646 THE CIRCULATION IN THE BRAIN. [BOOK m. 



In certain respects the circulation in the brain is peculiar. 

 The skull forms a fairly complete inextensible envelope, presenting 

 a strong contrast to the extensible elastic capsules which invest 

 such organs as the spleen and kidney. As a consequence of this, 

 when at any time an extra quantity of blood is sent from the 

 heart to the brain, room must be made for it by the increased exit 

 of the fluids already present. For any pressure on the brain- 

 substance beyond a certain limit is injurious to its welfare and 

 activity, as is seen in certain maladies, where blood passing by 

 rupture of the blood-vessels out of its normal channels remains 

 effused on the surface of the brain or elsewhere, and thus taking 

 up the room of the proper brain-substance leads, by ' compression ' 

 as it is called, to paralysis, loss of consciousness, or death. Within 

 the limits of the normal cerebral circulation, the characteristic venous 

 sinuses serve to regulate the internal pressure ; they form temporary 

 reservoirs from which a comparatively large quantity of blood can 

 be rapidly discharged from the cranium, the flow from the sinuses 

 being greatly assisted by the inspiratory movements of the chest. 



The arterial supply of the brain as a whole is undoubtedly 

 regulated by vaso-motor nerves, and in all probability the special 

 distribution of blood to the various parts of the brain is determined 

 by the same agents. 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 thus lead to cerebral disturbances 

 which would in ourselves be revealed by affections of our con- 

 sciousness. 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 by age or otherwise have become imperfect are 

 giddy when they attempt such rapid changes, is in itself adequate 

 evidence of the importance of the vaso-motor arrangements of the 

 brain. But our information concerning this matter, is at present of 

 a very vague and general character. As yet we have no detailed 

 knowledge, and are especially ignorant as to how far special 

 parts of the brain are supplied with independent vaso-motor 

 mechanisms. 



Many writers have insisted on the mechanical importance of 

 the cerebro-spinal fluid. By the foramen of Majendie at the apex 

 of the roof of the fourth ventricle, the fluid within the various 

 ventricular cavities of the brain is continuous with the fluid in the 

 subarachnoid labyrinth of the spinal cord. And it has been argued 

 that when an extra quantity of blood is driven into the skull, the 

 transference of a corresponding quantity of cerebro-spinal fluid 



