96 THE STRUCTURE OF MAN 



nearly attained its definitive position. 1 The brachial transition 

 zone is consequently more restricted and stable than the lumbo- 

 sacral, rarely extending backwards beyond the second thoracic 

 nerve. If, however, the upper limb preserves its original position 

 (the seventh cervical rib persisting), the brachial plexus receives 

 (either no contribution or at best an insignificant one from the 

 I first thoracic nerve (Eisler). 



Even if this conception of the " metameric transformation 

 of nerves," deduced by Fiirbringer, affords a partial explanation 

 of the existence and present condition of the nerve plexuses, the 

 actual causa movens lies deeper, i.e. in the original polymeric 

 origin of the limbs. In the region from which they develop we 

 meet with traces of a gradual fusion of originally distinct segments 

 (somites), with further clear traces of the shifting which they 

 have undergone during phylogeny. An excellent illustration of 

 the commencement of fusion among the body segments is yielded 

 by the transitional zones just defined. Quite apart from the 

 already-mentioned variations of the nerves, the primitive segmenta- 

 tion of the ventro-lateral body muscles is gradually being obliter- 

 ated, and the myocomniata with the ribs are becoming vestigial 

 in fact the whole ventral body-wall is affected by this process 

 of fusion (Eisler). 



1 That a further shifting of the human fore-limb in an antero-posterior direction 

 may be expected is evident, firstly, from the varying relation of the brachial plexus to 

 the anterior thoracic nerves ; and, secondly, from the very rare, yet occasional, retro- 

 gressive condition of the first thoracic rib before mentioned (ante, p. 43). 



