512 THE INNOMINATE VEINS. 



with the commencement of the deep cervical, and with the posterior spinal veins. 

 It passes downwards through the foramina in the transverse processes of the upper 

 six cervical vertebrae, forming in this part of its course a close plexus around the 

 artery of the same name ; l then inclining forwards, it descends under cover of the 

 internal jugular vein, and across the subclavian artery, to open into the commence- 

 ment of the innominate vein posteriorly, where its orifice is guarded by a single or 

 double valve. The vertebral venous plexus receives branches from the neighbouring 

 muscles, from the dorsal spinal veins of the neck, from the spinal canal through the 

 intervertebral foramina, and from a plexus over the anterior surface of the cervical 

 vertebrae ; and near its termination the trunk is joined by the anterior vertebral and 

 deep cervical veins, as well as in many cases by a small vein from the first inter- 

 costal space, which accompanies the superior intercostal artery. 



The anterior vertebral vein issues from the plexus over the cervical vertebrae, 

 and passes downwards in company with the ascending cervical artery, receiving 

 branches from the prevertebral and scaleni muscles, to join the lower end of the 

 vertebral vein. 



The deep cervical vein (posterior vertebral), a vessel of large size, commences in 

 the suboccipital region, and descends between the complexus and semispinalis 

 muscles to the lower part of the neck, where it turns forwards below the transverse 

 process of the seventh cervical vertebra, to unite with the vertebral vein just before 

 that enters the innominate trunk. It is joined by the occipital veins and by 

 branches from the deep parts at the back of the neck, and it has numerous com- 

 munications with the dorsal spinal veins. 



Varieties. The vertebral vein not unf requently passes through the foramen in the trans- 

 verse process of the seventh cervical vertebra, or sends a considerable offset through that 

 foramen to join the deep cervical vein. 



The inferior thyroid veins are of large size, and are formed by branches 

 which emerge from the lateral lobes of the thyroid body, where they anastomose 

 with the superior and middle thyroid veins. They form a plexus on the front of 

 the trachea below the isthmus of the thyroid body, and then descend along that 

 tube, under cover of the sterno-thyroid muscles, receiving on their way downwards 

 oesophageal, inferior laryngeal and tracheal branches. The vein of the left side 

 joins the left innominate trunk ; that of the right side either terminates in 

 common with the foregoing, or it inclines somewhat outwards across the inno- 

 minate artery, and opens into the angle of union of the two innominate veins, or less 

 frequently into the lower end of the right innominate. There is also in some cases 

 an additional median vein descending from the isthmus of the thyroid body. 



The internal mammary veins are t\yo in number on each side, and 

 accompany the artery of the same name. They receive tributaries corre- 

 sponding to the branches of the artery from the abdominal and thoracic 

 walls and from the mediastinal space ; and at the upper part of the thorax the 

 two veins join into a single trunk which terminates in the innominate vein, or on 

 the right side frequently in the beginning of the superior vena cava. 



The superior intercostal vein is a short vessel which receives the 

 veins from two or three intercostal spaces below the first, and has a different 

 termination on the two sides. The vein of the right side inclines down- 



1 C. Walther, " Recherches anatomiques sur les veines du rachis," These. Paris, 1885 ; N. Riidinger, 

 "Ueber die Hirnschlagadern und ihre Einschliessung in Knochencanalen," Arch. f. Anat., 1888 ; 

 Trolard, " De 1'appareil veineux des arteres encdphaliques," Journ. de 1'Anat., 1890. Trolard 

 describes the vertebral vein as a single large vessel, which resembles a sinus of the dura mater, 

 surrounding the artery except at its postero- internal third, and the lumen of which is to some extent 

 irregularly divided into secondary channels and cellular spaces by septa and trabeculse. 



