1132 THE ARTERIES OF THE BRAIN. [BOOK in. 



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 ventricle 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 directions 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 coming off from the top of the aorta, offers a more straight 

 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 ante- 

 rior, are distributed almost exclusively to the cortex and to the sub- 

 jacent 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 systems 

 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 supplying 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 distribution of these arteries 



