146 PHYSIOLOGY 



and one square centimetre in cross- section. The determination of 

 these diffusion coefficients presents many difficulties. The task is, 

 however, rendered easier by the fact, first ascertained by Graham, 

 that diffusion of salts occurs as rapidly through a solid jelly of gelatin 

 or agar-agar as through water. It is therefore possible to make the 

 plug in the diagram solid by the admixture of one of these two sub- 

 stances, and to maintain a constant concentration on the two sides of 

 it by the circulation of fluid without affecting the* rate of diffusion 

 through the cylinder by setting up accidental currents. 



More important from the physiological point of view than diffusion 

 through fluids is the exchange of fluids (water and dissolved substances), 

 which may take place across membranes. Such processes are of 

 constant occurrence in all parts of the body and are concerned in such 

 functions as the formation and absorption of lymph, the absorption 

 from and secretion into the intestines, absorption from serous cavities, 

 and so on. In many of these functions we shall have to consider later 

 whether the transference across the membrane is determined solely 

 by the nature and concentration of the fluids on the two sides of it or 

 is effected by the active intervention, involving the expenditure of 

 energy, on the part of living cells forming constituent elements of the 

 membrane itself. It is worth our while, therefore, to consider at some 

 greater length the purely physical factors which may be concerned in 

 the passage of water and dissolved substances across membranes. 



In the case of fluids containing only one substance in solution, the 

 exchange across the membrane will be determined entirely by the 

 osmotic pressures. Thus, if two watery solutions, with the same osmotic 

 pressure, are separated by a membrane through which diffusion can 

 take place, no change in volume occurs on either side of the membrane. 

 If the solutions on either side of the membrane are of unequal osmotic 

 pressure, water passes from the side where the pressure is lower to 

 the side where it is higher, and there is a simultaneous passage of the 

 solute from the side of greater to the side of less concentration. 



If, however, the solutions on the two sides contain dissimilar 

 substances, with different diffusion coefficients, the conditions are 

 more complicated, and may tend even jto produce a movement of 



