CHAP, iv.] THE VASCULAR MECHANISM. 349 



We may add that if we accept the view that the widening of 

 the blood vessels which accompanies muscular contraction, is due not 

 to the advent of impulses from the central nervous system but to 

 the changes in the tissue itself acting directly on the blood vessels, 

 we may regard such an event as another indication of the peri- 

 pheral blood vessels being able to change their condition apart from 

 the interference of the central nervous system. And as we have 

 said, it has been maintained that the vascular change accompanying 

 functional activity in organs other than the muscles may be 

 similarly explained. 



It has been supposed that the intrinsic tone of which we are 

 speaking is dependent on some local nervous mechanism, on peri- 

 pheral ganglia for instance ; in the ear at least no such mechan- 

 ism has yet been found ; and indeed, as we have already urged, it 

 does not seem necessary to appeal to any such special peripheral 

 nervous mechanism. In the case both of a vessel governed by 

 vaso-dilator fibres and of one governed by vaso- constrictor fibres, 

 we may suppose a certain natural condition 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 condition 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 constric- 

 tion even when separated from the central nervous system. And 

 apart from the influences of the central nervous system the equili- 

 brium may be disturbed by the changes going on in the tissue 

 itself in which the blood vessels lie. 



But to return to the bulbar 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 spinal bulb 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 i 

 centre, whence alone can issue tonic constrictor impulses or 

 whither afferent impulses from this or that part of the body must 

 always travel before they can affect the vaso-constrictor impulses 

 passing along this or that nerve. We are rather to suppose 

 that the spinal cord along "its whole length contains, interlaced 

 with the reflex and other mechanisms by which the skeletal 

 muscles are governed, vaso-motor centres and mechanisms of varied 

 complexity, the details of whose functions and topography have yet 



