CHAPTER X 



THE MEANING OF PERCEPTION 



What we have done so far has been to make a summary of the 

 main results of physiology. Now it is obvious that this summary 

 has been a very bare one, but we have so designed it that the 

 reader should have little difficulty in greatly amplifying his 

 knowledge by the study of a good, recent textbook. Assuming, 

 then, that he has done so, it will be seen that the outcome of 

 both medieval and modern anatomical and physiological research 

 has been to give us an analysis of the means of acting of the 

 animal mechanism — an analysis that has become more and more 

 refined as our chemical and physical methods of investigation 

 have become more powerful and penetrating. 



It is quite easy to show this by tracing out the history of 

 physiology with respect to any one particular mode of function- 

 ing — for instance, conduction within the nervous system. Thus 

 Descartes made an explanation of action which was based 

 entirely upon anatomical studies, and which, as we have just 

 seen, involved only mechanical ideas. The nerve joining a sense 

 organ — the eye, for example — with the brain contained one kind 

 of fibres, which were, in effect, threads stretched between the 

 receptors and certain valves in the walls of the cerebral ven- 

 tricles. "When these threads were stimulated they were jerked 

 in some kind of way, and this jerk opened the valves and allowed 

 animal spirits to escape from the reservoir in which they were 

 stored. The spirits flowed outwards along nerves which con- 

 tained another kind of fibres, which were tubes communicating 

 with muscles. When they entered the latter, an expansion in 

 one direction and a shortening in another one occurred, so that 

 the muscle exerted a pull on the movable bone to which it was 

 attached. Clearly we have here the original conception of 

 afferent and efferent nervous impulses. Much later (in 1821) 



' Bell verified this hypothesis in part by direct experiment. Each 

 spinal nerve contains two kinds of fibres, one carrying impulses - 



• to the centre, and others carrying impulses from the centre to 

 the periphery. Further, each spinal nerve has two roots, and 



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