208 VASO-MOTOR NERVES. [BOOK i. 



pallor being similarly due to an increase beyond what is normal of 

 these same impulses. In other words, the nervous influences 

 leading to arterial dilation and constriction differ in degree only, 

 not in kind, and may be considered as being merely phases (of 

 decrease or of increase as the case may be) of the same action. 

 And if we turn to the splanchnic nerve we find a similar interpre- 

 tation equally valid. Stimulation of the splanchnic nerve causes 

 constriction of the arteries governed by that nerve, apparently 

 because the stimulation supplies artificially the constrictive im- 

 pulses which, so long as the nerve is intact, pass down it from the 

 central nervous system, giving the requisite tone to its vascular 

 area, and the loss of which by division of the nerve gives rise to 

 dilation. So that were we to stop our inquiries at this point, our 

 explanation of vaso-motor action would be very simple. We might 

 speak of constrictive impulses as passing from the central nervous 

 system to the various vascular areas, to such an extent as to 

 constitute normal tone, but as being susceptible either of in- 

 hibition, complete or partial, thus leading to greater or less 

 arterial dilation, or of augmentation, thus leading to excessive con- 

 striction. 



But this simple view appears insufficient when we push our 

 studies further. 



In the first place such a conception does not cover all the facts 

 connected even with the two nerves just mentioned. For the 

 dilation or loss of tone which follows upon section of the cervical 

 sympathetic (and the same is true of the splanchnic) is not 

 permanent ; after a while, it may be not until after several days, it 

 may be sooner, the dilation disappears and the arteries regain 

 their usual calibre. This recovery is not due to any regeneration 

 of vaso-motor fibres in the sympathetic, for it may be observed 

 when the whole length of the nerve including the superior cervical 

 ganglion is removed. When recovery of tone has thus taken place, 

 dilation or increased constriction may be occasioned by local treat- 

 ment : the ear may be made to blush or to pale by the application 

 of heat or cold, by gentle stroking or rough handling and the like ; 

 but neither the one nor the other condition can be brought about 

 by the intervention of the central nervous system. So also the 

 spontaneous rhythmic variations in the calibre of the arteries of 

 the ear of which we spoke on p. 198, though they cease for a time 

 after division of the cervical sympathetic, eventually reappear, even 

 if the superior cervical ganglion be removed. And the analogous 

 rhythmic variations of the veins of the bat's wing have been proved 

 experimentally to go on vigorously when all connection with the 

 central nervous system has been severed; they may continue in 

 fact in isolated pieces of the wing. From this it is clear that what 

 we have spoken of as the tone of the vessels of the face, though 

 influenced by and in a measure dependent on the central nervous 

 system, is not simply the result of an effort of that system. The 



