DIGESTION IN THE STOMACH 291 



In several species of animals a diastatic enzyme is wanting in the saliva, 

 and in these the physiological importance of saliva is restricted to the above- 

 mentioned purely mechanical action. In general it has been supposed that 

 even where the ptyalin is present the formation of sugar induced by it plays 

 only a subordinate part in digestion, either because of the short time the 

 food remains in the mouth, or because the swallowed saliva would quickly 

 lose its diastatic power on account of the acid of the gastric juice. The latter 

 conclusion presumes either that the acid content of the gastric juice is suffi- 

 ciently high to neutralize the alkaline reaction of the saliva at once or that 

 the stomach contents are very quickly permeated by the gastric juice. But 

 neither presumption is warranted by the facts. Hensay has shown that as 

 much as eighty per cent of the carbohydrates raised from the human stomach 

 at the end of half an hour is maltose or the closely related dextrin. Accord- 

 ing to Cannon and Day, an acid reaction in the interior of the stomach con- 

 tents of the cat can only be observed after one to one and one-half hours from 

 the time of feeding, and during this time the ptyalin has every opportunity 

 to act on the starch. This was confirmed also by direct experiments in which 

 human saliva was given. 



2. DIGESTION IN THE STOMACH 



The first division of the alimentary canal in which the food is chem- 

 ically changed to any considerable extent, is the stomach. Here the carbo- 

 hydrates are split up partly by the ptyalin of the swallowed saliva, partly by 

 the acid and the Bacteria of the stomach contents. Starch paste is changed 

 by the acid of the stomach to soluble starch, and from this with the help of 

 acid fermentation under the influence of Bacteria, dextrin, sugar and lactic 

 acid are formed. 



Emulsified fat is split to a considerable extent by the stomach steapsin 

 (cf. page 250). Casein is coagulated under the influence of the rennin, and 

 the curd thus formed is dissolved again by the gastric juice. Moreover, what 

 is of particular importance for the nourishment of children, almost the 

 entire quantity of phosphorus from the curd passes, according to experiments 

 in vitro, into solution in organic combination. The passage of the casein, 

 which is the most important constituent of milk, into the intestine is delayed 

 by its coagulation in the stomach. 



The pepsin-hydrochloric acid dissolves all kinds of true proteids, the gela- 

 tin-forming substances and elastic tissues; but keratin is not acted upon. 

 The influence of gastric juice on the gelatin-forming substances appears to be 

 especially significant for the whole process of digestion; it is even said that 

 they are dissolved more easily and more completely by the gastric juice than 

 are proteids (Bikfalvi). 



This is quite in line with all that we know about the function of the stomach. 

 This function in brief is to transform the ingested food into a soupy mass, the 

 chyme to prepare it, in other words, for entrance into the intestine. The abil- 

 ity of the gastric juice to dissolve gelatin-forming substances aids in this direc- 

 tion; for the tissue elements which bind together the cells of the animal foods 

 are composed of just such substance, and as soon as they are dissolved the cells 

 are set free and the chyme is formed. 



