1294 



HUMAN ANATOMY. 



their position as regards one another, while the cords are denominated according to 

 their relation to the axillary artery, the outer lying lateral to, the inner mesial to, and 

 the posterior behind, the artery. 



Variations. — Considerable variety exists as regards the length of the component nerve- 

 bundles in their several portions, division and union taking place at different levels in different 

 individuals. The fifth cervical nerve may pass in front of or through the scalenus anticus. The 

 sixth, though not so frequently as the fifth, may traverse the scalenus anticus. The seventh 

 cervical nerve, as the middle trunk, may break up into three branches, one going to each of 

 the three cords. The fibres of the posterior cord may arise from only the seventh and eighth, 

 or the sixth, seventh and eighth cervical nerves. Plexuses have been seen in which only two. 

 cords, a smaller and a larger, were present, the latter taking the place of either the inner and 

 outer or the inner and posterior cords. 



Communications. — The five nerves comprising the source of the plexus are 

 connected to the sympathetic system by gray rami communicantes and there is 

 possibly a white ramus communicans passing from the first thoracic nerve to the 

 first thoracic ganglion of the sympathetic. 



Fig. 1092. 



Musculo-cutaneous nerve 



Median nerve 



Ulnar nerve' 



I. cervical nerve 

 II. cervical nerve ^ 



III. cervical nerve * 



IV. cervical nerve 



Vertebral artery 



V. cervical nerve 



Scalenus niedius muscle 



VI. cervical nerve 



VII. cervical nerve 



VIII. cervical nerve 



I. thoracic nerve 



External anterior thoracic nerve 



Suprascapular nerve 

 Upper subscapular nerve 

 Outer cord of plexus 

 Posterior cord of plexu.- 

 Circumflex nerve ^ 

 Deltoid muscle 



Internal anterior 

 thoracic nerve 

 Insertion of 

 scalenus anticus 

 Posterior 

 thoracic werve 



I. rib 



Serratus magnus, 

 first serration 



Musculo-spiral nerve 

 Internal cutaneous nerve 

 Lesser internal cutaneous nerve 



II. rib 

 Inner cord of plexus 



Middle subscapular nerve 

 Lower subscapular nerve 



Deep dissection of neck, showing constitution of right brachial plexus. 



Practical Considerations. — Sensory disturbances are rather rare in the 

 distribution of the brachial plexus of nerves, but motor troubles are comparatively 

 common, and are sometimes associated with disturbances of sensation. The whole 

 plexus, or only an individual branch, may be involved. The most common cause 

 is injury, such as dislocation of the head of the humerus, a fracture of the clavicle, 

 or a forced apposition of the clavicle to the first rib. Other causes are the 

 pressure of tumors or the constitutional effects of poisons and infections. The 

 plexus is so superficial above the clavicle that it can be felt or even seen in 

 thin people. 



Branches. — These fall naturally into two groups, those given off from the 

 supraclavicular and those from the infraclavicular portion of the plexus. 



