THE HEART AND VASCULAR SYSTEM. 179 



From the sterno-clavicular articulation to their division at 

 the upper border of the thyroid cartilage the common carotid on 

 either side pursues the same course. At the lower part of the 

 ne'ck the two vessels are separated by a very short interval, which 

 is occupied by the trachea, but at the upper part they diverge 

 widely, the larynx, pharynx, thyroid body, and several muscles 

 being interposed between them. It ascends the neck in a direc- 

 tion indicated by a line drawn from the sterno-clavicular articula- 

 tion to midway between the mastoid process and the angle of the 

 lower jaw. 



Relations. It is in relation in front with the sterno-mastoid, 

 sterno-thyroid, sterno-hyoid, and omo-hyoid muscles, the anterior 

 jugular and the superior and middle thyroid veins, the sterno-hyoid 

 artery, and the descendens and communicans noni nerves; behind it 

 rests upon the longest colli and rectus capitis anticus major muscles 

 and is in relation with the sympathetic nerve, the recurrent laryngeal 

 nerve, and the inferior thyroid artery; on the outer side, with the 

 pneumogastric and the internal jugular vein; to the inner side, the 

 larynx, trachea, pharynx, thyroid gland, inferior thyroid artery, and 

 the recurrent laryngeal nerve. 



THE EXTERNAL CAROTID ARTERY. 



From its commencement at the superior border of the thy- 

 roid cartilage it passes upward and forward and then backward, 

 beneath the anterior margin of the sterno-mastoid muscle, in a 

 direction indicated by an imaginary line drawn from the supe- 

 rior border of the thyroid cartilage to a point midway between 

 the meatus and the neck of the condyle of the lower jaw. It 

 ascends through the space known as the superior triangle of the 

 neck. 



Relations. It is in relation in front with the parotid gland and 

 the facial nerve and temporo-maxillary passing through it, covered by 

 the skin, superficial platysma, and deep fascia, and is crossed by the 

 digastric and stylo-hyoid muscles, hypoglossal nerve, lingual and facial 

 veins; behind it is separated from the internal carotid by the parotid 

 gland, stylo-glossus and stylo-pharyngeus muscles, and the glosso- 

 pharyngeal nerve, and at its lower part the superior laryngeal nerve 

 ascends behind it; to its inner side it is in relation with the pharynx, 

 hyoid bone, superior laryngeal nerve, and the ramus of the lower jaw, 

 from which it is separated by the parotid gland. 



The branches of the external carotid are eight : 



1. Superior thyroid, 5. Posterior auricular, 



2. Lingual, 6. Ascending pharyngeal, 



3. Facial, 7. Temporal, 



4. Occipital, 8. Internal maxillary. 



