494 THE HUMAN BODY. 



which are highly irritable tissues (p. 31), so constructed 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-centre and a 

 sensory nerve-fibre connecting this with the terminal appa- 

 ratus; 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 dis- 

 tinct from the group of general nerve stimuli (Chap. XIII), 

 while those most frequently, or naturally, acting upon our 

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

 which act as general nerve stimuli when directly applied to 

 nerve-fibres. The end-organs, however, as already pointed 

 out, so increase the sensitiveness of the parts containing 

 them that degrees of change in the exciting forces, which 

 would be totally unable to directly stimulate the nerve-fibres, 

 are appreciated. These terminal apparatuses are therefore 

 as truly mechanisms enabling changes, which would not 

 otherwise stimulate nerves, to excite them, as are the end- 

 organs in the eye or ear. 



The Cause of the Modality of our Sensations. Seeing 

 that the external forces usually exciting our different sensa- 

 tions differ, and that the sensations do also, we might at first 

 be inclined to believe that the latter difference depended on 

 the former: that brightness differed from loudness because 

 light was different from sound. In other words, we are apt 

 to think that each sensation derives its specific character 

 from some property of its external physical antecedent, and 

 that our sensations answer in some way to, and represent 

 more or less accurately, properties of the forms of energy 

 arousing them. It is, however, quite easy to show that we 

 have no sufficient logical warrant for such a belief* Light 

 falling into the eye causes a sensation of luminosity, a feel- 

 ing belonging to the visual group or modality; and, since 

 usually nothing else excites such feelings and light entering 

 the healthy eye always does, we come to believe that the 

 physical agent light is something like our sensation of 



