FUNCTIONS OF THE SPINAL CORD 9<>5 



the foot when the sole is tickled. The knee-jerk is obtained in 

 undiminished strength when the nerves of the ligamentum patellae 

 have been divided. It is therefore not a reflex movement caused by 

 stimulation of afferent nerves coming from the tendon, and the name 

 ' tendon-reflex ' is clearly a misnomer. But that it is related in some 

 way or other to afferent impulses is certain, for division of the 

 posterior roots that enter into the anterior crural nerve abolishes 

 the knee-jerk. The phenomenon, according to these authors who 

 deny that it is a true reflex, comes under the head of what is called 

 myotatic irritability that is, it depends on mechanical stimulation 

 of the slightly-stretched muscle by the pull of the tendon when it is 

 struck. It is necessary for this stimulation that the muscle should 

 be to a certain extent tonically contracted. So that when the 

 afferent fibres are interrupted, or the grey matter of the cord dis- 

 organized, and the reflex tone abolished, the knee-jerk disappears. 

 The strongest objection to considering it an ordinary reflex is the 

 shortness of the interval which elapses between the tap and the jerk, 

 which, according to some observers, is not much greater than the 

 latent period of the quadriceps muscle for direct electrical stimula 

 tion, as measured under the ordinary conditions of its contraction. 

 There is no doubt that the interval is very brief, although somewhat 

 conflicting results have been obtained by different observers for the 

 corrected latency that is, the period between stimulus and response 

 minus the latent period of the muscle. In man this period seems 

 to be about 0-02 second. In a dog with divided spinal cord the 

 interval was found to be 0-014 t 0-02 second (Applegarth) ; in the 

 rabbit only 0-008 to 0-005 second (Waller and Gotch). Recent 

 observations in which the electrical response of the muscle as re- 

 corded by the string galvanometer was employed instead of the 

 contraction have yielded results not very different from those 

 obtained by the older methods, o-on to 0-015 second according to 

 Synder. Taking account of the newer observations on the velocity 

 of the nervous impulse (p. 793), it would appear that the interval is 

 not really too short to prevent the knee-jerk from being classified 

 as a true reflex contraction, although a very brief one.* The rein- 

 forcement of the knee-jerk is referred to under another heading 

 (p. 911). It is admitted that, in addition to the direct stimu- 

 lation of the muscle on the same side, the tendon-tap may cause 

 also a true reflex knee-jerk on the opposite side, the interval between 

 tap and contraction being about second. 



Spread or Irradiation of Reflex Action. As the strength of the 

 stimulus which has been evoking a given reflex movement is in- 

 creased, the reflex effect becomes more and more extensive, spreading 

 out or irradiating in various directions. If, for example, the reflex 



* There is really now no good reason for regarding the knee-jerk as anything 

 else than a true reflex. The term pseudo-reflex and other terms implying 

 that the knee-jerk is not a reflex should therefore be dropped. 



