INTRINSIC ACTIVITY OF SECRETING CELLS 157 



and that the coefficient of distribution of the substance x is such 

 between the three fluids, that the concentrations of /3x in B and 

 of yx in C correspond to the concentration ax in A, so that there 

 is equilibrium when the ratio of concentrations is ax : /3x : yx in 

 the three fluids A, B, and 0. Now if at the commencement there 

 is none of x in B or C but x is present in A, diffusion into 

 B will take place, and as soon 

 as the concentration of x in B 

 commences to rise there will be 

 diffusion from B into C. Also, 

 the higher the value of the 

 ratio fi : a, the more rapid, other 

 things being equal, will be the 

 rate of entry of x into B ; but if 

 /3 be increased so as to increase 

 the ratio /? : a, the ratio of y : /3 



which determines the rate of out- FIG. 4 



put into C will be correspondingly 



diminished, and hence the rate of passage from A to is entirely 

 independent of the solubility in B, and depends only upon the 

 rate of transmission or diffusion through B. 1 Also the final con- 

 dition of equilibrium is independent of the solubility in B, for 

 A and C are each in equilibrium with B, and hence are in equilibrium 

 with one another, so that when the final concentrations are attained 

 the ratio of concentrations in A and C must be the same as if B 

 were left out and A and C had been placed in contact and allowed 

 to come into equilibrium. The only factor which affects the result 

 apart from equilibrium is the rate of diffusion of the substance x 

 through the layer B, and this may not bear any constant relationship 

 to the solubility of x in B. If the solubility in B is very low, 

 so also in all probability will be the rate of diffusion through B ; 

 but here, as pointed out above, the extreme thinness of the layer B, 

 in the case of the cell or any cell membrane, and the extent of 

 surface, renders any such factor in most cases of but secondary 

 importance. Certainly, however, solubility in B is not the determining 

 factor with regard to the distribution of the substance except in B 

 itself, and in no case can a high solubility in B determine a higher 

 concentration in C than is given by the coefficient of distribution 

 between A and C. 



1 The matter may also be put thus the rate of solution from A into B is 



8 y 



given by fcj -, that of solution from B into C by k 2 L therefore that from A into 



By y 



C is given by ^ ^ . k 2 , or K ^ 



