THE EXTERNAL CAROTID ARTERY. 733 



to be placed as near the root of the neck as possible. The internal jugular vein — 

 especially on the left side— the inferior thyroid artery, and the recurrent laryngeal 

 nerve must be avoided. The needle is passed from without inward. 



The collateral circulation is carried on from the proximal or cardiac side through 

 (a) the branches of the external carotid on the opposite side, (<^) the inferior thy- 

 roid, (<:) the profunda cervicis (from the superior intercostal and thus from the sub- 

 clavian), (flf ) the internal carotid and the vessels of the opposite segment of the 

 circle of Willis, and {e) the vertebral, by anastomosing respectively with (^) the 

 external carotid branches, (<^) the superior thyroid, (r) the princeps cervicis (from 

 the occipital), and i^d ) and (e) the vessels of the circle of WilHs on the affected side. 



THE EXTERNAL CAROTID ARTERY. 



The external carotid artery (a. carotis externa) (Figs. 692, 693) arises from the 

 common carotid at about the level of the upper border of the thyroid cartilage — a 

 level which corresponds to the body of the fourth cervical vertebra. Thence it is 

 directed upward and slightly backward towards the angle of the jaw, where it enters 

 the substance of the parotid gland and continues upward in that structure to just 

 below the root of the zygoma. Here it gives rise to a large branch, the internal 

 maxillary, and is then continued upward over the root of the zygoma upon the side 

 of the skull, this terminal portion of it being termed the superficial temporal artery. 



Relations. — In the first portion of its course the external carotid lies in the 

 superior carotid triangle (page 548), and is there crossed by the hypoglossal nerve 

 and the facial vein. Higher up it passes beneath the posterior belly of the digastric 

 and the stylo-hyoid muscles and also beneath the temporo-maxillary vein, and enters 

 the substance of the parotid gland. Posteriorly it is separated from the internal 

 carotid artery by the stylo-glossus and stylo-pharyngeus muscles and the glosso- 

 pharyngeal nerve ; the internal carotid artery lies laterally to it at its origin ; 

 internally it is in relation with the inferior and middle constrictors of the pharynx 

 and the superior laryngeal nerve. 



Branches. — From the anterior surface of the external carotid arise, from 

 below upward, (i) the sicperior thyroid, (2) the lingual, (3) ihe facial, and (4) 

 the internal maxillary arteries. From its posterior surface, in the same order of 

 succession, arise (5) the ascending pharyngeal, (6) the sterno-mastoid, (7) the 

 occipital, (8) \he posterior auricular arteiies. Finally, (9) \}^& superficial temporal 

 artejy is to be regarded as a branch which is the continuation upward of the 

 main stem. 



Variations. — Occasionally the external carotid artery is absent, its branches arising from 

 the common carotid, which is continued directly into the internal carotid. The number of its 

 branches may be reduced by certain of them, the lingual and facial, for instance, arising by a 

 common stem, or they may be increased by the occurrence of various accessory branches pass- 

 ing to regions supplied by the regular ones. 



Practical Considerations. — The external carotid is rarely the subject of 

 aneurism, except as a result of trauma. The tumor is situated below the angle of 

 the jaw. Pressure on the hypoglossal and glosso-pharyngeal nerves and on the 

 internal jugular vein causes various symptoms which are not usually definitely diag- 

 nostic. In one case there was unilateral atrophy of the tongue (Heath) probably 

 from involvement of the hypoglossal. If the aneurism is situated near the origin of 

 the vessel, it may be indistinguishable from aneurism of the common carotid at its 

 usual location, just below the bifurcation. The vessel is not infrequently tied for 

 wound of the neck, for aneurism of one of its branches, and occasionally as a pre- 

 liminary to certain operations, as excision of the superior maxilla or removal of a 

 malignant tonsillar or parotid tumor. In cases of stab or cut-throat wound it is 

 better, when possible, to find and tie both ends of the bleeding vessel, as the free 

 anastomosis between the branches of the two external carotids renders a recurrence 

 of hemorrhage probable after ligation of the main trunk. 



Ligation. — That part of the line for the common carotid extending from the 

 level of the angle of the lower jaw to that of the middle of the thyroid cartilage is the 



