266 PHYSIOLOGICAL PHYSICS. tchap. xxn. 



substances which, by their action on the animal 

 membranes, alter or interfere with their affinity for 

 certain substances. Fat, for instance, would so modify 

 the attractive power of a tissue. Thus it may be that 

 certain substances taken with food, even though not 



o 



interfering with the digestion, would seriously retard 

 the process of absorption, and set up all the trouble- 

 some sensations of bad digestion. 



Absorption by lymphatics may be considered as 

 presenting, so far as the physics are concerned, similar 

 features to that by blood-vessels. 



The mechanism of secretion is much more obscure 

 than that of absorption. The laws of diffusion, and 

 of osmosis, are to a certain extent applicable, but 

 other elements enter into the consideration of the 

 question which physics are unable to account for. 

 That, however, to some extent conditions similar to 

 those of absorption are present, seems without doubt, 

 a thin animal membrane separating on one side the 

 current of the circulation, and on the other the fluid 

 of the gland. 



TraiiKiidatioii or filtration must be carefully 

 distinguished from osmotic action. Experiments on 

 animal membranes show that under varying degrees 

 of pressure various solutions can be forced through 

 them. Thus a pressure of mercury will cause water 

 to pass out of a tube through a membrane closing its 

 mouth, the water gathering in minute drops on the 

 outer surface, and coalescing into larger ones. Brine 

 requires a greater pressure, and fat can be forced 

 through with a greater pressure still, while alcohol 

 requires even a larger amount. The readiness of 

 the passage of the fluid is thus dependent upon 

 the nature of the fluid ; and it depends also 

 upon the character of the membrane, being easier 

 when the membrane is thin, and not very dense. 

 Transu elation or filtration, therefore, is a passage 



