428 ANIMAL HEAT. 



fluence of oxygen, and for extricating its carbonic acid ; thus in Birds 

 and Mammals, the hlood is distributed, in a minute capillary network, 

 on the walls of the pulmonary air-cells, the gaseous contents of which 

 are continually renewed ; and in Insects, the air is carried into every. 

 part of the body by the ramifying tracheae. We constantly find a pro- 

 portion between the amount of heat evolved, and that of carbonic acid 

 generated ; this is peculiarly evident in Insects, whose respiration and 

 calorification vary so .remarkably ( 123); but it is also proved by 

 comparing the amount of carbonic acid generated by warm-blooded ani- 

 mals, when the external temperature is low, and when more heat must 

 be evolved to keep the temperature of their bodies up to its proper stan- 

 dard, with that generated by the same animals in a warmer atmosphere, 

 when the proper animal heat is diminished in amount ( 691). 



765. The sources of the Carbonic Acid thrown off by the lungs, have 

 been already pointed out (CHAP, vin.) : it is partly derived from the 

 metamorphosis of the tissues ; but partly, in all but purely carnivorous 

 animals, more directly from the non-azotized portion of the food. The 

 precise mode in which the carbon thus supplied is united with the oxy- 

 gen derived from the atmosphere, is not yet known ; but it is certain that, 

 in whatever manner the combination may take place, a certain measure 

 of caloric must be generated. It appears, however, from various ex- 

 periments, that the whole quantity of caloric generated by an animal in 

 a given time, is greater than that which would be evolved by the com- 

 bustion of the carbon, included in the carbonic acid evolved during the 

 same time. Hence it is evident that other chemical processes occurring 

 within the body are concerned in the maintenance of the temperature ; 

 and it is not difficult to point to some of these. It is probable, in the first 

 place, that some of the Hydrogen of the food may be "burned off" by 

 union with the oxygen of the atmosphere, so as to form part of the 

 water which is exhaled from the lungs. Again, the sulphur and phos- 

 phorus of the food are converted, by oxygenation, into sulphuric and 

 phosphoric acids ; in which process, heat must be generated. In the 

 composition of urea, moreover, oxygen is present in much larger propor- 

 tion, than it is in the proteine-compounds by the metamorphosis of which 

 it is formed ; so that in its production too, caloric will be generated. In 

 fact it may be stated as a general truth, that the whole excess of the oxy- 

 gen absorbed, over that which is contained in the carbonic acid exhaled 

 ( 689), must be applied to purposes in the laboratory of the system, in 

 which caloric will be disengaged. Still, the amount of Carbonic Acid 

 exhaled must always be the measure of the chemical processes, by which 

 heat is generated in the body; because it is itself the result of the 

 chief of these processes (the union of carbon and oxygen), and because 

 the surplus amount of oxygen which is absorbed, and which is applied 

 to other purposes, is closely related to it. 



766. The power of maintaining a high independent temperature is 

 usually much less in young warm-blooded animals, than in adults. 

 There are considerable variations in this respect, however, amongst 

 different species ; for where the young animal is born in such an ad- 

 vanced condition, as to be thenceforth almost independent of parental 

 assistance, it is capable of maintaining its own temperature ; but where 



