CHAP, i.] DIGESTION. 311 



the behaviour of various inorganic salts, when taken as food or 

 medicine, illustrate very clearly the influence of osmosis. When 

 the intestine contains a large quantity of watery matter, the 

 surplus water passes by diffusion into the blood, just as it passes 

 through the membrane of a dialyser, with blood or serous fluid on 

 the one side, and water on the other. When an albuminous fluid 

 of the specific gravity of blood-serum is exposed in a dialyser to 

 water, about 200 parts of water pass through the membrane of the 

 dialyser from the water into the albuminous fluid for every one 

 part of the albumin which passes from the fluid into the water. 

 Moreover, in the living body, the blood in the mesenteric capillary, 

 thus diluted by diffusion from the intestinal contents, is con- 

 tinually being replaced by fresh blood concentrated by its passage 

 through the skin, lung, or kidney. By the help of the circulation 

 an almost unlimited quantity of water can be absorbed from the 

 alimentary canal. 



It is a matter of common experience that such inorganic and 

 organic salts as are readily diffusible, pass with great rapidity into 

 the blood (and thus into the urine) when taken by the mouth ; and 

 the rapidity with which they are absorbed is in large measure pro- 

 portionate to their diffusibility. Of course, coincident with this 

 passage of the salt from the intestine into the blood, there is a 

 proportionate current of water in the contrary direction from the 

 blood into the intestine; but this, though opposed to, is, under 

 ordinary circumstances, too small to diminish to any serious extent 

 the passage of water from the intestine into the blood, of which we 

 spoke just now, as caused by the osmotic influence of the albuminous 

 constituents of the blood. But, under certain circumstances, the 

 former may overcome the latter. Thus, when a concentrated solu- 

 tion of a highly diffusible salt, such as magnesium sulphate, is in- 

 troduced into the alimentary canal, the flow of water from the blood 

 into the intestine accompanying the osmotic transit of the salt from 

 the intestine into the blood, is so great as largely to exceed the 

 current in the contrary direction ; and the intestine becomes filled 

 with water at the expense of the blood. This is probably the cause of 

 the purgative action of large doses of many saline substances. And 

 even the purgative action of more dilute solutions may be explained 

 in the same way, since in the case of some salts at least the transit 

 of water as compared with the transit of the salt is relatively more 

 rapid with very dilute solutions than with more concentrated solu- 

 tions. Salts such as these, which, when introduced into the intes- 

 tine, produces diarrhoea, bring about a contrary condition when 

 injected directly into the blood ; and magnesium sulphate, with its 

 higher endosmotic equivalent, is more purgative in its action than 

 sodium chloride with its lower equivalent. 



