50 LOMBARD :
tinuously applied, we recognize the sensation, not as constant, but as of varying intensity, and that Mitchell and Lewis found that the extent of the reenforcement depended upon the moment at which the blow was delivered. If the blow falls at the moment that the reenforcing influence is at its height, the resulting movement is more marked than if the knee-jerk is called out a little earlier, or a little later. Thus, in the second example given, the skin was itching all the time, but the intensity of the sensation was much greater at one moment than at another, and the reenforced knee-jerks show a similar difference. The above examples illustrate a fact which was demonstrated many times in the course of our experiments. It was noticed, again and again, that not only such a positive source of irritation to the skin, but anything causing discom- fort, as, for instance, a crease in the clothing, or an uncomfortable position, was sufficient to increase the extent of the knee-jerk. These observations corroborate the results of Mitchell and Lewis, who found that painful impressions brought to the skin, as heat, cold, the electric wire brush, etc., were capable of reenforcing the knee-jerk.
Keenforcements Produced by Voluntary Actions. Mitchell and Lewis also found that any voluntary movement, however slight, tended to reenforce the knee-jerk, and in our experiments we saw this fact illustrated over and over again. Thus at the exam- ination at 6.15 P. M., on April 11th, the average knee-jerk was 27 mm., and the movement which resulted from a blow which chanced to fall at the moment the subject was swallowing, was 50 mm.
