NORMAL KNEE-JERK. 15
how far the extent of the knee-jerk is dependent on the force of the blow. No one who has ever tried to call out the phenomenon can have any doubt that its amount varies with the force of the blow, but in our experiments the closeness of the re- lation did not at once appear, because we were confronted forthwith by the puzzle which has faced us throughout our work, and which still remains, to a great extent, undeciphered. We found, namely, that if a number of blows of the same force were struck at definite intervals, from exactly the same direction, and on the same part of the ligamentum patellae, no two of the resulting knee-jerks were of the same extent. Naturally, the sources of reinforcement described by Dr. L. Weir Mitchell and Dr. Morris J. Lewis were looked to for an explanation, but none such could be found. The subject was lying com- pletely at rest in a comfortable position and was conscious of no irritation. His eyes were closed, all the muscles were passive, and the vhole body was, as far as possible, in a state of rest. It was then suggested that the force of the blow be increased. This was done, and though similar vari- ations in the extent of the knee-jerk were seen, the movements were found, as a whole, to be greater than before. It was soon ascertained that though blows of the same strength called forth knee-jerks of very different amounts, the averages gained from several groups of twenty or more experiments each, made by striking blows of a certain force, were almost exactly the same, and furthermore, that if the force of the blow was altered, the averages of such groups of experiments made with blows of different strengths, were greater or less, according as the force of the
