256 ABSORPTION LYMPH AND CHYLE 



lation. When it is remembered that the small arteries may become so 

 contracted under the influence of the vasomotor nerves that* their calibre 

 is nearly obliterated, of course retarding in a corresponding degree the 

 capillary and venous circulation in the parts, and again, that the- same 

 vessels may be so dilated as to admit to a particular part many times 

 more blood than it ordinarily receives, it becomes apparent that absorp- 

 tion may be profoundly affected through this system of nerves. It has 

 been ascertained that while a section of some of the nerves distributed 

 to the alimentary canal will slightly retard the absorption of the poison- 

 ous substances the process is never entirely arrested. 



OSMOSIS 



If liquids pass through the substance of an animal membrane, it is 

 evident that the membrane itself must be capable of taking up a certain 

 portion by imbibition ; and this must be considered as the starting-point 

 in absorption. Imbibition is,, indeed, a property common to animal 

 tissues ; but it is a well-known fact that the tissues do not imbibe all 

 solutions with the same degree of activity. Distilled water is the liquid 

 taken up in greatest quantity, and saline solutions enter the substance 

 of the tissues usually in an inverse ratio to their density. In regard 

 to mixtures of alcohol and water, imbibition is in an inverse pro- 

 portion to the quantity of alcohol present. Among the other con- 

 ditions that have a marked influence on imbibition, is temperature ; 

 and it is a familiar fact that dried animal membranes may be more 

 rapidly softened in warm than in cold water. While nearly all the 

 structures of the body, after desiccation, will imbibe liquids, the mem- 

 branes through which the processes of absorption are most active are, 

 as a rule, most easily permeated ; and the character of the liquid, the 

 temperature, etc., have a great influence on the activity of this process. 



Mechanism of the Passage of Liquids through Membranes. The 

 passage of liquids through membranes is called osmosis, and this is in 

 obedience to what is known as osmostic pressure. In the case of two 

 liquids passing in opposite directions, the stronger current is called 

 endosmotic, and the weaker, exosmotic. In the passage of liquids into 

 the vessels, in physiological absorption, the process usually is called 

 endosmosis. 



It is now definitely ascertained that the following conditions are 

 necessary for the operation of endosmosis and exosmosis : 



i. That both liquids be capable of "wetting" the interposed mem- 

 brane, or in other words, that the membrane be capable of imbibing 

 both liquids. If but one of the liquids can wet the membrane, the 

 current takes place in only one direction. 



