VI] THE NERVOUS SYSTEM 123 



(p. 15). The three parts composing it are (1) the protocerebrum, 

 or ganglion of the first head segment, which forms the greater 

 portion of the whole brain, (2) the deutocerebrum, or ganglion of 

 the second head segment, which gives off nerves to the antennae, 

 (3) the tritocerebrum, or ganglion of the third or intercalary segment, 

 from which the sympathetic nervous system takes its rise. These 

 three parts are closely fused together, so that no definite boundaries 

 can be made out. 



As the optic ganglia, which are connected with the protocere- 

 brum by means of the optic nerves, are exceedingly highly 

 developed in the Dragonfly (exceeding both in bulk and complexity 

 the whole of the true brain or cerebrum), we shall deal with them 

 in a separate section. 



In order to grasp the relative positions of the three principal 

 parts of the brain, we must recall what we have already said 

 about the changes in the position of the parts of the head. When 

 the mouth became anterior in position (p. 10) the originally ventral 

 portions of the second and third segments (i.e. the clypeus and 

 labrum) became pushed up dorsally above it. In like manner, 

 the deutocerebrum and tritocerebrum have become pushed up 

 above the oesophagus, and have come to lie in front of the proto- 

 cerebrum. Also, just as the clypeus lies above the labrum, so the 

 deutocerebrum lies above the tritocerebrum. In fact, the latter 

 does not really lie above the oesophagus in the Dragonfly, but 

 partly on either side of it and partly beneath it (fig. 56 B). 



Fig. 55 shews a diagram of the brain of Austrolestes leda, con- 

 structed from transverse sections through the head of that species. 

 These were 200 in number, and 10 /A in thickness. The numbers 

 given for the seven separate sections shewn in fig. 56 refer to 

 these sections, so that the reader will be able to place them, 

 mentally, at their correct respective levels. 



The Protocerebrum. This consists of two large lateral lobes, 

 the procerebral lobes (pi), formed of an immense number of ganglion 

 cells (the cortex) enclosing a mass of nerve-fibres (the medulla), 

 The cortical cells are most abundant dorsally, and are of the usual 

 unipolar form, with large nuclei. The two lobes are connected 

 across the middle line by two commissures, (i) the central body (cb) 

 and (ii) the pons or bridge of the procerebral lobes (br). The central 



