218 NUTRITION 



Saline and other aqueous solutions are absorbed by the capillaries. 

 The principles which underlie the absorption of these substances are 

 doubtless those of the obscure physical chemistry of filtration, and 

 especially of osmosis. The "selective power" of the epithelium plays an 

 important part, but this selective power is probably only a matter of the 

 complex physical aspects of chemism. While it is possible that the 

 stomach absorbs some of the salines (as it does alkaloids and alcoholic 

 extracts), -both the small and large intestines are the chief sites of the 

 absorption of these substances. 



Fat is distinguished from all the other nutrient principles by being 

 absorbed through the club-shaped lymphatics of the intestinal villi, and 

 normally by no other route. It is likely that the lacteals of the jejunum 

 and the ileum do most of this work. Much if not all of the fat absorbed 

 is first split into fatty acids and glycerin, but these change back into 

 neutral fat probably before leaving the epithelium of the gut on their 

 way into the circulation. It is likely enough that the leukocytes of the 

 villi have something to do with the transfer of fat-particles inward. 

 During the digestion of a fatty meal the lymph of the great ducts may 

 contain 15 or 20 per cent, of fat for hours. The capillaries also take up 

 the fat-globules to some extent when the amount ingested is excessive. 

 It has been supposed that the bile-salts assist in the absorption of the fat, 

 but apparently they do so only indirectly through their emulsifying 

 activities. 



Carbohydrates are absorbed almost wholly as dextrose and levulose 

 by the capillaries lining the intestines. Probably the intestinal epithelium 

 changes the colloidal dextrin, cane-sugar, milk-sugar, and even starch to 

 dextrose while they pass through it. Reach has shown that sugar is 

 readily absorbed by the rectum, and this is a matter of some practical 

 importance in therapeutics. Everything recently discovered goes to 

 show that the intestinal epithelium is a very versatile tissue, acting by 

 means of the potent enzymes as well as by those selective ("vital") 

 powers afforded by its chemical and perhaps physical composition. 



Protein and albuminoids (which may be discussed together so far as 

 their absorption is concerned) also appear to be taken up wholly by the 

 portal capillaries. In what exact chemical forms these complex sub- 

 stances enter the epithelium and in what shapes they leave it, is still 

 under active discussion. The main question at issue discusses how far 

 the food-proteids and albuminoids are broken down, and therefore as to 

 the exact changes produced by the gut-epithelium before the proteic s 

 and albuminoids reach the circulation. The question cannot be exactly 

 answered as yet. It is the capillaries of both the large and small intestines 

 which absorb these substances. 



METABOLISM. 



Metabolism consists of both the up-building process, anabolism, and 

 the down-tearing process, katabolism. These terms apply especially to 



