ABSORPTION FROM THE SKIN, THE LUNGS, ETC. 369 



magnesium cations and the sulphate anion are least diffusible. These 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 the 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. 



ABSORPTION FROM THE SKIN, THE LUNGS, ETC. 



The dry corneous stratified epithelium covering the human body pos- 

 sesses great resistance to the absorption of most substances. The sebaceous 

 secretion keeps the surface slightly oily. Watery sclutions do not readily 

 wet the surface and therefore do not penetrate. There is some absorption 

 of water on prolonged contact with the skin, but the amount is insignificant. 

 Medicated baths, especially hot baths, may be accompanied by some slight 

 absorption of the substances dissolved in the waters; though it must be 

 confessed that the good effects of such treatment come from other sources. 

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