HIBERNATION ; etym. hiberno, to win ter, to pass the winter; syn. lethargy ; errone ously, torpor ; Fr. sommeil hivernal; Germ. Winterschlafand Sommerschkf; a term chiefly applied to express that condition in which cer tain animals pass the winter season.
How often have I been struck with admira tion in observing how variously the Creator has provided for certain of the insectivorous tribes, the swallow and the bat, for example, against the period when the sources of their daily food are cut off, when spring and summer yield to autumn and winter, and insects disappear I The first emigrates to a more genial climate where its nutriment still abounds ; the second sinks into a deep sleep, in which food is unnecessary, and which continues through the dreary season of cold and famine.
It has not hitherto been distinctly ascertained to what extent the state of hibernation prevails in the animal kingdom; the bat, the hedgehog, and the dormouse, are the genera which present us with the most marked examples of this sin gular physiological condition in this country ; to these the elegant authoress of " Sketches of Natural History" has added the water-rat and the wood-mouse, observing of the former " And when cold winter comes and the water plants die, And his little brooks yield him no longer supply, Down into his burrow he cozily creeps, And quietly through the longwinter-time sleeps." But before we proceed to discuss this ques tion of natural history, we must consider that of the physiology of hibernation.
There is, in my opinion, an ultimate law of animal existence, which seems to regulate the different forms in which the different classes of animals present themselves. The quantity of respiration is inversely as the degree of irritabi lity of the muscular fibre, the former being marked by the quantity of oxygen consumed in a given time, ascertained by the pneumatoine ter,* the latter by the force of galvanism neces sary to demonstrate its existence. The bird tribes have a high respiration and a low irrita bility ; the reptiles have a high degree of irrita bility and little respiration. This law obtains not only in the different tribes of animals, but also in the different stages or states of the same individual, the structural changes from one stage to another being always a change from a lower to a higher respiration, and from a higher to a lower degree of irritability, and the change of state, a change in the opposite direction : thus the changes from the egg to the bird, from the tadpole to the batrachian form, from the larva to the chrysalis and the insect condition, are changes in which, whilst a due ratio is con stantly maintained, the quantity of respiration is augmented and the degree of irritability diminished; un the other hand, the physiolo gical changes in the degree of activity in ani mals, during sleep, for example, but especially in that remarkable change which is the subject of this article, the respiration is diminished whilst the degree of irritability is, pari pass°, augmented.
On what this susceptibility of change de pends, and especially on what the power of taking on an augmented irritability depends, is at present unknown. But 1 think I may affirm that it is upon this power that the capability of passing into the state of hibernation reposes. I suppose that all animals have the faculty of sleeping; during sleep the respiration is slightly diminished, the irritability probably proportio nately augmented—probably one ultimate ob ject of this state of repose; but the phenomenon has its appointed limit which it cannot pass. In certain animals, that limit is not so con fined,—the quantity of respiration is still further diminished, the degree of irritability still further augmented, and the deeper sleep, or lethargy, of hibernation takes place.
During this lethargy, the law of the inverse ratio of the respiration and of the irritability still prevails, and the animal merely puts on a reptile state in these respects. Were the respi ration to be diminished without the appointed augmentation of the irritability, the heart would cease to be stimulated, and the animal would die, as in the cases of torpor and slow asphyxia; were the respiration augmented without the proportionate diminution of the irritability, the heart would be over-stimulated, and death would alike ensue, as in the case of a hiberna ting animal too suddenly roused from its lethar gy, and as (probably) in the case of an animal placed in pure oxygen gas.