there are few subjects in experimental physiology that are more difficult of study. Notwithstanding that most of us doubtless believe that we know the symptoms of nervous fatigue well, physiologists have been able to discover only scraps of fact in this field. The isolated nerve of a cold-blooded animal, when artificially stimulated in the laboratory, can perform its work for many hours without showing the least sign of a diminution of power. Only when placed under unfavorable conditions, such as in light anesthesia, or when deprived of oxygen, does the nerve exhibit with continued stimulation a gradual loss of conductivity. From this it is inferred that the nerve fiber itself under normal circumstances is highly resistant to fatigue, and that any unfavorable dissimilative changes which it undergoes in activity are compensated for at once by an equal assimilation. This highly interesting and suggestive conclusion is perhaps equally true of nerve centers. Hodge and others, it is true, have demonstrated morphological changes in nerve cells as the result of artificial stimulation and of normal daily activity. Thus the nuclei of the brain cells of a honey bee may show a diminution in volume of 75 per cent., at the end of a day's labor; and the English sparrow, though popularly regarded as less typical of industry, reveals almost as much cerebral activity. Notwithstanding these evidences of metabolism, no one has yet succeeded in obtaining, by direct or reflex artificial stimulation of the nerve ganglia, the spinal cord or the brain of animals, indisputable physiological evidence of the genuine fatigue of the nerve structures involved. Many attempts have been made to detect fatigue in the nervous system by testing the muscular power, as by the employment of the ergograph, the instrument in which a muscle or set of muscles is made to perform a series of voluntary contractions and lift a given weight, the progress of fatigue being indicated by the rate at which the lift diminishes. But endeavors to arrive at an exact analysis of the result of such an apparently simple experiment have given rise to a controversy as to the location of the fatigue, some investigators claiming it for the muscles, others for the brain.
A still further attempt at the investigation of brain fatigue is through the study of certain mental processes during or following long-continued effort. Mental fatigue is characterized by a diminution of attention, a difficulty in concentrating one's thoughts, slowness in reacting to sensory stimuli, in memorizing or in reasoning, difficulty in recalling memorized passages, errors or slowness in mathematical calculations, and other phenomena. While these are obvious in the fatigued individual, all attempts at exact measurement of them and the deduction therefrom of the degree of psychical or physical fatigue have failed.
Thus, while some of the characteristics of nervous fatigue are known, all methods heretofore adopted to study the fatigue of the
