MKCUAMSM or SECRETION 185 



1. Microscopical exaniinatioii of the oland shows that during 

 inactivity the lumen (storehouse) becomes lilled 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 explana- 

 tion of the second of these phenomena. The postulation of a 

 semi-permeable membrane is sufficient. The first fact presents 

 difficulties. 



(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 found that the 

 pressure of secretion was as high as 240 mm. Hg. with an arterial 

 pressure of 130 nnn. 



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 w^hich had been killed and fixed almost 

 instantaneously. 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 ^d pow er 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 Avhich the secretion and the 

 leaching water pass. 



He found that, during activity, there w'as the densest con- 

 densation of potassium at (3), the cell-lumen interface, less 

 potassium was found at the cell-cell interface and least at (1), the 

 cell-lymph interface, while when the gland was at rest there was 

 no marked difference between the interfaces. According to the 

 Gibbs-Thompson principle these results may be taken as an 

 indication — 



{a) That during rest there is no marked difference of surface 

 tension at the gland interfaces, and 



[h) That during activity a high tension develops at the surface 

 between cell and lymph, a low tension between cell and lumen 

 and that the cell-cell interface has a value intermediate. 



4. Blood Supply. It is well know^n that during glandular activity 



