718 PHYSIOLOGY 



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 submaxillary 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 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 conditioned 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 sub- 

 maxillary 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 0-25 per cent, hydro- 

 chloric acid or of meat powder. The amount of saliva secreted under the two circum- 

 stances is approximately the same, but that evoked by the introduction of meat powder 

 contains about twice as much solid contents as that which follows the introduction of 

 acid into the mouth. Babkin has shown that the same differences are found after 

 complete section of the sympathetic and that there is the same acceleration of the 

 circulation through the gland whether the secretion is aroused by introduction of 

 meat powder or of acid. It is impossible therefore to explain the difference in the 

 composition of the saliva obtained under these two circumstances as due to differences 

 in the blood supply to the gland, and we must conclude either that the chorda tympani 

 contains different kinds of fibres which are excited to varying extent according to the 

 nature of the reflex stimulations, or that one and the same nerve fibre can convey 

 specifically different impulses. The latter explanation would not be in accord with 

 the generally accepted Miiller's law of specific irritability, but is the explanation to 

 which Babkin himself inclines. At any rate it is certain that according to the nature 

 of the reflex stimulus either the secretion of water and salts or the secretion of organic 



