THE COMMON CAEOTID AKTEKIES. 889 



thoracic course, and, so far, its relations call for separate consideration ; whilst in 

 the rest of its course it passes upwards in the neck, like the right common carotid, 

 and has almost similar relations. 



Thoracic Portion of the Left Common Carotid. The thoracic or mediastinal 

 portion of the left common carotid artery extends from the upper aspect of the 

 aortic arch, immediately posterior and to the left of the origin of the innominate 

 artery, to the left sterno-clavicular articulation, where the cervical portion com- 

 mences. It is from 25 to 37 mm. (1 or 1J inches) in length, and it runs 

 upwards and slightly laterally through the upper part of the superior mediastinum. 

 It lies on a more posterior plane than the innominate artery. 



Relations. Posterior. The vessel is in contact posteriorly, and from below upwards, 

 with the trachea, the left recurrent nerve, the oesophagus, and the thoracic duct ; and the 

 thoracic part of the left subclavian artery is a postero-lateral relation. 



Anterior. The left innominate vein runs obliquely across the anterior aspect of the 

 artery, upon which cardiac branches from the left vagus and sympathetic descend 

 vertically. These structures, together with the remains of the thymus and the anterior 

 margins of the left lung and pleura, separate the artery from the manubrium sterni, 

 and from the origins of the sterno-hyoid and sterno-thyreoid muscles. 



Medial. The innominate artery below, and the trachea above, are on the right side. 



Lateral. The left pleura, and, on a posterior plane, the left phrenic and vagus nerves 

 and the left subclavian artery are on its left side. 



Cervical Portion of the Left Common Carotid Artery. The cervical part of 

 the left common carotid artery is about 85 mm. (three and a half inches) long ; it 

 extends from the left sterno-clavicular articulation to the level of the upper border 

 of the thyreoid cartilage and the lower border of the third cervical vertebra, where 

 it ends by dividing into the external and internal carotid arteries. 



Course. It runs upwards, laterally, and backwards, through the muscular and 

 in the lower portion of the carotid divisions of the anterior triangle of the neck. 

 Below it is separated from its fellow of the opposite side by the trachea and the 

 oesophagus, and above by the relatively wide pharynx. 



Relations. It is enclosed, together with the internal jugular vein and the vagus 

 nerve, in a sheath of deep cervical fascia the carotid sheath. 



Posterior. The longus colli and scalenus anterior, below, and the longus capitis, above, 

 are separated from the posterior surface of the artery and its sheath by the pre vertebral 

 fascia and the sympathetic trunk. The vertebral artery arid the thoracic duct are posterior 

 to it at the level of the seventh cervical vertebra ; the inferior thyreoid artery crosses 

 behind it, either between it and the vertebral or between it and the transverse process of 

 the sixth cervical vertebra, and the vagus nerve lies postero-lateral to it. 



Superficial. The descendens branch of the hypoglossal nerve lies superficial to the 

 artery, usually outside the sheath, but sometimes enclosed in it (Fig. 759). Opposite the 

 sixth cervical vertebra the omo-hyoid muscle and the sterno-mastoid branch of the superior 

 thyreoid artery cross superficial to the carotid artery, which is overlapped, above the omo- 

 hyoid muscle, by the anterior border of the sterno-mastoid and by cervical lymph glands. 

 It is frequently crossed, in that part of its extent, by the superior thyreoid vein (Figs. 759, 

 36). Below the omo-hyoid the artery is covered by the sterno-thyreoid, the sterno-hyoid, 

 and the sterno-mastoid muscles, and it may be overlapped by the lateral lobe of the 

 thyreoid gland ; it is also crossed, deep to the muscles, by the middle thyreoid vein, whilst 

 occasionally a communication between the common facial and anterior jugular veins 

 descends anterior to the artery along the anterior border of the sterno-mastoid. Just 

 above the sternum the anterior jugular vein is in front of the artery, but separated from 

 it by the sterno-hyoid and sterno-thyreoid muscles. 



Medial. The trachea and oesophagus, with the recurrent nerve in the angle between 

 them, are medial to the lower part of the artery ; the larynx and pharynx are medial 

 to its upper part. The carotid gland or glomus carOticum lies on the medial side of the 

 termination of the artery. 



Lateral. The internal jugular vein occupies the lateral part of the carotid sheath. 

 The vein lies not only to the lateral side of the artery, but also slightly in front of it, 

 especially in the lower part of the neck. 



Branches. As a rule no branches are given off from either of the common carotid 



