THE VERTEBRAL ARTERY 559 



here cross in front of the artery. In very muscular subjects the clavicular head of the sterno- 

 mastoid may be larger than usual, and in such a case will form one of the coverings of the artery. 



Behind, the artery is in contact with the scalenus medius, and with the lower trunk of the 

 brachial plexus. 



Below, the artery rests in the posterior of the two grooves on the upper surface of the first 

 rib. 



Above is the brachial plexus of nerves and the posterior beUy of the omo-hyoid muscle. 

 The trunk formed by the fifth and sixth cervical nerves is also above the artery, but on a some- 

 what anterior plane. It is close to the vessel, and has been mistaken for the artery in the appU- 

 cation of a Ugature. 



As a rule there is no branch given off from the third portion of the subclavian. 

 At times, however, the transverse cervical or the descending branch of the 

 transverse cervical (posterior scapular artery) may arise from the third portion of 

 the subclavian instead of from the thyreo-cervical trunk (thyreoid axis) and from 

 the transverse cervical respectively, as here described. 



There is considerable variation in the branches of the subclavian artery and 

 Bean (Am. Jour. Anat., Vol. 4, p. 303) has shown that the branches are arranged 

 in a different way on the two sides of the body. The usual form on the right side 

 is for the vertebral, internal mammary, the superficial cervical and the common 

 trunk of the inferior thjo-eoid and transverse scapular arteries to arise from the 

 first part of the subclavian. In this case the ascending cervical is a branch of 

 the inferior thyreoid, while the transverse cervical and costo-cervical arise from 

 the second portion. There are no branches from the third portion. On the left 

 side the usual form is for the vertebral and internal mammary, and thyreo-cer- 

 vical trunk, to arise from the first part. The thyreo-cervical trunk divides into 

 inferior thyreoid, transverse scapular, and transverse cervical arteries; the super- 

 ficial cervical is absent, and the costo-cervical trunk arises from the first part. 



There are three more types of origin of the branches; in one, the vertebral, 

 internal mammary, costo-cervical, and inferior thyreoid come from the first part, 

 while the transverse cervical arises from the second part, and the transverse 

 scapular comes either from the third part or the axillary arter}^; in the second, 

 the inferior thyreoid, transverse scapular and transverse cervical arise in a com- 

 mon stem from the first part; while in the third, which is the rarest form, the in- 

 ferior thj^reoid and superficial cervical arteries come by a common trunk from 

 the first part, while the transverse scapular artery arises from the internal 

 mammary. 



1. THE VERTEBRAL ARTERY 



The vertebral artery [a. vertebralis] (fig. 458) the first, largest, and most con- 

 stant branch, arises from the upper and posterior part of the first portion of the 

 subclavian, on the right side, about 2 cm. (f in.) from the origin of the latter ves- 

 sel from the innominate, on the left side, from the most prominent part of the 

 arch of the subclavian, close to the medial edge of the scalenus anterior muscle. It 

 first ascends vertically to the foramen transversarium of the sixth cervical 

 vertebra, and, having passed through that foramen and those of the next succeed- 

 ing cervical vertebrae as high as the epistropheus (axis), it turns laterally and 

 then ascends to reach the foramen in the transverse process of the atlas; after 

 passing through that foramen it turns backward behind the articular process, 

 lying in the groove on the posterior arch of the atlas. It next pierces the posterior 

 occipito-atlantoid membrane and the dura mater, and enters the cranium through 

 the foramen magnum. Here it passes upward, at first lying by the side of the 

 medulla, then in front of that structure, and terminates at the lower portion of the 

 pons by anastomosing with the vertebral of the opposite side to form the basilar. 



The vertebral artery may be divided for purposes of description into four parts: 

 the first, or cervical, extending from its origin to the transverse process of the 

 sixth cervical vertebra; the second, or vertebral, situated in the foramina trans- 

 versaria; the third, or occipital, contained in the suboccipital triangle; and the 

 fourth, or intracranial, within the cranium. 



The first or cervical portion. — The artery here lies between the scalenus anterior and longus 

 coUi muscles. In front it is covered by the vertebral and internal jugular veins, and is crossed 

 by the inferior thj-reoid artery, and on the left side, in addition, by the thoracic duct, which runs 

 over it medio-laterally. Behind, the artery Hes on the transverse process of the seventh cervical 

 vertebra and the sympathetic nerve. To its medial side is the longus coUi. To its lateral 



