86 THE CIRCULATION OF THE BLOOD 



this point is the isolated fact that stimulation of the cervical 

 sympathetic in the dog causes vaso-dilatation of the gums 

 and soft palate. 



After the administration of the drug, ergotoxine, stimula- 

 tion of the abdominal sympathetic causes vaso-dilatation. 

 This may be interpreted in two ways. The drug may 

 paralyse the vaso-constrictors, and so bring out the action 

 of the vaso-dilators, previously masked by the greater 

 power of their opponents. On the other hand, it may be 

 argued that ergotoxine acts by converting an excitor into 

 an inhibitor effect, in the same way as strychnine converts 

 an inhibitor into an excitor efiect. 



Passing to the somatic system, we find more certain 

 evidence of the existence of vaso-dilator nerves. 



It is possible to demonstrate that in the nerves supplying 

 the Hmbs, vaso-dilator as well as vaso-constrictor impulses 

 are conveyed. In the first place, the two sets of fibres are 

 susceptible to different modes of stimulation. When the 

 peripheral end of the cut nerve is stimulated by the ordinary- 

 interrupted current, vaso-constriction occurs; when by 

 slowly repeated induction shocks, vaso-dilatation is the 

 result. Again, when the nerve is stimulated two or three 

 days after section, vaso-dilatation invariably occurs, 

 pointing to a difference in the rate of degeneration between 

 the two sets. 



How do the vaso-dilator fibres emerge from the cord? 

 Are they part of the sympathetic or not ? When the 

 posterior root of a segmental nerve is cut and its peripheral 

 end stimulated, vaso-dilatation occurs over the area of 

 distribution of the nerve. This cannot be due to stimula- 

 tion of the sympathetic, since sympathetic fibres join the 

 nerve more distally. There is here, then, a contradiction 

 of Bell's Law, according to which the posterior root was 

 regarded as purely afferent. The question which we now 

 have to decide is this : Does the posterior root contain 

 two kinds of fibres, afferent conveying sensation and 

 efferent conveying vaso-dilator impulses, or are there but 



