KNEE-JERK 203 



but not always, with a subnormal tension of the tendon, while 

 a hypertonic state gives rise to an exaggerated jerk. 



When the nerves of the ligamenttim patellse have been divided 

 the knee-jerk is obtained in undiminished strength; nevertheless, 

 some sort of reflex is involved, for division of the posterior roots 

 that enter into the anterior crural nerve abolishes it. Accord- 

 ing to some authors it depends upon mechanical stimulation of 

 the highly stretched muscle by the pull of the tendon when 

 struck. It has been claimed that the time involved, 0.03 of 

 a second, is characteristic of a simple muscle twitch, but too 

 short for a reflex, the briefest of which requires more than one- 

 fourth again as much time. However, it is admitted that the 

 tendon tap may cause an undoubted reflex knee-jerk on the 

 opposite side, the interval between the tap and the contraction 

 being about one-eighth of a second. 



The variations found in the tendon reflexes are very consider- 

 able. In some perfectly healthy individuals no knee-jerk at 

 all can be obtained. Local fatigue of the extensor muscles 

 diminishes it, while general fatigue at first increases but later 

 diminishes it too. Shutting off the blood supply will cause the 

 knee-jerk to disappear in a quarter of an hour. Stimuli applied 

 to the skin, or a clinching of the fists, increase it if applied not 

 more than 0.2 to 0.6 second before the tendon is struck. This 

 phenomenon is termed reenforcement. If applied sooner they 

 cause an inhibition which reaches its maximum at an interval 

 of from 0.6 to 0.9 second before the kick. Sound always reen- 

 forces the jerk, while light causes very little if any inhibition. 

 Inhalation of anesthetics (chloroform, ether) extinguishes the 

 reflex. It has been found in man that the knee-jerk was present 

 immediately after decapitation, but usually injury to the cord 

 permanently abolishes it. Lombard has shown that all the 

 ordinary events of daily life are portrayed faithfully in changes 

 in the knee-jerk. In deep sleep the knee-jerk is absent, but 

 sensory stimuli too feeble to awaken the sleeper are manifested 

 in an exaggeration of the tendon reflexes. An increased knee- 

 jerk is a symptom of some spinal diseases. After removal of 

 the right half of the cerebellum the knee-jerk on the homony- 

 mous side is more vigorous. A similar result follows section of 

 the cerebellar peduncles. 



