1134 THE VENOUS SINUSES. [BOOK m. 



valves. This arrangement is correlated to the peculiar surroundings 

 of the brain, which is not like other organs protected merely by 

 skin or other extensible and elastic tissue, but is encased by a 

 fairly complete inextensible envelope, the skull. As a conse- 

 quence 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, los of consciousness, or 

 death. Some room may, as we have seen- ( 696), be provided by 

 the escape of cerebro-spinal fluid from the skull. But, within the 

 limits of the normal cerebral circulation, the characteristic venous 

 sinuses especially 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 low or 

 negative pressure obtaining in the veins of the neck at each 

 inspiratory movement of the chest. 



699. The supply of blood to the brain seems at first sight 

 not to correspond to the importance of this the chief organ of the 

 body. In the rabbit it would appear that hardly more than one 

 per cent, of the total quantity of the blood of the body is present 

 at any one time in the brain, a quantity but little more than half 

 that which is found in the kidneys ; and while the weight of blood 

 in the brain at any one time amounts to about five per cent, of the 

 total weight of the organ, being about the same as in the muscles, 

 in the kidney it amounts to nearly twelve per cent., and in the liver 

 to as much as nearly thirty per cent. Making every allowance for 

 the relative small size and functional importance of the rabbit's brain, 

 the blood-supply of even the human brain must still be small ; and 

 making every allowance for rapidity of current, the interchange 

 between the blood and the nervous elements must also be small. 

 In other words, the metabolism of the brain-substance is of im- 

 portance not so much on account of its quantity as of its special 

 qualities. 



The circulation in the brain may be studied by help of various 

 methods. A manometer may be connected with the peripheral 

 end of the divided internal carotid artery, a second manometer 

 being attached in the usual way to the central portion. Since the 

 peripheral manometer records the blood-pressure in the circle of 

 Willis transmitted along the peripheral portion of the carotid 

 artery, variations of pressure in the circle of Willis may thus be 

 studied ; and a comparison of the peripheral with the central 

 manometer will indicate what general changes are taking place 



