106 PRINCIPLES OP VETERINARY SCIENCE 



esses; others believe that although these processes are operative, 

 the controlling factor lies in the vital action of the epithelial cells 

 in the intestinal walls. 



Mineral substances are absorbed from the intestines by the 

 blood in proportion to the requirements of the body. 



Ammonia is formed, according to Folin, in the large intestine 

 as a result of putrefaction of the residue of protein digestion. 

 It is absorbed by the blood and carried to the liver where it is 

 largely disposed of by combining with hydrogen to form urea, the 

 chief waste substance excreted in the urine. When ammonia or 

 urea accumulate they irritate the tissues and interfere with their 

 work. 



Toxic materials are absorbed in considerable amounts from 

 the intestines by the blood. Upon reaching the liver they are 

 destroyed or rendered harmless by being dissolved, oxidized, or 

 precipitated. This important function of the organ was clearly 

 demonstrated by Schiff, who injected a dose of nicotine through 

 a peripheral vein and caused death, while the same dose intro- 

 duced into the portal vein was without effect. 



II. Materials Absorbed by the Lacteals. — These consist of 

 products of fat digestion and water. They form an emulsion 

 called chyle which is emptied into the receptaculum chyli. Chyle 

 is moved forward by the rhythmical contractions of the muscles 

 in the walls of the intestines and villi. It is prevented from re- 

 turning by valves in the lacteals. 



Fat is not absorbed until it is converted into fatty acid and 

 glycerin by the action of the digestive juices. As neither of 

 these is found free in the blood, the living epithelial cells in 

 the villi must synthetize or reconvert these substances into 

 a neutral fat. Shortly after chyle is poured into the blood its 

 fat disappears, having been taken up by the cells of the body for 

 their use. 



Water is taken up by the lacteals and forms the transporting 

 medium for the products of fat digestion. 



The order in which the various food nutrients mentioned above 

 are drawn upon for the production of energy under ordinary 

 conditions is as follows: first the available glucose is used, then 

 the fats are oxidized, and finally the amino-acids are made use 

 of by the body cells. 



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