INFLUENCE OF SEASONS TN TUE TION OF ANIMAL IIEAT.
The temperature of an animal is the result, 1st, of the heat which it produces ; 2d, of that which it receives ; 3d, of that which It loses. The proportion of heat which is lost depends on two principal conditions, the relatively colder temperature of the atmosphere, and the amount of evaporation that takes place from the surface of the animal. In cold and tem perate climates these two conditions of cooling are in inverse relations to one another in the opposite seasons of winter and summer. In winter the temperature of the air is lower summer the amount of evaporation greater. These two conditions of refrigeration, therefore, tend to compensate one another, and conse quently to maintain the equilibrium of tempe rature as regards the body in the two opposite seasons. They have unquestionably a consi derable share in this business; and it was long believed that the simple difference indicated in the external conditions sufficed to preserve the temperature of the body alike during the two periods. But in reflecting on the phenomena presented by cold-blooded animals with the changes of the seasons, which have already been spoken of at length, we find such an opi nion or view to be inadmissible. For in ex amining those species of cold-blooded animals which from their structure are liable to lose more by evaporation than any other animal, we see that no such compensation takes place. Frogs, for example, the skin of which is so soft and permeable, and whose bodies besides are so succulent that they must be presumed in the most favourable circumstances to sustain loss by evaporation, ought to preserve the same temperature in winter and in summer if the low temperature in winter were compensated by the excess of evaporation in proportion as the heat of the season augments. But we know that the temperature of these creatures follows, to a very great extent, that of the at mosphere, between 0° and c. (32° and 77° F.), dtficring at no time from it by more than degree or two. The phenomenon here is sim ple, by reason of the slight evolution of caloric by the frog, and leaves no doubt upon the mind. We must, therefore, have recourse to other conditions, in order to explain the slight difference that is observed in the summer and winter temperature of man and other warm blooded animals. Since external conditions do not appear to explain the phenomena, it must undoubtedly mainly depend once rtain changes effected in the animal itself, Now, since the internal conditions which influence the temperature of the body are those also that regulate the production of heat, it is here that the change must be effected.
It is obvious that the cause of refrigeration in winter being more active, to meet the greater expenditure there must be the means provided for furnishing a larger supply—the calorific faculty must be more active in winter than in summer. The inverse of this takes place in summer; so that the temperature of the body in the two seasons is determined in the follow ing manner :—in winter there is a more (+dire production with a greater lass; in summer a less roduclion, with a smaller loss of heat. In this way is there compensation, and a perfect equilibrium maintained at all seasons. To render this relation more evident, it may be expressed in another manner ; as, for example, in summer the body receives more heat from without, and produces less; in winter it receives less and produces more.
These considerations carry us farther. As this difference in the production of heat lasts as long as the various seasons, and takes place progressively, it is to be presumed that it be longs to an intimate and more or less en during change effected in the state of the body. In other words, the constitution alters, and the faculty of producing heat changes in the same degree. The fact thus expressed is immediately susceptible of an interesting ap plication. If the faculty of producing heat is less in summer, the temperature of the body will not he maintained to the same point in the two seasons under sudden exposure to the same degree of cold. By subjecting animals to the test of experiment in the two seasons, it is easy to judge of the justice of the preceding deductions, as well as of the principles which led to them. To have the mode of refrigera tion precisely the same, attention must be had not merely to the thermometric temperature of the air, but also to its humidity, which ought to be the same in both instances. A difference in the hygrometric state of the air will certainly produce a difference in the effects of refrigera tion. The apparatus employed consisted of earthen vessels plunged amidstaquantity of melt ing ice. Air thus cooled soon reaches the point of extreme humidity. The air being at zero c. F.), the animal is introduced, placed upon a stage of gauze to prevent its coming in con tact with the moist and rapidly conducting surface of the vessel. A cover, also piled over
with ice, is then placed over the apparatus, but so arranged as still to permit the ready reno vation of the air contained in the interior. Still farther to secure the purity of the included air, a solution of potash, which of course ab sorbed the carbonic acid produced with avidity, occupied the bottom of the vessel. In winter, in the month of February, the experiment was made at the same time upon five adult spar rows, which were all included in the apparatus. At the end of an hour they were found one with another to have lost no more than 4 c., or less than half a degree ; some of them having suffered no depression of temperature whatsoever, others having lost as much as, but none more than, 1° c. The temperature of the whole then remained stationary to the end of the experiment, which was continued for three hours. In the month of July the same expe riment was performed upon four full-grown or adult sparrows. The temperature of these birds at the end of an hour had undergone a depression, the mean term of which was 3°, 62, and the extremes 5 and 2° c. At the end of the third hour the mean term of the refrigeration suffered was the extremes being 12° and 3°, 5 c. It ought to have been stated that in the experiment in the winter month, the birds had been for some time kept in a warm room, so that the sudden transition was the same in both instances, in the winter as well as the summer experiment. The diver sity in the constitution of these birds, conse quently, with reference to the powers of pro ducing heat, was an effect of the difference of the seasons. Each month the temperature of which differs in any degree from that of the month before or after it, has an obvious ten dency to modify the temperament or constitu tion in the manner which has been indicated. In summer we may presume, nay we may be certain, that the differences obtain in degree according to the mean intensity of the heat proper to each. This is even to be proved by direct experiment. The month of August, as commonly happens, was not so hot as the month of July, and six sparrows treated in the same manner as those that were the subject of the July experiment already detailed, were found not to suffer refrigeration to the same extent. After the lapse of an hour the mean temperature of the six had sunk 62, and after three hours 4°, 87 c., from which it is ob vious that with the successive declensions of the external temperature the faculty of engendering heat increases. This is demonstrated by the experiments quoted. The animals that were the subjects employed suffered a relatively less degree of refrigeration in the cooler month than they had done in the hotter, when exposed to the same measure of cold. In the first set of experiments performed in one of the coldest months of the year, the power of resisting cold was made particularly manifest. The sparrows, kept for three hours in an atmosphere at the temperature at which ice melts, scarcely suffered any loss of heat at all. The results of the three series of experiments detailed confirm, in every particular, the conclusions which had been come to analogically and a priori. They do more than this. They bear out equally the principles which had been deduced with refe rence to the constitutions more or less favoura ble to the production of heat. It is apparent, in the first place, that the influence of the summer and that of the winter act on the con stitution in the same manner as the two opposed periods of early youth and adult age. Let us therefore inquire in what manner these different conditions tend to produce analogous effects. We have seen that the constitution of early life differed from that of adult age, especially in the inferior energy of the functions of innerva tion and nutrition. Now this is that which constitutes or causes the principal difference between the winter and summer constitution of man. We generally feel ourselves weaker in summer than in winter, and our digestive powers are then also decidedly less vigorous. What completes the analogy is that the motions of circulation and respiration are accelerated in summer; and as a complement of the whole of these data, the temperature is somewhat higher in summer ; just as we have seen that there is an epoch in youth when the tem perature exceeds that which is proper to com plete manhood. Thus, the parity between the constitution of youth (in the second period of childhood,) and that of the body in sum mer, contrasted with the constitution of the adult age and that of the body in winter, exists in the three following relations i—lst, a lessened faculty of producing heat ; 2d, greater activity in the motions of circulation and respiration ; 3d, a higher temperature of the body.