754 PHYSIOLOGY 



therefore natural that electrical changes should accompany the 

 intense chemical activity which is associated with secretion. The 

 interpretation of these changes is difficult, owing to the simultaneous 

 operation of another factor which may determine electrical differ- 

 ences of potential, namely, the movement of fluids through porous 

 membranes. If the hilum of the sub-maxillary gland and its outer 

 surface be connected with a galvanometer, a resting difference of 

 potential is nearly always found, generally in such a direction that 

 the outer surface is positive to the hilum. The current through the 

 gland is therefore from within out. On exciting the chorda tympani 

 nerve a diphasic effect is generally obtained, the resting difference 

 being first increased and later on diminished. On excitation of the 

 sympathetic nerve we generally obtain a purely negative variation 

 of the resting difference. These results were interpreted by Bayliss 

 and Bradford as due to the co-operation of the two factors, chemical 

 change in the gland- cells and movement of fluid through the cells. 

 The positive variation, i.e. the current from within out, was ascribed 

 to the movement of fluid, whereas the negative variation of the 

 resting difference was thought to be due to the chemical changes 

 in the gland- cells. 



THE SIGNIFICANCE OF THE DOUBLE NERVE-SUPPLY 



TO THE GLANDS 



According to Heidenhain, although the parotid gland gives little 

 or no secretion on stimulation of the sympathetic nerve, prolonged 

 stimulation of this nerve causes histological changes in the gland 

 even more marked than those produced by the cranial nerve. Similar 

 histological changes were found by him in the sub-maxillary gland. 

 He was therefore led to put forward the hypothesis that the salivary 

 glands are supplied by two fundamentally different classes of fibres, 

 namely : (1) trophic fibres, which determine the chemical changes 

 in the gland responsible for the production of the specific constituents 

 of the secretion, and (2) secreto-motor fibres, excitation of which 

 causes the cells to take up water and salts from the lymph and blood, 

 and pass them in lar'ge quantities into the duct. According to this 

 view the sympathetic nerve-supply to the gland would consist almost 

 entirely of trophic fibres, whereas secreto-motor fibres would pre- 

 dominate in the cranial nerve-supply. The action of atropine would 

 appear at first sight to favour this hypothesis. In minute doses 

 it entirely annuls the action of the chorda tympani nerve or the 

 corresponding nerve to the parotid, while it is without effect on 

 the sympathetic nerve-supply unless given in huge doses. The pre- 

 ponderating effect of the sympathetic on the histological structure 

 of the ^land-cells has not been confirmed by later observers, and 



