BRACHIAL PLEXUS OF NERVES IN MAN 307 



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VARIATIONS IN THE FORMATION AND DIVISION OF THE TRUNKS 

 AND OF THE FASCICULI OF THE PLEXUSES 



Variations of the above arrangement fall into two main groups. 

 In the first, no true cephalic or caudal trunks are formed but 

 some or all of the nerves divide into dorsal and ventral branches 

 and these combine to form the lateral, medial and dorsal fascic- 

 uli, or no true dorsal or lateral fasciculi are formed but branches 

 from the dorsal and ventral rami of the nerves or trunks unite 

 to form the branches of the plexus, or the cephalic and inter- 

 mediate trunks fail to divide into dorsal and ventral branches 

 but the trunks unite to form a single lateral cord which then 

 divides into dorsal and ventral branches. In all of these varia- 

 tions the fasciculi or their branches receive fibers from the same 

 spinal nerves as they would in the usual arrangement. 



There is another group of variations of the plexus in which 

 the lateral fasciculus receives fibers from nerves caudal to the 

 seventh cervical or in which the medial fasciculus receives fibers 

 from nerves cephalic to the eighth cervical nerve. A new ele- 

 ment is, in these cases, introduced into either the lateral or medial 

 fasciculus. These therefore are distinctly different than the usual 

 and warrant the subdivision of the groups into subgroups or 

 types. There are only 11 such atypical plexuses or 6.28 per cent 

 of the 175 studied. 



We shall consider the first group of variations, dealing with 

 each trunk and fasciculus separately, and shall then consider 

 the subdivision of the plexuses into subgroups. 



The cephalic trunk 



The cephalic trunk is formed by the union of the fifth and 

 sixth cervical nerves in 157 plexuses or in 89.71 per cent of the 

 175 plexuses studied. The fourth cervical nerve in all cases where 

 this enters the plexus joins the fifth before this has united with 

 the sixth. In 153 of the 157 cases the cephalic trunk divides into 

 dorsal and ventral divisions, (fig. 1), but in 4 it does not divide 

 but is joined by the intermediate trunk and the nerve cord thus 

 formed then divides into dorsal and ventral divisions (fig. 21). 



