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 HX00020389 
 
 A MANUAL 
 
 DENTAL ANATOMY 
 
 HUMAN AND COMPARATIVE 
 
 CHARLES S. TOMES, M.A., F.Pv.S. 
 
 WITH 191 ILLUSTRATIONS 
 
 SECOND EDITION 
 
 PHILADELPHIA 
 PRESLEY BLAKISTON 
 
 1882 
 

 LONDON : 
 BRADBURY. AGXEW, & CO. PRTNTERS, WHITEFRIARS 
 
PREFACE. 
 
 In introtlucing a Second Eilition of tins Manual but 
 few "words are necessary. It has been entirely revised, 
 some portions, notably the chapter on the dental tissues, 
 have been in great part rewritten, and many new illus- 
 trations have been added here and elsewhere in the 
 book. 
 
 My indebtedness to Professor Owen's " Odonto- 
 graphy," to my fiithcr's "Dental Surgery,"' and to 
 his " Lectures on Dental Physiology and Surgery," to 
 Professor Flower's " licctures on the Teeth " (published 
 in the B)-itis]i Medical Journal, 1871^, to Kolliker's 
 and to Strieker's Histologies, I will again acknow- 
 ledge, since it is both impossible and undesirable to 
 encumber a student's book with references to au- 
 thorities for every statement. And if I have any- 
 where failed to give due acknowledgment to another 
 whose writings I may have made use of, I crave forgive- 
 ness for my omission. I gladly embrace this opportimity 
 
PREFACE 
 
 of expressing my thanks to Professor Hollaender for the 
 lionoro" he has done to my book in rendering it into 
 German, and to Dr. Cruet for translating it into 
 French ; and at the same time I gratefully acknow- 
 ledge the kindness of many of my friends who have sent 
 me corrections which they have noted as being required 
 in the text of the first edition. 
 
 CHARLES S. TOMES. 
 
 r, Cavendish SyuARH 
 December, 1881. 
 
Digitized by the Internet Archive 
 
 in 2010 with funding from 
 Columbia University Libraries 
 
 http://www.archive.org/details/manualofdentalaOOtome 
 
A MANUAL 
 
 DENTAL ANATOMY 
 
TABLE OF CONTENTS. 
 
 CHAPTER I. 
 
 PACE 
 
 The Nature of Teeth — Description oi'' thr Teeth of Man . 1 
 
 CHAPTER II. 
 The Maxillary Boxes, axd Associated Parts . . . . 23 
 
 CHAPTER III. 
 
 The Dental Tissues : Ei^amel, Dentine, Cemextum, Tooth 
 
 Pulp, &c 40 
 
 CHAPTER IV. 
 
 The Development of the Teeth — in Fish — in Reptiles — m 
 
 Mammals — Calcification op the Dental Tissues . . . 113 
 
 CHAPTER V. 
 
 The Development op the Jaws and the Eruption and Attach- 
 ment op the Teeth 17( 
 
 CHAPTER VI. 
 The Teeth of Fishes 214 
 
CONTENTS. 
 
 CnAPTER VII. 
 
 PACK 
 
 Thk Tkkth of Batkaciua and Kki'Tilia 23!> 
 
 CHAPTER YIII. 
 
 TiiK Tketh op Mam>ials — Introductory Kkmaiiks — Homologies 
 
 o? THIS Teeth — Milk Dentition . . ... . . 264 
 
 CHAPTER IX. 
 
 TuK Tketh of Monoire.mata, Edentata, and Cetacea . . 304 
 
 CHAPTER X. 
 TuE Tkktii of Ungclata 314 
 
 CHAPTER XL 
 
 TuK Tekth of Sirenia, Hvracoidea, Proboscidea, and Kodeniia 344 
 
 CHAPTER XII. 
 TuK Teeth of Caknivora 374 
 
 CHAPTER XIII. 
 ToK Tekth of Insectivora, Ohikoi'tera, and Primates . . 394 
 
 CHAPTER XIV. 
 The Teeth of Marsuitalia . . . , 
 
MANUAL OF DENTAL ANATOMY 
 
 HUMAN AND COMPAKATIYE. 
 
 CHAPTER I. 
 
 THE TEETH OF MAX. 
 
 The range of the subject of Dental Anatomy tui'ns 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 undei-stood 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 orifice 
 of the alimentary canal, i.e., the mouth ; by these hard 
 masses, sometimes of bony and sometimes of horny nature, 
 various offices in connection with the prehension or com- 
 minution of food are performed, and to them the term 
 '' teeth " is applied. In many animals teeth have come to 
 be used for other purposes, such as for sexual wai-fare ; but 
 it can hardly be doubted that teeth have primarily 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 
 
2 A MANUAL OF DENTAL ANATOMY. 
 
 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 
 is continuous with — is, indeed, a part of — 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 under lip, but that its skin turns in over 
 its rounded jaw without interruption. The skin outside 
 carries spines (placoid scales), (') and these spines are con- 
 tinued over that part of it which enters the mouth and 
 bends over the jaws ; only they are a little larger in this 
 latter position. If the growth of the dog-fish be followed, 
 these spines of the skin which cover the jaws become deve- 
 loped to a far greater size than those outside, 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 of the shark 
 are nothing more than highly developed spines of the skin, 
 and thei-efore we infer 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) remain imbedded in 
 tough mucous membrane, and never acquire any connection 
 with the 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 it has been well expressed by Dr. Harrison Allen 
 
 (^) "The placoid scale lias tlie structure of dentine ; is covered by 
 €namel, and is continued at its base into a plate formed of osseous tissue." 
 Oegenbaur's Comparative Anatomy, translated by F. Jeifery Bell, p. 424. 
 
THE TEETH OF MAN. 
 
 ('Anatomy of the Facial Region'), "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." 
 
 In their simpler forms, then, teeth are met with as ver}' 
 numerovis spines, differing but little from the spines of the 
 skin except in size, and still less from one another. In 
 many fish the teeth, though more specialised, are scattered 
 over almost every one of the numerous bones which form 
 part of the walls of the mouth and pharynx ; in reptiles 
 they are much more limited in position, and in mammals 
 are absolutely confined to the intermaxillaiy, maxillaiy, 
 and mandibular (lower maxillary) bones. In fish and rep- 
 tiles it is the exception for the teeth in different parts of 
 the mouth to differ markedly from each other; in mam- 
 mals it is the rule. 
 
 Teeth owe their hardness to an impregnation with salts 
 of lime ; the organic matrix may be of albuminoid charac- 
 ter, 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, in which case the tooth is more richly 
 impregnated with salts, and is spoken of as " calcified." 
 
 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. 
 
 Without further prelude we may pass to a description of 
 the human teeth, this comrse appearing to me, after some 
 little consideration, to afford to the student the most ad- 
 
 B 2 
 
A MANUAL OF DENTAL ANATOMY. 
 
 vantageous introduction to the subject, as he must 
 sarily ah-eady possess some knowledge of their forms, while 
 to the matters alluded to in the preceding pages more full 
 reference will be made hereafter. 
 
 In the human subject no tooth rises above the level of 
 its fellows, and the teeth are arranged in close contact, with 
 no interspaces between them. The teeth are ranged around 
 the margins of the jaws in a parabolic cui*ve, or something 
 approximating to one ; in the lower races of mankind the 
 curve tends to a squarish, oblong form, owing to the 
 prominence of the canines (compare the figure of the denti- 
 tion of Simia Satyrus), whilst a deviation in the opposite 
 direction is daily becoming more common in the most 
 highly civilised races, resulting in a contour to which in 
 extreme cases the name of V-shaped maxilla is applied. 
 
 It may be stated, as generally true, that the teeth are 
 somewhat larger on their labial than on their lingual aspect, 
 a result which necessarily follows from their standing with- 
 out interspaces along a curved line. And as great variations 
 in size and shape, as well as in colour, are found to exist 
 between different individuals, it is only possible to give 
 such a description as shall apply to the generality of teeth. 
 
 The teeth of the upper jaw are ranged along a curve of 
 larger dimensions than those of the lower, the incisors pass- 
 ing in front of the corresponding lower teeth, and the ex- 
 ternal cusps of the bicuspids and molars closing outside 
 those of the lower teeth. 
 
 There are, however, some points of detail to be noted in 
 the relation borne by the upper to the lower teeth, besides 
 that comprised in the general statement that the former lie 
 outside the latter, by which it is brought about that each 
 tooth is antagonised by portions of two teeth in the other 
 jaw, and has not only 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, 
 
THE TEETH OF MAN. 
 
 shut over and in front of the lower teeth, concealing the 
 upper thirds of their crowns ; while the external tubercles 
 of the bicuspids 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 alloM'- 
 ing the external tubercles of the upper teeth to close ex- 
 ternally to the outer tubercles of the lower row. 
 
 From this aiTangement of the tubercles, we are enabled 
 in mastication to use the wdiole surface of the crowns of the 
 opposing teeth ; the act of mastication being performed by 
 bringing the external tubercles of the under molars opposite 
 to those of the upper row ; whence, by the lateral motion 
 of the under jaw^ inwards, their external tubercles pass down 
 the inclined surfaces of the external, and up those of the 
 intei-nal tubercles of the upper teeth, crushing in this action 
 any interposed substance. 
 
 It will also l)e observed that, from the difference of width 
 in the incisors of the two jaws, the centi-al incisors of the 
 upper extend over the centrals and half of the laterals of 
 the under row, and that the superior laterals lie over the 
 remaining half of the inferior laterals and the anterior half 
 of the canines of the lower jaw. The canines close over the 
 halves of the canines and first bicuspids, while the first bi- 
 cuspids impinge on the half of the first and half of the second 
 bicuspids of the lower row. The second upper bicuspids 
 close upon the anterior third of the opposing first molars 
 and the posterior half of the second bicuspids. 
 
 The first molars oppose 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 
 wisdom teeth. The wisdom tooth of the upper being 
 smaller in size than that of the lower jaw is perfectly 
 opposed by that portion of the latter left unoccupied by 
 the second upper molar tooth. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 By this admirable arraugement 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 alter- 
 nate 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 is verti- 
 cally downwards and slightly forwards, while those of the 
 lower jaw are placed vertically, the molars tending slightly 
 inwards. 
 
 It is usual to represent the dentition of any animal by 
 what is termed a dental formula, which enables the reader 
 at a glance to see the number of teeth of each variety pos- 
 sessed by the creature. Thus, instead of writing out at 
 length that man has two incisors on each side in both upper 
 and lower jaws, one canine, two bicuspids or premolars, &c., 
 it is written thus : — 
 
 T 2 1 2 3 „.^ 
 
 I. — c. — prm. — m. — = 61 ; 
 2 12 3 
 
 or in the deciduous set : — 
 
 I. ^ c. 1 dm. Ji = 20. 
 
 2 1 2 
 
 For the purpose of description three parts of the tooth 
 are distinguished by name, viz., the crown, neck, and root. 
 
 This distinction of parts which we make in describing 
 human teeth, when we speak of crown, neck, and root, is 
 applicable to the great majority of mammalian teeth, 
 though there are some few simple forms of teeth 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 
 
THE TEETH OF MAN. 
 
 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 divi- 
 sion into thi-ee 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 crow^ns, as, owing to the curvature of the alveolar 
 border, terms which had reference to front, back, or sides 
 would, in different parts of the mouth, indicate difterent 
 surfaces, and so lead to confusion. 
 
 The lips and tongue and the median line of the mouth, 
 however, are not open to this objection, so the sm-faces 
 which are directed outwards towards the lips are called 
 " labial ; " and those inwards towai-ds the tongue " lingual ; " 
 the interstitial sm-faces are called " median " and " distal," 
 the word median being applied to the surface which would 
 look towaixls the middle line of the mouth had the alveolar 
 border been straightened out. In other words behind the 
 canine, the " median " is equivalent to anterior, and " distal '' 
 to posterior surface. 
 
 Forms of the several Teeth. — It is usual to speak of 
 the teeth as being modified cones, and to attribute their 
 variations to deviations from this typical shape. In a broad 
 sense this may be true of the simplest teeth, such as are met 
 with in some fish and reptiles and monophyodont mammals, 
 which are little more than simple cones ; but there are in- 
 dications which would point to something more complex 
 than this ' as the fundamental form of a mammalian tooth, 
 for even among the monophyodonts, as I have elsewhere 
 
8 A MANUAL OF DENTAL ANATOMY. 
 
 pointed out, the armadillo has a bilobed tooth germ, the 
 one cusp predominating over the other. But I do not think 
 that we have at present the data upon which to cer- 
 tainly determine the fundamental form of the mammalian 
 tooth. 
 
 There is evidence that all the teeth in the jaw of a 
 mammal may have been derived from a single form ; in 
 other words, marked though the distinction between in- 
 cisors, canines, bicuspids, and molars seems to be at first 
 sight, a closer inspection reveals various gradational or 
 transitional characters linking them together, though there 
 are gaps in the chain not bridged over by forms known to us. 
 This may be seen by a careful study of the human teeth, as 
 I shall endeavour to show; but it is much more conspi- 
 cuously seen in an extinct animal (Homalodontotherium, 
 an extinct ungulate from Patagonia, described by Professor 
 Flower, Philos. Trans. 1874), which ajDparently possessed 
 the full typical number of mammalian teeth, viz., forty-four. 
 The point in which its dentition is chiefly instructive is that 
 the teeth, in close juxtaposition one with another, present 
 an exceedingly perfect gradation of form from the front to 
 the back of the mouth, no tooth differing mai'kedly from 
 its neighbour, though the difference between, say, the first 
 incisor and first molar, is exceedingly great. In Professor 
 Flower's words, " it is only by the analogy of other forms 
 that they can be separated into the groups convenient for 
 descriptive purposes, designated as incisors, canines, premo- 
 lars, and molars." 
 
 In viewing the gradational characters which do exist be- 
 tween the various human teeth, it must not be forgotten 
 that some links in the chain have dropped out and are 
 absent. Mention has already been made of the full typical 
 number of mammalian teeth being 44, i.e. 
 
 T 3 1 4 3,, 
 
 I. _ c. - pnn. _ m. - := 44. 
 3 1 4 3 
 
THE TEETH OF MAN. 
 
 The human subject does not possess the third incisor, nor 
 the first two premohirs, so that a somewhat abrupt change 
 of form in passing from the incisors to the canines, and 
 from the latter to the bicuspids, is no more than might be 
 anticipated. 
 
 Incisors. — Of these there are four in each jaw ; two cen- 
 tral, two lateral incisors. Their working surfaces form 
 wedges, or obtuse and blunt-edged chisels, calculated to 
 divide food of moderate consistency. 
 
 Upper Incisors. — The centrals are very much larger 
 than the laterals, and viewed either from the back or front 
 taper with some regularity from the cutting edge to the 
 point of the I'oot, the neck not being marked by strong con- 
 striction. The crown of the tooth, as seen from the front, 
 is squarish, or more strictly, oblong, its length being 
 greater than its breadth. 
 
 The median side, by which it is in contact with its fellow, 
 
 Fig. 1 {') 
 
 is a little longer than the distal, so that the median angle of 
 
 the crown is a little lower, and, as a necessary consequence, 
 
 a little more acute than the distal angle of the cutting edge. 
 
 Near to its base the crow^ns narrov/ rather abruptly, so that 
 
 near to the neck a space is left between the contiguous 
 
 teeth. 
 
 (') Front and side view of a left upper central incisor. 
 a Distal surface. 6 Neck. c Koot. 
 
10 A MANUAL OF DENTAL ANATOMY. 
 
 The labial sufface is slightly convex in each direction, 
 and often presents slight longitudinal depressions, which end 
 at the cutting edge in slight notches. 
 
 In recently-cut teeth the thin cutting edge is elevated 
 into three slight cusps, which soon wear down and disappear 
 after the tooth has been in 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 often presents longitudinal depressions similar to those 
 on the labial surface. The lingual surface towards the gum 
 terminates in a distinct prominence, oftentimes amounting 
 to a bounding I'ing 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 often 
 left in the enamel, Avhich is a favourite site for caries. The 
 crown, or what amounts to the same thing, the enamel, ter- 
 minates on the lingual and labial aspect of the tooth in a 
 curved line, the convexity of the curve being directed 
 upwards towards the ginn ; on the insterstitial surfaces, 
 both median and distal, 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 dentist wiU do well to remember 
 the disposition of the enamel in this situation, as it is a 
 point of some importance in shaping the cervical edge of a 
 cavity preparatory to filling it. 
 
 The transverse indentations of the enamel met with both 
 on lingual and labial surfaces, though more especially in the 
 lattei", are marks of an-est of development, and, common as 
 they are, are to be regarded as abnormalities. 
 
 The centi-al incisors are larger than the laterals, though 
 
THE TEETH OF MAN. 
 
 not in so great degree as is the case in the anthropoid 
 apes. 
 
 The pulp cavity bears a general resemblance to the ex- 
 ternal contour of the tooth ; towards the cutting edge it is 
 very thin, and is prolonged at its two comers to a slight 
 extent into " cornua ;" at the neck it is cylindrical, and is 
 also cylindrical in the root, tapering gradually till it ap- 
 proaches close to the apex, when it becomes suddenly con- 
 stricted. 
 
 Upper lateral incisors are in every dimension some- 
 what smaller than the centrals. 'They widen somewhat 
 abruptly near to the cutting edge, but below this they taper 
 pretty regularly to the end of the root ; the labial surface 
 
 Fig. 2 ('). 
 
 f I 
 
 is convex in each direction, while the lingual surface is 
 perhaps rather flatter than that of a central incisor. 
 
 The outer (distal) angle of the crown is far more rounded 
 or sloped away than in the centrals, and the distal siuface, 
 looking towards the canine, is in a slight degree convex; 
 the median sui-fiice may be slightly concave. 
 
 The enamel terminates towai'ds the gum in contours pre- 
 cisely similar to those which obtain in the centrals : but 
 the basal ridge, or cingulum, is often more strongly pro- 
 nounced, and the presence of a central tubercle upon it is 
 less infrequent. From this greater prominence of the cin- 
 
 (^) Front and side view of a left uiii^er latei'al incisor. 
 
12 A MANUAL OF DENTAL ANATOMY. 
 
 gulum and consequent more marked depression in front of 
 it, caries is more frequent ujion the lingual surfaces of upper 
 lateral than upon those of upper central incisors. 
 
 The pulp cavity is, relatively to the whole tooth, perhaps 
 a little larger than in the central incisors ; in other respects 
 the same description will suffice. 
 
 Iiower central incisors are very much narrower than 
 those of the upper jaw ; not more than half the width at 
 their cutting edges, which again ai-e much wider than the 
 necks of the teeth. 
 
 From before backwards they are deep at the neck ; hence 
 the fangs 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. 
 
 Fig. 3 ('). 
 
 Iiower lateral incisors are, unhke the upper teeth, dis- 
 tinctly larger than the centrals in each one of their dimen- 
 sions, but more especially in the length of their fangs, 
 which are much flattened, and often present on their sides 
 a median longitudinal depression, sometimes amounting to 
 an actual groove. 
 
 The distal angle of the crown is rounded off like that of 
 the upper lateral incisors, though not so markedly. 
 
 Canines, Cuspidati, Eye Teeth, are, in all respects, 
 
 (') Front aud side view of lower central incisor. 
 
THE TEETH OF MAN. 13 
 
 stouter teeth than the incisors ; not only are the crowns 
 thicker and stronger, but the roots are veiy 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 before and behind the point or cusp, but as that side 
 of the tooth which lies next to the bicuspid is convex, and 
 as it were produced towards that tooth, the slope is longer 
 on the distal than on the mesial half of the crowai. The 
 croAvn thus not being perfectly symmetrical renders 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 
 ridge runs down it from the apex of the cusp ; this ridge 
 
 Fui. 4 ('). 
 
 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 
 cusp. 
 
 In transverse section the neck is nearly triangular, the 
 outer or labial being much wider than the lingual aspect. 
 
 (^) Lingual; labial, and distal surfaces of an upper canine, showing the 
 basal cusp and the three ridges which converge towards it. 
 
14 A MANUAL OF DENTAL ANATOMY. 
 
 Lower canines are less pi'onounced in form than the cor- 
 responding upper teeth : the point is more blunted, the 
 fang shorter, the perpendicular labial ridge not being 
 traceable, and the want of symmetry between the mesial 
 and distal halves of the crown less marked. The lingual 
 surface has perhaps a greater tendency to concavity. 
 
 Premolars, Bicuspids, are eight in number, two on each 
 side of both upper and lower jaws, and they correspond to 
 the third and fourth premolars of the typical mammalian 
 dentition, the first and second premolars not being re- 
 presented in man. 
 
 Upper Premolars. — The crown, as seen looking upon its 
 grinding surface, is roughly quadrilateral, its outer or 
 lingual border being, however, larger and thicker than its 
 inner, and the teeth are carried round the curve of the 
 
 Fig. 5 (i). 
 
 alveolar border mainly by means of this difference in size 
 in the external and internal portions of the canines and the 
 two bicuspids. 
 
 As is implied by its name, the crown has two cusps, of 
 which the outer is the larger and 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. 
 The inner and outer cusps arc not joined by a transverse 
 ridge ; instead of this there is a deep transverse fissure ; in 
 point of fact the cingulum has been elevated to form the 
 inner cusp, and forms slight elevations bordering the anterior 
 
 (^) Grinding surface of an upper bicuspid. 
 
THE TEETH OF MAN. 15 
 
 and posterior (mesial aud distal) edges of the grinding 
 siu-face. 
 
 The root is single, and much compressed from side to 
 side : very often, however, it is double for the greater part 
 of its length, and if not so divided is often marked by a 
 groove upon each side indicating a tendency towards such 
 division. The outer border of the root is also often marked 
 by a longitudinal fuiTow, which may amount to complete 
 division. In fact a bicuspid may have three perfectly dis- 
 tinct roots, like a molar, or it may have any form of root 
 intermediate between this and its typical single laterally- 
 flattened root. The first bicuspid is more variable in respect 
 of its roots than the second. 
 
 The second upper bicuspid differs from the first in that 
 the diflference in size between its outer and inner cusps is 
 less, the inner cusp being relatively considerably larger, 
 and, indeed, often preponderating over the labial cusp in 
 length. 
 
 The pulp cavity in the crown is furnished with distinct 
 cornua ; at the neck it is very much flattened from side to 
 side, being often reduced to a mere fissure, which is how- 
 ever considerably larger at its two extremities than in its 
 middle. Hence the pulp cavity of an upper bicuspid is 
 difficult to fill ; a difl&cult^'^ again increased by the impos- 
 sibility of always discovering what number of fangs it has, 
 as their division sometimes takes place rather high up. 
 
 lower premolars are smaller teeth than those of the 
 upper jaw, and are quite distinct in shape. The outer or 
 labial cusp is bent inwards, and the labial surface of the 
 crown is very convex. The inner cusp is but feebly deve- 
 loped, and is connected with the outer by a low ridge ; it is 
 also narrow. 
 
 The root is rounded, a little larger on its outer side than 
 on its inner, and tapers regularly towards its point ; the 
 pulp cavity is cylindi'ical at the neck, and also tapers regi>- 
 
16 A MANUAL OF DENTAL ANATOMY. 
 
 larly in the root. The coruu of the pulp which corresponds 
 to the inner cusp is but feebly developed. 
 
 The second lower bicuspid differs a good deal from the 
 first ; its crown is much squarer and larger in all its dimen- 
 sions. The inner cusp reaches to a higher level and is 
 stouter, and the greater development of the ridge which 
 bounds the posterior (distal) border of the grinding surface 
 makes it attain to such a large size as to make the tendency 
 towards a transition from the bicuspid type to the quadri- 
 cuspid type of a true molai- very evident. 
 
 Having completed the brief description of the forms of 
 these several teeth, it is worth while to note one or two- 
 general characters of the series. The differences between a 
 well-marked 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 
 
 Fig. 6 ('). 
 
 it is far from uncommon to meet with teeth which possess 
 in a marked degree transitional characters. If the external 
 or distal angle of a lateral incisor be sloped off more than 
 usual, while at the same time its cingulum and basal pro- 
 minence be well marked, it makes no bad imitation of a 
 
 (1) Lower first bicuspid, seen from the iuner side, and sliowiiig the pre- 
 ponderance of its outer over its inner cusp. 
 
THE TEETH OF MAN. 17 
 
 diminutive canine ; and such laterals ai'e often to be met 
 with by any who search for such deviations from the normal 
 form. 
 
 Thus the form chai'acteristic of a lateral incisor, if it be a 
 little exaggerated, very nearly gives vis the form of a canine, 
 and if we look at the teeth of an Orang the lateral incisor is 
 to all intents a diminutive canine ; and in the present dis- 
 cussion 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 bicuspids a similar relation- 
 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 is to be found an 
 elevation of the cingulum, in many instances amounting to 
 
 Fia. 7 ('). 
 
 a low cusp, has been already noted ; and it has already been 
 pointed out that the inner cusp of the first lower bicuspid is 
 both smaller and lower than the outer, A longitudinal 
 section through the crowns of the two teeth will demon- 
 strate without the necessity of further description that the 
 
 (') Section of a lower canine and first bicuspid, showing the characteiB 
 common to the two. 
 
 C 
 
18 A MANUAL OF DENTAL ANATOMY. 
 
 basal cusp of the canine and the mner cusp of the bicuspid 
 are the same thing, differing only in degree, while it is 
 interesting to note that the pulp chamber in the bicuspid 
 has hardly any prolongation towards the small inner cusp, 
 so that the resemblance between the two teeth is thus made 
 more complete. 
 
 This close relationship of canines and bicuspids will be 
 again considered in the chapter on the Homologies of the 
 Teeth ; for our present purpose it will suffice to merely point 
 out its existence. The transition from the bicuspids to the 
 molars is more abrupt ; at least it is not so easy to point 
 out exactly how a modification of the one would arrive at 
 the form of the other. But it merely needs an exaggera- 
 tion of the differences existing between a canine and a first 
 bicuspid to make a good imitation of a second bicuspid. 
 
 If any one will take the trouble to make mental note of 
 the deviation in form wdiich he meets with in teeth, he will 
 find that they almost invariably consist of approaches to- 
 wards the form of the teeth on either side of them ; and 
 will infallibly be led to the conclusion that incisors, canines, 
 and bicuspids are not three patterns of teeth perfectly 
 distinct, and each sid generis, but that they are modifications 
 of one and the same pattern. I may add, that comparative 
 odontology teaches us the same thing, and demonstrates 
 clearly the substantial identity of the three forms, as also 
 of the true molars. 
 
 Upper molar teeth have crowns of squarish form, the 
 angles being much rounded off It may be premised that 
 the first molar is more constant in shape than the second, 
 and this latter than the third ; with this proviso the first 
 and second may be described together. 
 
 The masticating surface carries four subeqiial cusps, two 
 labial or external and two lingual or internal ; the anterior 
 internal cusp is distinctly the largest, and it is connected 
 with the posterior external cusp by a thick oblique ridge 
 
THE TEETH OF MAN. 
 
 of enamel, the remaining two cus^^s having no such con- 
 nection. 
 
 This oblique ridge on the upper molars is met with in 
 man, the anthropoid apes, and certain New World monkeys. 
 
 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 ; where they terminate, however, 
 there is often a pit, which is a very favourite situation for 
 
 Fig. 8 (i). 
 
 caries, especially on the labial aspect of the teeth. It ia 
 very rare to see the grooves passing down upon the mesial 
 or distal surfaces of the crown, a raised border of enamel 
 generally cutting them short in this direction. 
 
 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 in- 
 wards towards the roof of the palate, is subcylindrical, and 
 often curved. 
 
 The external roots are less cylindrical, being mutually 
 compressed, so that their largest diameter is transverse to 
 the dental arch ; the anterior is rather the larger of the two, 
 and is more strongly pronounced on the side of the neck of 
 the tooth. The anterior labial root is occasionally confluent 
 
 (^) Masticating surface of a first upper molar of the left side ; the 
 oblique ridge connects the anterior internal with the posterior external 
 cusp. 
 
20 A MANUAL OF DENTAL ANATOMY. 
 
 with the palatine root, but still more freqiiently the pos- 
 terior labial and palatine roots are confluent : occasionally, 
 also, four distinct roots may be met with. 
 
 Lower molars. — The first loAver molar is the most con- 
 stant in form, and is somewhat the largest ; its grinding 
 surface presents five cusps. 
 
 Four cusps are placed regularly at the four corners 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 is 
 thus to be described as median and posterior. 
 
 Fig. (1), 
 
 The transverse fissure passes over the limits of the grind- 
 ing surface, and on the outside or labial siu-face of the tooth 
 ends in a pit, which is a common site for caries ; although it 
 occasionally passes over the lingual siirface, it is here less 
 pronounced. Tliey are implanted by two fangs, jilaced 
 anteriorly and posteriorly ; the roots are much flattened 
 from 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 fangs 
 may be produced ; or this may happen with the one root 
 only, so that a three rooted tooth is the result. 
 
 Q) Masticating surface of a first lower molar, riglit side, the five cusps 
 of which are indicated by figures. 
 
THE TEETH OF MAN. 
 
 The second Biolar 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. 
 
 Fig. 10 (1). 
 
 Third molars, denies sapientia^, wisdom teeth, of the upper 
 jaw, resemble in a general way the first and second molars ; 
 that is, when they are well developed and placed in a roomy 
 dental arch. But amongst more civilised races it may 
 almost be said to be exceptional for the wisdom teeth to be 
 vegidar either in form or position, so that extreme varial)ility 
 prevails among these teeth. 
 
 The two inner tubercles are often l)lended 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. 
 
 Third lower molar. — This tooth is seldom so small as 
 the corresponding upper tooth, and its crown is often large 
 even w^hen 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 
 division into two fangs. 
 
 It is stated by Prof. Owen (" Odontography," page 454) 
 that although the wisdom tooth is the smallest of the three 
 molars, the difference is less marked in the Melanian than in 
 
 (1) Second lower molar of right side, the four cusps being indicated by 
 figures. 
 
22 A MANUAL OF DENTAL ANATOMY. 
 
 the Caucasian races, adding also that the trii)le implantation 
 of the upper and the double implantation of the lower is 
 constant in the former races. More extended observations 
 have overthrown this statement as a positive dictum to bo 
 accepted without excejjtions, but it may nevertheless bo 
 taken as expressing a general truth. 
 
 Fig. 11 (1). 
 
 The milk teeth differ from the permanent teeth by being 
 smaller, and having the enamel terminating at the neck 
 with a thick edge, so that the neck is more distinctly con- 
 stricted. The incisors and canines are somewhat similar to 
 their successors, the canines, however, being relatively shorter 
 and broader than their successors. 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 mor6 
 or less completely enclosing between them the crypts in 
 which the latter are developing. 
 
 (^) Third lowei' molar of the left side. 
 
CHAPTER II. 
 
 THE M.VXILLARY BONES. 
 
 The teeth are implanted iu a part of the jaw bones 
 specially developed for the piu'pose, the bone being moulded 
 around the roots of the teeth subsequently to their being 
 formed and moved into position. 
 
 The manner of attachment of the human teeth is that 
 termed "gomphosis," i.e., an attachment comparable to the 
 fitting of a peg into a hole ; the bony sockets, however, 
 allow of a considerable degree of motion, as may be seen by 
 examining the teeth in a dried skull, the fitting being iu 
 the fresh state completed by the interposition of the dense 
 periosteum of the socket. This latter, by its elasticity, 
 allows of a small degree of motion in the tooth, and so 
 doubtless diminishes the shock which would be occasioned 
 by mastication were the teeth perfectly immovable and 
 without a yielding lining within their bony sockets. When 
 this becomes inflamed and swollen by exudation the tooth is 
 pushed to a certain extent out of the socket, and so being 
 to a less extent limited in its range by the bony socket, 
 acquires an increased mobility. 
 
 The teeth are in all mammalia confined to the bones which 
 carry them in man, namely, the intermaxillary and maxillary 
 bones and the lower maxillaiy bone or mandible. 
 
24 A MANUAL OF DENTAL ANATOMY. . 
 
 While full description of these bones (>) will be found in any 
 general anatomical work, there are so many points in their 
 anatomy which directly concern the dental studeiit that a 
 brief enumeration of some of their relations can hardly be 
 dispensed with, 
 
 Superior maxillary bone. — To facihtate description of 
 its parts, anatomists divide it into a "body" and "pro- 
 cesses," of which latter there are four, the nasal, malar, 
 alveolar, and palatine. As the body of the bone is hollowed 
 out by an air cavity, the antrum, its shape is similar to that 
 of that cavity, namely, roughly pyramidal, the base of the 
 pyramid being inwards towards the nasal chamber. 
 
 The nasal process springs directly upwards from the body 
 in a vertical line with the canine tooth : it is a strong plate 
 of bone, roughly triangular when viewed from the side. 
 
 The malar process forms the apical portion of the pyramid 
 already alluded to ; it starts out nearly horizontally from 
 the body just behind and below the nasal process, and is 
 characterized by its great strength and stoutness. Never- 
 theless it has been known to be fractared by a blow, and 
 separated from the body of the bone. The antrum may be 
 prolonged into it. 
 
 The palatine process foi'ms a horizontal table projecting 
 inwards from the body; as the floor of the nose is nearly flat, 
 and the palate is arched from before backwards, the front of 
 the palatine process is necessarily much thicker than the 
 back, where it is quite a thin plate. 
 
 The alveolar process is a strong wide ridge of bone, curved 
 so as to form with that of the other maxillary bone the 
 elliptical figure characteristic of the dental arch in the higher 
 races. It may be described as consisting of two j^lates, an 
 outer and an inner, which arc connected by numerous trans- 
 
 (*) Much that is of great interest, and that is not to be found in text 
 books, is embodied in a series of papers on " The Facial Region," by Dr. 
 HaiTison Allen (American Dental Cosmos, 1873-7-1). 
 
THE MAXILLARY BONES. 25 
 
 verse septa, the sockets of the teeth being formed by the 
 interspaces between these septa. The internal alveolar plate 
 is the stronger, the external the thinner and weaker, a fact 
 of which we take advantage when we extract a tooth by- 
 bending it slightly outwards. On the outer surface of the 
 alveolar process are eminences corresponding to the roots 
 of the teeth, and depressions in their interspaces, apt to bo 
 
 especially marked over the canine teeth ; while between 
 the teeth the alveolar processes attain to a lower level, so 
 that the margins of the bone are festooned. Looking 
 down into an empty socket, the bone is seen to be every- 
 where very porous, and to be perforated by foramina of 
 considerable size, while at the bottom there is the larger 
 foramen admitting the vessels and nerves of the tooth. 
 
 The alveolus of each individual tooth consists of a shell of 
 comparatively dense bone of small thickness, which is im- 
 bedded in a mass of loose spongy bone; this dense shell 
 comes into relation with the dense cortical bone of the jaw 
 
 (') Superior maxillary boue of riglit side. 1. Boily. 2. Tuberosity. 
 7. JIalar process. 8. Nasal process. 12. Alveolar process. 
 
26 A MANUAL OF DENTAL ANATOMY. 
 
 mainly at its free margin, near to the neck of the tooth. 
 Over very prominent roots a portion of alveolus is at times 
 "wanting, so that in a macerated skull the root is exposed to 
 view. 
 
 The upper maxilla serves to give form and support to the 
 soft parts of the face, and also to carry the upper teeth. 
 These have to be rigidly fixed, while the teeth of the lower 
 jaw are brought forcibly against them with more or less of 
 shock. And whilst these blows have to be received, and 
 resisted, and ultimately borne by the cranium, it is obviously 
 desirable that they should be disti-ibuted over a sufficiently 
 wide area, so as not to be felt unpleasantly. 
 
 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 ; it is buttressed at the back 
 by the pterygoid processes. 
 
 Taking next the various surfaces of the bone, there are 
 four, or, if we include the palatine aspect, five : the external, 
 forming a large part of the face, the superior or orbital, the 
 internal or nasal, and the posterior or zygomatic. Upon the 
 external or facial surface we have to note the eminence 
 caused by the socket of the canine tooth (" canine emi- 
 nence"), and immediately behind this a depression, the 
 canine fossa, through which the antrum is sometimes punc- 
 tured. The alveolar border, from the situation of the third 
 molar to that of the second bicuspid, gives attachment to 
 the buccinator muscle ; while immediately beneath the 
 margin of the orbit is the infra-orbital foramen, whence 
 issues the infra-orbital nerve ; hence this is one of the situa- 
 tions to which neuralgic pain really dependent on the teeth 
 may be referred. 
 
 The orbital and nasal surfaces concern us only through 
 
THE MAXILLARY BOXES. 
 
 their relation to the autrura, to be presently described ; in 
 the zygomatic sui'face, which is convex and forms part of 
 the zygomatic fossa, are several orifices transmitting the 
 posterior dental nerves and vessels ; a groove which, con- 
 nected by the apposition of the palate bone into a canal, 
 forms the posterior palatine canal ; and at the bottom, a 
 rounded eminence, the maxillary tuberosity, -which lies 
 behind the wisdom tooth, and has been occasionally broken 
 off in extracting that tooth. 
 
 The body of the bone is excavated by an air-chambei', the 
 antrum, which is coated in life by a continuation of the nasal 
 mucous membrane, and this frequently becomes secondarily 
 involved in dental disease, so that its anatomical relations 
 are of great importance to the dentist. 
 
 Like the somewhat similar air cavities in the frontal bone 
 the maxillary sinus does not attain to its full size, relatively 
 to the rest of the bone, until after the age of puberty, 
 although it makes its appearance earlier than the other 
 nasal sinuses, its presence being demonstrable about the 
 fifth month of foetal life. Hence it follows that its walls are 
 thicker in the young subject than in the adult ; and, ac- 
 cording to the observations of Mr. Cattlin {^), it is somewhat 
 larger in the male than in the female. 
 
 It is very variable in size, so that out of one hundred 
 adult specimens the above-mentioned writer found one 
 which would only contain one drachm of fluid, while in 
 contrast with that was another which held eight drachms ; 
 two and a half drachms being the average capacity. Al- 
 though it is exceedingly variable in form as well as in size, 
 it tends towards a roughly pyramidal shape, the apex of the 
 pyramid being directed towards the malar bone, which it 
 has been seen to encroach upon, and the base towards the 
 nasal cavity; it is, however, useless to minutely describe 
 
 (') "Transactions of tlie Odontological Society," vol. ii. 1857. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 its form, inasmuch as the two antra in the same individual 
 are sometimes quite dissimilar. The floor of the cavity is 
 rendered uneven in most specimens by prominences corre- 
 sponding to the roots of the molar teeth, which ordinarily 
 are but thinly covered by its bony walls, while it is not by 
 any means rare to find some of them actually bare. 
 
 The cavity is also more or less completely subdivided by 
 loony partitions springing from its walls, as is well exempli- 
 fied in the accompanying figure ; these partitions are for 
 the most part thin, but they occasionally attain to consider- 
 able thickness, and they are stated to occur most frequently 
 at the anterior or posterior angles of the base of the 
 pyramid. 
 
 On the base of the pyramid is the orifice by which it 
 opens into the middle meatus of the nose ; this orifice 
 being partly closed in by the ethmoid, palate, and inferior 
 turbinated bones, and also by soft parts, so that in a recent 
 
 Fig. 13 ('). 
 
 subject it will barely admit a goosequill ; and it should be 
 noted that this orifice opens into the antrum near the top, 
 so that it does not afibrd a ready means of egress to fluids 
 accumulated in the cavity. 
 
 Through this orifice the mucous membrane Hning the 
 
 (1) Section of an antmm of the left side, divided into many iwuches, 
 ijy bony septa, and extending into the malar bone. Drawn from a 
 specimen in the collection of Dr. Maynard, in the possession of the Bal- 
 timore Dental College. 
 
THE MAXILLARY BONES. 
 
 antrum is contiuuous with that of the nasal fossa?, and, 
 like that, it is ciliated; but it differs from the latter in 
 being thinner and less vascular. 
 
 The teeth which usually come into the closest relation 
 with the antrum are the first and second molars, but any of 
 the teeth situated in the maxillary bone may encroach upon 
 its walls, and I have seen an abscess, originating at the apex 
 of the fang of a lateral incisor, pass backwards and perforate 
 the antrum. 
 
 Its walls have four aspects, namely, toAvards the orbit, 
 the nose, the zygomatic fossa, and the face, while its floor is 
 formed by the alveolar border. With the exception only of 
 the latter, its walls are very thin ; and this exception has 
 an important practical bearing in the diagnosis of tumors in 
 this region, as accumulations of fluid or morbid growths 
 really situated in the antrum bulge any or all of its walls 
 in preference to the alveolar l:)order, whereas tumors sjiring- 
 ing from the base of the sphenoid or elsewhere and encroach- 
 ing upon the antnim, push down and distort the alveolar 
 border as easily as any of the other walls of the cavity, 
 inasmuch as the pressure caused by them is not transmitted 
 equally in all directions, as is the case when the medium 
 transmitting the power is a fluid. 
 
 The lower maxilla or mandible consists of a body and 
 two rami, which ascend almost pei-peudicularly from its 
 posterior extremity. The horizontal portion or body is 
 curved somewhat in a parabolic form ; it has a convex ex- 
 ternal and concave internal surface, and an ixpper (alveolar) 
 and a lower border. On the convex facial surface we have 
 to note the ridge marking the position of the symphysis, 
 and below this the mental prominence. Externally to this, 
 below the line of contact of the first and second bicuspids 
 (or a little before or behind this point) is the mental fora- 
 men, which constitutes the termination of the inferior 
 dental canal. Running obliquely upwards, and first visible 
 
30 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 at a point a little distance from the mental prominence is 
 the external oblique line, which becomes merged in the base 
 of the coronoid process. Where it rises as high as the 
 alveolar border, i.e., opposite to the third and sometimes the 
 
 Fig. 14 (ij. 
 
 second molar, the outer alveolar plate is strengthened by it, 
 so that it becomes less yielding than the inner plate. The 
 student should bear this fact in mind when extracting a 
 lower wisdom tooth. 
 
 The buccinator is attached to the alveolar border oppo- 
 site to the molar teeth ; the platysma myoides to the outer 
 side of the lower border along a region somewhat further 
 forward : the masseter over the whole outer face and border 
 of the ascending ramus and the temporal to the apex and 
 side of the coronoid process. The other muscles attached 
 to it are facial muscles of expression. 
 
 On the inner surface of the body are four tubercles, 
 situated in pairs in the median line, about opposite to the 
 ends of the roots of the incisors, but somewhat variable both 
 
 (^) Lower Maxillary Bone. 2. Eamus, where masseter is attached. 
 3. Symphysis. 5. Mental foramen. 6. External oblique line. 8. Angle 
 of jaw. 9. Internal oblique line. 10. Coronoid process. 11. Condyle. 
 12. Sigmoid notch. 13. Inferior dental foramen. 
 
THE MAXILLARY BOXES. 
 
 in position and in size in different individuals. The upper 
 pair of tubercles give attachment to the genio-hyo-glossus, 
 the lower to the genio-hyoid muscles ; they are interesting to 
 the dental student not only as giving attachment to muscles 
 conceraed in deglutition, but as affording convenient fixed 
 points for measurements of the relative growth of pai'ts of 
 the jaw. Beneath these genioid tubercles lie the slight 
 depressions which give attachment to the anterior belly of 
 the digrastric muscle, while between the two points alluded 
 to commences the interaal oblique line, which runs ob- 
 liquely upwards and backwards, becoming more pronounced 
 as it extends backwards, and terminating at the inferior 
 dental foramen. This internal oblique ridge marks the line 
 of gi'owth of the condyle (see Development of the Jaws), and 
 gives attachment to the mylohyoid muscle, which forms the 
 floor of the mouth, in all its length. Thus the bone above 
 the ridge belongs strictly to the mouth, that below it has 
 more relation with cervical structures. The depression for 
 the sublingual gland is above this line, consequently this 
 gland is visible from the mouth ; that for the submaxillary 
 gland is beneath it and further back. 
 
 The inner surface of the ascending ramus gives attach- 
 ment to the following muscles : at the neck of the condyle 
 to the external pterygoid ; on the inner face of the coro- 
 noid process, as far down as the level of the top of the 
 crown of the wisdom tooth, to the temporal ; on the inner 
 side of the angle, over a large surface, to the internal 
 pterygoid. 
 
 The orifice of the inferior dental canal is rough and 
 spinous, giving attachment to the internal lateral ligament 
 of the jaw, while beneath and behind it is the groove for 
 the mylohyoid vessels and nerves ; the canal runs forward 
 in the bone a little distance beneath the ends of the roots of 
 the teeth, and emerges at the mental foramen, turning out- 
 wards at an angle to reach it, and sending onwards small 
 
32 A MANUAL OF DENTAL ANATOMY. 
 
 cauals to the incisors, not traceable far. It is nearer to the 
 enter than to the inner surface of the jaw in the latter half 
 of its course, and is apt to be very close to the ends of the 
 roots of the wisdom teeth, and to those of the bicuspids. 
 The alveolar processes of the lower jaw, at their posterior 
 part, diverge more widely than those of the ixpper jaw, the 
 relative antagonism between the upper and lower teeth 
 being preserved in this region by the former having an in- 
 clination outwards, the latter inwards. The ascending rami 
 join the body at an angle which is very obtuse in the foetus, 
 nearly a right angle in the adult, and once again obtuse in 
 advanced old age ; the explanation of this change will be 
 given \inder the head of the Development of the Jaw. 
 
 The articulation of the human lower jaw is peculiar, and 
 allows of a degree of play unusual in a joint. The ovoid 
 condyles, when the jaw is at rest, are lodged in depressions, 
 the glenoid fossce of the temporal bone, formed partly by the 
 squamous and partly by the vaginal portions of the bone. 
 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. Between the condyle of the 
 lower jaw and the temporal bone lies a moveable inter- 
 articular fihro-cartilage, which is an irregular bi-concave oval 
 plate, the edges of which are united with the capsular liga- 
 ment, so that the joint is divided into two cavities, furnished 
 with separate synovial membranes (unless when, as some- 
 times is the case, the fibro-cartilage is perforated in its 
 centre). 
 
 The joint is described as having foiu- ligaments : the 
 capsular, stylo-maxillary, internal and external lateral 
 ligaments. 
 
 The capsular ligament is but feebly pronounced, and 
 hai-dly deserves the name ; the stylo-maxillary reaches from 
 
THE MAXILLARY BONES. 
 
 the apex of the styloid process to the angle of the jaw; the 
 internal lateral from, the spine of the sphenoid to the mar- 
 gins of the inferior dental foramen ; the external lateral, 
 which alone is a ligament strictly proper to the articulation, 
 reaches from the outer side and tubercle of the zygoma to 
 the outer sm-face of the neck of the condyle. 
 
 The form of the articulating surfaces and the compara- 
 tive absence of retaining ligaments combine to allow of a 
 variety of movement unusual in any other than a ball and 
 socket joint. The articulation acts as a simple hinge when 
 the jaw is simply depressed, and this is the only motion 
 possible in many animals, as in typical carnivora. When, 
 however, the mouth is opened to the fullest possible extent, 
 the condyle leaves the glenoid cavity, slides forward, and 
 rests on the articular eminence, the interarticular fibro- 
 cartilage being carried forward with it. The passage of the 
 condyle on to the articular eminence, although always 
 taking place when the lower jaw is excessively depressed, 
 takes place sometimes without any depression of the lower 
 jaw, which then passes horizontally forward; or it may take 
 place on the one side only, giving to the jaw the lateral 
 movement so useful in mastication. In the mastication of 
 food the various movements are combined, or succeed one 
 another with great rapidity ; the lateral movements are not 
 very extensive, the outer cusps of the lower teeth of one 
 side being brought to antagonise the outer cusps of the 
 upper teeth, and then being made to slide forcibly down the 
 sloping surfaces of the latter till they return to their normal 
 antagonism ; when one set of muscles is tired the same pro- 
 cess is gone through on the other side of the mouth. 
 
 The closure of the jaw, and the rotatory and oblique 
 motions, are accomplished by four pairs of veiy powerful 
 muscles; these are antagonised by muscles comparatively 
 feeble ond indirect in their application. 
 
 The closure of the jaws is effected by the masseters and 
 
34 A MANUAL OF DENTAL ANATOMY. 
 
 the temporals, attached to the outer sides of the jaw ; and 
 the external and internal pterygoids, attached to its inner 
 sides. 
 
 The massetor, temporal, and internal pterygoid muscles 
 close the jaws and press the teeth against one another, and 
 
 Fio. 15 (1). 
 
 this is their principal action. They are antagonised by the 
 digastric, the mylohyoid, and the geniohyoid muscles, which, 
 aided perhaps hy the platysma, depress the lower jaw when 
 the hyoid bone is fixed by its own depressor muscles. 
 
 The external pterygoid draws the jaw forward, and so in 
 some measure tends to open it ; as the two muscles do not 
 always, or indeed generally, act together, they give a lateral 
 movement to the jaw. The superficial portions of the 
 masseter and the internal pterygoid are ordinarily supposed, 
 as their direction is slightly backwards, to assist in drawing 
 the jaw forwai-ds, but Langer, one of the most recent inves- 
 tigators of their action, attaches very little importance to 
 this, and indeed considers that, when the jaw has been 
 pulled forwards by the external pterygoid, the combined 
 action of the internal pterygoid, the temporal, and the mas- 
 seter, may bring it back again, 
 
 (^) Pterygoid muscles. 1. Upper, and 2. Lower heads of external 
 pterygoid muscle. 3. Internal pterygoid muscle. 
 
THE MAXILLARY BOXES. 35 
 
 In ordinary mastication the various movements are com- 
 bined in every possible manner. 
 
 When the mouth is widely open the condyles play upon 
 the articular eminence in front of the glenoid cavity, and 
 the external pterygoid, which assists in widely opening the 
 mouth, draws not only the condyle, but also the inter- 
 articular fibro-cartilage forwards, so that the latter still 
 intervenes between the condyle and the articular eminence. 
 The interarticular cartilages do not, however, accompany 
 the jaw in its extreme movement, but are believed only 
 to pass forwards as far as that part of the eminence which 
 is slightly hollowed out. As, however, in dislocation they 
 accompany the condyles, this supposition may be incorrect. 
 
 The position of repose is neither complete closure nor 
 opening of the jaws : in persons with enlarged tonsils the 
 habitual position is one with the mouth somewhat more 
 widely open, owing to the difficulty of breathing through the 
 nose; a fact which often causes an irregularity in the 
 disposition of the teeth. 
 
 The axis on which the jaw moves is, owing to the bend 
 of the ramus, far behind the glenoid cavity ; it lies veiy 
 nearly in a plane formed by prolonging the plane of the 
 masticating sui-face of the teeth. 
 
 The motions executed in mastication differ much according 
 to the nature of the food ; hence it happens that in different 
 animals the muscles of mastication are very variously 
 developed. 
 
 Thus, in the Herbivora, which move their jaws greatly 
 from side to side, as any one may observe for himself, the 
 pterygoids, 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 forwards in gnawing, the masseter is 
 enormously developed, and has a very marked general 
 backward direction. 
 
 D 2 
 
36 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 Although it is not strictly true, the masseter and temporal 
 may be said in mammals to be developed in an inverse ratio 
 to one another : when one is large the other is not. 
 
 The masseter is at a maximum in Caniivora, which have 
 
 Fig. 1G ('). 
 
 little lateral movement possible to their jaws ; the temporal 
 is also highly developed in many of the class. 
 
 In the great apes, the temporal becomes enormously 
 developed only at the period of second dentition ; this fact, 
 conjoined with its size, which in herbivora seems to have some 
 relation to the presence or absence of canines, would incline 
 one to suppose that it was useful in that rapid closure of the 
 mouth appropriate to biting when animals fight or seize prey. 
 
 The form of the glenoid cavity also bears an intimate 
 relation to the 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 
 surrounding elevations ; its axis is transverse, and little 
 rotary motion is made use of. In the adult it is deeply 
 
 (*) Condyle of the lower jaw, and glenoid fossa of a tiger. 
 
THE MAXILLARY BONES. 37 
 
 sunk : the axis of the condyle is oblique, and rotary move- 
 ments are largely made use of in triturating food. 
 
 In the Felidie, it is strictly transverse ; their teeth, 
 adapted for slicing but not grinding, would gain nothing 
 by lateral motion, vs^hich 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 Herbivora the condyle is roundish, the ascending ramus 
 long, the pterygoid muscles large, and the glenoid cavity 
 shallow ; in the whale, which of course does not masticate at 
 all, there is no interarticular cartilage, and no synovial mem- 
 brane ; the articulation is reduced to a mere ligamentous 
 attachment. 
 
 The harder a substance is, the farther back between the 
 molars . it is placed ; and as the food escapes from between 
 the teeth it is constantly being replaced by the lips, cheeks, 
 and tongue, the buccinator muscle being largely concerned 
 in this work of preventing morsels of food from escaping 
 from the teeth during its mastication. 
 
 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 Ruminants ; 
 in Herbivorous Marsupials it is larger, in the carnivorous 
 section smaller, than the submaxillaries. When an espe- 
 cially viscid fluid is required, as, for example, that which 
 lubricates the tongue of an ant-eater, this is furnished 'by 
 exceedingly large submaxillary glands. 
 
 The nerves of the teeth are derived from branches of the 
 
38 A MANUAL OF DENTAL ANATOMY, 
 
 fifth nerve, the nerve of sensation of the whole side of the 
 face and head : the lower teeth through the inferior maxil- 
 lary nerve, the upper through the anterior and posterior 
 dental branches of the superior maxillary nerve. The 
 nerves are given off from the nerve trunks in bundles 
 corresponding in number to the roots of the teeth for 
 which they are destined. For the details of the distribu- 
 tion of the fifth nerve the student must refer to works 
 treating of anatomy, as it would be out of place to enter 
 upon the subject at length in these pages, in which merely 
 one or two matters of special interest to the dental student 
 will be touched upon. 
 
 In the case of the inferior maxillary nerve the roots of 
 the teeth come into very close proximity with the main 
 trunk of the nerve ; this is especially the case with the 
 lower wisdom teeth. Within a few days of writing these 
 lines I extracted a lower wisdom tooth (with forceps) for 
 a gentleman, who, immediately after the extraction, in- 
 quired if he could have bitten his lip, as it felt swollen; 
 on testing it I found slight but well marked numbness on 
 that side of the lip and chin, which did not wholly subside 
 before he left me. In this case a groove upon the under 
 surface of the much curved roots appeared to indicate that 
 the nerve trunk was in close contact with the tooth. 
 
 No reason is at present known why the tooth pulp should 
 be so richly supplied with nerves, as no obvious advantage 
 results therefrom. Teeth with persistent pulps which go on 
 growing throughout the life of the animal, have always large 
 nerves : thus a very large trunk goes to the pulp of a rodent 
 incisor. But although in this case the rich nervous supply 
 doubtless has to do with nutrition, and presides over the 
 great formative activity of the tissue, this does not fully 
 account for the pulps of the teeth of limited growth being so 
 amply supplied with nerves. 
 
 As has been mentioned in the description of the lower 
 
Fic. 17 ('). 
 
 (1) KiAi'^UAM nj. Tin: DisTKiBriios iij irii: }VnANrin> oi tiii; KifTii NiRvr. 
 fFrnni Torac-,' '; Lectures on Dental Dijsiiili.gy iuid f^uigrry "— dian-n by Mr. C. De Mnrgan,) 
 Orhthalinlc division :— 1. Frontal. 2. Nasal and long ciliary. 3. Branches to ciliary ganglion. 
 Superior maxillary division :— 1. Orbital | Jnl,"^',"*'''^ } •"''. Sphenopalatine (WckrrsT) ganglion. 
 
 IV rostrrlnr drufcil, iiasslnu down. 7, S. Anterior d.-ntal. -.i. Int'ra-orhital. 
 Inferior niiixlllary divlsron :- 10. Aurieulo-tcmporHl. 11, Alasseteric. VJ. Deep tcMiporal. ix Pteiv. 
 -Old. 14. Buccalto bucdnator, &c. 10. Gustatory, l(j. Jlylohyoid branch. 17. Inferior dent -il 
 IS. Mental. 
 
 [To/m-cp.dS. 
 
THE MAXILLARY NERVES. 39 
 
 maxillary bone, the inferior dental nerve emerges from the 
 bone by the mental foramen, near to the end of the roots of 
 the bicuspid teeth. Pain due to distant causes is often 
 refei'red to the point of emergence of a nerve, as is so 
 frequently exemplified in supraorbital neuralgia; in the 
 same way pain due to diseased teeth far back in the lower 
 jaw (especially to wisdom teeth), is frequently i-efen-ed to 
 the bicuspid region. Curiously enough, though there is no 
 apparent close parallel in the disposition of the nerves, a 
 similar reference of pain to the bicuspid region is occasionally 
 observed in the upper jaw. And it may be added that there 
 is very probably some closer parallel in the minute disposi- 
 tion of the nerve fibres going to the teeth in the uj)per and 
 lower jaws than is recognisable by rough anatomical pro- 
 cesses, for while, to all appearance, the nerve trunks are 
 differently arranged, it is a matter of almost everyday ob- 
 servation to find pain due to one tooth referred with pre- 
 cision to its fellow in the other jaw. 
 
 The lower teeth derive their vascular supply from the 
 branches given off to each tooth by the inferior dental artery, 
 itself a branch of the internal maxillary ; the upper teeth 
 derive their ai'tex'ies from the superior dental, a part of 
 the alveolar branch of the internal maxillary, which supplies 
 the molar and bicuspid teeth ; and the front teeth from the 
 descending branch of the infraorbital, the vessels thus having 
 an arrangement somewhat analogous to that of the nerves. 
 
 The distribution of the veins corresponds closely to that 
 of the arteries. 
 
 No lymphatics have been traced into the teeth. 
 
 Tomes, J. Lectures on Dental Physiolog-y and Surgery. 1848. 
 HA.ERIS0N Allek. Anatomy of the Facial Region, Dental Cosmos, 
 
 1874. 
 Cattlin. Anatomy of Antram. Trans. Odontological Society, 
 1857. 
 
CHAPTER III. 
 
 THE DENTAL TISSUES. 
 
 It is usual to speak of there being two kinds of teeth, 
 namely, horny or albuminous, and calcified teeth ; but of the 
 development of the former nothing is accurately known, and 
 it is hence impossible to determine in what relation they 
 really stand to other, or calcified, teeth. 
 
 These latter are composed of one or more structures, 
 which are in great measure peculiar to the teeth (although, 
 what is to all intents and purposes dentine, is to be found 
 in the skeletons and in the dermal appendages of some fish, 
 and other exceptions might be found to the absolute accu- 
 racy of the statement), and hence are called " dental tissues." 
 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 
 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 surface 
 of the tooth ; this may be very partial, as in the eel or the 
 newt, in which animal 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 
 
THE DENTAL TISSUES. 
 
 in man. Perhaps the most usual condition is that the 
 enamel invests the whole crown of the tooth, stopping short 
 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 cu'cumference 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 greater 
 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 subclass Edentata compris- 
 ing sloths, armadillos, and ant-eaters have it not ; the narwal, 
 certain cetaceans, some reptiles, and many fish 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 difiiculty in making sections which show 
 it satisfactorily, but when it is very thin it is apt to 
 break otf 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 of the dentine being 
 also clear and structureless, it is very hard to decide whether 
 the appearance of a double boundary hne is a mere optical 
 eflfect 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. 
 
 My own investigations upon the development of the teeth 
 of fish and reptiles have led me to suspect that rudimentary 
 layers of enamel exist upon many teeth on which their 
 presence has not been recognised, for I have found that the 
 formative enamel organs occur universally. Upon the teeth 
 of snakes, which were stated by Professor Owen to be com- 
 posed only of dentine and cement, I have endeavoured to 
 
42 A MANUAL OF DENTAL ANATOMY. 
 
 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 I am hardly positive that there is 
 enamel upon its teeth, although there is an appearance of a 
 thin coat of distinct tissue. I have also demonstrated that 
 the armadillo has an enamel organ, but have failed to discover 
 any enamel or anything like it upon its teeth, and Professor 
 Turner has made a similar observation upon the narwal. 
 
 At all events we may safely say 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 I have shown the presence of an 
 enamel organ to be universal at an early stage. 
 
 Hence I feel some hesitation in endorsing Professor Owen's 
 generalisation that the dentine is the most and enamel the 
 least constant of dental tissues ; it is possible that it may be 
 so, but recent researches into the development of teeth have 
 very materially modified the conceptions formed as to the 
 relations of the dental tissues to one another, and must lead 
 us 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 the teeth, 
 and reaches up as far as the enamel, the edge of which it 
 overlaps to a slight extent ; 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 teeth 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, cow, horse, &c.) 
 
 It is probably entirely absent from the teeth of snakes, and 
 
THE DENTAL TISSUES. 43 
 
 indeed of very many reptiles ; in the reptilian class, at all 
 events, it appears to me to be confined to those in which the 
 teeth are lodged either in sockets or in a deep bony groove, 
 as I am unacquainted with any tooth anchylosed to the jaw 
 in which it exists, imless we are inclined to include under the 
 term cementum the tissue which I have designated " bone 
 of attachment," (See " Implantation of Teeth.") 
 
 Upon the outer surface of the dentine the enamel forms 
 a cap of a very much harder and denser material. In 
 its most perfect forms it is very far the hardest of all 
 tissues met with in the animal body, and at the same 
 time the poorest in organic matter. In the enamel of a 
 human adult tooth there is as little as 3| to 5 per cent, 
 of organic matter, and, judging from its brittleness and 
 transparency, there is probably even less in the enamel of 
 some lower animals ; the lime salts consist of a large quan- 
 tity of phosphate, some carbonate, and a trace of fluoride 
 of calcium ; in addition, there is a Httle phosphate of 
 magnesium. 
 
 Von Bibra gives two analyses of enamel : 
 
 Calciiun Phosphate and Fluoride 
 Calcium Carbonate . 
 Magnesium Phosphate 
 Other Salts .... 
 Cartilage .... 
 Fat 
 
 Organic .... 
 Inorganic .... 
 
 The cap of enamel is of varying thickness, being thicker 
 in the neighbourhood of cusps than elsewhere ; in teeth 
 
 ADULT 
 
 ADULT 
 
 MAN. 
 
 WOMAN. 
 
 89-82 
 
 81-63 
 
 4-37 
 
 8-88 
 
 1-34 
 
 2-55 
 
 •88 
 
 •97 
 
 3-39 
 
 5-97 
 
 •20 
 
 a trace 
 
 3-59 
 
 5-97 
 
 9G-41 
 
 94-03 
 
A MANUAL OF DENTAL ANATOMY. 
 
 of limited growth it terminates by a thin edge at the 
 neck of the tooth, where it is overlapped to some slight 
 extent by the cementum. When a thick coating of cemeu- 
 tum exists over the whole crown, this lies outside the 
 enamel, the proper jjlace of Avhich is therefore between the 
 cementum and the dentine. 
 
 The external surface of the enamel is finely striated, the 
 course of the strife being transverse to the long axis of the 
 crown ; in addition to this very 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, to 
 all appearance, structureless ; such is the nature of the 
 little caps which, like spear points, surmount the teeth of 
 fishes of the eel tribe, cod tribe, or of the newt, and which 
 from their extreme brittleness are often lost in preparing 
 sections, so that their very existence has long been over- 
 looked. But the absence of structure, if such it really be, 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 
 fibrous structure, very apparent in imperfect teeth, but far 
 less so in well-formed ones, and the enamel of the eel is, in 
 the manner of its development, fibrous ; so that even though 
 we cannot distinguish its constituent fibres when it is com- 
 pleted, this is merely an indication that calcification has 
 progressed a little farther than in human teeth : if calcifica- 
 tion only goes far enough, all structure, if not destroyed, 
 will at all events be masked from sight. 
 
 The structure of hviman enamel has been stated to be 
 fibrous ; that is to say, it has a cleavage in a definite direc- 
 tion, and is capable of being broken up into fibres or prisms, 
 which seem in transverse section to approximate more or 
 less closely to hexagonal forms brought about by their 
 mutual apposition. The general direction pursued by the 
 prisms is one from the dentine towards the surface ; this is. 
 
THE DENTAL TISSUES. 
 
 45 
 
 however, subject to many minor modifications. The curved 
 and decussating course of the human enamel prisms renders 
 them difficult to trace 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 prisms pursue a 
 perfectly straight course, is of comparatively rare occurrence, 
 but among the rodents the com-ses pursued by the enamel 
 prisms are simple, and produce very regular patterns, which 
 are constant for particular families (J. Tomes). Thus, in 
 the Sciuridce, a section of the enamel, whether longitudinal 
 
 Fig. is (i). 
 
 or transverse, appears divided into an outer and inner 
 portion, in which the prisms, although contimious from the 
 dentine to the free surface, piu'sue a different direction. As 
 seen in the longitudinal section, the enamel prisms 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 at an angle 
 of 45 degi'ees with their original course. In transverse 
 section the enamel prisms are found to be arranged in hori- 
 
 (') Section of dentine and enamel of a Beaver : in the inner half the 
 prismfi of contiguous layei's cross each other at right angles, iu the outer 
 they are parallel. 
 
46 A MANUAL OF DENTAL ANATOMY. 
 
 zontal layers, each layer being a single fibre in thickness ; 
 in alternate layers the prisms pass to the right and to the 
 left, 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 of the enamel, ■where 
 the prisms bend abruptly upwards, those of superimposed 
 layers no longer pass in opposite directions, but are all 
 parallel ; in fact no longer admit of distinction into laminee. 
 
 Thus each enamel prism passes in a veiy definite direction 
 and, seen with those of other layers, forms a very charac- 
 teristic pattern ; but the enamel prisms are not in any part 
 of their course curved. 
 
 In the beaver, from which the foregoing figure is taken, 
 the an-angement of the enamel prisms is dissimilar in the 
 upper and lower teeth, the lamination taking place in 
 difierent directions, so that a longitudinal section of the one 
 might, so far as this is concerned, be mistaken for a trans- 
 verse section of the other. As regards the decussation of 
 the prisms of alternate layers, it is similar to that of the 
 Sciuridce, but it difi^ers in the laminae being slightly flexuous 
 instead of pursuing perfectly straight lines. 
 
 Among the porcupine family very much more complex 
 patterns are met with, the enamel prisms being individually 
 flexuous, and their curves not being confined to one plane ; 
 the individual prisms pursue a serpentine coiurse, and cannot 
 be followed far in any one section. Near to the surface, 
 however, they all become parallel, the enamel thus conform- 
 ing with that of other rodents in being divided into two 
 portions (at least so far as the course pursued by, and the 
 pattern traced by, its fibres in its inner and outer parts can 
 be said to so divide it). The Leporidce, or hares, form an 
 exception to this ; their enamel has no such lamelliform 
 arrangement, but is built up merely of slightly flexuous 
 prisms. 
 
 By tracing the coiirses of enamel prisms from the simple 
 
THE DENTAL TISSUES. 47 
 
 pattern found in tlie Manatee through that of the squirrel 
 and dormouse, and the porcupine, we see how a very definite 
 arrangement, at first simple, becomes modified into some- 
 thing a little more complex, till at last it reaches a degree 
 of complexity that looks like mere disorder. No one un- 
 familiar with the enamel of other rodents, looking at the 
 enamel of the porcupine, Avould be able to unravel the very 
 indefinite looking chaos of prisms before him ; but had he 
 studied forms in some degree transitional he could not doubt 
 that the tortuous, curving course which he saw the prism 
 to be pursuing was nevertheless perfectly definite and precise, 
 and formed part of a regular pattern. 
 
 In perfectly healthy human enamel the fibrillar arrange- 
 ment is not so very strongly marked ; the prisms ai'e solid, 
 
 Fig. 19 (i). 
 
 are apparently in absolute contact with one another, without 
 visible intervening substance. 
 
 But Bodecker, basing his conclusions upon the examination 
 of thin sections, stained with chloride of gold, holds that 
 enamel is built up of columns of calcified substance, between 
 which minute spaces exist. These are filled by a material 
 
 (') Human enamel, from the masticating surface of a molar. The 
 figure is merely intended to show the general direction of the fibres. 
 
48 A MANUAL OF DENTAL ANATOMY. 
 
 Avhich takes the stain deeply, and is probably analogous to 
 the cement substance of epithelial foi-mations. As seen in 
 sections, it gives off exceeding fine thorns, which apparently 
 pierce the prisms at right angles to their length, so that it 
 forms a close network very intimately mixed up with the 
 calcified portion of the enamel. 
 
 It is not of uniform thickness, but is beaded, and BiJdecker 
 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 has hitherto been considered to 
 be. He believes it to become continuous with the soft 
 contents of the dentinal tubes through the medium of large 
 masses of protoplasmic matter found at the margins of the 
 enamel and dentine. 
 
 But although there are various reasons for suspecting that 
 enamel is not completely out of the pale of nutrition from 
 the moment that a tooth is cut, yet further observations are 
 needed before the activity and importance of the cement 
 substance demonstrated by Bodecker can be held to be fully 
 established. Klein 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 prisms ; 
 in the enamel of a developing tooth the interstitial substance 
 is larger in amount than in the fully formed organ. It is 
 improbable that nucleated protoplasmic masses are con- 
 tained in the interstitial substance of the enamel of a fully- 
 formed tooth, as is maintained quite recently by Bodecker." 
 The study of the development of marsupial enamel, to be 
 alluded to at a future page, by showing that the enamel is 
 penetrated by soft tissue differently placed, also tends to 
 throw doubt upon Bodecker's interpretation. W. J. Barkas 
 {3Ionthhj Review of Dental Surgery, 1874) has also perhaps 
 had imder observation this cementing substance ; he also 
 
THE DENTAL TISSUES. 
 
 believes that the enamel prisms are tubular, minute canals 
 running along their axes. 
 
 On the whole the prisms are parallel, and run from the sur- 
 face of the dentine continuously to that of the enamel. Their 
 paths are not, however, either perfectly straight or perfectly 
 parallel, for alternate layei's appear to be inclined in opposite 
 directions, while they are also wavy, forming several curves 
 in their length. The curvature of the enamel prisms is most 
 marked upon the masticating surface ; while the layers, 
 alternating in the direction of their inclination as just 
 described, are in planes transverse to the long axis of the 
 cro^\^l, and correspond to the fine striee 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 prism is more or less of 
 a spiral. 
 
 Although most prisms 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 prisms do not u.ndergo any alteration in size as 
 they pass outwards, many supplemental fibres are present in 
 the outer portions of the enamel which do not penetrate far 
 inwards. 
 
 The individual fibres are to all appeai*ance structureless 
 in perfectly formed human enamel, but a faint transveree 
 striation, fainter, but otherwise not unlike that of voluntary 
 muscle, is so general that it cannot be regarded as patho- 
 logical, although it is most strongly developed in imperfect 
 brownish enamel. The striation in question may be seen 
 even in a single isolated fibre, and is not necessarily con- 
 tinuous over adjacent fibres, though it often is so ; it is 
 rendered more apparent by the slight action of diluted acids 
 upon the fibre. Very various interpretations of this appear- 
 ance have been given. It has been attributed to " an inter- 
 mittent calcification" of the enamel fibi-e (Hertz), but is 
 
50 A 3fANUAL OF DENTAL ANATOMY. 
 
 with more probability referred to varicosities in the indi- 
 vidual fibres (Kulliker, Waldeyer) ('). It is very marked in 
 the enamel of the common rat, which shares with that of 
 other mnridte the peculiarity of having the individual fibres 
 almost serrated, those of adjacent crossing layers being fitted 
 to one another Avith great exactness. In human enamel the 
 adjacent fibres if united without any intermediate cement- 
 ing 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 bo the case. Thus the 
 "decussation of the fibres" is a plausible explanation of this 
 appearance of striation ; indeed isolated filires do present an 
 appearance of slight varicosities, repeated at regular in- 
 tervals. The penetration, at regular inten^als, of the prisms 
 by the " thorns " of cement substance (sec page 48), affords 
 another explanation. 
 
 Although the perfect enamel fibre appenrs to be entirely 
 
 Fig. 20 (-). 
 
 a -.. 
 
 homogeneous, it is not really so, for acids act with far greater 
 rapidity upon the central or axial portion of the fibi-e than 
 upon its periphery. The accompanying figujre, taken from 
 
 (^) The striation of voluntary muscle has been very recently proved to 
 he due to this same cause (Dr. Haycraft, " Proceedings of Royal Society," 
 Feb. 1881). 
 
 (") From human enamel, softened in chromic acid, until it coi^ld be 
 j'eadily cut with a knife. 
 
THE DENTAL TISSUES. 51 
 
 enamel softened by prolonged maceration in a 1 per cent, 
 solution of chromic acid shows this well ; the central por- 
 tions of the fibre are dark, and are stained green by the 
 rednced chromium sesquioxide, while the clear interspaces 
 are colourless. Again, if dilute hydrochloric acid be applied 
 to a section of enamel, the axial parts of the fibres are first 
 attacked and are dissolved away, so that, if the section be 
 transverse, a fenestrated mass remains. During the forma- 
 tion of enamel the hardening salts are deposited first in the 
 periphery of the enamel cells, so that the youngest layer of 
 enamel is full of holes, each one of which corresponds to the 
 
 Fig. 21 ('). 
 
 centre of a fibre. Although calcification goes on to obliterate 
 the visible difference between the centre and the periphery 
 of the enamel fibre, yet the action of an acid reverses the 
 order of its formation and once more makes it fenestrated, 
 indicating that there is not absolute identity of substance 
 in the inner part of the fibre. In imperfect enamel, indeed, 
 a central narrow canal has sometimes been observed in the 
 interior of an enamel fibre. 
 
 In fractured enamel, the line of fracture is said to run 
 through the centre of the fibres, 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 almost 
 never, quite parallel with the outer surface of the enamel, 
 but which nevertheless preserve some sort of parallelism 
 with it and the surface of the dentine. These are known 
 
 (1) Transverse section of enamel, the axial portion of the jirisms ha\dng 
 been removed by dilute hj'droohloric acid. 
 
 E 2 
 
A MANUAL OF DENTAL ANATOMY. 
 
 as the " brown strife of Retzius," and as they coincide with 
 what was at one time the outer surface of the enamel cusp, 
 
 Fig. 22 ('). 
 
 are in some sense marks of its stratification, in its original 
 deposition. 
 
 Pigment is seen in the enamel of many rodents ; it is in 
 
 (') Cavities in human enamel, whicli communicate with the dentinal 
 tubes. 
 
THE DENTAL TISSUES. 53 
 
 the outer layers of the enamel, but has no sharply defined 
 boundaiy, fading away gi-adually into the colourless tissue 
 lying within it. Some authors have supposed that the pig- 
 ment lay in a thin coating of cementum, or in a very dis- 
 tinct layer of enamel, but as has above been stated, such is 
 not the case. 
 
 Cavities of irregular form sometimes exist in the enamel 
 near to the surface of the dentine, and when such spaces 
 exist the dentinal tiibes sometimes communicate with them, 
 but these are perhaps to be regarded as pathological ; 
 Bodecker regards them as filled up l)y protoplasm. Irregular 
 fissures and cavities also occur upon the outer surface of the 
 enamel, which also have no special significance save as pre- 
 disposing causes of dental caries. 
 
 In man, however, dentinal tubes may occasionally be seen 
 to enter the enamel, passing across the boundary between 
 the two tissues, and pursuing their course without being lost 
 in irregular cavities ; though this appearance is seldom to be 
 found. As was pointed out by my father, the passage of 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 
 occiu-s in 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 
 recent genera. 
 
 The enamel of the wombat is peculiar also in another 
 respect, being covered by a strong and remarkably uniform 
 layer of cementum. 
 
 The penetration of the enamel l)y dentinal tubes is not, 
 however, a peculiarity quite confined to the marsupials, for 
 it is to be found in some rodents (e.ff. the jerboa), and in 
 some insectivoia (e.r/., the soricida)). 
 
5i 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 Fia. 23 (•). 
 
 
 
 ^1 "ill 
 
 (1) Enamel and dentine of a Kangaroo {Macropiis 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 
 namel is shown in the figure. 
 
THE DEXTAL TISSUES. 
 
 Waldeyer and. Hertz doubt the passage of the tubes of the 
 dentiue into the enamel ; as KiJlliker ubsen^es, it is difficult 
 to see how they can doubt it, even after mere observation of 
 a single specimen ; moreover, it is also capable of experi- 
 mental 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 fi*eed tubes are left 
 hanging out from the edge of the dentine, thus putting the 
 matter beyond aU possibility of doubt, while the develop- 
 ment of the marsupial enamel makes the nature of the con- 
 tents of the tubes quite clear. 
 
 The most marked variation in the structui-e of enamel, 
 which is on the whole a tissue differing but little in various 
 animals, is met with in the class of fish. 
 
 In the Sargus, or sheep's-head fish, for example, the enamel 
 is penetrated by a system of tubes which are not continued 
 
 Fis. 2-1 ('). 
 
 K 
 
 MWMMm 
 
 Dut of or derived from the dentine, but belong to the enamel 
 itself. 
 
 The tubes, as seen in the figure, run at right angles to 
 the external surface of the enamel, px-oceed inwards without 
 branch or bend for some little distance, and then, at about 
 
 (') Enamel and dentine of the Sheep's-head fish (Sargus oris). 
 The enamel is penetrated by a system of channels which enter from its 
 free exposed surface, pass in for a certain distance in straight lines, and 
 then abruptly Lending at an angle cross one another, and produce a com- 
 plicated pattern in the inner third of the enamel. 
 
56 A MANUAL OF DENTAL ANATOMY. 
 
 the same point, bend abruptly at an angle, and give oft 
 numerous branches. The meshwork produced by the cross- 
 ing 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 canals. Von Boas (Zeits, f. wissen. 
 Zoolog. Bd. xxxii.), describing the similarly constructed 
 enamel of scaroid fishes, says that I was in error in sup- 
 posing that the canals open upon the outer siu'face of the 
 enamel. But I do not understand his reasons for dissenting 
 from my opinion, which re-examination of many specimens 
 has tended to confirm. I have not been able to satisfy 
 myself whether the tubes occupy the interspaces of the 
 enamel prisms, or their axes. 
 
 It would appear also as if these tubes were empty during 
 life, as in sections they appear to be more or less blocked up 
 with dirt. The existence of the prisms at all is not certain, 
 and this led Kolliker to say that true enamel does not 
 appear to exist in fishes (Mik. Anat. p. 114); the enamel of 
 fish is, however, developed from an enamel organ homologous 
 with, and exactly like, that of amphibia and reptiles, so 
 that these anomalous tissues must 1)e regarded as l)eing un- 
 questionably enamel. 
 
 The greater part of every tooth is made up of dentine, 
 which thus, even after the removal of the other tissues 
 would preserve somewhat its characteristic form. Several 
 varieties of dentine exist in which those peculiarities of 
 structure which differentiate it from bone become less 
 marked, so that a point is sometimes reached at which it 
 is hard to say whether a particular structure should more 
 rightly be regarded as dentine, or as bone. It will be most 
 convenient to commence with the description of that variety 
 
THE DENTAL TISSUES. 57 
 
 of dentine which differs most markedly from bone ; 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 mammalia, (though it is by no means 
 confined to that class,) and known under the name "hard" 
 or "unvascular" dentine may be selected. 
 
 Dentine is a hard, highly elastic substance, in colour 
 white with a slight tinge of yellow, and to some extent 
 translucent, its transparency being often made more striking- 
 by contrast with the opacity which marks the first advent of 
 dental caries, ^yheu broken a silky lustre is seen upon the 
 fractured surfaces, 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 the dentine consists of an organic matrix 
 richly impregnated with calcareous salts ; this matrix is 
 everywhere permeated by parallel tubes, which run, with 
 some deviations, in a direction at right angles to the siu'face 
 of the tooth. 
 
 The Matrix. — The exact chemical composition of the 
 matrix is not known ; in man the proportion borne by the 
 organic to the inorganic constituents varies in diffei'ent 
 individuals, and very probably in the same individual at 
 different ages, so that analyses can only give approximate 
 results. In a fresh human tooth 62 per cent, of its weight 
 was found to be inorganic salts, the tooth cartilage being 
 28 per cent., leaving a residue of 10 per cent, of water. 
 
 Von Bibra gives the following analysis of perfectly dried 
 dentine : — 
 
 Organic matter (tooth cartilage) . 27"61 
 
 Fat 0-40 
 
 Calcium phosphate, and fluoride . 06" 72 
 Calcium carbonate . . . . 3 "36 
 Magnesium phosphate . . . . 1"18 
 Other salts 0-83 
 
58 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 Von Bibra gives another analysis : — 
 
 
 Cartilage 
 
 20-42 
 
 Fat 
 
 •58 
 
 Salts 
 
 1-00 
 
 Magnesium phosphate . 
 
 2-49 
 
 Calcium phosphate, and fluoride 
 
 G7-54 
 
 Calcium carbonate . 
 
 . 7-97 
 
 And Berzelius gives 
 
 Gelatine and water . 
 Sodium salts 
 Magnesium phosphate 
 Calcium phosphate 
 Calcium fluoride 
 Calcium carbonate 
 
 28-00 
 1-50 
 1-00 
 
 62-00 
 2-00 
 5-50 
 
 The dentine of many mammals is very much more rich 
 in magnesium phosphate than human dentine is; even 
 the latter, 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 prepon- 
 derates ; from 3| to 8 per cent, consists of calcium carbo- 
 nate ; a much smaller proportion consists of magnesium 
 phosphate, while calcium fluoride exists in traces only. 
 
 The organic basis of the matrix is closely related to that 
 of bone, with which however it is not identical ; it is of 
 firmer consistence, and does not really yield gelatine on 
 boiling, but, according to Kolliker (who quotes Hoppe), the 
 dentine of the pig yields a substance resembling glutin, 
 the dentinal globules remaining undissolved. The animal 
 basis of the dentine is called " dentine cartilage," and is 
 readily obtainable by submitting a tooth for several days to 
 the action of diluted acids. The form and most of the 
 
THE DENTAL TISSUES. 59 
 
 structural characteristics of a tooth so treated are main- 
 tained, the dental cartilage forming a tough, flexible, and 
 elastic semi-transparent mass. 
 
 In the matrix of a perfect tooth no trace of cellular 
 structure can be detected ; it is uniform and perfectly 
 transparent. 
 
 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 com-se 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 per- 
 pendicular to the surface, towards the periphery of the 
 dentine, which, however, it does not reach, as it becomes 
 smaller, and breaks up 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 room between them for tlie 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, but 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 ; the primary curves 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 many places at all events, an elongated spiral, as may 
 be very w^ell seen in thick sections transverse to the tubes ; 
 by alterations in the focus of the microscope the appearance 
 of the tube making a spiral turn is made veiy striking. 
 
60 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 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- 
 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 
 
 Fig. 25 (')• 
 
 is to say, each tube bends at the same distance from the 
 surface, and the bend makes a difference in the optical pro- 
 perties of the dentine at that point. 
 
 Schreger described these latter: the lines of Schreger, 
 
 (') Dentine and cementum of a Ntirwal, showing contour lines due to 
 TOWS of interglobular spaces. 
 
THE DENTAL TISSUES'. 
 
 61 
 
 therefore, are markings, ranged parallel with the exterior of 
 the dentine, which 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 the dentinal tubes, and those due to laminrc of 
 interglobular spaces, such as are met with in the teeth of 
 Cetacea. Retzius had seen and described contour markings 
 due to interglobular spaces, though his name is not usually 
 associated with them, the " brown strise of Retzius " being- 
 markings in the enamel. 
 
 The tubes as they pass outwards often divide into two 
 equally large branches ; they also give off fine branches, 
 which anastomose with those of neighbouring tubes. In the 
 
 Fig. 26 C). 
 
 cro^vn of a human tooth these fine branches are compara- 
 tively 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 distinguishing 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 para,llel course. 
 
 The tubes are subject to slight varicosities, and their 
 
 (') Temiinatioii of a dentiual tube in tlie midst of the dentine — human. 
 
62 A MANUAL OF DENTAL ANATOMY. 
 
 coui'so is sometimes apparently interrupted by a small inter- 
 globular space, as is to be seen in an extreme degree in the 
 dentine of Cetacea. 
 
 Owing to their breaking up into minute branches, some 
 of the tiibes 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 my 
 father, while others again pass out altogether beyond the 
 boundary of the dentine and anastomose with the canaliculi 
 of the lacunoQ in the cementum. 
 
 The enamel also may be penetrated by the dentinal 
 tubes, thovigh this Avhen occurring hi the human subject 
 must be regarded as exceptional and almost pathological 
 in its nature (see Fig. 22). As has, however, been men- 
 tioned in speaking of the enamel, in most of the Marsupials 
 and in certain other animals it is a perfectly nomial 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. 
 
 Bentinal 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 is, 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 
 Lave definite walls, and are not simple channels in the 
 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 
 
THE DENTAL TISSUES. 
 
 demonstrated in fossil teeth, in teeth boiled in caustic 
 alkalis, or in teeth which have been allowed to putrefy. 
 
 Although KolHker was, I believe, 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- 
 structible tissues are, however, to be met with surrounding 
 the Haversian canals and the lacunse of bone. It is 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 I should 
 rather say, isolated, to any extent in dentine, unless it has 
 been decalcified. Their existence has been recently denied 
 in toto by Magitot. 
 
 Transverse sections of dentine present fallacious appear- 
 ances, owing to the thickness of the section giving to the 
 tube a double contour which may be easily mistaken for a 
 
 Fig. '27 {^). 
 
 special wall. Immediately I'ound the opening of the canal,. 
 or "lumen," as it is called, there is however generally a 
 thin yellowish border, which is the sheath of Neumann. 
 In the earlier stages of caries, before the dentine is much 
 softened, the walls of the canals become strikingly appa- 
 
 (1) Transvei-se section of dentine. The appearance of a double contour 
 is so much exaggerated as to make the figure almost diagrammatic. 
 
64 A MANUAL OF DENTAL ANATOMY. 
 
 rent. The canals which everywhere permeate 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 thin section ; 
 neither are they, as was at one time supposed, tenanted 
 merely by fluid. 
 
 Dentinal Fibrils. — Each canal is occupied by a soft 
 fibril, which is continuous with an odontoblast cell upon the 
 
 Fir,. 28 (1). 
 
 surface of the pulp ; the existence of these soft fibrils was 
 first demonstrated by my father, Avho thus, to use the words 
 of Waldeyer, "oj)ened the way to a correct interpretation 
 of the nature of the dentine." 
 
 Henle, in his "AUgemeine Anatomic," (1841), a transla- 
 tion of a portion of which is to be found in the "Archives 
 of Dentistr}^," (1865), figured and described projections from 
 the edges of fragments of dentine in continuity with the 
 dentinal tubes. These he distinctly describes 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 Avith the tubes. 
 
 Miiller, (as translated in Nasmyth on the " Structure of 
 
 (^) A fragment of dentine [a), through which run the softer fibrils (c), 
 which are seen to be continuous with tlie odontoblast cells [h). (After 
 Dr. Lionel Beale.) 
 
THE DENTAL TISSUES. 65 
 
 the Teeth," 1839), says, "m breaking fine sections of the 
 teeth perpendicularly to the fibres, he has 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 my father's discovery la}'- in the 
 fact that dentine is permeated by soft, uncalcified structures ; 
 and what is yet more significant, that these soft fibrils, per- 
 meating the hard dentine, proceed from the pulp. In no 
 sense, therefore, did Henle anticipate this discovery. 
 
 In 1854 Lent figui-ed processes from the dentinal cells 
 (odontoblasts) which he rightly conceived to be concerned 
 in the formation of dentine ; but in the earlier editions of 
 the "Histology" of his friend and teacher, Prof Kolliker, 
 although Lent's discoveries are described and adopted with- 
 out reservation, no mention of the real structure of dentine 
 occurs. But in the last edition, Prof KiJlliker 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 ai-e as follows, and may be taken as 
 proofs of the 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 (Wurzburg Nat. Zeitschrift, Bd. VI. p. xi.) and. 
 others. 
 
A MANUAL OF DENTAL ANATOMY 
 
 In fresh dentine every formative cell sends a process into 
 the dentinal tubes (Tomes, Kiilliker, Lent, Waldeyer, Neu- 
 mann), and it has been fovmd possible to demonstrate both 
 the sheaths and the fibres in the same sections (Neumann, 
 Boll). 
 
 In transverse and even in longitudinal sections of decalci- 
 fied dentine the fibrils may be recognised in situ (Kcilliker). 
 
 The contrast between the dentinal sheaths and the fibrils 
 is this : — the sheaths are very indestructible, and can be 
 demonstrated in teeth which have undergone all sorts of 
 
 Fig. 29 (i). 
 
 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, we have (i.) a 
 matrix permeated by tubes ; (ii.) special walls to these tubes 
 or "dentinal sheaths;" and (iii.) soft fibrils contained in 
 these tubes, or " dentinal fibres ; " and it now remains to 
 consider these in farther detail. 
 
 In fortunate sections of small fragments of dentine taken 
 from the 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 these being extensile 
 they may be stretched or drawn out from the tubes for some 
 little distance without being broken across. In the same 
 
 (') Section of dentine from tlie edge of which hang out the dentinal 
 sheaths, and beyond these again tlie fibrils (after Boll). 
 
THE DENTAL TISSUES. 67 
 
 way they may be seen stretching across like harp strings 
 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 (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 the accompanying 
 figui'e, in which some hang out from the edge of the dentine, 
 
 PflllPf 
 
 
 while others have been pulled out from the dentine and are 
 seen attached to the odontoblast cells. 
 
 The dentinal fibril is capable of being stained with car- 
 mine, though with some difl&culty; in young dentine it 
 is more easily stained, especially near the pulp cavity, and 
 the accompanying drawing is taken from such a section of 
 
 (^) Surface of the pulp, with the odontoblast layer in situ. The dentine 
 fibrils pulled out of the dentinal tube hang like a fringe from the odonto- 
 blast 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. 
 
 P 2 
 
A MANUAL OF DENTAL ANATOMY. 
 
 dentine from a half-formed human incisor. The matrix is 
 slightly stained with the carmine, indicating that it has not 
 yet become fidly 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. I have observed precisely similar 
 appearances in the thin young dentine of calves' and pigs' 
 teeth ; Kblliker also mentions that the dentinal fibril may 
 be recognised in situ in transverse sections of fresh dentine. 
 
 Fig. 31 (1). 
 
 Bodecker finds that the dentinal fibrils stain darkly with 
 chloride of gold ; when viewed in transverse sections under 
 a magnifying power of 2,000 diameters they do not appear 
 round but somewhat angular, and give oflF tiny lateral 
 offshoots which seem to penetrate the dentine. In the 
 matrix itself there is an appearance of a faint network when 
 it has been stained with gold, and from this Bodecker infers 
 that the dentine is penetrated everywhere by a network 
 of living plasm, derived from, though far finer than, the 
 dentinal fibrils. 
 
 Probably the angularity of the fibril, wdiich, 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. 
 
 (') 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 are somewhat shrunken, 
 owing to the action of the glycerine in which the section is mounted. 
 
THE DENTAL TISSUES. 
 
 According to Neumanu, 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 
 cavity. But here they would naturally be more minvite, 
 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 outside of the 
 dentine. 
 
 Dr. Beale has seen prolongations of the nucleus of the cell 
 toAvards the base of the fibril, though in the example which 
 he figures it does not enter it. 
 
 Dentinal fibrils have been demonstrated in the Reptilia 
 and Amphibia by Santi Sirena and myself; and by myself 
 in the few fish that I have examined with that purpose. 
 
 Of their real nature some doubts are entertained : they 
 are certainly processes of the formative cells of the dentine, 
 and their substance seems identical with that of the proto- 
 plasm of the cell. Nerves, in the ordinary sense of the 
 word, they are not, and have never been supposed to be ; 
 but there are many examples of cellular structures which 
 are comiected 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 nerve of the pulp, the 
 termination of which have never been satisfactorily traced. 
 
 Mr. Coleman once suggested that it was possible that 
 the odontoblasts might have some tactile function ; but 
 M. Magitot has recently claimed for them a very definite 
 connection with the nerves of the pulp. According 
 to his observations and figures the nerves of the pulp 
 become continuous with a layer of reticulate cells which lie 
 beneath the odontoblasts ; and these freely communicate 
 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. M. Magitot speaks very 
 
70 A MANUAL OF DENTAL ANATOMY. 
 
 positively as to the accuracy of his views, which as yet, 
 however, have not been confirmed by other investigators. 
 
 Fig. 3-2 f»). 
 
 U4-L^:;ii}A 
 
 Yet another view of the nature of the dentinal fibril is 
 advocated by Klein ("Atlas of Histology," p. 183), who 
 holds that the odontoblasts are concerned only in the forma- 
 tion 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 dental canals. 
 
 In a recent paper (Comptes Rendus, 1880,) 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 
 
 (^) After Magitot. «. Dentinal fibrils. h. Amori^lious matrix. 
 c. OdontoljJasts. cl. Nuclei of odontoblasts, e. Stellate cells. /. Nerve 
 extremities which are continuous with the branched cells. 
 
THE DENTAL TISSUES. 71 
 
 precisely the same tissue. He would prefer to speak of 
 dentine as being a fibrillar tissue included in a hard and 
 homogeneous matrix. 
 
 These views, however, do not differ substantially from 
 those in the text, save that M. Magitot does not recognise the 
 existence of that transitional tissue which others believe to 
 be there, and call the sheaths of Neumann. 
 
 No true nerve fibril has ever been seen to enter the den- 
 tine ; nothing but the dentinal fibril has ever been proved 
 to pass from the pulp into the hard substance of the 
 tooth ; nevertheless, the observation of Boll is very sug- 
 gestive. 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 fi'om a plexus of 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. 
 
 Boll's observations likewise are awaiting confirmation or 
 disproof, and so far stand alone. 
 
 Be that as it may, there can be no question that the 
 sensitiveness of the dentine is due to the presence of soft 
 organized 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 conducted to the pulp ; moreover, 
 it is within the experience of every operator that after the 
 removal of a very sensitive layer of caries, you often come 
 down upon dentine, which, though nearer to the pulp, is 
 far less sensitive, a condition quite inexplicable, except 
 upon the supposition of a different local condition of the 
 contents of the tubes. Irritation applied to the dentinal 
 
72 A MANUAL OF DENTAL ANATOMY. 
 
 fibrils may be propagated to the pulp, and irritation of the 
 pulp set up without any real exposure of the latter. 
 
 With reference to the probabilities of actual uei've 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 fine- 
 ness 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 difiicult substances conceivable for the demonstra- 
 tion of fine nerve fibrils, if svich exist in it. 
 
 Interglobular Spaces. — In the layer of dentine which 
 underlies the cement an immense number of these spaces 
 exist, giving to the tissue as seen under a low power an 
 appearance of granularity. On this account my father gave 
 
 to this the name of the " granular layer " of dentine ; on 
 account of the far 'greater abundance of the spaces in that 
 situation it is far more marked beneath the cement than 
 beneath the enamel, and many of the dentinal tubes end in 
 these spaces. 
 
 Although the name " interglobular spaces " is strictly ap- 
 plicable to the structm-es 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 
 
 (^) Dentinal tubes terminating in the spaces of the granular layer. 
 
THE DENTAL TISSUES. 
 
 73 
 
 spaces, clustered together and usually most abundant at a 
 little distance below the surface, ai-e often to be seen, parti- 
 cularly if the section has been made from a brownish, im- 
 perfectly developed tooth. 
 
 These spaces have a ragged outline, furnished with short 
 pointed processes, and in favoxn-ably-prepared sections it 
 may be seen that their outlines are formed by portions of 
 the surfaces of closely opposed spheres, and globular con- 
 tom'S may often be detected iii the solid dentine near to 
 them, as is seen in the accompanying figure, taken from a 
 section boiled in wax in order to render it very transparent. 
 
 Although these large spaces are very common, 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 calcificfition, will again be alluded to in connection with 
 the development of teeth ; but although the term " inter- 
 
 ('; Interglobular spaces in dentine. 
 
74 A MANUAL OF DENTAL ANATOMY. 
 
 globular " is thus strictly applicable, the use of the word 
 " spaces " is not so correct. lu dry dentine it is true that 
 they are, as Czerinak 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 soft nature of tlie contents as com- 
 pared with the rest of the dentine, is well illustrated by a 
 section in my possession which was taken from a carious 
 
 Fia. 35 (1). 
 
 tooth, near to the alfected surface. In this the fungus, 
 leptothrix, had effected an entrance into some of the tubes, 
 giving to them a varicose beaded appearance, and causing 
 their enlargement. But when it reached the interglobular 
 space, the less amount of resistance, or possibly the more 
 favourable pabulum accessible, led to its more rapid deve- 
 lopment, so that the tubes within the confines of the space 
 lire many times more enlarged than those outside ; never- 
 theless the continuity of the tubes across the space is well 
 
 C) Section of carious dentine, in which some of the tubes are beaded 
 by the ingress of the leptothrix, which has develoioed with greater freedom 
 in one or two of the tubes where they cross the interglobular spaces. 
 
THE DEXTAL TISSUES. 75 
 
 demonstrated by the gro^vth of leptothrix having followed 
 them -with exactitude. 
 
 It sometimes happens that indications of spherical forms 
 and faintly discernible contours resembling those of the 
 interglobular spaces may be seen in chied sections, in which 
 no actual spaces occur. The appearances are perhaps pro- 
 duced by the formation of an interglobular space, the con- 
 tents of which have subsequently become more or less 
 perfectly calcified ; and the appearance described as " areolar 
 dentine" is probably to be explained in this manner. 
 
 The exact nature of the contents of the interglobular 
 spaces is not very certain : they may, with some difficulty, 
 be tinted by cai-mine, and it is said that they may, like 
 the dentinal sheaths, be isolated by the destruction of the 
 rest of the matrix in acids ; that this may be done I do 
 not doubt, although I have never succeeded in so isolating 
 them myself. 
 
 Bodecker finds 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 lacunee and canaliculi of 
 the cementum on the other. 
 
 In the dentine so far described, which is that variety known 
 as hard or uuvascular dentine, some degree of nutrition is 
 perhaps provided for by the penetration of the whole thick- 
 ness of the tissue by protoplasmic fibres, the dentinal fibrils, 
 but this nutrition may be eff'ected 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 I would classify dentines as 
 (i.) Hard or uuvascular dentine. 
 (ii.) Plici-dentine. 
 (iii.) Vaso-dentine. 
 (iv.) Osteo-dentine. 
 
76 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 Ordinary hard dentine has been sufficiently described ; of 
 it plici-dentine is a variety not very distinct in its essential 
 nature, though at first sight widely dissimilar. 
 
 Flici-deutine. — In ordinary dentine the dentinal tubes 
 radiate out from a pulp and pulp chamber of simple form ; 
 render complex that form by foldings of its walls, the den- 
 tinal tubes still running off at right angles to that portion 
 of pulp to which they immediately belong, and we have 
 a " plici-dentine." It is merely an ordinary dentine and its 
 
 Fio. 36 (1). 
 
 pulp folded up and wrinkled into a greater or less degi'ee 
 of complexity. 
 
 In the teeth of the Varanus (monitor lizard) 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 a single central 
 
 i}) Section of Plici-ilentine with the pulj) in situ (Lepidosteus). 
 Odontoblasts, p. Connective tissue framework of jjulp. d. Dentine. 
 
THE DENTAL TISSUES. 77 
 
 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 dippings in of 
 the dentine ; and the dentine is, as it were, in folds. The 
 
 Fig. .37 {'). 
 
 pulp on section might be compared to a paddle-wheel, the 
 floats of which correspond to the thin flat radiating pro- 
 cesses of pulp ; but as yet the central pulp chamber is 
 unaltered. A little lower down, as represented in the 
 accompanying figure, there is no longer a central simple 
 pulp chamber; the inflections round the periphery have 
 become relatively much deeper, and the centre of the tooth 
 is occupied by a tissue irregular, but not otherwise unlike 
 the dentine of Myliobates ; that is to say, there are a 
 number of columns of pulp, each of which forms the 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 
 
 (>) Transverse section across the cro^^•n of tlie tootli of Yaranus, near to 
 its base. The central pulp cavity is produced out into processes, and it 
 might be said the dentine is arranged in plates with some little regularity 
 round its periphery. 
 
78 
 
 A MANUAL OF DENTAL ANATOMY 
 
 of Lepidosteus (j^orth American bony pike) ; or sometimes 
 there are several branches. 
 
 Fig. 38 (>). 
 
 In Lepidosteus oxyurus there are simple inflections, and 
 a central pulp cavity; in L. spatula the inflections are 
 branched, and the central pulp cavity all filled vip. 
 
 In the foregoing figure of the base of a tooth of Lepi- 
 dosteus some few of the outrunning pulp chambei's are 
 
 (^) Transverse section across the tooth of Lepidosteus spatula. At the 
 exterior are regularly disposed radiating plates of dentine, each with its 
 own pulp cavity, while the central area is composed of more or less cylin- 
 drical pulp chambers, each of which forms the stai-ting point for its own 
 system of dentinal tubes. The pulp chambers are made dark in the figure 
 for the sake of greater distinctness. 
 
THE DENTAL TISSUES. 
 
 79 
 
 seen to be bifurcated, Avhile the central mass of the tooth 
 is composed of dentine permeated by pulp canals which 
 pursue a longitudinal coiu'se ; a slight further modification 
 brings us to the structure of the dentine of the Labyrin- 
 thodon, in which a maximum of complexity is attained, 
 although a clue to its intimate structure is afforded by the 
 teeth of Varanus or of Lepidosteus. 
 
 The laminae of pulp, w^ith their several systems of den- 
 
 Fig. 39 (i). 
 
 tinal tubes, instead of passing out in straight lines like the 
 spokes of a wheel, pursue a tortuous course as they run 
 from the central small pulp chamber towards the surfiice. 
 Not only do they undulate, but they also give off lateral 
 
 (^) Transverse section of a tootli of Labyrinthodon. (After Owen.) 
 fl -^The letter a> is placed in the centre jjulp chamber ; the letter b marks 
 the lines of separation between the system of dentinal tubes which belong 
 to each lamina of pulp ; these lines of demarcation were formerly sup- 
 posed to be occupied by cemeutum. 
 
80 A MANUAL OF DENTAL ANATOMY. 
 
 processes ; and at their terminations near to the surface of 
 the tooth, the thin laminae of pulp (so thin that the radi- 
 ating pulp chambers are mere fissures) become dilated ; so 
 that on section circular canals are seen at these points, as 
 is also the case at the points where subsidiary processes 
 branch off. 
 
 The wavy course pursued by the radiating plates of den- 
 
 "//^ -"- ' "^^^smf'i ~ ■^^- 
 
 A\ 
 
 ■t 
 
 iiitffi£f~z=z^ --■>///|!' 
 
 tine, and the disposition of the tubes round the dilated 
 portions of pulp chamber, render the general aspect of the 
 dentine structure very complicated ; the several "systems" (-) 
 
 (^) From tooth of Lalija-iiitliodon, showing the nature of the connection 
 between the contiguous dentinal systems. (After a di-awing of my father's. ) 
 
 (2) 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 Labyrinthodon is made up of many dentinal system.? ; the 
 same thing may be said of the tooth of Myliobates. 
 
THE DENTAL TISSUES. 
 
 are united to one another by an inosculation of the ter- 
 minal branches of the tubes in some few places, but more 
 generally by a clear layer containing radiate spaces, some- 
 thing like the lacunae of cementum. Hence Professor Owen 
 has described the tooth as consisting of radiating plates of 
 dentine, between which pass in equally convoluted plates 
 of cementum. But, as was pointed out by my father (Phil. 
 Trans. 1850), 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 
 present, the cementum is always outside the enamel, whereas 
 at the upper part of the tooth of the Labyrinthodon the cha- 
 racteristic inflections take place within a common investment 
 of enamel w^hich does not dip in. Thus the whole of the 
 tissue constituting the very complex pattern of the Laby- 
 rinthodon tooth is dentine, and the cementum does not, as 
 was usually supposed, enter into its composition at all. 
 
 Another form in which plici-dentine may exist is exempli- 
 fied in the teeth of Myliobates, a lai*ge Ray ; or in the teeth 
 of the rostrum of the saw-fish (Pristis). 
 
 In the Myliobates (Fig. 41) the flat pavement-like 
 tooth is permeated by a series of equidistant parallel 
 straight canals, running up at right angles to the surface ; 
 from the upper end and sides of these channels systems of 
 dentinal tubes radiate, just as the tubes radiate from the 
 single pulp chamber of a human tooth, save that they rim 
 for a comparatively short distance. In transverse sections 
 the tubes are seen radiating from these channels, 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 pai't of the tooth, but intimately 
 united together at its base, where the disposition of the 
 channels ceases to be regular, and, as a consequence, the 
 
 G 
 
82 A MANUAL OF DENTAL ANATOMY. 
 
 systems of dentinal tubes pass from them in various direc- 
 tions without producing the symmetrical patterns which 
 characterise the upper part of the crown. 
 
 Fig. 41 
 
 When the tooth comes into use and its immediate surface 
 gets worn off, the ends of the perpendicular pulp channels 
 would be laid open, were it not that they become blocked 
 by the deposition of a transparent homogeneous tissue 
 Avithin them, analogous to the similar tissue which closes 
 Haversian canals of an antler about to be shed. 
 
 Such is an example of plici-dentine in a simple form, in 
 which the tooth might be said to be built up of a series of 
 small parallel denticles ; and a similar structure is pre- 
 sented by the rostral teeth of the saw-fish, and by the teeth 
 of the Orycteropus or Cape ant-eater. 
 
 Vaso-dentine. — In the dentine of human teeth it occa- 
 sionally happens that a larger canal is found, having no 
 clear relation to the course of the dentinal tubes, which it 
 crosses at various angles ; this larger canal contained the 
 (^) Transverse section of the dentine of Myliobates. 
 
THE DENTAL TISSUES. 
 
 blood-vessel, the remains of which may be found even in 
 a dried section. But in human dentine vascular canals do 
 not often occur, and when they do, are to be regarded as 
 decided abnormalities. 
 
 The accompanying figure, representing a canal of large 
 
 Fig. 42 ('). 
 
 size, was drawn from a specimen shown to me at the Cam- 
 bridge (Massachusetts) Museum Ijy Dr. Andrews. 
 
 In some mammalian teeth these vascular canals are 
 disposed with regularity, running out in loops from the 
 pulp cavity, and lying, for a considerable i:)art of their course, 
 at right angles to the dentinal tubes. 
 
 lu the Manatee for example the dentinal tubes radiate 
 out with perfect regularity from the central pulp chamber, 
 and, so to speak, take no notice of the vascular canals, 
 
 (,') Vascular canal in dentine. From a liuman tooth. 
 
 G 2 
 
84 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 which are to be met with (especially in the root) in large 
 numbers. 
 
 Where they are numex'ous the vascular canals form loops, 
 so as to constitute a sort of plexus beneath the cementum. 
 
 The Tapir, whose teeth in external configuration are not 
 very dissimilar to those of the Manatee, also has vascular 
 canals in the dentine ; a curious diftereuce in this respect 
 
 Fia. 43 (•). 
 
 
 was pointed out by my father (Proc. Zoolog. Soc. 1851) 
 between the Indian and the American Tapir, the former 
 having the canals in the dentine of the crown of the teeth, 
 the latter having them not. The great extinct ^Megatherium 
 possessed dentine very rich in these canals : to the left of 
 the figure is seen the inner portion of the dentine, rich in 
 them ; in the middle a fine tubed dentine, forming the 
 external layer of the dentine of the whole tooth, and to the 
 right cementum, also rich in vascular tubes. 
 
 In those teeth in which the whole pulp is converted into 
 solid material, and no pulp cavity remains, the last portions 
 of the pulp are often converted into dentine which has not 
 the same character as that of the rest of the tooth. Thus 
 
 {}) Dentine and cementum of Megatherium : the latter to the right of 
 the figure. 
 
THE DENTAL TISSUES. 
 
 in teeth of perpetual growth, such as the incisors of rodents, 
 the axial portion of the tooth is that latest calcified, and 
 consists of a dentine containing vascular canals, which are 
 not present in the other part of the tooth. When a change 
 thus occm-s in the character of the tissue formed at a later 
 time than the rest of the dentine, the name "secondary 
 dentine " is applied to the resultant tissue. 
 
 But secondary dentine may partake of several different 
 varieties of structure, so that the term must not be taken as 
 
 Fig. 44 (')■ 
 
 denoting anything more than the circumstances under which 
 it was formed. 
 
 It is in the class of Fish, in which vaso-dentine is rather 
 common, that the most instructive examples of its nature 
 are to be found. 
 
 The conical teeth of the common Flounder, and indeed of 
 most flat fish (Pleuronecticke) have their pointed tips formed 
 
 (*) Tooth of a Flounder, a, Dentinal tubes near apex of tooth ; b. 
 Vascular canals ; c, Spear points of enamel. 
 
86 A MANUAL OF DENTAL ANATOMY. 
 
 of ordinary hard dentine, surmounted by enamel tips. In 
 this part of the tooth the dentinal tubes are numerous, and 
 regular in their disposition, radiating out from the axial i)ulp 
 chamber. 
 
 Lower down in the teeth the dentinal tubes become less 
 numerous, and at the same time much lai'ger looped canals 
 make their appearance, and as these become more numerous 
 and regular so do the dentinal tubes become less so. These 
 larger tubes contain blood-vessels, and red blood circulates 
 through them during the life of the tooth. 
 
 We may suppose that tha nutrition of the dentine maybe 
 
 Fig. 45 Q). 
 
 provided for either by protoplasm carried for a long distance 
 from the pulp by the dentinal tubes, or by blood circulating 
 through the larger vascular channels, but that both are not 
 required, and so do not exist together. 
 
 And whilst the teeth of the Manatee, the Tapir, and of 
 
 (^) Tooth of Ostracion. a, Enamel ; h. Capillary channels ; c, Axial 
 pulp chamber. 
 
THE DENTAL TISSUES. 87 
 
 the Flounder teach that hard dentine and vaso-dentine are 
 not very dissimihir in their nature, and tliat the one passes by 
 imperceptible gradations into the other, the dentine at the 
 base of the Flounder's tooth provides us with an example of 
 typical vaso-dentine : that is to say, dentine in which the 
 dentinal tubes are quite absent, having had their place 
 taken by a complete system of vascular channels. 
 
 The teeth of the Ostracion (Fig. 45), or of the Hake 
 (Figs. 46 and ^^), afford good examples of this form of tissue. 
 
 Fig. 46 ('), 
 
 '^;~\ 
 
 ■{ 
 
 o/t-^^.---:; 
 
 ■^ ' 
 
 r^^^f.^ __. 
 
 C]i 
 
 The matrix is solid, so far as penetration by fine tubes 
 goes, but it contains a system of larger canals which carry 
 only blood, and no pulp tissue, out to near the surface of 
 the dentine, where they form a plexus. 
 
 I have not been able to satisfy myself of the existence of 
 any definite structure in the matrix ; sometimes it looks 
 granular, and sometimes has a finely reticulated look, re- 
 calling the appearances described by Bodecker in human 
 dentine. (See page 68.) 
 
 Q) Section of Dentine from a freshly caught Hake (Merlucius). d. 
 Dentine matrix ; cp, Capillary blood-vessels hanging out from its edge, 
 containing here and there abundant blood-corpuscles. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 The an-angement of the vascular canals is regular and 
 striking, reminding one of the appeai'ance of the capillary 
 network in an injected intestinal villus. In fact, an intes- 
 tinal villus petrified, whilst the capillary network remained 
 pervious and carried red blood circulating thi-ough it, would 
 form no bad representation of a conical vaso-dentine tooth. 
 
 For these canals do actually contain capillaries, and blood 
 actively circulates through them ; a section cut from the 
 fresh, brilliantly red tooth of a Hake often shows the coats 
 of the capillary hanging out from the edge, and the canals 
 full of blood-corpuscles {Fig. 46). 
 
 In all vaso-dentine teeth with which I am acquainted the 
 pulp chamber is of simple form, the pulp coated by a distinct 
 layer of odontoblasts, and no pulp-tissue other than the 
 capillaries passing out into the dentine, so that each capil- 
 lary fits and wholly fills its channel in the dentine. 
 
 Vaso-dentine is less dense and hard than ordinary dentine, 
 and consequently generally gets protection by a harder 
 tissue when exposed to hard work. 
 
 The teeth of the Hake, used simply for piercing and 
 catching fish, are merely tipped with enamel (Fig. 86) ; 
 those of Ostracion, put to severer work, are coated with 
 enamel, while the teeth of the Wrasse (Labrus), which are 
 composed of ordinary dentine are, though very hard worked, 
 unprotected by enamel. 
 
 Osteo-dentine. — This is a tissue far more sharply marked 
 off from hard dentine, plici-dentine and vaso-dentine, than 
 these are from one another, and approaches closely to bone, 
 from which it has few points of essential diff"erence. 
 
 The distinction can hardly be fully emphasized until the 
 development of dentine has been described, but it may be 
 mentioned that it is not developed on the surftice of the pulp, 
 from an odontoblast layer, but within its whole substance. 
 Consequently in a completed osteo-dentine tooth there is no 
 single simple pulp, which can be withdrawn from the 
 
THE DENTAL TISSUES. 
 
 89 
 
 tooth, but pulp and calcified tissue are quite inextricably 
 mixed up. 
 
 And though there are numerous large channels, often 
 much larger than those of vaso-dentine, they are less regular, 
 
 Fi;;. 47 ('). 
 
 do not in their arrangement suggest the idea of capillary 
 loops, and in a fresh tooth contain masses of pulp-structure 
 as Avell as blood-vessels. 
 
 The Pike's tooth aftbrds a good example of osteo-dentine. 
 Its surface is formed of a layer of fine tubed tissue, almost 
 
 (1) Tooth of Common Pike. 
 inner mass of osteo-dentiue. 
 
 a, Outer la3'er of fine tubed dentine ; b, 
 
A MANUAL OF DENTAL ANATOMY. 
 
 like ordinary dentine, but this soon gives place to a coarsely 
 channeled tissue, containing elongated spaces filled with pulp, 
 from which canaliculi, like those of a Ijone lacuna, branch 
 off in all directions, but do not run far. 
 
 Veiy many sharks have teeth composed of osteo-dentine, 
 
 Fig. 4S (i). 
 
 witli an outer dense layer : the tooth of Lamna here figured 
 shows a central core of osteo-dentine, which constitutes the 
 bulk of the tooth ; external to this a somewhat thin layer of 
 hard dentine, in which all the dentinal tubes run out at 
 right angles to the surface, but are derived from the channels 
 of the osteo-dentine and not from any single pnlp chamber ; 
 Avhile the outermost layer, which is clear and structureless, 
 
 {') Tooth of a species of Lamna, consisting of a central mass of vaso- 
 dentine, passing towards its surface into a fine-tubed unvascular dentine. 
 The clear structureless layer on the surface may probably be regarded as 
 enamel. 
 
THE DENTAL TISSUES. 91 
 
 may be merely the outer part of the hard dentine, or may 
 be a thin layer of enamel. It is to be regretted that special 
 names have been given to this layer ; it is sometimes called 
 vitro-dentine, sometimes gauoin or fish-enamel ; but there is 
 no reason why it should have a special name at all. The 
 similarity of the channels of pulp in osteo-dentine to Haver- 
 sian canals in bone is very close ; in fact, -when teeth con- 
 sisting 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 difficidty is intensified by the fact that the bone of 
 many fishes lacks lacunse, and is almost exactly like dentine. 
 
 Osteo-dentine was defined by Professor Owen as dentine 
 in which the matrix was arranged in concentric rings around 
 the vascular canals, and in which lacunse similar to those of 
 bone were found. 
 
 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 made of osteo-den- 
 tine ; and so they cannot be made use of for purposes of 
 definition, although lacunse and lamination of the matrix 
 axe far moi*e often present in osteo-dentine than in the other 
 varieties of dentinal structure. 
 
 The varieties of dentine may be grouped thus : — 
 
 (A.) Dentines developed upon the surface of a pulp, by 
 calcification of a specialised layer of odontoblast 
 cells. 
 
 (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 foldings in 
 
92 A MANUAL OF DENTAL ANATOMY. 
 
 of its walls. Example — Lepidosteus, 
 Labyrinthodon. 
 (iii.) Vaso-dentine, dentinal tubes few or 
 absent, but capillary channels with 
 blood circulating through them abim- 
 dant. Example — Hake. 
 (B.) Dentines developed by calcifications shooting- 
 through the interior of a pulp, not by calcifi- 
 cation of a specialised surface layer of cells, 
 (iv.) Osteo-dentine ; with no true dentinal 
 tubes, but minvxte tvibes analogous to 
 bone canaliculi, and large irregular 
 channels containing pulp-tissue (not 
 blood-vessels only). Example — Tooth 
 of Pike. 
 
 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 from 
 an odontoblast layer 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 pulp 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 secon- 
 dary dentine, produced in a pulp which ordinarily forms 
 hard dentine, may partake of the character of vaso- or of 
 osteo-dentine. 
 
 Thus the pulp of a sperm whale's tooth becomes oblite- 
 rated by a development of secondary dentine, which some- 
 times forms irregular masses loose in the pulp chamber, and 
 sometimes is adherent to and continuous with the dentine 
 
THE DENTAL TISSUES. 
 
 previously formed. The structure of these masses is very 
 confused. Tubes, of about the same diameter as dentinal 
 tubes, abound ; but thev are often arranged in tufts or in 
 
 bundles, and without any apparent reference to any com- 
 mon points of radiation. Irregular spaces, partaking of the 
 character of interglobiilar spaces or of bone lacunte, 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 much contracted 
 in size, and is also very frequently formed as a i^rotection to 
 the pulp when threatened by the approach of dental caries, 
 or by the thinning of the walls of the pulp cavity through 
 excessive wear. The accompanying figure, representing one 
 of the cornua of the pulp chamber from a molar tooth 
 affected by caries, is a good example of secondary dentine. 
 
 (') Section of a mass of secondary dentine from the tooth of a sperm 
 whale. 
 
94 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 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. Moi'e often. 
 
 Fig. 50 {}). 
 
 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 secondaiy dentine, 
 the tubular structure again asserts itself more strongly : this 
 is well seen in the specimen figured. 
 
 (') Secondary dentine filling up one of the cornua of the puli^ cavity. 
 From a human molar affected by caries. 
 
TEE DENTAL TISSUES. 
 
 95 
 
 CEJIEXTUM. 
 
 The cement forms a coating of variable thickness over 
 the roots of the teeth, sometimes, when the several roots are 
 very close to another, or the cement is thickened by disease, 
 uniting the several roots into one. 
 
 The cement is ordinarily said to be absent from the 
 crowns of the teeth of man, the carnivora, ifec, and to com- 
 mence by a thin edge just at the neck of the tooth, over- 
 lapping the enamel to a slight extent ; it is, in the healthy 
 state, thickest in the interspaces between the roots of molar 
 or bicuspid teeth : it is, however, often thickened at the end 
 
 Fig. 51 ('). 
 
 
 of a root by a dental exostosis. In compound teeth, the 
 cementum forms the connecting substance between the den- 
 ticles (see the figures of the tooth of the Capybara, the 
 Elephant, &c.), and, before the tooth has been subject to 
 wear, foi'ms a complete investment over the top of the 
 crown. The cementum also covers the croAras of the com- 
 (') Thick laminated cementum from tlie root of a human tooth. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 plex-pattemed crowns of the teeth of ruminants ; and, in 
 my opinion, is present in a nidimentary condition upon the 
 teeth of man, &c., as Nasmyth's membrane. The cementum 
 is the most external of the dental tissues : a fact which ne- 
 cessarily follows from its being derived more or less directly 
 from the tooth follicle. 
 
 Both physically and chemically, and also in respect of the 
 manner of its development, the cementum is closely allied 
 to bone. It consists of a calcified matrix or Viasal substance, 
 to a slight extent laminated, and lacuna?. Vascular canals 
 corresponding to the Haversian canals of bone, are met with, 
 but it is only in thick cementum that they exist ; and, in 
 man, perhaps in exostosis more often than in the thick 
 healthy tissue. 
 
 The matrix is a calcified substance, whicli, when boiled 
 
 Fig. 52 ( ). 
 
 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 granvdar, or interspersed with small globules. 
 
 The lacun?e of cementum share with those of bone the 
 following characters : in dried sections they are irregular 
 
 (') Lacuna of cementum which communicates with the terminations of 
 the dentinal tubes. 
 
THE DENTAL TISSUES. 97 
 
 cavities, elongated in the direction of the lamellse 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 lamellse (see 
 Fig. 51), and, again, in cementum, are more abundantly- 
 directed towards the exterior of the root than towards the 
 dentine. The lacunae of cementum 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 cement of Cetacean teeth are 
 very remarkable. 
 
 Many of the lacunae in cementum are connected, by means 
 of their canaliculi, with the terminations of the dentinal 
 tubes (Fig. 52) ; they, by the same means, freely intercom- 
 municate 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. 
 
 Hei-e and there lacunae are to be found which are fur- 
 nished with comparatively short processes, and are contained 
 within well-defined 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 cement of the teeth of the horse (they are 
 specially abundant in the teeth of the solidung\ilata). By 
 cautious disintegration of the cementum in acids these en- 
 capsuled lacunae may be isolated ; the immediate walls of 
 the lacunae and canaliculi, just as in bone, being composed 
 of a material which has more power of resisting chemical 
 re-agents than the rest of the matrix. 
 
 The enca:psiiled lacunae are to be regarded as individual 
 osteoblasts, or nests of osteoblasts, with a common connective 
 
 H 
 
A MANUAL OF DENTAL ANATOMY. 
 
 tissue iuvestment, which have to some extent preserved their 
 individuality during calcification. 
 
 In the fresh condition it appears probable that the lacunas 
 are filled up by soft matrix, which shrinks up, and so leaves 
 them as cavities in dined sections. It can hardly as yet be 
 said that the question of the contents of lacunse has been 
 finally settled, though the researches of Bcidecker and Heitz- 
 mann have gone far towards doing so. 
 
 According to them each lacuna contains a protoplasmic 
 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. 
 
 The cement corpuscles communicate freely with one another 
 by offshoots, those of large size occupying the conspicuously 
 visible canaliculi of the lacuna;, whilst the finer offshoots are 
 believed by them to form a delicate network through the 
 whole basis substance or matrix. The cement corpuscles near 
 to the external surface give off numerous offshoots which 
 communicate with protoplasmic bodies in the periosteum. 
 By this means the cementum can remain alive even when 
 the pulp of the tooth is dead, and thus the tooth is in no 
 way a mere foreign body, dead and inert. 
 
 Like bone, cementum is also sometimes found to contain 
 Shai'pey's fibres ; that is to say, rods running through it at 
 right angles to its own lamination, and, as it were, perforat- 
 ing it. These are probably calcified bundles of connective 
 tissue. 
 
 Where the cementum is very thin, as, for instance, where 
 it commences at the neck of a human tooth, it is to all 
 appearance structureless, and does not contain any lacunse, 
 and therefore no protoplasmic bodies : nevertheless lacunae 
 may be sometimes found in thin cementum, as, for example. 
 
THE DENTAL TISSUES. 
 
 in that thiu la3-er which invests the front of the enamel of 
 the rodent-like tooth of a wombat. 
 
 The cementum at the neck is also devoid of lamellae ; it 
 appears to be built up by direct ossification of osteoblasts, 
 the prismatic 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 a fine protoplasmic 
 network ci'osses it. Bodecker 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 lacunse ; on the surface it is slightly nodular, and 
 might 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 characterizes dentine at an early 
 stage of its development. 
 
 The cementum is veiy closely, indeed inseparably, con- 
 nected 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 diflicult 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. 
 
 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 hiunan or other mammalian tooth„ 
 the crown of which is not coated by a thick layer of cemen- 
 tum, 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 sui-face of the 
 enamel. When thus isolated it is found to form a continu- 
 
 H 2 
 
100 A MANUAL OF DENTAL ANATOMY. 
 
 0118 transparent sheet, upon -which, by staining with nitrate 
 of silver, a reticulated pattern may be brought out, as 
 though it Avere made up of epithelial cells. The inner 
 surface of Nasmyth's membrane is, however, pitted for the 
 reception of the ends of the enamel prisms, which may have 
 something to do with this reticulate appearance. It is 
 exceedingly thin, Kolliker attributing to it a thickness of 
 only one twenty-thousandth of an inch, but, nevertheless, 
 it is very indestructible, resisting the action of strong nitric 
 or hydrochloric acid, and only swelling slightly when boiled 
 in caustic potash. Notwithstanding, however, that it resists 
 the action of chemicals, it is not so hard as the enamel, and 
 becomes worn off tolerably speedily, so that to see it well a 
 young and umvorn tooth should be selected. 
 
 Observations upon the presence or absence of Nasmyth's 
 
 Fig. 53 ('V 
 
 membrane in fish and reptiles are very much needed ; my 
 own recent investigations upon the development of the 
 
 (•) From a section of a bicuspid tooth in which the cemcntum, c, is con- 
 tinued over the outside of the enamel, a ; the dentine is indicated by the 
 letter h. 
 
THE DENTAL TISSUES. 
 
 teeth in these classes make me doubt whether the d priori 
 conclusion of Waldeyer, who believes that the cuticle will 
 be found on all teeth, is not based upon an interpretation of 
 its nature which is incon-ect. 
 
 The observation of Professor Huxley, who believed that 
 he found it upon the teeth of the frog, &c., may be suscep- 
 tible of another explanation, to which I shall have to I'ecur, 
 merely premising here that its presence is only certain in 
 Primates, Carnivora, and Insectivora. 
 
 The singular power of resistance to re-agents 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 lacunse, &c. 
 
 In my father's opinion (Dental Surgery, 1859) it is to be 
 
 
 Fir.. 54 {'). 
 
 "..^^ 
 
 '^^"'^^^^^^M'T^iil 
 
 
 ^M'Mi 
 
 \ 
 
 /^.f ?;' ' / 
 
 \/ 
 
 ,J: >'':■ ■ ' "^ , , 
 
 regarded as a thin covering of cementum, and I have given 
 additional evidence in support of the view in a paper 
 refen-ed to abeady in the list of works which heads this 
 chapter. 
 
 It now and then happens that the cementum upon a 
 more or less abnormal tooth, instead of ceasing at the neck, 
 (') Encai>suled lacuna occupying a pit in the enamel. 
 
102 A MANUAL OF DENTAL ANATOMY. 
 
 is continued up over the exterior of the enamel. This has 
 occurred less uncommonly than is generally imagined, and 
 the accompanying figure represents a portion of the crown 
 of such a tooth. 
 
 If the section be made of the grinding surfaces of such 
 teeth as present rather deep fissures in these situations, 
 well marked and unmistakeable lacunal cells, or encapsuled 
 lacuna), will be met with with considerable frequency. Now 
 
 Fig. .55 ('). 
 
 cl . 
 
 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 
 
 {}) Nasmyth's membrane, set free by the partial solution of the enamel. 
 a. Nasmyth's membrane, h. Dentine, d. Mass occupying a pit in the 
 enamel, e. Enamel. «'. Torn end of Nasmyth's membrane. 
 
THE DENTAL TISSUES. 
 
 colour. The occurrence of lacunse in these situations is far 
 from rare : my father's collection contains more than a dozen 
 good examples of them in these positions. 
 
 Nasmyth's membrane, thin though it is over the exterior 
 of the 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. 55). 
 
 In these places, then, where the encapsuled lacunse ai'e 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 lacuna; in Nasmyth's membrane 
 is due to the fact that it is not thick enough to contain them; 
 just as the thinnest layers of unquestionable cementum also 
 are without lacunse. 
 
 In sections of an unworn bicuspid which was treated with 
 acid subsequently to its having been ground thin and placed 
 upon the slide, I have several times been fortunate enough 
 to get a view of the membrane in situ ; it then appears to 
 be continuous with an exterior layer of cementum, which 
 becomes a little discoloured by the acid employed to detach 
 Nasmyth's membrane from the enamel. I am therefore 
 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 
 Hcrbivora ] and I am very glad to learn from my friend 
 Dr. Magitot, who has made many as yet unpublished re- 
 searches upon this subject, that he entirely concurs in this 
 view, which has also the support of Professor Wedl. 
 
 The evidence offered that Nasmyth's membrane is cementum, 
 although strong-, does not amount to absohite proof ; it is therefore 
 desirable to briefly recapitulate the other explanations of its nature 
 which have been offered. 
 
 Xasmyth, who fii-st called attention to its existence, regarded it 
 as •' persistent dental capsule ; " a view of its nature not very ma- 
 terially differing from that advocated in these pages. 
 
104 A MANUAL OF DENTAL ANATOMY. 
 
 Professor Huxley described ib as being identical with the mein> 
 brana performativa ; that is to say, with a membrane which covered 
 the dentine papilla prior to the occun-ence 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 ob- 
 jections to Professor Huxley's theory of the development of the- 
 teeth, that they cannot profitably be detailed in this place ; it will 
 suffice to say, that evidence and the weight of authority alike point. 
 to there being no such true membrane as this membrana performa- 
 tiva in the place in question. 
 
 Waldeyer holds that it {\.c., Nasmyth's membrane) is a product of 
 a part of the enamel organ. After the completion of the formation 
 of the enamel he believes that the cells of the external epithelium 
 of the enamel organ become applied to the surface of the enamel 
 and there corn ijird ; in this way he accounts for its resistance ta 
 reagents, and for its peculiar smell when it is burnt. 
 
 Its extreme thinness, so far as it goes, is an objection to this 
 supposition : a more weighty argument against it is the absence of 
 analogy for such a peculiar change, by which one portion of the 
 same organ is calcified, and the rest cornified ; and again, what be- 
 comes of these cells in those teeth in which cementum is deposited 
 in bulk over the surface of the enamel .' According to the statement. 
 of Dr. 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 confinned, is fatal to Waldeyer's 
 explanation. And Dr. Magitot, in his most recent paper on the 
 subject (Joiu-nal de Tanatomie, &c., 1881), gives his adherence to 
 the view that it is cementum. 
 
 KoUiker, 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 of forming the fibrous enamel was complete ; a 
 sort of varnish over the surface, as it were. 
 
 This would not account for the occui-rence of lacunaj in it. 
 
 The Tooth Pulp. 
 
 The Tooth Pulp, occupying the central chamber, or pulp 
 cavity, is the formative organ of the tooth, and consequently 
 varies to some extent in its anatomical character according 
 to its age. As well as being what remains of a formative 
 organ, it is the vascular and nervous source of supply 
 whence the dentine mainly derives its vitality. 
 
THE DENTAL TISSUES. 
 
 The pulp may be described as being made up of a mucoid, 
 gelatinous matrix, containing cells in abundance, which are 
 especially numerous near to its periphery. In it some 
 fibrous connective tissue is discoverable, though this is not 
 abundant until the period of degeneration has set in. Nerves 
 and vessels also ramify abundantly in it. 
 
 The cellular elements of the pvdp 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 membrana 
 ehoris, because it usually adheres more strongly to the 
 dentine than to the rest of the pulp, and is therefore often 
 left behind upon the dentine when the pulp is torn away, 
 consists of a single row of large elongated cells, of darkish 
 granular appearance, with a large and conspicuous nucleus 
 near to the end farthest from the dentine. 
 
 The sharp contours which the odontoblasts possess iu 
 pulps which have been acted on by chromic acid, alcohol, 
 or even water, are absent in the perfectly fresh and unaltered 
 condition, and it is believed that they have no special in- 
 vesting membrane. They are furnished with three sets of 
 processes. The dentinal process (which is equivalent to the 
 dentinal fibre) enters the canal in the dentine, and the in- 
 dividual odontoblast may be furnished with several dentinal 
 processes. By means of lateral processes the cells communi- 
 cate with those on either side of them, and by means of 
 their pidp processes with cells lying more deeply; these 
 deeper cells again are to some extent intermediate in size 
 between the odontoblasts and the internal cells of the pulp. 
 The membrana eboris covers the surface of the pulp like 
 an epithelium. The odontoblasts vary much in form at 
 different periods ; in the youngest puljis, prior to the forma- 
 tion of dentine, they are roundish, or rather pyriform ; 
 
106 A MANUAL OF DENTAL ANATOMY. 
 
 during the period of their greatest functional activity the 
 end directed towards the dentine is squai'ish, though tapering 
 to a slight extent into the dentinal process ; while in old 
 age they become comparatively inconspicxious, and assume 
 a rounded or ovoid shape. 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 pulp, finally form a capillary plexus immediately 
 beneath the cells of the membrana eboris. 
 
 No lymphatics are known to occur in the tooth pulp. 
 
 The nerves enter usually by one largish trunk and three 
 or four minute ones : after pursuing a parallel course, and 
 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 Raschkow 
 and many subsequent writers. 
 
 But here our exact knowledge ends, for the nature of the 
 terminations of the nerve fibres in the pulp is not with 
 ■certainty known : the primitive fibrils, which are extra- 
 ordinarily abiindant near to the surface of the pulp, often 
 form meshes, but this does not appear to be their real 
 termination. 
 
 Boll, as has been mentioned at a previous page, investi- 
 gated 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-meduUated nerve fibres, which he suc- 
 ceeded in tracing into continuity with the larger meduUated 
 fibres, may be discerned near to the surface of the pulp. 
 The ultimate destination of these nerve fibres is un- 
 certain ; but he has seen them passing through the mem- 
 
THE DENTAL TISSUES. T07 
 
 brana eboris, and taking a direction parallel to that of the 
 dentinal fibrils in such numbers that he infers that they 
 have been pulled out from the canals of the dentine. Still, 
 whatever may be the probabilities of the case, he has not 
 seen a nerve fibre definitely to pass into a dentinal canal, 
 nor has any other observer been more fortunate. 
 
 Boll's observations have not however been fully confirmed 
 by any subsequent worker in the field, nor have they been 
 definitely controverted until Magitot recently stated that he 
 had fully satisfied himself that tlie 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. 32.) 
 
 If this view of their relation to the nerves be correct the 
 sensitiveness 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. 
 
 It has already been mentioned that the pulp undergoes 
 alterations in advanced age, its diminution in size by its 
 progressive calcification and the addition thus made to the 
 walls of the pulp cavity being the most conspicuous change 
 which occurs. In pulps w^hich have undergone a little 
 further degeneration, the odontoblast layer becomes atro- 
 phied; fibrillar connective tissue becomes more abundant, 
 coincidently with, the diminution in the quantity of the cel- 
 lular 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 aveolar abscess ; but a tooth in wdiich the pulp 
 has undergone senile atrophy is seldom fast in its socket. 
 
 The pulps of the teeth of some animals become eventually 
 
108 A MANUAL OF DENTAL ANATOMY. 
 
 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 tliroughout the life of the 
 creature retain their j)ulp in an active and vascular condition. 
 
 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 hardness is in part due to the abundant tendinous fasci- 
 culi which it itself contains, 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 
 very 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 laminae of tesselated cells, much 
 flattened near to the surface ; but cylindrical cells form the 
 deepest layer of the epithelium, the rete malpighi. 
 
 Small round aggi-egations of pavement epithelium are 
 met with at a little depth, or even bedded in the smface ; 
 these, the "glands" of Sei-res, have no known significance. 
 In the neighbourhood of developing tooth-sacs ejiithelial 
 aggregations of similar appearance are to be met with, and 
 in such spots are remains of the neck of the enamel organ 
 (cf page 137), which has undergone this curious change 
 subsequently to the completion of its original function. 
 The gums are rich in vessels, but remarkably scantily sup- 
 plied with nerves. 
 
THE DENTAL TISSUES. 
 
 At the necks of the teeth tlie gum becomes continuous 
 with the periosteum of the internal surface of the alveoli, 
 into which it passes without any line of demarcation. 
 
 The Alveolo-Dextal Membrane. 
 
 The Alveolo-dental 'Periosteum, or Eoot 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 peri- 
 osteum of the alveolar process, and near to the apex of the 
 root. The general direction of the fibres is transverse ; that 
 is to say, they n;n across from the alveolus to the cemen- 
 tum, 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 dis- 
 tinguished ; and the study of its development 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 membrane. On its inner 
 aspect, where it becomes continuous with the cementum, it 
 consists of a fine network of interlacing bands, many of 
 which lose themselves 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 
 
110 A MANUAL OF DENTAL ANATOMY. 
 
 insensibly into the bands of the fine network of the inner 
 part, and there is no break of continuity whatever. 
 
 At tlie surface of the cementum it is more richly cellular, 
 
 Fig. 56 ('). 
 
 >*w.ftV 
 
 and here occur abundantly large soft nucleated plasm 
 masses, which are the osteoblasts concerned in making 
 cementum, and which by their offshoots communicate with 
 plasm masses imprisoned within the cementum. 
 
 Q) Portion of tlie side of the root of a tooth, the gum and alveolar 
 dental membrane, and the edge of the bone of the alveolus. 
 
 A band of fibres is seen passing over the surface of the alveolus and 
 dividing, some to pass upwards into the gum, others to pass more directly 
 across to the cementum. Numerous orifices of vessels cut across trans- 
 versely are seen between the tooth and the bone. 
 
THE DENTAL TISSUES. Ill 
 
 I have never seen the fibres, whether in longitudiual or 
 in transverse sections, pass straight in the shortest possible 
 line from the bone to the cementum, but they invariably 
 pursue an oblique course, which probably serves to allow for 
 slight mobility of the tooth without the fibres being stretched 
 or torn. 
 
 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 dental 
 nerves running to the dental pulps ; other filaments come 
 from the inter-alveolar canals (canals in the bone, contain- 
 ing 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 how it comes about that their vascular and nervous 
 supplies are so nearly identical. 
 
 The human tooth is, accepting as correct the researches 
 of Bodecker, which appear in every way deserving of credence, 
 connected with the living organism veiy intimately, even 
 though its special tissues are extra-vascular. 
 
 For blood vessels and nerves enter the tooth pulp in 
 abundance ; the dentine is organically connected with the 
 pulp by the dentinal fibrils ; these are connected with the 
 soft cement corpuscles, which again are brought by their 
 processes into intimate relation with similar bodies in the 
 highly vascular periosteum. 
 
 So that between pulp inside, and periosteum outside, there 
 is a continuous chain of living plasm. 
 
112 A MANUAL OF DENTAL ANATOMY. 
 
 K(»LLIKER. Gewebelehre. 
 
 Manual of Histology, annotated by Messrs. Busk and 
 Huxley, 1853. 
 Waldeykr. Strieker's Histology. 1870. 
 Prey. Manual of Histology. 1874. 
 Owen. Odontography. 
 
 CzERMAK. Zeitschrift f. Wiss. Zoologie. ISoO. 
 Neumann. Zur Kentniss der Xoi-malen Zahngeweber. 1 8o3. 
 Boll. Untersuchungen iiber die Zahnpulpa. Archiv. f. Mikros. 
 
 Anatom. 18(;8. 
 Klein. Atlas of Histology. 1 880. 
 Salter. Dental Pathology. 1874. 
 Tomes, J. Lectures on Dental Physiology and Surgery. 1848. 
 
 On the Dental Tissues of Eodentia and Marsupialia. 
 
 Philos. Transac. 1840, 1850. 
 On the Presence of Soft Fibrils in Dentine. Philos. 
 Transac. 1853. 
 Tomes, Charles S. On Vascular Dentine. Phil. Trans. 1 878. 
 
 On the Implantation of Teeth. Proc. Odontol. 
 
 Soc. 1874—1876. 
 On Nasmyth's Membrane. Quart. Jour. Micros. 
 Science, 1872. 
 Magitot et Le«ros. Journal de I'anatomie de M. Ch. Robin. 
 
 1881, 
 Petzius. Mikrosk. Undersok, &c. Stockholm. 18.S7, and Transla- 
 tion in Nasmyth on the Teeth, 183II. 
 Kasmyth. On the Teeth. 1 839. 
 
 Hertwig. Ueber der Bau der Placoidschuppen Jenaische Zeit- 
 schrift, B. viii. 
 Von Boas. Zahne der Scaroiden, Zeits. f. Wiss. Zoologie, B. xxxii. 
 BoDECKER. Dental Cosmos. 1878. 
 
 Lankester. Ray. On the Teeth of Micropteron. Quart. Journal 
 Micros. Science. 1857. 
 
CHAPTER IV. 
 
 THE DEVELOPMENT OP THE TEETH. 
 
 The development of the teeth is a process which, while 
 subject to modifications in the different groups of verte- 
 brates, retains nevertheless in all certain essential charac- 
 ters, 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. All, or a part only, of 
 the soft stmctures making up a tooth germ, become con- 
 verted into the dental tissues by a deposition of salts of lime 
 within their own substance, so that an actual conversion of 
 at least some portions of the tooth germ into tooth takes 
 place. The tooth is not secreted or excreted by the tooth 
 germ, but an actual metamorphosis of the latter takes 
 place. The details of this conversion can be better dis- 
 cussed at a later i)age ; 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 tool It 
 germ, and that we are hence accustomed 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 
 descriptions await confirmation. 
 
 In many anatomical works which the student may have 
 occasion to consult, the process of tooth development may 
 
114 A MANUAL OF DENTAL ANATOMY. 
 
 still be found to be divided into periods, under the names 
 of " papillary," " follicular," "and " eruptive " stages. 
 
 These stages are based upon a false conception, upon 
 theories now known to be incon-ect, and may advantageously 
 be 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 Kulliker, 
 Thiersch, and Waldeyer ; in the case of reptiles and fishes,, 
 upon those of Huxley and Santi Sirena, and upon Hertwig's 
 and my own), will be found to conflict with the accounts 
 published by a deservedly great authority, Professor Owen. 
 I cannot reconcile these discrepancies, except upon the as- 
 sumption that modern methods of research have disclosed 
 facts heretofore not demonstrable ; yet twenty years ago. 
 Professor Huxley demonstrated in a remarkable paper the 
 incorrectness of certain of the theories then promulgated. 
 Of the general accuracy of the following description I am 
 however fully satisfied, and most of the facts may be easily 
 verified by any one desirous of so doing. 
 
 True tooth gei-ms are never formed quite upon the surface ('),. 
 but are always situated at a little distance beneath it, lying in 
 some creatures at a considerable depth. 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, the latter 
 from the more deeply lying parts of the mucous membrane. 
 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 genus never develop any addi- 
 tional parts. The existence of an enamel organ in an early 
 stage is therefore perfectly independent of any subsequent 
 
 (') The placoid scales of embiyonic sliarks are, however, formed on the 
 surface, and the "germs" covered in by epithelium only (Hertwig). 
 
TEE DEVELOrMEXT OF THE TEETH. 115 
 
 formation of enamel by its own conversiou into a calcified 
 tissue, for I have shown that it is to be found in the germs 
 of teeth which have no enamel ; in fact, in all tooth germs 
 "whatever. 
 
 That part of the tooth germ destined to become dentine 
 is often called the dentine pajDilla, 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 den- 
 tine 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 contemporaneous with, if not antecedent to, that of the 
 dentine germ. The most general account that I am able 
 to give of the process is, that the deeper layer of the oral 
 epithelium sends down into the subjacent tissue a process, 
 the shape and structure of which is, in most animals, dis- 
 tinguishable and characteristic before the dentine gei'm has 
 taken any definite form. This process enlarges at its end, 
 and, as seen in section, becomes divaricated, so that it bears 
 some resemblance to an inverted letter Y ; or it might more 
 truthfully be compared to a bell jar with a handle ; this 
 constitutes the early stage of an enamel germ (see Fig. G-i), 
 while beneath it in the mucous tissue, the dentine gerai 
 assumes its papilliform shape. The details of the process 
 varying in different creatures, I will at once proceed to the 
 description of the development of teeth in the various 
 groups. 
 
 In Elasmobrancli Fishes. — If a transverse section 
 through the jaw of a dog-fish (Scyllium canieula) be ex- 
 amined, we shall find that the forming teeth lie upon the 
 inside of the semi-ossified jaw-bones, the youngest being at 
 the bottom (Fig, 57) ; progi-essing upwards, each tooth is 
 more fully calcified till, on passing over the border of the 
 jaw, we come to those teeth whose period of greatest useful- 
 ness is passed and which arc about to be cast off in the 
 
 I 2 
 
116 A MANUAL OF DENTAL ANATOMY. 
 
 course of that slow rotation of the whole tooth-bearing 
 mucous membrane over the border of the jaw, which is 
 constantly going on. 
 
 In the section figured there are four teeth advanced in 
 calcification, while beneath them are four tooth germs in 
 earlier stages ; of the former two only are fully protruded 
 throixgh 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 pro- 
 tected by a reflexion upwards of the mucous membrane (c in 
 the figure), which serves to protect them during their calci- 
 fication. 
 
 But although this may be termed a fold reflected upwards, it is 
 not, as was supposed by Professor Owen, a free flai^, 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. 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 passing across from the jaw to cover it is well seen in 
 the figure, in which the epithelial layer is represented as broken 
 just at the point (between the third and fourth teeth) where it 
 leaves the jaw to cross over on to the surface of the flap. 
 
 The conditions met with in the Elasmobrauch fishes are 
 peculiarly favourable for the determination of the homolo- 
 gies 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 
 papillye arise blends insensibly with that making up the 
 substance of the thecal fold on the one hand, and on the 
 
 (}) Compare the description of the placoid dermal spine (page 2). 
 
THE DEVELOrMEXT OF THE TEETH. 
 
 117 
 
 other, with that clothing the convexity of the jaw and giving 
 attachment to the teeth. 
 
 No sharp line of demarcation at any time marlcs 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 e;erms are cellular, the cells being large 
 
 Fig. 57 ('). 
 
 z.-^^ 
 
 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. 
 
 (') Trausvei'se section of lowei" jaw of a Dog-fish. a. Oral epithelium. 
 h. Oral epithelium jjassing on to flap. c. Protecting flap of mucous mem- 
 brane (thecal fold), d. Youngest dentine pulp. e. Youngest enamel 
 organ. /. Tooth about to be shed. (j. Calcified crust of jaw. 
 
118 A MANUAL OF DENTAL ANATOMY. 
 
 The dentine germs, and consequently the dentine, are 
 indisputably derived from the connective tissue of the 
 mucous membrane immediately subjacent to the epithe- 
 lium, 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 clear- 
 ness than those other animals in whom the original nature 
 of the process is more or less masked by the introduction of 
 further complexities. 
 
 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.) But although the epithelium is re- 
 flected across on to the thecal fold, it is also continued 
 downwards along the inner side of the developing teeth and 
 tooth germs, giving to each a complete investment, and 
 filling up the whole interval between the tooth germ and the 
 thecal fold. 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 so 
 arranged as to have relation only to the tooth germs ; it is 
 termed '•' enamel organs " because over the tooth germs these 
 epithelial cells assume a marked columnar character, and 
 are very different in appearance from the epithelium else- 
 where. 
 
 The terminal portion of this epithelium, or, in other 
 words, the youngest enamel germ, forms a bell-like cap over 
 
TEE DEVELOPMENT OF THE TEETH. 119 
 
 the eminence of mucous membrane 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 attached extremities, while the rest of 
 its substance is made up of much smaller cells, some of 
 which have inosciilating processes, so that they constitute 
 a sort of finely cellular connective tissue, very difi'erent in 
 appearance from anything met with in mammalian enamel 
 organs. It is sufficiently consistent to keep up the con- 
 tinuity of all the enamel organs, even when displaced in 
 cutting sections, so that the whole might be described as 
 forming one composite enamel organ. The columnar cells 
 already alluded to invest the whole siu"face which is 
 directed towards the forming teeth, but they atrophy some- 
 what in the interspaces of the tooth germs. 
 
 Before proceeding further in the description of the deve- 
 lopment of the tooth germs, it will be well to refer to a 
 somewhat 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 ; 
 hence, as is seen in the figiire, 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 outside 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 
 
120 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 shed and reproduced, so that it is less easy to find them in 
 all stages of their growth ; I believe, however, that they 
 follow a course esseutially similar to that of the teeth. 
 
 Fig. 58 (i). 
 
 It is stated by Gegenbaur 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 Ganoids and Osseous fishes 
 being occupied by conspicuous teeth; and Hertwig has. 
 shown that the dei-mal spines are developed in a manner 
 precisely analogous to that described in the teeth, save that 
 the germs are even less specialised. 
 
 In 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, derived fi-om that 
 of the next older tooth, in the latter each enamel germ often 
 arises independently and, as it were, de novo. At all events, 
 
 (') Section of lower jaw of young Dog-fish, showing the continuity of 
 the dermal sijines of the skin under the jaw, with the teeth which lie 
 above and over its end. 
 
THE DEVELOPMENT OF THE TEETH. 
 
 121 
 
 SO far as my own investigations go, no connection has been 
 traced between the germs of teeth of different ages ; but 
 Heiucke says that in the Pike new enamel organs may be 
 derived from older ones. 
 
 This independent origin of an indefinite number of teeth, 
 having no relation to their predecessors, is only certainly 
 known to occur in the osseous fish : of the development of 
 the teeth of Ganoid fish nothing is known. 
 
 The oral epithelium, which varies much in its thickness 
 and in other characters in difierent 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. 
 
 Fig. 59 (\). 
 
 the cells of the enamel organ remain small and insignificant, 
 as in the mackerel. If, on the other hand, a partial invest- 
 ment of enamel is found upon the perfected tooth, such 
 for instance, as the little enamel tips upon the teeth of the 
 eel (see Fig. 90), then the after-development of the enamel 
 organ is very instructive. 
 
 (') Tooth-igerra of an eel. d. Neck of enamel organ, e. Enamel cells. 
 «. Cap of enamel, h. Cap of dentine, e. Eudiraentary enamel cells 
 opposite to that part of tlie dentine germ where no enamel will be formed. 
 
122 A MANUAL OF DENTAL ANATOMY. 
 
 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 -^]y^ of an inch in 
 length 3 below this the investing caj) of enamel organ does 
 not cease, but it is continued in a sort of rudimentary con- 
 dition. 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 genu, 
 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 euamel 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 chai-acter of the lower 
 portion of that represented in the figure of the tooth germ 
 of the eel. 
 
 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 
 I have examined, that we may at once pass on to the con- 
 sideration of the development of the teeth of reptiles, merety 
 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. 
 
 In Reptiles. — So far as the appearances presented by 
 the individual germs go, there are few difierences worthy 
 of note to be found in the present class by which they are 
 distinguishable from those of either fish or mammals. The 
 enamel organ is derived from the oral epithelium, the den- 
 tine organ from the submucous tissue in a manner very 
 similai* ; nevertheless, there ai'e 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 
 
THE DEVELOPMENT OF THE TEETH. 123 
 
 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 " area of tooth deve- 
 lopment ; " this is bounded on the one side by the bone and 
 
 Fig. go (i). 
 
 A 
 
 teeth which it carries, and on the other by a more or 
 less sharply defined wall of fibrous connective tissue. In 
 the newt, for example (Fig. 60), to the left of the tooth in 
 use are seen four 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 simultaneoiisly new ones 
 are constantly being developed beyond them. In the newt, 
 
 •« 
 (^) Section of upper jaw of Triton crlstatus (newt). To the inner .side 
 of the tooth attached to the bone are tliree younger tooth germs. 
 
124 A MANUAL OF DENTAL ANATOMY. 
 
 the ^ingrowth of the epithelium is obviously the first step 
 apparent; this ingrowth of a process of epithelium takes. 
 place in close relation with the "neck" of an older enamel 
 organ (i.e., the contracted band of epithelium which remains, 
 for some time connecting the new enamel organ with the 
 epithelium whence it was derived). New enamel organs are 
 therefore not derived directly from the epithelium of the 
 surface, but from the necks of the enamel organs of their 
 predecessors. 
 
 In the newt, 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 ai'rangement so disclosed may be 
 advantageously compared with that seen in the dog-fish (see 
 Fig. 57). 
 
 The tooth sac of the newt is a good example of the sim- 
 plest form of tooth sac, consisting solely of an enamel organ 
 and a dentine germ, without any especial investment. The 
 *' sac" 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 differing 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 ne 
 lip, passing altogether inside the upper jaAv and its svip- 
 ported teeth, and so confining the area of tooth develop- 
 ment within very narrow limits. Consequently I have been 
 unable to satisfy myself whether the new tooth germs, or 
 rather their enamel organs, are derived from those of their 
 predecessoi's, or spring up de novo — analogy would indicate 
 the former, but appearances tend towards the latter sup- 
 position. 
 
TEE DEVELOVMENT OF THE TEETH. 125 
 
 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 at first 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 enatnel cells are developed upon the side of the 
 dentine germ to a much lower point than in the newt. The 
 
 Fig. 61 (•). 
 
 germs also acquire an adventitious capsule, mainly derived 
 from the condensation of the connective tissue around 
 
 {}) Transverse section of the lower jaw of common English Snake, e. 
 Inward dipping process of epithelium. /. Oral epithelium. 1, 2, 3, &c. 
 Tooth germs pf various ages. 8. Tooth in place, cut somewhat obliquely, 
 so that its tip aj^parently falls short of its surface, and does not project 
 above the mucous membrane. 
 
126 A MANUAL OF DENTAL ANATOMY. 
 
 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. 
 
 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 I do 
 not know of any data by which the prolmble durability of 
 an individual tooth could be estimated, the large number 
 of teeth which are developing in reserve, all destined to 
 succeed to the same spot upon the jaws, would indicate that 
 it is short. 
 
 I have 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, more- 
 over, contained in a sort of general investment of connective 
 tissue ; a species of bag to keep them from displacement 
 during the expansion of the mouth. 
 
 The inward growing process of oral epithelium enters this 
 case 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 
 dentine papilla may be observed, in no essential point differ- 
 ing from that which is to be seen in other animals. 
 
 That the derivation of each enamel organ is from a part of 
 that of its predecessor 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 
 
THE DEVELOPMENT OF THE TEETH. 
 
 127 
 
 alteration iu position takes place ; instead of preserving a 
 vertical position, they become recumbent, so that the form- 
 ing tooth lies moi-e 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 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. 
 
 Q) Developing teeth of a Snake. /. Oral epithelium, c. Neck of the 
 enamel organs, b. Dentine pulp. o. Enamel cells, d. Dentine. 1, 2. 
 Very young germs. 3, 4. Older germs. 
 
128 A MANUAL OF DENTAL ANATOMY. 
 
 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 exterior layer 
 upon the teeth of snakes is true enamel, and not, as has 
 been usually supposed, cementum. 
 
 Many points in the development of the teeth of reptiles I 
 have passed over very briefly for the want of space ; a more 
 full account of my observations will be found in the Philo- 
 sophical Transactions for 1875. 
 
 In Mammalia the earliest changes which will ultimately 
 result in the formation of a tooth are traceable at a very 
 
 Fig. 63 ('). 
 
 early period ; before the commencement of ossification, the 
 lower jaw consisting solely of Meckel's cartilage imbedded in 
 embryonic tissue, and the lateral processes which become 
 the upper maxillary bones having but just reached as far 
 as the median process which constitutes the intermaxillary 
 Iwne. That is to say, about the fortieth or forty-fifth day 
 (in the human subject), in the situation corresponding to 
 the future alveolar border, there appears a slight rounded 
 depression, extending the whole length of the jaw, it and 
 
 {}) Embryo at end of fifth week after Carpenter. 1, 2. First two 
 visceral arches, a. Saperior maxillary process, t. Tougne. b. Eye. 
 c. Lateral nasofrontal process, nf. Nasofrontal process. 
 
THE DEVELOPMENT OF THE TEETH. 129 
 
 its elevated borders being formed by tin increase in the 
 thickness of the layer of epithelial cells ; while in trans- 
 verse sections the proliferation of epithelial cells is found to 
 have been even more energetic in a direction downwards 
 into the substance of the jaw than it is upwards, so that a 
 cul-de-sac of epithelium dips into the embryonic sub-mucous 
 tissue/ 
 
 In a certain sense, then, there is a dental groove, hwi it 
 is not the same thing as that described as such in the text- 
 books, and it therefore better to abstain from applying that 
 or any other name to the shallow fuiTow of which we are 
 now speaking, which is almost filled up with spherical or 
 flattened cells, the deepest layer being, however, columnar 
 cells. From the bottom, or the side near the bottom of the 
 depression, an inflection of epithelial cells takes its origin, 
 which does not dip downwards vertically, but inclines in- 
 wards. This secondary narrow inflection of epithelium, 
 which in section closely resembles a tubular gland, consti- 
 tutes the rudiment of the future enamel organ ; a prolife- 
 ration of the cells at its deepest extremity speedily takes 
 place, so that it expands, attaining somewhat the form of a 
 Florence flask (Fig. 64). It should, however, be noted, that 
 while the inflection of epithelium takes place around the 
 entire circumference of the jaw, so that that which appears 
 in sections like a tubular gland is really a continuous sheet 
 or lamina of epithelium, the dilatations of its extremity, 
 which I have compared to a Florence flask, occur only at the 
 several points where teeth will nltimately be developed. 
 
 The cells upon the periphery are columnar, polygonal 
 cells occupying the central area of the enlargement. Very 
 
 (') The epithelium having been I'emoved by maceration or by keeping a 
 specimen in dilute spirit, a groove would result, and this is pi-obably what 
 was seen and described by Goodsir as the "primitive dental groove" : 
 but, as the student will gather from the text, there is at do 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. 
 
130 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 soon the terminal enlargement, as it grows more deeply 
 
 iiito the jaw, alters in form ; its base becomes flattened, 
 
 Fio. 64 (1). 
 
 / 2 
 
 "" ^#^^ 
 
 l- 
 
 c - 
 
 'm^T^^-^ 
 
 J- 
 
 A 
 
 ^^ 
 
 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 compared to a bell, sus- 
 
 i}) Three stages in the development of a mammalian tooth germ (from 
 Frey). a. Oral epithelium heaped np over germ. 6. Younger epithelial 
 cells, c. Deep layer of cells, or rete Malpighi. d. Inflection of epithelium 
 for enamel germ. t. Stellate reticulum. /. Dentine germ. g. Inner 
 portion of future tocth sac. h. Outer portion of future tooth sac. 
 i. Vessels cut across. A-. Bone of jaw. 
 
THE DEVELOPMENT OF THE TEETH. 131 
 
 pended from above by the thin cord of epithelium -which 
 still coimects it with the epithelium of the surface, or it 
 might in section be described as crescentic, the horns of the 
 crescent being long, and looking 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 a little farther 
 the development of the enamel organ. 
 
 The cells on its periphery remain prismatic or columnai', 
 but those in its centre become transformed into a stellate 
 network, in which conspicuous nuclei occupy the centre of 
 ramified cells, the processes from which anastomose freely 
 with those of neighbouring cells (see Fig. 65). This conver- 
 sion of the cells into a stellate reticulum is most marked 
 quite in the centre of the enamel organ ; near to its surfaces 
 the processes of the cells are short and inconspicuous. 
 
 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 den- 
 tine papilla; a narrow layer of unaltered cells which remain 
 between the stellate cells and the columnar enamel cells 
 constituting the " stratum intermedium." 
 
 Thus far 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 
 or convex surface of the enamel organ do not enlarge ; 
 indeed, according to some autliors, they even commence to 
 atrophy even at this early period. The cells which lie like 
 a cap over the dentine germ or "papilla" as they elongate 
 and their nuclei recede to their extremities, take on the 
 character to be presently described as belonging to the 
 
132 A MANUAL OF DENTAL ANATOMY. 
 
 " enamel cells," (enamel epithelium, internal epithelium of 
 the enamel organ). 
 
 The enamel organ, then, consists (proceeding from with- 
 out inwards) of an " external epithelium," a " stellate reti- 
 culum," a " stratum intermedium," and an " internal epithe- 
 lium," the external and internal epithelia being continuous 
 at the edges or base of the enamel organ, while at its summit 
 the external e]Dithelium remains still, through the medium 
 of the " neck of the enamel organ," in continuity 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." But it is the enamel organ alone 
 which is directly derived from the epithelium ; the origin of 
 the dentine germ is quite distinct. 
 
 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.(^) This appears as a mere increase in 
 the opacity of the part, without at first any visible structural 
 change, and it occupies the concavity of the enamel organ. 
 Thus the dentine germ appears early, indeed almost simul- 
 taneously with the formation of a definite enamel organ, but 
 the enamel organ is far in advance of it in point of structural 
 diiferentiation, and the earliest changes which result in the 
 formation of the enamel organ are strikingly visible before a 
 dentine germ can be discovered. According to Dursy the 
 
 Q) The term "dental papilla," although eminently convenient, is asso- 
 ciated with an erroneous feature of the older views upon tooth develoiJ- 
 ment ; where it is employed in the following pages, the student must 
 guard against the misconception that free papillte at any time exist in any 
 animal. 
 
THE DEVELOPMENT OF THE TEETH. 133 
 
 dark halo which becomes the dentine bulb is, like the inflec- 
 tion of epithelium which forms the enamel germ, continuous 
 all round the jaw, while eventually it develops into promi- 
 nences 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- 
 ward aiid slightly upwards, so that they in a measure embrace 
 the free edge of the enamel organ, and at a somewhat later 
 period they grow uj^wards till they fairh- embrace the whole 
 enamel organ, 
 
 These prolongations of the base of the dentine bulb are 
 the rudiments of the dental sac. In their origin, therefore, 
 the dental sac and the dentine organ ai-e 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 earlj^ mammalian tooth g-erm con- 
 sists of three parts, one of wdiich, 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 enamel organ is formed by a rapid increase of cells at 
 the bottom of a process which dips in ft'om the stratum 
 Malpighi of the oral epithelium ; the dentine germ and the 
 dental sac are formed in close contiguity to this enamel 
 organ from the submucous tissue. 
 
 If there was a " basement membrane " demonstrable at 
 this early period (which there is not) the enamel organ and 
 the dentine organ would lie upon the opposite sides of it. 
 
 The description of the appearance of the several parts of 
 the tooth germ has brought us to the period at which cal- 
 cification first commences, but before proceeding further, 
 it will be well to examine more minutely the structure of 
 the several organs in which calcification takes place. 
 
A MANUAL OF DENTAL ANATOMY 
 
 ENAMEL ORGAN. 
 
 The enamel organ, as has ah-eady been stated, forms a 
 cap-like investment to the dentine bulb, and it is itself 
 thickest over the apex of the latter, tliiuning down some- 
 what 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 much 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 greater bulk of the enamel organ consists of a stellate 
 tissue, which passes gradually through the medium of a 
 layer of rounded cells, the stratum intermedium, into the 
 enamel cells, or internal ejnthelitim. The essential portion 
 of the enamel organ is this layer of " enamel cells," which 
 by their calcification give rise to the enamel, and in lower 
 animals, such as most if not all reptiles, the whole enamel 
 organ is rei^resented by little else than this layer of " enamel 
 cells." 
 
 The cells of the internal epithelium (enamel cells) form 
 an exceedingly regular and perfect columnar epithelium, 
 the individual cells becoming by result of their mutual 
 apposition very symmetrical hexagons. 
 
 They are four or five times as long as they are broad, and 
 the nucleus, which is large and oval, occupies that end 
 which is farthest from the dentine. It is said by Waldeyer 
 that the sides of the cells only are invested by membrane, 
 the protoplasm being without investment at its two ends. 
 
 Towards the base of the dentine germ, where the internal 
 epithelium merges into the external ej)ithelium, the cells are 
 not so much elongated, and they then pass gradually into 
 the cubical form of these latter cells. At their attached 
 extremities the enamel cells are prolonged into processes 
 which are continuous with the cells of the stratum inter- 
 
THE BEVELOPMEMT OF THE TEETH. 135 
 
 medium, so that it may fairly be 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 intennedium" consists of cells intermediate 
 in character between those of the bordering epithelium and 
 the stellate reticulum ; they are branched, but less conspicu- 
 ously so than the stellate cells, with which on the one hand 
 they are continuous, on the other with the enamel 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. 
 
 Fig. 65 [^). 
 
 The function and destination of this portion of the enamel 
 organ is not veiy clear : enamel can be very Avell formed 
 without it, as is seen amongst reptiles and fish, and even 
 in mammalia it disappears prior to the completion of the 
 enamel, so that the external and internal epithelia come into 
 contact. It has been supposed to have uo more important 
 function than to fill up the space subsequently taken up by 
 the growing tooth. (See page 156). 
 
 {}) Cells of the stellate reticulum of the enamel organ. From Frey's 
 Histology. 
 
136 A MANUAL OF DENTAL ANATOMY 
 
 The external epithelium of the enamel organ is com] 
 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 disap- 
 pearance of the enamel pulp and the stratum intermedium, 
 it becomes applied to the enamel cells, and on the comple- 
 tion of the enamel becomes cornified and converted into 
 Nasmyth's membrane. Xolliker and Legros and Magitot 
 dissent from this opinion, the latter stating that the atrophy 
 of these cells commences early, and that they actually dis- 
 appear \mov to the complete atrophy of the organ. For 
 reasons which I have given elsewhere, I do not agree with 
 Waldeyer in this matter, but rather with Magitot. The 
 external ej^ithelium was seen by Nasmyth, Huxley, and 
 Guillot, but it was not very fully described until investigated 
 by Robin and Magitot. 
 
 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 ; 
 Dr. Lionel Beale, on the other hand, stating that a vascular 
 network lies in the stratum intermedium. 
 
 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 indented by numerous papillary projec- 
 tions, into which enter blood vessels of the dental sacculus. 
 These papillae are homologous with, and continuous with 
 those of the gum ; they may sometimes be traced along the 
 neck of the enamel germ, and it is believed that they 
 exercise an important influence on the formation of the 
 enamel, to which I shall again recur. 
 
 The narrow attenuated line of cells by which the enamel 
 organ retains its connection with the stratum Malpighi, 
 whence it was derived, varies much in length and direction 
 in different animals ; in man it is short and straight, in 
 the calf it is laroer, and undulates in its course. It does 
 
THE DEVELOPMENT OF THE TEETH. 
 
 137 
 
 not remain quite that simple line of cells of which it con- 
 sisted when first formed, but varicosities, made up of poly- 
 hedral cells, bud out from it. 
 
 The origin of the dental germs of the permanent teeth 
 remains to be described : the twenty teeth which have 
 deciduous predecessors being derived from parts of the 
 germs of these, the twelve true molai's having a distinct 
 origin. About the sixteenth week of intra-uterine life, from 
 the neck of cells which connects the enamel organ of the 
 temporary enamel germ with the stratum Malpighi, there 
 buds out a secondary inflection of epithelium, similar in 
 appearance to the first rudiment of the enamel germ of 
 
 Fig. 66 ('). 
 
 the milk tooth ; it passes down to the inner side of the 
 temporary tooth sac, and by undergoing a series of changes 
 in all respects analogous with those resulting in the forma- 
 tion of the temporary tooth germ, gives rise to the i:)erma- 
 nent tooth germ, 
 
 (') Dental germ of temporary tooth of an Armadillo, showing its enamel 
 organ, and the enamel germ of the successional permanent tooth to the 
 left of it. 
 
138 A MANUAL OF DENTAL ANATOMY. 
 
 The first permanent molar germ, howevei', is developed 
 about the sixteenth week by a similar budding out of epi- 
 thelium, from that same primary epithelial lamina, -whence 
 the temporary enamel germs originated : whilst the second 
 pei*manent molar oinginates from the neck of the enamel 
 organ of the first molar after a long interval, i.e., about the 
 third month after birth. 
 
 The enamel germ of the wisdom tooth is similarly derived 
 from the neck of that of the second permanent molar, again 
 after a long interval ; (about the third year. Magitot). 
 
 The accompanying figure represents the enamel germ 
 
 Fig. 6" ('). 
 
 c a J 
 
 * / / 
 
 for a permanent tooth budding ofi" from the neck of the 
 enamel organ of the temporary tooth. Many differences of 
 detail, such as the point at which they arise, the depth to 
 
 (1) From the upper jaw of a kitten about the time of birth, a. Oral 
 epithelium, i. Bone of jaw. c. Neck of enamel organ, d. Dentine 
 papilla, c. Enamel cells. /. Stellate reticulum, h. Tooth germ of the 
 permanent tooth, the enamel organ of which is derived from the neck of 
 that of its predi 
 
THE DEVELOPMENT OF THE TEETH. 139 
 
 which they penetrate into the surrounding parts, and other 
 such characteristics exist not only between the germs of teeth 
 of different animals, but even between those of teeth situated 
 in different parts of the mouth of the same animal, so that 
 but little importance is to l)e attached to them. 
 
 Dentine Organ. 
 
 The dentine germ, or dentine bulb, of which the origin 
 has been abeady described, at first was nothing more than 
 
 Fig. 63 ('). 
 
 
 y 
 
 a part of the submucous tissue of the jaw which had become 
 more rich in vessels and cells than other neighbouring parts, 
 
 (') Tooth sac of a calf. a. Tooth sac. a^ a^. Its outer and middle 
 portions, b. Stellate cells of enamel organ, c. External epithelium of 
 enamel organ, d. Internal ei^ithelium of enamel organ, e. Odontoblasts. 
 /. Dentine bulb in papilla, g. Vessels in dentine bulb. t. Points where 
 the sac becomes fused with the base of the dentine papilla. 
 
140 A MANUAL OF DENTAL ANATOMY. 
 
 but 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, 
 simjDly conical, if a tooth with two cusps, bicuspid; and 
 coincidently with these changes the layer of cells forming 
 its surface, which is in close relation with the enamel cells, 
 becomes differentiated from the parts beneath it. 
 
 These cells, which become dentine by their calcification, 
 form a very distinct layer, which, after the commencement 
 of calcification, adheres more strongly to the formed cap of 
 dentine than to the i-est of the pulp, and so is often pulled 
 away with the former when the two are separated ; hence 
 this layer of cells has obtained the name of " membrana 
 eboris," or membrane of the ivory ; but the student must • 
 beware of falling into the mistake of supposing that it really 
 is a " membrane " properly so called. 
 
 The individual cells, which collectively constitute the 
 membrana eboris, are called odontoblasts ; they are variable 
 in shape, differing even in the same animal according as 
 the formation of dentine is going on actively or not, but 
 are always much longer than they are broad, and are fur- 
 nished with oval nuclei near to their deeper ends. The 
 odontoblast cells are furnished with several processes, named 
 according to their direction ; thus the process which pene- 
 trates the formed dentine is called the ''dentinal process," 
 that wliich, at its opposite pole, passes into the deeper 
 part of the pulp where it communicates with other cells, 
 the " pulp process," and those which communicate with 
 neighbouring odontoblasts, " lateral processes." Before 
 entering upon a detailed description of the transformation 
 which the various cells undergo in their conversion into 
 enamel, dentine, or cementum, it will not be o\it of place 
 to say a few vrords relative to the process of calcification 
 generally. 
 
 But before doing ro it may perhaps assist the student, who may 
 
THE DEVELOPMENT OF THE TEETH. 141 
 
 be perplexed in endeavouring to reconcile the statements of various 
 authors, to give a succinct history of the views from time to time 
 set forth. (') 
 
 Before the time of Goodsir (1838), the development of the teeth 
 was described by Raschkow 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 papers of Goodsii- giving, in the place of somewhat vague and 
 general notions, a very definite and intelligible description of ob- 
 servations, was accepted without question by most anatomists, if 
 not by all. Accordingly we find in all the text-books at and after 
 that period, and ia some even at the present daj, the description 
 given by Goodsir reproduced almost without alteration, so that it 
 wiU be worth while to briefl'y relate what his views were. 
 
 He believed that at an early period in fcetal life there appeared 
 a continuous open groove, running round the whole circumference 
 of the jaws ; that from the bottom of this groove tliere arose iso- 
 lated and uncovered papillae, corresponding in number to the milk 
 teeth ; that these papillaj became covered in by the deepening of 
 the groove and the meeting of its two edges over their tops, whilst 
 at the same time transverse septa were formed, so that the several 
 papillae became enclosed in their own separate follicles. With the 
 details of the process as described by him we are not concerned ; it 
 will suffice to remember that he distmguished the four stages ; a 
 primitive dental groove, a papillary stage, a follicular stage, and an 
 eruptive stage (the latter of coiu-se 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 OdontogTaphies of Professor 
 Owen and Professor Giebel may be found accounts of the develop- 
 ment of the teeth in reptiles and fish which are perfectly in accoixl 
 with Goodsir's theoiy, 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 fii-st 
 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 mammalia. Marcusen (^') (1849) gave upon the whole a 
 correct account of the process, referring the enamel to the oral epi- 
 thelium, and Professor Huxley (1854), whilst demonstrating that 
 the stage of free papillse was not to be found in certain fish and 
 
 (') After the present suminary had been partly prepared, the autbor 
 met with the veiy excellent resume given by Messrs. Legros and Magitot, 
 from which he has received much assistance. 
 
 {^) In the rhaim given by Messrs. Legros and Magitot, before referred 
 to, due credit is not given to the papers of Marcusen and Huxley (1849, 
 1854) (although they are alluded to), and it appears to roe that too much 
 is given to that of Xatalis (juillot (1S58). 
 
142 A MANUAL OF DENTAL ANATOMY. 
 
 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. And whilst regretting that their 
 hold upon the minds of anatomists has been so strong as to en- 
 courage deductions therefrom going wider and wider of the mark, I 
 would not be understood to set small value upon the observations 
 of Arnold and Goodsir. They were a step in advance, and were 
 probably as accurate as the methods of investigation then in use 
 would allow of : moreover, the error in observation is very easy to 
 account for, as, the epithelium having peeled ofP as a result of de- 
 composition, or the use of weak spirit, the state of things left does 
 not widely differ from that described by Goodsir. 
 
 The subject rested for many years without further advances, but 
 in 18G3 Professor 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 Wal- 
 deyer, KoUmann, Hertz, Legros and Magitot, Wedl, and others, but 
 only in minor particulars have they been modified. 
 
 The development of the teeth of reptiles was found by a pupil of 
 M. KoUiker's, M. Santi Sirena, to have several features in accord 
 with that of mammalian teeth ; my own researches on the teeth of 
 Batrachia and Fish and Reptiles, elsewhere detailed, have proved a 
 striking general similarity in the process throughout the vertebrate 
 kingdom, though they are not in accord with the views of Pro- 
 fessors Owen and Giebel. 
 
 Dental Follicle. — Tu the foregoing accouut little mention 
 has been made of the tooth follicle or tissue forming a 
 capsule-like investment around the dentine germ and enamel 
 organ. At an early period of development the tissue form- 
 ing the dentine papilla of a mammalian tooth is seen to be 
 prolonged outwards and upwards from its base (see h in 
 Fig. 64) ; these processes appear to grow rapidly upwards, 
 so as to embrace the enamel organ; but whether 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. This up-growth from the base of the dentine papilla 
 is the first appearance of a special dental sacculus, which is 
 thus derived from a source identical with that of the forma- 
 tive organ of the dentine. 
 
Fio. fi9 (1) 
 
 rni.ied from Wil.Uxd (IMil. ^ /Mt- 
 
 tiithful to mturt , it is sotcii dia^'i mil 
 
 magnifying power einplnyed is iiitrmlii 
 
 :. DentiiiP. germ, ^vitli its iHnilfi- df 
 
 4. Points where flie inii'i' ipitli"; 
 
 continuous. :>. Kimmel rells (.r i 
 
 organ. 7. Htellate retieuliim <>( 
 
 organ. 9. Connective tissue aroun 
 
 gum (Of/); this eonsti 
 
 jaw. 11. Bone of low 
 
 J \w \N!) Ui \i 1 oi iM 1>MK fooiH Of \ Lamb, 
 t. lUt. Jled. IbUs). In its outline'^ tlie figure ib 
 that more ot structuic than could be seen with the 
 
 ■d deiil 
 
 liat is c-iil.M (lii 
 
 1-J. IVno>trlll,l 
 
 theyouiig tooth. 14. External skin with its 
 of mouth. 
 
 ormed enamel, 
 ii.niiel become 
 iiiii of enamel 
 ito the enamel 
 ontiiiuuu.s above with that of the 
 ic. 10. Vessels and nen-es of the 
 
 iiiiii. ii. K 
 N. Tapillai-y projei 
 
 .inw, 
 
 13. Heap of epithelium over 
 JIusciilar bundles from floor 
 
 [Tijfiw'p. 14;;, 
 
THE DEVELOPMENT OF THE TEETH. 143 
 
 While these changes are going on, the tooth sac is becoming 
 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 Fig. 69) the gum be stripped off the jaws, the 
 developing tooth capsules are torn off with it, from 
 which they ai'e inseparable except by actual cutting, thus 
 leaving the gutter of bone quite bare and empty. In fact 
 the capsule or sac which encloses the tooth germ consists of 
 almost the whole of the connective tissue which intervenes 
 between the special dentine and enamel germs and the bone. 
 
 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 foct, 
 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 (8 
 in Fig. 69), projecting 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 prisms, 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 Ruminants, where there is to be a deposition 
 of coronal cement. This differentiation of a portion of the 
 dental sac is thought by Messrs. Legros, Robin, and Magitot 
 to be sufficiently pronounced to justify its designation as a 
 distinct " cement organ." 
 
A MANUAL OF DENTAL ANATOMY. 
 
 The Cement Organ. 
 
 Cementum is, according to tliese 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 
 dentar periosteum), but where it is to form a thick layer 
 
 Fig. 70 f'l 
 
 
 over the crown, as in lluuiinants, a cartilaginous cement 
 organ is formed, and we have a calcification analogous to 
 formation of bone in cartilage. 
 
 Thus the cement organ is found in those animals only 
 which have coronal cement, such as the Herbivora. In a 
 calf embryo about the time that dentine calcification is 
 commenced, 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. 
 
 {}) Cement organ of a calf (aftei- Magitot), a. Fibroid matrix. 
 h. Cartilage cells and cap.si;]< s. 
 
THE DEVELOPMENT OF THE TEETH. 145 
 
 But though it exists at this early period, it is not till 
 later, wheu, after the completion of the dentine and enamel 
 immediately beneath it, its own function is about to come 
 into play, that it attains to its characteristic structnre. 
 This M, Magitot designates as fibro-cai-tilaginous, as there 
 appear in it characteristic cartilage cells or chondroplasts, 
 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 into 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, con- 
 jointly with the surrounding connective tissue, is converted 
 into the alveolo-dentar periosteum, but I cannot myself 
 recognise 'the justice of this distinction in practice. 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 very detailed 
 description. 
 
 Another structure, once thought important, and now 
 known to be a mere bundle of dense fibrous tissue, is the 
 " guhernaculum." The permanent tooth sacs, during their 
 growth, have become invested by a bony shell, which is 
 complete, save at a point near their apices, where there is a 
 foramen. Through this foramen passes a thin fibrous cord, 
 very conspicuous w4ien the suiTounding bone is broken 
 away, which is called the " guhernaculum," from the notions 
 entertained by the older anatomists that it was concerned 
 in directing or effecting the eniption of the tooth. The 
 gubemacula of the front permanent tooth sacs perforate 
 the alveolus and blend with the gum behind the necks of 
 the corresponding milk teeth; those of the bicuspids unit- 
 ing with the periosteum of the alveoli of their deciduous 
 predecessors. 
 
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A MANUAL OF DENTAL ANATOMY. 
 
 Calcification. 
 
 A tissue is said to be " calcified " when the organic stnic- 
 tures of which it is composed are hardened and stiffened by- 
 impregnation with salts of lime. The impregnation with 
 lime salt 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 up only from one to three per cent, of the whole, so 
 that practically the enamel wholly disappears under the in- 
 fluence of an acid j or the organic matrix may persist in 
 fiufl&cient 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 become 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 mollusks, 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 Rainie, and has been more recently 
 worked out by Professor Harting and Dr. Ord. 
 
 If a solution of a soluble salt of lime be slowly mixed 
 with another solution capable of precipitating the lime, the 
 
 (1) All observers are not, however, agreed as to the formation of the 
 enamel. (Cf. page 157.) 
 
THE DEVELOPMENT OF THE TEETH. 
 
 resultaut lime salt will go down as au amorphous powder, 
 or, imder 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 amor- 
 phous powder there are found various curious but definite 
 forms, quite unlike the character of crystals produced with- 
 out the intervention of the organic substance. 
 
 Mr. Rainie found that if calcium carbonate be slowly 
 formed in a thick solution of mucilage or albumen the re- 
 sultant salt is in the form of globules, laminated in structure, 
 so that the globules may be likened to tiny onions ; these 
 globules, when in contact, becoming agglomerated into a 
 single laminated mass, it appearing as if the laminae in 
 immediate apposition blended with one another. Globular 
 masses, at one time of mulberry-like form, lose the in- 
 dividuality of their constituent smaller globules, and become 
 smoothed down into a single mass ; and Mr. Rainie suggests 
 as an explanation of the laminated stnictm-e that the smaller 
 masses have accumulated in concentric layers which have 
 subsequently coalesced ; and in the substitution of the 
 globular for the amorphous or crystalline form in the salt of 
 lime when in contact with various organic substances, 
 Mr. Rainie claimed to find the clue for the explanation of 
 the development of shells, teeth, and bone. At this point 
 Professor Harting took up the investigation, 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 (i) might be produced. But the most im- 
 portant addition to our knowdedge made by Professor 
 Harting lay in the very peculiar constitution of the " calco- 
 spherites," by which name he designated the globular forms 
 seen and described by Rainie. That these are built up of 
 
 (M Thus lie was successful in artificially producing "dumb-bell"' 
 crystals. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 concentric laminae like an onion has already been mentioned, 
 and Mr. Rainie was aware that albumen actually entered 
 into the composition of the globule, since it retained its 
 form even after the application of acid. 
 
 But Professor 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 chitine, 
 the substance of which the hard skins of insects consist, 
 rather than any other body. 
 
 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 the last traces of lime are 
 retained very obstinately 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. 
 
 Now it is a very suggestive fact that in the investigation 
 of calcification we constantly meet with structures remark- 
 able for their indestructibility : for example, if we destroy 
 the dentine by the action of very strong acids, or by 
 variously contrived processes of decalcification, puti*efaction, 
 &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 
 remain behind large tubes, found to be the linings of the 
 haversian canals (Kolliker), and small rounded bodies, 
 recognisable as isolated lacunse ; and in the cuticula dentis 
 we have another excellent example of this peculiarly inde- 
 structible tissue. 
 
 In point of fact, as will be better seen after the develop- 
 ment of the dental tissue has been more fully described, on 
 
TH-E DEVELOPMENT OF THE TEETH. 151 
 
 the borderland of calcification, between the completed fully 
 calcified tissue and the formative matrix as yet unimpreg- 
 nated with lime, there very constantly exists a stratum of 
 tissue which in its physical and chemical properties very 
 much resembles " calcoglobulin." 
 
 It should also be noted that globulai*, spherical forms are 
 very constantly to be seen at the edges of the thin cap of 
 forming dentine, and may be also traced in and around the 
 interglobular spaces (see Fig. 34) ; moreover, isolated 
 spherules of lime salt have been described by Messrs. Robin 
 and Magitot as occun-ing abundantly in the young pulps of 
 human teeth, as well as those in the herbivora, where their 
 presence was noted by Henle. 
 
 Calcification of the Examel, 
 
 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 a somewhat simpler and 
 more easily intelligible process. 
 
 As has already been mentioned, I am distinctly of opinion 
 that the enamel is formed by the actual conversion of the 
 cells of the enamel organ into enamel, but as this view is 
 not held by all who have written upon the subject, I will 
 first mention the alternative theory, namely, that the enamel 
 is in some sense secreted or shed out by these cells. In 
 support of this latter theory the names of no less authorities 
 than Professor Huxley, KoUiker, Wenzel, and Magitot, may 
 be adduced, but the grounds on which their decisions are 
 based are appearances susceptible of a different interpreta- 
 tion. 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 fibres therefore corre- 
 
152 A MANUAL OF DENTAL ANATOMY. 
 
 spouding in size and being continuous with tiie enamel cells 
 whence they were shed out. 
 
 Professor Huxley's reason for doubting the direct con- 
 version of the enamel cells into enamel was that a membrane 
 could be raised from the surface of growing enamel, at any 
 period of its development, by the use of acid reagents, this 
 membrane necessarily intervening between the formed 
 enamel and the enamel cells ; hence he denied that the 
 enamel organ contributed in any way directly, though it 
 might indirectly, to the development of the enamel 
 
 To the nature of this "membrane " I shall have again to 
 refer, so that for the present it will suffice to say that the 
 structure in question cannot be demonstrated, and in fact 
 has probably no existence, prior to the use of the reagent. 
 
 The cells of the internal epithelium of the enamel organ 
 or enamel cells have been already in some measure de- 
 scribed : they are elongated cells, forming a very regular 
 columnar epithelium, and are hence rendered hexagonal by 
 mutual apposition ; they vary in their length and diameter 
 in different animals. 
 
 To secure uniformity of nomenclature, the name adaman- 
 toblast has recently been proposed for them, as being better 
 comparable with the term odontoblast and osteoblast. 
 
 Although they are connected with the cells of the stratum 
 intermedium by a process at their base, 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 sac 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, which were first described by 
 my father, and go by the name of " Tomes' processes." The 
 cells are also slightly enlarged at these extremities, especially 
 if they have been immersed in glycerine or any such fluid 
 which causes their shrinkage, for this end of the cell having 
 
THE DEVELOPMENT OF THE TEETH. 
 
 153 
 
 received a partial impregnation with lime salt at its peri- 
 phery, and so being rigid, is unable to contract with the rest 
 of the cell. These enlarged, everted ends, often show a very 
 
 Fig. 71 (}). 
 
 sharp contour, their trumpet-like mouths tending to confirm 
 the statement of Waldeyer that the protoplasm of the cell 
 is not covered in by membrane at its ends. The impregna- 
 tion with calcareous salts commences at the free end of the 
 
 Fw. 72 (-). 
 
 enamel cell, and at the periphery before the central portion, 
 and it is to this fact that the existence of " Tomes' pro- 
 cesses " is due, for when the enamel cell is dragged away 
 from the foi-med enamel prism, it separates across the line 
 of calcification ; and thus the axial part of the cell, when 
 torn away, projects out further than its peripheiy, in con- 
 
 (') Enamel cells with Tomes' processes. 
 
 (^) Enamel cells ; the two on the right have been shrunk by immersion 
 in glycerine, and present the open tmmpet-shaped ends described in the 
 text. 
 
154 A MANUAL OF DENTAL ANATOMY. 
 
 sequence of calcification having extended less far at this 
 central portion of the cell. 
 
 In other words, if the forming enamel were freed from 
 the adherent enamel cells, its surface would be pitted, 
 each little pit marking the centre of an enamel prism ; and 
 if a thin section of this immediate surface could be taken 
 off, it would be pierced with holes at regular intervals. 
 
 The enamel cell with its process is like an odontoblast with 
 a very short dentinal fibril, which has been pulled out of the 
 formed dentine, and the nature of the " Tomes' processes " is 
 well illustrated in the enamel organs of marsupials. It 
 will be remembered that their enamel is permeated by a 
 large number of canals, which become continuous at the 
 junction of the dentine and enamel, with the dentinal tubes. 
 Accordingly the enamel cell of a marsupial, engaged in the 
 formation of a permanently tubular enamel, is just like an 
 odontoblast in that it has a long, fine process, pulled out of 
 the already formed enamel. 
 
 As the youngest part of the enamel has by no means 
 attained to its full hardness, it is quite possible to obtain, in 
 small pieces, sections parallel to its surface ; the nearer they 
 are to the surface, the larger will be the perforations, show- 
 ing what has already been stated respecting calcification 
 commencing at the periphery of each cell to be true. And 
 it is possible, by the use of an acid, to obtain such sections 
 upon a larger scale, for under the influence of such a reagent, 
 this youngest layer of the enamel peels off in a sheet, 
 bringing with it in places enamel cells, in places enamel 
 prisms, adhering to its opposite sides. When destitute of 
 adherent enamel cells or prisms, this so-called membrane is 
 foraminated ; and the processes of the ends of the enamel 
 cells are fitted into and passed through these perforations. 
 
 The real nature of the membrane which could be raised 
 from the surface of growing enamel was first demonstrated 
 by my father, and his explanation has been accepted by 
 
THE DEVELOPMENT OF THE TEETH. 155 
 
 Waldeyer and other authorities ; it will be seen that this 
 sheet, produced solely by the destructive action of reagents, 
 corresponds with the membrana preformativa of some writers 
 (see page 171), and with the membrane described by Pro- 
 fessor Huxley as intervening between the enamel cells and 
 the enamel. Hence it will be seen that the fact of acids 
 raising a membrane from the surface of the enamel does not 
 really militate against the theory that the enamel is due to 
 the direct convei'sion of the enamel organ into enamel. 
 
 The ends of the enamel cells near to the formed enamel 
 are granular, this granularity being due to the deposition of 
 particles of lime salts, as is indicated by its clearing up 
 when treated with acid. 
 
 The cells on the one side of the membrane readily separate 
 from one another, adhering, however, slightly by their 
 dilated ends (vide supra), and the fact that we are able to 
 isolate the youngest layer of enamel as a thin sheet is 
 probably to be explained by its chemical nature. It ap- 
 pears to belong to that class of peculiarly resistant sub- 
 stances which are to be found on the borders of calcification, 
 and behaves very much like Professor Hartiug's " calco- 
 globulin" (see page 150); at all events it may safely be 
 said to have undergone some chemical change preparatory 
 to the i-eception of its full amount of lime salts. 
 
 The calcification of the enamel should be so complete 
 that itrf fibrous structure is but slightly apparent in longi- 
 tudinal sections, and the individual fibres should appear 
 structureless, with the exception of the feebly marked 
 striation (see page 50). In enamel of imperfect structural 
 character the centre of the fibre is not completely calcified, 
 the arrest of development having taken place short of its 
 full conversion. 
 
 The stellate tissue of the enamel organ disappeai-s some 
 time before the whole thickness of the enamel is formed, 
 and changes go on in the latter up to the time of the erup- 
 
156 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 tion of the tooth ; the enamel of a tooth prior to its erup- 
 tion having a chalky, opaque surface. 
 
 The enamel of the teeth of reptiles is developed from an 
 enamel organ which at no time possesses any stellate tissue ; 
 this is also the case in all fish which I have hitherto ex- 
 amined. In the poison fangs of snakes the enamel cells, 
 
 over the interior of the poison tube, appeared to be trans- 
 formed into a stellate reticukim, which change in this case 
 would appear to be a retrograde metamorphosis. 
 
 The nuclei of the enamel cells, which lie at the estre- 
 mities furthest from the enamel, appear to recede as calcifi- 
 cation goes on ; they do not exercise any special influence 
 on the process as far as can be seen. 
 
 (') Transverse section of the tooth sac of a poison fang (Viper). The 
 crescentic pulp (a) is surrounded by a layer of dentine (cZ) ; external to 
 this is a layer of columnar enamel cells, which, upon the exterior of the 
 tooth, upon which a thin layer of enamel is to be formed, are large con- 
 spisuous cells. Where 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 place taken by stellate cells (/). No enamel is formed 
 in this latter position. 
 
THE DEVELOPMENT OF THE TEETH. 157 
 
 As has been already mentioned, Kolliker dissents from the above 
 account of the calcification of the enamel, partly on the gi-ound 
 that enamel cells may be seen of the same size and form at all 
 stages of the foiTuation of enamel. 
 
 The process he regards as one of secretion, the enamel being shed 
 out. so to speak, from the free end of each enamel cell ; hence the 
 prisms of the enamel will con-espond in size and number with the 
 cells of the enamel epithelium ; the processes of the enamel cells 
 he regards as being fragments of this hardened secretion which are 
 still clinging to the parent cell. 
 
 M. Magitot (Journal de Tanatomie de M. Ch. Robin, 1879) has 
 reyived this view, describing each cell as tenninated, towards the 
 forming enamel, by a little plate of dense material through which 
 by some process of exosmosis the constituents of enamel travel out. 
 He notes that these plates often cohere so as to form a sheet (cf. 
 page IJJi), but says 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 excre- 
 tion theory. 
 
 The reasons for adopting the opposite view will have been ga- 
 thered from the text ; they are, in brief, the occurrence of the 
 •' Tomes' processes," especially in marsupials ; the rigidity of the 
 open mouths of the enamel cells ; the pitted surface of the youngest 
 layer of enamel, the foraminated membrane which can be raised 
 from it, and the relation of these facts to the occurrence of the 
 processes of the enamel cells. 
 
 Schwann believed that the enamel cell was constantly increasing 
 at its free end (/.<"., that next to the enamel), and that the new 
 growth, or youngest part of the cell, is calcified as fast as it is 
 foi-med ; this view differs little fi-om that of Kolliker, who prefers 
 to express it by saying that this end of the cell is constantly shed- 
 ding off or secreting a material which becomes external to itself. 
 My father, Waldeyer, Hertz, and many others, believe that the cell 
 growth takes place not at this free end, but at the attached nucle- 
 ated end, and that it is the oldest portion of the ceU itself which 
 receives an impregnation with salts and forms the enamel. 
 
 Professor Huxley's opinion (page 152) is, I take it, based on the 
 fact that a membrane could be raised from the surface of young 
 enamel, which must have intervened between the enamel cells and 
 the enamel prisms ; if my father's explanation of the nature of this 
 membrane be accepted, the difficulty vanishes. 
 
 My o^vn researches upon the development of the teeth of fishes 
 also furnish evidence tending in the same direction : as has been al- 
 ready mentioned, the enamel cells in some parts of the enamel org-ans 
 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 cells, 
 
158 A MANUAL OF DENTAL ANATOMY. 
 
 three times as long' as the corresponding cells lower down upon the 
 dentine papilla, are in the position of the terminal cap of enamel 
 which characterises these teeth. Moreover 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. 
 
 Calcification of the Dentine. — The dentine is formed 
 upon the aurface of the dentine bulb, or papilla, from with- 
 out 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 
 described ; it remains to consider somewhat more minutely 
 the nature of its surface. The cells constituting the mem- 
 brana eboris, to which Waldeyer has given the convenient 
 name of " odontoblasts," form an exceedingly sharply defined 
 layer upon the surface of the dentine wall, being aiTanged 
 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 what- 
 ever like the linear succession of formative cells, which, by 
 coalescence at their ends went to form the dentinal tubes, 
 as described l)y the older waiters, is to be seen. 
 
 The odontoblast cells vary in form according as the den- 
 tine formation is actively going on or not, but at the period 
 of their greatest activity they are broad at the end dii-ected 
 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, and are, according to 
 Waldeyer and Boll, destitute of all membrane ; the nucleus 
 is oval, lies in that extremity of the cell which is farthest 
 from the dentine, and is sometimes prolonged towards the 
 dentinal process so as to be ovoid or almost pointed. 
 
THE DEVELOPMENT OF THE TEETH. 159 
 
 The dentinal 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, 
 
 Fig. 74 (i). 
 
 which constitute the dentinal fibrils, may be seen stretching 
 across the interval in great numbers. 
 
 The odontoblasts, as may be seen from figm-es 30 and 31, 
 are fitted closely together, and there is no 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 appear- 
 ance of the thin edge of such a pulp suggests the idea that 
 they are bedded in a transparent and structureless jelly, 
 which projects a little beyond them. To render my mean- 
 ing more clear by a homely illustration, the surface of the 
 pulp at this stage reminds one of the clear jellies put upon 
 the table with strawberries or the like buried in them, near 
 to, but beneath, the surface. But no such substance can 
 be seen when once calcification has actively set in. 
 
 When the pulp has completed, for the time being at all 
 events, the formation of the dentine, the odontoblast cells 
 become more elongated and more rounded in their outline 
 and taper off towards and into the dentinal process, instead 
 of having truncated ends. 
 
 The cells figured by Lent as the formative colls of dentine 
 I regard as odontoblasts taken from an adult tooth, the 
 
 (') Isolated odontoblast cell. 
 
160 A MANUAL OF DENTAL ANATOMY. 
 
 period of formative activity being past, and I am inclined to 
 think that his views on the subject of development are open 
 to criticism, as being based upon the appearances presented 
 by such old cells. 
 
 Fig. 75 Q). 
 
 The dentine is, I believe, formed by the direct conversion 
 of the odontoblast cells, just as is the enamel by that of 
 the enamel cells, and is derived from them, and from them 
 alone. 
 
 According to this view, which is supported by Waldeyer, 
 Frey, Boll, Dr. Lionel Beale, and many other writers, the 
 dentinal fibrils, the dentinal sheaths, and the matrix between 
 these latter, are alike derived from the metamoi-phosis of 
 the odontoblast cell. In other words, the three structures 
 in question may be taken as being three stages in the con- 
 version of one and the same substance : thus we have 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 substances which lie on the 
 borders of calcification ; and lastly, the matrix, the com- 
 pleted, wholly calcified tissue. 
 
 That some such relation exists seems to be indicated by 
 the fact that dentinal tubes once formed are capable of 
 
 (') Odontoblasts in situ. After 'Wakleyer. 
 
THE DEVELOPMENT OF THE TEETH. 161 
 
 further calcification, by -^-hich their calibre becomes sensibly 
 diminished. Thus, my father states (speaking of the incisor 
 teeth of rodents), " the tubes which proceed from the pulp 
 cavity near the base of the tooth, are, in most cases, per- 
 ceptibly 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 
 Scim-idse 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." 
 
 And Dr. Lionel Beale calls attention to the fact that the 
 hollows of the canals are largest 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 oblite- 
 ration of the peripheral tubes. There is, too, the statement 
 of Robin 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 difiicu^lt to see how a dentinal tube once formed can 
 become contracted to a third or half of its diameter unless. 
 we believe that that which was at first the soft tissue (den- 
 tinal fibril) occupying its canal may become at its periphery 
 metamorphosed into " dentinal sheath," while that which 
 was ci-iginally this latter has passed into the condition of 
 matrix. Other illustrations of this fact, observed by inde- 
 pendent writei-s, suggest themselves to me ; the converti- 
 bility of the dentinal fibril into dentinal sheath and of the 
 latter into matrix, seems to be of necessity implied by the 
 narrowing of the calibre of a tube once formed, for the tubes 
 thus naiTowed present no special character ; their walls do 
 not appear any thicker, nor do they in any way become dif- 
 ferent save in the one matter of diameter. The phenomena 
 of dental caries also appear to lend some support to this. 
 
162 A MANUAL OF DENTAL ANATOMY. 
 
 view, that dentinal fibril, dentinal sheath, and matrix, are 
 but three ages of the same tissue. 
 
 For under the influence of caries the walls of the tubes, 
 invisible, or almost so, in perfectly healthy dentine, become 
 apparent. 
 
 As I have elsewhere expressed it, 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 perma- 
 nently soft fibril and the general calcified matrix, is that 
 jDortion which immediately surrounds the fibi-il, namely, the 
 dentinal sheath; as expressed by Dr. Lionel Beale they are 
 protoplasm, formed material, and calcified formed material. 
 
 That the whole of the dentine is derived from a conversion of the 
 •odontoljlast cells is not agreed to by all writers. Thus Kolliker and 
 Lent believe that while the canals and their contents are continua- 
 tions of the odontoblasts, the matrix is a secretion either from these 
 cells or from the rest of the pulp, and so is an " intercellular " sub- 
 stance. Their view is therefore intermediate between the excretion 
 und conversion theories ; and KoUiker 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 Professor KoUiker think- 
 ing and writing of those aged, spent cells which his jnipil Lent 
 figured ? No one could speak of a young, active odontoblast as 
 " dra'rni out into the dentinal fibril." A good section of young 
 developing dentine shows that the cells are square and abrupt 
 towards the dentine : they do not taper into the deirtinal process in 
 the smallest degi-ee, and there is no room for any intercellular sub- 
 stance whatever. 
 
 Hertz coincides with KoUiker in regarding the matrix as a " se- 
 cretion from aU the dentinal cells in common which stands in no 
 definite histological relation to the individual cells," but his figui-e 
 also I believe to be representative of an adult inactive surface of 
 pulp, in which dentine foiination lip.d almost ceased. 
 
 KoUiker and Lent are of opinion that a single cell is suflJcient to 
 form the whole length of a dentinal fibril, not having seen evidence 
 of active cell gTowth in the siibjacent layer of the i)ulp. from which 
 they would infer that the membrana eboris was supplemented by 
 
THE DEVELOPMENT OF THE TEETH. 163 
 
 new cells from below. In the latest edition, however, KoUiker 
 speaks with much more hesitation on this point. 
 
 Magitot now (1881) holds that the whole of the dentine is '• a 
 product elaborated by the odontoblasts," but neither secreted by nor 
 formed by the conversion of the odontolilasts, and he denies the 
 existence of the sheaths of Xeumann in iotn. 
 
 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 
 fonned by the transfoi-mation of the odontoblast cells, but that the 
 tubes were intcrspucfs between these latter, not corresponding with 
 the axes of the cells. 
 
 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 at- 
 tained a somewhat greater thickness, globules begin to 
 appear in it, which are small 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 strictl}^ 
 analogous to that demonstrated by Mr. Rainie and Professor 
 Harting (see page 148), is going on. Thus, in the formation 
 of the first skin of dentine, a stage of metamorphosis pre- 
 paratory to impregnation with calcareous salts distinctly 
 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 narrowiug of tlie forma- 
 
 M 2 
 
164 A MANUAL OF DENTAL ANATOMY. 
 
 tive pulp takes place, but the additions to the base of the 
 tooth are of constatit, or ever-increasing dimensions, as is 
 the case in some tusks, -which arc of conical form. 
 
 It is said that tlie 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. I am not however able, from my own ob- 
 servations, to thrown any light upon this matter. 
 
 THE CALCIFICATION OF VASO-DENTINE. 
 
 During the conversion of the membrana eboris into 
 ordinary hai'd un vascular dentine the vessels of the formative 
 pulp recede, so that, whilst at all stages a cajiillary plexus 
 is to be found just below the odontoblast layer, no vessels 
 are to be found amongst the cells which constitute it. 
 
 Nevertheless a moment's reflection will show that (except 
 in the eai-liest stages, before any dentine is formed) the 
 plexus must at a prior time have occupied the place now 
 taken possession of l"»y the inward marching odontol)lasts 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 caiTy blood 
 circulating through it, even after calcification has crept up 
 to and around it. 
 
 So that if we imagine a vascular papilla to have its 
 stroma suddenly petrified whilst its circulation went on all 
 the same, we should have something like a vaso-dentine. 
 
 Just as in hard dentine, the odontoblast layer is distinctly 
 
TEE DEVELOPMENT OF THE TEETH. 165 
 
 marked off from, the rest of the dentine oi'gan, and the 
 dentine is wholly derived from its conversion into calcified 
 material, so that the difference between vaso-dentine and 
 hard dentine is not one of a very fundamental character. 
 
 Indeed, as we have seen (p. 85), the same formative pulp, 
 the same odontoblast layer, is able at one time to form hard 
 tubular dentine, at another vaso-dentine. All, therefore, 
 that has been before said of the calcification of odontoblasts 
 will apply equally to those of a vaso-dentine pulp, save only 
 that in a typical tissue of this latter kind each cell calcifies 
 solidly, and does not leave the axial portion soft, to remain 
 as a dentinal fibril. 
 
 Of the development of Plicidentine nothing more need 
 be said, as it presents no peculiarities which are not the 
 obvious result of the folding of the surface of its formative 
 pulp- 
 
 THE CALCIFICATION OF OSTEO DENTINE. 
 
 With the exception of the thin external layers (see fig. 47), 
 which are develo])ed from a superficial layer of not very 
 highly specialised cells, osteo-dentine is built up in a 
 manner fundamentally different from that in which hard 
 dentine, plicidentine, and vaso-dentine, are constructed. 
 
 For it is not, like these, a surface formation ; it is not 
 laid down in a regular manner upon the exterior of a pulp, 
 and it has no relation to an odontoblast layer, if we except, 
 perhaps, its thin exterior shell. 
 
 So soon as this has been formed its inner surface becomes 
 roughened by trabecules shooting inwards into the substance 
 of the pulp, which speedily becomes traversed completely 
 by them, as Avell 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, 
 
A MANUAL OF DENTAL ANATOMY. 
 
 from the interior of the dentine cap. Osteoblasts clothe, 
 like an epithelium, the trabeculfe and the connective tissue 
 fibres attached to them, and by the calcification of these the 
 osteodentine is formed. 
 
 The process is exactly like the calcification of any mem- 
 brane 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 continuously 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 Sharpy's fibres to be 
 elastic, that the hinged teeth of the pike (see fig. 88) 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 im- 
 possible to say whether the tooth imder observation was 
 going to be anchylosed, or to be a hinged tooth tied down by 
 elastic strings. 
 
 The Calcification of Cementum. — Just as is the case 
 with ])ones elsewhere in the body, cenientum may 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 vascvdar membrane which 
 is in fact the follicle wall, and Avhich is to become, when the 
 cement is formed, the alveolo-dentar periosteum. 
 
 The inner or dentinal face of this membrane presents a 
 layer of large cells, the osteoblasts of Gegenbaur, and it i& 
 
THE DEVELOPMENT OF THE TEETH. 167 
 
 by their calcification that bone or cenjentum is directly 
 formed. These osteoblasts are themselves a special de- 
 velopment where bone is about to be manufactured, as is 
 cleaiiy explained in the following extract from a paper by 
 my father and the late Mr. De Morgan, 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 be- 
 tween them. Everywhere about growing hone a careful ex- 
 amination will reveal cells attached to its surface, tvhile the 
 surface of the hone itself will jyresent a series of similar bodies 
 ossified. To these we propose to give the name of osteal 
 cells, as distinguished from lacunal and other cells." 
 
 Externally to the osteoblast layer, but still very near to 
 the perfect cementum, lies a reticulum or network made up 
 by the inosculating branches of cells. The cells have largish 
 round nuclei, and are each furnished with three or four homo- 
 geneous processes, so that the tissue, save in very thin sec- 
 tions, looks hopelessly confused from the interlacing of the 
 cell processes. Many of these processes pass into, and are loit 
 in the clear, structureless matrix of the already formed 
 cementum ; the functions which they perform in its deve- 
 lopment are not very apparent, as they do not correspond 
 to anything which can be traced in the completed tissue. 
 
 Externally to the fine-meshed net-work which has been 
 Avell figured and described by Dr. Lionel Beale, the soft 
 tissue surrounding the root partakes more of the character 
 of ordinary fibrous tissue, and may be teased out nito 
 fibrils. The fibrou.s bands run mainly in a direction from 
 the alveolus towards the tooth. Many of them pass through 
 the whole thickness of the soft structures, extendincr from the 
 
168 A MANUAL OF DENTAL ANATOMY. 
 
 bone of the alveolus to the cementum of the tooth, becoming 
 lost at each extremity in the one tissue or other. 
 
 The osteoblasts form both matrix and bone corpuscles : 
 in Professor Klein's words " 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 we then find a row of oblong or round 
 territories, each composed of matrix, and in it a nucleated 
 branched cell. The outlines of individual territories are 
 gradually lost, and we have then a continuous osseous 
 lamina, with its bone-cells. 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 com- 
 mencing at the ' point of ossification.' The bone cells, with 
 their pi'ocesses, are situated in corresponding lacunae and 
 canaliculi, j ust as in the adult osseous substance. " 
 
 Thus jiist as calcification in an enamel cell or in an odon- 
 toblast commences upon its surface, and proceeds inwards 
 till it has more or less completely pervaded it, so in the case 
 of the osteoblast the deposition of calcareous salts proceeds 
 from without inwards. To use a rough comparison we 
 might imagine a calcifying osteoblast as like an egg-shell, 
 the central cavity of which was being gradually obliterated 
 by the addition of successive layers on its interior (it is not 
 to be understood that any such lamination is to be detected 
 in an individual osteoblast). In a certain number of osteo- 
 blasts this process of calcification does not proceed with 
 such regularity as to obliterate their centres, and at the 
 same time to fuse together their exteriors, but as it pro- 
 gresses with some degree of irregularity towards the centre, 
 tracks of uncalcified matrix are left, and finally it stops 
 short of obliterating the central portion of the cell. Al- 
 though for the purpose of description I have spoken of the 
 
THE DEVELOPMENT OF THE TEETH. 169 
 
 centre of the osteoblast cell as a ' space,' of course it is not 
 hollow, but consists of uncalcified matrix, and in this situa- 
 tion lies the nucleus of the cell. 
 
 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 lacunte of 
 a supposed Pterodactyle bone from the Wealden, a section 
 from which was ligured by my father in the paper referred 
 to. Exactly as calcification, advancing with irregularity in 
 
 Fig. 76 ('). 
 
 the interior of an individual cell, fails to render it homo 
 geneous by pervading its whole substance, so it may fail so 
 completely to unite contiguous cells as to obliterate their 
 contours. A lacuna, suiTounded by such a contour line, 
 mapping the limits of the original cell, or cluster of cells, is 
 what is termed an " encapsuled lacuna." 
 
 That Avhich determines the formation of a lacuna, or an 
 encapsiiled lacuna, at any particular spot, is unknown : all 
 that can certainly be said upon the subject is embodied in 
 
 (') Encapsuled laciinse 
 
170 A MANUAL OF DENTAL ANAT03IY. 
 
 the following extract from the paper by my father and Mr. 
 De Morgan, above alluded to : — " We see the bomidary of 
 the original lacunal cells only in those cases where the 
 lacunar have but few, or are entirely devoid of canaliculi. 
 It would appear to be a law, to which there are few, if any, 
 exceptions, that when anastomosis is established between 
 adjoining lacunee, the lacunal cells blend with the con- 
 tiguous parts, and are no longer recognisable as distinct 
 bodies." 
 
 According to Kolliker, the cemeutum first is deposited iu 
 isolated scales, which coalesce with one another, rather than 
 in a continuous sheet. In the teeth of the Primates, the 
 Carnivora, Insectivora, &c., the cementum, at least in any 
 appreciable thickness, is confined to the roots of the teeth. 
 Various reasons, however, exist, for regarding Nasmyth's 
 membrane as an exceedingly thin layer of cement, which 
 have been entered into in the section relating to that 
 structure, and need not be recapitulated here. It will 
 suffice to say, that it appears to be one of those structures 
 midway betwixt full calcification and full vitality, and 
 shares with such substances the power of resistance to 
 chemical reagents which characterises them. 
 
 M. Magitot states that the calcification of the cartilaginous 
 cement organ of Herbivora 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 
 jjortion of the organ, coalesce, and come to invade its entire 
 thickness ; and further that the cement at the period of 
 eruption is constituted of " osteoplasts " regularly grouped 
 round vascular canals, and included in a ground substance 
 finely striated. {Joimial deVanatomie, 1881, p. 32.) 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 
 
THE DEVELOPMENT OF THE TEETH. 171 
 
 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. Magi tot does 
 not describe in much detail this 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. 
 
 Membrana Preformativa. — To the student of dental 
 development few things are more perplexing than the con- 
 flicting statements wdiich he reads in various works as to 
 the nature and position of the Membrana preformativa, of 
 which I have hitherto studiously avoided all description; 
 Avhile it is not encouraging, after having mastered with 
 difiiculty some one description of its character, to find that 
 many of the most recent authors altogether deny its exist- 
 ence. I will endeavour, therefore, so far as I am able, 
 although not myself believing in its presence, to put the 
 matter in a clearer light, and to point out wherein lie the 
 discrepancies of statement. 
 
 According to the older theories of tooth development, 
 under the thrall of which most authors have written, the 
 tooth germ was in the first instance a free, uncovered papilla 
 of the mucous membrane, which subsequently sank in and 
 became encapsulated, &c., &c., (see page 129). Moreover, it 
 was taught by the older histologists that fine homogeneous 
 " basement membranes " were to be found in a great variety 
 of situations, amongst others beneath the epithelium of the 
 mucous membrane, and that these were of (physiologically) 
 umch importance, inasmuch as they formed defining limits, 
 through which structures did not pass. As a necessary con- 
 sequence of these views, it was assumed as a matter of 
 
172 A MANUAL OF DENTAL ANATOMY. 
 
 course that the " dentine papilla " was covered over by a 
 " basement membrane," or membrana prcformativa. 
 
 Thus this membrane necessarily intervened between the 
 enamel organ and the dentine papilla, and hence gave rise 
 to difficulties in the understanding of the calcifying process. 
 Henle considered that evidences of its presence speedily 
 became lost, but that ossification proceeded in opposite 
 directions upon the two sides of this membrane : from 
 within outwards in the case of the enamel, from without 
 inwards in the case of the dentine. 
 
 Prof. Huxley, starting on the same hypothesis as to its 
 position, namely, that it was between the enamel organ and 
 the dentine papilla, came to a different conclusion as to its 
 after fate ; relying upon the fact that a continuous sheet of 
 tissue or membrane can be raised from the surface of the 
 developing enamel (see page 152), he concluded that this 
 was the original membrana preformativa, that it afterwards 
 became the Nasmyth's membrane, and that enamel was 
 developed without the direct participation of the enamel 
 organ, seeing that a membrane separated the two. My 
 reason for doubting the correctness of these conclusions has 
 been there given; the membrane so demonstrable is, I 
 believe, artificial, and does not represent any naturally 
 existing structure. 
 
 KoUiker strongly affirms the existence of the membrana 
 preformativa, and in the older edition of his Histology, held 
 that if became converted into Nasmyth's membrane ; al- 
 though he now gives a different explanation of the origin 
 of Nasmyth's membrane, I have not found a definite state- 
 ment as to his recent views of the ultimate fate of the 
 membrana preformativa. 
 
 We have thus three destinations assigned to the mem- 
 brane covering the dentine papilla, or membrana prefor- 
 mativa. 
 
THE DEVELOPMENT OF THE TEETH. 173 
 
 (i.) Between the dentiue and the enamel (Henle). 
 (ii.) Between the enamel and tlie enamel organ, or out- 
 side the enamel (Huxley), 
 (iii.) Between the dentine and the pulp (several writers of 
 less authority). 
 
 "VVe come next to those writers who deny its existence 
 altogether, explaining on other gi'ounds the appearances 
 observed. 
 
 Markusen believed that it was nothing more than the part 
 of the papilla first ossified ; and Dr. 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. 
 
 Messrs. Robin and Magitot have offered a plausible ex- 
 planation of the appearance 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), and which reminds the observer 
 of the tissue contained in an umbilical cord. This is some- 
 what more dense towards the surface, where it forms a 
 matrix for the odontoblasts and projects beyond them, so 
 as to look, in section or at a thin edge, like a sort of 
 varnish to the papilla. From its greater density near the 
 surface, it may become corrugated, and so look like a 
 folded or torn membrane. I am quite inclined to agree 
 with the foregoing explanation. 
 
 I am inclined to think, that but for the eiToneous theories 
 that the dentine germ originated as a free papilla on the 
 surface, which would according to the prevalent view have 
 been necessarily invested by a basement membrane, we 
 should never have heard of a membrana preformativa. At 
 all events it is difficult to imagine that such a membrane 
 exists upon papilla formed at such a great distance from the 
 surface as those of tlie snake or the lizard (P'igs. 61 and 62) ; 
 
174 A MANUAL OF DENTAL ANATOMY. 
 
 and if there be such a membrane, it must be a secondary 
 development upon the surface of the mass of cells -which 
 primarily constitute the rudiment of the dentine papilla, and 
 in that case is not a part of the general basement membrane 
 of the oral mucous membi'ane ; or else it must have been 
 carried above as a sort of cul de sac in front of the inward 
 growing process of epithelium, to which in that case it would 
 belong rather than to the dentine germ. Neither of these 
 suppositions commend themselves as probable ; and a still 
 greater obstacle to the acceptance of a membrane in this 
 position is afforded by the structure of Marsupial teeth (see 
 fig. 23), in which the membrane would be everywhere per- 
 forated by the soft contents of the dentine and enamel 
 tubes. 
 
 Robin et Mag-itot. Journal de I'anatomie. 1866. 
 Legkos et Magitot. FoUicule Dentaire. Journal de ranatomie 
 de M. Ch. Robin, 1873. 
 Morphol. du foUicule dentaire. 1879. 
 Fonnation de I'organe dentaire. 1881. 
 Klein. Atlas of Histology. 1880. 
 Waldeyer. Strieker's Histology. 1870. 
 Huxley. Quart. Jour. Micros. Science. 1853. 
 Kollikek. Gewebelehre. 
 Tomes, J. Quart. Jour. Micros. Science, 1853. 
 
 Dental Surgery. 1859. 
 Tomes, Chakles S. Develop, of Vascular Dentine. Philos. Trans. 
 1878. 
 Develop, of Teeth of Batrachia. Ophidia, Se- 
 lacliia, and Teleostei. Phil. Trans. 1875 — 
 187G. 
 On Nasmyth's Membrane. Q. J. Microsc 
 Science, 1872. 
 Owen. Odontography. 1845. 
 
 Anatomy of Vertebrates. 1870. 
 Nasmyth. Med. Chirurg. Transact. 1839. 
 
 Observations on the Teeth. 1835. 
 Mabcxjsen. Bulletin de I'Acad. de S. Petersburg. 1849. 
 GOODSIB. Edinbm-gh Med. and Surg. Journal. 1838. 
 Beale, Dr. Lionel. Structure of the Simple Tissues. Archives 
 
 of Dentistry, vol. i. 
 Dursy, EiiiL. Entwickelungsgeschichte des Kopfes. 1869. 
 
THE DEVELOPMENT OF THE JAW. 
 
 Heetwig. Entwickelung der Placoidscliuppen uud Ziihnc. Jen- 
 aische Zeitsclirift. 1874. 
 Zahnsystem der Amphibien. Ai-cliiv. f. Mik. Anat. 
 187i. 
 Raschkow. Meletemata circa Dentium Evohitionem. 1835. 
 Heincke. Zeitsclirift f. "Wiss. Zool. Bd. :ixiii. 187:;. 
 WeI)L. Pathologie der Zahne. 1870. 
 Tomes axd De 5lOEaAX. On Development of Bone. Phil. Trans. 
 
 1852. 
 Gegenbal'E. Manual of Comparative jinatomj. Translated by 
 
 Jeffrey Bell. 1878. 
 ROLLET. Connective Tissues, in Strieker's Histology, 
 Haetixg. Quart. Journal Micros. Science. 1872. 
 Eainie. Brit, and Foreign Med.-Chimrg. Review. 1S.")7. 
 Dean. 31. S. Annotated Translation of Robin and Magitot on the 
 Origin of the Dental Follicle. Chicago, 1880. 
 
CHAPTER V. 
 
 THE DEVELOPMKXT OF THE JAWS — THE ERUPTION AND THE 
 ATTACHMENT OF THE TEETH. 
 
 At an early period in the development of the embryo 
 there is a single primitive bnccal cavity, which is subse- 
 quently divided into a nasal and an oral cavity by the 
 palatine plates growing horizontally across it ; the phaiynx 
 l)ehind the hinder end of the primitive buccal cavity 
 remains undivided. Both upper and lower jaws make their 
 appearance about the twentieth day as little buds from the 
 first visceral arch, and grow inwards towards the middle 
 line : those which form the lower jaw reach to the middle 
 and there coalesce, those for the upper jaw stop short, and 
 the gap left between them is filled by a double downward 
 sprouting process from the forehead, which afterwards forms 
 the intermaxillary bone. A failure in the coalescence of the 
 maxillary processes with this intermaxillary process, on 
 one or both sides, results in a single or double hare-lip. 
 
 In the lower jaw or mandibular processes there ap2:)ears, 
 about the end of the first month, a dense cord of cartilaginous 
 consistence, Meckel's cartilage, which seems to serve as a 
 scaffolding, giving form and consistency to the lower jaw 
 prior to the occurrence of calcification. Meckel's cartilage, 
 formed as two distinct halves, soon unites in the middle, and 
 then forms a continuous curved bar, the hinder ends of 
 which reach up to the tympanum. 
 
 About the fortieth day a centre of ossification appears in 
 the mandibular process, which, spreading rapidl}', soon forms 
 
THE DEVELOPMENT OF THE JAJF. 177 
 
 a slight osseous jaw outside Meckel's cartilage, which is not 
 however in any way implicated in it, and very soon begins 
 to waste away, so that by the end of the sixth month it has 
 disappeared : that end of it alone which extended up to the 
 tympanum does not so waste away, but becomes ossified into 
 the malleus. There are, however, observers who hold that 
 in some animals, at all events, Meckel's cartilage plays a 
 more active part in ossification of the jaw. 
 
 In the upper jaw the suture separating the intermaxillary 
 from the maxillary bones becomes obliterated ver}' early on 
 the exterior surface, but it long remains distinguishable on 
 the palatine aspect of the bones. 
 
 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 an-ive at a tolerably coiTect 
 appreciation of the process. In the first edition of my 
 father's " Dental Surgery," the results of a very extensive 
 series of observations can-ied out upon maxillae collected by 
 himself, were detailed, confirming in the main Hunter's 
 conclusions, but adding many new points to orn* knowledge ; 
 and from this work I have borrowed largely in the present 
 chapter. Professor 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 
 towai'ds 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 princiiile underlies the 
 
178 A MANUAL OF DENTAL ANATOMY. 
 
 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 intimate relation with the teeth, 
 the other serving a distinct purpose, and being only secon- 
 darily connected with the teeth, 
 ^ 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 con- 
 siderable development. Once formed it is never removed, 
 save that when in advanced old age the muscles of mastica- 
 tion are no longer in full use, it becomes, to a slight extent 
 only, wasted. 
 
 In order to understand the drift of the following descrip- 
 tion, 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 formed 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 thus formed is 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 fuiTOW is widely open at the top. 
 
 Passing on to the condition of the mandibles at the time 
 of birth, the two halves are as yet not anchylosed, but are 
 united only by fibro-cartilage. " The alveolar margins are 
 deeply indented with lai-ge open ciypts, more or less per- 
 
THE DEVELOPMENT OF THE JAW 
 
 179 
 
 fectly formed. The depth of these bony cells is only suffi- 
 cient to contain the developing teeth and tooth pulps, the 
 
 Fig. 77 ('). 
 
 (') Upper and lower jaws of a nine months fcetus, 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 lateral 
 incisors, h. Alveoli of canines, c. Alveoli of second temporary and first 
 permanent molars. A bristle has been passed through the inferior dental 
 canal. 
 
 N 2 
 
180 A MANUAL OF DENTAL ANATOMY. 
 
 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 sei'ies of cells that which at 
 an earlier nge was but a continuous groove, are less perfect 
 at the back than at the front part of the mouth. The 
 alveoli of the central 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 pulp of the 
 corresponding permanent 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 lateral incisors occupy a position slightly 
 posterior to those for the central teeth, 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 central incisors are rendered 
 broader in front than behind, and the relative dimensions 
 of the sockets of the lateral teeth are reversed, as shown in 
 Fig. 77. The crypts of the canine teeth are placed a little 
 anteriorly to those of the laterals, and nearly in a line with 
 those of the central incisors, giving to the jaws a somewhat 
 flattened anterior aspect." 
 
 While the main bulk of the lower jaw is 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 distinct 
 cavity cannot be said to exist, being merely represented 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. 
 
THE DEVELOPMENT OF THE JAW. 181 
 
 A full half of the length of the crowns of the central 
 incisors, about half that of the laterals, and the tips only of 
 the canines are calcified ; the first temporary molars are 
 complete as to their masticating surfaces ; the second tem- 
 porary molars 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 not being yet formed. 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 central incisor, somewhat less of the lateral, 
 and the extreme tip of the canine will be honeycombed, 
 ■while the remainder of the tooth will be perfect, thus per- 
 petuating 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 maxillse 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 con-espondence 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 sutvu-es uniting the two halves of the jaws 
 to one another, which is compensated by the inclination 
 in wards, of the alveoli of the central incisors. In the lower 
 jaw the articidar process, at first hardly raised above the 
 level of the alveolar border, rises rapidly up, the direction 
 
182 A MANUAL OF DENTAL ANATOMY. 
 
 of the ramus at first remaining oblique, though the angle of 
 the jaw becomes developed as a stout process for the attach- 
 ment of muscles. At the age of six months the symphysis 
 is still well marked, and the mental prominence first becomes 
 noticeable. 
 
 Fig. 78 ('). 
 
 An additional bony crypt for the first permanent molar 
 has also appeared, though its separation from that of the 
 second temporary molar, from which it was at first in no 
 way distinct, is yet incomplete, especially in the lower jaw. 
 In the upper jaw the first permanent molar crypt has no 
 posterior wall ; bony cells for the permanent central incisors 
 are well marked, but those for the laterals are mere deep 
 pits in the palatine wall of the crypts of the temporary teeth. 
 
 At the age of eight months, or thereabouts, the process 
 of the eruption of the teeth, or "teething," has fairly set in ; 
 anchylosis has taken place at the symphysis of the lower 
 jaw, the mental prominence is well marked, and in the upper 
 jaw the antrum has become a deep depression, extending 
 under the inner two-thirds of the orbit. 
 
 Postponing for the moment the consideration of the erup- 
 tion of the teeth, in order to follow up the growth of the 
 jaws, it becomes necessary to take some fixed points as 
 standards from which to measure the relative alteration of 
 other portions of the bone. In most bones, processes for 
 the attachment of muscles would be very unsuitable for the 
 pui-pose, because they would alter with the general altera- 
 
 (1) Lower jaw of a nine months foetus. 
 
THE DEVELOPMENT OF THE JAW. 183 
 
 tion in the dimensions of the bone : thus a process situated 
 at a point oue-third distant from the articular extremity of 
 a large bone, will still be found one-third distant from the 
 end, though the bone has doubled in length. The four 
 little tubercles which give attachment to the genio-hyo- 
 glossus and genio-hyoid muscles are not, however, open to 
 these objections, as they are already, so to speak, at the end 
 of the bone, or, at least, of each half of it ; and their general 
 correspondence in level with the inferior dental canal, which 
 can hardly be imagined to undergo much alteration, in- 
 dicates that their position is tolerably constant. 
 
 The points selected as landmarks are then, the spinte 
 mentales, the inferior dental canal and its orifice, and the 
 mental foramen. The mental foramen itself does undergo 
 slight change in position, but this change can easily be 
 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 tem- 
 porary molar ; later on it corresponds with the root of the 
 first bicuspid, which is thus shown to succeed, in exact 
 vertical position, the first temporary molar. 
 
 On the inner surface of the jaw the tubercles for the 
 attachment of the genio-hyo-glossus and genio-hyoid muscles 
 ai-e in the foetus, opposite to, and very little below the base 
 
]S4 A MANUAL OF DENTAL ANATOMY. 
 
 of the alveoli of the central 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 genei-al result arrived at by measurements taken 
 from these fixed points is that the alveolar arch occupied 
 by the teeth which have had deciduous predecessors, namely 
 the incisors, canines, and bicuspids, corresponds very closely 
 with the whole alveolar arch of the child in whom the tem- 
 porary dentition is complete ; and that the difterences which 
 do exist are referable, not to any fundamental alteration in 
 form or interstitial growth, but to mere addition to its 
 exterior sm'face. Or more briefly, that the front twenty of 
 the permanent succeed vertically to the places of the tem- 
 porary 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 other points on the surface. 
 
 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 temporary 
 molars, and at about the level of the genio-hyo-glossus 
 tubercles, it will be found that the increase is slight, if any, 
 notwithstanding that in other dimensions there is a very 
 great difl:erence between the jaws of a nine months foetus 
 and of a nine months child. 
 
 Again, if an imaginary line be sti'etched across between 
 these two points, and from its centre a line be drawn for- 
 wards to the spina mentalis in the same two jaws, this will 
 be found to differ but little in length in the two specimens. 
 
 But, if instead of measuring to the spina mentalis, the 
 line had been carried to the anterior alveolar plate, a great 
 dift'crence would have been observable ; in point of fact, con- 
 temporaneously with the development of the crypts of the 
 permanent teeth inside them, the temporary teeth and their 
 outer alveolar plates are slowly pushed outwards, a process. 
 
THE DEVELOPMENT OF THE JAJF. 
 
 185 
 
 the results of which we see in the separation which cc 
 about between each one of the temporary teeth, prior to 
 their being shed, where 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 
 results, which I have 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 
 
 (') Diagram representing a jaw of a nine months fcetus, superimposed 
 upon an adult jaw, to show in what directions increase has taken place. 
 
186 A MANUAL OF DENTAL ANATOMY. 
 
 one another at considerable intervals ; and by additions to 
 its exterior surfoce 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 men- 
 tioned, 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 
 temporary teeth 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 fcetus to the seven 
 years old child, in whom the temporary dentition is complete, 
 the framework of the jaw below our imaginary line has 
 
 far/t/.f _p/77c>/!///j 
 
 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, how- 
 ever, is far deeper in the adult than in the child (this 
 difference is not sufficiently well marked in the figure), and 
 
 (1) Lower jaw. The horizontal line marks the level of the inferior 
 dental canal. 
 
THE DEVELOPMENT OF THE JAW. 
 
 187 
 
 in fact constitutes almost the whole increase in vertical 
 dimensions down the passage from the child's to the adult's 
 form of the jaw. 
 
 Fig. 81 ('). 
 
 In the lower jaw we may take it as i^roven that the 
 basal 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 
 
 (^) Lower jaw of an adult. 
 
 ('-) Lower jaw of an aged person, the dotted lines indicating the outline 
 of the parts removed by absorption, as the jaw assumes the form charac- 
 teristic of advanced age. 
 
188 A MANUAL OF DENTAL ANATOMY. 
 
 which I shall again return in speaking of the erui)tion of 
 the teeth. 
 
 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 the comi^lete anchylosis taking 
 place : the share taken by this in increasing the size of the 
 jaw would, however, appear to be but small, after the ter- 
 mination 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. 
 
 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 im- 
 portant part. The manner in which the jaw is formed 
 might almost be described as wasteful ; a very large amount 
 of bone is formed which is subsequently, at no distant date, 
 removed again by absorption; or we might compare it to a 
 modelling process, in which thick, comparatively shapeless 
 masses, are dabbed on to be trimmed and pared down into 
 form. 
 
 To bring it more clearly home to the student's mind, if 
 all the bone ever formed were to remain, the coronoid pro- 
 cess would extend from the condyle to the region of the first 
 bicuspid, and all the teeth behind that would be buried in 
 its base : there would be no " neck " beneath the condyle, 
 but the internal oblique line would be a thick bar, corre- 
 sponding 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 Ihie ; 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 Avas redundant. 
 
THE DEVELOPMENT OF THE JAW. 
 
 On the outer surface of the jaw we can frequently discern 
 a shght 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 imme- 
 diately on the inner side of the condyle ; and microscopic 
 examination of the bone at this point shows that the lacuna3 
 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 acting on 
 its anterior, and deposition on 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 
 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 pos- 
 terior border, so that the above statements I'est upon a basis 
 of observation, and are not merely theoretical. Two cases of 
 arrested development of the jaw ("Dental Surgery," p. 108) 
 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. 
 
 AVedl inclines to this latter view, and the question cannot, 
 I think, be held to be absolutely settled. Although it is 
 difficult to form any definite conception of interstitial growth 
 in a tissue so dense and unvielding as bone, so that the doc- 
 
190 A MANUAL OF DENTAL ANATOMY. 
 
 trines promulgated in the foregoing pages have the support 
 of ct. priori probability, there are some rather paradoxical 
 facts to be met with in comparative odontology. Never- 
 theless, there can be no doubt, that backward elongation as 
 teeth are successively added, &c., is sufficiently near the 
 truth in the case of human and most mammalian jaws for 
 practical jDurposes. 
 
 It remains to notice the changes in form which the 
 ascending ramus and the angle of the jaw undergo. In 
 the foetus 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 supex'ficial and not an interstitial absorption, cor- 
 responding with a wasting of the muscles, of the pterygoid 
 plates of the sphenoid bone, &c. 
 
 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 theoiy 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 elonga- 
 tion of their fangs. 
 
ERUPTION OF THE TEETH. 191 
 
 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 
 root — are often ervipted. Again, a tooth may have the 
 whole length of its roots completed, and yet remain buried 
 in the jaw through half a person's life, and then, late in life, 
 be erupted. Moreover, when a healthy normal tooth is being 
 erupted, the distance travelled by its crown materially ex- 
 ceeds the amount of addition to the length of its roots which 
 has gone on during the same time. 
 
 To turn to comparative anatomy, the tooth of a crocodile 
 moves upwards, tooth pulp and all, obviously impelled by 
 something different from mere elongation; and my own 
 researches upon the development and succession of rep- 
 tilian teeth clearly show that a force quite independent of 
 increase in their length shifts the position of, and " erupts" 
 successive teeth. But what the exact nature of the impulse 
 may be, is an unsolved riddle : the explanations which I have 
 read, being, to my mind, less satisfying than the admission 
 that we do not know. 
 
 Towards the eighth month of childhood the bony crypts 
 which contain the temporary teeth in the front of the mouth 
 begin to be renewed. The process of absoi-ption goes on 
 with greater activity over the fronts of the crowns than over 
 their apices, so that almost the whole outer wall of the 
 alveoli is removed. At the back of the mouth the crypts 
 still retain their inverted edges ; indeed, development 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 Ijeing still i-equired as forming 
 part of the crypt of the developing successional tooth. But 
 no sooner has the crown passed up through the very wide 
 and fi-ee orifice so formed, than absorption gives place to 
 
192 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 deposition, and the bone rapidly developes so as to loosely 
 embrace the neck of the tooth. 
 
 Additions to the margin of the alveoli keep pace with 
 
 Fig. 83 ('). 
 
 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 iiniformly 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 iip and the jaw 
 is deepened posteriorly ; meanwhile the elongation of the 
 
 Q) Jaws of a male nine montlis old, in wliicla the eruption of the teeth 
 is jnst commencing. 
 
ERUPTION OF THE TEETH. 193 
 
 rami has been going on slowly, but -without interruption. 
 Thus is brought about a condition of parts allowing of 
 the whole series of teeth coming into their proper mutual 
 antagonism. 
 
 It was pointed out by Trousseau that the eruption of the 
 teeth is not a contimious process, which, once commenced, is 
 carried on without intermission to its completion, but that it 
 is interrupted by period? 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 nume- 
 rous ; the following may lie taken as an approximation to 
 the tiiith : — 
 
 The lower centrals 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 laterals 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 
 place. To this, as well as to the greater length of their 
 root (though 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 that of any other, but Dr. West thinks that the 
 eruption of the first molars causes the most suffering. It 
 may also be noted that the canines during their develop- 
 ment 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. 
 
194 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 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 not till between the fourth and sixth 
 years that the temporary set of teeth can be called abso- 
 lutely complete. 
 
 At the sixth year, preparatory to the appearance of any 
 
 of the permanent teeth, the temporary teeth maybe 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 
 
 (^) Normal well-formed jaws, from wliicli the alveolar plate has been in 
 great part removed, so as to expose the developing permanent teeth in 
 their crypts in the jaws. 
 
ERUPTION OF THE TEETH. 195 
 
 temporan- 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 slight degree of overlapping of the edges of the per- 
 manent central and lateral incisors exists. 
 
 The bicuspids lie in bony cells which are embraced pretty 
 closely by the roots of the temporary molars, and it hence 
 happens that extraction of the 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 new 
 bone is rapidly formed which embraces the neck, prior to 
 any considerable length of root being formed. 
 
 Last, then, follows the absorption of the root of the tem- 
 porary teeth, a matter first accurately investigated by my 
 father. The root at or near to its end, becomes excavated 
 by shallow cup-shaped depressions ; these deepen, coalesce, 
 and thus gradually the whole is eaten away. Although ab- 
 sorption usually commences on that side of the root which 
 is nearest to the successional tooth, it by no means invari- 
 ably does so ; it may be, and often is, attacked on the oppo- 
 site side, and in many places at once. 
 
 The cementum is usiially 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 independent of pressure ; the varying position 
 of the excavation has already been noticed, and it may be 
 added that in many lower animals, for examjjle, the frog or 
 the crocodile, the growing tooth sac passes bodily into the 
 
196 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 excavation made before it in the base of the tooth which has 
 preceded it, while if pressure had had any share in the matter 
 the cells of its enamel organ, &c., must have inevitably been 
 crushed and destroyed. 
 
 Again, when the absorption and shedding of the first teeth 
 
 Fig. 85 C). 
 
 have taken place early, before their successors are ready to 
 appear, perfect little sockets are formed behind the lost 
 
 (^) Jaws of a six year old child. In the upper jaw complete sockets 
 = een where the temporary incisors have been shed. 
 
ERUPTION OF THE TEETH. 197 
 
 temporary teeth, cutting them off from the permanent teeth 
 destined to follow them. Absorption, too, may attack the 
 roots of permanent teeth, which is another reason for regard- 
 ing the process as not necessarily dependent upon the approach 
 of a displacing tooth. Closely applied to the excavation 
 jiroduced by absorption is a mass of veiy vascular soft tissue, 
 the so-called absorbent organ. The surface of this is com- 
 posed 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 
 hemispherical indentations, the "lacunae of Howship," into 
 each of which one of the giant cells 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 amoebiform processes, others that they secrete 
 an acid fluid, but nothing very definite is known ; a curious 
 parallel is afforded by the manner in which a fungus can 
 di-ill 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 sur- 
 face eroded ; probably by the agency of the absorbent cells 
 themselves, which are capable of being calcified in the exca- 
 vations they have individually made. 
 
 These alternations of absorption and deposition, so com- 
 mon a result of inflammations of the pulp, or of the alveolo- 
 dentar periosteum, as to be diagnostic of the former occurrence 
 of these maladies, often occur during the normal process of 
 the removal of the deciduous teeth, and result in the deposi- 
 tion of. a tissue not unlike cementum in excavations made 
 in the dentine, or even in the enamel. 
 
198 A MANUAL OF DENTAL ANATOMY. 
 
 The eruption of the permanent teeth is a process closely 
 analogous to that of the temporary set. Rapid 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 temporar}' tooth, the action of the lips and tongue, 
 &LC., are all potent agencies in modifying the arrangement of 
 the teeth. 
 
 The temporaiy teeth stood vertically, the })ermanent 
 teeth in front of the mouth stand obliquely, thus opening 
 a space between the lateral incisors and the first bicuspid 
 for the canine, which during development was out of 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 simul- 
 taneously. 
 
 The permanent teeth usually make their appearance in 
 the following order : — First permanent molars, about the 
 seventh year ; a little later, the lower central incisors, upper 
 centrals and laterals, the first bicuspids, the canines, the 
 second bicuspids, the second permanent molars, the third 
 permanent molars. 
 
 The period of eruption is variable. From a comparison 
 of several tables, I find the principal discrepancies to relate 
 to the date of the appearance of the canines and the second 
 bicuspids. The canine would certainly appear to belong to 
 the eleventh and twelfth years ; but some authors consider 
 that the second bicuspid is usually cut earlier, otliers 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 
 
ERUPTION OF THE TEETH. 199 
 
 the temporary teeth : and next they were rebuilt about the 
 necks, to form the sockets, of the deciduous teeth. 
 
 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 teeth come down through 
 the gaining orifices. 
 
 When they have done so, the bone is reformed 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 to the floor of the antrum 
 and the 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 subsei'vuent to the position of the 
 tooth ; wherever the tooth may chance to get to, there its 
 socket will be built up round it. 
 
 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. 
 
 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 
 tlie 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 
 
200 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 period than is natural, they sometimes become elevated to 
 the general level of the permanent teeth (which is consider- 
 ably higlier 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 the tooth a higher level than before. 
 
 Fig. 86 ('). 
 
 
 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 
 regulation cases : it remains to say a few words as to the 
 forces which do 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 
 
 {}) From a child aged fourteen. The specimen well exemplifies the fact 
 that the height of the alveolar edge corresponds exactly to the position 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. 
 
ERUPTION OF THE TEETH. 201 
 
 miich 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 a 
 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 is, with equal 
 symmetry, pushing them outwards : between the two forces, 
 the lips and the tongue, the teeth naturally become moulded 
 into a symmetrical arch. That the lips and tongue are the 
 agencies which mainly model the arch is very well illustrated 
 by that which happens in persons who have from childhood 
 suffered from enlargement of the tonsils, and are conse- 
 quently obliged to breathe through the mouth, which is thus 
 pretty constantly open. This causes a slight increase in the 
 tension of the lips at the corners of the mouth, and is im- 
 pressed upon the alveolar arch as an inward bending of the 
 bicuspids at that point ; thus persons with enlarged tonsils 
 will be found, almost invariably, to present one of the forms 
 of mouth known as V-shaped. 
 
 But Dr. Norman Kingsley attaches far more importance 
 to disturbed innervation than to any mechanical causes, and 
 refers most dental iiTegularities to unhealthy conditions of 
 the child's nervous system. 
 
 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 an- 
 tagonism ; 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. 
 
202 A MANUAL OF DENTAL ANATOMY. 
 
 THE ATTACHiMENT OF TEETH. 
 
 Although the various methods by which teeth are fixed iu 
 their position upon the bones which carrj^ them pass by 
 gradational 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 toy means of Fibrous Membrane. — An 
 excellent illustration of this manner of implantation is 
 afforded by the Sharks and Rays, in which the teeth have 
 no dii'ect connection with the cartilaginous, more or less 
 calcified, jaws, but are imbedded solely in the tough fibrous 
 mucous 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. description of the denti- 
 tion of the sharks). That the whole fibrous giim, with the 
 attached teeth, does really so slide over the surface 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 mucous membrane which furnished the dentine papillte ; 
 and the gradual assumption of the fibrillated structure by 
 that portion of the mucous 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, and 
 very dense bands being attached to the bases of the com- 
 pleted calcified teeth. 
 
 Attachment by an Elastic Hinge. — The possession 
 
THE ATTACHMENT OF TEETH. 203 
 
 of moveable teeth, able to yield to pressure and subsequently 
 to resume the upright position, was formerly supposed to be 
 confined to the Lophius (Angler) and its immediate allies. 
 I have however found hinges in the common Pike (Esox), 
 and in the Gadidce (Cod tribe) ; so that, as they occiu' in these 
 fish so widely removed from one another in other respects, it is 
 probable that further investigation will bring 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 posterior 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 from the bone. The elasticity of 
 the ligament is such that when it has been compressed by 
 the tooth being over towards it, it returns it instantly into 
 position with a snap. Many of the teeth of the Angler are, 
 like most fishes' teeth, anchylosed firmly. 
 
 The Hake (Merlucius, one of the Gadidce) possesses two 
 rows of teeth, the inner and shorter of which are anchylosed, 
 whilst the outer and longer are hinged. 
 
 In some respects these hinged teeth are more highly 
 specialised than those of the Angler, which they resemble 
 in being attached by an elastic hinge fixed to their inner 
 sides, the elasticity of which is brought into play by its being 
 compressed, or at all events bent over, upon itself. 
 
 The pulp is highly vascular, and its vessels are so arranged 
 that, by entering the pulp through a hole in the ligament, 
 which is about at the axisof motion, they escape being stretched 
 or torn during the movements of the tooth. But the base 
 
204 A MANUAL OF DENTAL ANATOMY. 
 
 of the tooth itself is modified so as to be pai'ticularly fitted 
 for resisting the jars to which a moveable tooth must at times 
 be exposed, and so is the bone upon which it is set. 
 
 As is seen in the figure, the base of the tooth, or the side 
 
 Fio. 87 (')• 
 
 
 opposite to the hinge, is thickened and rounded, the advan- 
 tages which such a form must possess over a thin edge when 
 bumping upon the bone being sufiiciently obvious. This 
 
 (') Hinged tooth of Hake. a. Vaso dentine, h. Pulp. c. Elastic 
 hinge, d. Buttress of bone to receive /, formed out of bone of attach- 
 ment. €. Bone of jaw. f. Thickened base of tooth, g. Enamel tip. 
 
THE ATTACHMENT OF TEETH. 205 
 
 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 moveable in a less degree, whilst others 
 again have teeth rigidly fixed. So that within the limits of 
 a single family we have several steps in a gradual progres- 
 sion 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 con- 
 cerned 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 interior with a development of osteodentine, which, by 
 becoming continuous with the subjacent bone, cements them 
 upon it. The manner of development of this is by rods of 
 calcifying material shooting down through the central pulp 
 (see page 16.5) ; in the hinged teeth also these trabeculse shoot 
 down, and become continuous with the subjacent bone, only 
 instead of 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, so that the teeth depressed 
 and suddenly released return with an audible snap, but it 
 resides solely in these strings, for if these be divided by 
 carefully slipping a cataract needle under the tooth without 
 
206 A MANUAL OF DENTAL ANATOMY. 
 
 injuring the binge, the tooth will stay in any position into 
 
 which it is put. 
 
 Fio. 88 ('). 
 
 The points most noteworthy are, (i.) that hinged teeth have 
 arisen independently in three 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 a 
 different mechanism, and by the least modification possible 
 of the existing fixed teeth of the family. 
 
 (1) Hinged tooth of Pike. a. Dentine, h. Elastic rods, formed of un- 
 calcified trabeculoe which might have become bone. c. Hinge, not itself 
 elastic, d. Bone of attachment, c. Bone of body of jaw. 
 
THE ATTACHMENT OF TEETH. 207 
 
 Attachment by Anchylosis. — In both the socketed and 
 the membranous manners of attachment an organised, more 
 or less vascular membrane, intervenes between the tooth and 
 the jaw-bone ; in the method now under consideration 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 
 oflf under the application of only 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 
 cone is composed of osteodentine. The method by which the 
 entire fusion of this tissue with the bone beneath it takes 
 place has ah'eady been alluded to, the similarity of its 
 method of calcification with that of bone rendering the 
 fusion easy and complete. 
 
 And in certain extinct fish, whose nearest ally is the now 
 anomalous Australian shark, the Cestracion philippi, the 
 lower part of the tooth is composed of osteodentine, which 
 so closely resembles bone itself that it is impossible to say 
 at which point the bone may be said to commence and the 
 tooth to end ; but even where this intimate resemblance in 
 histological character does not exist, there is often to be 
 found more or less blending of the basal dentine with the 
 bone beneath it, so that there is even here a sort of transi- 
 tional region. 
 
 From the accounts which pass current in most text books 
 it would be supposed that the process of attachment by 
 anchylosis is a vezy 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. 
 
208 A MANUAL OF DENTAL ANATOMY. 
 
 In the few animals which I have examined ('), however, I 
 have found that this conception 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 ; but the union 
 takes j)lace through the medium of a portion of bone (which 
 may be large or small in amount) which is specially developed 
 
 to give attachment to that one particular tooth, and after 
 the fall of that tooth is itself removed. 
 
 For this bone I have proposed the name of "bone of 
 attachment," and it is strictly analogous to the sockets of 
 those teeth which have sockets. It is well exemplified in 
 the Ophidia, a description of the fixation of the teeth of 
 
 Q) Transactions of the Odontological Society, Dec. 1874. — "Studies on 
 the Attachment of Teeth." 
 
 (-) Section of tooth and a portion of the jaw of a Pytlion, showing the 
 marked difference in character between the bone of attachment and the 
 rest of the bone. 
 
THE ATTACHMENT OF TEETH. 209 
 
 which will serve to convey a good idea of its geucral cha- 
 racter. If the base of one of the teeth, with the subjacent 
 jaw-bone, be submitted to microscopic examination we shall 
 find 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 lacunce, with their very numerous fine 
 canaliculi, very regular, and the lamination obviously refer- 
 able 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 lamin- 
 ation is roughly parallel with the base of the tooth. The 
 dentine of the base of the tooth also bends inwards (Fig. 89), 
 and its 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 in- 
 timate relation with the tooth with which it is continuous, 
 for it is wholly removed with the Ml of the tooth, and is 
 specially developed again 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 ossifi- 
 cation, apparently contributes little. 
 
 In the frog the teeth are commonly described as being- 
 attached by their bases and outer surfiicc to a continuous 
 groove, of which the external wall is the highest. Such is, 
 however, an inadeqiiate description of the process, the 
 tooth, as seen in section, being attached on its outer side 
 by a new development of special bone, which extends for a 
 short distance up over its external surface ; and for the 
 support of its inner wall there springs up from the sub- 
 jacent bone a pillar of bone, which is entirely removed when 
 that tooth falls, a new pillar being developed for the next tooth. 
 
210 A MANUAL OF DENTAL ANATOMY. 
 
 When the teetli arc, as in many fish, implanted upon 
 what to the naked eye appears nothing more than a plane 
 surface of bono, a microscopic examination generally, in fact 
 in all specimens which I have examined, reveals 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. 90 ('). 
 
 (Fig. 90), in which each tooth surmounts a short hollow cylin- 
 der of bone, the lamination, &c., of which differs strongly 
 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 magiiifying power 
 
 (') From lower jaw of an Eel. a. Bone of jaw. h. Bone of attach- 
 ment, d. Dentine. /. Enamel, y. Space vacated by a shed tooth. 
 
THE ATTACHMENT OF TEETH. 
 
 the bone at this point would be found to be excavated by 
 " How'ship's lacunae." As an anchylosis, the implantation 
 of the teeth is less perfect than that of those of the snake, 
 for the dentinal tubes at the base of the tooth are not de- 
 
 flected, and do not in any sense blend witli the bone beneath 
 them. Accoi'dingly, the teeth are fir loss firmly attached, 
 and bi-eak off quite readily. 
 
 A transition towards the socketed type of implantation is 
 furnished by some of the cod tribe. In the haddock, for 
 example, the teeth surmount hollow cylinders of " bone of 
 attachment," resembling in many particulars those of the 
 eel; the teeth do not, however, simply surmoimt the bonv 
 cylinders, but are continued for a short distance within 
 them, definite shoulders being formed which rest on the 
 rims of the cjdinder. The base of the tooth does not, how- 
 ever, 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 con- 
 
 (^) From lower jaw of a Haddock, 
 inent, rf. Dentine of tootJi. 
 
 a. Bone of jaw. /;. lione of attach- 
 
212 A MANUAL OF DENTAL ANATOMY. 
 
 tinuous with the cavity of the osseous cylinder, into which 
 it is for a shoi't distance continued. 
 
 The bony supports of the teeth originate in many osseous 
 trabecula) whicli spring up simultaneously from the lione 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 my own researches enable 
 me to say, there is this much in common in all forms of 
 attachment by anchylosis, 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, which is more or less directly an 
 outgrowth from the jaw-bone itself, which is in some im- 
 seen manner stimulated into activity by the proximity of the 
 tooth. In amount this specially formed bone varies greatly^ 
 but in all instances it is not the tooth capsule, but tissue* 
 altogether external to this, which serve to secure the tooth 
 in its place by their ossification. 
 
 The teeth of the maclcerel present an interesting variety of 
 attachment by anchylosis. The margins of the jaws are very 
 thin, and by no means fleshy, and in this thin margin there 
 is a deep groove between the outer and inner plate of the 
 bone. In this groove are the teeth, their sharp points pro- 
 jecting beyond the edges of the bone, and they are held in 
 their place 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, 
 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. The 
 manner in which the sockets are, so to speak, plastered 
 
THE ATTACHMENT OF TEETH. 213 
 
 around the roots of the teeth, and are perfectly subservient 
 to and dependent on them, has already been described ; little, 
 therefoi-e, need be added here, save that the soft tissue 
 intervening between the bone and the tooth is not sepa- 
 rable, either anatomically or from the point of view of 
 -development, into any two layers, but is a single mem- 
 brane, termed the "aiveolo-den tar-periosteum." That it is 
 single, 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 nature and develojiment of the sockets in those 
 few reptiles and fishes whicli have socketed teeth require 
 further examination. I am not, from what I have seen in 
 sections of the jaws of a young crocodile, 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 
 subsex'viency to each individual tooth ; on the contrary, 
 successive teeth come up and occupy a socket which is 
 •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, no example of anchylosis 
 occm-ring between the tooth and the bone of the socket has 
 €ver been met with in man, or indeed in any mammal ex- 
 •emplifyiug a typical socketed implantation of the teeth. 
 
 Hunter. On the Anatomy of the Human Teeth. 
 
 Tomes, J. Dental Surgery. 1859. 
 
 HUMPHERY. Ti-ansact. Camb. Philos. Soc. 1863. 
 
 Wedl. Pathology of the Teeth. 
 
 Heudker. Beitrage zur Lehre von der Knochenentwickelung, 
 &c. 
 
 Tomes, Charles S. On Vascular Dentine and Hinged Teeth. 
 Philos. Transac, 1878, and Quart. Journal 
 Micros. Science, vol. xvii. new series. 
 Ti-ansac. Odontolog. Soc. 1874 — 187G. 
 
CHAPTER VI. 
 
 THE TEETH OF FISHES. 
 
 In the following pages nothing more than a brief account 
 of a few typical forms can be attempted ; the limits of space 
 forbid the mention of many creatures, or the insertion of 
 detailed descriptions of the dentition even of the few which 
 are included in its pages. In the class of fish the task of 
 selection of the 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 forms from which he 
 may gather, as best he can, a general idea of piscine 
 dentition. 
 
 Fish may be grouped into — 
 
 I. Phabyngobranchii. IV. Gaxoidei, 
 
 II. Marsipobranchii. v. Teleostei. 
 
 III. Elasmobbanchii. VI. Dipnoi. 
 
 I. PlinrymjobrancMl comprise only the Amphio.inis. 
 II. MarxipobrancMi comprise the Lampreys and the parasitic 
 Myxine. 
 
 III. Elii.sinohranc?i,ii comprise the Sharks and Eays (Phir/iostovii) 
 
 and the Chimera and its allies {IMocapJiali). Their skele- 
 tons are cartilaginous, with an ossified crust. 
 
 IV. (rtuinidri. A large number of extinct fish — of existing fish the 
 
 Lrpithititm.^. or Bony Pike, is the most familiar. 
 V. Tclcoxtei comprise the ordinary Fish in our seas and rivers. 
 VI. B'lpmn. The LepUlosircnK, or Mud Fishes, capable of living for 
 a long time in moist mud. 
 
 The Marsipohrancldi need not detain us long ; they are 
 
THE TEETH OF FISHES. 215 
 
 destitute of true calcified or dentinal teeth, the armature of 
 the mouth consisting of horny cones or serrated plates. 
 
 The parasitic Myxine, which is found in the interior of 
 other larger fish, is furnished with a median curved conical 
 tooth, of horny consistency, which is believed to act as a 
 holdfast, while the serrated edges of the horny plates upon 
 the tongue are brought into play in boring a way into the 
 interior of its host. 
 
 The concave circular disc which suiTounds the mouth of 
 the Lamprey is covered with concentrically disposed horny 
 teeth, of simple conical form ; in addition to these there 
 are lingual and palatal horny plates. 
 
 The teeth of Elasmohranchii present rather more of sim- 
 plicity and uniformity of plan than do those of most fish, 
 and it will hence be convenient to describe their teeth first, 
 although ill most respects they stand at the head of the 
 class of fishes, and present many indications of affinity with 
 the Batrachia. 
 
 In the Plagiostomi 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 snovit. 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 cartilage, neither true 
 maxillae nor premaxillse being present, are cartilaginous in 
 the main (although covered with a more or less ossified crust), 
 and therefore shrink and become much distorted in drying. 
 The shape of the jaws differs in the various groups of 
 Playiostomi, in some each of the two jaws being a tolerably 
 perfect semicircle, while in others they are nearly straight 
 and pai'allel to one another (see fig. 92 and fig. 96) ; but in all 
 the rounded working surface of the jaw is clothed or encased 
 by teeth, which are arranged in many parallel concentric 
 rows. 
 
216 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 The teeth, which are situated upon the edge or exposed 
 border of the jaw, arc 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 trojjical 
 
 Fig. 92 (';. 
 
 t n. s. 
 
 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-fishes the inner surfice of the 
 jaws forms an even rounded surface along which the rows of 
 teeth are disposed in every intermediate position between 
 those fully recumbent at the innermost pai-t 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 those teeth which 
 
 (^) Lower jaw of Lamna. a. Edge of flap of mucous membrane wliich 
 covers in the teeth not yet completed. 
 
THE TEETH OF FISHES. 
 
 217 
 
 are not as yet fully calcified and firmly attached to the 
 gum. 
 
 In Lamna, which may be taken as fairly illustrative, the 
 teeth are arranged round the jaws in concentric rows with 
 gi-eat regularity, the teeth of the successive rows con-espond- 
 ing 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 with it the teeth, slides bodily upwards over the 
 inner face of the jaw, and outwards over its border, beyond 
 
 (') Transverse section of lower jaw of a Dog-fish. «. Oral epithelium. 
 i. Oral epithelium passing on to flap. c. Protecting flap of mucous mem- 
 brane (thecal fold), d. Youngest dentine pulp. e. Youngest enamel 
 organ. /. Tooth about to be shed. g. Calcified crust of jaw. 
 
218 A MANUAL OF DENTAL ANATOMY. 
 
 which it, to borrow a phrase from geological science, has an 
 " outcrop." 
 
 In Lamna the second and third rows of teeth arc only 
 partially erect, the rows behind these lying recumbent, and 
 being in the fresh state covered in V)y the fold of mucous 
 membrane, which, being dried and shrunk in the specimen 
 figured, falls short of its original level. 
 
 Thus rows of teeth originally developed at the base of 
 the jaw arc 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 under- 
 stand why sharks' teeth are so abundantly found in a fossil 
 condition, although other indications of the existence of the 
 fish are rare enough ; 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 bedded 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 bedded in a very tough fibrous mem- 
 brane ; the natiu-e of then- fixation has been more exactly 
 described at another page (page 202). 
 
 The sheet of fibrous gum slides bodily over the curved 
 surface of the jaw, continually bringing up from below 
 fresh rows of teeth, as was proved by Andre's specimen, 
 and it may be worth while to condense from Professor 
 Owen the description of the manner in which it was thus 
 proved that an actual sliding or rotation of the membrane 
 does really take place, and that the whole bony 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 caine 
 under observation the spine had remained in this situation, 
 transfixing the jaw, for a long time, as was evidenced by all 
 
THE TEETH OF FISHES. 219 
 
 the teeth of these two rows, both above and below it, being 
 stunted aad 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 
 understand in what manner this could have affected them 
 had they not, at an earlier period of their growth, lain in 
 more immediate proximity to it. But if the membrane, 
 with the teeth attached, does move slowly along the surface 
 of the jaw, this difficulty at once disappears. 
 
 The forms of the teeth in various sharks are different 
 and characteristic ; nevertheless they vaiy somewhat with 
 age in some species, and present differences in size and 
 form in the upper and lower jaws, or in different parts of the 
 mouth of the same individual. For instance, in Lamna, in 
 the upper jaw, the third teeth of each horizontal row, count- 
 ing 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. 
 
 Thus, although it is often possible to refer a particular 
 tooth to its right genus or even species, much care is re- 
 quisite in so doing. 
 
 The teeth of the bloodthirsty white shark (Carch;u-ias) 
 are tx'iangular flattened plates, rounded on their posterior 
 aspect, with trenchant slightly serrated edges ; it is pointed 
 out by Professor 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 wdiales. 
 
 The intimate relationship between the teeth and the 
 dermal spines, which from the standpoint of development, 
 has been illustrated at page 2 and page 119, is apparent also 
 in their histological structure. There are many dermal 
 spines to be met with in the sharks, which seen alone could 
 
A MANUAL OF DENTAL ANATOMY. 
 
 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 
 page 90 is a fair example of a structure very common among 
 the sharks, viz., a central body of osteo-dentine, the outer 
 portion of which has dentinal tubes so fine, regular, and 
 closely packed as to merit the name of hard unvascular 
 •dentine, and over this again a thin varnish of enamel. C^) 
 
 And yet no observer from its structure alone could feel 
 sure whether it was a large dermal spine, or a tooth. Dental 
 tissues occur in other parts of the mouths of Selachia than 
 upon the jaws, not only in the embryonic stages, but in the 
 adult. Thus Professor Turner lias described (Proc. Koy. 
 Society, Edinburgh, 1880), very numerous comb-like ap- 
 pendages 5 inches long upon the branchial arches of the 
 Basking Shark (Selache maxima), which apparently perform 
 the same function as whalebone in straining the water. 
 These combs are formed of a variety of dentine C? osteo- 
 dentine), and closely resemble in structure the true teeth, 
 which are however very small in this shark. 
 
 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 oif 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 description. Those which have come into use 
 are, towards the back of the mouth, always much worn ; 
 their shedding and renewal takes places, as in other sharks, 
 
THE TEETH OF FISHES. 221 
 
 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 the Cestracion are fitted for the trituration 
 of hard substances, and for such they are used, its food 
 
 consisting of shell-fish, etc. The teeth consist of vaso- and 
 osteodentine, protected by what is appai-ently a structure- 
 less layer of enamel. 
 
 The extinct Cestracionts extended far back in time, beinf 
 met with in palaeozoic strata, and they were equally widely 
 distributed in space ; the size of many of the teeth also 
 indicates the existence of forms much larger than the recent 
 timid and inoffensive Cestracion Philippi. Many of the 
 extinct forms are known only by isolated teeth ; of others 
 
 (') Lower jaw of Cestracion Philippi. 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 tlie inner 
 side, and, just as in other sharks, are covered in by a flap of mucous 
 membrane. 
 
222 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 ijs.s. 
 
 portions of tlie jaw with teeth in situ have been discovered ; 
 thus fragments of the jaw of Acrodiis, 
 Fig. 95 ('). ^^g isolated fossil teeth of which have 
 
 been compared to fossil leeches, with 
 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 re- 
 markable, 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 regular intervals, and im- 
 planted 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 closely re- 
 semble the teeth of Myliobates (see 
 page 82), being made up of parallel 
 denticles, in the centre of each of 
 wiiich is a pulp cavity or medullary 
 canal. 
 
 What xise 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 — the one that they are dermal 
 spines, having a structure all but iden- 
 
 (') Rostrum and under side of the head of a small Pristis. o. Mouth. 
 i. Rostrum, c. One of the rostral teeth. 
 
 The teeth, with which the margins of the jaws are covered, are so small 
 that they cannot be represented in this figure. 
 
THE TEETH OF FISHES. 
 
 tical with that of the actual teeth of another ray, the 
 Myliobates ; the other that they are socketed, a manner of 
 implantation not at all common amongst the teeth of fishes ; 
 and yet another, 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 foiTu something ap- 
 proaching to a contniuous pavement over the jaws, with but 
 little interspace left between the teeth. 
 
 Fig. 96 ('). 
 
 :N.S 
 
 The dentigerous surface of the jaw is very much rounded, 
 and in some is completely encased under a pavement of teeth. 
 
 (^) Upper and lower jaw of Myliobates. At a, the mosaic pavement 
 fonned by the broad flattened plates whioli constitute its teeth is seen, 
 these being the oldest teeth which are about to be shed off in consequence 
 of the rotation of the whole sheet of mucous membrane over the surface 
 of the jaws. The letter 6 indicates the under surface of one of the plates, 
 which is seen to be finely fluted on its edge. 
 
224 A MANUAL OF DENTAL ANATOMY. 
 
 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, in the manner of their formation 
 and renewal conforming with the teeth of other Plagiostomi ; 
 the severe use to which they are put being indicated by the 
 extent to which the gi-inding 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 armatiu-e of the 
 jaw. 
 
 The structure of the teeth of Myliobates has already been 
 described and figured (see page 82). 
 
 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 gi-eat that few, if 
 any, general statements can be made about them. It is not 
 uncommon to find teeth crowded upon every one of the 
 bones which form a part of the bony framework of the 
 mouth and pharynx, and the teeth are sometimes in count 
 less numbers. And so great is the variability that even 
 within the limits of single families differences in the teeth 
 are to be found. 
 
 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 
 
THE TEETH OF FISHES. 
 
 means of these largest teeth ; theu, after holding it thus for 
 a time, and so 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 firml}- that when the angler '•' strikes " the 
 hooks do not get driven into the fish's mouth ; but aftei- 
 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 tectli, 
 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 Avide 
 parallel bands of teeth, those in the median band (on the 
 vomer) being directed backwards, those upon the lateral 
 bands (on the palatine bones) 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 
 they can only bend exactly in one direction. Those of the 
 vomerme 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, about at an angle of 45 with the median line of 
 the mouth, or somewhat more directly backwards. To a 
 body sliding over them in - one direction they ofter no 
 resistance, bending down as it passes, and sjjringiug up as 
 the pressm-e 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 
 
 Q 
 
2-20 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 the throat. The hinged teeth on the paLate seem ad- 
 mirably arranged for getting the fish into a longitudinal 
 
 position and keeping it there ; for, if we imagine the fish's 
 
 (^) 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 pahitine 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 l)ones. 
 
 The diagram beneath represents the direction in which the hinged teeth 
 of the vcmerinc and palatine bands can bci;d. 
 
THE TEETH OF FISHES. 221 
 
 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 eveiy 
 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 fish kept up against the palatine teeth, even its own 
 struggles will be utilised by every movement tending to 
 place it aright being allowed, and every other 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 
 (page 206). 
 
 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,) carry 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 its living prey has of escape, when once 
 it has been seized. 
 
 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 ; while the outermost portion of 
 
228 A MANUAL OF DENTAL ANATOMY. 
 
 all is a very dense and hard, and apparently structureless, 
 enamel film. The teetli are anchylosed to the bone, and are 
 very frequently renewed, their successors being developed at 
 one side of their bases. 
 
 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 piscatorius), another predatory fish, 
 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 ap- 
 j)roach 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, 20-3, as to 
 iillow of their bending in and towards the mouth, but not in. 
 the opposite or 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 sufliciently shown ; 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 difterent fish, the Hake (Mer- 
 lucius, one of the Gadicke). This fish, the most active and 
 l^redatory 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 ar- 
 ranged 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 riclily vascular vaso- 
 deutine. 
 
THE TEETH OF FISHES. 
 
 229 
 
 Another curious dentition is possessed by the Wolf-fish 
 (Anarrhicas 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 tupped crushing teeth in a double 
 
 (') Bones of the mouth of the Wolf-fish (Anarrhicas lupus). The letter 
 a. indicates the divergent pointed teeth which occupy the intermaxillaiT 
 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 (c). Strong crashing teeth are found also upon tlie 
 I>alatine bones (i), and upon the vomer (c). 
 
230 A MANUAL OF DENTAL ANATOMY. 
 
 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 tlic 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 gi'eat power, and it seldom happens that a specimen is 
 examined in which some of the teeth are not broken. It 
 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 " Gj'mnodonts " {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 observators 
 to have but two teeth. A kindred fish in which the division 
 of each jaw in the middle line is conspicuous, is similarly 
 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 jaws, 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 jDlates of dentine, which 
 are, as they wear down, replaced by the development of 
 fresh plates, which are added from beneath, where they are 
 developed in cavities situated low down in the substance of 
 the bone. 
 
TEE TEETH OF FISHES. 
 
 231 
 
 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 sulistance of the jaw, do not 
 
 Fig. 99 ('). 
 
 3\ S 
 
 come into use by the ordinary process of displacing their 
 predecessors, and being in turn themselves replaced, but 
 fresh plates only come into use by the actual wearing a-way 
 of all that is above them, both dentine and bone, so that 
 they come to be the topmost 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. 
 
 Tetrodou has not the roxmded triturating disk of the 
 Diodon, or has it but feebly rcpi-eseutcd ; and the margins 
 of the jaws are sharpen 
 
 In the Parrot-fishes (Scai'us), which are not very nearly 
 allied to the Gymnodonts, somewhat similar beaks are found, 
 
 (') Jaws of the Diodon. a. Base of tlie dental plates, where new lamellm 
 of dentine are being developed, h. Margin of jaw, formed mainly by the 
 sides of the denticles, c. Compound tooth, made up of the superimposed 
 lamelloe of dentine anchylosed together. 
 
232 A MANUAL OF DENTAL ANATOMY. 
 
 the individual teeth being more conspicuous. The whole 
 outer surface of the jaw near to its working edge is covered 
 by a 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 i)romincnt and more 
 or less sharp. 
 
 The structure and succession of these teeth have been care- 
 fully described by J. von Boas (Zeits. f. Wissen. Zool. xxxii.), 
 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 me to be that tissue 
 which I have termed " bone of attachment." See page 208. 
 In a section of a jaw in my possession, which I 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 
 superimposed cones with the points upwards ; they consist 
 of dentine and enamel, and the jDoint of the subjacent cone 
 fits closely up 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, so that the working denticle comes to be little 
 more than a hollow cone of enamel. This is not the case in 
 my specimen in which there is a quantity of dentine left 
 in each denticle. This vertical series of superimposed 
 sharp cones lie in the midst of the somewhat thin jaw bone, 
 fused together by cementum (J bone of attachment), and 
 enclosed between the inner and outer plates of the jaw. 
 The bone being much softer than the denticle, wears 
 
THE TEETH OF FISHES. 233 
 
 down much faster, so that the edge is always formed by a 
 prominent 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 
 similarly armed. It carries teeth which are anchylosed to 
 it, and which are so disposed as to keep the surface con- 
 stantly rough. When they are freshly formed the teeth 
 
 Fig. 100 (i). 
 
 ,r 
 
 c- 
 
 have flattened thin edges, something like human incisors. 
 The teeth are coated with enamel, and thus, Avhen calcifi- 
 cation has proceeded so far as to obliterate their central 
 pulp cavities, after the tooth is worn to a certain point 
 (c in Fig. 99) it presents a ring of enamel, inside which 
 comes a ring of dentine, and inside this a core of secondary 
 dentine, as seen in the figure. Owing to the different hard- 
 
 (•) Lower pharjTigeal bone of Pseudoscarus. a. Posterior Ijorder, at 
 Avhich the teeth are umvorn. c. Oval areas formed by teeth, the points 
 of which are worn off. b. Anterior edge of bone, at which the teeth ai'c 
 iilmost completely worn away. 
 
234 A MANUAL OF DENTAL ANATOMY. 
 
 ucss of the three tissues a constant roughness of surface is 
 maintained. The upper pharyngeals are similarly armed ; 
 and as the teeth and the supporting 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 j)osterior 
 instead of its anterior extremity. 
 
 The teeth are developed in bony crypts,beyond the youngest 
 functional teeth, and perforations in the roofs of the crypts 
 give passage to the connecting band l^etween the tooth sac 
 and the mucous membrane. 
 
 No more fitting place will occur for noticing the stout 
 pharyngeal teeth which are met with in so many fish. 
 Some fish, which are edentulous so far as the mouth is 
 concerned, have the j)haryngeal bones armed ■s\'itli teeth ; 
 in the carp and its allies, edentulous so far as the mouth 
 proper is concenied, 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. 
 
 But 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 difi"er 
 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 fur 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 
 
THE TEETH OF FISHES. 235 
 
 and size iu all parts of the mouth. As a general rule, fish 
 do not comminute their food very fully, but make use of 
 their teeth simply for the apprehension of prey, not sub- 
 mitting the food to any mastication whatever ; their teeth 
 are hence often mere sharp cones, slightly recurved, or set 
 looking backwards. Thus, though the mouth of the common 
 pike is beset with an immense number of sharp teeth, its 
 food is swallowed whole, and ver}' often is alive w^hen 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, but it does occur : for example the Barracuda 
 pike (Sphymena) has its lancet-shaped teeth implanted in 
 distinct sockets, to the walls of which they are said 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 direc- 
 tion, as happens in the pharyngeal teeth of the Wrasses, or 
 the curiously human-looking incisors of the Sheep's-head fish 
 (Sargus) ; the Lepidosteus also has its teeth aflixed in incom- 
 plete sockets, to the walls of which they are anchylosed ; 
 this is not a very uncommon airaugement with the teeth of 
 fish when they are socketed at all. 
 
 The teeth of fish are of all degrees of size and of fineness ; 
 in some (Chtetodonts). the teeth are as fine as hairs, and arc 
 so soft as to be flexible. 
 
 Teeth which are very fine and very closely set are termed 
 "dents en velom-s;" when they are a little stouter, "dents 
 en brosse," and when still stronger and sharper, "dents en 
 cardes." Teeth that are conical, wedge-shaped, spheroidal, 
 and lamelliform, are all to be met with; in fact there is 
 infinite diversity in the form of fishes' teeth. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 And there are some fish, e.g., some of the large Siluroicl 
 fishes, which have very strong, large teeth, an inch and a 
 half or more long, and very firmly anchylosed to the bone. 
 
 It is not common for sexual differences to be met with 
 between the teeth of the male and female, though a slight 
 difference exists between the sexes in some species of Skate. 
 And 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 produced, and turned 
 upwards at its point; the stout cartilaginous hook thus 
 formed is of such dimensions that it has to be accommo- 
 dated in closure of the mouth in a deep cavity formed 
 for it l)etwcen 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 breeding season, A fish in which it is strongly 
 developed is a foul fish, and is called a Kelt. It is used 
 apparently as a battering ram, and such salmon are con- 
 stantly found killed, with their sides deeply gashed by the 
 charges of their opponents. 
 
 jS'ot 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 vasodentine or 
 osteodentine ; this is often glazed over upon its exterior by 
 a thin film of enamel, so thin as often to appear structure- 
 less. 
 
 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. 
 
 Dentine of very complex structure (labyrintho-dentine) 
 is met with in some fish ; and an example from the Lepi- 
 
TKE TEETH OF FISHES. 237 
 
 dosteus (Americau garpike, a ganoid fish) has been figured at 
 page 78. 
 
 Enamel is often present in a very thin layer, glazing the 
 exterior of the dentine (see Fig. 48) ; sometimes it forms a 
 mere tip, a sort of spear-point to the tooth as in the Eel and 
 the Hake (see Figs, 87 and 90), and sometimes it is very 
 thick, and itself permeated by systems of tubes (see Fig. 24). 
 
 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 ; whilst the dermal scales and 
 protective spines of fish are often made up of a tissue much 
 resembling dentine (cf Professor "Williamson, Philos. Trans. 
 1849). AVe may say, then, that just as in the external skin, 
 bony or dentinal plates are developed for the purpose of 
 protecting it from destruction by attrition, so for a similar 
 purpose teeth are developed in that portion of the mucous 
 membrane which covers the jaws. 
 
 Near the borderland between fish and amphibia is the 
 Lepidosiren, or ^Mud-fisli, which is a fish rather than an 
 amphibian. The armature of its mouth is peculiar, the 
 margins of the lower jaws being formed by dental plates 
 anchylosed to the bone. These plates have upon their 
 edges five deep angular notches, the prominence of the 
 upper plate con-esponding to the notches of the lower ; 
 and the edge is kept somewhat sharp by the front surface 
 being formed of very dense hard dentine, while the bulk 
 of the tooth is permeated by large medullary canals, which 
 render it softer. The cutting plates of the upper jaw are 
 developed in tlie median line of the palate, and there arc 
 in front of them 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 
 
A MANUAL OF DENTAL ANATOMY. 
 
 as holdfosts, while the cutting edges of the deeply-notched 
 plates are brought into play to slice np the food. 
 
 Both in structure and general disposition the dental plates 
 in Lepidosiren arc paralleled by the teeth of Ceratodus, for 
 some time known only as a fossil, but of which recent 
 examples have been captured near Queensland ; this re- 
 semblance was suspected some years ago by my friend, 
 i\Ir. Moseley, of the Challenger, and has been since worked 
 out by other observers. 
 
CHAPTER VII. 
 
 THI:; TEETH OF BATRACHIA AXD RErTILES. 
 
 In these classes the teeth are never so numerous nor so 
 widely distributed upon the bones of the mouth as in iish ; 
 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 arrange- 
 ment rather common amongst Batrachia. Almost all 
 Batrachians and Reptiles have an endless succession of 
 teeth ; but there are a few lizards {e.g., Hatteria), in which 
 the manner of succession, if there be any, has not l)een 
 definitely ascertained. The outer of the two rows of teeth 
 in the upper jaw is situated upon the premaxillary and 
 maiillary bones, and usually extends further back than 
 the vomerine or inner row. 
 
 From this type of dentition there 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 jioints 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, itself 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 
 
A MANUAL OF DENTAL ANATOMY. 
 
 space required for the attainment of their full size, by the 
 absorption of the older solid bone and the tooth which 
 has preceded them, in the following manner. The teeth 
 are attached to the Iwne by anchylosis, each tooth being 
 perched upon a little pedestal of bone which is specially 
 formed for it ; and the successional teeth, the germs of 
 which originally lay at the inner sides of the old teeth, 
 commonly undermine the side of the pedestals and the 
 bases of the latter, and move bodily beneath them, so that 
 the new tooth 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 exist- 
 ence of which has been doubted by some writers ; but a 
 study of the tooth sac of the animal renders it probable that 
 the transparent layer which is xnidoubtedly there 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. 90), save that they are bifurcated, the 
 one point being larger and longer than the other. 
 
 The tadpole has its jaws armed with tough horny plates 
 something like a turtle's bill, which are shed off, prior to 
 the development of any true teeth; at all events I have 
 myself been xmsiiccessful in discovering any tooth germs at 
 the ^Deriod Avhen its horny bills are still in use. 
 
 Some extinct batrachia were of large size ; the Laby- 
 rinthodon, the structin-e of whose teeth has already been 
 described (page 79), was furnished with a marginal row of 
 teeth in the upper jaw, of which some few were of larger 
 size xmd greater length than the others. In the lower jaw, 
 the teeth, which are similar to those of the uptper, are 
 disposed in some sense in an incomplete double row, the 
 series of smaller teeth not being interrupted by the occur- 
 rence of the larger tusks, but passing in unbroken series 
 
THE TEETH OF REPTILES. 
 
 outside them. The Labyrinthodoii was possessed also of 
 palatine teeth. 
 
 The teeth were auchylosed to slight depressions or sockets, 
 and the successioual teeth were probably developed, as in 
 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. 
 
 In many reptiles teeth are developed for the merely tem- 
 porary 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, which afterwards are 
 lost, developed on the premaxillary bones (Owen), 
 
 The Chelo:^ia, comprising the Tortoises and Turtles, have 
 no teeth, but the margins of the jaws are sheathed in horny 
 cases, which are variously shaped in accordance with the 
 habit of the animal, being sharp and thin edged in carnivor- 
 ous, and blunt and rugged in herbivorous species. 
 
 Saurian reptiles (lizards, &c.), have, as a rule, rather 
 simple teeth, which are confined to the margin of the jaws, 
 the occun-ence of palatal teeth being less usual. The 
 teeth are of various forms, being blunt and rounded in many 
 genera, whilst in others they are long and pointed. They 
 are generally made up 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 auchylosed by its outer side to an 
 external parapet of bone, the creature is said to be " pleu- 
 rodont," when by the end of its base it is attached to the 
 summit of a parapet it is " aci'odont." 
 
 The succession of teeth in the 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 successioual tooth has attained to a certain 
 stage in its development. 
 
 The accompanying figure of the lower jaw of a Monitor 
 
242 A MANUAL OF DENTAL ANATOMY. 
 
 lizard will give an idea of a dentition common in the- 
 group. The teeth are not 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 
 
 Fig. 101 (1). 
 
 r'\ 
 
 \ 
 
 foramina which lead into the spaces in which new teeth are 
 being developed. 
 
 Amongst the lizards considerable vai'iety 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 
 bard dentine, with a simple central pulp cavity, but at 
 their bases of plicidentiue with numerous svibdivisions of 
 the pulp cavity, as is seen in the Monitor lizards (Varanus, 
 see p. 77). One Mexican lizard (Helodermus), has the re- 
 putation 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. 
 
 (^) Lower jaw of a Lizard (Varanus GouMiij. a. Foramina leading to 
 cavities of reserve. 
 
THE TEETH OF REPTILES. 243 
 
 Vaso-dentiue also occurs in the teeth of some sauriaus, as 
 for example, in those of the gi'eat extinct Iguanodon, in 
 which it, roughly speaking, formed the inner half of the 
 crown, the outer moiety consisting of hard dentine. In 
 addition to this peculiarity, the teeth of Iguanodon were 
 remarkable for the partial distribution of the enamel, which 
 was strongly ridged, the ridges being seiTated, and was con- 
 fined to the outer side of the crown. Thus at the outside 
 came the hardest tissvie, the enamel; next the harder dentine 
 and on the inside, the softer vaso-dentine. Hence, as the 
 tooth woi'e down, a sharp edge was long preserved. 
 
 There is a New Zealand lizard, to which the several names 
 of Hatteria, Sphenodon, and Rhyncocephalus have been given, 
 which has a very peculiar dental armature (Dr. Giiiither, 
 Phil. Trans., 1867). 
 
 The inter-maxillary bones are armed with two teeth, so 
 large as to be co-extensive with the whole bone in width, 
 and of a form which recalls that of the gnawing incisors 
 of Rodents ; the other teeth are quite small, and "acrodont " 
 in their attachment. 
 
 But the great peculiarity of Hatteria is that the alveolar 
 margins of the jaws are sharp, and when the teeth are worn 
 down, which would happen in adult specimens, the actual 
 sharp margins of the bone come into play as masticatory 
 organs, near to the front of the mouth. It occurred to me 
 as probable that the surface thus exposed might be coated 
 with dentine, but a microscopic examination of one of the 
 specimens in the British Museum, which I was, by the 
 kindness of Dr. Gllnther, enabled to make, proved that tlie 
 dense ivory-like surface which serves the purposes of masti- 
 cation is true bone, and has no relation to dental structure. 
 
 There are very few other instances of actual bone, un- 
 coated by dental tissues, being used for masticatory pui'poses. 
 
 The great extinct Dicynodon, an African fossil, also 
 had sharp trenchant margins to its jaws ; it is not 
 
 R 2 
 
244 A MANUAL OF DENTAL ANATOMY. 
 
 known whether these were sheathed in horny cases like 
 those of the turtles, or whether the bones thcmseh'es came 
 into use, as in Hatteria. But the most striking peculiarity 
 of Dicynodon was the co-existence with such jaws of a pair 
 of very large caniniform tusks, extending downwards and 
 forwards from the upper jaw, and growing from persistent 
 pulps, a thing altogether exceptional in the reptilian class. 
 
 The dentition of Ophidian reptiles (snakes) is very uni- 
 form; they may be conveniently divided into two groups, the 
 poisonous and the non-venomous snakes. 
 
 Non-venomous snakes 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 in both groups strongly recurved, and are 
 firmly anchylosed to the bone ; they consist of a central 
 body of vmvascular 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 base of the skull through the medium of an 
 elongated movable quadrate bone, is also such as to allow 
 of their being widely separated from the skull and from one 
 another, which allows of the dilatation rendered necessary 
 by the large size of the creatures which a snake swallows 
 whole. 
 
 The teeth of the snake are simply available for seizing 
 prey and retaining it, as the 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 im- 
 possibility, the mouth and pharynx have to undergo great 
 dilation. Tlie arrangements which combine to give to the 
 
THE TEETH OF REPTILES. 
 
 lower jaw its mobility have just been alluded to ; the 
 successional tooth germs, which are very numerous, are 
 also arranged in the snake in an unusual position, which by 
 
 bringing them very close to the surface of the bone, to 
 which they lie parallel, renders them less liable to displace- 
 ment and injury than they would have been had they been 
 placed vertically, as they are in all other creatures ; while 
 in addition to the advantage of protection by position, they 
 are wrapped i-ound by a sort of adventitious capsule of con- 
 nective tissue. 
 
 (^) Developing teeth of a Snake. /. Oral epithelium, e. Neck of the 
 enamel organs, h. Dentine pulp. c. Enamel cells, d. Dentine. 1, 2. 
 Very young germs. 3, 4. Older germs. 
 
246 A MANUAL OF DENTAL ANATOMY. 
 
 As the teeth during their development are thus lying down 
 parallel with the length of the jaw-bone, when the period 
 for their replacing a predecessor arrives, they have not only 
 to move \ipwards, but also to become erected ; how this 
 is done remains a mystery, for I have been quite unable to 
 discern the means by w^hich it is accomplished. 
 
 When a snake has seized its food, which it retains by 
 means of its many sharp recurved teeth, it slowly swallows 
 
 Fig. 103 ('). 
 
 it by advancing first its lower, then its upper jaw, till it 
 thus, so to speak, forces itself over the body of its prey. 
 When this latter is large, deglutition is a very lengthy 
 
 (^) One half of the skull of a Python (without the lower y<.m) seen from 
 below, a. Intermaxillary bone. h. Maxillary bone, carrying the outer 
 row of teeth, c. d. Palatine bone and pterygoid bone, the teeth upon 
 which constitute the inner or second row of teeth. 
 
THE TEETH OF REPTILES. 247 
 
 process, but an English snake can swallow a moderate-sized 
 frog with considerable rapidity. 
 
 There is an African snake (Rachiodon) which has none but 
 rudimentary teeth; its food consists of eggs, which thus 
 escape breakage until they reach the oesophagus, into which 
 spinous processes from the under surface of the vertebrse 
 project, and there serve to break the egg; snakes with 
 their dentitions similarly modiiied exist also in India (e.g., 
 Elachistodon). 
 
 It has already been mentioned that the non-venomous 
 
 Fig. 104 (i). 
 
 have two complete rows of teeth in the upper jaw, 
 the outer row being situated on the maxillary bones, the 
 inner upon the palatine and pterygoid bones. The teeth of 
 such snakes as the Pythons are all simple recurved cones, 
 and ai-e none of them either o-rooved or canaliculated."* 
 
 (') Head and jaws of Hydropliis. The maxillary bone (6), instead of 
 
 carrying a complete series of teeth, is armed with a few teeth only near to 
 
 the front. The foremost tooth is canaliculated, and forms the poison fang. 
 
 ('-) It has been proposed to divide the Ophidia into groups, distinguished 
 
 by the presence or absence of gi-ooved teeth, thus : — 
 
 i. Aglyphodontia. No grooved or canaliculated maxillary teeth, 
 ii. Opisthof/lyphia. Some of the posterior maxillary teeth grooved, 
 iii. Protcroylyphki, Anterior maxillary teeth grooved. 
 
 Posterior maxillary teeth solid. 
 iv. Solenpglyphia. Maxillary teeth few, canaliculated — poisonous 
 
248 A MANUAL OF DENTAL ANATOMY. 
 
 Some of the harmless snakes, however, have particular- 
 teeth which are developed to a greater length than the rest^ 
 and others have the posterior teeth on the maxillary bones 
 grooved ; but the statement that this grooving serves to 
 convey an acrid saliva into the wound inflicted rests on in- 
 sufficient foundation. The poisonous snakes are charac- 
 terized by a shortening of the series of teeth carried upon 
 maxillary bone, and by the front tooth of the series being 
 developed to much greater length than those which lie 
 behind it. Thus Hydrophis, a genus of poisonous sea-snakes, 
 has five or more teeth upon the maxillary bone, the fore- 
 most of which is much the largest, and this largest tooth is 
 so deeply grooved upon its anterior surface as to be converted 
 into a tube, the tube serving to convey the poison into the 
 wounds inflicted by it. 
 
 Poisonous snakes which have several teeth upon the 
 maxillary bone for the most part present some little external 
 resemblance to the harmless snakes, and are called "colu- 
 brine poisonous snakes " (coluber being the name of a genus 
 of harmless snakes) ; they present transitional characters 
 between these and the more specialised or "viperine" poison- 
 ous snakes. The Cobra is a familiar example of a colubrine 
 poisonous snake, and almost all the venomous snakes of 
 Australia belong to this group. Their poison fangs are not 
 very long, and they remain constantly erect, being anchy- 
 losed to the bone (the maxilla) which is long and not move- 
 able, and which also carries a varying number of small 
 insignificant teeth behind the poison fang. 
 
 In the viperine poisonous snakes (PufF-Addei', Ptattle- 
 snake, Vipers, &c.,) the poison apparatus is yet more 
 specialised. The maxillary bone carries no teeth at all 
 behind the poison fang ; it is so reduced in length as to be 
 of squarish form, and is so articulated to the skull as to be 
 movable. 
 
 The poison fang is of great length, so that if constantly 
 
THE TEETH OF REPTILES. 
 
 249 
 
 erect it would be much in the way ; -when it is out of use, 
 however, it is laid flat aloug the roof of the mouth, and is 
 only erected for the purpose of striking ; when in repose it 
 is altogether hidden by a fold of mucous membrane, which, 
 when it is erected, becomes tightly stretched over a part of 
 its anterior surface, and serves to direct the poison do\vn 
 the poison canal by, to a great extent, preventing its escape 
 around the exterior of the tooth. 
 
 The mechanism by which the poison fang is erected is 
 thus described by Professor Huxley (Anatomy of Verte- 
 
 mi. 
 
 brated Animals, p. 241) : — "When the mouth is shut the 
 axis of the quadrate bone is inclined downwards and back- 
 wards. The pterygoid, thrown back as far as it can go, 
 straightens the pterygo-palatine joint, and causes the axis 
 of the palatine and pterygoid bones to coincide. The trans- 
 verse, also earned back by the pterygoid, similarly pulls the 
 posterior part of the maxilla and causes its proper palatine 
 face, to which the great channeled poison fangs are attached, 
 
 (') Side and front view of tlie skull of Craspedocepkalus mdas. A 
 bristle is passed down the poison canal. 3fx. Maxillary bones. 3fn. 
 Mandible. PL Palatine bones. Pt. Pterygoid bones. Qu. Quadrate 
 bone. T. Transverse bone. 
 
 A. Side view. B. Front view. 
 
250 A MANUAL OF DENTAL ANATOMY. 
 
 to look backwards. Hence these fixngs lie along the roof of 
 the mouth, concealed between folds of the mucous mem- 
 brane. But when the animal opens its mouth for the 
 purpose of striking its prey, the digastric muscles, pulling 
 up the angle of the mandible, at the same time thrust the 
 distal end of the quadrate bone forwards. This necessitates 
 the pushing forward of the pterygoid, the result of which 
 is twofold : firstly, the bending of the pterygo-palatine joint; 
 secondly, the partial I'Otation of the maxillary upon its 
 lachrymal joint, the hidden edge of the maxillary being 
 thrust downwards and forwards. 
 
 "In virtue of this rotation of the maxillary through about 
 a quarter of a circle, the dentigerous face of the maxilla 
 looks downwards and the fangs are erected into a vertical 
 position. The snake ' strikes ' by the simultaneous contrac- 
 tion of the crotaphite muscle, part of which extends over 
 the poison gland, the poison is injected into the wound 
 through the canal of -the fang, and this being withdrawn, 
 the mouth is shut, all the previous movements reversed, 
 and the parts return to their first position." 
 
 The poison fang is a long, pointed, slightly recm-ved 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. This tube conveys the poison into 
 the puncture, its upper orifice being in close relation with 
 the end of the duct of the poison gland. 
 
 It has been mentioned that some snakes which have not 
 definite poison fimgs have a few of the large posterior teeth 
 grooved upon their front surfaces, the object of this grooving 
 being, as a matter of conjecture, to convey a more or less 
 poisonous saliva into the w^ounds inflicted by them. 
 
 By imagining such an anterior groove to be deepened, 
 and finally converted into a canal by its edges growing up 
 
THE TEETH OF REPTILES. 
 
 251 
 
 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 ; which might thus, as least in its canali- 
 culated 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 form of the harmless Python, and the venomous 
 Rattlesnake, so there are gradations in the form of the 
 poison tooth, in the degree in which the groove is converted 
 into a canal. 
 
 In colubriue poisonous snakes the canal is visible on the 
 
 Fig. 106 ('). 
 
 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 in a part of its length 
 
 (') Transverse section of tooth-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. 
 
252 A MANUAL OF DENTAL ANATOMY. 
 
 the canal is quite closed in, has a very marked line along 
 its front, and in section it looks much as would the dentine 
 in Fig. 106, if the two cornuahad their rounded extremities 
 brought together into actual contact, without, however, 
 their rounded outline being altered. 
 
 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 accompanying figure the pulp cavity is 
 seen to be a thin flattened chamber partly surrounding the 
 tube formed for the conveyance of the poison. 
 
 Fig. 107 ('). 
 
 ex 
 
 The poison-fang is exceedingly sharp, its point being con- 
 tinued some little distance beyond the place where the 
 poison canal opens on the front of the tooth ; this disposi- 
 tion of parts has been copied in the points of syringes for 
 making subcutaneous injections. 
 
 (') Transverse section of the poison fang of a Eattlesnake. ". Pulp- 
 cavity, d. Dentine. 
 
THE TEETH OF REPTILES. 253 
 
 The dentine is continued down to a very fine point, and 
 it is cased by an exceedingly tliin layer of enamel, not 
 much more than -gi^ 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 ; in Hydrophis 
 I believe that it does. As the point is simple, the tooth 
 germ of a poison-fang only becomes distinguishable from 
 that of another ophidian tooth after the tip of the tooth has 
 been formed, when a groove appears in its side (see 8 and 9, 
 in Fig. 108). 
 
 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 
 being broken off by the 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 
 succession of teeth is conducted upon a plan which is 
 unique, and which is excellently adapted to save loss of 
 time in the replacement 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, 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 itself stood, so that the places on the right 
 and the left of the bone are occupied alternately by the tooth 
 
254 A MANUAL OF DENTAL ANATOMY. 
 
 in use. Thus, in Fig.' 105, the poison [fang of the snake's 
 right side is seen occupying a position on the extreme out- 
 side of the maxillary bone, while its left poison fang is fixed 
 on the inside of the maxillary bone. 
 
 The upper boundary of Fig. 108 is formed by the flap 
 of mucous membrane which covers in the poison feng when 
 at rest. Isos. 1 and 2 lie in the pouch formed by it, the 
 
 Fia. 108 ('). 
 
 ^' -.0 
 
 section happening to be tal-jju fiuiu 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 Hap hanging free across this space serves apparently to 
 
 (') Ti-ansverse section of the reserve poison fangs of a Viper. 1. Tootli 
 at present in use, in its recumbent position ; were it erect, it would be 
 witiidrawn from view, or else seen in longitudinal section. 2. Tooth which 
 will next succeed to No. 1. 3, 4, 5, &c. Tooth -sacs numbered in the order 
 in which thev will succeed. 
 
THE TEETH OF REPTILES. 255 
 
 keep teeth of the oue 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 nnmber 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 series, as is indicated by the consecutive numbers 
 in 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 a viperine snake would. 
 
 But in the colubrine venomous snakes the successional 
 poison fang sometimes makes its way to a spot a little 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 {'). 
 
 This arrangement of hro distinct chains of younger de- 
 veloping 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. 
 
 Like other ophidian teeth the poison fangs become an- 
 
 (1) I have given a more detailed account of the suecession of poison fangs 
 in the Philos; Trans., 1876, Part i. 
 
256 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 chylosed 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. 
 
 The poison is secreted by a salivary gland homologous 
 with the parotid ; by an especial arrangement of the muscles 
 and fascia about it the erection of the poison fang and the 
 infliction of the bite cause a copious stream of poison to be 
 
 Fig. 109 ('). 
 
 ejected. The duct terminates in a sort of papilla, close to 
 the superior orifice of the tube in the fang : the passage of 
 a considerable portion of the poison down the tube is secured 
 by the close apposition of a shield of mucous membrane, 
 which is strained over the erected tooth. 
 
 In Crocodilia tlie teeth are confined to the margins of 
 
 (') Jaws of the Crocodile. Tlie first, fourth, and eleventh teeth in the 
 lower jaw, and the third and ninth in the upper, arc seen to attain to a 
 larger size than the others. 
 
THE TEETH OF REPTILES. 
 
 the jaws, where they are very formidable in size and shar])- 
 ness. 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, 
 to the walls of which they do not become anchylosed, and 
 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 teeth are spe- 
 
 FiG. 110 (1). 
 
 ^/ 
 
 cially large, while in the extinct African Galesaurus the 
 diflPerence is so marked that both in the upper and lower 
 
 (') Transverse section of tlie lower jaw of a yonng Alligator, a. Oral 
 epithelium, h. 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 a;erm. 
 
268 A MANUAL OF DENTAL ANATOMY. 
 
 jaws the teeth might be grouped as incisors and canines, 
 so far as size and probable function go in such a classification. 
 
 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 por- 
 tions. As already mentioned, they are implanted in tubu- 
 lar sockets ; new successional teeth are being continually 
 developed at the inner side of their bases, and as these at- 
 tain to a certain size, absorption attacks the base of the 
 older tooth, and its successor moves into the space so 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 ultimately 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 predecessor ; 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 
 armatu^re 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. 
 
 The huge Dinosauria, some of which must have been 
 thirty feet in length, had teeth implanted in imperfect 
 sockets, the outer alveolar wall being considerably higher than 
 the inner, and the transverse septa not very complete. The 
 I'oots of the teeth were more or less perfectly cylindrical, 
 and the enamelled crowns compressed and expanded, with 
 trenchant edges. The tooth of the Iguanodon will sen-e as a 
 fair example of a Dinosaurian tooth : the crown is greatly ex- 
 panded, and presents anterior and posterior sharp notched 
 margins ; the enamel is laid over the ou^ter 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 main- 
 
THE TEETH OF REPTILES. 259 
 
 tained. Moreover the maintenance of a sharp edge is further 
 secured by the dentine on the enamelled side of the crown 
 being of the hard unvasciilar variety, that on the inner being 
 vaso-dentine and therefore softer. The remnant of the pnlp 
 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 (American Journal of Science, March, 
 1880) has described and figured a pecuHar Diuosaurian den- 
 tition, 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 roots of the teeth in use were no less than five suc- 
 cessional teeth, in graduated stages of development, ready 
 to ultimately take its place ; so large a number of successional 
 teeth has uot hitherto been met with in a Dinosaur. 
 
 A very remarkable carnivorous reptile as large as a lion 
 has been described by Professor Owen (Quart. Journal 
 Geolog. Society, 1876,) under the name of Cynodraco major 
 for the reception of which he proposes a new reptilian order, 
 that of Theriodontia. Its dentition is not completely known, 
 but it possessed in the lower jaw eight incisors, of Avhich the 
 first is the smallest, and a canine of moderate size. The 
 upper incisors are not known, but there were a pair of upper 
 cabines of such size that they extended down along the 
 outside of flattened poi'tion of the lower jaw, like the 
 canine teeth of Machairodus. The hinder margins of these 
 canines were trenchant, and finely serrated. 
 
 The Pterosauria, or flying reptiles, have, since the 
 discovery 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 
 
260 A MANUAL OF DENTAL ANATOMY. 
 
 10 to 15 inches iu total length of body. In the Pterodactyls 
 the jaws are furnished with long, slender, sharp teeth in 
 their whole length : but in Ramphorhynchus the anterior 
 extremities of the jaws are without teeth, and it has been con- 
 jectured that tliese portions were sheathed in horny beaks. 
 
 And Prof. Marsh (American Journal of Science, 1876,) has 
 discovered, in the same formation in which he found the 
 toothed birds, several species of Pterodactyls wholly without 
 teeth, for which the generic name Pteranodon is proposed. 
 
 The jaws, which are more like those of birds than those of 
 any known reptile, show no traces of teeth, and the pre- 
 maxillaries seern to have been encased in a horny covering. 
 
 The Teeth of Biiids. 
 
 Piior to the discovery by Professor Marsh of Yale College, 
 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 Mr. Wood- 
 ward (Popular Science Review, 1875,) 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 
 functions of teeth are discharged by this horny armature of 
 the jaws. 
 
 It is noteworthy that the margin of the bone of the jaws 
 is also serrated, each serration corresponding to a similar 
 serration in the bill. In the fossil bird described by Pro- 
 fessor 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, far more conspicuous than those of the 
 
THE TEETH OF BIRDS. 261 
 
 Merganser. And 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 recalled dental pulps. Then there is also the 
 famous fossil Ai'chseopteryx, an anomalous oolitic bird, with 
 a long and jointed tail, which is by many zoologists believed 
 to have possessed teeth. There is a flaw in the evidence, 
 however, inasmuch as the toothed jaw is not in situ, and 
 therefore may possibly have belonged to some other animal 
 than that perpetuated in the rest of the fossil impression, 
 though i:)robability is altogether in favour of its really 
 belonging to the Archseopteryx. 
 
 In successive expeditions, conducted under great ditiiculties 
 owing to the extremes of heat and cold, and to the hostility 
 of the Indians, the remains of no less than one hundred 
 and fifty diflFerent individuals referable to the sub-class 
 Odontornithes have been obtained by Prof. 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 (Odontotoruse, illustrated by the genus Ichthyornis 
 as a type) ; the other group consisting of very large swimming- 
 birds, without wings, and having teeth in grooves (Odontolca;, 
 type genus Hesperornis). 
 
 In Ichthyornis the teeth were about twenty-one in num-ber 
 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 them ; the premaxillaries were probably edentulous, 
 and perhaps covered with a horny bill. 
 
 In the lower jaw the largest teeth occur about the middle 
 
A MANUAL OF DENTAL ANATOMY. 
 
 of the I'iimus, those at its posteiior 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 Hesperoruis, probably 
 diving birds, includes species 6 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 pre- 
 maxillary is edentulous, but the teeth 
 extend (piite to the anterior extremity 
 of the lower jaw : in one specimen 
 there arc fourteen sockets in the max- 
 illary 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, migrated into the excavations 
 ., grew large, and ultimately expelled their prede- 
 cessors, 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 osteodentine. 
 
 so formed, 
 
 Q) Mandible of Ichfcliyornis (after Prof. Marsh). A. Side view, show- 
 ing the teeth in situ. B. View of upper surface, showing the sockets in 
 
 which tlie teeth were implanted. 
 
THE TEETH OF BIRDS. 
 
 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. 
 
 In form the teeth of Hesperornis present a close resem- 
 blance to those of Mosasaiu'us, a great extinct lizard. 
 
 Indeed, as Prof. Marsh observes, "in all their main 
 
 Fig. 1]2(1). 
 
 features the teeth of Hesperornis are essentially reptilian, 
 and no anatomist v/ould hesitate to refer them to that class, 
 had they been found alone. Combined with the other 
 reptilian characters of Hesperornis .... 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, ikc, by 
 O. C. Marsh, Prof, 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. 
 
 (0 (After Prof Marsh.) A. Hesperornis regalis, with successional tooth 
 in an excavation at its base ; enlarged eight diameters. B. Tooth of Mos- 
 isauriis princeiJS, half natural size. 
 
CHAPTER VIII. 
 
 THE TEETH OP MAMMALS. 
 
 The class Mammalia is divided into three groups : — 
 
 I. Ornlthodclphla. 
 
 Animals with a common genito-ui-inaiy chamber, and separate 
 coracoid bones ; no vagina ; no teats ; comprises a single order, 
 Monntrriinita, which contains only two genera, the Ornithorhynchus 
 and the Echidna. 
 
 II. Didrlphia. 
 
 Animals with a vagina, &c. ; of which the young are bom in an 
 exceedingly early condition, probably without the formation of any 
 placenta, and are transferred to the nipple of the mother, where, 
 in almost all, they are protected by a fold of the abdominal integu- 
 ment, which forms the marsupium, or pouch ; comprises the single 
 order J/(cr.'<iij)ialia, animals now most largely represented in Aus- 
 tralia and its zoological region ; some few exist also in America. 
 
 The kangaroos, wombats, opossums, &c., are familiar examples 
 of Marsupials. 
 
 III. I\I(infl(h'lphia. 
 
 Placental mammals : i.e., animals in which the foetus acquii-es a 
 connection with the parent through the medium of a vascular pla- 
 centa, by means of which it is nourished for a long time, and is 
 ultimately born in an advanced condition. 
 
 The relations which the different orders of placental Mammalia 
 bear to one another are rather complex, and it is not possible to 
 place them satisfactorily in a consecutive series, because many of 
 the orders present affinities with, and are indeed linked by transi- 
 tional forms to, not one, but several other orders. Professor Flower 
 (Osteology, page 6.) has arranged them in the following tabular 
 manner, each order being placed near to those to which it presents 
 most resemblance. 
 
THE TEETH OF MAMMALS. 
 
 Relation of 
 
 the 
 
 existing Mammalian orders 
 
 to on( 
 
 another : — 
 
 
 Hominina. 
 Primates. 
 
 
 
 Chiropteea. 
 
 Suniina. 
 Lemurina. 
 
 
 
 Insectivora. 
 
 Carnivora. 
 Fissipedia. Pinnipedia. 
 
 
 
 
 Hyracoida. 
 
 Cetacea. 
 
 RODENTIA. 
 
 
 
 SiRENIA. 
 
 
 DiNOCEEATA (0 Perissodactyla. 
 
 Peoboscidea. Ungulata. 
 
 Suina. Tylojnnhi. 
 
 Ai"tiodactyla. 
 Tragvllna. Pecora. 
 
 Edentata. 
 
 Primates include man and the monkeys, the Lemiu's connecting 
 them with both the Insectivora and Chiroptera. 
 
 ChirojHera — Bats. 
 
 Imcctivora — Moles, Hedgehogs, &c. 
 
 Carnivora Ji,ss'n>idla — Cats, Dogs, and Bears, kc. 
 
 Carnivora jiinnipcilia — Seals, Wabus, &c. 
 
 Cefacra — WTialebone \\Tiales, Sperm "WTiales, Porpoises, &c. 
 
 Sircnia {Ilcrhivoroiis Cctacra) — Manatee, Dugong, &c. 
 
 Cngiihita (Hoofed Mammals) — 
 
 (i.) Perissodactyla, or odd-toed — Horse. 
 
 Tapir, Rhinoceros, &c. 
 (ii.) Artiodactyla, or those with an even number of toes — Pigs 
 and their allies. Camels, Ruminants, &c. 
 
 Dinoccrata — Gigantic Fossil Mammals, somewhat intermediate be- 
 tween Perissodactyl Ungulata, and Proboscidea. 
 
 Hyracoidca — The anomalous Hyrax (Biblical " Cony ") alone. 
 
 P/"oJf«c<V/frt— Elephants, extinct Mastodons, &c. 
 
 Bodentia — Hares, Rabbits, Rats, &c. 
 
 Edentata — Sloths, Armadillos, Ant-Eaters, &c. 
 
 To illustrate the meaning of the table, the gap existing between 
 the typical Carnivora and the Cetacea is bridged over by the seals, 
 which, though tnie carnivora, are some of them near the Cetacea 
 
266 A MANUAL OF DENTAL ANATOMY. 
 
 in many particulars ; or, again, the gap between the Monkeys and 
 the Insectivora is bridged over by the Lemurs, which are inter- 
 mediate forms. 
 
 We do not yet know enough of extinct Mammalia to feel quite 
 sure of the true line of aflfinities between all the orders, but the 
 foregoing table serves to give a more true idea of our present know- 
 ledge than any arrangement in linear series can convey. There is 
 no animal to which we can point and say that we know its whole 
 line of descent ; but the ancestry of some of the Ungulata has been 
 greatly elucidated of late years, and the chain of progressive modi- 
 fication by which so highly specialised a form as the Horse has been 
 arrived at, starting from a very much more generalised foim, is now 
 pretty complete. 
 
 In a treatise dealing only with the teeth, in which the orders 
 must necessarily be taken in succession, it will be convenient to 
 deviate somewhat from the natural order for the sake of taking 
 first those animals whose dentitions are of the simplest character. 
 Thus it is convenient to describe in succession the Edentata and 
 the Cetacea, which have little or nothing to do with one another, 
 because they alike have teeth of simpler form than the rest of the 
 Mammalia. But, as far as possible, the arrangement indicated in 
 the table, which the student will do well to impress upon his mind, 
 will be followed in these pages. 
 
 INTRODUCTORY REMARKS. 
 
 Not many years ago it was customary to explain the 
 various facts which were revealed by the study of com- 
 parative anatomy upon the supposition that there was some 
 sort of type or standard organization, and that all others 
 were anived at by modifications and departures from this 
 type, these modifications being introduced with a direct 
 pvirpose in view, in order to fit the creature to a special 
 habit of life. 
 
 Among the matters which this " type " theory sought to 
 account for was this : when an animal possesses some pecu- 
 liar organ, it is found on close examination that it, however 
 specialised, is after all only something which allied animals 
 also possess, only it has been exaggerated or developed in an 
 unusual manner and degree ; or, on the other hand, that 
 when an organ is wanting, the suppressed organ is not 
 absolutely abolished, but is to be found stunted and in a 
 
THE TEETH OF MAMMALS. 267 
 
 rudimentary condition, instead of in its ordinary size and 
 functional activity. 
 
 This is as true of teeth as of any other organs ; indeed 
 the study of odontology reveals many admirable examples 
 of the law. 
 
 Thus the tusks of the boar or of the Sus babirussa, large 
 and peculiar though they be, are not new developments, but 
 ai-e merely the canine teeth which in these species attain to 
 unusual dimensions. In the same way the enormous 
 straight tusk of the Xarwal (see Fig. 133) is nothing more 
 than an incisor tooth of one side, the fellow to which has 
 been checked in its develoi^meut ; but this is not missing, for 
 it remains throughout the 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 ; fm-thermore, as has been shown by 
 Professor Turner, in young specimens, a second pair of 
 rudimentaiy aborted incisors are to be found, which in the 
 adults have disappeared. 
 
 The modem school of biologists, rejecting this "ai-che- 
 type " theory as a far-fetched and unsatisfactory hypothesis, 
 refer these resemblances detected between dentitions upon the 
 whole dissimilar to one another to a more intelligible cause, 
 namely, inheritance. Assuming, as the balance of evidence 
 compels us to assume, that the many divergent forms which 
 we observe have been derived by progressive modifications 
 and difierentiations from fewer ancestral forms, we shall have 
 no difficulty in seeing how, by such processes as we full well 
 know to occur, namely, the dwindling of disused organs and 
 the exaggerated development of those used in an unusual 
 degi-ee, great differences may ultimately result. 
 
 To illustrate what is meant by this so-called "adapti^-e 
 modification," this suppression of things that are not 
 needed, and increased development of those most used, we 
 
268 A MANUAL OF DENTAL ANATOMY. 
 
 may recur to the dentitions of non-venomous and venomous 
 snakes. 
 
 In these we saw, in the non-venomous snakes, the max- 
 illary bones covered by a row of teeth sub-equal in size ; 
 then in the ' Colubrine ' poisonous snakes the front tooth of 
 those standing upon the maxillary bone taking upon itself a 
 special and important office, namely, the conveyance into a 
 wound of a poisonous saliva, and coincidently with this 
 tooth attaining its increased size and importance, the teeth 
 behind it on the maxillary bone reduced both in number 
 and in size. Going a step further, to the Viperine poisonous 
 snakes, the now useless small maxillary teeth have all disap- 
 jjeared, leaving the poison fang alone, and of vastly increased 
 dimensions, to occupy the whole bone. 
 
 But in many poisonous colubrine snakes three or four 
 small and useless teeth lingering upon the maxillary bone, 
 though their function was gone, seemed to indicate to us 
 in some measure the gradual process by which that singu- 
 larly perfect adaptation of means to an end, the poison 
 apparatus of the viper was arrived at. 
 
 It would be impossible in these pages to go through the 
 arguments by wdiich Mr. Darwin has established his main 
 propositions ; it must suffice to say here, that he has fully 
 convinced all those who are not in the habit, from the fixity 
 of early impressions, of putting many matters upon another 
 footing than that established by the exercise of i"eason, 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 genera- 
 tions, until great and material differences have more or less 
 masked the resemblance to the parent form. 
 
 Just as man, by favouring the breeding of those modifica- 
 tions of form, &c., 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 dui'ing which the 
 
TEE TEETH OF MAMMALS. 269 
 
 surface of land and sea has been of pretty nearly its present 
 form, to say nothing of the enormously longer earlier geo- 
 logical epochs — to profoundly modify the breeds of dogs, of 
 horses, of numbers of plants, all of which are absolutely 
 known to have had a common origin, so in nature forces are 
 and ever have been in perpetual operation, which effect the 
 same thing. 
 
 A pigeon fancier wants a pigeon of particular plumage, 
 with a few feathers a little different 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 of what he wants, 
 he will get what he wants, and will even tell you with confi- 
 dence that in so many years he will make a breed with the 
 peciiliarity desired. And exactly as the plumage that was 
 wanted is got, so in nature the tooth that is " wanted," 
 i.e., the dentition that is excellently well adapted to do its 
 work is manufactured by the operations of that law known 
 as " survival of the fittest." 
 
 It is quite enough that one of the small variations for 
 ever arising in animals shall be of advantage to it, for us to 
 see that the peculiarity is likely to be transmitted and 
 intensified in successive generations. 
 
 The question has been w^ell presented by Mr. Wallace, 
 who points out that wc must not think so much of varia- 
 tions 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. Now if a little extra height were of udvantage, 
 many or most of the fifty would experience it ; though some 
 might not. In the same way if we grouped one hundred 
 
 Q) An eminent i^igeon fancier, Sir J. Sebright, told ilr. Darwin that he 
 could produce any given feather in three years. 
 
270 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 animals whose teeth varied a little in respect of strength into 
 the fifty weaker and the 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, and would be more 
 certain to propagate their kind, and would repeat in most of 
 their pi'ogeny those peculiarities which had helped them- 
 selves to live. 
 
 Thus the doctrine of natural selection or survival of the 
 fittest, is as fully applicable to the teeth of an animal as to 
 
 Fig. 113 (i). 
 
 .^:: 
 
 
 any part of its organisation, and the operation of this natural 
 law will 1)0 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 very little 
 use, and thus rudimentary teeth we may understand to be 
 teeth which are in process of disappearance, having ceased 
 to be tiseful to their possessors, but which are still for a 
 time lingering upon the scene. Some teeth have disap- 
 
 (^) Skull of a placental roflent (Capybara), showing general character of 
 a rodent's dentition. 
 
THE TEETH OF MAMMALS. 
 
 271 
 
 peared utterly ; thus the upper incisors of Ruminants are 
 gone, and no rudiments exist at any stage (') (see page 334); 
 others still remain in a stunted and dwindled form, and do 
 not persist throughout the life-time of the animal, as for 
 instance the first premolars of a horse, or two out of the 
 four premolars of most bears. 
 
 Before leaving this section of our subject, an instructive 
 illustration of the operation of these agencies may be given. 
 Fig. 114 (2). 
 
 It is very easy for us to see how a " rodeut " tjj)e of 
 dentition is beneficial to its possessor by rendering acces- 
 sible articles of food wholly unavailable for creatures which 
 
 {}) Statements to the contrary have been made, and copied from book to 
 book wthout verification. 
 
 ('^) A. Milk teeth of the Lemurine Cheiromj's, with the permanent in- 
 cisors just coming into place. It diflfers from any Eodent by having many 
 milk teeth, i. Permanent incisor. i 2. Posterior deciduous incisor. 
 c. Deciduous canine, d, d 2. Deciduous molars. I. Lower permanent 
 incisor. I 2. .Lower deciduous cauine. d a, d b. Lower deciduous molar. 
 B. Reduced outline figure of its permanent dentition, in which it closely 
 mimicks the true rodents. 
 
272 A MANUAL OF DENTAL ANATOMY. 
 
 have no means of gnawing through a shell or other hard 
 body. Now it happens that in three regions of the world, 
 pretty completely cut off from one another, three animals, 
 in parentage widely dissimilar, have arrived at dentitions of 
 " rodent " type. 
 
 Thus in Australia, a region practically wholly monopo- 
 lised by Marsupials ; a marsupial, the Wombat, has a den- 
 tition 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 fcAv Muridse, a Lemurine 
 animal, the CheiromyvS, has a dentition modified in a similar 
 direction, though probably employed to get at a different 
 food ; and elsewhere, scattered all over the world, we have 
 the ordinary Rodents. 
 
 In fact, three creatures, as widely different from each other 
 in parentage as they well could be, have been modified by 
 natural selection until they have dentitions, not identical, 
 but for practical purposes not unlike. 
 
 It is impossible to conceive that these three creatures 
 have had anything in the way of common origin : 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 "rodent" 
 type of dentition. Of extinct Lemui's little is known, and 
 of the ancestry of Cheiromys nothing ; but in the compact 
 oi-oup of Marsupials, still living in Australia, we are able to 
 dimly see some of the progressive steps which seem to tend 
 towards a rodent form of dentition. In Australia, roughly 
 speaking, there were nothing but Marsupials ; in Madagascar 
 more Lemurs than anything else ; and in each case out of 
 the material at hand, natural selection manufactured its 
 " 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 priind facie 
 
THE TEETH OF MAM3IALS. 273 
 
 resemblance in the teeth, there iire points in their several 
 dentitions whereby the wombat shows its marsupial affini- 
 ties, and the Aye-aye its quadinimanous affinities. 
 
 In addition to those modifications which are of direct use 
 to the individual in the way of assisting in the procuring of 
 food, &c., 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 exemjilified in the teeth 
 of animals. The males of many frugivorous monkeys have 
 canine teeth much larger than those of the females ; they 
 are cut late coincidently 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 ai'e 
 insignificant. 
 
 But the most striking instance of the teeth being modi- 
 fied, 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 versd — a generalisation 
 which, although subject to slight exceptions, remains upon 
 the whole ti'ue. 
 
 The male musk-deer (Moschus moschiferus) has canine 
 teeth of enormous lengih, while it is quite without horns 
 (see fig. 115); the female has no canine teeth. The male 
 muntjak, which has very short horns, has canine teeth, but 
 of much smaller size than those of the musk-deer. Other 
 
2/4 
 
 A MANUAL OF DEXTAL ANATOMY. 
 
 examples of hornless deer furnished with canine teeth are to 
 he 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 Tragu- 
 Udce. It is obvious that males furnished with weapons 
 more jiowerful than their fellows, will be more likely to 
 
 LSalS, 
 
 prove victorious in their battles, to drive away the other 
 males, to monopolise the herd of females, and so to transmit 
 their own peculiarities to offspring, which will again be 
 favoured in the same wa3\ Thus it is very easy to see how, 
 amongst gregarious animals, the development of teeth 
 serving as sexual weapons is likely to be favoured, genera- 
 tion after generation, nntil canines as highly specialised as 
 those of the musk-deer, or the wild boar, are attained to. 
 
 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 
 evidences of descent from ancestors whose teeth departed 
 less from the typical mammalian dentition are clearly trace- 
 able by the existence of rudimentary teeth and other such 
 
 (^) Cranium of Moschus, showing tlie long canine tcotli. 
 
THE TEETH OF MAMMALS. 275 
 
 characters. And, althougli it is by no means probable that 
 we have recognised more than a part of the agencies which 
 are at work, natural selection and sexual selection appear 
 to be competent to produce most of the phenomena of 
 modification observed. There remains one other influence, 
 much more obscure in its nature, to be touched upon, 
 namely, " correlation of growth " or " concomitant varia- 
 tion." "When we fi)id 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 imable 
 to conceive the precise manner in which the one thing in- 
 fluences 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 apparatvis, or in the fact that 
 two kinds of weapon are not developed in the same animal. 
 
 But there are some correlations of growth of a still more 
 recondite nature, in which the connection is less obvious. 
 Of this nature is the relation which exists between pecu- 
 liarities of the skin and of the teeth : the Edentata, abnormal 
 in their skins, are different from most other Mammalia in 
 their teeth ; fa^tal Avhales, yet more aberrant in the nature 
 of their skins, have only rudimentary teeth, in tlic place of 
 which, after birth, plates of whalebone are found. 
 
 Mr. Darwin ("Animals and Plants under Domestication,") 
 has collected a number of curious instances of relations exist- 
 ing between hair and teeth. In general terms it may be said 
 that any great abnormality in the hair goes hand in hand 
 with an abnormalit}- of the teeth. Thus, there is a breed of 
 dogs found in Turkey which are almost hairless, and which 
 have very few teeth, their dentition being red need to a single 
 molar on each side, together with a few imperfect incisors ; 
 and in the human subject inherited baldness has been found 
 to be associated with inherited deficiency of the teeth. 
 
 But Avp must not go further than to say, that great 
 abnormality of hair goes hand in hand with abnormality 
 
 T 2 
 
276 A MANUAL OF DENTAL ANATOMY. 
 
 of teeth, for examples have just been i^iveu of absence of 
 hair and absence of teeth ; and, on the other hand, redund- 
 ance of hair has in several cases been accompanied by 
 absence of teeth. 
 
 Thus, in the case of the now famous hairy family of 
 Bui'mah, the j)eculiarity of silky hair being developed over 
 the face Avas transmitted to a third generation, and in each 
 case the teeth were very deficient in number, A year or 
 two ago a hairy man and his son, said to have come fi'om 
 the interior of Russia, were exhibited in London, and they 
 were also almost toothless. Q) 
 
 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 extensive nvimber 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 jNIr. Hepbuna, 
 which are indisputably known to be models of her mouth, 
 and 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 aflected with general hypertrophy 
 of the gums and alveolar j^rocesses 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 palillte 
 of the glim, and the redundant hau's on the face, we have 
 evidence of a disposition to hypertrophies of the integument 
 affecting in different places the different tegumentary appen- 
 dages which happen to be there. And that the teeth are 
 
 (1) The man's moutli 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 backwai-d elongation, -nhich 
 allows for the successive development and eniption of the molars behind 
 the sjjace occui^ied 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. 
 
THE TEETH OF MAMMALS. 277 
 
 dermal appendages has been shown at a previous page (see 
 page 2). 
 
 He would indeed be a rash man -who ventured to assert 
 that we had recognised all the agencies which are at work 
 in the modelling of animal and vegetable forms ; l)ut it is 
 safe to say that, at the present time, we are acquainted with 
 " natural selection," or " sui-vival of the fittest," an agency 
 by which variations beneficial to their possessors will be 
 l^reserved and intensified in successive generations ; of 
 " sexual selection," which operates principally^ by enabling 
 those possessed of certain characters to pi-opagate their race, 
 while others less favoured do not get the opportunity of so 
 doing ; of " concomitant variation " between different parts 
 of the body, an agency much more recondite in its opera- 
 tions, but by which agencies affecting one pai*t may second- 
 arily 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 of parts which 
 have become useless, for a time at all events, and by the 
 occasional reappearance of characters which have been lost. 
 
 Allusion has been made to these great biological questions 
 with the view of helping the student to have patience to 
 master descriptions of minute points, of which he does not 
 at the moment see the bearing, by giving him confideuce 
 that there are no characters so trivial but that they may 
 throw very important light upon the remote parentage and 
 the line of descent of the creature under examination. And 
 as a further incentive to painstaking and minute observa- 
 tion, it may be added, that things which are rudiraentarj- 
 and therefore inconspicuous, are often just the things which 
 happen to teach us most ; for being of no present use, they 
 are not undergoing that rapid change in adaptation to the 
 creature's habits which may be going on in organs which are 
 actively employed. 
 
278 A MANUAL OF DENTAL ANATOMY. 
 
 THE ]IOMOLOGIK.S OF THE TEIJTH. 
 
 A superficial survey of the teeth of tliose mammals which 
 possess two sets of teeth (diphyodonts) will indicate that, 
 notwithstanding the apparent anomalies brought about by 
 adaptive modifications, a close correspondence between the 
 several teeth of different animals exists. 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 
 certainty which of them it is that are absent. 
 
 As it is impossible, or at least inconvenient, to avoid the 
 use of the term "typical" dentition, it will be well to explain 
 at the outset what is, and what is not, meant by it. 
 
 That the gTeat majority of biologists reject utterly the 
 " archetype " theory, by which all those resemblances which 
 really exist were refeired to the influence of a sort of gene- 
 ralised " pattern " animal, according to the model of which 
 all other animals were fashioned, has already been mentioned : 
 this, then, is what is not meant by a "typicol" dentition. 
 What is meant, is a form so simplified, so little modified in 
 in any special direction, that we can conceive it to be near 
 to a common parent form Avhencc, by progressive modification 
 in successive generations, other forms have been derived. 
 We cannot point to any mammalian dentition at present 
 known to us, and say this may have been the parent ; 
 tliis is a typical form of mammalian dentition ; but we 
 do know many fossil forms which approximate to it far 
 moi-e closely than do any at present in existence, and as 
 transitional forms of animals, and animals of highly gene- 
 ralised characters, are every day coming to light, we do not 
 doubt that such forms once did actually exist, and may 
 one of these days be found. Absolute proof would be ob- 
 tainable only if we could refei- to its place every mammal 
 that had ever existed, and show every step in the series 
 
THE TEETH OF MAMMALS. 
 
 of modifications by which the ultimate divergence of den- 
 tition Avas efiected. But evidence far short of absokite 
 ocuhxr demonstration serves to satisfy vis on most points, 
 and there is sufficient evidence available to enable us to 
 say with some confidence that our "tyiMcal" or parent 
 mammalian dentition was, so far as the numbers of the 
 several kinds of teeth go, 
 
 .3 1 4 3 , , 
 
 a - c - prni m ,^ zz 4-i. 
 
 And when there are less than forty-four teeth, as has been 
 already mentioned, we can in most cases say which they 
 are that are absent. 
 
 Thus, taking a certain bear and a baboon {each having 
 two premolars only on each side), we are able to decide, by 
 comparison with allied creatures, that, in the case of the 
 bear, it is the second and third premolars which are wanting, 
 the first and fourth remaining ; while in the baboon it is the 
 first and second which arc wanting, the third and fourth 
 being present. By homology we mean such correspondence 
 as is above indicated ; a correspondence which might almost 
 be expressed as a relationship by descent. 
 
 Homology, then, is almost equivalent to identity of oi-igin, 
 or, at all events, to similarity of origin ; but it by no means 
 necessai-ily involves identity or even similarit}* in the pur- 
 pose to which a thing is ultimately applied — a fact which will 
 be further illustrated in speaking of canine teeth. 
 
 The homologies of the teeth may be treated under two 
 heads : the one, the homologies of the. teeth in their relation 
 to other parts of the body, and the other, their more special 
 homologies, or their relation to one another. 
 
 The relation of the teeth to the skin, which we express by 
 calling them " dermal ai^pendages," as well as the epidermic 
 nature of the enamel, and the dermic nature of the dentine, 
 have been sufiiciently discussed at former pages, so that- 
 
A MANUAL OF DENTAL ANATOMY. 
 
 \vc may at once pass to the homologies of the teeth with 
 one aiiotlier. 
 
 Teeth are divided into incisors, canines, ])remokrs, and 
 molars, but these classes do not all admit of quite satis- 
 factory definition. Incisors are defined as teeth implanted 
 in the intermaxillaiy bone, a definition which has the merit 
 of being precise ; and on the whole there is a certain resem- 
 blance running through incisor teeth in most animals, but 
 the definition of lower incisors as being the corresponding- 
 teeth in the lower jaw is a good deal less satisfactory, because 
 they are not situate upon any distinct bone. And it has 
 even been denied that there can be a true homology between 
 a maxillary and a mandibular tooth. 
 
 Molars arc teeth at the back of the mouth, which come 
 up behind the milk teeth (when there are any), and which 
 are generally subservient to grinding the food. 
 
 Premolars are teeth in front of the molars, usually differ- 
 ing from them by being more simple in form and being- 
 smaller, and in most animals by having displaced deci- 
 duous predecessors. But they are not always simpler in 
 form, nor smaller (e.ff., the horse, fig. 138), nor do they 
 always displace deciduous predecessors (e.o., they do not all 
 do so in the Marsupials), so that this definition is not abso- 
 lutely j)recise. Still, as a matter of practice, it is usually 
 easy to distinguish the premolars, and the division into 
 premolars and molars is useful. 
 
 Any objection that can be raised to the name of premolar 
 on the score of a short logical definition being impossible, 
 applies with tenfold . force to the canines. (Cf. Messrs. 
 Mosely and Lankester, .Journ. Anat. and Physiology, 1869.) 
 
 The neai'est approach to a good definition is that which 
 describes the canine as the next tooth behind the intermax- 
 illary suture, provided it be not far behind it; and the 
 lower canine as the tooth which closes in front of the upper 
 canine. 
 
THE TEETH OF MAMMALS. 281 
 
 A great deal of coufusiou has arisen out of the twofold 
 sense iu -which the word " canine " is used : if it were always 
 applied to designate the first tooth in the maxilla of the 
 typical mammalian dentition quite irrespective of its size, 
 etc., and of the lower tooth closing in front of it, no objec- 
 tion to its employment could be made, inasmuch as it wovild 
 designate truly homological 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 hence we are dissatisfied with calling the first 
 maxillary tooth " canine," when it is some other tooth 
 which is doing its work. 
 
 On the other hand, if we are to leave out of court all 
 considerations 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 teeth 
 behind it. So we must be content with some such state- 
 ment as the following. 
 
 A veiy large number of animals, notably the Carnivora, 
 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. Tlie 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 we 
 use the term canine we 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 or 
 incisor — the first of the premolars, if we allow five premolars 
 instead of four. 
 
 It would practically be very inconvenient to abolish the 
 
282 A MAXUAL OF DENTAL ANATOMY. 
 
 term canine ; but it .should be borne in inind tliat its signi- 
 ficance is mei'ely equivalent to " caninifonu premolar," and 
 that in describing the dog's dentition (fig. lG3)we should be 
 less liable to be misinterpreted, were we to say that it has 
 five premolars, of which the first is caniniform. 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. 
 Then, as animals diverged and became modified in accordance 
 with their requirements, their teeth would become so far 
 difiin-entiated that they would admit of being classified. 
 Thus the Carnivora would have attained to a stage of 
 diff"erentiation in which the canine is functionally certainly 
 deserving of a distinction, whereas along other lines of 
 descent, differentiation having not proceeded so far, or having 
 proceeded in a somewhat different direction, it would not 
 merit a distinctive appellation. 
 
 But as it so happens, that all the works on odontography 
 have started upon the basis that there was a '' type " denti- 
 tion, and as a canine figured in that dentition, it will be 
 necessary to point out a few instances of the propositions 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 intermaxillary bone, whether that or any other 
 tooth be large and pointed, " caniniform " or not. 
 
 In typical Ruminants, the upper jaw lacks both incisors 
 aiid canines (with certain exceptions, for which see p. 335), but 
 in front of the lower jaw there are grouped together eight 
 teeth, closely fitted together, 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 more pointed; (ii.) because this tooth 
 shuts in advance of the upper canine when the mouth is 
 
THE TEETH OF MAMMALS. 
 
 closed (iu those allied creatures -wliicli have au undoubted 
 upper cauine) ; (iii.) because it is cut later than the others 
 (Owen). 
 
 These three reasons arc weak, because (i.) form is a 
 veiy unsafe guide to homology, and as to the lateness of its 
 development (iii.), it succeeds to the third incisor, by Pro- 
 fessor Owen's own showing, after about the same lapse of 
 time which separated the eruption of the second and third 
 
 incisors. Moreover, Oreodon, an extinct Ruminant with 
 caninifonn teeth, has the eight incisors in the lower jaw in 
 addition to a caniniform tooth, ivhich is the fifth tooth coimtiwj 
 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 
 should 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 eight procumbent teeth 
 
 (') Oreodon Culbertsonii (after Leidy). It will Le observed tliat iu tlic 
 Tipper jaw the four premolars of the typical mammalian dentition are 
 behind the "cauine," 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. 
 
284 A MAXUAL OF DENTAL ANATOMY. 
 
 occupying the front of the lower jaw, of which the outer- 
 most 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 tlie upper jaw, 
 for they are just like the other incisors. 
 
 But it is in the Imectlrora that the greatest difficulties 
 occur. 
 
 To the mole no less 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 is implanted (and its deciduous 
 predecessors also lie) (Spencer Bate) within the limits of 
 the premaxillary bone. And besides this, the lower tooth, 
 which answers the purpose of, and looks like, a canine, 
 closes behind instead of in fi'ont of the great upper tooth. 
 
 Fig. 117 C). 
 
 ErictiJm has a sharp, long-, two-fanged tooth, in pattern of crown, 
 an enlarged premolar, in position of upper canine, and no canini- 
 form tooth in lower jaw. 
 
 Ceiitefcs has tyiiical canines, like a Cai-nivore. 
 
 Ilcmlccntctcit. the so-called canine, differs in no respect from the 
 premolars behind it. 
 
 Ei-iiiacnix. So called ujiper canine two-rooted, aud like the pre- 
 molars which follow behind it. 
 
 (') Upper and lower teetli of the common mole. lu it, just as in 
 Oreodon, the teeth which fulfil the functions of canines are not corre- 
 sponding teeth in the upper and lower jaws. 
 
THE TEETH OF MAMMALS. 
 
 Gj/mnura. Upper canine-like tooth has two roots ; a single-rooted 
 
 lower pointed tooth closes in front of it. 
 Maeroncclis and Pctrofh-oiinis. The third or outermost incisor is 
 
 two-rooted, long, and sharp, and plays the part of a canine. 
 PotnmogaJc. A small tooth, in no respect different from the other 
 
 premolars, is called a " canine." 
 
 In some of the groups no tooth has been lengthened and 
 pointed, so as to serve as a canine ; in others it is the Avrong 
 tooth, i.e., not the same tooth as in the Carnivora, or as in 
 other Insectivora. Conseqixentl}-, 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 premolai", or no tooth at all. 
 
 It appears to me that the result of all investigations into 
 the homologies of mammalian teeth may be summed up 
 somewhat in the following manner. 
 
 The evidence of a common pattern, which is traceable in 
 incisors, canines, i^remolars, and molars (see ]3age 8), wop.ld 
 seem to indicate that their special forms have been all 
 derived from modifications of some much more simple form, 
 and that if we are ever to find what might be called a parent 
 mammalian dentition, it will be nearly " homodont : " that 
 is to say, the several teeth will not differ much from one 
 another in size and shape, just as we see to be the case in 
 the dolphin (see fig. 131), or the armadillo. 
 
 It becomes open to question whether the term canine is 
 desirable in dental formulae or in homological determinations ; 
 if we put on one side its functional modifications there is 
 nothing left to distinguish it from other premolars : if we 
 bear in mind its functional development we encounter the 
 anomalies noticed above. 
 
 If we were able to place in unbroken series all the den- 
 titions through which, by progressive modification, the 
 original almost homodont dentition had passed into a 
 highly specialised dentition, like that, say, of the cat, it 
 •would be a matter of impossibility to fix upon any point 
 
286 A MANUAL OF DENTAL ANATOMY. 
 
 where we should bo justified in assertiiiu' that here the 
 hoinodont dentition has recently become heterodont : at 
 this point, for the first time, we have incisors, canines, 
 molars. 
 
 As a matter of fact, a large number of extinct Ungiilata 
 had the full t3'pical number of mammalian teeth, viz., forty- 
 four, and in some the individual teeth, incisors, canines, 
 premolars, and molars, passed into one another by insensible 
 gradations, and contiguous teeth were but little differen- 
 tiated from one another. Professor Flower has described 
 and figured such an extinct Ungulate under the name of 
 Homalodontotherium (Philos. Trans., 1874). It is exceedingly 
 interesting to find that back in geological time the dentitions 
 were more generalised, both carnivorous and herbivorous 
 mammals of the Eocene period usually j^ossessing the full 
 typical number of teeth, and displaying less of special 
 modification ; but the few forms of life which have been 
 handed down in a fossil state do not as yet offer us by any 
 means an unbroken chain of forms differing from one another 
 by progressive modification, except in a few cases : thus the 
 ancesti'y of the horse is now comparatively completely known 
 to us. Bearing in mind that the several kinds of teeth have 
 probably a common origin, the homological differentiation 
 in the incisors, premolars, and molars may be advantageously 
 admitted, and made use of as a basis for comparing and 
 classifying the teeth of different animals. It is usually said 
 that when incisors are missing from the full typical number, 
 they are lost from the outer end of the series : that 
 is to saj', if there is but one incisor it is Ij^ ; if two, I^ 
 and L. 
 
 There are many exceptions to this : e.g., the first incisor 
 is the first to disappear in the otter, walrus, and some few 
 others. 
 
 When premolars are missing, it is said that they are lost 
 from the front of the scries. This is generally true, but the 
 
THE TEETH OF MAMMALS!. 
 
 following exceptions may be given. In bears the second 
 premolar is often absent, the first being very constant ; the 
 same thing is true of many bats ; in Dasyurus the third, or 
 hindmost (it being a ]\Iarsupial) is absent, the first two lacing 
 present. Q) 
 
 A difiiculty at times occm-s in deciding whether a tooth 
 is to be regarded as a premolar, or as a milk tooth, as there 
 are many so-called permanent teetli which are lost early in 
 the lifetime of the animal. 
 
 Professor Flower gives an instance of this in the hippo- 
 potamus : the first premolar appears with the milk teeth ; 
 it probably has no predecessor, and is shed in middle life. 
 But in allied forms the corresponding tooth remains in place 
 throughout the creature's life. 
 
 The wart-hog is a conspicuous example of the early loss 
 
 of teeth which clearly belong to the permanent series (see 
 
 page 328), all the teeth (premolar and molar) in front of the 
 
 last great molar being cast off, and the dentition ultimately 
 
 reduced to — 
 
 .21 1 
 
 1 _ c ^ m _ 
 
 :'. 1 1 
 
 That general correspondence, whicli is found to exist 
 between the dentitions of various animals, extends also to 
 the patterns of individual teeth, so that we are able to trace 
 out the various stages by which complexity of pattern has 
 been arrived at. 
 
 In what might be termed a typical tooth we should have 
 a single central pulp cavity surrounded by a body of hard 
 dentine ; over the crown this is coated by enamel, whilst the 
 whole, crown and root, would be invested b}^ a layer of 
 cement. 
 
 The layer of coronal cement may Vie so thin as to be 
 
 (1) This is ascertained by tlje examination of allied forms, in which the 
 hird premolar is found to lie so small as to l>e rudimentary. 
 
288 A MANUAL OF DENTAL ANATOMY. 
 
 merely rudimentary, as in Man or the C'arnivora ; or the 
 investment -with enamel may be only partial, as upon the 
 front of a Rodent incisor; or a tooth may be composed 
 solely of a mass of hard vinvascular dentine, as in the teeth 
 of the Wrasses. 
 
 And just as endless varieties of teeth may be produced 
 by the suppression, or partial svippression, of certain of the 
 tissues, so differences may be brought about by the occvir- 
 rence of other than the three usual tissiies. Thus the 
 remains of the central pulp cavity often becomes occupied 
 by calcified pulp, forming " osteodentine ; " this, which 
 occurs in man as an almost pathological condition, is per- 
 fectly 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 
 medullary canals. 
 
 It is not so much the complexities induced by variation 
 in minute structure that concerns ns here, as those brought 
 about by the arrangement of the different tissues. 
 
 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 Ave imagine a tooth with four long similar 
 cusps, 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 basis 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 inter- 
 spaces 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 
 
THE TEETH OF MAMMALS. 
 
 to a lower level is well and sim])ly illustrated by the 
 " mark " of the incisor teeth of a horse. 
 
 Ill an uncut, and therefore perfectly unworn tooth, such 
 as is represented in the figure, the condition of the apex 
 may be compared to the fii^ger of a glove, the tip of which 
 
 Fig. 118 Q). 
 
 has been pushed in or invagiiiated. The depression so 
 formed is, like the rest of the sin-face, coated with enamel, 
 and with a thin layer of cementum. 
 
 When the tooth is worn down to a considerable extent, 
 we have a field of dentine, in the centre of which is an 
 oval ring of enamel; within this a space filled with the 
 debris of food, tfcc. This constitutes the mark (see next 
 jjage), 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. 
 
 Not only may inflections of the surface and of the enamel 
 take place from the grinding surface, but they also abun- 
 dantly occur upon the sides of the tooth. The inflection 
 of the surface, which in the incisors of the horse is of the 
 simplest possible form, may be crucifoi-m, or variously 
 waved and broken up, thus leading to all sorts of com- 
 plications 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 tlie tooth, thus cutting it into 
 
 Q) Apex of crown of an upper incisor of a Horse, not yet completely 
 formed. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 of plates of dentine and enamel, fused into one tooth by 
 the cementum (sec fig. 12-")). 
 
 Interestinf? as liave been the discoveries made of late 
 
 Fio. 119 (' 
 
 Fig. 120 (-). 
 
 years n\ Mammalian paleontology, it is not as yet by any 
 means possible to determine from what common pattern or 
 patterns all complex mammalian teeth may be considered 
 to have been derived ; though the pattern of some, for 
 example, of the molars of the horse, may be traced back 
 in increasing simplicity through a number of parent forms. 
 Enough has, however, been done to indicate that by careful 
 study many complexities of pattern may be referred to a 
 few particular types, and thus may be simplified by a com- 
 parison with other allied forms, in which essential charac- 
 teristics are not masked by minor complications. Starting 
 
 (') Horse incisor, in longitudinal section. 
 
 (-) Horse's incisors, showing the mark at various ages. 
 
THE TEETH OF MAMMALS. 291 
 
 from the human tooth, as being familiar to us all, a quadrate 
 crown, with four cusps at its corners, is common to many 
 animals ; the oblique ridge, present in some apes, not in 
 others, is met with also in some Insectivora, e.g. the Hedge- 
 hog. Ax'ound the neck of many teeth runs a pronounced 
 ridge, the " cingulum," and tliis may be produced up into 
 additional cusps. 
 
 A very instructive series of comparisons of the molar 
 teeth of Insectivora has been made by Mr. jMivart (Journal 
 of Anat. and Physiol.: 1868); pointing out that within 
 the limits of this group a great variety of patterns is met 
 with, the several modifications being connected by transi- 
 tional forms. 
 
 It would appear that upon the molar teeth (npper) 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 mice 
 (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. 
 
 Thus in Urotrichus, a Japanese creature having affinities 
 with the mole, the external cingulum is elevated into thi-ee 
 distinct pointed cusps, united by ridges with the two prin- 
 cipal cusps, an aiTangemeut which gives a sort of W pattei'n 
 to the sm-face, while to the inner side the cingulum forms 
 another cusp, so that there ai-e 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 distinctives. 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 
 intermediate forms which render it easy to identify its 
 
 x; 2 
 
A MANUAL OF DENTAL ANATOMY. 
 
 reversed part with those corresponding- to tlicni in the mole 
 or in Urotrichns. 
 
 Speaking generally, it may be said that new cnsps are 
 added to the number already existing, by tlie cingulum 
 
 Fig. 121 
 
 ■>*1f 
 
 becoming elevated into points ; it is not very unusual to 
 see subsidiary cusps, obviously originating in this way, upon 
 human molars. 
 
 Ridges may variously connect the cusps ; and the coales- 
 cence of two or more cusps to form an exceedingly 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. 
 
 A simple j^attern of tooth is formed by the junction of 
 the two anterior and two posterior cusj)s by simple ridges ; 
 and the cingulum may connect the outer ends of tliese two 
 ridges ; sucli a tooth is seen in the Tapir and in the 
 Palseotherium. By the varied obliquity of these ridges, 
 
 C) Upper molar teeth of (A) Urotriclm.s ; (IJ) Mole ; and (C) Chryso- 
 cliloris. The four principal cusps are lettered a, b, c, d, in each of the 
 figures. In A the cingulum has been elevated so as to form four additional 
 cusps on the exterior of the tooth, and one additional cusp on the interior. 
 B and C show the fusion of certain of these cusps, and the consequent 
 diminution in their number. (From Mivart. ) 
 
THE TEETH OF 21 AM MA LS. 
 
 and by the introduction of secondary inflections, patterns 
 apparently dissimilar are arrived at. 
 
 In the molar tooth of the horse, arrived at hy a modifi- 
 cation of the Palseotherium type, we have a surface con- 
 stantly kept rough by the varying hardness of its difterent 
 constituents. 
 
 In a worn tooth, we have upon a general field of dentine 
 
 Fi(?. 122 fix 
 
 two islands of cementum, Iiouuilcd 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 sm-face 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, it will suffice for the present to point out that 
 such correspondences do exist, and that all the complexities 
 of pattern found, may, in practice, be reduced to some few 
 types. 
 
 (') Molaf tooth of Horse, showing the cliaract eristic pattern of its 
 grinding surface. 
 
294 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 Tlic development of additional cusps from up-growths of the 
 cingulum, and the suppression or fusion of pre-existing cusps, 
 may be traced b}' a comparison of the teeth of allied animals, 
 and thus connecting links are foimd between jjatterns at 
 first sight very dissimilai-. The order Proboscidea affords, 
 however, so instructive an example of the manner in which 
 an exceedingly complex tooth lias l)een derived from a 
 simple one, that it may l)e mentioned in this place us an 
 exam]>le. 
 
 The tooth of the elephant is so strikingly unlike other 
 teeth tliat it might at first sight be supposed that it is 
 more essentially different than is really the case. The clue 
 
 Fig. 123 ('). 
 
 bed e 
 
 to its nature is afforded by the teeth of an extinct Pro- 
 boscidian, the Mastodon. If we take as our starting point 
 the second true molar of one of the Mastodons (Tetralo- 
 phodon) Ave find its crown to be made up of four sti'ongly 
 pronounced transverse ridges, the summits of which are 
 made up of rounded eminences (whence the name Mastodon, 
 
 (') Second ujiper molar of IMastudon (longirostri.s), from Falconer. 
 About one-eighth natural size. The four transverse ridges, h, c, d, c, are 
 seen to be, to some extent, divided into outer and inner divisions 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 tooth ; by its further elevation additional 
 ridges or cusjis would be formed. 
 
THE TEETH OF MAMMALS. 295 
 
 from /Aaaros, ix nipple). The three transverse ridges coalesce 
 at their bases, and the crown is siipportcd upon a number 
 of roots corresjionding to the ridges. 
 
 If we take the next tooth, or the third true molar, the 
 general character remains the same, save that there are five 
 ridges, and indications of as many roots ; still the general 
 correspondence of the ridges with the cusps of less aberrant 
 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 is not a very aberrant one ; it is obviously 
 nothing more than a tooth in which the somewhat numerous 
 cusps are connected bj- transverse ridges, and are very long 
 and strongly pronounced. 
 
 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 inter- 
 spaces between them with cementum, and to stunt the 
 roots. The completed tooth of an elephant is a squarish 
 or rather oblong mass, from the base of which sjDring con- 
 tracted and stunted roots. It consists of a common pulp 
 cavity, small in proportion to the bulk of the tooth, and 
 
 (') Molar tooth of an Asiatic Eleiihant, sliowiiig the transverse plates of 
 Jentiue borJereil by enamel. 
 
296 A MANUAL OF DENTAL ANATOMY. 
 
 deep down in the mass, from which many thin laminee are 
 sent up towards the surface, each consisting of an oblong 
 area of dentine enclosed by enamel ; and the interspaces of 
 these exaggerated cusps are solidly lillcd in by cementum. 
 
 Between the Mastodon and the Indian Elej)bant 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 Indian Elephant. 
 
 The numerous transverse })lates of the elephant's grinder."* 
 are united by dentine at their bases, and a common pulp 
 cavity and truncate roots are formed ; but in this last 
 respect the molar teeth of the ca})ybara depart still farther 
 from the ordinary type, for Ijcing molars of persistent 
 
 Fig. 125 ('). 
 
 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. 
 
 It has been suggested (J. A. ilyder, Proc. Acad. Nat. 
 Sciences, Philadelphia, 1878), 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, and that hence the greatest modification is to be 
 
 i}) Molar of CajDybara, sliowing the transverse plates of dentiue and 
 enamel united to one another by cementum. 
 
THE TEETH OF MAMMALS. 297 
 
 found nearest to the articulation, where the greatest force 
 is exei'ted. 
 
 Thus "bunodont" animals, z'.e. those that have rounded 
 conical cusps ujjon their short-rooted teeth, have a cylindrical 
 condyle ; selenodouts, or those with crescentic ridges on the 
 molars, have a condyle which is expanded and plane, while 
 lophodonts, or those with transversely ridged teeth, have a 
 globular condyle. 
 
 This correspondence pointed out between the condyle, the 
 movements of the jaw, and the form of the teeth does exist, 
 but it is less easy to see how it is brought about. The 
 simple mechanical explanation that the teeth are drawn out 
 into these forms, hardly conveys much information, seeing 
 that the tooth before it is subjected to these influences, is 
 quite finished, and its form, such as it is, is unalterable : 
 while to effect an alteration in the form of a masticating 
 surface an influence must be brought to bear iipon the 
 tooth germs at an exceedingly early period. It might with 
 equal justice be said that the crown of the tooth being 
 foi'med thus had influenced the excursions of the jaw, and 
 so modified the condyle. 
 
 THE JIILK DEXTITIOX. 
 
 Some thirty years ago Professor Owen called attention tc> 
 the fact that those mammals in whom thd teeth situated in 
 different parts of the mouth were alike in form (homodonts), 
 developed only one set of teeth, and to indicate this charac- 
 teristic he proposed for them the term " monophyodonts." 
 Those, which, on the contrary, had teeth of different size 
 and form in various parts of the mouth (heterodonts), de- 
 veloped two sets of teeth ; a " milk " set, which was dis- 
 placed by a permanent set, and this peculiarity he expressed 
 by the term " diphyodonts." As originally set forth, the 
 terms homodont and monophyodont were interchangeable. 
 
A MAXUAL OF DENTAL AXATOMY. 
 
 for they designated the same groups of animals ; in the 
 same way heterodont was an equivalent for diphyodont. 
 
 But although this is true of a large number of animals, it 
 is not true of all, and it becomes necessary to note some of 
 the exceptions. 
 
 The nine-banded armadillo (Tatusia puba) is a true homo- 
 dont : its teeth are all very nearly alike, they are simple in 
 form, and they grow from persistent pulps. Yet it has 
 been shown by Rapp, Gervais, and Professor Flower, to 
 have a well developed set of milk teeth, retained until the 
 animal is of nearly full size. 
 
 Thus it is a true diphyodont, at the same time that it is 
 a true homodont mammal. But no milk dentition has been 
 observed in the sloths, uor indeed at present has it been 
 seen in any other armadillo (except the doubtfully distinct 
 T. Klapperi) ; nor have milk teeth been found in any ceta- 
 cean, so that the rest of the homodont animals are, so far as 
 we know, really monophyodont. 
 
 Nor is it absolutely true that monopliyodonts are all 
 homodont : thus the rudimentary teeth of balfcnoptera are 
 heterodont (see p. 311). 
 
 Upon the whole, our information respecting the " milk " 
 or deciduous dentition is defective ; but much light has 
 been thrown upon the subject by the investigations of 
 Professor Flower (Journal of Anatomy and Physiology, 
 1869, and Transactions Odontological Society, 1871), of 
 whose papers I have made free use in this chapter. 
 
 The perpetual replacement of teeth lost, or shed in regular 
 course, which characterises the dentition offish and reptiles, 
 finds no parallel in the case of mammals, none of whom 
 develop more than two sets of teeth. 
 
 Just as homodont mammals as a rule develop but one set 
 of teeth, so heterodont mammals as a rule develop two sets 
 of teeth, though exceptions to this rule may be found. 
 
 The deciduous or milk set of teeth may be of any degree 
 
THE TEETH OF MAMMALS. 299 
 
 of completeness ; the milk teeth in man answer the require- 
 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 disapjiear very 
 early indeed, as in the mole (see Fig. 117) ; whilst there are 
 many instances of the milk teeth being absorbed in utero. 
 fSo that in the extent to which the milk teeth are developed, 
 the greatest variability is found to exist. 
 
 A perfectly typical milk dentition represents, upon a 
 reduced scale, the adult dentition of the animal, with the 
 exception only that sexual differences are but feelily marked, 
 if indeed the}- are at all present. 
 
 Thus, 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 displace them, which latter are generallj^ of 
 simpler form. 
 
 In what may be termed the normal arrangement, each 
 tooth of the milk series is vertically displaced b}^ a tooth of 
 the permanent series ; but plenty of examples may be 
 found of particular milk teeth which have no successors, 
 and, on the contrary, individual permanent teeth which 
 have never had a deciduous predecessor. 
 
 It has already l^een mentioned that amongst homodonts 
 no succession of teeth has been observed in the Cetacea, 
 nor in any other of the Edentata, save the armadillo ; 
 amongst heterodonts there are several Rodents which have 
 no deciduous teeth, e.g., the rat ; the dugong has probably 
 deciduous incisors, but no other milk teeth ; the elephant 
 has no vertical succession, save in the incisors. 
 
 Among Marsupials, which are true heterodonts, there is 
 only one milk molar on each side in each jaw; this is always 
 displaced by the third or last premolar ; but the milk tooth 
 varies in the extent to which it is developed from being 
 
A MANUAL OF DENTAL ANATOMY. 
 
 rudimentary in Thylacinus, probably absent altogether in 
 
 Dasyurus, and Phascolarctus, to being a large tooth retained 
 
 in full use till the animal is nearly full grown in Hypsiprymnus. 
 
 Within the group Carnivora, the dog and many others 
 
 Fig. 126 (i). 
 
 
 Fiti. 127 (2). 
 
 have a thoroughly well developed set of milk teeth, which 
 do service for some time ; in the l)ear the milk teeth are 
 relatively smaller, and are shed very early ; in the seal the 
 milk teeth arc rudimentary, functionless, and are absorbed 
 before birth, so that in the specimen figured the milk incisors 
 had already disappeared (see Fig. 127). 
 
 Q) Permaneut and milk dentitions of a Dug ; tlic iatter was well de- 
 veloped. Nat. size. 
 
 (-) Permanent and milk dentition of a seal (Plioca Greeulandia). 
 Nat. size. 
 
THE TEETH OF MAMMALS. 
 
 301 
 
 In the elephant seal the milk teeth are yet more rudi- 
 mentary, and the difference between its dentition and that 
 of the monophyodont homodont cetacean (Grampus) is not 
 
 Fig. 128 ('). 
 
 ■5 ncd P.IZO 
 
 Fir.. 129 (-) 
 
 iial size 
 
 gi-eat ; an observation which is the more interesting, inas- 
 much as this seal in other characters than its teeth ap- 
 proaches towards the cetacean group. From these facts, 
 which are well indicated in the accompanying figures, Pro- 
 fessor Flower arg-ues that the permanent set of teeth of 
 
 (') Permanent aiul milk ilentition of an Elephant Seal (Cystophora 
 proboscidea). 
 
 (-) Teeth of the truly moiioiihyoilal (jrampiis (Orca capensis). (These 
 four figures are copied from Prof. Flower's paper). 
 
302 A MANUAL OF DENTAL ANATOMY. 
 
 diphyodonts correspond to the single set of monophyodonts, 
 so that the milk dentition, when it exists at all, is some- 
 thing superadded. 
 
 Whether this 1)e so is a question difficult to determine ; 
 from the facts advanced by Professor Flower, while they 
 stood alone, most people would, with little hesitation, concur 
 Avith his conclusions ; but the history of the development of 
 the teeth interposes a fresh difficidty. 
 
 The tooth germ of the milk tooth is first f(jrmed, and the 
 tooth germ of the permanent tooth is derived from a portion 
 (the neck of the enamel germ) of the formative organ of the 
 milk tooth (see Fig. 67). Again, in most of those animals 
 in which there is an endless succession of teeth, such as the 
 snake, the newt, or the shark, each successive tooth germ is 
 derived from a similar part of its predecessor, the natural 
 inference from which would be that the permanent set, 
 being derived from the other, was the thing added in the 
 diphyodonts. 
 
 The question cannot be finally settled imtil we know more 
 of the development of the teeth of the monophyodont cetacea : 
 thus it might turn out that in them also there ai-e abortive 
 germs of milk teeth formed, which do not go on so far as 
 calcification, but which do bud off, as it were, germs or per- 
 manent teeth ; if such should prove to be the case, this 
 would bring their teeth into close correspondence with those 
 of the elephant seal. 
 
 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 (Phacochserus), which 
 loses successively all its premolars and the first and second 
 true molars, the last true molar alone being truly persistent. 
 
 Sometimes nothing but a careful comparison of the teeth 
 
THE TEETH OF JL-LUMALS. 303 
 
 of allied creatures will eiial)le us to decide whether a parti- 
 cular tooth is to be referred to the milk or to the permanent 
 series ; as occasionally teeth of the latter set are cut very 
 early, at a time when the milk teeth are all in place, and 
 are shed during adult life. Professor Flower gives as an 
 example of this, the first premolar of the hippopotamus. 
 
 CuviER. Dents des Mammiferes. 
 
 De Blaixville. Osteographie. 1839 — lSr)4. 
 
 Owen. Odontography. 184.5. 
 
 G-IEBEL. Odontographie. 18.i.>. 
 
 Flower. Lectures on Odontology (British Med. Journal, 187 1). 
 
CHAPTER IX. 
 
 THE TEETH OF MONOTREMATA, EDENTATA, AND CETACEA. 
 
 :monotremata. 
 
 The Echidna, or Spiny Ant-eater has no teeth whatever, 
 and the strange Ornithorhyncus (duck-ljilled Platypus) is 
 also destitute of true calcified teeth. 
 
 In the place of teeth its flattened hill is furnished with 
 eight horny plates, two on each side of each jaw. We 
 may therefore pass at once to the orders Edentata and 
 Cetacea, which it is convenient to take first, as their 
 dentitions are of that simple form designated by the term 
 "Homodont." 
 
 THE TEETH OF EDENTATA (BrUTa). 
 
 Sloths, Armadillos, Ant-eaters. 
 
 The term Edentata was applied to the animals of this 
 order to indicate the absence of incisors (teeth in the inter- 
 maxillary bone) : though this is true of most of them, a few 
 have some upper incisors, but the central incisors are in all 
 cases wanting. 
 
 Some of them are quite edentulous ; this is the case in 
 the Mutica, or 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. 
 Food is taken in by the protrusion of an excessively long, 
 whip-like tongue, which is covered by the viscid secretion 
 
THE TEETH OF EDEXTATA. 305 
 
 of the great sub-maxillary glands, and is wielded with 
 much dexterity. The Mauis, or Scaly Ant-eater is also 
 edentulous. 
 
 The Edentata belong to the monoj^hyodont or homodont 
 section of Mammalia ; but, in some, certain teeth are more 
 largely developed than others, so that we have teeth which 
 might be termed canines ; and it has already been mentioned 
 that one armadillo, at all events, is diphyodont. 
 
 The teeth are of simple form, and do not in any marked 
 degree differ in the different parts of the mouth, except 
 only by their size (to this the canine-like 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 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 Megathe- 
 rium hard dentine, a peculiar vaso-dentine, and richly 
 vascular cementum co-exist (see Fig. 4.3). 
 
 I am not aware that enamel has been seen upon the teeth 
 of any Edentate animal, but I found some years ago that 
 the tooth germs of the nine-banded armadillo were provided 
 with enamel organs ; this, however, proves nothing, for 
 (Philos. Trans., 1876) I believe the presence of enamel 
 organs to be uniA-ersal and quite independent of any after 
 formation of enamel. 
 
 The teeth of the nine-banded armadillo (T. 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 
 roundish form on section, and those of the upper and lower 
 jaws alternate, so that by wear they come to terminate in 
 wedge-shaped grinding surfaces : before they are at all 
 
 X 
 
306 A MANUAL OF DENTAL ANATOMY. 
 
 worn they are bilobed, as may be seen in sections of the tooth 
 germs. 
 
 In the accompanying figure the milk teeth are represented, 
 and beneath them their permanent successors : the divari- 
 cated bases of the milk teeth are due to the absorption set 
 up by the approach of their successors, and not to the 
 
 X2 I\.S. 
 
 formation of any definite roots. Successional teeth have 
 been detected in this armadillo only (except also in T, 
 kappleri, which is perhaps a mere variety) ; but material 
 does not exist in our museums which would enable us to 
 positively deny their occm-rence in other forms. 
 
 Professor Flower has failed to discover any succession of 
 teeth in the sloths, and I have myself, through the kindness'of 
 the late Professor Garrod, examined microscopically the jaws 
 of a foetal Choloepus, in which the teeth were but little 
 calcified, and failed to detect any indication of a second set 
 of tooth germs. The probability is, therefore, that they are 
 truly Monophyodont. 
 
 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. 
 
 Sloths have fewer teeth than armadillos, and these softer 
 in character, the axis of vaso-dentine entering more largely 
 
 (1) Lower jaw of a young Armadillo (Tatusia peba), showing the milk- 
 teeth (a) in place, and their successors (h) beneath them. From a specimen 
 in the Museum of the Royal College of Surgeons. 
 
THE TEETH OF CETACEA. 307 
 
 into their composition, and foimiug as much as half the 
 biilk of the tooth. 
 
 The Orycteropus, or Cape Ant-eater, the pecu]iarities of 
 ■whose teeth have ah-eady been alluded to, has about twenty- 
 six teeth in all ; the true ant-eaters are edentulous. The 
 teeth of some of the gigantic extinct Edentates -were a little 
 more complex in foi'm and structure ; thus the teeth of the 
 Glyptodon were divided by longitudinal grooves, which in 
 section rendered it trilobed ; and the teeth of the Mega- 
 therium were likewise marked by a longitudinal furrow. 
 
 In their persistent growth, uniformity of shape, and 
 absence from the inter-maxiUary bone, they strictly con- 
 formed with the teeth of recent Edentata. 
 
 THE TEETH OF CETACEA. 
 
 No cetacean is known to develop more than one set of 
 teeth, and these, when present in any considerable numbers, 
 closely resemble one another in form. 
 
 They are usually composed of hai'd dentine, with an in- 
 vestment of cement ; after the attainment of the full 
 dimensions of the tooth what remains of the pulp is very 
 commonly converted into secondary dentine ; tips, and even 
 entire investments <jf enamel, are met with in many of the 
 order. 
 
 The dentine of many Cetaceans, e. g. of the sperui whale, 
 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 lacunse in it are 
 very abundant ; its lamination is also very distinct. 
 
 In the dolphin the teeth are very numerous, there being 
 about 200 ; they are slender, conical, slightly curved in- 
 
 X 2 
 
308 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 wards, and sharply pointed ; as they interdigitate 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. 
 
 Fig. 131 (i). 
 
 Many variations in the number and form of the teeth 
 are met with ; the porpoise has not more than half the 
 
 r^ 
 
 TTiat" sxze 
 
 W\r\i\) 
 
 ■^ 
 
 number of teeth possessed by the dolphin, wliile the gram- 
 pus has still fewer. The teeth of the grampus become 
 worn down on their opposed surfaces, and coincidently 
 with their wearing away the pulps become calcified. In 
 the Oxford museum there is a grampus in which, owing to 
 a distortion of the lower jaw, the teeth, instead of inter- 
 digitating, 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 
 
 (1) Jaws of a common Dolpliin. 
 
 (-) Teetli of upper jaw of a Gramims (after Professor Flower). 
 
THE TEETH OF CETACEA. 309 
 
 obliteration of the pulps by calcification,' so that the pulps 
 died and abscesses around the teeth had resulted. 
 
 In the sperm whale the teeth are numerous in the lower 
 jaw, but in the upper jaw there are only a few curved, 
 stunted teeth, which remain bvu-ied in the dense gum. The 
 teeth of the lower jaw are retained in shallow and wide de- 
 pressions of the bone by a dense ligamentoiis gum, which, 
 when strijiped away, carries the teeth with it. Every inter- 
 mediate stage between this slight implantation and the 
 well-developed stout sockets of the grampus, is met with in 
 the Cetacea. 
 
 In the bottle-nosed whale (Hyperoodon bidens) the only 
 large teeth present are two conical, enamel-tipped teeth 
 (sometimes four) 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. (Esch- 
 richt, Lacepede.) 
 
 In the narwal (Alonodon monoceros) two teeth alone per- 
 sist, 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 at- 
 tains 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 speci- 
 mens, in which the two tusks had attained to equal and 
 considerable length, the spirals on the two wound in the 
 some direction ; that is to say, as regards the sides of the 
 head, the spirals were not symmetrical with one another. 
 
 (1) Trans. Odonto. Society, 1873. When I published this paper I was 
 not aware that Eschricht had previously published a similar observation. 
 
310 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 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. 
 
 Fig. 133 Q). 
 
 
 Professor Turner has lately noted the occurrence of two 
 stunted incisor rudiments in a foetal narwal : these ob- 
 viously repi-esent 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 jiair of teeth which attain to 
 
 (') Cranium of Narwal (Monodoii monoceros). a. Stunted tooth, witt 
 its basal pulp-cavity obliterated, h. Long tusk. The small figure, giving 
 the whole length of the tusk, shows the proportion which it bears to the 
 vest of the skull. 
 
THE TEETH OF CETACEA. 
 
 more considerable dimensions. All trace of this second pair 
 of incisors is lost in adult skulls. 
 
 The Cetaceans classed together as Ziphoids have no teeth 
 in the upper jaM', and in the lower jaw only two (in a 
 single species there are four) teeth which attain to any- 
 considerable size, though other rudimentary teeth have been 
 formed in the dense gum. 
 
 The structure of these teeth is very peculiar : a tooth of 
 a species of Ziphius in the Oxford University museum, 
 which was described by Professor Ray Lankestei", consists in 
 great part of cementum and osteo-dentine, the true dentine 
 being merely a little fragment situated at the top, and not 
 forming more than a tenth of the whole bulk of the tooth. 
 
 Zipbius Layardii has teeth nearly a foot long projecting 
 upwards from the lower jaw, which arch towards one 
 another, above the upper jaw, so that the}" must prevent 
 the mouth from being opened to any considerable extent. 
 
 The whalebone whales are, in the adult condition, des- 
 titute of teeth, but prior to birth the margins of both upper 
 and lower jaws are covered with a series of nearly globular 
 rudimentary teeth, which become calcified, but are speedily 
 shed, or, rather, absorbed. 
 
 The foetal teeth of the Baleenoptera rostrata have been 
 cai'cfully described by M. Julin (Archives de Biologic, 1880), 
 the Bakenopte7'idce having been previously supposed to be 
 without rudimentary teeth. The ramus contained 41 tooth 
 germs, each furnished with an enamel organ and dentine 
 bulb, with a slight capsule ; these were lodged in a con- 
 tinuous groove in the bone above the vessels, thus re- 
 calling 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 a,re simple near the front, bifid in the middle, and 
 trifid at the back of the mouth ; in other words, these 
 
312 A MANUAL OF DENTAL ANATOMY. 
 
 rudimentary teeth seem not to be rudiments of a homo- 
 dont dentition as might have been expected, but of a 
 heterodont dentition ; and it is suggestive of a resemblance 
 to such forms as Squalodon, an extinct cetacean, peculiar in 
 having heterodont teeth. At all events it seems to indicate 
 that the homodont dentition of Cetacea is a case of degrada- 
 tion from ancestral forms, a conclusion likewise pointed to 
 by the gradual suppression of milk dentitions (see p. 300). 
 
 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, so as to be fringed with stiff hairs, and their 
 fringed edges 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 w^hale 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 crea- 
 tures, while the water is expelled. Then the tongue sweeps 
 the entangled food from the fringe of the baleen plates and 
 it is swallowed. Each plate consists of two dense but 
 rather brittle lamiuse, which enclose between them a tissue 
 composed of bodies analogous to coarse hairs. By the pro- 
 cess of wear the brittle containing lamince break away, 
 leaving projecting from the edge the more elastic central 
 tissue, in the form of stiff hairs. 
 
 Each plate is developed 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- 
 
THE TEETH OF CETACEA. 313 
 
 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 Prof. Turner 
 (Proc. Roy. Soc. Edinbm-gh, 1870), the whalebone matrix 
 having been produced by the cornification of the epithelial 
 coverings of its various groups of papillte, is an epithelial or 
 epiblastic structure, and morphologically corresponds not 
 with the dentine, but with the enamel of a tooth. 
 
 The wdiole whalebone plate and the vascular ridges and 
 papillce 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. 
 
CHAPTER X. 
 
 THE TEETH OP UNGULATA. 
 
 In the two orders just considered, the Cetacea and 
 Edentata, a single set of teeth would seem to be the rule, 
 and most members of these orders are, so far as is known, 
 both monophyodont and homodont. But in all orders that 
 remain to be considered a Diphyodont dentition, the milk set 
 varying from the merest rudiments to full development, Avill 
 be the rule ; and being diphyodont, they are for the most 
 part heterodont, that is to say, the teeth differ from one 
 another, and we can distinguish them into incisors, canines, 
 premolars, and molai's. Hence we are able to assign to them 
 a dental formula, and an extended survey of mammalian 
 forms lends strong support to the idea that the typical dental 
 formula, in which the full normal mammalian number of 
 teeth is present, is 
 
 .31 4 3 
 
 1 — c — rirm — m — = 4-1. 
 3 1^43 
 
 Very many have less than this full number : only a few 
 have more ; and it is not a little interesting to find that 
 among extinct mammalia, and especially among extinct 
 ungulata, the typical dentition was more often present than 
 amongst recent animals. Indeed it may be said that most 
 mammals of the Eocene period had the full typical mam- 
 malian dentition. 
 
THE TEETH OF VNGULATA. 315 
 
 Ungulata, or hoofed animals are grouped thus : — 
 
 (i.) Ai-tiodactyles, or ) Hippopotamus. Pigs. Anoplotherium.&c, 
 even-toed Ungulata j Cows, Sheep. Deer, and other Ruminants. 
 
 (ii.) Perissodactyles. or) Horses. Tapirs. Rhinoceros. Pateotherium. 
 I ngulata with au / 
 odd number of toes ) 
 
 The distinction between the two groups is strongly marked, if 
 living animals alone be considered ; but, as Professor Huxley has 
 pointed out. increasing knowledge of fossil fonns is tending to 
 break do^^-n the line of demarcation. 
 
 The recent forms bear, in all probability, but a very small pro- 
 portion to the extinct Ungulata. of which our knowledge is as yet 
 but fragmentary ; though the discoveries of Professors Marsh and 
 Cope in the '■ mauvaises teiTes " of Wyoming have brought to light 
 a very large nmiiber of strange and interesting ungulates : and this 
 fragmentary condition of oui- knowledge makes it as yet impossible 
 to give a connected account of the dentition of ungulates, seeing 
 that the forms known to us are only isolated and often widely 
 separated links in the chain. 
 
 The Teeth of Perissodactyle Ungulates.— Perisso- 
 dactyle (odd-toed) Ungulates are far less numerous than the 
 even-toed section, and among recent animals only comprise 
 the Horse, the Rhinoceros, the Tapir, and their allies. Their 
 premolars, or at least the last three of them, are eqiially 
 complex in patteni with the true molars ; and canines, tusk- 
 like but not A'ery large, are of frequent occurrence. The 
 lower molai's of almost all perissodactyles have a character- 
 istic form, their grinding surfaces being made up of two 
 ci'escentic ridges. 
 
 The ungulate animals are all possessed of molar teeth, 
 which are kept iu an efl&cient state of roughness by the 
 enamel dipping deeply into the crowns ; by the cusps, in 
 fact, being of very great depth. It consequently happens 
 that after the immediate apex is worn away, the flattened 
 working face of the tooth is mapped out into definite 
 patterns, which, on account of the light thus thrown upon 
 fossil remains, often consisting of little else than the teeth, 
 have been studied with great care. The result has been to 
 
316 A MANUAL OF DENTAL ANATOMY. 
 
 establish a general community of type, so that, dissimilar 
 as they at first sight appear, it is possible to derive all, or 
 almost all, the configurxitions of their crowns from one or 
 two comparatively sim2Dle pattei'ns. But odoutologists are 
 not yet agreed, or rather do not yet know enough of the 
 vast number of extinct Ungulates which there is reason to 
 believe once existed (of which many have lately been dis- 
 covered) to decide with certainty what the parent pattern 
 was. 
 
 Bihinoceros. — It is difficult to assign a dental formula to 
 this genus, as the incisors are variable in the different 
 species, but all agree in the absence of canines. 
 
 . 2, 4 3 
 1 — ] c — p — m — 
 
 In the African Rhinoceros, in which the adult has no 
 
 incisors, the young animal has eight incisors ; other species 
 
 retain the incisors through life ; and it is noteworthy that 
 
 2 
 in the Indian Rhinoceros, which has i — , the outer incisors 
 
 in the upper javv, are, as is usual, the ones that are absent, but 
 in the lower jaw it is the central incisors which are missing. 
 The first premolar, just as in the Horse, is small, has no 
 milk predecessor, and is not long retained ; the other pre- 
 molars do not markedly differ from the true molars. The 
 premolars and molar teeth, though not diflfering much in 
 character, increase in size from before backwards. The 
 crowns of the teeth are of squarish outline, larger on their 
 outer than their inner side, and are implanted by four roots. 
 The pattern of their grinding surfaces is very characteristic; 
 but it will be best understood by first digressing to say a 
 few words on the dentition of the Tapir. 
 Tapir. — The dental formula is 
 
 .3143 
 
 o l 6 o 
 
THE TEETH OF PEEISSODACTYLE UXGULATA. 317 
 
 In a brief survey, like that to which the present work is 
 necessarily confined, it will suffice to mention that there is 
 no great peculiarity about the incisors, or the canines, save 
 that the lower canine ranges with the lower incisors ; behind 
 the canine comes an interval, after which come the pre- 
 molars and molars, which are interesting, as being of simpler 
 pattern than those of most Ungulates, and it will be necessary 
 to very briefly allude to the various patterns characteristic 
 of ungulate teeth, with a view of showing how they may 
 have been derived the one fi'om the other. 
 
 In the Tapir four cusps are traceable, but ridges uniting 
 the two anterior and the two posterior cusps are strongly 
 developed, at the cost of the antero-posterior depression, i. e. 
 of one of the arms of the cross which separates the four cusps 
 in other quadricuspid molars. There is therefore left only a 
 deep ti-ausverse fissure (hence it is called a bilophodont 
 tooth), and the quadricuspid form is disguised. A low wall 
 on the outside of the tooth connects the two ridges. 
 
 In the Hog we have a simple fourcusped molar, with a 
 
 
 Fig. 
 
 134(1 
 
 ). 
 
 
 
 
 , 7nj 
 
 
 7"", 
 
 ]l 
 
 
 m^ 
 
 
 
 
 
 i 
 
 ' ^^^-:r' 
 
 
 <w^ 
 
 ^. 
 
 
 "> 
 
 ' '- ■■":.• ''■ 
 
 
 
 
 
 1> 
 
 ~^KJ'-^ 
 
 ^vJ^ 
 
 ^'^ 
 
 c 
 
 
 d 
 
 crucial depression separating the cusps ; in the Hippopot- 
 amus the same pattena is repeated, though not quite so 
 simply, as each cusp is fluted in a definite manner. 
 
 In Rhinoceros the two external cusps are united by a 
 
 (1) Grinding surfaces of upper molar series of a Rhinoceros, a. Posterior 
 sinus, which at a' has become an island, c. Posterior ridge, d. Anterior 
 ridse. 
 
318 A MANUAL OF DENTAL ANATOMY. 
 
 longitudinal ridge, possibly the ciugulum, and the transverse 
 ridges become oblique ; consequently the valley between 
 the ridges c and d is also oblique in direction, and a 
 second valley "a" behind the posterior ridge is introduced 
 (Fig. 134). 
 
 The simplicity of the pattern is also departed from by 
 the margins of the ridges, and therefore the boundaries of 
 the depressions, being waved and irregular. 
 
 The lower molars of the Rhinoceros are made up of two 
 crescentic ridges, one in front of the other, with the hollows 
 turned inwards. It is less obvious how this pattern is 
 derived from that of the Tapir, but it may be that the trans- 
 verse ridges of the Tapir type of tooth may have become 
 ciirved and crescentic, so that the original outer edge of the 
 posterior ridge abuts against the exterior of the ridge in 
 front of it. The valleys between the processes of enamel 
 and dentine of the tooth of the Rhinoceros, termed " sinuses," 
 are not filled up solidly with cementum. The more complex 
 pattern which characterises the molar of the Horse may be 
 derived from a further modification of the Rhinoceros molar. 
 
 To use the words of Professor Huxley : " Deepen the 
 valley, increase the curvature of the (outer) wall and laminse 
 (transverse ridges), give the latter a more directly backward 
 slope ; cause them to develop accessory ridges and pillars ; 
 and the upper molar of the Tapir will pass thi'ough the 
 structure of that of the Rhinoceros to that of the Horse." 
 
 By a further increase in the obliquity of the ridges and 
 in their curvature (c and d), they become parallel to the 
 external or antero-posterior ridge (wall), and bend round 
 until they again touch it, thus arching round and completely 
 encircling the sinuses {a and the space between c and d) in 
 the Rhinoceros tooth. In this way the unsymmetrical pattern 
 of the Rhinoceros tooth may be supposed to become trans- 
 foi-med into the comparatively symmetrical one of the Horse 
 or of the ruminant. 
 
THE TEETH OF PERISSODACTYLE VXQULATA. 319 
 
 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 
 middle, and they ciu've backwards as they go to such an 
 extent as to include crescentic spaces (between themselves 
 
 Fig. 135 ('). 
 
 and the outer wall). To this we must add a vertical pillar, 
 which is slightly connected with the posterior end of each 
 crescentic edge (this pillar is in Hippariou quite detached). 
 
 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 sxirface. The 
 interspaces of the ridges and pillar are in the horse solidly 
 filled in with cementum. The extinct ancestors of the horse 
 have the molar pattern considerably simphfied, but yet 
 recognisable as being built up on the same model. 
 
 But in an elementary handbook such as this it will only 
 serve to perplex the reader to enter into a discussion of 
 the relative probabilities of the various and incompatible 
 explanations given of the homologies of the parts of the 
 ungulate molar : suffice it that such coiTespondences do exist, 
 and if we had before us perfect chains of molars from eveiT 
 
 (1) Ivlolar tooth of a Horse, stowing tlie pattern of its grinding surface. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 ungulate which ever lived, there would be no doubt as to 
 the relationship of the various patterns : as it is, we are em- 
 barrassed by the lack of the material, which leaves gaps too 
 great to bridge over without some amount of speculation. 
 As it is, Professor Flower divides the principal varieties 
 (Phil. Trans., 1874,) into three :— 
 
 (i.) That in which the outer wall is feebly develoi:)ed, 
 and transverse ridges become the prominent features, as in 
 the tapir. 
 
 (ii.) That in which the outer wall is gi-eatly developed 
 and more or less smooth, the transverse ridges being oblique, 
 as in the rhinoceros. 
 
 (iii.) That in which the outer surface and edge of the 
 outer wall is zigzagged, or bicrescentic, as in the horse and 
 palceotherium. 
 
 Equus. — The horse is furnished with the full mammalian 
 number of teeth, the dental formula being — 
 
 .3 1 4 .3 
 
 The canines, however, are rudimentary in the female, whilst 
 in the male they are well developed (in the gelding they are 
 of the same size as in the entire horse) ; and the first pre- 
 molai', which has no predecessor, is also rudimentary, and is 
 lost early. A considerable interval exists between the 
 incisors and the premolars and molars, which latter are very 
 similar to one another, both in shape, size, and in the 
 pattern of the grinding surface. 
 
 The incisors of the horse are large, strong teeth, set in 
 close contact with one another ; the teeth of the upper and 
 lower jaws meet with an " edge to edge bite," an arrange- 
 ment which, while it is eminently adapted for grazing, leads 
 to great wearing down of the crowns. An incisor of a horse 
 or other animal of the genus may be at once recognized by 
 that peculiarity which is known as the ''mark. " 
 
THE TEETH OF PERISSODACTYLE UXGULATA. 321 
 
 From the grinding surface of the crown there dips in a 
 fold of enamel, forming a cid cle sac. As this pit does 
 not extend the whole depth of the crown, and the in- 
 cisors of a horse are submitted to severe wear, the fold 
 eventually gets worn away entirely, and the worn surface 
 of the dentine presents no great peculiarity. But as this 
 wearing down of the crown takes places at something like a 
 regular rate, horse dealers are enabled to judge of a horse's 
 age by the appearance of the mark upon the diffei-ent incisors. 
 The " mark " exists in Hippariou, but not in the earlier 
 progenitors of the horse. 
 
 FiQ. 136 (1). 
 
 FiG.137('). 
 
 A horse attains to its adult dentition very slowly ; the 
 first permanent incisors appear about the end of the third 
 year, and the other two pairs follow at intervals of about 
 
 (1) Incisors of the Horse, showing the marks at various stages of wear. 
 
322 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 six mouths. As the rate of wear is equal, the mark gets 
 worn out soonest upon the central incisors (about the sixth 
 year) ; in the middle incisors next (about the seventh), while 
 it has totally disappeared by about the eighth year. 
 
 After the " mark " is worn away the centre of the tooth 
 is marked by a difference of colour, due to the presence of 
 secondary dentine, into which the remains of the pulp has 
 been converted. 
 
 Fig. 138 ('). 
 
 The molars of the horse are remarkable for their great 
 length ; they do not grow from persistent pulps, but never- 
 theless they do go on growing until a great length of crown 
 of uniform diameter is made, subsequently to which the 
 short and irregular roots are formed. As the upper working 
 surface of the crown becomes worn, the tooth rises bodily in 
 its socket, and when by an accident its antagonist has been 
 lost, it rises far above the level of its neighbours. This 
 elevation of the tooth takes place quite independently of 
 
 (1) Side view of the dentition of a Stallion. At a short interval behind 
 the incisors are seen the canines ; then, after a considerable interval, the 
 premolar and molar series. 
 
THE TEETH OF PERISSODACTYLE VNGVLATA. 323 
 
 growth from a persistent pulp, and, in fact, happens after 
 the formation of its roots. 
 
 The pattern of the horse's molar has been already de- 
 scribed ; it should be added that the last molar differs from 
 the rest in its posterior moiety being less developed than in 
 the other teeth. 
 
 As each ridge and each pillar of the tooth consists of den- 
 tine bordered by enamel, and the arrangement of the ridges 
 and pillars is complex ; as, moreover, cementum fills up the 
 interspaces, it will be obvious that an efficient rough grinding 
 surface will be preserved by the unequal wear of the several 
 tissues. 
 
 When a bit is put into a horse's mouth it rests in the 
 intei-val, or diastema, which exist between the incisors and 
 the commencement of the molar series, and the great con- 
 venience of the existence of such a space has led many 
 authors to assume that the horse was moulded in accordance 
 with man's special requirements, so that it might be suited 
 for its subserviency to his wants. 
 
 But the wide diastema appeared in the remote ancestors 
 of the horse long ages before man's appearance on the earth, 
 and the advocates of this theory of design would, as Professor 
 Huxley suggests, have to tell us what manner of animal rode 
 the Hipparion. 
 
 The milk teeth of all the Ungulata are very complete, 
 and are retained late ; they resemble the permanent teeth 
 in general character, but the canines of the horse, as might 
 have been expected, their greater development in the male 
 being a sexual character, are rudimentary in the milk 
 dentition. 
 
 To the Perissodactyle Ungulates which are specially inte- 
 resting on account of their dentition, must be added Homa- 
 lodontotherium, a tertiary mammal, the remains of which were 
 described by Professor Flower (Phil. Trans., 1874). 
 
 It had highly generalised characters ; its teeth were 
 
 Y 2 
 
324 A MANUAL OF DENTAL ANATOMY. 
 
 arranged without any diastema, and the transition in form 
 from the front to the back of the mouth was exceedingly 
 gradual, so that no tooth diifered much ft-om those on either 
 side of it. Taking the pattern of its molar teeth alone into 
 account, it would have been without hesitation declared to 
 be very nearly allied to rhinoceros, on which type they 
 are formed, but the resemblance fails in the canine and 
 incisor region, and it must be considered to be one of those 
 generalised ij\)Q'& related to rhinoceros, to Hyracodon and 
 perhaps connecting them with such aberrant forms as 
 Toxodon, 
 
 The largest of Perissodactyles equalled the elephant in 
 size, and have been named by Prof. Marsh Brontotheridce. 
 The dental formula was 
 
 .21 43 
 
 1 ^ c - pm - m - . 
 
 The incisors were small and sometimes deciduous, and the 
 canines short and stout, the lower being the more conspicuous 
 owing to its being separated by a slight diastema from the 
 premolars, which is not the case in the upper jaw. 
 
 The premolars in both jaws increase in size from before 
 backwards, and do not differ from the molars next them. In 
 the lower jaw the premolars and molars all consist of two 
 crescents, save the last, which have three crescentic cusps. 
 The molar teeth stand apart from those of any recent peris- 
 sodactyles in their huge size, the squarish last upper molar, 
 for example, measuring four inches antero-posteriorly and 
 more than three transversely (Prof. Marsh, American Journal 
 of Science and Arts, 1876). 
 
THE TEETH OF ARTIODACTYLE UNGULATA. 325 
 
 THE TEETH OF ARTIODACTYLE UNGULATA. 
 
 Artiodactyle, or even-toed Ungiilata, comprise pigs, hippo- 
 potami, camels, sheep, oxen, &c., amongst living animals. 
 
 They are divided into Ruminant and Non-ruminating animals : 
 the latter gxoup, equivalent to the "Suina" of the table on 
 page 265, includes the Pigs (Siud(i'), Ilipixijwtamkhc, and Anu- 
 j)lot?itrld(P. 
 
 The Ruminants are divided into three groups : (i.) The Tvurfu- 
 lidte (small deer of Southern Asia), which foiTu a connecting link 
 between the Anoplotherium (itself a link between the Pigs and the 
 true Ruminants) and the Pccura ; (ii.) Pccora (sheep, oxen, &c.) ; 
 (iii.) Cumdida. 
 
 In Artiodactyle Ungulata the premolars differ markedly 
 both in size and pattern fi-om 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 i_c— p — m— . 
 3 1^3 3 
 
 The position of the upper incisors is peculiar, the two 
 central 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 
 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 ; the 
 peculiarities as to size and direction which characterise the 
 
326 A MANUAL OF DENTAL ANATOMY. 
 
 tusks of the adult animal are not represented in the canines 
 
 of the milk dentition, al)out which there is not much that 
 
 4 
 is noteworthy, save that the yoimg pig has dec. m — , of 
 
 which the first remains in place till the permanent dentition 
 is nearly complete, and then falls out without having any 
 successor ; or it may perhaps be regarded as a permanent 
 tooth which has had no predecessor. 
 
 The form and direction of the canines are alike peculiar ; 
 
 Fig. 139 ('). 
 
 ^NM. Size 
 
 the upper canine, which in its curvature describes more 
 than a semicircle, leaves its socket in a nearly horizontal 
 direction, with an incHnation 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 
 
 (') Upper and lower teeth of Wikl Boar (Sus scrofa). In tliis specimen 
 the tusks are not so largely developed as they sometimes may be seen to be. 
 
THE TEETH OF ARTIODACTYLE UXGULATA. 327 
 
 directed forwards, and its sides being nearly flat. Enamel 
 is confined to the internal and external anterior snrfaces ; 
 the posterior surface, which plays against the upper canine, 
 is devoid of enamel ; 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 w^art-hog (Phaco- 
 choerus), which attain to an immense size. 
 
 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. Mr. Darwin 
 suggests that 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 backw^ards : thus the first pre- 
 molar or milk molar has a simple wedge-shaped crown, and 
 two roots ; the second and third by ti'ansitional characters 
 lead to the fourth premolar, which has a broad crown with 
 two principal cusps, and has foiu- roots. 
 
 The first true molar has four cusps divided from one 
 another by a crucial depression ; and the cingulum in front, 
 and yet more markedly at the back, is elevated into a 
 posterior transverse ridge. In the second molar the trans- 
 verse ridge is more strongly developed, and the four cusps 
 are themselves somewhat divided up into smaller accessory 
 tubercles. 
 
 The last molar measures, from front to l)ack, nearly twice 
 as much as the second ; and this great increase in size is 
 referable to a gi-eat development of the part corresponding 
 to the posterior ridge or cingulum of the second molar, 
 
328 
 
 A JfANUAL OF DENTAL ANATOMY. 
 
 which has become transformed into a great many subsidiary 
 tubercles. 
 
 That such is a correct interpretation of its nature is indi- 
 cated by our being able to trace the four principal cusps, 
 though modified and not divided off, in the front part of the 
 tooth, of which, however, they do not constitute more than 
 a small part. Those Ungulates in which the surfaces of the 
 molar teeth are covered by rounded or conical cusps, are 
 termed " bunodonts," in contradistinction to those which 
 present crescentic ridges on the masticating surface of their 
 molars, and which go by the name of " selenodonts." 
 
 In the Wart-hog (Phacochoerus), the genus with very large 
 
 Fig. 140 C). 
 
 ^cO 
 
 canines, the disproportion between the last true molar and 
 the other teeth is yet more striking. 
 
 (^) Upper and lower teeth of Phacochoerus. In the upper jaw, the last 
 two premolars, and the much- worn fii-st true molar remain. In the lower 
 all have been shed off, save the last two true molars. From a specimen 
 in the Museum of the Eoyal College of Surgeons. 
 
TRE TEETH OF ARTIODJGTYLE VNQVLATA. 329 
 
 In antero-posterior extent the third molar equals the first 
 and second true molars and the third and fourth premolars 
 (the -whole number of teeth of the molar series possessed by 
 the animal) together. 
 
 "Wlien a little worn its surface presents about thirty islands 
 of dentine, surrounded by rings of enamel, the interspaces 
 and the exterior of the whole being occupied by cementum. 
 Of course, prior to the commencement of wear, each of these 
 islands was an enamel-coated cusp. 
 
 The Wart-hog's dentition has, however, another instructive 
 peculiarity ; the first true molar is in place early, and be- 
 comes much worn down (this is true, in a less degree, of the 
 common pig, and indeed^of most Ungulata). Eventually it 
 is actually shed ; the same fate later befalls the third pre- 
 molar and second true molar, so that the dentition in an 
 aged specimen is reduced to the fourth premolar and the 
 third true molar alone, and eventually to the last true 
 molai's alone. Thus, both in the great complexity of the 
 back molars and the fact that the anterior teeth are worn 
 out and then discarded, the Wart-hog affords a parallel to 
 the anomalous dentition of the elephant. 
 
 As has already been noticed, the upper canines in the 
 boar turn outwards and finally iipwards, so as to pass out- 
 side the upper lip ; this peculiarity in direction, yet more 
 marked in Phacochoerus, attains its maximum in the Sus 
 babirussa. 
 
 This creature, strictly confined to the Malay Archipelago, 
 where it frequents woody places, has (in the male) the upper 
 and lower canines developed to an enormous extent. The 
 upper canines are turned upwards so abiaiptly that they 
 pierce the upper lip, instead of passing outside it as in 
 other Suidce, preserve a nearly upright direction for some 
 little distance, and then curve backwards, so that their 
 points are directed almost towards the eyes. 
 
 The lower canines are less aberrant in direction and in 
 
330 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 shape, being somewhat triangular in section, but they also 
 are of very great length, and pass upwards, far above the 
 level of the snout ; their points are also directed backwards, 
 
 FtG. 141 {'). 
 
 iNat.Siz, 
 
 but have in addition an outward inclination. The canines 
 are devoid of enamel, and grow from persistent pulps, a 
 fact which sometimes has a disastrous result, for the tip 
 of the tooth, occasionally taking a wrong direction, re-enters 
 the head or the jaws of the animal. 
 
 Their length is very great ; the animal is smaller than 
 the domesticated pig, but its canines attain a length of 
 eight or ten inches. Their use is a matter of conjecture; 
 the position of the upper tusks has suggested the idea that 
 they may serve as a protection to the creature's eyes, as it 
 seeks its food, consisting of fallen fruits, amongst the 
 brushwood. But were that the case the female also would 
 probably have them, which is not the case ; and although 
 
 {}) Skull of Sus babirussa (male). The upper incisor.s have been lost 
 from the specimen figured : they are much like those of a pig. 
 
THE TEETH OF AETIODACTYLE UXGULATA. 331 
 
 in old animals they are often broken ott", it is not certain 
 that they are mucli employed in fighting. Its other teeth 
 are in no resj^ects remarkable. 
 
 Hippopotamus. — The dental characters, as well as others, 
 indicate the athnity of the Hippopotamus to the SuidcB. 
 
 .2 1 4 3 
 1 e — p m — . 
 ■1 1^4 3 
 
 The incisors are tusk-like, and bear but little resemblance to 
 those of most other mamuialia ; they are nearly cylindrical, 
 blimtly pointed at their apices by the direction of wear, 
 which is in some measm-e determined by the partial distri- 
 bution of the enamel, which is laid on in longitudinal 
 bands in the upper teeth, but merely forms a terminal 
 cap on the lower incisors. 
 
 The upper, standing widely apart, are implanted nearly 
 vertically : the lower incisors, of which the median pair are 
 exceedingly lai'ge, are implanted horizontally. 
 
 The canines are enormous teeth ; the lower, as in the 
 Hog, is trihedral, and is kept pointed in the same manner ; 
 the upper canines are not so long, and the portion exposed 
 above the gum is but short. 
 
 The incisors and canines are all alike teeth of persistent 
 growth. 
 
 The premolars, of which the first is lost early (being 
 perhaps a milk molar like the similar tooth in the pig) 
 are smaller and simpler teeth built up on the same type 
 as the ti'ue molars. 
 
 These latter, especially when worn, have a very charac- 
 teristic double trefoil pattern ; the four cusps, in the first 
 instance, were separated by a deep longitudinal and a still 
 deeper transverse groove ; each cusp was, moreover, tri- 
 lobed; the first resiilt of wear is to bring out the appear- 
 ance of four trefoils ; next, when the longitudinal furrow is 
 worn away, two four-lobed figures result ; and finally all 
 
A MANUAL OF DENTAL ANATOMY. 
 
 pattern becomes obliterated, and a plain field of dentine 
 snrrounded by enamel alone remains. 
 
 The teeth of the Hippopotamus are subject to a great 
 amount of attrition, as is well shown by a specimen pre- 
 sented to the museum of the Odontological Society, in which 
 the molar teeth are all excessively worn. The Hippopotami 
 use their incisors and canine tusks for the purpose of up- 
 rooting aquatic plants, of which their food mainly consists : 
 the roots of these are of course mixed up with much sand, 
 which wears down the teeth with great rapidity. The larger 
 incisors and the canines are, and for centuries have been, 
 articles of commerce, the ivory being of very dense substance 
 and useful for the manufacture of small objects. 
 
 Anoplotheridse are an extinct (Eocene and Miocene) 
 family, linking together the Pigs and the Pecora. 
 
 Fig. 142 {'). 
 
 Anoplotherium is a genus of interest to the odontologist 
 because jt possessed the full typical mammalian dentition, 
 as far as the number of the teeth went ; the teeth were 
 of nearly uniform height, none strongly differentiated from 
 those nearest to them ; and they were set in close contiguity 
 with one another, so that there was no "diastema." 
 
 The lower molar teeth of the anoplotherium are built up 
 
 ^) Side view of the dentition of Anoplotherium (after Owen). 
 
THE TEETH OF ARTIODACTYLE UNGULATA. 
 
 333 
 
 on the same type as those of the rhinoceros (page 317), 
 and present the double crescent ; the upper molars are also 
 referable to the same fundamental forms, though the dif- 
 ference is greater. The lamiute (transverse ridges) oblique 
 in the rhinoceros, are in anoplotherium still more oblique, so 
 that they become more nearly parallel with the outer wall, 
 and an accessory pillar is developed at the inside of the 
 anterior laminte. 
 
 Not very widely removed from the anoplotherium is the 
 Oreodon, an Ung-ulate of Eocene age. 
 
 Fig. 145 (-). 
 
 Like a good muny tertiary Ungulates (both artiodactyle 
 and perissodactyle) it had the full typical number of teeth, 
 forty -four ; but its interest to the odontologist is enhanced 
 by the co- existence of strongly marked canines with molars 
 very much like those of ruminants, a gToup almost always 
 devoid of canines. 
 
 In the upper jaw oieodon had 
 
 .31 4 3 
 
 1 — c pm — m — 
 
 i.e. the typical numljer of each kind of teeth. But in the 
 lower jaw the fii"st four teeth are like incisors, and the 
 tooth which is like a canine is not the tooth cori'esponding 
 to the upper canine, but to the small upper fii'st premolar. 
 
 C) Upper and lower teeth of Oreodou Culbertsonii after Leidy (Smith- 
 oniau Coutributions, 1852). 
 
334 A MANUAL OF DENTAL ANATOMY. 
 
 This is a fair illustration of the fact that although in 
 nature it is generally the same tooth which is modified to 
 perform the function of a canine, it is not invariably the 
 same ; for here in the same animal are two different teeth 
 in the upper and lower jaw thus respectively modified. 
 
 And as they are diff'erent teeth, it happens that the upper 
 canine closes in front of the lower. 
 
 There is reason to believe that there was some difierence 
 in the size of canines between the male and female oreodon. 
 
 The hollow-horned ruminants (sheep and oxen and ante- 
 lopes), and likewise almost all the solid homed ruminant 
 (deer) have the following dental formula : — 
 
 .00 3 .3 
 
 '■6' 1?P 3-^3' 
 
 The lower incisors are antagonised not by teeth, but by 
 a dense gum w^hich clothes the fore part of the upper jaw ; 
 if a sheep is watched as it feeds, it will be seen to grasp the 
 blades of grass between the lower teeth and the gum, and 
 then to tear them off" by an abrupt movement of the head, 
 as it would be impossible for it to, strictly speaking, bite 
 it ofl:: 
 
 The anomaly of the entire absence of upper incisors w^as 
 held to have been diminished by the statement of Goodsir, 
 who believed that uncalcified tooth germs were to be found 
 in the fojtuses of many species. As this was precisely what 
 might have been expected, it has since that time passed 
 current as an established fact ; but recently M. Pietkewickz, 
 working in the lalioratory of M. Ch. Robin, has absolutely 
 denied the occurrence of even the earliest rudiments of 
 tooth germs in this situation, after an examination of a 
 series of foetuses of the sheep and cow, ranging even from 
 the earliest periods. (Journal d' Anatomic, par C. H. Robin, 
 1873, p. 452.) Since meeting with this statement I have 
 had no opportunity of vei'ifying this matter myself. 
 
THE TEETH OF ARTIODACTYLE UNGULATA. 335 
 
 Grouped with the six incisors of the lower jaw, and in no 
 respect differing from them, rise the pair of teeth which are 
 very arbitrarily termed " canines." As I cannot attempt to 
 do more in these pages than give the most bare outline of 
 generally well-known facts, I have retained the usual dental 
 
 Q 
 formula, i -^ ^ ~Y ' ^^^^^^ under protest, as I do not con- 
 sider the " canine " to have any such distinct existence as 
 would justify our calling a tooth which is so obviously refer- 
 able to the incisors by any distinctive name. 
 
 Although the absence of upper canine teeth is a very 
 general characteristic of ruminants, rudimentary canines 
 exist in some deer, and I am indebted to the kindness of 
 Sir Victor Brooke, a high authority iipon the Cerriche, for 
 the following : — 
 
 " The upper canines are present in both sexes in all the 
 species of cervidce, with the exception of Alces, Rangifer, 
 Dama, some smaller species of Rusa, Axis, Capreolus, Caria- 
 cus, Blastocerus, Coassus, and Pudu. The upper canines, 
 when present, are with the notable exception of Moschus, 
 Elaphodus, Cervulus, and Hydropotes, small laterally com- 
 pressed rudimentary teeth. Their crowns are in about the 
 same stage of reduction as the crowns of horses' canines, 
 but their roots are relatively much more reduced." Hence 
 they are often lost in dried skidls, and it has generally been 
 supposed that but few deer possessed canines at all. 
 
 The hornless musk deer possesses iipper canines of most 
 formidable dimensions, while the female has very small 
 subcylindrical canines. 
 
 The male pigmy musk deer (Tragulus) has large canines 
 of persistent growth, the female small canines with closed 
 roots. 
 
 The Indian Muntjac deer (Cervulus) has somewhat small 
 horns, which are perched upon persistent bony pedicles, 
 and it has upper canines which are curved outwards from 
 
336 A MANUAL OF DENTAL ANATOMY. 
 
 beneath the upper lip, much as are the tusks of a boar ; 
 they do not, however, grow from persistent pulps, and are 
 absent in the female. 
 
 Cuvier first pointed out that there was a relation between 
 the presence of horns and the absence of canine teeth ; the 
 latter, serving as weapons for sexual combat solely, and 
 
 Fig. 144 (i). 
 
 being, probably, in no other way of service to the animal, 
 are not required by an animal provided with powerful antlers 
 or horns, whereas the absolutely hornless musk deer would 
 be totally unprovided with weapons of offence w^ere it not for 
 his canines. To the musk deer and the muntjac must be 
 added Swinhoe's water deer, Hydropotes inermis, and Michie's 
 deer, Elaphodus cephalopus, another small hornless species, 
 of which the males are furnished with formidable canine 
 teeth. 
 
 Although, with the foregoing exceptions, all the deer, 
 oxen, sheep, antelopes, and the giraffe, animals constituting 
 the greater number of the " Ruminautia," are without canine 
 teeth, yet in the remaining family, the Camelidce, tusk-like 
 canines are met with. 
 
 It is a character of the Artiodactyle Uugulata that the 
 
 (') Cranium of male Musk Deer (Mosclius moschiferus). 
 
THE TEETH OF ART WD ACT YLE UNGULATA. 337 
 
 premolar teeth are of decidedly simpler form than the 
 molars ; indeed in the ruminants the premolars may be 
 said each to correspond to one half of a true molar. 
 
 The dentition of the ordinary ruminant having been 
 more or less illustrated by the example of the musk deer 
 (minus its great canines), the Camel may be selected as 
 illustrating the peculiarities of the molar series. 
 
 The Camel is possessed of an upper incisor, and, as has 
 already been noticed, of canines. 
 
 113 3 
 
 c n — m — . 
 3 1^2 3 
 
 The first two pairs of upper incisors are absent, but the 
 
 Fig. 145 (i). 
 
 imt.si=, 
 
 third or outermost pair ai'e preseiit, and are rather caniniform 
 in shape. In quite young skulls six upper incisors are present, 
 but the two inner pairs are lost very early. The canines are 
 strong pointed teeth, and the lower canine stands well apart 
 from the three incisors of the lower jaw, unlike the fourth 
 tooth in front of the mandible of typical pecora (see Fig. 
 115). 
 
 (') Upper and lower teeth of a Camel, 
 
338 A MANUAL OF DENTAL ANATOMY. 
 
 The first premolars are absent altogether ; the second 
 premolars, following the canines after an interval, ax*e 
 pointed caniniform teeth. The third premolar is some- 
 times lost early, but the fourth persists. 
 
 The molars of the Camel are of the " Selenodont " type ; 
 their derivation from forms already alluded to will be 
 sufficiently obvious to the reader who has mastered the 
 descriptions, and their double crescentic crowns, may 
 be taken as fair examples of simple ruminant patterns, 
 accessory pillars, etc., being added in some of the other 
 groups. 
 
 In all true Ruminants the last true molar of the lower 
 jaw has a third lobe ('), and the line of the outer surface of 
 the row of teeth is rendered irregular by the anterior edge 
 of each tooth projecting outwards slightly more than the 
 posterior border of the one in front of it. And the devia- 
 tions in the patterns of the surfaces of the molar teeth are 
 so constant and so characteristic that, although the common 
 ruminant pattern is preserved in all, it is often possible to 
 refer an individual tooth to its right genus. 
 
 The Paiminants all have a well-developed milk dentition, 
 which serves the animal for a long time, indeed iintil after 
 it has attained to its adult dimensions ; thus a sheep has not 
 completed the changing of its teeth till the fifth year, and 
 a calf till the fourth year. But the first permanent molar 
 is in them, as in so many other animals, the first of the 
 permanent set to be cut, and comes up in its place at the 
 sixth month (in the lamb), and hence has a long period of 
 wear before any of the other second teeth are cut. Conse- 
 quently the first permanent molar is, as is seen in Fig. 145, 
 invariably worn down to a much greater extent than the 
 other permanent teeth ; in the specimen figured it has been 
 
 (') Sir Victor Brooke informs nie that Neotragus liemprichii, a small 
 Abyssinian antelope, has only two lobes to the third lower molar. 
 
TOXODONTIA. 339 
 
 worn down below the inflections of enamel, so that it has 
 lost its roughened grinding sm-face, and is reduced to a 
 smooth area of dentine. 
 
 Not much is known of the structure of the dentul tissues of 
 the Ungulata which calls for mention in an elementary work. 
 The thick cement of the crown of the teeth of the Horse, 
 and indeed of most of the group which possess thick cement, 
 contains many " encapsuled lacunee," and is developed from 
 a distinct cement organ of cartilaginous consistence (see 
 page 144). 
 
 TOXODOXTIA. 
 
 The existing ungulate animals form only a small propor- 
 tion of those once peopling the earth, and many extinct forms 
 have been discovered, which while having afl&nities with the 
 Ungulata, can yet hardly be classified under any existing 
 order. For example, Toxodon, a creature equalling the 
 Hippopotamus in size, which was discovered by !Mr. Darwin 
 in late tertiaiy deposits of Soiith America, has a dentition 
 recalling in some respects the Bruta, in others the Rodents. 
 
 It possessed in the upper jaw two pairs of incisors, the 
 median pair small, the outer exceedingly large, with per- 
 sistent pulps, and long curved sockets extending back to the 
 region of the molars, just as in existing Rodents. 
 
 In the lower jaw there were three pairs of incisors, sub- 
 equal in size, and gi'owing from persistent pulps ; they 
 resemble the incisors of Rodents in haviug a partial invest- 
 ment with enamel, but differ from them in being prismatic 
 in section, and in having the enamel disposed on two sides 
 of the prism. 
 
 The molai's were also very remarkable ; they grew from 
 persistent pulps, and had curved sockets, but the cmwature 
 of these was in the reverse dnection to that which obtains 
 
 z 2 
 
340 A MANUAL 01' DENTAL ANATOMY. 
 
 in Rodents, i.e., the convexity was outwards, and the apices 
 of their roots almost met in the middle line of the palate ; 
 it was this peculiarity that suggested the name. 
 
 Another peculiarity in the molar teeth, in which they 
 stand quite alone, is that, like incisors, they have a partial 
 investment with enamel ; those refeiTed to the premolar 
 series having it confined to their outer surfaces, while the 
 three back teeth of the molar series had a plate also laid on 
 to their inner surfaces ; there were seven molar teeth above, 
 and six below. 
 
 In the interval between the incisor and molar series 
 canines have been found in the lower jaw ; they were sharp 
 edged, and had a pai'tial distribution of enamel over their 
 surface. In an upper jaw alveoli for canines were found, but 
 the teeth themselves are not known. 
 
 DINOCERATA. 
 
 In the same region which yielded the toothed birds 
 (Eocene formations of Wyoming), the remains of many huge 
 animals have been discovered, for which new orders have been 
 proposed by Prof. Marsh (" American Journal of Science and 
 Art," 1876), it being impossible to classify them under any 
 existing order. The Dinocerata were creatures nearly as 
 large as Elephants, and presenting some general resemblance 
 to them in general form ; they were remarkable for the 
 relative smallness of their brains, which coidd aj^pai-ently 
 have been drawn through the canal of the vertebral column. 
 They present points of resemblance to the Perissodactyle 
 Ungulata, and also to the Proboscidea, to which they were 
 at first referred, though their affinities are rather with the 
 former. 
 
 The dental formula was 
 
 .01 33 
 
 x_c-prm-m-^. 
 
DINOCERATA. 
 
 341 
 
 In Prof. Marsh's words " The superior caniues are loug, 
 decurved, trenchant tusks. They are covered with enamel, 
 and their fangs extend upwards into the base of the maxil- 
 lary horn-core. There is some evidence that these tusks 
 were smaller in the females. Behind the canines there is a 
 moderate diastema. The molar teeth are very small. The 
 crowns of the superior molars are formed of two transverse 
 crests, separated externally, and meeting at their inner 
 extremity. . The first true molar is smaller in this specimen 
 than the two preceding premolars. The last upper molar is 
 mvich the largest of the series. 
 
 " The lower jaw in Dinoceras is as remarkable as the skull. 
 Fig. 116 ('). 
 
 Its most peculiar features are the posterior direction of the 
 Q) Upper and lower jaws of Dinoceras (Marsh). 
 
A MANUAL OF DENTAL ANATOMY. 
 
 condyles, hitherto unknown in Ungulata, and a massive 
 decxirved process on each ramus extending downward and 
 outward below the diastema. 
 
 "The position of the condyles was necessitated by the long 
 upper tusks, as, with the ordinary ungulate articidation, the 
 mouth could not have been fully opened. The low position 
 of the condyle, but little above the line of the teeth, is also 
 a noteworth}^ character. The long pendant processes were 
 apparently to protect the tusks, which otherwise would be 
 very liable to be bix»ken. Indications of similar processes 
 are seen in Smilodon and other Carnivores Avith long upper 
 canines. With the exception of these processes the lower 
 jaw of Dinoceras is small and slender. The symphysis i& 
 completely ossified. The six incisors were contiguous, and 
 all directed well forward. Just behind these, and not 
 separated from them, were the small canines, which had a 
 similar direction. The crowns of the large molars have 
 transverse crests, and the last of the series is the largest." 
 
 It would appear possible that the eminences shown in the 
 figure, and spoken of as " maxillary horn-cores," may be 
 merely the extended sockets of the teeth, which w^ould other- 
 wise have had an implantation inadequate to their length ■. 
 they are, however, described as solid, except at their bases, 
 where they are perforated for the fang of the canine tusk, 
 which wotdd look as though they were truly horn-cores ; 
 moreover the Brontotheridse had horn-cores equally peculiar 
 in position {i.e., on the maxillary bones). 
 
 Yet another new oi'der, Tillodoxtia, comprising several 
 genera, has been proposed by Prof. Marsh for the Wyoming 
 fossil remains, to receive forms which, though not amongst the 
 biggest, are " amongst the most remai'kable yet discovered in 
 American strata, and seem to combine characters of several 
 distinct groups; viz., Carnivora, Ungulata, and Rodentia, 
 In Tillotherium, Marsh, the type of the order, the skull has 
 the same general form as in the Bear, but in its structure 
 
DINOCERATA. 
 
 343 
 
 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 
 gi-owing from persistent pulps, as in the Rodents. The 
 second pair of incisors are small, and have not persistent 
 pulps. The adult dentition is as follows — 
 
 1 3 3 
 
 -pmi— m 
 
 " There are two distinct families, Tillotheridce (perhaps 
 identical with Ancldppodontidce), in which the large incisors 
 
 Fig. 147 (')• 
 
 grew from persistent pulps, while the molars had roots ; and 
 the Styli7iodontid(e, in which all the teeth have persistent 
 pulps. 
 
 One genus (Dryptodon), known only by the lower jaw, had 
 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. 
 
 (') Upper and lower jaws of Tillotherium (Marsli). 
 
CHAPTER XI. 
 
 THE TEETH OF SIRENIA, HYRACOIDEA, PROBOSCIDEA, AND 
 RODENTIA. 
 
 THE TEETH OF SIRENIA. 
 
 More nearly connected with the Unguhita than with any 
 other order, but still rather widely removed from them, 
 stands the limited order of Sirenia, aquatic mammals 
 formerly termed Herbivorous Cetacea, a term rather ob- 
 jectionable, as they are not very nearly allied to the true 
 Cetacea. 
 
 The order is now represented by two genera only, the 
 Dugongs (Halicore) and the Manatees (Manatus), but a 
 third gemis (Rhytina) has only become extinct within about 
 a century. Their teeth, and other points in their organiza- 
 tion indicate that they are more nearly allied to the Ungu- 
 lata than to any other group, though their peculiarities are 
 such as to elevate them to the rank of a distinct order. 
 They are of large size, and frequent shallow water, such 
 as the mouths of great rivers, their food consisting of sea- 
 weed and aquatic plants. 
 
 The dentition of the Dugong is in several respects a very 
 interesting one : the front part of the upjjer jaw, consisting 
 in the main of the intermaxillary bones, bends abruptly 
 downwards, forming an angle with the rest of the jaw. 
 This deflected end of the jaw carries two tusks, of each of 
 which the greater part is buried within the alveolus. The 
 tusk has an investment of enamel over its front and sides, 
 but on the posterior surface of cementum only, so that in 
 
THE TEETH OF SIRENIA. 
 
 345 
 
 the disposition of the three structures it recalls the charac- 
 teristics of a Rodent incisor, like which it is worn away 
 obliquely so as to keep a constantly sharp edge, and like 
 which it gi-ows from a persistent pulp. 
 
 In the female, the tusks (incisors) do not project from the 
 
 Fig. 148 ('). 
 
 gum, their pulp cavities are closed, and the investment of 
 enamel is complete over the crown of the tooth. 
 
 The sloping surface of the upper jaw is opposed b}- the 
 region of the symphysis of the lower jaw, which is of un- 
 usual depth. In this deflected part of the lower jaw there 
 are eight, or ten (four or five on each side) shallow and 
 rather irregularly-shaped sockets, in which curved distorted 
 
 (') Side view of cranium and lower jaw of a Dugong (Halicore Indicus). 
 From a specimen in the ^Museum of the Royal College of Surgeons. The 
 surface of the deflected portion of the lower jaw, with its sockets for 
 rudimentary teeth, shown both in front and in profile view, is indicated hy 
 the letter a ; the con-esponding surface of the upper jaw by the letter h. 
 
346 A MANUAL OF DENTAL ANATOMY. 
 
 teeth may be fouud in a fresh specimen, but it must not 
 be from too aged an animal, as they become eventually eaten 
 away by a process of absorption. 
 
 These abortive teeth are excellent examples of rudi- 
 mentary teeth, as not only are they stunted, and even 
 ultimately removed by absorption, but they are actually 
 covered in by a dense horny plate which clothes this part 
 of the jaw, and so are absolutely functionless ('). 
 
 These horny plates, in their structure analogous to whale- 
 bone, are possessed also by the Manatee and Rhytina ; on 
 the free surface they are beset with stiff bristles, and are 
 throughout built up of hair-like bodies welded together by 
 epithelium. 
 
 Behind the region covered in by the horny plates, the 
 Dugong has five molar teeth on each side, of simple form, 
 like those of the Edentata, and consisting of dentine and 
 cementum only. 
 
 By the time the last molar is ready to come into place, 
 the first of the series is being removed by absorption of its 
 root and of its socket. In aged specimens only two molars 
 remain on each side of the jaws. 
 
 The Dugong is also peculiar as having a single deciduous 
 tooth : namely, a predecessor to the incisive tusks ; but it 
 has been doubted whether it be not rather a rudimentary 
 incisor than a milk tooth. 
 
 The molar teeth of the Manatee are much more nume- 
 rous and more complex in form, and they approach to the 
 configuration of the teeth of the Tapir very closely. 
 
 The Manatee has as many as forty-four molars, which 
 are not, however, all in place at one time, the anterior ones 
 being shed l)cfore the posterior are come into place ; no 
 
 (1) Similar rudimentary teeth are found in the corresponding deflected 
 part of the jaw of the young Manatee, to the number of twelve. (Gervais, 
 "Histoire Nat. des Mammiferes," vol. ii., p. 312.) 
 
THE TEETH OF SIEENIA. 
 
 vertical succession is known to occur among them. There 
 are no incisors nor canines, but there are horny plates in 
 the front of the mouth like those of the Dugong. 
 
 The extinct Rhytina, formerly abundant about Behring's 
 Straits, was altogether without teeth. 
 
 It has been mentioned that the teeth of the Manatee are 
 tapiroid in external form ; they also possess peculiarities in 
 minute structure, which are unusual in mammalian teeth, but 
 which are common to them and to the Tapirs. In examining 
 some teeth, I found that the dentine, to all intents and pur- 
 poses, of the hard unvascular variety, was pemieated by a 
 system of larger, or " vascular " canals, which were ar- 
 ranged with miich regularity, and passed out from the pulp 
 cavity to the periphery of the dentine, where they commu- 
 nicated with one another. The dentinal tubes did not 
 radiate from these vascular canals ; they, so to speak, take 
 no notice of them, so that there is an ordinaiy unvascular 
 dentine with a system of capillary- conveying channels as 
 well. It is interesting to find that the primd facie external 
 resemblance of the teeth of the Tapir is fully borne out by 
 minute histological structure, and it certainly suggests that 
 the resemblance is not accidental, but has some deeper 
 significance. 
 
 The enamel of the Manatee is also somewhat remarkable 
 for the absolute straightness of its enamel prisms in many 
 parts of the tooth. 
 
 The molar teeth of the Dugong consist of a central axis 
 of vaso-dentine, a much lai'ger mass of ordinary unvascular 
 dentine, and a thick layer of cementum, but they do not 
 share the peculiarities of the jManatee's tooth. 
 
348 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 THE TEETH OF HYRACOIDEA. 
 
 The Biblical coney (Hyrax), an animal as large as a 
 rabbit, must not be passed over without mention, as its 
 dentition has been indirectly the source of much contro- 
 
 Fid. 149 (1). 
 
 versy. So far as the pattern of its molar teeth goes, it 
 corresponds closely with Rhinoceros, and was hence classed 
 in close proximity to that genus by Cuvier. But a more 
 extended sui'vey of its characters has led to its being placed 
 in a separate order; it is a good example of the danger 
 which attends relying upon any single character, such as 
 the pattern of the teeth, as being alone a sufficient basis for 
 classification. 
 
 Q) Skull of the Hyrax. 
 
THE TEETH OF PROBOSCIDEA. 349 
 
 All observers, however, are not agreed as to its position ; 
 it certainly presents affinities -with Perissodactyla, and also 
 with the Rodents, also, perhaps, with the Insectivora, The 
 
 dental formula is i — c — prm — m — . 
 
 Seen from the side, the dentition bears some resemblance 
 to that of a Rodent, because the large size of its central 
 incisors, which grow fi-om persistent pulps, are chisel-edged, 
 and are fui-nished with a very thick coat of enamel on their 
 anterior faces : the second pair of incisors, which are small, 
 are soon lost. But Hyi-ax has the full typical number, pre- 
 molars and molars, and the patterns of the teeth are closely 
 similar to those of the Rhinoceros. 
 
 In the lower jaw the middle incisors are small, and the 
 outer ones largely developed, and all persist : their crowns 
 are in a manner trilobed, and they pass in ordinary closure 
 of the mouth behind the upper incisors, where they are met 
 by a dense pad of gum. 
 
 THE TEETH OF PROBOSCIDEA. 
 
 At the present day the Elephant stands alone, removed 
 by many striking peculiarities from the Ungulata, to which 
 it is more nearly allied than to other orders ; but in former 
 days the order Proboscidea was represented by a good many 
 genera, was widely distributed over the globe, and tran- 
 sitional forms linking, the elephant with somewhat less 
 aberrant mammalia were not wanting. In this group the 
 incisors grow from persistent pulps, and form conspicuous 
 
 tusks ; the Elephant has i — , the Mastodon has i _, the 
 ^ 1 
 
 Dinotherium i — 
 1 
 
 Two striking features characterise the dentition of the 
 
A MANUAL OF DENTAL ANATOMY. 
 
 Elephant ; the enormous length of the incisor tusks, and 
 the peculiar displacement from behind forwards of the molar 
 teeth, by which it results that not more than one whole 
 molar, or portions of two, are in place at any one time. 
 
 The upper tusks are preceded by small deciduous teeth ; 
 this is well established, though it has been recently denied by 
 Sanderson ("Wild Beasts of India" ). When first cut they 
 are tipped with enamel, but the enamel cap is soon worn off, 
 and the remainder of the tusk consists of that modification 
 of dentine known as " ivory," and of a thin external layer of 
 cement. 
 
 In the Indian elephant the tusks are not so large as in the 
 African species : and the tusks of the female are rery much 
 shorter than those of the male. In the African elephant, no 
 such difference in size has been established ; and amongst In- 
 dian elephants males are sometimes met with which have tusks 
 no larger than the females of corresponding size ; they go 
 by the name of " Mucknas." This peculiarity is not always 
 transmitted, and it is known that in Ceylon tuskless sires 
 sometimes beget " tuskers." Amongst the Ceylon elephants 
 the possession of large tusks by the male is an exceptional 
 thing, Sanderson stating that only one in three hundred has 
 them, while amongst 51 Indian elephants only five were 
 tuskless. The tusks are formidable weapons, and great 
 dread of a " tusker," is shown by elephants less well armed. 
 
 A male makes use of his tusks for all soi-ts of piu-poses ; 
 thus when a tamed one is given a rope to pull, he will, by 
 way of getting a good purchase upon it, pass it over one 
 tusk and grasp it between his molar teeth. 
 
 The largest tusks were possessed by the Mammoth, the 
 remains of which are so abundant in Siberia ; these, which 
 were strongly curved, and fonned a considerable segment 
 of a circle with an outward inclination, so as to well clear the 
 sides of the head, attained the lengih of 13 feet, and a weight 
 of 200 lbs. each. 
 
THE TEETH OF PROBOSCIDEA. 
 
 A pair of African tusks exhibited at the Great Exliibition 
 of 1851 weighed 325 lbs., and measured eight feet sis inches 
 in length, and 22 inches in circumference, but the average 
 tusks imported from Africa do not exceed from 20 lbs. to 
 50 lbs. weight. Indian elephants seldom have tusks attain- 
 ing very large dimensions ; one was, however, shot by Sir 
 Victor Brooke with a tusk 8 feet long, weighing 90 lbs. 
 
 The surfaces of the tusks of the female are often deeply 
 excavated about the level of the edge of the gum, and are 
 sometimes so weakened from this cause that they break off. 
 My friend Mr. Moseley tells me that he was informed by the 
 late Major Rossall, who as a sportsman had great knowledge 
 of Indian elephants, that the tusks of all the females he has 
 ever seen are so affected, and that the larva) or pupse of a 
 dipterous insect are often found bedded in the gum, and 
 attached to the surface of the tusk. There is a specimen of 
 a female elephant's tusk with the pupre attached in the 
 Museum of the Ro3-al College of Surgeons. It would be a 
 matter of interest to ascertain whether the larva really eats 
 away the tusk, or whether the wasting of the tusk be due 
 to absorption set up by the irritated gum. 
 
 The tusks of the elephant are implanted in long and 
 stout sockets, and grow from persistent pulps throughout 
 the lifetime of the animal. 
 
 In the Indian elephant about one half of the length of the 
 tusk is implanted, and in young animals the pulp cavity 
 extends beyond the implanted portion, but in older animals 
 it does not extend nearly so far. A knowledge of its extent 
 is necessary, seeing that the tusks of captive elephants have 
 to be shortened from time to time ; this operation is by 
 some done frequently, by others only at long intervals, such 
 as ten years, in which case a large and valuable segment of 
 ivory is cut off, and the end of the tusk bound with metal 
 to prevent it from splitting. 
 
 Tusks sometimes exemplify on a large scale the results of 
 
J MANUAL OF DENTAL ANA TOM V. 
 
 injury to the growing pulj), as it is of no unfrequent oc- 
 currence that elephants which have been shot at and 
 wounded escape. 
 
 The thin walls of the tusk near to its open end do not 
 offer very much resistance to the entrance of a bullet ; the 
 result of such an injury is not, as might have been expected, 
 the death of the pulp, but in some cases abscess cavities 
 become formed in the neighbourhood of the injury, while 
 in others less disturbance is set up, the bullet becomes 
 enclosed in a thin shell of secondary dentine, or sometimes 
 lies loose in an irregular cavity, and round this the normal 
 *' ivory " is deposited ; upon the outside of the tusk no indi- 
 cation of anything unusual is to be seen, so that the bullets 
 thus enclosed are found by ivory turners only when sawing 
 up the tusk for use. 
 
 As the tusk grows, that which was once in the pulp 
 cavity, and within the alveolus, comes to be at a distance 
 from the head, and in the midst of solid ivory. 
 
 As an example of the extent of injury from which a 
 tooth pulp is capable of recovery, may be cited a specimen 
 now deposited in the museum of the Odontological Society, 
 by Mr. Bennett, to whom I am indebted for permission to 
 figure it. 
 
 It is to be presumed that a trap was sot with a heavily 
 loaded spear, or that it was dropped by a native from a 
 tree, with the intention of its entering the brain of the 
 elephant as it was going to water, both of these methods of 
 killing elephants being practised in Africa. But in this case 
 the spear penetrated the open base of the growing tusk, 
 which looks almost vertically upwards (see fig. 150), and 
 then the iron point appears to have broken off". This did 
 not destroy the pulp, but the tooth continued to grow, and 
 the iron point, measuring no less than 7i by 1-J- inches, 
 became so completely enclosed that there was nothing upon 
 the exterior of the tusk to indicate its presence. 
 
THE TEETH OF PROBOSCIDEA. 
 
 353 
 
 I am told by Mr. Erxleben that he is acquainted with 
 another instance in which a spear head had become com- 
 pletely enveloped in ivory. 
 
 Fig. 150('). 
 
 There is also a specimen of a javelin head solidly im- 
 ,bedded in ivory in the Museum of the Royal College of 
 Surgeons. 
 
 Ivory is one of the most perfectly elastic substances 
 known, and it is on this account that it is used for billiard 
 balls ; it owes its elasticity to the very small size of the 
 dentinal tubes and the frequent bends (secondary curva- 
 tures) which they make ; to the aiTangement of the tubes 
 the peculiar engine-turning pattern of ivory is due. It 
 diflfers from other dentine in its containing from 40 to 43 
 per cent, of organic matter (human dentine contains only 
 about 25), and in the abundant concentric rows of inter- 
 globular spaces. Along these ivory -when it decomposes 
 breaks up, so that a disintegrated segment of a tusk con- 
 sists of detached concentric rings ; in this condition many 
 mammoth teeth are found, although sometimes where they 
 have remained frozen and protected from the air until the 
 time of their discovery they are hardly affected by the lapse 
 
 {}) Iron spear-head, irremovably fixed in the interior of a tusk, believed 
 to be from an African Ele^jhant. From a specimen in the possession of 
 Mr. Bennett. 
 
354 A MANUAL OF DENTAL ANATOMY. 
 
 of the thousands of years which haA'e gone by since their 
 possessors perished. 
 
 The trade in ivory is quite an important one, the Boaixl 
 of Trade returns for 1879, giving 9,414 cwts., of the value of 
 ^406,927, as the quantity brought to this coiuitry. 
 
 The best ivory is that which comes from equatorial 
 Africa ; Indian ivory is not so highly esteemed, and Mam- 
 moth ivory is so uncertain in its degree of preservation 
 that it does not find a ready sale, even though some samples 
 almost attain the quality of recent ivory. 
 
 The last remains of the pulp are converted into dentine 
 in which a few vascular canals persist ; this of ct)urse occu- 
 pies the centre of the task, and is small in amount. 
 
 Six molar teeth are developed on each side of the jaw by 
 the elephant, and, arguing from analogy, they are some- 
 
 3 3 
 
 times classified thus — milk molars - true molars - ; 
 
 3 3 
 
 occasionally a rudimentary tooth in front brings up the 
 number to seven on each side. But the peculiarity of their 
 mode of succession renders such a classification merely 
 arbitraiy, so far as the elephant itself is concerned, and it 
 depends upon analogy with the teeth of the mastodon. 
 Though the elephant has, during the course of its life, 
 twenty-four molars, they are not all in place, nor indeed 
 are they all actually in existence at the same time. Only 
 one whole tooth on each side, or portions of two (when the 
 front one of the two is nearly worn out), are in use at the 
 same time. After a tooth has been in use for some time, 
 and is worn down, a new tooth comes up to take its place 
 from behind it, and absori;)tion in the old tooth being set up, 
 it is shed off, and the new tooth pushes forward into its 
 place (see fig. 151). Each successive tooth is of greater 
 size than its predecessor ; thus in the Indian elephant the 
 first tooth having, on an average, four transverse plates ; 
 the second eight, the third twelve, the fourth twelve, the 
 
THE TEETH OF PROBOSCIDEA. 
 
 355 
 
 fifth sixteen, the sixth from tweuty-four to twenty-seven- 
 In the African elephant, in which the individual plates are 
 much broader, they arc fewer in number (sec page 361). 
 
 Fig. 151 ('). 
 
 10 \at Size 
 
 A reference to the accompanying figure will indicate how 
 the succession takes place. The tooth in reserve occupies a 
 position at an angle to that in use ; as it moves forwards 
 
 (') Side view of skull of young Indian Elephant. The teeth in use are 
 the second and third of the molars whicli displace one another from be- 
 hind forwards ; the anterior of these, corresponding to a milk molar in 
 other animals, is nearly worn out ; the residiial fragment is separately re- 
 presented on the left. The tusk, of which only a short piece can be 
 shown, is indicated within the socket by dotted lines, by which also the 
 form of the pulp cavity is majiped out. 
 
 A A 2 
 
356 A MANUAL OF DENTAL ANATOMY. 
 
 and (in the iipper jaw) downwards its track forms almost 
 the segment of a circle. Thus its anterior corner is the 
 first to come into use, at a time when the position of the 
 "whole tooth is still exceedingly oblique, and the greater part 
 of it is still within the socket. 
 
 The teeth as first formed consist of detached plates of 
 identine coated with enamel, the tops of which are mammil- 
 lated ; these only coalesce after a considerable portion of 
 their depth has been formed, and that portion of the tooth 
 has lieen reached in which there is a common pulp cavity ; 
 here dentine is continuous from end to end of the tooth. 
 
 Just as the cusps of a human molar are separate when 
 first calcified, so these exaggerated cusps or plates of an 
 elephant's tooth are separate from one another till a great 
 part of their length is completed, and they only coalesce 
 when they reach the level of the common pulp chamber ; in 
 point of fact the elephant's tooth is mainly made up of its 
 cusps, the remaining portion being insignificant. 
 
 Several of these detached plates, such as the one here 
 figured, are to be found at the back of the largest teeth 
 even at a time when the front corner has been erupted and 
 has come into wear. 
 
 That the tooth is thus being built up only as it is required 
 is of obvious advantage to the animal in diminishing the 
 weight to be carried, and is also an economy of space. 
 
 The teeth when they begin to be erupted do not at once 
 come into use over their whole surface, but they come for- 
 ward in an oblique position so that the front of the tooth 
 has been in use for some time, and its plates have been con- 
 siderably worn down, before the back of the tooth has 
 become exposed at all. Nay more, in the case of the larger 
 molars the front of the tooth is actually in use at a time 
 when its back is not yet completed. 
 
 In the elephant there is no vertical succession of teeth 
 ■whatever; the manner of succession usual amongst mammals 
 
THE TEETH OF PROBOSCIDEA. 
 
 357 
 
 has in them given place to a succession from behind, the 
 older teeth being pushed out forwards. Had the elephant 
 always been as isolated a form as it now appears to be, it 
 would have been very uncertain how its six molars shoidd 
 
 be classified. But it happens that proboscideans formerly 
 existed in which this pecular succession from behind was to 
 be found, at the same time that the ordinary vertical suc- 
 cession was not quite lost, and amongst these creatures (the 
 mastodons) we are able to say with certainty which of the 
 teeth are milk molars, which are pre-molars, and which are 
 true molars. And as the mastodons pass by insensible gra- 
 dations into the elephants, so that the line of demarcation 
 between the tv/o genera is an arbitrary one, we can tell 
 which of the mastodon's teeth correspond to each one of the 
 six molars of the elephant. 
 
 Mastodon. — In the later tertiary .periods this genus, ap- 
 proximating in its dental and other characters to the true 
 
 (') Isolated plate (= exaggerated cusp) of an Elephant's tooth, prior to 
 its coalescence with neighbouring jjlates ; at the top are seen its terminal 
 mammillated processes, one of which has been ciit ofi' to show the central 
 .irea of dentine, surrounded by enamel ; at the base would be the open 
 I)ulp cavity, not shown in the figure. 
 
358 A MANUAL OF DENTAL ANATOMY. 
 
 elephant, was widely distributed over the world. The dental 
 
 formula is not quite the same for all the genus, for in some 
 
 no premolars existed. 
 
 10 2 3 3 
 
 i. _ c. - prm. - milk molars - m. - 
 
 1 0^ 2 3 3 
 
 The upper incisors formed nearly straight tusks, seven or 
 eight feet in length ; the lower incisors also grew out hori- 
 zontally from the front of the jaw, but in some species the 
 lower tusks are rudimentary, are lost early, or are altogether 
 absent, thus more nearly approaching to the condition met 
 with in the elephant. 
 
 The several molar teeth of the Mastodon increased in 
 size from before backwards. The crowns were built up of 
 deep and strongly pronounced transverse ridges, of which 
 the last molar had the largest number. The apices of the 
 ridges, before being at all worn, were divided up into several 
 blunt nipple-like (mastoid) processes, upon which the 
 enamel was thick and dense, but the cement was thin, so 
 that the interspaces of the processes were not filled up level 
 by the latter tissue, as in the elephant. 
 
 Very definite roots were formed to the molars, the wear- 
 ing down of the teeth being met by the worn teeth being 
 shed oflP altogetlier from the front of the series, whilst new 
 teeth were added to the back. Thus, just as in the elephant, 
 the whole number of teeth were not in place at one time. 
 jSTot more than three were in use at one time, and by the 
 time the last and largest molar was cut, there was but one 
 tooth remaining in front of it, and even this Avas soon lost, 
 the dentition thus being reduced to a single molar on 
 each side. 
 
 As the succession of the molars in the Mastodon affords 
 a clue to the nature of the grinders of the elephant, it is 
 necessary to add a few words about it. Some Mastodons 
 had three milk molars, of which the last two were vertically 
 displaced by premolars, just as in most other mammals, 
 
THE TEETH OF PROBOSCIDEA. 359 
 
 but the first milk molar was not so replaced (Mastodon 
 angustidens). There ajjpear to have been Mastodons in 
 which no vertical succession at all took place, i.e., in which 
 there were no j^remolars, and others in which there was 
 but one. 
 
 No doubt can be entertained as to the homologies of the 
 teeth, even in those Mastodons which are not known to 
 have any vertical succession, because analogy with those 
 other species in which the second and third molai's, counted 
 from the front, were vertically displaced by nearly func- 
 tionless premolars, tells us that the three front molars ai"e 
 milk molars. Now elephants develop six molar teeth on 
 each side ; the elephant is in the same case, quoad its molars, 
 as the Mastodon Ohioticus, which had do vertical succes- 
 sion, so that we thus know the elephant's grinders to be 
 
 , 3 3 
 
 dm. - m. - 
 
 3 3 
 
 Dr, Falconer mentions an elephant from the Sewalik Hills 
 (E. planifrons) in which two rudimentary pre-molars, of no 
 functional importance, actually existed, and so the deter- 
 mination of the elephant's working teeth as 
 
 , 3 3 
 
 dm. _ m. - 
 
 3 3 
 
 rests not only upon analogy, but upon actual observation. 
 
 The Dinotherimu, a large animal, not unlike the Sirenia 
 in the character of its cranium, which was probably of 
 aqiiatic habits, was remarkable for possessing large tusks, by 
 analogy known to be incisors, in its lower jaw, none being 
 present in the upper jaw. The tusks projected do waiwards 
 at right angles with the body of the jaw, and were curved 
 backwards. The portion of jaw about the symphysis was 
 <leflected downwards, so as to aftbrd an adequate implanta.- 
 tion for these anomalous tusks. 
 
360 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 The Dinotherium was as largo as an elephant, and the 
 
 downward pointmg tusks were about 2 feet in length ; as, 
 
 however, tusks of only half this length were found in somo 
 
 jaws of identical dimensions and in other respects similar, 
 
 it is believed that the male Dinotherium had larger tusks 
 
 than the female. The molar teeth, much like those of a 
 
 tapii-, need not detain us. 
 
 .002 
 
 1. - c. -p. - m. - 
 
 The succession was vei'tical, as in other mammals, and it 
 
 had dm. - 
 o 
 
 But the Dinotherium, Mastodon, and Elephant, present 
 us with a very instructive series of modifications in which 
 we see how the excessively complex grinder of the Indian 
 elephant was attained to by degrees. 
 
 The molar of the Dinotherium resembles that of a tapii- 
 somewhat; it has not any very great exaggeration of its 
 cusps, and does not deviate very widely from the form of 
 many other mammalian teeth. 
 
 The tooth of Mastodon has its cusps or ridges more 
 numerous and more pronounced, as is seen in the accom- 
 panying figiu-e. 
 
 Fig. 153 (>). 
 
 h c R c 
 
 Other Mastodons have more numerous ridges upon the 
 teeth, and the African elephant has as many as ten upon 
 
 (') Molar tootli of Mastodon. 
 
THE TEETH OF PROBOSCIDEA. 
 
 S61 
 
 i-\is. 
 
 Fir,. 156 (^'). 
 
 (') Molar of African Elephant. E. Enamel. D. Dentine. C. Cementuni. 
 
 (-) Molar tooth of African Elephant, showing the form of its roots, &c. 
 n. Dentine, c. Cementum. e. Enamel. 
 
 (•*) Molar tooth of an Asiatic Elephant, showing the transverse plates of 
 dentine bordered by enamel. 
 
362 A MANUAL OF DENTAL ANATOMY. 
 
 its last 01- larger molar, although in it the ridges are in- 
 dividually wide and strongly pronounced. 
 
 In the Indian elephant the ridges or plates are still more 
 numerous, the roots very inconspicuous and the whole 
 formed into a solid block by cementura. 
 
 The gradual increase in complexity in the " ridge formula" 
 (or number of ridges in each tooth), of the molars, is well 
 seen in the following table, from Prof. Flower's Hunterian 
 lecture ("Nature," March 2, 1876); it is a corrected table 
 taken from Dr, Falconer's " Pahcontological Memoirs." 
 
 Milk Molars. TnieMolars. Total. 
 I. II. III. I. II. III. 
 
 Dinotherium giganteum . 
 
 . 1 
 
 2 3 
 
 3 2 2 
 
 13 
 
 Mastodon (Trilophodoii) americanus . 
 
 . 1 
 
 2 3 
 
 3 3 4 
 
 16 
 
 ,, (Tetralophodon) arvemensis 
 
 . 2 
 
 3 4 
 
 4 4 5 
 
 22 
 
 (Pentalophodon) sivalensis . 
 
 . 8 
 
 4 .5 
 
 .5 .0 6 
 
 28 
 
 Elephas (Stegodon) insignis . 
 
 . 2 
 
 5 7 
 
 7 8 10 
 
 39 
 
 ,, (Loxodon) africanus 
 
 . :i 
 
 fi 7 
 
 7 8 10 
 
 41 
 
 ,, „ raeridionalis 
 
 . 3 
 
 (i 8 
 
 8 9 12 
 
 46 
 
 ,, (Euelephas) antiquus 
 
 . 3 
 
 10 
 
 10 12 16 
 
 57 
 
 ., ., primigenius 
 
 . i 
 
 S 12 
 
 12 16 24 
 
 76 
 
 „ ,. indicus 
 
 . 4 
 
 « 12 
 
 12 16 24 
 
 76 
 
 Some variability exists in the number of ridges, especially 
 when they are very numerous, but the above may be taken 
 as averages ; and some species intermediate in the " ridge 
 formula " have been since discovered, thus M. pentelici and 
 M. andium bridge the distinction between Trilophodon and 
 Tetralophodon, and Elephas melitensis comes between 
 Loxodon and Euelephas (Flower). 
 
 It remains to describe, somewhat more in detail, the 
 structure of an elephant's tooth, and this has been deferred 
 till the last, because it can be the more easily understood 
 when the manner of its origin has been mastered. In the 
 Mastodon the molar consists of a crown with strong cusps, 
 standing apart, and with marked roots ; in the African 
 elephant that part which consists of cusps has become the 
 
THE TEETH OF PROBOSCIDEA. 
 
 greater bulk of the tooth, the roots are comparatively in- 
 significaut, and the interspaces of the cusps are filled up 
 with cementum. The molar of the Indiau elephant con- 
 sists of a larger number of yet more elongated and flattened 
 cusps, so that the greater i)art of the tooth is made up of 
 these flattened plates, fused together with cementum, and so 
 forming a strong and solid mass ; the roots are compara- 
 tively inconspicuous. 
 
 When the tooth is a little worn each plate consists of an 
 area of dentine sm-rounded by enamel. The interspaces 
 of the series of plates are wholly filled up by cementum ; 
 the summits of each plate Avere originally mammillated, 
 and divided up into moi'c numerous blunt processes than 
 the corresponding pai-ts of the tooth of a Mastodon ; when 
 the tooth comes into use tlie rounded tips are soon worn 
 ofi', and the grinding surface of the tooth then consists of 
 narrow transverse bands of dentine, suiTounded by enamel, 
 and of cementum in their interspaces. The difference in 
 hardness between these three tissues preserves a constant 
 rough surface, owing to their unequal rate of wear. In 
 their wild condition elej^hants eat trees with succulent juicy 
 stems, and oftentimes grass torn up by the roots, from 
 which they roiighly shake out the adherent earth. In con- 
 finement, the food containing less that is gritty, the teeth 
 become polished by working against one another, but the 
 rate of wear is insufficient to keep their surfaces rough ; for 
 the softer cementum does not get worn down in the inter- 
 spaces of the plates of dentine and enamel, but remains on 
 a level with them. 
 
 Great though the size of the Proboscideans be, they have 
 some points of affinity with the Eodents in the great 
 development of the incisors, the vacant interval between 
 these and the molar teeth, and, as was pointed out by the 
 late Professor Rolleston, the enamel of the elephant's molar 
 having, in its inner portions, a pattern produced hy the 
 
364 A MANUAL OF DENTAL ANATOMY. 
 
 decussation of the prisms which is very similar to that de- 
 scribed by my father as characteristic of all the Rodents 
 save the Leporidse (Hares) and Hystricid^o (Porcupines). 
 
 THE TEETH OF RODENTIA. 
 
 The animals belonging to this order, which is sharply 
 defined, are scattered almost all over the world ; the island 
 of ]\Iadagascar is, however, remarkable for being almost 
 without indigenous Rodents, as is the case also with Aus 
 trulia, 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 ai'isen a form so modified as to mimic the dentition of 
 the true Rodents, viz., the Cheiromys in Madagascar, and 
 the Wombat in Australia. 
 
 The species of Rodents are exceedingly numerous, and 
 the great majority of them are of small size ; the aquatic 
 Capybara is far the largest of recent Rodents. 
 
 In general features the dentitions of the numerous 
 species comprising this order are very uniform ; the incisors, 
 (save in the halves and rabbits, in which there is an ac- 
 cessory 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 the back 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 
 
THE TEETH OF RODENT I A. 365 
 
 relieve from direct pressiire their growing jDulps, 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 very large size, and, owing 
 to the open end of tlie tooth having formerly occupied a 
 more anterior position in the jaw, runs forward beneath the 
 
 Fi3 157 ('). 
 
 tooth, and then bends abruptly backwards to reach the 
 tooth-pulp. In many Rodents the enamel of the front of 
 the large incisors is stained of a deep orange colour ; this 
 colour is situated in the substance of the enamel itself. 
 
 The scalpriform incisors terminate by cutting edges, the 
 sharpness of which is constantly maintained b}" 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 said by Hilgendorff (Berlin Akad. d. Wiss. Monats- 
 bericht, 1865), that the incisors of Hares differ from those of 
 
 ') Side view of skull of a Rodent, giving a general idea of tlie denti- 
 tion of the order. 
 
366 A MANUAL OF DENTAL ANATOMY. 
 
 all other Rodents in having enamel all round them, although 
 it is veiy thin at the back. I have not been able to satisfy 
 myself that the thin clear layer at the back of the tooth is 
 enamel, and am 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 enamel organ is confined to the 
 anterior surface. 
 
 When a rodent incisor has been exposed to -weai-, the 
 anterior layer of enamel is left projecting beyond the level 
 of the dentine, and this an-angement results in a very sharp 
 edge being constantly maintained. The dentine also is 
 harder near to the front of the tooth than towards the back 
 of the tooth. 
 
 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 
 contiimed over the whole anterior surface of the scalpriform 
 incisors. 
 
 The molar teeth are not very numerous; the mouse 
 3 
 family have usually - ; the porcupines have constantly 
 
 4 G 
 
 -, and the hares - ; the Australian water-rat CHx- 
 4 5 
 
 dromys) is altogether exceptional in having so few as ^. 
 
 Observation has established that the last three of these 
 teeth are always true molars, and that when there are more 
 than three, the rest are premolars, and have had deciduous 
 predecessors. 
 
 But the extent to which the milk teeth are developed 
 varies much. Mr, Waterhouse (Nat. Hist, of Mammalia — 
 Rodents, p. 4), has found the milk molar 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. Deciduous incisors have 
 
THE TEETH OF EODENTIA. 367 
 
 not been found in any of the group, save in the hares and 
 rabbits. 
 
 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 incisors ; but in A'ery 
 young specimens there are six incisors, of which the one pair 
 are soon lost. 
 
 Prof. Huxley (Nature, vol. 23, p. 228) has recently written 
 that " 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 addi- 
 tion, two anterior upper and two lower deciduous incisora. 
 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 lai-gcr. It 
 would be interesting to examine foetal guinea-pigs in rela- 
 tion to this point ; at present they are known to possess 
 only the hindmost deciduous molars, so far agi'eeing with 
 the Marsupials." 
 
 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. 
 
 Other rodents, such as the rat, which has only three 
 teeth of the molar series on each side, and the Australian 
 water-rat (Ilydromys) have no milk teeth, and are hence 
 truly Monophyodont. 
 
 Moz-e diversity exists in the premolar and molar teeth ; 
 in rodents of mixed diet, such as the common rat, the back 
 teeth are coated over the crown with enamel, which nowhere 
 forms deep folds, and have distinct I'oots, i.e., are not of 
 persistent growth ; the molars of the rat have some sort of 
 i-esemblance to minute human molars. In aged specimens 
 the enamel is consequently worn off the grinding surface of 
 
368 A MANUAL OF DENTAL ANATOMY. 
 
 the crown, -which comes to be an area of dentine, surrounded 
 by a ring of enamel. 
 
 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 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 the areas of dentine completely 
 np, the result being a tooth like that of the Capybara, which 
 
 Fig. 158 ('). 
 
 is composed of a series of plates of dentine, or ' denticles,' 
 surrounded by layers of enamel, and all fused together by 
 the ccmentum. The result of this disposition of the struc- 
 tures is that the working surface is made up of enamel, 
 dentine, and cementum, three tissues of diftercnt hardness, 
 which will consequently wear down at different rates, and 
 so maintain its roughness. Various intermediate 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 never- 
 theless after a time form roots and cease to grow. When 
 the molar teeth grow from persistent pulps, they are always 
 curved, like the incisors, with the eff'ect of relieving the 
 pulps from direct pressure during mastication ; and the last 
 
 (') Molar of Capybara, showing the transverse plates of dentine and 
 ' united to one another by cementum. 
 
THE TEETH OF llOBENTIA. 
 
 remains of the jiulps 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 it is altoofcther small in amount, the 
 
 (^)- 
 
 formation of true dentine going on till the pulp at that 
 jiarticular point is almost obliterated. 
 
 As has already been mentioned, when the molar series 
 consists of more than three teeth, those anterior to the 
 three true molars are jDrcmolars, which have disjilaced milk 
 teeth ; but they do not differ materially in size or form from 
 the true molars. 
 
 The form of the condyle and of the glenoid cavity in 
 Rodents are characteristic ; they arc much elongated in an 
 autero-posterior direction, so tliat the range of backward 
 
 (') Condyle and glenoid cavity of the Capybara, showing their longitu- 
 dinal direction. 
 
A MANUAL OF DENTAL ANATOMY. 
 
 and forward motion, made use of in gnawing, is very con- 
 siderable. The Lepariihjd are exceptional in having more 
 lateral play than most Rodents. And the power of the 
 teeth is mai-vcllons ; rats will sometimes gnaw holes in 
 water-pipes, or in gas-pipes, in which they have heard water 
 bubbling. 
 
 The general character of a llodcnt's dentition may be 
 illustrated by a description of that of the Capylnira. 
 
 ■iKat.^izc^ 
 
 The incisor teeth are squarish. They are wider than 
 they are deep, and are slightly grooved on their anterior 
 .surface. 
 
 There are four grinding teeth on each side, of which the 
 first three are small, and with few cross plates of dentine 
 and enamel, but the fourth is a very complex tooth, with 
 twelve or more such plates, which are fused into a solid 
 mass by cementum. 
 
 The tooth being one of persistent growth, there is no 
 common pulp cavity, but each jilate has its own. 
 
 It has already been mentioned (page 160) that the den- 
 
 (') Cranium of CapyLara. 
 
THE TEETH OF IIUDEXTIA. 371 
 
 tiual tubes at that part of the liodent't> 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 surflxce 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 of a Rodent's 
 tooth 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 dentinal tubes which have ceased to be in continuity with 
 a vascular living pulp may be, there are, so far as I know, 
 no observations to indicate. 
 
 As Avas shown by my fother (Phil. Trans. 1850), 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 con-ectly refer a tooth to a par- 
 ticular 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 com-ses in these two portions. 
 
 Thus in the enamel of the beaver, in the inner half, nearest 
 to the dentine, the prisms of contiguous layers cross each 
 other at right angles, whereas in the outer portion they are 
 all parallel with one another. 
 
 In the genera Soiurus, Pteromys, Tamias, and Spermo- 
 philus the enamel fibres, as seen in longitudinal section, 
 start from the dentine at right angles to its surface ; in 
 Castor they incline upwards at an angle of G0°, but preserve 
 the distinction between the outer and inner layers very 
 distinctlv. 
 
372 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 In tlie MuriclcB the decussation of the layers in the inner 
 part, and their parallelism in the outer part of the enamel 
 
 Frr,. \a\ (1). 
 
 are also found, but in addition to this the borders of the 
 individual prisms are slightly serrated, the serrations of 
 contiguous fibres interlocking. 
 
 In the porcupine suborder the fibres of the inner portion 
 of the enamel pm-sue a serpentine course, nevertheless 
 showing indications of a division into layers ; they become 
 parallel in the outer portions as in other Rodents. Small 
 interspaces are found amongst the enamel fibres of the 
 Porcupines. 
 
 In the hares {Leporidcn) 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. 
 
 Many minor differences in the arrangement of the enamel 
 prisms exist, for a description of which I must refer the 
 reader to the original paper, but in general terms it may be 
 said that the " enamel lamelke have a different and distinc- 
 
 (') Transverse section of an incisor of a Eeaver (Castor fiber). The 
 enamel jirisms of suiierimposed layers cross each other at right angles in 
 the inner portion of the euaniel, but all becoize parallel in the outer. 
 
THE TEETH OF RODENT I A. 373 
 
 tive character in each of the larger groups, and that the 
 variety of structure is constant throughout the membci's of 
 the same grouj) ; we may take, for example, the Sciuriche, 
 the Muridoe, and the Hystricidce, in each of which the 
 structure of the enamel is different ; and in each is highly 
 distinctive." And further, that the varieties in the struc- 
 ture of the dental tissue, so far as they are known, with a 
 few isolated exceptions, justify and accord with the classifi- 
 cation of the members of the order given by Mr. Waterhouse 
 in his Natural Historv of the ]\Iammalia. 
 
CHAPTER XII. 
 
 THE TEETH OF CARXIVORA. 
 
 The auiinals grouped together under the name of 
 Carnivora are divided into two sections, the Aquatic and 
 the Terrestrial Carnivora. 
 
 The teiTestrial Carnivora were fomierly classed as •• digitigrade " 
 and " plantigi-ade," a classification exceedingly inconvenient, as ifc 
 left the greater number of the animals to be classified in the de- 
 bateable ground between the two extreme types. As a linear 
 classification is impossible, tliey are now grouped around three 
 centres : the Q^luroidea, or cat-like ; the Cynoidea, or dog-like ; 
 and the Ai'ctoidea, or bear-like Carnivora ; and, instead of taking 
 the Felidaj, or Cats, as the tyi^e of the group, it is generally con- 
 sidered that the Dog tribe are the most generalised foi-m, and that 
 the Cats are an extreme modification in one dnection, the Bears in 
 another. 
 
 The Cynoidea comprise the Dog. and its immediate allies, the 
 
 "Wolves and Foxes. 
 The OEluroidea, or cat-like Carnivora, comprise the Yiven-idaj (Civets), 
 
 Hya3nas, and Cats. 
 The Arctoidea, or bear-like Carnivora, comprise the Mustelidaj 
 
 (Weasels), Procyonidaj (Eacoons), and the true Bears. 
 
 The order Carnivora 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. 
 
 In carnivorous animals one tooth on each side of both 
 upjDer and lower jaws is of considerable length, is sharply 
 pointed, and is called a canine ; the upper canine is sepa- 
 rated by an interval from the incisors, the lower canine 
 
THE TEETH OF CAIINIVUIIA. 375- 
 
 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 sometimees- 
 large and pointed so as to be like 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 Felidce, the true molars are reduced to a minimum, and 
 the back teeth ai'e thin edged, " sectorial " teeth ; in the 
 
 bears, on the otlier hand, some of which are purely her- 
 bivorous, the molars are little short of the full typical mam- 
 malian number, and are furnished with obtuse and broad 
 gi'iuding surfaces. 
 
 The accompanying figi;re will serve to give the general 
 aspect of the teeth and jaws of a typically carnivorous 
 
 {') Side view of the cranium of a Tiger, with the mouth slightly opened 
 to show the relative position of the great canines. 
 
376 A MANUAL OF DENTAL ANATOMY. 
 
 animiil, and to show the great development of the processes 
 for the attachment of muscles, and the stout wide arch of 
 the zygoma. 
 
 To a particular tooth in the upper jaw, and to its antago- 
 nist in the lower jaw, Cuvier gave the name of " carnassial ;" 
 these, conspicuous in the true flesh-feeders, become less dif- 
 ferentiated in the Arctoidea or bear-like Carnivora, and in 
 the bears themselves are indistinguishable from the other 
 teeth, save by a determination of their homologies by a 
 process of compai'ison with the teeth of intermediate forms. 
 
 The sectorial or carnassial 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 
 iuitero-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, 
 situated to the inner side of the anterior end of the blade 
 (see fig. 16G). In those which arc most purely flesh-feeders, 
 the " blade " is well developed, and the tubercle of small 
 size ; an increase in the tul)ercular character of the tooth is 
 traceable through those genera which are mixed feeders. 
 
 The lower tooth which antagonises the upper carnassial, 
 passing a little behind it, is the first true molar; in the 
 Felidce it consists solely of the blade, which is divided into 
 two large cusps, behind which is a very small and rudimen- 
 tary third division (which in the Hya;nidce, for example, is 
 of conspicuous dimensions). In existing Carnivora but one 
 " sectorial " tooth is to be found on each side of the jaws, but 
 in the Hysenodon and some other extinct tertiary mannuals 
 there were three teeth partaking of this character. 
 
 In a general sense we may say that the characters which 
 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 implanta- 
 tion, and wide separation from one another of the canines ; 
 
THE TEETH OF CARNIVORA. 377 
 
 the reduction in number of the molar scries, those that 
 remcain being without broad crushing surfaces, in the phacc 
 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 subsists upon a mixed diet ; and a gradation may 
 be traced even in individual teeth, such as the carnassials, 
 in which a gradual increase in 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 ex- 
 amination of the teeth of Felidce, to those of mixed feeders, 
 such as the Arctoidea. 
 
 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 teiTestrial caruivora, so far as is 
 known, all have the same milk dentition. 
 
 .31 .3 
 
 1 _ c _ m - . 
 
 3 13 
 
 Cynoidea. — The dog presents almost the full typical 
 number of teeth, one upper molar (present in an extinct 
 dog-like animal, the Amphicyon) alone being wanting. 
 
 .31 4 2 
 
 1 _ c - pm _ m _ . 
 
 3 1^-1: 3 
 
 The incisors are small, the outermost being the largest ; 
 the upper incisors have, as in a great many Caniivora, a 
 tri-lobed shape, 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 canines, large and conical, are somewhat compressed 
 
378 
 
 A MANUAL OF DEXTAL AXATOMY. 
 
 from side to side, and have an anterior and a posterior 
 sharp ridge ; they arc also slightly flattened on their inner 
 surfaces. 
 
 The premolars are flattened from side to side, pointed, 
 
 Fig. 1(3:3 (•). 
 
 #2Vr«i .b'z^c 
 
 Fig. 164 (-) 
 
 
 r> p' / 
 
 V^v ^A /•* 
 
 \j 
 
 increasing in size from before backwards, and have small 
 basal accessory cusps (see fig. 163). The fourth upper pre- 
 molar is the sectorial tooth, and is very much larger than 
 the third premolar ; the blade is well pronounced, and the 
 
 (') Dentition of Australian Dog (Canis dingo). 
 
 ('-) Milk and permanent teeth of Dog (after Prof. Flower). 
 
THE TEETH OF CAENIVOUA. 37& 
 
 tubercle small. The fourth lower premolar does not 
 greatly (lifter 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, 
 lias a well-marked blade, which ai-ticulates with the blade 
 of the upper carnassial tooth ; but towards the posterior 
 border there is a somewhat thick and blunt tuberculate 
 portion, barely represented in the corresponding tooth of 
 the Felidce ; the tubercular jiortion articulates with the 
 bi'oad flat first upper molar. The second lower molar is 
 smaller, not being one-fourth the size of the first ; the third 
 smaller still; both are blunt-crowned tuberculated teeth 
 (the third lower molar, rudimentary in all dogs, is alto- 
 gether absent in the Canis primsevus). 
 
 The dentition of the dog, closely similar as it is to that 
 of the wolves and foxes, is such as to allow of a considerable 
 i-ange 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, 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 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 pro- 
 bably have been the diet of the animal ; an approxi- 
 mate idea may often be reached, but the sources of 
 fallacy are sufficiently numerous to render the conclusion 
 uncei-tain. 
 
 Amongst the various breeds of dogs some slight differ- 
 ences exist. Thus in the long-muzzled races considerable 
 intervals exist between the premolars, as is to some extent 
 seen in C. Dingo (fig. 163), while in the short-muzzled races 
 
380 A MANUAL OF DENTAL ANATOMY. 
 
 the teeth are in contact, and set somewhat obliquely, so as 
 to be almost imbricated. 
 
 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-nuizzled varieties. 
 
 In some long-muzzled races supernumerary teeth arc 
 sometimes found ; thus De Blainville (Ost^ographie, 
 Canidce) figures two examples, the supernumerary tooth 
 being in one case a premolar, in the other a true molar. 
 
 CEluroidea. — With a dental formula not differing much 
 from the dog (and not all from Canis primpovus) the 
 ViverridcB (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 
 
 .31 4 2 
 
 1 _ c - 1) - m - . 
 
 3 1 W 2 
 
 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 Viverridce recall the characters of 
 insectivorous dentitions rather than those of true flesh- 
 feeders ; furthermore, there are other Viverridcn which are 
 not at all savage, and which subsist on a diet of fruits, 
 eggs, etc., such as the Binturong or the Paradoxurus, the 
 teeth of which have almost lost the carnivorous character. 
 Little use can therefore be made of the Viverridce as illus- 
 trating 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 vary so 
 
THE TEETH OF CARXIVOEA. 
 
 381 
 
 as to adapt its members to different forms of food and 
 habits of life. 
 
 Hycenida'. — lu the Hyroua the jaw is short and stont ; 
 the canines are set far apart, and the teetli of tlie mohir 
 series are reduced in number. 
 
 .314 1 
 
 1 _ c - ]i - m - . 
 o I ^ \ 1 
 
 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 Felidce. 
 
 The premolars are all stout pointed teeth, with a very 
 
 Fig. 165 {}). 
 
 well pronounced basal ridge or cingulum, serviceable in 
 protecting the gxims when the creature is crushing up 
 
 (') t-'l'liei- and lower teeth of Hy^na. The strongly marked cingulum 
 is seen upon the lower teeth. In the upjjer jaw the fourth premolar 
 (carnassial tootli) has a strong Made, divided into three cusps, and a 
 small tubercle opposite to and within the anterior cusp ; it is a good 
 typical carnassial tooth. 
 
yi MANUAL OF DENTAL ANATOMY. 
 
 bones ; they increase in size from before backwards in the 
 upper jaw, tlie fom-th npper premohir being a well marked 
 carnassial tooth witti its blade and tubercle. 
 
 The lower cai-nassial 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 hya;na distinctly 
 more marked than in the Felidce (cf figs. 1G5 and 16G). The 
 only upper true molar is the rudimentary tooth, placed 
 inside the back of the fourth premolar. 
 
 The main featm-e of the dentition of the hysena is the 
 great stoutness and strength of the teeth ; they are admir- 
 ably adapted to the habits of the animal, which feeds rather 
 upon the portions of carcasses left by the fiercer carnivora 
 than upon those which it kills for itself, and consequently 
 bones form a large proportion of its food. 
 
 There is a curious hya)na-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 iaivXy well 
 developed ; the molars and premolars quite stunted. 
 
 The deciduous dentition ( dm. _. j is similar to the 
 
 adult, as respects the teeth being stunted. It is a cowardly 
 animal, and is siipposed to feed on putrid flesh ; it is said to 
 eat young lambs, and to bite the large tails of the Cape 
 sheep, which are remarkal)le for containing an abundance of 
 semi-fluid fat, 
 
 Felidce. — The dentition of this fomily is singularly 
 uniform. 
 
 .3131 
 1 _ c - p - m _ . 
 
 Thus the molar series is reduced below that of hysena 
 
THE TEETH OF CARXIVORA. 
 
 383 
 
 by the loss of a premolar in both jaws. The iucisors are 
 very short, the cauines very large, -widely apart, aud sharply 
 pointed, with a pronounced longitudinal ridge very charac- 
 teristic of the Felidce ; 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 (really the second of the typical mam- 
 malian dentition) premolar is almost a rudimentary tooth ; 
 
 Fio. 166 (1). 
 
 
 the second, a far larger tooth, is sharply pointed ; the third 
 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 small touth, placed trans- 
 versely, aud 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 
 
 (') Side view of lower, and p;datal aspect of upper jaw (Leopard). 
 
38-1 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 the " blade " only ; it is divided by a V-shaped notch into 
 two lobes, and the posterior tubercle is hardly represented. 
 
 In an extinct feline animal, the ^lachairodus, found in 
 tertiary strata, and very widely distributed (in France, 
 Italy, India, Brazil, Buenos Ayres) the first of the pre^ 
 molars left in the upper jaw of Felis, and there almost 
 rudimentary (see fig. 166), has disappeared; the dental 
 formula is thus : 
 
 . 3 
 
 The upper canines are of immense length, and the ridge 
 of enamel which runs down the front and back surface of 
 the teeth is distinctly serrated ; hence the name of saw- 
 toothed Tiger which has been given to the animal. 
 
 Fig. 167 ('). 
 
 The lower canines were quite small, and ranged with the 
 incisors. The enoi'mous length of the upper canine renders 
 it difficult to sec in what manner it was made use of, as the 
 
 {}) Side view of the jaws and cranium of Macliairodus (Drepanodon) 
 after Owen. 
 
TEE TEETH OF CAnXIVOllA. SSf. 
 
 mouth could hardly have been opened to an extent sufficient 
 to enable its point to do more than clear the lower jaw. 
 
 The extinct Hyajnodon had feline affinities, but diffi)red 
 in that it presented the typical mammalian formula of 
 
 .3 1 4 3 
 
 1 _ c - 11 - m - , 
 3 1^4 b' 
 
 its gi-eat peculiarity l)eing tliat one and all of these teeth 
 were of "carnassial" form. Yet the elongated form of its 
 jaw is, so far as it goes, opposed to the idea of its having 
 been highly carnivorous ; its food at all events must probably 
 have consisted of animals very much smaller than itself. 
 
 Arctoidea. — Amongst the Carnivora grouped together by 
 many characteristics as ' bear-lilce,' a tolerably complete 
 gradation of character in the matter of dentition may be 
 traced. 
 
 Some of the group, such as the stoats and martins, are 
 very carnivorous ; others are mainly herbivorous. Of the 
 Mustelida) the dental formula is 
 
 .3 1 4 1 
 
 1 _ c _ 1) _ m - 
 
 3 1^4 2 
 
 There is a sort of primd facie resemblance to the feline 
 dentition, for the sectorials are very much like those of the 
 Felidae, but the last tooth in each jaw is a broad topped 
 tubercular molar, even in the most carnivox-ous members 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 l)e a broad 
 grinding tootlu 
 
 In the Procyonidtc (Racoons and Coatimundis, ifcc), we 
 
 c c 
 
A MANUAL OF DENTAL ANATOMY. 
 
 have a further departure from the carnivorous character, 
 in the increased development of the molar series : the dental 
 formula is 
 
 3 14 2 
 - c _ 1) _ m _ 
 3 14 2 
 
 In the Coatimundi, for cxam^ile, the upper sectorial 
 
 Ai.^iiyjCx^[^ 
 
 has a very large ' tubei-cle,' and posteriorly to this there is 
 a small additional tubercle ; the ' blade ' has no large or 
 conspicuous thin, flat, sharp edge, but presents two pro- 
 nounced cusps. 
 
 The lower sectorial is no longer recognisable as a car- 
 nassial tooth, but 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 loAvor jaw strongly curved, and marked by a 
 deep groove near the front of their anterior surface. 
 
 (') Upper and lower tooth of a Coatimmuli (Xasua socialis). The fourth 
 upper premolar (carnassial tooth) has lost its sectorial character Ly tlie 
 T)lade heing much less, and the tubercle much more developed than in the 
 Qilluroidea ; there is an additional internal tubercle at the back of the 
 tooth. 
 
THE TEETH OF CARXIVOnA. 
 
 387 
 
 In the Bears the teeth are yet further modified to suit 
 the requirements of mixed or vegetable feedei-s. 
 The dental formula is generally 
 
 
 1 
 
 4 2 
 _ m _ 
 4 3 
 
 The incisors of tiie upper jaw present the notch across 
 the crown, so common in caruivora, and the outermost is 
 large and not iinlike a canine; the canines are, relatively 
 to the other teeth, not so large as iu dogs or Felidtc ; never- 
 theless they are stout strong teeth, upon which the anterior 
 and posterior ridges of enamel are well marked. 
 
 Tlio first three premolars are small dwarfed teeth; the 
 
 Fi«. 169 ('). 
 
 first premolar is very close to the canine, and has a crown 
 of peculiar form, produced out towards the canine. 
 
 All four of the premolars seldom persist through the life- 
 
 (') Teeth of a Bear (Ursus tluljetanus). Tlie figure is drawn from a 
 young speciiden, in which the canines have liariUy attained to their full 
 length. In this bear the fooi- ijremolars are ail persistent. 
 
388 A MANUAL OF DENTAL ANATOMY. 
 
 time of the animal ; the first premolar, however, is rarely 
 (if ever in recent sj^ecies) lost, the second being the first to 
 foil out, and then the third. As the fourth is never lost, in 
 most adult bears the first and fourth premolars are foiuid, 
 with a wide interval between them. The i)remolars of 
 bears thus form an exception- to the rule that when a tooth 
 is lost from the premolars, the loss takes idace from the 
 front of the scries. 
 
 The fourth ui.jjcr premolar (carnassial tooth) retains 
 something of its carnassial character ; the first lower molar 
 very little, save that it is a narrower and more elongated 
 tooth than the other true molars. 
 
 The true molars are squarish or oblong teeth, raised into 
 blunt tubercular cusps ; they vary in difierent species. 
 
 In the sloth bear (Ursus labiatus) the incisors are small 
 and the median pair are lost early ; it is varioiisly stated 
 to be frugivorous and to feed on ants, the latter jirobably 
 being the more truthful account. 
 
 Carxivora Pinxipedia (seals). 
 The aquatic Caniivora are divided into three families : — 
 
 I. Tlie Otariida3. or Eared Seals, comprising- the single genus Otaria 
 
 known as Sea Lions, or Sea Bears. These are the " fur Seals,'" 
 from which seal skin is jirocured, and they are less removed 
 from tlie terrestrial camivora than are the other seals : the 
 limbs are better adapted for walking, there ai'e external 
 ears, kc. 
 
 II. The Phocidte, to which familj' the Seals of our own coasts 
 
 (Phoca greenlandica, kc.) and the Great Proboscis Seals of the 
 southern seas (Cystophora) belong. 
 
 III. The Trichechidfe, or Walruses, an aberrant Arctic family con- 
 sisting 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. 
 
 The canines are generally well marked by being larger 
 
THE TEETH UF CARXIVORA. 
 
 than tlie other teeth, but the molars and premolars are 
 very similar to each other, and are simple in pattern. Tlie 
 milk dentition is very feebly developed in the seals ; in the 
 Otaria (fur seal), which of all the seals most approaches to 
 
 Fig. 170 ('). 
 
 the terrestrial caniivora in other characters, the milk teeth 
 iu-e retained for a few weeks, but in most others they arc 
 
 (') Jaws of Otaria, in wliicli tlie tectli are afiectt'cl !iy tbe fona of erosion 
 alluiletl to in the text. After Dr. Minie. Odciit. Suc.[Traii!^., 1870. 
 
300 A MANUAL OF DENTAL ANATOMY. 
 
 shed about the time of birth (of. page 300). Thus Pro- 
 fcsaor Flower tells us that in a l*hoca greenlandica a week 
 old .scarcely a trace of the milk teeth was left. 
 
 The teeth of Otaria and of some other seals become 
 much M'orn down, and they also seem to become eroded at 
 the level of the gums, as they are often deeply excavated at 
 jjoints which seem unlikely to have been exposed to friction, 
 but the nature of this erosion has not been adequately in- 
 vestigated. 
 
 The common seals (Phoca) have a dental formula 
 
 . n 1 4 1 
 
 1 _ c _]) _ m - . 
 ;i 1 M 1 
 
 The incisors are of simple form, and the outer are the 
 larger. The canine is a strong recurved tooth, with a 
 large root ; behind it follows a series of molars, each of 
 
 Fifl. 171 ('). 
 
 which (with the exception of the iirst) 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 lai-ger, 
 more deeply separated from one another and recurved ; and 
 in others the accessoiy cusps being multiplied, so that the 
 
 Q) Teeth of Plioca greenlandica. 
 
THE TEETH OF CARXIVORA. 
 
 391 
 
 name of ' saw-toothed seal ' has been given to their pos- 
 sessors. 
 
 In the Hooded seals (Cjstophora) the incisors are i*e- 
 duced to one in the lower jaw and two in the upper ; the 
 
 Fig. 172 ('). 
 
 -£)jat iiizo 
 
 canines are of great size, hut the molars are small and 
 simple in form, so as to approximate to the teeth of true 
 Cctacea. 
 
 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 foi-m 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 tu turn over obstacles, the walrus feeding upon Crus- 
 tacea, and also upon seaweed, &c. ; they are also used to 
 assist the animal in clambering over ice : as they are of 
 almost equal size in the female, they cannot be regarded as 
 weapons of sexual offence, but tliey are undoubtedly used 
 in the combats of the males. 
 
 ") Permanent and milk teetli of Elei)l)aut Seal (Cystopboraproboscidea). 
 
A MANUAL OF DENTAL ANATOMY. 
 
 In addition to the great tusks the walrus ordinai-ily has a 
 row of four or five short simple teeth, worn down to the 
 level of the gums ; of these, the one placed innnediatcly 
 within the base of the great canine is in the inter-maxillaiy 
 bone, and is an incisor : the ordinary dental formula is given 
 by Professor Flower as 
 
 . 1 1 
 
 1 _ c - 
 
 1 
 
 But there is some difficulty in assigni 
 Fig. 173 ('). 
 
 a definite dental 
 
 
 formula : for in front of the sulitu)y incisor are often the 
 
 (') Siilc view of ujiper ami lowei- jaws of a Waliiis (Tricheclius rosmarus). 
 The ujiper 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 deter- 
 mination of the teeth being open to question, they have been simply 
 umbered. 
 
THE TEETH OF CAllNIVORA. 
 
 sockets (or even the teeth themselves) or two others, which 
 are for various reasons rather to be regarded as noii-per- 
 sistcut teeth of the permanent set than as milk teeth ; and 
 there are also small teeth sometimes to be met with behind 
 the molars, which seem to be rudimentary permanent teeth. 
 The teeth above alluded to may persist through life, and 
 probably often do ; but they arc sure to be lost in macerated 
 skulls, as they have but little socket. Of the milk den- 
 tition four teeth have been traced in each jaw ; they are 
 rudimentary, are lost about the time of birth, and corres- 
 pond in position to the more largely developed teeth of the 
 adult. Hence tlie question if those small rudimentary 
 teeth above alluded to are to be regarded also as milk 
 teeth which are long retained, or as rudimentary permanent 
 teeth ; ,it present this requires further elucidation. 
 
CHAPTER XIII. 
 
 THE TEETH OF IXSECTIVORA, CHIROrTEi:A, AND PRIMATES. 
 
 The insectivora fonn rather a heterogeneous order of Mammals. 
 aud 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, aud their teeth are generally adapted for this 
 by being furnished with many points. The best known animals in 
 the order are the Hedgehogs, the Shrews, and the Moles ; to these 
 are to be added the Galeopithecus, or " Flying Lemi^r," and the 
 Macroscelidaj (Elephant mice). Insectivora are more abundant in 
 Africa. Asia, and South America than in Europe. The Shrews 
 approximate in some measm-e towards the Rodents, and the Tupaia 
 is very lemurine in its characters. 
 
 The common English Hedgehog (Erinacens)hasthe dental 
 foimula 
 
 . 3 4 3 
 
 In the upper jaw there is a wide interval between the 
 fii'rit pair of incisors, -which are much the largest, and are 
 caniniform in shape. The next two teeth (incisors) are 
 quite small, and resemble premolars in their form. The 
 next tooth has two roots, and a crown with one cusp, and is 
 also like the premolars behind it. This tooth, the root 
 of which shows indications of division, is sometimes called 
 a canine, because it comes next behind the intermaxillary 
 suture ; behind this come two small premolars. 
 
 The fourth upper premolar is totally different in size and 
 form from the third : its crown is large, squarish, and 
 
THE TEETH OF IXSECTIVOEA. 395 
 
 furnished with fo\ir cusps, of which the antero-cxtei'nal one 
 is far tlie longest and sharpest. 
 
 The first upper true molar has a square crown, upon 
 which are four sharp cusps : it is implanted by four roots. 
 
 Fig. 17J('). 
 
 .-' -€^^®)^ 
 
 ^ 
 
 V>- ^..^ - \rU. 
 
 The second true molar is also square, quadricusjjid, and 
 luis four roots ; but it is nuich 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, less widely separated 
 than the iipper, 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. The fourth tooth from 
 the front is a small single tooth, like the third, but upon a 
 smaller scale. Xext behind it, comes a tooth which is very 
 much larger, and its crown carries two principal cusj^s with 
 a small subsidiary cusp. The next tooth (first true molar) 
 has an oblong crown beset with five sharp cusj^s, of Avhich 
 four are arranged at the corners of a square, while the fifth, 
 obviously 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 
 
 (') Upper and lower teeth of the Iledgohog. 
 
A MANUAL OF DEXTAL ANATOMY. 
 
 true molar is a dwarfed tooth with but ouc cusp. Several 
 dental formula) have been assigned to the Hedgehog : there 
 is little room for difierence of opinion as to the nomen- 
 clature of its upper teeth : though some authors {e.g.. 
 Professor Mivart) prefer to call the Hrst premolar a canine. 
 But in the lower jaw some :uithors give i -c-pm-, 
 
 others i _ c - pm -, and othci's again, i - pm -. 'I'he 
 
 last given seems the least artificial, and corresponds liest 
 
 with the relations between the ui)})er and lower teeth wlien 
 the mouth is closed. 
 
 Rousseau describes the existence of twenty-four rnilk 
 
 teeth, which he classifies thus: (i - dm _) : that is to say, 
 ' 4 1 ■^ 
 
 all the teetli in front of the true molars had deciduous pre- 
 decessors, but his grouping of them into incisors and molars 
 is quite arbitrary. 
 
 The milk teeth are not shed and replaced until the 
 animal has attained to almost its full dimensions, and all 
 three true molars are in place. 
 
 The teeth of the Hedgehog fairly represents some of the 
 features of Insectivorous dentitions, for the forcep-like in- 
 cisors, the stunted or non -developed canines, and the molars 
 bristling with pointed cusi)S, are conmion to very many 
 Insectivora. 
 
 The Sln-ews have nimierous sharply-pointed teeth, the 
 points interdigitating and fitting veiy^ closely together 
 when the mouth is shut. 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 hicisors and the true 
 molars are several small teeth which, by analogy, are called 
 premolars. The true molars are not very different in 
 pattern from those of the mole (B in Fig. 176), and present 
 the W-contour so common in the molars of Insectivora. 
 
 1'he most marked ])GCuliai-ity in the dentition of the 
 
THE TEETH OF INSECTIVORA. 397 
 
 Shrews lies in the form of their incisors. The first upper 
 iucisor is alwuys very large indeed : it looks vertically 
 downwards, is a little 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 to in some measure encase this latter. The lower 
 iucisor is at least one-third as long as the whole alveolar 
 border. The incisor teeth of the Sln-ew would appear to 
 form a very efficient pair of i)incers, with which to pick up 
 the minute creatures on which it feeds. Of the milk teeth 
 of Shrews little is known : they are said to be absorbed 
 before birth, but accurate observations upon them arc miicli 
 needed, their very existence being doubtful. 
 
 The dentition of the Mole (Talpa) has been the subject of 
 much controversy, the determination of its canines, kc, 
 presenting such difficulty that no less than five different 
 dental formuke have been assigned to it. 
 
 In the front of the upper jaw come three small teeth, the 
 first being somewhat the largest, which are well within the 
 limits of the intermaxillary bone, and are doubtless incisors. 
 But the next tooth, which is very big, also appears to be 
 implanted in the intermaxillary bone, the suture passing 
 across its socket close, to the back of its posterior root. 
 According to its implantation it therefore would be au 
 incisor (') but it is very unlike an incisor ; and it is two- 
 
 (') I confess I cannot fdllo.v ilr. Fpenoe Bute when, in liis vahialile 
 paper on the milk teeth of tlie mole, lie says, " This tooth is implanteil 
 within the limits of the premaxillary bones, the suture separating them 
 from the maxillary, passing through the posterior portion of its alveolus : 
 thus demonstrdt'uuj that this deciduous tooth is the true honiolocjue of that 
 of the mnine in the mammalian ti/pc." Surely it would go to prove the 
 (.ontrar}', if accepted as evidence at all upon this point. 
 
398 A MANUAL OF DENTAL ANATOMY. 
 
 rooted, a thing anomalous either in an incisor or a canine, 
 though found in the canine of (ijnuun-a, which is l)eyond 
 (juestion in the maxiUary bone. 
 
 Next come three minute premolars, and a fourth, which 
 is much larger than the others : these all have single crowns, 
 consisting of little more than a single sharply-pointed cusp. 
 
 The first two upper molars are large teeth bristling with 
 cusps : the third is much reduced in size and siiiiplifiod 
 
 
 ~ NaV. Size. 
 
 in pattern, in the lower jaw tiie four fruut 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 thi'ce 
 teeth behind it in configuration, that it is obviously only one 
 of these premolars developed to a greater length than the 
 others. It closes hehind the caniniform upper tooth, so 
 cannot on this ground be called a canine by those who 
 attach importance to the term. 
 
 Q) Upper and lower teeth of the common Mole. The functionless milk 
 teeth (after Spence Bate) are p'aced above the permanent teeth which dis- 
 place them. 
 
THE TEETH OF IXSECTIVORA. 399 
 
 The remcaiuing three premolars are rather small and 
 single ; the true molars are of considerable size, and their 
 points are very long and sharp. 
 
 I have purposely avoided giving any dental formula for 
 the Mole : everything turns upon the value which we 
 attach to the term canine ; and I have already given reasons 
 (p. 284r) for attaching but little homological importance to 
 its determination. 
 
 Mr. Spence Bate's paper (Trans. Odontol. Society, 1867), 
 valuable as it is in contribiiting to our whole knowledge of 
 the milk dentition of the creature, does not appear to me to 
 help us much in the matter of determining 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, as is indicated in 
 Fig. 175. The milk incisors were about one-twentieth of 
 an inch in length, :uid one two-hundredth in diameter, and 
 were rudimentary in form, consisting of long thin c\'lindrical 
 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 intermaxil- 
 lary suture is very distinct, and there is no doubt that the 
 fom-th upper milk tooth, the predecessor of the caniniform 
 tooth, is in the intermaxillary bone. 
 
 The teeth had not fairly cut the gum, and the advanced 
 state of the permanent teeth beneath them make it doubtful 
 whether they ever do become erupted. At all events, they 
 can be of no use. 
 
 In many of the order Insectivora the milk dentition is 
 xmkuown, but we have exemplified amongst them cveiy 
 grade of completeness in its development. Thus in the 
 Hedgehog and Ccntctos (an allic>l animal from Madagascar) 
 
// MANUAL OF UENTAL ANATOMY. 
 
 the milk dentition is tolerably complete, while in the 
 Shrews it has all bnt, or quite, disappeared. 
 
 The W-patteni characterising the molars of Insectivora 
 has already been alluded to ; it is well exemplified in the 
 molar of Urotrichus. 
 
 In this tooth, as has been clearly shown by Prof. Mivart 
 (Osteology of Insectivoi-a, Joimi. of Anat., 1868), the four 
 cusps of the typical teeth [a, h, c, d) have been added to 
 by the elevation of the cingulum into three or four external, 
 and one internal cusp, making up the total number to nine. 
 Thus it is that the molars of this order often fairly bristle 
 with cusps. 
 
 In the Mule the number of cusps is diminished by the 
 coalescence of h and d into a ridge, and tlic disappearance of 
 Fig. n<M'»- 
 
 ^ .. . ^i , 
 
 AFC 
 the inner cusp of the cingulum, Avhile the simplification is 
 carried yet farther in the Cape Mole (c in I'^ig. 176). 
 
 It would be impossible to notice the somewhat varied 
 dentitions of other Insectivora in these pages, but mention 
 must be made of the very anomalous teeth of the Galeo- 
 pithecus, formerly placed with the Lemurs under the title of 
 "Flying Lemur." 
 
 Its lower incisors are divided by a number of vertical 
 divisions running down through a great part of the length 
 of the crowns, so that they can be compared to combs, or to 
 
 (') A. Upper molar of Urotricluis ; I!. Jtolc : C. Cape Iridescent Mole, 
 (Chrysochloris). 
 
THE TEETH OF INSECTIVORA. 
 
 hands with the fingers slightly separated. What the pur- 
 pose served by these comb-like teeth ma}^ be remains 
 nncertaiu : no other animal has similar teeth. (Jaleopithe- 
 cus has a well developed milk dentition, the milk teeth 
 being very similar to their successors. 
 
 The teeth of Insectivora are remai-kable for the thickness 
 of their enamel, which in the Shrews is to some extent 
 penetrated by the dentinal tubes. The enamel is deeply 
 coloured in some Shrews, the pigment being actually in the 
 substance of the enamel, and not in any distinct layer. 
 
 THE TEETH OF CHIROPTEltA. 
 
 The Bats, sharply distinguished fi-om all other mammals 
 by the possession of wings, are divided into two groups, 
 respectively insectivorous and frugivorous. 
 
 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 shai-p cusps, and generally present the W-pattcrn. In 
 fact, in general character, their teeth resemble those of the 
 Insectivora. 
 
 The incisors are sometimes reduced in uumbei-, and 
 spaces left betvreen 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 lai'ge 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, is stunted. The molar teeth are, however, sharp, 
 though small, and there is no marked distinction into 
 molars and premolars. 
 
 The frugivorous bats (of which the Pteropus, or flying 
 
 D D 
 
402 A MANUAL OF DENTAL ANATOMY. 
 
 fox, is an example) have much Larger muzzles, and the molar 
 
 teeth are set with intervals between them. 
 
 2 12 3 
 The dental formula is i - c - p - m -, hut in some the 
 2 1 ^ 3 3' 
 
 molar scries is reduced below tliis number. 
 
 The incisors are small, and the canines rather hu-gc. 
 
 Both molars and premolars are of somewhat simple form, 
 being long, and compressed from side to side. The outer 
 borders of the crown of the molars are elevated into distinct 
 but not exceedingly 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 bats. All the Pteropi have deciduous canines, 
 and four deciduous molars, of simple pointed form, but the 
 number of deciduous incisors is very variable. 
 
 The milk dentition of bats has been very carefully and 
 thoroughly investigated by Leche (Lund's Universit. Ars- 
 skrift, Tom. XII. and XIV., 1878), and at the present 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 dentition of the Vampire (Desmodus) (•) 
 
 (') In a skull of Desmodus, in the ijossession of Air. R. F. Tomes, the 
 third milk tooth appears to correspond in position to the iiernianent 
 canine ; the same is the case in the specimen iigured by Messrs. Gervais 
 and Castelmain (Expcd. dans les part. cent. d'Amerique du Sud). 
 
THE TEETH OF FIUMATES. 403 
 
 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 fi-ont of the upper jaw, six teeth, each 
 of which is very long and slender, and has a strongly hooked 
 point : it has been suggested that these feeble hooked teeth 
 may assist it in holding on to the mother. 
 
 THE TEETU OF PRIMATES. 
 
 The order Primates embraces Man, Monkeys, and the 
 Lemm-s. 
 
 Some naturalists have been disposed to separate the Lemuridfe 
 from the rest of the Primates, on the ground that some Lemurs 
 approximate rather closely to the Insectivora. while again the order 
 Insectivora contains some forms which recall the Lemurs. 
 
 But although the Lemm-ida; are undoubtedly inferior to the 
 Monkeys, and stand apart from them more widely than do the 
 Monkeys from Man. most authors now place them in the order 
 Primates, which is to be divided as follows : — 
 
 I Lemuridiv. Lemurs. 
 
 Primates <,' Simiada;. Old and new world Monkeys. 
 
 ( AnthropidaB. Man. 
 
 Isemuridse. — The Lemurs for the most part are foimd in 
 Madagascar, and to a less extent on the mainland of Africa 
 and in southern Asia. In their dentition, just as in other 
 characters, they ditter somewhat from the true monkeys, 
 though, on account of there being several very aben-ant in 
 form, it is difficult to give any general account of them. 
 Most of them have the upper incisors very small, and widely 
 separated from one another ; in the lower jaw these are 
 antagonised by six long, thin, narrow procumbent teeth, 
 generally regai-ded as being two pairs of incisors and the lower 
 canines: in both upper and lower jaws the next tooth is 
 large aud pointed like a canine, but the lower caniniform 
 tooth bites behind the upper, and therefore is held not to 
 
404 A MANUAL OF DENTAL ANATOMY. 
 
 corrcsjiond to it, but to be the first jn-emolar (of. page 283). 
 The premolars are compressed from side to side, and are 
 very sharp : the molars are armed with long sharp cusps, 
 which are worn down in old animals. 
 
 The upper molai-s iu many lemm-s are armed with four 
 cusps, connected by an "oblique ridge" like those of man 
 and the anthropoid apes. 
 
 There is a very aberrant lemur, the x^ye-aye ((Jheiromys), 
 which in its dentition imitates the rodents. 
 
 .10 1 3 
 
 1 _ c - pm _ m - • 
 
 In both upper and lower jaws the incisors form a single 
 pair of large curved teeth, growing from persistent pulps, 
 and wearing obliquely so as to constantly preserve a sharp 
 cutting edge. The enamel is very much less thick, if not 
 idtogether absent, upon the backs of these incisors. 
 
 After a considerable interval, which is devoid of teeth, 
 there follow four upper and three lower teeth, Avhich are 
 not of persistent growth, but have definite roots, and re- 
 semble the molars of many omnivorous rodents. 
 
 Being a somewhat rare and strictly nocturnal animal, 
 little is known of its food ; some have believed that it made 
 use of its rodent incisors to cut away portions of wood in 
 order to get at the grubs contained in it, drawing them out 
 of their hiding place by means of its curiously elongated 
 finger, whilst others believe that it gnaws the sugar cane. 
 But whatever the nature of its food may be, it is certain 
 that its scalpriform incisors are put to hard work, and so 
 kept worn down, for in a specimen kejit for a time in the 
 Zoological Gardens, which was supplied wnth soft food, 
 the incisor teeth grew to an excessive length, and idtimately 
 caused the animal's death by the points of its lower incisors 
 perforating the palate. The accompanying figure represents 
 the muzzle of this specimen, and altho\igh the upper teeth 
 
THE TEETH OF PRIMATES. 
 
 have grown to an inordinate length, and have divei'ged frora 
 one another, it will serve to show the rodent-like aspect of 
 its mouth. 
 
 Although, functionally, its teetli are those of a rodent, 
 yet despite this adaptive rcsemMance, the milk dentition 
 
 Fig. 177('). 
 
 retains certain characteis which indicate the Icmuriue origin 
 of the creature. 
 
 In the upper jaw the milk dentition consists of two small 
 incisors, a canine and three molars; in the lower jaw of two 
 small incisors and two small molars ; it is said that in an 
 early stage a third milk incisor is to bo found. 
 
 The permanent incisors push their way up between the 
 first and second milk incisors ; at a certain stage all three 
 
 ('; Aye- Aye (Cheiromys), which died in the Zoological Gardens (after 
 Dr. Murie). The upper incisors, from want of sufficient use, have grown 
 long and diverged from the middle line. 
 
406 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 are to be seen at once, but the large size of the permanent 
 incisors causes the speedy loss of the milk incisors. 
 
 No known rodent has so many milk teeth, nor indeed any 
 
 Fig. 1/8(1). 
 
 milk incisors at all ; the aye-aye thus affords an excellent 
 example of a milk dentition presen-ing characters which arc 
 the extremely modified adult dentition. 
 
 lost 
 
 The special interest which attaches to the dentition of 
 Cheiromys has been already alluded to (see page 271) ; to 
 briefly recapitulate, it is this : in Madagascar, an isolated 
 area separated by a wide tract of deep sea from other areas, 
 true rodents are almost absent, but lemurs abundant. But 
 
 (1) Upper and lower jaws of Cheiromys. A. Milk dentition, witli the 
 permanent incisors jnst emerging, i, I. Upper and lower permanent in- 
 cisors. i2, 12. Upper and lower milk incisors, c. Milk canines, rfl, 
 d 2, d a, d b. Upper and lower milk molars. (Twice natural size.) B. 
 Reduced figiu-e of permanent teeth (after Peters). 
 
THE TEETH OF PllUIATES. 407 
 
 one of the lemiirine animals which are to be found there has 
 been so modified that its teeth to all intents and purposes 
 are those of a rodent. Yet with all this modification it 
 retains characters (notably its milk dentition) -which are 
 quite unlike those of true rodents, but which recall the 
 manner of its origin from higher lemurine fonns. 
 
 Simiadse. — The true monkeys are divided into two 
 great divisions, the new world monkeys and the old world 
 monkeys. The former difter in many respects from the 
 latter; for the most part they have prehensile tails, and 
 their nostrils are set somewhat wndely apart, whence 
 they are called Platijrrhine, or wide-nosed monkeys, and 
 they differ also in their dental formula, which is — 
 
 .2 1 3 3 .,„ 
 1 _ c - p - m - = oo. 
 2 1^3 3 
 
 The little marmoset monkeys have only 32 teeth, but they 
 agree with the other new world monkeys in having three 
 pi'emolars on each side, the molars being reduced to two in 
 number. The upper molars of many new world monkeys 
 have the antero-interual and extcro-posterior cusps joined 
 by an oblique ridge, a character which is shared in the old 
 world groups by man and the anthropoid apes only. 
 
 All the Quadrumana have a well developed milk den- 
 tition. 
 
 Old world or (Jaturrhine monkeys all have the same 
 dental formula as man — 
 
 *> 1 2 3 
 i^c-p_m-; 
 
 As an example the Macaque monkey may be taken. The 
 upper and lower incisors, but especially the former, are 
 directed obliquely forwards, and the lateral incisors are very 
 much smaller than the centrals. In the upper jaw a con- 
 siderable interval separates the incisors from the canine, 
 
408 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 whicli is a veiy large tooth, somewhat triangular in section, 
 with a sharp edge directed l)ackwards, and with a deep 
 groove on its anterior surface. 
 
 The upper premolars are implanted by three distinct 
 roots, as arc also the true molars ; the latter are quadri- 
 cuspid, but lack the oblique ridge. 
 
 Nal. S'fzc 
 
 The lower canine is a sharp and powerful tooth, though 
 it is very much smaller than the upper ; the first lower 
 premolar, by its front surface, articulates with the upper 
 canine, and is of curious form. It is implanted by two 
 roots, but the anterior root is produced forwards, so that the 
 antero-posterior extent of the tooth is much increased. 
 
 The apex of the cusp of the tooth is almost over the 
 posterior root, and from this point the crown of the tooth 
 slopes obliquely forwards down to its anterior root. This 
 peculiarity in the form of the first lower premolar is eminently 
 
 (') Upper and lowor teeth of a Monkey (Macacus neiiiestiinus, male). 
 The length and sharpness of the canines, and the peculiar form of tho 
 anterior lower premolar, contrasts with the aspect of the coiresponding 
 teeth in the Anthropoid Apes or in Man. 
 
THE TEETH OF PIHMATES. 409 
 
 characteristic of the baboons. There is nothing to note of 
 the second premolar save that it is implanted by two roots, 
 like the true molars, which are quadricuspid ; of them the 
 thu'd is lai'ger than the first two, and is quinquicuspid. 
 
 There is considerable difterence in the size of the canine 
 in the two sexes, that of the male being very much the 
 larger ; this ditterence does not exist in the deciduous den- 
 tition, in which the canines are relatively small. 
 
 The Anthropoid Apes are the Gibbons (Hylobate.s), the 
 Chimpanzee (Simla Troglodytes, or Troglodytes niger), the 
 Orang (Simla or Pithccus Satyius), and the Gorilla (Troglo- 
 dytes Gorilla). 
 
 Upon the whole the gibbons are the lowest, and the 
 gorilla the highest of the anthropoid apes, which arc all 
 confined to tropical areas. Thus the gorilla and chimpanzee 
 are confined to tropical Africa, and the orang is limited to a 
 part of the Malay archipelago. The gibbons are more 
 widely distributed over the Malay archipelago and tropical 
 Asia. 
 
 Although upon the whole the gorilla approaches most 
 nearly to man, this can hardly be said to be the case Avith 
 its dentition. The jaws are veiy square, and there is a large 
 diastema in front of the upjier canine, which in the male 
 gorilla is of great size and strength, its top descending far 
 below the level of the aheolar border of the lower jaw when 
 the mouth is shut. 
 
 In the lower jaw there is no diastema, but the teeth are 
 all in contact with one another ; the first of the pre-molai-s 
 is a very strong pointed cone, showing plainly the close 
 relationship between canines and premolars alluded to at a 
 previous page (p. 17). 
 
 The molars increase in size from before backwards, the 
 third molars attaining to a very large size. 
 
 Nevertheless, though the teeth are coarser and stronger, 
 there is a general resemblance to those of man. 
 
410 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 It has been pointed out by the late Professor lioUeston that 
 the canine tooth of the male anthropoid apes is a little later 
 in coming into place than in the female. Thus in the male 
 chimpanzee and orang, it is not cut initil after the third 
 molars (wisdom teeth) are in place, whereas in the female 
 it follows the second, but precedes the third molars. The 
 sexual difference in the canine teeth is very well marked in 
 all the anthropoid apes, and its later eruption in the males 
 
 sxVol Si-f 
 
 is explicable both upon the ground that, being a sexual 
 weapon, it is not needed prior to the attainment of sexual 
 maturity, and also that being of very large size its formation 
 might be expected to take a longer time. No such difference 
 pertains to the milk dentition, in which the order of eruption 
 is exactly that met with in man. 
 
 Dr. Magitot (Bulletin de la Societc d'Anthropologie de 
 Paris, 1869) combats the idea that there is any difference in 
 the order of the eruption of the permanent teeth between 
 man and the anthropoid apes, but, while his observations 
 have been both careful and widely extended, he lays much 
 
 (') Upper and lower teeth of an Antln-(ipoi 
 Orang Outan). 
 
 Ape (Siniia Satyi-us, or 
 
THE TEETH OF PMIMATES. 411 
 
 stress upon an observation made upon a female gorilla skull, 
 in which, as has just been mentioned, the order of succession 
 is not quite the same as in the male. 
 
 The dentition of the orang approaches tolo-ably closely to 
 tliat of man, and the points of resemblance and of difference 
 may be fairly well seen in the accompanying figure. 
 
 The central upper incisors are similar to those of man, 
 but are larger ; the laterals are, relatively to the centrals, 
 much smaller, and are very caniniform in shajje, both inner 
 and outer angles of their cutting edge being sloped off" to 
 such an extent that a central pointed ciisp remains, in 
 place of a thin cutting edge. The canines are strong, 
 pointed teeth, the cinguluni and the ridge joining it with 
 the apex of the cusp being well marked upon their imrer 
 sides. In the female the upper canine is about half as 
 long again as any of the other teeth ; in the male it is 
 longer. 
 
 The first Incuspid is a little more caniniform than that 
 of man : its outer cusp is long and pointed, and a i-idge 
 unites it with the anterior part of the inner cusp, which is 
 feebly pronounced; the second is a blunter and broader 
 tooth. The premolars are implanted by three roots. The 
 molars are not unlike the human teeth in pattern. 
 
 In the lower jaw the incisors are large and stout ; the 
 canines sharply pointed, with a well marked cingulum, 
 and a well marked median ridge on the inner side of the 
 crown. The first premolar is a shorter, stouter, and 
 lilunter copy of the canine, and can hardly be said to 
 liavc an inner cusp. In the second premolar the inner 
 cusp is as high as the outer, and the cingulum is elevated 
 both before and behind till it almost forms two additional 
 cusps. 
 
 Indeed, [ am not acquainted with any dentition which 
 exemplifies the transition from incisors to canines, from 
 
412 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 canines to premolars, and from premolars to true molars, 
 better than that of the oranf'. 
 
 Fig. 181 Q 
 
 Fig. 182 {-). 
 
 Fig. ] 83 C'}. 
 
 The lower molars resemble those of man, save that their 
 
 (') Skull of a .young male Orang. The ujipcr canine dues not nearly 
 reach to the lower alveolar border. 
 
 (-) Skull of adult male Orang, in which the canine is largely developed . 
 (•') Side view of skull of an idiot. 
 
THE TEETH OF PRIMATES. 
 
 surface is marked by that finely wrinkled pattern -wliicli is 
 common to all the unworn teeth of the orang. One is 
 struck b}'- the great backward elongation of tlie jaws, by 
 their squareness, by the parallelism of the two sides 
 which converge slightly at the back, and by the large size 
 of the teeth in proportion to the bulk of the whole animal. 
 
 The large size of the canines being in a measure a sexual 
 cliaracter, is, as is so often the case, not very noticeable in 
 the young animal : the two acompanying illustrations of 
 a young and an adult male orang may serve to show this, as 
 well as some other differences developed by age. 
 
 The differences which serve to distinguish the dentition 
 <jf the most anthropomorphic apes from that of man are 
 mainly these. Relatively to the size of the cranium, and of 
 the whole creature, the teeth and jaws are very much larger 
 in all their dimensions ; hence the creatures are progna- 
 thous, and the facial angle small, even when compared 
 with the jaws and cranium of an idiot. As might be ex- 
 pected this difference is not so great in the yoinig as in the 
 adult animal. 
 
 In place of the teeth being arranged in a swcejiing curve, 
 the jaws are squarish, the incisors being arranged in some- 
 thing approaching to a straight line between the two great 
 outstanding canines, behind which the premolar and molar 
 series run in straight lines, converging somewhat as they go 
 backward. There is a '' diastema " Q) or intei-val in front of 
 the ujjper canine, into which the point of the lower canine 
 passes, when the mouth is closed. Both the greater square^ 
 ness of the jaws, and the existence of a diastema, are direct 
 results of the gi-eat size of the canines, and are consequently 
 not marked in young specimens. 
 
 The upper premolars are implanted by three roots, the 
 
 {') Pometliing approaching to a diastema is said to liavc been observed 
 by Vogt and Broca in early Eiu-opean skulls. 
 
414 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 lower by two roots, just like the true molars, and the prc- 
 
 Fii:. IS I M). 
 
 .Ti\^./A,S/^. 
 
 molars when unworn partake more of the pointed character 
 than they do in man. 
 
 {}) Upper teeth of a Caffir. The oblique ridge of the upper molar i-- 
 distinct, not only upon the first and second, but also upon the third uiolai- 
 or wisdom tooth, which in this skull has the normal three roots well 
 marked. 
 
 (-) Lower jaw of a Caffir, in which the quinquicuspid form of the first 
 and third molars is well seen, it being somewhat les.s strongly indicated iu 
 the second molars. 
 
THE TEETH OF PRIMATES. 415 
 
 The wisdom teeth present the same pattern of grinding 
 surface, are larger than the other molars in the gorilla 
 and the orang, and thei-e is abimdant space for them, so 
 that they play an important part in mastication. The 
 molar teeth of these apes are also squarer, their cusps 
 sharper and longer, and the characteristic patterns more 
 strongly prouomiced, than in man. 
 
 Anthropidse. — In passing from the highest of the apes 
 to the lowest of mankind, there is a sudden change in the 
 chai-acter of the dentition ; but while it cannot but bo 
 admitted that there is a gap, yet the differences are rather 
 of degree than of kind. 
 
 Even in the lowest of human races the facial angle is 
 greater, that is to say, they are much less " prognathous " 
 than the apes, and the upper and lower incisors are more 
 nearly vertical in position, not meeting one another at such 
 an angle as in the apes. Mr. Perrin (Monthly Review Dent. 
 Surgery, 1872) states that in a gorilla skull there is an inch 
 of bone in front of the anterior palatine foramen : in a negro 
 half an inch, and in a Greek skull it was close behind the 
 incisors. 
 
 It is generally said that in man the molars decrease in 
 size from before backwards ; that is to say, that the first 
 molar is the largest, while in anthropoid apes the contrary 
 is the case. Though this is on the whole true, it requires 
 some qualification : thus in certain lower races, such as the 
 Australian blacks, the second and third molars are not 
 smaller than the first, and of the chimpanzee the same 
 thing may be said. 
 
 There is no diastema ; no sexual difference in dentition ; 
 no tooth projecting beyond its fellows, and the teeth are 
 arranged in an unbroken arch. The premaxillary bones 
 become fused with the superior maxillary early in life, 
 whereas in the Quadrnmaua they remain distinct. 
 
 In general terms it may be said that the dentition of th 
 
416 A MANUAL OF DENTAL ANATOMY. 
 
 lower races of mankind differs from that of the higher in the 
 following particulars : the arch is not so rounded, but is 
 squarer in front ; the teeth are larger, and are disposed 
 with greater regularity ; the wisdom tooth has ample space 
 to range with the other teeth, and is a characteristic upper 
 or lower molar, the pattern of its grinding surface (quadri- 
 cuspid if it be an upper, quinquicuspid if it be a lower 
 tooth) and the disposition of its roots corresponding with the 
 first and second molars, which do not greatly exceed it in 
 size. Specimens of negro skulls may be found in which 
 there is scanty room for the wisdom tooth, and in which 
 consequently it is a little stunted in its development : on 
 the other hand, plenty of well formed and well placed 
 wisdom teeth may be picked out of European mouths, 
 though as a rule the wisdom tooth is much smaller than 
 the other molars, does not bear the characteristic pattern 
 of cusps and grooves, has its roots connate, and it is not 
 very infrequently a mere rudimentary peg. The stunted 
 development of the wisdom tooth would seem to be a 
 consequence of want of space during its formative period ; 
 the upper wisdom tooth is especially apt to be cramped in 
 this way. 
 
 There is some little evidence that tlie wisdom tooth is in 
 process of disappearance from the jaws of civilized races : 
 in anthropoid apes the wisdom tooth is nearly or quite as 
 large as the other molars, and shows no variability, whilst 
 it comes into place almost simultaneously with the canine : 
 in the lowest races of mankind the wisdom tooth appears to 
 vary but little, is of large size, and is seldom misplaced ; in 
 highly civilised races it is very variable in size, form, and in 
 the date of its appearance, is often misplaced, and is not 
 uncommonly quite rudimentary. It seems to be a legitimate 
 inference that a further modification of the race in the same 
 direction will i:esult in the disappearance of the wisdom 
 tooth altogether. 
 
THE TEETH UF I'RIMATES. 
 
 Some exception must however be taken to such general 
 statements : thus the Esquimaux not uncommonly have the 
 wisdom teeth small and sometimes crowded out of place ; 
 and amongst the African races instances on the one hand of 
 the wisdom teeth being small, and on the other, of fourth 
 true moUirs existing, are to be met with. 
 
 Nevertheless, for the present, a case in which the wisdom 
 teeth are very small can hardly be called a typical well-de- 
 veloped European mouth. 
 
 In many low races (Bosjesman, Xegro, Australian, New 
 <'aledonian, Caffir) the second lower molar has five cusps, 
 just like the first : this is so in the anthropoid apes, but in 
 European races the fifth cusp is generally wanting in the 
 second lower molar. 
 
 It is not a little interesting thus to find that the difter- 
 ences which serve to distinguish the teeth of the lowest 
 .savage from those of an European, are to a certain extent the 
 .same with those that mark the step from a Quadrumanal to 
 a human dentition, though of coxu-se the divergence of the 
 dentition of the savage from that of the ape is far greater 
 than is that of the European from the lowest savage. 
 
 It is very possible that the larger development of the 
 jaws of the savage may be simj)]y due to the harder work to 
 which they are put Avhile he is growing up. And after the 
 attainment of adult proportions, the teeth of such a man 
 become greatly worn down by reason of the hard and often 
 gritty nature of his food. 
 
 It was pointed out by Mr. Mummery, in a very insti'uctive 
 paper ("Transactions of the Odontological Society," vol. ii., 
 new series, 1869), that destructive wearing down of the 
 teeth was of very common occurrence amongst rude (') races, 
 ivhile the contrary is true of highly civilised races ; tliis was 
 
 i^) To , those races mentioued l>y Mr. Mummery may be adiled the 
 mound builders of North America, whose teeth were ah^ays worn down to 
 au excessive extent. 
 
 E E 
 
418 A MANUAL ni< DEXTAL AXATOMT. 
 
 very likely due to the admixture of earth aud other foreign 
 matter with the food. And, furthermore, that the occurrence 
 of dental irrcgiilarities, due to an insufficient size of the 
 arches, was comparatively speaking unknown among the 
 ruder races, whilst it has been common amongst peoples of 
 more luxurious habits for a long period of time. 
 
 The range of variation in the size of the jaws of healthy, 
 otherwise well-developed adults is great : thus the smallest 
 jaw (occurring in a man of stout build, above middle height) 
 with which I am acquainted measures in width only 1| inch, 
 and in length from back to front 1^ inch ; whilst the largest 
 (occurring in a gentleman of lesser stature ; of Basque ex- 
 traction, moreover, which makes it the more striking) (') 
 measures no less than 2^ inches in width and 2i 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 wisdom teeth, 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 tliero are fewer 
 constant differences between the teeth of the various races 
 of mankind than might have been a priori expected ; in fact, 
 we may almost say that the teeth of savage man are pretty 
 much what we should look upon as an exceedingly perfectly 
 formed set of teeth if we were to see them in the mouth of 
 an European. 
 
 (1) Magitot (Bullet, de la Soc. Anthropol. de Paris, 1869) says :— " Los 
 Basques, par exemple, remarqiiables par la petitesse extreme de leurs 
 ilents." 
 
CHAPTER XIY. 
 
 THE TEETH OF MARSUPIALIA. 
 
 The great sub-class of Marsupials, consisting of animals very 
 sharply marked off from placental Mammals by many striking 
 peculiarities, and amongst others, by the very helpless condition in 
 which the foetus is born, was once very widely distributed over the 
 globe. Xow. however. Marsupials are numerous only in Australia, 
 where they are almost the sole representatives of the Mammalian 
 class : there are a few Marsupials elsewhere, as in America 
 (Opossums) and Xew Guinea : but there are no Marsupials in 
 Europe, most parts of Asia, and Africa. 
 
 The Marsupials of America are all Opossums (I)ulr7j)litd(p'), and 
 this family is not represented in Australia. There is evidence to 
 indicate that the Marsupials of Anierica have nothing at all to do 
 with the Australian Marsupials, but were derived from a different 
 source, at the time when Marsupials abounded all over Eiu'ope. 
 
 The Marsupials of Australia almost monopolise that country ; 
 thus Mr. 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 (^C/iirnjifcrii), and the equally 
 cosmopolite Rodents. Of these latter, however, only one family is 
 represented — the Murida3 — (comprising the Rats and Mice), and 
 the Australian representatives of these are all of small or moderate 
 size — a suggestive fact in appreciating the true character of the 
 Australian fauna. 
 
 " In place of the Quadrumana. Camivora. 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 mo<t diversified forms, adapted to different modes 
 of life. Some are carnivorous, some herbivorous, some arboreal, 
 others ten-estrial. There are insect-eaters, root-gnawers, fruit- 
 eaters, honey-eaters, leaf or grass-feeders, 
 
 E E 2 
 
420 A MAXVJL OF DENTAL ANATOMY. 
 
 " 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 ])urposes 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. ?,'.)\. 
 
 What Mr. Wallace says, speaking of the creatures in their 
 entirety, is equally applicable to their 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 mauy genera and 
 species which have the habits of, and, as it were, fill the 
 place of such creatures as the Carnivora and Insectivora and 
 Eodents amongst the placental mammalia. And not only 
 do they possess something of their habits and external con- 
 figuration, but in those characteristic structures which are 
 subservient to the creature's immediate wants, the marsupial 
 i-epresentatives 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. 187 and 188). 
 
 And although marsupial dentitions do vary very much, 
 yet there are many transitional forms by which we are some- 
 times able to trace the successive modifications through 
 ■which extreme divergence has been ultimately attained. 
 
 Just as we ascribe to placental mammals the formula — 
 
 .3 1 4 3 ,, 
 1 _ c - r> - ni - = 44 
 3 K 4 3 
 
 as the typical or parent dental formula, so to the Marsupials 
 
 we must ascribe the follov/ing — 
 
 .313 4 ,, 
 i_ c-p-m- = 44. 
 3 K 3 4 
 
THE TEETH OF MAltSUPIALlJ 
 
 That is to say, tliongh the total number of teeth is the 
 same, the marsupial has ouly three premolars and has four 
 true molars. The premolars (false molars) differ from the 
 true molars in the greater simplicity of their crowns, just as 
 in most placental mammals ; but, although, looking at the 
 complete adult dentitions, no hesitation woiild be felt in 
 classing the teeth under the heads of premolars and true 
 molars, yet there is a curious anomaly in the succession of 
 the teeth which applies to the whole of the sub-class Mar- 
 supialia, and to some extent im-alidates the definition of 
 •'premolar" as applied to their teeth. Only one of the 
 premolars (the hindmost) has vertically displaced a milk 
 tooth ; indeed, the whole milk dentition of Marsupials con- 
 sists of four milk molars (one on each side of each jaw), 
 there being no milk incisors nor canines in any known 
 marsupial. It is further pointed ovit by Professor Flower, 
 who was the first to fully describe these peculiarities in the 
 succession of marsupial teeth (" Phil. Trans.," 1867), that the 
 extent to which the solitary milk molar is developed varies 
 much in the different families ; no trace of any succession 
 has been observed in the Wombat ; m tlie Thylacine (a dog- 
 like creature) tlie small milk molar is calcified, but is 
 absorbed or shed prior to any other teeth being eruj)ted, 
 whilst in the Kangaroos it is retained till a much later 
 period (see page 430), and in the Kangaroo Rat (Hyp- 
 siprymnus) the milk molar has not yet given place to its 
 successor at the time when tlie last permanent molar has 
 come into place, so that it for a long time ranges with the 
 other teeth and does work. It is difficult to obtain veiy 
 young Marsupials, and material for the complete elucidation 
 of the subject is wanting ; but I have had the opportunity 
 (jf making sections of the jaws of several young specimens 
 ( Perameles and Halmaturus), taken from the pouch by my 
 friend Prof. Moseley, and I liave not so far succeeded in 
 
422 
 
 A MANUAL OF DENTAL ANATOMY. 
 
 Fig. ISO ('). 
 
 K 
 
 111111! Ik 
 
 Wm 
 
 (') Enamel and dentine of a Kangaroo (Macroiius 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. 1!. Enamel penetrated by the tubes. 
 C. Individual dentinal tube. 
 
TUE TEETH OF MARSUPIALIA. 
 
 423 
 
 finding any uncalcified germs or other indications of any of 
 the missing teeth of the milk dentition. 
 
 A furtlier peculiarity of the Marsnpials is the structure of 
 
 Fill. 1S7 ('). 
 
 their enamel, which is penetrated by the dentinal tubes. 
 
 (') Upper and lower teeth of tlie Tliylaciiic. 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. 
 
 (-) 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. 
 
42i A MANUAL OF DENTAL ANATOMY. 
 
 My father, some years ago, described and figured the teeth 
 of a large number of Marsupial genera (" Philos. Transac.," 
 1850), and found that although 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 con- 
 spicuously present in the whole class, with the sole exception 
 of the Wombats, in whom nothing of the kind is to be found. 
 Prof. Moselcy's specimens have aftbrded to me the opportu- 
 nity of studying the development of this tubular enamel, and 
 the result of my investigations vvill be detailed elsewhere : 
 but it may be mentioned that the formation of the enamel 
 tube appears to be precisely analogous to that of a dentine 
 tube, and at a certain period the enamel cells have appended 
 to them long processes like the dentinal fibres. The dila- 
 tation noticeable at the boundary line of the enamel and 
 the dentine (see Fig. 186) 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. 
 
 There exists one genus of flesh-eating marsupials whose 
 ferocity is such as to have gained for them the names of 
 wolf and tiger, wdiile 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 is in dentition even more 
 close than in external form : whilst retaining certain mar- 
 supial attributes, the teeth of the Thylacine are, so far as 
 their working capabilities go, almost exactly like those of the 
 dog. The dental formula is — 
 
 . 4 1 ■^ 4 
 
 1 _ c - 1) - m _. 
 
 3 1 .3 4 
 
 The incisors are small, close set, and sharp edged, the 
 
 (') It lias of course no real relationshii) to the true opossums, which are 
 not found in Australia. 
 
THE TEETH OF MAllSUl'IALIA. 425 
 
 outermost being somewhat cuuiniform. The canines are 
 stout, pointed teeth, not quite so long relatively as those 
 of the dog. The premolars are conical teeth, implanted by 
 two root«, and very similar to those of the dog ; they are 
 followed in the upper jaw by four molars, increasing in size 
 from the first to the third, but the last true molar is again 
 a smaller tooth. 
 
 The uj^per molars ai-c all of the " carnassial " pattern ; 
 there is a " blade " elevated into subsidiary cusps, and 
 internally to this a " tubercle," supported by a third root. 
 
 The lower molars also bear some resemblance to the car- 
 nassial teeth of the dog, consisting of a strong, sharp-edged 
 blade, with anterior and posterior subsidiary ciisps, the latter 
 being somewhat broad and tubercular. 
 
 An allied animal (Dasyurus ursinixs), though smaller than 
 the Thylacine, and having teeth of a less sectoi'ial character, 
 is so destructive to sheep, and so fierce and untamable, that 
 it has earned the name of " Tasmanian Devil." 
 
 AVithin the limits of the same genus, a species (Dasyurus 
 viverrinus) is to be found in which the molar teeth are 
 studded over with long sliarp cusps, like the teeth of 
 Insectivora, a group which it resembles both in its habits 
 and food. 
 
 A number of smaller ^larsupials 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 Dasyurida; and the 
 herbivorous Kangaroos and Wombats. 
 
 Amongst the Opossums the larger species have large 
 canines, and a dentition iu its general features approxi- 
 mating to the Dasyuridie ; they feed upon birds and small 
 mammals, as well as upon reptiles and insects, while the 
 smaller species are more purely insectivorous. 
 
 Myx-mecobius, a small Australian Marsupial of insectivo- 
 rous habits and dentition, is remarkable as having teeth in 
 
426 A MANUAL OF DENTAL ANATOMY. 
 
 excess of the number of tlie typicul mamraaliau dentition, 
 having 
 
 . 4 1 :'. G 
 
 1 _ C - p _ 111 _. 
 
 In the Phalangers, nocturnal arboreal animals found in 
 Australia and a part of the ^lalay Archipelago, tlic canines, 
 though present, are feeble ; an interspace also sei)arates the 
 incisors from the molar series. 
 
 The lower incisors, reduced to a single pair, are procum- 
 bent, and grow from persistent pulps ; and a slight exagge- 
 ration of the peculiarities of the dentition of the Phalangers 
 brings us to that of the Kangai'oo Kats. 
 
 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 
 
 .3 1 1 4 
 
 1 0^1 4 
 
 The first })air of upper incisors are sharply })ointed, are 
 directed nearly vertically downwards, and grow from per- 
 sistent 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 oldiquely-worn 
 surface behind the cutting edges impinges against the second 
 and third upper incisors. 
 
 The arrangement of the incisors, and the sharpness of 
 
THE TEETH OF MARSUPIALIA. 
 
 427 
 
 the'r cutting edges, ayg calculated to effect the same objects 
 as those attained by the iucisors 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 incisor.s, and a compensating 
 extra development of the first pair. 
 
 The canines are not large ; yet they are not so small as 
 
 s\al Sf=e^ 
 
 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 third premolar, the tooth to which attention has 
 
 (') Upper and lower teeth of Hypsiprymnus (Bettongia) (Graii ?). Tbe 
 flentition represented is that of the adult animal, the permanent premolar 
 {pm 3) being already in place. 
 
423 A MANUAL OF DENTAL ANATOMY. 
 
 already been drawn on account of its size and other pecu- 
 liarities, by virtue of its great size displaces not only the 
 milk molar, to which it is the legitimate successor, but also 
 turns out the second premolar, a tooth belonging to the 
 " permanent " series. 
 
 In this particular the succession of the teeth in the 
 Hyj)siprymnus is the same as that of the true kangaroos, 
 which may be understood by a reference to fig. 190. 
 
 There are two extinct 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 elongated and sharp- 
 edged blades, held that Plagiaulax 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 first, and 
 Professor Flower in the case of the Thylacoleo, having shown 
 this view to be untenable, or at least unsupported by adequate 
 evidence. 
 
 The clue to the nature of the great blade-shaped teeth of 
 these two extinct creatures is afforded by the form of the 
 premolar of the herbivorous HyjDsiprymnus (see fig. 189). 
 The incisors were reduced in numl^er and were large ; the 
 teeth between them and the large premolar were 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 fiat in 
 aged animals), and by the rudimentary condition of its true 
 molars. But its incisors, lying forwards and closely ap- 
 proximated in the middle line, are particularly imsuitable 
 for catching and holding anything alive and struggling, 
 whilst the nearest resemblance to the blade-shaped tooth is 
 to be found in harmless herbivorous creatures, so that the 
 balance of evidence is much against Professor Owen's vicAv. 
 It has been cited here merely as an instance of how the 
 evidence afforded by teeth alone may bo misleading. 
 
THE TEETH OF JUESUFIALIJ. 
 
 429 
 
 The Kangaroos, comprising many species of very varying 
 size, are all docile creatures ( with the exception of a few old 
 males), of herbivorous habits ; they in some particulars 
 recall the ruminants. 
 Their dental formula is 
 
 () 1 4 
 
 0^1 4 
 
 The three pairs of upper incisors are more equal in size 
 
 . 3 
 
 1 - 
 1 
 
 Fig. 190 (' 
 
 A'irl Sizf^ 
 
 than in the Hypsiprymnus, and the central pair do not 
 grow from persistent pulps. The lower incisors are very 
 peculiar teeth : they grow from persistent pulps, are pro- 
 €ambent, projecting forwards almost horizontally, and arc 
 very much flattened from side to side, their outer surfaces 
 being but slightly convex, and their inner surfiices flat, with 
 a median ridge. Their margins are almost sharp. There is 
 an unusual amoimt of moliilitv between the two halves of the 
 
 (') Upper and lower teeth of Hahnaturns ualabatus. The penuancnt 
 premolar is not yet ernxjted, and is sho\vn in its crypt : when it comes 
 into its place it will displace the milk molar, and one of the anterior pre- 
 molars as well. In the upper jaw a rudimentary canine is shown. The 
 point of the lower incisor wouhl lit, in closure of the mouth, behind the 
 long anterior upper incisor, but the width of the page did not admit of the 
 teeth being placed in their true relative positions without reduction in 
 
430 A MAXVAL OF DENTAL ANATOMY. 
 
 lower jaw, so that these two teeth can be to a slight extent 
 separated from one another. 
 
 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 pei-plexing 
 to the student, for two reasons : the one, that the L\st or 
 third permanent j)remolar not only displaces the solitary 
 milk molar, but also, as in Hypsiprymnus, on account of its 
 gi'cater size, the second permanent premolar, which was in 
 front of the milk molar ; and, besides this, 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. 
 
 Thus the dentition of the kangaroo at successive ages 
 may be thus represented : 
 
 .3 111 4 
 
 1 _- c - p - dm - ni _, 
 
 1 0^1 1 4 
 
 or, in all, six molar teeth. Then the third premolar dis- 
 places the second true permanent premolar as well as the 
 milk molar, and we have 
 
 .301, ,4 
 
 1 _ c - p - (a new one) m -, 
 
 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 Phacocluei-us (see page 
 328), till all that is left is 
 
 3 2 
 
 ^ T W. 1^ ^^ 2- 
 
 The milk molar of the kangaroo is a fully-developed 
 tooth, which takes its place Avith the other teeth, and is not 
 distinguished from them by any special characters, so that 
 mere inspection of the jaw of a young Kangai'oo having 
 it in place, at the same time with a premolar in front of it 
 
THE TEETH OF MARSUl'IALIA. 
 
 aud four true molars behind it. would not lead au observer 
 to suspect its real nature. 
 
 No existing creature serves to connect tlie Kangaroos 
 closely with the wombat, but the extinct Diprotodon appears 
 to have iu a measure bridged across the gap. 
 
 The Wombats (Phascolomys) are heavily-built, inoffensive 
 creatures, which burrow iu the ground aud subsist largely 
 upon roots. In their dentition they closely simulate the 
 
 Frn. IPl ('). 
 
 -fXaJ.Si'ze. 
 
 Jim 
 
 C^ 
 
 Rodents, as they possess but a single pair of chisel-edged 
 iucisors in either jaw, growing from persistent pulps, and 
 embedded in veiy deep and cun^ed sockets. These differ from 
 the corresponding " dentes scalpi-arii " of true Rodents in that 
 there is a complete investment of cement, which passes over 
 the enamel in front of the tooth as Avell 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 
 
432 A MANUAL OF DENTAL ANATOMY. 
 
 fire very deeply grooved upon tlieir sides, so that their 
 
 grinding surfaces are uneven. 
 
 Their dental formula is, 
 
 10] 4 
 
 1 J c- p 3 m -^. 
 
 The first tooth of the molar series is a single column, 
 whereas the deep grooving of the others divides them into 
 two columns, so that its simpler appearance, as well as 
 analogy, would indicate that it is a premolar. But no 
 succession whatever has been ol)Scrved in the wombats. 
 
 The adai)tive resemblance to the dentition of the true 
 Rodents is exceedingly close, though the Wombat is an 
 undoubted Marsupial ; and the very closeness of the imita- 
 tion is an exemplification of the fact that adaptive charac- 
 ters are very apt to mislead, if used for the purposes of 
 classification. 
 
 Extinct wombats, of very much larger size than the 
 recent species, are found in the later tertiary dei)OsitH 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 re- 
 tained, as are a few small teeth which are opposed to them 
 above. 
 
 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 cannot better conclude the very brief survey of the 
 teeth of Mammalia which hasl)ecn attempted in this volume 
 
 (') Uppei- and knver teeth of 'Wombat (riiasculomys wombat). 
 
THE TEETH OF MARSUPIALIA. 
 
 than by calling the reader's attention again to the character of 
 the Mai'supial fauna : this microcosm, in which every place 
 is filled by a Marsupial that mimics the placental Mammal 
 which it represents, for nowhere can we more plainly see 
 the workings of natural selection than in areas thus isolated 
 and deprived of immigrant creatures for countless ages. 
 
 In the foregoing pages much stress has been laid upon 
 the variability of animals, and the agencies by means of 
 which the variations have been preserved and intensified, 
 so to speak, so that ultimately permanent hereditary modi- 
 fications have been the result ; and it is possible that in 
 laying this aspect of the matter prominently before the 
 reader an impression of too great instabilit}- may have been 
 conveyed ; and thus the forms of creatures made to appear 
 more plastic and more shifting than they really are, for it 
 is hardly possible to realize the enormous lengths of time 
 during which the agencies have been at work, and without 
 which they would have been powerless to produce profound 
 alterations. 
 
 The process which we term inheritance is constantly 
 reproducing animals which are minute copies of their 
 parents ; copies which are even more exact than we can at 
 first sight realise. 
 
 Thus, even amongst different species of the same genus, 
 whose teeth are apparently quite similar so far as their 
 number and pattern goes, differences exist which are con- 
 stant for the species, and which may be brought into 
 prominence by any method of investigation wliich is 
 sufficiently accurate. 
 
 A plan of representing in the f>rm of diagrams certain of 
 the characteristics of an animal's dentition, by means of 
 which difTerences of proportion so slight as to be barely 
 recognisable by an inspection of the teeth are brought con- 
 spicuously into view, has been devised by Mr. Busk ("Proc. 
 Roy. Soc. 1870 ")■ 
 
434 A MANUAL OF DENTAL ANATOMY. 
 
 If " odontogmms " of various Felidic be constructed, differ- 
 ences between them will be apparent at a glance, although 
 the forms of the several teeth in this family are so very 
 closely similar, that nothing but the veiy closest observa- 
 tion would have detected the smallest diflerence between 
 tliem. 
 
 I have not practically tested the applicability of this 
 method of comparison, but it is said that these diagrams, 
 embodying as they do only one set of facts about a dentition, 
 have proved less useful than might have been anticipated, 
 and occasionally may even prove misleading. 
 
INDEX. 
 
 A. 
 
 Absorption of teeth 
 
 -Verodus, teetli of . . . . 
 
 Adaptive moditieatiou, meiuiiii^^ 
 
 of term 
 
 Alveolar proeesses . . . . 
 „ process, development uf 
 Alveoli, attachment by means of 
 Alveolo-dental membrane 110, 
 Anarrhicas, teeth of . 
 -Vuchylosis of teeth 
 -Vngler, teeth of . 
 Anoplotherium, teeth of. . . 
 Anteatei-s, teeth of . 
 -Vnthropoid apes, teeth of 
 
 „ „ order of erup- 
 
 tion of teeth in 
 ,, „ comparison of 
 
 with man 
 
 Antrum 
 
 „ teetli ill relation with 
 Archreopteryx, teeth of . . . 
 ^Vi-maiUllo, teeth of . 
 Ai-ticulation of the lower jaw 
 „ ■ „ teeth with one 
 
 another 
 .b-ticular process, share of in 
 
 growth of jaw . . . 
 Artiodactyle unsulat;i . . . 
 -Vttachment of teeth . 
 
 „ „ by membrane 
 
 ,, „ by hinges 
 
 ., „ by anchylosis 
 
 „ ,, bv sockets 
 
 Aye-aye, teeth ^f. " . .271, 
 
 PAr.B 
 Barracuda pike, teeth of . . 235 
 Basal ridge, orcingulum . . 10 
 
 Basement membrane of tooth- 
 
 Babiinssa, teeth of 
 Baboon, teeth of 
 Bala;nidai, whalebone of 
 
 
 germ .... 
 
 . 171 
 
 267 
 
 Bats, teeth of 
 
 . 401 
 
 24 
 
 „ milk teeth of 
 
 . 402 
 
 192 
 
 Bear, teeth of 
 
 . 387 
 
 ib. 
 
 Beaver, teeth of 
 
 45, 372 
 
 212 
 
 Betton-ia, teeth of 
 
 . 427 
 
 229 
 
 Birusjiids, Iniman 
 
 . 14 
 
 207 
 
 l{il(i])li<idont, meanhig of tcnu 
 
 . 317 
 
 228 
 
 Birds, teetli of 
 
 . 260 
 
 ;i32 
 
 Boll, on dentinal fibrils 
 
 66-68 
 
 30.5 
 
 Bone, primary 
 
 . 168 
 
 409 
 
 „ development of . 
 
 . ib. 
 
 
 Brown striie of lletzins 
 
 . 52 
 
 ib. 
 
 Buccinator, attachment of . 
 
 . 30 
 
 
 Bunodont, meaning of term 
 
 . 328 
 
 410 
 27 
 
 
 
 29 
 261 
 
 ('. 
 
 
 305 
 
 Calcified teeth 
 
 . 3 
 
 32 
 
 Calcification, dates of in the st 
 
 ve- 
 
 
 ral teeth . 
 
 . 146 
 
 5 
 
 „ process of 
 
 . 148 
 
 
 „ of enamel . 
 
 . 151 
 
 188 
 
 ,, of dentine 
 
 . 158 
 
 325 
 
 ,, of vaso-dentine 
 
 . 164 
 
 202 
 
 ,, ofosteo- „ 
 
 . 165 
 
 ib. 
 
 ,, ofcementum 
 
 . 166 
 
 203 
 207 
 
 Calcoglobulin . 
 
 . 150 
 
 Calcosphcrites 
 
 . 149 
 
 212 
 
 Camel, teeth of . 
 
 . 337 
 
 404 
 
 Canaliculi of cementum . 
 
 . 97 
 
 
 Canidie, teeth of . 
 
 . 379 
 
 
 Canine teeth of ruminants 
 
 . 282 
 
 
 ,, of lemurs 
 
 . 283 
 
 
 „ oforeodon . 
 
 . ib. 
 
 329 
 
 „ of mole . 
 
 . 284 
 
 408 
 
 „ of insectivora 
 
 . ib. 
 
 312 
 
 Canine, definition of . 
 
 . 280 
 
436 
 
 INDEX. 
 
 t'liuiuc, sexual devflopiiient of . 
 
 I'AdE 
 
 Lecus-satiou of enamel prisms . . 
 
 ■AliE 
 
 50 
 
 ., true signification of term 
 
 281 
 
 Deer, teeth of . 
 
 335 
 
 Canines, human . . . . 
 
 12 
 
 Ueficien 
 
 •ies of teeth in hairy men 
 
 276 
 
 Capybaru, teeth of . . ;:G8-370 
 
 
 „ in Turkish 
 
 
 ,, nioliir teeth of 
 
 296 
 
 
 dogs. . 
 
 275 
 
 < 'archarias, teeth of . 
 
 219 
 
 Dentine 
 
 calcification of 
 
 158 
 
 Caniassial tooth . . . . 
 
 376 
 
 j^ 
 
 composition of . . . 
 fibrils of . 
 
 68 
 
 Carnivora, milk dentition of 
 
 ;i77 
 
 j' 
 
 64 
 
 ,, teeth of . ... 
 
 374 
 
 ., 
 
 genu . . 113, 
 
 132 
 
 Caraivorous dentition, general 
 
 
 \ 
 
 globular 
 
 163 
 
 iharacter of 
 
 376 
 
 ,, 
 
 granular layer of . . 
 
 72 
 
 Cement oigan .... 
 
 144 
 
 ^ 
 
 interglobular spaces of . 
 
 ib. 
 
 ,, doubtful existence 
 of 
 
 145 
 
 ;; 
 
 matiix of ... 
 modifications of in la- 
 
 58 
 
 Cement, over crowTi ol' tooth 
 
 100 
 
 
 byrinthodon 
 
 79 
 
 Cemeutuni 
 
 95 
 
 ,, 
 
 ,, in lepidosteus 
 
 78 
 
 „ rudimentary . . . 
 
 96 
 
 
 in manatee 83, 347 
 
 ,, structure of 
 
 ih. 
 
 
 „ in megathe- 
 
 
 distribution of . . 
 
 42 
 
 
 rium. 
 
 84 
 
 ,, calcification of . 
 
 166 
 
 ^ 
 
 „ in myliobates 
 
 81 
 
 Ceratodus, teeth of. . . . 
 
 238 
 
 1' 
 
 ,, in varanus . 
 
 76 
 
 IJcstracion, teeth of . 
 
 221 
 
 
 osteo- .... 
 
 88 
 
 Cetacea, teeth of . 
 
 ;;07 
 
 ^, 
 
 plici- .... 
 
 76 
 
 tJhsetodonts, teeth of . 
 
 235 
 
 ,, 
 
 secondary . 
 
 93 
 
 Cheirom^■s, niillc teeth of . . 
 
 406 
 
 
 sensitiveness of . . 
 
 71 
 
 „ ' teeth of , 
 
 271 
 
 
 sheaths of Neumaim in 
 
 63 
 
 Chelonia, teeth of ... 
 
 241 
 
 
 lei-mination of tubes of 
 
 72 
 
 Chiroptera, teeth of . . 
 
 101 
 
 '. 
 
 theories as to forma- 
 
 
 Cingulum, detinitiou of . . 
 
 11 
 
 
 tion of . . . . 
 
 162 
 
 ,, developed into addi- 
 
 
 
 lubes .... 
 
 61 
 
 tional cusps . 
 
 292 
 
 J, 
 
 unvascular . . . 
 
 56 
 
 Coatimuudi, teeth of . 
 
 ;i86 
 
 ,, 
 
 varieties of . 
 
 91 
 
 Cobra, teeth of .... 
 
 247 
 
 ,, 
 
 vascular . . . . 
 
 82 
 
 Complex teeth, manner of forma- 
 
 
 ,, 
 
 •\-aso- .... 
 
 ib. 
 
 tion of . . . 288 et 
 
 seq. 
 
 Dents en vcloui-s, en brosse, en 
 
 
 Contour lines of Owen 
 
 61 
 
 cardcs 
 
 
 235 
 
 Coronoid process, use of in growth 189 
 
 Dermal 
 
 ■spines of fish . . 2, 
 
 219 
 
 Correlations of growth . , . 
 Crocodiles, teeth of . . . 
 
 275 
 
 Desmodus, teeth of ... 
 
 401 
 
 256 
 
 Development of the teeth 113 et 
 
 seq. 
 
 „ implantation of teeth 
 
 
 ^, 
 
 commencement of 
 
 
 of . . . . 
 
 213 
 
 
 113 
 
 128 
 
 succession of teeth in 
 
 258 
 
 ^^ 
 
 in eel . . . 
 
 121 
 
 Crypts of developing teeth . 
 
 180 
 
 '^ 
 
 in fishes 
 
 115 
 
 Curvatures of dentinal tubes 
 
 60 
 
 '.' 
 
 in lizards . . . 
 
 125 
 
 Cusps, formation of . . . 
 
 292 
 
 
 in mammalia . 
 
 128 
 
 Cuticula dentis .... 
 
 99 
 
 ,, 
 
 in reptiles . . . 
 
 122 
 
 Cynodraco 
 
 259 
 
 '' 
 
 in snakes 
 
 126 
 
 Cystophora, teeth of . 
 
 o91 
 
 
 of the true molars . 
 
 138 
 
 Czermak, interglobular spaces of 
 
 74 
 
 Diastem 
 
 of the jaws . . 
 of the alveolar pro- 
 cesses . . . 
 
 176 
 
 198 
 413 
 
 1). 
 
 
 Dicynod 
 
 on, teeth of . . . 
 
 243 
 
 
 
 Dinoceras, teeth of . . 
 
 341 
 
 
 
 Dinotherium, teeth of . ."49, 
 
 360 
 
 Dasyurus, teeth of . . . . 
 
 425 
 
 Diodon, 
 
 teeth of . . . . 
 
 231 
 
 Deciduous dentition . 
 
 297 
 
 Diprotoc 
 
 on, teeth of . . . 
 
 431 
 
437 
 
 PAOE 
 
 . 378 
 . ib. 
 2, 217 
 . 343 
 . 345 
 . ih. 
 
 Dog, teeth of 
 ,, variation in toutli of 
 Dog-fish, teeth of . 
 Dryptodon, teeth of . 
 Dugong, teeth of . 
 ,, tusks of male 
 
 Edentata, teeth of . . . 304 
 
 Eel, development of teeth of . 121 
 
 „ enamel-tipped teeth of . 40, 210 
 
 Elasmobranih ti?h, teeth of 11-5, 215 
 
 Elephant, milk teeth of . . 354 
 
 ,, molars of . 356, 361 
 
 ,, sueeessiou of teeth iu . 354 
 
 tusks of . . .350 
 
 Enamel . . . . 41 ct scy. 
 
 ,, al)sence of . .41 
 
 ,, eavities in ... 52 
 
 „ eakitication of . . 151 
 
 cells 153 
 
 „ „ calci6cation of . ib. 
 
 „ cutrj- of dentinal tubes 
 
 into . . . . 53 
 „ fracture of . . .44 
 „ germ . ... 114 
 
 „ ofsargus . . .55 
 „ of marsupials . . . 53 
 „ organ, development of . 134 
 „ ,, external epithelium 
 
 of . . 132, 153 
 „ „ internal epitheliuiii 
 
 of . . 132, 153 
 „ ,, neck of . . . 132 
 ,, pigment iii . . . 52 
 „ prisms of . . . 44 
 „ rudimentary .41 
 „ organ, stellate reticu- 
 lum of . . . 135 
 ,, „ striation of . 50 
 „ theories of formation of . 154 
 ,, tubular . . . 53 
 Eruption of teeth, date of . . 193 
 „ „ mechanism of 190 
 External pterygoid muscle, action 
 of 34 
 
 Felidw, teeth of . . . . 375 
 
 Fibrils of dentine . .64 
 
 Fishes, teeth of . . . . 214 
 
 ,, classification of . . ib. 
 
 „ structure of teeth of . . 236 
 
 Foetus (nine mouths), teeth of . 179 
 
 
 PAGE 
 
 Follicle, dental . . .133 
 
 , 142 
 
 Frog, teeth of 
 
 239 
 
 G. 
 
 
 Galeopithecus, teeth of 
 
 401 
 
 Genu, tooth .... 
 
 113 
 
 Glenoid cavity, form of iu carni- 
 
 
 vora 
 
 34 
 
 ,, .. form of in herbi 
 
 
 vura . 
 
 . i't. 
 
 Globular dentine 
 
 163 
 
 Goodsir, special views of 
 
 129 
 
 Grampus, teeth of 
 
 308 
 
 Granular layer of dentine 
 
 72 
 
 Growth of the jaws 
 
 177 
 
 Gubei-naculum 
 
 145 
 
 Gum, the .... 
 
 108 
 
 Gymnodouts, teeth of 
 
 230 
 
 II. 
 
 
 Haddock, teeth of . . 
 
 . 211 
 
 Hair and teeth, correlation be 
 
 
 tween .... 
 
 . 275 
 
 Hairy men .... 
 
 Hairless dogs, teeth of . 
 Hake, dentine of 
 
 . 276 
 
 . 275 
 
 . 87 
 
 „ hinged teeth of . 20C 
 
 ,228 
 
 Halicore .... 
 
 . 345 
 
 Halmaturus, teeth of 
 
 . 429 
 
 Hare, teeth of . . . 
 
 . 367 
 
 Hatteria, teeth of . . 239, 243 
 
 Hedgehog, teeth of 
 
 . 394 
 
 Helodermis, teeth of . 
 
 . 242 
 
 Hesperornis, teeth of . 
 
 . 263 
 
 , Hinged teeth 
 
 . 202 
 
 Hipparion, teeth of 
 
 . 321 
 
 Hippopotamus, teeth of 
 Homalodontotherium 
 
 . 331 
 
 . 2S(; 
 
 Homologies of the teeth 
 
 . 278 
 
 : Horny teeth .... 
 
 . 3 
 
 Horse, teeth of . 
 
 . 321 
 
 Human teeth, forms of . 
 
 . 414 
 
 Huxley, Prof., special views on 
 
 development . . .15 
 
 2, 157 
 
 Hya>na, teeth of . . . 
 
 . 381 
 
 Hva;nodon, teeth of . 
 
 . 385 
 
 Hydrophis, teeth of 
 
 . 247 
 
 Hydromys, teeth of . 
 
 . 366 
 
 Hypsiprymnus, teeth of . 
 
 . 427 
 
 Hyrax, teeth of 
 
 ! I. 
 
 Ichthyosaurus 
 
 . 348 
 
 . 258 
 
 Ichthyornis, teeth of . 
 
 . 261 
 
433 
 
 Ig;uanodon, teeth of 
 Incisors, definition of . 
 
 ,, liuniiin, description of 
 Insectivora, teeth of . 
 
 ,, characteristic niolai 
 
 of 
 Internal pterygoid muscle 
 Interglobular spaces . 
 
 Kangaroos, teeth of 
 Kangaroo, enamel of 
 
 L. 
 
 403 
 
 •283 
 
 237 
 
 7G 
 
 74 
 
 60 
 
 241 
 
 228 
 
 •53 
 
 I.alnrintliodou, teeth of 
 
 ,, dentine of 
 
 Lacuna!, of cemcntum . 
 
 „ development of 
 
 „ encapsuled 
 
 in pits of enamel 
 
 ,, of Howship 
 Lanma, teeth of . 
 
 „ dentine of . 
 Lemurs, teeth of 
 
 ,, canines of 
 Lepidosiren, teeth of . 
 Lepidosteus, dentine of . 
 Leptothrix, in interglohu 
 
 spaces 
 Lines of Schreger . 
 Lizards, teeth of 
 Lophius, teeth of . 
 Lumen, appearance of 
 
 M. 
 
 ZSlachairodus, teeth of . . . 384 
 ;Mammalia, teeth of . 260 ct fcq. 
 „ tj'pical dentition of . 279 
 
 ^Mannuoth, tusks of 
 Man, teeth of . 
 
 „ teeth of difierent races 
 Manatee, teeth of 
 „ enamel of 
 ,, dentine of . 
 ^Mandible 
 
 Mark of horses' incisors 
 Marsupiatia, teeth of 
 
 ,, milk teeth of 
 
 ,, peculiar enam 
 
 iUasseter muscle . 
 Mastication, muscles of . 
 
 IXDEX. 
 
 
 PACE 
 
 
 |'a<;e 
 
 . 1'43 
 
 Mastodon, teeth of 
 
 . 357 
 
 . 280 
 
 „ molars of 
 
 . 360 
 
 ;» 
 
 milk teeth of . 
 
 . 359 
 
 . 394 
 
 Maxilbe, description of . 
 
 . 24 
 
 s 
 
 „ development and growth 
 
 . 400 
 
 of . . . 
 
 . 176 
 
 . 34 
 
 „ Y-shaped . 
 
 . 201 
 
 2, 74 
 
 Meckel's cartilage 
 
 . 177 
 
 
 Megatherium, dentine of 
 
 . 84 
 
 
 Membrana eboris . 
 
 158, 140 
 
 
 „ preformativa 
 
 . 171 
 
 
 Mental foramen, position of 
 Milk dentition, character of 
 
 . 183 
 
 . 430 
 
 . 297 
 
 . 53 
 
 „ rudimentarj' 
 
 . 301 
 
 
 Molars, definition of . 
 
 18, 280 
 
 
 Mole, teeth of 
 
 . 397 
 
 
 „ milk teeth of . 
 
 . 3f<9 
 
 
 Monkeys, teeth of . 
 
 . 407 
 
 . 240 
 
 Monodon, teeth of 
 
 . 309 
 
 . 80 
 
 Monotiemata, teeth of . 
 
 . 304 
 
 . 96 
 
 Muscles of mastication 
 
 . 33 
 
 . 168 
 
 iliislc deer, teeth of 
 
 . 336 
 
 . 169 
 
 Mustelidic, teeth of . 
 
 . 385 
 
 . 54 
 
 Myliobates, dentine of . 
 
 . 82 
 
 . 197 
 
 teeth of . 
 
 . 223 
 
 . 216 
 
 Mynnecobius, teeth of . 
 
 . 425 
 
 . 90 
 
 Myxiiie, teeth of 
 
 . 215 
 
 350 
 
 . . 1 
 
 of . 415 
 
 . 347 
 
 . . 45 
 
 83, 347 
 
 . . 30 
 
 . 289 
 
 . . 419 
 
 . 421 
 
 I of 53,423 
 
 . 34 
 
 . . 33 
 
 X. 
 
 Narwal 309 
 
 ,, teeth of . . . . ib. 
 
 Nasmyth's membrane, nature of . 99 
 
 Neck of enamel organ . . .132 
 
 „ of tooth . . . . 7 
 
 Nerves of dentine . . .71 
 
 „ of the pulp . . 71, 106 
 
 „ of the teeth ... 37 
 
 Neumann, sheaths of . . . 63 
 
 Newt, teeth of . . . .240 
 
 Oblique ridge of human molars . 19 
 Odontoblast cells . . 69, 158 
 
 Odontoptervx, teeth ot . . . 260 
 Odontorniths, teeth of . . 261 
 
 Ophidia, development of teeth of 125 
 Opossum, teeth of . . . 425 
 Orang, teeth of . . . . 410 
 Oreodon, teeth of . . . 332 
 ,, canines of . . . ib. 
 Oryeteropus, dentine of . .82 
 Osseous fish, teeth of . . . 224 
 Osteoblasts . . . .166 
 
INDEX. 
 
 439 
 
 Osteoclasts 197 
 
 Osteodentine . . . .88 
 ,, in tt'cth of rodents . 371 
 
 ,, in teeth of si)erni 
 
 whales . 92, 309 
 
 Ostracion, dentine of . . . 86 
 
 Otaria, erosion of teeth of . . 389 
 
 „ teeth of . . . . ib. 
 
 Papilla, formative . 
 
 PaiTot fish, teeth of . 
 
 Periosteum, alveolo-dental 
 
 I'erissodactj-le iiuy;ulata, teeth 
 
 Permanent teeth, eruption of 
 ,, „ development 
 
 Persistent dental capsule . 
 
 Phacochierus, teetli of . 
 
 Phalauger, teeth of 
 
 Pharyngeal teeth . 
 
 „ ,, of carp . 
 
 „ „ pike 
 
 ,, ,, racliiodon 
 
 ,, „ scarus . 
 
 rhofid;e, teeth of . 
 
 Pigment in enamel 
 
 Pig, teeth of ... 
 
 Pike, teeth of . . . '1 
 
 Plagiostomi, teeth of 
 
 Plagiaulas, teeth of . 
 
 Plicidentine . . . . 
 
 Poison fang, development of 
 „ mechanism of 
 
 ,, structure of 
 
 „ succession of 
 
 Poison gland 
 
 Porcupine, enamel of 
 
 Premolai-s, human 
 
 ,, detir.ition of . 
 
 Primates, teeth of 
 
 Pristis, teeth of . ; . 
 
 Proboscidea, teeth of . 
 
 „ affinities with i 
 
 dents . 
 
 Procvonida?, teeth of 
 
 Proteles, teeth of 
 
 Pseudoscarus .... 
 
 Pterodactyls, teeth of . 
 
 Pterosauria .... 
 
 Pulp, the .... 
 „ degeneration of 
 „ nerves of 
 „ vessels of . 
 
 Python, teeth of . 
 
 139 
 231 
 109 
 31.5 
 198 
 137 
 
 328 
 426 
 
 234 
 227 
 247 
 233 
 388 
 401 
 325 
 ,225 
 215 
 428 
 76 
 156 
 248 
 252 
 254 
 256 
 47 
 12 
 280 
 403 
 222 
 349 
 
 363 
 
 385 
 382 
 230 
 259 
 
 104 
 107 
 105 
 ib. 
 246 
 
 Rachiodon, teeth of . . . 247 
 Rattlesnake, teeth of . . . 248 
 Rhinoceros, teeth of . . . 316 
 Rhyncocephalus, teetli of . . 243 
 Rhytina, teeth of . . .347 
 Ridge fonnul-.e, of proboscidea . 362 
 Rodentia, teeth of . . . 364 
 „ milk teeth of . . . 366 
 ., enamel of . . 45, 371 
 Root membrane . . . . 109 
 Rostral teeth of s^w tish . . 222 
 Rudimentary teeth, examples of 
 
 267, 271,277,311,347,382,430 
 Rudimentary milk teeth. e.\- 
 
 amples of . 301, 399, 402, 423 
 Ruminants, teeth of . . . 334 
 „ absence of upper in- 
 
 Salivary glands . . . .37 
 Salmon, se.xual weapons of . . 236 
 Sargus, enamel of . . . 55 
 „ teeth of . . . . 235 
 Saurians, teeth of . . . 241 
 Saw-fish, teeth of . . . . 222 
 Scalpriform incisoi-s of rodents . 365 
 Scarus, teeth of . . . .231 
 Schreger, lines of . . . . 60 
 Seals, teetli of ... . 388 
 Second dentition . . . . 194 
 Secondary dentine . . .92 
 
 Selenodoiit 338 
 
 Serres, glands of . . . .108 
 Sexual weapons, teeth used as . 273 
 Sexual differences in teeth of 
 
 Babirussa 330 
 ,, ,, in teeth of 
 
 boar . 325 
 
 „ „ in teeth of 
 
 deer . 274 
 
 ,, „ in teeth of 
 
 dugong . 345 
 „ „ in teeth of 
 
 elephant . 350 
 „ ., in teeth of 
 
 horse . 320 
 „ „ in teeth of 
 
 monkeys 
 
 409, 413 
 „ „ in teeth of 
 
 narwal . 309 
 
INDEX. 
 
 P\GE 
 
 Sharks, development of teeth of . 117 
 
 „ teeth of . . . . 21.5 
 
 Sharper, fibres of . . . 98 
 
 Sheep's" head tish, teeth of . . 23-5 
 
 Shrews, teeth of . . . . 396 
 
 Simiina, teeth of . . . . 407 
 
 Sirenia, teeth of ... 344 
 
 Sloths, teeth of . . . . 306 
 
 Snakes, development of teeth of . 12-5 
 
 „ uon-venomous, teeth of . 244 
 
 „ colubrinc, poisonous . 217 
 
 „ viperme, poisonous . .218 
 
 Socketed teeth . . . .212 
 
 Sphenodon, teeth of . . . 243 
 
 Sphyraena, teeth of . . . 235 
 
 Stellate retifulum of enauael organ 
 
 131, 135, 156 
 Stratum intermedium of enamel 
 
 organ . . . 135 
 
 Stratum Malpighi, the . . . 108 
 
 Stria5 of enamel prisms . . 50 
 
 „ of Retzius . . . . 51 
 
 Succession of teeth in armadillo . 306 
 
 ,, „ in lizards . 239 
 
 „ „ in mammals . 297 
 
 ,, ,, in marsupials . 421 
 
 „ „ in osseous fish 121 
 
 ,, ,, in proboscidea 354 
 
 „ ,, in reptiles . 122 
 
 ,, ,, in sharks . 115 
 
 ,, „ in snakes . 125 
 
 ,, „ in poisonous 
 
 snakes . 253 
 
 Supernumerary teeth in dogs . 380 
 
 Sus babirussai teeth of . . . 330 
 Sus scrofa, teeth of . . .325 
 
 Tapir, teeth of . . . . 316 
 Tarsiijes, teeth of . . . 432 
 Teething . . . . . 182 
 
 Teeth, equivalent to dermal spines 2 
 Teleostei, teeth of . . . 224 
 „ development of teeth of 120 
 Temporal muscle, action of . . 32 
 Temporary teeth, eruption of . . 193 
 Tetrodon,"teeth of . . . 231 
 Theriodonts, teeth of . . . 259 
 Thvlacinus, teeth of . . . 422 
 'Ihylacoleo, teeth of ... 428 
 
 PAOE 
 
 Tillodonts, teeth of . . . 342 
 Tillctlicrium, teeth of . . . 343 
 Tallies' til.rils . . . . 64, 68 
 Tomes' i)roresscs of enamel cells . 153 
 
 j-ooiii, ueuiiiiion oi 
 Tooth sac 
 
 
 142 
 
 Tooth germ 
 
 
 113 
 
 Tortoises, teeth of . 
 
 
 241 
 
 Toxodon, teeth of 
 
 
 339 
 
 Trichechus, teeth of 
 
 
 391 
 
 Tusks of wild boar 
 
 
 325 
 
 „ of elephant, foreign 
 
 iodic. 
 
 
 in . 
 
 
 352 
 
 Typical tooth 
 
 • 
 
 27S 
 
 U. 
 
 Ungulata, teeth of . 
 
 
 314 
 
 ,, molar patterns 
 
 jf 
 
 319 
 
 Yampire, teeth of 
 
 . 401 
 
 Varan us, teeth of . 
 
 . . 242 
 
 „ dentine of . 
 
 . 76 
 
 Vaso-dentiue 
 
 . 82 et seq. 
 
 Viper, teeth of . 
 
 . . 248 
 
 succession of teeth in 
 Viverrida?, teeth of 
 
 Walrus, teeth of . 
 
 . . 
 
 391 
 
 Wart-hog, teeth of . 
 
 
 328 
 
 AVhaleboue . 
 
 311 
 
 ,312 
 
 AVhale, rudimentary teeth of 
 
 
 311 
 
 Wisdom teeth 
 
 
 22 
 
 ,, of lower races 
 
 of 
 
 
 man 
 
 
 416 
 
 „ of monkeys . 
 
 410 
 
 415 
 
 Wolf-fish, teeth of . " . 
 
 
 229 
 
 Wombat, teeth of . 
 
 
 432 
 
 „ enamel of 
 
 
 53 
 
 Wrasse, teeth of . 
 
 
 235 
 
 Ziphoid cetacca, teeth of 
 
 BRADBOnV, 
 
 iNEW, <fe CO., PUINTERS, WHITEFmARS. 
 
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