422 THE SUBCLAVIAN ARTERIES. 



are furnished to the medulla oblongata, which they penetrate mainly along the 

 roots of the nerves and in the anterior median fissure. 



Varieties. Origin. While the vertebral artery arises, in the great majority of cases, on 

 the right side from the subclavian between half an inch and an inch from its beginning 1 , and 

 on the left side from the same trunk as it bends outwards, its range of origin extends over 

 the whole of the first part of the subclavian artery. In those cases in which the right 

 subclavian artery is given off independently from the posterior part of the arch of the aorta, 

 the right vertebral artery sometimes arises from the common carotid of the same side. In 

 some rare instances the right vertebral artery has been seen arising from the aorta, between 

 the right subclavian and carotid (p. 387), between the innominate and left carotid, or beyond 

 the left subclavian artery and passing to its usual place behind the oesophagus. 



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 it has been observed 

 arising last of the branches from the arch. 



Either vertebral artery may as a rare occurrence arise by two roots, both proceeding from 

 the subclavian artery, or one from the subclavian and the other from the aorta or from the 

 inferior thyroid artery. One example of three roots to a vertebral artery (right) is recorded 

 (R. Quain, pi. 24, fig. 2). 



Course and division. The vertebral artery not unfrequently enters the foramen of the 

 fifth or fourth cervical vertebra ; more rarely the third, or even the second. On the other 

 hand, it has been seen .to enter the foramen of the seventh vertebra. Rare varieties that 

 have been met with are division of the artery close to its origin into two branches which 

 join again into a single trunk (R, J. Anderson, Journ. Anat., xiv. 250) ; division of the trunk 

 above the axis, one part entering the spinal canal with the second cervical nerve, while the 

 other pursues the normal course and unites with the first after piercing the dura mater 

 (Herberg, Kadyi) ; and the formation of a similar, but smaller, loop around the hypoglossal 

 nerve within the cranium (Tarenetzky). 



Size. One vertebral artery, more frequently the left, is sometimes much larger than the 

 other. 



Branches. The vertebral artery has been found, though very rarely, to give off the inferior 

 thyroid or superior intercostal artery. The inferior cerebellar artery is often wanting on one 

 side. 



Arteries of the spinal canal. It will be convenient here to refer generally 

 to the arrangement of the intraspinal arteries, which pass in from various sources 

 through the intervertebral foramina, and are distributed in a similar manner 

 throughout the whole length of the canal. They are derived from the vertebral and 

 deep cervical arteries in the neck, the intercostal arteries in the dorsal region, and 

 below this from the lumbar, ilio-lumbar, and lateral sacral arteries. A branch enters 

 each intervertebral foramen, and there divides, one part being distributed to the 

 walls of the canal, and the other piercing the sheath of dura mater on the issuing 

 nerve. The latter division in the majority of cases is small, and ends by supplying 

 the nerve-roots ; but some of these branches are larger, and are continued to the 

 spinal cord, where they enter the longitudinal anastomoses in the manner described 

 above. There are no arteries passing to the cord below the third lumbar nerve, 

 and only exceptionally are there medullary arteries on the anterior and posterior 

 roots of the same nerve (Kadyi). 



The larger parietal portion of the spinal artery immediately divides into two prin- 

 cipal branches which are distributed to the bones and ligaments in the following 

 manner : one curves upwards on the back of the body of the vertebra above, near 

 to the attachment of the pedicle, while the other descends in a similar manner on 

 the vertebra below ; and each communicates with a corresponding branch from the 

 neighbouring spinal artery. As this arrangement prevails on both sides and through- 

 out the whole length of the spine, there is formed a double series of arterial arches 

 behind the bodies of the vertebras, the convexities of which are turned towards each 

 other. From the arches on opposite sides offsets are directed inwards at intervals 

 to reinforce a median longitudinal vessel, which extends along the spine like the 

 single artery on the front of the spinal cord. The arches are moreover joined 



