758 HUMAN ANATOMY. 



The collateral circulation after ligation of the third portion of the subclavian 

 artery is carried on from the proximal or cardiac side of the ligature by {a) the 

 suprascapular and posterior scapular ; (^) the aortic intercostals, the superior inter- 

 costals, and the internal mammary ; and (c) numerous subdivisions of subclavian 

 branches running through the axilla, anastomosing respectively with {a) the sub- 

 scapular, and the acromio-thoracic ; ((^), the subscapular, long thoracic, infrascap- 

 ular, and dorsalis scapulce ; (r) the axillary trunk or its branches. 



I. The Vertebral Artery. — The vertebral artery (a. vertebralis) (Figs. 695, 

 704), the first and largest branch of the subclavian artery, is destined chiefly for the 

 supply of the spinal cord and the brain, joining with the internal carotid arteries to 

 form the remarkable intracranial anastomotic circle of Willis. In view of its peculiar 

 course, the vertebral artery may be conveniently divided into four parts. 



The first portion arises from the upper surface of the first part of the sub- 

 clavian artery, opposite the interval between the longus colli and scalenus anticus, 

 and courses upward and somewhat backward, between these muscles and in front 

 of the transverse process of the seventh cervical vertebra, to the foramen in the 

 transverse process of the sixth cervical vertebra, which it enters. The artery is 

 surrounded by a plexus of sympathetic nerve-fibres, and in front is crossed by the 

 inferior thyroid artery and covered by the vertebral and internal jugular veins. 

 The second portion includes the ascent of the artery through the foramina in the 

 transverse process of the upper si.x cervical vertebrae, surrounded by plexiform net- 

 works of sympathetic ner\'e-fibers and of veins, and lying in front of the trunks of 

 the cervical nerves. As the artery traverses the foramen in the axis it abandons its 

 previous almost vertical course and passes upward and outward to reach the foramen 

 in the atlas. As it emerges from this opening, passing between the suboccipital 

 nerve and the rectus capitis lateralis muscle, its third portion begins, bending hori- 

 zontally to the outer side and back of the superior articular surface of the atlas to 

 enter the suboccipital triangle (Fig. 522) where it rests in the vertebral groove 

 upon the posterior arch of the atlas, being separated from the bone, however, by 

 the suboccipital nerve. The artery then perforates the lower border of the posterior 

 occipito-atlantoid ligament and enters the spinal canal. The fourth portion of 

 the artery pierces the spinal dura mater, passes between the roots of the hypoglossal 

 nerve and the dentate ligament and enters the cranial cavity by traversing the fora- 

 men magnum. Passing forward along the medulla oblongata and gradually inclin- 

 ing towards the mid-ventral line, at the posterior border of the pons the vertebral 

 artery unites with its fellow of the opposite side to form the basilar artery (a. basi- 

 laris), which extends forward along the median line of the pons to the anterior 

 border of that structure, where it terminates by dividing into the two posterior 

 cerebral arteries. 



Branches.— In its course up the neck the vertebral artery gives off, opposite each inter- 

 vertebral space which it passes, lateral and medial branches which represent the original seg- 

 mental arteries by the anastomoses of whose branches the vertebral was formed (page 721). 



(a) The lateral or muscular branches pass to the muscles of the neck and form anasto- 

 moses with the ascending and deep cervical branches of the subclavian and with the arteria 

 princeps cervicis of the occipital. 



{b) The medial or spinal branches ( rami spinales) pass through the intervertebral foramina 

 into the spinal canal, accompanying the spinal nerves, and are distributed to tlie bodies of the 

 vertebrce and to the membranes and substance of the spinal cord. Each branch gives off an 

 ascending and a descending ramus upon the posterior surface of tlie spinal cord, and tliese, 

 anastomosing with each other and with twigs from the spinal branches of the intercostal, lum- 

 bar, and lateral sacral arteries below and with the posterior spinal branches of the upper part 

 of the vertebral, assist in the formation of the posterior spinal arteries, which run the entire 

 length of the spinal cord wpon its posterior surface on each side of the median line. Ante- 

 riorly the spinal branches of the vertebral unite with the anterior spinal artery, reinforcing 

 that vessel. 



(c) The posterior meningeal artery (ramus meningeus) arises from the vertebral, just after 

 it has pierced the dura mater, and sui^plies tiie portion of that membrane wliich lines the pos- 

 terior portion of the posterior fossa of the skull. 



