418 DIGESTION 



swallowed, especially liquid food or water, the pyloric sphincter 

 may relax and allow the stomach to propel a portion of its contents 

 into the intestine; and such relaxations occur at intervals as diges- 

 tion goes on, although it is not for several hours (three to five) that 

 the greater portion of the food reaches the duodenum. During this 

 period the acidity has at first been constantly increasing, although 

 for a time the hydrochloric acid has combined, as. it is formed, with 

 the proteins of the food. Then comes a stage where the hydrochloric 

 acid has so much increased that, after combining with all the proteins, 

 some of it remains over as free acid. After a time the total acidity 

 begins to fall, the partially digested proteins continually passing on 

 through the pylorus, while a considerable proportion is so fully 

 digested as to be absorbed by the gastric mucous membrane itself. 

 Thus, in one experiment on the digestion of meat in a dog, it was 

 found that 30 per cent, was absorbed in the stomach, while 40 per 

 cent, passed through the pylorus as peptone, over 20 per cent, as 

 undissolved or soluble protein (acid-albumin), and a little more than 

 8 per cent, as proteose (Tobler). The large proportion of peptone 

 is noteworthy, as indicating some kind of selective passage of the 

 different digestive products from the stomach into the duodenum. 

 For the gastric contents contain plenty of proteose, although only 

 traces of peptone. The total ' titratable acidity ' goes on diminishing 

 till the third or fourth hour, the proportion of free to combined acid 

 continuing, nevertheless, to rise, since nearly all that is now secreted 

 remains free. In addition to a certain amount of protein, small 

 quantities of soluble and easily diffusible substances, like sugars and 

 some of the organic crystalline constituents of meat e.g., kreatin 

 may also be absorbed into the blood by the gastric mucous membrane. 

 The substances which reach the duodenum are (i) The greater 

 part of the fats. The partial digestion in the stomach of the enve- 

 lopes and protoplasm of the cells of adipose tissue, and of the protein 

 which keeps the fat of milk in emulsion, prepares the fats which are 

 not split up by the gastric juice for what is to follow in the intestine. 

 (2) All the proteins which have not been carried to the stage of 

 peptone, and much peptone. (3) All the starch and dextrins and 

 glycogen, if any be present which have not been converted into 

 sugars, and probably a portion of the sugars. (4) Nucleins, haematin, 

 cellulose, and other substances not digestible, or digestible only with 

 difficulty, in gastric juice. (5) The constituents of the gastric juice 

 itself, including pepsin. Most of the pepsin is soon destroyed in the 

 unfavourable environment of the intestinal contents. But it has 

 been shown that a certain amount of active pepsin may be present 

 for a rime in the intestine, even in the free condition, and still more 

 when enclosed in the interior of masses of protein which protect it, 

 and which still continue to be digested by it. This is particularly 

 true of certain materials, like elastin and connective tissue, which are 



