ABSORPTION IN GENERAL 125 



proteids of the blood is comparatively small. But since they 

 are here chiefly as colloids and tend to maintain the concen- 

 tration 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. Hy- 

 potonic solutions are most easily absorbed ; isotonic 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 may be called ex osmotic. 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 di- 

 rection 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 

 membrane 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 solu- 

 tion, 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 re- 

 sult 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 chlor- 

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

 fluid for urea, for example. This is suggestive when an at- 

 tempt is made to explain the products of glandular secretion, 

 excretion, etc. It may be that the capillary walls are per- 

 meable to certain substances in certain situations and not in 

 others. 



