ANATOMY OF THE NERVE CENTRES. 435 



a. The Thoracic Nerve Centre of the Blow-fly Imago. 



The thoracic nerve centre (PL XXVII.) is far more simple 

 in structure than the cephalic. It is fusiform, and exhibits 

 three pairs of hemispherical enlargements on its ventral surface 

 (PI. XXVII. , a, b, c), corresponding with the three pairs of 

 thoracic segmental ganglia (PL II., Fig. i, /, 2, j) of the 

 generalised nervous system. Its dorsal surface exhibits a 

 central band of longitudinal fibres, a continuation of the 

 cephalo-thoracic nerve cord. On either side of this band 

 there are three dorsal enlargements (PL XXVII. c, f, h) of 

 the central stroma (p. 66). These I term the dorsal meso- 

 and meta-thoracic and the abdominal ganglia. There are also 

 two pairs of deeply seated masses of stroma (seen in the 

 section PL XXVII., Fig. 4, d and /). The anterior pair 

 I shall term the dorsal pro-thoracic lobes and the posterior 

 pair which give origin to the great nerves of the halteres 

 may be distinguished as the great or central ganglia of the 

 halteres. 



Nerves. Nine pairs of nerves arise from the thoracic 

 ganglion. Three ventral pairs (PL XXVI., v', v", v"') from the 

 ventral enlargements which supply the three pairs of legs, and 

 six pairs of dorsal nerves. Three of the latter are very large, 



they are the dorsal pro-, meso- and meta-thoracic nerves (d', 



jn j"'\ 

 d ,d ). 



The pro-thoracic dorsal nerves supply the muscles of the 

 neck and the anterior lateral wing-muscles. The second pair, 

 the meso-thoracic dorsal nerves, supply the remainder of the 

 thoracic muscles, the dorsales, sterno dorsales, and the pos- 

 terior muscles of the wing root. The third pair of dorsal 

 nerves are entirely sensory and terminate in the small ganglia 

 at the bases of the halteres (the peripheral ganglia of the 

 halteres). 



The pro- and meso-thoracic dorsal nerves each arise by 

 two roots, a dorsal bundle of fibres which may be traced into 

 the median dorsal band, and a second, or ventral, bundle about 



