370 SUBCLAVIAN ARTERIES. 



fourth, or more rarely of the third vertebra, or even, according to several anatomists, 

 of the second. On the other hand, the vertebral artery has been seen to enter the 

 foramen of the seventh vertebra. 



Branches. In. the neck, the vertebral artery has been found, though very rarely, 

 to give branches which are usually derived from the subclavian, such as the superior 

 intercostal and the inferior thyroid. 



BASILAR AND POSTERIOR CEREBRAL ARTERIES. 



The basilar artery, the single trunk formed by the junction of the right 

 and left vertebral in the middle line, extends from the posterior to the 

 anterior border of the pons Varolii, along the median groove of which it lies 

 under cover of the arachnoid. The length of this artery is therefore about 

 equal to that of the pons, at the anterior border of which it divides into 

 two terminal branches, the posterior arteries of the cerebrum. 



BRANCHES. Besides numerous small branches to the substance of the 

 pons, the basilar artery gives off the following : 



(a) The transverse arteries, several on each side, pass directly outwards. One, the 

 artery of the acoustic nerve, accompanies that nerve into the internal auditory 

 meatus and labyrinth of the ear. 



(6) The anterior inferior cerebellar arteries pass backwards, one on each side, to 

 the anterior part of the under surface of the cerebellum, anastomosing with the 

 inferior cerebellar branches of the vertebral arteries. 



(c) The superior cerebellar arteries arise so close to the bifurcation of the basilar, 

 that this artery is described by several anatomists as dividing into four branches. 

 Each one turns backwards and outwards immediately behind the third nerve, and, 

 entering the groove between the pons Yarolii and the crus cerebri, turns round the 

 latter, close to the fourth nerve, to reach the upper surface of the cerebellum, where 

 it divides into branches. Of these some extend outwards, and one or more back- 

 wards along the superior vermiform process, to reach the circumference of the cere- 

 bellum, where they anastomose with the branches of the inferior cerebellar arteries ; 

 while others run inwards to supply the vermiform process and the valve of Vieussens, 

 and in part the velum interpositum. 



The posterior cerebral artery on each side, resulting from trie division of 

 the basilar, passes outwards, parallel to the superior cerebellar artery, and 

 separated from it at its origin by the third nerve, which comes forwards be- 

 tween the two vessels. It turns backwards round the crus cerebri, and then 

 runs outwards and upwards on the under surface of the posterior lobe of the 

 cerebrum, passing near the posterior extremity of the corpus callosum. It 

 divides beneath the posterior lobe into many branches, which, ramify upon 

 the under, median, and outer surfaces, and anastomose with tlie other 

 cerebral arteries. 



BRANCHES. Immediately after its origin the posterior cerebral artery gives off 

 numerous small parallel branches, which perforate the substance of the brain between 

 the crura, at the point which is called from this circumstance the posterior perforated 

 spot. As it turns backwards, a short distance from its origin, this artery is joined 

 by the posterior communicating artery, and in this way contributes as already 

 described (p. 363) to form the circle of Willis. Lastly, the posterior cerebral gives 

 origin to a small branch, the posterior choroid, which, arising external to the junction 

 of the communicating artery, turns backwards over the crus cerebri and the tubcr- 

 cula quadrigemina, supplying these with branches, and ending in the velum inter- 

 positum and choroid plexus in the interior of the brain. 



PECULIARITIES. Traces of a septum are sometimes found in the interior of the 

 basilar artery. (Davy, " Researches," &c., vol. i. p. 301.) This trunk has also been 

 found perforated by a small foramen, owing to a partial fissuring of the vessel along 

 the median line. 



