POSTERIOR TRIANGULAR SPACE. 65 



area are several muscles, which are placed in the following order from 

 above down, viz., splenius capitis, levator anguli scapulae 6 ; and the mid- 

 dle scalenus 5 ; and at the lower and outer angle, somewhat beneath the 

 trapezius lies the upper part of the serratus magnus. Covering the space 

 are the structures already examined, viz., the skin and superficial fascia, 

 the platysma over the lower half or two-thirds, and the deep fascia. 



The small omo-hyoid muscle, *, crosses the lower part of the space, so 

 as to subdivide it into two a lower or clavicular, and an upper or occi- 

 pital. 



The clavicular part is small in size and close to the clavicle, and con- 

 tains the subclavian artery. It is triangular in form, with its base directed 

 forwards ; and is bounded in front by the sterno-mastoid, l ; above by the 

 omo-hyoid muscle, 8 ; and below by the clavicle. This small space meas- 

 ures commonly about one inch and a half from before backwards, and 

 about half that in front at its base. 



Crossing the area of this portion, rather above the level of the clavicle, 

 is the trunk of the subclavian artery, 7 (its third part), which issues from 

 beneath the anterior scalenus muscle, and is directed over the first rib to 

 the axilla. In the ordinary condition of the vessel the companion subcla- 

 vian vein is seldom seen, owing to its situation being lower down beneath 

 the clavicle. Above the artery are the large cords of the brachial plexus, 9 , 

 which accompany the vessel, and become closely applied to it beneath the 

 clavicle. Behind the artery and the nerves is the middle scalenus 

 muscle, 5 . And below the vessel is the first rib. 



Along the lower boundary of the space, and rather beneath the clavicle 

 lie the supra-scapular vessels ; and crossing the upper angle, at the meet- 

 ing of the omo-hyoid and sterno-mastoid muscles, are the transverse cer- 

 vical vessels. Entering the space from above' is the external jugular 

 vein, 8 , which descends over or under the omo-hyoideus near the anterior 

 part, and opens into the subclavian vein ; in this spot the vein receives 

 the supra-scapular and transverse cervical branches, and sometimes a small 

 vein, over the clavicle, from the cephalic vein of the arm. 



The size of the clavicular part of the space from before back is influ- 

 enced by the attachment of the trapezius and sterno-mastoid muscles along 

 the clavicle : in some bodies these muscles occupy nearly the whole length 

 of that bone, leaving but a small interval between them ; and in others 

 they meet so as to cover the underlying vessels. This space may be fur- 

 ther increased or diminished from above down by the position of the omo- 

 hyoideus in the neck ; for this muscle may lie close to the clavicle, being 

 attached thereto, or it may be distant one inch and a half from that bone. 



In depth the space varies naturally ; and in a short thick neck with a 

 prominent clavicle, the artery is farther from the surface than in the op- 

 posite condition of the parts. But the depth may be altered much more 

 by the position of the clavicle, according as the limb may be raised or de- 

 pressed. And lastly, the artery may be concealed entirely in its usual 

 position by forcing upwards the arm and shoulder, as the collar bone can 

 be raised above the level of the omo-hyoid muscle. 



The situation of the trunk of the subclavian artery may vary much, for 

 the vessel may be one inch and a half above the clavicle, or at any point 

 intermediate between this and its usual level just above the prominence of 

 that bone. Further, its position to the anterior scalenus may be changed ; 

 and instead of the vessel being beneath, it may be in front of, or even 

 between the fibres of that muscle. 



