POSTERIOR TRIANGLE 151 



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 small occipital nerve turns round its lower border, 

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

 third and fourth cervical nerves. 



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 the small occipital, 

 the great auricular, the nervus cutaneus colli, and the supra- 

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

 the nerves to the scalenus medius and posterior, the nerve 

 to the levator scapulae, the branches to the trapezius and the 

 communication to the accessory nerve. 



The Third Part of the Subclavian Artery. Only a 

 portion of this part of the subclavian artery is in the triangle ; 

 the lower and lateral part is behind the clavicle. The part 

 in the triangle is situated deeply in the anterior inferior 

 angle and below the omo-hyoid muscle. It is covered with the 

 skin, superficial fascia, the platysma, deep fascia, the external 

 jugular vein, the ends of the transverse scapular, and trans- 

 verse cervical veins, and the nerve to the subclavius muscle. 

 Behind it is the lowest trunk of the brachial plexus, which 

 separates it from the insertion of the scalenus medius. Below, 

 it rests upon the first rib, against which it can be compressed, 

 and, more medially, on the cervical pleura. 



The Brachial Plexus and its Supraclavicular Branches. 

 Only the upper portion of the brachial plexus lies in the 

 region of the posterior triangle, i.e. the roots, the trunks, and 

 some of the branches ; the remainder lies either posterior to 

 the clavicle or in the axilla. The cervical portion lies in the 

 lower and anterior part of the posterior triangle partly in 

 the occipital and partly in the supraclavicular areas. The 

 detailed study of the plexus should be left till the fifth day 

 after the body has been placed upon its back, when the 

 dissector of the head and neck will assist the dissector of the 

 upper extremity to disarticulate the clavicle and to lay bare 

 the whole of the plexus (p. 160). 



The fourth day after the body has been placed upon its 



