INTRINSIC ACTIVITY OF SECRETING CELLS 249 



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

 by substances in solution in the plasma. Such specific substances 

 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. Also by the observation that after ad- 

 ministration 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 before the pressure 

 in the ductules can exceed that in the bloodvessels, and so the 

 stoppage of secretion is a purely mechanical effect. 



These experiments prove that, although secretion under normal 

 conditions may be aided by filtration, yet the process in its nature 

 is not one of passive filtration. 



That it is not passively dependent upon osmotic pressure is 

 shown (1) by the fact that the total osmotic pressure of the secretion, 

 as shown by depression of freezing-point, is in many cases greater 

 than that of the plasma; (2) that even in cases such, for example, 



