the second animal showed evidences of fatigue. Their power of contraction on stimulation was diminished. A similar experiment with an extract made from resting muscle had no such effect.

About twenty-five years later, the Italian scientist Mosso showed that the depressant action of fatigue substances is not confined to the tissues in which they arise. He demonstrated that the blood becomes charged with these chemical wastes produced in the muscles, and carries them to all parts of the body. He proved this by injecting the blood of a dog fatigued by long continued running into the vessels of a second dog from which an equivalent amount of blood had been drawn. Upon this, the second dog showed the usual signs of fatigue.

Products of muscular activity are thus shown to cause symptoms of fatigue when injected into resting tissue. In the study of muscular fatigue we may learn how these waste products are created and how they affect the organism. Muscular fatigue has been longest studied since fatigue of the muscles can be most easily observed and registered by certain instruments of precision or measurement. The observation of fatigue or diminished power of reaction in frog muscles preceded Mosso's famous studies of human fatigue.

The myograph, designed by H. von Helmholz, shows how the loss of energy in wearied frog muscles results from noxious substances in the muscles, produced during work. The leg muscle of a frog is separated from the rest of the body and hung by one end upon a support. To the other end of the muscle a lever is attached which comes in contact with a revolving cylinder covered with sooty paper. If the leg is at rest, a straight line is traced upon the revolving cylinder. If the muscle is electrically stimulated to contract, the lever records the contractions by upward and downward marks upon the sooty surface of the revolving cylinder, the height of the curves being determined by the force of the contracMosso, Angelo: Arch. für Anatomie u. Physiologie. Physiologische Abtheilung, 1890, p. 89.

Series of 550 contractions of a frog's gastrocnemius muscle, excised and stimulated at intervals of two seconds. Every contraction is recorded, except at the places indicated by the black bands, at each of which the records of fourteen contractions are omitted. The record of the first contraction is at the bottom of the figure: that of the last one at the top. Fatigue is shown in the progressive decrease in height and the increase in length of the curves.

tion. As the muscle tires, the contractions grow smaller and smaller until finally the lever cannot be raised at all.*

It can be shown that this fatigue of the muscles is due to the paralyzing action of the accumulated fatigue products See Figures 1, 2, and 3. The illustrations are from The Nature of Fatigue, by Professor Frederic S. Lee. Popular Science Monthly, Feb., 1910. (Reproduced by permission.)

Series of contractions of a rat's gastrocnemius muscle, excised and stimulated at intervals of two and one-half seconds. Fatigue is shown in the progressive decrease in height of the curves.

in the blood. For if at any time after fatigue has set in, the muscle, while suspended, is washed out through its bloodvessels with a normal salt solution, its power to contract returns. As soon as the fatigue products are washed away, the muscle is rested.*

See Figure 4.

Series of contractions of the frog's gastrocnemius muscle, excised and stimulated at intervals of two seconds. Every fiftieth contraction is recorded. Fatigue is shown in the progressive lengthening of the descending limb of the curves.

Series of contractions of a frog's gastrocnemius muscle in situ and stimulated at intervals of two seconds. The flow of blood through the muscle was stopped by ligating the artery, and the record of fatigue was made. At the break in the series, the muscle rested five minutes, during which time the ligature was removed and the blood was allowed to circulate through the muscle. The record of contractions at the right of the break was made immediately after the resting period, and while the blood was still circulating.

Using the same principle described above, Mosso devised an apparatus called the ergograph, to study muscular contraction in man.

By its means," writes Professor Frederic S. Lee, himself one of the foremost American investigators in this field, "(Mosso) began the long series of studies of voluntary contractions in man, which has made the Turin School famous and has immeasurably extended our knowledge of fatigue in living human beings."*

The ergograph is an instrument constructed so as to record the contractions of a single muscle or group of muscles. Thus, for instance, the arm and hand, except the middle finger, may be supported and held fast. The person experimented upon contracts his middle finger at regular intervals, thereby lifting a known weight to a definite height or stretching a spring of known tension. As in the myograph, contractions are recorded by curves upon a revolving cylinder, and show a steady diminution of the lifting power of the muscles, the rate and regularity of the diminution differing with individuals. If the highest points of the curves recorded on the cylinder are joined together, the result is a curve of characteristic form for each individual, known as the curve of fatigue. This curve remains practically the same for each person whether his contractions are voluntary or due to electric stimulation. Some persons obviously tire less quickly than others; some work at high pressure for a short time, giving out suddenly, while others work more slowly and regularly. All this is borne out by the record of the ergograph, which shows graphically on paper how great are the varieties of individual working capacities. (See Figure 5.)

In industrial occupations, obviously, the working time cannot be measured off for each individual according to his special capacity. But the testimony of the ergograph to the infinite varieties of endowment in strength and staying capacity emphasizes the need of setting a fair maximum working period which shall not over-reach the natural limits of the majority of individual workers.

By the use of the ergograph we learn more of the funda

Lee, Frederic S., Ph. D. (Professor of Physiology, Columbia University, New York): Fatigue. Harvey Lectures, 1905-06, p. 172. Philadelphia and London, Lippincott, 1906.