CHAPTER VI. 

 THE OKGANS OF SENSE. 



SECTION I. 



THE organs of sense, with their nerves, form the medium by which 

 afferent impressions are conveyed to the cortex of the cerebral hemi- 

 spheres. If the sensory mechanism is concerned with impulses excited 

 by stimuli from without, it is described as exteroceptive ; if the stimuli 

 arise in the viscera, the mechanism is called enteroceptive ; if they arise 

 in the muscles or sense organs affected by the position of the body, 

 the mechanism is proprjoceptivf-. Thus the exteroceptive system 

 includes the structures which have to do with sensations of pressure, 

 taste, smell, sight, and hearing ; the enteroceptive system includes the 

 mechanisms for hunger and thirst ; and the proprioceptive system has 

 to do with sensations of position of the head and limbs and of the 

 degree of muscular contraction. 



The structures concerned in the production of sensation are : (1) 

 an end-organ, (2) a chain of neurons which transmits the impulse, and 

 (3) the sensory, psychic, and association areas in the cortex to which 

 the impulse is transmitted. The end-organs for each sense are 

 structurally adapted to receive the stimulus for that particular sense, 

 this being known as the adequate stimulus. Thus the rods and cones of 

 the retina are stimulated by waves of light, but not by waves of sound, 

 while the hair cells in the organ of Corti are excited by sound waves 

 but not by those of light. The nerve fibres which transmit the im- 

 pulses, on the other hand, appear to be able to transmit any variety of 

 stimulus, and they all give the same type of electrical variation when 

 stimulated by an electric current. 



Although the various sense organs differ widely in structure, certain 

 general principles can be formulated which are applicable to 

 them all. 



(1) Stimulation of the end-organs of any particular sense gives 

 rise to the sensation peculiar to that sense, and to that sensation only ; 



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