ANATOMY OF THE NERVE CENTRES. 437 
in the female and the vesiculz seminales in the male. After 
a most careful search I have been unable to find any traces of 
these: ganglia. 
Longitudinal Sections of the Thoracic Ganglion (Pl. XXVII.) show 
that the various ganglionic enlargements of the core are united 
by numerous distinct bundles of fibres, some of these pass into 
the dorsal band and unite the thoracic and cephalic centres, 
others are merely commissural between the component ganglia 
of the thoracic nerve centre. 
The great cephalo-thoracic nerve cord divides into three sets 
of fibres—a dorsal median set (Fig. 3, e) which form the dorsal 
band; a second set which lie in a lower plane, and are con- 
nected with the dorsal ganglia (Figs. 1 and 4); and a third set 
which enter the ventral ganglia (Fig. 2). 
The commissural fibres between the component ganglia are 
vertical, transverse, or longitudinal ; so that all the ganglia are 
connected in the most complex manner. These connections 
have not, however, been worked out in detail ; some are repre- 
sented in the figures. 
Owsjannikow [172] describes the longitudinal nerve fibres 
of the segmental ganglia in the Crustacea as of two kinds: a 
dorsal set of large fibres which are superficial—these, he says, 
are united with the large nerve cells of the cortex and are 
motor in function—and a deeply-seated set of small fibres, 
which are connected with the small cells; these he regarded 
as sensory. 
In the Blow-fly the dorsal band of fibres clearly correspond 
with the superficial dorsal fibres of Owsjannikow, and they are 
probably directly connected with the large nerve cells, with the 
dorsal nerve roots, or with both; but they also have deep 
connections with the central stroma. 
The deeper longitudinal fibres scarcely differ in size, how- 
ever, from the dorsal set; they are most probably sensory. 
As the fibres which constitute the dorsal band are far more 
numerous than those of the cephalo-thoracic nerve cord, it is 
certain that they do not all come from the cephalic centres, 
and possibly none of the cephalo-thoracic fibres are connected 
