62 



THE MECHANISM OF LIFE 



soup through a piece of porous blotting-paper ; in the latter case 

 the liquid soup goes through unchanged, except for the solid 

 particles, which are kept back by the blotting-paper, while the 

 living cells of the mucous and other membranes act on the 

 digested food matters. We have seen that the digestive enzymes 

 change the composition of the proteids thus: 



Proteids >■ amino- acids. 



But the same enzymes are contained in the cells of the mucous 

 and other membranes, and as the amino-acids pass through 



External layer 

 , (serous, coat) 



y Artery 



Mucous 

 membrane 



r ^^ Capillary network 

 in submucous layer 



Interior 



X °f 

 aiimentcinf 



Canal 



Vein 



r 



^Muscular layers 



Fig. 12.- 



-Part of a Transverse Section through the Wall of the 

 Intestine. Very Diagrammatic. 



they are again acted upon by the enzymes thus : 



Amino-acids ^ proteids. 



Thus the activity of the enzymes is reversible. Now it is no 

 use presenting all this to the reader as a finished picture of the 

 processes of digestion and absorption of the food substances, for 

 we have only the vaguest notion of what is the nature of the 

 reversibility; one thing is, however, certain — the proteids are 

 split up into amino-acids in the cavity of the alimentary canal, 

 and as these substances are soluble they can pass through the 

 intestinal wall into the capillary blood-stream. In passing into 



