828 THE VENOUS SINUSES. [Boon in. 



coming direct from the circle of Willis, or from the very begin- 

 nings of the three cerebral arteries. It is stated that these two 

 systems make no anastomoses with each other ; but different in- 

 dividuals in respect to this appear to vary much. We may add 

 that the anterior cerebral artery supplies the cortex of the dorsal 

 aspect of the frontal lobe as well as the front and middle portions 

 of the whole mesial surface of the hemisphere; while the middle 

 cerebral, always large, is distributed to the side of the brain, that 

 is, the parietal lobe, with the ventral part of the frontal lobe and 

 the dorsal part of the temporal lobe ; the posterior cerebral sup- 

 plying the rest of the cortex, that is to say, the occipital lobe 

 including the hind part of the mesial surface of hemisphere, 

 together with the ventral part of the temporal lobe. The dis- 

 tribution of these arteries therefore does not correspond to 

 functional divisions, for while the middle cerebral supplies a 

 large part of the motor region, it does not supply the whole 

 of it, and does supply parts outside it. Though the small 

 arteries as they run in the pia mater on the surface of the cortex 

 anastomose freely, there is very little anastomosis between the 

 small arteries which leaving the pia mater dip down into the 

 substance of the brain ; hence when these latter arteries are 

 blocked, the nutrition of the part of the cortex supplied by 

 them is apt to be impaired. 



520. The venous arrangements of the brain have very 

 special characters. 



The channels for the venous blood of the brain are not 

 veins but sinuses, not so much tubes for maintaining a uniform 

 current as longitudinal reservoirs, which while affording an 

 easy onward path can also be easily filled and easily emptied, 

 and in which the blood can move to and fro without the 

 restrictions of 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 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. Some 

 room may, as we have seen ( 518), 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. The injurious effects of too 



