THE SUBCLAVIAN 505 



l)rain afte-r Ik^ature of both the carotids and the other vertebral artery. Further, 

 the circle of Willis is the only medium of connnunication l)et\veen the ganglionic 

 or central and the hemispheral or cortical branches of the cerebral arteries, and 

 between the various ganglionic Ijranches themselves. The ganglionic and the cor- 

 tical 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 anasto- 

 mose with the neighlMjuring ganglionic Ijranches. 



THE SUBCLA VIAN ARTERY 



The subclavian artery on the right side arises at the 1 )ifurcation of the innomi- 

 nate behind the right sterno-clavicular articulation. On the left side it arises from 

 the arch of the aorta, and as far as the inner ])order of the scalenus anticus is situ- 

 ated deeply in the chest. The first portion of the left subclavian artery is described 

 separately. 



Beyond the inner l^order of the scalenus anticus the artery has the same rela- 

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

 curve across the root of the neck to the outer ])order of the first rib. there to end 

 in the axillary artery. Thus the course of the artery in the neck will l)e indicated 

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

 to the middle of the clavicle. The height the artery rises in the neck varies. It is 

 perhajts most commonly about half an inch above the clavicle. If the curved line 

 above mentioned is drawn to represent part of the circumference of a circle having 

 its centre at a point on the lower margin of the clavicle an inch and a half 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 ril) by passing between 

 the scalenus anticus and medius muscles. The artery is accompanied by the 

 subclavian vein, the latter vessel lying in front of the scalenus anticus, anterior to 

 the artery, and on a slightly lower plane. 



The subclavian artery is divided into three portions — as it lies internal to, 

 behind, or external to, the scalenus anticus muscle. 



THE FIRST OR THORACIC PORTION OF THE LEFT 

 SUBCLAVIAX ARTERY 



The left subclavian artery Tfig. 328) arises from the termination of the trans- 

 verse portion of the arch of the aorta. The first part of the left subclavian is con- 

 sequently longer than the first jjart of the right, which arises at the liifureation of 

 the innominate opposite the right sterno-clavicular joint. The artery at its origin 

 is situated deeply in the thorax, and as it arises from the aorta is on a plane ]m»s- 

 terior 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 

 curs'es outwards over the apex of the left pleura and lung to the interval between 

 the anterior and middle scalene muscles. Beyond the inner l)order of the scalenus 

 anticus — that is, in the second and third portions of its course — its relations are 

 similar to those of the right suVjclavian artery. 



Relations. — In front it is covered by the left pleura and lung, wliilst more 

 superficial are the sterno-thyroid. sterno-hyoid. and sterno-mastoid muscles. It is 

 crossed a little above its origin h\ the left innominate vein, and higher in the neck 

 near the iicalenus anticus by the internal jugular, vertebral, and sulxlavian veins. 

 The ]>hrenic nerve crosses the artery innnediately internal to the scalenus anticus, 

 and then descends parallel to it Init on an anterior plane to cross the arch of the 



