The Teeth
The Herbivora.—Let us consider first the teeth of the herbivorous animal. The horse, the ox, and the sheep are typical of this class of animals—living, as they do, almost entirely upon grass and herbs. The character of their food is peculiar. It is bulky, coarse, and covered with sharp, cutting edges—ill suited for tender mouths and gums. It must be mashed and ground thoroughly between the teeth and in the mouth before it is fit to be swallowed; and teeth of a peculiar construction and mutual relation are necessary in order properly to perform this function. Just such teeth they possess. There are twenty-four molars, six on each side in each jaw; and in the lower jaw, in front, eight incisors, or cutting teeth, with none on the upper jaw. In place of any of these teeth on the upper jaw, there is simply a horny plate upon which the long incisors of the lower jaw impinge when the jaws are closed. This renders possible the tearing, grinding motions necessary for biting off and masticating the food upon which these animals live. Not only that. The actual structure of their teeth is peculiarly suited to their food and its mastication. Unlike our teeth, they are not covered with enamel, but are composed of alternate layers of enamel and dentine—a soft, bony substance lying between the layers of enamel, and wearing away more rapidly than it does. The result is that there is soon formed a series of jagged edges, which form cutting, grinding surfaces, and are especially adapted for the food which these animals feed upon. No such formation is present in any other class of animals, since their food is different from that of the herbivora. It is a wise provision of nature, precisely adapted to the desired ends.
The Insectivora.—The insect-eaters are more nearly related to the Rodentia than to the Carnivora. The form of teeth varies with the species. The incisors and canines are not especially prominent, but the molars are always serrated with numerous small-pointed eminences, or cusps, adapted to crushing insects. The three leading families of the Insectivora are the moles, the shrew-mice, and the hedgehogs. They are of small size, and are found in all countries, except in South America and Australia.
The Rodentia.—The Rodentia is a peculiar order of animals, characterised by two very long and strong teeth in each jaw, which occupy the place of the incisors and canines in other animals. Back of these there is a toothless space, and then four or five molars, which, when they have a roughened crown, indicate a vegetable, but when pointed, an insectivorous, diet. Their principal foods are grains and seeds of all kinds, and with these, often, fruits, nuts and acorns. To this order belong the families of the squirrel, marmot, all species of mice, the beaver, porcupine, hare, and others.
An especial dietetic subdivision of the Rodentia is the Rhizophaga, or root-eaters, which includes some species of the Marsupials, and of mice. The food often consists exclusively of the roots of the beet, carrot, celery and onion.
The Edentata.—Occasionally, though rarely, animals of this class have rudimentary back teeth. Their food consists of leaves, blossoms, buds, and juicy stalks. Some also devour insects, especially ants. To this order belong the sloth, armadillo, pangolin, and great ant-eater.
The Omnivora.—Omnivorous animals have very distinctive teeth. The canines are markedly developed, forming regular tusks at the side of the mouth. These are used for attack and defence, and also to dig up roots, upon which these animals largely feed. The hog is typical of omnivorous animals of this character, and we all know his disposition no less than his anatomy! Animals of this class can live upon both animal and vegetable food, and man is supposed to be included in this category! The great argument brought forward by those who recommend a “mixed” diet (i.e. one composed of flesh, vegetables, fruits, roots, grains, etc.), is that man can live on all these foods, and retain life and some degree of health, and that therefore he is omnivorous. We shall come to consider this argument somewhat at length presently. For the moment, it is enough to say that (so far as his teeth are considered, at any rate) man is in no way similar to the hog, or to any omnivorous animal whatever, but totally different from all of them. The most casual glance at the mouth and teeth of the hog should convince us that we are not in that class! The other considerations we shall come to later.
The Carnivora.—The next great class that we must consider is the carnivora. Their teeth are very distinctive, and their shape and arrangement are entirely different from those of any of the other animals. There are the incisor teeth in front, and molars behind; but the most distinctive teeth are the canine, which especially distinguish this class of feeders. There are four of these—two in each jaw, placed upon the sides, and they are long, sharp, and pointed. The more nearly the animal is purely carnivorous, the more are these teeth developed, and the less meat the animal eats, the less are they developed. Thus, the feline species, which is perhaps the most typically carnivorous of all, have canine teeth very well developed; in them, they are most marked. In such animals, the canine teeth are also set considerably apart from other teeth. In the dog, however, the teeth are less prominent than they are in the cat; his claws, eyes, etc., are also less distinctly carnivorous, and it will be observed that his habits are decidedly less like those of the preying animal than are those of the cat: he sleeps at night instead of in the daytime; does not adopt the stealthy methods for catching birds, etc., which the cat follows. All this has its significance when it is remembered that dogs are much more easily weaned away from a flesh diet, on to one of milk, bread, biscuits, etc., than are cats, who are very difficult to wean from their carnivorous habits. This, however, is by the way.
In the bear family, again, the carnivorous characteristics are still less marked. The canine teeth are less and the molars and incisors more developed—the latter having a flat but roughened crown. All this indicates a still nearer approach to a vegetable and fruit diet—as is actually the case. The bear, as is well known, is fond of berries, fruits of all kinds, milk and honey.