392 THE ARTERIES OF THE HEAD AND NECK. 



Varieties. Origin. The rigid carotid artery occasionally arises directly from the aorta, 

 either alone or in conjunction with the left carotid ; and in the latter case it has been seen 

 beginning on the left of the middle line, and crossing the front of the trachea above the 

 upper border of the sternum to gain its usual position on the right side. When it arises 

 from the aorta, it is usually the first large vessel from the arch, the subclavian being dis- 

 placed ; but it has been found to occupy the second place, the right subclavian or, in cases 

 of a right aortic arch, the left carotid being the first. 



The place at which the right carotid artery commences varies with the point of bifurca- 

 tion of the innominate artery. A change from the usual position on a level with the upper 

 border of the clavicle was found by R. Quain in the proportion of about one case in eight 

 and a half of those observed by him ; and it was found to occur somewhat more frequently 

 below than above that point. 



The left carotid artery varies in its origin much more frequently than the right. In the 

 greater number of its deviations from the ordinary place of origin, this artery arises from, 

 or in conjunction with, the innominate artery ; and in those cases in which the right sub- 

 clavian is a separate branch of the aorta, the two carotids most frequently arise by a common 

 trunk. 



In cases of transposition, or of right aortic arch without other abnormality, the left 

 common carotid springs from a left innominate artery, which is the first vessel to arise from 

 the arch, and the right carotid is the second vessel. 



Place of division. This often deviates somewhat from its usual position ; it does so more 

 frequently in an upward than in a downward direction. It is often as high as the hyoid 

 bone, and occasionally much higher. It is found occasionally opposite the middle of the 

 larynx, and, in rare instances, opposite the lower margin of the cricoid cartilage, or even 

 lower. One case was observed by Morgagni, in which the carotid artery, measuring one inch 

 and a half in length, divided at the root of the neck. The carotid artery has been found, as 

 a very rare occurrence, to asoand in the neck without dividing into the two usual branches ; 

 either the external or the internal carotid being altogether wanting. 



In a few recorded cases there was no common carotid artery, the external and internal 

 carotids arising directly from the arch of the aorta (Malacarne, v. p. 386 ; Power, Macalister, 

 on right side ; G-ottschau, on left side), or from the termination of the innominate artery 

 (Kosinski). 



Relation, to nerves. The pneumo -gastric nerve has been observed to descend in front of 

 the artery. 



Occasional branches. The common carotid artery somstimes gives origin at its upper end 

 to the superior thyroid or ascending pharyngeal artery. In rarer cases it furnishes a laryn- 

 geal, or an inferior or accessory thyroid branch, or from its lower part the vertebral artery. 



SURGICAL ANATOMY OF THE COMMON CAROTID ARTERY. 



As the common carotid does not in ordinary cases furnish any branch, a ligature may b3 

 applied to any part of the vessel except close to its comnnncement or termination. It is 

 usually tied either immediately above or balow the omo-hyoid muscle, the former situation 

 being preferred if possible, since the artery is here more supsrficial, and the operation is 

 consequently free from the difficulties caused by the muscles lower down. An incision is 

 made along the anterior border of the sterno-mastoid muscle, through the integuments and 

 fascia, and in doing this the communicating vein above referred to (p. 391), if present, must 

 be avoided, or it may if necessary be secured with two ligatures and then divided. The small 

 branch of the superior thyroid artery to the sterno-mastoid muscle will also probably be cut. 

 The sterno-mastoid is next everted, and the anterior belly of the omo-hyoid displayed and 

 drawn inwards and downwards. The sheath is now exposed and is to be opened over the 

 artery near the trachea, in order to avoid the internal jugular vein. The special sheath is 

 next to be separated from the artery, and the aneurism needle should be passed from the 

 outer side, for thus the vein and the pneumo-gastric nerve will be most effectually avoided. 

 In opening the sheath the possible occurrence of a middle thyroid vein, crossing the artery 

 at the level of the oricoid cartilage, should be borne in mind, and the descending branch of 

 the hypoglossal nerve, if it comes into view, must be carefully preserved. Should the jugular 

 vein overlap the artery, as it sometimes does, especially at the lower part of the neck on the 

 left side, it will be a source of much difficulty in completing the operation, and great caution 

 will be required in passing the needle round the artery. If the operation is performed at 

 the lower part of the neck, some fibres of the muscles will require to be cut across in order 

 to lay the artery bare with facility ; and the necessity for this step increases in approaching 

 the clavicle. Near the clavicle also the transverse lower portion of the anterior jugular vein 

 crosses the line of the incision. 



Collateral circulation. After ligature of the common carotid trunk the blood is conveyed to 



