VI] THE NERVOUS SYSTEM 127 



behind the pons. They then diverge again to enter the ocellus 

 on either side. The diverging portions enclose between them a 

 fair number of nerve-cells. 



The Deutocerebrum. This consists of two rather small lobes, 

 the antennary lobes (al), forming slight rounded bosses anteriorly 

 on either side of the middle line (fig. 55 A). These are connected 

 by a transverse commissure lying above and a little in front of 

 the anterior limit of the central body, and representing the so-called 

 dorsal lobe of the insect brain. The antennary nerve (ant) arises 

 from the antennary lobe, and passes transversely, and a little 

 obliquely, outwards and forwards to the base of the antenna, 

 where it divides into two. The nerves to the median ocellus, 

 which are also supposed to arise from the deutocerebrum, are 

 dealt with on p. 126. 



The Tritocerebrum. The two lobes of the tritocerebrum are 

 known as the oesophageal lobes (oel). Each lies below the corre- 

 sponding antennary lobe, with which it is closely fused. The two 

 oesophageal lobes are connected by a slender transverse commis- 

 sure, in the form of an inverted arch, called the commissure of the 

 oesophageal ring (tc). This passes between the brain and the 

 suboesophageal ganglion, beneath the oesophagus. Hence it 

 cannot be seen in fig. 55 B. 



The oesophageal lobes give off (i) the arched nerves to the 

 frontal ganglion, known as the frontal commissures (fig. 55, fc), 

 (ii) slender oblique commissures to the anterior ganglia of the 

 paired sympathetic system, and (iii) the very thick and short 

 circumoesophageal commissures (co fig. 55, coe fig. 56) which con- 

 nect the brain with the suboesophageal ganglion. 



The Optic Ganglion (fig. 58). 



As we have already stated, the optic ganglion or optic tract is 

 the largest and most complicated portion of the cephalic nerve- 

 mass. This, of course, is only to be expected, when we consider 

 the marvellous development of the sense of sight in the Dragonfly, 

 since that sense supplies the stimulus to almost every act of the 

 insect's life. 



We shall follow Viallanes [186] in dividing the optic ganglion 



