STEUCTUEE OF SENSE-OEGANS. 467 



end organs in the eye, others end organs in the ear, and so 

 on; while others, less changed, remain in the skin as organs 

 of touch and temperature; and so, from a general exterior 

 surface responding equally readily to many external natural 

 forces, we get a surface modified so that its various parts 

 respond with different degrees of readiness to different ex- 

 ternal forces; and these modified parts constitute the essen- 

 tial portions of our organs of special sense. Every sense 

 organ thus comes to have a special relationship to some one 

 natural force or form of energy is a specially irritable 

 mechanism by which such a force is enabled to excite sen- 

 sory nerves; and is, moreover, commonly supplemented by 

 arrangements which, in the ordinary circumstances of life, 

 prevent other forces from stimulating the nerves connected 

 with it. Not all natural forces have sense-organs with ref- 

 erence to them developed in the Human Body; for exam- 

 ple, we have no organ standing to electrical changes in the 

 same relation that the eye does to light or the ear to 

 sound. 



The Essential Structure of a Sense-Organ. In^every 

 sense-organ the fundamental part is one or more end 

 organ&r which are highly irritable tissues (p. 31), so con- 

 structed and so placed as to be normally acted on by some 

 one of the modes of motion met with in the external world. 

 A sensory apparatus requires in addition at least a brain-cen- 

 tre and a sensory nerve-fibre connecting this with the ter- 

 minal apparatus; but one commonly finds accessory parts 

 added. In the eye, e.g., we have arrangements for bringing 

 to a focus the light rays which are to act on the end 

 organs of the nerve-fibres; and in the ear are found similar 

 subsidiary parts, to conduct sonorous vibrations to the end 

 apparatus of the auditory nerve. 



Seeing and hearing are the two most specialized senses; 

 the stimuli usually arousing them are peculiar and quite 

 distinct from the group of general nerve stimuli (seep. 188), 

 while those most frequently, or naturally, acting upon our 

 other sense-organs are not so peculiar; they are forces wiiich 

 act as general nerve stimuli when directly applied to nerve- 

 fibres. The end organs, however, as already pointed oufr 



