NORMAL KNEE-JERK. 45
practicing physician, because it teaches him not to expect a vigorous knee-jerk from a patient who has walked a mile to his office.
How far the lessening of the movement seen in such cases is due to fatigue of the muscles which extend the knee, and how far it is dependent on fatigue of the central nervous mechanisms, is a problem, the solution of which would require a special research, which we have as yet had no time to under- take. That the extent of the knee-jerk is intimately dependent on the activity of the spinal centers can- not be doubted, and this dependence probably ac- counts to a great extent for the diurnal variations which we have called attention to, but it is not at all clear that it is the wearying of the spinal centers which accounts for the low knee-jerk which is found to result from a walk.
Effect of Mental Fatigue. — In our experiments we find that the brain exerts an indirect, but never- theless very considerable, influence over the extent of the knee-jerk, as will be shown when we come to study the subject of reinforcements. It is rarely, if ever, that the mechanisms of the brain act singly, and consequently it is most difficult to trace the reenforcing influences to their proper source. Ap- parently, however, it is those centers which are the seat of the will, and of the emotions, rather than those by which we perform such forms of mental work as adding, memorizing and planning, that are chiefly concerned in reenforcing the knee-jerk. In our experiments we have not found that short periods of mental work have any effect on the ex- tent of the knee-jerk, and when the work extended
