MAIS-UAL OF CATIXTS-FEEDIKG. 17 



grouped cells, riesli-fibriii behaves, also, somewliat differ- 

 ently to chemical reagents from the coagulated blood-fibrin, 

 but, like all insoluble modiiications, it is easily converted 

 by the action of the digestive fluids into a soluble form. 



Casein is found in considerable quantity only in milk ; 

 it is a product of the milk-glands and therefore not to be 

 looked upon as a constituent of the body in general. It 

 does not coagulate on heating ; the tenacious skin which 

 forms on the surface of milk when it evaporates is a sub- 

 stance altered by the action of the air. 



On the other hand, the casein separates almost com- 

 pletely in a coagulated state when a small quantity of 

 rennet is added to the milk, as in making cheese, or when 

 the milk is gently warmed with dilute acids or various 

 other substances, as well as in the natural souring of 



xXXXXxs— 



Composition. — ^AU the albuminoids contain, m essen- 

 tial constituents, carbon, hydrogen, oxygen, nitrogen, and 

 sulphur ; and these constituents are present in such coti- 

 stant quantities that it is impossible to distinguish the 

 various albuminoids from each other by their percentage 

 composition, samples of the same albuminoid from different 

 Sources often showing as great differences as exist between 

 members of different groups. The following numbers 

 show the extremes of variation : 



Carbon 53-54 per cent. 



Hydrogen 7 " '' 



Nitrogen 15-17 '' '' 



Oxygen 24-21 '* "' 



Sulphur 1-1.5 " " 



Generally the average amount of nitrogen is assumed to 

 be 16 per cent,, and the total quantity of albuminoids in a 



