CH. xv.] NERVE IMPULSES. IJ3 



Another method, largely employed by Bemstein, is to take the 

 electrical change as the indication of the impulse. The rheotome 

 is the instrument used. If fig. 1 69 (p. 1 45 ) is referred to, and a long 

 nerve substituted for the muscle-nerve preparation, the stimulus 

 is applied at one end, and the change in the electrical condition of 

 the nerve is recorded by the galvanometer, which is connected to 

 the other end of the nerve. The time measurement is effected by 

 the adjustment of the rheotome, which must be such as to tap off 

 the electrical change at the moment it occurs. 



The rate of the transmission of nervous impulses discovered by 

 these methods is, in a frog's motor-nerve, 28 to 30 metres a second ; 

 in human motor-nerves, 33 metres a second ; in sensory nerves, 

 30 to 33 metres a second. 



Direction of a Nerve Impulse. 



Nerve impulses are conducted normally in only one direction : 

 in efferent nerves from, in afferent nerves to, the nerve-centres. 

 But there are some experiments which 

 point to the conduction occurring under 

 certain circumstances in both directions. 



Thus, in the rheotome experiment just 

 described, if the nerve is stimulated in the 

 middle instead of at one end, the electrical 

 change (the evidence of an impulse) is 

 found to be conducted towards both ends 

 of the nerve. 



Kiihne's gracilis experiment proves the 

 same point. The gracilis muscle of the 

 frog (fig. 184) is in two portions, with a 

 tendinous intersection, and supplied by 

 nerve-fibres that branch into two bundles ; K * 'flf^? waikr) fr *' 

 excitation strictly limited to one of these 



bundles, after division of the tendinous intersection, causes both 

 portions of the muscle to contract. 



Another striking experiment of the same kind can be performed 

 with the nerve that supplies the electrical organ of Malap- 

 tercorus. This nerve consists of a single axis cylinder and its 

 branches ; stimulation of its posterior free end causes the " dis- 

 charge " of the electrical organ, although the nervous impulse 

 normally travels in the opposite direction. 



Older experiments, designed to prove the same points, were performed by 

 Paul Pert. He Drafted the tip of a rat's tail either to the back of the same 

 r;it. or to the nose of another. When union had U'c'ii effected, the tail was 

 ;mi|iiii;itr.| near its base. After a time, irritation of the end of the trunk- 



