438 THE XERVOUS SYSTE.If. 



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 



I M-.SI Kir riON < >! Pi. A 1 1: \\VII. 



Details of the thoracic nerve centre. 



Kir.. I. A lateral section through the ventral ganglia, showing the origin of the 

 ventral nerve routs. 



Fn;. 2. A similar section nearer to the ventral surface. 



Fie. 3. A vertical longitudinal (sagittal) section, slum ing the origin of the dorsal and 

 ventral nerve roots. 



I [G. 4. A similar section nearer the medium plane and somewhat oblique. 



./, /', <\ pro-, meso-, and meta-thoracic ventral ganglia ; if, dorsal prothoracic 

 ganglion ; e, dorsal mesi (-thoracic ganglion ; f, root of (he nerve to the halter 

 an'l great ganglion of the same ; .;, in I-'ig. }, meta-thoracic ventral ganglion, in 

 l-ig. 4, dorsal ganglionic enlargement corresponding to the abdominal ganglia; 

 ''. ganglion at the root of the lirst abdominal nerve. 



