CHAP, ii.] THE BRAIN. 829 



great a pressure on the brain-substance are seen in certain mal- 

 adies, where blood passing by rupture of the blood vessels out 

 of its normal channels remains effused on the surface of the 

 brain or elsewhere, arid thus taking up the room of the proper 

 brain-substance leads, by ' compression ' as it is called, to paral- 

 ysis, loss of consciousness, or death. They are also shewn by 

 experiments on animals. When by driving an excess of fluid 

 into the subdural cavity through a hole in the cranium or by 

 other means a certain amount of pressure is established in the 

 cranial cavity both the respiration and the circulation are 

 affected. The breathing is slowed and eventually arrested, but 

 may in certain cases be quickened. The heart is slowed by 

 vagus inhibition, and a rise of blood pressure due to vaso-con- 

 striction is observed unless the slowing of the heart be sufficient 

 to neutralize this. These phenomena point of course especially 

 to an influence exerted on the spinal bulb; but besides these 

 changes in the pupil and other effects are met with. 



521. 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 pres- 

 ent 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 func- 

 tional importance of the rabbit's brain, the blood-supply of even 

 the human brain must still be small ; and making every allow- 

 ance 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 importance 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 manom- 

 eter 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 in the circulation through the brain. Thus a fall 

 of pressure in the peripheral manometer unaccompanied by any 

 corresponding fall in the central manometer would shew that 

 the " peripheral resistance " in the brain was being lowered, in 

 other words, that the vessels were being dilated. 



