46 DIGESTION AND FOOD. 



and acids ; and this is accounted for by the fact, that tho 

 proportion of the organic elements is the same in both." 

 (GREGORY.) 



III. CASEINE. This, as Dr Gregory says, is the third 

 great form of sanguigenous matter found in vegetables, and 

 in largest proportion in leguminous seeds, hence called 

 legumine. It is the azotised constituent of milk, and dis- 

 tinguished from albumen by not coagulating under the in- 

 fluence of heat, but by dilute acids, which are unable to 

 coagulate albumen. " In milk, which is alkaline, caseine is 

 dissolved, along with sugar of milk, salts, and suspended oil 

 or butter. When milk turns sour, its alkali is gradually 

 neutralised and overpowered by lactic acid, produced by the 

 fermentation of sugar of milk, and the caseine at last coagu- 

 lates from the presence of free acid. It is absolutely certain 

 that caseine, in the animal body, can yield albumen and 

 fibrine, because young animals, fed on milk alone, produce 

 blood and muscle, and milk contains no other sanguigenous 

 compound than caseine. Cheese is coagulated and pressed 

 caseine, and when made from well-skimmed milk, is nearly 

 pure ; but that made from sweet milk or cream contains also 

 much butter. The infusion of the lining membrane of a calf s 

 fourth stomach, or rennet, as it is called, contains albumen or 

 some other substance of a like nature, in a state of decay, 

 that is, of decomposition. It acts on the sugar in milk, con- 

 verts a part of it into lactic acid, and thus causes coagulation ; 

 but as curd is formed before the milk has become sour, we 

 must infer either that the caseine coagulates as soon as the 

 milk becomes neutral, or that the ferment or rennet coagu- 

 lates it by an action of contact. Perhaps both are true. Nay, 

 it has been found that milk, even when made distinctly alka- 

 line, coagulates with rennet if warmed rather more than 

 without the alkali. Indeed, it would appear that the curd, a 



