This section is from the book "Health", by W. H. Coefield.
Now, as to the first set of teeth. In front there are two teeth with edges like a chisel, cutting edges; these are called the incisor teeth, because they are used in cutting up the food, and they are especially developed in animals which have to eat hard foods. There is a certain class of animals which go by the name of rodents or gnawing animals, some of which we know very well, such as hares, rabbits, squirrels, rats, mice, etc., and these all have the front or cutting teeth very remarkably developed. Then, behind these two, there comes a tooth which is pointed; it has no cutting edge like the first two; and because it is the tooth that corresponds to the tooth which is most easily seen in a dog's mouth, it goes by the name of the canine tooth, although in us it projects little beyond the rest of the teeth. The function of this tooth is to tear the food, and these teeth are most developed in carnivorous animals, such as the lion, the bear, the dog, and the cat. Behind this tooth there come two teeth which have large square surfaces with points upon them, and they are especially adapted for grinding the food, and so go by the name of molar teeth, because they, more or less, resemble mill-stones in their action. These teeth are especially developed in animals which eat food which requires grinding, such as the horse, the cow, sheep, etc. They are developed sometimes almost to the exclusion of the rest of the teeth.
These just described belong to the first set or milk teeth, and they are the temporary teeth. Of these there are five teeth on each side of each jaw; twenty in alL
These teeth are replaced at about five years of age in the following way: the two incisor or cutting teeth are replaced by incisor or cutting teeth; the canine is replaced by a canine stronger and larger, but still a canine tooth; the two molars are replaced by two teeth which have not one point like canines, but two points on their summits, and so go by the name of bicuspid. The canines have only one root, the bicuspid have really one root, but it is divided into two at the point. Then, besides these, there are three more molars developed. In the adult there are on each side of each jaw two incisor or cutting teeth, one canine or tearing tooth, two bicuspids, which have replaced the molars of the infant, which are partly tearing and partly grinding teeth-being in construction between the canine or tearing and the molars or grinding teeth - and three true molars. There are then in a complete set eight teeth on each side, making thirty-two in alL The last molar, on each side of each jaw, is developed at a late period of life, and they commonly go by the name of the wisdom teeth.
You will probably think it strange that in human beings the teeth should be of different kinds, although almost all of the same length; but this is sufficient to show us that human beings are adapted to eat various kinds of food. Man is not like the animals,-as horses, cows, sheep, etc.-which live only on vegetable foods, nor like lions, tigers, etc, which eat animal food only.
Animals that live on one kind of food have their teeth developed in the way which best enables them to eat that particular kind of food, but we, as you see, have teeth corresponding to the teeth of various classes of animals; so it is quite clear, from the mere construction of our mouths, that we are adapted to live upon various kinds of food.
I said the mouth was lined by a mucous membrane; in this mucous membrance there are very numerous depressions; these form what we call glands, and, because they are in the mouth, they go by the name of buccal glands; and here I may take the opportunity of saying, that when I speak of a gland, that is what I mean-I mean a depression from the skin, or from the mucous membrane.
Tou will remember, when speaking about lymphatic glands, I told you that they were not glands properly so called, and when speaking of a true gland, a depression from the skin or mucous membrane is meant.
It does not matter how blunt that depression may be, how far it may go, or how far it is pushed out in different directions. These buccal glands are merely little depressions.
But, besides, there are six important glands, three on each side, connected with the mouth; these secrete fluid, which is poured in the mouth, and goes by the name of saliva or spittle. These six glands secrete fluid, which, together with the fluids secreted by the numerous little glands I have mentioned before, has extremely important purposes.
When we take food into our mouths it is subjected to certain processes in the mouth, which altogether go by the name of mastication and insalivation (or mixing with the saliva). This is done by means of the movement of the lower jaw upon the upper jaw (because the lower jaw alone is movable, moving up and down, across, forwards, and backwards), the movements of the tongue, and the movements of the cheeks. While the lower jaw is moving, so as to crush the food between the teeth, the tongue and cheeks act in opposite directions, so as to keep the pieces of food between the teeth. When the food gets on one side, either the tongue or the cheeks contract, and push it between the teeth again; so that, by a series of extremely complex actions, the food that we have in our mouth is kept, while necessary, between the teeth, and so divided into very minute fragments. Tou will see how this acts, and the way in which the food is kept between the teeth, when you consider how frequently, when eating game, you find a shot in your mouth, and you feel it half a dozen times between your teeth before you can secure it, thus showing the extremely perfect way in which the muscles of the cheeks and the tongue act to keep the particles of food between the teeth, so as to ensure their being completely crushed. While this is going on, and because this is going on, the glands that I have mentioned are secreting their fluids; the very fact that the tongue and cheeks are moving, is sufficient to make the salivary glands secrete their fluids, and they secrete comparatively little fluid at any other time, only just sufficient to keep the mouth moist. Whenever anything is in the mouth they are excited to secrete more fluid, as any of you who have been up a mountain can testify. The mouth then, or when going for a long walk, always becomes dry; and it is a very capital plan to keep a small pebble, a marble, or a plum-stone in the mouth, so as to secure, by continually moving it about, a constant flow of saliva.