Page:The American Journal of Psychology Volume 1.djvu/53

 NORMAL KNEE-JERK. 47

his whole attention to planning an apparatus, for instance, he could, after a little time, forget the irritating sensation. When the thoughts were thus engaged on other matters, it would seem that the spinal cord would be more free from cerebral control than when the mind was wholly interested in the knee-jerk, and yet the disagreeable sensa- tion and the exaggerated movements ceased, which proved the excitability to be in the brain rather than in the cord. It was never found during this research that it was possible to inhibit the extent of the knee-jerk by an act of the will, but the subject noticed again and again that when the knee-jerk was being reenforced by unusual cerebral activity, especially if of an emotional character, the extent of the movement could be reduced by direct- ing the thoughts to some indifferent subject, for instance, by quietly concentrating the attention on the warmth of the skin of the hand.

As far as the writer can judge, from his experi- ments, fatigue, whether bodily or mental, is accom- panied by a decrease of the knee-jerk, and the exceptions recorded above, when excessive mental weariness was found to increase the extent of the phenomenon, was due to the fact that the mind was in an irritable condition, and reenforced the knee-jerk. This matter will become clearer after a review of the ways in which the knee-jerk can be reenforced.

[Since the above was written the attention of the author has been called to a short article by Maximilian Sternberg, in the Cen- tralblatt fiir Physiologie, May, 1887, in which the writer relates his experiments, and states his conclusion that an increase of the ten- don reflex is a sign of general fatigue, whether produced by long- continued physical or mental exertion, and explains the fact as possibly resulting from the withdrawal of cerebral inhibition. This result is the opposite of that reached by the author of this