186 HUMAN ANATOMY. 



anterior communicating. The posterior communicating unites 

 the internal carotid with the posterior cerebral, the latter being 

 the terminal branches of the basilar, which in its turn is formed 

 by the union of the two vertebrals. 



THE SUBCLAVIAN ARTERY 



on the right side arises from the arteria innominata, behind 

 the right sterno-clavicular articulation, and on the left side 

 directly from the arch of the aorta. They pass outward to the 

 outer margin of the first rib, where they become the axillary, 

 being divided in their course by the scalenus anticus muscle into 

 three portions the first portion to the inner side of the muscle, 

 second portion behind it, and the third portion between its outer 

 margin and the lower border of the first rib. The vessels differ 

 only in their first portions, the left ascending more vertically. 



Relation. The first portion of the right subclavian is in relation 

 in front to the internal jugular and vertebral veins, the sterno-hyoid 

 and sterno-thyroid and the clavicular portion of the sterno-mastoid 

 muscles, and the pneumogastric, phrenic, and cardiac nerves; behind 

 it is in relation with the longus colli muscle, the transverse process 

 of the first dorsal or seventh cervical vertebrae, the sympathetic nerve, 

 and the recurrent laryngeal nerve beneath with the pleura. 



The first portion of the left subclavian artery is in relation in 

 front with the left internal jugular and innominate veins, the left 

 carotid artery, the sterno-thyroid, sterno-hyoid, and sterno-mastoid 

 muscles, the pleura and left lung, and the pneumogastric, phrenic, and 

 cardiac nerves; behind, with the vertebral column and longus colli 

 muscle, the O3sophagus, and thoracic duct, and the inferior cervical 

 ganglion of the sympathetic; on the inner side with the oesophagus, 

 thoracic duct, and trachea, and on the outer side with the pleura. The 

 second portion of the subclavian artery on either side is in relation with 

 the subclavian vein, the scalenus anticus muscle, and the phrenic nerve ; 

 behind with the middle scalenus muscle and pleura; above by brachial 

 plexus, and below by the pleura. The third portion of the subclavian 

 artery on either side is in relation in front with the external jugular 

 vein and its branches, the clavicle, subclavius muscle and suprascapular 

 artery, the descending branches of the cervical plexus and the cervical 

 fascia; behind with the scalenus medius, above with the omo-hyoid 

 muscle and the brachial plexus, and below with the first rib. 



Its branches are: 



1. Vertebral, 5. Transverse cervical, or 



2. Thyroid axis, transversalis colli, 



3. Inferior thyroid, 6. Internal mammary, 



4. Suprascapular, 7. Superior intercostal. 



These branches are all given off from the first portion, ex- 

 cept the superior intercostal, which arises on the right side, from 

 the second portion. 



