964 THE VASCULAE SYSTEM. 



of the artery to join the anterior trunk, thus forming a common terminal vein which 

 ends in the usual manner. 



Venae Thyreoideae Inferiores. Each inferior thyreoid vein commences by the 

 union of tributaries which issue from the isthmus and the corresponding lobe of 

 the thyreoid gland. The two veins descend, along the front of the trachea, into 

 the superior mediastinum, where the right inferior thyreoid vein terminates 

 either in the right innominate vein or in the junction of the two innominate 

 veins, and the left in the upper border of the left innominate vein ; or the two 

 veins unite to form a single trunk, which ends, usually, in the left innominate 

 vein, but, occasionally, in the right. As they descend in the neck the inferior 

 thyreoid veins anastomose together, and sometimes the anastomoses are so frequent 

 that a venous plexus is formed in front of the lower cervical portion of the trachea. 



VEINS OF THE HEAD AND NECK. 



Vena Jugularis Interna (Figs. 756, 759, 787, 800 and 801). Each internal 

 jugular vein commences, in the posterior compartment of the jugular foramen, 

 as the direct continuation of the transverse sinus, and terminates, behind the 

 medial part of the clavicle, by uniting witji the subclavian vein of the same 

 side to form the innominate vein. 



Its commencement, which is dilated, forms the superior bulb of the jugular vein. 

 In the upper part of the neck it lies postero-lateral to the internal carotid artery 

 and the last four cerebral nerves. As it descends it accompanies first the internal 

 and then the common carotid artery. It inclines forwards as it descends, and 

 gradually passes from its original position, behind and to the lateral side of the 

 internal carotid artery, until it lies more completely to the lateral side of the internal 

 and common carotid arteries, and, indeed, somewhat overlaps the latter anteriorly. 

 This is more especially the case on the left side, for both internal jugular veins 

 trend slightly towards the right as they descend ; consequently, at the root of the 

 neck, the right vein is separated from the right common carotid artery by a small 

 interval filled with areolar tissue, whilst the left vein is more directly in front of 

 the corresponding common carotid artery. 



A dilatation, the inferior bulb, is present at the inferior extremity of the vein ; 

 it is bounded, either above or below, by a valve of two or three semilunar 

 cusps. Sometimes both the superior and inferior ends of the bulb are bounded 

 by valves. 



Relations. The vein lies anterior to the transverse processes of the cervical verte- 

 brae, the rectus capitis lateralis, longus capitis, and scalenus anterior muscles, the ascend- 

 ing cervical artery, which runs upwards in the interval between the attachments of the 

 two latter muscles, and the phrenic nerve ; the transverse scapular and the transverse 

 cervical arteries intervene between it and the scalenus anterior. At the root of the neck 

 the vein lies in front of the first part of the subclavian artery and the origins of the 

 vertebral artery and the thyreo-cervical trunk, and on the left side it is anterior to the 

 terminal part of the thoracic duct. 



On the antero-medial side of the internal jugular vein, immediately below the skull, 

 are the internal carotid artery and the last four cerebral nerves ; in the rest of its extent 

 it is in relation, medially, first with the internal and then with the common carotid artery 

 whilst to its medial side and somewhat posteriorly, between it and the large arteries 

 lies the vagus nerve. 



Each internal jugular vein is covered, superficially, in the whole of its length, by th( 

 . sterno-mastoid muscle; near its upper end it is crossed by the styloid process, the stylo 

 pharyngeus and stylo-hyoid muscles, and the posterior belly of the digastric, whilst in 

 its lower half, the omo-hyoid, the sterno-hyoid, and the sterno-thyreoid muscles are 

 superficial to it, under cover of the sterno-mastoid. Just below the transverse process of th( 

 atlas, and under cover of the sterno-mastoid, the vein is crossed, on its lateral side, by 

 the accessory nerve and by the occipital artery ; about the middle of its course it is 

 crossed by the communicans cervicis nerve, and near its lower end by the anterior jugular 

 vein ; the latter vessel, however, is separated from it by the sterno-hyoid and sterno 

 thyreoid muscles. Superficial to the vein are numerous deep cervical lymph glands. 



