DIGESTION. 269 



The composition of the gastric juice of various animals is 

 given by Hammarsten thus : 



Human, mixed Dog, free from QV> 

 with saliva. saliva Shee P' 



Water ... ... 99-44 97-30 98-62 



Solids ... ... -56 2-70 1-38 



Organic bodies ... -32 1'71 -41 



Common salt ... -15 -25 -44 



Calcium chloride ... -01 -06 '01 



Potassium chloride ... '06 -11 -15 



Free hydrochloric acid -02 -31 -12 

 Phosphates of iron, cal- 



cium, and magnesium -01 -20 -21 



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 inter- 

 vention of phosphates, thus 



Na 2 HP0 4 + C0 2 +H 2 - NaHCO 3 +NaH 2 P0 4 

 NaH. 2 P0 4 + NaCl = Na. 2 HPO 4 



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 albumoses. A solution con- 

 taining about -1 to '3% of hydrochloric acid is most favourable 

 for its action. 



The proteid swells up and becomes transparent and finally 

 dissolves. 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 the food is kneaded and incorpo- 

 rated with the gastric juice, the pulpy mass produced being 



