ABSORPTION IN GENERAL 125 



the blood is comparatively small. But since they are here 

 chiefly as colloids and tend to maintain the concentration of the 

 circulating fluid, their effect is a permanent factor influencing 

 absorption into the blood-vessels. 



Isotonic and hypotonic solutions are those having equal and 

 less densities respectively as compared to blood serum. Hypo- 

 tonic solutions are most easily absorbed; hypertonic least easily. 

 Application of these principles explains the rationale of giving 

 some medicines in dilute and others in concentrated form. As 

 to the direction of the current, the one of greater volume may 

 be called the endosmotic and the one of lesser volume the exos- 

 molic. For example, the current in ordinary absorption from 

 the alimentary canal is usually termed endosmotic, though it 

 may be reversed, as when magnesium sulphate is given. 



When it is said that the greater current is from the less dense 

 to the more dense fluid, no reference is had to the direction of 

 the solids in solution. If there be only one solid concerned, it 

 will be the one responsible for the difference in density and if it 

 be a crystalloid, it will pass through the numbrane until the 

 density on the two sides is equal, and its direction will be opposite 

 to that of the water. If on the side of less density there be 

 another crystalloid in solution, but in less quantity than the solid 

 on the side of greater density, it will pass in the direction of the 

 greater current of water until conditions of equal concentration 

 with respect to this solid are established. In the laboratory 

 the final result in any case of dissolved crystalloid or crystalloids 

 is two liquids absolutely identical in composition. A rectal 

 enema, hypertonic with sodium chloride, will give up sodium 

 chloride to the blood, but it may at the same time draw upon that 

 fluid for urea, for example. This is suggestive when an attempt 

 is made to explain the products of glandular secretion, excretion, 

 etc. It may be that the capillary walls are permeable to certain 

 substances in certain situations and not in others. 



In the body it may be said that well-nigh all the vital functions 



