84 THE CHEMISTEY OF THE FARM. 



the solid constituents of the food into a form suitable for 

 absorption into the blood. Of the carlo -hydrates of the 

 food some, as sugar, are already soluble and diffusible, 

 and need no digestion; others, as starch and cellulose, 

 are naturally insoluble. The digestion of carbo-hydrates 

 commences with the action of the saliva, which has the 

 property of converting starch into sugar (maltose). 

 Phis action, in the case of ruminants, is prolonged by the 

 temporary sojourn of the food in the first two stomachs, 

 and its return to the mouth in chewing the cud. The 

 digestion of the cellulose by a fermentive process com- 

 mences in the paunch. The further solution of starch 

 and cellulose is effected in the intestines. The pancre- 

 atic juice has a powerful action on starch. In the intes- 

 tines maltose is converted into dextrose. The cellulose is 

 dissolved in the colon by a fermentive process, due 

 apparently to bacteria, in which acetic and butyric acids, 

 carbonic acid, and a little marsh gas are the products. 



The albuminoids of the food are attacked by the 

 gastric juice of the stomach (the fourth stomach of rumi- 

 nants), and converted into peptones, bodies similar to 

 albuminoids in composition, but which, unlike them, are 

 diffusible through a membrane. The pancreatic juice of 

 the small intestines also converts albuminoids into pep- 

 tones, and partly into amides, leucine and tyrosine. 



The digestive agents in saliva, gastric juice, and pan- 

 creatic juice, are unorganised ferments (enzymes), known 

 respectively as ptyalin, pepsin, and trypsin. 



Fat, liquefied by the heat of the body, is probably 

 capable of absorption without change. The digestion of 

 fat in large quantities is greatly assisted by the bile and 

 pancreatic juice. 



