THE EFFECT OF COLD ACCLIMATION ON THE SIZE OF ORGANS AND TISSUES OF THE RAT, WITH SPECIAL REFERENCE TO MODES OF EXPRESSION OF RESULTS

Abstract

In experiments in which two groups of animals of different mean body weight are compared, individual organ weights of the animals can be expressed as absolute weights, as fractional weights, or as absolute weights statistically regressed onto constant body weights. The second, and commonest, mode of expression involves the assumption that the part is directly proportional to the whole, and this is shown to be unlikely for all organs except the muscle mass. Practical as well as theoretical justifications for the use of regressed weights (which utilize the actual slope of the line relating the organ weight to the whole) are given.The experimental data are from white rats kept for 4 weeks in a warm (30 °C.) or a cold (6 °C.) environment. It is shown that cold adaptation had no effect on brain, genitals, and lung weights, but that it reduced the growth of muscle, pelt, fat, skeleton, spleen, and thymus, and that it hypertrophied the liver, intestine, kidney, heart, and adrenals. Apparently cold acclimated rats are smaller than the controls mainly because they have a smaller muscle mass.