From what has been said, it is obvious that oxygen con-sumption, 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 sub-stance 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 71 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 for one minute, show the effect on oxygen consumption of walling on a level and climbing, the subject being a man of 55.5 kilos body-weight: 1

1 Atwater and Sherman. Loc. cit.. p. 61.

2 L. Schnjder: Muskelkraft und Grawecluel. Zeitschrift fur Biologie, Band 33. p 288.

1 G. Kaizenalein: Ueber die Einwirkung der Muskeltliitigkeit auf den Stoffverbrauch des Memchen. Pfluger's Archiv fur die gesammte Physiologie, Band 49. p 330. Also Magnus-Levy: v. Noorden's Handbuch der Pathologic der Stoffwechel, p. 233.

Remembering that these figures represent the oxygen consumption for only one minute of time, it is easy to see the striking effect of moderate and vigorous exercise ou respiratory interchange. Simply walking along a level suffiies to increase the consumtion ofoxygen threefold over what occurs when the body stands at rest. When the more vigorous exercise attendant on lifting the body up a steep incline is attempted, most striking is the great increase in the amount of oxygen consumed. We thus see another forcible illustration of the influence of muscular activity upon the exchange of matter in the body, and a further confirmation of the statement, so many times made, that oxidation - especially the oxidation of fats and carbohydrates by which large quantities of heat are set free, easily convertible into mechanical energy - is a primary factor in the metabolic processes, by which the machinery of the living man is able to work so efficiently.

Finally, we cannot avoid the conclusion that the outgoings of the body, in the form of matter and energy, are subject to great variation, incidental to the degree of activity of the day or hour. The ordinary vicissitudes of life, bringing days of physical inaction, followed perhaps by periods of unusual activity; changes in climatic conditions, with their influence upon beat production in the body; alterations in the character and amount of the daily dietary, etc., - all seemingly combine as natural obstacles to the maintenance of a true nutritive balance. Outgo, however, must be met by adequate amounts of proper intake if there is to be an approach toward a balance of nutrition. In some way the normal, healthy man does maintain, approximately at least, a condition of balance; not necessarily for every hour or for every day, but the intake and outgo if measured for a definite period, not too short, say for a week or two, will be found to approach each other very closely, Body equilibrium and approximate nitrogen balance may be reasonably looked for, as well as a balance of total energy, in the case of a healthy man leading a life which conforms to ordinary physiological requirements. The man who, on the other hand, consciously or unconsciously, continues an intake way beyond the outgo, whose daily income of nitrogen and total fuel value far exceeds the requirements of his body, obviously lives with an accumulating plus balance, which ordinarily shows itself in increasing body-weight and with a storing away of fat.

Equally conspicuous is the effect of an inadequate income of proper nutriment; a food supply which persistently fails to furnish the available nitrogen and total energy value called for by the body under the conditions prevailing, will inevitably result in a minus balance, which, if continued too long, must of necessity tax the body's store to the danger limit At the same time, the well-nourished individual, without being unduly burdened by a bulky store of energy-containing material, is always supplied with a sufficient surplus to meet all rational demands, when from any cause the intake fails, for brief periods of time, to be commensurate with the needs of the body. It is reasonable to believe, however, that in the maintenance of good health, and the preservation of a high degree of efficiency, the body should be kept in a condition approaching a true nutritive balance.