The determination of total metabolism in the mouse.

The apparatus to be described was designed to investigate certain phenomena of pregnancy by enabling (a) the measurement of the total ingesta and excreta of a mouse, including the exchange of water and gases, for periods of approximately 24 hr. under conditions as normal as possible, and (b) the repetition of such daily measurements for long periods (up to the duration of pregnancy) with a minimum break between successive daily runs. It was hoped to obtain a complete picture of the metabolism of the mouse, i.e. the daily intake of foodstuffs, their utilization, the water balance and the average metabolic rate. This has proved to be possible, though only non-pregnant animals have so far been used. The considerable work involved is offset by the advantage of being able to measure interdependent factors simultaneously; furthermore, the relatively long periods of measurement, usually impracticable in specialized metabolic studies, smooth out those sporadic fluctuations of metabolic rate which would otherwise require for their elucidation numerous repeated determinations. For similar reasons, it is to be expected that a series of consecutive long-period determinations will reveal gradual changes not easily demonstrable otherwise. The main advantage of a closed-circuit design for such studies, over the more frequently used open-train Haldane-Pembrey type, is that all determinations, including that of oxygen utilization, are made by direct measurement. The balance of all such separately determined factors therefore provides an infallible guide to the accuracy of the apparatus. No inaccuracy can be masked by the indirect determination of any factor by difference. The construction of an apparatus fulfilling the above requirements, although straightforward in principle, was beset by many technical difficulties. It is felt, therefore, that a detailed description of the design now in use may be of value to those undertaking similar work. The authors are unaware of any previous published description of such an apparatus, although the closed-circuit form with measurement of water excretion has been used for mice by Asmdi (1921), Barbour & Trace (1937) and Newton (1937). Total metabolism studies in man, including the intake and utilization of foodstuffs, were conducted by Benedict & Milner (1907). Their method of obtaining food balance was, however, by means of the elementary composition of total ingesta and excreta.