SUBCLAVIAN ARTERIES. 



821 



foramina, and which are conducted by the 

 nerves to the spinal cord, where they join and 

 reinforce the proper spinal arteries. 



3. In the space between the atlas and occi- 

 pital bone, the vertebral artery sends several 

 long branches down the neck, which are con- 

 cealed by the splenius and complexus muscles. 

 Accompanied by smaller branches of the occi- 

 pital, these arteries keep up an important 

 anastomosis on the back of the neck with the 

 cervicalis profunda. 



4, 5. The proper spinal arteries. These 

 are two in number on each side, an anterior 

 and a posterior branch from each vertebral 

 trunk. 



The posterior spinal arteries arise lower 

 down than the anterior, and pass downwards 

 and backwards to reach the posterior surface 

 of the medulla oblongata ; from this the 

 artery of each side descends parallel to its 

 fellow, to which it is connected by numerous 

 transverse branches. Opposite the second 

 lumbar vertebra, the posterior spinal arteries 

 cease to exist as distinct trunks. 



The anterior spinal arteries are given off 

 from the vertebral near to its termination ; 

 these arteries in descending approximate to 

 each other, and at last unite opposite the 

 lower edge of the medulla oblongata ; the 

 single trunk thus formed (anterior median artery 

 f the spinal cord) descends tortuously in front 

 of the medulla spinalis, and, passing through 

 the very centre of the fibres of the cauda 

 equina, reaches the lowest portion of the 

 vertebral canal, when it anastomoses with 

 branches of the sacral arteries. 



The anterior and posterior spinal arteries 

 are connected in every region of the spine 

 with branches of arteries which enter the 

 spinal canal through the " foramina of con- 

 junction ;" these reinforcing branches, as they 

 may be termed, of the spinal arteries (derived 

 from the cervicalis ascendens and vertebral 

 in the neck, from the intercostal in the back, 

 and from the lumbar and sacral arteries in 

 the lower portion of the spinal column), 

 cause these arteries which they join to pre- 

 serve a remarkable uniformity of size through- 

 out their entire course. Both the spinal 

 arteries furnish small branches to the dura 

 mater and to the spinal cord. 



Branches of the basilar artery. 1. The 

 inferior (or posterior) ccrebcllar artery. 

 This artery seldom arises in precisely the 

 same manner at opposite sides of the same 

 subject. It most frequently springs on one 

 side from the vertebral, and on the other 

 from the basilar trunk. Inclining outwards 

 and backwards, in front of the pyramidal 

 body, the vessel in question passes (according 

 as its origin is from the basilar, or from the 

 vertebral artery) before or behind the sixth 

 nerve; it then runs through the filaments 

 going to form the ninth nerve, and between 

 the pneumogastric and spinal accessory divi- 

 sion of the eighth pair ; it is ultimately dis- 

 tributed internally to the inferior vermiform 

 process and sides of the median fissure (a 



branch or two may be traced into the choroid 

 plexus of the fourth ventricle), and externally 

 to the inferior surface and circumference of 

 the cerebellum, where it communicates with 

 the superior artery of the cerebellum. 



2. The superior (or anterior) cercbcllar 

 artery. This artery arises near the antcro- 

 superior edge of the pons varolii ; it passes in 

 a curved direction outwards and backwards, 

 around the line of junction of the pons with 

 the crus cerebri. It is at first parallel to the 

 posterior artery of the cerebrum, but separated 

 from it by the third nerve. The fourth nerve 

 in its trajet forwards is strictly parallel to the 

 artery as it runs backwards on the side of the 

 pons varolii ; the nerve, however, is contained 

 in a canal between the layers of the tentorium, 

 whilst the artery proceeds beneath that par- 

 tition, and in contact with the upper surface 

 of the cerebellum. 



The branches of the superior cerebellar 

 artery are numerous ; they are distributed to 

 the upper surface and circumference of the 

 cerebellum, anastomosing with the inferior 

 cerebellar artery, and also to the pons varolii, 

 velum interpositum, superior vermiform pro- 

 cess, and valve of Vieussens. One small 

 branch of this artery accompanies and separates 

 the facial and auditory nerves, entering with 

 them into the internal auditory meatus. 

 Lastly, some of its branches pass on the upper 

 surface of the tentorium, and are distributed 

 to the inferior surface of the cerebrum. 



3. Posterior artery of the cerebrum. The 

 two posterior arteries of the cerebrum are 

 the terminating branches of the basilar trunk ; 

 each artery passes at first forwards, then 

 backwards and outwards, following the course 

 of the great cerebral fissure, and partly en- 

 circling the crus cerebri. For a considerable 

 portion of its course the posterior artery of 

 the cerebrum is parallel to the posterior (or 

 inferior) cerebellar artery; the two arteries, 

 however, are separated from one another, at 

 first, by the third nerve (which latter, in its 

 further course, hooks round the posterior 

 artery of the cerebrum), and subsequently by 

 the tentorium. At the point where the 

 posterior artery of the cerebrum changes its 

 direction in order to pass backwards and 

 outwards, it is joined by the " posterior com- 

 municating artery," and by this means a com- 

 munication is established between the internal 

 carotid and basilar arteries. 



The posterior artery of the cerebrum is 

 chiefly distributed by long slender branches 

 to the inferior surface of the posterior lobe of 

 the cerebrum, but it furnishes, in addition, 

 the following collateral branches:!. Nume- 

 rous small twigs which enter the floor of the 

 third ventricle, through the apertures in the 

 locus perforatus medius, or are distributed to 

 the crura cerebri, corpora albicantia, and 

 tuber cenereum ; and 2. a choroid branch, which 

 winds round the cms cerebri, enters the 

 cerebral fissure, and is lost in the velum inter- 

 positum, corpora qnaclrigemina, and choroid 



plexuses. 



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