662 



ANIMAL HEAT. 



taining the temperature of the air about them, 

 an inherent capacity to produce heat is apparent. 

 Did they evolve no caloric, they would fall 

 below the temperature of the air, in conse- 

 quence of the evaporation which goes on from 

 the surface of their bodies. They must of 

 necessity produce as much as is necessary to 

 repair the loss which takes place from this 

 cause. 



What we have said of animals is equally 

 applicable to vegetables. To explain the pro- 

 gression of the temperature of cold-blooded 

 animals, which we have exposed above, regard 

 must be had to the relation which connects the 

 quantity of vapour formed with the degree of 

 external temperature. Within moderate limits, 

 which may be styled temperate, the vapour 

 formed will be nearly as the degrees of tem- 

 perature of the air. But under higher tempera- 

 tures, evaporation will go on in a greater ratio 

 than that of the external temperature. Thus 

 when the air is cool or moderately warm, eva- 

 poration is trifling, and among the superior 

 classes of cold-blooded animals lieat enough is 

 produced to maintain their temperature above 

 that of the air. But when the air becomes 

 warmer, as in the height of summer, evapora- 

 tion and the cold which results from it increase 

 in a far greater ratio than the temperature of 

 the body, so that the body remains at a tem- 

 perature inferior to that of the air, and this 

 by so much the more as the external tempera- 

 ture rises higher. Twenty-five degrees is the 

 limit at which this change commences in regard 

 to cold-blooded animals. But it is obvious 

 that a higher degree must be necessary to ob- 

 serve such phenomena in man and the warm- 

 blooded tribes, inasmuch as the heat from 

 without is for a long time added to that pro- 

 duced internally, and which among the warm- 

 blooded tribes is so much greater in amount 

 than it is among the cold-blooded. 



Relations of the bulk of the body with 

 animal heat. If the temperature of the larger 

 animals be compared with that of the smaller, 

 it will be found that the former do not mark 

 so high a degree as the latter. In the elephant 

 and horse, for instance, no higher a temperature 

 than 37, 5 c. (100 F.) has been observed, 

 whilst in the rat and squirrel temperatures of 

 38, 8, and of 39, 4 (102 and 103 F.) have 

 been noted. To prove that the difference is 

 less owing to the order or species than to the 

 simple size, we shall contrast several animals 

 belonging to the same order, selecting the 

 ruminants. The temperature of the air being 

 the same, namely, 26 c. (79 F.), the tem- 

 perature of the ox was found to be 38, 9 

 (102 F.), whilst that of a castrated he-goat 

 was 39 5 (103 F.), and that of the she-goat 

 and sheep 40 (104 F.).* 



It is evident that smallness of size must in 

 itself be one of the conditions unfavourable to 

 height of temperature among animals, when 

 this is merely viewed in relation with the am- 

 bient medium. As the external temperature is 

 almost always lower than that of the bodies of 



* Vide Observ. of Dr. Davy. 



animals, the ambient medium tends to lower 

 their temperature ; and small bodies having a 

 more extensive surface in reference to their mass 

 than large bodies, small animals must have a 

 greater tendency to lose heat than larger animals. 

 But, on the other hand, the circulation and 

 respiratory motions generally increase in rapidity 

 in proportion to the smallness of size ; and we 

 have seen that acceleration of these motions 

 had an influence in keeping up the temperature. 

 With a small size of the body, consequently, 

 we find associated a higher activity of function 

 which tends to compensate the disadvantage 

 resulting from inferior size in reference to tem- 

 perature. In fact it constantly happens that 

 this higher activity more than compensates the 

 cooling disposition from inferiority of size, and 

 causes the balance to incline towards the side of 

 higher temperature. It must be apparent, 

 however, that there is no occasion for such a 

 preponderance always existing in the case of 

 small animals. And then we know that the 

 motions of circulation and of respiration cannot 

 be greatly ^accelerated without causing incon- 

 venience and even danger to health and life. 

 It follows that the external temperature being 

 liable to fall disproportionately low, small ani- 

 mals have not, under like disadvantageous cir- 

 cumstances, the same power as larger animals 

 of supporting their temperature. The relations 

 of size naturally lead us to consider those that 

 depend on age. 



Relations of age with animal heat. The 

 size of the body changes with the age. The 

 same relations between bulk of body and de- 

 velopment of heat ought therefore to be ex- 

 hibited in youth as compared with adult age. 

 In early life the greater rapidity of the motions 

 of circulation and respiration, all things else 

 being equal, ought to increase the heat. At 

 the same time the constitution differs in other 

 respects, and if these were unfavourable to the 

 evolution of heat, it would be impossible to 

 foresee the result of these two opposite ten- 

 dencies. Nevertheless it is probable, from 

 what we have seen to happen in warm-blooded 

 animals of different sizes, that there might 

 occur a period in early life when the heat would 

 be higher than in adult age. A confirmation 

 of this inference may be found in comparing the 

 different observations of Dr. Davy, who has 

 given a table of the temperatures of fifteen chil- 

 dren from four to fourteen years, the mean age 

 of the whole being nine years and nine months. 

 The mean temperature of the bodies of these 

 children was 38, 31(101 F.). But the mean 

 temperature of twenty-one adults was no higher 

 than 37, 82 (100 F.); a difference that seems 

 the more worthy of being confided in from the 

 temperature of the air having, at the time of 

 the observations, been more favourable for the 

 adults than for the children, this having, in re- 

 ference to the former, been 26 and 26, 7 

 (79 and 80 F.), whilst when the latter were 

 made the subjects of investigation, it was but 

 24 and 26 (75, 5 and 79 F.). 



It seems impossible, therefore, to doubt from 

 what precedes, that size is not an element 

 which has much influence in the particular 



