714 FUNCTIONS OF THE SPINAL CORD [CIL XL1X. 



Closely related to this is the phenomenon known as reinforcement 

 of the knee-jerk, which was first described by Jendrassik in 1883, and 

 has since been studied by numerous observers. The extent of the 

 jerk may be increased if at the time the patella tendon is struck, a 

 strong voluntary contraction, such as clenching the fists or the jaw, 

 is made by the individual. In many normal persons the knee-jerk 

 is difficult to elicit, but in these it may usually be obtained by the 

 reinforcing action just described. 



After the reinforcing action has occurred it is followed by an 

 interval in which the knee-jerk is lessened (inhibition or negative 

 reinforcement). Many explanations have been offered of the 

 phenomenon ; one is the " overflow " theory, that is, motor impulses 

 from the brain which produce a contraction of hands or jaw will not 

 only affect the lower centres concerned in such movements, but will 

 also overflow to other regions, for instance, those which come into 

 play in the knee-jerk and influence motor irritability there. The 

 " drainage " theory of M'Dougall to be described a few paragraphs 

 ahead may possibly explain reinforcement ; the drainage of nervous 

 potential to one part will lessen the resistance of the synaptic 

 junctions and cause a drainage of nervous energy from other parts, 

 and so allow reflex actions to be more readily elicited there. 



We have devoted most space to the knee-jerk because this tendon reflex is the 

 one which has been longest known and most worked at. The recent investiga- 

 tions of Babinski have, however, shown that other tendon phenomena may be 

 quite as easily investigated, and in some cases are more valuable aids to diagnosis 

 than the knee-jerk. Amongst these we have already referred to the ankle-jerk. 

 Another phenomenon allied to these is usually called UabinskVs sign, or the extensor 

 plantar reflex. If the sole of the foot is stroked, the usual response is flexion of 

 the toes, and especially of the big toe. If the pyramidal tract is diseased, extension 

 of the great toe occurs. This is a very delicate reaction, and occurs long before 

 any exaggeration of the knee-jerk takes place. In new-born children the normal 

 response is not obtained because their pyramidal tracts are not then fully developed. 

 The weakness of the test, however, is that in many people it is difficult to elicit 

 the reflex, and minor conditions such as cold feet interfere with it. 



Among numerous other reactions there is one which Hertz proposes to call 

 BabinskVs second sign ; it is this. If a normal person lies down with the legs widely 

 separated and the arms folded, and tries to sit up, both legs rise from the ground 

 to an equal extent. On letting himself fall back sharply both legs again rise 

 equally from the ground. In hemiplegia due to organic disease, as soon as the 

 patient is strong enough to make such an effort, the paralysed leg rises higher than 

 the healthy limb. In hysterical hemiplegia, which often simulates the genuine 

 disease, the paralysed leg remains flat on the ground, both on sitting up or lying 

 down, as this is what an uninitiated person would expect. This has been found of 

 much service in distinguishing the two conditions. 



Reciprocal Action of Antagonistic Muscles. This is an 

 interesting branch of muscle physiology, which we owe to the 

 researches of Sherrington. In brief, he shows that the inhibition 

 of the tonus of a voluntary muscle may be brought about by 

 excitation of its antagonist. 



