Page:The American Journal of Psychology Volume 1.djvu/55

 NORMAL KNEE-JERK.

49

to not try to answer this question, and to attack the subject from another side.

As has been said, it was not the object of our research to determine the causes of the reinforce- ment of the knee-jerk, but we soon found that we could not study the subject at all without taking this question into consideration. It is not too much to say, that every knee-jerk which one obtains, is the resultant of a vast number of reenforcing influ- ences, which are for the most part unrecognizable, but which occasionally reveal themselves, though singly, when some source of reinforcement is so active as to attract attention.

Reenforcement Caused by Irritation of the Skin. — For instance, a sensory irritation, such as a prickling or itching of the skin, causes a marked re- enforcement. Thus, at the examination at 6.30 P. M., April 5th, the average knee-jerk was 14 mm., and the reenforcement which resulted from a blow which chanced to be given at the moment when the ear itched was 63 mm. (Fig. 4.) Again, at the examination at 12 P. M., on April 12th, the average knee-jerk

fell t'-.rough ! instead of 40 °

ments, one even when

was 13 mm., and itch- ing of the skin caused a group of reinforce- ments, viz: 37, 14, 25, 25, (Fig. 5.) With regard to the extent of the reenforce- must remember that an irritant is con-

Fig. 5.

April 12th. 12.00 P.M.

M. M.