284 PHYSIOLOGY 



estimated at from 60 to 120 metres per second. The higher of these 

 figures is probably nearer the truth. 



On the other hand, in invertebrata the velocity of propagation along nerve 

 fibres may be quite slow. The following Table represents the velocity of trans- 

 mission along a number of different fibres, as determined by Carlson, compared 

 with the duration of single muscle-twitch in the same animal. 



The velocity of propagation in sensory nerves is more difficult 

 to determine 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 contraction. There is another method of deter- 

 mining 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 imme- 

 diately 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 same 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 



