444 ARTERIES. 



thorax. It will, therefore, be more convenient to describe first the course and 

 relations of that portion of the left carotid which intervenes between the arch 

 of the aorta and the left sterno-clavicular articulations (see Fig. 272). 



The left carotid within the thorax ascends obliquely outwards 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, oesophagus, and thoracic duct. Internally, it is in relation with the 

 arteria innominata ; externally., with the left pnemogastric nerve, and left sub- 

 clavian artery. 



PLAN OF THE KELATIONS OF THE LEFT COMMON CAROTID. 

 THORACIC PORTION. 



In Front. 



Sternum. 



Sterno-hyoid and Sterno-thyroid muscles. 



Left innominate vein. 



Eemains of thymus gland. 



/^~~\ 



T /Left CommonX Externally. 



Internally. Carotid ) Left pneumogastric nerve. 



Arteria innominata. \ **> ) 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 upwards, 

 from behind the sterno-clavicular articulation, to a level with the upper border 

 of the thyroid cartilage, 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 respectively. 

 The course of the common carotid is indicated by a line drawn from the sternal 

 end of the clavicle below, to a point midway between the angle of the jaw and 

 the mastgid process above. 



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

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

 the upper part, the thyroid body, the larynx and pharynx project forwards 

 between the two vessels, and give the appearance of their being placed further 

 back in that situation. The common carotid artery is contained in a sheath, 

 derived from the deep cervical fascia, which also incloses the internal jugular 

 vein and pneumogastric 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 inclosed in a separate fibrous investment. 



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

 deeply seated, being covered by the superficial fascia, Platysma, and deep fascia, 

 the Sterno-mastoid, Sterno-hyoid, and Sterno-thyroid muscles, and by the Omo- 

 hyoid opposite the cricoid cartilage ; but in the upper part of its course, near 

 its termination, it is more superficial, being covered merely by the integument, 

 the superficial fascia, Platysma, and deep fascia, and inner margin of the Sterno- 

 mastoid, and is contained in a triangular space, bounded behind by the Sterno- 

 mastoid, above by the posterior belly of the Digastric, and below by the anterior 

 belly of the Omo-hyoid. This part of the artery is crossed obliquely from 



