THE BASILAR ARTERY 561 



(3) The posterior spinal artery [a. spinalis posterior] runs downward obliquely along the 

 side of the medulla to the back of the cord, down which it passes behind the roots of the spinal 

 nerves, being reinforced by spinal branches accompanying these nerves, in the neck, the thoracic, 

 and in the lumbar region. It can be traced as low as the end of the spinal cord. 



(4) The anterior spinal artery [a. spinahs anterior] comes off from the vertebral a little 

 below its termination in the basilar artery. Descending with a medial slant in front of the 

 medulla, it unites on a level with the foramen magnum with its fellow of the opposite side. 

 The single vessel thus formed runs downward in front of the spinal cord beneath the pia mater 

 as far as the termination of the cord, being reinforced by the spinal branches on the way down. 

 The spinal arteiies are described in detail with the anatomy of the spinal cord. 



(5) The posterior inferior cerebellar [a. cerebeUi inferior posterior] (fig. 456) — the largest 

 branch of the vertebral — arises from that vessel just before it joins its feUow to form the basilar 

 artery. At times it may come off from the basilar itself. It runs, at first laterally ar-ross 

 the restiform body between the origin of the vagus and hypoglossal nerves, and, descending 

 toward the vallecula, there divides into two branches, medial and lateral, (a) The medial 

 branch runs backward between the vermis and the lateral hemisphere of the cerebellum. It 

 supplies the vermis, and anastomoses with the artery of the opposite side, and with tlie superior 

 vermian of the superior cerebellar. (6) The lateral branch runs laterally and, ramifying over 

 the under surface of the cerebellar hemisphere, supplies its cortex and anastomoses along its 

 lateral margin with the superior cerebellar arteries. 



From the undivided trunk of the posterior inferior cerebellar artery branches are given 

 to the medulla oblongata, supplying the chorioid plexus and the fourth ventricle. 



THE BASILAR ARTERY 



Tlie basilar artery [a. basilaris] is formed by the confluence of the right and 

 left vertebral arteries, which meet at an acute angle at the lower border of the 

 pons. It runs forward and upward in a slight groove in the middle line of the 

 pons, and divides at the upper border of that structure at the level of the tentorial 

 notch into the two posterior cerebral arteries, which take part in the formation of 

 the circle of Willis (fig. 456) . 



Branches of the Basilar Artery 



The branches of the basilar artery are: — 1. Pontine; 2. internal auditory; 

 3. anterior inferior cerebellar; 4. superior cerebellar; 5. posterior cerebral. 



(1) The pontine branches [rami ad pontem] are numerous small vessels which come off at 

 right angles on either side of the basilar artery, and, passing laterally over the pons, supply that 

 structure and adjacent parts of the brain. 



(2) The internal auditory artery [a. auditiva interna], a long slender vessel, accompanies 

 the auditory nerve into the internal auditory meatus (fig 514). It here hes between the 

 facial and auditory nerves, and at the bottom of the meatus passes into the internal ear, and 

 anastomoses with the other auditory arteries. (See Internal Ear.) 



(3) The anterior inferior cerebellar [a. cerebelh inferior anterior] arises from the basilar 

 soon after its origin, passes laterally and backward across the pons, and then over the brachium 

 pontis to the front part of the under surface of the cerebellum. It anastomoses with the 

 posterior inferior cerebellar artery (fig. 456). 



(4) The superior cerebellar [a. cerebeUi superior] comes off from the basilar immediately 

 behind its bifurcation into the posterior cerebral arteries. It courses laterally and backward 

 over the pons, in a curve roughly corresponding to that of the posterior cerebral artery, from 

 which it is separated by the third cranial nerve; but, soon sinking into the groove between the 

 pons and the pedunculus cerebri, it curves round the latter onto the upper surface of the 

 (JprebeUum, lying nearly parallel to the fourth nerve. Here it divides into two branches 

 medial and lateral, (o) The medial branch courses backward along the superior vermis, anas- 

 tomosing with its feUow of the opposite side, and, at the posterior notch of the cerebellum, with 

 the inferior vermian branch of the posterior inferior cerebellar artery, (b) The lateral runs 

 to the circumference of the cerebellum, anastomosing with the lateral branch of the inferior 

 posterior cerebellar artery. 



Branches are given off from the main trunk of the superior cerebellar artery, or from its 

 medial branch to the anterior velum (valve of Vieussens), the corpora quadrigemina, the pineal 

 bodj% and the chorioid plexus. 



(5). The posterior cerebral arteries [aa. cerebri posteriores] are the two terminal branches 

 into which the basilar bifurcates at the upper border of the pons, immediately behind the 

 posterior perforated substance. Each artery runs at first laterally and a little forward across 

 the pedunculus cerebri immediately in front of the third nerve, which separates it from the 

 superior cerebellar artery. After receiving the posterior communicating artery, which runs 

 backward from the internal carotid, the posterior cerebral turns backward onto the under surface 

 of the cerebral hemisphre, where it breaks up into branches for the supply of the temporal and 

 occipital lobes. 



The branches of the posterior cerebral artery are described below in connection with those 

 of the other cerebral arteries. 

 36 



