THE ABSORPTION OP POODS 



117 



u^til nearly all the digested portion of the food has passed out 

 of the intestine into the blood. 



But meantime, according to our illustration, water 

 has been passing from the blood into the intestine. Here, 

 however, the illustration partly fails; for while doubtless 

 some of the water of the blood does enter the intestine, it 

 does not do so as fast as it would in the illustration. Exactly 

 why this is so, physiologists do not fully know. So it must be 

 conceded that the real secret of food absorption is not entirely 

 understood, and we have merely to say that it occurs because 

 of the nature of the living cells in the intestinal walls. The 

 membrane of the villi is made of living cells and these cells, 

 when alive, act differently from those of non-living tubes. 



CHANGES IN FOOD AFTER ABSORPTION 



We can now understand that the 

 is to bring the foods into such a 

 pass through the intestinal walls. 

 Further changes occur in them, 

 however, after their absorption. 

 The proteids, for example, are 

 by digestion broken into very 

 simple compounds, but after these 

 are taken into the blood they are 

 built up into proteids again. Just 

 where and how this occurs is riot 

 yet known; but it is known that 

 proteids are abundant in the 

 blood although only the simple 

 products of proteid digestion are 

 absorbed. Somewhere, therefore, 

 they are reconverted. A further 



first purpose of digestion 

 condition that they can 



FlG. 64. A HIGHLY MAGNI- 

 FIED VIEW OF THE TIP OF 

 A VILLUS 



Showing the absorption of fat. 

 The black dots are fat. 



change also takes place 



