VERTEBRAL. 397 



mmbar, ilio-lumbar, and lateral sacral arteries in the lower part of the spine. They 

 unite, bj 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 oft' 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 ob- 

 longata; 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 re- 

 inforced 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 com- 

 mencement, it gives off an ascending branch, which terminates on the side of the 

 fourth ventricle. 



The inferior cerebellar artery, the largest branch of the vertebral, winds back- 

 wards round the upper part of the medulla oblongata, passing between the origin 

 of the spinal accessory and pneumogastric nerves, over the restiforrn body, to the 

 under surface of the cerebellum, where it divides into two branches ; an internal 

 one, which is continued backwards to the notch between 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 Basilar 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 Yarolii, 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 Yarolii 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 to the fourth nerve, and, arriving at the upper 

 surface of the cerebellum, divide into branches which supply the pia mater, 

 covering its surface, anastomosing with the inferior cerebellar. It gives several 

 branches 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 surface of 

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

 anterior and middle cerebral arteries. Near their origin, they give oft' numerous 

 branches, which enter the posterior perforated spot, and receive the posterior 

 communicating arteries from the internal carotid. 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 branches 

 of the internal carotid and vertebral arteries at the base of the brain, constitutes 

 the circle of Willis. It is formed, in front, by the anterior cerebral and anterior 

 communicating arteries ; on each side, by the trunk of the internal carotid, and the 



