NORMAL KNEE-JERK. 11
able, not only to corroborate his results, but to show that the knee-jerk was subject to the most extensive variations, even during health, and that these varia- tions probably occurred by means of alterations in the activity of the nerve centres, upon whose integ- rity the knee-jerk has been found to be dependent. Although the work of these observers is, as far as the writer has been able to test it, correct in every particular, it has been received with a certain amount of scepticism, because their remarkable results are based on observation alone, and not upon any record which can be a proof to others. It is, indeed, wonderful that, trusting as they did to the ability of the hand to deliver a series of blows of constant force, and to the eye to observe slight differ- ences in the extent of the knee-jerk, they should have been able to discover so many facts and to prophesy truly the discovery of so many others. Men who have not their keen power of observation obtain their results with difficulty, and regard them with doubt, and are half inclined to deny all except the results that can be obtained by the coarsest experiments.
THE EXPERIMENTS OF THE AUTHOR.
It seemed to the author that before the knee-jerk could take its proper place as an aid to physical diagnosis, or, still more, as a means of investigating the influences which affect the activity of the central nervous system, there must be devised, first, a method of striking the ligamentum patellae a blow of known force, and, second, a method of recording the extent of the resulting knee-jerk. If one could be sure of giving the same stimulus throughout a
