126 THE INVOL UNTAR Y NER VO US S YSTEM 



of secretion. The evidence at the present time is distinctly against 

 this view, for both nerves cause apparently the same kind of meta- 

 bolism in the gland cell, and both nerves can cause a secretion. 

 If then we are bound to conclude that there is no real difference 

 in the function of the two nerves, how can we explain the double 

 innervation ? The simplest explanation to my mind is that the 

 nerves supply with secretory fibres glandular structures which are 

 different morphologically, part being epidermal skin glands like 

 the sweat glands and part being endodermal glands like the 

 gastric and pancreatic glands. The former being supplied by 

 sympathetic fibres would be affected by adrenaline and ergotoxine, 

 while the latter would be supplied by connector nerves from the 

 facial and glossopharyngeal nerves belonging to the bulbar group. 



Langley states that, after long-continued stimulation of the 

 chorda tympani, there are always gland cells to be found in 

 the condition of rest upon microscopical investigation. This 

 fact points to the conclusion that there are gland cells which 

 are not innervated by the chorda tympani ; the same reason- 

 ing applies also to the sympathetic. As yet no one has at- 

 tempted to find out whether the gland cells affected either by 

 adrenaline or stimulation of the sympathetic nerve are always 

 the same and can be plotted out into definite groups. When we 

 remember how definitely the Islets of Langerhans in the pancreas 

 have been dissociated from the pancreatic cells themselves as the 

 result of long-continued patient research, it is quite possible that 

 similar research may succeed in differentiating between gland 

 cells supplied by the chorda tympani and sympathetic nerves 

 respectively. At present we do not know enough about the 

 development of these glands or about their phylogenetic origin to 

 be able definitely to say whether they belong to the endodermal 

 or the epidermal group of glands or to both. It is possibly of 

 significance that the two foremost gills in Ammoccetes, those 

 belonging to the facial and glossopharyngeal segments, are mark- 

 edly differentiated from the rest of the gills by the large amount 

 of mucous glands in them, which have taken the place of the re- 

 spiratory epithelium ; indeed the gills of the foremost or facial re- 

 spiratory segment have almost entirely lost all trace of respiratory 

 epithelium and become converted into mucous glands. 



The existence of secretory nerves for the gastric glands has 

 been placed beyond question by the work of Pawlow. He 



