228 THE VASCULAR MECHANISM. 



whose medulla has been divided cannot be kept alive for any length of time. 

 We cannot, therefore, put the matter to the simple experimental test of extir- 

 pating the supposed medullary vasomotor centre and seeing what happens 

 when the animal has completely recovered from the effects of the operation ; 

 we have to be guided in our decision by more or less indirect arguments. 

 And against the argument that the effects are those of shock, we may put 

 the argument, evidence for which we shall meet with in dealing with the 

 central nervous system, that when one part of 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 medullary vasomotor 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 

 dilatation or loss of tone which follows upon section of the cervical sympathetic 

 (and the same is true of the abdominal 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 not be until after several days, the dilatation 

 disappears and the arteries regain their usual calibre ; on the other hand, in 

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

 This recovery, when it occurs, cannot always be attributed to any regenera- 

 tion of vasomotor fibres in the sympathetic, for it is stated to have been 

 observed when the whole length of the nerve including the superior cervical 

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

 dilatation or increased constriction may be occasioned by local treatment ; the 

 ear may be made to blush or 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 speak, though they cease for a time 

 after division of the cervical sympathetic, may in some cases eventually reap- 

 pear 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 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 

 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 vasomotor fibres ; they appear to have an intrinsic tone 

 of their own, and it seems natural to suppose that when the central nervous 

 system causes dilatation 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 have said 

 above ( 176), 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 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 muscu- 



