NORMAL KNEE-JERK. 69
mechanisms of the central nervous system, except when very strong, is not accompanied by the devel- opment of reenforcing influences ; this is far from certain, however, and, inasmuch as the origin of but few of the more delicate reenforcing influences were discovered, this interesting question must be left open to future study.
In general, then, it may be said that the knee-jerk is increased and diminished by whatever increases and diminishes the activity of the central nervous system as a whole, and that it is even more notice- ably altered by temporary changes in the activity of certain mechanisms of the spinal cord and brain.
In the experiments described in this paper it was found that the movements of the foot, caused by knee-jerks that were produced by the usual blow, i. e., when the hammer fell through an arc of 40°, varied from millimetres to 130 millimetres. Still greater movements would undoubtedly have been seen had vigorous ^enforcements occurred at the time when the average knee-jerk was higher. The average movement gained from the results of the 2,320 experiments of this series was forty milli- metres. The least blow which was seen to produce a movement of the foot was obtained by letting the hammer fall through an arc of 20°.
The Results of Experiments of Series 11.
The results of the experiments of Series I were so remarkable that it seemed to the writer that he ought not to publish them without assuring himself of their correctness. He accordingly undertook a second series of experiments, which extended like the first over two weeks, and which differed from 6
