474 ARTERIES. 



cession of small branches, which enter the spinal canal through some of the 

 intervertebral foramina; these branches are derived from the vertebral and 

 ascending cervical in the neck ; from the intercostal, in the dorsal region ; and 

 from the lumbar, ilio-lumbar, and lateral sacral arteries in the lower part of 

 the spine. They unite, by means of ascending and descending branches, to 

 form a single anterior median artery, which extends as far as the lower part of 

 the spinal cord. This vessel is placed beneath the pia mater along the anterior 

 median fissure ; it supplies that membrane and the substance of the cord, and 

 sends off branches at its lower part, to be distributed to the cauda equina. 



The posterior spinal arises from the vertebral, at the side of the medulla 

 oblongata; passing backwards to the posterior aspect of the spinal cord, it 

 descends on either side, lying behind the posterior roots of the spinal nerves ; 

 and is reinforced by a succession of small branches, which enter the spinal 

 canal through the intervertebral foramina, and by which it is continued to the 

 lower part of the cord, and to the cauda equina. Branches from these vessels 

 form a free anastomosis round the posterior roots of the spinal nerves, and 

 communicate, by means of very tortuous transverse branches, with the vessel 

 of the opposite side. At its commencement, it gives off an ascending branch, 

 which terminates on the side of the fourth ventricle. 



The inferior cerebellar artery (Fig. 281), the largest branch of the vertebral, 

 winds backwards round the upper part of the medulla oblongata, passing be- 

 tween the origin of the spinal accessory and pneumogastric nerves, over the 

 restiform body, to the under surface of the cerebellum, where it divides into 

 two branches ; an internal one, which is continued backwards to the notch be- 

 tween the two hemispheres of the cerebellum ; and an external one, which 

 supplies the under surface of the cerebellum, as far as its outer border, where 

 it anastomoses with the superior cerebellar. Branches from this artery supply 

 the choroid plexus of the fourth ventricle. 



The Ba&ilar Artery, so named from its position at the base of the skull, is a 

 single trunk, formed by the junction of the two vertebral arteries; it extends 

 from the posterior to the anterior border of the pons Varolii, where it divides 

 into two terminal branches, the posterior cerebral arteries. Its branches are, 

 on each side, the following : 



Transverse. Superior cerebellar. 



Anterior cerebellar. Posterior cerebral. 



The transverse branches supply the pons Varolii and adjacent parts of the 

 brain ; one accompanies the auditory nerve into the internal auditory meatus ; 

 and another, of larger size, passes along the crus cerebelli, to be distributed to 

 the anterior border of the under surface of the cerebellum. It is called the 

 anterior (inferior) cerebellar artery. 



The superior cerebellar arteries arise near the termination of the basilar. They 

 wind round the crus cerebri, close \o the fourth nerve, and arriving at the upper 

 surface of the cerebellum, divide into branches, which ramify in the pia mater, 

 and anastomose with the inferior cerebellar. Several branches are given to 

 the pineal gland, and also to the velum interpositum. 



The posterior cerebral arteries, the two terminal branches of the basilar, are 

 larger than the preceding, from which they are separated near their origin by 

 the third nerves. Winding round the crus cerebri, they pass to the under sur- 

 face of the posterior lobes of the cerebrum, which they supply, anastomosing 

 with the anterior and middle cerebral arteries. Near their origin, they receive 

 the posterior communicating arteries from the internal carotid, and give off 

 numerous branches, which enter the posterior perforated space. They also 

 give off a branch, the posterior choroid, which supplies the velum interpositum 

 and choroid plexus, entering the interior of the brain beneath the posterior 

 border of the corpus callosum. 



Circle of Willis. The remarkable anastomosis which exists between the 



