265

CHAPTER XXIX.

DENTAL SYSTEM OF MAMMALIA.

§ 218. General characters of the Teeth.-The present class includes a few genera and species that are devoid of teeth; the true ant-eaters (Myrmecophaga), the scaly ant-eaters (Manis), and the spiny monotrematous ant-eater (Echidna), are examples of strictly edentulous Mammals: Ornithorhynchus has horny teeth; the whales (Balana, Balanoptera) have transitory embryonic calcified teeth, fig. 219, succeeded by whalebone substitutes, fig. 217, in the upper jaw. The female Narwhal seems to be edentulous, but has the germs of two tusks in the substance of the upper jaw-bones: one of these so remains; the other becomes developed into a large horn-like weapon in the male Narwhal, fig. 220, A, and suggested to Linnæus the name, for its genus, of Monodon but the tusk is never median, like the truly single tooth on the palate of the Myxine; and occasionally both tusks are developed. In Hyperoödon the teeth are reduced in the adult to two in number, whence the specific name, H. bidens; but they are very small and confined to the lower jaw. Ziphius has two teeth of functional size and shape, one in each ramus of the lower jaw; and this is perhaps a sexual character. The Delphinus griseus has five teeth on each side of the lower jaw: but they soon become reduced to two. The Marsupial genus Tarsipes is remarkable for the paucity as well as minuteness of its teeth. The Elephant has never more than one entire molar, or parts of two, in use on each side of the upper and lower jaws, to which are added two tusks, more or less developed, in the upper jaw. Some Rodents, Hydromys, e. g., have two grinders on each side of both jaws, which, added to the four cutting teeth in front, make twelve in all; the common number of teeth in this order is twenty; but the hares and rabbits have twenty-eight teeth. The sloth has eighteen teeth. The number of teeth, thirty-two, which characterises man, the apes of the Old World, and the true Ruminants, is the average one of the class Mammalia; but the typical number is forty-four. The examples of excessive number of teeth are presented, in the order Bruta, by the Priodont

Armadillo, which has ninety-eight teeth; and in the Cetaceous order by the Cachalot, which has upwards of sixty teeth, though most of them are confined to the lower jaw; by the common porpoise, which has between eighty and ninety teeth; by the Gangetic dolphin, which has one hundred and twenty teeth; and by the true dolphins (Delphinus), which have from one hundred to one hundred and ninety teeth, yielding the maximum number in the class Mammalia.

Where the teeth are in excessive number, as in the species above cited, they are small, equal, or sub-equal, and of a simple conical form; pointed, and slightly recurved in the common dolphin; with a broad and flattened base in the Gangetic dolphin; with the crown compressed and expanded in the porpoise; compressed, but truncate, and equal with the fang, in Priodon. The compressed triangular teeth become coarsely notched or dentated at the hinder part of the series in the great extinct cetaceous Zeuglodon. The simple dentition of the smaller Armadillos, of the Orycterope, and of the three-toed Sloth, presents a difference in the size, but little variety in the shape of the teeth, which are subcylindrical with broad triturating surfaces; in the two-toed Sloth, the two anterior teeth of the upper jaw are longer and larger than the rest, and adapted for piercing and tearing, fig. 215.

Teeth are fixed, as a general rule, in all Vertebrates. In Mammals the movements of the teeth depend on those of the jaw bones supporting them, but appear to be independent in the ratio of the size of the tooth to the bone to which it is attached: the seemingly individual movements of divarication and approximation observable in the large lower incisors of the Bathyergus and Macropus,' are due entirely to the yielding nature of the symphysis uniting the two rami of the lower jaw, in which those incisors are deeply and firmly implanted.

In Man, where the premaxillaries early coalesce with the maxillary bones, where the jaws are very short, and the crowns of the teeth are of equal length, there is no interspace or diastema ’ in the dental series of either jaw, and the teeth derive some additional fixity by their close apposition and mutual pressure. No inferior Mammal now presents this character; but its importance, as associated with the peculiar attributes of the human organisation, has been somewhat diminished by the discovery of a like contiguous arrangement of the teeth in the jaws of a few extinct quadrupeds; e. g., Anoplotherium, Nesodon, and Dichodon.2 CLXXX. fig. 130.

XXV. vol. i. p. 285.

2

The teeth in Mammals, as in the foregoing classes, are formed by superaddition of the hardening salts to pre-existing moulds of animal pulp or membrane, organised so as to insure the arrangement of the earthy particles according to that pattern which characterises each constituent texture of the tooth, together with a course of vitalising plasma through its tissue.

The complexity of the primordial basis, or matrix,' corresponds, therefore, with that of the fully-formed tooth, and is least remarkable in those conical teeth which consist only of dentine and cement. The primary pulp, fig. 129, i*, which first appears as a papilla rising from the free surface of the alveolar gum, is the part of the matrix which, by its calcification, constitutes the dentine. In simple teeth, the secondary, or enamel pulp, covers the dentinal pulp like a cap; in complex teeth it sends processes into depressions of the coronal part of the dentinal pulp, which vary in depth, breadth, direction, and number, in the different groups of the herbivorous and omnivorous quadrupeds. The dentinal pulp, thus penetrated, offers corresponding complications of form; and, as the capsule follows the enamel pulp in all its folds and processes, the external cavities or interspaces of the dentine become occupied by enamel and cement the cement, like the capsule which formed it, being the outermost substance, fig. 237, c, and the enamel, ib. e, being interposed between it and the dentine, ib. d. The dental matrix presents the most extensive interdigitation of the dentinal and enamel pulps in the Wart-hog, Capybara, and Elephant.

The matrix of the mammalian tooth sinks into a furrow, and soon becomes inclosed in a cell in the substance of the jaw-bone, from which the crown of the growing tooth extricates itself by exciting the absorbent process, whilst the cell is deepened by the same process, and by the growth of the jaw, into an alveolus for the root of the tooth. Where the formative parts of the tooth are reproduced indefinitely, to repair, by their progressive calcification, the waste to which the working surface of the crown of the tooth has been subject, the alveolus is of unusual depth, and of the same form and diameter throughout, figs. 215 and 216, except in the immature animal, when it widens to its bottom or base. In teeth of limited growth, the dentinal pulp is reproduced in progressively decreasing quantity after the completion of the exterior wall of the crown, and forms, by its calcification, one or more roots or fangs, which taper to their free extremity. The alveolus is closely moulded upon the implanted part of the tooth; and it is worthy of special remark, that the complicated form of

socket, fig. 256, which results from the development of two or more fangs, is peculiar to animals of the class Mammalia.

In the formation of a single fang, the activity of the reproductive process becomes enfeebled at the circumference, and is progressively contracted within narrower limits in relation to a single centre, until it ceases at the completion of the apex of the fang, which, though for a long time perforated for the admission of the vessels and nerves to the interior of the tooth, is, in many cases, finally closed by the ossification of the remaining part of the capsule.

When a tooth is destined to be implanted by two or more. fangs, the reproduction of the pulp is restricted to two or more parts of the base of the coronal portion of the pulp, around the centre of which parts the sphere of its reproductive activity is progressively contracted. The intervening parts of the base of the coronal pulp adhere to the capsule, which is simultaneously calcified with them, covering those parts of the base of the crown of the tooth with a layer of cement. The ossification of the surrounding jaw, being governed by the changes in the soft but highly organised dental matrix, fills up the spaces unoccupied by the contracted and divided pulp, and affords, by its periosteum, a surface for the adhesion of the cement or ossified capsule covering the completed part of the tooth.

The matrix of certain teeth does not give rise, during any period of their formation, to the germ of a second tooth, destined to succeed the first. This, therefore, when completed and worn. down, is not replaced; all the true Cetacea are limited to this simple provision of teeth. In the Armadillos, Megatherioids, and Sloths, the want of germinative power, as it may be called, in the matrix, is compensated by its persistence, and the consequent uninterrupted growth of the teeth. In most other Mammals, the matrix of certain of the first developed teeth gives origin to the germ of a second tooth, which displaces its predecessor and parent. All those teeth which are so displaced are called temporary, deciduous, or milk teeth, fig. 293, di, d 1-4. The mode and direction in which they are displaced and succeeded, namely from below upward in the lower jaw, in both jaws vertically, are the same as in the crocodile; but the process is never repeated more than once in the present class. A considerable proportion of the dental series is thus changed; the second, or permanent teeth, ib. i 1-p, 2-4, having a size and form as suitable to the jaws of the adult as the displaced temporary teeth were adapted to those of the young animal. Those permanent teeth, ib. m 1-m 3, which

assume places not previously occupied by deciduous ones, may be regarded as a continuation of that series, and are posterior in their position; they are generally the most complex in their form. The successors of the deciduous incisors and canines differ from them chiefly in size. The successors of the deciduous molars may differ likewise in shape, in which case they have less complex crowns than their predecessors. The bicuspids' in Anthropotomy, fig. 258, p 3, p 4, and the corresponding teeth called ' premolars' in lower mammals, fig. 293, p 2-4, illustrate this law.

210

The Mammalian class might be divided, in regard to the succession of the teeth, into two groups-the Monophyodonts, or those that generate, as a rule, one set of teeth, and the Diphyodonts, or those that generate two sets of teeth. The Monophyodonts include the Monotremata, Cetacea and Bruta; all the other orders are Diphyodonts.

The teeth of Mammalia, especially of the Diphyodonts, have n usually so much more definite and complex a form than those of fishes and reptiles, that three parts are recognised in them: the fang or root (radix, fig. 210, f) is the inserted part; the crown (corona, ib. k) is the exposed part; and the constriction which divides these is called the neck (cervix, ib. n). The term 'fang' is properly given only to the implanted part of a tooth of re

stricted growth, which fang gradually tapers to its extremity. Those teeth which grow uninterruptedly, fig. 236, have not their exposed part separated by a neck from their implanted part, and this generally maintains to its extremity the same shape and size as the crown.

It is peculiar to the class Mammalia to have teeth implanted in sockets by two or more fangs, figs. 256, 293; but this can only happen to teeth of limited growth, and generally characterises the molars and premolars: perpetually growing teeth require the base to be kept simple and widely excavated for the persistent pulp, figs. 215 and 216. In no mammiferous animal does anchylosis

1 Vol. ii. p. 268.