Section I. From London Lancet, July 11, 1903
The Effects of Mastication
The primary object of mastication is to break up the food so as (1) to facilitate the swallowing of it, and (2), still more important, to insure its intimate admixture with the digestive juices, not only within the mouth, but throughout the entire digestive tract. Mastication has, however, other important and far-reaching effects. Thus it promotes the flow of saliva and, when properly performed, secures a due insalivation of the food; it increases the quantity of alkaline saliva passing into the stomach; it stimulates the heart and circulation; and it finally influences the nutrition of the jaws and their appendages by stimulating the local blood and lymph circulation. Now to consider these various objects and effects of mastication.
Mastication facilitates swallowing.—Many foods cannot be swallowed without first going through some preparation in the mouth. Soft, moist, pultaceous foods, such as milk pudding and porridge, can be and often, indeed, are swallowed with little or no preliminary chewing. On the other hand, it is a mechanical impossibility to swallow large lumps of tough food, or very dry food, even though, like flour, it be in a finely divided state. Dry food needs first to be well moistened; and it is not surprising that it promotes a more abundant flow of saliva than moist food, though the secretion thus excited may be poor in ferment. Hence it follows that if we desire to give foods which compel mastication, they should be tough or dry. On the whole, vegetable foods necessitate more thorough mastication than animal. The carnivora can scarcely be said to masticate at all the flesh which they consume; they simply tear off portions, and forthwith swallow them whole. Cooked flesh, however, does require mastication, owing to the coagulation of its proteids. The herbivora, on the other hand, unlike the carnivora, have to subject their food to considerable mastication before it can be swallowed; but they generally masticate it far more than is needful to render swallowing mechanically possible, as is exemplified in the act of rumination, the object here being to facilitate the admixture of the digestive juices with the food.
According to Van Someren, if the habit of masticating efficiently is once acquired, the food is not swallowed before it is converted into the liquid state, the swallowing of unmasticated lumps being effectually prevented by a pharyngeal reflex.
Mastication, by breaking the food up into small particles, enables it to be brought into intimate contact with the digestive juices.—Such comminution is especially needful in the case of raw vegetable foods of the tougher kind, in order to break up their cellulose framework, and to set free the contained starch, proteids, and fats. Foods of this kind, unless masticated, yield practically no nutriment to the organism. I cannot too strongly emphasise the fact that before man learned to break up the cellulose framework of his vegetable food by cooking he was compelled to subject it to laborious mastication. But, while thorough comminution is especially needed for vegetable food when raw, it is also needed for many cooked forms of it also,—as, for example, solid batter pudding and new underbaked bread, heavy lumps of which, passing into the stomach, may seriously hamper the work of that organ. Such substances are indigestible essentially by virtue of their impermeability to the digestive juices, and they gain in digestibility in proportion as they are comminuted. The indigestibility of new bread would appear to be wholly due, not to any peculiarity of chemical composition, but to its tendency to elude the teeth and form a sodden mass impermeable to the digestive juices, while the more powdery stale bread is more easily broken up both in the mouth and within the stomach. Cabbage, again, owes its indigestibility to the fact that it is allowed to pass into the stomach in large masses, while the well-known digestibility of cauliflower and minced spinach is due to the fineness of their division; were cabbage as finely minced as spinach usually is it would be equally digestible.
Turning now to animal food it has to be remarked that while in the raw state it may be readily digestible with little or no previous mastication, since massive pieces of it are readily attacked by the digestive juices, the like is much less true of animal food the proteids of which have been coagulated and rendered less permeable by cooking. Large lumps of hard-boiled egg or overdone meat, for instance, may obstinately resist gastric digestion; indeed, as with vegetable so with animal foods, their relative digestibility depends more upon physical consistence than chemical composition; beef is generally more indigestible than mutton and pork or veal than either, not so much by virtue of chemical composition as of physical consistence; the indigestibility of cheese illustrates the same truth; the individual nutritive ingredients of this substance—the proteids and fats—are not in themselves indigestible; casein in the form of protein or plasmon is known to be easy of digestion, and butter is one of the most digestible of fats; but in cheese the two are welded together into a comparatively impermeable mass, which is apt to escape comminution by the teeth and to pass down into the stomach in the form of solid lumps. A plain, wholesome cheese well masticated or intimately mixed with other foods, as in macaroni cheese, is quite easily digested by the majority.
I do not, of course, deny the influence of the chemical factor. Undoubtedly food may disturb digestion by virtue of its chemical composition, apart altogether from its physical characters; thus, while cooked goose-fat sets up violent irritation in some, others cannot tolerate eggs in any shape or form, and innumerable idiosyncrasies in respect of special articles of diet are met with which are essentially referable to chemical composition; but making due allowance for this chemical influence there can, I think, be little doubt that the digestibility of the more common articles of diet, both animal and vegetable, depends in the main upon their physical constitution, all of them tending to be equally digestible when reduced to the same degree of comminution. This, if true, is, I need scarcely say, a fact of the greatest practical importance, for it amounts to this: that we may often allow to those with very weak digestions foods which are generally considered indigestible, provided that they be thoroughly comminuted, whether by mastication or artificial means.
Mastication promotes the flow of saliva and the insalivation of the food.—The more efficiently food is masticated the greater is the salivary flow, and the more intimately is it mixed with the saliva, or, as we say, insalivated. The saliva has apparently no effect on fats; whether it acts on proteids seems more doubtful, though by some authorities the penetration of these by the alkali of this fluid is said to aid in their subsequent digestion; on starch, however, the saliva acts very potently, and hence mastication plays a special part in promoting the digestion of starchy foods. Indeed, if only mastication be persisted in long enough, starch may be wholly converted into maltose within the mouth, and it need scarcely be said that it is better for the individual himself to manufacture his maltose in this way than that he should take it ready made for him in the form of one of the many “malt extracts” on the market. Patients are often forbidden starchy food, while they are allowed the maltose which they can quite well manufacture in their own mouths. Provided they be sufficiently insalivated, there are few starchy foods which are indigestible, not even excepting the proverbially indigestible new potato. These remarks are especially applicable to children, as will be more particularly insisted on later.
Mastication increases the amount of alkaline saliva passing into the stomach, and this not only prolongs the period of starch digestion within this organ but, by its influence upon the reaction of the gastric contents, influences all the digestive processes taking place there. I shall have occasion to point out later that a deficient supply of alkaline saliva in the stomach predisposes to certain forms of indigestion.