96 THE STRUCTURE OF MAN 
nearly attained its definitive position 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 
first thoracic nerve (Hisler). 
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 hes deeper, z.¢c. in the original polymeric 
origin of the hmbs. 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 myocommata with the ribs are becoming vestigial— 
in fact the whole ventral body-wall is affected by this process 
of fusion (Hisler). 
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- 
5 ’ y, ; ’ 
gressive condition of the first thoracic rib before mentioned (ante, p. 48). 
