THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA PRESENTED BY PROF. CHARLES A. KOFOID AND MRS. PRUDENCE W. KOFOID k-RK50 H8 1865 F Biology Library KofoUi >s\^ THE NATURAL HISTORY OF * THE HITMAN TEETH EXPLAINING THEIR STRUCTURE, USE, FORMATION, GROWTH, AND DISEASES. IN TWO TARTS. By JOHN HUNTER, F.R.S. WITH NOTES BY FRANCIS C. WEBB, M.D., F.L.S., MEMBER OF THE ROYAL COLLEGE OF PHYSICIANS, LONDON, PHYSICIAN TO THE GREAT NORTHERN- HOSPITAL, ETC. ; AND ROBERT T. HULME, M.R.C.S., F.L.S., LECTURER ON DENTAL SURGERY AT THE METROPOLITAN SCHOOL OF DENTAL SCIENCE, AND DENTAL SURGEON TO THE NATIONAL DENTAL HOSPITAL. LONDON: ROBERT HARDWICKE, 192 PICCADILLY. MDCCCLXV. M'GOWAN AND DANKS, PRINTERS, GREAT WINDMILL STRKET, HAYMARKET, LONDON NOTICE. The present edition of Hunter on the Teeth originally appeared m the pages of the "Dental Eeview" for the years 1861 — 63. The Notes appended to the First Part were furnished by Dr. "Webb, and formed the basis of a Course of Lectures on the Comparative Anatomy of the Teeth, delivered to the Students of the Metropolitan School of Dental Science. Such notes were necessarily a work of compilation rather than of original research, and have been principally derived from the rich stores of infor- mation to be found in the works of Owen, Tomes, Nasmyth, and Kolliker. Obligations to these and other authors are duly acknowledged. The Notes to the Second Part were furnished by Mr. Hulme, and are restricted to short comments on Hunter's views. To have attempted more, and to have brought this portion of the work up to the present advanced state of Dental Surgery, would have necessitated the writing an entire treatise on the subject. In the present edition, the Annotators' Notes are distinguished from those of the Author by alphabetical references and by being placed within brackets. CONTENTS. PART I. PAGE Of the Upper Jaw 1 Lower Jaw ....... 3 Alveolar Processes ..... 7 Articulation of the Lower Jaw 9 Motion in the Joint of the Lower Jaw . 13 Muscles of the Lower Jaw .... . 16 Temporals ....... . 19 Internal Pterygoid Muscle .... 22 External Pterygoid Muscle .... 22 Digastric Muscle 2G Structure of a Tooth ..... 34 Enamel ........ . 34 Bony Part of a Tooth . 44 Cavity of the Teeth . . 53 Periosteum of the Teeth 55 Situation of the Teeth . 5G Number of Teeth ...... Incisores ....... . Gl Cuspidatus ....... . 70 Bicuspides ....... 78 Grinders ....... 81 Articulation of the Teeth .... . 105 Gums ........ . 10G Action of the Teeth arising from the Motion of th j Lower Jaw ........ . 107 General Comparisons between the Motion of the Jaw ii i Young and in Old People . 109 Of the Formation of the Alveolar Process . . 110 Formation of the Teeth in the Foetus . 115 Cause of Pain in Dentition .... . 120 Formation and Progress of the Adult Teeth . 121 Manner in which a Tooth is formed . 127 Ossification of a Tooth upon the Pulp . 129 Formation of the Enamel .... . 135 Manner of Shedding the Teeth . 140 VI CONTENTS. 01' the Growth of the Two Jaws The Reason for the Shedding of the Teeth Of the Cavity filling up as the Teeth wear down Continual Growth of the Teeth Sensibility of the Teeth .... Supernumerary Teeth .... The Use of the Teeth so far as they affect the Voice Of the Class among Animals of the Human Teeth Diseases of the Teeth .... Cleaning the Teeth .... Transplanting the Teeth 141 143 145 146 149 149 151 152 153 154 15G PART II. Introduction. Chap. I. — Of the Diseases of the Teeth and the Consequences op them . . . . . . m .101 1. The Decay of the Teeth arising from Rottenness — Symptoms of Inflammation — Stopping of the Teeth.— 2. The Decay of the Teeth by Denudation. — 3. Swelling of the Rang.— 4. Gum Boils. — 5. Excrescences from the Gum. — 6. Deeply- seated Abscesses in the Jaws. — 7. Abscess of the Antrum Maxillare. Chap. II. — Of the Diseases of the Alveolar Processes and the Consequences of them 199 III. — Of the Diseases of the Gums and the Conse- quences of them 204 1. The Scurvy of the Gums, vulgarly so-called. — 2. Callous Thickenings of the Gums. Chap. IV. — Of Nervous Pains in the Jaws .... 208 V. — Of the Extraneous Matter upon the Teeth . 210 VI. — Of the Irregularity of the Teeth . . .215 VII. — Of Irregularities between the Teeth and Jaw . 221 Of Supernumerary Teeth . . . . . .221 VIII.—Of the Under Jaw 222 IX. — Of Drawing the Teeth ..... 224 Transplanting Teeth — Of the State of the Gums and Sockets — Age of the Person who is to have the Scion Tooth — the Scion Tooth — Replacing a Sound Tooth when drawn by mistake — Transplanting a Dead Tooth — The Immediate Fastening of a Transplanted Tooth. Chap. X. — Of Dentition ....... 242 Of the Cure — Of Cutting the Gums — Cases. INDEX. Abscess of Antrum, 196, N. tv, 198. Alveolar Abscess, N. s, 192. Processes, 3, 4, 7. Characters of, in African Negro, N. t", 5. Diseases of, 199. Formation of, 110, N. s, 111. Antrum Maxillare, Abscess of, 196, N. w, 198. Bicuspides, 78. Bone, 34. Bony Part of a Tooth, 44. Canines, Comparative Anatomy of, N. g, 70. Uses of, N. i, 77. Cases, 249. Cavity of the Teeth, 53. Cement, N. I, 34. Composition of, N. I, 37. Condyle, 6. Characteristics of, in Mammalia, N. j, 6. Coronoid Process, 6. Crusta Petrosa, N. I, 34, Cuspidatus, 70. Decay of the Teeth, 161. By Denudation, 181, N. o, 183. Theories respecting, N. e, 166. Dental Pulps or Bulbs, Origin of, N. v, 115. Dental Tissues, N. I, 34. Canals, N. q, 45 Dentinal Pulp, N. v, 54. Uses of, N. u, 53. Dentine, N. I, 34. Composition of, N. r, 50. Structure of, N. q, 45. Dentition, 242. Cause of Pain in, 120. Digastricus, 26. Comparative Anatomy of, N. d, 27. Diseased Teeth, Changes in, 161, N. a, 161. Treatment of, 175. 11 . INDEX. Enamel, 34, 40, N. n, 40. Composition of, N, p, 43. Formation of, 135, N. /,136. Epulis, X. 2, 2i>7. Exostosis, X. s, 51. Extraction of Teeth, 224. Hemorrhage after, 226, N. d, 227. Fang, Swelling of, 184. Glenoid Cavity, 9, X. ;;, 9. Boundaries of, 10. Comparative Anatomy of, X. p, 9 Grinders, 81. Comparative Anatomy of, X. o. 81. Gum Boil, 186. Treatment of, 190. Gums, 106, X. r, 106. Diseases of, 204. Excrescences from, 193. Lancing of, 247, X. g, 247. Thickenings of, 206. Incisores, 61, X. d, 61. Comparative Anatomy of, X. d, 61. Interarticular Cartilage, 11, X. s, 11. Intermaxillary bones, X. a, 1. Jaw Bone, Lower, 3. Upper 1. Jaws, Abscess in, 194. Growth of, 141. Movements of, in Young and Old People, 109. Xervous Pains in, 208. Lower Jaw, 3. Articulation of, 9. Interarticular Cartilage of, 11. Motion in the Joint of, 13. Muscles of, 16. Ossification of, 4. Masseter Muscle, 17. Comparative Anatomy of, X. y, 18. Maxillary Bones, Superior, X. a, 1. Xeuralgia, X. a, 209. Osteo-Dentine, X. I, 34. Palate, Breadth of, 3. Part II. — Introduction, 159. Periosteum of the Teeth, 55. Premaxillary Bones, X. a, 1 . Premolars, X. j, 78. Comparative Anatomv of, X. o, 81. Pterygoid Muscles, 22. Comparative Anatomy of, X. b, 26. INDEX. Pterygoideus Extermis, 22. Interims, 22. Scurvy in the Gums, 204, N. y, 206. Smith, Dr. John, on the Teniporo-Maxillary Joint, N. x, 14. Sound Teeth, Replacement of, 234, N. e, 236. Spina Ventosa, 51, 184, N. p, 185. Stopping Teeth, 178. Material used for, 179, N. /, 180. Suture-Maxillo-intermaxillary, N. a, 1. Tartar, N. r, 155. Composition of, N. b, 214. Teeth, Action of, 107. Articulation of, 105. Characters of, in Man, 5, 6. Cleaning of, 154. Continual Growth of, 146. Decay of, 161. Diseases of, 153, 159, 161. Drawing of, 224. Extraneous Matter upon, 210. Formation of, 115. Formula of, in Man, N. b, 59. Human, in Relation to Voice, 151. Under what Clas3 they come, 152. Inflammation of, 171. Irregularity of, 215. Liability to Decay, N. r, 188. Names of, 58. Number of, 57, N. a, 57. Permanent Formation of, 121, 127. Eruption of, N. c, 125. Sensibility of, 149. Shedding of, 146. Situation of, 56, N. z, 56. Stopping of, 178. Supernumerary, 149, 221, N. o, 150. Temporary, when cut, N. y, 120. Transplanting of, 156, 230, N. s, 157. Teeth and Jaw, Irregularities between, 221. Temporal Muscle, 1 9. Comparative Anatomy of, N. z, 21. Temporo-Maxillary Articulation, 13, N. w, 13. Tooth Cavity, Filling up of, 145, N. m, 145. Definition of, 34. Ossification of, 129, N. i, 129. Structure of, 34. Transplanted Teeth, Fostering of, 237. Transplanting of Teeth, 156, 230, N. s, 157,/. 241. Dead Teeth, 237. Under Jaw, 222. Vaso -dentine, 34 Wisdom Teeth, Cutting of, 248, N. h, 248. ERRATA. Page 102 -24th line from top— for p. 192, Vol. III., read p. 35. 122 — 4th line from bottom — for note v, p., read note v, p. 115. 128 — 8th line from bottom — for note v, p. 337, read note y, p. 55. 141— last line— for p. 343, read p. 121. 184 — at foot note — for p. 37, read p. 51. 197 — 15th line from top -for finus, read sinus. PART THE FIRST. THE NATURAL HISTORY OF THE HUMAN TEETH. OF THE UPPER JAW. Befoee we enter into a description of the Teeth themselves, it will be necessary to give an account of the Upper and Lower Jaw-bones, in which they are inserted ; insisting minutely on those parts which are connected with the Teeth, or serve for their motion and action, and passing over the others slightly. The Upper-Jaw is composed of two bones, (a) which generally (a) [The superior maxillary bones (maxillae super lores). In addition, it may be stated that the palatine bone on each side is generally reckoned as constituting part of the upper jaw, although in the cavity of the mouth it is confined to the palate, and forms no portion of the alveolar process. In the class Mammalia, the upper jaw consists of three bones on each side,— the palatine, superior maxillary, and inter- or pre- maxillary bones. The last-named bones constitute the anterior portion of the jaw, and form the alveoli of the superior incisors. Whatever modification in size or shape the teeth implanted in the premaxillary bones may display, they are, nevertheless, classed as incisors in the dental formulae of naturalists. Anchylosis between the superior and premaxillaries takes place sooner or later in life in the typical Quadru- niana. In the Chimpanzee, the facial portion of the maxillc-inter- maxillary suture disappears at or about the period of the first dentition ; in the Orangs, the suture remains until the development of the great canine teeth ; in the Semnopitheci and Inui, the separation continues 13 Z OF THE UPPER-JAW. remain distinct through life, (b) They are very irregular at their posterior and upper parts, sending upwards and backwards a distinct until the period of old age. In Man, on the contrary, the pro- cess of anchylosis (with the exception of a portion of the suture on the palate, which usually remains until after birth) is com- pleted before the termination of intra-uterine existence. It is, there- fore, only in the fcetal condition that Man can be said to possess an intermaxillary bone ; and even this has been denied by many writers, on the ground that the fissure of separation cannot be traced through the external alveolar plate. On the other hand, analogy, the constant existence of the nasal and palatal portions of the maxillo-interrnaxillary fissure in intra-uterine life, and the entire separation of the incisive portion of the superior maxillary bone in certain cases of deformity (double hare-lip), are strong arguments for admitting the premaxillary as an original element of the human skull. Direct observation appears, however, to have definitively settled the question. Some years since Dr. Leidy announced to the Academy of Natural Sciences of Philadelphia (1) that he had found the inter- maxillary entirely separable in an embryo of one inch eleven lines in length. The division in this case was traceable through the alveolar ridge at a point corresponding to the separation between the incisor and canine alveoli. Figures of the preparation tf ken from those published in the Proceedings of the Academy will be found amongst the Engravings which illustrate this work. The more recent researches (2) of Robin and Magitot strongly confirm Dr. Leidy's observation, these observers have discovered the existence of a separate formative cartilage for each premaxillary, in which an osseous point is developed two or three days after the first deposition of ossific matter in the alveolar edge of the formative cartilage of the superior maxillary. It is stated by Scemmerring, that vestiges of the maxillo-inter- maxillary suture are to be noticed in Negro crania. Dr. Prichard long since denied any constancy of difference between the melanous and leucous varieties in this respect, and extended observation will verify bis statement. In skulls of all nations, it is not very uncommon to rind Si .me small vestige of the suture both on the palate and in the floor of the nares, extending outwards from the anterior palatine canal ; more rarely it may be traced on the palate to within a short distance of the alveolus. An instance of the permanency of any portion of the suture on the face has never fallen under the Editor's notice.] (b) [Anchylosis between the superior maxillary bones is exceptional in (1) Proceed. .Acad. Nat. Sciences, Philadelphia, Jan., 1849, p. 145. (2) Vide Journal de la Physiologic Edited by Dr. Brown-Sequard. Janvier, 1860, p. G. OF THE LOWER-JAW. 3 great many processes, that are connected with the bones of the Face and Skull. The lower and anterior parts of the Upper- Jaw are more uniform, making a kind of a circular sweep from side to side, the convexity of which is turned forwards ; the lower part terminates in a thick edge, full of sockets for the Teeth. This edge is called in each bone the Alveolar Process. Behind the Alveolar Processes there are two horizontal lamellae, which uniting together, form part of the roof of the Mouth, which is the partition between the Mouth and the Nose. This plate, or partition, is situated about half an inch higher than the lower edge of the Alveolar Process ; and this gives the roof of the Mouth a considerable hollowness. (c) The use of the Upper-Jaw is to form part of the Parietes of the Mouth, Nose, and Orbits ; to give a basis, or supply the Alveolar Process, for the superior row of Teeth, and to counter- act the Lower-Jaw ; but it has no motion itself upon the bones of the Head and Face.(d) OF THE LOWER-JAW. As the Lower-Jaw is extremely moveable, and its motion is indispensably necessary in all the various operations of the Man. Obliteration of the facial portion of the mesian premaxillary suture commonly takes place as life advances in the Anthropoid Apes.] (c) [The hollowness and breadth of the palate vaiy in individual crania. It will be also found of greater comparative length in prognathic skulls. In the larger number of Australian crania, and in those of other dark-skinned races which approach the Australo-Tasmanian type, the bony palate is relatively of great breadth and length.] (d) [The fixity of the bones constituting the upper jaw is a character common to Mammalia, and contrasts with the moveable condition of the upper mandible in birds — the distensibility resulting from the elastic ligamentous connection which subsists between the premaxillary and maxillary bones in the constricting serpents — the moveable articulation of the superior maxillary in the poisonous serpents'— and with the loose connection of the corresponding bones in fishes, which permits, in addition to the ordinary movements, those of protrusion and retraction.] 4 OF THE LOWER-JAW. Teeth, it requires to be more particularly described. It is much more simple in its form than the Upper, having fewer processes, and these not so irregular. Its anterior circular part is placed directly under that of the Upper- Jaw; but its other parts extend farther backwards. This Jaw is at first composed of two distinct bones ; (e) but these, soon after birth, unite into one, at the middle of the chin. (/) This union is called the Symphysis of the Jaw. Upon the upper edge of the body of the bone is placed the Alveolar Pro- cess, a good deal similar to that of the Upper-Jaw. The Alveolar Process extends all round the upper part of the bone, () [It is evident that the description in the text refers only to the articulating portion of the glenoid cavity, or that anterior to the Glaserian fissure. The posterior portion of the cavity lodges a process of the parotid gland, and does not enter into the articulation. In Man, as in all other Mammalia, the lower jaw articulates with the squamous element of the temporal bone ; in them the tympanic element is solely subservient to the organ of hearing. But this is not the case in the other vertebrate classes. In birds, reptiles, and osseous fishes, the mandible articulates with bones, which are the homologues of the i\ oapanic ring in Man. In cartilaginous fishes, the articulating surface is formed by the pterygoid bone, the homologue of the internal pterygoid plate in the human subject. In the Lepidosiren, which in other re- c 10 OF THE ARTICULATION OF THE LOWER-JAW, eminence, (q) placed in the same direction, convex upon the top, in the direction of its shorter axis, which runs from behind for- wards ; and a little concave in the direction of its longer axis, which runs from within outwards. It is a little broader at its outer extremity ; as the outer corresponding end of the Condyle describes a larger circle in its motion than the inner. The surface of the cavity, and eminence, is covered with one con- tinued smooth cartilaginous crust, (r) which is somewhat liga- mentous, for by putrefaction it peels off, like a membrane, with the common Periosteum. Both the cavity and eminence serve for the motion of the Condyle of the Lower-Jaw. The surface of the cavity is directed downward ; that of the eminence down- ward and backward, in such a manner that a transverse section of both would represent the Italick letter S. Though the emi- spects presents a curious association of the characters of the osseous and cartilaginous fishes, the pterygoid hone contributes the inner, the tympanic the outer portion of the recipient articular surface. (1) The articulating portion of the glenoid cavity is hounded posteriorly in the Anthropoid Apes, and generally in the lower Mammalia, by a prominent ridge or process. In the Rodentia, a similar ridge bounds the articulating cavity internally. In Man this posterior boundary is absent, or but slightly indicated. The downward development of the human cranium posterior to the articulating surface, affording, as it does, a support against backward dislocation, obviates the necessity for the development of a post-glenoid process. (2) When its rudiment exists in Man, it is known as the middle root of the zygoma. I believe that the Author of the article Temporo-Maxillary Articulation is mis- taken when he asserts that this process is more frequently indicated in the lower races of mankind. I have seen it at least as frequently in the skulls of the natives of Europe and Asia as in African and Austra- lian crania.] (q) [Anterior root of the zygoma.] (■/•) [It is only that portion of the glenoid cavity concerned in the articulation of the joint which is covered with cartilage.] (1) Owen, Trans. Lin. Soc, vol. xviii., p. 336. Article Temporo-Maxillary Articulation, in Todd's Cyclopaedia of Anatomy, by S. R. Pittard. (2) Owen, Zool. Trans., vol. i., p. 340. OF THE ARTICULATION OF THE LOWER-JAW. 11 nence may, on a first view of it, appear to project considerably below the cavity, yet a line drawn from the bottom of the cavity, to the most depending part of the eminence, is almost hori- zontal, and therefore nearly parallel with the line made by the grinding surfaces of the Teeth in the Upper- Jaw: and when we consider the Articulation farther, we shall find that these two lines are so nearly parallel, that the Condyle moves almost directly forwards, in passing from the cavity to the eminence ; and the parallelism of the motion is also preserved by the shape of an intermediate cartilage. In this joint there is a moveable cartilage, (s) which, though common to both Condyle and cavity, ought to be considered rather as an appendage of the former than of the latter, being more closely connected with it ; so as to accompany it in its motion along the common surface of both the cavity and eminence. This cartilage is nearly of the same dimensions with the Condyle, which it covers; is hollowed on its inferior sur- face, to receive the Condyle : (t) on its upper surface it is more unequal, being moulded to the cavity and eminence of the articulating surface of the Temporal Bone, though it is con- siderably less, and is therefore capable of being moved with the Condyle, from one part of that surface to another. Its texture is ligamento-cartilagineous. This moveable cartilage is con- nected with both the Condyle of the Jaw, and the articulating surface of the Temporal Bone, by distinct ligaments, arising from its edges all round, (u) That by which it is attached to the (s) [The inter-articular fibre-cartilage. It is the only inter-articular fihro-cartilage into which muscular fibres are inserted — viz., a portion of the external pterygoid, which it receives along its anterior border, ami which is instrumental in moving the fibro-cartilage in the antero- posterior motion of the condyle. The inter-articular fibro-cartilage is constantly found in animals which suckle their young, but not in the inferior vertebrata.] (t) [The inter-articular fibro-cartilage is thicker at the edges than in the centre ; it is not unfreijuently perforated in the latter situation.] (u) [Thin and short ligamentous fibres [memhrawx articularis, C2 12 OF THE ARTICULATION OF THE LOWER- JAW. Temporal Bone, is the most free and loose ; though both liga- ments will allow an easy motion, or sliding of the cartilage on the respective surfaces of the Condyle, and Temporal Bone. These attachments of the cartilage are strengthened, and the whole articulation secured, by an external ligament, -which is common to both, and which is fixed to the Temporal Bone, and to the neck of the Condyle. On the inner surface of the liga- ment, which attaches the cartilage to the Temporal Bone, and backwards, in the cavity, is placed what is commonly called the Gland of the Joint ; at least, the ligament is there much more vascular than at any other part, (v) Weitbrecht) surround the greater portion of the joint, cover the synovial membranes, and serve to connect the fibro-cartilage with the osseous margins. These thin ligamentous structures are not distinguished in the text from the synovial bursae, neither is it usual to find the latter noticed specially in anatomical works of the period. They had been, however, described at some length by Weitbrecht, whose work appeared in 1742. (1) They are two in number — one placed above the fibro-cartilage, the other below it. The superior is the larger and looser ; it lines the upper surface of the inter- articular cartilage and the smooth part of the glenoid cavity. The inferior is interposed between the condyle and the lower surface of the fibro-cartilage. When the latter structure is perforated in the centre, the synovial bursa) communicate and form one cavity.] (c) [Dr Clopton Havers,(2) in 1G91, described the vascular processes of the synovial membrane, which are commonly found projecting more or less into the cavities of joints. When these processes are of any size, they generally contain fat. They are frequently cleft, so as to present a fringed appearance at their free border, which is very vascular. Dr Havers regarded them as special structures for the secretion of synovia, and named them the "mucilaginous glands of the joints." Subsequent anatomists have generally denied them a special function, although, as extensions of the synovial membrane, they must necessarily increase the amount of secretion. Havers's view was revived by Mr Rainey, in a paper published in the Proceedings of the Royal Society, in 1846. He bases his advocacy on the constant occurrence of such vascular processes, not only in the joints, but also in synovial sheaths, on a peculiar convo- luted condition of their blood-vessels, and on the arrangement of the epithelium covering them, which, " besides enclosing separately each (1) Syndesmologia, p. 80, 1742. (2.) Ostcologia Nova, p. 187, 1G91. OP THE MOTION IN THE JOINT OF THE LOWER-JAW; 13 OF THE MOTION IN THE JOINT OF THE LOWEll-JAW. The Lower-Jaw, from the manner of its articulation, is sus- ceptible of a great many motions, (w) The whole Jaw may be brought horizontally forwards, by the Condyles sliding from the cavity towards the eminences on each side. This motion is per- formed chiefly when the Teeth of the Lower-Jaw are brought directly under those of the Upper, in order to bite, or hold any thing very fast between them. packet of convoluted vessels, sends off from each tubular sheath second- ary processes of various shapes, into which no blood-vessels enter." Mr Kainey's account of the structure of these processes has since been confirmed in most particulars by Kolliker. (1)] (w) [The form of the osseous surfaces entering into the temporo- maxillary articulation, and the diversities of motion which it admits, are so modified to suit the functions and necessities of the differ- ent families of Mammalia, that there is no portion of the skeleton calculated to yield more certain information to the naturalist. In the frugivorous Quadrumana the articulation is loose, as in Man, and permits within certain limits each kind of movement— viz., in the vertical, the anteroposterior, and lateral directions. The principal differences between bimanous and quadrumanous structure in this joint are the somewhat greater flatness of the glenoid surface, the minor development of the anterior articular eminence, and the constancy and size of the post-glenoid ridge in the latter. In the Carnivora the con- dyle of the lower, jaw is an oblong cylindroid process of considerable length, placed transversely, almost, if not quite, in a straight line with its fellow of the opposite side. The receiving surface is bounded anteriorly and posteriorly by salient ridges, which increase its depth, and limit the chief motion of the jaw to the vertical direction, per- mitting only in addition a slight lateral gliding of the condyles. The development of the anterior ridge varies in different species. In the badger both processes project in such a manner as almost to embrace the con- dyle, so that in the skeleton the inferior maxilla is retained in situ without any artificial fastening. In the placental Rodents (the Hares excepted), on the other hand, the glenoid cavity is a deep groove ex- cavated longitudinally under the base of the zygoma, and corresponds with the long diameter of the usually oval condyle, which is set antero- posteriorly instead of from side to side. In accordance with this con- formation, the conspicuous movement of the lower jaw in the Rodents is in the anteroposterior direction. The "nibbling" motion allowed is in (1) Vidt Quams Anat, sixth edit., vol. i., p. ccxhx. 14 OF THE MOTION IN THE JOINT OF THE LOWER-JAW. Or, the Condyles only may be brought forwards, while the rest of the Jaw is tilted backwards, as in the case when the Mouth is open ; for on that occasion the angle of the Jaw is tilted backwards, and the chin moves downwards, and a little backwards also. In this last motion, the Condyle turns its face a little forwards ; and the center of motion lies a little below the Condyle, in the line between it and the angle of the Jaw. By such an advancement of the Condyles forwards, together with the rotation mentioned, the aperture of the Mouth may be considerably enlarged; a circumstance necessary on many obvious occasions. The Condyles may also slide alternately backwards and for- wards, from the cavity to the eminence, and vice versa ; so that while one Condyle advances, the other moves backwards, turning the body of the Jaw from side to side, and thus grinding, be- tween the Teeth, the morsel separated from the larger mass by the motion first described, (x) In this case, the center of motion lies exactly in the middle between the two Condyles. And it is to be observed, that in these slidings o* the Condyles forwards exact conformity with the structure of their teeth and the disposition of the muscles of mastication. In the Ruminantia, again, we find both glenoid surface and condyle almost flat : the former is bounded behind by a transverse crest which passes inwards from the zygoma, but has no corresponding anterior limitation. Such a joint is admirably adapted for an extensive lateral movement of the jaw in chewing the cud, whilst the power of gaping is proportionately limited. (1)] (.r) [In a paper read before the Royal Society of Edinburgh, Dr John Smith has advanced the following theory of the mechanism of the Temporo-Maxillary Joint. He considers that in the movement of simply opening and shutting the mouth, "the condyles cannot act as a simple hinge, as they lie, not at right angles to the plane of motion of the lower jaw, but obliquely to it, each condyle looking inwards and forwards. Their more perfect action, therefore, cannot occur in this movement, but seems to belong to that of mastication. The articulating surface, strictly speaking, on each condyle appears to constitute the (1) Vide Cuvier, Le?ons d'Anatomie Comparee, Tome iv., P. 1, p. 36, edit. 1835 ; Todd's Cyclopaedia of Anatomy, Article Tem^ioro-Maxillary Articulation. OF THE MOTION IN THE JOINT OF THE LOWER-JAW. I •"> and backwards, the moveable cartilages do not accompany the Condyles in the whole extent of their motion ; but only so far as to adapt their surfaces to the different inequalities of the Temporal Bone : for as these cartilages are hollow on their lower surfaces where they receive the Condyle, and on their opposite upper surfaces are convex where they lie in the cavity ; but for- wards, at the root of the eminence, that upper surface is a little hollowed ; if they accompanied the Condyles through the whole extent of their motion, the eminences would be applied to the eminences, the cavities would not be filled up, and the whole articulation would be rendered very insecure. This account of the motion of the Lower-Jaw, and its car- tilages, clearly demonstrates the principal use of these cartilages ; namely, the security of the articulation ; the surfaces of the cartilage accommodating themselves to the different inequalities, in the various and free motions of this joint. This cartilage is also very serviceable for preventing the parts from being hurt by the friction ; a circumstance necessary to be guarded against, thread, or rather part of the thread, of a conical screw passing over an axis lying at or about right angles to the plane of motion in simple opening and closing of the jaws. This spiral course of the articular surface is perhaps best seen in some of the large Carnivora, such as the lion, but is also obvious in a well-developed human condyle. " The action of this conical screw or tap within the glenoid cavity, con- sidered as the conical die, takes place with accuracy only when one joint alone acts with the condyle within the glenoid cavity — the other condyle being beyond it, and gliding on the surface of the zygoma, as during mastication. The food is in this process crushed between the molar teeth of that side whose condyle remains within the glenoid cavity, this condyle screwing the jaw back, so to speak, to its natural position at each closure of the teeth. "By this construction a great amount of friction is avoided; what would otherwise be a rubbing being thus converted into a rolling motion between the condyloid and glenoid surfaces ; while, by one or other con- dyle always remaining in the glenoid cavity during mastication, greater steadiness and security is afforded to the joint." (1) It need scarcely be remarked here, that however the above explanation (1) Edinb. riiilosopli. Journal, N.S., vol. vi 16 OF THE MUSCLES OF THE LOWER- JAW. where there is so much motion. Accordingly, I find this car- tilage in the different tribes of Carnivorous Animals, where there is no eminence and cavity, nor other apparatus for grinding ; and where the motion is of the true ginglimus kind only. In the Lower-Jaw, as in all the joints of the body, when the motion is carried to its greatest extent, in any direction, the muscles and ligaments are strained, and the person made uneasy. The state, therefore, into which every joint most naturally falls, especially when we are asleep, is nearly in the middle, between the extremes of motion ; by which means all the muscles and ligaments are equally relaxed. Thence it is, that commonly, and naturally, the Teeth of the two Jaws are not in contact ; nor are the Condyles of the Lower-Jaw so far back in the Temporal Cavities as they can go. OF THE MUSCLES OF THE LOWER-JAW. Having described the figure, Articulation, Motion, and use of the Lower-Jaw, it will be necessary, in the next place, to give some account of the Muscles that are the causes of its motion. There are five pair of Muscles, each of them capable of pro- ducing various motions, according to the situation of the Lower- Jaw, whether they act singly, or in conjunction with others ; and two or more of them may be so situated, as to be capable of moving the Jaw in the same direction ; and every motion is produced by the action of more than one Muscle at a time. Thus, if the Jaw is depressed, and brought to one side, either the Masseter, Temporal, or Pterygoidgeus internus of the opposite may be applicable to the movements of the condyle in the human sub- ject, it cannot be extended to the motion of the joint in the large Carnivora ; in which animals, nevertheless, a spiral direction of the articular surface is well seen. This spiral appearance of the articular surface in the Felidce is clearly dependant on the disposition of the anterior and posterior articular processes, the post-glenoid being most developed internally, whilst the anterior ridge is necessarily more external to permit the play of the contiguous coronoid process. f OF THE MUSCLES OF THE LOWER- JAW. 17 side will not only raise the Jaw, but bring it to its middle state. It will be necessary in the description of each Muscle, to give its use in the different situations of the Jaw ; by which means, after they are all described, their compound actions will be better understood. I shall first describe those which raise the Jaw ; then those which give it the lateral motion ; and lastly, those which depress it ; proceeding in each class as they rise in dissection. The most superficial is the Masseter : it is situated upon the posterior and lower part of the Face, between the cheek-bone, and angle of the Lower-Jaw, directly before the lower part of the Ear. It is a thick, short, complex Muscle, and a little flattened : it appears to have two distinct origins, an anterior outer, and a posterior inner ; but that is owing only to its outer edge at its origin being slit, or double ; and the fibres of these two edges having a different course, decussating each other a little. The anterior, and outer portion of the Muscle begins to rise from a small part of the lower edge of the Malar Process of the Maxillary Bone, adjoining to the Os Malse ; and con- tinues its origin all along the lower horizontal edge of this last bono, to the angle where its Zygomatic Process turns up, to join that of the Temporal Bone. The external layer of fibres in this portion are tendinous at their beginnings, while the internal are fleshy. The posterior and inner portion of this Muscle begins to rise partly tendinous, and partly fleshy, from the same lower edge of the Os Malse ; not where the origin of the other portion ter- minates, but a little farther forwards ; and this origin is con- tinued along the lower edge of the Zygomatic Process of the Temporal Bone, as far backwards as the eminence belonging to the articulation of the Lower-Jaw. From this extent of its origin, the Muscle passes downwards to its insertion into the Lower- Jaw. The anterior external por- tion is broader at its insertion than at its origin ; for it occupies a triangular space of the Lower-Jaw above the angle, and on the outside, of about an inch in size, to about an inch and a 18 OF THE MUSCLES OF THE LOWER-JAW. half from the angle towards the Chin. In consequence of this extent of insertion, the fibres of this portion divaricate very considerably. They are mostly fleshy at their insertion, a few only being tendinous, particularly those that are inserted back- wards. The posterior and inner portion of the Masseter is narrower at its insertion than at its origin ; its posterior fibres running forwards, as well as downwards, while its anterior run almost directly downwards. It occupies in its insertion the remaining part of the scabrous surface, above the angle of the Lower-Jaw, which lies between the anterior portion and the two upper processes, viz., the Condyle and Coronoide. As the anterior fibres of this portion rise on the inside of the posterior fibres of the other portion ; and as its posterior fibres run forwards as well as downwards, and its anterior run almost directly downwards, while the fibres of the other portion radiate both forwards and backwards ; these two portions in some measure decussate, or cross one another. The anterior fibres, which run farthest and lowest down, are tendinous at their insertion, while the posterior and shortest are fleshy. The use of the whole Muscle is to raise the Lower-Jaw ; and when it is brought forwards, the posterior and inner portion will assist in bringing it a little back ; so that this Muscle becomes a rotator, if the Jaw happens to be turned to the opposite side. We may observe, that this Muscle is intermixed with a number of tendinous portions, both at its origin and its insertion ; which give rise to a greater number of fleshy fibres, and thereby add to the strength of the Muscle, (y) (y) [The masseter muscle exists in all the inferior Mammalia. It may be said generally to vary in size and strength with the amount of re- sistance it is destined to overcome in the act of mastication. In those animals, also, in which the other elevators of the jaw, especially the temporal, are of small size, the masseter takes on increased development and power (e. g., Rodentia, Ruminantia). The size, strength, and shape of the zygomatic liar, together with the condition of the angle ami postero- external surface of the inferior maxilla, may he taken as indices of the development and powers of this muscle. In the typical Carnivora, the itic arch i. ; of yery greal strength and depth ; below it is formed TEMPORALIS. 19 TEMPORALIS. It is situated on the side of the Head, above, and somewhat before the Ear. It is a pretty broad, flat, and radiated Muscle ; broad and thin at its origin ; narrow and thick at its insertion ; and is covered with a pretty strong Fascia, above the Jugum. This Fascia is fixed to the bones round the whole circum- ference of the origin of the Muscle. Above, it is fixed to a smooth white line, that is observable upon the Skull, extending from a little ridge on the lateral part of the Os Frontis, con- tinued across the Parietal Bone, and making a turn towards the Mammillary Process. It is fixed below, to tho ridge where the Zygomatic Process begins, just above the Meatus Auditorius ; then to the upper edge of the Zygomatic Process itself, and anteriorly to the Os Malse. This adhesion, anteriorly, above, and posteriorly, gives, as it were, the circumference of the origin of the Temporal Muscle. almost entirely by the malar bone, above by the temporal, so that the suture between them extends obliquely nearly the whole length of the arch. It is arched upwards as well as outwards, the upper margin being convex, the lower concave, and the latter is deeply excavated for the attachment of the muscle. It is to be observed that the more purely carnivorous the animal, the greater is the convexity of the vertical arch described by the zygoma. The short and very thick masseter arising from the vaulted arch above passes downwards, backwards, and inwards, to be inserted into the posterior part of the lower border of the jaw, and into the lower portion and inferior boundary of the deep fossa which occupies its external surface behind the great molar tooth (carnasstere), whilst the most external of the fibres turn round the inferior margin of the bone, to be fixed in a tendinous rapM common to it and the external pterygoid. (1) The postero-internal portion of the muscle which rises in front of the glenoid cavity has a direction downwards and forwards, ami is inserted into the external excavated surface of the coronoid. In the Carnivora, the masseter is at its maximum of development and power as an elevator of the lower jaw. With them we may compare another order — Rodentia — in which this muscle has another purpose to fulfil besides that o'f raising the jaw — viz., the production of that motion from behind forwards for which we have already seen the articular surfaces of the (1) Vide Straus-Durckheim, Anatomie du Chat, T. ii., p. 217. 20 TEMPORALIS. This Muscle arises from all the bones of the side of the Head, that are within the line, for insertion of the tendinous Fascia, viz., from the lower and lateral part of the Parietal Bone, from all the squammous portion of the Temporal Bone, from the lower and lateral part of the Os Frontis, from all the Temporal Process of the Os Sphenoides, and often from a pro- cess at the lower part of this surface, (which portion, however, is often common to this Muscle, and the Pterygoidseus externus) condyle and glenoid cavity are specially adapted. In the Rodents, the lower border of the zygoma is convex, the curve of the vertical arch being always in the opposite direction to that which is constant in the Carnivora ; in some species — e. g., the capybara, the agoutis, and pacas — the convexity of the arch descends to a level below that of the superior grinders. The superior maxillary bone contributes also to its formation, the malar being frequently, as it were, suspended between its apophysis and the zygomatic process of the temporal bone. In these animals the masseter is of great strength when compared with the temporal, and it is divided into several distinct portions, which in some species may rank as separate muscles. A large portion of the muscle will be found to arise from the superior maxillary and fore part of the arch, and to pass obliquely backwards, its action being to bring the jaw directly forwards. In many instances a muscular fasciculus commences by a strong tendon under the sub-orbital foramen, and passes nearly directly backwards ; in the agouti this portion of the masseter covers the posterior half of the jaw, and terminates on the internal surface of its posterior border. Other fibres have a more per- pendicular arrangement, and in some species — e.g., the rabbits — the fibres of a small portion of the muscle will be found to run in the oppo- site direction to the larger mass. They assist in moving the jaw upwards and backwards. (1) In Man and the Quadrumana, the zygomatic bar is nearly transverse. In the latter it is of greater length, in conformity with the relative di- mensions of the cranium and face ; it is also somewhat more curved, the upper border being slightly convex behind and concave in front. In the larger Anthropoid Apes (TV. Gorilla and Pith. Satyrus) the massive dimensions of the zygoma, dependant on the great size of the lower jaw and the development of the canine teeth, present a striking contrast with the human configuration. (2)] (1) Vide Cuvier, Logons d' Anatomic Comparcc, T. iv., partie 1, lecon xvi., edit. 1835. (2) Vide Owen, Zool. Trans. TEMPORALIS. 21 and from the posterior surface of the Os Malae. Outwardly, it rises from the inner surface of the Jugum, and from the whole inner surface of the Fascia above described. At this origin, from the Jugum it is not to be distinguished from the Masseter, being there, in fact, one and the same Muscle ; and indeed the Masseter is no more than a continuation of the same origin, under the edge of the Jugum ; and might properly enough be reckoned the same, both as to its origin and insertion, and in some measure in its use also. The origin is principally fleshy ; and the Muscle passes from it, in general, downwards, and a little forwards, converging, and forming a thin middle tendon. After which the Muscle runs downwards, on the inside of the Jugum, and is inserted into the Coronoid Process of the Lower-Jaw, on both sides tendinous and fleshy, but principally tendinous. It reaches farther down upon the inside of the Coronoid Process, than upon the outer side, where the insertion is continued as low as the body of the bone. The posterior and inferior edge of this Muscle passes over the root of the Zygomatic Process of the Temporal Bone, as over a pulley, which confines the action of the Muscle to that of raising the Lower- J aw, more than if its fibres had passed in a direct course from their origin to their insertion. The use of the Temporal Muscle, in general, is to raise the Lower- Jaw ; and as it passes a little forwards to its insertion, it must bring the Condyle at the same time backwards, and so counteract the Pterygoidseus externus of the opposite side ; and if both Muscles act, they counteract both the Pterygoidaei, by bringing back the whole of the Jaw. (z) (z) [The size and development of the temporal muscle (crotaphite) is indexed by the extent and depth of the temporal fossa, the outward or horizontal curve described by the zygoma,togetherwith the height and the excavations on the internal and external surfaces of the coronoid process. As the powerful elevator of the lower jaw, it is par excellence the biting muscle ; and it may be generally stated that its dimensions and force will be found closely related to the development of the piercing and cutting teeth, especially the canines. In those of the inferior Mammalia 22 PTERYGOID^US internus and externus. PTERYGOID^US INTERNUS. It is situated upon the inside of the Lower-Jaw, opposite to the Masseter, which is upon the outside. It is a strong short Muscle, a little flattened, especially at its insertion. It arises tendinous and fleshy from the whole internal surface of the external Ala of the Sphenoid Bone ; from the external surface of the internal Ala, near its bottom ; from that process of the Os Palati that makes part of the Fossa Pterygoids ; likewise from the anterior rounded surface of that process, where it is connected to the Os Maxillare superius. From thence the Muscle passes downwards, a little outwards and backwards, and is inserted tendinous and fleshy into the inside of the Lower- Jaw, from the angle, up almost to the groove for the admission of the Maxillary Nerve, where the surface of the bone is remarkably scabrous. The Use of this Muscle is to raise the Lower Jaw ; and from its direction, one would suspect that it would bring the Condyle a little forwards ; but this motion is contrary to that of the Lower-Jaw, for it is naturally brought back when raised. PTERYGOIDJEUS EXTERNUS, Is situated immediately between the external surface of the external Ala of the Pterygoid Process, and the Condyle of the in which this muscle attains considerable size, it is common to find the depth of the fossa from which it arises increased by the production of its boundaries in the form of osseous ridges or crista, which spring from the surfaces of the frontal, the parietal, and the occipital bones. The direct relation which exists between the development of the temporal and the condition of the dental apparatus is well illustrated by a comparison of the male and female and immature and adult crania of the large Anthropoid Quadrumana. Amongst these the maximum of extension and depth of the temporal fossa is to be seen in the skull of the adult male Gorilla, where a lofty sagittal crest is produced by the union of the frontal ridges at the coronal suture, and behind is con- tinued into two thick salient occipital crista?. The same thing is re- peated, although in a somewhat inferior degree, in some skulls of the PTERYGOID.EUS EXTERNUS. 23 Lower-Jaw ; lying, as it were, horizontally along the basis of the Skull. It is somewhat radiated in some bodies ; broad at the origin, and small at the insertion ; but the greater part of adult male Orang {Pith. Satyrus). In other specimens of this species, the parietal ridges, although considerahly developed, are not blended together in the form of a median crest, hut are separated by an interval of varying width. These differences in the crania of the male Orangs do not seem to indicate a difference in race ; they are rather the indices of age and muscular energy. (1) In the males of Loth Tr. Gorilla and Pith. Satyrus the laniaries attain a development almost rivalling their proportions in the more powerful Carnivora. In the females of these species, on the othei hand, which are distinguished by the compara- tively small size of the canines, the osseous cristse are proportionately of minor development ; the parietal ridges do not unite to form a high sagittal crest, but are continued separately backwards over the upper surface of the cranium, albeit in the female Gorilla they are only divided by a narrow groove. (2) Again, in the smaller species of Chim- panzee (Tr. nigcr) the muscular processes of the male skull are less developed than in Tr. Gorilla, the laniaries also attaining smaller dimensions. But in all these great Apes it is only on the completion of the second dentition that the surface for the attachment of the biting muscles becomes thus extended. In the immature Chimpanzees and Orangs, whilst the cranial portion of the skull is comparatively large, the facial part and especially the jaws are but little developed, the sur- face of the cranium is uninterrupted, and the temporal fossae lack the depth which they afterwards acquire, with the full attainment of the powers of combat and mastication. It is at this stage of his existence that the young Ape is so eminently anthropoid : but, "as growth pro- ceeds, the milk teeth are shed, the jaws expand, the great canines suc- ceed their diminutive representatives, the biting muscles gain a propor- tional increase of carneous fibres, their bony fulcra respond to the call for increased surface of attachment, and the sagittal and occipital crests begin to rise." (3) It is scarcely necessary to say that no such extension of the temporal fossa is ever found in Man ; although in the low, uncivilized races, where the teeth and jaws are put to rough work, it is not unusual to find the arched border of the fossa more strongly defined than in the higher (1) Vide Owen, Zool. Trans., vol. iv., p. 165. (2) Descriptive Catalogue of Osteological Scries in Museum of R. Col. of Surgeons, vol. ii., p. 803. (3) Owen, Class, and Distrib. Mammalia, Appendix on Gorilla, p. 76. 24 PTERYGOID^US EXTERNUS. it forms a round strong fleshy belly ; so that the part that makes it of the radiated kind is thin. varieties. In both Man and the Quadrumana the zygoma is slightly curved outwards ; amongst the latter the outward curve is well marked in the Baboons (Cynocephali). It is in the order Carnivora that the temporal acquires its highest development. This is evidenced by the great depth of the fossa, the outward curve of the zygoma, the development of frontal, parietal, and occipital crests, and the height of the coronoid process of the lower jaw, together with its deeply-excavated external surface. The skulls of the hyaena, wolf, and some of the larger kinds of dog— e. g., the mastiff, blood-hound, &c— are remarkable for the development and height of the sagittal crest, produced by the union of the frontal and parietal ridges. Amongst the Felidce, the single crest is best expressed in the cranium of the lion ; in the smaller species the parietal ridges only meet in the posterior part of their extent. On removing the skin from the upper surface of the head in these animals, we find nothing on each side but the great mass of the temporal, which completely covers the superior and lateral regions. In the Cat, the muscle is described by Straus-Durckheim as consisting of three portions — an antero-external of large size, which arises from the anterior part of the strong temporal aponeurosis and the anterior part of the fossa, and two posterior portions, of which the internal is the larger, and occupies the two posterior thirds of the same fossa. The three heads unite to be inserted into the coronoid process of the jaw, their fibres covering its surfaces and borders, being attached on the inner side as low down as the dental foramen, on the outer to the upper portion of its surface, and to an aponeurosis common to this muscle and the masseter. (1) If we contrast with the Carnivora the Ruminants and Rodents, we find a wide difference in the development of the temporal, as indicated by the size and depth of its fossa. In the latter orders this space is narrow and shallow, the muscular ridges are but slightly expressed ; and in the Ruminants the zygomatic arch is short, and projects but little. (2) As another instance of the close relation which exists between the develop- ment of the temporal muscle and the dental type, we may adduce the Camel. In the Camel, canines are present in both jaws, together with a pair of laniarifonn incisors and premolars in the upper jaw. Concomitant on this higher type of dentition, we find the skull furnished with a sharp and deep but thin occipital crest, a low but sharp parietal crest, (1) Vide Straus-Durckheim, Anatomic Descriptive et Comparative du Chat, T. ii., p. 210. (2) Cuvier, op. cit., lccon xvi. PTERYGOIDiEUS EXTERNUS. '25 The thick and ordinary portion of it arises tendinous and fleshy, from almost the whole external surface of the external Ala of the Pterygoid Process of the Sphenoid Bone, excepting a little bit of the root at the posterior edge ; and towards the lower part, it arises a little from the inner surface of that Ala. The thin portion arises from a ridge of the Sphenoid, that is continued from the process towards the Temple, just behind the Foramen Lacerum inferius, which terminates in a little pro- tuberance. This origin is sometimes wanting ; and in that case, the Temporal Muscle arises from that protuberance; and very often this origin is common to both. These two origins of this Muscle are sometimes so much separated, as to make it a Biceps. From these origins the Muscle passes outwards, and a little backwards, converging ; that is, the superior fibres passing out- wards and backwards, and a little downwards ; while the inferior, or larger portion of it, passes a little upward. It is inserted tendinous and fleshy into a depression on the anterior part of the Condyle and Neck of the Lower-Jaw, upon the inside of that ridge, which is continued from the Coronoide Process. A little portion is likewise inserted into the anterior part of the moveable cartilage of the joint. When this Muscle acts singly it is a rotator ; for it brings the Condyle of the Jaw forwards, and likewise the moveable cartilage, which throws the Chin to the opposite side ; but if it acts in conjunction with its fellow of the opposite side, instead of being turned to one side, the whole Jaw is brought forwards, and thus these counteract the Temporal, &c. These two Muscles generally act alternately ; and when they do so, one acts at the time of depression, the other at the time " whilst the zygomatic arches are longer and span across a wider tem- poral fossa than in the ordinary Ruminants," (1) in which the upper in- cisors are absent, being replaced by a callous pad, the upper canine is in many instances wanting, whilst the lower canine resembles and ibrrns one of the same series with the lower incisors.] (1) Osteoloft Catalogue R. Col. Surgeons, vol. ii., p. 26 DIGASTRICUS. of elevation ; so that these Muscles act, both when the Lower- Jaw is raised, and when it is depressed : yet they do not assist either in raising or depressing it. (b) DIGASTRICUS.. It is situated immediately under, and a little upon the in- side of the Lower-Jaw, and outside of the Fauces, extending from the Mastoid Process to the Chin, nearly along the angle made by the Neck and Chin, or Face. The name of this Muscle (b) [The pterygoids vary in size and function in the different families of Mammalia. They are usually largely developed in herbivorous and frugivorous animals, where they are in direct relation to the lateral and antero-posterior motions of the jaw in mastication. In the Chimpanzees and Orangs, the force and size of the internal pterygoid is evidenced by the roughness and elevations which exist on the internal surface of the posterior border of the ascending ramus and angle of the jaw. (1) In the Rodents, the internal pterygoid, like the masseter, is employed in draw- ing forward as well as in raising the mandible. To increase the obliquity of its fibres and the backward extent of surface for their insertion, the angle of the jaw is prolonged posteriorly. The Ruminants, and grass- feeding animals generally, have these muscles of large size. In the typical Carnivora, on the other hand, the principal pterygoid is simply an elevator of the jaw, not a rotator or protrusor. In the Cat, the larger muscle (external Pterygoid, Straus-Durckheim ; internal Pterygoid, Cuvier) is flat and of considerable size ; it has its origin from the whole length of the lateral border of the guttural part of the palatine bone, and from its corresponding orbital surface, and passes outwards, backwards, and downwards, to be inserted into the inner surface of the lower border of the jaw, into the tendinous raphe common to it and the masseter, and into the inner surface of the coronoid. The smaller slip (internal Pterygoid, Straus-Durckheim) is almost blended with the preceding ; it is placed behind, and is in part internal to and under the principal muscle. None of the fibres of the pterygoid in the Cat are attached to the condyle or to the interarticular cartilage. These muscles, as do the temporal and masseter, present an admirable instance of adaptation to the type of dentition, and to the structure of the joint. (2) (1) Vide Owen, Zool. Trans., vol. iv. (2) Vide Straus-Durckheim, op. et loc. cit. ; Cuvier, op. cit., p. DIGASTRICUS. 27 expresses its general shape, as it has two fleshy bellies, and of course a middle tendon, (d) Yet some of its anterior belly does not arise from the tendon of the posterior, but from the Fascia, which binds it to the Os Hyoides. These two fleshy bellies do not run in the same line, but form an angle, just where the tendon runs into the anterior belly ; so that this tendon seems rather to belong to the posterior, which is the thickest and longest. This Muscle arises from the Sulcus made by the inside of the Mastoid Process, and a ridge upon the Temporal Bone, where it (eQ [The name " digastricus " is not equally applicable to this muscle in all the Mammalia in which it is found. In the Apes, as in Man, it consists of two bellies, united by a middle tendon, which passes through the stylo-hyoid. But the digastric of the higher Quadrumana differs from the same muscle in Man in the greater size and strength of the anterior belly. Duvernoy describes this portion of the muscle in the Gorilla as being large and flat In the Chimpanzee, the anterior fasciculus, according to Vrolik, is relatively stronger than in Man, and is less separated from its fellow of the opposite side. Thereby, he observes, a tendency is shown to the condition which exists in the other Apes, " in which the anterior fasciculi of the digastric are so developed as to fill up all the interspace between the two rami of the lower jaw." This expansion of the anterior portion of the muscle he has observed in the Orang and Gibbon, but it is most remarkable in the Macaques and Cynocephali. In the latter, the term " digastric " might be applied to the posterior bellies of opposite sides, for their tendons unite in front of the os hyoides to form an arch, from the convexity of which the two anterior ventres, which are in close proximity the one to the other, take their origin by an aponeurotic expansion. By this arrangement the power of the muscle, as a depressor of the jaw, must be considerably augmented, and it would appear to be a provision in direct relation with the more carnivorous and ferocious habits of the Baboons and Mandrills. Professor Vrolik states that in the Loris (Stenops) the structure of the digastricus points to a transition from the Quadru- manous type to the simple form which it takes in the Carnivora, there being only an indication of the intermediate tendon. In the Rodents (with the exception of the Rabbit), and in the Ruminants generally, it is biventral. On the contrary, in the Carnivora, the Kangaroo, the Sloths, the Elephant, the Hog, the Hyrax, it is only a single-bellied muscle. Straus-Durckheim, however, describes it in the Cat as being interrupted a little behind the angle of the jaw by a tendinous intersection, which gives rise to new fleshy fibres ; and Vrolik has also noticed the occurrence of aponeurotic fibres in the 28 DIGASTRICUS. is united with the Os Occipitis. (e) The extent of this origin is about an inch : it is fleshy upon its outer part, viz. that from the Mastoid Process, and tendinous on the inside from the ridge. From its origin it passes forwards, downwards, and a little inwards, much in the direction of the posterior edge of the Mammillary Process ; and forms a round tendon first in its center and upper surface. This tendon passes on in the same direction ; and when got near the Os Hyoides, it commonly perforates the anterior end of the Stylo-Hyoideus Muscle ; and from the lower edge of this tendon, some fibres seem to go off, which degenerate into a kind of Fascia, that binds it to the Os Hyoides; and some of it goes across the lower part of the Mylo-Hyoidgeus, and joins its fellow on the opposite side ; bind- ing the Os Hyoides by a kind of belt. At this part the tendon middle of the muscle in other Carnivora. It would almost seem as if the unity of the muscle in the Carnivores was produced by the reunion of the two fasciculi which are common to Man and the Apes. The straight line pursued by the simple nr.scle in these animals, as compared with its deflected course in Man, is evidently conducive to more powerful and direct action in the abduction of the maxilla. According to Cuvier, the digastric is absent in the Ant-eaters and Armadillos, its place being supplied by a long and slender muscle which takes its origin from the sternum, and is inserted into the inferior border of the jaw.] (1) (e) [It is observed by Tyson that in the Chimpanzee the digastric does not arise from the mastoid process of the temporal, as in Man, but from the occipital bone. He adds, " Drelincourt describes it (the digastricus) in Apes thus : Tendinem habet intermedium pollice longum, etgracilem, enascitur antem non ab apophyse Styloide, sed ab osse Basilari." According to Vrolik, however, in the Chimpanzee it is attached to the tuberosity which represents the mastoid apophysis. In the Gorilla, Duvernoy traces its attachment to a groove behind the mastoid apophysis.] (2) (1) Vide Cuvier, op. cit., p. 91 ; Duvernoy, Archives du Museum d'Histoire Naturelle, Tome viii., p. 182 ; Vrolik, Recherches d'Anatomie Comparee sur le Chimpanse, Amsterdam, 1841, pp. 17, 26 ; Article Quadrumana, Todd's Cyclopsedia of Anatomy ; Straus-Durckheim, op. cit., p. 218. (2) Orang Outang, sive Homo Sylvestris, or the Anatomy of a Pygmie, by E. Tyson, M.D., F.R.S., p. 86 ; Vrolik ; Duvernoy, op. et loc, cit. DIGASTRICUS. 29 becomes a little broader, makes a turn upwards, inwards, and forwards, and gives origin to the anterior belly, which passes on in the same direction, to the lower part of the Chin, where it is inserted tendinous and fleshy, into a slight depression on the under, and a little on the posterior part of the Lower-Jaw, almost contiguous to its fellow. (/) Besides the attachment of the middle tendon to the Os Hyoides, there is a ligamentous binding, which serves, in some measure, as a pulley, (g) This is more marked in some subjects than in others ; and this depends on the strength of the tendinous expansion, which binds the tendon of the Digastricus to the Os Hyoides. When we say that these parts are attached to the Os Hyoides, we do not mean that they can be traced quite into it, like some other tendons in the body ; but the Os Hyoides seems to be the most fixt point of attachment. Very often we find two anterior bellies to each Muscle ; the uncommon one, which is the smallest, does not pass to the Chin, but joins with a similar portion of the other side, in a middle tendon, which is often fixed to the Os Hyoides. At other times, we find such a portion on one side only ; in which case it is commonly fixed to the middle tendon of the Mylo-Hyoidaeus. The use of these Muscles with regard to the Lower-Jaw, is (/) [The surface for insertion of the digastric varies in position in Mammalia. In Carnivora it is attached for a varying distance to the inferior border of the horizontal ramus. In the Cat it is inserted, according to Cuvier, into the angle of the jaw. Straus-Durckheim, however, describes it as reaching the anterior half of the lower border. In the Rodents it is prolonged to the back part of the arch of the chin, into which it is fixed ; in the Elephant it is attached to the posterior border of the maxilla ; in the Horse the principal portion of the muscle is inserted into the angle ; in the Ruminants it extends as far as the mid-length of the ramus.] (1) {g) [The existence of an apparatus analogous to the pulley was denied by Monro primus {vide infra, note h). Many writers describe the tendon of the digastricus as passing through' an aponeurotic sheath lined by a synovial membrane. Such a structure, although in some instances apparent enough, is by no means constant. (1) Cuvier ; Straus-Durckheim, op. cit. 30 DIGASTRICUS. principally to depress it ; but according as one acts a little more forcibly than the other, it thereby gives the Jaw a small rotation ; and becomes, in that respect, a kind of antagonist to the Pterygoidaeus Externus. Besides depressing the Lower-Jaw. when we examine the dead body, they would appear to raise the Larynx. But although they have this effect, a proper attention to what happens in the living body, will probably shew, that their principal action is to depress the Lower- Jaw, and that they are the Muscles which are commonly employed for this purpose. Let a Finger be placed on the upper part of the Sterno- Mastoidgeus Muscle, just behind the posterior edge of the Mastoid Process, about its middle, touching that edge a little with the finger ; then depress the Lower- Jaw, and the posterior head of the Digastric will be felt to swell very considerably, and so as to point out the direction of the Muscle. In this there can be no deception ; for there is no other Muscle in this part that has the same direction ; and those who are of opinion that the Digastric does not depress the Lower- Jaw, will more readily allow this, when they are told, that we find the same head of the Muscle act in deglutition ; but not with a force equal to that which it exerts in depressing the Lower-Jaw. (h) Further, if the Sterno-Hyoidei, Sterno-Thyroidei, and Costo- Hyoidei, acting at the same time with the Mylo-Hyoidei, and Genio-Hyoidei, assisted in depressing the Jaw, the Os Hyoides, and Thyroide Cartilage, would probably be depressed, as the (h) [Hunter here probably refers to a paper by Alexander Monro primus on the Articulation Muscles and Luxation of the lower jaw, which appeared in the first volume of the Edinburgh Medical Essays. Monro opposes the commonly-received opinion as to the function of the digastric by the following arguments : That the shortness of the bellies of the digastric does not admit so large a contraction as is often required ; —that " the proportional force of the digastric to that of the elevators of the jaw is considerably less than is seen in other parts of the body where antagonist muscles are, which proportional force of these muscles is on some occasions greatly lessened by the angle of insertion of these digastric muscles into the jaw, decreasing as the mouth is opened." He denies the existence of any mechanism akin to a pulley, such as is seen in the larger oblique muscle of the eye. He describes the connection of the digastric with the os hyoides as taking place simply by a firm DIGASTRIOUS. 31 bellies of the Sterno-Hyoidei, and of the other lower muscles, are by much the longest ; but on the contrary, we find that the Os Hyoides, with the Thyroide Cartilage, is a little raised in the depression of the Jaw, which we may suppose to be done by the anterior belly of the Digastric : (i) and secondly, if these aponeurosis, which comes off from the round tendon of the muscle, and is in part united to the os hyoides and in part joins the aponeurosis of the opposite side. He denies that there is anything like a sheath in which the middle tendon slides. Lastly, he describes some experiments on the living and dead subject, which he believes disprove the agency, at least, to any great extent, of the anterior or posterior belly of the digastricus in depressing the jaw. He believes the action of the muscle to be principally exercised in deglutition, in which act it pulls the os hyoides upwards, so as to press the root of the tongue against the soft palate. The real depressors of the maxilla he considers to be the sterno-hyoidei and genio-hyoidei, together with the coraco-hyoidei and mylo-hyoidei, which may be looked upon as digastrics whose middle intersection is at the hyoid bone ; and also the sterno-thyroidei, thyro- hyoidei, hyo-glossi, and genio-glossi, which, in their united action, are to be considered as two many-bellied muscles acting on the lower jaw. (1) The opinion which is now generally held, and which is probably correct, may be taken as a mean between the opposite views of Hunter and Monro. Anatomists generally allow that the principal instrument in the depression of the jaw is the digastric ; but in order to the pro- duction of that effect it is necessary that the os hyoides should be fixed, which is done by the agency of the muscles that pass between that bone and the sternum and scapula, so that the latter become indirectly depressors of the maxilla. It is also generally admitted that all the other muscles which pass from the os hyoides to the maxilla when the former is fixed, may be accessories of the digastric in this act. When, on the contrary, the jaw is fixed, the contraction of the digastric must raise the os hyoides and the parts attached to it. This action of the digastric is called into play in deglutition when, together with the stylo-hyoid, mylo-hyoid, and genio-hyoid, it elevates the hyoid bone, and with it the base of the tongue. In swallowing the mouth is first closed.] (i) [Monro states that the os hyoides moves downwards and forwards when the mouth is opened. In observations on myself and others I have never been able to arrive at the conviction that the hyoid bone and thyroid cartilage are raised when the lower jaw is depressed, as is asserted in the text.] (1) Medical Essays and Observations by a Society in Edinburgh, vol. i., 1733. 32 DIGASTRICUS. Muscles were to act to bring about this motion of the Jaw, these parts would be brought forwards, nearer to the straight line between the Chin and Sternum, which is not the case in this action ; whereas we find it to be the case in deglutition, in which these evidently act. By applying our fingers upon the Genio- Hyoideus, and Mylo-Hyoideus, near the Os Hyoides, between the two anterior bellies of the Digastric, (not near the Chin, where the action of these two bellies may occasion a mistake) , we find these Muscles quite flaccid ; which is not the case in deglutition, nor in speaking, in which they certainly do act ; nor do we find the Muscles under the Os Hyoides at all affected, as they are in the motion of the Larynx. It has been observed, that when we open the mouth, while we keep the Lower-Jaw fixed, the fore-part of the Head or Face is necessarily raised. Authors have been at a good deal of pains to explain this. Some of them considered the Condyles of the Jaw, as the center of motion ; but if this were the case, that part of the Head, where it articulates with the Spine, and of consequence the whole body, must be depressed, in proportion as the Upper-Jaw is raised ; which is not true in fact. Others have considered the Condyles of the Occiput as the center of motion ; and they have conceived the Extensor Muscles of the Head to be the moving powers. The Muscles which move the Head in this case, are pointed out by two circumstances, which attend all muscular motion ; in the first place, all actions of our body have Muscles immediately adapted to them ; and secondly, when the mind wills any particular action, its power is applied by instinct to those Muscles only, which are naturally adapted to that motion ; and further, the mind being accustomed to see the part move which is naturally the most moveable, attends to its motion in the volition, although it be in that instance fixed, and the other parts of the body move towards it ; and although the other parts of the body might be brought towards it by other Muscles, and would be so, if the mind intended that they should come towards it, yet these Muscles are not brought into action. Thus the Flexors of the Arm commonly move the hand to the body ; but if the hand be fixed, the body is moved by the DIUASTRICUS. 33 same Muscles to the hand. In this case, however, the mind wills the motion of the hand towards the body, and brings the Flexors into action ; whereas if it wished to bring the body towards the hand, the Muscles of the forepart of the body would be put into action, and this would produce the same effect. To apply this to the Lower-Jaw ; when we attempt to open the Mouth, while the Lower-Jaw is immoveable, we fix our attention upon the very same Muscles (whatever they are) which we call into action, when we depress the Lower- Jaw ; and we find that we act with the very same Muscles ; for our mind attends to the depressing of the Jaw, and not the raising of the face ; and under such circumstances the mouth is actually opened. We find then by these means the head is raised ; and the idea that we have of this motion, is the same that we have in the common depression of the Jaw ; and we should not know, except from circumstances, that the Jaw was not really depressed ; and we find at this time too, that the Extensors of the head are not in action. On the contrary, when the Jaw is fixed in the same situation, if we have a mind to raise the head, or Upper- Jaw, which of course must open the mouth, we fix our attention to the Muscles that move the head backwards, without having the idea of opening our mouth ; and at this time the Extensors of the head act. This plainly shows, that the same Muscles which depress the Jaw, when moveable, must raise the head, when the Jaw is kept fixed. This is a proof too, that there are no other Muscles em- ployed in depressing the Lower-Jaw, than what will raise the head under the circumstances mentioned. This will further appear from the structure of the parts ; wherein four things are to be considered, viz. the articulation of the Jaw; the articulation of the Head with the Neck ; the origin, and the insertion of Digastric Muscle. Suppose A, the Upper- Jaw, to be fixed, and the Lower- Jaw B, to be moveable on the Condyle C : if the Digastric contracts, its origin E, and insertion F will approach towards 34 HUNTER ON THE TEETH. one another ; in which case it is evident, that the Lower- Jaw will move downwards and backwards. But if the Lower- Jaw be fixed, as in the case supposed, and the Vertebrae G G G be also fixed, the Condyle will move upwards and forwards upon the eminence in the joint, the fore-part of the head will be pushed upwards and backwards by the Condyle, and the hind-part of the head will be drawn down ; so that the whole shall make a kind of circular motion upon the upper Vertebra; and the Digastric Muscle pulling the hind-part of the head towards the Lower Jaw, and at the same time pushing up the Condyles against the fore-part of the head, acquires, by this mechanism, a very considerable additional power. OF THE STRUCTURE OF A TOOTH. AND, FIRST, OF THE ENAMEL. A Tooth is composed of two substances, viz. Enamel and Bone. (I) The Enamel, called likewise the vitreous, or cortical (I) [It may l>e stated, in general terms, that a tooth is a hard body- primarily distinct from the skeleton, situated in the mouth or at the commencement of the alimentary canal, composed of organic and inor- ganic matter, and enclosing a vascular pulp which occupies a cavity in its interior. In examining the teeth of the greater number of vertebrate animals, we are able to discriminate two or more tissues, of differing structure and hardness. The principal dental tissues are the dentine or ivory (bone of the text), the enamel, and the cement or crusta pdrosa. Of these, the dentine forms the chief mass of the body of the tooth, the cement the outer crust, whilst the enamel, where it exists, is external to the dentine, or between it and the cement. Besides the three principal dental tissues, various modifications of the dentine not unfrequently occur, to which names have been applied indicative of their structural peculiarities — e.g., osteo-dentine, vaso-dentine. Although structures ana- logous to dental organs are met with in the invertebrate classes, it is amongst the vertebrata alone that true calcified teeth are found. In the teeth of vertebrate animals, the number of component tissues, and the manner of their arrangement, exhibit considerable variation. Amongst the class Pisces, there are a few instances (e.g., the Wrasse Ldbrus) in which the teeth consist of but one tissue, which is always a modification of dentine. It is more common, in the same class, to find the tooth OF THE STRUCTURE OF A TOOTH, ETC. 35 part, is found only upon the body of the Tooth, and is there composed of ordinary dentine, and of the same structure, permeated by vascular canals (vaso-dentine) : in such instances, the hard unvaseular dentine is external, and supplies the place of the enamel of higher animals. The molars of the Dugong afford examples of teeth composed of dentine and cement alone, the latter forming a thick crust on the surface of the former. At first sight, the human teeth and those of Carnivora might he supposed to consist of but two tissues— dentine and enamel. This is the statement of the text, and until a comparatively late period it passed as an acknowledged fact. It was on account of this apparent simplicity of structure that these teeth were distinguished as " simple " by the Cuviers. More recent observation, however, has shown that, besides the enamel and dentine, the third principal dental tissue — the cement — forms a component part of the human and the other so-called simple teeth. The teeth of Mammalia, which received from Cuvier the name of " Compound" or " Complex," differ from those of Man and the Carnivora only in a different proportion and disposition of the same constituent tissues. In some instances — e.g., the molars of the Elephant, the molars of the African Wart-hog {Phacoclicerus), the pec- tinated incisors of the Flying Lemur (Galeopithecus) — a tooth of a high degree of complexity is produced by the aggregation of distinct denticles. The great grinding tooth of the Elephant is composed of a series of den- ticles, each having the form of a plate, vertical to the grinding service, and transverse to the long diameter of the tooth. Each plate consists of dentine coated by an investment of enamel, whilst the interspaces between the denticles are completely filled by cement. In this fully- formed tooth, the bases of the several divisions of the crown become fused into a common body of dentine ; but at an earlier period, before the calcification of the plates is complete, they are held together by the cement alone. The incisors of Galeopithecus, on the other hand, differ from the tooth just described in the circumstance that the denticles project as distinct processes from the base of the crown : each consists of dentine with a covering of enamel, which is again coated with a layer of cement of extreme thinness ; at the base the denticles are united, and into each a prolongation is continued from the common pulp cavity. But a high degree of complexity of dental structure is not restricted to Mammalia. In the grinding apparatus of the Parrot-fishes (Scarus), for instance, which" browse upon the lythophitas that clothe as with a richly- tinted carpet the bottom of the sea, just as the Ruminant Quadrupeds crop the herbage of the dry land," (1) dental masses, bearing a close analogy to the complex molars of the Elephant, are formed by an aggre- gation of denticles. Moreover, each denticle of the Parrot-fish consists of dentine with a thick covering of enamel : the denticles are united (1) Owen, Art. Teeth, Todd's Cyclop. Anat and Phys. db HUNTER OX THE TEETH. laid all around, on the outside of the bony, or internal substance. together by a partial filling of their interspaces by cement, whilst a fourth substance is added, in the form of a vascular osteo-dentine, which, produced by the ossification of the base of the pulp, serves to unite the denticles with each other, and with the subjacent pharyngeal bone. The tooth, thus composed of four substances, exhibits the highest degree of complexity yet observed in the animal kingdom. Many other varia- tions in the structure of the teeth, and in the arrangement of the dental tissues, are to be found amongst vertebrate animals ; but sufficient examples for the purpose of illustration have been adduced. (1) In the human tooth, the dentine forms the principal mass both of the fang and crown : in the latter situation it is covered by enamel, in the former by cement. As no separate mention is made of this last- named substance in the text, it will be, perhaps, convenient to describe its disposition and structure in the human tooth before commenciug our commentary on the enamel. The Cement, Crusta Petrosa, Substantia Ostoidea, is a layer of true bone which in Man covers the fang or fangs of the tooth. In some instances it appears to commence where the enamel ceases, but in many others it manifestly overlaps the enamel for a short distance from its edge. The layer of cement is extremely +hin at the neck of the tooth ; but, in tracing it down the fang, it is found to become thicker as it descends, until it attains its maximum of depth at the apex. That con- fluent condition of the fangs which is so common in certain of the molar teeth amongst the leucous and xanthous varieties of Man, is produced by the cement which fills the interspaces, just as in the com- pound molar of the Elephant it unites a series of denticles into a single mass. It is, of course, found of considerable thickness along the sulci which indicate the division of fangs so enjoined. Accord- ing to some of the best observers, a thin layer of cement may be traced in the unworn tooth covering the enamelled crown. In a paper read before the Medico-Chirurgical Society in 1839, Mr. Nasmyth announced the discovery of a membrane separable by the action of dilute hydrochloric acid from the surface of the enamel, which, he expressly states, is continuous with the cement, and which, he believed, was at least analogous to, if not identical with, a layer of that substance in a rudimentary condition : this layer he terms the persistent capsular investment. (2) Nasmyth' s observation was confirmed by Professor Owen, who, in writing of the so-called simple teeth, "says their crowns are originally and their fangs are always covered by a thin coat of (1) Owen, op. cit., Odontography. (2) Mod. -Chir. Trans., vol. xxii., pp. 312-315. Researches on the Develop- ment, Structure, and Diseases of the Teeth, by Alex. Nasmyth, p. 79 ; 1S19. OF THE STRUCTURE OF A TOOTH, ETC. 37 It is by far the hardest part of our body ; insomuch that the cement." (1) Other observers, amongst whom are Huxley and Kolliker, although allowing the existence of a thin calcified pellicle (by the former named ' NasmytJis membrane] by the latter the ' cuticle of the enamel') on the surface of the enamel, do not recognise in it a prolonga- tion of the cement. It would appear that the layer in question is remarkable for its great capability of resisting chemical reagents ; it differs in this respect from ordinary cement, and is proportionately better fitted to protect the crown of the tooth. The original surmise of Nasmyth has, however, received some confirmation from certain observations recorded by Mr Tomes in his ' System of Dental Surgery.' This gentleman, in describing the disposition of the cement, writes : " In a few rare instances it may be traced, not only over the terminal edge of the enamel, but for some little distance upon the coronal portion of the tooth ; and specimens are now and then found in which it fills up the deep fissures situated between the tubercles of the molar teeth." Again, in reference to Nasmyth's membrane, he says : " In several specimens which have been decalcified after being reduced sufficiently thin for microscopic examination, this membrane is obviously con- tinuous with the cementum of the fang ; and in other specimens which have not been treated with acid, I find the membrane thickened in the deep depressions of the crowns of molar teeth, and there tenanted by a distinct lacuna." (2) As life advances, the cement increases in thickness, especially towards the apex of the fang. In those cases in which its formation extends beyond a certain limit, it constitutes the disease known as " dental exostosis." The inner surface of the cement is so intimately connected with the dentine, that under a high degree of magnifying power its boundary line is not very sharply defined. Its external surface, which is uneven and not unfrequently marked with circular striae, is in close contact with the alveolar periosteum and the gum ; but it is more firmly united with the former than with the latter. Its tissue is less hard than either dentine or enamel : consequently, its surface when exposed to attrition becomes worn down at an earlier period, as may be seen in the complex grinders of the Elephant, the Masked Boar, or Capybara. In chemical composition the cement is almost identical with bone. The following is its analysis according to Bibra : (3) In Man. In the Ox. Organic substance - - - 29-42 32-24 Inorganic substance - - - 70'58 67'76 100-00 100-00 (1) Article Teeth, Todd's Cyclop., p. 867. (2) Tomes, 'A System of Dental Surgery,' pp. 257, 271 ; 1859. (3) Manual of Human Microscopic Anatomy, by A. Kolliker, p. 297 ; 1860. 38 HUNTER ON THE TEETH. hardest and sharpest saw will scarce make an impression upon In the Ox the proportions of the several ingredients were — Phosphate of Lime and Fluoride of Calcium - 58-73 Carbonate of Lime - - - - 7-22 Phosphate of Magnesia - 0"99 Salts - - - - - - 0-82 Cartilage - - - - - 31 31 Fat - - - - - - 0-93 100-00 The cement may be readily deprived of its earthy salts by the action of acids. The organic residue yields ordinary gelatine on boiling. Cement, we have already said, is a layer of true bone, with but little modification. But it is only in situations in which it attains great thickness {e.g., the teeth of the Horse, Sloth, Ruminants), that it exhibits canals for the passage of vessels (Haversian canals). In the human tooth the existence of Haversian canals is to be considered exceptional, although they are occasionally to be met with in situations where the cement is of unusual thickness. When, for instance, the fangs of a molar are united by cement, a vascular canal is not unfrequently found traversing the medium of union (Tomes.) Their existence, therefore, in the human tooth is not to be regarded ai necessarily an indication of morbid action, although they are most frequently to be noticed in the thickened cement of old teeth and in cases of dental exostosis. When present, they penetrate from without ; in some instances they branch once or twice, and anastomose, or end by blind extremities, or a canal may end by a dilatation, " as though occupied by a vessel that had turned upon itself and gone out by the same channel through which it entered." (1) Their diameter is too • narrow (0-003'" to 0-01"', Kolliker) to admit the presence of marrow as well as of blood-vessels. As in bone, the vascular canal in cement is surrounded by concentric laminae. Cement, like other osseous tissues, is composed of a matrix, in which are imbedded more or less numerous microscopic cells {corpuscles or lacuna), from which proceed, in various directions, fine, delicate ray-like canals (canaliculi), which by anastomosing constitute a system of tubes for the convection of nutrient fluid through the tissue. The matrix or basal substance usually appears to consist of lamellae, concentrically placed, the centre of the tooth being the common centre. According to Kolliker, the basal substance is sometimes granular, sometimes striated in the transverse direction, sometimes more amorphous, frequently lamellated. Mr Tomes describes it as granular, and likens it to a mass of coherent fig-seeds or to oolite. The cells or lacuna: are scattered through the matrix with some degree of regularity, being frequently, (1) Tomes, Lectures on Dental Physiology and Surgery, p. 58 ; 1848. OF THE STRUCTURE OF A TOOTH, ETC. 39 it, and we are obliged to use a file in dividing or cutting it. (m) although not always, disposed in series ; or they appear as if placed between concentric laminae. They are always absent near the neck of the tooth, where the cement is thinnest ; they generally appear about the middle of the fang, and increase in number with the thickness of the cement towards the apex, where they often present the serial appear- ance to which we have just referred. They possess the essential characters of the lacunae of bone. They appear dark by transmitted, and white by reflected light ; a circumstance due simply to their being filled with air. In the lacunae of fresh osseous tissue, a delicate- walled cell, containing a clear fluid and a single nucleus, has been dis- covered by Virchow. From this included cell, numerous processes are pent into the canaliculi which branch out on every side from the lacuna. These processes anastomose with like processes from the included cells of neighbouring lacunae. In shape the lacuna is generally oval, but occasionally round or fusiform ; in fact, in this respect they are subject to great variety. Equally variable is their size ; according to Kolliker, - 005'" to 0-02'", even to 0'03'", in diameter. In the thick layers of cement which are found on old teeth, these recesses are generally of an elongated form and very irregular. Some lacunae, either separately or in groups, may be seen to be surrounded by a tissue of more than the ordinary transparency. This clearer substance is well defined by a 6lightly-waved border, which Kolliker supposes may be in some way related to the cell from which the lacuna is formed. The canaliculi which are given off by the lacunae of cement are of unusual number and length. They are frequently so numerous as to give the idea of a pencil or plume of processes, which again branch into still finer rays. The majority of the canaliculi are directed outwards towards the surface of the tooth, or inwards towards the dentine : some, however, follow the course of the length of the tooth ; or, where a vascular canal is present, they take a direction towards its surface. In crossing the matrix, they give it a finely-striated appearance ; they anastomose freely with the canaliculi of neighbouring lacunae, and near the surface of the dentine they may be seen to inosculate with the terminations of the dentinal tubes. Other cavities may be observed in cement ; some of them are of a pathological nature. But, besides these, Mr Tomes notices tubes of occasional occurrence which pass across the cement towards the surface of the tooth. They equal in size the dentinal tubes, but have fewer branches. Occasionally the canaliculi enter them. Kolliker, in referring to what appear to be the same tubuli, says that their branches are frequently connected with the terminations of the dentinal canals, and with the canaliculi. On the fangs of deciduous teeth the cement is thin, and exhibits comparatively few lacunae.] (1) (m) [Such is the hardness of the enamel, that it emits sparks when (1) Kolliker, Tomes, Owen, op. cit. 40 HUNTER ON THE TEETH. When it is broken it appears fibrous or striated ; and all the fibres or striae are directed from the circumference to the center of the Tooth. This, in some measure, both prevents it from breaking in mastication, as the fibres are disposed in arches, and keeps the Tooth from wearing down, as the ends of the fibres are always acting on the food, (n) The Enamel is thickest on the grinding surface, and on the cutting edges, or points, of the Teeth ; and becomes gradually struck with steel. (1) It is also more brittle than the cement and dentine. We have already referred to the worn surface of the Elephant's molar, as well exemplifying the relative density of the dental tissues. It presents a series of transverse cylindrical ridges of enamel, each enclosing a depressed surface of dentine, whilst the plates so formed are separated from each other by still more deeply depressed valleys of cement.] («.) [The external surface of the enamel appears smooth, but almost always presents, on careful examination, delicate, closely-approximated transverse ridges, and in some instances .umular elevations. We have already adverted to the fact that it is covered by a delicate calcified pellicle (Nasmyth's membrane, cuticle of the enamel). According to Kolliker, " Nasmyth's membrane " is a calcified structureless membrane, 0-0004"' to 0-0008'" in thickness, which on its internal surface is fur- nished with depressions for the reception of the extremities of the enamel fibres. Its connection with the enamel is so intimate, that it can only be demonstrated by the employment of hydrochloric acid. Professor Huxley believes that the membrane in question is the altered membrana preformativa of the dental pulp. To this subject we shall have, however, to recur, when stating the various opinions current on the subject of the development of the dental tissues. In very thin sections, enamel is translucent and of a bluish colour. Its substance is generally allowed to be composed of the so-called enamel fibres (enamel-prisms, enamel-needles) — hard, dense, microscopic structures, consisting almost entirely of earthy matter. These fibres were described by Retzius as being solid hexagonal prisms. They are not, however, regular in outline. Mr Tomes observes, that in a trans- verse section he has as frequently found them approach a square or an irregular circle as any other form. They are long, extending for the most part throughout the thickness of the enamel, the inner end resting on the dentine, the outer being in contact with Nasmyth's membrane. The position of the enamel fibres being generally vertical to the surface (1) Nasmyth's Researches, p. 81. OP THE STRUCTURE OF A TOOTH, ETC. U thinner on the sides, as it approaches the neck, where it tcrmi- of dentine on which they rest, it follows that their direction will be horizontal ou the sides of the tooth, vertical on the summit of the crown. Fibres occupying all the intermediate positions between the vertical and horizontal will be seen in examining the enamel as itpassea from the grinding or cutting surface downwards to the side of the tooth. We have said that the fibres, for the most part, extend throughout the entire breadth of the enamel, and this is undoubtedly true of the greater number ; but, as the surface of the dentine is more or less conical, and no separation of the fibres takes place as they proceed outwards, neither has any increase in their thickness been observed, we cannot doubt the existence of shorter supplemental fibres, which fill up the intervals that otherwise would necessarily occur. Such supplemental fibres, how- ever, are not easily demonstrated, owing to the waved course of the fibres, and the consequent difficulty in tracing a series of them through- out their entire length. It is almost impossible to isolate for any distance the enamel fibres in the adult tooth, although they can be readily seen in transverse and longitudinal sections : it may, however, be more readily accomplished in the softer enamel of young or develop- ing teeth. The breadth of the enamel fibre is, according to Kolbker, 01)0 15'" to 0-0022"'. Isolated prisms obtained from young enamel are seen, especially after treatment with dilute hydrochloric acid, to have a slightly varicose outline, and to be marked with transverse lines, or strise, which occur at tolerably regular intervals, and produce an appearance similar to that of a " colossal muscular fibrilla." (1) This striated aspect led Mr Nasmyth to the opinion that enamel consisted of "nothing more than a mass of cells arranged in rows, and fitted closely together, but held only slightly in contact by a thin web of membrane." (2) Eetzius and others have also believed the transverse marks to be the indications of pre- existing walls of coalesced cells. Kolliker, on the other hand, maintains that they " are the expression of the growth of the fibres through appo- sition, or are not the expression of their composition of cellules." (3) Mr Tomes observes that the transverse strise are much more strongly marked in some specimens than in others, and that they are most evident in those portions of enamel which, when seen by transmitted light, are of a brown colour. This condition will sometimes be found in certain un- healthy subjects to pervade the entire enamel. In such cases he has found the markedly striated appearance to result from the occurrence, at regular intervals, of minute granular masses in the central portion of the enamel fibres. These masses are comparatively opaque, and the alternation of opaque and transparent parts gives the appearance of striatum. This appearance is rendered more distinct by the use of (1) Kolliker, op. cit. , p. 205. (2) Nasmyth, op. cit., p. 83. (3) Kolliker, op. et loc. cit. 42 HUNTER OX THE TEETH. nates insensibly, though not equally low, on all sides of the dilute hydrochloric acid, which has the effect of removing the granular masses from that portion of the tissue with which it comes more directly in contact, whilst in deeper parts the opaque masses become more dis- tinct in consequence of the removal of all opacity from the superficial portion. A transverse section thus treated with dilute hydrochloric acid has the appearance of a portion of honeycomb from which the honey has been removed. As the result of his observations, Mr Tomes does not admit that the structure of fully-formed enamel is in the strict sense of the term " fibrous." " A honeycomb," says this observer, " if the cells were filled with a material of greater opacity or density than the wax of which the cells themselves are formed, would not be regarded as fibrous ; yet the arrangement of the parts would resemble those of the enamel." To the argument that the cleavage of the enamel demonstrates its fibrous nature, he replies that the lines of fracture do not run in the longitudinal interspaces between the fibres, but through the lines of granular masses. He, however, allows that in the young tooth, during the process of formation, the fibrous arrangement is sufficiently distinct, and may be demonstrated by hydrochloric acid. In the most perfectly- developed enamel, he finds the sheaths of the fibres completely blended ; the longitudinal and transverse markings are comparatively faint, and appear under a high magnifying power, with a good light, not as dark,, but as light lines, enclosing spaces occupied by a more dense and opaque material. He describes several imperfect forms of development in this tissue, all of which are to be regarded as predisposing causes of caries : the fibrous condition may be maintained in consequence of the imperfect blending or fusion of the sheaths of the fibres ; or the central portion or contents of the fibre may be imperfectly developed, remaining in the condition of fine globular masses or granules ; or fine cavities arranged in single lines may occupy the centre of the fibre — in some cases these may coalesce, and convert the fibre into a tube ; or, lastly, both longitudinal and transverse markings may be replaced by a general granular condition of the tissue. (1) There can be no doubt that the conjunction of the enamel fibres is very intimate. Neither intermediate substance nor regular canals have been demonstrated between them. Cavities, however, frequently exist in enamel, and they are referrible either to extension of the dentinal tubes and of elongated cavities produced by the enlargement of the dentinal tubes into the enamel, or to irregular fissures in the outer and middle parts of that tissue. These latter are frequently found leading down from the depressions between the cusps of the molar and premolar teeth, and are to be placed in the foremost rank amongst the predis- posing causes of caries. Like the tubes of dentine, the enamel fibres do not pursue a straight (1) Tomes, System of Dental Surgery, p. 258—277. OF THE STRUCTURE OF A TOOTH, ETC. 43 Teeth, (o) On the base or grinding surface it is of a pretty equal thickness, and therefore is of the same form with the bony sub- stance which it covers. It would seem to be an earth united with a portion of animal substance, as it is not reducible to quick lime by fire, till it has first been dissolved in an acid. When a Tooth is put into a weak acid, the Enamel, to appearance, is not hurt; but on touching it with the fingers, it crumbles down into a white pulp. The Enamel of Teeth, exposed to any degree of heat, does not turn to lime : it contains animal mucilaginous matter ; for when exposed to the fire, it becomes very brittle, cracks, grows black, and separates from the inclosed bony part of the Tooth, (p) It is capable, however, of bearing a greater degree of heat than the bony part, without becoming brittle and black.* This substance * From this circumstance we can shew the Enamel better by burning a Tooth, as the bony part becomes black sooner than the Enamel. The method of burning, and shewing them after they are burnt, is as follows.— Let one half of a Tooth be filed away, from one end to the other, then burn it gently in the fire ; after this is done, wash the filed surface with an acid, or scrape it with a knife. By this method you will clean the edge of the Enamel, which will remain white, and the bony part will be found black. course. On the contrary, they are everywhere slightly and irregularly waved. On the summit of the crown they present two or three larger waves. Decussations of layers of the enamel prisms take place in transverse planes of the tooth. Hence it is that, on the addition of hydrochloric iicid, a longitudinal section will have a striated appearance, the prisms ■cut transversely will appear darker than those which present their longitudinal aspect. Layers of the enamel fibres may be seen on the summit of the crown to run in an annular form, describing circles on the molar, ellipses on the cutting teeth. Besides the striated appearance indicating the stratified arrangement of the fibres, Kolliker describes certain brownish or colourless lines crossing the enamel fibres in different directions, which in perpendicular sections are seen as obliquely ascending lines or arches, but in transverse sections as circles in the outer layers of the tissue, rarely occurring throughout its entire sub- stance. He regards them as " the expression of the lamellated mode of formation of the enamel."] (1) (o) [The enamel extends lower upon the inner and outer than upon the opposed surfaces of the crowns.] Q)) [Enamel in chemical composition differs from the other tissues of (1) Kolliker, op. cit., pp. 296, 297. 44 IIUNTEK ON THE TEETH. has no marks of being vascular, and of having a circulation of fluids : the most subtile injections we can make never reach it ; it takes no tinge from feeding with madder, even in the youngest animals ; and, as was observed above, when soaked in a gentle acid, there appears no gristly or fleshy part, with which the earthy part had been incorporated.* We shall speak of the use and formation of the Enamel here- after, when they will be better understood. OF THE BONY PART OF A TOOTH. The other substance of which a Tooth is composed, is bony ; but * In all these experiments I never could observe, that the Enamel was in the least tinged, either in the growing or formed Tooth. This looks as if the Enamel were the earth more fully depurated, or strained off from the common juices in such a manner, as not to allow the gross particles of madder to pass. Here it may not be amiss to remark, that the names given to animal substance, such as Gluten, &c. are not in the least expressive of the thing meant ; for there is no such thing as glue in an animal, till it has either undergone i putrefactive process, or been changed by heat. And here too I woidd be understood, that I call earth no part of an animal ; nor does it make up any part of an animal substance. the tooth in the exceedingly small proportion of animal matter which it contains. The organic matter differs from gelatine, and is in every respect analogous to the substance of the epithelium. The following is Pi bra's analysis : (1) Molar Tooth in an Adult Man. Phosphate of Lime, with some Fluoride of Calcium - 89 - 82 Carbonate of Lime - - 4-37 Phosphate of Magnesia 1-.34 Salts - - - - o-88 Organic substance - - 3\39 Fat - - - - - 020 [00-00 Organic substance Inorganic parts - (1) Kollikcr, pp. cit., p. OF THE BONY PART OF A TOOTH. 45 much harder than the most compact part of bones in general. (7) ((/) [In the following description of the minute structure of dentine (termed, in the text, the bony part of a tooth), the account given by Kolliker has been principally although not solely followed '. The dentine, ivory, substantia eburnea, ebur, is of a yellowish white colour, and appears in the fresh specimen to a certain extent transparent or translucent. In the dried tooth it is white, and has a silky lustre, from the circumstance that it is permeated in every part of its substance by a series of microscopic tubes which, in that condition, contain air. It consists of a matrix and of the tubes just mentioned, dentinal tubules, dental canals, canaliculi dentium. The dentine alone bounds the pulp cavity, with the exception of a small portion at the extremity of the root ; and in the uninjured tooth it is never exposed, being covered everywhere by enamel or cement. The matrix is perfectly homogeneous, showing, according to Kolliker, no trace of organised structure. The dentinal canals measure in width O'OOOG'" to 0-001'", some of those at the root 0-002'" : they commence by open mouths at the pulp cavity, and proceed outwards throughout the entire thickness of the tissue. Each canal appears to have a special wall in the shape of a yellowish ring, which is generally visible when the tubes are transversely divided. Kolliker believes, however, that the yellowish ring surrounding the tube is not to be considered as the real wall. " The apparent walls," he writes, " of the dentinal tubes, which are usually seen upon the transverse sections, are not the true walls of the canals, but rings, the appearance of which arises from this, that a certain length of the canals is always seen with the microscope in the thickness of the section, never sufficiently fine to obviate this effect ; and the short tubular segments being curved in direction, a greater apparent thickness is thus given to the walls than they really possess. If, upon a transverse section, the openings of the canals be brought accurately into focus, we perceive, instead of a dark ring, only a yellowish, very narrow edge; and it is this that I regard as being the true wall." (1) During life, the canals are generally believed to be permeated by a clear, pellucid fluid; in the dried state, they become filled with air, and appear as dark lines by transmitted, and shining filaments by reflected light. The dentinal tubes undulate in their course ; they present two or three larger flexures, and a great number of smaller ones — according to Retzius as many as 200 in a line. As they proceed outwards, they divide, branch, and anastomose. The divisions, which are frequent near the origin of the tubes, are generally dichotomous ; they may be repeated from two to five times or more — so that one tube may form four, eight, sixteen, or more tubes. Their calibre becomes diminished by division ; and they run in a nearly parallel (1) Kolliker, op. cit., p. 292. 40 HUNTER ON THE TEETH. This substance makes the interior part of the body, the neck, manner, and close together towards the surface of the dentine. In the eiown of the tooth, the dental tubes, at about the middle or outer third of their course, commence sending off fine ramifications, which are mostly simple, but sometimes branched. In this situation, a tube will frequently appear to terminate by dividing into two fine branches. In the fang, the ramifications are much more numerous than in the crown ; they commence earlier in the course of the tube, and sometimes give it a plumose, or, when the ramifications are long and branched, a brushdike appearance. These ramifications, by anastomosing, serve to connect with each other neighbouring or more remote canals. The terminations of the dentinal tubes are fine in proportion to their amount of ramifica- tion ; they frequently appear as extremely delicate pale lines, like the fibrils of connective tissue, and beeome at length so attenuated that they cannot be farther traced. Where, however, the tubes and their ramifications can be followed to their termination, they are either found to form loops in the substance of the dentine itself by the junction of one tubule with another (terminal loops of the dental canals), or to end in a granular layer to be hereafter noticed ; — or, passing across the boun- dary of the dentine, they may be traced into the enamel or crusta petrosa. (1) Dentine presents indications of lamelhtion. In a longitudinal section of a tooth, arched lines more or less parallel to the circumference of the crown, and situated at a varying distance from each other, are to be observed. In transverse sections they appear as rings. These are the contour lines of Professor Owen ; so called from their general similarity with the contour of the tooth. Mr Salter, who has described their appearance and arrangement with great minuteness, states, how- ever, that the course of the contour line never exactly corresponds with the circumference of the crown of the tooth. " The contour of the two," lie writes, "is not identical, for the markings (in whatever part examined) are more divergent than the outline of the tooth, and, passing from within outwards, abut in succession upon the external surface of the dentine. In comparing the absolute contour of any tooth, it will be found that the angle formed by its sides is more acute at its summit, or the summit of any particular cusp, than the contour markings within." (2) In teeth which have more than one cusp, the upper contour markings are confined to their own cusps, and their extremities abut against their sides; the lower ones are continuous with those of continuous cusps. These contour lines or markings are curved in their course : according to the observer just quoted, the curves are "in pro- (1) Vide Kolliker, op. cit., p. 281. (2) On certain Appearances occurring in Dentine, by S. J. A. Salter, Quart, Journ. Micros. Sci., vol. L, p. 254. OF THE BONY PART OF A TOOTH. 47 and the whole of the root of a Tooth. It is a mixture of two portion to the primary curves of the dentinal tubes at any particular spot, and cross them at right angles." (1) These lines vary in human teeth in intensity and in number. In the teeth of some animals they present a beautiful appearance ; especially is this the casein some of the Cetacea and Pachydermia, and in the Walrus. A tendency to break into laniellso is not unfrecpiently found in these and in f< issil teeth, and is some- times indicated in fresh human teeth and in tooth cartilage. In the human tooth, the extremities of the better defined contour lines terminate in irregularly-shaped cavities interglobular spaces, Czermak), situated at, the surface of the dentine, immediately within the cement and enamel. Mr Salter describes these spaces or cavities as being more or less club- shaped, with the butt end of the club towards the surface, and the pointed or attenuated end stretching obliquely inwards and upwards towards the pulp cavity. They vary in size ; their walls are formed by spherical masses or globules of dentine, which project into the cavity ; and they are traversed by the dentinal canals, the number of which will vary with the size of the cavity. The dental canals permeate the dentine globules, five or six traversing a large globule : in the dried specimen, when the cavity contains air, the individual tubules may be traced from one globule to another, skipping, so to speak, the inter- lo- bular space. Kolliker asserts that, during life, the spaces are filled with a soft substance resembling tooth cartilage, which is permeated by the tubules. He says that this soft substance " offers more resistance to hydrochloric acid than the matrix of the true ossified tooth, and on this account can be isolated exactly like the dentinal tubes." (2) In the smaller spaces, the spherical character of the tissue composing the walls is not so apparent ; the cavity has a more jagged outline, and resembles in appearance a lacuna of bone, the more so as it is traversed by the dentinal canals. In the fang, these smaller interglobular spaces and globules constitute what has been termed the 'granular layer' by Tomes. (3) True lacunae have been rarely seen by Kolliker in dentine, and never at a distance from the cement boundary. Interglobular spaces and globules sometimes occur in the interior of the dentine of the fangs ; and on the surface of the pulp cavity, the projecting spheres of dentine may produce irregularities or stalactite-like formations, visible to the naked eye. In many teeth, the interglobular spaces are absent, or but little marked: they are al ways most conspicuous in those specimens in which the enamel exhibits irregular development, and there appears to be a relation between the contour markings in the dentine and the grooves and irregularities in (1) Op. cit., p. 255. (2) Kolliker, op. cit., p. 294. (3) Tomes, Lectures on Dental Physiology and Surgery, p. 48. 48 HUNTER ON THE TEETH. substances, viz. calcarious earth and an animal substance, which the enamel. (1) In connection with this condition, also, Czermak has pointed out the existence of opaque white line>, forming rings round the fang: they are, of course, abnormal, and vary in breadth from the l-50th to the l-100th of an inch. The existence of the contour lines has been explained by referring them to the darker appearance produced by a series of secondary curves in successive dentinal tubes, or by a widen- ing of those tubes ; but it appears more probable that they are due, for the most part, to the same cause which has produced the interspaces and the globular condition of the dentine — viz., to the mode in which the animal material of dentine is calcified, and to occasional arrests in the process. Mr Salter observes that the contour markings, as well as the fracture lines, which readily occur in the intermediate normal dentine, and are parallel to them, exactly correspond to the pulp surface in the progressive formation of the dentine — are identical, in fact, with the juncture line of the pulp and internal dentine surface at any particular time of growth. The existence of interglobular sjiaces and persistent globules may be, with high probability, attributed to arrest at various times in the process of calcification. The observations of Czermak led him to the fact that the organic material of dentine is, during the pro- cess of calcification, impregnated with earthy salts in globular forms ; and that, by a deeper process of calcific impregnation, the whole tissue is imbued with the hardening element, and the globules are fused. It is easy, therefore, by supposing a temporary interference with this process from some constitutional cause, to account for the appearances we have been describing. (2) Some light is, perhaps, thrown on the process of calcification by some observations of Mr Rainje, quoted in Mr Tomes's 'System of Dental Surgery :' " Mr Eainie finds, that if carbonate of lime is formed in a thick solution of mucilage or albumen by the decomposi- tion of carbonate of soda or potash, the newly-formed salt takes a globular instead of a crystalline form. The globules produced are com- posed, however, not only of carbonate of lime, but also of a certain portion of mucilage or albumen in which the combination has taken place." Phosphate of lime, if produced under similar circumstances, supposing a small proportion of carbonate of lime be present, will also assume the globular form. The globules are laminated, and increase by the addition of new layers on the surface ; and if two or more globules are in contact, they become fused into one laminated mass by the union of the laminae which are in contact. "The globules themselves have been produced by the coalescence of smaller masses, which again are made up of still smaller spherules of similar material, the Individuality (1) Salter, op. cit., p. 253. (2) Vide Suiter, op. cit. OF THE BONY PART OF A TOOTH. 49 we might suppose to be organized and vascular. The earth is in very considerable quantity ; it remains of the same shape of the constituent bodies being ultimately lost in the uniform fusion of the whole into one compact mass." " In the discovery of the substitution of the globular for the crystalline form of these two salts of lime, Mr Rainie considers he has found an explanation of the process of calcifica- tion, not only of bone and teeth, but also of the formation of shells." (1) In many teeth, the original dentine globules are indicated by faintly- traced arched lines ; but the more perfectly calcified the tooth, the more completely will fusion have taken place. The dentinal canals are generally held to be permeated during life by a nutrient fluid. Mr Tomes, however, has announced the. discovery that each dentinal tube is permanently tenanted by a soft fibril, which passes from the pulp into the tube, and follows its ramifications. The dentinal fibril he describes as consisting of an almost structureless tissue, trans- parent, and of a comparatively low refractive power. He has not hitherto been able to determine whether it is tubular or solid. " In some cases," he writes, "there is an appearance of tubularity ; but, being cylindrical, this may be a mere optical effect. When accidentally stretched between two fragments of dentine, the diameter of the fibril becomes much diminished; and when broken across, a minute globule of transparent but dense fluid may sometimes be seen at the broken end, gathered into a more or less spherical form." These appearances may lead to the surmise that the fibril, like the white fibrillte of nerves, consists of a sheath containing a semi-fluid matter ; but such a conclusion is not to be accepted without farther evidence. Mr Tomes is unable to state the manner in which the fibrils he describes are connected with the pulp. He has traced them for a short distance into its substance, but at present cannot decide whether they terminate in cells, or are in any way connected with the nerves of the pulp. Although he does not take upon himself to affirm that these fibrils are of the nature of nerves, he yet regards them as the means by which sensibility is com- municated to the dentine. (2) These observations are highly interesting and important, but at present they require confirmation. Nasmyth took a different view of the structure of dentine. From his observations, he was led to deny the existence of the dentinal tubuli. He believed that " the so-called tube was in reality a solid fibre, com- posed of a series of little masses succeeding each other in a linear direc- tion, like so many beads collected on a string." The matrix — or, as he terms it, inter-fibrous substance — he describes as being originally cellular in composition ; and he maintains that the baccated fibres are, in fact, rows of persistent nuclei belonging to the cells of which the inter-fibrous (1) Tomes' s System of Dental Surgery, pp. 298, 299. (2) Tomes, op. cit, pp. 282-288. 50 HUNTER ON THE TEETH. after calcination, so that it is in some measure kept together by cohesion ; and it is capable of being extracted by steeping in the muriatic, and some other acids. The animal substance, when deprived of the earthy part, by steeping in an acid, is more compact than the same substance in other bones, but still is soft and flexible, (r) substance is composed. (1) His observations have not, however, been confirmed by those of any other writer on the dental tissues. Dentine exhibiting Haversian canals (the vaso-dentine of Owen) is very rarely seen in Man, although it occurs in many animals. The dentine which is produced in cases of obliteration of the pulp cavity in. iv, however, exhibit a few Haversian canals, and rounded cavities resembling lacunce: this has been described by Prof. Owen under the name of osteo-dentine. (2)] (r) [The organic basis of dentine (tooth cartilage) is identical chemically with that of bone : it is, by boiling, readily converted into gelatine. When separated by treating dentine with hydrochloric acid, it retains not only the form, but the internal structure of the tissue : the tubes, however, are not so easily visible. Kolliker states that if tooth cartilage " be macerated in acids or alkalies until it is quite soft, the matrix is found in the act of disintegration ; but the dentinal tubes, with their walls, are still preserved, and may be isolated in large quantities." The tubes may also be isolated after long-continued boiling. By prolonged maceration in acids or alkalies, the whole of the organic basis is dis- solved. The following is the chemical composition of dry dentine according to VonBibra:(3) Molar Tooth of a Man. Phosphate of Lime and some Fluoride of Calcium 6672 Carbonate of Lime 336 Phosphate of Magnesia 1-08 Salts 0-83 Cartilage 27-61 Fat 0-40 100-00 Organic substances ------ 28-01 Inorganic substances - 71-99] (1) Nasmyth, Researches, pp. 93, 94. (2) Vide Kolliker, op. et loc. cit. (3) Kolliker, op. cit., p. 292. OF THE BONY PART OF A TOOTH. 51 That part of a Tooth which is bony, is nearly of the same form as a complete Tooth ; and thence, when the Enamel is removed, it has the same sort of edge, point, or points, as when the Enamel remained. We cannot by injection prove that the bony part of a Tooth is vascular : but from some circumstances it would appear that it is so ; for the Fangs of Teeth are liable to swellings, (s) seemingly of the spina ventosa kind, like other bones ; and they sometimes anchylose with the socket by bony and inflexible continuity, as all other contiguous bones are apt to do. (f) But there may be a deception here, for the swelling may be an original formation, and the anchylosis may be from the pulp that the Tooth is formed upon being united with the socket. The following considerations would seem to shew that the Teeth are not vascular : first, I never saw them injected in any preparation, nor could I ever succeed in any attempt to inject them, either in young or old subjects ; and therefore believe that there must have been some fallacy in the cases where they have been said to be injected. Secondly, we are not able to trace any vessels going from the pulp into the substance of the new-formed Tooth ; and whatever part of a Tooth is formed, it is always completely formed, which is not the case with other bones. But what is a more convincing proof, is reasoning from the analogy between them and other bones, when the animal has been fed with madder. Take a young animal, viz. a pig, and feed it with madder, for three or four weeks ; then kill the animal, and upon examination you will find the following appearance : first, if this animal had some (s) [Exostosis of the fang, produced by hypertrophy of the cement.] (t) [Mr Tomes states that " although numerous instances may be found where two teeth become united by cementum developed under circumstances which constitute its formation a disease, yet in no well- authenticated instance has the cementum become continuous with the bone of the socket." (1) He believes in the existence of a law which prohibits the union of the tooth to the jaw in Man. Anchylosis of the teeth to the jaw is the normal condition in reptiles.] (1) Tomes, op. cit., p. 445. 52 HUNTER ON THE TEETH. parts of its Teeth formed before the feeding with madder, those parts will be known by their remaining of this natural colour ; but such parts of the Teeth as were formed while the ani- mal was taking the madder, will be found to be of a red colour. This shews, that it is only those parts that were forming while the animal was taking the madder that are dyed ; for what were already formed will not be found in the least tinged. This is different in all other bones ; for we know that any part of a bone which is already formed, is capable of being dyed with madder, though not so fast as the part that is forming ; there- fore as we know that all other bones when formed are vascular, and are thence susceptible of the dye, we may readily suppose that the Teeth are not vascular, because they are not susceptible of it after being once formed. But we shall carry this still farther ; if you feed a pig with madder for some time, and then leave it off for a considerable time before you kill the animal, you will find the above appearances still subsisting, with this addition, that all the parts of the Teeth which were formed after leaving off feeding with the madder will be white. Here then in some Teeth we shall have white, then red, and then white again ; and so we shall have the red and the white colour alternately through the whole Tooth. This experiment shews, that the Tooth once tinged, does not lose its colour ; now as all other bones that have been once tinged lose their colour in time, when the animal leaves off feeding with madder (though very slowly) , and as that dye must be taken into the constitution by the absorbents, it would seem that the Teeth are without absorbents, as well as other vessels. This shews that the growth of the Teeth is very different from that of other bones. Bones begin at a point, and shoot out at their surface ; and the part that seems already formed, is not in reality so, for it is forming every day by having new matter thrown into it, till the whole substance is complete ; and even then it is constantly changing its matter. Another circumstance in which Teeth seem different from bone, and a strong circumstance in support of their having no circulation in them, is that they never change by age, and seem OF THE CAVITY OF THE TEETH. 53 never to undergo any alteration, when completely formed, but by abrasion; they do not grow softer, like the other bones, as we find in some cases, where the whole earthy matter of the bones has been taken into the constitution. From these experiments it would appear, that the Teeth are to be considered as extraneous bodies, with respect to a circula- tion through their substance; but they have most certainly a living principle, by which means they make part of the body, and are capable of uniting with any part of a living body ; as will be explained hereafter : and it is to be observed, that affections of the whole body have less influence upon the Teeth than any other part of the body. Thus in children affected with tho rickets, the teeth grow equally well as in health, though all the other bones are much affected ; and hence their Teeth being of a larger size in proportion to the other parts, their mouths are protuberant. (?',) OF THE CAVITY OF THE TEETH. Every Tooth has an internal Cavity, which extends nearly the whole length of its bony part. It opens, or begins at the point of the fang, where it is small ; but in its passage becomes larger, and ends in the body of the Tooth. This end is exactly of the shape of the body of the Tooth to which it belongs. In general it may be said, that the whole of the Cavity is nearly of the shape of the Tooth itself, larger in the body of the Tooth, and (?/) [The dentinal tubules and their ramifications, the lacunae and canaliculi of the cement, and even the inter-fibrous spaces of the enamel, are probably all provisions by which fluid of a nutrient character may permeate the dental tissues. But there is no proper circulation through the calcified structures of the human tooth, unless in those exceptional instances in which the Haversian canals may be traced in the cement or dentine. Change of material is much slower than in bone, as is proved by the above experiments of Hunter, which have been confirmed by the observations of Flourens and others. (1) There can be no doubt, how- ever, that the formation and development of the teeeth are considerably influenced by constitutional states. Early caries having its origin in a faulty condition of the dental tissues is one of the most frequent results of a weakly, ill-nourished childhood.] (1) Kulliker, op. cit., p. 309. 54 HUNTER ON THE TEETH. thence gradually smaller to the extremity of the fang ; simple, where the Tooth has but one root ; and in the same manner compounded, when the Tooth has two or more fangs. This Cavity is not cellular, but smooth in its surface : it con- tains no marrow, but appears to be filled with blood-vessels, and, I suppose, nerves, united by a pulpy or cellular substance, (v) The vessels are branches of the superior and inferior Mamillaries ; and the nerves must come from the second and third branches of the fifth pair. By injections .we can trace the blood-vessels distinctly through the whole Cavity of the Tooth ; (w) but I could never trace the Nerves distinctly even to the beginning of the Cavity, (x) (v) [The dentinal pulp is that portion of the fetal tooth-papilla which remains after the development of the dentine. It is connected below with the periosteum lining the alveolus, passes up the aperture in the fang, and accurately fills the central cavity of the tooth, being everywhere in close contact with and adherent to the inner surface of the dentine. It is a soft red substance, highly vascular, with the exception of a layer on its surface, and well supplied with nerves. In structure that portion which contains vessels consists, according to Kolliker, of " an indistinctly fibrous connective tissue, with very numerous round or elongated nuclei interspersed." The same authority describes the pulp as being covered by a delicate structureless membrane, under which is seated "a layer (ii)2'", 0-03'", to 0-04'" thick, consisting of several rows of cylimlrical or conical nucleated cells, 0-012"' long, 0-002'" to 0-003'" broad, disposed perpendicularly upon the surface of the pvdp like a columnar epithelium. Further inwards, these cells are arranged less regularly, and ai pass into the vascular tissue of the pulp without well-defined limits by the medium of shorter and more roundish cells." (1). These cells corre- spond with those termed by the same author "the formative cells of th< dentine," and he believes that they furnish materials for any deposition of dentine on the walls of the cavity which may occur in the adult.] From three to ten small arteries may be observed entering tin- pulp of each tooth. By ramifying in the pulp substance, they produce a loose net-work of capillaries, from which the veins arise. No lymphatics have been observed in the dentinal pulp. (2)] (x) According to Kolliker, the nerve supply to each tooth consists of a principal trunk, 0-03'" to 0"04'" in size, and of six or even more fine (1) K. 'Hiker's Manual of Human Microscopical Anat.j pp. 299, 300. (2) Op. cit., p. 300. OF THE PERIOSTEUM OF THE TEETH. 55 OF THE PERIOSTEUM OF THE TEETH. The Teeth, as we observed, are covered by an Enamel only at their bodies ; but at their fangs they have a Periosteal)), which, though very thin, is vascular, and appears to be common to the Tooth which it incloses, and the socket, which it lines as an investing internal membrane, (y) It covers the Tooth a little beyond the bony socket, and is there attached to the Gum. twigs, 0-OT" to 0-Q2"'. These ascend into the pulp cavity without forming at first considerable anastomoses, but giving off separate fibrils. On reaching the thicker part of the pulp, they form a rich plexus " with elongated meshes and collections of nerve tubes, and thus gradually break up into tine primitive fibres, O'OOl'" to 0-0016'" in diameter." These primitive fibres form evident loops, but it is uncer- tain whether they represent the absolute terminations. (1) Mr Tomes states that he has been unable to trace any connection between the ultimate nerve fibres of the pulp and the dentinal fibrillse which be describes. (2) It would appear that at least in some instances the size of the nerve supplying the tooth-pulp is proportionate to the size and importance of the tooth. Thus Professor Owen notices the large size of the nerve supplying the laniary in the Dog and other Carnivora. In the Boar, he observes that the nerve supplying the developed tusk is still larger, having relation not oidy to the size of the tooth, but also to the continual reproduction of the matrix at its base. (3)] (y) [A difference of opinion appears to exist between writers on the subject of the dental periosteum. Hunter, Bichat, and Kblliker describe but one layer of periosteum as common to the tooth and the socket. The latter author writes, "the periosteum of the alveoli is very accurately applied to the surface of the fangs." Other observers, amongst whom are Fox, Bell, Spence Bate, and Hulme, believe that the periosteum consists of two layers — one lining the alveolar cavity, the other covering the fang. Mr Bell asserts that he has frequently removed a tooth in the dead subject, and found not only the fang covered, but the socket lined with periosteum. (4) Mr Spence Bate applies the term " peridenteum" to the layer covering the fang, to distinguish it from the periosteum of the alveolus. He regards the peridenteum as a dermal tissue distinct in origin from the periosteum, and on their complete separation, he believes, depends the (1) Kolliker, op. cit., p. 300. (2) Tomes, Manual of Dental Surgery, p. 286. (3) Odontography, Introduction, p. lxvi. (4) The Anatomy, Physiology, and Diseases of the Teeth, byT. Bell, F.R.S., p. 43, 2nd edition. 50 HUNTER ON THE TEETH. OF THE SITUATION OF THE TEETH. The general shape and situation of the Teeth are obvious, (z) The opposition of those of the two Jaws, and the circle which each row describes, need not be particularly explained, as they may be very well seen in the living body, and may be supposed to be already understood, from what was said of the Alveolar Processes. We may just observe, with regard to the situation of the two rows, that when they are in the most natural state of contact, the Teeth of the Upper-Jaw project a little beyond the lower Teeth, even at the sides of the Jaws ; but still more remark ably at the fore part, where in most people the upper Teeth lie before those of the Lower- Jaw : and at the lateral part of each row, fact that osseous union is never observed between the tooth-fang and the alveolus. (1) Mr Hulme, in his published Lectures on Diseases of the Dental Periosteum, adduces the case of some herbivorous quadru- peds, as for instance the common Ox, in which the distinction of the layers is easily shown in support of a similar view. The latter author does not allow that the layer covering the fang is merely a reflection of that lining the socket : he believes, with Mr Bate, that there is a distinct origin for each membrane. " The alveolus," he writes, " is provided with its periosteum long before the crown of the tooth is completed ; whereas in the tooth there is no periosteal layer until the formation of its fang and its investing layer of cement. At the period when the fang of the tooth is about to be formed, the sac of the tooth- germ becomes adherent to the neck of the tooth ; its outermost layer continues to grow in the same ratio as the fang, and becomes the formative organ of the cement. When the tooth is completed, this membrane still remains, and constitutes its periosteum. At the apex of the fully-developed fang, the periosteum of the tooth becomes intimately associated with that of the alveolus, and the contiguous surfaces of the two membranes are connected together by the passage of blood-vessels and nerves." (2) Kolliker states that the alveolar periosteum is suiter than that of other bones, that it contains no elastic elements, but rich plexuses of nerves having thick nerve-tubes. (3)] (2) [In the class Mammalia, true teeth implanted in sockets are (1) On the Peridental Membrane, in its relation to the Dental Tissues, - British Journal of Dental Science, vol. i., p. 7. (2) Dental Review, vol. hi., p. 203. (13) Kolliker, op. cit., loc. cit. OF THE NUMBER OF TEETH. 57 the lino, or surface of contact, is hollow from behind forwards, in the Lower-Jaw ; and in the same proportion it is convex in the Upper-Jaw. The edge of each row is single at the fore part of the Jaws ; but as the Teeth grow thicker backwards, it there splits into an internal and external edge. The canine Tooth, which we shall call Cuspidatus, is the point from which the two edges go off ; so that the first grinder, or what we shall call the first Bicuspis, is the first Tooth that has a double edge. OF THE NUMBER OF TEETH. Their number in the whole, at full maturity, is from twenty- eight to thirty-two : I once saw twenty-seven only, never more than thirty-two. Fourteen of them are placed in each Jaw, when the whole number is no more than twenty-eight; and sixteen, when there are thirty-two. If the whole be twenty- nine or thirty-one in number, the Upper-Jaw sometimes, and sometimes the Lower, has one more than the other ; and when the Number is thirty, I find them sometimes divided equally between the two Jaws ; and in other subjects sixteen of them are in one Jaw, and fourteen in the other. In speaking of the Number of Teeth, I am supposing that none of them have been pulled out, or otherwise lost ; but that there are from eight to twelve of those large posterior Teeth, which I call Grinders, and that they are so close planted as to make a continuity in the circle : and in this case, when the number is less than thirty- two, the deficiency is in the last grinder, (a) restricted to the maxillary, premaxillary, and inferior maxillary bones. They form a single row in each jaw, and in most Mammals, as in Man, occupy each of the above-mentioned bones. They may, however, project only from the premaxillary bones, as in the Narwhal ; or only from the lower maxillary bone, as in Ziphius ; or be implanted only in the superior and inferior maxillaries, and not in the premaxillaries, as in the true Ruminantia and most Bruta (Sloths, Armadillos, Orycte- ropes). (1)] (a) [The normal number of the permanent teeth in Man is thirty-two. (1) Owen on the Characters of the Class Mammalia,— Journal Tioc. Lin. Soc, vol. ii., p. 6. 58 HUNTER ON THE TEETH. The Teeth differ very much in figure from one another ; but those on the right side in each Jaw resemble exactly those on the left, so as to be in pairs ; and the pairs belonging to the Upper-Jaw nearly resemble the corresponding Teeth of the Lower-Jaw in situation, figure, and use. Each Tooth is divided into two parts, viz., first, the body, or that part of it which is the thickest, and stands bare beyond the Alveoli and Gums ; secondly, the fang, or root, which is lodged within the Gum and Alveolar Process : and the boundary between these two parts, which is grasped by the edge of the Gum, is called the Neck of a Tooth. The bodies of the different Teeth differ very much in shape and size, and so do their roots. The difference must be considered hereafter. The Teeth of each Jaw are commonly divided into three classes, viz. Incisors, Canine, and Grinders ; but from consi- dering some circumstances of their form, growth and use, I chuse to divide them into the four following classes, viz. Inci- It occasionally happens that the last molars, or wisdom teeth, remain enclosed in their bony recesses until a late period of life. In some few- instances they may be absent. In the class Mammalia, examples of the greatest number of teeth are observed amongst those forms which generate but a single set of teeth {Monophyodonts), and occur in the orders Cetaceaand Bruta. Thus the priodont Armadillo has ninety-eight teeth ; the Cachalot, u] twards i if sixty, most of them being confined to the lower jaw ; the common Porpoise has between eighty and ninety ; the Gangetic Dolphin, one hundred and twenty ; whilst the true Dolphin presents the maximum number in Mammalia, having from one hundred to one hundred and ninety teeth. Where the teeth are excessive in number, they are of small size, equal or nearly so, and usually conical in form. In placental Mammalia which have two sets of teeth the one succeed- ing the other (Diphyodonts), the number of the permanent teeth never exceeds forty-four. A few genera and species of Mammalia are strictly edentulous. Thus the true Ant-eaters (Myrmecophaga), the scaly Ant-eaters (Manis), and the spiny monotrematous Ant-eaters (Echidna) possess no true teeth, although in Echidna homy processes are present analogous to the lingual and palatal teeth in fishes. (1)] (1) Owen, op. cit., pp. 5— 9, and Art. Odontology, Enc. Brit., 8th edit., 438. OF THE NUMBEB OF TEETH. o ( J sores, commonly called Fore Teeth ; Guspidati, vulgarly called ( 'anine ; Bicvspides, or the two first Grinders ; and Molares, or the three last Teeth. The number of each class, in each Jaw, for the most part, is four Incisorcs, two Cuspidati, four Bicus- pides, and four, five, or six Molares. (b) There is a regular gradation, both in growth and form through these classes, from the Incisorcs to the Molares, in (b) [The permanent human dentition is indicated by the subjoined formula: 2_2 1—1 2-2 3-3 2-2 1-1 2-2 3-3 The following is the definition and classification of teeth in diphyodont Mammalia proposed by Professor Owen. The permanent teeth in diphyodont Mammalia are referrible to four classes — viz., incisors, canines, premolars (called in Man bicuspides), and molars. The terms ' incisor,' ' canine,' and ' molar' had originally reference to the shape and office of the teeth : they are now, however, used in Comparative Anatomy to distinguish teeth according to their relative position, connec- tions, and development ; the latter circumstances constituting a truer guide to the homologies of organs than shape, size, or office. The incisors are those teeth which are implanted in the premaxillary bones and in the corresponding part of the lower jaw. Thus the term incisor comes to include the tusk of the Elephant, the pectinated lower front teeth of the Flying Lemur, the broad tuberculate front teeth of some ( !arni vora. The tooth in the superior maxillary bone which is situated at or near to the suture between it and the premaxillary, is the upper canine ; the lower canine is that tooth which, in opposing the upper, passes in front of its crown when the mouth is closed. The premolars are those which succeed the deciduous molars. The permanent teeth, the most posterior in position, which do not occupy places previously occupied by deciduous teeth, but which are themselves a backward continuation of the first series of teeth, being developed in the same primary groove of the fetal gam, are the true molars. From the dentition of early forms of both Herbivorous and Carni- vorous Mammalia which existed during the eocene tertiary periods, Professor Owen has been led to regard three incisors, one canine, and seven succeeding teeth on each side of both jaws as the type formula of diphyodont dentition. Of the seven teeth, four may be premolars succeeding four deciduous molars, and three molars ; or there maybe three premolars succeeding three deciduous molars, and four true molars. The latter type of dentition is peculiar to the Marsupials and Monotremes ; the former is the typical formula of the placental diphyo- dont series. Amongst the few existing Mammals which have retained 60 HUNTER ON THE TEETH. which respect the Cuspidati are of a middle nature, between the Incisorcs and Bicuspides, as the last are between the Gus- pidati and Molarcs ; and thence the Incisores and Molares are the most unlike in every circumstance.* (c) * It is here to be understood, that the Teeth from which we take our description, are such as are just completely formed, and therefore not in the least worn down by mastication. Our description of each class is taken from the Lower-Jaw; and the difference between them, and their corresponding classes in the Upper immediately follows that description. the typical number of teeth, are the Hog and the Mole. The permanent teeth in the genus Sus are indicated by the following formula : 3-3 1-1 4-4 3-3 i c p m =44 3-3 1-1 4-4 3-3 " When the premolars and the molars are below this typical number, the absent teeth are missing from the fore part of the premolar series and the back part of the molar series. The most constant teeth are the fourth premolar and the first true molar ; and these being known by their order and mode of development, the homologies of the remaining molars and premolars are determined by counting the molars from before backwards,— e.g., ' one,' ' two,' ' three,'— and the premolars from behind forwards—' four,' ' three,' ' two,' ' one.' The incisors are counted from the median line, commonly the foremost part, of both upper and lower jaws, outwards and backwards. The first incisor of the right side is the homotype, transversely, of the contiguous incisor of the left side in the same jaw, and vertically, of its opposing tooth in the opposite jaw ; and so with regard to the canines, premolars, and molars ; just as the right arm is the homotype of the left arm in its own segment, and also of the right leg in a succeeding segment. It suffices, therefore, to reckon and name the teeth of one side of either jaw in a species with the typical number and kinds of teeth : e. g., the first, second, and third incisors ; the -first, second, third, and fourth premolars ; the first, second, and third molars ; and of one side of both jaws in any case." (1) The fol- lowing formula therefore represents the homologies of the human teeth in relation to the typical formula : i. 1, i. 2 ; c ; p. 3, p. 4 ; m. 1, m. 2, m. 3; — the third incisor and the first and second premolars being sup- pressed in Man, as they are also in the Catarhine Quadrumana. (2)] (c) [The large size of the molars as compared with the incisors, canines, and premolars, and the regular gradation in size of the human teeth from the incisors to the molars, are amongst the dental character- istics which distinguish Man from the higher Apes.] (1) Owen, op. cit., p. 9. (2) Owen, Art. Teeth, Todds Cyclopaedia, p. 904. : UK [NCISORES; 61 OF THE INCISURES. The Incisores are situated in the anterior part of the Jaw ; (>?) the others more backwards on each side, in the order in which we have named them. The bodies of the Incisores are broad, having two flat surfaces, one anterior, the other posterior. These surfaces meet in a sharp cutting edge. The anterior sur- face is convex in every direction, and placed almost perpendicu- (rf) It has been already stated that in scientific nomenclature the term incisor is restricted to those teeth which are implanted in the pre- maxillary hones, and in the corresponding part of the lower jaw. In the Mammalian series, (1) if we commence with the order Bruta, we find many instances of the total absence of teeth thus implanted. This is the case in the Phyllophagous Bruta — the Sloths or Tardigrades; and amongst the Insectivorous Bruta, in the Cape Ant-eater {Orycteropus Capensis), and in the Great Armadillo (Priodon gigas). In the Armadillos of the sub-genus Euphractus, however, a single tooth which resembles the succeeding molars is implanted in the premaxillary bone ; and in the lower jaw the two anterior teeth being in advance of the premaxillary tooth, are also to be considered incisors. A portion of the lower jaw of the great extinct Armadillo (Glyptodon) shows that the teeth extend close to the symphysis, a corresponding implantation in the premaxillaries above being thereby indicated. The premaxillary bones are generally toothless in the true Cetacea : the Dolphin, however, is an exception, one pair of its numerous teeth being premaxillary. The Narwhal (Monodon monoceros) may be also consi- dered another exceptional instance. In the female Narwhal, a tusk is concealed in each premaxillary bone at its junction with the maxillary, but remains throughout life an abortive germ; in the male, the tooth on the left side continues to grow, and is developed into the so-called "horn," acquiring a length of nine or ten feet. In common with many Bruta, the Carnivorous Cetacea exhibit an infe- riority in their dental system not only in the acquisition of but a single set, but in the uniformity of shape which characterises the dental organs ; but on turning to the Aquatic Pachyderms, designated by Cuvier Herbivorous Cetacea, we find a higher type of dentition attained. The Dugong (Halicore) has incisors distinguished both by shape and position from the molars ; in the upper jaw are two deciduous incisive tusks (1) In compiling the information contained in the above and following notes on the Comparative Anatomy of the TWtli in Mammalia, the Editor has been chiefly indebted to the writings of Prof. Owen, F.R.8., especially the article 'Odontology,' in the last edition of the 'Encyclopaedia Britannica.' I 62 HUNTER ON THE TEETH. larly ; and the posterior is concave and sloping, so that the cutting edge is almost directly over the anterior surface. These surfaces are broadest and the Tooth is thinnest at the cutting edge, or end of the Tooth, and thence they become gradually narrower and the Tooth thicker towards the neck 7 which are displaced vertically and succeeded by permanent tusks. It is, however, in the male sex and in the upper jaw only that these teeth project from the gum ; in the female Dugong both upper and lower incisors remain concealed throughout life. The superior incisors are two in number in both sexes ; in the male the extremity only of the tusk projects from the jaw, at least seven-eighths of its extent being lodged in the socket. The male tusk is subtrihedral, moderately and equally curved, and its extremity is bevelled off to a cutting edge like the scalpriform incisors of the Rodents. In the young American Manatee (Manatus Americanus) each premaxillary bone supports a deci- duous tusk, which, however, is not replaced In the gum which covers the deflected portion of the ramus of the lower jaw in the new-born Manatee, six depressions for rudhnental teeth are to be observed; in one of these an incisor tooth was observed by Stannius. Incisors are present in all the species of the order Marsupialia, but they vary in number, in some instances exceeding that of the Mammalian type. In the carnivorous Dog-headed Thylacine and Ursine Dasyure, there are eight incisors in the upper jaw, and six in the lower ; in the Dasyures these teeth are simple in structure, and are arranged in a regular semicircle. The existing Australian genus Myrmecobius, and the extinct Marsupial genera Phascolotherium and Amphitherium, found in the oolitic slate at Stonesfield in Oxfordshire, afford examples in which the incisors are separated from each other and from the canines by vacant spaces. Ten incisor teeth in the upper jaw and eight in the lower are found in the Opossums (Didelphys) ; the two middle upper incisors are longer than the others, from which they are separated by a short interspace. In the Tapoa {Phalangista fuliginosa), the Koala {Phascolarctos cinereus), the Kangaroo Rats {Hypsiprymnus), and the Kan- garoos (Macropus), there are six incisors in the upper jaw, whilst the lower are reduced to two. The two anterior upper incisors are more than twice as large as the lateral in the Koala; the same teeth also attain large dimensions as compared with the others of the series in the Kangaroo Rats : in the latter genus their pulps are persistent. The feeble development of the lateral superior incisors in Phascolarctos and Hypsi- prymnus is succeeded by their total suppression in the Marsupial Rodent, the Wombat. In the Wombat (Phascolomys), the incisors are two in the upper and two in the lower jaw ; they are genuine dentes scalprarii with persistent pulps, although inferior, especially in the lower jaw, in length and curvature to those of the true Rodents OF THE INC1SORES. G3 where the surfaces are continued to the narrowest side, or edge of the fang. The body of an Incisor, in a side-view, grows gradually thicker, or broader, from the edge or end of the A transition between the dentition of the Kangaroos and the Wombat is presented by a gigantic extinct Australian herbivorous Marsupial, the Diprotodon. In this species the general dentition approached that ot the Kangaroo ; but the median upper incisors were large curved scalpri- forni tusks which worked against a pair of procumbent tusks below. In the different species of the order Insectivora, the incisors differ in number and size : in some species the anterior ones approximate more or less to the scalpriform teeth of the Rodents. In the Cape Mole (Chryso- chloris Capensis), the Shrew Moles of America (Scalops), in the Solenodon paradoxus of Hayti, there are three incisors in each premaxillary, and the median one is of large size. In Scalops this tooth is scalpriform, in Chrysocldoris and Solenodon laniariform. In the lower jaw the anterior incisor in these genera is of small size and procumbent, whilst the second incisor is large and laniariform: a third lower incisor of small size is present in Chrysocldoris and Solenodon, but is absent in Scalops The incisor teeth in the European Mole {Talpa) are six in number in each jaw, small, and simple in conformation ; the fourth tooth on each side below, although resembling the incisors in shape, is to be considered a canine — its crown passing in front of the upper canine when the mouth is shut. The typical Shrews (Soricidce) manifest their analogy to the Rodents by the great preponderance in size of the first two incisors in both jaws, and the sub-genera of Shrews are partly founded on variations in the shape of these teeth. In the European Hedgehog (Erinaceus Europmus) these teeth are six in each jaw, the anterior pair both above and below being larger and longer than the rest and very deeply implanted. The number is reduced to; four in each mandible in the tropical Hedgehogs Echinops and Ericulus; in the Tenrecs or Tailless Hedgehogs of Madagascar (Centetes) there are four small incisors above, whilst the typical number six is retained below. The incisors in Cheiroptera may be entirely wanting or present in the numbers 1. 1. to 2. 2. in the upper jaw, and 1. 1. to 3. 3. in the lower ; they are always very small, and in the upper jaw unequal and separated by a median interval. In the Suctorial or Vampire Bats (Desmodus) the anterior teeth are modified in accordance with their habits. The upper incisors are two in number, one in each premaxillary bone, closely approximated, with a very large compressed curved and sharp- pointed crown, implanted by a strong fang which extends beyond the pre- maxillary into the maxillary bone. They are succeeded by similarly- formed canines. In the lower jaw, the incisors, two in number on each side, are much smaller than in the upper, and have bilobed crowns. In I 2 G4 HUNTER ON THE TEETH. Tooth to its neck ; and these coincide with the flat, or broad side of the fang ; so that when we look on the fore part, or on the back part of an Incisor, we observe it grows constantly the Colugos (Galeopithecus), which in their general organisation approach the Lemurs, although placed by Cuvier at the end of the Bats, we find two incisors in the upper jaw on each side, of which the two anterior are separated by a wide interval ; in the Philippine Colugos they are small with bilobed crowns. In Galeopithecus Temminckii the crown is an expanded plate with three or four tubercles ; the second upper incisor in both species is implanted by two fangs. The lower incisors are three in number on each side. The crowns of the first two present the form of a comb, — a configuration which, unique in Mammalia, depends upon the prolongation of notches deeper and more numerous, but yet analogous to the indentations on the cutting edge of the newly-formed human incisor. In the extensive order of Rodents, we find the incisors represented by a large curved ever-growing pair of teeth in each jaw, which are separated by a wide interval from the short molar series. To this there is but one exception — the family of the Leporidce, Hares, Rabbits, and Picas, in which a minute second incisor occurs on each side behind the large upper scalpriform tooth. The upper incisors describe a larger segment of a smaller circle ; the lower, a smaller segment of a larger circle : the lower are the larger teeth, and their sockets extend to the back part of the jaw on the inner side of the molars. These teeth are of unlimited growth ; and if by accident the opposing teeth are lost, they continue to grow until they describe a complete circle, perforating the skin, and causing absorption of the bone with which they come in contact. Throughout life the base of the tooth continues widely open, and contains a long conical persistent pulp, which is surrounded by a pro- gressively ossifying capsule. Near the crown an enamel-pulp is attached to the inner side of that part of the capsule which covers the convex surface of the curved incisor. The tooth consists of a body of dentine with a plate of enamel on its anterior surface, and a general investment of cement, which is, however, very thin upon the enamel. The mutual action of the lower and upper incisors produces an oblicpiely worn surface on the extremity of the crown, which slopes backwards from the anterior edge of hard enamel : the resemblance so produced to the cutting edge of a chisel has given the name of " dentes scalprarii" to these teeth. The varieties the scalpriform incisors present in the different species of Rodents are limited to their size, and the colour and sculpturing of their anterior surface. In the Coypu, Beaver, Agouti, ami siinie other species, the enamelled surface isof an orange or brownish- red colour. In some genera the same surface presents a longitudinal groove. OF THE [NCISORES. 65 narrower from its cutting edge to tlie extremity of its fang. But in a side-view it is thickest or broadest at its neck, and thence becomes gradually more narrow, both to its cutting edge, and to the point of its fang. On turning to the order Quadrumana, we find a genus (Cheiromys) in which, as in the Marsupial Wombat, the Suctorial Bats, and the Insecti- vorous Shrews, the dentition is modified in analogy with the Rodent Type. In the Aye-aye (Cheiromys Madagascariensis) canines are want- ing, and a wide interval separates two large curved scalpriform incisors from the molar series. These teeth differ from the incisors of the Rodents in having an entire investment of enamel, which is, however, thickest on the anterior surface. In the slow Lemurs (Stenops), and in the true Lemurs, or Makis, there are four incisor teeth in each jaw. In the Stenops tardigradus the first upper incisor is larger than the second : in the true Lemurs the upper incisors are small and vertical, with expanded crowns ; an interval separating the two on the right side from those on the left. The inferior incisors with the canine are long, narrow, compressed, and procumbent. In the Platyrhine Quadrumanes of America, as in the Quadrumana of the Old World, the incisors are f : f : ; the Sakis manifest the lemurine character of long narrow inferior incisors. In Cebus the inferior incisive teeth have broad, thick, wedge-shaped incisive crowns; and this character is generally retained throughout the Quadrumanous series. The Simice of the Old World present the nearest approach to human conformation offered by the animal kingdom. The large black tailless Apes of Africa (Troglodytes), and the red Orangs of the Indian Archipelago (Pithecus), afford the closest approximation. It will be interesting here to compare this part of the dental series in the two best-known African species — the Gorilla and Chimpanzee, and in the two species of Orang, with the human permanent incisors. The characteristics of the human incisors are their smallness of size in proportion to the other teeth and to the entire skull ; their near equality of size with each other ; their thin wedge-like form, and their vertical, or nearly vertical, implantation. In the upper jaw the two middle incisors slightly exceed the lateral in breadth ; the posterior surfaces of both are smooth and slightly concave. The middle incisor presents a slightly-developed basal ridge on its posterior surface; but sucli elevation is entirely absent on the posterior surface of the lateral one. The external angle of the crown of the lateral incisor is not rounded off, and is in contact with the canine. In the lower jaw the incisors are somewhat smaller than in the upper, and the breadth of the central is slightly inferior to that of the lateral; the outer angle of the crown of the latter is entire. In the Gorilla (see plate I) and Chimpanzee the inoism teeth are not only absolutely .much larger (thicker ui hi broader) than in man, but they are of i'ar greater size in proportion to the molar series 66 HUNTER ON THE TEETH. The Enamel is continued farther down, and is thicker on the anterior and back part of the Incisorcs than on their sides, and is even a little thicker on the fore part than upon the back part of the Tooth. If we view them laterally, either when intire, or and to the entire skull. In the Gorilla, the transverse measurement of the four incisor teeth is not greater than in the Chimpanzee ; they are therefore proportionately smaller ; and in this respect, although still very far removed, the Great Ape makes a nearer approach to human dimensions. A further deviation from human type is seen in the greater inequality of the incisors of the upper jaw, the central incisors being considerably broader than the lateral. This inequality is greatest in the Gorilla, but in both species it is a marked point of differentiation. Each incisor has a well-developed prominent basal ridge on its posterior surface ; and the outer angle of the crown of the lateral, instead of being entire, as in Man, is rounded off. In the lower jaw these teeth are equally characterised by their great size. As in Man, the lower lateral are broader than the central ones ; but the former have the external angles of the crowns rounded off. In both jaws the direction of the incisive teeth departs from a vertical or nearly vertical position. In the incisors of the Orangs we notice a still greater divergence from human type in the superior breadth of the entire series, in the greater size of the upper central teeth, and the greater amount of inequality between them and the lateral. The central incisor of the upper jaw in Pith. Satyrm and Pith. Mario is of unusual size and strength, being double the width of the outer one, whdst its thickness is nearly equal to its transverse measurement. From their constant use in overcoming the resistance of hard and tough vegetable substances, they become worn down ; and in the old Orang they present a broad abraded surface, which extends obliquely backwards from the cutting edge to the posterior margin of the base of the crown. The lateral incisors have their external angles obliquely truncated ; in the lower jaw they exceed in transverse measurement the dimensions of the central. In the obliquity of their implantation in both jaws they exhibit the ordinary Quadrumanous character. In the Gorilla and Great Orang, as in the Quadrumana generally, a well-marked interval or diastema separates the upper incisors from the canine. The incisors are six in number in each jaw in the typical Carnivora. In the Lion they form a transverse row; the outermost above is the largest, and resembles a small canine ; the intermediate ones have broad and thick crowns indented by a transverse cleft. These teeth are tin- ployed in gnawing the gristly ends of bones, and scraping off the attachments of periosteum and muscles. The crowns of the incisors in the Hyamas form a similar transverse line ; the crown of the external one above is laniariform; the crowns of the intermediate teeth are OP THE INCISURES. 67 when cut down through the middle, but especially in the latter case, it would seem as if the fang was driven like a wedge into, and had split the body or Enamel of the Tooth. They stand divided by a transverse cleft into a strong anterior corneal lobe and a posterior ridge, wkich is notched vertically, giving to the tooth a three- lobed configuration. The inferior incisors increase in size from the first to the third ; the second and third have the crown indented externally, but they do not present a posterior notched ridge ; the conical points oi* these teeth fit into the depressions separating the lobes of the incisors above. The incisive teeth in the Dog form a segment of a circle in both jaws ; they increase in size from the first to the third ; the edge is divided by two notches into a large middle and two smaller lateral lobes. In the most aquatic and piscivorous of the Mustelidce — viz., the Sea-Otter (Enhydra) — the number of incisors is reduced by the absence of the two central in the lower jaw ; their number is still farther diminished in some of the Phocidce: in ths great proboscidian and hooded Seals (Cystophora) the incisor formula is ff ; they are, however, of large size, and laniariform, the two outer ones above being largest. In the young Wahais there are three teeth in each premaxillary above, and two on each side below ; but they soon disappear, with the exception of the outer and upper one on each side, which remains on the inner side of the enormous canine tusk. The normal number of incisive teeth, fj, which we find in the extinct herbivorous Dichodon of the eocene tertiary deposits, is replaced in the existing typical Ruminants by the total suppression of incisors in the upper jaw, whilst the number six is retained in the lower. In the hollow-horned Ruminants Antilopidce, Ovidce, and Bovidaz, a callous pad supplies the place of the upper incisors, albeit their rudiments have been observed by Professor Goodsir and others in the embryo Sheep and Cow. The incisors in the lower jaw have low, broad crowns. Upper incisors are also absent in the solid-horned Cervidce: in the Camels one laniariform incisor is present in each premaxillary bone. Professor Owen has described six deciduous upper incisors in the new-born Dromedary, of larger size than any rudiments of these teeth which exist in the hollow-horned Ruminants. The Suidce, or Hog tribe, retain the typical dental formula which is exemplified in the Chieropotamus, Anthracotherium, and other extinct non- ruminant Axtiodactyles. In the Hog, the upper incisors decrease in size from the first to the third ; the central upper incisors are inclined to each other, and touch by their prolonged inner surfaces ; their crowns are short, strong, and obtusely pointed : the crown of the second is as broad as that of the first, but shorter and thinner ; it has a trenchant and dentated edge, which soon, however, becomes worn clown. The third is a small tooth separated by a short interval from the second. In the 68 HUNTER OX THE TEETH. almost perpendicularly, their bodies being turned a little for- wards. Their fangs are much shorter than those of the lower jaw, the incisors are long, straight, and sub-compressed : the third is the smallest, the second rather larger than the first. An excessive development of the canine and incisor teeth marks the genus Hippopota- mus. Of the incisors there are four in each jaw; the lower attain the most remarkable development. The two median lower incisive tusks are cylindrical, of great size and length, and are worn obliquely on th* outer and upper side of their extremity ; the deeply-implanted basal portion is grooved longitudinally : the two outer incisors are likewise cylindrical and straight, but of smaller size ; they exhibit an abraded surface towards the inner side of the apex. A large persistent matrix occu- pies the excavated base of these teeth, and provides for constant growth and reparation. An ancient form of Hippopotamus which formerly inhabited India retained the typical number of incisors J; f . . Amongst Perisso-dactyle Ungulata that number is retained in the genera Equus and Tapirus. In the Horse, the crowns of the incisors form the arc of a circle at the extremity of each jaw. They are distin- guishable from the incisors of all other animals by the vertical fold of enamel which dips down into the substance of the crown from its broad, flat upper surface, like the inverted finger of a glove. In the moderately- worn tooth, the fold of enamel remains surrounded by a cavity, which is partly filled by cement and partly by the debris of the food. This constitutes the "mark" of horsedealers. The mark disappears in the aged animal when the tooth becomes worn still lower than the fold of enamel. In the different species of Rhinoceros, the incisor teeth present consider- able variation. They may be absent, and when present they differ much in form and proportions, and it has been remarked that their development has a close relation to the development of the nasal weapon. The two-horned Rhinoceroses of Africa, which have one or both of the horns largely developed, and a great extinct species (Bh. tichorinus) in which the horns attained a prodigious size, are instances of the total absence of these teeth in the adult condition. The Sumatran two-horned Rhinoceros, in which the horns are but moderately developed, exhibits incisors in both jaws ; they also exist in both the one-horned species ; but these teeth attained their largest development in an extinct hornless Rhinoceros (Rh. incisivus.) The Tapir presents six incisors in each jaw. The upper median has a broad trenchant crown separated by a transverse channel, into which the wedge-shaped crown of the Lower incisor fits, from a basal ridge. The outermost pair of the upper incisors are large and canine-like, the lower are unusually small. In the extinct genus Dinotfu rium the incisors are represented by two large tusks implanted in the deflected extremity of the lower jaw The crowns of these tusks graduallj decrease to a point ; they are bent OF THE INCISOKES. *"»^ ( 'uspidabi, but pretty much of the same length with all the other Teeth of this Jaw. (c) In the Upper-Jaw they are broader and thicker, especially the two first : their length is nearly the same with those of the Lower- Jaw. They stand a little obliquely, with their bodies turned much more forwards (the first especially) and they gene- rally fall over those of the Under- Jaw. The two first Incisores cover the two first, and half of the second of the Lower-Jaw, so that the second Incisor in the downwards and backwards ; the base, like that of the tusk of the Elephant and Mastodon, is widely excavated for a persistent pulp. No corresponding teeth have been discovered in the upper jaw. In the existing Proboscideans, two deciduous tusks in the upper jaw, replaced by a pair of permanent tusks of large size, are the sole representatives of incisors ; but in the extinct Mastodons, two lower incisive tusks were also present, although it is uncertain whether they were pre- ceded by deciduous teeth. The tusk of the Elephant, meeting no opponent to its growth, increases to an enormous length, following the curve originally impressed upon it by the form of its socket. Its growth may be compared to the abnormal increase of the scalpriform incisor of the Eodent when accident has deprived it of an opposing tooth. In the female of the Indian species, the tusk is shorter and straighter, and less deeply implanted, than in the male. According to Cuvier, large tusks are present in both sexes in the African Elephant ; at least, this is the case in certain localities. Professor Owen is of opinion that the smaller fossil tusks of the Mammoth {Elephas primi ■ genius) which have been discovered belonged to the females of that extinct species. (1)] (e) [A degree of departure from the perpendicular direction in the incisor teeth, especially in the upper, is a usual concomitant of the prognathic conformation ; and although most common in the Melanian races, is occasionally observed in every variety of Man. Amongst the African nations, the prognathic development of the maxillae, and the consequent obliquity of implantation of the incisor teeth, appear to be most remarkably expressed in the tribes of the West Coast. The Quaiquco race, including the Bushmen and Hottentots, exhibit the prognathic character in a less degree than do the true Negroes. (2)] (1) Prof. Owen, Art. Odontology, Encyclopedia Britannica, 8th edit., vol. xvi., pp. 440-477. (2) ' On the Teeth in the Varieties of Man,' Dental Review, 1860. 70 HUNTER ON THE TEETH. Upper-Jaw covers more than half of the second, and more than the half of the Cuspidatus of the Under-Jaw. The edges of the Incisor it is not very unusual to find a fourth supernumerary molar, sometimes in the upper, sometimes in the lower jaw, on one or both sides. It is worthy of remark, that supernumerary teeth occurring in the human subject are generally incisors or canines. The molar series in the true Carnivora presents a peculiar type. Its number varies in different genera, and the typical characters are most strongly marked in those which exhibit most strongly carnivorous pro- pensities. The formula in the Felidce is p. § [ -f I , m. $ [ f" . In the upper jaw, the first premolar (p. 2) is small and rudimentary ; the second (p. 3) is much larger, implanted by two fangs, with a trenchant, conical crown, having its cutting edge slightly notched, indicating a division into three lobes, with a posterior basal ridge ; the third (p. 4) is the upper "dent carnassiere " of Cuvier — the scissor, or sectorial tooth. It is a large tooth, implanted by two fangs. The crown consists of a trenchant portion (the blade) which is divided into three lobes, of which the anterior is the smallest, the middle the longest and most pointed, and the posterior largest. On the inner side of the base of the anterior third of the crown, a thick tubercle is developed, which projects inwards. The two posterior thirds of the inner surface of the crown are smooth, and against this smooth portion works the outer surface of the blade of the lower carnassiere. The fourth tooth (m. 1) is small and rudimentary. It is implanted behind and on the inner side of the carnassiere. In the lower jaw, the premolar series is reduced to two by the absence of p. 2. Of the two premolars, the anterior is the smaller. They have both compressed, trenchant, conical crowns, divided by slight indenta- tions in the cutting edge into three lobes, of which the middle is the largest. The second (p. 4) has a posterior basal ridge. The third tooth is the lower carnassiere — the first molar. It is implanted by two fangs, and its crown is nearly equally divided into two trenchant lobes. It is to be remembered that the upper carnassiere in the Carnivora always succeeds a deciduous tuberculate molar — it is, therefore, essentially a premolar ; whilst the lower carnassiere is developed behind the deci- duous series — it is, therefore, the first true molar. The upper carnassiere is always a little anterior to the lower. In the Hyamas, the molar formula is P' ¥. %. m - t. t. • The teeth of the molar series in both jaws are larger and stronger than in the Felidce, and an additional premolar tooth, p. 1 in the upper, p. 2 in the lower jaw, is retained. As in the Felidce, the upper true molars are rudimentary. The premolars are remark- HUNTER ON THE TEETH. first and second are placed directly under the Maxillary Sinus. I once saw the second Grinder naturally wanting on one side of the Lower-Jaw. able for the strong basal ridges they present, which serve to protect the gums in the rough work of splitting and crushing bones. The first upper premolar is very small and conical, the second is much larger, and the third still larger, and of great strength. The strong cones of the second and third are each belted by a posterior and internal basal ridge, and the posterior part of the cone is also traversed by a longitu- dinal ridge. The fourth is the carnassiere, and presents a blade divided into three lobes, and a strong internal trihedral tubercle. In the lower jaw, the first premolar (p. 2) fits into the interspace letween the first and second premolars above. It presents a ridge anteriorly, and abroad basal talon behind. The second (p. 3) is the largest ; it has an anterior and a posterior basal ridge, and its strong, rounded cone presents also an anterior and posterior vertical ridge. In the third (p. 4), the posterior basal ridge is developed into a small cone. The lower carnassiere of the Hyaena resembles that of the Felidce, except that the points of the two lobes are less produced, and from a small posterior talon a ridge is continued along the inner side of the base. In the family of the Viverridce (Civets, Genets, Ichneumons, &c), the molar series is p. £ £ \ , to. fi fi . This increase in the number of these teeth forms a link between the genus Canis and the genera already referred to. The sectorial teeth are more tubercular than trenchant. In some aquatic species, however, as in the sub-genus Cynogale, the premolars have compressed, triangular, trenchant, sharp-pointed crowns, the edges of which are minutely serrated, like the teeth of certain sharks. In the Indian Musangs (Paradoxuri), which are but little carnivorous in their habits, subsisting principally on the fruit of the palm-tree, these teeth manifest the opposite or tuberculate character. The lower sectorial (to. 1) has a pointed lobe developed on the inner side of its two anterior lobes ; its posterior lobe is tritubercu- late, as in the dog. The crown of the last lower molar (m. 2) is oval, with four small tubercles, resembling m. 2 in the Dog. The molar formula in the genus Canis is p. \ ' *[ , m. § ' § ; . The premolars have, strong, sub-compressed conical crowns ; they increase in size from before backwards, the larger ones presenting one or two small posterior tubercles. The upper sectorial (p. 4) is of much larger size than the other teeth of the series. It is divided by a wide notch into two lobes, of which the anterior is much the larger and more produced ; from the inner side of its base the tubercle is developed. The upper true molars are tubercidate ; the second is very small. The lower sectorial (m. 1) has the blade formed by twu conical lobes, of which the posterior is the larger ; behind this the base of the crown extends into a broad quadrate tri tuberculate talon. The second molar presents two anterior opposite OF THE GRINDERS. 97 The third Grinder is commonly called Dens Sapientice ; it ig a little shorter and smaller than the others, and inclined a little more inwards and forwards. Its body is nearly of the same cusps, and a broad posterior talon. The third molar is very small. Professor Owen remarks — "The succession of two tubercular teeth behind the permanent sectorial tooth in the adult, or permanent dentition of the lower jaw, carries the genus Cams farther from the type of its order, and stamps it with its own proper omnivorous character, and this contributes to adapt the Dog to a greater variety of climates and food, and of other circumstances, all of which tend, in an important degree, to fit that animal for the performance of its valuable services to Man." In the Weasel tribe (Mustelldce) there are usually three pre- molars in the upper jaw on each side ; the Otter, however, has four. In the lower jaw on each side there are four or three premolars. The true molars are usually $'. £: . In the Otter the sectorial and molar teeth present increased grinding surfaces, in relation to the coarser nature of their a nim al diet, and the necessity of crushing the bones of fish before they are swallowed. In the great Sea-Otter (Enhydra) the upper sectorial (p. 4) is remarkably modified ; the two lobes of the blade are hemispheric tubercles. The last tooth (m. 1) is larger than the sectorial, and has a similar broad crushing form. The crown of the lower sectorial (m. 1) presents many gradations in this genus from the cutting form observed in the Weasel and Glutton, to the crushing type of the Eatel and Sea-Otter. In a South American Skunk, the second upper premolar is absent. The molar series, p. £ \ f ; , m. | ] |; , in the subfamily of Badgers (Melidce) contrasts in its tuberculate and omnivorous character with the sectorial type found in the predaceous Weasel. The upper true molar in the European Badger attains an enormous size as compared with that of the same tooth in any of the preceding Carnivora ; it presents three external tubercles, and an extensive horizontal surface traversed by a low ridge, and bounded by an internal belt. The molar series in the Bears (Ursidoe) is p. || £ , ire. |; £.' ; but the number of premolars presents some variation in different species. The true molars in both jaws present a tuberculate grinding surface. The teeth which correspond with the true molars in the Seals (Phocidce) are more numerous than in the digitigrade family of Carnivores ; in the upper jaw, they occasionally rise to the typical number of three on each side. The entire molar series in the Seals is usually five or six teeth on either side of the upper jaw, and five on each side of the lower. In some genera — as the Eared Seals (Otarice), and Elephant Seals (Cystophora), they are each supported by a single fang, in other genera by two fangs, which are usually connate in the first or second teeth. The fangs of all the teeth in the Seals are of remarkable thickness. The crowns of the molars are generally compressed, conical, and pointed ; they present the ' cingulum,' and OS HUNTER ON THE TEETH. figure, but rather rounder, and its fangs are generally not so regular and distinct, for they often appear squeezed together ; and sometimes there is only one fang, which makes the Tooth anterior and posterior basal tubercles more or less developed. In a few, however, of the larger species the molars are simple and obtuse. This is especially the case in the Walrus, in which the molar series is reduced to a smaller number than in the true Seals, being, in the adult, three on each side in the upper jaw, and four in the lower. On turning to the extinct Carnivora, we find in Machairodus two premolars, p. 3 and p. 4, and one small rudimentary tubercular molar in the upper jaw ; in the lower the molar series accords with that of Felis. The more ancient carnivorous forms of the older tertiary formations exhibit the typical formula of Placental Diphyodont dentition. In Hycenodon each of the three lower molars present the carnassial form as truly as the lower carnassiere of the Felidoe. In another early carnivore (Amphicyon), with the same typical number of teeth, the molars have the tubercular con- formation — the prototypes of the tubercular molars of the Viverridce and Canidce. The study of the grinding teeth in the various vegetable feeders, is assisted and simplified by a reference to the forms of those teeth in the fossil remains of early herbivora, occurring in the eocene or most ancient tertiary formations. In the Anoplotherium, the crown of the upper molar is divided into a front and back lobe, by a valley which extends from its inner side two thirds across. A second valley, crosses its ter- mination at right angles, and forms a crescentic depression in each lobe, concave towards the outer side of the crown — the side of the crown being impressed by two parallel excavations. There is a large conical tubercle at the entrance of the transverse valley on the inner side of the crown. This type is continued into the Dichodon, Dichobunes and existing Ruminants. The dentition of the Dichodon, an extinct genus, the remains of which have been found in the upper eocene of Hampshire, forms a transitional link between that of the Anoplotherium and the present Ruminants. The Dichodon presented the typical formula p. %\ ^[ , m. f] £ \ . The crowns of the first three premolars are extended from before backwards, each presents three progressively developed and pointed cusps on the same line ; to the third in the upper jaw, a fourth inner and posterior cusp is added. The fourth pre- molar is thicker and shorter from before backwards, and has two pairs of cusps. The crowns of the upper true molars have also each two pairs of sharp-pointed cusps, and also a series of five low accessory points, developed from the outer part of the cingulum. The lower molars have crowns of the same complex character as those above, but the convex side of the principal cusps are turned in the opposite direction to those of the upper, and the accessory basal points arc OF THE GRINDERS. 99 conical : it is much smaller than the rest of the Grinders. In the Upper-Jaw this Tooth has more variety than in the Lower, and is even smaller than the corresponding Tooth of the Lower, developed from the inner instead of the onter side of the crown. Com- pared with the molar of the Anoplotherinm, the outer lobes of that of Dichodon are thicker and sharper, the inner ones are more nearly equal to the outer, and are more distinctly divided from them ; the transverse valley extends completely across the tooth, and is crossed by the antero-posterior doubly crescentic depression. In existing Ruminants, the cusps of the upper molars are . less pointed and lower than in Dichodon. When worn, the crown presents the inner and outer pair of crescentic lobes of dentine, whilst the double crescentic valley separating them, containing a thicker layer of cement, forms two detached crescents. The premolar resembles one half of the true molar. The permanent formula of the grinding series in existing Rumi- nants is p. |; |; , m, f \ l\ . The lower molar of the Ruminant resembles the upper one reversed. The single median longitudinal depression which in the upper tooth is internal, in the lower is external ; whilst the two concavities of the outer side of the upper molar are repeated, although less deep on the inner side of the lower. The lower molars are thinner than the upper, and in the worn crown, the crescentic islands are narrower and less bowed. The outer contour of the grinding series in Ruminants is slightly zigzag, the anterior and outer angle of each tooth projecting beyond the posterior and outer angle of the tooth before it. The premolars form a continuous series with the molars. They are smaller and more simple. In the Hyracotherium, another eocene herbivore, the grinding sur- face supports four principal cusps. Each transverse pair is connected by a ridge which supports a smaller conical tubercle, and the crown is girt with a cingulum. On this type are formed the upper grinders of the existing Hog tribe and of the Hippopotamus. The genus Sus is an instance of the retention of the typical formula p. \\ %\ , m.% \ || . In the Wild Boar, the teeth of the grinding series increase in size from the first to the last. The first premolar has a simple, conical crown, thickest behind, and is implanted by two fangs. The second has a broader crown, with a posterior lobe, having a depression on its inner surface ; each fang exhibits a tendency to division. The crown of the third is still broader, and it is implanted by four fangs. The fourth premolar has two principal tubercles, and some irregular vertical depressions on the inner half of the crown. The first true molar originally bears four principal cones, with smaller irregular sub-divisions, and an anterior and posterior ridge ; but owing to its early development, the tubercles become worn down, and a smooth surface of dentine is exposed by the time the last molar is in place. A crucial depression, dividing four 100 HUNTER ON THE TEETH. and thence stands directly opposed to it ; but for this circum- stance the Grinders would reach farther back in the Upper-Jaw than in the Lower, which is not commonly the case. cones, with more complex shallow divisions, and a larger tnberculate posterior ridge characterise the second true molar. The last molar is of great antero-posterior extent, owing to the development of the posterior ridge into a cluster of tubercles. The four primary cones are distin- guishable on the fore part of the surface of the crown. In the Wart- hogs (Phacochcerus), the molar series is reduced by the suppression of p. 1 and p. 2. The last true molar in the Wart-hogs is very remarkable for its extent and complexity. It is, perhaps, the most peculiar and complex tooth in the Mammalian series. The surface of the crown presents three series of enamel islands, ranged in the long axis of the grinding surface ; each row consists of eight or nine islands. Those of the middle row are elliptic and simple ; those of the inner and outer rows are sometimes sub-divided into smaller islands. Each island or lobe consists of an abraded column of dentine encased by thick enamel, and the whole are united together to form the crown by abundant cement which fills up the interspaces, and gives an external covering to the whole tooth. The molar series in the Hippopotamus consists of P- 1! £ ! » m. f" f| . The first premolar has a simple corneal crown and a single root. It comes early into place at some distance in advance of the second, and is soon shed. In the existing Hippopotamus, the other premolars and molars form a continuous series ; but in the Hippopotamus Major, of the pliocene strata, the second premolar was in advance of the third by an interval equalling its own breadth. The third and fourth premolars retain the conical form, but are of larger size, and present one or two longitudinal grooves on the outer surface, which give the crown, when worn, a lobed appearance. The true molars are each divided into two cones or lobes by a wide transverse valley. Each cone is again sub-divided by a narrow, antero-posterior cleft, into two half cones, with their flat sides next each other. When moderately worn, each half cone presents a trefoil of enamel ; but when worn to the base, the surface of each lobe presents a quadrilobate figure. The crown of the third molar has a fifth smaller cone developed behind the two normal pairs of half cones. In the Paloeotherium, another eocene herbivore, the crown of the molar is divided into an anterior and posterior part, by an oblique fissure, continued from near the middle of the inner surface of the crown obliquely outwards and forwards, two thirds across the tooth. This fissure enlarges at its termination, so as to mark a division of the anterior part of the crown into internal and external lobes. The pos- terior division of the crown is likewise divided into inner and outer lobes by a short, wide valley or fissure, which extends forward from the OP THE GRINDERS. 101 In the Upper-Jaw this third Grinder is turned but a very little outwards ; is frequently inclined somewhat backwards ; and it projects over that of the Under- Jaw. It often er be- comes loose than any of the other Teeth. posterior surface. This type is the fundamental pattern of the upper molars of the Horse and Ehinoceros. The formula of the molar series in the Horse i&p.% '. f '. in. f \ f \ . The crowns of the upper molars bear some resemblance to those of the Ruminants in their complexity. The re- semblance of the crowns of the lower molars to those of the Rhinoceros and Palceotherium is more marked. The grinders of the Horse may be distinguished from the complex teeth of other Herbivora of an equal size by the great length of the tooth before it divides into fangs. Abrasion of the crown goes on to a considerable extent before the division begins ; hence, except in old horses, a considerable portion of the whole molar is implanted by an undivided base. In the Rhinoceros, the likeness of the crown of the molar to that of the Palceotherium is very obvious. The differences in the upper molar are chiefly that two concavities which exist on the outer side of the crown in Palceotherium are almost levelled in the Rhinoceros ; that the termination of the oblique transverse fissure in the Rhinoceros is more expanded, and in some species bifurcates and deepens, so that in the worn crown one branch may form an insulated circle of enamel. The posterior valley is also deeper and more extended. A basal ridge girts the internal and anterior and posterior sides of the crown. The formula of the grinding series in the Rhinoceros is • t. t. "*■ y- 7- • A fourth type is furnished by the molars of the Lophiodon. They are nearly allied, in configuration, to those of Palceotherium, but they have a more decided transversely ridged character. This configuration obtains in the Dinotherium and the existing Tapies. The Proboscidean Family. Certain extinct species of proboscidians received from Cuvier the name of " Mastodon," in consequence of the peculiar conformation of their grinding teeth, which appeared to place them in a distinct group from the existing Elephant. Subsequently, how- ever, other fossil remains of proboscidians have been discovered in the ter- tiary deposits of tropical Asia, which establish the transitional characters between the lamello-tuberculate teeth of the elephants and the mam- milated molars of the typical Mastodous, showing that the distinctions deducible from their molar teeth rather establish differences of species than of genera. In the Mastodon giganteus and M. angustidens, the grinding surface of the molar, instead of being cleft into numerous thin plates, as in the Elephant, was divided into wedge-shaped, transverse ridges, the summits of which were again divided into conical protu- berances, more or less resembling the teats of a cow. The crown of the N 102 HUNTER ON THE TEETH. They are placed under the posterior part of the Maxillary Sinus, and there the parts which compose the Sinus are thicker than in the middle. The variations as to the natural number of the Teeth, depend commonly upon these Dcntes Scqricntiw. (p) tooth is formed of dentine, thickly coated with dense and brittle enamel. A thin covering of cement is continued from the fang upon the crown, but it does not fill up the interspaces of the divisions of the crown as in the Elephant. In the Mastodons, there were three deciduous molars (d. m, 2, 3, and 4) on each side, in both jaws ; the second was replaced by one premolar (p. 3), ami there were three true molars on each side, both above and below. The grinders of the Proboscidians follow each other, from before backwards, at long intervals, and are never simultane- ously in place. In the Mastodons not more than three were in use at any period on one side of either jaw ; all the molars, save the penul- timate, were shed by the time the last molar had cut the gum, and in the old Mastodon, the dentition was at last reduced to m. 3 on each side of both jaws. In the existing Elephants, the grinders are d. m. -?/ 3 ' m. no premolar replaces either of the deciduous molars. The grinding teeth are large and complex, and there is never more than one, or two partially, in place and use at the same time. The series is constantly in progress of growth and destruction, shedding and replacement. No premolar succeeding the deciduous molars, all the grinding teeth follow each other horizontally from behind forwards. The structure of the Elephants tooth has been before described (Vide note 1., p. 192, vol. iii.) In the extinct species which, at one time, was a denizen o! Northern Europe and Asia — the Mammoth — (Elephas primigenius), the enamelled plates were more numerous in proportion to the size of the crown, and thinner than in the existing Asiatic species. In the African Elephant, on the other hand, the plates of enamel-covered dentine are fewer and thicker : they expand from the margins to the centre of the tooth, and present a lozenge form when worn down by mastication. The final blending of the plates by a common base of dentine does not take place simultaneously along the whole extent of the tooth in the Indian Elephant ; the anterior plates which are first formed, are worn down, and the base of dentine is exposed, whilst the posterior divisions of the crown are still distinct, adhering only by cement. The African Elephant, by the complete basal confluence of the plates before the anterior ones are worn out, manifests a closer affinity to the Mas- todon. (1) (p) [In the Melanian varieties of Man, the molar teeth are of large size, and the fangs of the wisdom and penultimate molars are not, as a rule, connate or conjoined. The great size of the molar teeth is most (1) Professor Owen, Article Odontology, Enyclopsedia Britanmca, Sth edit. OP THE ORINDEBS. 103 Thus from the Incisorcs to the first Grinder, the Teeth become gradually thicker at the extremity of their bodies, and smaller from the first Grinder to the Dens Sapientice. From the Cuspidatus to the Dens Sapientice the fangs become much shorter ; the Incisor es are nearly of the same length with the Bicuspides. From the first Incisor to the last Grinder, the Teeth stand less out from the sockets and Gums. The bodies of the Teeth in the Lower-Jaw are turned a little outwards at the anterior part of the Jaw, and thence, to the third Grinder, they are inclined gradually more inwards. The Teeth in the Upper-Jaw project over those of the Under, espe- cially at the fore-part, which is owing to the greater obliquity of the Teeth in the Upper- Jaw ; for the circle of the sockets is nearly the same in both Jaws. This oblique situation, however, becomes gradually less, from the Incisores backwards, to the last Grinder, which makes them gradually project less in the same proportion. remarkable in the Australian variety, in which race also the wisdom teeth attain large relative dimensions, and are generally distinguishable from those of Europeans, not only by a complex implantation by distinct fangs, but by the fuller development of their posterior tubercles. A careful examination, however, of the dentition in a large collection of crania will prove that the dimensions of the molar teeth in Australian skulls vary ; that in some cases they are equalled in size by the same teeth in other races, and that in exceptional instances the development and implantation of the upper third molar does not differ from the ordinary standard. In the Museum of the Royal College of Surgeons is an Australian skull, in which the upper wisdom exactly resembled the same tooth in the cranium of a Celtic Scot, with which it was com- pared. It was considerably smaller than the penultimate, whilst the diminution, as in Europeans, principally depended on the minor deve- lopment of the posterior internal tubercle. In another instance, in the same race, the writer found that the three fangs of the upper third molar were conjoined, and he has noticed the same thing in a skull of an allied variety, the Papuan of New Guinea. In the West Coast African Negro, the molars also usually attain large, but not exceptional, dimensions. In skulls of the Hottentot and Bushman race which have come under the writer's notice, the development of these]; teeth has not been above the ordinary standard. (1)] (1) ' On the Teeth in the Varieties of Man,' Dental Review, 1860. 104 HUNTER ON THE TEETH. The Teeth in the Upper- Jaw are placed farther back in the circle than the corresponding Teeth of the Lower ; this is owing to the two first Incisores above being broader than the corre- sponding Incisores below. All the Teeth have only one fang, except the Grinders, each of which has two in the Lower-Jaw, and three in the Upper.* The fangs bear a proportion to the bodies of the Teeth; and the reason is evident, for otherwise they would have been easily broken, or pushed out of their sockets. The force com- monly applied to them is oblique, not perpendicular ; and they are not so firmly fixed in the Upper-Jaw, that is, the Alveolar Process in that is not so strong as in the Under-Jaw : it is per- haps on this account, that the Grinders in that Jaw have three fangs. This particular structure in the Alveolar Process of the Upper- Jaw, is perhaps to give more room for the Antrum H iyhmori- anum; on this supposition the fangs must be made accordingly, i.e. so that they shall not be pushed into that cavity ; now, by their diverging, they inclose as it were the bottom of the Antrum, and do not push against its middle, which is the weakest part ; and the points of three diverging fangs will make a greater resistance (or not be so easily pushed in) than if they were placed parallel. If there had been only two, as in the Lower- Jaw, they must have been placed opposite to the thinnest part of the Antrum ; and three points placed in any direction but a diverging one, would have had here much the same effect as two ; and as the force applied is endeavouring to depress the Tooth, and push it inwards, the innermost fang diverges most and is supported by the inner wall of the Antrum. That all * Those Anatomists who allow the Teeth to have more fangs, have been led into a mistake, I suppose, by often observing two canals in one fang ; and thence concluded, that such a fang was originally two, and that these were now growing together, (q) (q) [This observation of Hunter's is itself a mistake. The fang of the upper premolar really consists of two connate fangs.] OF THE ARTICULATION OF THE TEETn. 105 this weakness in the Upper-Jaw is for the increase of the Antrum is probable, because all the Teeth in the Upper- Jaw are a good deal similar to those in the Lower, excepting those that are opposite to the Maxillary Siinus ; and here they differ principally in the fangs, without any other apparent reason ; and what confirms this, is, that the Dentes Sajcrientice in both Jaws are more alike than the other Grinders ; for this reason, as I apprehend, because the Dens Sapientice in the Upper- Jaw, does not interfere so much with the Maxillary Sinus. What makes it still more probable that the two first superior Grinders have three fangs on account of the Maxillary Sinus, is, that the two Grinders on each side of the Upper-Jaw, in the child, have three fangs, and we find them underneath the Antrum; but those that succeed them have only one fang, as in the Lower- Jaw ; but by that time the Antrum has passed further back, or rather the arch of the Jaw has projected, or shot forwards, as it were, from under the Antra, so that the Alveolar Processes that were under the Antrum at one age, are got before it in another. That the edge of every fang is turned towards the circum- ference of the Jaw, in order to counteract the acting power, we shall see when we consider the Motion of the Jaw, and the Use of the Teeth. OF THE ARTICULATION OF THE TEETH. The fangs of the Teeth are fixed in the Gum and Alveolar Processes, by that species of Articulation called Gomphosis, which, in some measure, resembles a nail driven into a piece of wood. They are not, however, firmly united with the Processes, for every Tooth has some degree of motion ; and in heads which have been boiled or macerated in water, so as to destroy the Periosteum and adhesion of the Teeth, we find the Teeth so loosely connected with their sockets, that all of them are ready to drop out, except the Grinders, which remain as it were hooked from the number and shape of their fangs. 106 HUNTER ON THE TEETH. OF THE GUMS. The Alveolar Processes are covered by a red vascular sub- stance, called the Gums, which has as many perforations as there are Teeth ; and the neck of a Tooth is covered by, and fixed to this Gum. Thence there are fleshy partitions between the Teeth, passing between the external and internal Gum, and, as it were, uniting them ; these partitions are higher than the other parts of the Gum, and thence form an arch between every two adjacent Teeth. The thickness of that part of that Gum which projects beyond the sockets is considerable ; so that when the Gum is corroded by disease, by boiling, or otherwise, the Teeth appear longer, or less sunk into the Jaw. The Gum adheres very firmly in a healthful state both to the Alveolar Process and to the Teeth, but its extreme border is naturally loose all around the Teeth. The Gum, in substance, has something of a cartilaginous hard- ness and elasticity, and is very vascular, but seems not to have any great degree of sensibility ; for though we often wound it in eating, and in picking our Teeth, yet we do not feel much pain upon these occasions ; and both in infants and old people, where there are no Teeth, the Gums bear a very considerable pressure, without pain, (r) The advantage arising from this degree of insensibility in the Gums is obvious, for till the child cuts its Teeth, the Gums are to do the business of Teeth, and are therefore formed for this purpose, having a hard ridge running through their whole (r) [The gum (gingiva) owes its resistance principally to the hardness of the subjacent parts ; in itself it is soft. It is composed of a firm, dense, whitish, submucous tissue, and a proper mucous membrane, covered by a layer of pavement epithelium. The mucous membrane bears papilla; of considerable size (0-15'" to 0-3'"). The epithelium is 0-23'" to 0-4'" in thickness between the papillae. According to Kolliker, there are no glands in the gum. He observes : " We must be careful not to take for the orifices of glands, certain rounded depressions of the epithe- lium, O08"' to Crl5'" in diameter, with cornified epithelial cells."(l)] (1) Kolliker, op cit., p. 300. OF THE ACTION OF THE TEETH, ETC. 107 length. Old people, who have lost their Teeth, have not this ridge. When in a sound state, the Gums are not easily irritated by being wounded, and therefore are not so liable to inflammation as other parts, and soon heal. The Teeth being united to the Jaw by the Periosteum and Gum, have some degree of an yielding motion in the living- body. This circumstance renders them more secure ; it breaks the jar of bony contact, and prevents fractures both of the sockets and of the Teeth themselves. OF THE ACTION OF THE TEETH ARISING FROM THE MOTION OF THE LOWER-JAW. The Lower-Jaw may be said to be the only one that has any motion in mastication ; for the Upper-Jaw can only move with the other parts of the head. That the Upper-Jaw and head should be raised in the common act of opening the mouth, or chewing, would seem, at first sight, improbable ; and from an attentive view of the mechanism of the joints and muscles of those parts, from experiment and observation, we find that they do not sensibly move. We shall only mention one experi- ment in proof of this, which seems conclusive : let a man place himself near some fixed point, and look over it, to another distant and immoveable object, when he is eating. If his head should rise in the least degree, he would see more of the distant, object over the nearest fixed point, which in fact he does not. The nearer the fixed point is, and the more distant the object, the experiment will be more accurate and convincing. The result of the experiment will be the same, if the nearest point has the same motion with the head ; as, when he looks from imder the edge of a hat, or any thing else put upon his head, at some distant fixed object. We may conclude then that the motion is entirely in the Lower-Jaw : and, as we have already described both the articulation and the motion of the bone, we shall now explain the action of mastication, and, at the same tune, consider the use of each class of Teeth. With regard to the action of the Teeth of both Jaws, in 108 HUNTER ON THE TEETH. mastication, we may observe, once for all, that their action and re-action must be always equal, and that the Teeth of the Upper and Lower-Jaws are complete, and equal antagonists both in cutting and grinding. When the Lower-Jaw is depressed, the Condyles slide for- wards on the eminences ; and they return back again into the cavities, when the Jaw is completely raised. This simple action produces a grinding motion of the Lower- Jaw, backwards on the Upper, and is used when we divide any thing with our fore Teeth, or Incisores. For this purpose, the Incisores are well formed; as they are higher than the others, their edges must come in contact sooner ; and as the Upper project over the Under, we find in dividing any substance with them, that we first bring them opposite to one another, and as they pass through the part to be divided, the Lower-Jaw is brought back, while the Incisors of that Jaw slide up behind those of the Upper-Jaw, and of course pass by one another. In this way they complete the division, like a pair of scissars ; and at the same time they sharpen one another. There are exceptions to this ; for these Teeth in some people meet equally, viz. in those people whose Fore-Teeth do not project further from the Gum, or socket, than the back Teeth ; and such Teeth are not so fit for dividing ; and in some people the Teeth of the Lower-Jaw are so placed, as to come before those of the Upper- Jaw; this situation is as favourable for cutting as when the over-lapping of the Teeth is the reverse, except for this circumstance, that the Lower- Jaw must be longer, and therefore its action weaker. The other motion of the Lower-Jaw, viz. when the lateral Teeth are used, is somewhat different from the former. In opening the mouth, one Condyle slides a little forwards, and the other slides a little further back into its cavity; this throws the Jaw a little to that side, just enough to bring the lower Teeth directly under their corresponding Teeth in the Upper- Jaw: this is done, either in dividing, or holding of substances; and these are the Teeth that are generally used in the last OF THE MOTION OF THE JAW, ETC. 109 mentioned action. When the true grinding motion is to be performed, a greater degree of this last motion takes place ; that is, the Condyle of the opposite side is brought farther forwards, and the Condyle of the same side is drawn farther back into the cavity of the Temporal Bone, and the Jaw is a little depressed. This is only preparatory for the effect to be produced ; for the moving back of the first mentioned Condyle into the socket, is what produces the effect in mastication. The lateral Teeth in both Jaws are adapted to this oblique motion ; in the Lower they are turned a little inwards, that they may act more in the direction of their axis ; and here the Alveolar Process is strongest upon the outside, being there supported by the ridge at the root of the Coronoid Process. In the Upper-Jaw the obliquity of the Teeth is the reverse, that is, they are turned outwards, for the . same reason ; and the longest fang of the Grinders is upon the inside, where the socket is strengthened by the bony partition between the antrum and nose. Hence it is, that the Teeth of the Lower- Jaw have their outer edges worn down first ; and, vice versa, in the Upper-Jaw. GENERAL COMPARISONS BETWEEN THE MOTION OF THE JAW IN YOUNG AND IN OLD PEOPLE. In children who have not yet Teeth, there does not seem to be a sliding motion in the Lower-Jaw. The articular eminence of the Temporal Bone is not yet formed, and the cavity is not larger than the Condyle; there- fore the centre of motion in such, must be in the Condyle. In old people, who have lost their Teeth, the centre of motion appears to be in the Condyles, and the motion of their Jaw to be only depression and elevation. They never depress the Jaw- sufficiently to bring the Condyle forwards on the eminence, because in them the mouth is sufficiently opened when the Jaw- is in its natural position. Hence it is that in old people, the gums of the two Jaws do not meet in the fore-part of the mouth, and they cannot bite o HO HUNTER ON THE TEETH. at that part so well, as at the side of the Jaw ; and, instead of the grinding motion, which would be useless, where there are no Grinders ; they bruise their food rather by a simple motion of the Jaw upwards and downwards. It is from the want of Teeth in both those ages, that the face is shorter in proportion to its breadth. In an old person, after the Teeth are gone, the face is shorter, while the mouth is shut, by almost the whole lengths of the Teeth in both Jaws ; that is, about an inch and an half. From the want of Teeth too, at both those ages, the cavity of the mouth is then smaller ; and the tongue seems too large and unmanageable, more especially in old people. In these last we observed also, that the chin projects forwards, in pro- portion as the mouth is shut ; because the basis of the Lower- Jaw (which is all that now remains) describes a wider circle than the Alveolar Process in younger people. The Jaws do not project so much forwards in a child, as in an adult ; hence the face is natter, especially at the lower part. In proportion as the last Grinders are produced, the sides of the curve formed by the Jaws become longer, and push forward the fore-part, none of the additional part passing backwards. The fore-part also continues nearly of the same size, so that the whole Jaw is longer in proportion to its breadth, and projects farther forwards. OF THE FORMATION OF THE ALVEOLAR PROCESS. Having considered the Alveolar Processes in their adult, or perfect state, we shall next examine and trace them from their beginning. "We observe the beginning of the Alveolar Process at a very early period. In a fcetus of three or four months it is only a longitudinal groove, deeper and narrower forwards, and becom- ing gradually more shallow and wider backwards: instead of bony partitions, dividing that groove into a number of sockets, there are only slight ridges across the bottom and sides, with intermediate depressions, which mark the future Alveoli. OF TIIE FORMATION OF TIIE ALVEOLAR PROCESS. Ill In the Lower- Jaw the vessels and nerves run along the bottom of this Alveolar cavity, in a slight groove, which after- wards becomes a complete and distinct bony canal. The Alveolar Process grows with the Teeth, and for some time keeps the start of them. The ridges which arc to make the partitions shoot from the sides across the canal, at the mouth of the cell, forming hollow arches : this change happens first at the anterior parts of the Jaws, (s) As each cell becomes (s) [A very lull description of the formation of the alveoli is given by Drs. Robin and Magitot, in their " Treatise on the Genesis and Develop- ment of the Dental Follicles." According to these observers, the first traces of ossification in the lower jaw appear near the middle of the inferior maxillary cartilage ; they are elongated in form, and show themselves about the thirty-fifth day of intra-uterine existence. The ossifying process rapidly invades the whole cartilage. Two very thin ridges then appear on the upper margin of the bone ; these ridges do not exist in the cartilaginous state, but are formed according to the mode of ossification called " invasion." At an early period, they give the bone the appearance of being formed by two parallel osseous bands. These ridges limit the groove in which the dental follicles originate. The groove extends from the anterior edge of the ascending branch of the maxillary, encroaching a little on its inner face, as far as the anterior extremity of the corresponding branch of the maxillary ; consequently, the whole of its contents can be removed in one piece. The groove is of considerable depth. In Man, the solid part of the bone, from the canine as far as the symphisis, becomes higher than the groove is deep about the beginning of the fourth month. At the level of the molars, and in relation to the axis of the lower maxillary, the groove is situated inside the latter, but passes round it, in order to be continued on the side of the outer face in the whole portion containing the follicles of the canine and the incisors. The groove is widened, as if swollen in blisters about its posterior third, narrow in front, and more suddenly contracted behind. It opens at the inner face of the ascending branch of the maxillary by a fissure-shaped opening, broadened and rounded at the level of the bottom of the groove, and narrow above, where it soon closes ; there then remains only the lower part of the slit which forms the posterior dental foramen, or posterior orifice of the dental canal, which is traversed by the vessels and nerves of the same name ; they make part of the contents of the groove. The dental vessels and nerves are contained in a slight furrow at the bottom of the groove. The furrow is smooth and regular, and, at a later period, becomes the dental canal. From the time of the genesis of the dental follicles, the inner surface of each lamina forming the sides of the groove becomes thickened 112 HUNTER ON THE TEETH. deeper, its mouth also grows narrower, and at length is almost but not quite, closed over the contained Tooth. The disposition for contracting the mouth of the cell, is at intervals by little vertical projections, which stand facing each other on each side. These projections increase, and ultimately join each other, forming complete partitions, which divide the groove into cells, or alveoli. In Man, this takes place at an advanced period of gestation. As late as the ninth month of foetal life, the contents of the groove can be raised in one piece. The partitions bridge over the lower part of the groove containing the dental vessels and nerves. The rudiments of the partitions always unite, in the first place, between the first molar and the canine, and then between the latter and the second incisor. The rudiments of the first partition join to form a narrow, slender, and thin lamina, passing over the nerves and vessels at the bottom of the groove towards the end of the fourth month. The prolonga- tions between the canine and second incisor become united near the sixth month. They unite between the two incisors near the seventh month, but not between the molars until after birth. Before the projections unite throughout their whole height, they form a very slender bridge immediately above the vessels. This bridge grows in height by prolongations from the walls of the groove. The partitions are not completed until after birth. The following is the account given by the same observers of the formation of the alveoli in the upper jaw. About the fifty-fifth day (in the human subject) there appear at the outer edge of the superior maxillary bone two thin crests, an external and an internal one ; these crests limit a groove, which has at first the appearance of a shallow furrow. Shortly after, a similar groove is formed in the same manner upon the inter-maxillary. In the posterior, two-thirds of the groove in the superior maxillary, the suborbital vessels, and nerves are lodged. The external lip of the dental groove passes above them, near its anterior extremity, in order to allow them to reach under the skin. " This groove is thus produced immediately below the eye, a place occupied at that time by the edge of the upper maxillary, and which the nerves and vessels continue to occupy. The same thing occurs in the lower maxillary, where the groove appears before that of the other jaw. The groove of the upper maxillary is common to the dental follicles, which are about to appear, and to the vessels which remain suborbital." The bottom of the groove becomes in the course df development the suborbital canal, just as in the lower maxillary it becomes the dental canal, but the separation between the canal and the groove takes place much sooner in the superior than in the inferior maxilla. "Nevertheless, it is at the bottom of this groove, against the suborbital nerves and vessels, and consequently in the part which after- wards becomes the suborbital canal, that the dental follicles also OF THE FORMATION OF THE ALVEOLAR TROCESS. 113 chiefly in the outer plate of the bone, which occasions the con- tracted orifices of the cells to be nearer the inner edge of the Jaw. The reason, perhaps, why the bone shoots overj and almost covers the Tooth, is that the Gum may be firmly sup- ported before the Teeth have come through, (t) originate. Those of the molars are, however, the only ones that originate here, for the canal belongs only to the maxillary, and not to the incisive bone, and because it is already separated from the groove in front when the follicle of the canine makes its appearance." From the third month, the posterior half of the groove, which was confounded with the suborbital canal, becomes closed, and separates the dental follicles placed at the level of the vessels and nerves." The external and internal plates which bound the groove are thin, fragile, and slightly undulated. The plates sink in a little opposite the intervals between the dental follicles. About the beginning of the fourth month, the rudiments of partitions begin to form as in the lower jaw, but they arc produced from the bottom as well as the sides of the groove. " After the seventh month, the partitions reach in height nearly but not quite the level of the edges of the groove between the first molar and the canine, as well as between the incisors ; the rest of the groove remains undivided and common to the two molars. At this period, the nerve and suborbital vessels, which start at first at the level of the line of contact between the canine and the second incisor, show their orifice of egress at the level of the rudimental partition interposed hetween the first molar and the canine. That part of the upper maxillary, which separates the canal of the bottom from the follicles is, at this time, only in the condition of a thin osseous plate, not thicker at most than from a quarter to half a millimetre, and perforated by one or two very small orifices intended for the passage of the vessels destined for these organs." (1) ] (t) [Mr. Tomes, who has given a very minute and accurate description of the changes which take place in the child's jaw from the time of birth to the fifth year, states that the partial closure of the alveoli commences between the second and third month after birth. He says that, when the third month has been attained, " the alveoli exhibits a considerable change in character, their depth has increased, and the free edges which were before open, so that in a macerated preparation the teeth readily fall out, are now turned inwards towards the median line of the alveolar ridge, thereby contracting the orifices, and affording protection to the (1) Robin and Magitot in the ' Journal de la Thysiologie de l'Honime et des Animaux.' Edited by Dr. E. Brown-Sequard, 1860. Translated in the ' Dental Cosmos.' 114 HUNTER ON THE TEETH. The Alveoli which belong to the adult Grinders, are formed in another manner ; in the Lower- Jaw they would seem to be the remains of the root of the Coronoid Process ; for the cells are formed for those Teeth in the root of that Process ; and in proportion as the body of the bone, and the cells already formed, push forwards from under that Process, the succeeding cells and their Teeth are formed, and pushed forward in the same manner. In the Upper-Jaw there are cells formed in the tubercles for the young Grinders, which at first are very shallow, and become deeper and deeper as the Teeth grow ; and they grow somewhat faster, so as almost to inclose the whole Tooth before it is ready to push its way through that inclosure and Gum. There is a succession of these, till the whole three Grinders are formed, (u) enclosed teeth, which are no longer liable to fall out when the bone is examined." At the age of six months he has found the inversion of the edges of the alveoli, and consequent narrowing of the apertures less pronounced. He adds, " the increased size of the alveolar orifices must be regarded as the first of those changes which precede the eruption oi the teeth." (1)] (u) [At the time of birth, a large open socket exists, posterior to the alveolus for the first temporary molar in both jaws. In the upper jaw, this, at that time, has a very imperfect posterior wall. The rudiments of a septum, however, intended to divide the large socket into two alveoli, one for the second temporary, and the other for the first per- manent molar, are present. The division of the socket usually takes place a little earlier in the lower than in the upper jaw. At the age of eight months, the septum between the second milk and first permanent molar is still imperfect. In the upper jaw the posterior wall of the crypt of the first permanent molar is in progress of development, but not complete. At twelve months, the alveolus of the first permanent molar in the upper jaw has become more perfect. The alveoli of these teeth, in both jaws, communicate with the surface by small openings, in a line with the alveoli of the other teeth. At eighteen months, "the first permanent molars lie deep within their respective sockets, the orifices of which, in the lower jaw, are contracted by the inversion of the outer alveolar plate, and the base of the coronoid process, the teeth at this time being placed with their posterior two-thirds, internal to that portion of the jaw. The posterior edge of the socket is brought forward (1) Tomes's 'System of Dental Surgery,' pp. 13—16. OF THE FORMATION OF TEETII IN THE FCETUS. 110 OF THE FORMATION OF THE TEETII IN THE FCETUS. The depression, or first rudiments of the Alveoli observable in a Foetus of three or four months, is filled with four or five little pulpy substances, which are not very distinct at this nge.(r) over the back part of the crown, to the extent of one-fourth of its antero-posterior dimensions. On the upper surface of this, within a line of its edge, a depression in the bone may be seen. This is a com- mencement of a crypt for the second permanent molar. The corre- sponding teeth of the superior maxillae occupy the tuberosity, the posterior part of which is extremely thin, and in the median line im- perfect. This gives a long and curved opening to the socket, and a posterior direction to its further half. In the upper jaw we have, as yet, no indications in the bone, of preparations for the lodgement of the second permanent molar." (1) At the age of forty months, the opening of the crypt of the first upper permanent molar has become smaller, and is directed downwards instead of downwards and backwards. Behind it on the posterior surface of the tuberosity is a depression marking the commencement of the crypt for the second permanent molar. Nine months later, the crypts for the second permanent molars have become larger, with well-defined margins. " In the upper jaw they look backwards towards the pterygoid plates of the sphenoid ; in the lower, upwards and a little inwards, their floors lying immediately over the inferior dental canal, near its commencement." About the age of seven years, when the first permanent molars have gained the level of the temporary teeth, the second upper molars, which at first were directed backwards, now begin to descend into the dental line, and are directed obliquely downwards and backwards. Between the age of twelve and thirteen, the second permanent molars are advancing towards the surface of the gums, and the crypt for the third molars hold the positions which those for the second occupied when the first molars were emerging from the alveoli. (2) ] (y) [The dental pulps or bulbs. Two opinions are current with respect to the mode of origin of the dental pulps which may be here briefly stated. One is that the dental pulp commences as a papilla on the free siu-face of the mucous membrane covering the edges of the maxillary arches. This is held by Arnold and Goodsir, who came to their con- clusions independently of each other. The other is that the dental pulp, together with the follicle in which it is afterwards contained, and the "enamel organ," have their origin in the submucous tissue which fills the groove in the maxillary. This opinion is sustained by the researches of Robin and Magitot. (1) Tomes, Op. tit., p. 30. (2) Tomes, Op. cit. , pp. 5—103. 116 HUNTER ON THE TEETH. About the fifth month both the processes themselves and the The following description of the formation of the different parts of the dental follicle has been generally adopted on the authority of Goodsir. About the sixth week of foetal life a groove, the primitive dental groove, is formed along the edge of the jaw, in the mucous mem- brane of the gum ; from its floor ten papillae rise in succession in each jaw. The papillae appear in the following order: — the first milk molar (7th week) ; the canine (8th week) ; the two incisors (about the 9th week) ; the second milk molar (10th week). The papillae in the upper jaw appear a little before those in the lower. With the appearance of the last papilla the papillarij stage terminates. The next stage is the follicular. This consists in the thickening and deepening of the margins of the dental groove, and the separation of the papillae by the formation of membranous partitions which pass across from one margin of the groove to the other. This stage is completed about the fourteenth week. In the early part of this stage the papillae grow rapidly and begin to assume peculiarities of form according with those of the crowns of the future teeth. The follicular stage is converted into the saccular about the end of the fifteenth week. The change consists in the formation of small laminae or opercula of membrane which project from the sides of the follicle, and at last, by meeting and cohering, convert the follicle into a closed sac. The lips of the dental groove also become thickened and unite, so that the groove is at length entirely obliterated. The num- ber of opercida in each follicle is said to vary with the character of the future tooth, there being four or five in the case of the molar ; two, one external and one internal, in that of the incisor; and three, two internal and one external for the canine. About the fourteenth week a lunated depression is formed behind each milk follicle ; these escape the general adhesion of the margins of the groove. In them the pulps of the ten anterior permanent teeth are subsequently developed ; they are called by Mr. Goodsir " cavities of reserve." The formation of the pulp and the closure of the cavity by 02?ercula is similar to that already described in the case of the follicles of the milk teeth. The pulp of the first permanent molar arises as a papilla about the sixteenth week in a portion of the primitive dental groove which remains unclosed behind the sac of the second milk molar. The papilla seated at the bot- tom of this "posterior cavity of reserve'" becomes shut off and enclosed in a sac by the formation and cohesion of opercula, whilst the upper por- tion of the cavity lengthens backwards, and forms a second cavity of reserve in which the papilla for the second molar tooth appears about the seventh month after birth. Ultimately the same change again occurs, and gives rise to the formation of the papilla and sac of the wisdom tooth, which takes place about the sixth year. Considerable doubt, however, has been thrown by various observers on the account given by Arnold and Goodsir. Valentine and Rasch- OF THE FORMATION OF THE TEETH IN THE FXETUS, 117 pulpy substances become more distinct; the anterior of which kow (1) before the publication of Goodsir's observations had contradicted those of Arnold; and more recently M. Natalis Guillot has given an entirely different account of the formation of the tooth follicles, in which he denies their origin on the surface of the mucous membrane, neither does he admit that they depend on any folding of the mucous membrane which has carried the germ from the surface into the cavity of the alve- olus.(2) More recently the researches of Robin and Magitot have led them to a similar conclusion. According to these observers the dental follicles and bulbs originate in the Bnbmucous tissue which extends to the bottom of the osseous groove in the maxillaries. The term follicle is made to include the bulb, the organ of the enamel and the follicular wall ; of these parts the bulb originates first, then the follicular wall, which sub- sequently closes over, and, lastly, the organ of the enamel as soon as the follicle is closed. The follicles of the lower jaw precede those of the upper. In the human foetus the appearance of the follicles is nearly in the same order as the eruption of the corresponding teeth in each jaw. The internal incisor and the anterior molar first. These are followed closely by the external incisor, then the posterior molar, and, lastly, the canine. The changes which take place in the dental follicles follow an order in accordance with that of the appearance of the follicles, — i.e., in the follicle which appears first the follicular wall closes first, the organ of the enamel appears first, the ivory and enamel originate first. The first follicle in the lower jaw appears about the sixtieth day, that in the upper about the sixty-fifth day. The number of follicles of the first dentition in the lower jaw is complete about the seventy-fifth day ; in the upper jaw about five days later. The follicles of the first permanent molars appear in the lower jaw about the eighty-fifth day ; in the upper from the ninetieth to the ninety-fifth. The follicles of the teeth of replacement appear, some a little before birth or sometimes later, the others at longer or shorter intervals after birth. The bulbs do not com- mence close to the dental vessels and nerves, but a little above them in the thickness of the submucous tissue, nearly at the middle of the depth of the groove. The base of the follicle, as the organ -rows larger, be- comes nearer to the vessels and nerves. The phenomena of the genesis and completion of the follicles have been minutely described by Robin and Magitot. The following are the principal steps of the process as observed by them : — A little before the first appearance of a follicle, that point of the submucous tissue at (1) For an account of Raschkow's paper, see Xasinyth's "Historical Intro- duction." (2) Ann. des Sciences Nat. Ser. iw, T. ix. (1858), and Dental Rev., vol. ii., p. 401 P 118 HUNTER ON THE TEETH. are the most complete. About this age, too, the ossifications which it is about to originate becomes more opaque and vascular than the surrounding gelatiniform tissue. In the middle of the vascu- larity, which depends on a network of capillaries, a small, obscure, roundish mass appears. This is the bulb. When the bulb reaches a certain size, a dark, greyish band is formed round it. This represents the follicular wall. This band, after circumscribing the bulb, becomes raised above it, and eventually its free edge unites with itself, so as to convert the follicle into a closed sac. The cavity of the sac is soon divided into two equal parts, the upper of which is occupied by the organ of the enamel, which is formed immediately on the occlusion of the follicle — the lower by the bulb. At this time the bulb is in con- tinuity with the surrounding tissue at its base, the rest of its periphery although immediately contiguous to the surrounding tissue, is easily de- tached, and has a smooth and very clearly denned surface. The bulb, from the time of its appearance, is formed of finely-granulated ovoid nuclei, separated from each other by a small quantity of granulated amorphous matter. At the time of its origin, the bulb of the incisor, or canine, is shaped like a cone, or more or less elliptical ; that of the molar is blunt, more bulged, and broader at the base. The appearance of the follicular wall is accompanied by an increase of vascularity. The wall itself is traversed from its base to its summit by a capillary net- work, having regular polygonal meshes. The vessels and nerves of the bulbs are not developed until a little later, but always before the moment of the appearance of the first cells of origin of the dentine. The organ of the enamel has the appearance of a clear, transparent mass, situated between the inner surface of the wall and the progressing part of the bulb. It has no continuity of substance with the wall — it is only in contact with the surface of the bulb, from which a pale, white line septa- rates it shortly after its appearance. This line consists of a continuous range of the cells of the enamel. As the follicle progresses, its wall becomes better marked, until it forms quite a resisting envelope, corn- pletely distinct from the neighbouring tissues. The base of the bulb, which is at first its broadest or almost its broadest part, becomes after- wards constricted and lengthened out to form the radicular portion of the bulb, which receives the vessels and nerves. The bulb at first consists only of ovoid or rounded nuclei with a little intervening amorphous matter. These nuclei are less transparent than the embryo-plastic nuclei. The follicular wall includes some embryo-plastic nuclei, a little firmly- granulated amorphous matter, and some laminated fibres, in the con- dition ot fusiform bodies, or more completely developed. The organ of the enamel is a thin, gelatiniform bed, composed of star-studded fibro- plastic bodies, which are ramified and anastomosed. It soon exhibits, on its deep or bulbular surface, the continuous range id' the cells of the enamel. They are prismatic and vertically arranged, and when seen as OK THE FORMATION OF THE TEETH IN THE FCETUS. 1 1 !> begin on the edge of the first Tncisores. The Cuspidati are not in the same circular line with the rest, but somewhat on the outside, making a projection there at this age, there not being sufficient room for them. (w) In the sixth, or seventh month, the edges, or tips, of all these five substances have begun to ossify, and the first of them is a little advanced ; and besides these, the pulp of the sixth Tooth has begun to be formed ; it is situated in the tubercle of the Upper-Jaw, and under and on the inside of the Coronoid Process in the Lower- Jaw : so that at this age, in both Jaws, there are in all twenty Teeth begun to ossify, and the stamina of twenty-four, (x) They may be divided into the Iucisores, Caspidati, and Molares ; for at this age there are no Bicuspides, the two last teeth in each side of both Jaws having all the characteristics, and answering all the purposes of the true Molares in the adult, though Avhen these first Molares fall out, their places are taken by the Bicuspides. The Teeth gradually advance in their ossification, and about the seventh, eighth, or ninth month after birth, the Incisorcs begin to cut or pass through the Gums ; first, generally, in the Lower-Jaw. Before this time the ossifications in the third Grinder, or that which makes the first in the adult, are begun. a whole under the microscope, appear like a clear band. It is on the surface of the bulb beneath the deep face of the enamel organ that the first cells of the dentine appear. Such is a sketch of the germs of the dental follicles, as seen by these observers. We shall hereafter notice the subsequent changes which are said by these observers to take place in the process of development. (1)] (w) [An instance of an accordance in the embryonic state with the more general type.] (x) [More recent observations refer the commencement of dentine to a somewhat earlier period. By the end of the fourth month, thin caps of dentine may be found surmounting the pulps of all the milk teeth, and a little later on the first permanent molars. According to Robin and Magitot, the first appearance of the tooth, properly so called, takes place between the eightieth and eighty-fifth day in the lower middle incisor.] (1) Robin and Magitot, op. cit. 120 HUNTER ON THE TEETH. The Cuspidatus and Molares of the Foetus are not formed so Fast as the lncistyres; they generally all appear nearly about the same time, viz. about the twentieth, or twenty-fourth month: however, the first Grinder is often more advanced within the socket than the Cuspidatus, and most commonly appears before it.(?/) These twenty are the only Teeth that are of use to the child from the seventh, eighth, or ninth month, till the twelfth or fourteenth year. These are called the Temporary or Milk Teeth, because they are all shed between the years of seven and fourteen, and are supplied by others. OF THE CAUSE OF PAIN IN DENTITION. These twenty Teeth, in cutting the Gum, give pain, and produce many other symptoms which often prove fatal to children in Dentition. It has been generally supposed that these symptoms arise from the Tooth's pressing upon the inside of the Gum, and working its way mechanically ; but the follow- ing observations seem to be nearer the truth. The Teeth when they begin to press against the Gum, irritate it, and commonly give pain. The Gums are then affected with heat, swelling, redness, and the other symptoms of inflammation. The Gum is not cut through by simple or mechanical pressure, but the irritation and consequent inflammation produces a thinning, or wasting of the Gum at this part : for it often happens that when an extraneous, or a dead substance, is contained in the body, that it produces a (y) [The milk incisors usually are cut from the seventh to the ninth month after the birth ; the first milk molars at about the twelfth month ; the canines at about eighteen months ; and the second milk molars at about twenty-four months. The first teeth cut are the central incisors of the lower jaw ; these are followed by the same teeth in the upper ; and, as a general rule, the lower tooth is cut earlier than the corre- sponding one above. According to Mr. Tomes, it is not until the child has attained the age of four and a half years that the temporary teeth are entirely perfected. He found in a child of four years and one month that the incisors were the only teeth fully formed. The fangs of the others were slightly deficient in length, and hollow at their extremities.] OF THE FORMATION OF THE ADULT TEETH, ETC. 121 destruction of the part between it, and that part of the skin which is nearest it, and seldom of the other parts, excepting those between it and the surface of a cavity opening externally, and that by no means so frequently. And in those cases there is an absorption of the solids, or of the part destroyed, not a melting down, or solution of them into Pus. The Teeth are to be looked upon as extraneous bodies, with respect to the Gum, and as such they irritate the inside of that part in the same manner as the Pus of an abscess, an exfoliation of a bone, or any other extraneous body; and therefore produce the same symptoms, excepting only the formation of matter. (2) If, there- fore, these symptoms attend the cutting of the Teeth, there can be no doubt of the propriety of opening the way for them ; nor is it ever, as far as I have observed, attended with any dangerous consequence. OF THE FORMATION AND PROGRESS OF THE ADULT TEETH. Having now considered the first formation, and the progress of the Temporary Teeth, we shall next describe the formation of those Teeth which are to serve through life. (a) (2) [Any real analogy between the tooth and an extraneous body can- not be admitted. In a note on this section, Mr. Bell observes, " There can, indeed, be no doubt that the emancipation of the rising tooth is occasioned by absorption of the gum ; but it is also probable that this absorption is increased, if not wholly produced, by the pressure of its edge on the horizontal surface of the tooth. It appears probable, there- fore, that when, in consequence of the rapid elongation of the root, the crown of the tooth rises faster than this process for the removal of the containing parts goes on, an undue pressure takes place on the inside of the gum, and local inflammation, accompanied by much constitutional disturbance, is the result. The mere existence of the tooth in contact with the gum, ' as an extraneous body,' would not account for all this disturbance ; for, after the gums are lanced, the tooth is still in contact with the soft parts ; but because the pressure is thus taken off, the irritation immediately subsides. (1) " (a) [According to the more commonly-received view, based on the (1) 'Huuter's Works,' by Palmer, vol. ii., p. 35. 122 HUNTER ON THE TEETH. In this enquiry, to avoid confusion, 1 shall confine the description to the Teeth in the Lower-Jaw ; for the only diffc- observations of Goodsir, as has been already stated, the ten anterior permanent teeth in each jaw are formed in cavities of reserve behind the follicles of the ten milk teeth, whilst the three permanent molars on each side are formed in posterior cavities of reserve, formed by pro- longations backwards of the primitive dental groove. About the fourteentli week, behind each milk follicle a small lunated recess, similar to an impression made by the nail, is said to be formed. These depressions escape the general adhesion of the sides of the dental groove, but by the closure of the latter they become converted into cavities, which are formed successively from before backwards, and ultimately become the sacs for the incisor, canine, and bicuspid teeth. These cavities elongate and sink into the substance of the gum, above and behind the upper milk follicles, below and behind the lower. A papilla is formed at the bottom of each, that for the central incisor appearing first, about the sixth month. Opercula, as is alleged in the case of the temporary teeth, are developed from the sides of the sac, dividing it into two portions, the lower of which encloses the papilla, which becomes the pulp of the permanent tooth. The upper and narrower part of the cavity becomes gradually obliterated by the adhesion of its sides. The permanent sac then adheres to the back of '.hat for the temporary tooth. Both grow, and after a time it is found that each sac (the permanent and temporary) becomes lodged in a separate osseous recess or socket — that for the permanent tooth in the lower jaw being below and behind that for the temporary, in the upper jaw above and behind. A bony par- tition separates them. The permanent sac and its osseous recess present a pear-shape. The sac is connected with the gum by a solid mem- branous pedicle, which is contained in an osseous canal, which opens by an aperture on the edge of the jaw, behind the socket for the tem- porary tooth. The permanent tooth is formed in its sac in the same manner as the milk tooth, from which it is separated by a bony par- tition. In the rise of the permanent tooth through the gum, it presses on tlie bony partition and on the root of the milk tooth ; these latter become absorbed under the influence of the pressure ; the milk tooth, after the greater or less absorption of its fang, falls out, and the per- manent tooth takes its place. A description of the formation of the posterior cavities of reserve, and of the papilla of the true molars, has already been given in the previous note on the dental pulps. (1) Vide note v. p. Kolliker gives, on the whole, a similar account of the formation of the germs of tin- permanent teeth, lie observes, thai in the fourth (1) Vide 'Quain's Anatomy,' by Professors Sharpey and Quain. OF THE FORMATION OF THE ADULT TEETH, ETC. L28 rence between those in the two Jaws, is in the time of their appearance, and generally it is later in the Upper-Jaw. Their formation and appearance proceeds not regularly from the first Incisor backwards to the Dens Sapientta, but it begins at two points on each side of both Jaws, viz. at the first Incisor, and at the first Molaris. The Teeth between these two points make a quicker progress than those behind. The Pulp of the first adult Incisor, and of the first adult Molaris begin to appear in a Foetus of seven or eight months, and five or six months after birth the ossification begins in them ; soon after birth the Pulp of the second Incisor and Cuspidatus begin to be formed, and about eight or nine months afterwards they begin to ossify ; about the fifth or sixth year the first Bicuspis appears ; about the sixth or seventh the second Bicuspis, and the second Molaris; and about the twelfth, the third Molaris or Dens Sapicntim.ip) month, the cavities containing the papilke of the temporary teeth become narrower and narrower, and at length are perfectly closed, " but in such a manner, that over each cavity, or tooth-sac, another small recess is formed as cavity of reserve for the twenty anterior permanent teeth, of which, even in the fifth month of icetal life, the tooth germs are de- veloped. At first the new cavities lie over the tooth sacs of the milk teeth, but they gradually move to the posterior side of them ; and when the bony alveoli of the milk teeth appear, are received into small dilatations of them, which, in the incisor and canine teeth, become at last completely separate from the others ; in the two first molars (bicus- pides), on the other hand, open at the bottom of the alveoli of the milk teeth. The tooth sacs of all these teeth are drawn out at the apex in form of a solid cord, which extends either to the gum or on the two first molars (bicuspides), to the periosteum at the bottom of the two milk molars, and which has been incorrectly considered as being a con- ducting band (gubernaculum) of the teeth in their eruption. (1)" Robin and Magitot deny that the follicles of the permanent teeth are developed in depressions on the surface of the mucous membrane of the gum.] (b) [The papillae of the first true molars appear as early as the six- teenth or seventeenth week according to Kolliker and Goodsir ; some- what earlier — the eighty-fifth to the ninety-fifth day — according to Robin and Magitot. Kblliker, as before noticed, states that the tooth genus of the (1) Kolliker, op, oit., p. 301. 124 HUNTER ON THE TEETH. The first five may be called the permanent Teeth : they differ from the temporary in having larger fangs. The permanent Incisores and Cusjiidati are much thicker and broader, and the Molarcs are succeeded by Bicuspidcs, which are smaller, and have but one fang. All these permanent or succeeding Teeth are formed in dis- tinct Alveoli of their own ; so that they do not fill up the old sockets of the temporary Teeth, but have their new Alveoli formed as the old ones decay. twenty anterior permanent teeth are developed in the fifth month of foetal life. Robin and Magitot say they originate some before and some after birth. According to Goodsir, the papilla of the second molar appears about the seventh month after birth, that of the third about the sixth year. Calcification begins first in the anterior molar. The process is a little earlier in the teeth of the lower jaw than in the upper. The fol- lowing is the order in which the process of calcification has been ob- served to take place in the permanent teeth of the upper jaw. First molar, five or six months ; central incisor soon after ; lateral incisor and canine, eight or nine months ; two bicuspids, two years and over ; second molar five or six years ; third molar, or wisdom tooth, about twelve years.(l) Mr. Tomes gives the following description of the condi- tion of the permanent teeth at the period when all the temporary teeth are perfected, but the first true molars have not cut the gums : — " The crowns of the permanent incisors, both of the upper and lower jaws, are perfected, excepting perhaps at that part where the enamel terminates. There the dull and chalk-like appearance, which that tissue presents when the development is progressing may be observed. The canines are still less advanced, while the crowns of the first bicuspids have not attained to more than two-thirds, and those of the second bicuspid not more than a third, of their ultimate lengths. The crowns of the first permanent molars are, as respects their external surface, fully developed ; and the septa of dentine which extend across the base of the pulps mark- ing out the several roots yet to be developed are fully pronounced. The second permanent molars are at present represented by about two-thirds of their crowns, and invested with a thin layer of partially-developed enamel. The positions of the pulps of the wisdom teeth are but faintly indicated by slight depressions in the bone posterior to the sockets, which contain the forming second molars. These marks may, however, at this period, be altogether wanting.(2)] (1) Quain and Sharpey, op. tit. (2) Tomes' System of Dental S Surgi n . p. 60. OF THE FORMATION, ETC., OF THE ADULT TEETH. 125 The first Incisor is placed on the inside of the root of the corresponding temporary Tooth, and deeper in the Jaw. The second Incisor and the Guspidatus begin to be formed on the inside, and somewhat under the temporary second Incisor and Cuspidaius. These three are all situated much in the same manner, with respect to the first set ; but as they are larger, they are placed somewhat farther back in the circle of the Jaw. The first Bicuspis is placed under, and somewhat farther back than the first temporary Grinder, or fourth Tooth of the child. The second Bicuspis is placed immediately under the second temporary Grinder. The second Molaris is situated in the lengthening tubercle in the Upper- Jaw, and directly under the Coronoid Process in the Lower. The third Molaris, or Dens Sapimtim, begins to form imme- diately under the Coronoid Process. The first Adult Molaris comes to perfection, and cuts the Gum about the twelfth year of age, the second about the eighteenth, and the third, or Dens Sapiential, from the twentieth to the thirtieth : so that the Incisor -es and Cuspiclati require about six or seven years, from their first appearance, to come to perfection ; the Bicuspides about seven or eight ; and the Molar es about twelve, (c) (c) [The eruption of the permanent teeth takes place a little earlier in the lower jaw than in the upper. The following tables record the results of the observations of Dr. Blake and Mr. Cartwright as the period of eruption : — (1) Blake. Years, Molar, First 6| Incisors, Central 7 „ Lateral 8 Bicuspids, Anterior 9 „ Posterior 10 Canines 11 to 12 Molars, Second 12 to 13 ,, Third (or Wisdom) ... 17 to 25 Cartwright. Years. Molar, First 5 to 7 Incisors, Central Inferior 5 to 7 ,, ,, Superior ... 6 to 8 ,, Lateral 7 to 9 Bicuspids, Anterior 8 to 10 Canines 9 to 12 Bicuspids, Posterior 10 to 12 Molars, Second 12 to 1 4 ,, Third (Wisdom) 17 to 25 (1) From Quain's Anatomy, by Sliarpey and Quain. Q 126 HUNTER ON THE TEETH. It sometimes happens that a third set of Teeth appears in very old people ; when this does happen, it is in a very irregular manner, sometimes only one, at other times more, and now and The following table gives the results obtained from an examination of 3,074 cases by Mr. S. Cartwright, jun. : — (1) CO OS OS .ioiio;so,£ jaAioq; " CN - 1 r 00 o ■a _ - t~ CO joTjgisoj aaddjQ CO * •spidsnoig "* «« (M CO to CO § CO OS r— co joua^soj jgAicj •spidsnoig -H CM CO CO »o o oc -<* CN OS o t-- CO .xoua^soj jaddfi rt " •spidsnoig; i-H *- CO o CO c «o OS CO os 00 o CO OS CM «> CO CO O oo co co OS O co co •spidsno J9avo^ OS co w & 8 •spidsng jaddfi * CO o CO CO >. ~ >-f -r 3 m^P p ri 3 _i- irthdays. children, irthdays. children. ^o^*# — -: - - tiwzo _i co ^ "* - : i & SiH ~ ~~ _j o ' o 3S — tJ = 5"* r-H O 71 ■_ -* rH c iO =*-co =w rH O I-H O t3 ~<5 ^6 °jl - s - - - c - 5 - 5 5050 71 M o co a 1 ~ oo 33 (1) From Tomes' System of Dental Surgery. THE MANNER IN WHICH A TOOTH IS FORMED. 127 then a complete set comes in both Jaws. I never saw an in- stance of this kind but once, and there two fore Teeth shot up in the Lower-jaw. I should suppose that a now Alveolar Process must be also formed in such cases, in the same manner as in the production of the first and second sets of Teeth. From what I can learn, the age at which this happens is generally about seventy. From this circumstance, and another that sometimes happens to women at this age, it would appear that there is some effort in nature to renew the body at that period. When this set of Teeth which happens so late in life, is not complete, especially where they come in one Jaw, and not in the other, they are rather hurtful than useful ; for in that case we are obliged to pull them out, as they only wound the opposite Gum. (