To provide this necessary fluid, and to connect its supply directly with the process of mastication to which it is subservient, several glands for its secretion have been placed in the immediate neighbourhood of the mouth and jaws, in such a way that the latter cannot be opened and shut without affording them a stimulus, and still farther increasing the secretion which the presence of the morsel is itself sufficient to begin. From this arrangement it follows, that the more perfectly mastication is performed, the more thoroughly does the morsel become impregnated with the salivary fluid, and the better fitted is it rendered for subsequent deglutition and digestion.
The apparatus of mastication varies according to the kind of food on which the animal is destined to live; but in the higher orders of animals, it consists essentially of the parts already mentioned. In some animals, however, which live on soft gelatinous food—as the whale—no teeth are to be found, because their peculiar power is not required. In others—as the granivorous or grain-eating birds—the grinding or triturating process is effected not in the mouth but in the gizzard, where the food (mixed with gravel, which the animal is instinctively impelled to swallow for the purpose) is effectually bruised and softened down by the strong muscles which constitute the greater part of its substance. In these instances, the gravel is the grinding instrument, and without its presence digestion cannot be carried on any more than it could in man without the agency of teeth.
The degree of mastication required varies also according to the mode of life of the animal, and the digestibility of its food. Animal food, for example, being easy of digestion, requires less mastication than vegetable food, which is more difficult. This is so much the case, that most animals which live on fresh vegetable matter spend half their waking hours in ruminating or re-masticating the food, which they have already cropped and stored up for the purpose in one of their four stomachs. To this necessary act in them, Providence seems to have attached a high degree of gratification, for the very purpose of insuring its regular performance.
Man, being naturally omnivorous, or adapted for the digestion of both animal and vegetable substances, holds, as it were, an intermediate place in regard to the rapidity of mastication. He is neither obliged to ruminate like the cow, nor can he beneficially bolt his food with the rapidity displayed by birds of prey. His object is merely to reduce the alimentary mass to a soft and pulpy consistence, and digestion is promoted or retarded in exact proportion as he approaches or falls short of this point. Hasty mastication is consequently injurious, because it prevents the food from being sufficiently broken down and impregnated with saliva; and the more uncommon error of protracted mastication is also injurious, owing to the undue dilution which the mass sustains from the overflow of the salivary secretion.
Due mastication being thus essential to healthy digestion, the Creator, as if to insure its being adequately performed, has kindly so arranged, that the very act of mastication should lead to the gratification of taste—the mouth being the seat of that sensation. That this gratification of taste was intended, becomes obvious when we reflect that, even in eating, Nature makes it our interest to give attention to the process in which we are for the time engaged. It is well known, for example, that when food is presented to a hungry man, whose mind is concentrated on the indulgence of his appetite, the saliva begins to flow unbidden, and what he eats is consumed with a peculiar relish and is easily digested. Whereas, if food be presented to an individual who has fasted equally long, but whose soul is absorbed in some great undertaking or deep emotion, and who is consequently insensible to the gratification of taste, it will be swallowed almost without mastication, and without sufficient admixture with the saliva—now deficient in quantity—and therefore lie on the stomach for hours unchanged. In this point of view the peculiarly English custom of reading the newspapers or magazines during breakfast is more hurtful than one would suppose; and many dyspeptics have been surprised at the benefit resulting from its discontinuance. However, therefore, philosophy and morality condemn the undue cultivation of our bodily appetites, it cannot be denied that a certain degree of attention to taste, and to the pleasures of appetite, is both reasonable and beneficial; and it is only when these are abused that we oppose the intentions of Nature.
From the existence of this intentional relation between mastication and the salivary secretion, the latter is always most copious in those creatures whose food requires continued mastication. In ruminating animals, accordingly, the salivary glands are numerous and of great size, while they are at the same time so situated that the play of the muscles in the act of rumination communicates to them a proportionate stimulus. In those, again, which do not masticate at all, but swallow their food entire, there is scarcely any salivary secretion, and the glands appropriated to it are very small. Birds, and many fishes and reptiles, belong to the latter class.
From the foregoing explanation of the object and conditions of mastication, the reason will be apparent why fluids do not require to undergo that process, and also why dry mealy substances stand in need of protracted chewing before they can be easily swallowed. When hot spicy food is taken into the mouth, the secretion of saliva is immensely increased, obviously for the purpose of diluting the excess of stimulant before it shall be allowed to reach the stomach. But when the food is of a mild and unirritating quality, much dilution is unnecessary, and the secretion is accordingly moderate.
The chief purpose of mastication, then, is evidently the minute division of the aliment, so as to admit of its being easily acted upon by the gastric juice when received into the stomach. Dr Beaumont, however, seems to me to go too far in inferring, that “if the materia alimentaria could be introduced into the stomach in a finely divided state, the operations of mastication, insalivation, and deglutition, would not be necessary.” It would require a more extensive range of experiments than that which he has made, to prove that “aliment is as well digested and assimilated, and allays the sensation of hunger as perfectly, when introduced directly into the stomach (through an opening in the side) in a proper state of division, as when the usual previous steps have been taken.”[15] It is quite true that mastication and deglutition are chiefly mechanical processes; but it is difficult to believe that so much care would have been taken to provide a proper supply of fluid of a constant and peculiar character like saliva, if water were capable of answering the purpose as well, and if saliva were useful only in lubricating the food. There subsists, moreover, between the sense of taste and the power of digestion a certain relation, which renders it more than probable that the active gratification of the former during mastication, is favourable to the production and flow of nervous energy towards the stomach, and consequently in so far conducive to the healthy performance of digestion that even in that point of view insalivation could not easily be dispensed with. Dr Beaumont’s experiments, however, abundantly demonstrate that Montègre, and those who, along with him, consider the saliva as the principal agent in digestion, have not a shadow of foundation for their opinion.
When unmasticated food is introduced into the stomach, the gastric juice acts only upon its surface, and changes of a purely chemical nature sometimes commence in its substance before its digestion can be effected. Hence often arise, especially in children, those pains and troubles, that nausea and acidity, consequent on the continued presence of undigested aliment in the stomach. By a peculiarity of constitution, however, the stomach will not long retain food which it cannot dissolve. After a number of hours,—varying, according to the state of health, from one or two to ten, or even twenty,—it is either rejected by vomiting, or transmitted unchanged to the intestine, where its presence irritates and gives rise to colic, flatulence, bowel-complaints, and, in delicate children, not unfrequently to convulsions. Hence another proof of the importance of slow and deliberate mastication.