STIMULATION OF MUSCLES AND OF NERVES 417 



B. RATE OF TRANSMISSION OF A NERVE IMPULSE 



It is necessary for the sake of a more complete study of the excitation of 

 nerves that we discuss here the rate of transmission of the stimulus within 

 them. The first researches bearing on this subject we owe again to Helm- 

 holtz. The principle of his method is very simple. The latent period is 

 determined as above described, but instead of stimulating the muscle directly 

 the stimulus is applied to its nerve: (1) as near as possible to the muscle 

 and (2) as far as possible from it. We find that the latent period is greater 

 in the second case than in the first. If the two contractions are the same 

 size (Fig. 155), this difference can only be due to the greater length of nerve 



A B c D 



FIG. 155. Curves illustrating the method of determining the rate of conductivity in the sciatic 

 nerve of a frog. A, marks the point of stimulation. The first curve which leaves the base line 

 at B (really a little farther to the right than indicated) was obtained by direct stimulation; 

 the second curve (C) was obtained by stimulating the nerve as close as possible to the muscle; 

 the third curve (D) by stimulating the nerve as far away from the muscle as possible. The 

 lag of the third curve behind the second should give the time necessary for the stimulus to travel 

 from the second point of stimulation on the nerve to the first point in this case about 55 mm. 

 Since one complete vibration of the tuning fork (below) represents sooth of a second and 

 this is (almost exactly) the time from C to D, the rate of transmission in this particular 

 case is only about 11 meters per second (200 x .055). 



traversed by the stimulus in the second case. Knowing the difference of 

 length in millimeters and the difference of time in hundredths of a second, 

 we can easily calculate the rate of transmission in meters per second. In 

 the motor nerves of the frog at room temperature this rate is 20-26 m. per 

 second. At lower temperatures it is less ; besides, there is a certain dependence 

 upon the strength of the stimulus, a stronger stimulus increasing the rate 

 sometimes very considerably. 



In the invertebrates the rate is very much lower and appears to be less the 

 slower the normal movements of the animal. In a mussel (Anodonta) it is only 

 1 cm. per second, in an octopus 3-5 m. per second. The nonmedullated fibers 

 of the olfactory nerve of a fish (pike) transmit a stimulus at 20 C. at the rate 

 of 14-24 m. per second (Nicolai). 



By recording the contractions of the muscles in the ball of the thumb on 

 stimulation of the median nerve at different points the rate of transmission in 



