348 VASO-MOTOR CENTRE. [BOOK i. 



the central nervous system is removed or in any way placed hors 

 de combat, another part may vicariously take on its function ; in 

 the absence of the bulbar vaso-motor centre, its function may be 

 performed by other parts of the spinal cord which in its presence 

 do no such work. 



And we may, in connection with this, call attention to the fact 

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

 cervical sympathetic (and the same is true of the splanchnic) 

 is not always, though it may be sometimes, permanent ; in a 

 certain number of cases it has been found that after a while, it 

 may be not until after several days, the dilation disappears and 

 the arteries regain their calibre ; on the other hand in some cases 

 no such return has been observed after months or even years. 

 When recovery of tone has thus taken place, dilation or increased 

 constriction may be occasioned by local treatment : 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. Moreover, a similar 

 recovery is stated to have been observed not only after simple 

 section of the cervical sympathetic but even when the superior 

 cervical ganglion has been removed. From this ganglion as we 

 have seen ( 169) the vaso-constrictor fibres start afresh, as from 

 a new centre ; and it might be supposed, that the fibres, when 

 cut adrift from the spinal cord by the section of the cervical 

 sympathetic, were governed by this ganglion as by a functionally 

 active centre. But if the experiment be trusted, this is not the 

 case. So also the spontaneous rhythmic variations in the calibre of 

 the arteries of the ear of which we spoke in 164 though they cease 

 for a time after division of the cervical sympathetic, may in some 

 cases eventually reappear, and that even if the superior cervical 

 ganglion be removed ; in other cases they do not. And the 

 analogous rhythmic variations of the veins of the bat's wing have 

 been proved experimentally to go on vigorously when all con- 

 nection with the central nervous system has been severed; they 

 may continue in fact in isolated pieces of the wing provided that 

 the vessels are adequately filled and distended with blood or fluid. 

 From these and other facts, even after making allowance for the 

 negative cases, we may conclude 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 muscular walls of the 

 arteries are not mere passive instruments worked by the central 

 nervous system through the vaso-motor fibres ; they appear to have 

 an intrinsic tone of their own, and it seems natural to suppose 

 that when the central nervous system causes dilation or constric- 

 tion of the vessels of the face, it makes use, in so doing, of this 

 intrinsic local tone. 



