TENDON JERKS; SENSORY PATHWAYS IN SPINAL CORD 829 



to the tendon and the electrical response observed in the vastus internus 

 muscle by the string galvanometer, was found by Jolly in the spinal cat 

 to be 0.0055 of a second, whereas measured in the same way the latent 

 period of the flexion reflex was found to be just twice as long ; i. e., 0.0106 of 

 a second. These differences were explained by Jolly as indicating that 

 the knee-jerk is a simple reflex, involving but two neurons, whereas the 

 flexion reflex involves three and therefore has twice as long a latent 

 period. By subtracting from the total latent period the time occupied 

 in the transmission of the impulse along the nerves and the time lost at 

 the afferent and efferent nerve endings, we secure a figure giving the time 

 lost in .the .synapses between the neurons. This synapse time, as it is 

 called, was found by Jolly to be 0.0021 of a second for the knee-jerk 

 and 0.0043 of a second for the flexion reflex. 7 Snyder obtained somewhat 

 similar results in man by the same method. 



Some authors, particularly Gowers, do not, however, believe that the 

 knee-jerk is of the nature of a simple reflex, but explain it as being due 

 to a contraction of the extensor muscles brought about by direct 

 mechanical stimulation while the muscle is in a hyperexcitable condition 

 as a result of a reflex increase in its tonicity. Gowers believes that by 

 putting the extensor muscles on the stretch and the hamstring muscles 

 in the relaxed condition, afferent impulses are transmitted to the cord 

 which excite the efferent neurons of the extensor muscles, so as to throw 

 them into a hypertonic condition, during which the tapping of the ten- 

 don directly excites a contraction. Of course this hypothesis would ac- 

 count once and for all for the remarkably short latency of the knee- 

 jerk, but on the other hand it leaves us many difficulties to explain; 

 such, for example, as the fact that, although tapping the tendon produces 

 the jerk, similar tapping of the muscle itself has no effect. 



The effective stimulus of the jerk is a slight passive increase of the 

 tension to which the extensor muscle itself is subjected, and not a stimu- 

 lation of receptors in the tendon, for it still occurs after the tendon has 

 been denervated. The importance of the relationship of the hamstring 

 nerve to the knee-jerk becomes evident in connection with reciprocal 

 action; thus, when the flexor is contracted, as in the flexion reflex, the 

 knee-jerk disappears (page 814), whereas when the hamstring nerves are 

 cut, it is augmented. 



Whatever its nature may be, the knee-jerk is of value because of the 

 ease with which it can be altered not only by conditions affecting the 

 reflex arc concerned, but also by changes occurring elsewhere in the 

 central nervous system. The best known of these conditions is that 

 known as reinforcement. This is brought about by having the patient 

 make some voluntary muscular effort at the moment that the tap is ap- 



