ANATOMY OF THE NERVE CENTRES. 437 



in the female and the vesiculse 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 (PI. 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, i 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 



