314 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



Composition and Uses of Gastric Juice. — Acci- 

 dental woundings of the stomach have afforded means 

 of collecting the gastric juice of man in considerable 

 quantity ; that of animals has been collected in vivisec- 

 tion experiments. The secretion, therefore, has been 

 analyzed. It consists of mucus, with a little free acid, 

 hydrochloric and lactic, and a peculiar ferment, an en- 

 zyme called pepsin, which has the property of dissolving 

 albuminoids. This it does by hydration. The soluble 

 forms of albuminoids thus formed are called peptones, 

 and the process of change peptonization. In some 

 cases of weak digestion pepsin made from a calf's stom- 

 ach may be used as medicine with good effect. 



In both saccharization and peptonization the process 

 is purely chemical, and takes place just as well in a warm 

 flask as in the stomach. The true vital process is the 

 formation of the ferment, not the change effected by it 

 on the food. 



Effect on Milk. — The effect of pepsin on milk is 

 very characteristic. It first curdles, and then dissolves 

 it. The albuminoid — casein — in milk is in a liquid state 

 and apparently suitable for direct absorption. But albu- 

 minoids in their natural state are unstable and liable 

 to pass into a solid. Therefore the casein is first solidi- 

 fied and then changed into peptones, in which state it is 

 no longer liable to solidification. Advantage is taken 

 of this property of curdling milk in the manufacture of 

 cheese. Fresh milk is treated with a small quantity of 

 an extract of rennet (calf stomach) ; the quantity used 

 is sufficient to curdle, but not enough to dissolve the 

 casein. 



Absorption. — Water, alcohol, perhaps to some ex- 

 tent sugar, may be taken up directly by the capillaries 

 of the stomach into the blood. But absorption is the 

 special function of the intestines. The chyme is there- 



