GANGLIONIC NERVOUS SYSTEM 



625 



centre for optical and other sensations from the sense organs concentrated 

 in the head, it acts as an association centre and exercises an important 

 integrative, particularly inhibitory, control over motor activity throughout 

 the body. This is well seen in ablation experiments. After removal of this 

 ganglion either in Crustaceans or Insects, spontaneous locomotion and 

 coordinated feeding cease but local segmental reflexes persist, and owing to 

 the removal of inhibition these activities tend to be much exaggerated, 

 whether they control reflex movements, locomotion or the chromatophores. 

 Section of one circum-oesophageal comiective leads to unilateral effects and 

 circus movements (Jordan, 1918 ; Herter, 1931 ; ten Cate, 1931 ; Prosser, 

 1946). This inhibitory action of the cerebral ganglion over the ventral 



Figs. 694 to 695. — The Optic Lobes and Cerebral Tracts of the Insect. 



Fig. 694. — Vertical section through the head of a bee. 



Showing, centrally, the paired protocerebnim or cerebral ganglion, underneath 

 which are the sub- oesophageal ganglia. Joining the compound eye wnth the central 

 nervous sj^steni lie the optic lobes wherein the three nuclei — the lamina ganglionaris 

 externally, the external medulla and internal medulla internally — are well differen- 

 tiated (Xorman Ashton). 



P RO T O-C £ R E B R U M 



OPTIC LOBE 



Fig. 695. — Scheme of the visual paths from the eye to the protocerebrum in a 



typical insect. 



CE, compound eye, from which nerve fibres go directly to the lamina ganglionaris 

 (optic ganglion I), LG. Thence a decussation of fibres, the external chiasma, EC, leads 

 to the external medulla (optic ganglion II), EM. Thence a third relay of fibres, the 

 internal chiasma, IC, leads to the internal medulla (optic ganglion III), IM, which may 

 be divided into two parts. Thence filires are relajecl to the optic centres in the cerebral 

 ganglion — mainly the ojatic tubercle, OT, and the pedunculate body, PB — as well 

 as contributing decussating fibrt'S, DF, to the nuclei of the other side. For the 

 descending fibres, see Fig. 696. 



