There remains the point of view of func tional psychology. From this aspect, fatigue is commonly regarded as a process which inter feres with the efficiency of an organism in its continuous response to a particular situation. It is important to note that the organism is now functioning both physiologically and psy chologically, i.e., as a whole, and the response is any prolonged activity whether physical or mental.
In the interest of highest efficiency, numer ous attempts have been made to measure the amount of fatigue. The purpose has been not only to forestall serious injury to the organism from overwork, but also to alternate periods of work and periods of rest in such fashion that the largest amount commensurate with the best quality of output might be ac complished. The attempts at measurement have been based upon (1) a distinction between physical and mental work, and (2) certain physiological and mental processes which are regarded as symptoms. Since, however, the organism is functioning as a whole, it must not be supposed that physical fatigue is induced solely by muscular, and mental fatigue by mental effort. On the contrary, the difference is principally a matter of form of activity, and consequently some symptoms such as the feeling of fatigue and decrement in quantity and quality of work are common to both forms of fatigue. In physical fatigue there is a more constant relation between its degree and the reduction of output, and for this reason its measurement may be made most easily and directly by the measurement of the loss of efficiency. In the case of mental fatigue, on the other hand, the problem is much more com plex and the experimental attempts to measure it cover a period of two and a half decades. The mental symptoms such as the feeling of fatigue, blocking of associations and fluctuation of attention were early disregarded as unre liable because they do not appear under strong emotion, and they disappear under the influence of stimulants. Recourse was then had to the impairment of physical or mental efficiency. It was thought, for example, that we might meas ure objectively the amount of fatigue by the loss of physical ability in terms of the scale of the ergograph or dynamometer; or by the decrease in sensitivity as measured by the aesthesiometer; or, again, by the relative num ber of errors and total time consumed in the solution of some test introduced in the course of a period of mental activity; or, finally, that we might assume as an index of mental fatigue the decrement in quality and quantity of the fatiguing work itself. All of these methods have, in fact, been tried and no one found to be fully satisfactory; the curve of fatigue thus obtained is rarely found to co incide
the curve of physiological fatigue. The investigations themselves, however, have revealed the cause; other factors than fatigue determine the course of efficiency. Of these, practice and habituation are, perhaps, the most important; the former means less expenditure of energy, the latter freedom from worry and from ideas foreign to the task in hand. It has also been found that there may be a '
factors as competition, hope of reward and fear of penalty, encouragement and persuasion may stimulate the individual in such fashion as to mask the course of fatigue and to .maintain efficiency until exhaustion begins. Finally, the course of efficiency is conditioned, in part at least, by the great physiological rhythms. It has been found, for example, that the curve of diurnal mental, efficiency rises during the morn ing hours and reaches a maximum about 11 o'clock, it then falls during the noon hours, rises during the early, and falls again during the late afternoon hours. These fluctations are not necessarily the result of fatigue. On the contrary, they seem to be matters of habit; the organism is not accustomed to doing mental work early in the morning and at noon.• Nor mal sleep is another intercurrent rhythm which is not now, as formerly, thought to be due to fatigue. In view of these facts we must re gard true fatigue not as absolute but as relative —relative to the external and internal stimuli, to the competition of associative paths in the aervous system, to intercurrent rhythms, etc. Whether we shall be able ultimately to separate true fatigue from all other factors which con dition maximum efficiency, and thus
at a measure or index of fatigue, is a question which can only be answered by further:irivqstir gation.
Assuming that true 'functional fatigue is correlated with qualitative and physiologiCal fatigue, the following conclusions seem prob able: (1) Since the fatigue substances are dis solved in the blood and distributed throughout the body, fatigue tends to be general and not restricted to some particular locality; it is wrong to suppose, therefore, that more than local relief is afforded by change from one form of muscular work to another. (2) Physi cal and mental fatigue are inherently the same; rest from mental fatigue cannot, then, be gained by physical exertion. (3) Sleep is the best restorer because it furnishes the best con dition••for the removal of fatigue substances and for the processes of assimilation. (4) In the or dinary course of our daily lives we may trust the mental symptoms as the most adequate indicators of the degree of fatigue which we now know.
Bibliography.— Binet, A., and Henri, V:, 'La fatigue intdlectuelle) (Paris 1898); Dodge, R., 'The Laws of Relative Fatigue' (in Ps chological Review, XXIV, Princeton '1917) ; Gilbreath, F. B., 'Fatigue Study) (New York 1916); Herrick, C. J., 'Introduction to Neu rology) (Philadelphia 1915) • Howell, W. H., 'Text-book of Physiology' (Philadelphia 1916) ; Mosso, A., 'Fatigue) (trans. by M. Drummond and W. B. Hammond, New York' 1904) ; Offner,