TENDON REFLEXES 275 



cord is involved, since the jerk is abolished when the 

 motor nerve has been cut. But it might be that the cord 

 sends out a constant succession of impulses which keep 

 the quadriceps muscle in a state of tonus, and that the 

 jerk is the expression of a local irritabihty only present 

 when the muscle is in tonic contraction. Against this 

 view is the fact that while the cut peripheral end of the 

 motor nerve to the quadriceps is being stimulated so as 

 to keep the muscle in tetanic contraction, the knee-jerk 

 cannot be ehcitated. In favour of the view that the 

 knee-jerk is a true reflex is the fact that contraction of the 

 quadriceps is accompanied by relaxation of the hamstrings. 

 It was once alleged that the knee-jerk could not be a 

 true reflex, since its latent period was too short. But 

 recent and more accurate estimations have shown that the 

 time elapsing between the moment of stimulation and the 

 moment of contraction is of the same order as in the case 

 of actions known to be of a reflex nature. We are therefore 

 justified in regarding tendon-reflexes as true reflexes. 



Such being in general the nature of reflex action, it now 

 remains to find out which of the morphological components 

 is responsible for those characteristics of the reflex arc which 

 distinguish it functionally from the nerve-fibre. It cannot 

 be the nerve-cell (so-called), since we have no evidence of 

 any alteration in character of a nervous impulse as it 

 travels through the spinal gangha. The functions of the 

 cell-body have always been regarded as being to control 

 the nutrition of the whole cell, including the fibres, and 

 to serve as a junction or meeting-place of dendrites and 

 axons. 



Many of the features of reflex conduction resemble 

 those of conduction from nerve to muscle : the irrever- 

 sibility, the delay in transmission, the liabihty to fatigue, 

 and the susceptibihty to drugs. For this reason, as 

 well as by a process of exclusion, we are driven to the 



