The carbohydrates, fats, and proteins as they exist in foods* are in most cases not of a nature to be used by the body tissues in the exact form in which they are eaten, but must usually undergo more or less alteration in the digestive tract to fit them for absorption and utilization. In so far as the changes which the food undergoes in the alimentary tract are chemical they are brought about mainly by the action of digestive enzymes; but the efficiency of the digestive process is also largely dependent upon the mechanical factors of digestion which therefore will also be briefly considered in this chapter.

Historical

The idea that changes comparable to fermentation are involved in the processes of digestion apparently originated with von Helmont about 300 years ago. Sylvius, half a century later, cited alcoholic and acetous fermentations to illustrate the type of process by which he believed the foodstuffs to be digested. Descartes held that as the result of a peculiar fermentation there was generated in the stomach "an acid of great potency, comparable to nitric acid." From the standpoint of our present knowledge these early scientists appear to have made considerable progress toward a correct interpretation of the digestive process: but in their own times, before the beginning of the scientific development of organic or physiological chemistry, the views which they advanced appeared hazy and unscientific compared with those of the physiologists who were studying digestion from the mechanical point of view and by supposedly exact methods. Thus Dr. Archibald Pitcairn (1652-1713) proposed to explain gastric digestion, "without the aid of a Daemon or a Stygian Liquor," as due entirely to the triturating action of the stomach, the power of whose muscular walls he estimated as "equal to 12,951 pounds" (Gamgee).

* A table showing percentages of proteins, fats, and carbohydrates in foods is given in Appendix B.

The view that the digestion of food in the stomach is due solely to the mechanical action of the stomach walls was refuted by Reaumur, working with birds, and by Stevens, who experimented with a man who was accustomed to swallow small stones and regurgitate them at will. In Stevens' experiments this man swallowed hollow silver balls filled with food and perforated to permit access of the gastric juice but strong enough to resist the muscular contractions of the stomach walls. Food thus introduced was found to undergo digestion in the stomach although it was entirely protected from the triturating action of the stomach walls. Furthermore Stevens found that gastric juice obtained from a dog was able to digest meat outside of the stomach. At about the same time Spallanzani also showed clearly that gastric juice can act outside of the body. In addition, he pointed out its antiseptic properties and emphasized the difference between the digestive process and that of alcoholic, acid, or putrefactive fermentation. About fifty years after the work of Spallanzani came the classical observations (1825-*&33) of Dr. Beaumont upon Alexis St. Martin, who, as the result of a gunshot wound, was left after recovery from his injury with a gastric fistula which permitted both the collection of human gastric juice and the direct observation of the processes going on in the stomach of a healthy man "active, athletic, and vigorous, exercising, eating, and drinking, like other healthy and active people." Dr.Beaumont's full and interesting account of his experiments with St. Martin 1 greatly extended the knowledge both of the muscular behavior of the stomach and of the conditions governing the secretion of the gastric juice and the "chymification" of the food in the stomach. The year after the publication of Beaumont's observations, Eberle showed2 that by extracting the mucous membrane of the stomach with dilute hydrochloric acid he could obtain an artificial juice which showed the same digestive action which Spallanzani and Beaumont had observed with the natural secretion, and two years later Schwann3 concluded that gastric juice owed its peculiar activity to a substance presumably different from any substance previously known and to which he gave the name pepsin. Schwann did not claim to have isolated this peculiar substance in a pure state but did effect a partial separation. Subsequently several other investigators attempted to isolate pepsin.