EXTRINSIC MECHANISMS OF SECRETION 167 



centration of the hydrogen ion has to be increased from an almost 

 immeasurably low concentration in the plasma up to about the 

 strength of a deci-normal solution, and to do this by passive 

 absorption an enormous amount of water must enter the secreting 

 cell and be again rejected at the same side at which it entered. 

 Again, in the secretion of urea in the urine 75 kilograms would 

 have to enter and pass through the kidney cells and be reabsorbed 

 in order to concentrate and separate the daily output of urea. 

 Also, in absorption from the intestine, to take up a meal of 150 

 grm. of carbohydrate or fat in one per cent, solution, which is 

 probably in excess of the concentration at which these food- stuffs 

 are normally absorbed, it would be necessary for 15 kilograms 

 of water to be taken up by the absorbing cells, and either returned 

 by alternating back streams into the intestine free from carbo- 

 hydrate or fat, or else poured into the blood stream. Such an 

 amount is probably much in excess of the sum of the water taken 

 with the food and the combined digestive secretions. 



Hence we must suppose that the cell, whether absorbing or 

 secreting, does not undergo passive infiltration by the fluids in 

 contact with it, and allow these, or even the water, to stream 

 through passively, but is an active energy machine, and takes up 

 the various constituents and their solvent in definite and well- 

 regulated proportions. 



For the reason stated at the outset, the amounts of energy 

 involved in the formation by the cell of the new organic constituents 

 of the secretion not present in the plasma cannot at present be 

 estimated, and so we pass to a consideration of the extrinsic 

 mechanisms of secretion. 



THE EXTRINSIC MECHANISMS OF SECRETION 



Alterations in the Blood Supply to the Secreting Gland. Accom- 

 panying the increased amount of physiological work which the 

 secreting cells have to perform, there is always during secretion 

 an increase in the amount of blood supplied. This increase was 

 estimated by Chauveau and Kaufmann in the case of the sub- 

 maxillary salivary gland as amounting to three times the blood 

 supply in the resting condition of that gland, but according to 

 more recent experiments by Barcroft, it may in the dog be set 



