672 SYMPATHETIC AND OTHER SYSTEMS OE NERVES. 



cussed the several possible explanations of the recovery of tone, and 

 adopted the view that in the foot and in the body generally the tone of 

 the blood vessels was maintained by the action of local ganglion cells. 

 The progress of histological inquiry has shown that the hypothetical local 

 ganglion cells of the type of sympathetic nerve cells do not exist. And 

 unless we fall back upon " interstitial cells," the view of the maintenance 

 of tone by local nerve cell centres must be abandoned. It is very doubt- 

 ful whether the " interstitial " cells are nerve cells at all. And, on the 

 whole, the most probable explanation of such recovery of tone as occurs 

 is the inherent contractile power of the plain muscular tissue itself. 



It must be noticed that the degree of recovery of tone, after section 

 of all vasomotor fibres, is still a matter in dispute (cf. p. 655), and that 

 possibly the rapid recovery in the tone of the vessels of the foot, after 

 section of the sciatic, is due to vasomotor fibres taking the course of 

 the crural nerve (cf. p. 657). 



In the stomach and intestine, the nerve cells of the plexuses of 

 Auerbach and Meissner may well act as the local centres, but nothing 

 definite can be said at present with regard to this. 



QUESTIONS COMMON TO THE SYMPATHETIC AND SOME OTHEE 

 PARTS OF THE AUTONOMIC SYSTEM OF NERVES. 



We have considered separately the sympathetic, the cranial auto- 

 nomic, and the sacral autonomic systems, so far as this could be done 

 conveniently, but a separate treatment is inconvenient for certain 

 questions. These we shall consider in the following pages. 



Mode of Action of Inhibitoey Fibres. 



Claude Bernard was the first to attempt to explain the action of 

 inhibitory nerves. This he did with regard to the vaso-dilator fibres of 

 the chorda tympani. He 'considered that the fibres of the chorda 

 tympani inhibit the action of a local vaso-constrictor centre. On this 

 hypothesis the local vaso-constrictor centre is in' constant action ; 

 sympathetic fibres run to it and increase its action ; chorda tympani 

 fibres run to it and decrease its action ; thus the nerve cells would 

 receive fibres from two sources. This view of the mechanism of 

 inhibition was advocated by Vulpian, and is supported by Dastre and 

 Morat. 



There are many facts which show that this view is untenable. We 

 will consider the two chief instances of vaso-dilator fibres, the chorda 

 tympani and the nervus erigens. 



The local nerve centre with which the chorda tympani fibres is 

 connected is the submaxillary ganglion in the hilum of the gland. This 

 is shown by the action of nicotin. On Bernard's hypothesis, stimulation 

 of the fibres proceeding from the local centre should cause contraction of 

 the arteries. But, after the chorda tympani fibres have been paralysed 

 by nicotin, stimulation of the fibres past the ganglion causes dilatation 

 of the arteries of the gland, and does not cause contraction ; hence the 

 nerve fibres given off from the local nerve centre are not vaso-constrictor 

 fibres but vaso-dilator fibres. So that the chief assumption of the 

 hypothesis is incorrect. Moreover, when the sympathetic fibres are 

 followed by means of serial sections into the gland, it is seen that they 



