NORMAL KNEE-JERK.
53
Fig. 9.
April 12th 8.00 P. M.
M.M. 70
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were 71 mm. and 60 mm. (Fig. 9.) At the same time, the rattling by of carts, an accustomed sound, and one devoid of interest, had no ap- preciable effect. It was soon found that if the subject were spoken to, if a knock came at the door, or if in any other way the attention of the subject were attracted at the moment that the blow was struck, the knee-jerk was markedly in- creased.
Effect of Cerebral Inactivity and of Sleep. If the sudden awakening of the attention was capable of increasing the knee-jerk it might seem as if a quieting down of cerebral activity would produce the opposite effect, and this appeared to be the case. Not infrequently the average of the experiments at the beginning of an examination, when the mind of the subject, who had perhaps just stopped working, was in an active state, was considerably higher than the average of the experiments which were made toward the close of the examination, when quiet, or even a condition closely resembling sleep, had crept on. It is, perhaps, worth noting that the subject has always had the faculty of going to sleep at short notice, and that the jars caused by the regular blows of the hammer ceased to attract his attention after a few hundred experiments had been made upon him. The effect of the quieting down of the cerebral mech- anisms was illustrated in the examination at 8.15 P. M., April 2, when the average of the first fifteen experiments was 41 mm. and the average of the next
