Page:The American Journal of Psychology Volume 1.djvu/11

 THE VARIATIONS OF THE NORMAL KNEE-JERK, AND THEIR RELATION TO THE ACTIVITY OF THE CENTRAL NERVOUS SYSTEM.

BY WARREN PLYMPTON LOMBARD, M. D.

The research described in this paper was made with the purpose of studying the variations to which the knee-jerk is normally subject. The observations reported are based on experiments which were made with a hammer which struck a blow of known force, and with an apparatus that gave an accurate record of the extent of the resulting knee-jerks. The ex- periments were all made on the writer, who is a healthy man; they extended over six weeks; they were made at two hundred and thirty-nine different times; and they numbered six thousand, six hundred and thirty-nine. Of these experiments only five thousand, four hundred and seventy-six are reported in the tables accompanying this paper.

These experiments demonstrate not only that the extent of the knee-jerk varies with the force of the blow employed, but that when blows of the same force are used the extent of the knee-jerk varies greatly on different days, at different parts of the same day, and even in experiments which rapidly suc- ceed each other.

The cause for these variations in the extent of the knee-jerk are conditions which affect fche vigor of the muscles and nerves involved in the process, and,

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