328 VASO-MOTOR CENTRE. .[Boom. 



time after the operation the vaso-dilator nervi erigentes (which as 

 far as we know have no special connection with the medullary 

 vaso-motor centre) cannot be thrown into activity as part of a 

 reflex action ; their centre remains for some time inactive. After a 

 while however it recovers, and erection of the penis through the 

 nervi erigentes may then still be brought about by suitable stimula- 

 tion of sensory surfaces. Hence the question may fairly be put 

 whether the effects of cutting and injuring the structures which 

 we have spoken of as the medullary vaso-motor centre, are not in 

 reality simply those of shock, whether the vascular dilation which 

 follows upon sections of the so-called medullary vaso-motor centre, 

 does not come about because section of or injury to this region 

 exercises a strong inhibitory influence on all the vaso-motor centres 

 situated in the spinal cord below. Owing to the special function 

 of the medulla oblongata in carrying on the all-important work of 

 respiration, a mammal 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 extirpating the supposed 

 medullary vaso-motor 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 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 abdominal 

 splanchnic) is not always, though it may be sometimes, per- 

 manent; 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 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 regeneration of vaso-motor fibres in 

 the sympathetic, for it is stated to have been observed when the 

 whole length of the nerve including the superior cervical ganglion 

 had been 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 



