CHAP, iv.] THE VASCULAR MECHANISM. 329 



the ear of which we spoke, 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 re- 

 moved ; in other cases they do not. And the analogous rhythmic 

 variations of the veins of the bat's wing have been proved experi- 

 mentally 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 provided that the vessels are ade- 

 quately 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 depend- 

 ent 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 constriction of the vessels 

 of the face, it makes use, in so doing, of this intrinsic local tone. 

 It has been supposed that this intrinsic tone is dependent on some 

 local nervous mechanism ; in the ear at least no such mechanism 

 has yet been found ; and indeed, as we said above, 167, no such 

 peripheral nervous mechanism is really necessary. In the case both 

 of a vessel governed by vaso-dilator fibres and one governed by 

 vaso-constrictor fibres, we may suppose a certain natural con- 

 dition of the muscular fibres which we may call a condition of 

 equilibrium. In a vessel governed only by vaso-dilator fibres, if 

 there be such, this condition of equilibrium is the permanent con- 

 dition of the muscular fibre, from which it is disturbed by vaso- 

 dilator impulses, but to which it speedily returns. In a vessel 

 governed by vaso-constrictor fibres, and subject to tone, the 

 muscular fibre is habitually kept on the constrictor side of this 

 equilibrium, and, as in the cases quoted above, may strive of 

 itself towards some amount of active constriction even when 

 separated from the central nervous system. 



But to return to the medullary vaso-motor centre. Without 

 attempting to discuss the matter fully we may say that, after all 

 due weight has been attached to the play of inhibitory impulses 

 or ' shock ' as the result of operative interference, there still 

 remains a balance of evidence in favour of the view that the 

 region of the medulla of which we are speaking does really act 

 as a general vaso-motor centre in the manner previously explained, 

 and plays an important part in the vaso-motor regulation of 

 the living body. 



It is not however to be regarded as the single vaso-motor 

 centre, whence alone can issue tonic constrictor impulses or 

 whither afferent impulses from all parts of the body must 

 always travel before they can affect the vaso-motor impulses 



