CH. xxi.] CIRCULATION IN THE BRAIN. 279 



Is in the most dependent parts of the body. The flow of 

 blood through the brain will, under these conditions, cease, that 

 is to saj, so long as the body is in the erect posture. Thus, 

 to restore a fainting person the head must be lowered between 

 the knees. Muscular exercise, by returning blood to the heart 

 from the veins of the lower parts of the body, conduces to the 

 maintenance of an efficient cerebral circulation. 



It is not the volume of the blood but the velocity of flow 

 which is altered in the brain by changes in the general circu- 

 lation. The brain with its circulating blood almost entirely fills 

 the cranial cavity in the living animal ; that is, there is no more 

 cerebro-spinal fluid there than is sufficient to moisten the mem- 

 branes. Cerebro-spinal fluid escapes into the veins at any 

 pressure above the cerebral venous pressure ; the tension of this 

 fluid and the pressure in the veins are therefore always the same. 

 The fluid probably transudes from the vascular fringes of the 

 choroid plexuses in the ventricles of the brain, and is absorbed 

 by the pial veins. There is not enough of this absorbable fluid 

 present to allow of more than a slight increase of the volume 

 of blood in the brain. If the aortic pressure rises and the vena 

 cava pressure remains constant the conditions in the brain are as 

 follows : 



More blood in the arteries, less in the veins, increased velocity 

 of flow. 



While if the aortic pressure remains constant and the vena cava 

 pressure rises, the conditions are : 



Less blood in the arteries, more in the veins, diminished 

 velocity of flow. 



The brain presses against the cranial wall with a pressure 

 equal to that in the cerebral capillaries. A foreign body intro- 

 duced within the cranium, such as a blood-clot or depressed bone, 

 produces local aneemia of the brain, by occupying the room of the 

 blood. So soon as the capillaries are thus obliterated the pressure is 

 raised to arterial pressure. This local increase of cerebral tension 

 cannot be transmitted by the cerebro-spinal fluid, because this 

 fluid can never be retained in the meningeal spaces at a tension 

 higher than that of the cerebral veins, but is immediately 

 re-absorbed. The anatomical arrangements of the tentorium 

 cerebelli and the falciform ligaments are such as to largely 

 prevent the transmission through the brain-substance of a local 

 increase of pressure. There is complete pressure discontinuity 

 between the cranial and vertebral cavities. The serious results 

 that follow cerebral compression are primarily due to obliteration 

 of the blood-vessels, and consequent anaemia of the brain. A very 



