1 Lectures on Digestion, New York, 1881, p. 37.
2 "On the Destruction of Ferments in the Alimentary Canal," Journal of Physiology, London, Jan., 1882, p. 246.
When the food enters the stomach the nitrogenous (albuminoid) elements are attacked by the gastric juice, the bundles of muscular fibre are broken up, the fibrillæ are reduced to a granular mass, but not completely dissolved (Frerichs), the fat-globules are freed from their envelopes of connective tissue, milk is coagulated, and the casein is dissolved.
"The tangible, practical object of this change is to form out of a little-diffusible body (albumen) one easily diffusible (peptone), which is capable of absorption through animal membrane in a higher degree than ordinary albumen" (Ewald). Peptone is formed out of ordinary albumen, as grape-sugar is formed out of starch, by taking up water; it is therefore the hydrate of albumen.
The more tardy the digestion in the stomach the more highly charged with acid is the gastric juice. According to Wright, the degree of alkalinity of the saliva is in proportion to the acidity of the stomach fluids, and Bence Jones has observed that during the excretion of acid in the stomach the total alkalinity of all alkaline digestive fluids is increased. The lesson is thus learned that a too careful preparation of food, so as to shorten and lessen gastric labor, diminishes the activity of the gastric juice as well as that of all other digestive fluids.
Intestinal digestion begins when the softened mass passes through the pylorus. This mass (chyme) is composed of (1) the products of gastric digestion which have not been absorbed—peptone, dextrose, levulose, peptonized gelatin, with mucus and gastric juice; (2) all matters which have escaped digestion—the starch of vegetable substances, dissolved gelatin and albumen which have not been peptonized, and some unaltered muscle-structure; and (3) fat, fatty acids, and cellulose upon which neither saliva nor gastric juice has had any influence (Ewald).
This complex semi-fluid mass with an acid reaction enters the duodenum and comes in contact with fluids and ferments destined to work remarkable changes in its composition. The first of these fluids is the bile, which is alkaline and composed of the glycocholate and taurocholate of sodium, cholesterin, soaps, etc., phosphates and carbonates of lime and sodium, chlorides of potassium and sodium, bile-pigment, etc. The outflow of bile is excited by the contact of the chyme with the orifice of the bile-duct. When the alkaline bile is mingled with the acid mass in the duodenum, it neutralizes its acidity, precipitates the peptones, and therefore stops all further action of the gastric juice. Fats containing free fatty acids are emulsified, soaps being formed by a combination of the alkalies of the bile with the fatty acids. Lastly, bile hinders fermentation in the intestine and acts as a purgative by exciting peristalsis. Absorption is probably also favored by bile, as it has been found that emulsified fats pass more readily through an animal membrane which has been wet with bile.3
3 Ewald thinks this result is doubtful: in animals killed during digestion he has found an acid reaction in the contents of the intestine beyond the opening of the bile-duct, with no precipitation of the albumen (op. cit., p. 82).
As far as we now know, the function of the bile is to neutralize the acidity of the duodenal contents, and thus pave the way for the action of a digesting fluid of much greater potency and of much higher function.4
4 In order still further to demonstrate the necessity of bile-action as a preparation for pancreatic digestion, it may be mentioned that in artificial experiments, with a heat equal to that of the body, if antiseptics analogous to gastric juice and bile are not used, there is a too rapid change from alkalinity to acidity, and consequently all of the starch is not converted into sugar before it develops lactic acid with putrefactive disorganization. A deficiency of bile, therefore, is a cause of intestinal indigestion (Bartlett, op. cit., pp. 12, 13).