Page:The American Journal of Psychology Volume 1.djvu/74

 68 LOMBAKD :

times of day, being as a rule greatest in the morn- ing, soon after breakfast, and being very much less at night. The decline which occurs as the day ad- vances is very irregular, but, in general, the knee- jerk is larger after each meal. Finally, the extent of the knee-jerk may differ greatly on different days.

The causes of these variations of the knee-jerk are not only alterations in the muscles and nerves in- volved in the process, but, to a still greater degree, changes in the activity of the central nervous sys- tem, either as a whole or in part. Thus fatigue, hunger, enervating weather and sleep, conditions which decrease the activity of the whole central nervous system, decrease the average knee-jerk, while rest, nourishment, invigorating weather, and wakefulness, influences which increase the activity of the central nervous system, increase the average knee-jerk. These influences account for the diurnal variations of the knee-jerk, while the multitude of changes that are seen to occur within short intervals of time are due to temporary alterations in the ac- tivity of certain parts of the brain and cord. Thus voluntary movements and strong emotions, when synchronous with the blow, are found to increase the movement ; and this is noticed even during sleep when the dreams are vivid. Similarly, sensory irri- tations, even when not strong enough to produce visible reflex actions, may markedly reinforce the knee-jerk, but whether on account of their effect upon the brain, or upon the spinal cord, must be proved by future experiments.

Inasmuch as the normal respiratory movements and quiet thought were not seen to influence the pro- cess, it seems probable that the action of the many