CHAPTER VIII 

 THE ANALYSIS OF BEHAVIOUR 



Our analysis of the nervous system has, so far, been that of 

 a very complicated structure built up of superposed reflex arcs. 

 A reflex arc is the series of nervous parts that connect together 

 a receptor and an effector organ; thus the arc that is functional 

 in the simple act of winking connects the retina of the eye with 

 the visual centre in the mid-brain (by the optic nerve), the 

 visual centre with the oculo-motor centre (by an intracerebral 

 tract), and the oculo-motor centre with the muscles of the eye- 

 lids (by the oculo-motor and facial nerves). When an action 

 of any kind occurs, such a reflex arc or arcs become functional. 

 The receptor organ receives a stimulus — that is, something 

 happens outside the body: a flash of light, a noise, a current of 

 air carrying odoriferous particles in suspension, etc. — and this 

 event causes some change in the receptor organ. The nerve 

 terminations in the retina, the internal ear, or the mucous 

 membrane of the nose are thus stimulated, and they initiate a 

 nervous impulse which is propagated along the sensory nerve 

 to the centres of the brain. 



There the afferent nervous impulse breaks upon a series of 

 synapses, and so enters a number of nerve cells that constitute 

 the centre (or nucleus, or ganglion). Something now happens 

 to the impulse in these cells, and its physical nature is doubtless 

 changed in some way. At any rate it is transferred from the 

 cells to the axons which come from the latter. Those axons 

 constitute a nervous tract leading to another nucleus, where 

 there is also a series of synapses. The impulse, after further 

 modification in the cells of the second nucleus, is now transferred 

 to an efferent nervous tract which is constituted by the axons 

 passing out from those cells. After traversing this third nervous 

 tract (or effector nerve), the impulse is received by the effector 

 organ. Let us suppose that this is a muscle. The latter there- 

 upon contracts or relaxes. 



This is a scheme of what happens whenever a bodily action 

 occurs: a stimulus is initiated and propagated along an afferent 



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