THE SUBCLAVIAN REGION. 97 



The relations of the subdavian artery in the third part 

 of its course are : 



In front. Integument, superficial cervical fascia, pla- 

 tysma, external jugular vein, and the venous plexus 

 before mentioned, descending cervical nerves, subclavius 

 muscle, supra-scapular artery, and clavicle. 



Above. Brachial plexus, and posterior belly of omo- 

 hyoid muscle. 



Below. First rib, covered by the first serration of the 

 serratus magnus. 



Behind. The middle scalene muscle. 



Ligature of the Subdavian Artery in the third part of its 

 course. The point selected for placing a ligature upon 

 this vessel, is just where it lies on the first rib, at the 

 bottom of the hollow above described. It is not inclosed 

 in a definite sheath like the carotid, but is bound down 

 by a process of the deep cervical fascia, derived from the 

 aponeurotic investment of the omo-hyoid. The incision 

 is most advantageously made, by drawing the integu- 

 ment down, and cutting upon the clavicle, allowing it to 

 retract afterwards; but this proceeding must be modified 

 by circumstances. When the vessel is reached, just as 

 it emerges from behind the anterior scalene muscle, the 

 needle must be passed round it from before, backwards ; 

 and the end of the needle must be made to insinuate 

 itself round the vessel closely, so as to avoid the united 

 cord of the eighth cervical and first dorsal nerves, which 

 lie immediately behind it. 



The collateral circulation developed after ligature of the 

 third part of the subdavian artery is as follows : 



As the third portion rarely gives off any branches, the 

 blood would pass towards the arm by the supra-scapular 

 and posterior scapular branches of the thyroid axis, 



