116 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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

 of the cerebrum, the other in a muscle. In this respect and 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 terminate, whether in an explosive mixture, an arc light, 

 or solutions of electrolytes of various kinds. We have in physi- 

 ology 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 reac- 

 tion, 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 stimu- 

 late the optic nerve. But if we apply other forms of stimulation 

 to the nerve they will also, if effective, give a sensation of light. 

 Cutting the optic nerve or stimulating it with electrical currents 

 gives visual sensations. On the identity theory of the nerve im- 

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

 that each gives only one kind of response is referred entirely to 

 the characteristics 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 

 artificial stimuli are essentially similar. Moreover, nerves of 

 one kind may be sutured to nerves of another kind, and, after re- 

 generation has taken place, the reactions are found to be deter- 

 mined solely by the place of ending (see p. 76). 



The Nutritive Relations of the Nerve Fiber and Nerve Cell. 

 In recent times in accordance with the so-called neuron doctrine 

 (see p. 122) every axis cylinder has been considered as a process of 

 a nerve cell, and therefore as a part, morphologically speaking, of 



