DEVELOPMENT OF HEAT. 681 



perature is almost entirely dependent upon that of the surrounding element, the respiration 

 is feeble, being carried on, for the most part, through the medium of water. In the warm- 

 blooded Verti-brata, however, which have the power of keeping up the heat of their bodies to 

 an elevated standard, even when that of the surrounding air is far beneath it, the quantity of 

 oxygen consumed is very large; and that required by Birds is more, in proportion to their size, 

 than that employed by Mammalia; as we should expect from the more elevated tempera- 

 ture of the former. In the class of Insects, we have a very remarkable illustration of the 

 same general fact. It appears, from the researches of Mr. Newport, that Insects, during 

 their larva and pupa states, and even in their perfect condition when at rest, are to be re- 

 garded as truly cold-blooded animals ; their temperature rising and falling with that of the 

 surrounding medium, and being at no time more than a degree or two above it. In a state 

 of activity, however, the temperature of the body attains a considerable elevation, fre- 

 quently as much as 10 or 15 above that of the air. It must be remembered that, owing 

 to their larger extent of surface in proportion to their bulk, small animals are cooled much 

 more rapidly than large ones; and the temperature of Insects would probably rise much 

 higher, if it were not for the loss they are thus continually experiencing, which is greatly 

 increased by the action of the wings. In one of Mr. N.'s experiments, a single Humble-bee, 

 in a state of violent excitement, communicated to three cubic inches of air as much as 4 

 of heat within five minutes; its own temperature being raised 7 in the same time. When 

 several individuals in a state of excitement, however, are clustered together, so that the loss 

 of heat is prevented, the elevation of temperature is much more considerable; thus a ther- 

 mometer introduced among seven "Nursing-Bees" stood at 92$, whilst the external air 

 was only 70 ; and the temperature of a hive was raised by disturbing it, during winter, 

 from 4S^ to 102, the temperature of the air being only 34 J at the time! In all these 

 instances, the amount of Oxygen consumed bears an exact proportion to that of the Heat 

 evolved. 



890. In the higher animals, as in the lower, exercise has a considerable 

 effect in producing an elevation of temperature ; and, that this is not merely 

 due to the acceleration of the circulation, is shown by the very curious fact, 

 that the exercise of a particular muscle will cause an increase in the heat 

 liberated from it, as shown by needles plunged in its substance, and connected 

 with the thermo-multiplier.* It may, indeed, be stated as a general proposi- 

 tion, applicable as well to different parts of the same being, as to different 

 individuals, that the development of Heat is proportional to the activity of the 

 molecular processes which constitute the functions of Nutrition, Secretion, 

 &c. ; increasing with their activity, and diminishing with their torpor. It is 

 very easy to explain, on this principle, the known influence of the Nervous 

 system on the calorific function : since, although the molecular changes in the 

 organized fabric are not dependent upon the agency of that system, they are 

 very much influenced by it; and thus we can readily understand how a state 

 of nervous excitement may produce an elevation of temperature, whilst a de- 

 pression of nervous power occasions a cooling of the body. The experi- 

 ments of Sir B. Brodie, Chossat, and others, in which a greater or less 

 portion of the nervous centres was removed, and the animal cooled notwith- 

 standing the maintenance of the circulation, by no means prove that the 

 Nervous system is directly concerned in the production of heat; since in all 

 such experiments, there is a gradual loss of those other vital powers, which 

 are concerned in the function of calorification. From the experiments of Dr. 

 W. Philip and Dr. Hastings, it appears that an animal whose nervous centres 

 have been removed, cools much faster when left to itself, than when Artifi- 

 cial Respiration is practised; and that, if the cooling have made much pro- 

 gress before the artificial respiration is caused to commence, the temperature 

 may be raised ; and this, too, in spite of the very imperfect manner in which 

 natural Respiration is replaced by movements artificially effected. 



891. That the maintenance of Animal Heat is due in part to those molecu- 

 lar changes, to which the Cutaneous Respiration is subservient, appears from 



torn. vi. 



See the experiments of MM. Becquerel and Breschet, in Ann. des Sci. Nat. N. S. Zool., 



