The physiology of digestion considered with relation to the principles of dietetics

by Andrew Combe

The teeth are modified in different animals to suit their habits of life. In herbivorous animals, the canine teeth, for which they have no use, are comparatively undeveloped; whereas in carnivorous animals, which tear their prey in pieces, the canine teeth are large, powerful, and pointed, and the incisors comparatively small. In these animals they constitute what are properly called the tusks, and in some species they are of a truly formidable character. The molar or grinding teeth differ in like manner according to the nature of the food. In herbivorous and granivorous animals they are large and powerful, and to increase their efficiency the lower jaw admits of considerable lateral motion in a horizontal direction; whereas, in carnivorous animals, it admits of motion only upwards and downwards, as in opening and shutting the mouth. The lateral grinding motion is very evident in ruminating animals, such as the cow, which, after having filled its stomach with provender, is generally seen to lie down and ruminate, or chew the cud as it is called—the rumination consisting in bringing up small masses of herbage from the stomach, and submitting them to a thorough mastication or grinding between its molar teeth before being again swallowed and digested.

From this relation between the food and the organs of mastication, naturalists can tell with certainty, by simply inspecting the teeth, on what kind of food the animal to which they belong is intended to live; and as the teeth of man partake of the characters of those of both herbivorous and carnivorous animals, there cannot be a doubt that his diet was intended to be of a mixed kind, not confined exclusively to either the vegetable or the animal kingdom.

Hard and resisting as the teeth appear, they must still be regarded as living structures. Anatomically speaking, each tooth is divided into three parts: the fang or root, implanted in the socket of the jaw-bone; the neck, or portion encircled by the gum; and the white crown, appearing above the gum, and covered with enamel.

The root of each tooth is perforated longitudinally by a small canal, through which the bloodvessels and nerve are admitted to its central parts. From these bloodvessels the tooth derives its nourishment when growing; but they afterwards almost entirely disappear. From the nerve it derives that sensibility which makes us instantly aware of the contact of bodies either too hot or too cold with the teeth; and which, when the nerve is diseased, gives rise to the racking pain of toothach.

So effectually is life maintained in the teeth by this provision of vessels and nerves, that a tooth newly extracted from the socket of a young animal, and implanted in the fleshy comb of a cock, has been found to adhere and retain its vitality; and, in like manner, if, in early life, a tooth extracted by accident be immediately replaced in its socket, it will generally adhere and live.

The visible part or crown of the tooth is covered with a very hard white ivory-looking substance called enamel, which serves to prevent it from being worn down by friction, and into which neither bloodvessels nor nerves have been observed to penetrate. Owing to this structure, the tooth can be safely exposed without sustaining damage—a privilege on which most persons will be disposed to place a higher value after having experienced the pains consequent upon injury of the nerve from a portion of the enamel being broken off.

An obvious advantage attending the vitality of the teeth is, that it enables them to accommodate themselves to the growth of the jaw and the rest of the system at the different periods of life. In early infancy, when the human being is designed to live exclusively on his mother’s milk, which of course requires no mastication and consequently no teeth, the latter are still imperfectly formed and entirely hidden in the jaw: it is only at the end of some months that the front or cutting teeth begin to appear; and the whole set of milk, deciduous, or falling-out teeth, twenty in number, is not completed till about or after the third year. In the course of three or four years more, however, growth has advanced so far that the first set of teeth no longer fill the jaw; and they soon begin to be displaced by the second or permanent set, the gradual development of which commences at that period of life, and is not finished till the appearance of the last four grinders or wisdom-teeth, about the age of maturity.

It is a curious fact that the infant is born with the rudiments of both sets of teeth in the jaw at the same time, although neither makes its appearance till long after birth. The permanent teeth lie in a line under the milk-teeth, and it is from their growth causing the gradual absorption of the roots of the first teeth that the latter no longer retain their hold of the jaw, but drop out as soon as the others are ready to protrude. In the preceding wood-cut, the situation of the permanent teeth before they emerge from the jaw is rudely represented at A, where the outer surface of the jaw-bone has been removed on purpose, to shew the appearance of the roots. But nothing of this kind is to be found in the adult jaw, the parts marked A being inserted in the plate merely to illustrate what was once the position which the permanent teeth occupied.

The changes in the condition of the teeth, it may be remarked in passing, indicate clearly what species of food nature has intended for us at different ages. In early infancy, when no teeth exist, the mother’s milk is the only nutriment required; and in proportion as the teeth begin to appear, a small addition of soft farinaceous food prepared with milk may be made with propriety and gradually increased. But it is impossible to look at the small jaw, moderate muscle, and imperfect teeth of early life, without perceiving that only the mildest kinds and forms of animal food are yet admissible, and that the diet ought to consist essentially of soft and unirritating materials. It is not till the permanent teeth have appeared, that a full proportion of the ordinary kinds of butcher-meat becomes either beneficial or safe.

The teeth, being living parts and at the same time endowed with a mechanical function, are liable to injury in both capacities. Being composed chiefly of earthy matter, such as phosphate and carbonate of lime, the contact of strong acids decomposes their substance, and leads to their rapid decay. Hence the whiteness produced by acid tooth-powders and washes is not less deceitful than ruinous in its consequences; and hence also great caution is necessary in swallowing the acid drops frequently prescribed by the physician, which ought never to be allowed to come into contact with the teeth.