THE SOURCES OF ENERGY 



63 



jtrom ^Fore- limbs , 

 head and. neck 



To Hear 



Hejiati 



The Cava! veins 



Jliver 



Jhrfa./ vein 



the tissues which utiKse them, the same enzymes now reconvert 

 the amino-acids into proteid. But the proteid is now a different 

 one (or ones), for the amino-acids have been rearranged. 



Somewhat similar rearrangements take place in respect of the 

 fats. 



The blood circulating in the intestinal wall thus becomes 

 charged with proteid, fat, and carbohydrate, all three classes of 

 substances being now 

 thrown into forms which 

 are " native " to the 

 body of the animal in 

 which the digestive pro- 

 cess occurs. One eats the 

 proteids characteristic 

 of beef, mutton, pork, 

 fowl, fish, etc., but all 

 these become converted 

 somewhere into human 

 proteid, and all the 

 different fats that one 

 eats are similarly con- 

 verted into human fat. 

 That is, the processes 

 of digestion, absorption, 

 and assimilation are a 

 laborious and round- 

 about series of chemical 

 conversions which make 

 up a large portion of our 

 vital activity, and which 

 might be shortened with 

 much advantage. 



Little by little the 

 capillaries in the walls 

 of the stomach and intestine unite together to form veins, and 

 the smaller veins unite further until one large vessel — the portal 

 vein — drains away all the blood circulating in the alimentary 

 canal from the stomach backward. This great vein carries the 

 intestinal blood, laden with nutritive substance, to the liver, and 

 there it divides up again into small and smaller veins, and these 

 finally break up into capillaries which ramify among the cells of 



^CLiimenTcLry Canal 



From other 

 viscera. 



Fig. 



From hind, limbs and. trunl< 



13. — Diagram of the Great Veins 

 IN THE Body op a Mammal. 



