THE ANALYSIS OF BEHAVIOUR IN ANIMALS 157 



less distinct commissural portions composed of internuncial 

 axons and ganglionic regions in which the cell-bodies and cell- 

 connections are located. In Vertebrates the commissural 

 part (white matter) encloses the ganglionic (gray) matter in 

 the greater part of the C.N.S. ; while among invertebrates 

 the C.N.S. is built up of discrete ganglionic and commissural 

 parts. A peculiarity of the Vertebrate C.N.S. Hes in the fact 

 that all secreting cells and smooth muscle fibres are innervated 

 by motor neurones whose cell-bodies are located in subsidiary 

 (autonomic) ganglia receiving efferent impulses from the 

 cord but not themselves the centre of reflex activity. They 



Cerebral qanqljon 



Preganglionic 



neurone 



f?ccepfor 



Postgangllonrc neurone 



Fig. 39. — Diagram of pedal ganglion ot Razor-shell. 



are thus distributive centres for multiplying impulses to be 

 relayed to a large number of similar effectors of which simul- 

 taneous and identical action is required. According to experi- 

 ments by Drew (1908) on the nervous system of the Razor- 

 shell clam an analogous state of affairs is seen in the pedal 

 ganglion of the Lamellibranch (Fig. 39). Drew's experiments 

 indicate that the pedal ganglion of the Lamellibranch only serves 

 as a distributive centre for impulses from the cerebro-pleural 

 ganglia. But in the gasteropod Aplysia, the pedal ganglion 

 is, according to Frohlich (19 10), a reflex as well as a distributive 

 centre. The cardiac ganghon of Limulus is a structure which 

 has probably no close analogy in Vertebrates, unless to the 



