THE ANALYSIS OF BEHAVIOUR 



145 



The Nervous System as a Whole. 



Having made this very bare analysis of the structure of the 

 central nervous system, we are now in a position to consider its 

 working as a whole, but, first of all, we may profitably think 

 about it as a series of superposed mechanisms, thus following the 

 natural path of evolution. Fundamentally, then, the nervous 

 system mediates between the stimulation of the organs of sense 

 on the one hand, and the organs of activity on the other. 



Some physical change in the environment leads to the stimula- 

 tion of a receptor organ, and the initiation of a nervous impulse. 

 The latter is received by the nerve cells in the ganglion, and in its 



A * 



Shin 



Iffector 



Receptor 

 organ 



.Muscles 



FIG. 43. THE SIMPLEST POSSIBLE SENSORI-MOTOR ACTIVITY. 



A spinal reflex mechanism, the line AB, represents the junction of 

 spinal cord and medulla. 



turn initiates a number of impulses which pass out from the 

 ganglion along efferent nerves and set up a co-ordinated and 

 purposeful activity in the muscles to which those nerves go. 

 There need not be (and there usually is not) any perception in 

 such a stimulus and response. The latter is usually determined, 

 and can be predicted. 



Next, the development of the special sense organs in the head, 

 and the concentration of ganglia there, lead to a certain integra- 

 tion of sensory stimuli and their resulting responses. The head 

 ganglia were primarily the centres, or nuclei, of the great organs 

 of sense, but it happens they also become connected with the 

 lower spinal centres, as we have indicated in Fig. 28, and so we 

 get a superposed series of connections. 



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