If a muscle is so thoroughly exhausted that it refuses to respond to a •voluntary effort at con traction, it may nevertheless continue to work, if it is electrically stimulated. And, on the other hand, voluntary contraction ensues when electrical stimulation is inefficient. It is difficult, however, to draw conclusions from these eases; for in either event sonic new muscular element is apt to be brought into function by the new source of stimulation.
Various methods have been devised for meas uring the general intellectual fatigue which fol lows upon mental exertion. School children in particular have been subjected to tests for the determination of the fatigue-effects of various studies. Two types of method have been em ployed: (1) A period of instruction—e.g. the solving of simple problems in addition or multi plication—is made the test, and the extent of fatigue produced is determined by the amount of work done and the number of errors made. The results have shown a gradual increase in quantity in the latter part of the day's session, but at the same time a large increase in the number of errors. It is difficult to translate these results into terms of fatigue. because prac tice enters as a prominent factor. Further, the method does not give a fair test of the fatigue induced by regular school work, because the tests require an unusual tension, while at the same time their monotony renders them uninter esting and irksome, and invites carelessness. (2) Tests of the second type are introduced be tween the periods of study at various points in the day's work, e.g. at the end of every hour. The first of these methods is that of Criesbach. Two blunt compass-points are set down near together upon the skin. and gradually separated until the individual is just able to distinguish the points as two. This just diseriminable difference of locality is called the linen for localization. (See EXTENSION ; PSYCHOPIITSICS. ) It has been found slowly to increase under fatigue. The limen varies so much, however, from individual to individual. with the time of the day, the de gree of attention, and many other circumstances, that it is difficult to obtain significant results from the method. Another test is furnished by the 'memory method.' in which the pupil writes down a list of words previously read to him. The omissions and errors are taken as an index of fatigue.
Obviously the best test for fatigue will be one which involves the very processes which have been affected by work; hence it is necessary to make an analysis of the mental functions that are operative in ordinary school work. in order to de
termine the actual capacities which are drained by sustained application. Memory for words. e.g. may remain unimpaired. even though the in dividual is too fatigued for intellectual work; i.e. memory may he more or less mechanical. Now, the essence of mental work is the sorting and weighing and unifying of material. It is a combining or synthetizing process. The great in tellect is one that makes clear and comprehen sive systems of the facts with which it deals; but it only in degree from the ordinary mind that plods through its tasks. An adequate test of mental fatigue is, then, a combining or con structive test. The most satisfactory one so far employed is that used in the schools at Breslau hy IT. Ebbinghans. It consists in filling out mutilated texts. The individual is given a printed page. in which .words• letters, and syl lables are lacking, and is asked to reconstruct it so that it shall make sense. The complexity of the task is suited to the age and general capa bility of the student. The test is given at various periods during the day, and the measure of fatigue is obtained from the number of omis sions filled in, the number overlooked, and the number of sentences that make sense.
The word 'fatigue' is also used, in various meanings, in the psychology of sensation. Thus, in the Young-Helmholtz theory of vision (see VISUAL SENSATION) it denotes a decreased sus ceptibility of the retina toward light. When a red surface becomes grayish during continued fixation, the red fibre of the retina is said to be fatigued for red light, and therefore to function ate less actively than at the beginning of stimu lation. For this concept of fatigue, the rival theory of Hering substitutes that of adaptation. (See AFTER- IMAGE. ) Fatigue is also applied, less definitely, to nervous processes in the organ of hearing. A tone, e.g. heard continuously for a long time becomes slightly less intensive. Roth here, and in sensations of temperature, smell, and taste, where the effect of sustained stimulation is much more noticeable, it is well to substitute the word exhaustion for fatigue. Even in vision, the mental processes accompanying decreased excitability are entirely different from the fa tigue-experience which we have discussed above. Consult: Mosso, La fatigue intellectuclle et physique (Milan, 1894) ; A nierican Textbook of Physiology, ed. by Howell (Philadelphia, 1898) ; ritebener, Experi!liental Psychology (New York, 1901). For the method of exhaustion in olfac tometry, see SMELL.