CEREBRO-SPINAL BRANCHES OF THE VERTEBRAL. 369 



(a) Muscular branches of variable size are distributed to the deep-seated cervical 

 muscles. 



B. Cranial branches : 



(a) The posterior meningeal artery is a small branch which arises opposite the 

 foramen magnum, and ramifies between the dura mater and the bone in the occipital 

 fossa, and upon the falx cerebelli. There are sometimes two of these small vessels. 



(b) The posterior spinal artery, arising at an obtuse angle from the vertebral, 

 inclines backwards round the medulla oblongata to reach the back part of the spinal 

 cord ; aided by reinforcements from small arteries which ascend upon the cervical 

 and dorsal nerves through the intervertebral foramina, it may be traced along the 

 cord, lying behind the roots of the nerves, as a minute tortuous vessel, or rather a 

 series of little inosculating vessels, as far as the second lumbar vertebra, where it 

 terminates in ramifications on the cauda equina. 



(c) The anterior spinal 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 correspond- 

 ing vessel of the opposite side, so as to form 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 thyroid arteries, in the back from the 

 intercostal, and below this from the lumbar, ilio-lumbar, and lateral sacral arteries. 

 These small vessels enter the spinal canal at irregular intervals through the inter- 

 vertebral 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 thick- 

 ness, named the 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 posterior roots 

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

 spinal. 



(d) The posterior inferior cerebellar artery, the largest of the branches, arises from 

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

 wards 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 surface of the cerebellum. Here, running backwards 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 surface of the cerebellum as far as its outer border, 

 over which the ultimate 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 fourth ventricle. 



PEGULIAKITIES. 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 sub- 

 clavian 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 ver- 

 tebral artery has been placed on record. (R. Quain, plate 24, 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 artery 

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



