ANATOMY OF THE NERVE CENTRES. 439 



dorsal band. Such fibres, however, may be, and probably 

 are, connected with the cortex in other parts of the thoracic 

 ganglion. 



The arrangement of the cells and fibres and their relations 

 to the nerve roots in the thoracic ganglion are in consonance 

 with the view that the ventral chain, which it represents, has 

 functions similar to those of the medulla spinalis and medulla 

 oblongata in the Vertebrata. I shall hereafter show that the 

 evidence we possess indicates that the ventral chain in the 

 Arthropoda consists of a series of reflex centres and conducting 

 cords. The reflex character of the thoracic ganglia becomes 

 very apparent after the cephalo-thoracic cord has been divided 

 in the Diptera ; an indication that the cephalic centres exhibit 

 an inhibitory influence on their reflex functions. 



Morphology of the Thoracic Nerve Centre. Although sections 

 through the thoracic nerve centre exhibit considerable devia- 

 tions from the typical form of the ventral ganglionated cord of 

 the more generalised Arthropoda, there can be no doubt that 

 it is composed of the three pairs of ventral ganglia cor- 

 responding to the three thoracic segments, and of two or more 

 pairs of abdominal ganglia. The segmentation of the neuro- 

 blast only occurs in the early stages of the formation of the 

 nymph ; there is no such segmentation in the larva. The 

 four anterior segments, like those of the Pupiparas, become 

 far larger than any of the others, which apparently undergo 

 absorption or, after having become, greatly reduced in size, 

 are fused with the fourth segment (compare Figs. 8 and 9, 

 PI. II.). The subsequent development of the dorsal ganglia 

 takes place by the enlargement and sub-division into two parts 

 of each of the primitive thoracic ganglia (see ' Development 

 of the Nervous System '). 



The union of the abdominal ganglia with the thoracic is by 

 no means confined to the Diptera and Hymenoptera, but is 

 apparently characteristic of all the higher groups of insects. 

 Thus Herold [140] has shown that the first pair of abdominal 

 ganglia are fused with the thoracic in the butterfly, Pontia 

 Brassicas ; whilst the figure given by Straus Durckheim [40] 



