MECHANISM OF SECRETION 151 



of glucose used runs, as one would expect, parallel to the oxygen 

 consumption. 



The mechanism of secretion has been provocative of much 

 controversy. A regular pitched battle results when vitalists, 

 neovitalists and mechanists discuss the problem. What are the 

 facts ? 



1. Microscopical examination of the gland shows that during 

 inactivity the lumen (storehouse) becomes filled with granules 

 and the gland increases in volume. When the gland is excited 

 to secretion, these granules disappear with the secreted fluid and 

 the gland decreases in volume. 



2. Water passes from the blood into the gland and out with 

 the secretion. 



There seems to be no difficulty in giving a plausible explanation 

 of the second of these phenomena. The postulation of a semi- 

 permeable membrane is sufficient. The first fact presents diffi- 

 culties. 



(a) The osmotic pressure of the secretion is often greater than 

 the osmotic pressure of the blood. 



(b) The pressure in the duct against which the saliva may be 

 secreted was found by Ludwig, in 1851, to be greater than that 

 of the artery supplying the gland. Hill and Flack recently 

 estimated this difference in pressure. They found that the 

 pressure of secretion was as high as 240 mm. Hg. and the arterial 

 pressure 130 mm. 



3. Macallum demonstrated alterations in surface tension during 

 secretory activity. 



As mentioned in Chap. VI. this worker made use of the Gibbs- 

 Thompson distribution of salts to determine the relative values 

 of surface tension in cells which had been killed and fixed almost 

 instantaneously. He found, in the pancreas for instance, that 

 during rest there was an unequal distribution of potassium, but 

 when the gland was stimulated to activity potassium salts were 

 found equally distributed throughout it. 



Theoretically, in an active gland there must be at least three 

 different values for surface tension, viz. : 



(1) Cell-lymph interface, i.e. on the outer face through which 

 raw material and power enter. 



(2) Cell-cell interface where the cell wall is in contact with some 

 of the other cells of the gland. 



(3) Cell-lumen interface through which the secretion and the 

 leaching out water pass. 



