802 A MANUAL OF PHYSIOLOGY 



The Knee-jerk is sometimes termed a pseudo-reflex. For 

 certain authorities believe 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 

 objection to considering it an ordinary reflex is the fact that the 

 interval which elapses between the tap and the jerk ( T J T to yj^ 

 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 quadriceps muscle for direct electrical stimula- 

 tion, as measured under the ordinary conditions of its con- 

 traction. 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 some 

 authors, comes under the head of what is called myotatic irrita- 

 bility 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 disorganized, and the reflex tone abolished, the knee-jerk 

 disappears. It is admitted by all that, in addition to the direct 

 stimulation 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 i second. 



Spread or Irradiation of Reflex Action. As the strength 

 of the stimulus which has been evoking a given reflex movement 

 is increased the reflex effect becomes more and more extensive, 

 spreading out or irradiating in various directions. If, for 

 example, the reflex in question is the flexion reflex elicited by 

 stimulation of the plantar surface of the hind-foot in the spinal 

 animal, increase of the stimulus will cause, in addition to flexion 

 of the same hind-foot, extension of the opposite hind-limb, then 

 in the homonymous fore-limb (i.e., the limb on the same side) 

 extension at the elbow and retraction at the shoulder, then certain 

 definite movements, the details of which need not detain us here, 

 in the opposite fore-limb, and ultimately also definite movements 

 of the head and tail (Sherrington). Obviously there is a certain 

 orderliness in the spread of the reflexes ; they follow a certain 

 regular march ; the irradiation in the tangle of the spinal paths 

 is not an indiscriminate one. The same fact emerges quite as 



