436 THE NERVOUS SYSTEM. 
equal to the former in size, which divides into a number of 
fasciculi, some of which are lost in the cellular cortex, whilst 
others enter the corresponding reticular enlargement of the 
stroma. 
The nerve to the halter has only one root, which is similar 
to the ventral root of the pro- and meso-thoracic dorsal nerves; 
it arises from the great ganglion of the halter, partly from the 
central stroma and partly from the cortex (see ‘ Physiology of 
Nervous System’). 
The remaining nerves may be termed the anterior (ad’) and 
posterior (ad”) accessory dorsal, and the first pair of abdominal 
nerves (ab’). 
The accessory dorsal nerves arise from the lateral region 
of the thoracic ganglion, between the dorsal and ventral nerve 
roots. As only two pairs are developed, which terminate near 
the spiracles, it is highly probable that they supply the 
spiracular muscles, but owing to their extreme fineness I have 
been unable to trace them into these muscles. 
The abdominal nerves of the thoracic ganglion are two in 
number, one on each side (ab’), they pass beneath the meso- 
phragma, and are distributed to the muscles and skin of the 
basal part of the abdomen. 
The dorsal band, much reduced in size, is continued as a 
single nervous trunk, the abdominal cord, which gives off four 
pairs of nerves in the abdomen, and terminates in several 
branches. 
According to Meigen, quoted by Burmeister [8], the Blow- 
fly has two small ganglia on the abdominal nerve-cord, one in 
the second abdominal segment, and one between the ovaries 
DEscrRIPTION OF PLATE XXVI. 
The central nervous system of the imago seen from its dorsal aspect. 
a, antennal nerves; ad’, abdominal nerves arising from the thoracic nerve 
centre ; ab" to ab'”, abdominal nerves arising from the ventral abdominal nerve 
cord ; aa’, ad", anterior and posterior accessory dorsal nerves ; d’, a”, d’”, pro-, 
meso- and meta-thoracic dorsal nerves; mx, maxillary nerves; 0, ocelli; 
@, cesophageal opening between the crura ; 0 g, optic ganglia ; 0 7, optic nerve ; 
ph, pharyngeal nerves ; v', vv”, ventral thoracic nerves. The hemispherical 
caps on the optic nerves represent the retina. 
