Of the factors known to influence metabolism profoundly, muscular work and the ingestion of food are among the foremost. Both these factors have had such marked effects upon the metabolism that they have been the source of a great deal of study. Hence exact information is now at hand regarding the effects of various degrees of muscular work, and we have a fairly comprehensive idea of the characteristic metabolic reactions to the ingestion of different kinds and quantities of food. Doubtless the sense of fatigue following intense, sustained mental effort, comparable in many instances to the subjective impressions of exhaustion following severe muscular exercise, has led to the conclusion that mental effort may justly be compared to muscular work. Thus the idea of work as such is incorporated in the titles and the discussions of many of the earlier papers bearing upon problems of mental effort, and not infrequently "mental work" and "muscular work" are compared in the same publication. An interesting correlation between mental effort and muscular work was noted by Loeb in 1886 and some ten years later by Welch. Basing his idea upon the well-known fact that it is practically impossible to carry out severe muscular work and mental effort at the same time, Loeb proposed to measure mental activity by the amount of muscular work that can be performed simultaneously. The idea has long been held that the mental process is of dynamic nature. An exhaustive treatment of this phase of the subject has been given by Lehmann. Dodge, basing his opinion upon the psychodynamics of Lehmann, clearly considers the mental process to be dynamic in nature. Both to the psychologist and to the physiologist studies of mental effort are important. Rubner states, "Die schwierigste Arbeit in der Naturerkenntnis ist vielleicht dem Biologen zugefallen, da er die Lebenserscheinungen im weitesten Sinne, also auch die Geistestätigkeit erforschen soll."
Indeed, the mental state has such a pronounced influence in many ways upon physiological reactions, particularly in pathological cases and in true psychosis, that any information regarding the effect of mental effort is of value.
The term "work" (or its equivalent in languages other than English) as applied to mental effort has the justification of long usage, since it has been freely employed by writers on this problem and invariably in a more or less definitely dynamic sense. Indeed, some go so far as to compare the "psychoergogram" with the ordinary ergogram. The fact that the pulse rate usually increases during mental operations might at first lead to the conclusion that there is a dynamic factor in mental work. We have felt that the mental . . .