170

DIFFERENT KINDS OF TEETH.

and in many animals (especially those of carnivorous habits) project far beyond the former; they are adapted not to cut the food, but, by being deeply fixed in it, to enable the animal to tear it asunder: these are termed canine teeth. The teeth of the third kind have large irregular flattened surfaces, and are adapted to bruise and grind the food; these are called molar (or mill-like) teeth. The manner in which these different teeth are implanted in the jaw, varies with the form of their crowns, and is in accordance with their several The incisors, whose action tends as much to bury them in their sockets as to draw them forth, have but a single root or fang of no great length. The canine teeth, on which there is often considerable strain, penetrate the jaw more deeply than the incisors; especially when they are large and

uses.

long, as in the Cat tribe (fig. 94). And the molars, whose action requires great firmness, have two, three, or even four roots or fangs, which spread out from each other; and these at the same time increase the solidity of their attachment to the jaw, and prevent the teeth from being forced into their sockets by any amount of pressure.

182. The arrangement of the dental apparatus varies, in different Mammalia, according to the nature of the aliment on which they are destined to feed; and this correspondence is so exact, that the anatomist can generally determine by the simple inspection of the teeth of an animal, not only the nature of its food, but the general structure of the body, and even its ordinary habits. Thus, in those that feed exclusively on animal flesh, the molar teeth are so compressed as to form

DIFFERENT KINDS OF TEETH.

171

cutting edges, which work against each other like the blades of a pair of scissors (fig. 94); whilst in animals that live on insects, these teeth are raised into conical points, which lock

Fig. 95.

Fig. 94. TEETH OF CARNIVOROUS ANIMAI. TEETH OF INSECTIVOROUS ANIMAL. into corresponding depressions in the teeth of the opposite jaw (fig. 95). When the nourishment of the animal consists principally of soft fruits, these teeth are simply raised into rounded elevations (figs. 97, 98); and when they are

Fig. 96.

TEETH OF HERBIVOROUS ANIMAL. Fig. 97.-TEETH OF FRUGIVOROUS ANIMAL.

destined to grind harder vegetable substances, they are terminated by a large flat and roughened surface (figs. 96, 99). The roughness of this surface is maintained by the peculiar arrangement of the three substances of which the tooth is composed. The enamel, instead of covering its crown, is arranged in upright plates, which are dispersed through the tooth; and the space between them is filled up by plates of ivory and of cementum (§ 54). These last, being softer than the enamel, are worn down the soonest; and thus the plates of enamel are left constantly projecting, so as to form a rough surface admirably adapted to the grinding action which the tooth is destined to perform. The mode in which these plates are disposed, affords a most characteristic distinction between the two species of Elephant at present existing,

172

DIFFERENT KINDS OF TEETH.

namely, the African and the Indian; as also between each of these and the great extinct species known as the Mammoth (fig. 99). In the great gnawing teeth of the Rabbit,

&c., the front surface only is covered with enamel; and as this is worn away more slowly than the ivory, it stands up as a sharp edge (fig. 91), which is always retained, however much the tooth may be worn away.

183. Of all the teeth, the molars may be regarded as the most useful. They are seldom absent in the Mammalia; and their office is usually essential to the proper digestion of the food. Animal flesh (the most easily digested of all substances) needs but to be cut in small pieces; but the hard envelopes

of beetles and other insects must be broken up; and the tough woody structure of the grasses, and the dense coverings of the seeds and fruits on which the herbivorous animals are supported, must be ground down. The incisors and canines are chiefly employed among Carnivorous animals for the purpose of seizing their living prey, and are never deficient in them; but they are less required in Herbivorous animals; and either or both kinds are not unfrequently deficient. Sometimes, however, they are not

SUCCESSION OF TEETH.-WHALEBONE.

173

only present in the latter, but are largely developed, serving as weapons of attack and defence; as in the Boar (fig. 100).

184. In the Mammalia in general, as in Man, the teeth are not much developed at the time of birth, that they may not interfere with the act of sucking; and they do not make their appearance above the gum, until the time approaches when the young animal has to prepare its own food, instead of simply receiving that which has been prepared by its parent. The teeth which are first formed are destined to be shed after a certain period, and to be replaced by others. They are called milk-teeth; and in Man they are twenty in number, namely, four incisors in the front of each jaw, and two canines and four molars on each side. These begin to fall out at about the age of seven years; previously to which, however, the first of the permanent molars appears above the gum, behind those of the first set. The incisors and canines of the first set are replaced by incisors and canines respectively; but the molars of the first set are replaced by teeth like small molars, having only two fangs; these are called false molars, or, more properly, bicuspid teeth (fig. 93). The second of the true molars does not make its appearance until all the milk-teeth have been shed; since it is only then that the jaw becomes long enough to hold any additional teeth. The third does not usually come up until the growth of the jaw is completed; and as this time corresponds with that at which the mind as well as the body is matured, they are commonly known as wise or wisdom teeth. There are then thirty-two teeth in all, or sixteen in each jaw ;-namely, four incisors, two canines, four bicuspid, and six true molars.-In extreme old age, these teeth fall out like those of the first set; but they are not replaced by others, and their sockets are gradually obliterated.

185. There are a few Mammalia which do not possess teeth. This is the case with the common Whale, in which they are replaced by an entirely different structure. From the upper jaw (fig. 102) there hang down into the mouth a number of plates of a fibrous substance (fig. 101), to which we give the name of whalebone, though it is really analogous to the gum of other animals. The fibres of these plates are separate at their free extremities, and are matted (as it were) together, so as to form a kind of sieve. Through this sieve the Whale

174 ABSENCE OF TEETH IN WHALE, ANT-EATER, ETC.

draws water in enormous quantities, whenever it is in want of food; and in this manner it strains out, as it were, the minute gelatinous animals upon which it lives, from the water of the seas it inhabits. The water thus taken in is expelled from the nostrils or blow-holes, which are situated at the top

of the head. Most of the Whale tribe have short fringes of this kind in the roof of the mouth; but in none, except the Balana, or Greenland Whale, is it long enough to make it worth separating; all the other species having teeth, either in one or both jaws. It is a curious fact, that the rudiments of teeth may be discovered in both jaws of the young Greenland whale, although they are never to be developed. And the rudiments of incisor teeth in the upper jaw, and of canine teeth in both jaws, may also be discovered in the young of the Ruminant quadrupeds (oxen, sheep, &c.), though they never show themselves above the gum.

186. The Ant-eaters, also, are destitute of teeth, and usually obtain their food by means of their long extensible tongues,

which are covered with a viscid saliva; this being pushed into the midst of an ant-hill, and then drawn into the mouth, brings into it a large number of these insects, which are sufficiently bruised between the toothless jaws (fig. 103). Lastly, may be mentioned as a curious exception to the general rules respecting the teeth of Mammalia, the remarkable Ornithorhyncus of New Holland (ZOOLOGY, § 317), which feeds,

Fig. 103.-SKULL OF THE ANT-EATER.