THE METABOLIC MACIIINEHY OK ANIMALS 297 



possibly fats as well as carbohydrates. Storage of glycogen in the 

 liver has been demonstrated by taking two rabbits, which were fed 

 heavily on clover after a period of starvation. After allowing suit- 

 able time for digestion and assimilation, one rabbit was killed and 

 glycogen was demonstrated in the liver cells, while the other was 

 given strenuous exercise before being sacrificed to science. Upon 

 examination the second rabbit showed a greatly reduced quantity of 

 glycogen in the liver cells. 



The Secretions of the Small Intestine. There can be no 

 doubt of the importance of the part played by the pancreas and liver 

 in digestion which is supplemented by secretions of the intestinal 

 wall, called collectively intestinal juice, or succus entericus, a substance 

 containing five important enzymes secreted by small intestinal 

 glands of the mucosa (see figure of villus). The first, enterokinase, 

 acts as a co-ferment on proteins and was formerly thought to be an 

 activator for trypsinogen. Erepsin, while appearing to be the same 

 as that appearing in the pancreas, hydrolyzes peptides to amino acids ; 

 maltasc, as previously noted, converts maltose into dextrose, while 

 lactase hydrolyzes milk sugar into the simple compounds of galactose 

 and dextrose, and invertase converts ordinary table sugar into levu- 

 lose and dextrose. The la.st three are frequently spoken of col- 

 lectively as inverting enzymes. 



It should be remembered that the large intestine produces no 

 enzymes, wherefore it is as.sumed that little or no digestion takes 

 place there. The bacteria of the large intestine attack any protein 

 material which has escaped digestion and break it down by putre- 

 factive fermentation. 



Absorption and the Fate of Absorbed Foods 



In animals that possess circulatory systems the diffusible end-prod- 

 ucts of foods are passed through the epithelium of the gut into the 

 blood stream, or, in the case of fats, through the lymphatics to the 

 blood. In higher vertebrates most of the absor})tion takes place in 

 the walls of the small intestine. While diffusion and osmosis are im- 

 portant factors in the passage of food and water through the walls 

 of the intestine, many physiologists agree that the living matter in 

 the cells lining the intestine exerts energy which affects the absorption 

 of the substances that pass into the blood and lacteals. This is proved 

 by the fact that if these cells are injured or poisoned, absorption 

 follows the laws of osmosis and diffusion. Ordinarily the cells lining 



