THE BRANCHES OF THE ARCH OF THE AORTA 251 



however, a thymic or bronchial branch or the arteria thyroidea 

 ima arises from it. 



The common carotid arteries are identical in course, branches, 

 and relations in the neck, but differ in their origin. Thus, the 

 right is a branch of bifurcation of the innominate, while the left 

 is a primary branch of the transverse aorta. From its origin 

 the left carotid passes obliquely upward and outward to the 

 left sternoclavicular joint, and from that point follows a course 

 corresponding to that of the right carotid. A thoracic portion 

 of the left carotid artery is therefore described. 



Relations. In front, sternum, sternohyoid and thyroid, 

 thymic remains, left innominate vein; behind, trachea, 

 esophagus, thoracic duct; left side, left subclavian artery, left 

 vagus; right side, inferior thyroid veins and innominate artery, 

 which is also somewhat in front. 



In the neck each carotid ascends from the sternoclavicular 

 joint to the level of the upper border of the thyroid cartilage, 

 there dividing into the external and internal carotids. Each is 

 enclosed, with the internal jugular vein and vagus, in a sheath 

 of deep cervical fascia, the several structures being partitioned 

 from one another within the sheath. The artery is internal, 

 the vein external, the nerve between them, but in a posterior 

 plane. 



Relations. In front, integument, fascia, platysma, sterno- 

 mastoid, hyoid, thyroid, and omohyoid muscles, descendens and 

 communicans hypoglossi nerves, sternomastoid artery, superior, 

 middle thyroid, and anterior jugular veins, and a branch con- 

 necting anterior jugular with facial; behind, longus colli, rectus 

 anticus major, spinal column, inferior thyroid artery, sympathetic 

 and recurrent laryngeal nerves; outer side, internal jugular vein, 

 vagus ; inner side, trachea, esophagus, larynx, pharynx, thyroid 

 gland, inferior thyroid artery, and recurrent nerve. In the lower 

 part of the neck the internal jugular diverges from the artery on 

 the right side, but approaches, and may cross it, on the left. 

 The common carotid regularly gives off no branches, but a 

 vertebral, thyroid, or laryngeal branch may arise from it on 

 either side. 



The external carotid artery runs from the bifurcation of the 

 common carotid to the space between the neck of the condyle 

 of the mandible and the auditory meatus, and there divides into 

 the superficial temporal and internal maxillary. At its origin 



