COMMON CAROTID ARTERIES. 341 



or absence of the other thyroid arteries. It ascends to its destination in front of the 

 trachea, and its presence might therefore complicate the operation of tracheotomy. 



COMMON CAROTID ARTERIES. 



The common or primitive carotid arteries of the right and left sides of 

 the body are nearly similar in their course and position whilst they are in 

 the neck ; but they differ materially in their place of origin, and conse- 

 quently in their length and position, at their commencement. On the 

 right side the carotid artery commences at the root of the neck behind 

 the upper part of the sterno-clavicular articulation, at the bifurcation of the 

 innominate artery ; but on the left side the carotid arises within the thorax, 

 from the highest part of the arch of the aorta, very near the origin of the 

 innominate artery. The left carotid is therefore longer than the right, and 

 is at first placed at some depth within the thorax. 



In consequence of this difference, it is convenient to describe, at first, 

 the thoracic portion of the left carotid, or that part which intervenes 

 between the arch of the aorta and the sterno-clavicular articulation ; after 

 which, the same description will suffice for both vessels. 



Within the thorax, the left carotid ascends obliquely behind and at 

 some distance from the upper piece of the sternum and the muscles 

 (sterno-hyoid and sterno-thyroid) connected with that part of the bone ; it 

 is covered in front by the remains of the thymus gland, and is crossed by 

 the left innominate vein. This part of the artery lies in front of the 

 trachea, and of the oesophagus, which, at the root of the neck, deviates a 

 little to the left side ; the thoracic duct is also behind it. The left carotid 

 artery here lies between the innominate and the left subclavian arteries, and 

 the vagus nerve is to its outer side. 



In the neck the common carotid artery of each side reaches from behind 

 the sterno-clavicular articulation to the level of the upper border of the 

 thyroid cartilage, where it divides into two great branches, of which one is 

 distributed to the cranium and face, and the other to the brain and eye. 

 These divisions have, from their destination, been named respectively the 

 external and internal carotid arteries. 



The oblique course taken by the common carotid artery along the side of 

 the neck is indicated by a line drawn from the sterno-clavicular articulation 

 to a point midway between the angle of the jaw and the mastoid process of 

 the temporal bone. At the root of the neck, the arteries of opposite sides 

 are separated from each other only by a narrow interval, corresponding 

 with the width of the trachea ; but, as they ascend, they are separated by 

 a much larger interval, corresponding with the breadth of the pharynx and 

 larynx. The carotid arteries have the appearance of being placed farther 

 back at the upper than at the lower part of the neck, owing to the forward 

 projection of the larynx above. 



The common carotid artery is enclosed, together with the internal jugular 

 vein and the vagus nerve, in a common membranous investment, continuous 

 with the deep cervical fascia. Separated by means of this sheath from all 

 the surrounding parts, except the vein and nerve just mentioned, the 

 carotid artery is deeply placed at the lower part of the neck, but is com- 

 paratively superficial towards its upper end. It is covered below by the 

 sterno-mastoid, sterno-hyoid, and sterno-thyroid muscles, in addition to the 

 platysma and the layers of fascia between and beneath the muscles ; and it 

 is crossed opposite or near the lower margin of the cricoid cartilage by 



