166 ENERGY CHANGES INVOLVED IN SECRETION 



Or the energy changes may be brought about by chemical 

 combinations and dissociations in the cell. 



But whatever view be taken as to the mode of operation, it 

 is clear from the experimental study of the selective rates of 

 passage of dissolved substances through the cell that what might 

 be termed " polar " properties must be ascribed to the cell in 

 its phenomena of secretion and absorption. This is not theory 

 but experimental fact. It is seen that many substances pass 

 through the cell several times more rapidly than the solvent, while 

 others pass through more slowly. In the case of those which 

 pass through more rapidly, work in giving velocity to these mole- 

 cules or ions and in increasing osmotic energy must be done by 

 the cell. It is clear from this that the amount of solvent in which 

 any given quantity of a constituent is dissolved need never enter 

 the cell, but instead the dissolved substance be attracted and moved 

 through the solvent toward and into the cell by the energy of the 

 cell ; just as independent velocities are given to the ions towards 

 the electrodes by the electric potentials on the electrodes without 

 the solvent between the electrodes moving at the same rate towards 

 either electrode. Thus there may only be a slow current of water 

 through the cell, with a slow uptake of water from the lymph, and 

 a much more rapid current of dissolved substances and corre- 

 sponding increase in concentration of these in the secretion or 

 absorbed fluid. 



If the water containing the dissolved substances were taken 

 up at the same rate by the secreting cell, then in order that the 

 secretion could become more concentrated in any constituent 

 it would be necessary that at intervals water should be returned 

 or pressed out again at the side of the cell at which it entered, 

 containing the constituent which was to be concentrated in more 

 dilute solution. It hence appears more probable that instead 

 of such a to and fro movement of water, the dissolved substances 

 are taken up upon the entrance side of the cell more rapidly 

 than if they passively moved in with a corresponding amount 

 of water. 



That this view is probable is seen from the enormous amount 

 of water which would have to pass into and out of the cell 

 alternately if only passive absorption of water and its dissolved 

 substances formed the first stage in the process of secretion. Thus 

 in the secretion of hydrochloric acid in the gastric juice, the con- 



