NORMAL KNEE-JERK. 57
began to breathe again the irritation began to pass off, and the movement to become less, and in about forty seconds it had got back to its normal average.
When the breath was held the second time, the increase in the knee-jerk came much sooner, and as in the first case, the extent of the movement in- creased as the feeling of oppression increased. As in the previous case, it required about forty seconds after breathing had begun again, for the knee-jerk to get back to its normal amount.
How far the increase in the phenomenon seen in these experiments was due to the pain, and how far to the effects of temporary asphixia upon the central nervous system, is difficult to say.
Similar results were got when the breath was, as far as possible, expelled and kept out. During quiet — 52, 41, 47, 46, 41 — breath expelled and kept out— 65, 80, 85, 99— breathe again— 72, 80, 60, 69, 63, 67, 44. This was a much more painful experi- ment, and the effect of the lack of air was percepti- ble almost at once in the increase of the knee-jerk. At the end of forty seconds the pain was so intense as to bring tears to the eyes, and even after the breath was taken again, the painful feeling referred to the lower part of the chest lasted for some time. It is noticeable that in this case the knee-jerk returned to the normal more slowly than in the previous experiments.
These experiments are recorded here not because any definite conclusion can be drawn from them alone, but because they are suggestive, and because they illustrate one more of the many sources of reenforcement of the knee-jerk. Whether they should be grouped with reinforcements which result
