INTRINSIC ACTIVITY OF SECRETING CELLS 143 



produced in the passage through the cell, 1 the rate of secretion 

 or transmission is so much subject to variation apart from purely 

 physical factors, that the cell must be regarded as something more 

 than simply an inert membrane, because its permeability for 

 different dissolved substances, and for the solvent, vary from 

 time to time as the cell is acted upon either by the nervous system 

 or by substances in solution in the plasma. Such specific sub- 

 stances dissolved in the plasma possess the power of affecting 

 permeability for other and quite different substances in a manner 

 never seen in the case of non-living membranes or apart from 

 living cells. 



The consideration of the mechanism of secretion may accord- 

 ingly be divided into two parts, viz. (1) the intrinsic activity of 

 the cells concerned in secretion, and (2) the accessory mechanisms 

 by which the rate of secretion is varied and controlled. 



INTRINSIC ACTIVITY OF SECRETING CELLS 



That secreting cells do not act in a passive inert manner as 

 filtering mechanisms, or as membranes possessing different and 

 constant permeabilities for different dissolved substances in the 

 plasma, or as media in which different substances possess different 

 solubilities, is proven by many experimental observations. Thus 

 that the rate of .secretion is not merely passively dependent upon 

 blood pressure and blood supply (although under normal con- 

 ditions it is subject to variations corresponding to changes in 

 these physical factors) is shown by the observation of Ludwig 

 that the secretion pressure in the submaxillary salivary gland, 

 when the outflow is resisted by fluid in a manometer, may rise 

 much above the arterial pressure ; and also by the observation that 

 after administration of a drug, such as atropin, the blood supply 

 may be increased as much as before administration of the drug on 

 stimulation of the secretory nerve, without however calling forth 

 any flow of secretion. In other cases, such as the kidney, where 

 the secretion pressure cannot be raised above arterial pressure, 

 this is due to the nature of the minute anatomical structure, as a 

 result of which all supply of fluid is cut off from the secreting cells 



1 It is improbable that this condition ever is completely realised in the action 

 of living cells. 



