694 TOUCH. [CH. LI. 



instance, in the production of sensations of taste, touch, weight, 

 and alteration of temperature by conduction ; in the case of the 

 olfactory end-organs, the sensation is also excited by material 

 particles given off by the odoriferous body, and borne by the air 

 to the nostrils. In sound also, though there is no actual contact 

 of the ear with the vibrating body which emits the sound, the 

 organ of hearing is excited by waves of material substance, first 

 of air, then of bones, then of endolymph, and these excite the 

 nerve-endings of the internal ear. 



When the eye is excited by any other kind of stimulus than by 

 light, which is its adequate or homologous stimulus, the sensation 

 experienced is light all the same ; for instance, one sees sparks when 

 the eyeball is struck ; singing in the ears, the result of an accumula- 

 tion of wax against the membrana tympani, is a similar example. 



This brings us to the conclusion of this chapter by leading to 

 the question, Is there such a thing as specific nerve-energy ? It is 

 an old question, but the answer has still to be found. Sight is a 

 different thing from hearing, and both are different from taste 

 and smell. What is the difference really due to ? Can it be 

 explained by supposing that the nervous impulse along the optic 

 nerve is a different kind of molecular change from that which 

 accompanies gustatory or auditory impulses ? Or can it be 

 explained by supposing that the main difference is in the end- 

 organ, or in the psychical process which interprets the impulse 

 from the end-organ ? Until we know more about the nature of 

 the molecular change which constitutes a nervous impulse, it is 

 merely a matter of speculation whether specific nerve-energy exists. 

 (See Langley's experiments, p. 303.) 



CHAPTER LI. 



TOUCH. 



UNDER the general heading Touch we shall include the various 

 kinds of sensory impressions that start from the skin and muscles. 



Tactile End-organs. 



First, however, it is necessary to study the varieties of end- 

 organs concerned in the reception of the impressions. They are 

 of numerous kinds, but the following are the principal ones : 



Pacinian Corpuscles. These are named after their discoverer, 



