i 



536 THE BLOOD-VASCULAR SYSTEM 



nerve and vertebral artery are exposed. On the left side the vein approaches and somewhat 

 overlaps the artery, thus leaving no interval corresponding to that on the right side. 



The cricoid cartilage is, as a rule, taken as the centre of the incision in the operation for 

 ligature of the common carotid artery. The incision is made in the line of the vessel parallel 

 to the anterior margin of the sterno-mastoid muscle. The omo-hyoid forms one of the chief 

 rallying points in the course of the operation for ligature of the artery above that muscle, the 

 usual situation. The artery is found beating at the angle formed by the omo-hyoid with the 

 sterno-mastoid. 



Branches. — ^(1) External and (2) internal carotid arteries. The common 

 carotid gives off no lateral branch, and consequently does not diminish in size as it 

 runs up the neck. It is often a little swollen just below its bifurcation, a condition 

 that should not be mistaken for an aneurismal dilation. 



The collateral circulation (fig. 444), after ligature of the common carotid, is carried on 

 chiefly by the anastomosis of the internal carotid with the internal carotid of the opposite side 

 through the circle of Willis; by the vertebral with the opposite vertebral; by the inferior thy- 

 reoid with the superior thyreoid; by the deep cervical branch of the costo-cervical trunk (superior 

 intercostal) with the descending branch of the occipital; by the superior thyreoid, lingual, 

 external maxiUary (facial), occipital, and temporal, with the corresponding arteries of the oppo- 

 site side, and by the ophthalmic with the angular. The anastomosis between the deep cervical 

 branch of the costo-cervical trunk with the descending branch of the occipital is an important 

 one; it is situated deeply at the back of the neck, and is to be found lying between the semi- 

 spinaUs capitis (complexus) and cervicis muscles. 



THE EXTERNAL CAROTID ARTERY 



The external carotid artery [a. carotis externa] (fig. 445), the smaller of the 

 two branches into which the common carotid divides at the upper border of the 

 thyreoid cartilage, is distributed to the anterior part of the neck, the face, and 

 the cranial region, including the skin, the bones, and the dura mater. It is 

 at first situated medial to the internal carotid; but as it ascends in the neck it 

 forms a gentle curve, with its convexity forward, and, running slightly backward 

 as well as upward, terminates opposite the neck of the mandible just below the 

 condyle, by dividing into the internal maxillary and superficial temporal arteries. 

 It here lies superficial to the internal carotid, from which it is separated by a 

 portion of the parotid gland. At its origin it is overlapped by the anterior margin 

 of the sterno-mastoid, and is covered by the superficial fascia, platysma, and deep 

 fascia. Higher up the neck it is deeply placed, passing beneath the stylo-hyoid 

 muscle, the posterior belly of the digastric muscle, and the hypoglossal nerve; 

 and finally becomes embedded in the parotid gland, where it divides into its 

 terminal l)ranches. It is separated from the internal carotid artery posteriorly 

 by the stylo-pharyngeus and stylo-glossus muscles, the glosso-pharyngeal nerve, 

 the pharyngeal branch of the vagus nerve, a portion of the parotid gland, and the 

 stylo-hyoid ligament; or, if the styloid process is abnormally long, by that 

 process itself. It measures about 6.5 cm. (2| in.). 



Relations. — In front, in addition to the skin, superficial fascia, platysma, and deep fascia, 

 it has the hypoglossal nerve, the lingual, common facial and posterior facial veins, the posterior 

 beUy of the digastric and stylo-hyoid muscles, the superior cervical lymphatic glands, branches of 

 the facial nerve, and the parotid gland. The sterno-mastoid also overlaps it in the natural 

 state of the parts. 



Behind, it is in relation with the internal carotid, from which it is separated by the stylo- 

 glossus and stylo-pharyngeus muscles, the glosso-pharyngeal nerve, the pharyngeal branch of 

 the vagus nerve, the stylo-hyoid ligament, and the parotid gland. The superior laryngeal nerve 

 crosses behind both the external and internal carotid arteries. 



Medially, it is in relation with the hyoid bone, the pharyngeal wall, the ramus of the 

 mandible, the stylo-mandibular ligament which separates it from the submaxillary gland, and 

 the parotid gland. 



Laterally, in the first part of its course, it is in contact with the internal carotid artery. 



The branches of the external carotid are usually given off in the following 

 order, from below upward: — ■ 



1. Ascending pharyngeal. 



2. Superior thyreoid. 



3. Lingual. 



4. External maxillary (facial). 



5. Sternocleidomastoid. 



6. Occipital. 



7. Posterior auricular. 



8. Superficial temporal. 



9. Internal maxillary. 



