386 



DIGESTION 



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 percentage 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. 26) 

 vary only within narrow limits. Even the secretion 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 in- 

 fluence of the nervous system on secretion (p. 395). The following 

 tables illustrate this point : 



Pancreatic Juice of Dog (Pincussohn) . 



* The blood and gastric contents were obtained from the animals twenty- 

 four hours after the last meal. 



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



J See footnote on p. 27. The number in brackets is the temperature at 

 which the measurement was made. 



