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 surliice 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 :-1. Nume rous small twigs which enter the floor of the third ventricle, through the apertures in the locus perforatus medics, or are distributed to the crura cerebri, corpora albicantia, and tuber cenereum; and 2. a choroid branch, which winds round the erns cerebri, enters the cerebral fissure, and is lost in the velum inter positum, corpora quadrigemina, and choroid plexuses. _ _ Varieties occasionally observable in the verte bral arteries. —1. Of origin.—a. It has already been mentioned that the vertebral artery may arise from different portions of the first stage of the subclavian artery being sometimes nearer, and sometimes further removed, from the innominata; but, independently of these varieties, the vertebral artery on the right side is b. Sometimes furnished by the common
carotid artery. In all the cases where this anomaly has been observed, the right subcla vian artery was given off directly as a branch of the aorta. Again, c. The vertebral artery sometimes comes off from the arch of the aorta. This irre gularity is as unfrequent on the right, as it is common on the left side. When the left vertebral artery springs from the arch of the aorta, it usually arises between the left carotid and the left subclavian arteries, though some times its origin has been found to the left of all the other branches of the arch.
d. In a few instances, more than one vessel has been found to constitute the origin of the vertebral artery; thus, it may be formed by the union of two roots, both arising from the subclavian artery, or one from the subclavian and the other from the aorta. In one exam ple, where it was formed by three roots, two of these were derived from the subclavian, and the third from the inferior thyroid artery. These roots of the vertebral artery in some instances united before the artery had become engaged in the vertebral foramina, whilst in others the union took place subsequently.
2. Of size.—There is often a considerable difference in the size of the two vertebral arte ries, which is stated to be most frequently in favour of that of the left side ; thus, in 98 ob servations made by Mr. Davy, the left verte bral artery was the larger in twenty-six, and the right in eight instances only.
3. Of course and relations.—The vertebral artery may enter the transverse process of the last cervical vertebra (though the contrary has been asserted), or it may enter one of the fo ramina higher than that in the transverse pro cess of the sixth, which latter it usually selects. When the artery enters any vertebra higher than the sixth cervical, it always occupies an unusually superficial position in the neck, lying external and parallel to the common carotid artery, for which, consequently, it is liable to be mistaken. (Vide CAROTID AR