The difference between the hibernating and all other animals then is, an ultimate faculty of assuming an augmented degree of irritability of the muscular fibre—a power possessed by all animals within certain limits, but by the hiber nating animal beyond the usual limit.
Sleep, however inscrutable in itself, is the connecting link between the two physiological states; a disquisition on hibernation is, there fore, a disquisition on sleep—on profound sleep. It will shortly appear that one eminent philoso pher has fallen into the error of assimilating different physiological phenomena by neglect ing to take this fact into his consideration. Sleep and hibernation are similar periodical phenomena, induced by similar causes, leading to similar effects, and differing only in degree. hibernation appears more extraordinary only because less familiar than sleep. Most animals are, in fact, naturally awake and asleep every revolving day, some being diurnal, others noc turnal. But in summer the bat actually hiber nates, loses its respiration, and with its respira tion its temperature, acquires vastly augmented irritability, and presents the other phenomena of complete hibernation, regularly and periodi cally every twenty-four hours ; and the hedge hog and the dormouse present similar pheno mena, only after other intervals.
Sleep then is the first stage of hibernation. The faculty of passing into the second is iden tical with that of assuming a greatly augmented irritability of the muscular fibre. Such are the results of my long attention to this interesting physiological question. Much error has arisen from viewing hibernation as a simple effect of cold. The influence of cold in inducing hiber nation is merely its well-known influence in inducing sleep, concurring with the other causes of this condition. The direct effect of cold on the animal frame is, as I shall shortly have occasion to state particularly, totally different from hibernation. Hibernation is a physiolo gical condition ; the direct effect of cold, or torpor, is, on the contrary, a pathological and generally a fatal one.
The term hibernation has usually been ap plied to designate what its etymology implies, the condition in which certain animals pass the winter season. An error is, as I have already stated, involved in this view of the subject ; for the condition termed hibernation is not con fined to the winter season. Cuvier observes,
in speaking of the Tenrecs, " ce sont des ani maux nocturnes qui passent trois moil de l'ann6e en lethargie, quoique habitants de la zone torride. Burguihre assure meme que c'est pendant les grandes chaleurs qu'ils I fence the term Sommerschlof employed in Ger many. It is plain too, from this circumstance, that the state of hibernation is not necessarily connected with a low degree of external tempe rature, and we are surprised to find this cele brated naturalist, whom I have just quoted, observing, " la seule condition de la lethargie est le froid et l'absence des causes irritantes. ' I I must repeat that hibernation is, in every respect, but the parallel of ordinary sleep, vary ing only in force and duration. It is equally marked by an inexplicable periodicity ; it is equally modified by cooperating or opposing causes; and it is equally manifested in its pe culiar effects, only varying in degree and inten sity.
In giving a distinct idea of hibernation we must extend our views to the altered condition of each function in the animal economy, for this peculiar state is not limited to any special function or organ. It is, in fact, a treatise on physiology which should be written, compa ring the state of each function and of each organ, in the hibernant or lethargic and in the active condition, a disquisition on sleep, indeed, in its various degrees, and in its effects in mo difying the various functions.
The first question then is,—what is sleep? a question difficult, perhaps impossible, to an swer, if we mean by it what is its nature or essence, but highly interesting to prosecute, if we mean what are its special phenomena and their mutual relations.
In order to treat of sleep properly, I must first observe that, of the nervous system, of which it is primarily a modification, formerly divided into the cerebrospinal and the ganglio nic, the first division must now be subdivided into the cerebral and the true spinal, the former being the exclusive seat of sensation, volition, &c.; the latter, the special source of certain actions now designated excito-motory, and ob served in the orifices, the ingestors, the expul sors, the sphincters, &c. Now it is the cere bral system which sleeps, the true spinal re taining all its energies.