FOOD AND DIGESTION. 363 



The action of pepsin is one of hydrolysis and the products are hydrated 

 forms of proteid. The acid is not only essential to the action of pepsin, 

 but it also aids digestion by causing the proteids to swell. That this ac- 

 tion is important is proven, in laboratory experiments, by the increased 

 length of time required for digestion when fibrin has been wrapped with 

 thread and thus prevented from swelling. 



Reactions of Proteases. The proteoses cannot be coagulated by heat. 

 All are soluble in salt solution. All are precipitated by picric acid or by 

 saturation (after neutralizing) with ammonium sulphate. All give the 

 Biuret test, copper sulphate producing a precipitate which redissolves on 

 the addition of caustic potash and forms a rose red solution. The pri- 

 mary proteoses are precipitated by strong nitric acid, also by acetic acid 

 and potassium ferrocyanide, and by saturation with sodium chloride and 

 magnesium sulphate. The secondary proteoses are not precipitated by 

 these reactions just mentioned but are characterized by the fact that their 

 precipitates, when formed, disappear on warming and reappear on cool- 

 ing. Proto-proteose is distinguished by being soluble in water while 

 hetero-proteose is not. 



Peptone reacts to the same test as deutero-proteose, but is not precipi' 

 tated on saturation with ammonium sulphate. 



Circumstances favoring Gastric Digestion. 1. A temperature of about 

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

 together stopped. 2. An acid medium is necessary. Hydrochloric is the 

 best acid for the purpose. Excess of acid or neutralization stops the proc- 

 ess. 3. The removal of the products of digestion. Excess of peptone 

 delays the action. 



a. Fibrin is first dissolved, forming a solution of globulins. The in- 

 termediate products of the digestion of globulins are called globuloses ; 

 of vitellin, vitelloses; of casein, caseinoses; of myosin, myosinoses. 

 These are practically the same as albumoses, and are included under the 

 term proteoses. 



b. Proteids. All proteids are converted by the gastric juice into pro- 

 teoses and peptones, and, therefore, whether they be taken into the body 

 in meat, eggs, milk, bread, or other foods, proteoses and peptone are still 

 the resultant. 



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

 The curdling is due to a special ferment of the gastric juice, 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 (rennet), 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 dis- 

 tinct from pepsin, and is called rennin. 



