438 THE NERVOUS SYSTEM. 
with the nerve roots or ventral abdominal cord without com- 
municating with ganglion cells in the thoracic ganglion. 
Many of the fibres of the thoracic nerve centre are simply 
commissural, uniting its several constituent ganglia. Such 
fibres are probably concerned in the co-ordination of the 
movements of the thoracic appendages. 
The Vesicular or Gray Cortex covers the surface of the nerve 
centre. Its thickness is subject to considerable variation, it is 
thickest between the ganglionic enlargements and around the 
nerve roots; it is very thin on the dorsal aspect of the centre, 
and is entirely wanting above the central dorsal band. 
The nerve cells of which it is composed are of two kinds— 
very large stellate cells and smaller fusiform cells. The 
former are found in groups close to the nerve roots, and the 
latter cover the intervening portion of the centre, and are the 
more numerous. Between the large and smaller cells are 
others which are intermediate in dimensions. 
The gray cortex also passes into the interior of the ganglion 
between the component lobes of the central stroma; this is 
well seen in the figures. 
All the cells of the cortex are connected by fine processes 
with the central stroma, and other processes from many of 
them can be traced into the nerve roots. But the fibres of the 
nerve roots are not all of them connected with the cells of the 
cortex directly. Many of them can be traced into the medullary 
stroma, and others, as has been already stated, come from the 
DEscrIPTION OF PLATE XXVII,. 
Details of the thoracic nerve centre. 
Fic. 1.—A lateral section through the ventral ganglia, showing the origin of the 
ventral nerve roots, 
Fic. 2.—A similar section nearer to the ventral surface. 
Fic, 3.—A vertical longitudinal (sagittal) section, showing the origin of the dorsal and 
ventral nerve roots, 
Fic. 4.—A similar section nearer the medium plane and somewhat oblique. 
a, 6, ¢, pro-, meso-, and meta-thoracic ventral ganglia ; @, dorsal prothoracic 
ganglion ; e, dorsal meso-thoracic ganglion ; /, root of the nerve to the halter 
and great ganglion of the same ; y, in Fig. 3, meta-thoracic ventral ganglion, in 
Fig. 4, dorsal ganglionic enlargement corresponding to the abdominal ganglia ; 
A, ganglion at the root of the first abdominal nerve. 
