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A TEXTBOOK OF PHYSIOLOGY 





Tendon reflexes, of which the knee-jerk is an example, are not 

 usually regarded as true reflexes. They are elicited by placing a 

 muscle on the stretch, and sharplj- striking the tendon. As a result, 

 the muscle contracts. In the knee-jerk, the quadriceps extensor is 

 stretched by placing one knee over the other. The patellar tendon 

 of the upper knee is then sharply struck, preferably when the subject \ 

 attention is diverted. As a result, the foot is jerked up by the sudden 

 contraction of the quadriceps muscle. 



The value of this ' ; reflex " is that it shows the condition of the 

 reflex arc which supplies the quadriceps extensor. If it be not intact, 

 the muscle is toneless, and there is no response. An excessive re- 

 sponse indicates that the cerebral control of the reflex is missing. 

 The knee-jerk is. therefore, of great clinical value. It has been taught 



FIG. 410. FUOG WITH STRYCHNINE CONVULSIONS. 



that it cannot be a true reflex, on the grounds that the time occupied 

 between the striking of the blow and the muscular response is too 

 short for an impulse to have travelled into the cord and out again to 

 the quadriceps muscle. Reflexes as short are, however, now considered 

 possible, and the knee-jerk may be regarded as an example of a true 

 reflex. 



Another tendon reflex which is often investigated is that known 

 as " ankle clonus." This is evoked by bending the subject's knee 

 slightly while supporting it with one hand. With the other hand 

 the fore-part of the foot is suddenly '' dorsiflexed," and the pressure 

 maintained. As a result of the sudden strain, the calf muscles m;iy 

 contract and then relax ; but as the result of the continued pressure 

 contract again, and so on, the result being that a series of contractions, 

 or " clonus," results. Ankle clonus is nearly always a sign of disease. 



