350 VETERINARY PHYSIOLOGY 



synthesis of their proteins, and possibly in order to use their 

 non-nitrogenous part as a source of energy. 



(2) Carbohydrates. — These are absorbed as monosaccharids 

 in solution, and are carried away in the blood of the portal 

 vein. Any surplus, over that required by the body, may be 

 stored in the liver and subsequently sent to the tissues (p. 354). 



(3) Fats. — After being split up into their component 

 acids and glycerol, fats pass, as soluble soaps or as fatty acids 

 soluble in the bile, through the borders of the intestinal 

 epithelium. Here they ap])ear to be again converted into 

 fats by a synthesis of the acid with glycerol. Fine fatty 

 particles are found to make their appearance in the cells at 

 some distance from the free margin and to increase in size. 

 A similar synthesis occurs even when free fatty acids alone 

 are given, so the cells must be capable of j)roducing the 

 necessary glycerol to combine with the acids. The fats are 

 sent on from the cells, through the lymph tissue of the villi, 

 into the central lymph vessels, and thus on, through the 

 thoracic duct, to the blood stream. Unlike the proteins and 

 carbohj^drates, they are not carried directly to the liver. In 

 some animals they are stored in the fatty tissues, in others 

 to a certain extent in the liver. 



Since neither the character nor the amount of food 

 consumed are determined by the actual requirements of the 

 muscular and other tissues, it is of importance that there 

 should be some regulator which will control the amount and 

 character of the nourishment sent to the muscles. 



Such a regulator is found in the liver. 



When more food is taken than is at once required by the 

 tissues, one of three things may happen — 



1. It may be oxidised with the evolution of heat. 

 This is specially the case with proteins, the high specific 

 dynamic action of which markedly increase heat production 

 (p. 272). 



2. It may be excreted, unchanged in the urine as in the 

 ■case of sugar. 



3. It may be stored and sent to the muscles as it is 

 required, and thus the supply of energy-yielding material 

 may be regulated. 



