NERVES 161 



lung, being between the pericardium and the mediastinal 

 pleura, to the diaphragm, which it perforates, and is dis- 

 tributed on its abdominal surface. The right nerve lies to 

 right of right innominate vein and superior vena cava. The 

 left nerve in the neck is crossed by the thoracic duct, and 

 below crosses the arch of the aorta and left pneumogastric, 

 and is longer than the right. Filaments from each supply 

 the pericardium and pleura. 



DEEP BRANCHES (External) : 



Communicating : to spinal accessory in sterno-mastoid and 

 trapezius muscles. 



Muscular : to sterno-mastoid from the 2nd, levator anguli 

 scapulae from the 3rd and 4th, scalenus medius from the 3rd 

 and 4th, and trapezius from the 3rd and 4th. 



NERVES OF THE UPPER EXTREMITY. 



The BRACHIAL PLEXUS is formed by the union and subse- 

 quent division of the anterior primary divisions of the lower 

 four cervical and part of anterior division of the ist dorsal 

 nerves. The 5th and 6th cervical communicate with the 

 middle, and yth and 8th with inferior cervical ganglia. The 

 following is the usual method of union and redivision : 



The 5th and 6th cervical join together at outer border of 

 the scalenus anticus to form an upper trunk. 



The yth cervical forms a middle trunk. 



The 8th cervical and the ist dorsal form a lower trunk. 



Each of these trunks then subdivides into an anterior and 

 a posterior branch. 



The anterior branches from the upper and middle trunks 

 form the upper or outer cord of the plexus. 



The anterior branch of the lower trunk forms the lower or 

 inner cord of the plexus. 



The posterior branches of all the trunks unite to form the 

 posterior or middle cord. 



The subjoined scheme will make this arrangement clear : 



C 5 } TT__, _._i. ( Posterior 



^ 5 L Upper trunk .J ^"=V UI "I 



^ Outer cord I i 



>J 



g 8 ,}' 1 -- " k fet-i, lid. 



C, tMiddl. trunk {!? P c 



The plexus lies between scalenus anticus and medius, 

 and divides into branches for the upper limb below tlu 

 pectoralis minor. 



ii 



