OF MYR1APODA AND MACROUROUS ARACHNIDA. 
251 
transversely inwards, to that of the cord on which it is reflected, and passes lon- 
gitudinally backward ; thus forming a part of its external surface until it arrives at 
the root of the nerve to which it is to be distributed, and along which it again passes 
transversely outwards, bounding the anterior side of the nerve to its distribution on 
the lateral surface of the body. These fibres of reinforcement form a large propor- 
tion of the whole cord, and enter into the composition of the upper, anterior, and 
part of the inferior surface of the root of every nerve, in their course inwards to the 
cord; and of its posterior and inferior surface on their again proceeding outwards. 
In this manner these fibres of reinforcement connect all the nerves of the cord on 
one side of the body, as the corresponding fibres do those on the opposite side. 
They form, as it were, double, treble, or quadruple circles, one within the other. 
Thus the fibres that pass inwards along one nerve may proceed along the cord to 
pass outwards again on the front of a second, a third, or a fourth, thus linking the 
segments in one continued series of nervous communications, independent of the 
brain. But these communications exist only between nerves on the same side of the 
body, and not between those on the opposite. The commissural nerves connect the 
opposite sides of each individual segment, as those of reinforcement do the same sides 
of two separate segments. 
Every nerve from a ganglionic enlargement of the cord is thus composed of four 
sets of fibres, an upper and an under one, which communicate with the cephalic gan- 
glia ; a transverse or commissural, that communicate only with corresponding nerves 
on the opposite side of the body ; and a lateral set that communicate only with nerves 
from a ganglionic enlargement on the same side of the body, and form part of the 
cord in the interspace between the roots of the nerves. It is by the successive 
addition of these lateral portions of the cord that its size is maintained almost uni- 
formly throughout its whole length in the elongated bodies of the Myriapoda. On 
examining the cord very closely, I have reason to believe that the upper and inferior 
sets of longitudinal fibres, the ganglionic and the aganglionic, are somewhat smaller 
at their posterior than at their anterior extremity, a circumstance readily understood 
in the fact that successive series of filaments are given off from them at each distribu- 
tion of nerves from the ganglionic enlargements, while the relative size of the lateral 
portions of the cord appears to be greater in the posterior than in the anterior. On 
this account I have named these lateral fibres, fibres of reinforcement of the cord. 
In regard to the identification of these fibres, it may be well further to state, that 
their separate existence is indicated chiefly at the postero- lateral margin of the gan- 
glia, (fig. 7-f) where they are seen to form part of the nerves and cord without passing 
upwards to the brain. In other parts of their course they are not distinguishable by 
colour, and very rarely by any longitudinal line of separation from the fibres which 
form the inferior longitudinal series, or portion of the cord, to which they are ap- 
proximated ; but from which they are believed to be distinct from the fact, that they 
do not ascend with them to the brain. Their function must be regarded only as 
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MDCCCXLIII. 
