"Regarding differences in the availability of fats, it may be stated that, as a rule, the fatty matter contained in vegetable foods is less readily, or less thoroughly, digested than that present in foods of animal origin. In the latter, about 95 per cent of the fat is digested and absorbed. This figure, however, is generally taken as representing approximately the digestibility or availability of the fat contained in man's daily dietary, since by far the larger proportion of the fat consumed is of animal origin. Carbohydrates, on the other hand, are much more easily utilized by the body. Naturally, sugars, owing to their great solubility and ready diffusibility, offer little difficulty in the way of easy digestion; but starches likewise, though not so readily assimilable, are digested, as a rule, to the extent of 98 per cent, or more of the amount consumed."4

Before food taken into the body can be built up into living tissue or made to yield its content of heat and energy, it must first undergo the process of digestion. This process - contrary to the ideas of the old physiologists who thought that digestion began in the stomach - is now known to have its inception in the mouth. Saliva - once believed to be chiefly useful in rendering foods soft and moist enough to pass easily down the esophagus - is now known to be one of the most powerful digestive juices secreted by the body; and the act of mastication - once thought to serve the somewhat insignificant purpose of breaking up the food into particles small enough to be swallowed - is now known to perform a number of important offices in the work of digestion. The first of these is to accelerate the flow of saliva and to increase its digestive power by heightening its alkalinity.

4 Chittenden: "Nutrition of Man," pp. 12-13.

This is of the very highest importance to the digestion of the whole great class of foodstuffs known as carbohydrates. While the saliva has no appreciable chemical effect upon the fats and. proteids, it is capable of practically completing the digestion of the starches and sugars - if they are retained in the mouth long enough and subjected to sufficient mastication - before they are swallowed into the stomach.

"Need we comment, in view of the n&tural brevity of this process," says Prof. Chittenden, "upon the desirability for purely physiological reasons of prolonging within reasonable limits the interval of time the food and saliva are commingled in the mouth cavity? It seems obvious, in view of the relatively large bulk of starch-containing foods consumed daily, that habits of thorough mastication should be fostered, with the purpose of increasing greatly the digestion of starch at the very gateway of the alimentary tract. It is true that in the small intestine there comes later another opportunity for the digestion of starch; but it is unphysiological, as it is undesirable, for various reasons, not to take full advantage of the first opportunity which Nature gives for the preparation of this important foodstuff for future utilization. Further, thorough mastication, by a fine comminution of the food particles, is a material aid in the digestion which is to take place in the stomach and intestine.

Under normal conditions, therefore, and with proper observance of physiological good sense, a large proportion of the ingested starchy foods can be made ready for speedy absorption and consequent utilization through the agency of salivary digestion."5

Dr. Harry Campbell6 of the North-West London Hospital says:

"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 (a form of sugar) within the mouth. . . . Provided they be sufficiently insalivated, there are few starchy foods that are indigestible, not even excepting the proverbially indigestible new potato."

5Chittenden: "Nutrition of Man," p. 23. 6 Campbell: Lecture quoted in "The A. B. - Z. of Our Own Nutrition," pp. 101-102.

The chief factors in the production of salivary flow - aside from the mechanical movements of chewing - are the thought, taste, sight and smell of tempting food. "The secretory center may be stimulated, and likewise inhibited, in impulses which have their origin in higher nerve centers in the brain," says Professor Chittenden. . . "The thought and the odor of savory food cause the mouth to water. Similarly fear, embarrassment,and anxiety frequently cause a dry mouth and parched throat. . . . The application of these facts to our subject is perfectly obvious, since they suggest at once how the production of an important digestive fluid - upon which the utilization of a given class of foodstuffs may be quite dependent - is controlled and modified through the nervous system by a variety of circumstances. We might reason that the appearance, odor, and palatability of food are factors of prime importance in its utilization by the body; and that the aesthetics of eating are not to be ignored since they have an important influence upon the flow of the digestive secretions.

A peaceful mind, pleasurable anticipations, freedom from care and anxiety, cheerful companionship, all form desirable table accessories which play the part of true psychical stimuli in accelerating the flow of the digestive juices and thus pave the way for easy and thorough digestion."7

In the stomach the saliva continues its digestive activity until it is brought in contact with the gastric juice. This fluid, being an acid, promptly neutralizes the effect of the alkaline saliva, and checks temporarily the further digestion of the starchy foods. Therefore it is easy to understand that, as Dr. Harry Campbell8 points out: