156 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



of tonus. This tonus depends upon the reflex are, the sensory 

 impulses from the muscle servipg to keep it in that condition 

 of tension known as tone. On this view destruction of the reflex 

 arc renders the muscle less irritable, so that it will not respond 

 by a contraction to the sudden mechanical extension or pull 

 caused by the blow on the tendon. The adherents of this view 

 lay emphasis upon two facts: First, the knee-jerk is a simple 

 contraction, and not a tetanus, and, generally speaking, the 

 motor centers of the cord discharge a series of impulses when 

 stimulated. In answer to this objection it may be said that 

 while muscular contractions produced reflexly are usually te- 

 tanic, it does not follow that this is invariably the case. Sher- 

 ringtonf has shown, for instance, that an undoubted reflex 

 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 i 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 itself. 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. Ill), 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 



* Sherrington, "The Integrative Action of the Nervous System," 1906. 

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

 "Archiv f. Physiologic," 1910, 223. 



