328 PHYSIOLOGY CHAP. 



of the reflex contractions. He saw that even with perfect 

 equality of stimulation periodical variations in the height of 

 the contractions, corresponding to those which Fano observed 

 on the tortoise, were always present. He attributed these to 

 corresponding variations in the excitability of the spinal centres. 

 Fatigue was practically excluded under Scheven's experimental 

 conditions. In one experiment he recorded some 900 reflex 

 contractions, excited at intervals of one second, without fatigue, 

 as noted by Treves in his experiments on man, with the ergograph 

 (Chap. I. p. 51). The height of the contractions increased in 

 direct ratio with the height from which the hammer dropped to 

 arouse the reflex, and rose rapidly at first and then more slowly to 

 the maximum when the height of drop was about 30 cm. On 



FIG. l&O. Comparison of contraction from extensor muscle of rabbit 's Ir^ to direct electrical 

 stimulation (d.s.), and reflex mechanical stimulation (r.x.). (Scheven.) Tuning-fork 100 

 vibrations per second. 



further increasing the drop the height of the twitch declined, 

 probably owing to inhibition caused by the strong excitation of 

 the afferent cutaneous nerves. Specially important is the fact that 

 the height of the contraction depended to a large extent upon the 

 stimulation frequency ; the smaller the interval between two 

 stimuli, up to a certain point, the higher was the contraction. 

 This is undoubtedly an effect of summation of stimuli, and as 

 summation is a property of the nervous centres (see last chapter) 

 this fact also testifies strongly to the reflex nature of the knee-jerk. 

 Whether the knee-jerk be regarded as a reflex or not, it is in 

 any case dependent on the integrity of a spinal reflex arc the 

 afferent limb of which conducts from the sensory organs in the 

 muscle itself and its appendages to which is due the tone or state 

 of tension in the latter during rest. If the afferent nerves of the 

 muscle or its motor or sensory roots are divided, the knee-jerk is 

 abolished ; while it persists, and may even be increased, if all the 



