THE NERVOUS SYSTEM. 553 



nervous system from the periphery, or from the central nervous system 

 to the muscles or other tissues. The same fibre cannot be used for the 

 one purpose at one time, and for the other at another, simply because of 

 the nature of its terminal organs. Thus, when a cerebro-spinal nerve- 

 fibre is irritated in the living body as by pinching, or by heat, or by 

 electrifying it, there is, under ordinary circumstances, one of two effects, 

 either there is pain, or there is twitching of one or more muscles to 

 which the nerve distributes its fibres. From various considerations 

 it is certain that pain is always the result of a change in the nerve- 

 cells of the brain. Therefore, in such an experiment as that referred 

 to, the irritation of the nerve-fibre is conducted in one of two direc- 

 tions, i.e., either to the brain, which is the central termination of the 

 fibre, when there is pain, or to a muscle, which is the peripheral ter- 

 mination, when there is movement. 



The effect of this simple experiment is a type of what always occurs 

 when nerve-fibres are engaged in the performance of their functions. 

 The result of stimulating them, which roughly imitates what happens 

 naturally in the body, is found to occur at one or other of their ex- 

 tremities, central or peripheral, never at both ; and in accordance with 

 this fact, and because, for any given nerve-fibre, the result is always the 

 same, nerve-fibres have been commonly classed as sensory or motor. 



This is not altogether accurate, and the terms centrifugal or efferent 

 and centripetal or afferent are more properly used, since the result of 

 stimulating a nerve of the former kind is not always the production of 

 pain or other form of sensation, nor is motion the invariable result of 

 stimulating the latter. 



The term intercentral is applied to those nerve-fibres which connect 

 more or less distinct nerve-centres, and may, therefore, be said to have 

 no peripheral distribution, in the ordinary sense of the term. 



Impressions made upon the terminations or upon the trunk of a 

 centripetal nerve may cause (a) pain, or some other kind of sensation ; () 

 special sensation ; or (c) reflex action of some kind ; or (d) inhibition, 

 restraint of action. Similarly impressions made upon a centrif- 

 ugal nerve may cause (a) contraction of muscle (motor nerve) ; (b) it 

 may influence nutrition (trophic nerve) ; or (c) may influence secretion 

 (secretory nerve) ; or (d] inhibit, augment, or stop any other efferent 

 action. 



It is a law of action in all nerve-fibres, and corresponds with the con- 

 tinuity and simplicity of their course, that an impression made on 

 any fibre, is simply and uninterruptedly transmitted along it, without 

 itself being imparted or diffused to any of the fibres lying near it. It 

 is possible that the mere passage of a nerve impulse along a nerve-fibre, 

 however, may produce some effect upon the neighboring nerve-fibres. 



