PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



When an organic tissue has absorbed liquid by imbibition, its inter- 

 molecular spaces being filled with that liquid, the liquid, which becomes 

 superficial on the far side of the membrane, is in direct continuity with 

 the body of the liquid having the greater affinity for the membrane, and 

 in direct contact with the liquid of lesser affinity. Diffusion phenomena 

 therefore commence. 



When a membrane has a tendency to imbibe water, it will absorb 

 more water than salt solution, if placed between water and a salt solu- 

 tion, and will increase in volume. In every pore or intermolecular 

 space of the membrane, therefore, the layer of liquid in contact with 

 the sides of the pores, or with the solid molecules, will contain less 

 salt than water. 



From the affinity which the two liquids exert on one another this 



condition will not remain constant, 

 and the rapidity with which the inter- 

 change takes place will depend upon 

 the affinity of the two liquids for 

 one another ; but the interchange 

 will not occur in the same manner 

 as if no membrane were present, i.e., 

 equal quantities of salt solution and 

 water will- not substitute one an- 

 other. Such an interchange will 

 only occur in the centre of the pore, 

 while on the wall of the pores only 

 water will pass ; consequently the 

 salt solution will increase in volume 

 and the water will diminish corre- 

 spondingly, while the rapidity of 

 motion along the walls will be less 

 than in the centre of the pores on 

 account of the attraction of the 

 walls for the water. 



Therefore, the greater the concentration of the salt solution the 

 higher will be the osmotic equivalent, since the difference of affinity of 

 the membrane for the water as compared with the salt solution will be 

 the more marked. This, however, only applies to salts whose solutions 

 are also imbibed by the membrane ; where this is not the case increased 

 concentration produces a decrease in the osmotic equivalent, for in the 

 latter case there will be more tendency for the membrane to hold the 

 layer of water in contact with the walls of its pores. 



Different membranes will consequently modify the osmotic exchange 

 of liquids taking place through them. 



FIG. 39. DIAGRAM ILLUSTRATING 

 OSMOSIS. 



This figure represents a diagrammatic section of a 

 single, capillary pore in an organic membrane separating 

 salt solution and water. From the greater affinity of the 

 membrane for water than for salt solution, the former 

 passes in a layer in contact with the circumference of the 

 tube, while the salt solution passes only in the centre of 

 the tube. Therefore, more water than" salt solution will 

 traverse the membrane. 



