1136 THE CIRCULATION IN THE BRAIN. [BOOK m. 



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 plethysmo- 

 graphic method can give us indirect information only. 



700. 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 continually 

 undergoing changes brought about by various causes. Each heart- 

 beat makes itself visible on the cerebral as on the renal plethys- 

 mographic tracing, and as we have seen in speaking of respiration,, 

 the diminution of pressure in the great veins of the neck during 

 inspiration leads to a shrinking, and the reverse change during 

 expiration to a swelling of the brain. The plethysmograph also 

 shews variations, larger and slower than the respiratory undu- 

 lations, 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 ; undu- 

 lations corresponding to the Traube-Hering variations ( 387) of 

 blood-pressure may not unfrequently be observed. 



All the various methods shew that the flow through the brain 

 is largely determined by a vaso-motor action of some kind or 

 another. And this we might indeed infer from ordinary expe- 

 rience. 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 disturb- 

 ances, which in turn would in ourselves be revealed by affections 

 of our consciousness. That a perfectly healthy, and especially 

 young organism whose vaso-motor 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 vaso-motor mechanisms have by age or otherwise 

 become imperfect are giddy when they attempt such rapid 

 changes, is in itself adequate evidence of the importance of the 

 vaso-motor arrangements affecting the circulation through the 

 brain. The several methods agree in shewing 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 and the venous 

 outflow by the lateral sinus is quickened. 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 



