DIFFERENCES OP CONSTITUTION IN RELATION WITH TUE PRODUCTION OP IIEAT AMONO ANIMALS.
Since the body and the functions are pro gressively developed, and without interruption between the two grand periods named, there is in the coarse of this long interval as much dif ference in the state of the constitution as there are sensible degrees of development ; a circum stance that implies a long series of varieties. But these intimate differences are not mani fested externally by corresponding states of temperature of body. For we have seen that this undergoes but four sensible variations in this respect, and that,of these four modifications, two were of like import. It is every way worthy of attention to observe that, at the point which separates the first from the second period of infancy, the temperature should be equal to that of the adult.
It is difficult to imagine that this equality can exist under every variety of external cir cumstance, when we see that the elements upon which it depends are so different. Aod this leads us to consider the production of heat under a new point of view. Under what cir cumstances has this equality of temperature be tween the infant and the adult been observed ? It was when the external temperature was mild or even warm. Would the same thing have been observed had this been cold or severe? It is evident that if the faculty to produce heat is the same at this period of infancy as it is in adult age, the heat of the body will always re main the same, making abstraction of the diffe rences that depend on those of simple corpo real bulk. Thus, all things else being equal, a young animal at this epoch ought to cool to the same degree as an adult under the influence of external cold, if it have the same power of pro ducing heat. If, however, it be inferior in its calorific powers, it will not be competent to maintain its temperature to the same degree as the adult, and it will fall under this limit in a proportion determined by the difference which exists in the faculty of producing heat. On making application of the principles which have been already announced, let us try if we cannot predict the effects. By reason of the inferiority in energy of the nervous system in early life, it is difficult to suppose that a young animal will resist the action of intense cold in the same manner as an adult. This inference is fully borne out by the following experiment. A young guinea-pig a month old, the tempe rature of whose body was high and steady, the temperature of the external air being mild, was exposed along with an adult to the same degree of diminished temperature—the air was at 0°c. (32° F.). In the course of an hour the young creature had lost 9° c. in temperature, whilst the adult had only lost 2°,5 c. This experi ment, repeated several times with the same species of animal, always gave the same result.
Young and adult birds of the same species, treated in a similar manner, showed the same diversity in their powers of resisting the effects of external cold, from which we may infer that the law is quite general. Several young mag pies, for instance, whose temperature was sta tionary in a mild spring atmosphere, were placed with an adult in air cooled to + 4°c. After the lapse of twenty minutes, one of the young ones was found to have lost 14° of tem perature. The others, examined at intervals, none of which exceeded one hour and ten mi nutes in length, had cooled from 14° to 16° c. The adult bird, on the contrary, similarly cir cumstanced, did not suffer a greater depression of temperature than c. The loss of heat sustained by the young birds was so great as to be incompatible with life, if continued ; that endured by the old one was trifling in amount, and not inconsistent with health. It is quite true that the difference in point of size and quantity of plumage has an influence upon this inequality of cooling; but at the period of de velopment, when the experiment was tried, the difference was not remarkable in regard to either point ; nevertheless it is only proper to take notice of it. By prolonging the period during which the adults were exposed to the cooling process, the advantages they derive from their greater size and closer plumage may be counterbalanced or compensated. It is es sential to observe that in the course of the first hour the adult bird had only lost temperature in the proportion of one-fifth of that lost by the young birds, which obviously bears no ratio to the difference in point of size, plumage, &c. And then, the operation of the cold being con tinued, the adult suffered no further depres sion of temperature : it fell three degrees cen tigrade, and then became stationary. We can not, therefore, ascribe the entire difference in the cooling to that of the physical conditions of size and plumage ; a difference of constitution must go for a great deal ; there are inherent diversities of constitution, favourable or the re verse, to the production of heat. The truth of this conclusion appears much more clearly if we continue to subject young birds to the kind of experiment at successive epochs not so close to one another. The rapid progress they make in the power of evolving heat is, indeed, a very remarkable fact. A few days later, and they lose temperature in a much less considera ble degree when exposed to cold under the same circumstances, although there was little or po apparent difference in the external appear ance of the birds. And this is a new and con vincing proof that the inequality in the disposi tion to lose heat obvious at different periods of life under exposure to a low external tempe rature, is principally owing to inherent inequa lity in the faculty of producing caloric.