9 o 4 THE CENTRAL NERVOUS SYSTEM 



duced by the stimulation of an afferent path which is primarily 

 inhibitory for them. One of the most striking illustrations of this 

 possibility is seen in the action of strychnine. Stimulation of the 

 internal saphenous nerve below the knee say in a dog after removal 

 of the cerebrum is known always to produce inhibition of tl.e 

 portion of the quadriceps extensor whose contraction causes the 

 knee-jerk. 



If now the animal be poisoned by a small dose of strychnine, 

 stimulation of the nerve causes no longer reflex relaxation, but reflex 

 contraction of the muscle. This fact indicates that the essential 

 action of strychnine is something different from a mere reduction of 

 the resistance to the spread of impulses in the cord (Sherrington). 

 Tetanus toxin produces a similar effect, though more slowly. 



The reversal of the depressor reflex on the blood- pressure has been 

 previously alluded to (p. 188). A different type of reversal, and one 

 of most interest in connection with the co-ordination of reflexes, is 

 illustrated by such observations as the following: The extensor 

 thrust is only obtained by the adequate form of stimulation de- 

 scribed on p. 901, when the hind-limb is in a condition of flexion. 

 When the leg is passively extended at the time when the stimulus is 

 applied, the response is not the extensor thrust, but flexion of the 

 leg and thigh (direct flexion reflex). The passive assumption of a 

 condition of flexion at the knee and thigh appears, accordingly, to 

 favour the extensor reaction (Sherrington). The observations of 

 Magnus have shown that such relations are general ; for example, 

 the reaction is usually extension when the opposite posterior limb 

 is flexed at the time of stimulation, and flexion when the opposite 

 leg is extended. In the spinal cat, stimulation applied to the tail, 

 especially near the root, elicits always a stroke towards the side on 

 which at the time of stimulation the muscles are extended. The 

 phenomenon is dependent upon the integrity of the afferent nerves 

 of the passively extended or flexed muscles whose position influences 

 the reflex, and of the afferent nerves of the tendons and fascia re- 

 lated to them. The condition of the reflex centres is in some way 

 influenced by impulses conducted along those afferent paths. 



Not only is the tone of the extensors diminished or abolished 

 during the activity of the flexors, but the contraction of the knee 

 extensors evolved by striking the patellar tendon, which is called 

 the knee-jerk, either fails to appear, or appears but feebly, when the 

 flexion reflex is simultaneously elicited, even when the mechanical 

 antagonism of the flexor contraction has been eliminated by pre- 

 viously detaching the flexors from the knee. 



The Knee-jerk. This is sometimes termed a pseudo-reflex. For 

 certain authorities believe that the mechanism by which it is pro- 

 duced is different from that concerned in the reflex blinking of the 

 eyelid, or the reflex retraction of the testicle, or the drawing-up of 



