This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
Much controversy has arisen over the question as to how far the stomach performs the essential work of digestion, and how far the intestine is responsible for it. Some writers argue that the stomach is a comparatively useless organ except as a receptacle, and that the small intestine, with the different juices which are poured into it, is abundantly capable of doing alone the entire digestive work. The entire stomach has been excised from man and in several cases a fair measure of digestion has been retained. In these operations the oesophagus is united to the duodenum.
The only really important action of the stomach consists in digesting a single class of foods - namely, proteids - and this process is not always finished, whereas the intestine digests not only proteids, but fats, starches, and sugars. The digestion of starch, inaugurated by ptyalin in the mouth, is continued for a varying length of time in the stomach, until the hydrochloric acid reaction of the gastric juice becomes sufficiently strong to inhibit it. The period of this amylolytic digestion may be prolonged in the stomach by administration of diastase. The period of the unaided digestion of starch is usually stated to be only 15 or 20 minutes, but according to the researches of Austin it is much longer - one or two hours, so that but little starch digestion may be left for the pancreatic juice to complete. The stomach warms and macerates all the food, so that it relieves the small intestine of much preliminary work. Gastric digestion is hindered by either acids or alkalies used in excess, by metallic salts, strong alcohol, and by regurgitation of bile from the intestine.
Gillespie has found as many as twenty-four varieties of bacteria in the intestine, most of which are harmless.
No reliable estimates of the exact quantity of gastric juice, or, in fact, of any of the digestive fluids, are obtainable. At best, such estimates vary greatly according to different authorities. An abundant secretion is not necessarily an active one in ferment or acid, and the constant reabsorption of the water makes it quite impossible to say how much fluid has been secreted, for if the digestive juices be drained off and measured, the natural conditions are disturbed. Bile or pancreatic fluid allowed to drain off constantly through a fistula soon becomes altered in quality and weakened in digestive power.
The estimates of the total quantity of the digestive fluids secreted per diem extend from three or four quarts to three gallons, and it is almost impossible to make exact measurements.