COMPARISON BETWEEN SEASONAL AND THERMAL ACCLIMATION IN WHITE RATS: IV. MORPHOLOGICAL AND PATHOLOGICAL CHANGES

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Abstract
White rats have been shown to develop a similar degree of resistance to cold whether exposed individually to a continuous cold temperature or exposed in groups of 10 to the fluctuating environmental conditions prevailing outdoors during the winter.Under these two different types of cold exposure, the rats were observed to have a reduced muscle growth. However the enlargement of the adrenals, thyroids, pituitary, heart, and digestive tract and the reduction of the mesenteric and subcutaneous fat and of the pelt weight, which have repeatedly been found in the indoor cold-acclimated rats, did not take place in the "outdoor winter" rats. Therefore it is clear that increased resistance to cold can be brought about without some of the anatomical changes characteristically associated with acclimation to continuous cold exposure.Cold stimulation outdoors was not sufficiently severe or of sufficient duration to produce renal lesions, hypertension, or cardiac lesions of any importance, although the animals were exposed to an average temperature of −10 °C which would certainly produce these lesions in indoor individually exposed rats.