DIGESTION 



357 



No tissue lacks them ; no physiological process goes on without 

 them ; they are not high and special products. As we breathe 

 nitrogen which we do not need because it is mixed with the 

 oxygen we require, the secreting cell passes through its substance 

 water and salts as a sort of by-play or adjunct to its specific 

 work. But this is not the whole truth. The gland-cell is not a 

 mere filter through which water and salts pass in the same 

 proportions in which they exist in the liquids that the cell draws 

 them from. When, e.g., the salivary glands secrete against the 

 resistance of an abnormally high pressure in the ducts, the per- 

 centage of salts in the saliva increases. The secretions of 

 different glands differ in the nature, and especially in the relative 

 proportions, of their inorganic constituents. They differ also 

 in their osmotic pressure and electrical conductivity, which 

 depend so largely upon those constituents, notwithstanding the 

 fact that the osmotic pressure and conductivity of the blood- 

 serum (p. 25) vary only within narrow limits. Even the secre- 

 tion of one and the same gland is by no means constant in this 

 respect, as we shall have to note more especially when we come 

 to deal with the influence of the nervous system on secretion 

 (P- 367)- The following tables illustrate this point : 



Pancreatic Juice of Dog (P incus so hn). 



* The blood and gastric contents were obtained from the animals in the 

 writer's laboratory twenty-four hours after the last meal. 



f The depression of the freezing-point below that of distilled water. 



| K is the specific conductivity of a cube of the liquid of i centimetre 

 side, the conductivity of a similar cube of mercury being taken as unity. 

 The number in brackets is the temperature at which the measurement was 

 made. To obtain expressions for K in whole numbers, it is multiplied by io 4 . 



