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 NORMAL KNEE-JERK.

51

Fig. 6.

April 11th 6.15 P. M.

M. M

(Fig. 6.) Again, at the examination at 6.15 P. M., on April 8th, the aver- age knee-jerk was 44 mm., and the knee-jerks which Fig. 7.

were called out im- mediately after the regular experiments and which were re- enforced by talking, measured 71, 75, 86, (Fig. 7.) As has

82 and 74 mm. been said, to get the full effect of the reinforcement, the blow must be de- livered at just the right moment after the reenforcing act. "When this was done, such active reenforc- ing acts, as clenching the hands or teeth, enormously increased the movement. (See fig. 4.)

Reinforcements Produced by Exciting the At- tention. — All these reenforcing influences were of interest to us chiefly because of our wish to avoid them, and our desire to see blows of the same force call forth knee-jerks of the same extent. When the subject was lying entirely quiet, with closed eyes, in what he felt to be an absolutely com- fortable position, the knee-jerks continued to be of variable extent. A cause for some of these variations was, however, soon discovered. Dur- ing the examination at 6.15 P. M., April 2d, a child in the next room began to cry, but was immediately quieted ; in a few moments the child began to cry again and was again quickly quieted. The average