528 



THE EYE IN EVOLUTION 



boundaries are appreciated with difficulty (cerebral, pedal and pleuro- 

 visceral), all being well protected by investing cartilages. 



In the Cephalopods, the sub -oesophageal ganglionic mass contains centres 

 for regulating the locomotor and visceral activities ; here lie the centres 

 which control the ocular muscles, the pupil and the chromatophores. To 

 the supra-oesophageal (cerebral) ganglia come the sensory efFerents, here lie 

 the higher motor centres controlling movements of large groups of muscles, 

 and here also, situated in the upper part of the ganglion, lies a large associa- 

 tive and integrative area, ablation of which does not impair purely reflex 



Fig. 697. — The Nervous System of a Pulmonate Mollusc, Li My ^ a. 



A pair of cerebral ganglia, C, overlie the oesophagus, below which is a mass of 

 ganglia composed of 2 pedal ganglia, P, 2 pleural, PI, and 2 parietal, Pr, while ventrally 

 in the centre lies the visceral ganglion, V (after Spengel). 



activities or sensory impressions but abolishes initiative in behaviour and 

 plasticity in responses ; this is seen, for example, in such reactions as chasing 

 prey round blind corners or in attempting expedients to escape from artificial 

 restrictions {Se2na, Octopus — Buytendijk, 1933 ; Sanders and Young, 1940). 

 The course of the nerve-fibres associated with vision is shown in Fig. 

 698.1 The axons of the retinal ganglion cells leave the eye, decussate in the 

 very short optic nerve and enter the large optic lobes situated one on either 

 side of the paired cerebral ganglion which they dominate completely by 

 their size. Around the periphery of the optic lobe run two layers of granular 

 cells separated by a plexiform layer of fibres, while in the centre of the lobe 

 are two nuclei, a central and a peduncular nucleus. The axons of the 

 ganglion cells of the retina enter the plexiform layer between the two 

 granular layers and here they meet dendrites of these cells ; the pathway is 



1 Foi 

 Kappers i 



anatomy of the visual fibres of Molluscs, see v. Uexkiill (1895), Cajal (1917), 

 1936), and Sanders and Young (1940). 



