DIGESTION IN THE MOUTH 673 



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 funda- 

 mentally different classes of fibres, namely : (1) trophic fibres, which deter- 

 mine 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 large 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 predominate 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 preponderating effect of the sympathetic on the histological 

 structure of the gland- cells has not been confirmed by later observers, and 

 the varying effects of atropine on the two sets of nerve fibres may be con- 

 ditioned by morphological rather than by functional differences between 

 their nerve-endings. According to Langley and Carlson, the difference in the 

 action of the chorda tympani and of the sympathetic on the submaxillary 

 gland is due to the synchronous action of these nerves on the blood-supply 

 to the gland, the sympathetic causing vaso-constriction, while the chorda 

 tympani causes vaso-dilatation. In confirmation of this explanation they 

 have shown that clamping the carotid artery during chorda stimulation 

 diminishes the amount of saliva secreted but increases the percentage of 

 solids in the fluid. This theory is however inadequate to explain the 

 differences observed in the secretion of saliva reflexly aroused by introduction 

 of substances into the mouth. In a dog with a permanent submaxillary 

 fistula a copious flow of saliva may be caused by the introduction of -25 per 

 cent, hydrochloric acid or of meat powder. The amount of saliva secreted 

 under the two circumstances is approximately the same, but that evoked 

 by the introduction of meat powder contains about twice as much solid 



22 



