CHAPTER X 



NERVOUS SYSTEM 



In the early years of this century experiments on the nervous 

 system were made in many invertebrates, relying upon macro- 

 scopical studies on the effects of nerve section on behaviour, what 

 happened when the nerves regenerated and how the behaviour 

 altered in response. Histological investigations gave details of the 

 innervation of sense organs and motor systems and further 

 experiments revealed something of the properties of these systems. 



Since the 1930s, however, our knowledge has increased rapidly 

 with the introduction of electro-physiological techniques. With the 

 advance of amplifiers, oscilloscopes, stimulators, tape recorders and 

 all the other impedimenta of the neurophysiologist, our knowledge 

 of nervous activity has increased rapidly. Some of the first 

 experiments were carried out on invertebrates and the first micro- 

 electrode implanted into a single axon was placed in the squid 

 giant axon. A large volume of work has accumulated and is 

 excellently summarized in the recent book by Bullock and Horridge 

 (in press). By far the greatest volume of work has been carried out 

 on insects, Crustacea and molluscs, but other phyla have not been 

 ignored. 



In the realm of earthworm physiology the functions of the 

 nervous system have received more attention than any other 

 aspect. Much of the interest in this system has centred upon the 

 presence of three giant fibres which extend the entire length of the 

 body and conduct very rapid impulses from end to end. And yet 

 despite the considerable experimentation made upon them some of 

 the properties of these fibres still evade description. In other 

 aspects very little is known about sense organs, synaptic properties, 

 motor fibres and innervation of muscles. It is not proposed here to 

 go deeply into the anatomical and histological structure of the 

 nervous system since very full descriptions can be had from 



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