52 HEAD AND NECK 



medially and downwards from the clavicle to the sternum, the 

 anterior, superior, and posterior portions must be divided close 

 to the sternum, care being taken to avoid injury to the anterior 

 jugular vein, which passes laterally close to the upper and 

 posterior part of the joint. When the division is completed, 

 elevate the sternal end of the clavicle by depressing the 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 articular disc from the sternum and the 

 cartilage of the first rib, and to divide the lower part of the cap- 

 sule 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. 



Plexus Brachialis. The brachial plexus is fully described 

 on p. 39, 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. Those various nerves constitute the 

 roots of the plexus. 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 com- 

 munication from the fourth. The seventh nerve alone forms 

 the middle trunk ; 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 forma- 

 tion 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, 

 and 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 



