depressions in the bone, which is perforated by foramina for their vessels and nerves. When the creature is about twelve inches long the teeth are shed, and then the horny cups grow in underneath and become complete. Thus the curiously cupped and sculptured surface of the horny plates which have been so long familiar, has its form determined by once having formed the bed for a tooth with several roots, and although the horn grows underneath and fills up the holes for the roots to go through, yet the old general form is more or less maintained by the horny plate which serves as the organ of mastication throughout the rest of the life of the animal. So far as is known, this is an arrangement quite unique amongst mammals, or indeed any other tooth-bearing creatures. The horny plates are therefore not at all to be regarded as horny teeth, but they are epithelial structures which take the place of the teeth after these are shed, and therefore they are not absolutely similar to the horny teeth of lampreys and myxinoids. The true teeth consist of a body of dentine, with a central pulp cavity, capped with thin but hard enamel, and implanted THE TEETH OF MAMMALIA. — PROTOTHERIA. 397 by short roots, the breadth of crown greatly exceeding its vertical dimensions. The enamel is- apparently of simple structure, and the dentine is permeated by fine dentinal tubes and beset with a wonderful number of interglobular spaces, which in parts of the crown mask its tubular structure. In the principal cusp larger, apparently vascular, canals exist, and as one approaches the stunted roots a somewhat abrupt transition in structure takes place, all dentinal tubes disappearing and large lacunae appearing. The roots are hence of softer, coarser material than the crown, which itself is not of a high type of dentine structure. Thus the dentine structure of the tooth is somewhat that which we are accustomed to see as a result of pathological processes, and would suggest, so far as it goes, that the Fig. 202. — Molar tooth of Ornithorhynchus. c. Long cusp. Omithorhynchus -tooth has degenerated from some earlier and more complete tooth-form in which the roots consisted of properly developed dentine. The author is not acquainted with an example of this degeneration in the type of dentine formation as the root portion of the tooth is approached, in any other mammalian tooth. In those teeth which have no roots, if such an expression may be allowed, but which are about to become anchylosed to the bone, something of the kind may be seen. (Cf. also the tooth of Hespero7°nis p. 334.) The British Museum specimen described by Oldfield Thomas (loc. cit.)* shows that the teeth were subjected to severe attrition, as one, which was about to be shed, was worn as thin as paper. No tooth, fossil or recent, corresponds in naked-eye characters with that of Ornithorhynchus, though that of the mesozoic Microlestes, as has been already stated, does so in some degree. * An interesting discussion on the significance of the teeth of Ornitho- rhynchus, by^Professor St. George Mivart, will be found in the " Proc. Roy. Soc." 1888. 398 A MANUAL OF DENTAL ANATOMY. Microlestes has generally been looked upon as a marsupial, allied to Plagiaulax, though Cope has suggested that all the group of multituberculate toothed mesozoic mammals may have been monotremes. But a microscopic examination of a tooth of Microlestes, which Professor Poulton kindly allowed the author to make, shows no close resemblance to the recent marsupials, for the enamel is not clearly penetrated by dentinal tubes, nor does it present the peculiarities of structure found in Ornitho- rhynchus. Thus its structure gives no help, but rather goes to render still more indefinite its probable zoological position. Though there is no doubt that the Omithorhynchus reveals to us a tooth of a period very remote in point of time, it has yet by no means a simple crown, and it does not help us much towards knowing anything which can be regarded as a primitive parent form whence all mammalian teeth may have been derived. Still the discovery is of incalculable value, inasmuch as many of the early mammalia are known to us only by teeth, and the form and structure of a monotrematous tooth gives us something far lower in the evolutionary scale than had hitherto been known. Eeferences. (x) Poulton, E. B. " True Teeth in the Young Omithorhynchus para- doxus." Proc. Eoy. Soc, LoncL, 1888, vol. xliii., pp. 353—356. (2) " The True Teeth and the Horny Plates of Omitho- rhynchus" Quart. Journ. Micr. Sci., 1889, vol. xxix., N.S., pp. 9 — 48. (3) Stewart, 0. "On a Specimen of the True Teeth of Omitho- rhynchus" Quart. Journ. Micr. Sci., 1892, vol. xxxiii., N.S., pp. 229—231. (4) Thomas, 0. " On the Dentition of Omithorhynchus" Proc. Eoy. Soc. Lond., 1889, vol. xlvi., pp. 126—131. (6) Tims, H. W. Marett. " The Evolution of the Teeth in the Mammalia." Journ. Anat. and Phys., 1903, vol. xxxvii., pp. 131 — 149. (6) Wilson, J. T. and Hill, J. P. " Observations on Tooth-Develop- ment in Omithorhynchus." Quart. Journ. Micr. Sci., 1907, vol. li., pp. 137—165. CHAPTER XV. THE TEETH OF METATHERIA. MARSUPIALIA. Marsupialia Polyprotodontia Paucituberculata Diprotodontia f Didelphyida} J Dasyuridce | Notoryctidce I Peramelidce Epanorthida ... i f Phascolarctidoe [PhalangeridcB The great sub-class of marsupials, consisting of animals sharply marked off from placental mammals by many striking peculiarities, amongst others by the absence of a well-developed allantoic placenta and by the very helpless condition in which the foetus is born, was once very widely distributed over the globe. Now, however, marsupials are numerous only in Australia, where they are almost the sole representa- tives of the mammalian class. There are a few marsupials elsewhere, as in America (Opossums) and New Guinea ; but there are no marsupials at the present time in Europe, most parts of Asia, or in Africa. There is a good deal of uncertainty as to the relation of marsupials to theEutheria (placental mammals). It was formerly assumed that a good many Mesozoic mammals, known only by their jaws, were marsupials; but some of these may have been Monotremes, and doubt is felt about a good many others, which, though they have points of resemblance to the marsupials, cannot, for the want of sufficient material, be definitely referred to that sub-class. Recently the discovery of a rudimentary placenta in the Bandicoot (Perameles) as well as of some vestiges of a second dentition suggests the possibility that after all marsupials may be degenerate forms of placental mammals, or that the two may have had a common ancestry. The opinion held that (although marsupials in former ages were animals apparently very widely distributed) placental mammals are not descended directly from them, is gaining ground. The marsupials of America are all Opossums (Didelphidce), and this family is not represented in Australia. The ancestors of the marsupials of America are now regarded as a primitive form whence all other marsupials may have been derived. The marsupials of Australia almost monopolise that country ; thus Wallace says of it : " The Australian region is broadly distinguished from all the rest of the globe by the entire absence of all the orders of non-aquatic Mammalia that abound in the old world, except two : the Winged Bats (Cheiroptera) and the equally cosmopolite Rodents. Of these latter, however, only one family is represented — the Muridce, (comprising the Rats and Mice)— and the Australian representatives of these are all 400 A MANUAL OF DENTAL ANATOMY. of small or moderate size, a suggestive fact in appreciating the true character of the Australian fauna. "In place of the Quadrumana, Carnivora, and Ungulates, which abound in endless variety in all the other zoological regions under equally favourable conditions, Australia possesses two new orders or sub- classes, Marsupialia and Monotremata, found nowhere else in the globe, except a single family of the former in America. " The marsupials are wonderfully developed in Australia, where they exist in the most diversified forms, adapted to different modes of life. Some are carnivorous, some herbivorous, some arboreal, others terrestrial. There are insect-eaters, root-gnawers, fruit-eaters, honey-eaters, leaf or grass-feeders. "Some resemble wolves, others marmots, weasels, squirrels, flying squirrels, dormice or jerboas. " They are classed in six distinct families, comprising about thirty genera, and subserve most of the purposes in the economy of Nature fulfilled in other parts of the world by very different groups ; yet they all possess the common peculiarities of structure and habits which show that they are members of one stock, and have no real affinity with the old- world forms, which they often outwardly resemble " (" Geographical Distribution of Animals," p. 391). This passage from A. K. Wallace has been quoted because much of it is applicable to the teeth. In Australia, the present home of the marsupials, repre- sentative species abound ; that is to say, widely different though the animals really are, there are many genera and species which have the habits, and, as it were, fill the place of such creatures as the Carnivora, Insectivora and Eodents amongst the placental Mammalia. And not only do they possess something of their habits and external configuration, but in those characteristic structures which are subservient to the creature's immediate wants the marsupial representatives closely mimic the more highly organised placental mammals. Thus the teeth of an insect-eating marsupial very closely resemble those of a true insectivore, though retaining certain eminently marsupial characters ; in the same way the dentition of the marsupial Thylacine mimics that of a dog. (Compare Figs. 207 and 208.) And although marsupial dentitions do vary very much, yet there are many transitional forms by which we are sometimes able to trace the successive modifications through which extreme divergence has been ultimately attained. Just as we ascribe to placental mammals the formula — .314 THE TEETH OF METATHEKIA. — MARSUPIALIA. 401 as the typical or parent dental formula, so to recent marsupials the following has generally been attributed : — • .313 4 AA iTcTpTmT=44. This formula was arrived at prior to the existence of any data as to the nature of the succession of teeth in the marsupials, and was, therefore, necessarily based only upon the forms of the teeth, which appeared to indicate such a classification, and it has long held the field unquestioned. One fact at least is well established : in marsupials there is never any vertical replacement of teeth except at one point in the jaw, and this is always the same point. It is always the third cheek tooth from the front which is vertically replaced, in some instances both deciduous and replacing teeth being equally fully developed and in others reduction falling upon one or both. But the advance of palaeontology, by bringing to light extinct forms which may or may not have been actually marsupials, and the investigation of the peculiar succession found in recent marsupials and in creatures which appear to have been their ancestors, as well as in some Eutheria, throw doubt upon the correctness of this division into premolars and molars as differing from that found in placental mammals, and the difficulties thence arising have led one palaeontologist, Ameghino, to go so far as to abandon the division altogether and to number the cheek teeth in one category as one to seven from front to back. The close resemblance between the dentitions of marsupial and placental carnivorous animals was noted long ago by Gaudry, who, on the strength of this resemblance, assigned the same dental formula to the Thylacine, to Hycenodon, 4 3 and to the Dog, viz., pm— and m^-. Lydekker has pointed out (5) that if we place side by side the figures of the dentitions of Hycenodon, Borhycena, Thylacinus (recent), Prothylacinus, and Amphiproviverra, it is obvious that to assign to them any fundamental difference in dental formula must be artificial. Yet here we have Creodonts lying upon or near to the ancestral stem of the Carnivora, ancestral marsupials, and modern marsupials, and he seeks to find a dental formula for d.a. 26 402 A MANUAL OF DENTAL ANATOMY. the recent marsupials which will fit the demonstrated peculiari- ties of their tooth-change and at the same time harmonise them with the extinct forms. This he does in the first place by adopting the view that the functional teeth of the marsupials (with the exception of the one replacing tooth) are homologous with the milk teeth of placental mammals, includ- ing amongst milk teeth in both the true molars which are never replaced in any mammal. On this interpretation the fourth marsupial cheek tooth (first true molar, according to the usual designation) becomes the fourth milk molar, a tooth deciduous and replaced by a premolar in placental mammals, but not shed nor replaced in marsupials ; in other words, it is the homologue of the dm4 of Sus or Canis, and only remains because no pm4 is developed below it. It is pointed out by Lydekker that the tooth in this position differs little in form from the true molar behind it. This resemblance was the cause of marsupials having been considered as possessing four true molars. In his view the fourth permanent premolar in marsupials is not developed ; indeed, only one permanent premolar, pm3, is developed, and he would write the functional marsupial dentition thus : — . 3 1 mpirii mpm2 ppm3 mpm4 mi 1112 m3 mi^r mc-r- — — — , 3 1 mpm! mpm2 ppm3 mpm4 mi m2 m3 or briefly — i-|-c-i-mm-| + pm (cf. p. 359). But whatever explanation we adopt of the homologies of the teeth of marsupials, and of the replacing teeth in particular, some considerable difficulties remain, and the question is not as yet to be regarded as finally settled. For if the deciduous tooth and its successor are regarded as belonging to different serial dentitions, it is equally strange whether we consider the milk dentition to have been reduced to one functional tooth, the others having been suppressed so very completely (i.e., if we regard it as the last trace of a disappearing milk dentition), or if we consider the single successional tooth as the sole representative of a commencing second dentition. And this anomaly was established in very early geological times, in Triconodon and others. It will therefore be necessary to consider the various hypotheses which have been advanced to explain the anomaly, and in the present edition the lettering THE TEETH OF METATHEBIA. — MARSUPIALIA. 403 of the figures and to some extent the letterpress have been left unaltered, so that they follow the older theory, of there being three premolars and four true molars. An excess over the total numbers of the typical mammalian dentition is, however, more common in marsupials than in placental mammals. In Macropus billardieri Hopewell- Smith and Tims have found what they believe to be evidences of six incisors on each side in the upper jaw. In the lower jaw five ante-molar teeth are represented, but whether the most posterior is to be regarded as a canine, or as the outermost incisor, the canine having disappeared, these writers are unable to determine (2)* This affords some evidence of a polyprotodont ancestry. M. F. Woodward (15) examined by means of serial sections, as well as by clarifying whole jaws in clove oil, a very fine series of marsupial embryos. He found that in Petrogale (one of the kangaroo family) there are no less than six incisors in the premaxillary bone, of which three were minute calcified vestigial teeth, while the other three were in the stage of tooth-germ only, but were of far larger size. The opossums have ten incisors in the upper and eight in the lower jaw ; the Thylacine, very dog-like in its dentition, has eight upper incisors ; whilst in the little ant-eating Myrme- cobius the number amounts to a total of fifty-two or fifty-six teeth, having eight or nine molar and premolar teeth on each side of the jaw. But though after careful investigation Woodward was at that time fully convinced that the teeth did not belong to two dentitions, but that they, like the rest of the functional series, certainly all belonged to the milk series, he later on partly abandoned this view (15). Moreover, he holds that this animal possesses five premolars. This is less of a difficulty than it would otherwise be, as the recent Myrmecobius has eight or nine cheek teeth on each side, and the ancient Amphilestes had twelve, of which, judging by form, six would be classified as premolars. It was first pointed out by Flower (x) that in recent marsu- pials, and in such extinct forms as were then known, there is only a successional tooth developed at one point in the jaw. Only one of the premolars (the third, or hindmost according to the old view) has vertically displaced another 26—2 404 A MANUAL OF DENTAL ANATOMY. tooth. From this it was at first argued that the whole milk dentition consists of four milk molars (one on each side of each jaw), there being no succession among incisors nor canines in any known marsupial. This view is now partly obsolete. It was further pointed out by Flower that the extent to which the solitary displaced molar is developed varies much in the different families : no trace of any succession has been observed in the Wombat ; in the Thylacine the small deciduous molar is calcified, but is absorbed or shed prior to any other teeth being erupted ; whilst in the Kangaroos it is retained till a much later period (see p. 419), and in the Kangaroo Eat (Hypsiprymnus) the deciduous molar has not yet given place to its successor at the time when the last permanent molar has come into place, so that it, for a long time, ranges with the other teeth and is functional. This subject was further investigated by Oldfield Thomas, who found that the Dasyuridce present the same deciduous dentition as the other families, that is to say, that some have a well-developed deciduous molar, and that it occurs in various phases of reduction, some having none at all (6). This author, writing of Phascologale, one of the Dasyuridce, says, — " The normal state of a member of this group is to have three well-developed premolars, the last one of which has a (milk) predecessor. Then a tooth reduction has taken place, all of which has fallen upon what is evidently a peculiarly plastic tooth, i.e., pm4, and this, with the (milk) tooth preced- ing it, has been decreased in various degrees and in the end suppressed, as in the allied genera Dasyurus and Sarcophilus." Those species which have a large pm4 have preceding it a tricuspid (milk) tooth, but as pm4 gets reduced the tooth preceding it is reduced still faster till it quite aborts; even then the pin4 presents the peculiarity of not being erupted and indeed not being calcined, until later than the other teeth. It will be observed that he speaks of the replacing tooth as the fourth premolar, as in fact homologous with the last premolar of placental mammals, although marsupials develop but three functional premolars, as then believed. Even assuming that no living marsupial has more than three premolars, it had been presumed that four was the original number, as in placental mammals, and Oldfield Thomas had found in Dasyurus and in Phascologale dorsalis THE TEETH OF METATHEKIA. — MARSUPIALIA . 405 a fourth tooth, in the position of pm2 ; he infers, therefore, that it is pm2 which has been lost by recent marsupials, and further points out that a marked gap exists in this situation in Didelphys, Perameles, and others. If this be so, it to some extent militates against the acceptance of Lydekker's marsupial dental formula. Hopewell-Smith and Marett Tims have recently (loc. cit.) investigated the tooth-genesis in the Wallaby (Macropus billardieri) and have confirmed Oldfield Thomas' conclusion that pm2 is the missing tooth in the premolar series of the adult marsupial dentition. This tooth is represented by a ■: "/' " '- ' 7? . * , -p in2 - :. t ■ Fig. 203.— Showing the deeply situated bulbous rudiment of pm2 with the very long neck of the enamel-organ, reaching to the surface of the oral epithelium. (After Hopewell-Smith and Tims, Proc. Zool. Soc. Lond., 1911.) vestigial enamel-organ which is connected to the oral epithelium by a remarkably long neck. The enamel-organ is in fact crowded out of its normal position, and comes to lie deeply embedded in the jaw (Fig. 203). This also confirms the view that the marsupials originally possessed the typical mammalian number of four premolars, and that the vertical tooth replacement is concerned with pm4. In the Mesozoic Triconodon, which had seven cheek teeth, it was the fourth which had a vertical successor, and in Miocene formations in South America dentitions resembling the recent Thylacinus have been found in which there are 406 A MANUAL OF DENTAL ANATOMY. two replacing teeth, and not merely one (Prothylacinus, Borhycena), these being apparently premolars 2 and 3. To recapitulate, then, we have in recent marsupials three cheek teeth which, judging them by form alone, we should be inclined to call premolars, of which the last one only is vertically replaced. Behind these come four teeth which we should, again judging by form alone, be inclined to call true molars. But when we extend our survey to extinct forms this classification becomes less satisfactory, and to meet the difficulty Lydekker, who points out that Owen more or less anticipated him in this view, makes the following proposal. Premising that all true molars are homologically milk teeth, and that, with the exception of the one replacing tooth, all the teeth of marsupials belong to the same series, he regards their dentition as a persistent milk dentition plus one tooth of a successional dentition. He would call those milk molars of a placental mammal, which are displaced ultimately by suc- cessional teeth or permanent premolars (pp), milk premolars (mp). Hence he would write the formula of a marsupial thus : . 3 1 mpi mp2 pp3 mp4 1.2.3 mi - mc i-1 — v±±M — ^ m - — - — -, 6 1 mpi mp2 pp3 mp4 1.2.3 or shortly .31 .43 i - c - mp + pp - m -. There would then be no essential difference as to number of premolars and true molars between the marsupials and the placental mammals, and this view is adopted by Wortman. The question whether the tooth change of the marsupials is the remnant of a more complete change in an ancestral form, or is the dawning of a more complete change as believed by Flower, has been discussed at length by Oldfield Thomas. That it is the first formation of a change he formerly held strongly (i.) because the marsupials are at a lower stage of evolution than other mammals, and so it would be unlikely that they had once evolved a change, and then evolved it away again ; (ii.) because five out of the six marsupial families have precisely the same amount of tooth change, which would be unlikely if it were an atavism ; (iii.) and because no fossil marsupials then known presented any indication of a fuller change, but had just the same. THE TEETH OF METATHEEIA.— MAKSUPIALIA. 407 But in the face of the discoveries of Kiikenthal, Bose, Leche, and others, Oldfield Thomas abandoned this position and accepted the view as incontestable, that the functional dentition of marsupials is homologous withthe milk dentition of true Diphyodonts, there being a single permanent or successional tooth developed on each side of the jaw. But before entering further upon this question it is desirable to clear up if possible the homologies of the whole functional set of teeth of marsupials. With regard to this, the most diverse opinions still prevail. When it was discovered by Flower that there was a vertical succession of one tooth only, the last of the then apparent premolars, he at first thought that all the rest of the functional teeth, with this one exception, corresponded to the permanent teeth of other mammals ; and that there was a milk dentition consisting of one tooth only, variable in the extent to which it was developed, so that he regarded it as presenting the first appearance of a milk dentition. There were many difficulties in the way of accepting this view, which was nevertheless accepted by Oldfield Thomas in an elaborate paper on the Dasyuridce (6). But Kiikenthal and Bose, investigating serial sections of the jaws of young animals, found that the dental lamina presented swellings to the lingual side of the developing functional tooth-germs, which they interpreted as being aborted permanent teeth, and hence they came to the conclusion that the whole of the functional set must be regarded as the milk teeth, and that the permanent teeth were suppressed. And this view was cordially accepted by Thomas (who abandoned his earlier ideas on the subject), and more recently by Lydekker. The molars, originating from the dental lamina and never having had any predecessors, they regard also as belonging to the milk series, just as they regard them in other mammals. This general idea, that the functional teeth really represent the milk dentitions of other mammals (with, perhaps, the solitary exception of the one replacing premolar), has gained somewhat general acceptance ; and it has been sought to explain the various other phenomena presently to be described in accordance with this hypothesis. Woodward (15) in 1892 discovered that in some of the Macropidce there were a number of small calcified teeth, in addition to the tooth-germs of the functional teeth. In Petrogale he found, in addition to the tooth - germs of the three functional incisors, three minute but calcified rudiments in the upper jaw, and two in the lower. Whilst accepting the view that the functional incisors were of the milk dentition, he regarded the rudiments which he discovered as belonging to the same series ; that is to say, he considered that they represented additional suppressed teeth, and hence were indications that the polyprotodont marsupials had six incisors on each side in the upper jaw. Subsequently, however, he adopted Leche' s interpretation that they were pre -milk teeth. Leche (3 and 4) described in Myrmecohius the occurrence of calcified rudiments which lay to the labial side of the functional tooth-germs, 408 A MANUAL OF DENTAL ANATOMY. and therefore were referable to an antecedent set of teeth. Bound by the idea that the functional teeth are milk teeth, he was compelled to assume that these rudiments are referable to a premilk series, and this interpretation was accepted by Woodward, who discovered some additional calcified rudiments in Myrmecobius and held that he had found con- firmation of this by the discovery of similarly placed rudiments in Phascologale. It will be seen that so far the interpretation of the homologies of the functional teeth turns wholly upon the degree of significance which is attached to the observations of swellings of the tooth- band to the lingual side of the functional tooth-germs, that is to say, to the observations of Rose and Kukenthal, who interpreted them as unquestionable tooth- germs, and whose interpretation was for some time accepted almost without question. But if these are not real tooth-germs, that is to say, if they are only swellings such as may occur in the residual tooth-band without such definite significance, the whole case falls to the ground. And this question is strongly raised by Messrs. Wilson and Hill(14), who, in an exhaustive paper upon the development of the teeth in Perameles, have ably summarised the whole matter (of this summary the author has made much use). They hold that (using the criteria laid down by Leche, who, however, has not been led to this conclusion by his own criteria) there is no adequate evidence that the differentiation of these swellings is sufficient to entitle them to be considered as tooth-germs, and hence that there is no evidence that Leche's tooth rudiments in Myrmecobius are to be regarded as " premilk,'1 but that it is a far simpler and less far-fetched interpreta- tation that they, as well as the germs observed by themselves, by Woodward and others, are ordinary milk teeth. Hence their conclusion is, that the functional dentition of a marsupial is the permanent dentition of other mammals, and that the milk dentition is the one upon which suppression has fallen in varying degrees of com- pleteness. This conclusion had also been arrived at independently by Marett Tims (8 and 9). The marsupials, according to these writers, are to be regarded as truly diphyodont mammals, in whom the milk dentition, not wanted owing to the peculiar conditions of the birth of the young and their long sojourn in the pouch attached to the teats of the mother, has practically vanished. This view, if adopted, greatly simplifies the whole question, and is the one to which the author inclines from a study of the various papers published upon the matter ; it, of course, does away with the somewhat improbable assumption of the existence of a" premilk " dentition in the marsupials. The observations of Rose upon the early conditions of the Wombat are important in this connection ; he found that there were rudiments of calcified teeth entirely unrepresented in the functional dentition, and that they belong to a generation of teeth antecedent to the functional teeth. Moreover, he finds in the place subsequently occupied by the single per- sistent premolar, a precociously calcified tooth with a large germ to its THE TEETH OF METATHERIA. — MARSUPIALIA. 409 lingual side, which latter he regards as the germ of the functional tooth. To fit in this observation of Rose's with the widely accepted views as to the functional teeth of marsupials being the milk teeth leads into difficulties, and Rose himself frames the far-fetched hypothesis that in the Wombat there is a difference in the homologies of the teeth in the front and the back of the jaw, and so that this animal stands very much alone in a very exceptional condition as regards its dentition. Leche, on the other hand, sets aside Rose's difficulties (arising from Rose feeling compelled to regard the calcified precocious germs of the front of the jaw as true milk teeth), and relegates the functional teeth of the Wombat to the milk dentition. The view of Wilson and Hill {vide supra) reopens the question of the homologies of the replacing tooth and its predecessor. They regard the deciduous tooth as a true milk tooth, ranging with the other calcified and uncalcified milk rudiments (premilk of Leche and Woodward), and point to the fact that it, like them, has in some cases completely disappeared, as in Dasyurus, though even in this animal it is present as a calcified vestige in the mammary foetus, while in Thylacinus the successional premolar is a well- developed tooth, and is preceded by a minute and rudimentary milk tooth shed in infancy. Hence they hold that there is a process of reduction affecting first the milk tooth, and finally both the milk tooth and its successor, as in Dasyurus, and that this process of reduction is the same that has affected all the other milk teeth ; it is the hindmost member of the milk series, and upon it reduction has come last, and so less than upon the more anterior milk teeth. For the curious fact that there is a succession at one point in the jaw only, Oldfield Thomas suggested as an explanation that there first took place a retardation of a back permanent tooth, perhaps useful for " packing" purposes in a jaw as yet small, and that when the retardation had gone to a certain point a milk tooth was developed to fill the gap in the series which would otherwise have existed. A different light was thrown upon the subject by the very interesting paper of M. F. Woodward : he finds that even the successional tooth-germ itself develops from the primitive tooth-band, between pm.3 and pm4 as they are usually supposed to be ; he therefore believes that it really belongs to precisely the same series as the others, and that it is delayed by being crowded out. This suggestion, that it really belongs to the same series as the other teeth, i.e. the milk series, seems much more probable than that just one single tooth of a different series should attain to the very full development which it does in some genera. Rose comes somewhat near to the same idea, as he urges that in some species of Didelphys, Perameles, etc., it has been shown that no tooth change takes place after the rule laid down by Flower and Thomas, but that a late tooth thrusts itself up into the row between two of the others without the absorption of any of them, so that the question arises whether there is any real tooth change in marsupials, in the ordinarily accepted sense of the word. 410 A MANUAL OF DENTAL ANATOMY. Leche has apparently adopted the belief that the single tooth change is the herald of a new dentition, but he holds to the idea that the functional teeth are milk teeth, and regards the epithelial swellings of Kukenthal and Kose not as remnants of a lost second set, but as heralds of a second set not yet acquired by the marsupials. The whole question of the single tooth replacement has been reinvestigated by Marett Tims (10). He draws attention to the significant fact that the deciduous tooth, like the posterior deciduous premolar of the Dog, Man, and certain Insectivores, is molariform in pattern, and therefore unlike its successor ; and further, that the successional tooth develops in all these cases in front of its predecessor, the reverse of that which obtains in all other instances of tooth replacement. He there- fore regards, as Woodward had done, all the teeth of the marsupials as belonging to the same series, but differs from the latter writer in i c e b Fig-. 204. — Showing a Concentric Epithelial (c.e.b.) from the jaw of Macropus bittardieri. (After Hopewell-Smith and Tims, Proc. Zool. Soc., Lond., 1911.) regarding this series as the homologue of the successional series of the Eutheria (vide supra). According to this interpretation the deciduous tooth in all the animals above mentioned is in reality the first of the molar series pushed over the top of the replacing tooth, which is the last of the premolar series, due to a progressive shortening of the jaw. The degree to which this shortening has taken place determines the age at which the tooth succession occurs. In Myrmecobius with its very elongated narrow jaws no tooth change is definitely known to occur, and this animal possesses a larger number of molar teeth than any other mammal, a further evidence of its primitive character. In confirmation of this interpretation Tims believes that he has found in the Dog a vestige of the true dpm4, crowded between the ppm4 and its predecessor (Pm1). With regard to the molar teeth he agrees with M. F. Woodward, Lataste, and Magitot in regarding them as belonging to the successional series, the vestiges of their predecessors being still preserved as " concentric epithelial bodies." That such a projection of the anterior molar teeth over the top of the posterior premolars does take place, and, moreover, that it is a phenomenon of considerable antiquity, is evidenced by the accompany- ing Fig. 205. THE TEETH OF METATHEEIA.— MAKSUPIALIA. 411 The question is, however, far too lengthy for full discussion in these pages. For further information upon it the reader is referred to Messrs. Wilson and Hill's able summary, to Leche, and to the papers of Kiikenthal, Eose, Woodward, Thomas, and Marett Tims. A further peculiarity of the marsupials is the structure of their enamel, which is penetrated by the dentinal tubes. Sir John Tomes, some years ago (12), described and figured the teeth of a large number of marsupial genera, and found that, although pm3. i pm.4 Fig. 205. — Mandible of a young individual of Palre mastodon wintoni, showing the crown view of the right milk molars and a side view of the cheek teeth of the right side, the pm3 and pra4 replacing mm3 and mm4. The first true molar (ml) and the germ of the second (m2) are also seen. (After C. W. Andrews, Phil. Trans. Roy. Soc, Lond., vol. 199, series B, 1908.) in the different families the tube system of the enamel varied in its richness and in the depth to which the tubes penetrated, yet it was conspicuously present in the whole class, with the sole exception of the Wombats, in whom nothing of the kind is to be found. The dilatation noticeable at the boundary line of the enamel and the dentine (see Fig. 206) is a kind of clumsy joint brought about by the coalescence at this point of the tube- forming cells, on the one side odontoblasts, on the other enamel cells. 412 A MANUAL OF DENTAL ANATOMY. The dilatation is not universally present, but a well-marked curve at the juncture of the enamel and the dentine sometimes occurs, as well as various curves in the course of the tubes through the enamel. Fig. 206. — Enamel and dentine of a kangaroo (Macropus major'). The dentinal tubes in the dentine (A) are furnished with numerous short branches at the line of juncture with the enamel ; they are dilated, and a little bent out of their course, while beyond the dilatation they pass on through about two-thirds of the thickness of the enamel in a straight course and without branches. Only a part of the whole thickness of the enamel is shown in the figure. B. Enamel penetrated by the tubes. C. Individual dentinal tube. The extent to which the tube system exists in the enamel varies much in different families. It attains its utmost THE TEETH OF METATHERIA. — MARSUPIALIA. 413 development in the Macropoclidce, and is more feebly present in some Dasyuridce, whilst in Myrmecobius, and yet further in Tarsipes it is reduced to small limits. It is said by von Ebner that the tubes in marsupial enamel are not in the axes of the prisms, but run between them, but this is a point upon which it is a little difficult to be certain ; and the author is of opinion that the tubes are in the substance of the prisms. (See p. 51.) Recent marsupials are grouped into — Polyprotodonts. — Incisors numerous, small, and subequal ; canines pronounced and strongly tuberculated, molars in many, sharp-cusped, e.g., Didelphys. 3 3 Diprotodonts. — Incisors not more than -g-, often -y and sometimes as low as i. The first upper and the lower incisors are large, with cutting edges. Upper canines absent or small, lower absent. Molars bluntly tuberculated or ridged, e.g., Kangaroo. Under the Diprotodont division come the Wombats, which have all their teeth with persistent pulps, and their incisors, rodent-like, with enamel on their front surfaces only. Among the Polyprotodont series there exists one genus of flesh-eating marsupials whose ferocity is such as to have gained for them the names of wolf and tiger, while the resemblance of the head to that of a dog has given origin to the popular name of " dog-headed opossums." * The resemblance to the dog in dentition is even more close than in external form ; whilst retaining certain marsupial attributes, the teeth of the Thylacine are, so far as their work- ing capabilities go, almost exactly like those of the dog. The dental formula is — 4 1 7 i-^-c— , cheek teeth—. The incisors are small, close set, and sharp-edged, the outer- most being somewhat caniniform. The canines are stout, pointed teeth, not quite so long relatively as those of a dog. The premolars are conical, implanted by two roots, and very similar to those of the dog ; they are followed in the * It has, of course, no real relationship to the true opossums, which are not found in Australia. 414 A MANUAL OF DENTAL ANATOMY. upper jaw by four (?) molars, which increase in size from the first to the third, but the last true molar is again a smaller tooth. The upper molars are all of the "carnassial" pattern; there is a " blade" elevated into subsidiary cusps, and internally to this a " tubercle," supported by a thin root. Fig. 207. Fl'G. 207. — Upper and lower teeth of the Thylacine. The rudimentary milk molar, which is absorbed before birth, has been placed over the third or last of the premolars, which succeeds to it vertically. Pending agreement amongst naturalists as to the homologies of the premolars and molars, the old lettering has been left upon this and the ensuing figures. Fig. 208. — Upper and lower teeth of a dog, which are placed side by side with those of the Thylacine, to show the many points of resemblance between the two dentitions. The lower molars also bear some resemblance to the carnassial teeth of the dog, consisting of a strong, sharp-edged blade, with anterior and posterior subsidiary cusps, the latter being somewhat broad and tubercular. An allied animal (Dasyurus ur sinus), though smaller than the Thylacine, and having teeth of a less sectorial character, is so destructive to sheep and so fierce and untamable that it has earned the name of " Tasmanian Devil." THE TEETH OF METATHEEIA. — MAKSUPIALIA. 415 Within the limits of the same genus a species (Dasyurus viverrinus) is to be found in which the molar teeth are studded over with long sharp cusps, like the teeth of Insec- tivora, a group which it resembles both in its habits and food. A number of smaller marsupials approximate in their dentition more or less to the insectivorous type, whilst a tolerably complete chain of existing forms serves to bridge over the gap between the rapacious Dasyuridce and the herbi- vorous Kangaroos and Wombats. Amongst the Opossums the larger species have large canines, and the dentition in its general features approximates to the Dasyuridce ; they feed upon birds and small mammals, as well as upon reptiles and insects, though the smaller species are more purely insectivorous. Myremecobius, a small Australian marsupial of insecti- vorous habits and dentition, is remarkable as having teeth in excess of the number of the typical mammalian dentition,* having — .41 9 i-g-c^j-, cheek teeth-^-, though of course there is a doubt as to their division into pre- molars and molars. It is further remarkable in that its milk (or premilk ?) rudiments are carried further than those of most other marsupials. Leche described calcified predecessors to .1.2 i — , i — , and to , and an uncalcified predecessor to c •-g-, and to these Woodward added — , — , and—; in the lower jaw he found the same as Leche, and also well- dpm 4 differentiated germs of ~- 7, so that its full milk (or pre- b dpm 4 r milk ?) formula was — . 1234 dpm 4 1 23 C 1 dpm 4* In the Phalangers, nocturnal arboreal animals found in Australia and a part of the Malay Archipelago, the canines * A possible explanation of the large number of molar teeth has been suggested on a previous page (p. 408) 416 A MANUAL OF DENTAL ANATOMY. though present, are feeble ; an interspace also separates the incisors from the molar series. The lower incisors, reduced to a single pair, are procumbent and grow from persistent pulps. There is thus, functionally, some faint approach to the character of a rodent dentition, as may be seen by an inspection of the accompanying figure, though there is a strongly-marked transverse condyle to the lower jaw. Phascolarctos cinereus has been shown by Oldfield Thomas to have the same reduced deciduous tooth as Thylacinus. The very sharp cusps of the molars, in the lower jaw some- FiG. 208A. — Teeth of Phascolarctos cinereus. An outline figure of the skull is placed above to show the general " rodent" aspect of the skull. what concave on their inner aspect, are so formed as to long preserve their sharpness and accurate co-adaptation. The name " Kangaroo Rats " {Hypsiprymnus) is applied to a genus consisting of about a dozen species ; they are all small creatures, not much larger than rabbits, but having the general proportions of Kangaroos. They are quiet, gentle little creatures, of strictly herbivorous habits, and they are interesting to the odontologist as possessing a dentition which throws some light upon several anomalous extinct forms whose habits and affinities have been the subject of much controversy. The dental formula is — i — c -r, cheek teeth THE TEETH OF METATHEKIA. — MAESUPIALIA. 417 The first pair of upper incisors^ are sharply pointed, are directed nearly vertically downwards, and grow from persistent pulps. The second and third do not grow from persistent pulps, and their worn crowns do not attain to the same level as those of the first pair. All three pairs are antagonised by the single pair of large procumbent lower incisors, of which the sharp points meet the first pair of upper incisors, while the obliquely-worn surfaces behind the cutting edges impinge against the second and third upper incisors. The arrangement of the incisors and the sharpness of their cutting edges are calculated to effect the same objects as those attained by the incisors of a rodent. A still closer resemblance would be brought about by the dwindling (which occurs in other genera) and final disappearance of the second and third upper incisors, and a compensating extra develop- ment of the first pair. The canines are not large, yet they are not so small as to be termed rudimentary ; in the lower jaw they are absent. Only one premolar exists in the adult, and this is a very peculiar tooth ; its crown is very long from back to front (at least twice as long as any of the molars, and in some species as long as three of the molars), and consists of a finely furrowed blade with a sharp edge ; the blades of the upper and lower teeth slide over one another. Behind this there are four (?) true molars, with square quadricuspid crowns, which become much worn down by use. The last and only (?) premolar, the tooth to which attention has already been drawn on account of its size and other peculiarities, by virtue of its great size displaces not only the usually deciduous molar, to which it is the legitimate successor, but also turns out the premolar in front of it.* In this particular the succession of the teeth in the Hypsiprymrius is the same as that of the true Kangaroo, which may be understood by a reference to Fig. 210. There are some extinct animals, perhaps marsupials, known only by their jaws, which have been the subject of much controversy. Professor Owen, basing his arguments largely upon the presence of premolars which possessed * The old nomenclature has been here retained. If Lydekker's view be accepted, these teeth would be two premolars and only three true molars. D.A. 27 418 A MANUAL OF DENTAL ANATOMY. elongated and sharp-edged blades, held that Plagiaulax (cf. p. 380) and Thylacoleo were carnivorous, saying of the latter that it possessed the simplest and most effective dental machinery for predatory life known among mammalia. Dr. Falconer in the case of the former, and Sir William Flower in the case of the Thylacoleo, have shown this view to be untenable, or at least unsupported by adequate evidence. A clue to the nature of the great blade-shaped teeth of these extinct creatures is afforded by the form of the premolar of the herbivorous Hypsiprymnus (see Fig. 209). The incisors are reduced in number, and are large ; the teeth between them 7JV Fig. 209.— Upper and lower teeth of Hypsiprymnus (Bettongia) (graii ?). The dentition represented is that of the adult animal, the permanent premolar being already in place. and the large premolar are stunted ; but both these points are true of the herbivorous Kangaroos. The Thylacoleo differs, however, from all known animals by the immense size of the thin-edged premolar (worn flat in aged animals), and by the rudimentary condition of its true molars. But its incisors, lying forwards and closely approximated in the middle line, are particularly unsuitable for catching and holding anything alive and struggling, whilst the nearest resemblance to its blade-shaped tooth is to be found in harmless herbivorous creatures, so that the balance of evidence is much against Professor Owen's view. The dental formula of Thylacoleo was — 3 13 1/ 4 \ Tc o~pTmT \cheek teeth~37' The first upper incisor was very large and the second and THE TEETH OF METATHEEIA. — MAKSUPIALIA. 419 third very small, as were the canine and the first two upper premolars ; but the last upper and the only lower premolars * were great blade-shaped teeth like the large premolars of Hypsiprymnus, but considerably larger. Thus its useful teeth were only a pair of incisors above and below, and a pair of sectorial premolars. The Kangaroos, comprising many species of very varying size, are (with the exception of a few old males) all docile creatures of herbivorous habits, recalling in some particulars the ruminants. Their dental formula is — The three pairs of upper incisors are more equal in size than in the Hypsiprymnus, and the central pair do not grow Fig. 210. — Upper and lower teeth of Halmaturus valabatus. The succession al premolar is not yet erupted, and is shown in its crypt ; when it comes into its place it will displace the deciduous molar, and one of the anterior premolars as well. In the upper jaw a rudimentary canine is shown. The point of the lower incisor would fit, in closure of the mouth, behind the long anterior upper incisor, but the width of the figure does not admit of the teeth being placed in their true relative positions without reduction in size. from persistent pulps. The lower incisors are very peculiar teeth : they grow from persistent pulps, are procumbent, projecting forwards almost horizontally, and are much flattened from side to side, their outer surfaces being but slightly convex, and their inner surfaces flat, with a median ridge. Their margins are almost sharp. There is an unusual amount of mobility between the two halves of the lower jaw, so that these two teeth can be to a slight extent separated from one another. * Lydekker suggests that there were other lower premolars present, but they were soon lost. c jr-j cheek teeth n 27—2 420 A MANUAL OF DENTAL ANATOMY. The upper canine is often present as a very minute rudiment, but in no Kangaroo does it attain to a greater size. The dentition of the Kangaroo is somewhat perplexing to the student, for two reasons : the one that the premolar not only displaces the solitary deciduous molar, as in Hypsi- prymnus, on account of its greater size, but also the next pre- molar which was situated in front of the deciduous molar ; and, moreover, in animals past adult age, teeth are shed off from the front of the molar series till at last only the last two true molars on each side remain. The dentition of the Kangaroo at successive ages may be thus represented : 3 0 6 i -p c cheek teeth of which one is deciduous, or, in all, six molar teeth. Then the third premolar displaces the second premolar as well as the usually deciduous molar, giving— .30 5 i ' -=- c -p-, cheek teeth -=-, of which one is the successional tooth, 1 U 5 or, in all, only five molar teeth. Then, one after another, teeth are shed off from the front of the molar series, just as in the Pliacochcerus (see p. 512), till all that is left is — .3 0 0 2 ^c-^-p-m^. The deciduous molar of the Kangaroo is a fully-developed tooth, which takes its place with the other teeth, and is not distinguished from them by any special characters, so that mere inspection of the jaw of a young Kangaroo having it in place, at the same time with a premolar in front of it and four true molars behind it, would not lead an observer to suspect its true nature. No existing creature serves to connect the Kangaroos closely with the "Wombat, but the extinct Diprotodon appears to have in a measure bridged across the gap. The Wombats (Phascolomys) are heavily built, inoffen- sive creatures, which burrow in the ground and subsist largely upon roots. In their dentition they closely simulate the rodents, as they possess but a single pair of chisel-edged incisors in either jaw, growing from persistent pulps, and embedded in very deep and curved sockets. These differ from THE TEETH OF METATHEEIA. — MAESUPIALIA. 421 the corresponding dentes scalparii of true rodents in that there is a complete investment of cementum which passes over the enamel in front of the tooth as well as covering its back and sides. They are unlike the teeth of other marsupials in their structure, as the dentinal tubes do not penetrate the enamel, which is therefore probably harder and denser and so less readily worn away. The molar teeth also grow from persistent pulps and are very deeply grooved upon their sides, so that their grinding surfaces are uneven. The dental formula is — 0 cheek teeth — . 5 2" Nat. Sizt Fig. 211. — Upper and lower teeth of a Wombat (Phascolomys wombat). The first tooth of the molar series forms a single column, whereas the deep grooving of the other teeth divides each of them into two columns. The simpler appearance, as well as analogy, would indicate that the first tooth is a premolar. The adaptive resemblance to the dentition of the true rodents is exceedingly close, though the Wombat is an un- doubted marsupial ; and the very closeness of the imitation is an exemplification of the fact that adaptive characters are very apt to mislead, if used for the purposes of classification. Another peculiarity of the Wombats has been described by Kose, who confirms a long discredited statement of Owen's that deciduous teeth are found in these animals, and he further states that the Wombat has two dentitions, altogether like a placental mammal. Kose finds that there are calcified predecessors to the func- 422 A MANUAL OF DENTAL ANATOMY. tional incisors, a deciduous canine which has no successor, and deciduous premolar ; all of these appear to be absorbed or shed at a very early period. Extinct Wombats, of very much larger size than the recent species, are found in the later tertiary deposits of Australia. Amongst the marsupials there is a pretty little arboreal creature (Tarsipes), not larger than a small rat, which sub- sists upon insects and the nectar of flowers, which it reaches by means of a long protrusible tongue. Its molar teeth are rudimentary, variable in number, and are soon shed. The lower incisors, which are procumbent, are, however, retained, as are also a few small teeth which are opposed to them above. Another remarkable South American marsupial was de- scribed by E. F. Tomes (1863) under the name of Hyracodon, but, this name being already appropriated, it is now termed Coenolestes (Thomas) (7). Its dentinal formula is — 4 1 7 i-g c -j-, cheek teeth -y. It is remarkable that, though a true marsupial, it differs widely from the other American marsupials, and resembles those of Australia ; the Diprotodonts have a syndactyle foot (i.e. two toes contained in one skin), while among Polyproto- donts the Peramelidce have the syndactyle foot, but are not diprotodonts ; and American opossums are neither diprotodont nor have they the syndactyle foot. Coenolestes, however, has much of the diprotodont charac- ter in its dentition, and yet has not the syndactyle foot, so that it crosses the usual lines of classification and blends the characters of several groups. Its upper incisors are not unlike those of Didelphys, and it has a well-developed canine, but the front pair of lower incisors are much elongated, as in Diprotodonts. The premolars are compressed cutting teeth, the last being much the largest. The upper molars are low-crowned and nearly square in shape, the first two teeth having four cusps, the third three, and the last being a minute triangular tooth. In the lower jaw, behind the big procumbent incisor, four THE TEETH OF METATHERIA. — MARSUPIALIA. 423 minute teeth are situated ; probably two are to be regarded as incisors, a canine, and one premolar ; the other two premolars are narrow sharp teeth, and the lower molars are elongated from before backwards, and more or less ovate in general form. Within the last few years Mr. Ed. Stirling has discovered a marsupial mole, to which the name Notoryctes has been given. By Mr. Stirling's kindness the author has had the opportunity of examining the structure of its teeth and has found (13) that it presents the usual marsupial character of penetration of the enamel by tubes continuous with those of the dentine, the pattern being like that of Didelphyidce. Its teeth, at first pointed like those of an insectivorous animal, become worn down, probably by the sand in which it lives and which it FlG. 212.— Upper and lower teeth of Notoryctes. The worn surface of a lower molar is shown yet more magnified in the single tooth placed above. takes in with its food, so that they present areas of dentine surrounded by an upstanding ring of enamel, more or less elevated in places. The feeble implantation of the teeth, how- ever, points to the food not being really hard, and the animal appears to live upon larvae. The wonderful diversity of the forms into which the marsupials have branched out in Australia seems to prove that they have been established in that region, and have been without the competition of more highly organised placental mammals, for a prodigious length of time ; and one can- not better conclude the very brief survey of the teeth than by calling the reader's attention again to the character of the marsupial fauna — this microcosm in which every place is filled by a marsupial which mimics the placental mammal which it represents — for nowhere can we more 424 A MANUAL OF DENTAL ANATOMY. plainly see the workings of natural selection than in areas thus isolated and deprived of immigrant creatures for countless ages. Eeferences. (x) Flower, W. "On the Development and Succession of the Teeth in the Marsupialia." Phil. Trans. Roy. Soc, Lond., 1867, vol. clvii., pp. 631—643. (2) Hopewell-Smith, A., and Tims, H. W. Marett. " Tooth-Genesis in the "Wallaby Macropus hiUardieri." Proc. Zool. Soc, Lond., 1911, pp. 926—942. (3) Leche, W. "Nachtrage zu Studien iiber die Entwicklung des Zahnsystems bei den Saiigethiere." Morpholog. Jahrb., 1893. Bd. 19—20, pp. 113—143, 502— 552. (4) " Zur Entwicklungsgeschichte des Zahnsystems der Saiigethiere." 1895, Stuttgart. (5) Lydekker, E. ' ' The Dental Formula of the Marsupial and Placental Carnivora." Proc. Zool. Soc, Lond., 1890, pp. 922—928. (6) Thomas, Oldfield. " On the Homologies and Succession of the Teeth in the Dasyuridce, with an Attempt to Trace the History of the Evolution of Mam- malian Teeth in General." Phil. Trans. Eoy. Soc, Lond., 1887, vol. clxxviii. pp. 443—463. (7) " On Coenolestes, a still existing survivor of Epanorthidce of Ameghino, and the Eepresen- tative of a New Family of Eecent Marsupials." Proc. Zool. Soc, Lond., 1895, pp. 870-878. (8) Tims, H. W. Marett. " Notes on the Dentition of the Dog." Anat. Anzeiger, 1896. Bd. XL, pp. 537—546. (9) . "Tooth- Genesis in the Canidce." Journ. (Zoology) Linnsean Soc, Lond., 1896, vol. xxv., pp. 445—480. (10) — ■ " On the Succession and Homologies of the Molar and Premolar Teeth in the Mam- malia." Journ. Anat. and Phys., 1902, vol. xxxvi. , pp. 332—343. "Evolution of the Teeth in the Mam- malia." Journ. Anat. and Phys., 1903, vol. xxxvii, pp. 132 — 149. (12) Tomes, Sir J. " On the Structure of the Dental Tissues of Marsupial animals, and more especially of the Enamel." Phil. Trans. Eoy. Soc, Lond., 1849, vol. cxxxix., pt. ii., pp. 403—413. (13) Tomes, 0. S. "On the Minute Structure of the Teeth in Notoryctes." Proc. Zool. Soc, Lond., 1897, pp. 409—412. THE TEETH OF ME T ATHEEIA. — MAESUPIALIA. 425 (14) Wilson, J. T., and Hill, J. P. " Observations upon the Develop- ment and Succession of the Teeth in Perameles, together with a Contribution to the Discussion of the Homologies of the Teeth in Marsupial Animals." Quart. Journ. Micr. Sci., Lond., 1896 — 97, vol. xxxix., n.s. pp. 427 — 584. (15) Woodward, M. F. "On the Development of the Teeth in the Macropodidce." Proc. Zool. Soc, Lond., 1893, pp. 450—471. CHAPTEK XVI. THE TEETH OF EUTHEEIA. INSECTIVOKA, GALEOPITHECIDiE, CHEIROPTERA, CARNIVORA. There is great difficulty in arranging Placental Mammals in any definite order of sequence, inasmuch as those now existing and the known fossil forms are but a small fraction of those which have once existed. And although no modern naturalist can well doubt that all existing mammals are lineal descendants of a smaller number of forms previously existing, yet they do not admit of being arranged on any one stem, and are not all lineal descendants of one another. The mixed and uncertain affinities of some groups render any simple linear classification unsatisfactory, but two other methods may be attempted : the " circular," in which animals are grouped round generalised types ; and what may be termed the phylogenetic, in which animals are, as far as possible, arranged according to their descent, in which case there would be a branching parent stem, the branches subdividing again and again. If the lines of mammalian descent were really known, the latter is, of course, the more logical, but as no two authorities quite agree, it is for the present only partially practicable. Dr. Wortman writes : " We know, for example, that the Primates could not be the ancestral group, for the obvious reason that they do not extend beyond Miocene time ; nor the Carnivora, which appeared about the same time ; nor the Eodentia, which date from the Middle Eocene ; nor the Cheiroptera, which can be traced back no further than the Upper Eocene. We are, therefore, restricted in our choice to the Insectivores, Lemuroids, Creodonts, or Tillodonts, which alone of the entire series continue backwards to the base of the Eocene period. With reference to the Creodonts, I do not believe that any important distinctions exist between them and the Insectivores, while the line between this latter group and the Lemuroids and the Tillodonts becomes exceedingly shadowy at this point." THE TEETH OF INSECTIVORA. 427 That the Insectivora are a very ancient and generalised group all naturalists agree, and the Lemuroids also go very far back. Though questions of classification are foreign to the subject of this work, yet, inasmuch as the teeth are a very important criterion of affinity, it is desirable, but exceedingly difficult, to arrange an account of the dentitions in anything approaching to a logical sequence. The sequence and classification of the Metatheria and of the Eutherian sub-classes, &c, adopted in the present edition of this work, are based upon, though they do not strictly follow, that given in Professor Max Weber's " Die Saugetiere " as being one of the most recent and comprehensive works on the subject. The Insectivora form a somewhat heterogeneous order of mammals, and embrace very various forms. All of them are of rather small size, and some are very small indeed. Their diet consists for the most part of insects, and their teeth are generally adapted for this by being furnished with many pointed cusps. The best known animals in the order are the Hedgehogs, the Shrews, the Moles, and the Macro scelidae (Elephant mice). Insectivora are more abundant in Africa, Asia, and South America than in Europe. The Shrews approximate in some measure towards the Rodents, and the Tupaia is very lem urine in its characters. They all have small, rather smooth brains and long faces. The Insectivora are ancient and in many respects generalised mammals, so that they may be supposed not to have diverged so far from the parent forms as other mammalia. In recent Insectivora the canines are often not sharply differentiated from the incisors, and sometimes a tooth clearly The Teeth of Insectivora. f j Tnpaiidce. j Macroscelidce, Elephant shrews. Talpidce, Moles. Soricidce, Shrews. Erinaceidce, Hedgehogs. Insectivora < Lipotyphla < Potamogalidcz. CentetidcB. Chrysochloridce, Golden moles. 428 A MANUAL OF DENTAL ANATOMY. of the premolar group is differentiated to function as a canine, but as this does not occur in the earlier Insectivora, it is evidently a feature subsequently acquired. Hence it happens that different writers give different dental formulae to the Insectivora, the differences mainly turning upon whether a tooth is regarded as a canine or not. (Cf. p. 339.) But, as pointed out by Leche and Marsh, in the oldest known mammals this tooth was always two -rooted, and the reduction to a single root is to some extent correlated with the canini- form specialisation of its crown. Thus when it has but one root this is a character acquired, and not an original distinction. The extent to which a milk dentition is developed varies much ; in many of the group it is erupted and lost early, and is more or less suppressed; whilst in others, e.g., Centetes, Hemicentetes, Macroscelis, and Tupaia, it is well developed and functional for some time. The common Hedgehog (Erinaceus) has the following dental formula : — .123 1 0234 / 1204 \ 123 /w , ,N 1 0~2lTC TP "0204 l0r P 10047 m T23 (Woodward)' Leche, after an examination of 103 specimens belonging to seventeen species, gives us a formula common to the genus : — . 1.2.3 J_ 234 1.2.3 1 0.2.3 C 1 P 03 4 m 1-2.3' But a good many dental formulae have been assigned to it, in consequence of certain difficulties and obscurities in the identifi- cation of the teeth, which have only lately been cleared up. The differences chiefly arise because the canines are not markedly caniniform ; they are implanted by two roots, and they appear to be some little way behind the intermaxillary suture. But it has been found that they are preceded by single- rooted caniniform teeth, and that the distance from the suture is apparent and not real, owing to the maxilla growing forward and embracing the premaxilla both on its labial and palatal aspects. In a young specimen in the possession of the author, prior to this overlapping of the maxilla the canine is seen to be close to the suture. In the upper jaw there is a rather wide interval between the THE TEETH OF INSECTIVOKA. 429 first pair of incisors, which are much the largest, lean inwards towards one another, and are somewhat caniniform in shape. The second pair of incisors are much smaller, whilst the third are intermediate in size and caniniform. The canine, though it has an indication of division into two roots, is neither a very large nor important tooth. Leche (7) figures this tooth from four specimens of E. europmis showing that it varies in the same species between a tooth in all respects like a premolar and a powerful, though still two- rooted, canine. It is followed by two small single-rooted premolars, of which the second is variable and sometimes absent, and then by a large (fourth) upper premolar. The fourth upper premolar is totally different in size and form from the third : its crown is large, squarish, and furnished with four cusps, of which the antero-external one is far the longest and sharpest. The first upper true molar has a square crown, upon which are four sharp cusps, and in an unworn tooth a fifth small central cusp ; it is implanted by four roots. The second true molar is also square, quadricuspid, and has four roots ; but it is much smaller than the first, while the third upper true molar is quite a small, compressed, double-rooted tooth, with a thin-edged crown. In the lower jaw the first incisors, really the second of the typical mammalian dentition, less widely separated than the upper, are also the largest ; then follows another tooth termed incisor, on account of its relation to the upper incisors when the mouth is closed. The third tooth is much larger and of peculiar form, and is the canine according to recent writers. The fourth tooth from the front is a small single tooth, like Fig. 213. — Upper and lower teeth of the hedgehog. 430 A MANUAL OF DENTAL ANATOMY. the third, but upon a smaller scale. Next behind it comes a tooth which is very much larger, and its crown carries two principal cusps with a small subsidiary cusp ; this is certainly a true premolar. The next tooth (first true molar) has an oblong crown beset with five sharp cusps, of which four are arranged at the corners of a square, while the fifth, perhaps an elevation of the cingulum, lies a little in front and towards the inside of the tooth. In the second true molar the fifth cusp is but little indicated, while the last true molar is a dwarfed tooth with but one cusp. The teeth of the Hedgehog fairly represent some of the features of insectivorous dentitions, for the forcep-like incisors, the stunted or non-developed canines, and the molars bristling with pointed cusps are common to very many of the order. In the upper quinquecuspidate molars, the two outer cusps are the paracone and metacone, the two inner the protocone and hypocone, and there is a small central metaconule. Of these the order of development was — Paracone \ Metacone j Trigon. Protocone / Hypocone. Metaconule. In the lower molars the five cusps are those of the trigon with two extra cusps, the internal entoconid and external hypoconid. Owing to the partial suppression of the paraconid, and the protoconid and metaconid being joined by a ridge, as are the entoconid and hypoconid, the resultant tooth is almost square with two transverse ridges. The order of appearance of the cusps is — Protoconid. Metaconid. Entoconid. Hypoconid. Paraconid. Thus in their order of appearance in neither upper nor lower teeth do the cusps fit in with what might have been expected on the tritubercular theory. In a former edition of this book the persistent dentition of the Hedgehog was stated to be a mixture of replacing teeth with per- sistent milk teeth. This view, which was given on the authority of Leche, has been proved by that observer and by Woodward to be incorrect, and it has been found that all of the teeth which persist have been preceded by tooth vestiges in various degrees of reduction. THE TEETH OF INSECTIYOEA. 431 Some of the milk teeth are well developed and functional, others, notably the upper canine, very variable in the extent to which they are developed, whilst others again are functionless and never cut the gum, but are represented by tooth-germs which undergo development only to the extent of a trifling irregular calcification. The formula of the milk dentition may be written thus, the teeth which are functionless being printed in italics : — r12^ 1 , 0 2 3 4/ , 1 2 5 4\ dl 123 dc T^1)JYA0V dp 105%} The deciduous upper canine has been found to vary between that of a minute irregular calcification (Woodward, Leche) and a fully functional tooth. (Cf. Fig. 178.) In the molar region there are to be found both a lingual continuation of the dental lamina and a slight, but constant, outgrowth on the labial side of the neck of the enamel-organ of the functional tooth (Woodward), not distinguishable from those incisor germs which are slightly calcified. He therefore concludes that there are evidences of a vestigial set, of a functional set, and of a lamina capable of going on to form an additional set in the molar region. Gymnura, an animal of the Malay region, closely allied to the Hedgehog, but in appearance more like a very big Shrew, with a body a foot in length, is somewhat peculiar in possessing the whole typical number of teeth, its dental formula being . 12_3 l_ 1 ?234 J_2J} 1123clpl?234 m12 3* The teeth closely resemble those of the Hedgehog, as in it the upper canine is two-rooted, though preceded by a single-rooted deciduous tooth. Knowledge of its milk dentition is owing to Thomas (13) and to Woodward (l8) : — ,.125, 1 , 1234 dl T23 dc 1 dPmrT^4' Thus, although it has four premolars retained in its functional dentition there is a doubt whether p -j— are not retained milk teeth. As in so many other mammals, no vertical replacement of this tooth is known, and, according to Woodward, the relation of the germ of this tooth to the dental lamina is such as to suggest that it is a milk tooth, with the lamina continued beyond it so as to have a potential source for a successor. Woodward, however, remarks that there may possibly be a later develop- ment of a successor, as the pm-i- of the adult seems rather a stouter tooth than that of the young animal. The second milk premolars are vestigial, and their successors prema- turely developed ; the third and fourth premolars are strongly developed. 432 A MANUAL OF DENTAL ANATOMY. There is thus a fairly complete functional milk dentition, viz., — and only beiDg vestigial.* Centetes has typical canines like a Carnivore. Bemicentetes has canines which in no respect differ from the premolars behind it, and Ericulus has a sharp, long, two-rooted upper canine, in pattern of crown, an enlarged edition of the premolars behind it. There is no caniniform tooth in the lower jaw. The Shrews have numerous sharply-pointed teeth, the points interdigitating and fitting very closely together when the jaws are closed. There is no tooth, either in the upper or lower jaw, which is so elongated as to deserve the name of canine ; but between the incisors and the true molars are several small teeth which, by analogy, are called premolars. Fig. 214. — Teeth of the common shrew, X 3. The true molars are not very different in pattern from those of the mole (B in Fig. 216), and present the W contour so common in the molars of Insectivora. The most marked peculiarity in the dentition of the Shrews lies in the form of their incisors. The first upper incisor is always very large indeed ; it looks vertically downwards, is slightly hooked, and has a notch and a second low cusp behind the principal long pointed cusp. The tip of the lower incisor fits into this notch. The lower incisor is also very large ; it lies nearly horizontally, though the point is bent a little upwards. Along its upper edge there are, in most species, three or four small cusps, while its lower border is curiously prolonged outside the bone of the jaw, so as in some measure to encase this latter. The lower incisor is at least one- third as long as the whole alveolar border. The incisor teeth * For a possible explanation of certain peculiarities in the tooth succession in the posterior premolar region, the reader is referred to a previous chapter dealing with 'the same subject in the Marsupialia. THE TEETH OF INSECTIVOKA. 433 of the Shrew would appear to form a very efficient pair of pinchers, with which to pick up the minute creatures on which it feeds (Fig. 214)'. It is difficult to assign a dental formula to the Shrews, as the premaxillary suture is lost early, and there is no differen- tiated canine. Brandt considers that there are four upper incisors, because the fourth upper tooth appears to be in the premaxilla ; but, as Woodward points out, in all Insectivora the relation of the canine to the intermaxillary suture is more variable than in other orders, and, moreover, the gap between the maxilla and premaxilla was so large in a foetus 1*2 inches long that there were several tooth germs in the gap. On the whole its probable dental formula is .313 3 1TC o"pTm 3' though the upper canine is abnormally situated, and Dobson (4) does not allow the existence of a canine at all. A great deal of doubt has rested upon its milk dentition, some authors claiming that calcified milk teeth existed, and others (Leche) denying the existence of any milk dentition in any stage of suppression. Woodward, however, examining a foetus a trifle smaller than Leche's, describes and figures well-differentiated milk tooth-germs to many of the teeth. None of them, however, apparently undergo any calcification, though proceeding so far as the formation of the bell-shaped enamel-organs. All of the incisors, canines, and premolars show traces of these milk tooth-germs, though some are less differentiated than others. The large first incisors show an anomalous condition, namely, a large but not highly differentiated outgrowth on the lingual side of the functional tooth-germ ; yet it can hardly be supposed that these large and peculiar teeth are persistent milk teeth. There is thus a highly-reduced milk dentition, none of the germs being known to calcify : — di— dc— dpm The dentition of the Mole (Talpa) has been the subject of much controversy, the determination of its canines, &c, d.a. 28 434 A MANUAL OF DENTAL ANATOMY. presenting such difficulty that no less than five different dental formula? have been assigned to it. In the front of the upper jaw come three small teeth, the first being somewhat the largest, situated well within the limits of the intermaxillary bone, and are doubtless incisors. But the next tooth, which is of considerable size, also appears to be implanted in the premaxilla, the suture passing across its socket close to the back of its posterior root. The difficulty in accepting this tooth as the true canine was formerly considerable, but is now diminished by the established fact that the relations of this tooth to the suture are variable in Insectivores, and that the two-rooted tooth of Gymnura is without doubt the canine. The mole has forty-four teeth, and the dental formula usually assigned to it is — .12 3 1_ (1)23 4 1 23 1 123 C 1 P (1)2 3 4 m 12 3* M. F„ Woodward, however, gives reasons for the belief that p i is a persistent milk tooth, in which case its premolar formula should be written thus : — dpml 2.3.4 dpml pm 2.3.4' Next behind the canines come three small premolars, and a fourth which is much larger than the others ; the three have somewhat simple crowns consisting of single sharply-pointed cusps, whilst the fourth is more molariform. The first two upper molars are large teeth, bristling with cusps; the third is much reduced in size and simplified in pattern. Woodward calls them of tritubercular or trituber- culo-sectorial pattern. (See Fig. 216b.) In the lower jaw the four front teeth are all small, but the fourth or outermost of these incisors is called by some writers the lower canine, because, when the teeth are closed, it passes in front of the upper caniniform tooth. Nevertheless the tooth which does the work of a canine in the lower jaw is the fifth counting from the front ; this is a two-rooted tooth, and conforms so closely with the three teeth behind it in configuration, that it is obviously only one of these premolars developed to a greater length than the others. It closes behind the caniniform upper tooth, so cannot on this THE TEETH OF INSECTIVOKA. 435 ground be called a canine by those who attach homological importance to the term. The remaining three premolars are rather small and single- rooted. The lower true molars are of considerable size, and their points are very long and sharp ; they have a large heel, so large as to preponderate over the trigon. The late Spence Bate's paper valuable as it is in contributing to our whole knowledge of the milk dentition of the creature, does not finally determine the homologies of the canine. In a mole 3| inches long he found eight milk teeth on each side of both upper and lower jaws. The milk incisors were about one-twentieth of an inch in length, and one two- Fia. 215. — Upper and lower teeth of the common mole. The functionless milk teeth are placed above the permanent teeth which displace them, with the exception of the deciduous canine, which is placed too far back, over the first premolar. (After Spence Bate.) hundredth in diameter, and were rudimentary in form, con- sisting of long thin cylindrical tubes surmounted by slightly expanded crowns. All the milk teeth were of this simple form, save only the last in each jaw, which presented crowns with two cusps, and had their roots to some little extent divided into two. At the time when these teeth are present, the intermaxillary suture is very distinct, and he had no doubt that the fourth upper milk tooth, the predecessor of the canine, was in the premaxilla. None of the teeth had fairly cut the gums, and the advanced state of the permanent teeth beside them made it doubtful if they ever did become erupted. The subject has been reinvestigated by Woodward, who confirms in most respects the accuracy of Spence Bate's 28-2 436 A MANUAL OF DENTAL ANATOMY. account. But in one point he disagrees : he was unable to find (either in clarified jaws or in serial sections) any trace of the deciduous predecessor figured by Spence Bate over the than the other milk teeth, were uncalcified and more backward in their development, but occupying a position corresponding to the reduced milk series. Moreover, there was a lingual bud from the dental lamina, indicating that the tooth-germ under observation was really the milk tooth. He therefore con- cluded, after an examination of eight specimens, that there is no permanent tooth developed at this place, but that the functional persistent tooth is the milk tooth. The Insectivora may be divided into groups by the pattern of their molar teeth ; the majority present a W-shaped pattern (Tupaia, Macroscelis, Erinaceus, Sorex, Talpa), whilst the other group have narrower molars with a V pattern (Potamogale, Centetes, Chrysochloris). The V-shaped teeth, having the early generalised trituber- cular form, occur in the Insectivora of Africa, Madagascar, and the West Indies only. It is probable that these are the more generalised types, and that those with the W-shaped molars are the more specialised forms. The W pattern met with in the molars of Insectivora is well exemplified in the molar of Urotrichus. Mivart (n) held that the patterns of insectivorous teeth were arrived at sometimes by a process of addition, new cusps being formed by elevations of the cingulum so that the molars bristle with cusps, or by a process of lateral com- pression, the tritubercular tooth of the Cape Mole thus being due to a fusion of the more numerous cusps of such forms as the Mole. The trituberculate forms are said to approach the Jurassic Tritubercu- lata, but, as pointed out by Woodward, this hardly applies to the lower teeth ; and it is noteworthy that Woodward finds in two genera (Centetes and Ericulus) which he has examined that the protocone is the first cusp to appear, both in the upper and the lower jaw.* But though this fact is one which will be gratifying to the advocates of the tritubercular theory, unfortunately it is not true of other genera ; for those Insectivora * Marett Tims (16) has shown that there is a doubt whether the so-called Protocone of the molar teeth in these two genera is in reality the homologue of the protocone in most other mammals. — -; on the contrary, he found that dpml dpml were larger THE TEETH OF INSECTIYOKA. 437 which have more complex upper teeth, with four or five cusps, present a totally different order of appearance of the cusps, and the paracone and metacone appear first. Indeed, it is true of all mammals, which have been adequately examined, except the two mentioned, that the paracone appears first in the upper teeth, the protocone coming second or third in order. This suggests that the homologies as set forth by Osborn and Cope may be incorrect, and that the so-called paracone in the upper molar is the real protocone. If this be so, the awkward dilemma that in the premolars of various mammals the protocone stands outside, and in the molars it stands inside (Scott, 12), much alike though the last premolar and first molar may be, would be got rid of. Dr. Forsyth Major (8) gives strong reasons for not regarding the tricuspid teeth of Chrysochloris and Centetes as primitive forms, but, on the contrary, as due to reduction, thus reasserting Mivart's view and AFC Fig. 216. — Upper molar teeth of (A) Urotrichus ; (B) Talpa; (C) Chrysochloris. (After Mivart.) The four principal cusps, according to Mivart, are lettered a, b, c, d, the other cusps being elevations of the cingulum. Woodward would suggest as their homologies — b. Protocone. a. Paracone. o. Metacone. d. Hypocone. He therefore agrees with Mivart that the additional cusps upon the outer border are of less importance than the other or inner four. rejecting those of the advocates of trituberculism so far as these teeth are concerned. He holds that in the upper molars the three cusps present are certainly not the three cusps of the primitive triangle or trigon, but that the para- cone and metacone are fused together, and that the protocone is greatly reduced. He further remarks that in the interpretation of the homologies the cusp, which is really paracone -f- metacone, has been very generally confounded with the protocone. The lower molar, on the contrary, he regards as consisting essentially of the primitive triangle. On close examination and comparison with other forms he considers that the so-called tritubercular molars contain more components than the three cusps, and that the three main cusps with which he is dealing have different homologies in each of the three orders (Lemuroidea, Car- nivora, Insectivora) with which he deals ; hence he regards these sporadic appearances of trituberculism as phenomena of convergence. In a paper upon the great extinct Lemuroid Megaladapis, (9) Forsyth Major points out that both this creature and Centetes have tritubercular upper molars, but that when we come to look at the lower molars we find 438 A MANUAL OF DENTAL ANATOMY. that in the one it is the front part of the tooth which has undergone reduction, whilst in the other it is the back or heel. Now. according to the advocates of trituberculism, this heel or talon is a secondary and later introduction, but this he entirely disputes. In support of his argument that the tritubercular molar is a tooth which has been reduced, he cites the fact that the Viverridce, Centetidce, and Lemuridce all have it, though there can hardly be any close generic relationship between them, and there is, moreover, evidence that the reduction sometimes goes well beyond the stage of trituberculism in each of these families, e.g. , in the carnivorous Eupleres, the insectivorous Hemicentetes, and the Lemuroid Cheiromys. The structure of the teeth of Insectivora does not call for much detailed notice. Penetration of the enamel by dentinal tubes occurs in some — e.g. Sorex. In the Shrews the outer layer of the enamel is deeply pigmented. The enamel is abundantly entered by dentinal tubes, and there is no sharp line of demarcation at the junction of the two tissues ; the tubes in the enamel often bend abruptly and then resume their original direction. In Erinaceus the tubes do not enter so freely, nor do they run far into the enamel. In Gymnura and Tupaia there is little sign of penetration, and the dentinal tubes break up into brushes at the edge of the dentine. In Centetes no penetration occurs, but the enamel is very distinctly laminated. Teeth of Galeopithecidae. This sub-class contains but a single genus, Galeopithecus, the common but erroneous name of which is the " Flying Lemur " of the Malay region. Its systematic position has undergone many changes. Formerly classed with the true Lemurs it was afterwards included among the Insectivora. Kecent researches have shown the advisability of, at any rate provisionally, placing it by itself. The teeth, the genesis of which has been investigated by Deppendorf (3), are somewhat anomalous. The lower incisors are divided by a number of vertical clefts running down through a great part of the length of the crowns, so that they can be compared to combs, or to hands with the fingers slightly separated. What the purpose THE TEETH OF GALEOPITHECIDiE. 439 served by the secomb-like teeth may be remains uncertain ; no other animal has similar teeth. It has been supposed that these teeth are used by the animal in combing its fur, but Leche ridicules this interpretation, deeming it far more probable that they have relation to peculiarities of food, Indeed, one has been observed when eating a banana to suck it through its front teeth, using them as a sieve. Unlike the Insectivora, Galeopithecus feeds on fruits and leaves. The so-called canine, which Leche Fig. 217. — Section of lower incisor of Galeopithecus, showing arrangement of pulp cavity, dentine, and enamel. prefers to call the first premolar, is two-rooted, as happens in many Insectivora, but Galeopithecus stands almost alone among recent mammals in having i2 two-rooted ; Leche points out that this is no real distinction from Insectivora, as Petro- dromus has it two-rooted, whilst in the genus Erinaceus it may be found in different species sometimes distinctly two-rooted, sometimes one-rooted. For Galeopithecus Leche (6) gives the following dental formula : — * .2 3 3 i¥pm g-m-g . * This formula differs somewhat from that given by Deppendorf . 440 A MANUAL OF DENTAL ANATOMY. Galeopithecus has a milk dentition, more complete than most Insectivora, which is late in developing, no teeth being cut in a newly-born foetus, and in one which had a full coat of hair only the points of ^3 and r ... , — were through the gums. (lii ai2 clni3 At this time no successional teeth were far advanced, but all the true molars were as far advanced in calcification as the milk teeth, and lay but little deeper in the jaws. Hence it comes about that in Galeopithecus all the molars (except m3) are up in place and at work during the persistence of all the milk teeth. This appears to be, so far as it goes, an argument in favour of regarding true molars as homologous with the milk teeth. There is this further peculiarity, that the so-called di3, described by Owen, is really pi3, which is in place, and at work with the milk teeth for a considerable time, di3 being a small tooth lost early. The back teeth, both of first (dm) and of second dentitions (pm), present gradated stages of differentiation from the molar type ; thus dm2, contrary to the general rule, differs more widely from the molar type than does its successor ; dm3 and its successor pm3 are quite alike. From these peculiarities Leche concludes that the dentition of Galeopithecus is an ancient inheritance, and that for a very long time it has been fully adapted to its manner of life. The enamel of the teeth is very distinctly laminated, but no penetration by the dentinal tubes occurs. Beneath the enamel there is a distinct granular layer, but in no Insectivore which has been examined by the author does a granular layer occur in this situation. This is probably correlated with the frequency of penetration of the enamel by a tube system in the latter, and in this respect, as well as in many others, Galeopithecus stands somewhat apart from the other members of the group. THE TEETH OF CHEIKOPTERA. 441 The Teeth of Cheiroptera. f Megacheiroptera Pteropodidce \ ( Rhinolophidce Cheiroptera j , • , Phyllostomatidce Microcneiroptera < ^ * „ i H.vnhrtllrwt.ti.vt.n./v Vespertilionidce The Bats, sharply distinguished from all other mammals by the possession of wings, are divided into two groups, respectively insectivorous (Microcheiroptera) and frugivorous (Megacheiroptera). The insectivorous Bats, by far the most numerous section, are for the most part possessed of small incisors, rather large canines, and premolar and molar teeth which bristle with sharp cusps, and generally present the W-shaped pattern. In fact, in general character, their teeth resemble those of the Insectivora, but the dental formula never exceeds — The incisors are sometimes reduced in number, and spaces are left between them ; and some, as, for example, the Vampire (Desmodus), have teeth specially modified to accord with their blood-sucking habits. This Bat has only one permanent incisor on each side, and this is a large but thin and sharp-edged tooth, with which the wound is made ; the lower incisors are small teeth with feebly notched edges. The canines are large, and the molar series, which is not required in an animal existing upon blood, Fig. 217a. — Skull of Desmodus, showing milk teeth. .21 33 1 ~3~ c T pm T m "3* 442 A MANUAL OF DENTAL ANATOMY. is stunted. The molar teeth are, however, sharp, though small, and there is no marked distinction into molars and premolars. The dental formula is — .11 2 0 1 iTcTpm~m-or - The frugivorous bats (of which the Pteropus, or flying - m. Mi % : v • • J FlG. 218. — Section of molar teeth of Hhinolophus in situ, with extreme development of cingula. fox, is an example) have much larger muzzles, and the molar teeth are set with intervals between them. 2 1 2 3 The dental formula is i-^-c — pm — m -g, but in some the molar series is reduced below this number. The incisors are small, and the canines rather large. Both molars and premolars are of somewhat simple form, long, and compressed from side to side. The outer borders of the crown of the molars are elevated into distinct but not very sharp cusps, which become worn down by use. The insectivorous character of the presence of many sharp cusps upon the teeth is not to be found in any of the frugivorous THE TEETH OF CHEIKOPTEKA. 443 bats. All the Pteropi have deciduous canines, and four deciduous molars, of simple pointed form, the number of deciduous incisors Jbeing very variable. The milk dentition of bats has been very carefully and thoroughly investigated by Leche (5), and at the present time the Megadermata are the only family in which the milk teeth are unknown. The milk teeth are not of much functional importance, as they are shed soon after, if not absorbed before, birth, and they are not therefore implanted in very definite sockets. In their slight cylindrical elongated roots, surmounted by expanded crowns, these milk teeth often recall those of the Mole. Sometimes the milk teeth are to be found even after the permanent teeth are in situ; in other instances, as, for example, the deciduous molars of Molossus, they never cut the gum. The milk detention of the Vampire (Desinodus)* appears to consist of incisors only, or of incisors and canines, though the absence of observed molars may be due to the fact that they are, as in Molossus, shed very early. It has, near to the front of the upper jaw, six teeth, each of which is very long and slender, with a strongly hooked point (Fig. 217a). It has been suggested that these feeble hooked teeth may assist it in holding on to the mother. In general terms it may be said that the milk teeth of the majority of Cheiroptera do not at all resemble their permanent successors. An anomalous dentition has been found in a Solomon Island bat, in which, the canines, whilst having a long principal cusp, are rendered multituberculate by other cusps at their base, this pattern being more or less repeated in the other teeth (Oldfield Thomas) (13). This seems to indicate that the fruit-eating bats are descended from insectivorous forms. The bats illustrate well a character of insectivorous dentition in the production of the cingulum to such an extent as to completely cover and protect the gum between the teeth. * In a skull of Desmodus, in the possession of Mr. K. F. Tomes, the third milk tooth appears to correspond in position to the permanent canine ; the same is the case in the specimen figured by Messrs. Gervais and Castelmain (" Exped. dans les Part Cent. d'Amerique du Sud"). 444 A MANUAL OF DENTAL ANATOMY. The Teeth of Carnivora. f Viverridce, Civets. Herpestoidea^ Hycenidce, Hyamas. Carnivoka fC. fissipedia Arctoidea < MustdidcS} Weasels. {Felidce, Cats. [ Canidce, Dogs, j Procyonidce, Racoons. ^C.pinnipedia ( Ursidce, Bears, f Otariidce, Sea-lions. «j Trichechidce, Walrus. [Phocidce, Seals. The animals grouped together under the name of Carni- vora are divided into two sections, the Terrestrial (C. fissipedia) and the Aquatic Carnivora (G. pinnipedia). The order Oarnivora is a very natural one, and its name is, upon the whole, fairly descriptive of the habits of the majority of its members, though there are some creatures included in it which are mixed feeders and others which are purely vegetarian. The terrestrial Carnivora were formerly classed as " digitigrade " and " plantigrade," a classification exceedingly inconvenient, as it left the greater number of the animals to be classified in the debateable ground between the two extreme types. As a linear classification is impossible, they are frequently grouped around three centres : the JEleuroidea, or cat-like ; the Cynoidea, or dog-like ; and the Arctoidea, or bear-like Carnivora ; and, instead of taking the Felidce, or Cats, as the type of the group, it is generally considered that the Dog tribe are the most general- ised forms, and that the Cats are an extreme modification in one direction, the Bears in another. The Cynoidea comprise the Dog, and its immediate allies the Wolves and Poxes. The JEluroidea, or Cat-like Carnivora, comprise the Viverridea (Civets), Hyaenas, and Cats. The Arctoidea, or Bear-like Carnivora, comprise the Mustelidce (Weasels), Procyonidce (Bacoons), and the true Bears. This arrangement is a convenient one, so far as recent Oarnivora go, but it breaks down when the extinct forms are included, the Arctoidea being connected some with the Canidce, others with the Viverridce. Again, the JElueridea are connected with the Procyonidce. The connection which undoubtedly exists between the Cynoidea and Arctoidea is expressed in the classification given above, the Canidce being included among the Arctoidea, the Cynoids not being retained as a separate group. Some fossil forms long known, such as Hycenodon, remained of uncertain THE TEETH OF CAENIVORA. 445 position till the rich discoveries in North American deposits revealed a large number of forms which seem to require distinction as a sub-order, to which the name Creodonts has been given. Nevertheless, naturalists are not yet quite agreed as to the limits of this group, Wortman (19), for example, removing from it some forms which Matthew (10) would retain in it, namely, the more immediate ancestors of some of the modern Carnivora. In its more extended sense it undoubtedly contains the ancestors of the Carnivora, and possibly also of the Insectivora, while it is possible that the Condylarthra, the presumed ancestors of the Ungulates, may have had a common origin with the Creodonts, as resemblances occur at some points. Creodonts present resemblances also to the carnivorous marsupials, although they have a more complete tooth change, and their relationship to the marsupials is still uncertain. In a large number of Creodonts there is a full number of teeth, which, while adapted to a carnivorous diet, do not show that limitation of the slicing " carnassial" Fig. 219. — Sinqpa agilis. (After Wortman.) type to a particular tooth in the upper and to another in the lower jaw which characterises the Carnivora. Thus amongst the Hysenodonts, of which Sinopa agilis is here figured, the upper carnassial pm4 is sectorial in form, but, with the exception of the last, the true molars are also somewhat sectorial. In the lower jaw the first true molar has a shearing action, but it is smaller than those behind it, which participate in its sectorial character. Another interesting Creodont form is Mesonyx, which had teeth differentiated into incisors, large canines, premolars, and molars. The incisors were small, and their crowns more or less conical. The upper canines were large, strongly recurved, and sharply pointed, with a considerable diastema in front of them ; the first premolar was single-rooted, but the other three had two roots ; their crowns were compressed, having a principal cusp and a posterior lobe with a cingulum ; the fourth was a little more complex than the others, and was more typically tritubercular. 446 A MANUAL OF DENTAL ANATOMY. The upper molars were tritubercular, with a strongly pronounced cingulum at the back, the last molar, however, lacking the postero-external cusp. The lower premolars and molars are rather simple ; the first is single-rooted and conical, the others having two roots, and a principal cusp, to which are added small anterior and posterior cusps, elevations of the cingulum. From such a dentition, as Wortman has pointed out, more complex patterns can easily be derived by additions to the Fig. 220. — Upper and lower dentition of Mesonyx ossifragus. (After Cope.) One-third nat. size. front and back of the cones and by the elevation of a cingulum, from which additional cusps can be formed. Its dental formula was — .31 43 i-g- c y pm^ Hi 3. Of the teeth of early Creodonts it may be said that it would not be difficult to , imagine how the teeth of all other diphyo- dont mammals might be evolved from them, whilst Cope (2) (and passim; see also Dr. Wortman (20)) gives an admirable series of extinct genera in which the molar patterns become more complex, and approximate on the one hand to the THE TEETH OF CARNIVORA. 447 sectorial teeth of Carnivora, and on the other to the bristling- cusps of modern Insectivora. In others the car.nassial or shearing character became even in early times limited to the fourth upper premolar and the first lower molar. These, the Creodonta aclaptiva of Matthew, Wortman would separate off and group in the same sub-order as modern Carnivora. Thus, according to Wortman's view, the Creodonts proper failed and died out without giving origin to modern forms. [Several authors have drawn attention to the resemblances which exist between the Creodonts and the Polyprotodont Marsupials. The author has compared the minute structure of the teeth in these two groups (17) in order to see what light, if any, is thrown upon the problem. The structure of the enamel in marsupials presents characteristics very unusual in Placental mammals, the course pursued by the enamel prisms being very simple. The Carnivora also present well-marked enamel characteristics. From his investigations based upon the structure of the teeth alone, the author came to the conclusion (to quote his own words) that the Creodonts " certainly do not stand very near to any marsupial, and that if there be a marsupial ancestor, or an ancestor common to the Marsupials and to the Creodonts, it must be sought considerably further back than any of those examined."] In carnivorous animals one tooth on each side of both upper and lower jaws is of considerable length, is sharply pointed, and is called a canine ; the upper canine is separated by an interval from the incisors, the lower canine being received into the vacant space or " diastema " so formed. The incisors are short, almost always six in number, and stand nearly in a straight line, transversely across the front of the jaw, the outermost upper incisor being sometimes large and more pointed, so as to resemble a small canine. The incisors and canines may, on the whole, be said to be tolerably uniform throughout the order, but the variations in the premolar and molar teeth are both numerous and interesting. In the most purely carnivorous members of the order, the Felida, the true molars are reduced to a minimum, and the hinder ones are thin-edged, " sectorial " teeth. In the Bears, on the other hand, some of which are purely frugivorous, 448 A MANUAL OF DENTAL ANATOMY. the molars are little short of the full typical mammalian number, and are furnished with obtuse and broad grinding surfaces. The accompanying figure will serve to give the general aspect of the teeth and jaws of a typically carnivorous animal, and to show the great development of the pro- cesses for the attachment of muscles, the stout wide arch of the zygoma and the transversely elongated mandibular condyle. To a particular tooth in the upper jaw, and to its antagonist in the lower jaw, Cuvier gave the name of " carnassial." These, conspicuous in the true flesh feeders, become less differentiated in the Arctoidea, or bear-like Carnivora, and Fig. 221. — Side view of the cranium of a tiger, with the mouth slightly opened to show the relative position of the great caDines. in the bears themselves are indistinguishable from the other teeth, save by a determination of their homologies by a process of comparison with the teeth of intermediate forms. The carnassial or sectorial tooth in the upper jaw is always the fourth premolar ; its crown is divisible into two parts, the one a thin sharp-edged blade, which runs in an antero- posterior direction, and is more or less divided by one or two notches into a corresponding number of cusps ; the other part, the " tubercle," is a shorter and blunter cusp, and supported upon a distinct internal root situated at the inner side of the anterior end of the blade. (See Figs. 222.) In those which are most purely flesh-feeders, the " blade" is well- developed, and the tubercle of small size, an increase in the tubercular character of the tooth being traceable through those genera which are mixed feeders. THE TEETH OF CARNIVORA. 449 Thus in the bears the tubercle is said to be highly developed ; but it is to be noted that the large flattened inner portion of the bear's sectorial 'tooth is in a more posterior position than the tubercle of a cat's sectorial, and is not supported upon any separate root. The lower tooth which antagonises the upper carnassial, passing a little behind it, is the first true molar (Fig. 223). In the FelidcB it consists of the blade, which is divided into two large blade-shaped cusps, behind which is a very small and rudi- mentary third division (which in the Hyanidce, for example, is of conspicuous dimensions). In existing Carnivora but one " sectorial " tooth is to be found on each side of the jaws, but i tl m FlG-. 222. — Left upper carnassial teeth of Carnivora. I. Fells. II. Canis. III. Ursus. 1. Anterior added cusp. 2. Paracone. 3. Metacone. 4. Protocone. 5. Inner posterior cusp, without distinct root, characteristic of Ursidge. in the Hycenodon, which had the full number of forty-four teeth, and in some other extinct tertiary mammals, there were more teeth partaking of this character. When the blade of the upper carnassial tooth consists of only two cusps, as in the Dog, these are the paracone and metacone of the primitive triangle, according to Osborne's interpretation ; the inner tubercle supported on a special root is the protocone. The blade of the lower carnassial molar consists apparently of the paraconid in front and protoconid behind, the metaconid or postero -internal cusp having disappeared. According to the tritubercular theory, the first change from the primitive trituberculism was the addition of a low heel to the lower teeth, thus forming the tuberculo- sectorial tooth, which in the most specialised forms, such as the Felidce, has disappeared again. D.A. 29 450 A MANUAL OF DENTAL ANATOMY. It is pointed out by Marett Tims (15) that cusp identification in the Canidaz leads into various difficulties, and he hence proposes a fresh identification. He further points it out as remarkable that the " all- important protocone " is so reduced as to have almost disappeared in the upper carnassial tooth, and, according to usual identifications, in some other teeth. The accompanying illustrations, which are taken from Flower and Lydekker's " Mammalia," show the modification which the carnassial teeth undergo in various members of the class. In a general sense we may say that the characters which Fig. 223. — Left lower carnassial teeth of Carnivora. I. Felis. II. Cards. III. Herpestes. IV. Lutra. V. Meles. VI. Ursus. 1. Paraconid. 2. Protoconid. 3. Metaconid. 4. Hypoconid. It will be seen that the relative size of the two roots varies according to the development of the portion of the crown they have respectively to support. indicate a pure flesh diet are : the small size of the incisors as compared with the canines, and their arrangement in a straight line across the jaw; the large size, deep implantation and wide separation from one another of the canines ; the reduction in number of the molar series, those that remain being without broad crushing surfaces, in the place of which a pointed or sharp-edged form prevails. Thus the more numerous the teeth of the molar series, and the broader their crowns, the more likely it is that the creature THE TEETH OF CAKNIVOKA. 451 subsists upon a mixed diet ; and a gradation may be traced even in individual teeth, such as the carnassials, in which a gradual increase iri relative size of the internal tubercular cusps of the upper and of the posterior tubercles of the lower teeth may be traced as we pass from the examination of the teeth of Felicia to those of mixed feeders, such as some of the Arctoidea. All of the Canidce have triangular upper molars : all Ursidce have quadrate upper molars. In the Canidce the inner cusp is anterior ; in the Ursidce it is median in position (Matthew, loc. cit.). It is a familiar observation that immature animals differ less from their allies than do the respective adults, and this is exemplified by the milk dentition of the present order. With the exception of the Felidce, which have only two lower milk molars, the terrestrial Carnivora, so far as is known, all have the same milk dentition : A- 3 A 1 A 3 The shearing action between the upper and lower carnassial teeth, which serves to cut off chunks of flesh, was not very much developed in the earlier types of Creodonts, and was shared alike by all the molars. But in more specialised types it was chiefly between — ^ or — , till in others it came to be between m2 m3 as in all recent carnivores. mi Herpestoidea, Viverridse. — With a dental formula not differing much from the dog and not at all from Canis primcevus, the Viverridce (Civet cats, Ichneumons, &c.) approach the more typical carnivores in such points as the thinner and sharper blades of the premolar teeth and the greater relative length and sharpness of the canines. The dental formula is — 29—2 452 A MANUAL OF DENTAL ANATOMY. At the same time the lower carnassial tooth has no less than six sharply-pointed cusps, and it lacks the typical character of a sectorial tooth, while the long pointed cusps of the molars of some Viverridae recall the characters of insectivorous dentitions rather than those of true flesh feeders ; furthermore, there are other Viverridce which are not at all savage, and which subsist on a diet of fruits, eggs, &c. such as the Binturong or the Paradoxurm, the teeth of which have almost lost the carni- vorous character. Little use can therefore be made of the Viverridce as illustrating the transition between the dental characters of the other families of the order ; they rather serve to exemplify how within the limits of a single family, with an identical dental formula, the form and size of the teeth may Fig-. 224. — Left upper dentition of Indian civet (Viverra zibetha). (From Flower and Lydekker's " Mammalia.") vary so as to adapt its members to different forms of food and habits of life. Hysenidse. — In Hyaena the jaw is short and stout ; the canines are set far apart, and the teeth of the molar series are reduced in number. .314 1 1TcTpIm T The incisors are short and stout, but the outermost upper incisor is somewhat caniniform ; the canines are very strong, but are not so long relatively to the other teeth as in the Felidae. The premolars are all stout pointed teeth, with a very well pronounced basal ridge or cingulum, serviceable in protecting the gums when the creature is crushing up bones. They increase in size from before backwards in the upper jaw, the fourth upper premolar being a well-marked carnassial tooth, with its blade and tubercle. HEEPE STOIDE A. 453 The lower carnassial or first molar consists of little more than the notched blade ; but the little posterior tubercle, so strongly pronounced in the dog, is in the hyaena distinctly more marked than in the Felidce. (Cf. Figs. 221 and 225.) The only upper true molar is the rudimentary tooth, placed inside the back of the fourth premolar. The main feature of the dentition of the hyena is the great stoutness and strength of the teeth ; they are admirably adapted to the habits of the animal, which feeds rather upon the por- tions of carcases left by the fiercer Carnivora than upon those which it kills for itself, and consequently bones form a large proportion of its food. FlG. 225. — Upper and lower teeth of Hycena. The strongly marked cingulum is seen upon the lower teeth. In the upper jaw the fourth premolar (car- nassial tooth) has a strong blade, divided into three cusps, and a small tubercle opposite to and within the anterior cusp ; it is a good typical carnassial tooth. There is a curious hyaena-like animal found at the Cape (of which there are often specimens at the Zoological Gardens) called Proteles or Aardwolf, in which the teeth of the molar series are quite rudimentary. The incisors (much worn in old animals) and the canines are fairly well developed, the molars and premolars quite stunted. The deciduous dentition is similar to the adult as regards the stunted teeth. It is a cowardly animal, and is supposed to feed on putrid flesh ; it is said to eat young lambs, and to bite the large tails of the Cape sheep, which are remarkable for containing an abundance of semi-fluid fat. h 454 A MANUAL OF DENTAL ANATOMY. There is also another creature, from Madagascar, named Eupleres, which has weak jaws, and a muzzle so thinned down and a dentition so much reduced in power that it was at first regarded as an Insectivore. But its teeth are not so rudiment- ary in form as those of Proteles, and they are more numerous, numbering forty teeth. Felidae. — The dentition of this family is singularly uniform. Thus the molar series is reduced below that of hyaena by the loss of a premolar in both jaws. The incisors are very short, the canines very large, widely apart, and sharply pointed, with a pronounced longitudinal ridge very characteristic of the Felicia ; the premolars nearest to them are quite short, so that they stand practically alone, and so can penetrate the flesh of living prey more readily. The first upper premolar (really the second of the typical mammalian dentition) is almost a rudimentary tooth ; the second, a far larger tooth, is sharply pointed ; the last is a well-pronounced carnassial tooth, of which the " blade " is divided by two notches into three sharp lobes, with the middle one of which the " tubercle " is connected by a slight ridge. The solitary true molar is a very small tooth, placed trans- FlG. 226.— Proteles (Aardwolf). HEBPESTOIDEA. 455 versely and within the back of the premolar, so that, looking from the outside, it is not visible at all. In the lower jaw the carnassial (first molar) is reduced to the " blade " only ; it is divided by a V-shaped notch into two lobes and the posterior tubercle is hardly represented. Fig. 227. — Side view of lower, and palatal aspect of upper jaw (Leopard). In an extinct feline animal, the Machairodus, found in tertiary strata, and very widely distributed in France, Italy, India, Brazil and Buenos Ayres, the first of the premolars left in the upper jaw of Felis, and there almost rudimentary (see Fig. 228), has disappeared ; the dental formula is thus — .312 1 VTpIm T The upper canines are of immense length, and the ridge of enamel which runs down the front and back surfaces of the teeth is distinctly serrated ; hence the name of Saw-toothed Tiger has been given to the animal. The lower canines are quite small, and range with the incisors. The enormous length of the upper canine renders it difficult to see in what manner it was made use of, since it would appear as if the mouth could hardly have been opened to an extent sufficient to enable its point to do more than clear the lower jaw. But it is supposed from the form of the jaws and of the articulation that many of the Creodonts may have been able to open their jaws very widely, and Matthew suggests that Machairodus struck rather than bit with its long tusks, a 456 A MANUAL OF DENTAL ANATOMY. supposition borne out by the feebleness of the processes for attachment of the muscles which close the mouth. It may be fairly assumed that the great contemporary Ungulates had thick skins, and that the very long canine was useful for piercing them. Fig. 228. — Side view of the jaws and cranium of Machairodus (Drepanodoii). (After Owen.) Smilodon, a somewhat similar extinct animal, had a dentition still further reduced, viz. : .31 2 0 1 77 C t P o T HI 7- . 2 H 2 or 1 1 Professor Cope has described a rich series of extinct cats, mostly from Miocene beds (2). He summarises their characters thus : " It is readily perceived that the genera above enumerated form an unusually simple series, representing stages in the following modification of parts : — (1.) In the reduced number of molar teeth. (2.) In the enlarged size of the upper canine teeth. (3.) In the diminished size of inferior canine teeth. (4.) In the conic form of the incisors. (5.) In the addition of a cutting lobe to the anterior base of the upper sectorial tooth. (6.) In the obliteration of the inner tubercle of the lower sectorial. (7.) In the extinction of the heel of the same. (8.) In the development of an inferior flange at the latero- anterior angle of the front of the ramus of the lower jaw. HEKPESTOIDEA. 457 (9.) In the development of cutting lobes upon the pos- terior border of the large premolar teeth. " The succession of the genera above pointed out coincides with the order of geologic time, very nearly. " The relations of these genera are very close, as they differ in many cases by the addition or subtraction of a single tooth from each dental series. " These characters are not even always constant in the same species, so that the evidence of descent, so far as the genera are concerned, is conclusive. No fuller genealogical series exists than that which I have discovered amongst the extinct cats." Arctoidea. Amongst the Carnivora grouped together by many characteristics as " bear-like," a tolerably complete gradation of character in the matter of dentition may be traced. Some of the group, such as the Stoats and Martens, are carnivorous, others are mainly herbivorous. Canidse. —The Dog presents almost the full typical number of teeth, one upper molar (present in an extinct dog-like animal, the Amphicyon, and sometimes in C. cancrivorus) alone being wanting : .31 4 2,1 3 2 1 IT c T pm ¥ m 3' or T Pm y m ;p Thus in the Cynoidea the number of teeth varies from forty in Cyon to forty-six or forty-eight in Otocyon, a fox-like animal. The incisors are small, the outermost being the largest. The upper incisors have, as in a great many Carnivora, a trilobed shape (Figs. 229, 230), the surface of the crown being marked by a transverse groove into which the apex of the lower tooth fits, and the anterior of the lobes thus formed being notched so as to divide it into two. The small lateral cusps of the incisors are, as Tims has shown, entirely of the nature of cingulum-cusps, and are, in his opinion, the homologues of the Paracone and Metacone of the cheek teeth. The canines, large and conical, are somewhat compressed from side to side, and have an anterior and posterior sharp ridge ; they are also slightly flattened on their inner surface. 458 A MANUAL OF DENTAL ANATOMY. The premolars are flattened from side to side, pointed, increasing in size from before backwards, and have small Fig. 229. — Deciduous dentition of Canis famiharis. (After Marett Tims, Journ. Linnean Soc, vol. xxv.) Fig. 230.- — Permanent dentition of Cams faniiliaris. (After Marett Tims, Journ. Linuean Soc., vol. xxv.) basal accessory cusps. (See Fig. 231.) The first premolar has no predecessor ; it is a small tooth, and is often lost AKCTOIDEA. 459 early ; it is cut considerably before the other premolars, of which pra4 erupts first* ; and as its follicle appears about the second week, before there is any trace of the follicle of the other premolar teeth, Lesbre regards it as a persistent milk tooth. From this, however, Marett Tims dissents, regarding it as a permanent tooth. The fourth upper premolar is the sectorial tooth, and is very much larger than the third Fig. 232.— Milk and permanent teeth of dog. (After Professor Flower.) premolar ; the blade is well pronounced, and the tubercle small. The fourth lower premolar does not greatly differ from the third. The two upper true molars are blunt, broad- crowned tuberculated teeth, but the second is very small. In the lower jaw the first true molar or carnassial tooth has a well-marked blade, which articulates with the blade of the upper carnassial tooth ; but towards the posterior border there is a somewhat thick and blunt tuberculate * The peculiarity in the replacement of pm^ has already been referred to in Chapter XV. 460 A MANUAL OF DENTAL ANATOMY. portion, barely represented in the corresponding tooth of the Feliclce. This tubercular portion articulates with the broad, flat first upper molar. The second lower molar is smaller, being not one-fourth the size of the first, the third is smaller still ; both are blunt-crowned tuberculated teeth (the third lower molar, rudimentary in all dogs, is altogether absent in the Canis primcevus). The dentition of the dog, closly similar as it is to that of the wolves and foxes, is such as to allow of a considerable range of diet, there being tubercular molar teeth in addition to a full armament of such sharply-pointed teeth as are characteristic of flesh-feeding animals. Thus the Canidce, fairly uniform as they are in dentition, have somewhat different habits ; the Arctic fox, a flesh-feeder purely, has a dentition indistinguishable from the North Italian fox, which is reputed to be vegetarian in its diet ; the Canis cancrivorus of Guiana, which often possesses a fourth molar, eats small mammals, crabs, and also fruit. Hence it is necessary to be very careful in deducing from the character of the teeth what may probably have been the diet of the animal ; an approximate idea may often be reached, but the sources of fallacy are sufficiently numerous to render the conclusion uncertain. Amongst the various breeds of dogs some slight differences exist. Thus in the long-muzzled races considerable intervals exist between the premolars, as is to some extent seen in C. dingo (Fig. 231), while in the short-muzzled races the teeth are in contact and set somewhat obliquely, so as to overlap irregularly, sometimes the hinder edge of the anterior tooth passing outside and sometimes inside the front of the tooth next behind it. Otocyon, with its extra teeth, is probably a somewhat primi- tive form, and it is pointed out by Marett Tims that its teeth are more multicuspidate than those of most of the family. De Blainville mentions that in long-muzzled dogs super- numerary teeth are met with more often than in short, and on the whole it may be said that the teeth are less easily susceptible of modification in size than are the jaws, so that crowding of the teeth is induced by selective breeding aiming at the production of short-muzzled varieties. Dinocyon gidleyi, a huge extinct dog-like carnivore with a AECTOIDEA 461 massive skull larger than that of any living carnivore, has been described by Matthew (10b). The Mustelidae (Weasels) have the dental formula — There is a sort of prima facie resemblance to the feline dentition, for the sectorials are very much like those of the Felidce. The first premolar is in many of the Mustelidce rudi- mentary, and is lost early, but there is not enough known of it to enable us to decide whether it is a persistent milk tooth or a deciduous second tooth. The last tooth in each jaw is a broad- topped tubercular molar, even in the most carnivorous members Fia. 232a. — Upper and lower teeth of a Coatimundi QJVasua socialis). The fourth upper premolar (carnassial tooth) has lost its sectorial character by the blade being much less, and the tubercle much more developed than in the iEluroidea ; there is an additional interna' tubercle at the back of the tooth. of the group, while in those which are less so, such as the badger, the molar teeth are very broad and obtuse, the lower sectorial having a very small blade and a very large tubercular posterior talon, so that, without having really lost its typical formation, it comes practically to be a broad grinding tooth. In the Procyonidae (Bacoons and Coatimundis, &c.) we have a further departure from the carnivorous character in the increased development of the molar series ; the dental formula is — \ . 3 4 2 462 A MANUAL OF DENTAL ANATOMY. In the Coatimundi, for example, the upper sectorial tooth has a very large " tubercle," and posteriorly to this there is a small additional tubercle ; the " blade " has no large or conspicuous thin, flat, sharp edge, but presents two pronounced cusps. The lower sectorial is no longer recognisable as a carnassial tooth, and all the true molars are broad teeth with four or five cusps. The canines are very peculiar, those of the upper jaw being very straight and much flattened from side to side, those of the lower jaw strongly curved, and marked by a deep groove near the front of their anterior surface. Fig. 233. — Teeth of a bear ( Ursus thibetanus ?). The figure is drawn from a young specimen, in which the canines have hardly attained to their full length. In this bear the four premolars are all persistent. Ursidae. — In the Bears the teeth are yet further modified to suit the requirements of mixed or vegetable feeders. The dental formula is generally — .314 2 1 8 °1 P 4 m 8' The incisors of the upper jaw present the notch across the crown so common in Carnivora, and the outermost is large and not unlike a canine. The canines are, relatively to the other teeth, not so large as in dogs or Felidce ; nevertheless they are stout, strong teeth, upon which the anterior and posterior ridges of enamel are well marked. The first three premolars are small dwarfed teeth ; the first premolar is very close to the canine, and has a crown of peculiar form, produced out towards the canine. AKCTOIDEA. 463 All four of the premolars seldom persist through the life- time of the animal ; the first premolar, however, is rarely (if ever in recent species) lost, the second being the first to fall out, and then the third. As the fourth is never lost, in most adult bears the first and fourth premolars are found with a wide interval between them. The premolars of bears thus form an exception to the rule that when a tooth is lost from the premolars the loss takes place from the front of the series. The fourth upper premolar (carnassial tooth) retains some- thing of its carnassial character, though relatively to other teeth it is smaller than in the Felidce, the first lower molar very little, save that it is a narrower and more elongated tooth than the other true molars. The other true molars are square or oblong teeth, raised into blunt tubercular cusps : they vary in different species. In the sloth bear (Melursus labiatus) the incisors are small, and the median pair are lost early. This animal is variously stated to be frugivorous and to feed on ants, the latter probably being the more truthful account. Carnivora pinnipedia. The aquatic Carnivora are divided into three families : — I. The Otariidce, or Eared Seals, comprising the single genus Otaria, known as Sea Lions, or Sea Bears. These are the "fur seals" from which sealskin is procured, and they are less removed from the terrestrial carnivora than are the other seals : the limbs are better adapted for walking, there are external ears, &c. II. The Phocidce, to which family the seals of our own coasts (Phoca greenlandica, &c.) and the Great Proboscis Seals of the southern seas (Cystaphora) belong. III. The Tricliechidae,, or Walruses, an aberrant Arctic family consisting of one genus only. The dentition of the Seals is less highly specialised than that of other Carnivora, in some cases approximating to that of homodont Cetaceans. This reversion to the homodont type is probably correlated with a fish diet. The milk dentition is very feebly developed in all seals. In the Otaria (fur seal), which of all the seals most approaches 464 A MANUAL OF DENTAL ANATOMY. to the terrestrial carnivora in other characters, the milk teeth are retained for a few weeks, bat in most others they are shed about the time of birth. Thus Professor Flower tells us that in a Phoca greenlandica a week old scarcely a trace of the milk teeth was left. The canines are generally well marked by being larger than Fig. 234. — Jaws of Otaria, in which the teeth are affected by a form of erosion. (After Dr. Murie, " Odont. Soc. Trans.," 1870.) the other teeth, but the molars and premolars are very similar to each other, and are simple in pattern. The teeth of Otaria, and of some other seals, become much worn down, and they also seem to become eroded at the level of the gums, as they are often deeply excavated at points which seem unlikely to have been exposed to friction, but the nature of this erosion has not been adequately investigated. Otaria has a dental formula — .31 4 1 or 2 i 2 c T pm j m -y— . CARNIVOKA PINNIPEDIA. 465 The first premolar has no predecessor either in this animal or in the Phocidce. The incisors, however, vary in number in different groups, while the canines, premolars, and molars are constant. Fig. 235. — Teeth of Phoca greenlandioa. The common seals (Phoca) have a dental formula — .314 1 igClPimT The incisors are of simple form, and the outer are the largest. The canine is a strong recurved tooth, with a large root ; behind it follows a series of molars, each of which (with the exception of the first) bears a central principal cusp, with a smaller accessory cusp before and behind it. The forms of the crowns vary a good deal in different genera, in some the cusps being much larger, more deeply separated from one another, and recurved ; and in others the accessory cusps are multiplied, so that the name of " saw-toothed seal " has been given to their possessor. It has been suggested by Baume that the degree to which the teeth have become simplified perhaps corresponds with the antiquity of the aquatic habits of the genus, those which have taken to the water more recently having retained a greater complexity of tooth crown. The Leopard Seal (Ogmorhinus) has teeth with exceedingly long roots disproportionate, one would say, to the necessities of a fish feeder for firmness in the implantation of its teeth. In the Hooded Seals (Cystophora) the incisors are reduced to one in the lower jaw and two in the upper ; the canines are of great size, but the molars are small and simple in form, so as to approximate to the teeth of the Cetacea. But it is noteworthy that, whereas the Cetacea arrived at a monophyodont and homodont dentition by the suppression of D.A. 30 466 A MANUAL OF DENTAL ANATOMY. the second dentition, and retain their milk teeth,* the converse is true of the seals. The Walrus (Trichechus rosmarus), an aberrant Arctic form, is possessed of enormous upper canines, which pass down outside the lower lip, and are of such dimensions as to materially modify the form of cranium by the size of their sockets ; they grow from persistent pulps, and are composed of dentine with a thin investment of cement. The great tusks are employed to tear up marine plants and to turn over obstacles, the walrus feeding upon Crustacea, and also upon seaweed, etc. ; they are also used to assist the animal in clambering over ice. As they are of almost equal Fig-. 236. — Permanent and milk teeth of elephant seal (Cystophoi-a proboscidea.} size in the female, they cannot be regarded as weapons of sexual offence, but they are undoubtedly used in the combats of the males. The largest tusks seen by Nordenskiold were 30 inches in length, and 8 inches in circumference ; the tusks of the females attain to the same length, but they are much more slender. In addition to the great tusks, the walrus ordinarily has a row of four or five teeth, short, simple and worn down to the level of the gums. Of these the one placed immediately within the base of the great canine is in the intermaxillary bone, and is hence an incisor. The ordinary dental formulais given by Flower as — * This statement is open to doubt. Reference is made to this point in Chapter XVIII. CARNIVOKA PINNIPEDIA. 467 But there is some difficulty in assigning a definite dental formula, for in front of the solitary incisor are often the sockets (or even the teeth themselves) of two others, which are for various reasons rather to be regarded as non-persistent teeth of the permanent set than as milk teeth ; and there are small teeth sometimes to be met with behind the molars, which also seem to be rudimentary permanent teeth. Fig. 237.— Side view of upper and lower jaws of a walrus (Trichechus rosmarus). The upper jaw has been tilted a little to one side, in order to bring into view the molar teeth at the same time with the long tusks. The determination of the teeth being open to question, they have been simply numbered. In young specimens the dentition is— .31 , 5 lyCjpm + m ^ The teeth above alluded to may persist through life, and probably often do ; but they are sure to be lost in macerated skulls, as they have but little socket. Of the milk dentition four teeth have been traced in each jaw ; they are rudimentary, are lost about the time of birth, and correspond in position to the more largely developed teeth of the adult. Hence the question if those small rudimentary teeth above alluded to are 30—2 468 A MANUAL OF DENTAL ANATOMY. to be regarded also as milk teeth, which are long retained, or as rudimentary permanent teeth ; at present this requires further elucidation. Eeferences. (*) Bate, C. Spence. " On the Dentition in the Mole (Talpa europcea)." Trans. Odontol. Soc. Gt. Brit., 1867, vol. v., pp. 261—288. (2) Cope, E. D. " On the Mutual Eelations of the Bunotherian Mam- malia." Proc. Acad. Nat. Sci. Phil., 1883, pp. 77—83. (3) Deppendorf, T. " Zur Entwicklungsgeschichte des Zahnsy stems der Marsupialier." Semon's " Zoolog. For- schungsreisen in Australien und dem Malay- ischen Archipel." 1897-1910, Bd. iii., pp. 243— 202. (4) Dobson, G. E. "A Monograph of the Insectivora." Lond., 1882. (5) Leche, W. " Zur Kenntniss des Milchgebisses in der Zahn- homologien bei Chiroptera." II. Theil. Lunds Universitets Arsshrift, 1877-78. Bd. 14. (6) -• — . "Uber die Saugethier-Gattung Galeopithecus" Svenska Yelenskaps Akad., 1884-85. Bd. xxi., No. 11, pp. 1—92. (7) . "Zur Morphologie des Zahnsystems der Insec- tivoren." Anat. Anzeiger, 1897, Bd. xiii., pp. 1—11, 514—529. (8) Major, 0. T. Forsyth. " On the Change and Form of the Teeth in Centetidce, a Malagasy Family of Insectivora." Ann. and Mag. Nat. Hist., 1897, Dec, vol. xx., pp. 525—544. (9) . " On Megaladapis madagascariensis, an extinct gigantic Lemuroid from Madagascar, with Eemarks on the Associated Genera and on its Geological Age." Phil. Trans. Eoy. Soc. Lond., 1894, vol. clxxxv., pt. i., pp. 15 — 18. (10) Matthew, W. D. (a) "Additional Observations on the Creo- donta," Bull. Amer. Mus. Nat. Hist., 1901, vol. xiv., pp. 1 — 38. (b) "A Skull of Dinocyon from the Miocene of Texas." Bull. Amer. Mus. Nat. Hist., 1902, vol. xvi., pp. 129—136. (u) Mivart, St. G. "Notes on the Osteology of the Insectivora." Journ. Anat. and Phys., 1868, vol. ii., pp. 117 — 154. (12) Scott, W. B. " The Evolution of the Premolar Teeth in the Mam- mals." Proc. Acad. Nat. Sci. Philad., 1892, pp. 405 — 444. CAKNIVOKA PINNIPEDIA. 469 (13) Thomas, Oldfield. " The Mammals of the Solomon Islands based on the Collections made by Mr. C. M. Woodford during his Second Expedition to the Archipelago." Proc. Zool. Soc. Lond., 1888, pp. 470—484. (14) . " On the Insectivorous Genus, Echinops, Martin, with Notes on the Dentition of the Allied Genera." Proc. Zool, Soc. Lond., 1892, pp. 500—505. (15) Tims, H. W. Marett. " On the Tooth-genesis in the Ca?iidce." Journ. (Zoology) Linnsean Soc. Lond., 1896, vol. xxv., pp. 445 — 480. (is) , . «The Evolution of the Teeth in the Mammalia." Journ. Anat. and Phys., 1903, vol. xxxvii., pp. 131- — 149. (1T) Tomes, C. S. " On the Minute Structures of the Teeth of Creo- donts, with especial reference to their suggested resemblance to Marsupials." Proc. Zool. Soc. Lond., 1906, pp. 45—58. (18) Woodward, M. E. " Contributions to the Study of the Mam- malian Dentitions. Pt. II. On the Teeth of certain Insectivores." Proc. Zool. Soc. Lond., 1896, pp. 557—593. (19) Wortman, J. L. "Studies of Eocene Mammalia in the Marsh Collection, Peabody Museum." Amer. Journ. Sci. 1901-2 (series of papers). (20) . " The Comparative Anatomy of the Teeth of the Vertebrata." Amer. System of Dentistry, Phil., 1886, vol. i. CHAPTER XVII. THE TEETH OF EDENTATA, KODENTIA AND TILLODONTIA. THE TEETH OF EDENTATA. ' Tubulidentata Orycteropodidce, Aard-varks. The term Edentata was applied to the animals of this order to indicate the absence of incisors (teeth in the intermaxillary bone). Though this is true of most of them, a few have some upper incisors, the suture between the maxilla and the pre- maxilla passing, in some Armadillos, behind the first upper tooth, but the central incisors are in all cases wanting. It is also stated that in Dasypus sexcinctus there are some rudi- mentary teeth in the incisor region which never cut the gums. Some of them are quite edentulous ; this is the case in the South American Ant-eaters (Myrmecophaga and Cyclothurus), in which the excessively elongated jaws cannot be separated to any considerable extent, the mouth being a small slit at the end of the elongated muzzle. Marett Tims has shown (9) that the elongated muzzle is associated with greatly elongated lips the free margins of which have fused together leaving only a small circular mouth at the extremity of the muzzle. The insects are by this means prevented from escaping at the sides of the mouth, between the lips. Food is taken in by the protrusion of an excessively long, whip-like tongue, which is covered by the viscid secretion of the great submaxillary glands, and is wielded with much dexterity. The Manis, or Scaly Ant-eater, is also edentulous ; and it is said by Max Weber (13) that the reduction has gone so far that no Edentata Pholidota Manidce, Pangolins. ' Bradypodidce, Sloths. Myrmecophagidce, S. American Ant-eaters, v Dasypodidce. Xenarthra THE TEETH OF EDENTATA. 471 dental rudiments could be detected. Eose,* however, examin- ing Max Weber's sections, came to a different conclusion, and is of opinion that the slight epithelial thickening which he finds in them is to be interpreted as the preliminary to the formation Fig. 238. — Lower jaw. Ingrowth of epithelium in outer incisor region. Tooth vestige of Rose (after Tims. Journ. Anat. and Phys., vol. xlii.). of a tooth-band, and in the lower jaw he has found it to grow in to form a small rudimentary tooth-bancl (6). It, however, goes no farther, and the appearances would not be very significant but for the inherent probability that some remnants of original tooth formation would linger behind, on the ground that Fig. 239. — The deepest extremity of the same. Three sections behind preceding figure. The intervening sections show the downgrowth to be continuous (after Tims. Journ. Anat. and Phys., vol. xlii.). edentulous mammals are almost certainly degenerated forms which have lost their teeth. [In Manis javanica Marett Tims has confirmed Eose's con- clusion (loc. cit.) by finding what he considers to be a quite definite tooth-vestige which he regards as that of the outermost upper incisor (Figs. c238, 239). He also finds more posteriorly a number of curious structures of doubtful nature. Micro- * The species examined by Max Weber and Rose was 31. tricuspis. 472 A MANUAL OF DENTAL ANATOMY. scopically, they somewhat resemble developing hair, but their roots are implanted in the substance of the jaw, and are growing towards the alveolar margin. They closely resemble the imperfect enamel-organs seen in certain fish. Marett Tims, from a full consideration of all the facts, believes that these structures must be regarded as the last vanishing remains of teeth (Fig. 240). There are as many as thirteen or fourteen of these structures in the lower jaw on each side.] Fig. 240. — Tooth-vestiges (?) highly magnified (after Tims. Journ. Anat. and Phys., vol. xlii.). The Edentata belong to the homodont section of mammalia, but in some sloths certain teeth are more largely developed than others, so that there are teeth which might be termed canines. Whilst they are mostly monophyodont, at least one Armadillo has a succession of teeth, and so has the aberrant Orycteropus or Aard-vark. Leche states that he has found in Brady pus a calcined but aborted predecessor to one tooth in front of the mouth, and Kose states that in all (Dasy- pus novemcinctus, D. villosus, D. hybridus, and Orycteropus) which have been microscopically studied indications at least of successional germs have been seen. The teeth are of simple form, and do not differ in any material degree in different parts of the mouth, except only by their size (to this the caniniform tooth of the two-toed sloth is an exception). They are all of persistent growth, and therefore no division of parts into crown, neck, and root is possible. They consist generally of dentine THE TEETH OF EDENTATA. 473 and cement, with sometimes the addition of vaso-dentine, into which latter tissue the central axis of the pulp is converted ; while in some members of the order other peculiarities of structure exist : thus in the Orycteropus (Cape Ant-eater) dentine like that of Myliobates is found ; and in the Mega- therium hard dentine, a peculiar vaso-dentine, and vascular cementum co-exist. Enamel has not been seen upon the teeth of any existing Edentate animal, but the author found some years ago that the tooth-germs of the nine-banded armadillo were provided with enamel-organs. This however, proves nothing, for he showed the presence of enamel-organs to be universal and quite independent of any after-formation of enamel (u). Kose, however, whilst confirming the absence of true enamel, holds that there is a mere glaze of structureless material (schmelz- FlG. 241. — Lower jaw of a young Armadillo (Tatusia pebci), showing the milk teeth (&) in place, and their successors (V) beneath them. From a specimen in the Museum of the Koyal College of Surgeons. oberhautchen). And it is pointed out by Leche that in Tatusia the specialisation of the enamel organ goes on to the point of the internal epithelium becoming transformed into columnar ameloblasts, but that in Bradypus these cells remain rounded. It may be supposed, both from palaeontological and develop- mental evidence, that the Edentata, so far as teeth are concerned, are degenerated forms ; the shape of the tooth- germs and of the unworn teeth suggests that they branched off from an early triconodont stage with numerous molars, diphyodont, heterodont and furnished with enamel. They have suppressed the milk heterodont dentition, so that they are now more or less completely monophyodont, and have established a homodont second series. Armadillos. — The teeth of the Nine-banded Armadillo (Tatusia peba) will serve to illustrate the character of the dentition of the class. They are seven in number on each side of the jaw, of rounded form on section, and those of the upper 474 A MANUAL OF DENTAL ANATOMY. and lower jaws alternate, so that by wear they come to terminate in wedge-shaped grinding surfaces ; before they are at all worn they are bilobed, as may be seen in sections of the tooth germs. In the accompanying figure (Fig. 241) the milk teeth are represented, and beneath them their permanent successors. The divaricated bases of the milk teeth are due to the absorption set up by the approach of their successors, and not to the formation of any definite roots. Successional teeth have been detected in this armadillo only (except in T. kappleri, which is, perhaps a mere variety). In the armadillos the teeth are always of simple form and about thirty-two in number, except in Priodon, which has as many as a hundred teeth, a number altogether exceptional among mammals. The axis of the tooth is occupied by a sort of secondary dentine in which the tubes run almost longitudinally and are gathered into bundles. Sloths have fewer teeth than armadillos, and these softer in character, the axis of vaso -dentine (which is well pronounced and sharply marked off from the peripheral dentine) entering more largely into their composition, and forming as much as half the bulk of the tooth. 5 The two-toed Sloth has -—r teeth in each side of the jaws, and 4 these are nearly cylindrical in section and of persistent growth, while in the region usually occupied by a canine there is a tooth much longer and larger than the rest. Sir W. Flower and Pouchet and Chabry had failed to discover any succession of teeth in the sloths, and the author examined microscopically the jaws of a foetal Gliolapus and one other sloth in which the teeth were but little calcined, and failed to detect any indication of a second set of tooth germs. Semon also, though finding two rudimentary incisors in the lower jaw of a sloth, has seen no evidence of any succession. The probability is, therefore, that they are truly monophyodont, unless Leche, examining younger embryos than he has yet obtained, should find further traces of an antecedent dentition. The true ant-eaters are all edentulous, but Kose has seen very faint indications of a possible tooth-band in Myrmecophaga. THE TEETH OF EDENTATA. 475 The Orycteropus, or Cape Ant-eater (Aard-vark), the peculiarities of whose teeth have already been alluded to, has about thirty-six teeth in all ; but these are not all in place at one time, the smaller anterior teeth being shed before the back teeth are in place. Pouchet and Chabry found a rudimentary tooth in the incisor region. They are cylindrical teeth of persistent growth and simple form, and, like the teeth of the Armadillo, interdigitate with one another. The affinities of the Aard-vark are very uncertain ; it is grouped with the Edentata, but it is a very aberrant form, not only differing strongly in the structure of its teeth, which resembles that of Myliobates, but in other zoological characters as well. Oldfield Thomas has discovered that it also displays a succession in its teeth (7). The milk teeth are rudimentary and probably quite function- less, and though they become calcined, it is doubtful if they ever cut the gum. Unlike the permanent teeth, they are not of the persistent growth type, but one tooth at least, the most posterior, has two distinct roots. There are no teeth in the premaxilla, but seven have been found in the maxilla, and four on each side of the mandible. In structure they present an approximation to that of the functional teeth, and the last is molariform, the others being simple cones. So that Orycteropus retains traces of descent from a diphyodont and heterodont ancestor. In former ages many Edentates existed of much larger size, and the teeth of some of the gigantic extinct Edentates were a little more complex in form and structure ; thus the teeth of the Glyptodon were divided by longitudinal grooves, which in section rendered it trilobed ; and the teeth of the Megatherium were likewise marked by a longitudinal furrow, and presented greater complexity of structure. In their persistent growth, uniformity of shape, and absence from the intermaxillary bone, they strictly conformed with the teeth of recent Edentata. Amongst the Eocene fossils from the Puerco region, Cope has found forms which connect the Edentata with other mammals so far as their dentitions go. Thus Eemiganus (Cope) had certainly one, if not two, upper incisors with enamel on their anterior faces only, and with closed roots, and a 476 A MANUAL OF DENTAL ANATOMY. very large canine, with enamel all over its crown, though thin upon the back. The lower canine had closed roots and enamel limited to the front. (It may have been worn off the back.) Psittacotherium had two incisors above and one below ; the enamel was limited to the front on the upper incisor, which became worn to a chisel edge, and the lower incisor had enamel all over, but very thin on the back. The canines also had enamel on the front, and, like the incisors, had closed roots, though they went on growing long. Thus in Psittacotherium (Cope) the external lower incisors had disappeared, and a more or less rodent type of dentition had become established. In Calamodon, the lower canines were very large, scalpiform, with thick enamel on the front and none on the back, and of persistent growth. Pre -molars and molars somewhat hyposodont, with elongated roots, but not of persistent growth, and possessed of enamel in bands, thick cementum taking its place where it was deficient. In Stylinodon (Cope) the molars were cylindrical with long bands of enamel, and were of persistent growth. According to Wortman (15), these primitive Edentata (Ganodonta, a disputed term) in their earlier forms had teeth with closed and divided roots, and enamel more or less com- plete ; in later forms the teeth became hyposodont, without roots, of persistent growth, and with enamel limited to vertical bands, while incisors were present in both jaws. The animals belonging to this sharply denned order are scattered almost all over the world. The island of Madagascar is, however, remarkable for being almost without indigenous Rodents at the present day, though squirrels were numerous during the Miocene period. Indigenous rodents are also absent from Australia, two facts which are of no small interest to the student of odontology. For in each of these areas, out of the creatures which are there (in the one Lemurs, in the other Marsupials), there has arisen a form so modified as to mimic and take the place of the true Rodents, viz., the Cheiromys in Madagascar and the Wombat in Australia. In past times, however, a large number of animals, widely apart from one another in their affinities, have more or less approximated to the characteristics of a rodent dentition, and as this adaptive resemblance would give to them advantages in getting access to food which was denied The Teeth of Rodentia. j Duplicidentata — Hares and Rabbits. | Simplicidentata — All other Rodents. THE TEETH OF KODENTIA. 477 to animals with relatively blunt and weak front teeth, it is possible that some of these may have left descendants, and that hence the existing Rodents may have sprung from more than one stock. Matthew (5) has described an extinct rodent [Geratogaulus) which is peculiar in possessing horn cores situated on the nasal bones. Others which resemble it in dentition are without the horn cores. They all have one tooth (pm4 ?), exceedingly enlarged. The species of Kodents are exceedingly numerous, and the great majority of them are of small size ; the aquatic Capybara is by far the largest of recent Eodents. An average Kodent dentition would be — .10 10 3 iTcTpmTor?m 3; as extremes the Hare has — .2 0 3 3 iTcTpm^-m -g, and Hydromys — .10 0 2 1TcTpm'om 2' In general features the dentitions of the numerous species comprising this order are very uniform. The incisors (save in the hares and rabbits, in which there is an accessory small pair immediately behind the large ones) are reduced to four in number, are of very large size, and grow from persistent pulps. The jaws for some little distance behind the incisors are devoid of teeth, while beyond the interval {diastema) the cheek teeth, generally not more than four in number, are arranged in lines which diverge slightly as they pass backward. The large scalpriform, or chisel-like incisors, extend far back into the jaws and are much curved, the upper incisors, in the words of Professor Owen, forming a larger segment of a smaller circle than the lower, which are less curved. The length and curvature of these incisors relieve their growing pulps from direct pressure, which come to be situated far back in the jaw. the open end of the lower incisor, for example, being in many species actually behind the last of the molar teeth. The nerve going to supply the persistent pulps is of considerable size, and, owing to the open end of the tooth having formerly occupied a more anterior position in the jaw, runs forward beneath the tooth, and then bends abruptly backwards to reach the tooth- pulp. In many Eodents the enamel of the front of the large 478 A MANUAL OF DENTAL ANATOMY. incisors is stained of a deep orange colour ; this colour is situated in the substance of the enamel itself. The scalpriform incisors terminate in cutting edges, the sharpness of which is constantly maintained by the peculiar disposition of the tissues of the tooth. The investment of enamel, instead of being continued round the whole circumference of the tooth, is confined to its anterior and lateral surfaces, on the former of which it is thickest. It is, however, stated that the enamel-organ is continued round the roots, so that the connective tissue bundles by which attachment to the cementum is made have to grow through it to take their hold (v. Brunn). It is said by Hilgendorff (2), that the incisors of hares differ from those of all other Kodents in having enamel all round Fig. 242. — Side view of skull of a rodent, giving a general idea of the dentition of the order. them, although it is very thin at the back. It is doubtful whether the thin clear layer at the back of the tooth is enamel, and the author is disposed to regard it as cementum, the more so as it seems to be continued a little way upon the enamel, and in very young teeth the large cells of the enamel- organ are confined to the anterior surface.* When a Kodent incisor has been exposed to wear, the anterior layer of enamel is left projecting beyond the level of the dentine, and this arrangement results in a very sharp edge being constantly maintained. The dentine also is harder near to the front of the tooth than towards the posterior surface. It is said that no enamel was ever present on the tip of the incisor. * Of. E. G. Betts, " Observations on the Teeth of certain Rodents." Trans. Odontological Society, May, 1884. THE TEETH OF BODENTIA. 479 A thin external coat of cement is found upon the back of the tooth, but is not continued far over the face of the enamel. In the Marsupial wombat this layer of cement is continued over the whole anterior surface of the scalpriform incisors. Behind the great incisors in all rodents there comes a wide gap, no canines and but few premolars being present ; this gap leaves the growing incisors free scope in use. In many the lips dip deeply into this gap, and so separate the mouth into an anterior and posterior chamber. Possibly this is advantageous in preventing the chips of such a thing as a nutshell from passing back into the mouth. The grinding teeth are not very numerous ; the mouse 3 . 4 family have usually — ; the porcupines have constantly — and the hares -= ; the Australian water-rat (Hydromys) is NdtSiie Fig. 243. — Molar of Capybara, showing the transverse plates of dentine and enamel united to one another by cementum. 2 altogether exceptional in having so few as — . Observation A has established that the last three of the teeth are always true molars, and that when there are more than three the rest are premolars. A good deal of diversity exists in the premolar and molar teeth. In Eodents of mixed diet, such as the common rat, the back teeth are coated over the crown with enamel, which nowhere forms deep folds, and they have distinct roots, i.e., are not of persistent growth. In aged specimens the enamel is consequently worn off the grinding surface of the crown, which comes to be an area of dentine, surrounded by a ring of enamel. The molars of the rats have some sort of resemblance to minute human molars. But in those whose food is of a more refractory nature the molars, like the incisors, grow from persistent pulps (as is exemplified in the Capybara here figured), and their working surfaces are kept constantly rough by the enamel dipping in 480 A MANUAL OF DENTAL ANATOMY. deeply from the side of the tooth, as may also be seen in the common water-rat. The inflection of enamel may be so deep as to divide up completely the areas of dentine, the result being a tooth like that of the Capybara, which is composed of a series of plates of dentine, or " denticles," surrounded by layers of enamel, and all fused together by the cementum. The result of this disposition of the structures is that the working surface is made up of enamel, dentine, and cementum, three tissues of different hardness, which wear down at different rates, and so maintain its roughness. Various intermediate Fig. 244.— Condyle and glenoid cavity of the Capybara, showing their longitudinal direction. forms of the molar teeth are met with ; thus there are some in which complexity of the surface is maintained by folds of enamel dipping in for a little distance, but which, nevertheless, after a time form roots, and cease to grow. In general terms it may be said of the Eodents, just as of Ungulates, that the brachyodont bunodont type is the older, and that the laminated hypsodont type is of later acquisition. And, as is pointed out by Forsyth Major (4), the young, unworn, laminated hypsodont molar is at first tuberculated or bunodont, and the brachyodont tubercular molar, when worn down, partakes more or less completely of the laminated pattern on its grinding surface. When the molar teeth grow from persistent pulps, they are THE TEETH OF KODENTIA. 481 always curved, like the incisors, with the effect of relieving their pulps from direct pressure during mastication ; and the last remains of the pulps are converted into secondary or osteo-dentine, which thus forms the central axis of the incisors, or molars, as the case may be. In this tissue vascular tracts sometimes exist, but the secondary dentine is altogether small in amount, the formation of true dentine going on till the pulp at that particular point is almost obliterated. As has already been mentioned, when the molar series con- sists of more than three teeth, those anterior to the three true molars are premolars, but they do not differ materially in size or form from the true molars. The forms of the condyle and of the glenoid cavity in Fig. 245. — Cranium o£ Capybara. rodents are characteristic ; they are much elongated in an antero-posterior direction, so that the range of backward and forward motion, made use of in gnawing, is very considerable. The Leporidce are exceptional in having more lateral play than most rodents, and the power of the teeth is marvellous. Eats will sometimes gnaw holes in water-pipes, or in gas-pipes, in which they have heard water bubbling. The general character of a Rodent's dentition may be illus- trated by a description of that of the Capybara. The incisor teeth are quadrilateral, wider than they are deep, and are slightly grooved on their anterior surfaces. There are four grinding teeth on each side, of which the first three are small with few cross plates of dentine and enamel, but the fourth is a very complex tooth with twelve or D.A. 31 482 A MANUAL OF DENTAL ANATOMY. more such plates, which are fused into a solid mass by cementum. This tooth being one of persistent growth, there is no common pulp cavity, but each plate has its own. It has already been mentioned (p. 198) that the dentinal tubes at that part of the rodent's incisor which has come into use are much smaller than those near to its growing base, thereby proving that they have undergone a diminution in calibre at a time subsequent to their original formation. Near to the surface actually in wear they become cut off from the pulp cavity by the conversion of what remains of the pulp into a laminated granular mass, so that the dentine exposed on the surface must be devoid of sensitiveness, and the contents of the dentinal tubes must have presumably undergone some change. But what the nature of the change in the contents of the dentinal tubes which have ceased to be in continuity with a vascular living pulp may be there are no observations to indicate. As was shown by Sir J. Tomes (10), the enamel of rodents is peculiar, and some little diversity in the arrangement of the prisms exists in different families of the order, their character being in many cases so marked, that it is often possible to correctly refer a tooth to a particular family of rodents after simple inspection of its enamel. In general terms it may be said that the enamel is divided into two portions, an outer and an inner portion (this is true of all save the hares and rabbits), and that the enamel prisms pursue different courses in these two portions. Thus, in the enamel of the beaver, in the inner half, nearest to the dentine, the prisms of contiguous layers, as seen in an oblique transverse section, cross each other at right angles, whereas in the outer portion they are all parallel with one another, presenting, however, the peculiarity of not running straight on to the surface at right angles to it, but inclining towards the middle line of the skull. In the genera Sciurus, Pteromys, Tamias, and Spermophilus the enamel rods, as seen in longitudinal section, start from the dentine at right angles to its surface ; in Castor they incline upwards at an angle of 60°, but preserve the distinc- tion between the outer and inner layers very distinctly. In the Muridce the decussation of the layers in the inner THE TEETH OF KODENTIA. 483 part and their parallelism in the outer part of the enamel are also found, but, in addition to this, the borders of the individual prisms - are slightly serrated, the serrations of contiguous rods interlocking. In the porcupine sub-order (Hystricomorpha) the columns of the inner portion of the enamel pursue a serpentine course, nevertheless showing indications of a division into layers, becoming parallel in the outer portions as in other Kodents. Small interspaces are found amongst the enamel rods of the porcupines. In the hares (Leporidce) the lamelliform arrangement and the division into outer and inner layers alike disappear. The peculiarities in the disposition of the enamel fibres, which are so marked in the incisors, do not generally exist in the molars of the same species, except when these are of continuous growth. Many minor differences in the arrangement of the enamel prisms exist, for a description of which the reader is referred to the original paper, but in general terms it may be said that the " enamel lamellae have a different and distinctive character in each of the larger groups, and that the variety of structure is constant throughout the members of the same group. We may take, for example, the Sciuridce, the Muridce, and the Hystricidce, in each of which the structure of the enamel is different, and in each is highly distinctive." And further the varieties in the structure of the dental tissue, so far as they are known, with a few isolated exceptions, justify and accord with the classification of the members of the order usually given. It is stated by von Ebner that tubes exist in the enamel of hares and of Cavia, but that they do not communicate with the dentinal tubes. There is, however, no doubt that in the jerboa the tubes in the enamel are in continuity with those of the dentine. The milk dentitions of the rodent have been investigated by a number of observers, with rather discrepant results. At all events, it may be said that the extent to which the milk teeth are developed varies much. It has already been mentioned that in the hares and rabbits there are four incisors in the upper jaw, a small and apparently functionless pair being placed close behind the large rodent 31—2 484 A MANUAL OF DENTAL ANATOMY. incisors ; but in very young specimens there are six incisors, of which the one pair are soon lost. Professor Huxley (3) wrote : " The deciduous molars and the posterior deciduous upper incisors of the rabbit have been long known. But I have recently found that unborn rabbits possess, in addition, two anterior upper and two lower deciduous incisors. Both are simple conical teeth, the sacs of which are merely embedded in the gum. The upper is not more than one-hundredth of an inch long, the lower rather larger. It would be interesting to examine foetal guinea-pigs in relation to this point ; at present they are known to possess only the hindermost deciduous molars, so far agreeing with the Marsupials." These additional teeth he rightly interpreted as being milk teeth, and not as representatives of an additional pair of incisors. Huxley's observation has been confirmed by Pouchet and Chabry, Freund, Woodward, and others. The latter observer has reinvestigated the tooth development in the rabbit and finds (14) that never cut the gum, and are absorbed in di. i utero. -j"-2 is functional, and is shed about the third week after birth, dpm -x- are functional, and are shed about the same period as the last. The squirrel also has milk incisors (Freund) ; three rudi- ments are described by that author, which Woodward, as it appears to the author with good reason, interprets as a milk incisor, as a vestigial milk incisor representing the second incisors of the hares and rabbits, and as a third incisor (Freund regarding the last as a canine). Woodward (loc. cit.) describes a pair of minute calcified rudiments in the mouse, though their presence is not quite constant. Thus the squirrel, the rabbit, and the mouse present three stages in the reduction of the vestigial milk incisors. To these Marett Tims (8) adds a further stage in the guinea-pig (Cavia) in which a labial growth of the tooth band occurs, but no calcification has been observed. In other rodents no milk teeth are known in the front of the mouth. THE TEETH OF KODENTIA. 485 Waterhouse (12) has found the milk molars still in place in the skull of a half -grown beaver ; while in the hares they are shed about the eighteenth day after birth, and in the guinea- pig disappear before birth. Hares and rabbits have six milk molars in the upper and four in the lower jaw, which come into use, but differ from their successors in forming definite roots, and not growing from persistent pulps. Milk premolars have been described in the hare and rabbits, in Castor, Dasyprocta, Ctenodaetylus, Hystrix, Atherura, Erethizon, all porcupines, so that it is probable that further search will reveal traces of others. In Atherura, a brush- tailed porcupine, there is a single well-developed and functional milk premolar on each side of both jaws. Other rodents, such as the rat, which has only three teeth of the molar series on each side, and the Australian water-rat (Hydromys), have no known milk teeth, and are hence perhaps truly monophyodont. The change of teeth has been carefully investigated in the guinea-pig (Cavia) by Marett Tims (loc. cit.). It possesses as a functional dentition four cheek teeth on each side, which are generally regarded as 1 3 PTm Cuvier had discovered that before birth in the molar region there was a single milk tooth. In the foetal guinea-pig five cheek teeth are found in process of development. The deciduous tooth, Cuvier's milk molar, is the second, and the tooth which displaces it is not, as might have been expected, its morphological successor, but is the tooth which is developing in front of it and quite independently of it. This raises a doubt whether the deciduous tooth is a milk tooth at all. It appears as if it belonged to the same series as the functional teeth, in which case it must be regarded as a permanent tooth (probably the first of the molar series) which is shed before birth, and the guinea-pig would have no milk teeth all. A somewhat similar doubt attaches to the single deciduous tooth of Marsupials and other animals. (Cf. pp. 402 and 459.) Eodents are fairly well represented in Eocene Tertiary deposits, but their ancestry is by no means clear. Cope considers that the Tillodonta (see p. 487) may have been the ancestors of existing rodents, and 486 A MANUAL OF DENTAL ANATOMY. gives (*) an instructive series of reduction of certain incisors and hypertrophy of others in Esthonyx, Psittacotherium, Calamodon, and Anchippodus, in which one pair of very rodent-like incisors remain in the lower jaw. He holds with Eyder that continued severe use has resulted in hypertrophy of these teeth, and that the backward and forward excursions of the jaw have led to the disappearance of the postglenoid process in the articulation, allowing of the very posterior position of the lower jaw characteristic of rodents. Forsyth Major (4) states that the investigation of early rodents lends no support to the tritubercular theory, but that a brachyodont bunodont molar is obviously the more primitive type among them, and that the more brachyodont the tooth the more it is multitubercular. He holds that in the molars of all mammals a tendency to reduction and simplification, which proceeds from the inner side outwards in the upper jaw and from the outer side inwards in the lower jaw, is traceable, and that the true tritubercular molar having been arrived at in this way, is not a primitive, but a specialised type. (Cf. p. 381.) Marett Tims finds that there is a reduction in the number of cusps on the molar teeth of Cavia during development, thus bringing embryo- logical evidence in support of Forsyth Major's palseontological conclusions. Tillodontia. This order, comprising several genera, was proposed by Marsh to include some Wyoming fossil remains which, though not amongst the biggest, are " amongst the most remarkable yet discovered in American strata, and seem to combine characters of several distinct groups, viz., Carnivora, Ungulata, and Eodentia." In Tillotherium (Marsh), the type of the order, the skull has the same general form as in the bear, but in its structure resembles that of the Ungulata. Its molar teeth are of the ungulate type, the canines are small, and in each jaw there is a pair of large scalpriform incisors, faced with enamel, and growing from persistent pulps as in the rodents. The hinder pair of incisors are small and have not persistent pulps. It has been pointed out by Wortman that the greatly developed incisors are the second pair, and that the first and third are disappearing just as in the ancestors of the Proboscidea. Esthonyx (Cope), an Eocene mammal with a dentition roughly resembling that of a gigantic shrew, foreshadows this 2 condition, having i — , of which— is extremely reduced, while o 13 THE TEETH OF TILLODONTIA. 487 — is enlarged, and its enamel confined to its anterior and external aspects ; ii is absent, i2 enlarged, with no enamel on its back, but none of these teeth grow from persistent pulps. The adult dentition of Tillotherium is as follows : — .21 33 1 2 cTprm-2mF " There are two distinct families : Tillotherid ce (perhaps identical with Anchippoclontidcz), in which the large incisors grew from persistent pulps, while the molars had roots ; and the Stylinodontidice, in which all the teeth have persistent pulps." One genus (Dryptodon) , known only by the lower jaw, had Fig. 246. — Upper and lower jaws of Tillotherium (Marsh). six teeth, described as " clearly incisors," the two inner pairs of which are small and cylindrical, the outer of enormous size, faced in front only with enamel, and with persistent pulps carried back under the premolars. Whilst Flower endorses Marsh's view that Tillodonts have ungulate affinities and resemblances to rodents also, Cope on the other hand regarded the Tillodonts as being the ancestors of the rodents. This is disputed by Dr. Wortman (15), who says that he fails to discover any traces of ungulate relation- ship, and he prefers to refer them to those generalised forms with insectivorous affinities which Cope groups as Bunotheria. The adaptive character of incisors faced with enamel and of persistent growth he would appear to consider as not going far 488 A MANUAL OF DENTAL ANATOMY. towards establishing more than a superficial resemblance to the Kodentia. He holds that Tillodonts are probably descendants of Esthonyx, itself allied to the shrews, and that they may also have given origin to the Toxodonts. Eefeeences. (!) Cope, E. D. " Primary Factors of Organic Evolution." Chicago, 1896. (2) Hilgendoefe. "tjber das Gebiss der hasenartigen Nager." Konigpress, Akad. d. Wissensch. Monatsbericht d. Berlin, 1865, p. 673. (3) Huxley, T. H. Nature, 1881, vol. xxiii., p. 228. (4) Major, 0. I. Forsyth. "On some Miocene Squirrels, with Be- marks on the Dentition and Classification of the Sciurinoe." Proc. Zool. Soc, Lond., 1893, pp. 179—214. (5) Matthew, W. D. "A Horned Eodent from the Colorado Museum, with a Ee vision of the Mylogauli Beavers and Hares of the American Tertiary." Bull. Amer. Mus. Nat. Hist., 1902, vol. xvi., art. xxii., pp. 291 — 310, (6) Eose, C. (a) " Beitrage zur Zahnentwickelung der Edentaten." Anat. Anzeiger, 1892, bd. vii., pp. 495 — 512. (b) " tjber rudimentare Zahnanlagen der Gattung Manis." Ibid., pp. 618—622. (7) Thomas, Oldfield. " A Milk Dentition in Oryderopus." Proc. Eoy. Soc, Lond., 1890, vol. xlvii., p. 126. (8) Tims, H. W. Maeett. " On the Tooth-genesis in the Caviidce." Journ. (Zoology) Linngean Soc, Lond., 1901, vol. xxvinS, pp. 261—299. (9) . "Tooth Yestiges and Associated Mouth Parts in the Manidce.''1 Journ. Anat. and Phys., vol. xlii., pp. 375—387. (10) Tomes, J. " On the Structure of the Dental Tissues of the Order Rodentia." Phil. Trans. Eoy. Soc, Lond., 185T), pt. ii., 529—568. (n) Tomes, C. S. " On the Development of the Teeth of the Newt., Frog, Slow-worm, and Green Lizard." Phil. Trans. Eoy. Soc, Lond., 1876, vol. clxv., pt. i., pp. 285 — 296. (12) Waterhoese, G. E, " Natural History of Mammals." London, 1848. (13) Webee, Max. "Beitrage zur Anatomie und Entwickelung der Genus Manis." Zoolog. Ergebnisse einer Eeise in Niederlandisch Ost Indien., 1892, bd. ii. pp. 1—118. THE TEETH OF TILLODONTIA. 489 (14) Woodward, M. F. "On the Milk Dentition of the Eodentia, with a Description of a Vestigial Milk Incisor in the Mouse [Mus musculue)." Anat. Anzeiger, *1894, bd. ix., pp. 619—131. (16) Wortman, J. L. (a) "The Ganodonta and their Eelationships to the Edentata." Bull. Amer. Mus. Nat. Hist., 1897, vol. ix., pp. 59—110. (&) Amer. Syst. of Dentistry. Phil., 1886, pp. 350—575. i CHAPTER XVIII. THE TEETH OF CETACEA, UNGULATA, AMBLYPODA AND CONDYLARTHRA. Cetacea IMystacoceti . f Odontoceti . ' Physeteridce. Platanistidce. Delphinapteridce. Delphinidce. f Balcenidce. \ Rhachianectidce. \Balcenopteridce. This order is divided into two groups, namely, the toothed whales or Odontoceti, and the whalebone whales or Mystacoceti ; these two groups are sharply denned from one another. No Cetacean is known to develop more than one set of functional teeth, and these, when present in any considerable numbers, closely resemble one another in form. The teeth, however, of the extinct Zeuglodon and Squalodon, which have about 361 teeth, are heterodont in character. Squalodon had about sixty teeth. The four front teeth on each side were simple in form, the next four nearly simple, whilst the last seven had sharp cuspidate edges, and were implanted by two roots. Kukenthal has shown that, at all events in Beluga, Tarsiops and Globiocephalus, rudiments of successional teeth are to be found to the inner sides of the functional teeth, which latter are thus shown to be persistent milk teeth, though Leche considers it still an open question. [The conditions as to the number and relations of the dentitions present in the developing jaw are similar to those found in the Marsupials. In both cases there is much to justify the view that the functional teeth belong to the successional or replacing series of the mammalia generally (p. 362).] The Cetacea appear to have descended from an ancient heterodont diphyodont stock, probably with ungulate affinities, and to have reverted to a primitive homodont series, represented by a single dentition. THE TEETH OF CETACEA. 491 In Phoccena (common Porpoise) the enamel-organs are not much differentiated, the internal epithelium cells remaining rounded, but in Balaenoptera the differentiation goes further, although the teeth are never to be cut. The teeth of Cetacea are usually composed of hard dentine, with an investment of cementum. After the attainment of the full dimensions of the teeth what remains of the pulp is very commonly converted into secondary dentine ; tips, and even entire investments of enamel, are met with in many of the order. The dentine is remarkable for the very numerous interglobular spaces which it contains ; these are clustered in concentric rows, so as to give rise to the appearance of contour lines. The cement is often of great thickness, and the lacunae in it are abundant ; its lamination is also very distinct. The Dolphin (Delphinus) may be taken as representing Fig. 247. — Jaws of common dolphin (Delphinus). a generalised cetacean dentition. In it the teeth are very numerous, there being about 200 : they are slender, conical, slightly curved inwards, and sharply pointed. As they inter- digitate with one another, there is very little wear upon the points, which consequently remain quite sharp. The largest teeth are those situated about the middle of the dental series. Many variations in the number and form of the teeth are met with. The porpoise {Phoccena) has not more than half the number of teeth possessed by the dolphin. They are small, and have compressed crowns with cylindrical, contracted necks. In Inia, a fresh-water form akin to the dolphins, the posterior teeth have a distinct tubercle on the inner side. The extent to which sockets, seldom very complete, are developed also varies greatly in the Cetacea, in some there being little more than a continuous groove, in which the teeth are implanted. There is a little confusion in the nomenclature, the popular names Porpoise, Dolphin, and Grampus being applied somewhat vaguely to several different 492 A MANUAL OF DENTAL ANATOMY. genera. Thus the grampus of sailors is the genus Orca of naturalists. The genus Grampus of naturalists is a quite distinct creature, with few teeth, only three to seven on each side, and these only in the lower jaw, and is furnished with fewer teeth than the dolphin or the porpoise, but they are very large and stout, and exist in both jaws. The teeth of the grampus (Orca) become worn down on their opposed surfaces, and coincidently with their wearing away the pulps become calcined. In the Oxford Museum there is a grampus in which, owing to a distortion of the lower jaw, the teeth, instead of interdigitating, became exactly opposed to one another ; the consequence of this was that the rate of wear was greatly increased, and the pulp cavities were opened before the obliteration of the pulp by calcification had taken Fig. 248.— Teeth of upper jaw of a grampus {Orca). (After Professor Flower.) place, * so that the pulps died and abscesses around the teeth had resulted. These animals are strong and fierce ; they attain to a length of 20 feet, feeding on fish, seals, and other cetaceans smaller than themselves, and combining in numbers to attack even whales, which they thus succeed in killing. It occasionally happens in the Delphinidce that small additional teeth are interposed between the regular alternating teeth. The remaining toothed cetaceans which will be noticed all present some reduction from the dentitions thus far alluded to. Thus Beluga, the white whale, has from eight to ten teeth on each side of each jaw, but they are rather small, and are confined to the anterior half of the jaws. The Sperm Whale (Physeter) grows to a gigantic size, 50 to 60 feet in length, and is pursued for the sake of its * " Trans. Odontol. Society," 1873. When this paper was published the author was not aware that Eschricht had previously published a similar observation. THE TEETH OF CETACEA. 493 blubber, which yields sperm oil, and for the oil contained in the cells of its head (spermaceti). It feeds on large and even gigantic cuttle-fish, and upon fish, which as a rule it swallows whole without biting them, as might be expected from its inefficient dental armament. The teeth are numerous in the lower jaw, but in the upper jaw there are only a few curved, stunted teeth, which remain buried in the dense gum, and are generally lost from skulls. Kitchie and Edwards have however recently noted the occurrence of functional teeth in the upper jaw in more than one specimen (8). The teeth of the lower jaw are retained in shallow and wide depressions of the bone by a dense ligamentous gum, which, when stripped away, carries the teeth with it. Thus every intermediate stage between this slight implantation and the well-developed stout sockets of the grampus is met with in the Cetacea. In tertiary deposits in Patagonia a small sperm whale has been found with a full series of enamelled teeth in the upper jaw. In the Bottle-nosed Whale {Hyperoddon bidens) the only large teeth present are two (sometimes four) conical, enamel-tipped teeth, which remain more or less completely embedded within the gum, near to the front of the lower jaw. In addition to these there are 12 or 13 very small rudimentary teeth loose in the gums of both jaws (Eschricht, Lacepede). The Ziphoid Cetaceans present one of the most curious and inexplicable dentitions to be found in any animal. The upper jaws are edentulous, as in the Hyperoddon, and the lower jaws contain only a single tooth upon each side; but these teeth have attained to great proportions, measuring in full-grown specimens as much as ten inches in length ; they are thin, flat and strap-shaped, straight for some considerable part of their length and then curving over towards each other, may even cross each other above the upper jaw, so that they actually limit, to a considerable extent, the degree to which the jaws can be opened. A few small additional teeth are often found in ziphoid cetaceans, but these are generally lost from skulls. It is not merely difficult to see of what use these teeth can be, but it is hard to suppose that they can be otherwise than actually detrimental to their possessors in the pursuit of 494 A MANUAL OF DENTAL ANATOMY. food. There is some reason to suppose that the presence of well-developed tusks is a character of the male sex, though upon this point the evidence is not quite complete. Females have been found with their skins curiously scored in two parallel lines, especially near the pudenda, suggesting the idea that they are liable to be attacked by the males. The structure of these teeth is not less peculiar than their general form. The summit of the tooth, which starts off nearly at right angles to the shaft (and so, the shaft being curved over the top of the upper jaw, comes to stand nearly vertically), consists of a denticle bluntly pointed and made up of dentine coated with enamel. This denticle of triangular shape is only about a third of an inch in length, and in the adult specimen described by Professor Sir Wm. Turner had the enamel coat partially worn off. In the Challenger Reports (" Zoology," vol. i.) he says : " In the earlier stage their structure does not differ materially from the ordinary type of tooth one meets with, say in the human or carnivorous jaw, the crown being formed by enamel, the fang by cement, whilst the great body of the tooth consists of dentine, in which is a marked pulp cavity, com- municating with the exterior by a slit-like aperture at the root of the fang. The exceptional character these teeth exhibit in the erupted condition is due to the disappearance of the enamel from the crown, to the cessation in the develop- ment of the ordinary dentine, to the excessive formation of osteo-dentine, of modified vaso-dentine, and of cement, by means of which the pulp cavity becomes almost obliterated, and the fang assumes dimensions which, in the case of Mesoplodon layardii, lead to the production of a tooth having the very remarkable form and relation to the beak which I have described." As may be gathered from the above, the development of the tooth begins by the formation of the denticle, which is of an ordinary structure ; the enamel, however, soon ceases to be formed, and, but very little further down, the true dentine also, not, however, before cementum has begun to be formed upon its exterior. (See c in Fig. 249.) Then there comes an abrupt change in direction, and in the place of true dentine we find a coarser textured tissue which contains large vascular canals. THE TEETH OF CETACEA. 495 This Sir W. Turner and Sir E. Bay Lankester regard as a vaso-dentine, seeing that it is in all probability a product of the dentine pulp. Of this the great mass of the tooth consists, but it has throughout its length an investment of cementum of an ordinary type, which forms a complete external layer, laminated, full of lacunae and for the most part devoid of Haversian canals. Immediately beneath this layer there is, if the distinction be not exaggerated in the drawing, a definite stratum of tissue of material thickness which is characterised by an abundance of vascular canals arranged perpendicularly to the surface (a in Fig. 249), which is regarded by Sir William Turner FlG. 249. — Upper part of tooth of Mesoplodon. (After Professor Sir W. Turner.) a. Tissue of doubtful origin, permeated by vascular canals. c. Cementum. d. Dentine, e. Enamel, v. Vaso-dentine. as belonging to the dentine group of tissues, i.e., as being a vaso-dentine. But there is a difficulty in accepting this view, viz., that near to the denticle it is seen to lie distinctly out- side the true fine-tubed dentine (see Fig. 249), which it overlaps to a considerable extent. Now if this tissue was formed by the dentine pulp we should have the anomaly of a pulp first forming a very vascular vaso-dentine, then changing to form a fine-tubed normal dentine (which is exactly the reverse of what is met with in other creatures in which the pulp forms these two structures), and finally reverting to the building up of a vaso-dentine. Judging by analogy, this seems so improbable that, in the absence of more positive knowledge and simply judging from the 496 A MANUAL OF DENTAL ANATOMY. figure, one would be inclined to refer this layer to the cementum. Lower down in the shaft of the tooth anastomoses take place between the tubes of this layer and those of the unquestionable pulp products, but anastomoses between dentine tubes and enamel tubes and between dentine tubes and cement lacunae are of common occurrence in many animals, so that this com- munication does not prove anything as to their respective origin. However, Sir Eay Lankester lays stress upon the globular botryoidal structure of this layer, which he states shades off into the fine-tubed dentine, so that it may perhaps be regarded as an excessive development of the globular layer of dentine, rather than as a vaso-dentine. In reconciliation of the discrepancy between the two descriptions, it is suggested by Turner that the vascular canals seen by him in this layer may have become obliterated in the presumably much older specimen described by Lankester. The central pulp cavity becomes reduced to the merest traces, so that the completed tooth is almost solid. In the Narwal (Monodon monoceros) two teeth alone persist, and these are in the upper jaw. In the female the dental- germs become calcified, and attain to a length of about eight inches, but they remain enclosed within the substance of the bone, and their pulp cavities speedily fill up. In the male, one tusk (in some very rare instances both) continues to grow from a persistent pulp till it attains to a length of ten or twelve feet, and a diameter of three or four inches at its base This tusk (the left) is quite straight, but is marked by spiral grooves, winding from right to left. It is curious that in one of the specimens, in which the two tusks had attained to equal and considerable length, the spirals on the two wound in the same direction ; that is to say, as regards the sides of the head, the spirals were not symmetrical with one another. The tusk of the male narwal may fairly be assumed to serve as a sexual weapon, but little is known of the habits of the animal. Professor Sir Wm. Turner has noted the occurrence of two stunted incisor rudiments in a foetal narwal : these obviously represent a second pair of incisors, and attain to a length of half an inch, but are irregular in form ; they are situated a little behind the pair of teeth which attain to more consider- THE TEETH OF CETACEA. 497 able dimensioDS. All trace of this second pair of incisors is lost in adult skulls. Mystacoceti. — The remaining group of cetaceans, the Mystacoceti or whale-bone whales, are characterised by being, in the adult condition, destitute of teeth, but prior to birth the margins of both upper and lower jaws are covered with a series of nearly globular rudimentary teeth Fig. 2i9a. — Cranium of Narwal (Mono&on monoceros). a. Stunted tooth, with its basal pulp cavity obliterated, b. Long tusk. The small figure, giving the whole length of the tusk, shows the proportion which it bears to the rest of the skull. which become calcined, but are speedily shed or rather absorbed. The foetal teeth of the Balcenoptera rostrata (the Rorqual, perhaps the commonest of the whalebone whales, which is sometimes stranded on British coasts, and has short and inferior whalebone) have been carefully described by M. Julin (2), the Bakenopteridce having been previously d.a. 32 498 A MANUAL OF DENTAL ANATOMY. supposed to be without rudimentary teeth. The ramus con- tained forty-one tooth-germs, each furnished with an enamel- organ and dentine bulb, with a slight capsule ; these were lodged in a continuous groove in the bone above the vessels, thus recalling the condition of the parts in a human embryo at a certain stage. A very small amount of calcification takes place, a mere film of dentine being formed upon the dentine bulb. But what is very remarkable is that the dentine bulbs are simple near the front, bifid in the middle, and trifid at the back of the mouth ; in other words, these tooth germs would go to form heterodont teeth, not unlike those of some seals, or of Squalodon. Hence it has been suggested that the whale may be descended from some such ancestral form. Kukenthal states that as these trifid teeth become absorbed they may separate into three. Moreover, he states that there are traces of successional rudiments. From the upper jaw of an adult whalebone whale there hang down a series of plates of baleen, placed transversely to the axis of the mouth, but not exactly at right angles to it. The principal plates do not extend across the whole width of the palate, but its median portion is occupied by subsidiary smaller plates. The whalebone plates are frayed out at their edges and collectively form a concave roof to the mouth, against which the large tongue fits, so as to sweep from the fringes whatever they may have entangled. The whale in feeding takes in enormous mouthfuls of water containing small marine mollusca ; this is strained through the baleen plates, which retain the Pteropods and other small creatures, while the water is expelled. Then the tongue sweeps the entangled food from the fringe of the baleen plates, and it is swallowed. When the mouth is closed the baleen plates slope backwards towards the throat, but when it is opened by their elasticity they spring downwards to a vertical position, so as to still fill up the mouth, and as the mouth closes and expels the water, they act as strainers. Each plate consists of two dense, but rather brittle, lamina3,which enclose between them a tissue composed of bodies analogous to coarse hairs. By the process of wear the brittle laminae break away, leaving projecting from THE TEETH OF CETACEA. 499 the edge the more elastic central tissue in the form of stiff hairs. Each plate is deyeloped from a vascular persistent pulp, which sends out an immense number of exceedingly long thread-like processes, which penetrate far into the hard substance of the plate. Each hair-like fibre has within its base a vascular filament or papilla ; in fact, each fibre is nothing more than an accumulation of epidermic cells, con- centrically arranged around a vascular papilla, the latter being enormously elongated. The baleen plate is composed mainly of these fibres, which constitute the hairs of its frayed- out edge, but in addition to this there are layers of flat cells binding the whole together, and constituting the outer or lamellar portion. As has been pointed out by Professor Sir Wm. Turner (10), the whalebone matrix having been produced by the cornification of the epithelial coverings of its various groups of papillae is an epithelial or epiblastic structure, and morphologically corresponds not with the dentine, but with the enamel of a tooth. The whole whalebone plate and the vascular ridges and papillae which form it may be compared to the strong ridges upon the palates of certain Herbivora,* an analogy which is strengthened by the study of the mouth of young whales prior to the cornification of the whalebone. It must not be supposed that, because in the Dolphins, &c, there is a simple homodont dentition, they are in any way the parent form of other mammalia. On the contrary, the possession of some amount of hair on the skin, the nature of the rudimentary teeth and many other characters, point to their being extreme, and in some particulars degenerated modifications of other, and probably terrestrial, mammals, and Kiikenthal goes so far as to give the opinion that the two groups, the toothed whales and the whalebone whales, have had a distinct origin. Thus, like the Edentata, the Cetacea have become homodont and practically monophyodont by a retrograde process, and the superficial resemblance of their simple conical teeth to those of a reptile is misleading. * Doubt has been thrown on this by some recent writers, H. W. M. T. 32—2 500 A MANUAL OF DENTAL ANATOMY. Teeth of the Ungulata. Ungulata - Aetiodactyla Non-ruminantia f Hippopotamidw Suoidea. Tylopoda . Pecora . Traguloidea Perissodactyla { Suidce Camelidce (" Cervidce ! Bovidce I Girqffidce Trar/ulidce ( Tapiridce Rliinocerotidce Eqitidce This is a group of animals, of which a vast number of forms are extinct, and are only imperfectly known. Eecent discoveries, especially those of Marsh, Cope, Osborn, Wort- man, and many others, in the rich deposits of America have brought to light a very large number of strange and highly interesting forms, which, while they have not done away with the old division of Ungulates or hoofed mammals into the Artiodactyle (even-toed) and Perissodactyle (odd-toed) Ungu- lates, have brought within it, to a certain extent, Hyracoidea and Proboscidea, and have introduced a number of existing forms which will not fit into either of the old divisions. Provisionally, they may be grouped into Ungulata Vera and Subungulata. The Ungulata Vera comprise — (i.) Artiodactyles or \ Hippopotamus, Pigs, Anoplotherium, Cows, even-toed Ungulata J Sheep, Deer, and other Kuruinants. (ii.) Perissodactyles, or "j UDgulata with an > Horses, Tapirs, Ehinoceros, Paheotherium. odd number of toes ) . The distinction between the two groups is strongly marked, if living animals alone be considered ; but, as Huxley pointed out, increasing knowledge of fossil forms has partly broken down the line of demarcation. The ill-defined group of — (iii.) Subungulata or Ungulata ) Hyracoidea, Proboscidea, Amblypoda, Polydactyla, comprising } Toxodontia, Tillodontia, &c. But fortunately it is not necessary in an elementary work on Odontology to do more than present the descriptions of the several creatures somewhere near their right places and so the difficulties of the classification of the Ungulata need not be discussed. TEETH OF THE UNGULATA. 501 Of Eocene Ungulata it may be said that almost all possessed the full mammalian detention, i.e., .31 4 3,, l"3"cTpm Ym_3"= ' and a few had a larger number. A great many of them had five toes, and were otherwise less specialised than modern forms. Moreover, as we look at the patterns of the molar teeth, we find them far more simple, trituberculate or quadri- tuberculate crowns being the rule. In the earlier forms the molars were short and distinctly rooted (brachyodont). Those with much longer crowns, such as the horse, are termed hypsodont, and occur later. When the surfaces of the molars present blunt cones, they are termed bunodont, and when they are lengthened out antero-posteriorly and curved, they are called selenodont, e.g., as in the sheep. In the more generalised types, such as, amongst recent Ungulates, pigs and tapirs for example, the cheek teeth have low cusps, so that in all stages of wear the bottom of the intervening valleys is in view ; but in the long-crowned teeth of ruminants and the still longer ones of the horse the bottom of the valleys (in the latter case filled in with cementum) can- not be seen at all until the teeth are worn down. In all existing Ungulates the quadrituberculate type of tooth has been reached, but amongst the earlier ones the tritubercu- late pattern still persisted, or at all events persisted in their immediate ancestry, if such forms as Phenacodus be regarded as the parent forms of Ungulates. The modern Ungulata are all fitted for a vegetable diet, though possibly a few forms may occasionally prey upon much weaker animals. Indeed, among the extinct Ungulata there are some which tend to bridge the gap between existing Ungulata and Carnivora, wide though it now is. There is a tendency to the reduction of teeth in the front of the mouth, to the suppression of canines, and to a great development of cheek teeth, in relation to their vegetable diet. The milk dentition is generally well developed, and in the case of modern domesticated animals remains in use for a considerable time. In size they range from the little extinct Artiodactyle 502 A MANUAL OF DENTAL ANATOMY. Hypsodon described by Matthew, which was no larger than a rabbit, to the huge Amblypoda. It may save recapitulation to give at this point some account of the evolution of ungulate molars. The complicated-looking patterns found in the cheek teeth of LTngulates have long been studied with no little care, as it has been found that they are a guide to affinity, but unfortu- nately different writers have made use of very different termin- ologies in describing these teeth. Whether the tritubercular theory be adopted or not, its advocates, and notably Osborn, have had access to a great number of extinct forms till recently unknown, and have been able to determine the homologies of the various cusps and ridges in a manner previously impossible. Hence the terminology adopted by them is to be preferred to any of the older designations. The termination -cone is em- ployed for the principal or crown cusps, and its diminutive -conule for smaller- intermediate cusps. "With the exception of the hypocone, which, although derived from the cingulum, was very early established as a crown cusp, all peripheral cusps, derived from elevations of the cingulum or borders of the crowns are called styles. The crests which are so conspicuous features in ungulate teeth prior to any wear taking place are always built up of two or more cusps and styles, and are denoted by the termination -loph ; and in all cases the termination -id denotes the corre- sponding part of the lower teeth. To give an example of the application of these terms, the tooth of a Ehinoceros is truly lophodont, since all its cusps have lost their integrity, and are fused into -lophs. In general terms, whilst the trigon and talon serve in earlier mammals and in Carnivora to distinguish between the cutting and the crushing portions of the crown, in Ungulates the trigon is lowered to the same level as the talon, and loses its apparent distinction from it. At first the cusps remained distinct (bunodont type, which still persists in many e.g., the pigs), but gradually there came in the lophodont and selenodont forms, in which the cusps lost their integrity by being joined into ridges, which ridges again became curved or sinuous, and accessory styles being added still further increased the complexity of pattern. In what Schlosser terms the modernisation of the type he considers the obliquity of the ridges to be a survival of the obliquity of the sides of the TEETH OF THE UNGULATA. 503 primitive triangles. And, finally, the superadded ''styles" attain to a size and importance equal to the earlier cusps, and when they too become fused into ridges or lophs create a complexity which it would have been impossible to unravel without the help of the earlier transitional forms. Fig. 250. — Description of figure. (After Osborn, "Studies Biol. Lab. Columbia College," "Zoology," vol. i., No. 2.) Teeth of Euprotogonia, Hyracotherium,a,n& Anchitherium. a. Protocone. b. Paracone. c Meta- cone. d. Hypocone. e. Protoconule. f. Metaconule. According to Osborn's later nomenclature in Anchitherium, a and e — protoloph ; d and /, metaloph ; the whole ridge b to c, ectoloph ; ps, parastyle ; vis, mesostyle ; mts, metastyle. It will be gathered that the chief importance in the com- parison of various forms lies in the identification of the cusps and ridges, but as one often has to deal with a tooth in which the top is worn more or less level, it is desirable to have a name for the areas which intervene, and which are filled up 504 A MANUAL OF DENTAL ANATOMY. with cementum. For these Cuvier's term fossettes is retained. The foregoing illustration (Fig. 250), adapted from Osborn, will serve to show, in some measure, the stages by which such transformations of pattern are brought about. In the first the bunodont tooth, with its four principal cusps and near to the middle of its crown the subsidiary cusps, is seen to be tending towards a lophodont type by the joining of these with the protocone and hypocone. In Hyracotherimn this has gone further : the outer cusps are joined by a ridge (longitudinal), and the protocone and protoconule and the hypocone and metaconule are joined by ridges (transverse, or rather oblique, to the line of the alveolus), the protoloph and the metaloph, whilst the external Fig. 251. — Grinding surfaces of an unworn tooth of AncMtherium, of unworn tooth of horse, and of worn tooth of horse. In the latter the light areas are dentine, the shaded areas cementum, and the black border between them enamel. (After Flower and Lydekker.) cusps, the paracone and metacone, are merged in a continuous ridge, the ectoloph, fortified at three points by cingulum cusps or styles. At this stage the tooth is not far from that of the Tapir, or from that of Rhinoceros. In the third figure and also in Fig. 251 is seen the tooth of Anchitherium, one of the ancestors of the Horse. In this all *the ridges are more developed, so that the identity of the cusps is nearly lost; the external ridge has become sinuous, and pillars have been added, conspicuously at its left hand end and less so at its right, while an additional detached pillar appears to the right. By further increase in the obliquity and curvature of the ridges, and by the appearance of other detached pillars, the tooth pattern becomes transformed into that of Hipparion, in which two pillars on the inner side remain distinct, and into TEETH OF THE UNGULATA. 505 that of the Horse, in which they become more or less fused with the curved ridges. The outer ridge or* wall is in the upper molar of the horse doubly bent, the concavities looking outwards. The trans- verse ridges start inwards from its anterior end and from its Fig. 252. — Upper molar of horse. middle, and they curve backwards as they go, to such an extent as to include crescentic spaces between themselves and the outer wall. To this must be added a vertical pillar, which is slightly connected with the posterior end of each crescentic edge (these pillars are in Hipparion quite detached), and a second which is fused with the other crescent. The lower molars of the horse present the double crescent, just like those of the rhinoceros, save that vertical pillars are attached to the posterior end of each crescent, thus slightly complicating the pattern of the worn surface. The interspaces of the ridges and pillar are in the horse solidly filled in with cementum. The extinct ancestors of the horse are now, however, pretty well known (5). Hyracotherium found in the Lower Eocene is probably the most primitive stage known. It was no larger than a cat, and had four toes on each forefoot and three on each hindfoot ; a single splint bone represents one missing digit on each foot. The teeth carried rounded cusps just beginning to show indications of fusion into crests. Then came Eohippus, the size of a fox, and in most respects very like Hyracotherium, except that the cusps of the molars show more indication of fusion to form crests ; it had four well-developed toes and a rudiment of the fifth (forefoot). Next came the Eocene Orohippus (four-toed) : the Miocene Mesohippus, as large as a sheep (three-toed, with rudimentary splint) ; the Upper Miocene Miohippus (three- 506 A MANUAL OF DENTAL ANATOMY. toed); the Lower Pliocene Protohippus, as large as an ass (three-toed, but only the middle one reaching the ground), Recent. Equus. Pliocene. Pliohippus. Protohippus (Hippariori). Miocene. Miohippus (Anchitherhim). Mesoliippus. Eocene. Orohippus. Fig. 253.— Ancestry of the horse. (After Marsh.) The forefoot and surfaces of upper and lower molars. which was like the European Hipparion ; then the Pliocene Pliohippus, which has lost the small hooflets, whilst in the Upper Pliocene the true Equus first appears. TEETH OF THE UNGULATA. 507 Eohippus had forty-four teeth, the molars being quite distinct from the premolars, with short crowns and roots ; * in Orohippus the last premolar is like the true molars, while in Mesohippus two premolars are like the true molars. Coincidently with the elaboration of the crown pattern a great lengthening of the crown took place, so that there was much more material available for wear, but considerable crown elaboration had arisen before the teeth ceased to be brachyodont. The great lengthening which has taken place in the hypsodont molars of the horse since the time of the Miocene Miohippus (Anchitherium) is well seen in Fig. 254 : Fig. 254. — Outer view of second upper molar of Anchitherium (brachyodont) and of horse (hypsodont). It is perfectly obvious that the hypsodont type of tooth provides for a much longer period of severe attrition before the roots are formed than the brachyodont tooth of Miohippus, so that it is probable that the food of the modern horse is of a drier and harsher kind than that of its ancestors. Indeed, the small early ancestors seem to have been adapted to a forest life, whereas the modern horse inhabits level grass plains, often rather dried up, and its food needs more mastication. In Miohippus the surface had two nearly transverse and slightly curved ridges with valleys between them, but the valleys were shallow and were not filled up with cementum. * Ancient deer also had short crowns to their molar teeth. 508 A MANUAL OF DENTAL ANATOMY. As the crowns of the teeth became longer, and the valleys between the cusps very much deeper, they became filled up solidly with cementum, which not only added strength, but as the tooth wore down, preserved a rough surface by affording three tissues of different hardness ; for as the tips of the cusps wore down the enamel was worn off, and areas of dentine surrounded by a rim of enamel became exposed, and the intervening cementum also came into wear, the efficiency of the surface for grinding purposes being yet further increased by the sinuosity of the enamel rings. The Teeth of Artiodactyle Ungulates. Artiodactyle, or even-toed, Ungulates comprise, amongst living animals, pigs, hippopotami, camels, sheep, oxen, &c. They are grouped into (i.) Bunodonts, or Suina, comprising Pigs and Hippo- potami. (ii.) Selenodonts, comprising Anoplothericlce and the Euminants. (a.) The Tylopoda, or Camels, (b.) Pecora, or sheep, oxen, and deer, (c.) The Traguloidea (Chevrotains), small deer of S. Asia, which are somewhat intermediate between the Suidse and the deer. The primitive Artiodactyla all had forty-four teeth of brachyodont type ; in later geological times they became long-toothed (hypsodont), like recent sheep and oxen. The premolars differ markedly both in size and pattern from the true molars. Of those Artiodactyle Ungulates which are not ruminants the common Pig may be taken as an example. 3 13 3 The dental formula is 1 3 c P 3 m or PernaPs 4 3 Plm 8 ' The position of the upper incisors is peculiar, the two first upper incisors, separated at their bases, being inclined towards one another so that their apices are in contact ; the third pair are widely separated from the inner two pairs of incisors. The lower incisors are straight, and are implanted TEETH OF THE UNGULATA. 509 in an almost horizontal position ; in both upper and lower jaws the third or outermost incisors are much smaller than the others. The lower incisors are peculiar in having upon their upper surfaces a strongly pronounced sharp longitudinal ridge of enamel, which gets obliterated by wear. An interval separates the incisors from the canines, which latter are very much larger in the male than in the female, and in the wild boar than in the domesticated animal. Castration arrests the further development of the tusks ; and the peculiarities as to size and direction which characterise the tusks of the adult animal are not represented in the canines of the milk dentition. The form and direction of the canines are alike peculiar. The upper canine, which in its curvature describes more than a semicircle, leaves its socket in a nearly horizontal direction, with an inclination forwards and outwards. After rounding past the upper lip its terminal point is directed upwards and inwards. The enamel upon the lower surface of the tusk is deeply ribbed ; it does not uniformly cover the tooth, but is disposed in three bands. The lower canines are more slender, of much greater length, and by wear become more sharply pointed than the upper ones ; they pass in front of the latter, and the worn faces of the two correspond. The lower canine is in section triangular, one edge being directed forwards, and its sides being nearly flat. Enamel is confined to the internal and external anterior surfaces ; the posterior surface, which plays against the upper canine, is devoid of enamel, and the tooth is kept constantly pointed by the obliquity with which its posterior surface is worn away. The tusks of a boar are most formidable weapons, and are capable of disembowelling a dog at a single stroke, but they are greatly exceeded by those of the African Warthog (Phacochoerus) , which attain to an immense size, and are articles of commerce, being used for small articles as a substitute for ivory. In the domestic races the tusks of the boars are much smaller than in the wild animal, and it is a curious fact that in domestic races which have again become wild the tusks of the boars increase in size, at the same time that the bristles become more strongly pronounced. Darwin suggested that 510 A MANUAL OF DENTAL ANATOMY. the renewed growth of the teeth may perhaps be accounted for on the principle of correlation of growth, external agencies acting upon the skin, and so indirectly influencing the teeth. As in most Artiodactyles, the teeth of the molar series increase in size from before backwards ; thus the first pre- molar (? milk molar) has a simple wedge-shaped crown, and two roots ; the second and third by transitional characters lead to the fourth premolar, which has a broad crown with two principal cusps, and has four roots. The first premolar is a tooth about which there is a doubt whether it is a retained milk tooth or a permanent tooth which has not had a predecessor. Of this tooth Lesbre (3) says that it is cut about the fifth month, after the milk molar, but before the replacing teeth. Fig. 255. — Upper and lower teeth of wild boar ( Primitive heel. Postero-mesial = Hypoconulid ) The author commenced to make a record of the percentage of deviations in the cusps of the several races of mankind, but finding that Topinard (13) had already done so upon a very large series of skulls, and that the observations, so far as they had been carried, were in complete accord with his, desisted. Topinard, after rejecting all doubtful cases, includes in his record 595 human skulls, 120 anthropoid apes, and a very large number of lower apes. He finds that Mx was quadricuspid, with a well-marked oblique ridge in 99 per cent, of human skulls taken without distinction of race, so that Mi is constant in all races. M2 had 4 cusps in 66 per cent, of all races J5 ^2* 5) JJ 16 ,, ,, j? 3 }j •) 16 ,, ,, ., ,, (By 3^ is meant three well-developed cusps with an indica- tion of the fourth.) M2 has the normal four cusps in 79 per cent, of Malays, 83 of New Caledonians and New Hebrideans, 78 of Melanesians, 73 of Australians and Tasmanians ; bat the percentage falls to 68 in Chinese and Japanese and to 58 in European, Semitic, and Egyptian skulls. M3 had 4 cusps in 37 per cent, of all races Ri- 1 1 u2 " " XA " " " " j> ^ >> ^9 ,, j, ,, 2 ,, 5 ,, ,, jt and was totally irregular in 6 per cent. M3 had four cusps in 72 per cent, of Melanesians, 69 of Egyptian groups, and only in 36 per cent, of European skulls. It had three cusps in 56 per cent, of Chinese and in 47 per cent, of negroes and Australians. THE TEETH OF PEIMATES. 603 Thus, as he remarks, the third upper molars tend to be " disordered" in almost all races. For lower molars -he gives — Mi, 5 cusps in 82 per cent, of all races >> >> 4 , , ,, 4 10 55 55 -Lvy 55 5 5 5 5 5 5 Thus Mi retains its type strongly, though not so absolutely as Mi, M2, 5 cusps in 24 per cent, of all races ^2 >' 55 10 ,, ,, 4 64 ^ 55 55 55 55 5? 55 M2 is distinctly a quadricuspid tooth, and is quinquecuspid less frequently than Mi. It has the typical four cusps in 75 per cent, of Egyptians, in Europeans in 60 per cent., in negroes in 52 per cent., in Polynesians in 52 per cent., and in Japanese in 44 per cent. ; whilst it has five cusps in 33 per cent, of Melanesian skulls, in 30 per cent, of Malays, and in 37 per cent, of Chinese and Japanese. M3, 5 cusps in 46 per cent, of all races ^2 " " w ' ' " " » "1 55 55 ^1 5) >' !>> 55 And M3 is very variable. The race comparisons come out thus : — In Europeans Mi is getting away from its original type, which occurs in only 61 per cent. Polynesians have the full five cusps in 91 per cent., Chinese and Japanese in 87 per cent., Melanesians in 86 per cent., negroes in 88 per cent., and Egyptians in 77 per cent. From his observations he concludes that the teeth of man are in process of transformation, the lower molars tending towards a quadricuspid type with a crucial fissure, and the upper towards a tricuspid type. In the lower it is the fifth cusp (which, from comparison with the teeth of apes, Topinard holds to primarily originate in the middle line, in a bifurcation of the posterior arm of the cross), which tends to displacement to the outer aspect of the crown more and more, until it disappears. In the upper molars it is the poster 0 -internal cusp which disappears, so that the tooth terminates posteriorly with 604 • A MANUAL OF DENTAL ANATOMY. the oblique ridge. This manner of suppression of cusps brings the human tricuspid molar into close resemblance to the trituberculate lemurine tooth ; but, as is set forth in the footnote below, Topinard by no means agrees with Osborn in the determination of the homologies of the cusps severally.* It is therefore held by Cope to indicate that the upper molars of European races show a tendency to revert towards the Eocene trituberculate lemurine tooth. When it is further reduced to two cusps, it is by the fusion into one of the antero- and postero-external cusps. The range of variation in the size of the jaws of healthy, and otherwise well-developed adults is great. Thus the smallest jaw (occurring in a man of stout build, above middle height) with which the author is acquainted, measures in width only 1| inch, and in length from back to front If inch; while the largest (occurring in a gentleman of lesser stature, of Basque extraction moreover, which makes it the more striking) t, measures no less than inches in width and 2J inches in length : and even larger dimensions are recorded in the " Dental Cosmos " of September, 1876, the width being taken between the centre of the alveolar borders at the position of the third molars, and the length being measured on a line drawn from the incisors to another line joining the two wisdom teeth. On the whole, it must be said that there are fewer constant differences between the teeth of the various races of mankind than might have been a priori expected; in fact, it may almost be said that the teeth of savage man are such as we should look upon as an exceptionally perfectly formed set of teeth if we were to see them in the mouth of a European. FlowTer (6) has investigated the relation of the size of the teeth to that of the skull very closely, with the result of bringing out * Topinard holds that the antero-external cusp is the protocone, loth in upper and lower molars, therein differing from Osborn, and that the postero-external is the metacone, the paracone being absent in Primates, and both the internal cusps being secondary acquisitions. He states that in both jaws in development the antero-external is the first to appear, and long keeps a preponderance, the antero-interaal following, then the postero-external, and much later the postero- internal. f Ma^itot (" Bullet, de la Soc. Anthropol. de Paris," 1869) says : — " Les Basques, par exemple, remarquables par la petitesse extreme de leurs dents." THE TEETH OF PRIMATES. 605 certain race distinctions. As measures for comparison he takes the length of the cranio-facial organs, measured from the front edge of the occipital foramen to the naso-frontal suture, and the length from the front of the first premolar to the back of the third molar in situ. His "dental index" is arrived at thus : Dental Index - Length of Teeth X 100 Cranio-iacial axis This gives figures ranging from 42 (microdont), 43 (mesodont) 44 and upwards (megadont). As in females the skull is smaller, whilst the teeth are the same, a slightly higher index is arrived at for them. The Microdont races are — European. Egyptian. British. Polynesian. The Mesodont are — Chinese. Malays. American Indian. Negroes. And the Megadont — Melanesians. Australian. Andamanese. Tasmanian. As to this classification it is to be remarked that the teeth of Polynesians are actually larger than those of Europeans, but the cranio-facial axis is of extreme length, so that the index is reduced ; this is also the case with the American Indians, whilst the Andamanese are brought into the megadont series by the relative size of teeth to the basis cranii, though in this small people the teeth are actually small, considerations which diminish the value of the method. The dental index of megadont races being from 44 to 47, that of the chimpanzee is little more than the highest of these, namely, 47*9, whilst the orang rises to 55, and the gorilla to 54, but in the gibbon the index is as low as 41 -7. It was, however, the opinion of the African traveller, Mr. Stanley, that the teeth of African races vary in accordance with the build of the individual, and particularly with the size of the jaws, such small races as the Somalis having small teeth. It does not appear, however, that these impressions are based upon actual exact measurement, but only upon general 606 A MANUAL OF DENTAL ANATOMY. appearance. He mentions that many of the races who show no regard for cleanliness otherwise, assiduously clean their teeth. The account of the morphology of the teeth of the Primates has been thus briefly sketched in the preceding pages. The subject is one, however, which has recently attracted great attention, owing to the fact of the relative frequency with which the teeth have been found in the fossilised human remains. A considerable literature has therefore recently grown up, and the reader who wishes to study the subject more comprehensively is referred to the recent monographs on this subject by Bolk(2) and by de Terra (3). Eefekences. (*) Bateson, W. ' ' On Some Cases of Variation in Secondary Sexual Characters, statistically examined." Proc. Zool. Soc. Lond., 1902, pp. 585—594. (2) Bolk, L. " Die Ontogenie der Primatenzahne." Jena, 1913. (3) de Terra. " Vergleichende Anatomie des menschlichen Gebisses und der Zahne der Vertebraten." Jena, 1911. (4) Cope, E. D. "Primary Factors of Evolution." Chicago, 1896. (6) Duckworth, W. H. L. " Some Dental Rudiments in Human Crania." Trans. Odontol. Soc. Gt. Brit., 1901. Vol. 33. N.S., pp. 89—97. (6) Flower, W. " Size of the teeth as a Character of Pace." Journ. Eoy. Anthropol. Inst. 1884, June, vol. 14, pp. 183—187. (7) Keith, A. " Problems relating to the Teeth of Earlier Forms of Prehistoric Man." Proc. Eoy. Soc. Med., May, 1913. Vol. V., Odontolog. Sect., pp. 103—119. (8) Keith, A., and Knowles, F. H. S. "A. Description of Teeth of Palaeolithic Man from Jersey." Journ. Anat. and Physiol., 1912, vol. 46, pp. 12—27. (9) Klaatsch, H., and Hatjser, 0. " Homo mousteriensis Hauseri." Arch. f. Anthropologic, 1909. Bd. VII., N.F. Braunschweig, pp. 287 — 297. (10) Magitot. "L'homme et les singes anthropomorphes." Bull, de la Soc. d; Anthropol. de Paris, 1869. T. 4, 2nd Ser., pp. 113—145. (u) Eegnault, F. (a) " Des Malformations dentaires chez le singe." C. E. Soc. de Biol., Paris, 1893. T. 5, Ser. 9, pp. 931—935. (b) ' ' Variations dans la forme des dents suivant les races humaines." ibid., pp. 971 — 972. (12) Tomes, C. S. " Studies in the Growth of Jaws." Trans. Odontol. Soc. Gt. Brit., 1892. Vol.24. N.S., pp. 143— 158. (13) TOPINARD. " L'anthropologie," 1892 INDEX. A. PAGE Aard-vark . . . . . .475 Aardwolf .454 Absorption of teeth . . . 243 Acquired characters, inheritance of 274 Acrodont teeth .... 310 Adamantobiasts . . . . 142 Adaptive modification 263, 267, 568 Adder 318 iEluroidea 444 Age, as determinable by teeth development . . . .214 Aglypha .... 313, 316 Agnatha 276 Alligator 308 Allotheria 379 Alveolar arches, growth of 238, 240, 245 Alveoli, nature of . . . 29 Alveolo-dental membrane . . 126 Amblypoda 530 Ameghino, on homologies of cheek teeth 401 Ameloblasts, atrophy of . . . 142 Amphibia 305 Amphicyon 457 Amphilestes ..... 373 Amphitherium . . . . 379 Anaptomorphus . . . .374 Anarrhicas 296 Anchylosis, attachment by . . 255 Anchitherium 504 Andrews, views on enamel develop- ment 171 Angle, facial . . ... 581 Angler, the 294 Anomodon 330 Anomodont reptiles . . . 327 Anoplotherium, dentition of . .515 Ant-eaters .... 470, 475 Anthropidas 580 Anthropoid apes .... 572 Anthropopithecus . . . . 572 Apes, comparison of teeth with man ...... 579 Apes, variability in . . 267, 579 Archaeopteryx .... 332 Archetype theory . . . . 263 Arctoidea 457 PAGE Armadillo, succession in . . . 474 Articulation, temporo-maxillary . 25, 481, 531 Artiodactyle ungulata . . . 508 Ateles 570 Attachment of teeth . . .248 Attachment by fibrous membrane 248 „ ., elastic hinge . . 250 ,, ,, anchylosis . . 255 ,, bone of ... 256 „ on pedestals . . 258 ,, by sockets . . . 260 Aye-aye, teeth of . . . . 567 B. Babirussa, teeth of . . . . 512 Baboons 571 Balaenidae . . , . . 490 Balsenoptera .... 497 Bate, Spence, on milk teeth of insectivora 435 Barracuda pike .... 301 Basal layer of Weil . . . . 113 Basement membrane . . . 144 Bathysaurus . . ... 250 Batrachia . . . . 305 Bats 441 Bdellostoma 277 Beale, on dentinal fibrils . . . 76 Bears 462 Beaver, enamel of . . . . 38 Beluga 490 Bertz, on composition of enamel . 33 Berzelius ,, dentine . 64 Bettongia 418 Bibra, von, on composition of enamel 32 ,, ,, ,, ,, U.C1IL1UC Ul Bicuspids of man . . . ,12 Bilophodont, meaning of . . . 522 Binturong 452 Birds, teeth of 331 Black, on root membrane . .128 ,, on lime salts of teeth . . 65 Boas, von, on enamel structure . 58 Bodecker, on enamel . . . 47 „ on dentine . . . 75 Boll, on nerves of pulp . . . 79 Bone of attachment . . . 256 Bottle-nosed whale . . . . 493 608 INDEX. PAGE Bothrops 319 Bovidaa 518 Brachyodont, meaning of . .501 Bradypus . . . . 472, 474 Branchial arches, teeth on 3, 283, 301 Brontotheridse .... 529 Brown striae of Retzius . . . 48 Brunn, von, on enamel organ 129, 144 Buccal cavity, primitive . . 218 Bunodont, meaning of . . . 501 Bunotheria 487 C. Calamodon 476 Calcification . . . . . . 1H2 „ by conversion . . 167 ,, by excretion . 163, 167 ,, of cementum . . 207 ,, of dentine . . .188 „ of enamel . . . 166 „ „ in osseous fish 180 ,, ,, in Selachia . 180 ,, ,, in mammals . 170 „ of osteo-dentine . . 201 ,, of vaso-dentine . . 199 Calcification, Sims Woodhead on . 165 Calcoglobulin . . . .164 „ in ameloblasts . .169 Calcospherites . . . .163 Camel 516 Camper, facial angle of . .581 Canaliculi of cementum . . .117 Canidas 457 Canines, definition of . . 339 „ homologies of . 339, 428 „ of man . . . . 11 ,, sexual development of . 270 ,, sexual development of, in horse . . . . 528 „ sexual development of, in deer . . . 519 ,, sexual development of, in monkeys . . .573 Capybara, molars of . 267, 479, 481 Carcharias . . 99, 185, 283 Carnassial teeth . . 448, 449, 450 Castor .... 38, 485 Carnivora 444 ,, pinnipedia . . . 463 Cartilage, interarticular . . 26 Castration, effect of, on tusks 509, 525 Catarrhine monkeys . . . 571 Cebidae 570 Cementum, structure of . . .116 ,. matrix of . . .116 „ calcification of . . 207 ,, development of . . 206 ,, organ . ... 205 Centetes . . 342, 432, 436, 438 Ceratodus . . . 100,289 Cercopithecus . ...... 572 PAGE Cervidse 518 Cestracion . . . . . 284 „ enamel of . . . 57 Cetacea 490 „ milk teeth of . . 490 „ dentine of 491 ,, rudimentary teeth of 493,497 Chastodonts 290 Chamasleon 311 ,, absence of succession 154, 305, 311 572. Chauliodus . Chelonia .... Chevrotain . Chimpanzee Chin, absence in apes, etc. Cheiromys, tide Aye-aye. Cheiroptera Cholcepus Chrysochloris, molars of . Ciliaf orm teeth Cingulum, nature of ,, on human teeth ,, theory of cusps Classification of teeth . Cleft palate Coatimundi . Cobra .... Cod, dentine of . . . Ccenolestes Collagen Colobus, deficient teeth in Colubrine snakes . Concomitant variation Concrescence, theory of evolution . Condylarthra .... Connective tissue, theory of den tine development Constant, on eruption of teeth Contour lines of Owen Cope, doctrine of Kinetogenesis ,, ,, Trituberculism ,, on extinct cats . Coronoid process, share of ir growth of jaws Correlations of growth . Craspedocephalus . 330 307 517 577 582 . . 441 . 474 370, 437 . 291 12, 453 9, 12 370, 386 . 337 . . 19 . 461 . . 318 92, 195 . . 422 62, tooth , 290. 116 272 317 271 368 532 193 239 68 366 371 456 236 271 319 Creodonts Crocodilia 401, 445 . . 307 „ implantation of teeth .261, 308 Crypts of developing teeth . . 228 Ctenacodon 380 Cusps, development of 187, 486, 555 ,, homologies of . . . 382 „ modifications of . . . 383 „ theories of formation of . 290, 366, 371, 386 Cuticula dentis . . . . 120 Cyclostomata . . . .277 Cyclothurus 470 INDEX. 609 PAGE Cynodraco . . . . 330 Cynognathus . . . . . 328 Cynoidea . . . 444 Cystophora . . 350, 465 Czermak, interglobular spaces of . 80 Dasypeltis Dasypus Dasyurus Deer, sexual canines of Deficiency of teeth in hairy ,, ,, in hai: dogs Dendrodus Dental furrow ,, formula „ follicle „ ,, of Marsupials ,, index Dentinal fibrils chemical 404, 316 472 415 519 272 men :less . 272 . . 287 . 140 344, 359 . 202 . . 406 . 605 • • 72 com- position of 52 101, 83, ,, sheaths ,, tubes Dentine, adventitious . ,, calcification of ,, classification of chemical composition of ,, connective tissue in ,, development of distribution of ,, follicle . . . . ,, globular. . . 81, ,, lamination of . . . ,, matrix of .61 ,, organ . . . . ,, rate of growth of . 197, ,, secondary . . . . ,, sensitiveness of theories of formation of . . . 191, „ varieties of 61, 83, Dentine papilla . 134, 143, Dentition, milk .... „ post permanent . . „ premilk . . 355, ,, typical mammalian Dents en velours, en brosse, etc. . Dentz, on nerves in dentine . Deppendorf , on nerves in dentine . Dermal spines, relation to teeth 66 ,69 66 104 188 101 61 192 187, 188 . 61 . . 202 197 196 , 66 187 198 103 80 192 101 147, 187 346 352, 361 361 344 291 79 111 3, 133, Desmodus Development of the teeth ,, in amphibia ,, in fish ,, in reptiles ,, in mammals D.A. 147, 283 441, 443 . . 132 . 151 . . 145 151, 307 139, 156 Development of alveoli ,, of bone of attach- ment .. . . of cementum . of dentine . 187, of early germs of enamel . . . „ in Elas- mobranch fish PAGE 241 253 207 188 133 166 145 „ in osseous fish . 150, 180 , in Sela- chia . . . 183 m mam- mals of the jaws Diastema, the Diademodon Dingo . Dinocerata Dinosauria . Dinoceras Dinotherium. Dioelurodon Diodon . Diphyodonts . Dipnoi Diprotodonts . Dog . ,, dentition of Dog-fish Dolphin . Dromatherium Du^ons: . E. Echidna Ebner, von, on enamel ,, on bone Edentata . Eel . . ■ . Elachistodon Elaphodus . Elasmobranchii, teeth of . 218 . . .218 . 447 . . 329 . 459 . . 530 . 325 . . 531 . 549 . . 330 . 297 . . 346 . 277, 288 . . 413 . 457 . . 350 3, 146, 281 . . 491 . 372 . . 558 392 47 66 470 257 316 519 279 Elastin Elephant develop- ment of 145 . 62 . . 545 538 et jyassim 387, 551, 552 tusks of molars of succession of teeth in . 538, 548, 555 milk teeth of . 539, 548 Eminentia articularis . Enamel, calcification of . . . ,, cavities in ,, chemical constitution of ,, classification of . . . ,, development of „ „ ,, in osseous fish . 39 25 162 49 32 186 166 180 610 INDEX. PAGE Enamel, development of, in Sela- chia . . . 52, 183 mals m mam- . 156 in mar- . . 170 tubular 171 31, 36, 150 . 51 supials . development of distribution of marsupial pigmentation of . . 478 prisms of ... 37 striation of . . . 41 structure of . . .37 tubular . 49, 69, 438, 510 theories as to formation . 177 organ, formation of . 137, 142, 144, 147 „ as form-builder . 143, 144 137, 142 142 140 158 133 origin of . 133, 137 142 156 neck of stages of vascularity of universality of stratum inter- 141, of, 142, of, 142, 156 . . 117 . 586 . . 379 . 507 . . 317 2,218 506, 525 342, 428, 436, 438 medium ,, epithelium internal „ epithelium external Encapsuled lacunse . Eoanthropus Eocene mammals Eohippus Enhydris .... Epiblast Equus Erinaceus Erosion of teeth . . . . 464 Eruption of teeth .... 238 „ dates of . . . . 241 „ influence of nutrition . 274 Ericulus 342 Esthonyx 486 Eupleres 454 Euprotogonia, teeth of . . . 503 Eutheria 391 „ ancestry of . . . 426 Evans, on composition of enamel . 33 Evolution of tooth forms . . . 365 External pterygoid muscle, action of . . . • • .23 F. Face and maxillary bones . . 22 Face, formed by maxillse . . . 22 Facial angle 581 Feeding, effects of, on develop- ment of teeth . . . . 274 Felidse . „ ancestry of . Fibrils, dentinal . Fish, teeth of . Flounder, dentine of PAGE . 454 . 456 65, 72 . 276 . 93 Foetus (nine months), jaws of . . 229 Follicle, dental . . . .202 Foregut 218 Formula, dental . . . 344, 359 Forsyth Major, on teeth of insecti- vora . . . 437 „ „ on teeth of rodents 486 Fossette, meaning of term . . 504 Frog 305 „ development of teeth of . .153 G. Gadidse 295 Galeus . . . . . 55 Galeopithecus . . 89, 338, 438 Galippe, Dr., on percentage of lime salts in teeth . . . . 64 Ganoid fish . . . . .287 Ganodonta ..... 476 Garial . . . . ^ . .308 Gegenbaur, on dermal spines . . 149 Gelatine 62 Germ, tooth 132 Gibbons 572 Gibbs, J.H., on polyphyodontism 347 Glands, of Serres . . 126, 143 Glenoid cavity . . .25, 481 Globiocephalus . . . . 490 Globular dentine . . .81, 197 Glyptodon 475 Gnathic index . . . .581 Goodsir, views of . . . . 210 Gorilla 573, 579 Grampus .... 350, 492 Granular layer of dentine . . 80 Gubernaculum . . . . 207 Guinea-pig 485 Gum, structure of . . . . 125 Gymnodonts . . . .297 Gymnura . . 342, 431, 438 H. Haddock, teeth of 258 Hair, abnormalities in teeth and 272 Hake, dentine . . 90, 92, 194 „ hinged teeth of .91, 253, 294 ,, tooth germ of 181 Halicore 559 Halmaturus . . . .174, 419 Hapalidas . . . . . 570 Haplodont, meaning of . . 374 Harderian glands . . . . 316 Hare 477 Harelip 19 INDEX. 611 Hartin_ tion . Haversian canal Hatteria researches on calcifica- . 1 . . . .162 . . 165 . 153, 303, 312 342, of . 338, . 291, 505, Hedgehog . . . 351,428 Heincke, on development of teeth of pike . . . ... Heitzmann on cementum . . Heloderma ..... Hemicentetes . Hemiganus .... Herpestoidea . Hertwig, Epithelial sheath of „ on development dermal spines . Hesperornis Heterodont, meaning of Heterodontus . Hinged teeth Hipparion Hippocampus Hippopotamus . Holocephali Homalodontotherium Homo brelardensis heidelbergensis ,, neanderthalensis Homodonts Homologies of the teeth of mammalian teeth 337 ,, of marsupial teeth . 355 ., of milk dentitions 349, 359 „ of the molars . . 360 Hoppe Seyler, analyses of enamel 33 ,, „ analyses of dentine 33 „ ,, bone salt of 33, 166 Horny teeth . . 5,278,393 Horse, incisors of . . 384, 386 ,, evolution of molars of 504, 507 „ teeth a test of age . .527 „ ancestry of . . . . 506 Howship, lacunar of . . 244, 251 Huber on nerves of pulp . . .111 Huxley, views on development 210, 212 346, 2, 150 118 311 428 475 451 130 133 333 346 57 293 525 301 514 276 529 590 591 588 472 337 Hyasna Hyaenidae Hyaenodon Hydromys Hydrophis Hydropotes Hydrus Hylobates Hyperoodon . Hypoblast Hypocone Hypsiprymnus Hypsodont . Hyracodon Hyracoidea . Hyracotherium Hyrax . Hystrix 453 . . 453 . 449 479, 485 . 317 . . 519 . 317 . . 572 . 493 . 2. 218 . 374 404, 416 501, 507 . . 529 . 537 . . 504 1 76, 536 39, 485 I. Ichneumon lchthyornis . Ichthyosaurus . Iguana . Iguanodon Incisors, definition of , , human ,, missing, of Indri PAGE . . 451 . 332 307, 309 . 311 307, 326 . 338 . . 8 . 19 . . 567 Inheritance of acquired characters 274 Insectivora 426 „ cusp patterns of . 370, 436 ,, molars of . . 436 Interarticular cartilage . . 25 Interglobular spaces . . . 80 Intermaxillary bone . . . 220 Internal, pterygoid muscle . . 24 Introductory remarks . . . 263 Ivory .... 63, 540 Jaws of nine months1 foetus . 229, 233 „ at seven years . . . . • 234 ,, adult and old age . . 235 „ centres of ossification . . 220 arrested growth of . .237 ,, variation in size of . . . 604 „ development of . . .219 Jerboa 176 Kaffir, teeth of . . . .599 Kangaroo . . . 412, 419 ,, enamel and dentine of . 51 rat .... 416 Keratin 5 Kinetogenesis .... 366 Klein, on structure of enamel . . 47 ,, „ dentine . 77 Kolliker on enamel organ . .158 „ ,, development . 178 „ ,, bone of fishes . . 303 Krapina teeth 593 Labyrinthodentine Labyrinthodon ,, dentine of Lacertilia, teeth of . Lamprey Lamna ,, dentine of ,, enamel of ,, teeth of Lacunas, development of ,, encapsuled of cementum 302 306 86 309 277 184 99 55 281 209 123 119 i£6 . emur iren . teus dentine of ;us, enamel of £6 nts, of jaw . E Schreger • , Eurche . 3 piscatorius . ont . 428, 429 . 565 . . 567 . 289 . . 287 . 84 . . 53 39, 481, 483 . . 24 49, 68 . . 136 . 309 . . 294 522 359 . 503, :er, on dental formula . . on marsupial dental formula . . .401 atic glands, of jaws . .115 atics, absence of in pulp . 114 supposed, in root brane . . . .129 M. is 571 rodus .... 455 •el 259 »us . . . 172, 412, 419 celis . . 342, 427, 436 t, on dentine . . 77, 78 on cement organ 206, 210 on enamel develop- ment . . 158,178 on odontoblasts and as 77 ez, on epithelial rests . 144 on alveolo-dental mem- brane . . .127 alian enamel organ . .156 als, teeth of . . . 391 ancestry of . 329, 391 typical dentition of . 344 oth, tusks of . . . . 540 Lentition of 580 eeth of races of 579, 581, 601 alfeolithic . . . 580, 582 3e, teeth of . . . . 560 dentine of . . 96, 97 ale, development of . . 220 movements of .24 tooth rudiments of . .471 )f horses' incisors . . 525 >sets 570 obranchii, vide Cyclo- ata Dials 399 change of tooth in . 405, 406 Mastication, motions of . . . 21 Mastodon . . . .387, 547 „ molars of . . 386, 551 Matrix of bone . . . .62 ,, of cementum . . .116 ,., of dentine . . 61, 66 ,, of enamel . . . . 33 Maxillary bones, development of . 219 Mechanical theory of tooth forms 365 Meckel's cartilage . . 134, 225 Megaderma .... 443 Megadonts . . . . 605 Megatherium, cementum of . 94 ., dentine of . . 94 Melursus 463 Membrana eboris . . 108, 188 ,, preformativa . 107, 211 ,, alveolo-dental . .126 Mental foramen .... 232 Merganser, bill of . . . . 331 Merlucius (see Hake). Mesoblast 2 Mesodonts 605 Mesognathion .... 581 Mesohippus . . ... 507 Mesonyx 445 Mesoplodon 495 Mesotherium .... 535 Mesozoic mammals . . . . 379 Metacone . . . " . 376 Metatheria 399 Microdonts ..... 605 Microlestes 395 Milk dentitions . . . . 346 ,, „ absence of . . . 361 „ ,, origin of supposed . 360 of marsupial 404, 406 . 507 . . 554 in human . 602 . 337,358 . . 16 . 440 . . 374 342, 370, 434 . . 437 . 443 . . 311 565, 571 565, 570 . 496 347, 499 . 392 lp 111, 113 . 334 Miohippus Mceritherium Molars, cusp variation ,, definition of „ of man . „ homologies of ,, of primates . Mole, teeth of ,, marsupial Molossus Monitor . Monkeys, Catarrhine ,, Platyrrhine Monodon Monophyodonts Monotremata Morgenstern, on nerves in pu Mosasaurus . Moschus .... Mouth, development of Multituberculata 271, 519 . 3 . . 377 INDEX. 618 Multitubercular theory of tooth forms .... 381. Mummery, on dentine develop- ment . . . ,, on nerves of pulp Muscles of jaw . Musk deer, canines of . 270, Mustelidae . . . .444, Mycetes ... . . Myeloid cells .... Myliobates, dentine of . . 88, „' teeth of . Myrmecobius . . 356, 403, Myrmecophaga . Mystacoceti . . . 490, Myxine ...... N. PAGE 486 193 111, 113 23 519 461 570 244 101 287 415 470 497 277 496 120 461 265 Narwal ..... Nasmyth's membrane . Nasua Natural selection, theory of . Neanderthal man, vide Homo- neanderthalensis. Nerves, of pulp . . . 77,110 Neumann, sheaths of . . 62, 70 Newt 152, 306 Notoryctes 423 Nysticebus 567 Nythosaurus 329 O. Oblique ridge, of human molars . Occlusion of the teeth . Odontoceti . . Odontoblasts . . 77, 108, ,, description of ,, share in dentine de- velopment . 77, Odontopteryx Odontornithes Odontostomus, Ogmorhinus . Ophidia . Ophisaurus . Opisthoglypha Opossum Opoteroclonta Orang . Orca . hinged teeth of 154, 313, 574, 350, Ord, researches in calcification Oreodon . . . .341, Ornithorhyncus . . . 1, Ornithosauria . Orohippus . . . . . Orthognathism . 16 20 491 190 108, 188 113, 191 332 335 251 465 313 305 316 415 313 577 492 162 515 392 307 507 581 Orycteropus . Ossa mentalia Osseous fish, teeth of Ossification, centres of in jaws Osteoblasts . Osteoclasts Osteodentine ,, calcification of Ostracion, dentine of . Otaria . . . „ erosion of teeth in Otariidas .... Otocyon Owen, contour lines of P. Palaeichthyes Palaeolithic man Palaeomastodon . Pakeotherium . Papilla, dentinal . Paracone . Paradoxurus Parrot fish . Paul, on Nasmyth's membrane ,, on enamel organ Perameles Periodontal membrane Periosteum, alveolo-dental Perissodactyle ungulata Permanent teeth, eruption of „ development of Petrodromus Petrogale Petromyzon Petrodromus Phacochaerus . Phalangers . Pharyngeal teeth Phased arc tos Phascolomys . Phenacodus . Phoca Phocidae Phocaena . Physeter Phyllodus Pig Pike, dentine of „ hinged teeth of „ teeth of . Pinna shell . Pisces, teeth of Pithecanthropus Pithecidas Plagiaulax . Platurus .... Platyrrhine monkeys . Pleurodont Pleuronectid;e, dentine of 134, PAGE . 475 . 225 . 290 .219, 220 . 208 . 244 . 97 . 201 . 90 . 464 . 464 . 463 57, 460 . 68 279 580 549 530 143 372 452 298 120 158 409 129 127 521 238 352 342 407 277 342 511 415 301 416 420 533 465 463 491 492 289 508 98 254 292 43 . . 276 . 584 571 380, 418 . . 317 . 570 . . 310 . 93 26, 137, 403, 501, 353, 255, 614 INDEX. Plicidentine Pliohippus . Poison fang's Poly mastodon . Polyphyodontism in mammals Polyprotodonts . . . . Porcupine, enamel of . Post-permanent dentition, sup- posed . . . .852, Potamogale . . . 342, Premaxilla, development of . Premilk dentition, supposed PAGE . . 83 . 506 171, 316. 319, 320 . ' . . 381 348 413 39 Premolars 363. 338. definition of ,, of man „ forms of in apes . . ,, order of loss of . ,, homologies of first . . 431, 457 et passim, 508, 514, 517, 522, Primates Primative triangle (Trigon) . Priodon . . . . . . Pristis ...... Proboscidea . ... „ ancestry of Proctodeum . . ... Procyonidae . Prodycnodon . Prognathism Prosimiae . Proteles Proteroglypha . Protocone Protodont Protohippus . Prototheria Pseudoscarus Psittacotherium Pteranodon . Pterodactyles . Pteropus Pterosauria Puerco mammals ,, edentates Puff adder . Pulp, structure o Python 414, 238. 313, 256, 311, 361 436 220 357, 408 359 12 571 463 343, 465, 523 563 373 474 286 538 552 3 461 330 582 565 454 316 372 374 506 392 298 476 331 331 142 331 379 475 318 107 314 R. Rachiodon Racoon . . . . Raia . ..... Rainie, researches on calcification Ramphorhynchus Rat . . . . . 479, Rattlesnake Reptiles, teeth of 316 461 186 lfi2 331 485 318 287 307 for breaking eggs . 307 PAGE Reptiles, development of teeth . 151 Retzius, brown striae of . .48 „ on nerves of pulp . .111 Rhinoceros ..... 523 Rhinolophus . . . . 442 Rhizodus . . . . .287 Rhytina, dentition of . . 563 Ritctie, on dentition of sperm whale . . ... . 493 Robertson^ on nerves of pulp . .112 Rodentia 476 ,, milk dentition of . . 483 Rodents, ancestry of . 269, 476, 485 ,, enamel of . . . . 483 „ food of . . . ,, molars . . . . ,, extinct . . Romer, on dentine . . . . ,, on nerves of pulp . 111. Rorqual Rose, on classification of dentine 479 479 485 77 113 497 83, 102 71 154 ,, on dentinal sheaths . . ,, on tooth germs . . 133 „ on premilk and post per- manent teeth . . . 354 „ on enamel development . . 177 Rostral teeth . . . .285 Rudimentary teeth . . 263, 354 et passim, 373, 375, 382 et passim milk teeth . 350,352, 528 382 Ruminants 518 518 518 tooth germs . absence of incisors in milk teeth of S. Salivary glands . . . . 24 Salmon 302 Sargus .... 300, 302 „ enamel of . . . .59 Saw-fish, vide Pristis. Scales of dog-fish .... 3 ,, of Lepidostens . . . 4 Scarus 298 Schafer, Prof., on cementum . . 209 Schreger, lines of, in enamel . 49 „ ,, dentine . 68 Schwann, on enamel development 178 Seals 463 „ hooded .... 465 „ milk dentition of . 349, 463 Secondary dentine ,, „ in rodents Selache . Selenodont teeth . Semon, on teeth of ceratodus Serres, glands of . . 103 . 105, 480 . . 283 . 501 . . 290 126, 143 INDEX. 615 PAGE Setiform teeth 291 Seyler, Hoppe, on calcification . 166 Sexual weapons, teeth used as . 270, 520 „ differences in teeth . . 497, 509, 513, 520, 525, 532, 538, 558. 573 cervidse . 270, 520 ., ,, cetacea . . 497 ., ., dinotherium . 550 „ ., elephant . . 539 ,. ,., horse . . 525 ,, ,, protoceras . . 532 ., ., monkeys . . 573, 577 ,, sirenia. . . 558 „ selection . . . .273 Sharks, teeth of . . . . 280 Sharpey's fibres . . .118, 127 ,, „ in osteo-dentine . 202 Sheep's head fish, vide Sargus. Shrew 432, 438 ,. milk teeth of . . 361,433 Siluroids . . • . . .291 Simia 571 Simiae 5-0 Sinopa . . . . . . 445 Sirenia, teeth of . . . . 557 Sloth, teeth of 474 Smiiodon ..... 456 Smith, Hopewell, on cementum . 119 ,, „ nerves in pulp . 112 ,, ,, concrescence . 369 ,, ,, teeth of mar- supials . Snakes, classification by teeth 410 313, 316 154 316 260 313,317 432, 436 ,, development of teeth in ,. venom of . Socketed implantation of teeth Solenoglypha . Sorex .... Spalacotherium . . . . 372 Spatularia 287 Spee, von, globules of . 169, 177 Spence Bate, on teeth of insecti- vora 435 Sperm whale . . ... 492 Sphenodon . . . 153,303. 312 Sphyraena 301 Squalodon 490 Stages of tooth germs . . .213 ,, at various ages . . .214 Stellate reticulum . . 142,157 Stomodaeum . . .3 Stratum intermedium . 142, ,, Malpighi Sturgeon . . ... Stylinodon ..... Submaxillary gland . . . Subungulata .... Succession of teeth in sharks . . 157 144 301 476 24 500 282 Succession of teeth in mammals . 361 „ „ in proboscidea 546 Sudduth, on dentine . . .78 S us babirussa, dentition of . .513 ., scrofa 510 Sword-fish 291 T. Tadpole, horny jaws of . . 306 Talon, the 373 Talpa .... 370, 433, 436 Tapir . . . . . . 522 Tarsiops ..... 490 Tarsipes . . . . . 422 Tatusia 473 Taxeopoda ..... 538 Teeth, calcified . . . .30 ., horny . . . 1, 5, 6, 277 „ deciduous, human . .19 ,, homologies with dermal spines .... 2 „ classification of . . 338 „ dates of eruption . .241 „ as tests of age . . . 274 ,, tables of stages of tooth germs at various ages .214 „ of amphibia . . . 305 „ of reptiles . . . . 307 „ of birds .... 331 „ of fish 276 ,, of mammals . . 391 Teething .... 231, 240 Teleostei .... 277, 290 „ tooth germs of . . . 150 Temporal muscle . . . .23 Tenrec, vide Centetes. Tetrabelodon . . . 555 Tetrodon 298 Therosuchia ..... 328 Thylacine . . . 404,416 Thylacoleo 418 Tiger 448 Tillodontia 486 Tillotherium 486 Tims, Marett, on milk teeth of cavia . . . 485 ,, ,, on trituberculism 383 ., „ on premilk teeth . 354 ,, ., on cingulum cusps 370 ,, ,, on concrescence . 369 ,, ,, on evolution of tooth pattern . 383 ., ,, on dentition of canis . . 458 „ ,, on dentition of manis . . 471 ,, ,, on dentition of marsupials . 408 330 . . 30 Tinoceras Tissues, dental 616 INDEX. PAGE Tomes' processes of ameloblasts . 168. 169, 171 „ fibrils . ... 72 Tooth, parts of .... 7 band . . . .135, 147 ,, definition of . . . . 1 ,, germs (early) . . .132 ,, ,, at various ages ". . 214 „ „ forms, genesis of 262, 365 Topinard, on molars of man . . 602 Tortoises . . . . .307 Toxodon 534 Toxodontia ..... 534 Trabeculo-dentine, of Kose . .102 Traguloidea ..... 517 Tragulus 519 Triangle, primitive . . . 371 Trichechidge . . ... 462 Trichechus 466 Triconodon . . . 372, 405 Trigon 372, 430 Triton cristatus . . . . 152 Tritubercular teeth . . .374 ,, ,, in insectivora 430, 436 „ ,, lemurs . 566 ,, „ man . . 604 Trituberculism, theory of . . 372 ,, stages of . .374 ,, difficulties in ac- cepting . 381, 430, 436, 557, 604 Trituberculo-sectorial teeth . . 374 Trousseau, on eruption of teeth . 241 Tubular enamels . . . . 176 Tupaia 427, 438 Tusks of elephant . 539, 540, 554 ,, of wild boars • . . .509 ,, foreign bodies in . . . 544 Underwood, on Nasmyth's mem- brane TIngulata .... „ early forms of ,, artiodactyle ,, perissodactyle ,, molar patterns of ,, vera Uromastyx Urotrichus . . . . Ursidse .... Ursus V. Vampire Varanus dentine of 124 . . 500 . 501 . . 508 . 521 386. 502 .' 500 . . 312 371, 436 . . 462 . 462 441 311 84 Variation in teeth . . . . ,, causes of . ,, concomitant . . . Variability in monkeys' teeth ,, in human teeth . . „ in number of teeth . Vascular canals, in human dentine ,, ., in dentine of Sargus . ,, ,, in dentine manatee . Vaso-dentine . ,, calcification of: ,, development of Vestibulum oris . ... Villiform teeth Vipers Visceral arches Vitrodentine Viverridge W. PAGE 267 270 271 578 602 460 96 of . 97 66, 90 . 199 . 199 . 135 . 291 318 . . 218 52, 92 444, 451 170, WalkhofF, on enamel . Waldeyer, on enamel organ 157, Walrus Wart-hog. . . . . . Wedl, on enamel organ Weil, basal layer of . . . . Whalebone ..... Whales . . . . . . Wild boar, vide Sus scrofa. Williams, Leon, on enamel de- velopment 44, 141, 171, ,, on membrana preforma- tiva . Williamson, on dermal plates Wilson and Hill, on marsupial succession . . 354, 357, Wolf -fish . Wombat .... 408, Woodhead, Sims, on calcification Woodward, homologies of teeth . ,, on milk dentition 351, ,, on tritubercular teeth ,, on succession . ,, on milk teeth of in- sectivora . 431, ,, on teeth of marsupials 403, Wrasse 40 158 466 511 158 113 498 490 213 303 408 295 420 165 343 356 434 343 433 407 303 Z. Zahnwall . . . . . 135 Zahnleiste . . . .135; 147 Zeuglodon 490 Ziphoid cetaceans . . . 493 Zsigmondy, on enamel . . . 48 BRADBURY, AGNEAV, & CO. LD.; PRINTERS, LONDON AND TONBRIDGE. No. 3, J. & A. CHURCHILL Recent Works for Students and Practitioners of Medicine. LONDON : 7, Great Marlborough Street. MAY, 1914 INDEX. PAGE 2 Anatomy. Physiology. 3 Materia Medica. Phar= macy. 4 Hygiene. Bacteriology. 5 Pathology. Psychol = ogy. Dictionaries. 6 Medicine. 7 Medicine, 8 Surgery. o Surgery. Anaesthetics. 10 Neurology. Urinary Disorders. Tropical Diseases. 11 Midwifery. Gynaeco = logy. 12 MedicalJurisprudence. Ophthalmology. Der= matology. 13 Otology. Paediatrics. 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