580 



A TEXTBOOK OF PHYSIOLOGY 



from 33 to 100 metres per second. It may be estimated as follows, 

 using the sciatic -gastrocnemius preparation of the frog : The re- 

 cording drum is arranged at a fast rate, with a " striker " for com- 

 pleting the circuit of the primary current of the induction coil. To 

 the secondary coil are attached two Du Bois keys in the manner shown 

 in the diagram (Fig. 319). From these pass two pairs of electrodes, 

 one of which will be applied to the upper portion of the nerve, the 

 other to the lower portion. The latent period of the muscular con- 

 traction is determined, first for stimulation by the upper pair of 

 electrodes, the lower pair being short-circuited by closure of its 

 Du Bois key; then for stimulation by the lower pair of electrodes, the 

 upper pair being short-circuited. The difference in time of the latent 

 periods is determined by recording underneath the curves the vibra- 

 tions of a tuning-fork oscillating 100 times per second. This difference 

 repregents the time taken for the nervous impulse to pass along the 

 length of nerve between the two pairs of recording electrodes (Fig. 320). 

 The length is measured in millimetres, and the velocity of the 



FIG. 319. DIAGRAM OF THE EXPERIMENT ox THE RATE OF TRANSMISSION OF A 



NERVOUS IMPULSE. 



transmission of the nervous impulse thus calculated. The rate of con- 

 duction may be measured more accurately by means of an electro- 

 meter and determining how long it takes for the '' current of action " 

 to pass between two points of a nerve. The velocity may be ascer- 

 tained in man by estimating the time taken for the impulse to puss 

 along the length of nerve from the clavicle to elbow, by stimulating 

 first at one point, then at the other, and recording the contraction of 

 the thumb muscles by means of tambours. 



The impulse is conducted along a nerve in both directions. This 

 can be shown by the following experiments: The iliac end of a dis- 

 sected sartorius muscle is divided into two portions (Fig. 321). Stimu- 

 lation with a weak induction shock at a or a', where there are no 

 nerve-fibres, produces a contraction of the one half of the muscle; 

 excitation at b or &', where there are nerves, evokes contraction of 

 both halves. Again, the gracilis muscle of the frog is completely 

 separated into two portions by a tendinous intersection (Fig. 322). 

 Both halves of the muscle are supplied by a single nerve, the individual 

 fibres of which divide and supply both halves of the muscle. Stimula- 

 tion at a or a', where there are no nerve-fibres, causes only the 



