334 



PHYSIOLOGY 



most familiar example. If the leg is allowed to hang loosely in a position 

 of slight flexion at hip and knee and the patellar tendon be struck, the ex- 

 tensor muscles of the thigh contract and raise the leg. This phenomenon 

 is known as the knee-jerk. Similar ' tendon reflexes ' can be obtained in 

 other muscles, such as the tendo Achillis, the triceps, and the extensor 

 muscles of the wrist, but with not so great ease as is the case with the knee. 

 The knee-jerk is not altered by rendering the tendon anaesthetic by section 

 mi fiiiiiiirrumi ii ^ a ^ ^ s 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 sever- 

 ance at any point of the reflex are, 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 oi 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 has 

 therefore been thought to be due to the direct stimula- 

 tion 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 

 FIG 167 Hind art Direct stimulation to take place, and it seems to keep 

 of a spinal frog, the muscles in a state of wakefulness ready to respond 



}!? TTepoLrio? to the sli g htest local stimulation. The knee-jerk is 



roots of the nerves therefore of special importance as an index to the tonic 



limb* 6 have been condition of the muscles concerned, beingr brisk and 



divided. easily elicited when the tonus is pronounced, and slight 



(BECHTEREW.) *^~ t ^ ~ 



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 ham- 

 strings 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 ham- 

 string muscles, or by weak stimulation of the central end of the cut nerve to 

 the hamstrings (Fig. 168). 



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

 strings automatically abolishes the resistance which would be offered by 



