SEC. 9. THE VASCULAE AEEANGEMENTS OF THE 

 BEAIN AND SPINAL COED. 



519. In the brain two important features of the distribu- 

 tion of the arteries deserve special attention. In the first place, 

 the quadruple supply by the right and left vertebral and internal 

 carotid arteries is made one by remarkable anastomoses forming 

 the circle of Willis. Blood can pass along this circle in various 

 ways ; from the basilar artery along the right posterior commu- 

 nicating 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 radi- 

 ating 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 ante- 

 rior 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 

 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 



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