THE DIGESTION OF FEEDS 31 



feeds at a time as cattle, nor can they digest coarse feeds containing 

 considerable cellulose (fiber) so completely as the ruminants. 



To make up in a measure for its small stomach, the horse has "a 

 large sac, cmcum, which is about a yard long and corresponds to the 

 vermiform appendix in man. The large intestine is also of con- 

 siderable size and has several enlargements. The entire length of 

 the alimentary canal of the horse is about twelve times the length of 

 the body, that of the pig fourteen times and that of cattle and sheep 

 twenty times or more the length of the body. The length of time 

 during which the feed remains in the alimentary canal and is ex- 

 posed to the action of the digestive enzymes (or to bacterial action 

 in the intestines) will, therefore, vary in different animals. In 

 cattle and sheep the undigested part of the feed is voided in three 

 to five days, and in horses and pigs in one and one-half to two days. 



Absorption of Digested Materials.— -The soluble materials and 

 those that have been broken down into simpler, largely soluble com- 

 pounds through the various processes of digestion are absorbed to 

 some extent by the mucous membrane of the stomach, but for the 

 most part pass through the walls of the intestines. The intestines 

 are lined with innumerable fine projections called villi, inside of 

 which are found microscopic branches of two systems of vessels, 

 the capillaries of the blood-vessels, and the lacteals belonging to the 

 so-called lymphatic system. The digested materials in the form of 

 sugar, salts, soluble proteose and peptones, and similar compounds 

 pass over into the capillaries by the process of osmosis. The capil- 

 laries are exceedingly fine blood-vessels that converge to a large 

 vein called the portal vein, through which the materials absorbed 

 by the blood are taken into the liver. They are here distributed 

 through a second set of capillaries and then reunited, passing into' 

 the hepatic vein which leads to the heart. 



The emulsified fats and free fatty acids, or combinations of 

 these with alkali (soaps), on the other hand, are taken up by the 

 lacteals in the villi of the intestines. From these they pass into 

 the lymphatic system and are later emptied into the thoracic duct 

 which leads to one of the large veins before this enters the heart. 



The nutrients thus taken into the blood circulation come into 

 contact with the oxidizing agent of the blood, the oxyhemoglobin, 

 and are either directly oxidized in the blood or carried to the body 

 tissues to repair waste and supply materials for the formation of 

 new tissues. Very likely, both these processes occur simultaneously. 

 Some of the digested and assimilated nutrients, especially sugar 

 and lactic acid, soon disappear from the blood through oxidation, 

 and the carbon-dioxide and water formed in the process of oxidation 



