muscular work varying greatly with the condition of the subject, and his familiarity with the particular task involved. [40] Atwater and Sherman. Loc. cit., p. 51. [41] L. Schnyder: Muskelkraft und Gaswechsel. Zeitschrift für Biologie, Band 33, p. 289. From what has been said, it is obvious that oxygen consumption, as well as output of carbon dioxide, must vary enormously with variations in the muscular activity of the body. The one important factor influencing the quantities of oxygen and carbon dioxide exchanged in the lungs, _i. e._, the extent of the respiratory interchange, is muscular activity; and since, as we have seen, carbonaceous material is the substance mainly oxidized in muscle work, it follows, as carbon dioxide is excreted principally through the lungs, that the respiratory interchange becomes in good measure an indicator of the extent of chemical decomposition incidental to external work. If we recall that man, on an average, at each inspiration draws in about 500 cubic centimeters of air (30 cubic inches), and that for the 24 hours he averages 15 breaths a minute, it is easy to see that in one minute the average man will inspire 7.5 litres of air, or 450 litres an hour, with a total of 10,800 litres for the entire day, which is equivalent to about 380 cubic feet. This would be a volume of air just filling a room 7-1/3 feet in length, width, and height. Inspired air loses to the body 4.78 volumes per cent of oxygen, while expired air contains an excess of 4.34 volumes per cent of carbon dioxide. In muscular work, respiration is increased in frequency and in depth. The volume of air exchanged in the lungs during severe labor may be increased sevenfold, while oxygen consumption and carbon dioxide excretion are frequently increased 7–10 times. The following figures, being values for one minute, show the effect on oxygen consumption of walking on a level and climbing, the subject being a man of 55.5 kilos body-weight:[42] [42] G. Katzenstein: Ueber die Einwirkung der Muskelthätigkeit auf den Stoffverbrauch des Menschen. Pflüger’s Archiv für die gesammte Physiologie, Band 49, p. 330. Also Magnus-Levy: v. Noorden’s Handbuch der Pathologie der Stoffwechsel, p. 233.