THE COMMON CAROTID 475 



To the left side are the left common carotid, the remains of the thymus gland, 

 the inferior thyroid veins; and higher, the trachea. 



Variations in the Innominate Artery 



The variations in the innominate artery are of surgical interest, (a) It may divide lower 

 than normal, thus decreasing the available space for the application of a ligature to it, but at the 

 same time increasing the length of the first portion of the right subclavian artery, {b) It may 

 divide higher than usual, and may then incline abnormally to the left, mounting in front of the 

 trachea above the sternum. Under these circumstances it is in danger in the low operation of 

 tracheotomy, (c) When abnormally long and inclining to the left, it may pass behind the trachea 

 or the oesophagus to gain the right side. ((/) It may give off the thyroidea ima artery, and, more 

 rarely the vertebral, the internal mammary or a smaller twig, as a bronchial, thymic, pericardiac, 

 or tracheal branch. 



The branches of the innominate artery are: (1) The right common carotid; 

 and (2) the right subclavian. These nw terminal branches. There are usually no 

 collateral branches from this vessel, but at times the thyroidea ima may arise 

 from it. 



THE COMMON CAROTID ARTERIES 



The common carotid arteries pass up deeply from the thorax on either side 

 of the neck to about the level of the upper border of the thyroid 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, tlie 

 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. 

 Tlie 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 liifurcation of 

 the innominate behind the sterno-clavicular joint; on the left side from the 

 arcli of the aorta a little to the left of the innominate artery, and on a some- 

 what posterior plane to that vessel (fig. 321). The portion of the left common 

 carotid artery which extends from the arch of the aorta to the level of the sterno- 

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

 tion; 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 first bone 

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

 middle of the transverse portion 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 rela- 

 tions are similar to those of the right common carotid. 



Relations. — In front, but at some little distance, are the manubrium and 

 the origins of the left sterno-hyoid and sterno-thyroid nmscles; whilst in con- 

 tact 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 innom- 

 inate vein. 



Behind, it lies successively upon the trachea, the cpsophagus (which here 

 inclines a little to the left), the thoracic duct, and the left recurrent laryngeal 

 nerve. 



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

 trachea and the inferior thyroid veins. 



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



