306 HAND-BOOK OF PHYSIOLOGY. 



if a piece of ordinary bladder be used as the septum between water and 

 alcohol, the current is almost solely from the water to the alcohol, on 

 account of the much greater affinity of water for this kind of membrane; 

 while, on the other hand, in the case of a membrane of caoutchouc, the 

 alcohol, from its greater affinity for this substance, would pass freely into 

 the water. 



Osmosis by Blood-vessels. Absorption by blood-vessels is the 

 'consequence of their walls being, like the membranous septum of the 

 endosmometer, porous and capable of imbibing fluids, and of the blood 

 feeing so composed that most fluids will mingle with it. The process of 

 absorption, in an instructive, though very imperfect degree, may be ob- 

 served in any portion of vascular tissue removed from the body. If 'such 

 a one be placed in a vessel of water, it will shortly swell, and become 

 heavier and moister, through the quantity of water imbibed or soaked 

 into it; and if now, the blood contained in any of its vessels be let out, it 

 will be found diluted with water, which has been absorbed by the blood- 

 vessels and mingled with the blood. The water round the piece of tissue 

 also will become blood-stained; and if all be kept at perfect rest, the stain 

 derived from the solution of the coloring matter of the blood (together 

 with which chemistry would detect some of the albumen and other parts 

 of the liquor sanguinis) will spread more widely every day. The same 

 will happen if the piece of tissue be placed in a saline solution instead of 

 water, or in a solution of coloring or odorous matter, either of which will 

 give their tinge or smell to the blood, and receive, in exchange, the color 

 of the blood. 



Colloids and Crystalloids. Various substances have been classified 

 according to the degree in which they possess the property of passing, 

 when in a state of solution in water, through membrane; those which 

 pass freely, inasmuch as they are usually capable of crystallization, being 

 termed crystalloids, and those which pass with difficulty, on account of 

 their, physically, glue-like characters, colloids. (Graham.) 



This distinction, however, between colloids and crystalloids, which is 

 made the basis of their classification, is by no means the only difference 

 between them. The colloids, besides the absence of power to assume a 

 crystalline form, are characterized by their inertness as acids or bases, and 

 feebleness in all ordinary chemical relations. Examples of them are 

 found in albumin, gelatin, starch, hydrated alumina, hydrated silicic 

 acid, etc. ; while the crystalloids are characterized by qualities the reverse 

 of those just mentioned as belonging to colloids. Alcohol, sugar, and 

 ordinary saline substances are examples of crystalloids. 



Rapidity of Absorption. The rapidity with which matters may be 

 absorbed from the stomach, probably by the blood-vessels chiefly, and 

 diffused through the textures of the body, may be gathered from the his- 

 tory of some experiments. From these it appears that even in a quarter 



