THE NERVOUS SYSTEM. 17 



abdomen, will give rise to the well-known effort to clean it off, 

 by rubbing the tarsi together or against the part so irritated : 

 all these are clearly reflex acts. 



These facts go far to prove that the same acts are always 

 produced by external stimulus in the living fly, in other words, 

 that they are reflex and not voluntary ; they also show how 

 extremely sensitive the integuments are to external impressions, 

 and that their hardness does not prevent their being the seat of 

 the sense of touch to a very great extent. Lastly, as there is 

 nothing irregular in any of these acts after decapitation, it seems 

 that the co-ordination of muscular movements is due to this nerve 

 centre, a function, which, according to the experiments of 

 M. Faivre,* is performed by the sub-cesophageal ganglion in 

 Dytiscus, and probably in all beetles : but I believe the homo- 

 logue of this ganglion is transferred in the fly to the thoracic 

 nerve centre, as the inferior portion of the posterior cephalic 

 segment enters into the composition of the thorax, and not of 

 the head. Thus the thoracic nerve centre appears to be the 

 analogue of the cerebellum, medulla oblongata, and spinal cord 

 of the vertebrata. 



The nerves of the viscera are derived from a separate system 

 of ganglia, and differ in structure from those of animal life, like 

 the nerves of organic life, or the sympathetic system of vertebrates, 

 to which they may be justly compared. The chief ganglion of 

 this system is situated at the junction of the thorax and abdomen, 

 but numerous smaller ganglia varying in size are scattered over 

 the visceral organs, and the whole are connected by a complex 

 system of minute nerves, which form a close network over all 

 the viscera. Dr. Lockhart Clark tells me he has described a 

 similar system of nerves in the earthworm. 



* Ann. Sc. Nat. IV. Tom riii 245. 



