266 



THE NERVOUS SYSTEM 



central or synaptic nervous system. Out of the single 

 continuum of nervous tissue is evolved a system of nerve- 

 cells or neurones which form a complex chain, adjacent 

 hnks of which are functionally continuous, but, so far as 

 is known, histologically discontinuous, the gaps which 

 separate the neurones being known as si/napses. As an 

 impulse traverses a synapse it is hable, as we shall see, 

 to modification both in intensity and in character. It is 



EPITHtLIUM 



NUCLEUS 



MUoCLE FIBRE5 



Fig. 41. — Diagram of part of body-wall of Medusa (jelly-fish), 



after Bethe. 



owing to the special physiological characters of the synapses 

 that the animal is enabled to grade and to alter in kind 

 the form of its nervous response. 



Synapses or junctions between neurones are collected 

 together into groups, and in segmented organisms each 

 segment has its own synaptic centre, the centres of all 

 the segments being connected together by nervous strands. 

 In this way is formed the beginning of a spinal cord wliich 

 serves the purpose of conducting an impulse from one 

 segment to another, and thus of co-ordinating the activities 

 of all the segments for the good of the whole organism. 



