CHAP, ii.] THE BKAIH. 831 



cially 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 plethysinographic method can give 

 us indirect information only. 



522. 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 contin- 

 ually 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 speak- 

 ing 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 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 respira- 

 tory movements, and apparently phases of activity of the brain 

 itself, as in waking and sleeping ; undulations corresponding to 

 the Traube-Hering variations ( 315) of blood-pressure may 

 not unfrequently be observed. 



All the various methods show 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 ordi- 

 nary 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 posi- 

 tion, 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 re- 

 vealed 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 mechan- 

 isms 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. 



