Fatigue, that is, an unusual degree of susceptibility to fatigue, is a marked characteristic of a number of conditions of impaired vigor and usefulness that have become unexpectedly prominent during the war. Thus, in persons classed as subjects of neurocirculatory asthenia or the so-called effort syndrome and of "irritable heart," fatigue seems to be a conspicuous feature. The fatigue symptoms may be accentuated by even slight attempts at muscular activity. Sometimes it appears to be more marked in persons who may be described as emotionally sensitive. Fatigue plays a significant rôle in the efficiency of the human machine; so that its physiologic manifestations have become the subject of extensive investigation in relation to industrial conditions.1
Where psychic factors of sensation and personal impressions are involved it is manifestly difficult to secure data possessing the degree of accuracy that pulse rate, temperature, body weight, heat output, and other metabolic determinations