ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM 445 



meter. If the electrodes of a sensitive galvanometer be 

 placed on a nerve and an impulse then be transmitted along 

 the nerve the needle of the galvanometer will be deflected 

 in such a way as to reveal the presence of a current at the 

 moment the nervous impulse is passing. This current has 

 been called the current of negative variation. This wave 

 of variation travels, of course, with the same speed as the 

 impulse, and the time it takes to pass a certain point can be 

 easily determined. One has simply to note at what instant 

 the deflection of the needle begins and the instant it ceases. 

 Measurments of this kind have shown that the wave takes 

 only about .0007 of. a second to pass. Thus, knowing its 

 rate of speed and the time consumed in passing a given 

 point, it is a simple mathematical calculation to show that 

 it is a little over seven-tenths of an inch in length. The 

 needle further indicates that at first this current is very- 

 feeble, then rises to a maximum, then gradually falls and 

 disappears. 



This wave" of negative variation is but a result of the 

 nervous impulse and is caused by the molecular changes in 

 the nerve fiber as the impulse proceeds, much as in the 

 running of a train there might be along the rails accom- 

 panying the train currents of electricity caused by the fric- 

 tion of the running wheels, which current might be easily 

 detected by a galvanometer even though the train itself 

 should be invisible. 



KINDS OF NERVE FIBERS. 



Any real division of nerve fibers is obviously possible 

 only on the basis of the organs or centers with which they 

 are connected. On this basis nerve fibers are divided into 

 two classes, the first called the afferent or sensory fibers, the 

 second the efferent or motor fibers. The terms sensory and 

 motor are, however, not very fortunate ones, as there are 

 some afferent fibers that never carry sensations which reach 

 consciousness, while many of the motor fibers carry impulses 

 which do not reach muscles. A more detailed classification 

 is usually made as follows: 



