"38 A MANUAL OF ANATOMY 



from the muscle as a single cord, and the lower or third root of the 

 nerve descends in front of the scalenus medius, and joins the fore- 

 going cord about the level of the first rib. 



Scalenus Posticus — Origin. — By two or three short tendons from 

 the tubercles of the transverse processes (posterior tubercles) of the 

 lower two or three cervical vertebrae. 



Insertion. — ^The upper part of the outer surface of the second rib, 

 in front of the insertion of the highest slip of the serratus posticus 

 superior, and behind the origin of a portion of the first, and the 

 second, serrations of the serratus magnus. 



Nerve-supply. — ^The anterior primary divisions of the sixth, 

 seventh, and eighth cervical nerves, the branches of which pass 

 through the scalenus medius. 



The muscle is directed downwards- and outwards. 



Action. — (i) To elevate the second rib, and (2) to flex the neck to 

 one side. 



l^oidtXions^— Anterior. — ^The scalenus medius. Posterior. — The 

 lower two tendons of origin of the levator anguli scapulae. The 

 scalenus posticus is intimately connected with the scalenus medius, 

 of which it may be regarded as a detached part. 



Subclavian Artery. — ^The right subclavian vessel arises from the 

 innominate artery behind the right sterno -clavicular joint on a 

 level with its upper part, and the left subclavian vessel arises from 

 the upper aspect of the arch of the aorta towards its back part. On 

 each side the artery terminates at the outer border of the first rib by 

 becoming the axillary artery. In its course the vessel is crossed 

 superficially by the scalenus anticus muscle, which divides it into 

 three parts, first, second, and third. 



The first part extends from the origin of the vessel to the inner 

 border of the scalenus anticus ; the second part lies behind that 

 muscle ; and the third part extends from the outer border of the 

 muscle to the outer border of the first rib. The total length of the 

 vessel on the right side is about 3 inches, and, on the left, about 

 4I inches. The artery describes an arch over the apex of the 

 corresponding lung and pleura, and the height to which it rises 

 above the clavicle is about \ inch. 



First Part of the Right Subclavian Artery. — ^This part extends 

 from the bifurcation of the innominate artery behind the right 

 sterno-clavicular joint, on a level with its upper part, to the inner 

 border of the scalenus anticus muscle. Its direction is upwards 

 and outwards, and it lies very deeply. 



Relations — Anterior. — The skin, superficial fascia and platysma 

 myoides, deep cervical fascia, clavicular origin of the sterno-cleido- 

 mastoid, and sterno-hyoid and stemo-thyroid muscles. Three 

 veins are related to it superficially. The internal jugular and 

 vertebral veins cross it from above downwards close to the scalenus 

 anticus, the latter vessel being under cover of the former, and the 

 anterior jugular vein crosses it from within outwards, but super- .J 

 ficial to the sterno-hyoid and stemo-thyroid muscles. Important 



