IMBIBITION AND ENDOSMOSIS. 335 



The cohesive attraction of the constituent particles of insoluble solids is so great, that 

 the entrance of fluids is impossible, unless the substance be porous, and this always 

 involves the law of capillary attraction ; but, in liquids, the cohesive attraction is so 

 slight as to admit of the penetration and diffusion of certain other liquids. 



Homogeneous animal membranes, which are of a semisolid consistence, are capable 

 of imbibing certain liquids; and any liquid which can pass into such membranes will 

 pass through them. under proper conditions. The cohesive attraction of the particles of 

 the membrane is not such as to allow them to imbibe an indefinite quantity of any liquid ; 

 but it is one of the distinctive properties of organic tissues, that a limited quantity of 

 liquid can be taken up in this way. 



In view of these facts, it is not necessary to assume the existence of infinitely small 

 capillary openings in homogeneous membranes through which osmotic currents can be 

 made to take place, in order to explain the mechanism of these currents. In the case of 

 two liquids capable of diffusing with each other and separated by an animal membrane, 

 the mechanism of the endosraotic and exosmotic currents is very simple. In the first 

 place, the membrane imbibes both the liquids, but one is always taken up in greater 

 quantity than the other. If water and a solution of common salt be employed, the sur- 

 face of the membrane exposed to the water will imbibe more than the surface exposed to 

 the saline solution ; but both liquids will meet in its substance. The first step, therefore, 

 in the production of the currents is imbibition. Once in contact with each other, the 

 liquids diffuse, the water passing to the saline solution, and vice versa. This takes place 

 by precisely the same mechanism as that of the passage of liquids through porous septa. 



The osmotic currents may be modified with the same liquids by using different mem- 

 branes. This fact was well illustrated in some of the experiments of Matteucci and Cima, 

 in which comparative observations were made upon the currents through the skin of the 

 torpedo, the skin of the frog, and the skin of the eel. The results obtained with these 

 different membranes showed marked and constant variations. The same observers, 

 using the mucous membrane of the stomach of the lamb, found a marked difference in the 

 endosmotic phenomena when the surface exposed to the water was reversed. In two 

 experiments, with the epithelial surface of the membrane turned toward the interior of 

 the endosmometer, the elevations of the liquid in an hour and a quarter were forty- 

 four and fifty-six millimeters ; but, with the membrane reversed, so that the attached sur- 

 face was turned toward the interior, the elevations during the same period were sixty-six 

 and seventy-two millimeters. This difference is readily explained by the difference in the 

 constitution of the two surfaces of the membrane used. From these facts, it is evident 

 that, while the diffusion of liquids as they meet in the substance of a membrane is the 

 actual cause of the osmotic currents, which are continued as the liquids diffuse with each 

 other upon either side of the membrane, the determination of a predominating or endos- 

 motic current, the ordinary conditions being undisturbed, is effected by the greater at- 

 tractive force which the membrane exerts upon one of the liquids. 



Influence of Different Liquids upon Osmotic Currents. The action of the liquids be- 

 tween which endosmotic currents take place is, as we have seen, most intimately con- 

 nected with the force by which the liquids enter the membrane, be it capillary attraction 

 or imbibition ; but the attractive force exerted by the membrane is never capable, in itself, 

 of producing a current. It is evident, therefore, that the properties of the liquids must 

 have an important influence upon osmosis, both from differences in the attraction of the 

 membrane for the liquids and their different degrees of diffusibility. In order to appre- 

 ciate fully all the physical phenomena of osmosis, it will be necessary to study carefully 

 the laws of diffusion of liquids and the diffusibility of different solutions ; but it will be 

 sufficient, for our present purpose to state a few general propositions, which will be found 

 more or less applicable to physiological absorption. 



When two liquids, capable of mixing with each other, are brought together, they 

 diffuse with greater or less rapidity, until the constitution of the mixture becomes uniform. 



