) 



556 THE BLOOD-VASCULAR SYSTEM 



brain after ligature of both the carotids and the other vertebral artery. Further, the circle of 

 Willis is the only medium of communication between the ganglionic or central and tliQ peripheral 

 or cortical branches of the cerebral arteries, and between the various ganglionic branches them- 

 selves. The ganglionic and the cortical branches form separate and distinct systems, and do 

 not anastomose with each other; and the ganglionic, moreover, are so-called end-vessels, and do 

 not anastomose with the neighbouring ganglionic branches. The three cerebral arteries, 

 anterior, middle, and posterior may be regarded as branches of the circle of Willis. (For details 

 concerning the distribution of the cerebral arteries see p. 562.) 



THE SUBCLAVIAN ARTERY 



The subclavian artery on the right side [a. subclavia dextra] arises at the bifur- 

 cation of the innominate opposite the upper hmit of the right sterno-clavicular 

 articulation. On the left side it arises from the arch of the aorta, and, as far as the 

 medial border of the scalenus anterior, is situated deeply in the chest. The first 

 portion of the left subclavian artery is described separately. 



Beyond the medial border of the scalenus anterior the artery has the same rela- 

 tions on both sides. It courses from this point beneath the clavicle in a slight 

 curve across the root of the neck to the lateral border of the first rib, there to end 

 in the axillary artery. Thus the course of the artery in the neck will be indicated 

 by a line drawn from the sterno-clavicular joint in a curve with its convexity 

 upward to the middle of the clavicle. The height to which the artery rises in the 

 neck varies. It is perhaps most commonly about 1.2 cm. (| in.) above the clavicle. 

 If the curved line above mentioned is drawn to represent part of the circumference 

 of a circle having its center at a point on the lower margin of the clavicle 3.7 cm. 

 (ll in.) from the sternal end of that bone, the line of the artery will be sufficiently 

 well indicated for all practical purposes. In its course the artery arches over the 

 dome of the pleura and gains the groove on the upper surface of the first rib by 

 passing between the scalenus anterior and medius muscles. The artery is accom- 

 panied by the subclavian vein, the latter vessel lying in front of the scalenus 

 anterior, anterior to the artery, and on a slightly lower plane. 



The subclavian artery is divided into three portions — as it lies medial to, pos- 

 terior to, or lateral to, the scalenus anterior muscle. 



THE FIRST OR THORACIC PORTION OF THE LEFT 

 SUBCLAVIAN ARTERY 



The left subclavian artery [a. subclavia sinistra] (fig. 457) arises from the left 

 end of the arch of the aorta. The first part of the left subclavian is consequently 

 longer than the first part of the right, which arises at the bifurcation of the 

 innominate artery. The artery at its origin is situated deeply in the thorax, 

 and as it arises from the aorta is on a plane posterior to and a little to the left of 

 the thoracic portion of the left common carotid. It first ascends almost vertically 

 out of the chest, and at the root of the neck curves laterally over the apex of the 

 left pleura and lung to the interval between the anterior and middle scalene mus- 

 cles. Beyond the medial border of the scalenus anterior — that is, in the second 

 and third portions of its course — its relations are similar to those of the right sub- 

 clavian artery. 



Relations. -In front it is covered by the left pleura and lung, whilst more superficial are 

 the stcriio-tliyrfoid, stenio-hyoid, and sterno-mastoid nuiscles. It is crossed a little above its 

 origin by the left imiomiiiate vein, and higher in the neck near the scalenus anterior by the 

 internal jugular, vertebral, and subclavian veins. The phrenic nerve crosses the artery imme- 

 diately medial to the scaleims anterior, and then descends parallel to it, but on an anterior plane, 

 to cross the arch of the aorta. The vagus nerve descends i)arallel to the artery between it and 

 the left common carf)ti(l, coming into contact with its anterior surface just before crossing the 

 arcli of th(! aorta. Tlie left ('(irvical cardiat; nerves of the .sympathetic also descend in front of 

 it on their way to the, cardiac pU^xus. The left (inaa sul>ciavin also looi)s in front of the subclavian 

 artery. The left common carotid is situated anteriorly and to its right. The thoracic duct 

 arches over the artery just medial to the scalenus anterior, to emi)ty its contents into the 

 confluenc«! of the internal jugidar and .subclavian veins (fig. 442). 



Behind and somewhat UKMlial to it are the (esophagus, thoracic duct, inferior cervical gang- 

 lion of the syriipatlietic, longus colli nuisde, and vertebral column. To some extent it is over- 

 lapped posteriorly by tl)(! left plein-a and lung. 



()\\ its right side are the trachea and the inferior laryngeal nerve, and, higher up, the (xsopha- 

 gus and thoracic duct. 



On its left side are the left pleura and lung. 



