POSTERIOR TRIANGLE 143 



temporal bone and the superior nuchal line to the external occipital pro- 

 tuberance. (2) From the sternal to the acromial end of the clavicle, 

 following the line of that bone. (3) Join the anterior extremities of I and 

 2 by a vertical incision passing along the back of the external acustic meatus 

 and then down the middle of the sterno-mastoid muscle. Reflect the flap, 

 thus marked out, from before backwards, and note that the skin is thicker 

 over the upper and posterior part of the triangle than over the lower and 

 anterior part. 



When the skin is reflected the superficial fascia and the lower part of 

 the platysma muscle will be exposed. 



The superficial fascia in the region of the posterior triangle 

 is comparatively thin, and embedded in its lower and anterior 

 part is the lower and posterior part of the platysma. 



The Platysma. The platysma is a thin sheet of muscle 

 which commences in the superficial fascia of the infra-clavi- 

 cular region, whence it ascends across the clavicle and through 

 the superficial fascia of the side of the neck, to the face where 

 its upper border has been examined already (p. 126). It 

 covers the lower and anterior part of the posterior triangle, 

 and the upper and posterior part of the anterior triangle ; and 

 it is supplied by the cervical branch of the facial nerve, 

 which emerges from the lower end of the parotid gland. 



Dissection. Make an incision through the lower part of the platysma 

 along the line of the clavicle, and turn the part above the incision upwards 

 and anteriorly. Whilst making the incision and whilst reflecting the muscle, 

 be careful not to injure the supraclavicular cutaneous nerves and the 

 external jugular vein, which lie directly subjacent to it. 



After the platysma is reflected, clean the external jugular vein, which 

 emerges from the lower end of the parotid and passes downwards, in- 

 clining posteriorly, to the lower and anterior angle of the posterior tri- 

 angle, where it pierces the deep fascia. Whilst cleaning the vein, avoid 

 injury to the nervus cutaneus colli, which sometimes crosses superficial 

 to the vein about the middle of its length. Secure and clean the posterior 

 auricular vein, which descends behind the auricle and joins the external 

 jugular a little below the level of the angle of the mandible. Next, find 

 and clean the superficial branches of the cervical plexus as they pierce the 

 deep fascia. They are (i) descending branches, the anterior, middle, and 

 posterior supra-clavicular nerves. (2) A transverse branch, the nervus 

 cutaneus colli (O.T. transverse cervical). (3) Ascending branches, the 

 great auricular and the small occipital. 



The anterior and middle supra-clavicular nerves will be found piercing 

 the deep fascia immediately above the clavicle, the anterior at the posterior 

 border of the sterno-mastoid and the middle above the convexity of the 

 clavicle. They descend into the pectoral region as far as the lower border 

 of the second rib and their lower portions will be displayed by the dissector 

 of the arm. The posterior supra-clavicular nerves pierce the deep fascia at 

 a somewhat higher level. They descend across the lower and anterior part 

 of the trapezius to the acromial region, and to the skin of the arm over the 

 upper part of the deltoid, where they will be exposed by the dissector of 

 the arm. 



