134 



THE NERVOUS SYSTEM 



[CH. 



these delicate structures. The system may be divided into two 

 principal parts: 



1. The unpaired Frontal System. 

 This consists of a small triquetral 

 frontal ganglion (fig. 56 A, fig. 55, fg, 

 fig. 60, fr), situated in the middle line, 

 just in front of the brain. It is connected 

 with the tritocerebrum by a pair of 

 arched frontal commissures (fc), which 

 enter the brain a little below and inter- 

 nally to the origins of the antennary 

 nerves. The frontal ganglion gives off 

 two fine nerves (m) to the mouth parts, 

 and a long and very delicate posterior 



median nerve-cord (nv-.) which passes R g- 60 - Sympathetic ner- 



. vous system of Libellula. 



beneath the brain, and runs dorsally Explanation in text, br, 



along the oesophagus, crop and gizzard. Dotted ^es) bo^d- 

 The nerve-cells of the frontal ganglion Brandt, 

 lie dorsally over the nerve-fibres (fig. 56 A). 



2. The paired Splanchnic or Stomatogastric system. This is 

 formed by two pairs of small ganglia lying upon the oesophagus. 

 The anterior splanchnic ganglia (asg) are fused together in the 

 middle line. They lie very close behind the brain, and are con- 

 nected with the tritocerebrum by two very fine commissures. 

 A pair of oblique commissures (cob) connects them with the posterior 

 splanchnic ganglia (psg). These are separate, and lie one on either 

 side of the oesophagus, a little behind the anterior pair. They 

 give off (a) a pair of nerves (nv 2 ) obliquely backwards to the 

 muscles of the oesophagus, and (6) a second pair (nv 3 ) running 

 straight back along its lateral walls. 



Peripheral Nerves. 



In some insects, or at any rate in their larvae, the presence has 

 been demonstrated of a -peripheral nervous system quite uncon- 

 nected with any part of the central nervous system, but forming 

 a kind of general nerve-plexus beneath the hypodermis and the 

 muscle-layer. Zawarsin [202], after examining the various peripheral 



