256 
MR. NEWPORT ON THE NERVOUS AND CIRCULATORY SYSTEMS 
being those in which the structures themselves are nourished. The vessels distri- 
buted over the ganglia penetrate into their substance, and are more abundantly sup- 
plied to them than to any other parts of the nervous system, as will hereafter be 
seen in the Scolopendra. 
The structure of the ganglia in Polydesmus complanatus is well seen after the cord 
has remained for some time in spirit. When examined in the recent state it is far 
less distinct, but the nuclei, which enter largely into the composition of the ganglia, 
are well observed on the under surface. In specimens which have remained in spirit 
the whole of the fibres of the cord are rendered apparent, although the ganglia them- 
selves are more opake. In P. maculatus the aganglionic tract passes in a direct line 
over the ganglia (figs. 9, 10.), as in other Articulata, and gives off its branches as in 
lulus, at some distance anterior to the ganglion. The fibres of the inferior or gan- 
glionic tract (fig. arriving at a ganglion, are softened and somewhat enlarged 
in diameter, and take a slightly curved direction outwards, as far as the middle of the 
ganglion, and then are gradually reduced in size and again directed inwards, until they 
are about to leave it, when they again assume the longitudinal course and form the 
under surface of the cords. This curved direction of the fibres is owing in part to their 
own enlarged structure, and in part also to the presence of numerous gray nucleated 
cells, which assist to form the ganglion. Between these two series of longitudinal 
fibres are placed the commissural ones (g), which pass transversely through the gan- 
glia for the posterior pair of nerves id). The fibres of reinforcement which form the 
sides of the cord are distinctly seen at the sides of each ganglion beneath the trans- 
parent covering of the cord (Z>), bounding the sides of the ganglion in the interspace 
of two nerves (/'), and also at the posterior surface of the nerve where they join the 
cord, having between themselves and the commissural fibres, and fibres of the cord, 
a slight interspace, which is occupied by nucleated cells. Those fibres which belong 
to the anterior pair of nerves, which have been seen to be afterwards removed from 
the ganglia, have communications both with the anterior and posterior nerves, thus 
combining in action the nerves which are distributed to the muscles (c) and sides of 
the segment with those which are given to the legs ( d ). This fact is interesting from 
the circumstance that the commissural fibres which enter into the composition of 
these anterior nerves are placed above the superior aganglionic tract of the cord, and 
this will in great measure account for the removal of these nerves from above the 
ganglion, with which they are thus shown to be in connexion during the growth and 
elongation of the cord itself. It is also further worthy of notice, that this is a con- 
dition in these nerves of Polydesmus precisely analogous to that which exists in the 
respiratory nerves in the larva state of insects, in which, as I formerly* showed, there 
are commissural fibres running transversely across the segments and lying loosely 
above the aganglionic tract of the cord. In regard to the enlargement of the fibres 
of the cord, it may be remarked, that the ganglia are always softer and far more 
* Philosophical Transactions, 1836, Part II. p. 544. 
