ENDOCRINE (i LANDS 187 



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

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

 secretion. (Heidenhain, 18(38, and Habkin, ]013.) Hoth 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, {h) 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, 

 because the alkaline reserve of the blood is sufficient to keep the 

 cell-lymph interface normal or rather supernormal, while the cell- 

 cell interfaces obviously need not be considered. 



In short, the arrival of the appropriate stimulus causes the 

 cell to draw on its store of material, alter the packing of the 

 material and launch it into the duet on a current of water. The 

 stimulus may be nervous or it may be a hormone (chemical 

 messenger) formed in another organ and transported to the gland 

 by the blood. 



Bayliss and Hill have shown that the salivary gland does not 

 become heated during activity. From this we may deduce that 

 all the additional energy set free during the course of activity is 

 used in doing work (in elaborating the secretion and in setting it 

 free, etc.), and in maintaining to some extent the normal tempera- 

 ture of the body (cf. Muscle). Thus leaving alone the latter sink 

 of energy we may assume that a gland is 100 per cent, efficient, and 

 calculate the work done from the oxygen intake or carbon dioxide 

 output or from the diminution of sugar in the blood passing 

 through it. 



Of the mechanism of those glands which secrete directly into 

 the blood stream little is known. Seemingly, the secretion is 



