The left carotid artery arises from the arch of the aorta. The first stage of this artery may be divided into two portions,a thoracic and cervical: the thoracic extends from the origin of the vessel from the arch of the aorta, between the origins of the arteria innominata and left subclavian, and opposite to the second dorsal vertebra, to the upper and back part of the left sterno-clavicular articulation. This portion is therefore situated within the cavity of the thorax. Anteriorly, its origin is covered by the sternum, sterno-clavicular articulation, sternohyoid and sterno-thyroid muscles, and the commencement of the left vena innominata; and higher up, in its second or cervical portion, it has the same anterior relations as the artery of the right side. Internally it is related to the arteria innominata, trachea, oesophagus, and thymus gland which usually overlaps it. In close relation to it externally we find the internal jugular vein, and the pneumogastric nerve, which lies concealed deeply between the artery and the vein; the phrenic nerve, and the upper part of the left pleura and lung are also related to its outer side : the thoracic duct lies posterior to the artery at its origin, but afterwards passes to its external side. Posteriorly, it first rests on the left side of the trachea, and on the oesophagus, and afterwards upon parts similar to those which constitute the posterior relations of the right common carotid. Higher up it is separated from the spine and longus colli muscle by the sympathetic nerve and its cardiac branches, as at the right side. These are the usual relations of the left carotid, but they may vary, or their number be increased in consequence of irregularities.

Hence it appears that the right and left common carotids differ in the following respects in their first stage. The right comes from the arteria innominata, and the left from the arch of the aorta; consequently the left is longer than the right. The left lies within the cavity of the thorax, on the front of the trachea and oesaphagus, and is intimately connected with the thoracic duct. On the right side the internal jugular vein separates from the artery inferiorly, passing outwards from its external surface; a small vascular triangle is thus formed, bounded internally by the carotid artery, externally by the internal jugular vein, and inferiorly by the first stage of the subclavian artery. On the left side the jugular vein overlaps the outer edge of the carotid artery inferiorly, so that no such vascular triangle exists.

The student should now examine the large venous trunks which are related to the thoracic portion of the left carotid artery. The left internal jugular vein will be seen descending along the outer side of the artery, and in this situation uniting with the left subclavian vein to the right side of, and on a plane anterior to, the left subclavian artery, to form the left vena innominata. When the jugular vein is distended it overlaps the outer part of the left common carotid artery in this situation. The origin of the left vena innominata will be therefore anterior to a point corresponding to the narrow interspace between the lower parts of the thoracic portions of the left carotid and subclavian arteries: it then passes obliquely in front of the left common carotid, the trachea and arteria innominata, and unites, as already described, with the right vena innominata to form the vena cava descendens. In its course the left vena innominata receives the inferior thyroid, the left internal mammary, left phrenic, pericardial, and anterior mediastinal veins. The left vena innominata is retained in its position by a thin layer of the descending portion of the thoracic fascia.