756 THE VASCULAR SYSTEM. 
the left subclavian arteries. The innominate is a short trunk from the termination 
of which the right common carotid and the right subclavian arteries spring (Figs. 
552 and 556); thus there is at first a difference between the stem vessels of opposite 
sides, but beyond this the subsequent course and the ultimate distribution of these 
vessels closely correspond. 
THE INNOMINATE ARTERY. 
The innominate artery (a. anonyma, Fig. 552) arises behind the middle of the 
lower part of the manubrium sterni, from the convexity of the arch of the aorta 
near its right or anterior extremity, and terminates opposite the right sterno- 
clavicular articulation, where it divides into the right subclavian and right common 
carotid arteries. 
Course.—The trunk, which measures from one and a half to two inches (37 to 
50 mm.) in length, runs upwards, backwards, and outwards in the superior 
mediastinum. 
Relations.—Posterior.—It is in contact behind, with the trachea below and with 
the right pleural sac above. 
Anterior.—The left innominate vein crosses in front of the lower part of the artery, 
and above this the sterno-thyroid muscle separates it from the sterno-hyoid and the right 
sterno-clavicular joint. ‘The anterior margin of the right pleural sac overlaps the artery, 
and the remains of the thymus gland, which separate it from the manubrium sterni, are 
also in front. 
Lateral.—The right innominate vein and the upper part of the superior vena cava 
are on the right side of the artery. On its left side is the origin of the left common 
carotid artery, whilst at a higher level the trachea is in contact with it. 
Branches.—As a rule the innominate artery does not give off any branches except 
its two terminals, but occasionally it furnishes an additional branch, the thyroidea ima. 
The thyroidea ima is an inconstant and slender vessel When present it 
sometimes arises from the arch of the aorta, but it usually springs from the lower 
part of the innominate. It passes upwards in front of the trachea, through the 
anterior part of the superior mediastinum and the lower part of the neck, and gives off 
branches to the lateral lobes and isthmus of the thyroid body and to the trachea. 
THE ARTERIES OF THE HEAD AND NECK. 
The vessels distributed to the head and neck are chiefly derived from the 
carotid trunks; there are, however, in addition, other vessels which arise from the 
main arterial stems of the upper extremities, and it will be advantageous to 
describe the most important of these, viz. the vertebral arteries, with the carotid 
system. The smaller additional branches will be considered along with the 
remaining branches of the subclavian arteries. 
The carotid system of arteries consists on each side of a common carotid trunk, 
which divides into internal and external carotid arteries, from which numerous 
branches are given off. 
The internal carotid arteries are distributed almost entirely to the contents of 
the cranial cavity internal to the dura mater, and to the structures in the cavity 
of the orbit. The external carotid arteries, on the other hand, supply structures of 
the head and neck more externally situated. 
It is to be observed, however, that the vascular supply of the brain is not wholly 
derived from the internal carotid vessels, but that the vertebral arteries also 
contribute largely to it. 
THE COMMON CAROTID ARTERIES. 
The right and the left common carotid arteries are of unequal length. The 
right common carotid commences at the bifurcation of the innominate artery, 
behind the right sterno-clavicular articulation; the left arises in the superior 
mediastinum from the arch of the aorta, but each terminates at the level of 
the upper border of the thyroid cartilage; the left artery has thus a short intra- 
