832 THE CIRCULATION IN THE BEAIK [BOOK in. 



Seeing that the cerebral arteries have well-developed muscu- 

 lar 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 observations, 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 investi- 

 gating 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 



freater flow through the kidney according as the vessels of the 

 idney 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 in- 

 creased 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 coincident 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-constric- 

 tion 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 ( 151) less blood 

 necessarily flows along the cerebral vessels. And indeed one 

 may recognize here a sort of self-regulating action ; for dimin- 

 ishing 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, in- 

 cluding the bulb, and thus the injurious effects to the brain 

 threatened by an anaemic condition are warded off by the very 



