510 THE ARTERIES 



aiiticiis and longus colli muscles. In front it is covered by the vertel)ral and 

 internal jii<iular veins, and is crossed by the inferior thyroid artery; and on the left 

 side, in addition, by the thoracic duct, Avhich runs over it from within outwards. 

 Behind, the arterv lies on the transverse process of the seventh cervical vertebra 

 and the sympathetic nerve. To its inner side is the longus colli. To its outer 

 side is the scalenus anticus. It gives off as a rule no branch in this ])art of its 

 course. Occasionally, however, a small branch passes into the foramen of the 

 transverse process of the seventh cervical vertebra. 



The second or vertebral portion. — As the artery passes through the inter- 

 vertebral foramina, it is surrounded by a plexus of veins and by liranches of the 

 sympathetic nerve. The cervical nerves lie behind it. Between the transverse 

 processes it is in contact with the intertransverse muscles. 



The third or occipital portion. — The artery here lies in the suboccipital 

 triangle, bounded by the superior oblique, inferior oblique, and rectus capitis pos- 

 ticus major muscles. As it Avinds round the groove on the atlas, it has the rectus 

 capitis lateralis, the articular process, and the occipito-atloid ligament in front of 

 it; the superior ol)lique, the rectus capitis posticus major and the complexus 

 behind it. Separating it from the arch of the atlas, is the first cervical or sub- 

 occi[)ital nerve. 



The fourth or intracranial portion extends from the aperture in the dura 

 mater to the lower border of the pons, where it unites with its fellow to form the 

 basilar artery. It here winds round from the side to the front of the medulla, 

 lying in the vertebral groove on the basilar process of the occipital bone. In this 

 course it passes beneath the first process of the ligamentum dentatum, and between 

 the hypoglossal nerve in front, and the anterior roots of the sul)0cci])ital nerve 

 behind. 



Chief Variations of the Vertebral Artery 



(A) The right vertebral artery may arise from the first part of the subclavian, either nearer 

 to the innominate, or nearer to the anterior scalene muscle than normal. 



(B) It may come off from the arch of the aorta direct. (See Variations in the Chief 

 Branches op the Arch of the Aorta.) 



(C) It may arise from the right common carotid when the right subclavian is given off from 

 the aorta beyond the left subclavian. 



(D) It may pass behind the oesophagus. 



(E) The left vertebral artery may also arise from the arch of the aorta direct, or from the 

 left common carotid. (See Variations in the Chief Branches of the Aorta.) 



(F) Either vertebral may enter the foramen in the seventh cervical vertebra, or in that of the 

 fifth, fourth, third, or second. When entering one of the higher vertebral foramina, it maj' lie 

 behind the common carotid and cause some embarrassment in the ligature of the latter vessel. 



(G) Either vertebral may give ofiP the inferior thyroid, superior intercostal, deep cervical, 

 or occipital arteiy. 



(H) One or other artery may be much increased or diminished in size. 



Branches of the Vertebral Artery 



A. Cervical portion. — No branch. 



B. Vertebral portion. — 1. Lateral spinal; 2. muscular. 



C. Occipital portion. — 1. Muscular; 2. anastomotic. 



D. Cranial portion. — 1. Posterior meningeal; 2. posterior sjiinal; 8. anterior 

 spinal; 4. posterior cerebellar. 



Branches of the Second or Vertebral Portion 



1. The lateral spinal branches run through the intervertel>ral foramina into 

 the vertebral canal, and there divide into two branches: (a) The spinal, which 

 ramifies on the backs of the bodies of the cervical vertebra'; and (h) the medullary, 

 Avhicli runs along the spinal nerves, supplies the cord and its meml)ranes, and anas- 

 tomoses with the arteries above and below. 2. The muscular branches sui)ply 

 the deep muscles of the neck, and anastomose with the ascending cervical, occi2)i- 

 tal, and deep cervical arteries. 



