154 MANUFACTURING CELLS 



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

 material and launch it into the duct 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. 



Of the mechanism of these glands which secrete directly into 

 the blood stream little is known. Seemingly the secretion is 

 extruded from the cell and washed away by the blood itself. 

 Their oxygen consumption has never been measured nor yet their 

 utilisation of glucose. 



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 consider a gland as 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. 



A considerable amount of work has been done in connection 

 with the electrical changes which take place when a gland secretes. 

 Bayliss and Bradford found that two kinds of changes occurred 

 in the salivary gland. That associated with the secretion of 

 water produced changes in the opposite direction from that 

 associated with the output of the colloidal matter. The causes of 

 this phenomenon are not yet certain but two " pointers " may be 

 indicated. (1) The work of Loeb and his colleagues has shown 

 how the presence of colloidal matter may alter the sign of electrifi- 

 cation of water (p. 111). (2) In accordance with the accepted 

 theory of secretion, the onset of true secretion leads to an increase 

 in the permeability of the cell membrane at one end. It is obvious 

 that, at the more permeable end, we are in electrical contact 

 with the interior of the cell and so obtain the potential difference 

 of the Helmholtz double layer between permeable and imper- 

 meable parts of the surface. During a mere secretion of water 

 permeability is not supposed to undergo alteration. 



