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



I have repeated the observation substituting for severance other 

 modes of destruction of the conductivity of the root. The root is a 

 fairly long one, longer in the Monkey than in the Cat, and it is not 

 difficult to apply reagents to it. I find the jerk immediately 

 abolished by cooling the root to near the freezing point. To do this 

 I pass under the posterior root, well lifted from the anterior, one end 

 of a copper strip, the other end of which lies in an ice and salt 

 mixture. The application of C0 2 vapour to the root has a similar 

 effect, and on removing the vapour the "jerk " returns. The vapour 

 I have applied through a thin-walled india-rubber tube, made to 

 enclose the root. Cocain I have also applied, and found it abolish 

 the jerk in about 70 sees., when used as a 1 per cent, solution in 

 0*6 per cent, sodium chloride solution. I place under the root, before 

 applying the cocain, a thin strip of india-rubber sheeting, and apply 

 the solution with a fine camel's-hair brush by painting on the fila- 

 ments of the root. 



There seems, therefore, no doubt that abolition of the jerk can be 

 produced by lowering the conductivity of the fibres of this posterior 

 root. Whatever the nature of these afferent fibres which thus come 

 up from part of the quadriceps extensor of the thigh, and keep 

 the " knee-jerk " going, facts show that they are less hardy 

 under experimental interference than are those from the skin which 

 carry centripetal impulses subserving tactile sensation. A very 

 little interference with this posterior root abolishes the knee-jerk ; 

 a very great deal will often not obviously impair cutaneous reflexes 

 elicited through it. To lift the posterior root by a thread passed 

 under it will often suffice to interrupt the afferent fibres for "the 

 jerk," but at the same time leaves the afferents of tactile sense not 

 obviously impaired. Probably the former fibres are much the smaller 

 and more delicate. 



The irritation of this root, when cut, by its own demarcation 

 current does not cause inhibition of the jerk. I have tried on three 

 occasions to recover the "jerk," after its disappearance on section of 

 this root, by electrical excitation of the central end of the divided 

 root. The excitation, when too feeble to elicit any reflex contraction 

 of the muscles, did not obviously influence the briskness of the jerk 

 in either direction. The excitation very readily, however, causes 

 contraction of the hamstring muscles, which so alters the position of 

 the knee that the condition of the "jerk " can no longer be satisfac- 

 torily compared with what it was before. 



Excitation of the central end of the divided hamstring nerve does 



at once abolish, or greatly reduce, the briskness of the "jerk." I 



have elsewhere* described a curious fact concerning the "jerk," 



namely, that it can be rendered brisk by section of afferent, or of 



* ' Journ. of Physiol.,' loc. cit. 



