THE COMMON CAROTID ARTERY 533 



The thyreoida ima artery, which occurs iii about 10 per cent, of subjects, ascends on the 

 front of the trachea to the thyreoid gland. It may be large in which case it might complicate 

 the low operation of tracheotomy. It does not always arise from the innominate, but occa- 

 sionally from the arch of the aorta (fig. 443) or from the right common carotid. 



THE COMMON CAROTID ARTERIES 



The common carotid arteries [aa. carotides communes] pass up deeply from 

 the thorax on either side of the neck to about the level of the upper border of the 

 thyreoid cartilage, where they divide into the external and internal carotid 

 arteries. The external carotid supplies the structures at the upper part of the 

 front and side of the neck, the larynx, pharynx, tongue, face, the upper part of 

 the back of the neck, the structures in the pterygoid region, the scalp, and in 

 chief part the membranes of the brain. The internal carotid gives off no branch 

 in the neck, but enters the cranium and supplies the greater part of the brain, the 

 structures contained in the orbit, and portions of the membranes of the brain. 



The common carotid artery on the right side arises from the bifurcation of the 

 innominate at the upper limit of the sterno-clavicular joint; on the left side from 

 the arch of the aorta a little to the left of the innominate artery, and on a plane 

 somewhat posterior to that vessel (fig. 441). The portion of the left common 

 carotid artery which extends from the arch of the aorta to the upper limit of the 

 sterno-clavicular articulation lies deeply in the chest, and requires a separate 

 description; but above the level of the sterno-clavicular joint the relations of the 

 right and left carotids are practically the same, and are given under the account 

 of the right common carotid. 



THORACIC PORTION OF THE LEFT COMMON CAROTID ARTERY 



Within the thorax the left common carotid is deeply placed behind the 

 manubrium of the sternum, and is overlapped by the left lung and pleura. It 

 arises from the middle of the aortic arch, close to the left side of the innominate 

 artery, and a little posterior to that vessel, and ascends obliquely in front of the 

 trachea to the left sterno-clavicular articulation, above which its relations are 

 similar to those of the right common carotid (fig. 442). 



Relations. — In front, but at some httle distance, are the manubrium and the origins of the 

 left sterno-hyoid and sterno-thyreoid muscles, whilst in contact with it are the remains of the 

 thymus gland, and the loose connective tissue and fat of the superior mediastinum. Crossing 

 its root is the left innominate vein. 



Behind, it lies successively upon the trachea the left recurrent (inferior laryngeal) nerve, the 

 oesophagus (which here inclines a little to the left), and the thoracic duct. 



To its right side is the root of the innominate artery, and higher up are the trachea and the 

 inferior thyreoid veins. 



To its left side, but on a posterior plane, are the left subclavian artery and the left vagus 

 nerve; and, shghtly overlapping it, the edge of the left pleura and lung. 



THE COMMON CAROTID ARTERY IN THE NECK " 



The common carotid artery in the neck extends from the sterno-clavicular 

 articulation to the upper border of the thyreoid cartilage on a level with the fourth 

 cervical vertebra, where it divides into the external and internal carotid arteries. 

 A line drawn from the sterno-clavicular joint to the interval between the mastoid 

 process and the angle of the jaw would indicate its course. The artery is at first 

 deeply placed beneath the sterno-mastoid, sterno-hyoid, and sterno-thyreoid 

 muscles, and at the level of the top of the sternum is only 2 cm. (f in.) distant 

 from its fellow of the opposite side, and merely separated from it by the trachea. 

 As the carotid arteries run up the neck, however, they diverge in the form of a V 

 and become more superficial, though on a plane posterior to that in which they lie 

 at the root of the neck, and are separated from each other by the larynx and 

 pharynx. At their bifurcation they are about 6 cm. {2\ in.) apart. The com- 

 mon carotid is contained in a sheath of fascia common to it and the internal 

 jugular vein and vagus nerve. The artery, vein, and nerve, however, are not in 

 contact, but separated from one another by fibrous septa, which divide the com- 

 mon sheath into three compartments: one for the artery, one for the vein, and one 



