" J'avais place l'animal de maniere que je pouvais repeter, autant que je le jugeais aces saire, 1(..a experiences thermometriques. Des que sa temperature out baisse jusqu'A zero, (ee ne fut qu'a 2 heures du matin) je he retirai du bocal et le placai dans une temperature de 12° et plus au dessus de la glace; mais l'animal Cart mort."* To induce true hibernation, it is quite neces sary to avoid extreme cold ; otherwise we pro duce the benumbed and stiffened condition to which the term torpor or torpidity may be applied. I have even observed that methods which secure moderation in temperature, lead to hibernation : hedgehogs, supplied with hay or straw, and dormice, supplied with cotton wool, make themselves nests and become lethar gic ; when others, to which these materials are denied, and which are consequently more ex posed to the cold, remain in a state of activity. In these cases, warmth or moderated cold ac tually concur toroduce hibernation.
When we of insensibility, of a stiffened state of the muscles, and of a cessation of the circulation, as obtaining in hibernation, we may be certain that a state of torpor has been mis taken for that condition. The actually hiber nating animal exposed to continued severe cold is, as al. Saissy correctly observes, first roused from this state of ease and preservation into a painful activity, and then plunged into a fatal torpor.
This subject will come to be considered in a subsequent part of this inquiry, in which I purpose to trace the effects of cold in changing the relative quantity of respiration and degree of the irritability in animals of different ages which do not hibernate; in the meantime, the accurate distinction between mere torpor, which may occur in any animal, and which is a de struetivestate, from true hibernation, which is preservative, and the peculiarity of certain ani mals, will enable us to correct many inaccuracies into which Legallois,-I- M. Edwards,: and other physiologists have fallen. (See IRRITABILITY.) In conclusion, one of the most general effects of sleep is to impair the respiration, and with that function the evolution of animal tempera ture. The impaired state of the respiration in duces a less arterial condition of the blood, which then beconies unfit for stimulating the heart; accumulation of the blood takes place in the pulmonary veins and left auricle ; a sense of oppression is induced, and the animal is either roused to draw a deep sigh or awakes altogether.
Such are the phenomena in animals in which the heart has not the faculty of taking on an augmented state of irritability, with this lessened degree of stimulus. But in those animals which do possess this faculty, a property which con stitutes the power of hibernation, the heart con tinues the circulation of the blood, more slowly indeed, but not less perfectly, although its arte rial character be diminished and its stimulant property impaired. No repletion of the pul
monary veins and of the left auricle, no sense of oppression is induced, and the animal is not roused ; the respiration continues low, the tem perature falls, and the animal can hear, for a short period, the abstraction of atmospheric air.
All the phenomena of hibernation originate, then, in the susceptibility of augmented irritabi lity. The state of sleep, which may be viewed as the first stage of hibernation, induces an im paired degree of respiration. This would soon be attended with pain, if the irritability of the heart were not at the same time augmented, so as to carry on the circulation of a less arterial blood, and the animal would draw a deep sigh —would augment its respiration or awake. Occasional sighs are, indeed, observed in the sleep of all animals, except the hibernating. In these, the circulation goes on uninterruptedly, with a diminished respiration, by the means of an augmented irritability. There is no stagna tion of the blood at the heart ; consequently, no uneasiness; and the animal becomes more and more lethargic, as the circulation of a venous blood is more complete. This lethargy is even tually interrupted by circumstances which break ordinary sleep, as external stimuli or the calls of appetite.
It still remains for me briefly to discuss the question,—what are the hibernating animals ? I must first advert to the fact, on which I have already insisted, that hibernation does not pre sent itself in an equal degree in all the hiber nating tribes. All animals sleep periodically, in the night or in the day. Some sleep for several days together, especially after taking food, and in the cool seasons of the year, as the hedgehog. Perhaps the bat may be the only animal which sleeps profoundly the winter through, without awaking to take food.
These remarks prepare us for a more just view of hibernation and of hibernating animals than is, as I believe, usually taken.
Of the hibernating animals the most unequi vocal are the bat, the hedgehog, the marmot, the hamster, the dormouse. It has been said that the bear and heaver belong to the num ber, but this is extremely doubtful. It has been said also that the swallow belongs to the hibernating class, but this is incorrect. The cold-blooded animals, the Chelonian, the Sau rian, the Ophidian, and the Batrachian tribes, all, however, indubitably pass the winter in a state of apathy and lethargy. Some of the fishes also become lethargic during the cold season. The same remark applies to some of the molluscous and insect tribes.