ELECTRIC PHENOMENA 153 



interface ; allowing free passage to cat-ion and an-ion. That 

 is, at this surface the electrical potential recorded will be that 

 of the interior of the cell (cf. muscle-current of injury). 



An explanation of the current which flows during the elaboration 

 of secretion is more difficult. There seems no doubt that just 

 before being carried out through the duct, the granules undergo 

 some change. The large colloidal particles either break down 

 into smaller particles or go into solution. Either of these actions 

 is accompanied by the setting free of adsorbed salts and altera- 

 tions in the electrical charge. 



6. These two processes, water secretion and the elaboration of 

 the organic secretory material, seem to be controlled by different 

 sets of nerves. Secretory nerves when stimulated cause the 

 gland to be flooded with water, while " trophic " nerve fibres 

 have to do with the elaboration of the granular material of the 

 secretion. (Heidenhain, 1868, and Babkin, 1913.) Both sets of 

 fibres may go to the gland in the same nerve. It is interesting 

 to note that acid and other irritants excite secretory fibres only, 

 while normal excitants stimulate both secretory and trophic 

 fibres. 



Can we from these facts construct a picture of the mechanism 

 of secretion ? 



(1) The formation of granules in the cell may be similar to the 

 formation of starch in the plant. Substances are thus put out 

 of action. The colloidal granules not only have a low osmotic 

 pressure but they adsorb crystalloids and so prevent endosmosis. 



(2) On stimulation, these granules are broken down into smaller 

 particles and water rushes in. It may be that stimulation of the 

 gland follows the same course as in muscle and produces acid. 

 This acid would interfere (a) with the colloids present, especially 

 with their power to hold water (imbibition) and salts (adsorption) 

 and so bring about alterations in their size, electrical charge and 

 the osmotic pressure of their dispersion medium, (b) Acid has 

 a direct action on the electrical charge of any solution and, there- 

 fore, acts on its surface tension, lowering it. The only surface 

 where this can take place is between the cell and the lumen. The 

 alkaline reserve of the blood is sufficient to keep the cell-lymph 

 interface normal ; or rather supernormal, while the cell-interfaces 

 obviously need not be considered. 



(For a neat model of Mucoid secretion due to M. Fischer, see 

 Part II. p. 415.) 



In short, the arrival of the appropriate stimulus causes the 



