486 



NEUBOLOGT, 



AETERIAL CIRCULATION. 



Four large arteries, two internal carotids, and two cerebro- 

 spinal branches of the occipital arteries, convey the blood to the 

 encephalon. An examination of the tortuous course of, these 

 arteries shows how beautifully Nature l^as provided against injury 

 to the delicate brain-structure, by the blood impinging with too 

 great a force against its base. 



The Cerebro-spinal arteries enter the neural canal through 

 the internal foramina in the wings of the atlas, and wind round 

 the cord to its inferior surface, where they each divide into two 

 branches, one of which, the anterior or cerebral branch, passes 

 forwards and joins its fellow near the middle of the medulla 



oblongata, to form the basi- 

 lar artery. The ^posterior 

 branches pass backwards, 

 and also coalesce to form 

 the middle spinal artery, 

 which runs along the longi- 

 tudinal fissure of the cord. 

 The Basilar artery is 

 continued forwards in the 

 longitudinal furrow on the 

 inferior surface of the pons 

 Varolii, near the anterior 

 border of which it divides 

 into two branches, the pos- 

 terior cerebrals. In its course the basilar artery gives off — Ist, 

 Several small branches, which penetrate the surface of the meduDa 

 oblongata and pons Varolii ; 2nd, The posterior cerebellar s, right 

 and left, which, like the terminal branches, are given off at right 

 angles behind the posterior border of the pons. They are carried 

 outwards, ascending over the pons, and then/ inflected backwards, 

 are distributed over the lateral and posterior parts of the cere- 

 bellum. 2rd, The anterior cerebellar arteries, two or three on 

 each side, the principal being given off just in front of the pons, 

 on the outer side of which it ascends in an outward and slightly 

 backward direction, to be distributed to the anterior surface of 

 the cerebellum. 



The terminal branches, the Posterior cerebral arteries, are 

 the largest, and form the posterior border of that peculiar arterial 



Fio. 188. 



The encepludlc arteries, a. Anterior branch of the 

 right, and a', of the left carotid arteries ; h V, Posterior 

 communicating branches ; c, Inosculation of the an- 

 terior branches ; d d. Inosculation of i>osterior com- 

 municating, aqd posterior cerebral branches of basilar 

 arter7 ; e, Basilar trunk. 



