THE VERTEBRAL ARTERY. 385 



(c) The anterior sjj'inal artery, somewhat larger than the preceding-, arises near 

 the end of the vertebral artery, and descends obliquely in front of the medulla 

 oblongata. Immediately below the foramen magnum, it unites with the cor- 

 responding vessel of the opposite side, so as to fonn a single trunk, which 

 descends a short distance only along the middle line in front of the spinal cord, 

 forming the upper part or commencement of the anterior median artery of the 

 cord. This anterior spinal branch of the vertebral artery supplies therefore only 

 the upper part of the cord ; the remainder being provided with a series of small 

 arteries, which are derived in the neck from the vertebral and inferior thji-oid 

 arteries, in the back from the intercostals, and below this from the lumbar, ilio- 

 lumbar, and lateral sacral arteries. These small vessels enter the spinal canal at 

 iiTegular intervals through the intervertebral foramina, and, passing along the 

 roots of the nerves, communicate with each other along the middle line by means 

 of ascending and descending branches ; so that, by a succession of anastomoses, a 

 very slender single vessel, of varying thickness, named ih.e anterior median artery, 

 appears to extend from the one end to the other of the cord. This vessel, or 

 chain of inosculating vessels, supplies the pia mater and the substance of the 

 cord — some entering the anterior median fissure. At the lower end of the spinal 

 cord it sends branches downwards on the cauda equina. 



On a part of the spinal cord near the lower end, and in front of the posteiior 

 roots of the nerves, may be found another small artery, about equal in size to the 

 anterior spinal. 



(^) The posterior inferior cereheUar artery, the largest of the branches, arises 

 from the vertebral near the pons, and sometimes from the basilar artery : it 

 turns backwards and outwards, between the hypoglossal and pneumogastric 

 nerves, over the restiform body and near the side of the opening of the fourth 

 ventricle to reach the under sui'face of the cerebellum. Here, running back- 

 wards between the inferior vermiform process and the hemisphere, it divides into 

 two branches : one of which continues backwards in the sulcus between the 

 hemispheres ; while the other, turning outwards, ramifies on the under siu'f ace of 

 the cerebellum as far as its outer border, over which the ultunate divisions of 

 each branch anastomose with those of the superior cerebellar arteries. This 

 artery partly supplies the hemisphere and the vermiform process, and gives 

 branches to the choroid plexus of the f omiih ventricle. 



"Varieties. — Origin. — The right vertebral artery has been seen to arise from 

 the common carotid of the same side, in some of those cases in which the right 

 subclavian has been given as a separate vessel from the posterior part of the 

 aorta. In very rare instances, the right vertebral artery arises from the aorta. 



The left vertebral artery is not unfrequently derived from the aorta, in which 

 case it generally arises between the left carotid and subclavian arteries, but 

 sometimes it is the last of the branches from the arch. 



The left vertebral artery in a few instances, and the right vertebral in one, 

 have been found to arise by more than a single root ; and an example of three 

 roots to a vertebral artery has been placed on record. (R. Quain, plate 2-i, fig. 2.) 

 Two roots may proceed from the subclavian artery, or one from the subclavian 

 and one from the aorta. 



Course. — Instead of entering the foramen of the sixth vertebra, the vertebral 

 arteiy of one side not unfrequently enters higher up, through the foramen of the 

 fifth, or fourth, or more rarely of the thu-d vei-tebra, or even, according to several 

 anatomists, of the second. On the other hand, the vertebral artery has been 

 seen to enter the foramen of the seventh vertebra. 



Branches. — In the neck, the vertebral arteiy has been found, though very 

 rarely, to give branches which are usually derived from the subclavian, such as 

 the superior intercostal and the inferior thyroid. 



The basilar arteiy the single trunk formed by the junction of the right 

 and left vertebral arteries in the middle line, extends from the posterior 

 to the anterior border of the pons Varolii, along the median groove of 

 which it lies under cover of the arachnoid. The length of this artevy is 

 therefore about equal to that of the pons, at the anterior border of 



VOL. I, c G 



