290 



THE SPINAL NERVES. 



down they are placed, the first on the outer side, the second on the inner side, and 

 the third behind that vessel in its second part, whence they are continued into the 

 principal nerves for the arm. 



Varieties. Deviations from the arrangement above described, depending* upon alterations 

 in the level at which the several portions of the plexus separate and unite, are often met with. 

 The seventh cervical nerve is sometimes divided into three branches, one passing- to each of 

 the three cords of the plexus. The posterior cord has been observed arising* from the sixth, 

 seventh, and eighth, and in one case only from the seventh and eighth cervical nerves 

 (Turner). Cases are recorded in which the plexus consisted of only two cords, the larger one 

 representing- either the inner and outer, or the posterior and inner cords of the normal 

 arrangement. 



The fifth cervical nerve is not unfrequently, the sixth more rarely, directed outwards 

 through the fibres of the scalenus anticus ; the fifth nerve may even pass altogether in front 

 of that muscle. 



BRANCHES. The nerves proceeding from the brachial plexus are usually divided 

 into two classes, viz., A, those that are given off from its supraclavicular part, and 



w 



musculo-spiral ; 

 humeral nerve. 



circumflex ; i,i, intercostal nerves ; 



Fig. 192. PLAN OF THE CERVICAL AND 

 BRACHIAL PLEXUSES. (Allen Thom- 

 son). i 



The nerves are separated from the 

 spinal cord at their origin and are sup- 

 posed to be viewed from before : CI, the 

 first cervical or suboccipital nerve, and 

 the Roman numbers in succession from 

 II, to VIII, the corresponding cervical 

 nerves ; DI, the first, and II, and III, 

 the second and third dorsal nerves ; the 

 origin of the posterior piimary branch is 

 shown in all the nerves ; of these p 2, 

 indicates the great occipital from the 

 second, and p 3, the smallest occipital 

 nerve from the third. Cervical plexus : 

 1, anterior primary branch of the first 

 cervical nerve and loop of union with the 

 second nerve ; 2, small occipital nerve ; 

 3, great auricular nerve ; 3', superficial 

 cervical nerve : 3 n, communicating 

 branches to the ansa cervicalis from the 

 second and third ; 3 s, communicating to 

 the spinal accessory from the third and 

 fourth nerves ; 4, supraclavicular nerves ; 

 4', phrenic nerve. Brachial plexus : 

 V, to VIII', and D', the five roots of the 

 brachial plexi>s ; 5, tbe rhomboid nerve ; 

 5', suprascapular ; 5", posterior thoracic ; 

 6, nerve to the subclavius muscle ; 7, 7, 

 inner and outer anterior thoracic nerves ; 

 8, 8', 8", subscapular nerves. In the 

 larger nerves proceeding to the shoulder 

 and arm from the plexus, those of the 

 anterior divisions are represented of a 

 lighter shade, those belonging to the 

 posterior division darker ; ec, external 

 cutaneous or musculo-cutaneous ; m, 

 median ; n, ulnar ; ic, internal cu- 

 taneous ; w, nerve of Wrisberg ; r, 

 lateral branch of the same ; ih, intercosto- 



so arise before the formation of the three cords of the plexus ; and B, those arising 

 below the clavicle from the outer, inner, and posterior cords. 



To the former (supraclavicular) group belong some small muscular nerves to the 

 scaleni and longus colli muscles the nerve to the rhomboid muscles, the posterior 



