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In this article, so far, we have dealt with the natural-history side of hibernation, explained what is meant by the various terms used in connection with this state and in what respect the condition itself differs in the various animals subject to its peculiar manifestations: a condition provided by nature to tide an animal over a period when its very existence, owing to scarcity of food, becomes too difficult or even impossible to maintain; so by preserving the animal's life allows it to perpetuate its species.

We will now pass on to consider the purely physiological phases and phenomena of this state.

In hibernation all the activities of the body are greatly reduced, the temperature of the animal is lowered and even falls to a point slightly above that of the surrounding media. As it has been pointed out above, animals which hibernate do not belong to any one class, but examples are met with in mammals, reptiles (?), amphibians, insects, molluscæ, but curiously enough, no case is known among birds.

In some cases, previous to entering the hibernating state, the animal stores up food in its den or nest, on which it feeds when it wakes at intervals during its winter sleep. This is hardly pure hibernation, as in the true cases there is a special accumulation of fat in the animal's body before the commencement of the torpid state (the animal not waking to feed) and this serves as food during the hibernating period. A peculiar physiological change is here involved—a herbivorous animal becomes carnivorous, this being caused by the animal living on its own flesh, hence the excretions (small) of the animal become profoundly and completely altered in their chemical characters.

A low temperature is the cause generally assigned for the production of hibernation, but a more careful consideration of the facts long ago showed that cold could not be the sole cause of the phenomenon. Most observers who have worked on the subject have found that extreme cold will not cause an active animal to hibernate; although Saissy has observed that continued cold, and a limited amount of air for respiration caused a marmot to pass into a typical hibernating condition, even in summer. Against this we have Vernon Bailey's experiments with spermophiles (first cousins of the marmots or ground-hogs), which showed that in the case of hibernating animals a few degrees lower temperature changed the torpid state into one of death.

Mangili found that torpid marmots and bats were awakened by exposure to severe cold and that lessened or confined air would not cause hibernation. Dormice have been kept in a warm room throughout the winter and yet they hibernated and were not aroused when the extreme temperature was 20° C. The warmth, however, delayed the onset of torpidity by two months and made it less profound. Again, as has been mentioned before, hibernation may take place in the dry hot season.