We have accepted so far without discussion the almost self-evident fact that the food is the immediate source of the substance and energy of the animal body. It now remains for us to consider the way in which nutrition is accomplished. The first step in this direction is the digestion of food. It is necessary for food ingredients to be placed in such relations to the animal organism that they are available for use. This involves both condition and location. The various nutrients in the exercise of their several functions must be generally distributed to all the interior parts of the animal body. It is obvious that bread and meat as such cannot be so distributed, and so their compounds must, in part at least, be brought into soluble and diffusible condition, in order that they may pass through the membranous lining which separates the blood vessels and other vascular bodies from the cavity of the alimentary canal.

90. Digestion And Assimilation

In discussing physiological relations of food, two terms are employed: viz., digestion and assimilation. Digestion refers to the preparation of food compounds for use, by rendering them soluble and diffusible, - changes which are accomplished in what we call the alimentary canal, a passage that begins with the mouth, includes the stomach and intestines, and ends with the anus. Assimilation signifies the appropriation of nutrients, after digestion, to the maintenance of the vital processes and to the building of flesh and bone, - processes taking place in the tissues, to which the nutritive substances are conveyed by the blood. The two terms are entirely distinct in meaning, although they are confused in popular speech.

91. General Changes In Food Through Digestion

In digestion, food undergoes both mechanical and chemical changes. It is masticated, that is, ground into finer particles, after which, in its passage along the alimentary canal, it comes in contact with several juices which profoundly modify it chemically. That portion of it which is rendered diffusible is absorbed by certain vessels that are embedded in the walls of the stomach and intestines, and is conveyed into the blood. The insoluble part passes on and is rejected by the animal as worthless material, and constitutes the solid excrement or feces. The forms in which the nutrients are conveyed into the circulation are believed to be the following: The proteins, previous to absorption into the blood, are converted into soluble bodies, proteoses and peptones, or mainly into simpler nitrogen compounds resulting from a more extensive cleavage, or more probably into all these forms; the carbohydrates enter the blood as sugars, chiefly as dextrose. The fats are changed into a finely divided form either as such or as fatty acids and soaps. The function of digestion is to transform the various nutrients into these forms. A study of digestion includes, then, a knowledge of mastication, of the sources, nature, and functions of the several digestive juices, and a consideration of the various conditions affecting the extent and rapidity of digestive action.