POSTERIOR TRIANGLE 41 



sternomastoid it joins the intermediate tendon which connects 

 it with the anterior belly. Its nerve has already been seen 

 entering its deep surface (p. 36). As it crosses the posterior 

 triangle it lies superficial to the suprascapular nerve, the 

 transverse cervical artery and the brachial plexus. 



Nervus Accessorius (O.T. Spinal Accessory). The portion 

 of the accessory nerve which appears in the posterior triangle 

 consists of fibres which arise from the cervical part of the 

 spinal medulla, and with them are incorporated some filaments 

 derived from the second cervical nerve. Before appearing 

 in their present situation the spinal fibres entered-tbe cranium 

 throughjthe: foramen magnum and left it_bv passing through 

 the jugular foramen ; then they passed downwards and back- 

 wards, through the deeper fibres of sterno-mastoid, where 

 they received the communication from the second cervical 

 nerve. As already pointed out, the nerve usually enters the 

 posterior triangle at the level of the union of the upper third 

 with the lower two-thirds of the posterior border of the 

 sterno-mastoid or at a slightly lower level. It runs down- 

 wards and backwards, through the triangle, along the line of 

 the levator scapulae, and disappears under the trapezius at 

 the junction of the upper two-thirds with the lower third 

 of its anterior border. As it enters the triangle the lesser 

 occipital nerve turns round its lower border; and, as it 

 crosses the triangle, it is joined by twigs from the third and 

 fourtkervical nerves. It lies parallel with, but at a higher 

 level than, the dorsalis scapulae nerve (Fig. 12). 



The Branches of the Cervical Plexus. The dissector 

 should note that whilst many of the branches of the cervical 

 plexus lie within the area of the posterior triangle, the plexus 

 itself is under cover of the upper part of the sterno-mastoid, 

 where it will be exposed and studied when the sterno-mastoid 

 is reflected. The branches which appear in the triangle are 

 the superficial branches (i) the lesser occipital ; (2) the great 

 auricular; (3) the nervus cutaneus colli, and (4) the supra- 

 clavicular nerves ; and the deep posterior branches, that is, the 

 nerves to (i) the scalenus medius and (2) the scalenus 

 posterior; (3) the nerve to the levator scapulae; (4) the 

 branches to the trapezius, and (5) the communications to the 

 accessory nerve. 



The Third Part of the Subclavian Artery. Only a portion 

 of the third part of the subclavian artery is the triangle ; the 



