468 THE HUMAN BODY. 



(p. 190), so increase the sensitiveness of the parts contain- 

 ing 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 sen- 

 sations differ, and that the sensations do also, we might at 

 first be inclined to believe that the latter difference de- 

 pended on the former: that brightness differed from loud- 

 ness 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 physi- 

 cal 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 feeling 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 some- 

 thing like our sensation of luminosity. But, as we have 

 already seen (p. 191), no matter how we stimulate the optic 

 nerve we still get visual sensations; close the eyes and press 

 with a finger-nail on one eyelid; a sensation of touch is 

 aroused where the finger meets the skin; but the pressure 

 on the eyeball distorts it and stimulates the optic nerve- 

 fibres in it also, and the result is a luminous patch seen 

 in front of the eye in such a position as a bright body must 

 occupy in space to radiate light to that part of the expan- 

 sion of the optic nerve. Finding, then, the same kind of 

 sensation, a visual one, produced by the totally different 

 causes, pressure and light, we are led to doubt if the dif- 

 ferences of modality in our sensations depend upon the dif- 

 ferences of the natural forces arousing them; and this 

 doubt is strengthened when we find still other forces (p. 191} 



