254 



PHYSIOLOGY 



On the other hand, in in vertebra ta the velocity of propagation along nerve fibres 

 may be quite slow. The following Table represents the velocity of transmission along 

 a number of different fibres, as determined by Carlson, compared with the duration of 

 a single muscle twitch in the same animal. 



The velocity of propagation in sensory nerves is more difficult to deter- 

 mine owing to the fact that a sensory impulse, on arrival at the receiving 

 organ i.e. some part of the central nervous system does not at once give 

 rise to some definite recordable mechanical change, such as a muscular con- 

 traction. There is another method of determining the velocity of conduction, 

 which may be used also with sensory fibres. The passage of a nerve- 

 impulse down a nerve, just as the passage of a wave of contraction along a 

 muscle-fibre, is immediately preceded or accompanied by an electrical change, 

 which also travels along the nerve as a wave of ' negativity.' The velocity 

 of propagation of this wave may be measured, and is found to give the sain** 

 numbers as the velocity determined by the preceding method. 



The existence of this electrical change enables us to show that a nerve- 

 impulse, excited at any point in .the course of a nerve fibre, travels in both 

 directions along the fibre. The power of nerves to transmit impulses in either 

 direction is shown further by the experiment known as Kiihne's gnu -ills 

 experiment. The gracilis muscle of the frog is separated into two portions 

 by a tendinous intersection, so that there is no muscular continuity between 

 the two halves. The nerve to the muscle divides into two branches, one 

 to each half, and at the point of junction there is division of the axis cylinder* 

 themselves. If the section a in the diagram (Fig. 104), which is quite isolated 

 from the rest of the muscle, be stimulated, as by snipping it with scissors, 





