128 



THE NERVOUS SYSTEM 



[CH. 



into six main parts, which will be dealt with, in order, from the 

 compound eye inwards to the procerebral lobe. 



1. The layer of post-retinal fibres, or terminal anastomosis (pr). 

 Each ommatidium (p. 141) rests upon the fenestrate membrane (/), 

 and sends through it delicate nerve-fibres, which at once collect 

 together in bundles. These bundles quickly pass into a narrow 

 layer of nerve-cells, which form with them a plexus. From this 



op 



crtn 



Fig. 58. T.S. through right optic ganglion and eye of Au&trolestes leda Selys ( x 50), 

 with key below. The arrows indicate the part represented in the key. Explana- 

 tion in text. Original. 



plexus the bundles emerge irregularly, being separated from one 

 another by large and thin-walled tracheae (tr r ). They next enter 

 another small layer of nerve-cells, forming a second plexus, lying 

 just outside the ganglionic plate. All this complex structure is 

 called the terminal anastomosis. The portion lying nearest to 

 the fenestrate membrane is heavily charged with black pigment, 



