i CUTANEOUS SENSIBILITY 7 



(c) "The specific sensations of each sensory nerve can be 

 evoked by different internal and external stimuli." ..." Sensa- 

 tion is not the transmission to consciousness of a quality or 

 state of an external body but of the quality, or the state of a 

 sensory nerve as produced by an extrinsic cause, and these 

 qualities differ in the different sensory nerves." 



Many attempts have been made, both by the predecessors and 

 by the successors of Johannes Muller, to explain the capacity of 

 the different sensory nerves for receiving certain impressions, by 

 ascribing to them a specific excitability to certain stimuli. This 

 hypothesis is inadequate to explain the facts. We have seen 

 that each sensory organ has an " adequate stimulus," that is, is 

 specifically predisposed to become excited by a given stimulus. 

 But this does not prevent its being excited also by other stimuli 

 which we have termed "inadequate." Mechanical or electrical 

 stimulation of the chorda tympani of man at the point at which 

 it passes through the tympanic cavity excites sensations of taste 

 at the tip of the tongue. The electrical current is not an 

 adequate stimulus of any sense-organ ; there is no special sense- 

 organ for this physical agent, as, e.g., the eye reacts to light, or 

 the ear to sound. Yet electricity is capable of exciting every 

 sense-organ, and evokes different sensations in each. We are 

 therefore compelled " with Aristotle to attribute to each sensory 

 nerve distinct energies, which are its vital qualities, just as 

 contractility is the vital property of muscle. The sensation of 

 sound is thus due to the specific energy of the auditory nerve, 

 light and colour to that of the optic nerve, etc." (Muller). 

 When a certain number of air -vibrations impinge upon the 

 auditory organ, they produce a sensation of sound ; when ether 

 vibrations of a certain wave-length fall on the visual organ, a 

 sensation of light results ; but sound and light as sensations are 

 not comparable with the vibrations of the air or ether. The 

 same vibrations of a tuning-fork that produce a note in the ear 

 excite a sensation of vibration in the skin ; the same ether waves 

 streaming from a lamp produce light through the eye and a 

 sensation of warmth on the skin. In order to obtain sensations 

 of sound or light not only the vibratory movement of the air or 

 ether, but also the presence of an auditory or visual organ, is 

 indispensable. " Without the living ear there would be no sound 

 in the world, but only vibrations. Without the living eye there 

 would be no brightness, no colour, no night, only the oscillations 

 of the imponderable matter of light, or the absence of them" 

 (Muller). 



How does the excitation of the sensory nerves arouse the 

 different conscious sensations in the brain ? Of what character 

 is the active state of the sense-organs which generates in us the 

 different modalities of sensation? In every age philosophers 



