910 THE VASCULAR SYSTEM. 



The second part of the subclavian artery, on each side, extends from the 

 medial to the lateral border of the scalenus anterior, behind which it lies. 



Relations. Posteriorly and below it is in relation with the pleural sac. Anteriorly 

 it is covered by the anterior scalene and the sterno-mastoid muscles. The anterior scalene 

 separates it from the subclavian vein, which lies at a slightly lower level, from the trans- 

 verse cervical and transverse scapular arteries, from the anterior jugular vein, and, on 

 the right side, from the phrenic nerve. 



The third part of the subclavian artery is the most superficial portion. 

 It extends from the lateral border of the anterior t scalene to the outer border of 

 the first rib, lying partly in the clavicular portion of the posterior triangle of the 

 neck and partly behind the clavicle and the subclavius muscle. 



Relations. It rests upon the upper surface of the first rib. Immediately posterior 

 to it is the lowest trunk of the brachial plexus, which separates it from the middle 

 scalene muscle. Anterior to it, and at a slightly lower level, lies the subclavian vein. The 

 external jugular vein crosses the medial part of this portion of the artery, and just before 

 its termination it receives the transverse cervical and transverse scapular veins ; those 

 vessels also pass superficial to the artery, which is thus covered superficially by venous 

 trunks ; it is also crossed vertically, behind the veins, by the nerve to the subclavius 

 muscle. The lateral section of this part of the artery lies posterior to the clavicle and 

 the subclavius muscle. It is crossed anteriorly by the transverse scapular artery, but 

 the layer of deep cervical fascia which binds the posterior belly of the omo-hyoid to the 

 posterior border of the subclavian groove intervenes " between the two vessels. More 

 superficially the third part of the artery is covered by the superficial layer of the deep 

 fascia, the supra-clavicular branches of the cervical nerves, the platysma, and the skin. 



BRANCHES OF THE SUBCLAVIAN ARTERY. 



(1) The vertebral artery is distributed almost entirely to the head and neck 

 and its chief function is to supply the posterior part of the brain. Its description 

 has therefore been given with that of the other cerebral arteries (see p. 905). 



(2) Truncus Thyreocervicalis. The thyreo-cervical trunk (Figs. 757 and 759) 

 arises close to the medial border of the scalenus anterior, from the upper and front 

 part of the subclavian artery, directly above the origin of the internal mammary 

 artery. After a short upward course of about 4 mm. (two lines), it ends, under 

 cover of the internal jugular vein, by dividing into three branches viz., the inferior 

 thyreoid, the transverse cervical, and the transverse scapular. 



(A) Arteria Thyreoidea Inferior. The inferior thyreoid artery (Figs. 757, 759) 

 ascends along the anterior border of the scalenus anterior, and turns medially, 

 opposite the cricoid cartilage, to the middle of the posterior border of the corre- 

 sponding lobe of the thyreoid gland ; it then curves medially and downwards, and 

 descends to the lower end of the lobe of the thyreoid gland, where it divides into 

 ascending and inferior terminal branches. 



Relations. Posterior are the vertebral artery, and the longus colli muscle; 

 the recurrent nerve passes either anterior or posterior to the vessel, opposite the lower 

 border of the thyreoid gland. It is covered anteriorly by the carotid sheath, which 

 contains the common carotid artery, the internal jugular vein, and the vagus nerve; the 

 middle cervical ganglion of the sympathetic lies in front of the artery as it bends medially : 

 and on the left side the thoracic duct also passes in front of it. 



Branches. It gives off the following branches : 



(a) Muscular. Numerous small branches pass to the scalenus anterior, the longus 

 colli, the infra-hyoid muscles, and the inferior constrictor of the pharynx. 



(6) The ascending cervical usually springs from the inferior thyreoid near its origin 

 but it may arise separately from the thyreo-cervical trunk. It ascends, parallel with anc 

 medial to the phrenic nerve, in the angle between the longus capitis and the scalenue 

 anterior, to both of which it gives branches. It also gives off spinal branches whicl 

 pass through the intervertebral foramina to the vertebral canal. It anastomoses witl 

 branches of the vertebral, occipital, ascending pharyngeal, and deep cervical arteries. 



(c) (Esophageal. Small branches given to the walls of the ossophagus, whicl 

 anastomose with the oesophageal branches of the thoracic aorta. 



