126 THE HEAD AND NECK 



of the anterior border of the trapezius, in which it terminates. 

 Below, and parallel to the accessory nerve, the branches to the 

 trapezius from C. 3 and C. 4 cross the triangle. In operations 

 in this region they may be mistaken for the accessory nerve, 

 and, as it is important that the latter should be preserved, the 

 surgeon may require to nip the nerve with dissecting forceps to 

 make certain of its identity. In the case of the accessory, the 

 upper fibres of the trapezius will contract and the shoulder will 

 be elevated (p. 6) ; in the other case, the lower fibres of the 

 trapezius will respond, but the result cannot be appreciated 

 from in front. Both these motor nerves pass to the deep surface 

 of the trapezius, and in this way they can be distinguished from 

 the posterior supra -clavicular (supra -acromial) nerves, which 

 run superficial to the muscle (Fig. 35). 



In its lower part the space is crossed by the posterior belly 

 of the omo-hyoid, which runs medially and slightly upwards. 

 Laterally, the muscle is completely hidden by the clavicle, but as 

 it approaches the sterno-mastoid it rises to a higher level. When 

 operating in this region the surgeon often finds it convenient 

 to cut through the fascial sheath of the omo-hyoid (p. in) so 

 that the muscle may be retracted upwards. If necessary, it may 

 be cut away, without any subsequent disability. 



The Transverse Cervical Artery arises from the thyreo- 

 cervical trunk (thyreoid axis, p. 143), and runs laterally, deep to 

 the sterno-mastoid, but in front of the prevertebral fascia and 

 the scalenus anterior. It then crosses the floor of the posterior 

 triangle under cover of the posterior belly of the omo-hyoid. 

 In enlargement of the lymph glands in this region the transverse 

 cervical artery is increased in size, and the haemorrhage which 

 occurs during their removal is due to the division of its branches. 



The Transverse Scapular (Suprascapular) Artery has a 

 similar origin and course, but it lies at a lower level. It is placed 

 behind the clavicle, and crosses in front of the subclavian 

 artery and the brachial plexus. The veins which accompany 

 these arteries join the external jugular as it lies in front of the 

 third part of the subclavian artery (p. 131). 



Exposure of the lower part of the Posterior Triangle. A 

 curvilinear incision, extending downwards along the sterno- 

 mastoid and laterally along the clavicle, gives the best view of 

 the lower part of the posterior triangle. When the flap is turned 

 backwards the external jugular vein is exposed, near the apex 

 of the wound, as it pierces the deep fascia (p. 109). 





