1893.] Correlation of Action of Antagonistic Muscles. 559 



efferent, spinal roots immediately below that one on which the jerk 

 itself depends. I added, with regard to it, " Its explanation appears 

 to lie in the abolition of the tone of the hamstring muscles by section 

 of the afferent roots belonging to them." I wish now to support, and 

 somewhat enlarge, the explanation then offered. 



Severance of the great sciatic trunk produces, as Tschiriew* has 

 pointed out in the Cat, an increased briskness of the " jerk." This I 

 find to depend scarcely at all, if indeed at all, on section of the 

 external or internal popliteal divisions of the trunk, either singly or 

 together ; but to depend upon the cutting that portion of the trunk 

 which is destined for the hamstring muscles the portion referred to 

 in my previous paper as " the hamstring nerve." In Macacus this 

 '* hamstring " division of the sciatic sends afferent fibres into the 

 spinal cord by the posterior roots of the 8th, 7th, and 6th sub- 

 thoracic nerves. In Cat, the 8th and 7th posterior roots are those in 

 question. On severance of these afferent roots, the "tonus" of the 

 hamstring muscles is broken, and the "jerk " becomes more brisk ; 

 sometimes there is a short interval of depression immediately suc- 

 ceeding the operation. The motor fibres of the hamstring muscles 

 leave the cord by the anterior roots, "correspondent with the above- 

 mentioned posterior. Severance of these anterior roots causes im- 

 mediate increase in the briskness of th*e jerk. 



As to the manner in which the loss of tonus of the hamstring 

 muscles gives rise to increase of the knee-jerk, two possibilities at 

 once present themselves. One is purely mechanical ; the other is of 

 a physiological nature. The loss of tension accompanying the loss of 

 "tone " will leave the leg more free to swing at the knee joint. It is 

 for that reason that the posture of limb usually employed as the 

 most favourable when the jerk is to be elicited is with the ham- 

 strings relaxed and the leg at a right angle with the thigh. In this 

 way the points of bony attachment of the hamstring muscles are 

 approximated, and the knee can swing through a greater arc to that 

 point at which it is cut short by the mechanical check of the flexor 

 muscles passively tightened by the movement. The extensor move- 

 ment during the " jerk " has, in this way, further scope before the 

 hamstrings break it. As far as this explanation goes, the above- 

 mentioned increase of the jerk would occur even if the hamstring 

 muscles were replaced by india-rubber cords, the effect produced by 

 severance of the hamstring nerve being equivalent to any arrange- 

 ment which rendered those india-rubber cords less tight. 



I should have considered this simple explanation sufficient had it 



not been for certain additional facts. During experiments on the 



effects of stimulating the motor spinal roots of the lumbo-sacral 



nerves there is much risk of being deceived by escape of the excit- 



* ' Arch. f. Psych.,' vol. 8. 



