378 PHYSIOLOGY 



section of all its nerves. The essential feature is a slight passive 

 increase of the tension to which the muscle is already subjected. 

 Since the jerk is abolished by severance at any point of the reflex 

 arc. viz. muscle spindle, cord, muscle, it was thought at first to be 

 of the nature of a reflex action. The interval, however, which elapses 

 between the moment at which the tendon is struck and the response 

 of the muscle is generally considered to be too short to allow of an 

 impulse to travel from the tendon, or muscle, up to the cord and 

 back again to the muscle. The interval was found by Gotch to be 

 about -005 sec., whereas the latent period of contraction which 

 ensues upon direct stimulation of the vastus internus is also -005 

 sec. On the other hand, the latent period when the nerve 

 to the muscle was stimulated was -01 sec. The lost time of the 

 knee-jerk is less than one-quarter of that of the briefest reflex time. 

 The contraction must therefore be due to the direct stimulation of 

 the muscle by the sudden stretching produced on striking its tendon. 

 Mere tension of the muscle is not, however, the only factor. The tone 

 which is reflexly maintained in the muscle is necessary for this response 

 to direct stimulation to take place, and it seems to keep the muscles 

 in a state of wakefulness ready to respond to the slightest local 

 stimulation. The knee-jerk is therefore of special importance as 

 an index to the tonic condition of the muscles concerned, being brisk 

 and easily elicited when the tonus is pronounced, and slight or absent 

 when the tone of the muscle is depressed. 



Especially interesting is the relation shown by Sherrington to 

 exist between the tonic condition of antagonistic muscles, e.g. between 

 the hamstrings and the vastus internus of the quadriceps extensor 

 muscle. Section of the hamstring muscles (so as to relax them) 

 or even section of their nerve causes at once great increase in the 

 jerk elicited by tapping the patellar tendon. On the other hand, 

 the knee-jerk is abolished by stretching the hamstring muscles, or 

 by weak stimulation of the central end of the cut nerve to the ham- 

 strings (Fig. 169). 



In this way a voluntary flexion of the knee by contraction of 

 the hamstrings automatically abolishes the resistance which would 

 be offered by the tonic contraction of the extensor muscles. In the 

 absence of such an arrangement every movement of a joint, by 

 stretching the antagonistic muscles, would automatically increase 

 their tone, and thus set up a resistance to itself. The subject would 

 thus be muscle-bound. 



Very great exaggeration of the tendon phenomenon is observed 

 in cases where the pyramidal tracts are degenerated, and indicates 

 a heightened reflex excitability of the lower spinal centres, perhaps 

 reinforced by impulses from the cerebellum. The importance of 



