CHAP, ii.] THE BRAIN. 1137 



coats, the basilar artery in fact being conspicuous in this respect, 

 one would be led to suppose that the brain possessed special 

 vaso-motor nerves of its own ; and recognising the importance of 

 blood supply to rapid functional activity one would perhaps 

 anticipate that by special vaso-motor action, the supply of blood 

 to this or that particular part of the brain might be regulated 

 apart from changes in the general supply. The various obser- 

 vations, however, which have hitherto been made have failed to 

 demonstrate with certainty any such special vaso-motor nerves or 

 fibres directly governing cerebral vessels. It would be hazardous 

 to insist too much on this negative result, especially since the 

 observations have been chiefly directed to the nerves of the neck, 

 the experimental difficulties of investigating the presence of vaso- 

 motor fibres in the cranial nerves being very great. Still it may 

 be urged and indeed has been urged that the flow of blood 

 through the brain is so delicately responsive to the working of 

 the general vaso-motor mechanism just because it has no vaso- 

 motor nerves of its own. In such an organ as the kidney, an 

 increase of general blood-pressure, as we have more than once 

 insisted, may or may not lead to a greater flow through the kidney 

 according as the vessels of the kidney itself, through the action of 

 the renal vaso-motor nerves, are dilated or constricted ; and, as we 

 have seen, a constriction of the renal vessels may be one of the 

 contributors to the increased general pressure. In the brain, on 

 the other hand, an increase of general arterial pressure seems 

 always to lead to increase of flow. Thus in the Traube-Hering 

 undulations just mentioned, the expansions of the brain are coinci- 

 dent with the rises of the general pressure, whereas in the normal 

 kidney and in other organs the local Traube-Hering undulation 

 reverses the general one, the shrinkings are synchronous with the 

 rises of pressure, the local constriction being one of the factors of 

 the general rise. It is argued, that in the absence of vaso-motor 

 nerves of their own, the cerebral vessels are wholly, so to speak, in 

 the hands of the general vaso-motor system, so that when the 

 blood- pressure is high owing to a large vaso-constriction in the 

 abdominal viscera, more blood must necessarily pass to the brain, 

 and when again the blood-pressure falls through the opening of 

 the splanchnic flood-gates ( 173) less blood necessarily flows along 

 the cerebral vessels. And indeed one may recognize here a sort 

 of self-regulating action ; for diminishing the supply of blood to 

 the vaso-motor centre in the bulb acts, as we know, as a powerful 

 stimulus in producing vaso-constriction, and so leads to a rise of 

 blood-pressure ; but this very rise of blood-pressure drives more 

 blood to the brain, including the bulb, and thus the injurious 

 effects to the brain threatened by an anaemic condition are 

 warded off by the very beginning of the anaemia itself. All these 

 advantages are, however, quite compatible with the coexistence of 

 special vaso-motor mechanisms. 



