888 THE VASCULAR SYSTEM. 



They are three in number, viz., the innominate, the left common carotid, and 

 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. 

 756 and 757) ; thus there is, at first, a difference between the stem vessels of opposite 

 sides, but the subsequent course and the ultimate distribution of those vessels 

 closely correspond. 



ARTERIA ANONYMA. 



The innominate artery (Fig. 757) arises, posterior to the middle of the 

 manubrium sterni, from the convexity of the arch of the aorta near its right 

 or anterior extremity, and it ends opposite the right sterno- clavicular articulation, 

 where it divides into the right subclavian and right common carotid arteries. 



Course. The trunk measures from 37 to 50 mm. in length ; it runs upwards, 

 posteriorly, and laterally, in the superior mediastinum, to the root of the neck. 



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 that the sterno-thyreoid muscle separates it from the sterno-hyoid and the 

 right sterno-clavicular joint. The remains of the thymus, whicfr separate it from the 

 manubrium sterni, are also in front. 



Right Lateral. The right innominate vein and the upper part of the superior vena 

 cava are on the right side of the artery. 



Left Lateral. 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 thyreoidea ima. 



The thyreoidea ima is an inconstant and slender vessel. When present it 

 may arise from the arch of the aorta, but it springs usually from the lower part of 

 the innominate. It passes upwards, anterior to the trachea, through the anterior 

 part of the superior mediastinum and the lower part of the neck, and gives oft 

 branches to the lateral lobes and isthmus of the thyreoid body and to the trachea. 



THE AKTEEIES 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 tc 

 describe the most important of those, viz., the vertebral arteries, with the carotic 

 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 numerou 

 branches are given off (Figs. 759, 760, 761, 764). 



The internal carotid arteries are distributed, almost entirely, to the contents o 

 the cranial cavity, internal to the dura mater, and to the structures in the cavit; 

 of the orbit. The external carotid arteries, on the other hand, supply structures c 

 the head and neck more externally situated. 



It is to be noted, however, that the vascular supply of the brain is not wholl 

 derived from the internal carotid vessels, but that it is contributed to, largely, b 

 the vertebral arteries also. 



ARTERIA CAROTIDES COMMUNES. 



The right and the left common carotid arteries are of unequal length. Tl 

 right common carotid commences at the bifurcation of the innominate arte) 

 posterior to the right sterno-clavicular articulation ; the left arises in the superi 

 mediastinum, from the arch of the aorta; but each terminates at the level 

 the upper border of the thyreoid cartilage ; the left artery has thus a short intr 



