218 HUMAN PHYSIOLOGY 



axis-cylinders divide, so that each axis-cylinder gives a branch to 

 the lower and to the upper half of the muscle. If the muscle is 

 cut transversely without injury to the fork of the nerve and one of 

 the branches of the fork be stimulated, both halves of the muscle 

 contract. Hence the impulse of the stimulated nerve passes not 

 only in the centrifugal but also in the centripetal direction, and 

 then, in the other branch, passes in the centrifugal direction. 



The electrical phenomenon also spreads in both directions from 

 the place artificially stimulated. 



(c) Velocity of the impulse. The velocity of the nerve 

 impulse in an excised frog nerve, at room temperature, is 

 27 metres per second. In man it has been variously stated 

 (between 30 and 60 m per second). 



The velocity of the impulse is measured as follows : Take a frog 

 muscle-nerve preparation and stimulate the nerve in two place>, 

 one at a place as near to, the other as far removed from, the mus- 

 cle as possible. Determine the difference in latent period (the 

 lapse of time between the stimulation and the beginning of con- 

 traction). This can be done best by graphically recording the 

 contraction. The latent period following the stimulation of the 

 point far removed from the muscle is greater than that following 

 the stimulation of the point near the muscle. The difference 

 between the latent periods is the time it takes for the impulse to 

 travel the distance between the two points stimulated. From this 

 it can be calculated how great a distance the impulse travels in one 

 second. 



Experiments based on the same principle have been made upon 

 human beings, but the results are not constant. 



3. Stimulation and changes in irritability. The stimu- 

 lating influences in many cases also produce changes in 

 irritability and conductivity. Hence we may properly con- 

 sider these actions collectively. 



(a) Mechanical influences. Hitting, pulling, squeezing, 

 cutting, and drying stimulate the nerve, but also destroy its 

 irritability and conductivity. 



(b] Thermal influences. Temperatures above 45 C. and 

 below freezing-point destroy irritability and conductivity. 

 Within the limits of temperatures which are not injurious, 

 irritability and conductivity increase with the temperature. 

 Sudden great changes in temperature stimulate, e.g. touch- 

 ing a nerve with a red-hot needle. 



