THE COMMON CAROTID ARTERIES. 757 
thoracic course, and so far its relations call for separate consideration; whilst in 
the rest of its course it passes, like the right common carotid, upwards in the neck 
and has similar relations. 
Thoracic Portion of the Left Common Carotid. —The thoracic or mediastinal 
portion of the left common carotid artery (a. carotis communis sinistra) extends 
from the upper aspect of the aortic arch, a little behind and to the left of the 
origin of the innominate artery, to the left sterno-clavicular articulation, where 
the cervical portion commences. It is about one or one and a half inches in 
length (25 to 37 mm.), and it runs upwards and slightly outwards through the 
upper part of the superior mediastinum, lying farther back than the innominate 
artery and, therefore, being more overlapped by pleura. 
Relations.—Posterior.—The vessel is in contact behind and from below upwards with 
the trachea, the left recurrent laryngeal nerve, the esophagus, and the thoracic duct. 
Anterior.—The left innominate vein runs obliquely across the front of the artery, upon 
which cardiac branches from the left vagus and sympathetic descend vertically. These 
structures, together with the remains of the thymus gland 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-thyroid muscles. 
Lateral.—The innominate artery below and the trachea above are on the right side. 
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 three and a half inches long; it extends 
from the left sterno-clavicular articulation to the level of the upper border of the 
thyroid 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, outwards, and backwards, through the muscular and 
the lower part of the carotid divisions of the anterior triangle of the neck. In the 
lower part of its extent it is separated from its fellow of the opposite side by the 
trachea and the cesophagus, and in the upper part 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 anticus below and the rectus capitis anticus 
major above are separated from the posterior surface of the artery and sheath by the pre- 
vertebral fascia and the sympathetic cord. The inferior thyroid artery crosses close 
behind the vessel about the level of the first ring of the trachea ; lower down the vertebral 
artery and the thoracic duct are posterior to it, and the vagus nerve lies behind and to its 
outer side. 
Anterior.—The descendens cervicis nerve descends superficial to the artery, usually out- 
side the sheath, but sometimes enclosed in it. Opposite the sixth cervical vertebra the 
omo-hyoid muscle and the sterno-mastoid branch of the superior thyroid artery cross the 
carotid artery, which is overlapped, above the omo-hyoid muscle, by the anterior border of 
the sterno-mastoid, and is frequently crossed by the superior thyroid vein. Below the omo- 
hyoid the artery is covered by the sterno-thyroid, the sterno-hyoid, and the sterno-mastoid 
muscles, and it may be overlapped by the lateral lobe of the thyroid body ; it is also crossed 
beneath the muscles by the middle thyroid vein, whilst occasionally a communication 
between the common facial and anterior jugular veins descends in front of 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- 
thyroid muscles. 
Lateral.—The trachea and cesophagus, with the recurrent laryngeal nerve in the angle 
between them, are internal to the lower part of the artery; the pharynx and larynx are 
internal to its upper part. The carotid gland lies immediately to the inner side of the 
termination of the artery. 
The internal jugular vein occupies the outer part of the carotid sheath, and lies to the 
outer side of the artery, which it also overlaps in front, especially in the lower part of its 
extent. 
Branches.—As a rule no branches are given off from either of the common carotid 
arteries except the terminal branches and some minute twigs from each to the correspond- 
ing carotid sheath and carotid body. 
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