36 HEAD AND NECK 



Dissection. The accessory nerve, previously found at the 

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

 posterior border of the sterno-mastoid, must now be traced 

 downwards and backwards, through the triangle, to the point 

 where it disappears under cover of the trapezius, at the junction 

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

 border of that muscle. As the nerve is cleaned, attempt to secure 

 twigs from the third and fourth cervical nerves which com- 

 municate with it in the posterior triangle. 



Turn next to the posterior belly of the omo-hyoid muscle, 

 which crosses the lower part of the triangle. Note that it 

 divides the triangle into a large upper or occipital portion, and 

 a small lower or subclavian portion. Cut through the fascia 

 on the surface of the muscle, parallel with the muscle fibres, 

 and turn it upwards and downwards ; then turn the upper border 

 of the muscle laterally and find the nerve from the ansa hypo- 

 glossi which emerges from under cover of the sterno-mastoid 

 and enters the deep surface of the posterior belly of the omo- 

 hyoid to supply it. 



Now remove any parts of the fascial roof of the upper part 

 of the posterior triangle which are still present, and note a 

 number of lymph glands which lie embedded in the subjacent 

 areolar tissue ; they are placed along the posterior border of 

 the sterno-mastoid, superficial to the stems and branches of the 

 cervical nerves. At the apex of the triangle look for the 

 occipital artery, which either emerges between the adjacent 

 borders of the trapezius and the sterno-mastoid, or pierces the 

 trapezius a little further back. 



Between the accessory nerve above and the posterior belly of 

 the omo-hyoid below find : (i) the upper part of the brachial 

 plexus ; (2) its branch to the subclavius ; (3) its suprascapular 

 branch ; (4) its dorsalis scapulae branch ; (5) its long thoracic 

 branch ; (6) branches from the third and fourth cervical nerves 

 to the levator scapulae ; (7) branches from the third and fourth 

 cervical nerves to the trapezius, and others which communicate 

 with the accessory nerve in the posterior triangle ; and (8) the 

 upper and posterior part of the transverse cervical artery. Find 

 the transverse cervical artery as it appears from under cover of 

 the upper border of the omo-hyoid. It runs upwards and back- 

 wards. Next, secure the nerve to the subclavius, which lies 

 under cover of the deep fascia above the omo-hyoid immediately 

 behind the sterno-mastoid. Trace it upwards to its origin from 

 the trunk formed by the union of the fifth and sixth cervical 

 nerves. Clean the latter nerves and the upper part of the 

 seventh cervical nerve, which lies immediately below them. 

 Then find the suprascapular nerve, which springs from the 

 lateral border of the trunk formed by the fifth and sixth nerves. 

 It lies immediately above the anterior part of the posterior belly 

 of the omo-hyoid, and disappears under cover of the posterior 

 part. Turn the trunk formed by the fifth and sixth cervical 

 nerves forwards and find, posterior to it, the upper roots of the 

 long thoracic nerve, which spring from the fifth and sixth nerves, 

 and are emerging through the fibres of the scalenus medius 

 muscle. The nervus dorsalis scapulas (O.T. nerve to the rhom- 

 boids) lies at a slightly higher level than the suprascapular nerve. 

 It springs from the fifth cervical nerve, runs downwards and 



