SEC. 13. THE VASCULAR ARRANGEMENTS OF T1IK 

 BRAIN AND SPINAL CORD. 



697. The blood vessels reach the nervous structures by 

 means of the pia mater. In the spinal cord arteries coming from 

 the vertebral, intercostal and other arteries, and travelling along 

 the nerve roots join the pia mater, and then through the fissures 

 and septa reach all parts of the cord ; but as we have previously 

 remarked the capillary network is much denser, and therefore the 

 blood supply much greater in the grey than in the white matter. 

 The veins, also gathered up along the septa and fissures into the 

 pia rnater, those coming from the grey matter forming, before they 

 reach the external pia mater, a conspicuous longitudinal vein on 

 each side of the posterior grey commissure, pass from the pia 

 mater to the large venous sinuses of the dura mater and so to 

 adjoining veins. 



In the brain two important features of the distribution of the 

 arteries deserve special attention. In the first place, the quad- 

 ruple supply by the right and left vertebral and internal carotid 

 arteries is made one by remarkable anastomoses forming the circle 

 of Willis. The right and left vertebral arteries entering the 

 vertebral canal at the level of the 6th cervical vertebra, and 

 running forwards towards the brain, join beneath the ventral 

 surface of the bulb to form the single median basilar artery. 

 This, after giving off branches to the bulb, cerebellum, and pons 

 divides into the right and left posterior cerebral arteries. Each 

 internal carotid entering the skull reaches the base of the brain 

 in the region of the floor of the third ventricle, and, passing 

 ventral to and athwart the optic tract, gives off the large and 

 important middle cerebral artery along the fissure of Sylvius, and 

 then, turning forwards and towards the median line, passes dorsal 

 to the optic nerve to end in the anterior cerebral artery. Just 

 however as it gives off the middle artery, it sends backward.-, 

 inclining to the middle line, a relatively large branch, the posterior 

 communicating artery, which joins the posterior cerebral 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 forwards along the frontal 



