352 PHYSIOLOGICAL CHEMISTRY. 



Nutrition of animals. Before the food can be utilized for the 

 construction and sustenance of the animal body it has to undergo 

 digestion, which means that it has to be dissolved or otherwise 

 converted into such a form that it can be absorbed by the blood, 

 which carries it first to the lungs and from there to all the 

 various parts and organs of the body, which have to be con- 

 tinually repaired or reconstructed by the constituents of the 

 blood. 



The first step towards the digestion of the food is its disinte- 

 gration, which is accomplished by the teeth with the aid of the 

 saliva, an alkaline fluid acting chemically upon starch, converting 

 it into dextrine and sugar and also upon the fatty portions of the 

 food by emulsifying them at least partly. 



The masticated food thus prepared passes down into the 

 stomach, where it undergoes another change, consisting chiefly 

 in the conversion of insoluble albuminous matter into soluble 

 peptones. The agent producing this change is the gastric juice, 

 secreted by the lining membrane of the stomach. 



The action in the stomach lasts for several hours ; the par- 

 tially digested food resulting from this action, and known as 

 chyme, is then propelled by peristaltic motion into the com- 

 mencement of the intestines (the duodenum), where it meets 

 with and is acted upon by two other agents, the bile and pan- 

 creatic juice. The special function of bile in the digestion of 

 food has not been sufficiently explained, but it appears that, in 

 cooperating with the pancreatic juice, it acts upon the fats, 

 causing them either to form intimate mixtures (emulsions) with 

 water, or saponifying them partly. Pancreatic juice, moreover, 

 acts powerfully upon those starchy portions of the food which 

 may have escaped the previous action of the saliva, and it also 

 acts upon proteids, converting them into peptones. 



From the duodenum the almost digested food now passes into 

 other intestines, from which again juices are liberated further 

 acting upon the semiliquid mass, which, upon its arrival in the 

 small intestines, contains the soluble nutritious matters in the 

 form of a thin, milky fluid called chyle. This chyle is a mixture 

 of liquefied and chemically slightly altered albuminous, starchy, 

 and fatty matters, which, during their passage through the in- 

 testines, are gradually absorbed (by a simple process of diffusion), 



