Hibernation, or Winter Sleep, is the torpid condition in which certain animals pass the winter in cold countries. The phenomenon is of commonest occurrence in cold-blooded species whose temperature rises and falls with that of the surroundings, but it is practiced by a considerable number of warm-blooded mammals which normally possess the capacity of keeping their temperature at approximately the same level, irrespective of the temperature of the air. It is in these that the physiological accompaniments of hibernation have been chiefly studied, and the principal changes are as follows: The temper¬ature falls to within a few degrees of that of the air, and the heart-beat becomes slow and feeble; respiration almost stops; the alimentary canal and excretory organs cease to operate, but life is maintained by the absorption of fat stored in the tissues during autumn.
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