RESPIRATION. 



degree of heat but little above that of the medium in which 

 they live ; while the mammalia have a temperature confider- 

 ably higher ; and birds, whofe lungs bear the largelt propor- 

 tion to their bodies, are the warmeft of all animals. The 

 obfervation of thefe fads led naturally to the opinion, that 

 the temperature of animals was immediately conne&ed with 

 the funaions of the refpiratory organs ; and an abundance 

 of fafts, collected and arranged by Mr. Ellis, prove that 

 the fmall excefs of temperature, which not only the inferior 

 animals, but which vegetables alfo poffefs, is adually de- 

 rived from the decompofition of the air by thefe feveral 

 daffes of beings, fo long as living aftion continues. " No 

 explanation, however," fays Mr. Ellis, " of the mode in 

 which the air contributes to fultain animal heat, was at- 

 tempted, till after the great difcovery of latent heat by Dr. 

 Black. That excellent philofopher having already proved 

 that the change effected in the air by refpiration, confided 

 in the formation of carbonic acid, fimilar to what happens in 

 many examples of combuition, afcribed the production of 

 animal heat to the decompofition of the air in the lungs, 

 by which its latent heat was rendered fenfible, in the 

 fame manner as it is given out in combuilion. The 

 blood, in its paffage through the lungs, had, he con- 

 ceived, its temperature by this means railed ; and thus was 

 rendered capable of communicating heat to all parts of 

 the body, in the courfe of its circulation through the fyf- 

 tem. To this it was objeaed by Dr. Cullen, that, if true, 

 the temperature of the body ought to be greateft in the 

 lungs, and to diminifh gradually, as the diftance from the 

 lungs increafes, which is not according to faa. This diffi- 

 culty was removed by the ingenuity of Dr. Crawford, who, 

 by a happy extenfion of Dr. Black's doarine, maintained, 

 that the heat, liberated by the decompofition of the air in 

 the lungs, palled into the blood, and exiited in that fluid in 

 the form of latent, or, what is now termed, fpecific heat, in 

 confequence of which its temperature was not raifed ; and 

 that this heat, by other chemical changes, was given out by 

 the blood in a fenfible form during its circulation. 



" In what manner, then, does the air, breathed by the fupe- 

 rior animals, give out its heat, to fupport that high degree 

 of temperature above the furrounding medium, which they 

 all poffefs ? We have feen reafon to conclude, that the in- 

 fpired air is decompofed in the bronchial cells of the lungs, 

 and that all its oxygenous portion which difappears, is con- 

 verted into carbonic acid, by carbon emitted from the ex- 

 halent furface of thofe organs. During this gradual con- 

 verfion of the oxygen gas, a quantity of fpecific caloric, 

 much greater than what is neceifary to maintain the elaili- 

 city of the carbonic acid that is formed, is neceflarily fet 

 free ; and to this excefs of heat, thus conftantly liberated 

 in the lungs, by the decompofition of the air, do we look 

 as the fource of that fuperiority of temperature, above the 

 furrounding medium, which man and other animals, under 

 every viciffitude of climate, are enabled to exhibit and main- 

 tain. 



" But if a quantity of caloric be thus conftantly difengaged 

 in the lungs, it may be expeded that the blood, in its 

 tranfmiflion through thofe organs, fhould acquire a certain 

 portion of it. To afcertain this point, Dr. Crawford, 

 purfuing the difcoveries of Drs. Black and Irvine, mixed 

 together certain quantities of water, at the temperature of 

 53 , with feparate portions of arterial and venal blood ; and 

 then meafuring the heat of the mixture, at different fuc- 

 ceffive periods, till coagulation took place, he found that 

 the water containing arterial blo'.i preferved a fuperiority 

 ©f temperature over that mixed with venal blood ; and, from 



the refults of feveral trials, he concluded, that the fpecific 

 heat of the arterial blood of a dog, was to that of the 

 venal, as 114 to 100, and that of a fheep as 115 to 100, 

 or as ni to 10. (On Animal Heat, p. 279.) Thefe re- 

 fults derive confirmation from the experiments of Mr. Cole- 

 man, who, in order to difcover the relative fpecific heat 

 of arterial and venal blood, while yet retained in the fyf- 

 tem, ftrangled a cat, and immediately opened its cheft, 

 while the blood in the left ventricle was ftill florid. He 

 then introduced a thermometer, through an opening in the 

 pericardium on each fide of the heart, and it flood at 98 : 

 in the left ventricle the temperature was only 97 , and in 

 the right ventricle it was nearly 99". In fifteen minutes, 

 however, inftead of the right ventricle poHeffing two de- 

 grees of heat more than the left, it was found to have 

 four degrees lcfs. Mr. Aftley Cooper repeated this ex- 

 periment in different ways, and found invariably, that al- 

 though the venal blood was iuperior in temperature at 

 firft, yet before coagulation was complete, the arterial be- 

 came from three to fix degrees warmer. (On Sufpended 

 Refpiration, p. 42, et feq. ) Thefe fads afford clear and 

 decifive proof, that the fpecific heat of the arterial blood 

 exceeds that of the venal, and demonftrate, likewife, that 

 this excefs is obtained during the paffage of that fluid 

 through the lung;. 



« Admitting the lungs, then, to be the organs in which, 

 by a decompofition of the air, the blood, as it pafles 

 through them, obtains its heat, it is next required to fhew 

 the iufficiency of this decompofition, to fupply heat enough 

 for the maintenance of that fuperiority of temperature, 

 which the warmer blooded animals poffefs." (Inquiry, 

 p. 234 236.) For an account of the mode in which this 

 was explained by Dr. Crawford, fee Heat, minimal. 



Although this explanation of the produaion of animal 

 heat appears fimple and fatisfaaory, and has been held 

 adequate to account for the phenomena by fuch philofo- 

 phers as Black, Crawford, Lavoifier, and La Place, the 

 evidence 0.1 which it refts is not fo clear as to have com- 

 manded univerfal affent, or entirely fet afide objeaions. 

 It has, indeed, been generally allowed, that refpiration, and 

 the changes it produces in the air and animal fluids, areeflen- 

 tial conditions of the evolution of the caloric in animals ; 

 but it has been thought, that there are other circumftances, 

 hitherto perhaps not well underltood, which influence the 

 phenomena. 



In external appearance the blood is the fame in all the 

 veflels of the foetus ; is this any proof that its temperature 

 is owing to the converfion of oxygen gas into carbonic acid ? 



Is the uniformity of temperature in the higher animals, 

 under varying ftates of refpiration and circulation, and the 

 confumption of various quantities of oxygen, whether in 

 the fame or different individuals, confiftent with the theory ? 

 and can local variations of temperature be explained from it ? 



Some recent inveftigations of Mr. Brodie are particu- 

 larly calculated to increafe our doubts on the fubjeft. 

 Having pithtfd or decapitated animals, he kept up artificial 

 refpiration, and thus maintained their circulation. The 

 blood continued to be changed in the lungs from venous to 

 arterial, and from arterial to venous, in the general circu- 

 lation. 



The refpeaive colours of the two kinds of blood could 

 not be dittinguifhed from thofe which they exhibit in living 

 and healthy animals. Yet the temperature of an animal 

 thus heated, funk fafter than that of another fimply killed 

 and left to itfelf. The following table exhibits the refults 

 of fuch a comparative experiment. 



