286 On the Structure and Functions of the Organs of Respiration. 



referable to the principles of the mutual diffusion of gases, which have 

 been already noticed.* 



The maintenance of animal heat is a very important function, especially 

 in warm-blooded animals ; and we find the temperature of the body very 

 much dependent upon the activity of tlie respiratory power. Thus in the 

 mammalia, it is usually from 96° to lOl^, and in birds from 107° to \\\° ; 

 whilst in reptiles and fishes, the heat of the body is but a few degrees 

 higher than that of the surrounding medium. John Hunter observed that 

 snails and earth-worms maintained their temperature a little above that of 

 the air; whilst a thermometer introduced into a hive of bees stood at 82o, 

 when the air was at 40°. Part of the latter effect was, however, evidently 

 due to the number collected together. If we extend our observations to 

 hybernating animals, we shall find that their consumption of air lessens as 

 their temperature falls, and as the state of torpidity comes on ; and that if, 

 whilst in this state, they are mechanically stimulated to breathe, their 

 temperature rises in proportion to the rapidity of the respiration. It may 

 be observed as a general law, that the temperature of the various classes 

 of animals, and their vitiation of the air are always proportional ; and in- 

 versely to the length of time that they can live without oxygen. VVe find 

 also that in animals suddenly killed, the cooling of the body is much re- 

 tarded, if artificial respiration be carefully maintained. On the whole, 

 therefore, it may be concluded that the combination of the oxygen con- 

 tained in arterial blood, with the carbon whicli it meets with in tiie course 

 of the circulation, is the principal source of the heat developed in the 

 body J and it is probable that the influence of the nervous power in pro- 

 moting or retarding this combination, will be sufficient to explain those 

 phaenomena, which have caused some physiologists to consider the mainte- 

 nance of animal heat as entirely a vital function. It is remarkable that 

 carbon, which is thus constantly being exhaled from the body during life, 

 should be the substance which, in the spontaneous decomposition of the 

 body after death, is first rejected, uniting with the oxygen of the atmos- 

 phere ; and Spallanzani found that this process went on to a considerable 

 extent before decomposition was perceptible. He says moreover, that torpid 

 animals, whose respiration li-ad entirely ceased, also carbonated the air, 

 probably through the medium of the skin. 



In our examination of this important function, we cannot but be struck 

 with the simplicity and uniformity of the processes into which its appa- 

 rently complicated phaenomena may be resolved, and with the similarity of 

 the means by which they are carried into effect ; and the truth of the ge- 

 neral conclusion, that " unity of design and identity of operation pervade 

 the whole of nature," is thus forcibly impressed upon us; a conclusion that 

 clearly " points to one great and only cause of all things, arrayed in the 



* See page 219. 



