784 PHYSIOLOGY OF DIGESTION AND SECRETION. 



be digested, and, so far as the psychical or appetite secretion is 

 concerned, with the palatableness of the food. The apparatus 

 is adjusted in this respect to work economically. Different 

 kinds of food produce secretions varying not only as regards 

 quantity but also in their acidity and digestive action. The 

 secretion produced by bread, though less in quantity than that 

 caused by meat, possesses a greater digestive action. On a 

 given diet the secretion assumes certain characteristics, and 

 Pawlow is convinced that further work will disclose the fact 

 that the secretion of trie stomach is not caused normally by general 

 stimuli all affecting it alike, but by specific stimuli contained in the 

 food or produced during digestion, whose action is of such a kind 

 as to arouse reflexly the secretion best adapted to the food ingested. 



One of the curves, showing the effect of a mixed diet (milk, 600 

 c.c.; meat, 100 gms.; bread, 100 gms.) upon the gastric secretion, 

 as determined by Pawlow's method, is reproduced in Fig. 295. It will 

 be noticed that the secretion began shortly after the ingestion of the 

 food (seven minutes), and increased rapidly to a maximum that was 

 reached in two hours. After the second hour the flow decreased 

 rapidly and nearly uniformly to about the tenth hour. The acidity 

 rose slightly between the first and second hours, and then fell gradu- 

 ally. The digestive power showed an increase between the second 

 and third hours. 



Nature and Properties of Pepsin. Pepsin is a typical proteo- 

 lytic enzyme that exhibits the striking peculiarity of acting only in 

 acid media; hence peptic digestion in the stomach is the result of 

 the combined action of pepsin and hydrochloric acid. Pepsin is 

 influenced in its action by temperature, as is the case with the other 

 enzymes; low temperatures retard, and may even suspend its 

 activity, while high temperatures increase it. The optimum tem- 

 perature is stated to be from 37 to 40 C., while exposure for some 

 time to 80 C. results, when the pepsin is in a moist condition, in the 

 total destruction of the enzyme. Pepsin may be extracted from the 

 gastric mucous membrane by a variety of methods and in different 

 degrees of purity and strength. The commercial preparations of 

 pepsin consist usually of some form of extract of the gastric mucous 

 membrane to which starch or sugar of milk has been added. Labora- 

 tory preparations are made conveniently by mincing thoroughly 

 the mucous membrane and then extracting for a long time with 

 glycerin. Glycerin extracts, if not too much diluted with water or 

 blood, keep for an indefinite time. Purer preparations of pepsin 

 have been made by what is known as "Briicke's method/' in which 

 the mucous membrane is minced and is then self-digested with a 5 

 per cent, solution of phosphoric acid. The phosphoric acid is pre- 

 cipitated by the addition of lime-water, and the pepsin is carried 

 down in the flocculent precipitate. This precipitate, after being 



