Hibernation , (Lat. hibernare, to stay in winter quarters), generally understood as the condition of lethargy, in which many animals pass the cold season. The sources of their daily food being at this time cut off, they sink into a deep sleep, in which nutriment is unnecessary, and so remain until the warm weather of spring; a remarkable provision for the preservation of animals which would otherwise perish from cold and hunger. Among the animals in which this state has been noticed are the bat, hedgehog, dormouse, hamster, marmot, and other rodents; chelonians, sauri-ans, ophidians, and batrachians; and some fishes (like the eel), mollusks, and insects. The phenomena of hibernation, however, are not confined to the winter season, and are not necessarily connected with a low degree of external temperature; the bats, in the summer time, present these phenomena regularly every 24 hours; the tenrec, a nocturnal insectivorous mammal, though living in the torrid zone, according to Cuvier, passes three of the hottest months of the year in a state of lethargy. The influence of cold in producing this state is due only to its tendency to cause sleep, and if carried too far, instead of inducing the physiological condition of hibernation, leads to the pathological one of torpor, and even death.

According to Marshall Hall ("Cyclopaedia of Anatomy and Physiology," article " Hibernation1'), the quantity of respiration is inversely as the degree of irritability of the muscular fibre, the former being measured by the amount of oxygen inspired, and the latter by that of the galvanic force necessary to demonstrate its existence. Birds have a high respiration and a low muscular irritability; reptiles, on the contrary, have a high degree of irritability and a low respiration. This is true also of the progressive development of animals from the immature to the perfect state, in which the change is from a lower to a higher respiration, and from a higher to a lower muscular irritability. In sleep, and especially in the profound sleep of hibernation, the respiration is diminished and the irritability increased. Sleep and hibernation are similar periodical phenomena, differing only in degree, and the latter is extraordinary only because less familiar than the former; the ordinary sleep of the hedgehog and dormouse, and of the bat in summer, is a diurnal hibernation, ceasing daily at the call of hunger, and accompanied by a diminution of respiration and animal heat; and this sleep may pass into true hibernation as the blood becomes more venous in the brain, and the muscular fibres of the heart acquire increased irritability.

In perfect hibernation the process of sanguification is nearly or entirely arrested; the bat takes no food, and passes no excretions from the intestines or kidneys; but the dormouse awakes daily, and the hedgehog every two or three days, in a temperature of 40° to 45° F., and they take food and pass excretions, and subside again into their lethargy. Respiration is also very nearly or entirely suspended in perfect hibernation, as has been experimentally proved by the absence of all external respiratory acts, by the unchanged condition of the surrounding air, by the diminution of the animal heat to that of the atmosphere, and by the capability of supporting the entire privation of air or the action of carbonic acid and other ir-respirable gases. The circulation, though very slow, is continuous, and the heart beats regularly; the blood, from the absence of respiration, is entirely venous, but the increased muscular irritability of the left ventricle of the heart permits it to contract under the slight and usually insufficient stimulus of a non-oxygenated blood; it is the exaltation of this single vital property which preserves life and renders hibernation possible, forming the only exception to the general rule of the circulation in animals which possess a double heart; the slow circulation of a venous blood keeps up a state of lethargy induced by a diminished respiration.

Sensation and volition are quiescent, as the brain and its sensory ganglia are asleep, but the true spinal or excito-motory system is awake and its energies are unimpaired, as is shown by the facility with which respiration is excited by touching or irritating the animal; muscular motility is also unimpaired in this state; the action of the heart has been found to continue about ten hours in an animal in the state of hibernation, in which the brain had been removed and the spinal marrow destroyed, while in the same animal in a natural state it ceases after two hours. With such an irritable condition of the heart, the introduction into it of an arterial or oxygenated blood from respiration would soon cause death from over stimulation; and as trifling causes are sufficient to excite the respiratory act, hibernating animals adopt various means of securing themselves from disturbance; bats retire to the recesses of gloomy caverns, where they hang suspended by the claws of the hind feet, head downward; the hedgehog and the dormouse roll themselves into a ball; tortoises burrow in the earth, frogs and eels plunge under the mud, and snakes twist themselves together in natural or artificial crevices and holes in the ground.

The call of hunger and the warmth of returning spring arouse all these from their winter retreats, the irritability gradually diminishing as the respiration becomes active. Extreme cold will rouse a hibernating animal from its lethargy and speedily kill it; hence many animals congregate in carefully prepared nests, and others, like the snakes, entwine themselves for mutual protection from cold. The state of hibernation, or that in which the stimulus of venous blood is sufficient to continue the heart's action, finds a parallel in some cases of disease accompanied by lethargy, in which revival has occurred after supposed suspended animation, and in others in which actual death has been delayed for days after the apparent cessation of respiration and circulation; the causes of this condition, which might throw much light on the kinds and phenomena of death, have not been fully investigated in the human subject. The torpor produced by extreme cold, though sleep be always induced, is very different from true hibernation; the former is attended with diminished sensation and rigidity of the muscles, and if prolonged ends in arrest of the circulation and death; the latter, in which sensation and motility are unimpaired, has for its object the preservation of life; the hibernating bat or dormouse is aroused from its sleep by too great cold, and is destroyed by it like any other animal.

Most animals lay up a store of fat under the skin, which is slowly absorbed during hibernation; in the frogs, and probably in many reptiles, the adipose accumulation takes place within the abdominal cavity in the folds of the peritoneum, for a similar purpose. The phenomena of insect hibernation are very interesting in all stages of growth; many pass the winter in this condition, both above and beneath the surface of the ground; eggs and chrysalids have been known to withstand a temperature several degrees below the freezing point of water. It is well known that many species of fish may become stiff from cold and yet not perish, but actual congelation is fatal; in the so-called frozen fishes which have revived in warm water, there must have been a low degree of vital action in the organs of circulation. In batrachians the necessary respiration may be effected entirely through the skin, in the hibernating state. The lower animals generally seem to possess a remarkable power of resisting cold, and may be reduced to a condition of apparent death, without the irritability of hibernation, and yet not identical with the torpidity usually produced by cold.