186 MANUFACTURING CELLS 



there is an increase in the rate at which blood enters the gland. 

 In other words, raw material and power are taken into the factory 

 at an increased rate. The view was at one time held that the 

 secretion was due to this increased flow of blood. Barcroft's 

 experiments have shown that this cannot be true, because 



(a) The increase in the blood flow through the organ is initiated 

 after the secretion begins and is continued for some time after 

 secretion has ceased ; and 



(6) Vaso-dilatation may take place without secretion. 



The increase in blood flow or vasodilatation is a consequence 

 of secretion and not the secretion a result of the vasodilatation. 

 The actively secreting gland, as it were, sends out a call for 

 oxygen, for power and for material. This call is in part met by 

 this increase in the transport service (see Chap. XXV.). 



5. Electric Potential. Alterations take place in electrical poten- 

 tial of one part of the gland to another. These have been studied 

 principally by Bayliss and Bradford on the salivary gland and by 

 Orbelli on the skin glands of the frog. The results vary somewhat 

 with the means of investigation, but may be taken as indicating 

 two things. 



(a) The secretion of water, i.e., dispatch of secretion, is a different 

 function of the gland or a function of a different mechanism in 

 the gland from the elaboration of the true secretory material. 

 That is, we have to consider two phenomena, the preparation of 

 material and its flooding out of the cell by water. The latter is 

 accompanied by — 



(6) A large difference of potential between the cell-lymph inter- 

 face and the cell-lumen interface, the former by a small potential 

 difference of the opposite sign from the latter. 



The cause of the larger difference may be sought in the increased 

 permeability (lowered surface tension) of the cell-lumen 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. injured muscle). 



An explanation of the potential difference developed 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 

 alterations 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 



