560 Dr. C. S. Sherrington. On the Knee-jerk and the [Feb. 9, 



ing current to other motor roots besides the one to which the 

 electrodes are applied. When stimulating the motor root of the 7th 

 lumbar, I frequently observed contraction of the extensor muscles of 

 the knee as well as of the flexors, and imagining that the phenomenon 

 must be due to escape of the exciting current to the 5th lumbar root, 

 I was accustomed to reduce the strength of the exciting current 

 until the contraction of the extensors of the knee no longer occurred. 

 To avoid this supposed escape of current, it was necessary to reduce 

 the strength of stimulus sometimes to very slightly indeed above 

 minimal efficiency for the motor fibres to which the electrodes were 

 applied. The use of such weak currents has serious disadvantages, 

 and was extremely embarrassing for the experiment. It was not 

 until I had discarded a number of experiments on the ground of 

 escape of current, that three points concerning the contraction of the 

 extensor muscles produced by stimulating the motor nerve to the 

 flexors attracted my attention. (1.) If for the excitation of the 

 motor root to the flexors a series of induced currents are employed, 

 succeeding each other at a rate slow enough to produce not perfect 

 tetanisation, but tremulent contraction of the muscles, the contraction 

 obtained in the extensor muscles coincidently was, nevertheless, per- 

 fectly steady and tetanic, although not vigorous. (2.) If the flexor 

 muscles are severed from connexion with the knee joint, so that their 

 contraction cannot affect the joint, and if the " knee-jerk " be elicited 

 before, during, and after stimulation of the motor root to the flexor 

 muscles, during the excitation, when those flexor muscles were con- 

 tracting, the knee-jerk, brisk previously and brisk later, disappeared, 

 or almost disappeared. (3.) If the sensory spinal roots belonging to 

 the hamstring nerve are severed, the stimulation of a motor root to 

 the hamstring muscles is no longer accompanied by contraction of 

 the extensor muscles of the knee, even when strong stimulation is 

 employed. 



One next observed the effect on the extensors of the knee of 

 excitation of the central end of the nerve to the hamstring muscles 

 after that nerve had been ligated and cut through. It was 

 found that excitation with currents just perceptible at the tip of 

 the tongue causes immediate disappearance or diminution of the 

 " knee-jerk." " Exaltation " of the jerk follows the depression by 

 the excitation. If the excitation be continuously maintained for a 

 time, the jerk tends to return in spite of the continuance of the 

 stimulation. By use of stronger currents the extensors are imme- 

 diately thrown into a tonic contraction, lasting so long as the 

 stimulus is continued. The same effects on the knee-jerk, and on the 

 activity of the extensor muscles, are elicited by exciting the central 

 ends of the divided posterior roots of the 7th or 8th sub thoracic 

 nerve. 



