s 
VERTEBRAL ARTERY. 775 
ing twigs from the spinal branches of the vertebral, intercostal, and lumbar arteries. — It 
gives off branches which pierce the pia mater and supply the cord, and it unites below 
with the posterior spinal arteries. 
(4) The posterior inferior cerebellar (a. cerebelli inferior posterior) is the largest 
branch of the vertebral artery. It arises a short distance below the pons and passes 
obliquely backwards round the medulla, at first between the roots of the hypoglossal 
nerve, and then between the roots of the spinal accessory and vagus nerves, into the 
vallecula of the cerebellum, where it divides into external and internal terminal branches. 
The trunk of the artery gives branches to the medulla and to the chorioid plexus of 
the fourth ventricle. The internal ter minal, or vermiform branch, runs backwards 
between the inferior vermiform process and the lateral lobe of the cerebellum ; it supplies 
principally the former structure, and anastomoses with its fellow of the opposite side. The 
external or hemispheric branch passes outwards on the lower surface of the hemisphere 
and anastomoses with the superior cerebellar artery. 
Basilar Artery (a. basilaris)—This artery is formed by the junction of 
the two vertebral arteries; it commences at the lower border and terminates at the 
upper border of the pons Varolii, bifurcating at its termination into the two 
posterior cerebral arteries. 
Course and Relations.—It runs upwards in a shallow groove on the front of 
the pons Varoli, behind the sphenoidal section of the basi-cranial axis and between 
the sixth nerves. 
Branches.—(1) he transverse, a series of small arteries which pass round the sides 
of the pons, supplying it (rami ad pontem), the middle peduncles of the cerebellum, and 
the roots of the fifth cranial nerve. 
(2) The auditory (a. auditiva interna), a pair of long but slender branches which 
accompany the eighth cranial nerves. Each enters the corresponding internal auditory 
meatus with the seventh and eighth nerves, and, passing through the lamina cribrosa, is 
distributed to the internal ear. 
(3) The anterior inferior cerebellar (a. cerebelli inferior anterior), two branches which 
arise, one on each side, from the middle of the basilar artery. They pass backwards on the 
lower surfaces of the lateral lobes of the cerebellum, and anastomose with the posterior 
inferior cerebellar branches of the vertebral arteries. 
(4) The superior cerebellar (aa. cerebelli superiores) branches, two in number, arise near 
the termination of the basilar. Each passes outwards at the upper border of the pons, 
directly below the third nerve of the same side, and turning round the outer side of the 
crus cerebri below the fourth nerve, reaches the upper surface of the cerebellum, where it 
divides into an internal and an external branch. The internal branch supplies the upper 
surface of the vermiform process and the valve of Vieussens. The external branch is 
distributed over the upper surface of the lateral hemisphere, anastomosing at its margin 
with the inferior cerebellar arteries. 
(5) Posterior Cerebral Arteries (aa. cerebri posteriores, Figs. 557 and 559).— 
These are the two terminal branches of the basilar. They run backwards and 
upwards, between the crura cerebri and the uncinate convolutions and _ parallel 
to the superior cerebellar arteries, from which they are separated by the third 
and fourth cranial nerves. Each posterior cerebral artery is connected with the 
internal carotid by the posterior communicating artery; it gives branches to the 
tentorial surface of the cerebrum, and is continued backwards, ‘beneath the splenium 
of the corpus callosum, to the calcarine fissure, where it divides into calearine and 
parieto-occipital branches, which pass to the outer surface of the occipital lobe and 
supply the inner and tentorial surfaces of the occipital lobe and the posterior part 
of its outer surface. 
Branches.—(A) Central or ganglionic.—This group includes (a!) A postero-mesial 
set of small vessels which pass on the inner side of the crus cerebri to the posterior 
perforated space. They supply the crus, the posterior part of the optic thalamus, the 
corpora albicantia, and the walls of the third ventricle. 
(a?) A postero-lateral set of small vessels which pass round the outer side of the crus 
cerebri. They supply the corpora quadrigemina, the brachia, the crus, the posterior part 
of the optic thalamus, and the corpora geniculata. 
_(a°) A posterior chorioidal set of small branches which pass through the upper part of 
