SPECIFIC NERVOUS ENERGY 223 



some sort. A similar investigation may be held concerning those 

 nerve fibres which carry impulses from the periphery. Stinuilation 

 of the optic nerve gives rise to the sensation of light, of the cochlear 

 nerve to sound, and so on. On these facts about sensory stinuila- 

 tion is based Midler'' s " Doctrine of Specific Nervous Energy,'''' 

 which reads : " The specific sensations of each sensory nerve can 

 be evoked by different internal and external stimuli." " Sensation 

 is not the transmission to consciousness of a quality or state of an 

 external body, but of the quality or state of a sensory nerve as 

 produced by an extrinsic cause, and these qualities differ in the 

 different sensory nerves." They differ not because of any inherent 

 quality in the nerve, but because of the nature of the central body 

 to which they go. In the same way, as far as modern work goes 

 it shows that the concomitants of all nervous impulses are abso- 

 lutely similar. The sole difference between motor neurons and 



NERVE CELL 



DENDRITES IN 

 A 



> 



DENDRiTES IN 



CONDUCTING AAON R£CEIVIN(3 AXON V^ 



KOTO:^ ORGAN ^ RECEPTOR. ORCAN 



^ • CONTACT KEY 



GALVANIC CELL 



FiCt. 54. — A. Diagram of a unit of the nervous system compared with B. 

 B. Electrical Model to illustrate Miiller's Law and the " All or Nothing " hypothesis 

 as explained in the text. 



sensory neurons is in the nature of the organ to which the nervous 

 disturbance is propagated. 



No difficulty should be experienced in grasping this idea, 

 especially if an electrical model be kept in mind. 



Consider an electrical circuit such as shown in Fig. 54b, where 

 a galvanic cell or other electrical unit is connected by wires F^ and 

 F.)^ to M, an electric machine. A key closes the circuit, {a) It 

 does not matter how the key is closed, the current passing along F 

 will be the same, and (/;) the manifestation of the current will 

 depend on the nature of M. If M is a telephone receiver, the 

 closing of the key will cause a sound to be heard, if M is an incan- 

 descent globe, light will be seen, if M is a motor, motion will result, 

 and so on. The electrical energy of the electric generator can thus 

 be converted into any form of energy by an appropriate M. 

 Further, the magnitude of the force applied to the key makes no 

 difference to the magnitude of the resulting manifestation at M. 



