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 
(Pl. XXVII., a, b, c), corresponding with the three pairs of 
thoracic segmental ganglia (Pl. II., Fig. 1, 7, 2, 3) 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. ¢, f, 1) 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 f). 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. XXVL., 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’, 
Gevdao)s 
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 
