368 ARTERIES. 



pliagus, and thoracic duct. Internally, it is in relation with the arteria innominata ; 

 externally, with the left pneumogastric nerve, and left subclavian artery. 



PLAN OF THE EELATIONS OF THE LEFT COMMON CAROTID ; 

 THORACIC PORTION. 



In front. 

 Sternum. 



Sterno-hyoid and Sterno-thyroid muscles. 

 Left innominate vein. 

 Remains of thymus gland. 



Internally. ^ ^ Externally. 



Arteria innominata. / Xeft pneumogastric nerve. 



/ T.nfY \ T <! I 1 



Lett 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. Starting from each side of the neck, 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, the former from its distribution 

 to the external parts of the head and face, the latter from its distribution to the 

 internal parts of the cranium. The course of the vessel 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 mastoid 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 corresponds to the trachea ; but 

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

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

 in this 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 ter- 

 mination, 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 within 

 outwards by the sterno-mastoid artery ; it is also crossed by the facial, lingual, 

 and superior thyroid veins, which terminate in the internal jugular, and, descending 

 on its sheath in front, is seen the descendens noni nerve, this filament being joined 

 with branches from the cervical nerves, which cross the vessel from without 

 inwards. Sometimes the descendens noni is contained within the sheath. The 

 middle thyroid vein crosses it about its centre, and the anterior jugular vein below. 

 Behind, the artery lies in front of the cervical portion of the spine, resting first 



