"lo A MANUAL OF ANATOMY 



inner, and posterior, the latter being mesially placed. The outer 

 and inner compartments are separated from each other by a mesial 

 septum, the inner compartment containing the common carotid 

 artery and, it may be, the descendens cervicis nerve, and the outer 

 compartment the internal jugular vein. The posterior compart- 

 ment is situated within the back part of the septum, and contains 

 the pneumogastric nerve. The descendens cervicis nerve may lie 

 upoii the sheath, or within it, and the gangliated cord of the sympa- 

 thetic descends behind, and in intimate relation with, it. 



Common Carotid Arteries. — ^The right common carotid artery 

 arises from the innominate artery behind the upper border of the 

 right stemo-clavicular joint, and the left common carotid artery 

 arises from the upper surface of the arch of the aorta, in close 

 proximity to the origin of the innominate artery. The vessel of 

 the right side is therefore entirely cervical, whilst that of the left 

 side is partly thoracic and partly cervical. 



The thoracic part of the left common carotid artery has already 

 been described in connection with the thorax (see p. 961). 



In the neck the common carotid artery of each side extends 

 from the back of the corresponding stemo-clavicular joint to 

 the level of the upper border of the thyroid cartilage of the 

 lar5aix, which corresponds to the disc between the bodies of 

 the third and fourth cervical vertebrae. At this level the 

 vessel divides into the external and internal carotid arteries. 

 The place of bifurcation is sometimes opposite the body of the 

 hyoid bone, and, more rarely, on a level with the cricoid carti- 

 lage of the larynx. The vessel is about 3I inches long, and its 

 course is upwards and outwards in the direction of a line drawn 

 from the stemo-clavicular joint to a point midway between the 

 angle of the inferior maxilla and the mastoid process of the temporal 

 bone. This line, as high as the level of the upper border of the 

 thyroid cartilage, represents the course of the common carotid 

 artery. At the root of the neck the two common carotid arteries 

 are not very far apart, and the trachea lies in the intervening space. 

 As the two vessels ascend they become more divergent, on account 

 of the projection of the lateral lobes of the thyroid body and the 

 thyroid cartilage. 



The artery, along with the intemal jugular vein and pneumo- 

 gastric nerve, and, it may be, the descendens cervicis nerve, is con- 

 tained within the carotid sheath, already described. Opposite 

 the cricoid cartilage it is crossed by the anterior belly of the omo- 

 hyoid muscle. Below this level it lies deeply in the region of the 

 muscular triangle, being under cover of the stemo-hyoid and stemo- 

 thyroid muscles, in addition to the platysma myoides and the 

 anterior border of the sterno-cleido-mastoid. Above this level it 

 is situated in the carotid triangle, being here more superficially 

 placed, its only muscular coverings being the platysma myoides 

 and the anterior border of the sterno-cleido-mastoid. 



Relations — Anterior. — ^The skin ; superficial fascia and platysma 



