142 DIGESTION. 



dissolved, so that the muscular bundles become more distinct, and 

 separate from each other. They gradually fall apart, and a little 

 brownish deposit is at last all that remains at the bottom of the 

 tube. If the hard adipose tissue of beef or mutton be subjected 

 to the same process, the walls of the fat vesicles and the inter- 

 vening areolar tissue, together with the capillary bloodvessels, &c., 

 are dissolved ; while the oily matters are set free from their en- 

 velops, and collect in a white, opaque layer on the surface. In 

 cheese, the casein is dissolved, and the oil which it contains set 

 free. In bread the gluten is digested, and the starch left un- 

 changed. In milk, the casein is first coagulated by contact with 

 the acid gastric fluids, and afterward slowly liquefied, like other 

 albuminoid substances. 



The time required for the complete liquefaction of these sub- 

 stances varies with the quantity of matter present, and with its state 

 of cohesion. The process is hastened by occasionally shaking up 

 the mixture, so as to separate the parts already disintegrated, and 

 bring the gastric fluid into contact with fresh portions of the diges- 

 tible substance. 



The liquefying process which the food undergoes in the gastric 

 juice is not a simple solution. It is a catalytic transformation, 

 produced in the albuminoid substances by contact with the organic 

 matter of the digestive fluid. This organic matter acts in a similar 

 manner to that of the catalytic bodies, or "ferments," generally. 

 Its peculiarity is that, for its active operation, it requires to be dis- 

 solved in an acidulated fluid. In common with other ferments, it 

 requires also a moderate degree of warmth ; its action being checked, 

 both by a very low, and a very high temperature. By its opera- 

 tion the albuminoid matters of the food, whatever may have been 

 their original character, are all, without distinction, converted into 

 a new substance, viz., albuminose. This substance has the general 

 characters belonging to the class of organic matters. It is uncrys- 

 tallizable, and contains nitrogen as an ultimate element. It is pre- 

 cipitated, like albumen, by an excess of alcohol, and by the metallic 

 salts ; but unlike albumen, is not affected by nitric acid or a boil- 

 ing temperature. It is freely soluble in water, and after it is once 

 produced by the digestive process, remains in a fluid condition, 

 and is ready to be absorbed by the vessels. In this way, casein, 

 fibrin, musculine, gluten, &c., are all reduced to the condition of 

 albuminose. By experimenting as above, with a mixture of food 

 and gastric juice in test-tubes, we have found that the casein of 



