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GASTKIC JUICE 



The food next passes into the stomach, which in ruminating 

 animals is divided into several parts and from which the food can be 

 brought back into the mouth to undergo further mastication (" chewing 

 the cud "). In the stomach, gastric juice is brought into contact with 

 the food. This is a colourless or slightly yellow liquid secreted in the 

 stomach, containing free hydrochloric acid and chlorides and phos- 

 phates of calcium, magnesium and the alkalies. Its characteristic 

 constituents, however, are the enzymes, pepsin, which has the power 

 of converting proteids into albumoses and peptones, thereby rendering 

 them soluble and diffusible, and rennet or chymosin, which coagulates 

 casein. These enzymes have not been obtained in a pure state and to 

 some extent probably exist as "zymogens," i.e., substances which yield 

 the true enzymes on treatment with an acid. 



The composition of the gastric juice of various animals is given by 

 Hammarsten thus : 



How the free hydrochloric acid of the gastric juice is secreted from 

 the alkaline blood is not exactly known. Maly's theory is that the 

 carbon dioxide of the blood sets free minute traces of hydrochloric 

 acid from chlorides, possibly through the intervention of phosphates, 

 thus : 



Na 2 HP0 4 + C0 2 + H 2 = NaHC0 3 + NaH 2 PO 4 

 NaH 2 P0 4 + NaCl = Na 2 HP0 4 + HC1 



and that the acid so formed diffuses from the blood into the gastric 

 juice, being possibly held there in weak combination with pepsin. 



Pepsin is apparently a nitrogenous substance like other enzymes, 

 and is destroyed by boiling, though in the dry state it is said to be 

 able to bear a temperature of over 100 without losing its activity. 

 Its characteristic property is its power of converting, in acid but not 

 in alkaline solutions, albuminous bodies into soluble peptones and al- 

 bumoses. A solution containing about 0*1 to O3 per cent of hydro- 

 chloric acid is most favourable for its action. 



The proteid swells up and becomes transparent and finally dis- 

 solves. In the case of pepsin from most warm-blooded animals, 

 activity ceases below 3 and is at a maximum about 40. Salicylic 

 acid and phenol hinder digestion by pepsin, while arsenious acid is 

 said to promote it. By the movements of the walls of the stomach, 



