144 INTRINSIC ACTIVITY OF SECRETING CELLS 



before the pressure in the ductules can exceed that in the blood- 

 vessels, and so the stoppage of secretion is a purely mechanical effect. 



These experiments prove that, although secretion under normal 

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

 is not one of passive nitration. 



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, as the saliva, where the total osmotic pressure is less 

 than that of the plasma, the osmotic pressure of certain con- 

 stituents is higher than their pressure in the plasma for example, 

 in the saliva, the pressure of dissolved carbon-dioxide, of calcium 

 salts, and of the sulphocyanide ; (3) that new constituents appear 

 in the secretion as a result of chemical activity in the cell which 

 are entirely absent in the plasma, and are not sent into the plasma, 

 but into the gland duct, by means of cellular activity and in opposi- 

 tion to the operation of osmotic energy ; (4) the alteration in many 

 cases of chemical reaction by concentration of hydrogen or of 

 hydroxyl ions in the secretion high above the concentration which 

 they possess in the plasma, may be quoted as an example of 

 cellular activity producing an effect in opposition to osmotic 

 pressure. 



Now it is clear that while the source of energy residing in the 

 blood pressure might separate a secretion, with concentration 

 possessing any value up to that of the same dissolved constituent 

 in the plasma, it cannot produce a concentration in even a single 

 constituent exceeding the value of the concentration of that same 

 constituent in the plasma, and certainly cannot produce a new 

 constituent not present in the plasma. When the results of 

 experiment are taken in conjunction with this statement, it is 

 found that in every secretion in the body cellular activity must 

 be brought into action, in other words, the secreting cell must 

 furnish energy in the process of secretion, and this not only holds 

 obviously for the new constituents, but also for all those crystallised 

 and inorganic constituents which are found in the secretion at 

 a higher concentration than in the plasma, and hence possess 

 more osmotic energy. 



Not only does the increase in concentration of certain con- 

 stituents in the secretion above their concentration in the plasma 



