122 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



ous nerve fibers differ in kind is a question of great interest in physi- 

 ology. The usually accepted view , is that they are identical in 

 character in all fibers and vary only in intensity. According to 

 this view, a sensory nerve the auditory nerve, for instance car- 

 ries impulses similar in character to those passing along a motor 

 nerve, and the reason that in one case we get a sensation of hearing 

 and in the other a contraction of a muscle is found in the manner 

 of ending of the nerve, one terminating in a special part of the cortex 

 of the cerebrum, the other in a muscle. From this standpoint 

 the nerve fibers may be compared to electrical wires. The current 

 conducted by the wires is similar in all cases, but may give rise to 

 very different effects according to the way in which the wires ter- 

 minate, whether in an explosive mixture, an arc light, or solutions 

 of electrolytes of various kinds. We have in physiology what is 

 known as the doctrine of specific nerve energies, first formulated 

 by Johannes Miiller. This doctrine expresses the fact that nerve 

 fibers when stimulated give only one kind of reaction, whether 

 motor or sensory, no matter in what way they may be stimulated. 

 The optic nerve, for instance, gives us a sensation of light, usually 

 because light waves fall on the retina and thus stimulate the optic 

 nerve. But if we apply other forms of stimulation to the nerve 

 they will also, if effective, give a sensation jf light. Cutting the 

 optic nerve or stimulating it with electrical currents gives visual 

 sensations. On the identity theory of the nerve impulses the 

 specific energies of the various nerves that is, the fact that each 

 gives only one kind of response is referred entirely to the charac- 

 teristics of the tissue in which the fibers end. If, as has been said, 

 one could successfully attach the optic nerve to the ear and the 

 auditory nerve to the retina then we should see the thunder and 

 hear the lightning. 



The alternative theory supposes that nerve impulses are not 

 identical in different fibers, but vary in quality as well as intensity, 

 and that the specific energies of the various fibers depend in part at 

 least on the character of the impulses that they transmit. On 

 this theory one might speak of visual impulses in the optic nerves 

 as something different in kind from the auditory impulses in the 

 auditory fibers. With our present methods of investigation the 

 question is one that can not be definitely decided by experimental 

 investigation; most of the discussion turns upon the applicability 

 of the doctrine to the explanation of various conscious reactions 

 of the sensory nerves. 



So far as experimental work has been carried out on efferent 

 nerves, it is undoubtedly in favor of the identity theory. The 

 action current is similar in all nerves examined; the reactions to 



