THE COMMOX CAROTID ARTERIES. 



547 



The importance of avoiding the thyroid plexus of veins during the primary steps of the opera- 

 tion, and the pleural sac whilst including the vessel in the ligature, should be most carefully 

 borne in ruind. The niu*t frequent cause of death after operation is secondary haemorrhage, 

 which has occurred in almost every case. Other causes are pleurisy, pericarditis, and suppura- 

 tive cellulitk 



THE COMMON CAROTID ARTERIES. 



The common carotid arteries, although occupying a nearly similar position in 

 the neck, differ in position, and, consequently, in their relation at their origin. 

 The right carotid arises from the innominate artery, behind the right sterno- 

 clavicular articulation ; the left from the highest part of the arch of the aorta. 

 The left carotid is, consequently, longer, and at its origin is contained within the 

 thorax. The course and relations of that portion of the left carotid which inter- 

 venes between the arch of the aorta and the left sterno-clavicular articulation will 

 first be described (see Fig. 344). 



The left carotid within the thorax ascends obliquely outward from the arch of 

 the aorta to the root of the neck. In front, it is separated from the first piece of 

 the sternum by the Sterno-hyoid and Sterno-thyroid muscles, the left innominate 

 vein, and the remains of the thymus gland ; behind, it lies on the trachea, oesoph- 

 agus, and thoracic duct. Internally^ it is in relation with the innominate 

 artery, inferior thyroid veins and remains of thymus gland ; externally, with the 

 left pneurnogastric nerve, left pleura, and lung. The left subclavian artery is 

 posterior and external to it. 



PLAN OF THE RELATIONS OF THE LEFT COMMON CAROTID. 

 THORACIC PORTION. 



In front. 

 Sternum. 



Sterno-hyoid and Sterno-thyroid muscles. 

 Left innominate vein. 

 Remains of thymus gland. 



Internally. 



Innominate artery. 

 Inferior thyroid veins. 

 Remains of thymus gland. 



Externally. 



Left pneumogastric nerve. 

 Left pleura and lung. 

 Left subclavian artery. 



Behind. 



Trachea. 

 (Esophagus. 

 Thoracic duct. 



In the neck the two common carotids resemble each other so closely that one 

 description will apply to both. Each vessel passes obliquely upward from behind 

 the sterno-clavicular articulation to a level with the upper border of the thyroid 

 cartilage, opposite the third cervical vertebra, where it divides into the external 

 and internal carotid ; these names being derived from the distribution of the 

 arteries to the external parts of the head and face and to the internal parts of the 

 cranium and orbit respectively. 



At the lower part of the neck the two common carotid arteries are separated 

 from each other by a small interval, which contains the trachea; but at the upper 

 part, the thyroid body, the larynx and pharynx project forward between the 

 two vessels, and give the appearance of their bemg placed farther back in that 

 situation. The common carotid artery is contained in a sheath derived from the 

 deep cervical fascia, which also encloses the internal jugular vein and pneumo- 

 gastric nerve, the vein lying on the outer side, of the artery, and the nerve between 

 the artery and vein, on a plane posterior to both. On opening the sheath these 

 three structures are seen to be separated from one another, each being enclosed in 

 a separate fibrous investment. 



Relations. At the lower part of the neck the common carotid artery is very 



