The opposite effect takes place with the successive rises of the temperature from that of winter to that of summer ; so that among cold blooded animals the maximum of vitality, i. e. tenacity of life, corresponds to the depth of winter, the minimum to the height of sum mer. The slight and from moment to moment inappreciable effects produced by the external temperature, whethet tending to increase or to diminish the vitality, accumulate with their repetition through the period of each season, and produce a corresponding change in the con stitution with regard to tenacity of life. These accumulated effects of the different portions of the year constitute the influence of the seasons ou the constitution with respect to many of the most important relations of life. The first of these we have just examined cursorily—that is, the faculty of living in air according to the influence of the actual temperature, or of that of the past temperature, in other words the season that has immediately preceded. The second of these fundamental relations consists in the various proportions of air necessary to the maintenance of life according to their re lations with the temperature. We have seen that it is at the minimum of temperature that the cold-blooded animals possess the greatest tenacity of life, as regards the most essential relation, in other words they are in the con dition the most favourable to enable them to do without air; at this point they are in a state to live for the longest time without breathing. It is obvious that here they must require less air than under any other circumstances ; they must necessarily require so much the less, as their life will continue longer here than under any other circumstances without any access of air at all. It is, however, essential to appre ciate duly this fundamental relation, namely, that at the lower limit of the scale of tempera ture mentioned, cold-blooded animals require less air to live, and what is more, they con sume less air than under any other circum stances, and are even incapacitated from con suming more than they do. The minimum temperature of this scale consequently is an index of the maximum of vitality or tenacity of life, and at the same time of the minimum of respiration. In the same proportion as the temperature rises, the vitality or tenacity of life declines, which makes it necessary that this declension should be compensated by a corresponding increase in their relation with the air, in order that the vivifying influence of this fluid may neutralize the deleterious effects of the increase of heat. And this is what actually happens. With the rise in tempe rature the sphere of activity of the respiration extends, and the vivifying influence of the air, which increases with the quantity of the fluid consumed, compensates the successive decre ments in vitality or tenacity of life, dependent on successive increments of temperature. We shall therefore express in a very few words this fundamental relation between the tempera ture of the air and the maintenance of life among the invertebrate series of animals,—a relation entirely deduced from direct experi ment, which we can hut refer to here, but which we shall lay before our readers with all the requisite details in our article on RESPIRA 'nos. The rise of temperature in the scale from zero to 40 c. exerts upon the nervous system of cold-blooded animals an action the tendency of which is to diminish its vitality ; the air, on the contrary, exerts a vivifying in fluence on this system. It becomes necessary, therefore, to the maintenance of life that their respective relations with the economy he such that their effects compensate or counterbalance each other.
The principle relative to the influence of temperature on the vitality of cold-blooded animals just laid down, is applicable in every particular to the changes experienced and the phenomena presented by the hybernating tribes among the warm-blooded series of animals.
Their vitality changes with the wane of the year, i. e. under the influence of prolonged exposure to cold, in the same manner They are then in a condition to exist with a supply of air by so much the less as this influence has been more intense and more protracted:; and precisely as the cold-blooded tribes, if entirely deprived of air in winter, they will live for a much longer time in this deleterious position than they would have done in summer.
ItVluence of temperature on the vitality of warm-blooded animals and man in the states of health and disease.
These principles and considerations lead us to examine what happens among warm-blooded animals in the same circumstances. There being great and manifold analogies between them and the preceding tribe of animals, there must also be some community in the application of the principles laid down ; but as they also differ in many important respects, this application must be correspondingly restricted. In the first place, then, there is complete analogy between the one and the other with regard to the influence of the superior thermal limit on the vitality of the nervous system. To seize the analogy properly, it is however necessary to regard the temperature which modifies this system in each series, from a point of view that is common to both. Whether the temperature proceeds from without or from within, we may presume that it will influence or modify the nervous system in the same manner, if not to the same degree, inasmuch as this system presents differences. Warm-blooded animals having in general a high temperature at all seasons of the year, they must be compared in this respect with cold-blooded animals in the height of summer. On the one hand, heat within certain limits tends to increase sensibi lity and motility ; warm-blooded animals, therefore, with a few exceptions, which always present a high temperature, constantly exhibit also, with a few exceptions, a high degree of sensibility and motility. The same thing can only be said of the cold-blooded tribes during the continuance of the warm weather. On the other hand, again, high temperature tends to lessen the vitality proper to the nervous system, or the faculty of living without the agency of the ordinary stimuli. This is also the reason why, if respiration be interrupted among warm blooded animals at all times, and among cold blooded animals during the warmer seasons of the year, they all perish alike speedily or nearly so. The difference in the time that elapses before life is extinct still depends on, or is in relation with, the difference of temperature. For in the hotter season of the year, cold blooded animals never attain the temperature of the warm-blooded tribes, even in the most burning climates of the globe. Their nervous system will consequently have a higher degree of vitality in the sense already indicated ; that is to say, they will not perish so promptly in summer under deprivation of air ; but if they be immersed in water at the mean tempera ture of warm-blooded animals generally, which is about 400 c. (104° F.), they will die as sud denly—(at least this is the case with those of small size upon which the experiment has been made)—as the warm-blooded Vertebrata when deprived of the contact of air.
The analogy on either hand consists in the effects of temperature. But the differences that must necessarily occur between natures that vary in so many other respects are espe cially encountered in the dissimilar effects of cold. here we observe a general compensa tion which distinguishes in the most marked manner the Vertebrata having a constant or all but a constant temperature, from the hyber nating tribes or Vertebrata whose temperature varies, and the cold-blooded series generally. The relation of cold, or of a low temperature relatively to the standard of the more perfect beings of creation, is one of essential impor tance, and requiring our most careful investi gation.