THE VASCULAR ARRANGEMENTS OF THE BRAIN. 733 



branch, the posterior communicating artery, which joins the posterior cere- 

 bral near the origin of this from the basilar artery. Moreover, the two 

 anterior cerebral arteries, soon after they have crossed the optic nerves, just 

 as they are about to run straight forward along the frontal lobes, are joined 

 together by a short, wide branch, the anterior communicating artery. In 

 this way the vertebral arteries through the basilar artery join with the carotid 

 arteries to form around the optic chiasma beneath the floor of the third ven- 

 tricle an arterial circle, the circle of Willis. 



Blood can pass along this circle in various ways from the basilar artery 

 along the right posterior communicating artery to the right internal carotid, 

 and so by the right anterior cerebral artery and anterior communicating 

 artery to the left side of the circle, and similarly from the basilar artery 

 along the left side to the right, or from the right or from the left carotid 

 through the circle, to the right hand or to the left hand in each case. Since 

 the channel of the circle is a fairly wide one, the passage in various direc- 

 tions is an easy one ; all the vessels radiating from the circle, including the 

 basilar artery and its branches, can be supplied by the carotids alone, or by 

 the vertebrals alone, or even by one carotid or one vertebral alone. In this 

 way an ample supply of blood to the brain is secured in the face of any 

 hindrance to the flow of blood along any one of the four channels. 



In what may, perhaps, be considered the usual arrangement, the calibre 

 of the posterior communicating arteries is rather smaller than the other parts 

 of the circle, so that, other things being equal, most of the vertebral blood 

 will pass by the posterior cerebral arteries, while the carotid blood passes to 

 the middle and anterior cerebral arteries ; but many variations are met with. 

 We may also here, perhaps, call to mind the fact that the left carotid com- 

 ing off from the top of the aorta offers a shorter path for the blood than 

 does the right carotid which comes off from the innominate artery. 



Another special feature of the arterial supply to the brain is that the 

 three large cerebral arteries posterior, middle, and anterior are distrib- 

 uted almost exclusively to the cortex and to the subjacent white matter, 

 while the deeper parts of the hemisphere, the nucleus caudatus, thalamus, 

 and the like, with the capsule and other adjoining white matter, are supplied 

 by smaller arteries coming direct from the circle of Willis, or from the very 

 beginnings of the three cerebral arteries. It is stated that these two sys- 

 tems make no anastomoses with each other ; but this appears to vary much 

 in different individuals. 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 the hemisphere, together with the ventral 

 part of the temporal lobe. The distribution 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 of 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. 



610. The venous arrangements of the brain have very special cha- 

 racters. 



