Abstract
Summary.: 1. It is stated that hibernation is but one manifestation of a property very widely spread amongst living organisms, i.e. that of withdrawing from an unfavourable environment. It is urged that comparison of hibernation with other analogous conditions would be valuable. Evidence is given that somatic changes occurring for various unknown reasons may cause the assumption of lethargy at an unusual time. Therefore somatic changes are as important in determining the cause of hibernation as environmental changes.2. The metabolism of hibernating animals, which is the lowest metabolism required to maintain the existence of protoplasm, is discussed. The various values obtained in connection with the respiratory exchange are mentioned. It is pointed out that the chief points of contention concern themselves with the possible effects of temperature on the solubility of CO2. From experiments covering long periods it is urged that the observed phenomena cannot be explained in this way. The adaptation to changes in temperature is discussed to a certain extent.3. The governor of metabolism amongst the higher animals apart from the nervous system is the endocrine system. The value of various histological findings, and of various injections, is discussed. It is pointed out that lack of structural modification does not necessarily eliminate the possibility of functional modification. Special attention is paid to the importance of the pituitary.4. The effects of varying concentrations of water on the metabolic processes are discussed. It is also indicated how the water content can modify the behaviour of an animal, and that a reduction of it may have opposite effects in various cases.5. Observations are given which demonstrate the remarkable adaptivity, and primitive structure, of the nervous system of hibernating mammals. Particular stress is given to the comparatively slight effects of decerebration on the heat regulative and postural reactions of such animals.6. The various internal and external stimuli tending to terminate lethargy are mentioned, as well as the phenomena connected with the actual return to activity. Emphasis is laid on the need of direct calorimetry to elucidate this problem in the mammalia.

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