ABSORPTION OF MINERALS AND WATER IN THE INTESTINES 419 



the body temperature, are the more completely absorbed. The efficiency 

 of such absorption is as high as 96 to 98 per cent, for the oils, and decreases 

 sharply for such fats as the tallows. 



The large intestine is capable of absorbing fats, though not so readily as 

 the small intestine. 



Absorption of Minerals and Water in the Intestines. The salts 

 common in the foods are most of them readily soluble, dissociate quite com- 

 pletely in the dilute solutions, and diffuse and dialyze readily. Of the salts 

 of the foods, the sodium and potassium cations and chlorine anion are the 

 most readily dissociated and are most diffusible, while the calcium and 

 magnesium cations and the sulphate anion are least diffusible. The sub- 

 stances pass through the intestinal epithelial cells and the intercellular sub- 

 stance; at least salts easily recognized by microchemical means have been 

 found in both localities during absorption. It seems probable that the forces 

 concerned are largely osmosis and diffusion. 



Yet observers have not been able to show that the rate and character of 

 the absorption of even the salines obey the known physical laws. In fact, 

 there is evidence that some of the salts, iron for example, are taken up as 

 organic compounds (hematogens of Bunge). The activity of the epithelial 

 cells is to be taken into account, even in the absorption of salts. 



Water, which we have seen is not absorbed in the stomach, is readily 

 absorbed in the small intestine. Perhaps the bulk of water taken into 

 the system is absorbed in the upper part of the small intestine. In the large 

 intestine, too, it is absorbed with facility and in considerable quantities. 

 The content of the bowel is still quite fluid when it enters the ascending colon, 

 but the feces are quite firm on discharge from the rectum. There are many 

 analogies by which we may suppose a controlling influence of the epithelium 

 over the process of water absorption. Among the fishes there are species, 

 the salmon for example, in which the blood maintains a relatively constant 

 osmotic pressure, and therefore salt content. In the salmon this is about 

 the same as that of human blood. The blood is separated in the gills by 

 an extremely thin epithelium from the water in which the animals live, yet 

 these fishes go with impunity from sea- water, with two and a half times more 

 salt than the blood, to fresh water with practically no salt at all. The epi- 

 thelium of the gills permits the passage of oxygen, but it does not permit 

 the diffusion or dialysis of the salts or the water in either direction. It is 

 possible that there is a certain amount of resistance to the passage of water 

 through the walls of the stomach, while the intestinal epithelium permits 

 water to pass readily. 



The factors active in absorption are under searching investigation at the 

 present time, so that it is reasonable to hope that the near future will give 

 a more exact understanding of this intricate subject. 



