OF MYRIAPODA AND MACROUROUS ARACHN1DA. 
249 
described as the motor tract, and to which the function of volition seems still to be 
accorded by Valentin*, Carpenter-^-, and Baly^, is extended in lulus, as in other 
Articulata, as a separate fasciculus along the upper surface of the cord ; but in these 
Myriapoda it is much narrower in proportion to the whole width of the cord than in 
insects. This fact is interesting in reference to its presumed function. On a cursory 
inspection it does not appear to give off any branches, but seems to pursue its course 
uninterruptedly along the whole length of the cord. It does not indeed give off fila- 
ments to the nerves from a ganglion immediately opposite to their origin, while pass- 
ing over that ganglion, but immediately it has passed one ganglion it gives off the 
filaments that proceed to the nerves from the next ganglion. These filaments seem 
almost immediately to join with others that belong to the sides of the cord, and pass 
out with them into the nerve from the next ganglion along its anterior surface. This 
is almost precisely the manner in which the filaments from this aganglionic column 
in the Crustacea are united with those from the ganglionic, as formerly shown in my 
description of the nerves in that class, when the existence of the lateral fibres of the 
cord was unknown to me. 
The inferior longitudinal, or ganglionic set of fibres (fig. 5. a) of the cord, affords 
many interesting considerations. It is placed, exactly as in insects, on the under 
surface, but like the upper series it is narrower than the whole cord, of which it forms 
a part. It is formed of a longitudinal series of fibres, like the upper tract, beneath 
which it is placed, and from which it is divided by some of the fibres that pass trans- 
versely through the cord, and which enter into the composition of the nerves from 
the ganglion on either side. It appears also to receive filaments from the upper 
series, and perhaps others are sent from it to the upper, thus decussating each other 
in the middle substance of the cord, where these two longitudinal series are in close 
apposition ; since it is almost impossible, even in the large nervous cord of Scolo- 
pendra, to separate these two tracts from each other, although their distinctness is 
evinced in their relative size and longitudinal lines of separation. But there is one 
fact of great interest in regard to this ganglionic series of fibres. Almost the whole 
of the fibres of which it is composed are traceable, in the lulidse, directly through 
each enlargement of the cord, which they mainly assist to form. At the anterior part 
of each enlargement the diameter of each fibre, or fasciculus of fibres, appears to be 
slightly increased, and its structure becomes more softened and delicate. While 
passing through these ganglionic enlargements, occasioned chiefly by their own in- 
creased diameter, the fibres take a slightly curved direction outwards, and then in- 
wards, but are reduced to their original size, and assume the longitudinal direction 
on again forming the aganglionic portion of this tract of the cord. This structure of 
the fibres is well seen in the lulidce and Polydesmidce (fig. 7- i), as I shall hereafter 
* De Functionibus Nervorum, Bern. 1839. (Vide Baly’s Muller.) f Op. cit. 
X Muller’s Elements of Physiology, second edit. vol. i. 1840, p. 771. 
