THE NERVOUS SYSTEM 441' 



branches, each one or two in number, the former distributed 

 along the extensor, the latter along the flexor, aspect. In the 

 figures given, which are drawn from the ventral side, the dorsal 

 elements are represented as forming a deeper layer, and are 

 shaded for the purpose of rendering them more distinct. It 

 will be seen, also, that each of these final elements involves 

 more than one root, and also that the same roots furnish 

 fibers for more than one nerve. Furthermore, owing to the 

 embryonal relation of the chiridium to the body, the first digit 

 being anterior in both cases, the nerves supplying the inner 

 (radial or tibial) side of each limb are derived more from the 

 anterior portion of the plexus; those supplying the outer side 

 from the posterior. Based upon the principles given above, 

 the formation of a plexus possesses great morphological signifi- 

 cance; for its intercommunications and its branchings are, 

 in part at least, records of the past history of the limbs, rec- 

 ords which are so complicated that but little progress has as 

 yet been made in their interpretation. It may be supposed, 

 however, that if any two parts, two muscles, for example, 

 each supplied by its own nerve, should coalesce, their nerves 

 would also fuse into a single bundle, at least distally, and even 

 that the extent of this fusion, that is, the distance from the 

 origin at which these two nerves come together, would meas- 

 ure the relative length of time the parts have remained fused. 

 Similarly, if a single part should differentiate into two, a 

 phenomenon constantly occurring among limb muscles, the 

 nerve would branch; and, furthermore, the increase of the 

 differentiation between them, that is, a gain in the independ- 

 ence of action, would tend to separate the nerve still more 

 'and cause the point of bifurcation to move proximally. 



It is thus probable that the plexuses have a meaning for 

 him who is able to read it, the well-known conservatism 

 of nerves in regard to their course assisting greatly in the 

 preservation of these records. This conservatism is well shown 

 in the case of snakes, in which the limbs have been lost, but 

 where there are still traces of the plexuses, a fact attesting the 

 former presence of the limbs. In certain other cases, as in the 



