DIGESTION. 253 



They cannot be precipitated by heat, by nitric, or acetic acid, or by po- 

 tassium ferrocyanide and acetic acid. They are, however, thrown down 

 by tannic acid, by mercuric chloride and by picric acid. 3. They are 

 very soluble in water and in neutral saline solutions. 



In their diffusibility peptones differ remarkably from egg-albumin, 

 and on this diffusibility depends one of their chief uses. Egg-albumin 

 as such, even in a state of solution, would be of little service as food, in- 

 asmuch as its indiffusibility would effectually prevent its passing by ab- 

 sorption into the blood-vessels of the stomach and intestinal canal. 

 Changed, however, by the action of the gastric juice into peptones, albu- 

 minous matters diffuse readily, and are thus quickly absorbed. 



After entering the blood th.e peptones are very soon again modified, 

 so as to re-assume the chemical characters of albumin, a change as neces- 

 sary for preventing their diffusing out of the blood-vessels, as the pre- 

 vious change was for enabling them to pass in. This is effected, proba- 

 bly, in great part by the agency of the liver. 



Products of Gastric Digestion. The chief product of gastric diges- 

 tion 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 un- 

 doubtedly 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 anti-peptone and 

 hemi-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 tyrosin. Pepsin acts the part of a 

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

 bumin, peptone being a highly hydrated form of albumin. 



Circumstances favoring Gastric Digestion. 1. A temperature of 

 about 100 F. (37.8 0.) ; at 32 F. (0 C.) it is delayed, and by boiling 

 is altogether stopped. 2. An acid medium, is necessary. Hydrochloric 

 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 pro- 

 teids are converted by the gastric juice into peptones, and, therefore, 



