HOW THE ANIMAL LIVES 219 



glands found in the end of the stomach nearest 

 to the intestine. It acts on the nitrogenous 

 principles in the food, which are made to take 

 up water, and to change into a much more 

 stable and diffusible liquid called a peptone. 



390. Peptones of a great number of different 

 kinds are produced from the varied food prin- 

 ciplesfrom such as fibrin, albumin, gluten, 

 casein. The peptones all agree in certain com- 

 mon characters : (a) they are easily and com- 

 pletely soluble in water (fibrin, coagulated al- 

 bumin and casein themselves, are not soluble); 

 (b) they filter rapidly through animal mem- 

 branes, such as a bladder (the agents from which 

 they are derived do not) ; (r) they are not 

 thrown down as solids by boiling or by strong 

 acids (albumin and casein are precipitated by 

 strong acids, and albumin by boiling). 



391. Peptones are thus easily absorbed into 

 the blood, while the absorption of the original 

 principles from which they are derived would be 

 exceedingly slow and difficult. Pepsin acts much 

 more rapidly in an acid medium, BO that it is 

 specially adapted to cooperate with the muriatic 

 acid. 



392. The milk -curdling ferment is the product 

 of the gastric glands. It is utilized in the 

 manufacture of cheese. Like pepsin, it acts best 

 in the presence of muriatic acid. One part of 



