These hybernating animals, whilst they pre sent the structure of the warm-blooded tribes in general, still approach in a very remarkable degree to the cold-blooded tribes in their defective energy, or their indifferent powers of reaction. This is to be regarded as the prin cipal source of the phenomena they exhibit in the current of the year, phenomena which are unknown among the more perfectly con stituted warm-blooded animals, but which are absolutely of the same nature as those presented by the cold-blooded Vertebrata in the same circumstances, and which only differ in degree. This analogy or resemblance in the phenomena appears to arise from analogy not of structure but of constitution. Very opposite organiza tions may have analogous constitutions; cold blooded animals for example present the greatest diversities of structure, and all are affected and bear themselves in the same man ner under similar circumstances in very many respects. They have thus a common constitu tion which characterizes them, the fundamental principle or distinguishing feature of which is a defect of energy, or of power of reaction. This principle, so simple in itself, and which is but the true expression of the various facts reduced to unity, renders plain and obvious much that otherwise appears anomalous or contradictory. In studying the phenomena of animal heat under new relations, we shall find the confirmation of what precedes.
Or TIIE SYSTEM UPON W1I1CU TILE EXTERNAL TEMPERATURE ACTS PRIMARILY AND PRIN CIPALLY.
Our sensations admonish us that it is the nervous system that is acted upon primarily and principally by changes of external temperature. In the first place the impression is felt instan taneously ; in the second place the intensity of the sensation is in relation with the degree of external heat or cold ; in the third place the im pression is not limited to the various degrees of the corresponding sensation of heat or cold ; it extends to the other faculties of the nervous system, increasing or diminishing the general or special sensibility ; in the fourth place it acts powerfully in increasing or diminishing the activity of the muscular system, principally through the medium of the nervous system.
Influence of temperature on the vitality of cold-blooded animals.
If the functions of respiration and general circulation be destroyed by the excision of the lungs and heart of a cold-blooded animal, of one of the Batrachia for example, life may still continue for a time. Of the three principal systems of the economy the only one then left untouched is the nervous ; so that the animal may he viewed as living almost exclusively by the agency of this system. If several animals in this condition be plunged in water deprived of air, they will live in it different spaces of time according to the degree of its temperature, the extremes compatible with their existence being zero and 90 c. It is towards the inferior limit, zero, that they live the longest.
Towards the upper limit they die almost im mediately. Temperature, consequently, pre sents in the scale of variations just mentioned very remarkable relations with the vitality of these animals. Towards the lower limit or that of melting ice, it is obviously most favourable to life; towards the upper limit, it is most inimical to life, extinguishing it almost immediately. Ilere it is impossible to mistake the system upon which the variety of temperature exerts its first and principal effects•the nervous system.
If respiration only be annihilated by plunging these creatures under water deprived of air, the temperature of which is caused to vary as above, they will be found to present the same phenomena according to the degree of the heat or cold, but in a more striking manner. Temperature in this case has the same kind of influence, but the effects are more manifest, from the circulation of the venous blood prolonging life at every degree short of the one at the upper limit of the scale, at which life is extinguished quite as suddenly as in the former instance.
Such are the direct and instantaneous effects of temperature upon the vitality of cold blooded animals. But there are others which flow from its successive agency, during a con siderable length of time. If the series of ex periments just quoted he made in summer, and the different lengths of life at different degrees of temperature of the frogs immersed in water be noted, (between the limits which we have pointed nut above,) and the same expe riments be repeated in autumn, the length or tenacity of life manifested by the animals will be much greater at the same degrees of tem perature—they will in general be found to live twice as long now as they did in summer, at corresponding and equal temperatures of the medium in which they are immersed. The depression of atmospheric temperature in the autumn has modified their constitution, and actually increased their vitality, precisely in the manner indicated above. The slight effects of each successive fall in the general temperature have accumulated in the constitu tion so as to render their vitality or tenacity of life much greater, a fact which is made abun dantly manifest by the faculty of the animals to remain for a much longer time immersed in water without breathing than they could have done in summer. If a third series of experi ments of the same description he made in winter, the tenacity of life will be found to have increased in a very high degree. At the same degree of temperatUre frogs will be found to live immersed in water deprived of its air at least twice as long in winter as they could have done in autumn. The same cause—the depression of the atmospheric temperature— has continued to act with greater intensity and for a longer period, and the constitution, gradually modified by greater and longer con tinued cold, has acquired greater tenacity of life.