130 THE HEAD AND NECK 



medius is next separated from the upper surface of the rib in a 

 forward direction, and the bone is divided as close to its proximal 

 extremity as possible. The distal portion is drawn laterally, 

 and separated from the intercostal muscles. It is then traced 

 forwards as far as possible and again divided. 



The same method may be adopted in the removal of a first 

 thoracic rib, if it is producing pressure symptoms, but great care 

 must be exercised to avoid injuring the pleura. 



The Muscular Floor of the Posterior Triangle is formed in 

 its upper part by the splenius capitis, the fibres of which pass 

 forwards and upwards from the vertebral spines and ascend 

 under cover of the upper part of the sterno-mastoid to reach 

 the mastoid process. Below and parallel to the splenius capitis, 

 the levator scapula (Fig. 42) runs downwards and laterally from 

 the transverse processes of the upper cervical vertebrae to reach 

 the medial (superior) angle of the scapula. At a lower level 

 the scalenus medius and posterior form the floor of the triangle. 

 They are usually more or less blended with one another, and 

 their fibres have a more downward inclination, as they are in- 

 serted into the posterior portions of the first and second ribs re- 

 spectively. In their lower part they are crossed by the trunks 

 of the brachial plexus and the subclavian artery. 



The Scalenus Anterior is usually completely under cover 

 of the posterior border of the sterno-mastoid, but it is exposed 

 when that muscle is retracted medially. It arises from the 

 anterior tubercles of the transverse processes of the intermediate 

 cervical vertebrae, and runs downwards and slightly laterally, 

 to be inserted into the scalene tubercle on the inner margin of 

 the first rib. Like the muscles in the floor of the posterior 

 triangle, it lies behind the prevertebral fascia, by which it is 

 separated from the posterior belly of the omo-hyoid. 



The Phrenic Nerve arises mainly from C. 4, but it usually 

 receives additional fibres from C. 3 and .5. It runs vertically 

 downwards in front of the scalenus anterior but behind the 

 prevertebral fascia, and it is overlapped by the internal jugular 

 vein. Owing to the obliquity of the muscle, the phrenic nerve 

 crosses the scalenus anterior from its lateral to its medial border 



(Fig- 44). 



The nerves of supply to the levator scapulae and the scalene 

 muscles all lie deep to the prevertebral fascia (p. 112). 



The Dorsalis Scapulae Nerve (N. to the Rhomboids) arises 

 from C. 5, as it lies between the scalenus anterior and medius, 



