7o8 



A MANUAL OF PHYSIOLOGY 



the jaw-jerk and ankle-clonus) among reflex movements, it 

 might more properly be termed a pseudo-reflex, for there is 

 evidence that the mechanism by which it is produced is 

 different from that concerned in the reflex blinking of the 

 eyelid, or the reflex retraction of the testicle, or the drawing- 

 up of the foot when the sole is tickled. The strongest part 



of this evidence is the fact 

 that the interval which elapses 

 between the tap and the jerk 

 (TFTT to y^j- second) is distinctly 

 shorter than the reflex time of 

 the extremely rapid lid-reflex, 

 and is not much greater than 

 the latent period of the quadri- 

 ceps muscle for direct electrical 

 stimulation, as measured under 

 the ordinary conditions of its 

 contraction. 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 pos- 

 terior roots that enter into the 

 anterior crural nerve abolishes 



FIG. 254. DIAGRAM OF REFLEX the knee-jerk. The phenomenon 

 CENTRES IN CORD (AFTER HILL). , ,. 



probably comes under the head 



of what by some authors 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 seems to be 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 



