248 HAND-BOOK OF PHYSIOLOGY. 



digestion is undoubtedly peptone. We have seen, however, in the above 

 experiment that there is a by-product, and this is almost identical with 

 syntonin or acid albumin. This body is probably not exactly identical, 

 however, with syntonin, and its old name of parapeptone had better be 

 retained. The conversion of native albumin into acid albumin may be 

 effected 'by the hydrochloric acid alone, but the further action is undoubt- 

 edly due to the ferment and the acid together, as although under high 

 pressure any acid solution may, it is said, if strong enough, produce the 

 entire conversion into peptone, under the condition of digestion in the 

 stomach this would be quite impossible; and, on the other hand, pepsin 

 will not act without the presence of acid. The production of two forms 

 of peptone is usually recognized, called respectively a^-peptone and 

 ^em^-peptone. Their differences in chemical properties have not yet been 

 made out, but they are distinguished by this remarkable fact, that the 

 pancreatic juice, while possessing no action over the former, is able to 

 convert the latter into leucin and ty rosin. Pepsin acts the part of a 

 hydrolytic ferment (proteolytic), and appears to cause hydration of albu- 

 min, peptone being a highly hydrated form of albumin. 



Circumstances favoring Gastric Digestion. 1. A temperature 

 of about 100 F. (37 '8 C.); at 32 F. (0 C.) it is delayed, and by boil- 

 ing is altogether stopped. 2. An acid medium is necessary. Hydro- 

 chloric is the best acid for the purpose. Excess of acid or neutralization 

 stops the process. 3. The removal of the products of digestion. Excess 

 of peptone delays the action. 



Action of the Gastric Juice on Bodies other than Proteids. 

 All proteids are converted by the gastric juice into peptones, and, there- 

 fore, whether they be taken into the body in meat, eggs, milk, bread, or 

 other foods, the resultant still is peptone. 



Milk is curdled, the casein being precipitated, and then dissolved. 

 The curdling is due to a special ferment of the gastric juice (curdling 

 ferment), and is not due to the action of the free acid only. The effect 

 of rennet, which is a decoction of the fourth stomach of a calf in brine, 

 has long been known, as it is used extensively to cause precipitation of 

 casein in cheese manufacture. 



The ferment which produces this curdling action is distinct from 

 pepsin. 



Gelatin is dissolved and changed into peptone, as are also cliondrin 

 and elastin; but mucin, and the horny tissues, keratin generally are un- 

 affected. 



On the amylaceous articles of food, and upon pure oleaginous prin- 

 ciples the gastric juice has no action. In the case of adipose tissue, its 

 effect is to dissolve the areolar tissue, albuminous cell-walls, etc., which 

 enter into its composition, by which means the fat is able to mingle 

 more uniformly with the other constituents of the chyme. 



