736 THE BRAIN. 



be corrected by a simultaneous observation of the general arterial pressure 

 and of the blood-pressure in the veins of the neck. Moreover, the argument 

 which we used ( 353) in reference to the kidney may be applied here and 

 probably with equal force, namely, that the value of the blood stream for 

 the nutrition of the tissue is dependent not alone on the amount of blood- 

 pressure but also and especially on the rapidity of the flow; indeed, this 

 second factor is of particular importance in view of the need of supplying 

 the nervous elements with an adequate interchange of gases. Now of the 

 rapidity of flow the plethysmographic method can give us indirect informa- 

 tion only. 



612. By one or other or all of these methods certain important facts 

 have been made out. The volume of the brain as determined by the amount 

 of blood present in it, is continuously undergoing changes brought about by 

 various causes. Each heart-beat makes itself visible on the cerebral as on 

 the renal plethysmographic tracing, and as we have seen in speaking of res- 

 piration, the diminution of pressure in the great veins of the neck during in- 

 spiration leads to a shrinking, and the reverse change during expiration to 

 a swelling of the brain. The plethysrnograph also shows variations, larger 

 and slower than the respiratory undulations, and brought about by various 

 causes, such as the position of the head in relation to the trunk, movements 

 of the limbs, modifications of the respiratory movements, and apparently 

 phases of activity of the brain itself, as in waking and sleeping ; undulations 

 corresponding to the Traube-Hering variations ( 330) of blood-pressure may 

 not unfrequently be observed. 



All the various methods show that the flow through the brain is largely 

 determined by a vasomotor action of some kind or another. And this we 

 might indeed infer from ordinary experience. When the head is suddenly 

 shifted from the erect to a hanging position, there must be a tendency for 

 the blood to accumulate in the cranial cavity, and conversely when the head 

 is suddenly shifted from a hanging to an erect position, there must be a 

 tendency for the supply of blood within the cranium to be for a while less 

 than normal. Either change of position, and especially perhaps the latter, 

 would lead to cerebral disturbances, which in turn would in ourselves be 

 revealed by affections of our consciousness. That a perfectly healthy and 

 especially young organism, whose vasomotor mechanisms are at once effective 

 and delicately responsive, can pass swiftly from one position of the head to 

 the other without inconvenience, whereas those in whom the vasomotor 

 mechanisms have by age or otherwise become imperfect are giddy when 

 they attempt such rapid changes, is in itself adequate evidence of the im- 

 portance of the vasomotor arrangements affecting the circulation through 

 the brain. The several methods agree in showing that increased general 

 arterial pressure, such as that, for instance, induced by stimulation of a 

 sensory nerve, leads to a greater flow of blood to the brain ; the volume of 

 the brain is increased arid the venous outflow by the lateral sinus is quick- 

 ened. Conversely, a lowering of arterial pressure leads to a lessened flow 

 of blood to the brain. 



Seeing that the cerebral arteries have well-developed muscular coats, the 

 basilar artery in fact being conspicuous in this respect, one would be led to 

 suppose that the brain possessed special vasomotor nerves of its own ; and 

 recognizing the importance of blood-supply to rapid functional activity one 

 would perhaps anticipate that by special vasomotor 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 vasomotor nerves or fibres directly governing cerebral 



