II44 A MANUAL OF ANATOMY 



of the second rib to become the second intercostal artery. The 

 superior intercostal artery, therefore, furnishes the first and second 

 intercostal arteries for the first and second intercostal spaces, their 

 distribution being similar to that of the succeeding aortic inter- 

 costal arteries. Besides these two branches, the vessel gives off 

 the deep cervical artery. This branch, which is homologous with the 

 dorsal branch of an aortic intercostal artery, arises from the superior 

 intercostal before it descends in front of the neck of the first rib. 

 It passes backwards between the transverse process of the seventh 

 cervical vertebra and the neck of the first rib, and then ascends 

 between the complexus and semispinalis colli muscles to about the 

 level of the axis. In this latter situation it anastomoses with the 

 deep branch of the ramus cervicalis princeps of the occipital from 

 the external carotid. . The deep cervical in its course gives off 

 muscular branches which anastomose with branches of the vertebral 

 and ascending cervical arteries. It also furnishes a spinal branch 

 which enters the spinal canal through the intervertebral foramen 

 between the seventh cervical and first thoracic vertebrae. 



The superior intercostal artery on each side is developed from the anasto- 

 motic loop between the upper two or three thoracic segmental arteries. 



Subclavian Vein. — ^This vessel, which is the direct continuation 

 of the axillary vein, extends from the outer border of the first rib 

 to the back of the inner end of the clavicle, where it unites with the 

 internal jugular to form the innominate vein. It lies in front of, 

 and on a lower plane than, the subclavian artery, and it passes in 

 front of the scalenus anticus muscle and phrenic nerve. In rare 

 cases the vein has been met with passing behind the scalenus 

 anticus. Its principal tributary is the external jugular vein (some- 

 times also the anterior jugular). The external jugular vein joins it 

 close to the outer border of the sterno-cleido-mastoid. 



The thoracic duct opens at the angle of junction of the left sub- 

 clavian and left internal jugular veins, and the right lymphatic duct 

 opens into the angle of junction of the corresponding veins on the 

 right side. 



Vertebral Vein. — ^This vein commences in the suboccipital venous 

 plexus within the suboccipital triangle. It passes through the 

 costo-transverse foramen in the atlas, and accompanies the ver- 

 tebral artery through the succeeding costo-transverse foramina as 

 low as the sixth, forming a plexus around the vessel. This plexus 

 is ultimately replaced by a single vein, which emerges through the 

 costo-transverse foramen in the sixth cervical vertebra (sometimes 

 the seventh). It then descends in front of the first part of the 

 subclavian artery, under cover of the internal jugular vein, and 

 opens into the back part of the innominate vein, the opening being 

 provided with a single or double valve. 



The chief tributaries of the vertebral vein are as follows : muscular, 

 spinal, anterior vertebral, and deep cervical. It also, as a rule, 

 receives the first intercostal vein. 



No vein accompanies the cervical part of the internal mammary 



