454 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The substances which enter the blood by venous absorption are 

 probably all those which are soluble in water, such as salts, sugar, soaps, 

 and peptone, as we know that all of these will more or less readily diffuse 

 through organic membranes outside of the body. The process of osmosis 

 is generally accepted as explaining the mode of entrance of soluble sub- 

 stances 'into the blood, but many data are still required before this view 

 can be acknowledged as conclusively established. In the case of grape- 

 sugar the author has found that the osmotic equivalent of sugar ab- 

 sorbed from the stomach of the frog very closely corresponds with the 

 equivalent of diffusion in ordinary physical experiments, and some- 

 what similar observations have been made for various salts and for 

 peptone. There is still, however, a great deal of obscurity in the matter. 

 We have seen that by the time the blood has left the secreting surfaces 

 of the alimentary tube and reached the absorbing surfaces it has lost a 

 good deal of its water, Now, if we assume that the absorption (of 

 sugar, for instance) is governed by purely physical laws, we must admit 

 that for every gramme of sugar that is absorbed seven and one-tenth 

 grammes of water (the osmotic equivalent of sugar) will leave the blood 

 to enter the intestine. The blood will, therefore, become progressively 

 concentrated and the intestinal tube filled with fluid ; consequent!}', every 

 substance which has a high osmotic equivalent will be apt to prove a 

 cathartic ; and it has, indeed, been found that the higher the osmotic 

 equivalent of the purgative salts, the more marked is their cathartic 

 action, though the relation of cause and effect between these facts has 

 been denied (Hay). 



Consequently, though it is probable that osmosis is largely con- 

 cerned in the absorption of substances which are soluble in water, the 

 process is not a pure one, such as might occur between similar fluids 

 separated by a membrane outside of the body. In the living animal the 

 phenomena of filtration may greatty aid venous absorption. As we have 

 seen, the contents of the venous radicals are subjected to but low pres- 

 sure, and it is quite conceivable that the pressure exerted through its 

 contractions by the intestine on its contents may aid the passage of 

 fluids from the exterior to the interior of the veins. Here also we are 

 unable to conceive of an uncomplicated process of filtration, for a pres- 

 sure sufficiently great to force fluids through the walls of a blood-vessel 

 would undoubtedly so compress that vessel as to obliterate its lumen. 



From the above it follows that although the physical processes of 

 osmosis and filtration underlie the absorption of salts, sugar, and peptone 

 from the alimentary tract, neither process is entirely analogous to similar 

 operations outside of the body, it being probable that the excess of pres- 

 sure on the intestinal contents keeps down the osmotic equivalent, and 

 again aids in the resorption of the transuded water ; while, further, the 



