194 The American Naturalist. [February, 
twenty-five students, in whom the knee-jerks resulting from the blows 
of a hammer, the force being known and constant, were recorded ona 
drum. The peripheral theory assumes that the tension of the muscle 
determines its ability to respond to mechanical stimuli, and that this 
tension dependi on tonus impulses originating in the spinal cord. 
Th without proofs, and the theory is opposed by facts. 
“be knee-jerk may be present when muscle tonus appears to be 
wanting, and may be absent in the case of men who apparently have a 
normal amount of tonus. When the knee-jerk is lacking, it cannot be 
restored by any amount of tension which can be artificially supplied to 
the muscle. The tonus theory does not explain the difference which 
always exists in the size of the successive knee-jerks, for it is found ex- 
perimentally that the size of the knee-jerk is not influenced by slight 
variations in the tension of the muscle; nor can the changes in the 
amount of the knee-jerk be attributed to alterations of the irritability 
of the muscle dependent on fine variations in tonus, because experi- 
ments show that the irritability of the muscle does not change within 
short intervals of time. The peripheral theory does not explain the 
reenforcements of the knee-jerk [z.e., the increase of the latter when 
it is accompanied by voluntary motion in some part of the body or by 
sensations], because reenforcing acts, unless very violent, do not alter 
the tension or irritability of the muscles, The discovery of Mitchell and 
Lewis, that muscular contraction called out by electrical stimulation 
cannot be reenforced, is inexplicable by the peripheral theory, though 
readily explained by the reflex theory, Finally, occasionally the flex- 
ors, as well as the extensors, of the knee are seen to contract in re- 
sponse to the blow on the ligamentum patellae. This contraction of 
the flexor muscles is of reflex origin, and there is little reason to doubt 
that the extensors are irritated by the same reflex process. The idea 
that the flexors are mechanically stimulated by the strain brought on 
them by the sudden extension of the knee is untenable, because we 
know that muscles are not irritated by slight strains on their tendons, 
and the flexors are seen to contract when the knee has extended so 
little as to bring almost no strain upon them ; moreover, in spite of the 
fact that the muscle irritability does not change within short intervals 
of time, small knee-jerks may be seen to be accompanied by marked 
contractions of the flexors, “o immediately after, large knee-jerks by 
little or no flexor contraction.’ 


Pick! finds histological evidence in favor of the reflex theory. 
i f. Psychiatrie, XX., 3, p. 896; cf. Centralblatt f. Physiologie, 1889, No. 12, p. 
