10 WHAT IS LIFE 



which do not come into operation in connection 

 with the process of chemical osmosis. Again the 

 process of secretion by glands, such as those which 

 produce the saliva, is now known not to be one in 

 which the secreting cells " act in a passive inert 

 manner as filtering mechanisms, or as membranes 

 possessing constant permeabilities for different dis- 

 solved substances in the plasma, or as media in 

 which different substances possess different solu- 

 bilities". This "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 conditions 

 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 sub- 

 maxillary 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, how- 

 ever, calling forth any flow of secretion." l These 

 and other instances, such as that of oxidation, which 

 might also be cited, show that further knowledge 



J B. Moore in Recent Advances in Physiology and Bio- 

 Chemistry, ed. L. Hill, Arnold, 1906. 



