160 HEAD AND NECK 



acromial end, introduce the knife into the cavity of the joint, close 

 to the sternum, and carry it laterally below the clavicle, to detach 

 the lower part of the interarticular cartilage from the sternum and 

 the cartilage of the first rib, and to divide the lower part of the capsule 

 and the costo-clavicular ligament, which lies immediately lateral to it. If 

 the subclavius muscle has not already been detached, it also must be divided, 

 and then the clavicle can be displaced laterally, and the whole extent of 

 the plexus will be exposed. 



The Brachial Plexus. The brachial plexus is fully described on p. 28, 

 Vol. I., and only a brief resume of the main facts regarding it is given here. 

 The plexus is formed by the last four cervical nerves and the larger part 

 of the first thoracic nerve; it also receives a communication from the fourth 

 cervical nerve and not uncommonly a small twig from the second thoracic 

 nerve. These various nerves constitute the roots of the plex^ts. The roots 

 of the plexus emerge from between the scalenus medius and the scalenus 

 anterior, and unite to form three trunks, upper, middle, and lower, which 

 lie superficial to the scalenus medius, the lowest of the three being wedged in 

 between that muscle posteriorly and the third part of the subclavian artery 

 anteriorly. The upper trunk is formed by the fifth and sixth nerves and the 

 communication from the fourth. The seventh nerve alone forms the middle 

 trtmk ; and the lowest trunk is formed by the eighth cervical and first 

 thoracic nerves and the communication from the second thoracic. Almost 

 immediately after their formation the trunks divide into anterior and 

 posterior divisions, and the divisions reunite to form three cords, lateral, 

 medial, and posterior. The lateral cord is formed by the anterior divisions 

 of the upper and middle trunks, the medial cord by the anterior division of 

 the lowest trunk, and all three posterior divisions unite to form the posterior 

 cord. The cords descend behind the clavicle and subclavius muscle, 

 through the cervico-axillary canal, to the level of the coracoid process of 

 the scapula where the plexus terminates and each cord divides into two 

 terminal branches. The terminal branches of the lateral cord are the 

 lateral head of the median nerve and the musculo-cutaneous nerve. Those 

 of the medial cord are the medial head of the median and the ulnar nerve, 

 and the posterior cord divides into the axillary (O.T. circumflex) nerve 

 and the radial (O.T. musculo-spiral). In addition to the terminal branches, 

 collateral branches are given off from the roots, the trunks and the cords ; 

 and the roots are connected with the middle and lower ganglia of the 

 cervical part of the sympathetic trunk by grey rami communicantes. The 

 branches given off from the roots are twigs of supply to the longus colli, 

 the scalenus anterior, the scalenus medius, and the scalenus posterior, the 

 roots of origin of the long thoracic nerve, which supplies the serratus anterior 

 (O.T. magnus) and the dorsal scapular nerve (O.T. nerve to the rhomboids). 

 The roots of the long thoracic nerve spring from the fifth, sixth, and seventh 

 nerves ; the upper two pierce the scalenus medius and the lowest passes 

 anterior to that muscle. The three unite, behind the trunks of the plexus, 

 to form the stem of the nerve, which descends behind the cords of the 

 plexus into the axilla. The dorsalis scapulae nerve arises from the lateral 

 liorder of the fifth nerve ; it disappears under cover of the levator scapulae 

 and supplies the two rhomboid muscles, and, sometimes, the levator scapulae. 



The branches from the trunks of the plexus are the suprascapular nerve 

 and the nerve to the subclavius. They both spring from the upper trunk. 

 The collateral branches of the three cords of the plexus, are ( I ) from the 

 outer cord : the lateral anterior thoracic nerve ; (2) from the posterior cord : 

 the upper and lower subscapular nerves and the thoraco-dorsal nerve 

 (O.T. long subscapular) ; and (3) from the medial cord : the medial anterior 

 thoracic, the medial cutaneous nerve of the arm (O.T. lesser internal 



