158 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



designated by him as the "extensor thrust," which also involves 

 the extensor muscles of the hind leg, is very short lasting, requir- 

 ing perhaps only | sec., and judged by this standard is as much 

 of a simple contraction as the knee-jerk. The "extensor thrust " 

 is a sharp contraction of the extensor muscles of the hind leg 

 aroused by pressure upon the plantar surface of the hind foot. 

 On the frog also a single stimulus applied to the central end of the 

 divided sciatic nerve will call forth a reflex contraction, which is a 

 twitch, and not a tetanus. Second, the time for the jerk that is, 

 the interval between the stimulus and the response is too short 

 for a reflex. The determination of this time has been attempted by 

 many observers for the purpose of deciding the controversy, but 

 unfortunately the results have been lacking in uniformity, although 

 the best results from man indicate a latency between stimulus and 

 response of 0.023 sec. after deducting the latent period of the mus- 

 cle icself. Applegarth, making use of a dog with a severed spinal 

 cord, obtained for the time of the knee-jerk an interval of 0.014 to 

 0.02 sec. ; Waller and Gotch, using the rabbit, found the time to be 

 only 0.008 to 0.005 sec. Other figures would appear to indicate 

 that the latent period is shorter the smaller the animal, a fact which 

 in itself would imply that some factor other than the latency of 

 the muscle itself enters into the time required. And if we accept 

 the newer figures in regard to the velocity of the nerve impulse in 

 mammalian nerves at the body temperature (see p. 112), there 

 would seem to be sufficient time in all cases for the impulse to get 

 to the cord and back. Several observers* have attempted to 

 determine the time intervening between stimulus and response 

 by using the string galvanometer to indicate the electrical response 

 in the muscle, instead of attempting to record the contraction 

 itself. According to Snyder, the time interval lies between 0.0113 

 and 0.015 sec., while Hoffmann's results give an interval of 0.019 

 to 0.024 sec. The calculations of both observers indicate that the 

 time is sufficient for a reflex, and much too long for a direct excita- 

 tion. In the case of the Achilles jerk, Hoffmann finds that it may 

 be liberated by electrical stimulation of the n. tibialis and that 

 under these circumstances there is first a deflection of the galvano- 

 meter, due to direct stimulation of the gastrocnemius through 

 its motor nerve, and this is followed later by a second deflection, dut 

 to reflex stimulation. This latter accords in time interval with the 

 Achilles jerk, and gives a new proof that the phenomenon is a 

 genuine reflex. In view of these facts it would seem to be safe 

 to conclude that the knee-kick and similar phenomena are reflexes 



* Snyder, "American Journal of Physiology," 26, 474, 1910. Hoffmann, 

 "Archiv f. Physiologic," 1910, 223. 



