FUNDAMENTAL LAWS OF NERVOUS ACTIVITY 411 



time connected with a galvanometer, the action current appears in both. This 

 is true not only of mixed nerve trunks composed of both afferent and efferent 

 fibers, but can be demonstrated on the anterior roots of the spinal nerves 

 which contain only efferent fibers (Du Bois-Reymond). 



If a living nerve be severed, it of course no longer has the power to 

 transmit the stimulus. But the same is true if the nerve be simply tied off. 

 To be capable of conducting, a nerve must, therefore, be intact, not only in 

 the physical, but also in the physiological sense. 



The conductivity of a nerve may be diminished or abolished for a time by 

 various agents: external pressure e.g., when, as we say, the limbs "go to 

 sleep " ; chloroform ; alcohol, etc. ' All such agents have this in common, that 

 they reduce or even abolish the physiological continuity of the nerve, without 

 destroying its physical integrity. And yet the local excitability of the nerve 

 in the same place may persist. Under certain circumstances it may happen also 

 that a segment of the nerve, which for some reason is not excitable, still has the 

 power to transmit the stimulus received at some other point on the nerve. This 

 is witnessed, for example, in certain stages of regeneration of a nerve that has 

 been cut. Moreover, thfe excitability of the nerve does not always keep even 

 pace with its conductivity, possibly because the nerve responds in a different 

 manner to its natural stimulus propagated from one segment to another, from 

 what it does to the artificial stimuli. 



A stimulus once received by a nerve fiber is transmitted only within the 

 same fiber and its branches, never passing to the fibers running beside it in 

 the same trunk (law of isolated conduction). 



This law holds also for the conducting pathways in the central nervous sys- 

 tem. One can convince himself of its validity in a very simple way. If, for 

 example, he touch the tip of his tongue, at each of two places about 1 mm. apart, 

 with a sharp point, he can distinguish the two points very accurately, which of 

 course would not be possible if the two stimulated the same nerve fiber. 



In very close relation with this law belongs the discovery of the specific 

 character of the response to excitation i. e., that stimulation of a definite 

 nerve produces an effect in its own answering organ and in that organ only. 

 By the answering organ we mean that particular organ connected with the 

 nerve and specially influenced by it. The answering organ of an ordinary 

 motor nerve is the muscle which it innervates, the answering organ of a 

 secretory nerve is the gland which it controls, etc. The answering organs 

 of the afferent nerves are nerve cells situated in the central nervous system. 

 From these new nerve paths originate, and end in other nerve cells, and thus 

 stimulation of a single afferent nerve may rouse a whole series of different 

 nerve cells united together. Finally, a nerve cell connected with an efferent 

 nerve may be set in action by an afferent nerve, and a peripheral organ may 

 thus be stimulated without the participation of the will or even of conscious- 

 ness. Such a phenomenon we call a reflex (Chapter XXII). Because of the 

 manifold way in which the nerve fibers are combined with one another in the 

 central nervous system, very complex effects may result from a single afferent 

 stimulus, without in any way invalidating the law of specific response. 





