248 ELEMENTARY PHYSIOLOGY less. 



Gastric juice turns milk into curds ; whether this 

 coagulating power is due to a particular ferment called 

 rennin is less certain than it was formerly supposed 

 to be. It is in all probability the first action of the 

 pepsin on the milk-protein. This is the basis of cheese- 

 making, and the " rennet " used for obtaining the curd 

 in this process is really an extract of the mucous mem- 

 brane of the stomach of a calf. 



Peptone differs from most other proteins in its extreme 

 solubility. Many proteins, as fibrin, are naturally in- 

 soluble in water, and others, such as white of egg, though 

 apparently soluble, are not completely so, and can be 

 rendered (juite solid or coagulated by being simply heated, 

 as when an egg is boiled. A solution of peptone however 

 is perfectly fluid, does not become solid, and is not at all 

 coagulated by boiling. 



As far as we know gastric juice has no direct action on 

 fats, unless in a state of very fine division (emulsion, 

 p. 260) ; however, by breaking up the protein frame- 

 work in which animal and vegetable fats are imbedded, 

 it sets these free, and so helps their digestion by 

 exposing them to the action of other agents. Gastric 

 juice has no direct action on carbohydrates. Carbo- 

 hydrate digestion does take place however in the 

 stomach, especially when large quantities of food are 

 swallowed. The food mixed with the alkaline .saliva is 

 only penetrated slowly by the acid secreted by the 

 stomach wall. Whilst this is taking place the saliva has 

 the opportunity of converting starch into sugar. At the 

 end of about twenty minutes it is estimated that carbo- 

 hydrate digestion in the stomach is brought to a standstill 

 by the hydrochloric acid present. 



When food is swallowed it accumulates in the cardiac 

 end of the stomach which gradually distends to receive it. 

 The mass is gradually digested over its surface by the 

 gastric juice. A layer of fluid, of the consistency of pea 



