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constitutes the fifth chapter. In this difficult inquiry the 

 author thus expresses himself: 



" It is generally admitted that the cellular surface of the 

 lungs is furnished with exhalant vessels. These vessels, 

 like those of the skin and intestinesj^appear to be endued 

 with a power, not only of exhaling water, but likev.'ise of 

 emitting carbon ; for water and carbonic acid are expelled 

 from the lungs in respiration in the same manner as they 

 are produced by the skin when in contact with atmospheric 

 air. As, therefore, the products of respiration aad perspira- 

 tion are in kind precisely similar, we arc justified in ascrib- 

 ing their formation to similar laws ; and since it seems to 

 have been demonstrated, by direct experiment, that no tran- 

 spiration of aeriform fluids takes place through the skin, we 

 may presume that none is able to be carried on through the 

 ceils and blood-vessels of the lungs. Not only is the cel- 

 lular substance of the lungs furnished with absorbent and 

 exhalant vessels like that of the skin, but it is supplied from 

 within by the same blood, and exposed from without on the 

 saine atmospheric air. It has been shown also, that the co- 

 lourless fluids of various animals are able to efiect the same 

 change on the air as that which is produced by the blood ; 

 and that the serum of the blood itself (which is especially 

 destined to supply the exhalant function) produces on the 

 air the same identical change as it experiences in the lungs ; 

 a.11 which circumstances strongly incline us to suppose that 

 the function of the lunsrs resembles in kind that of the skin. 

 The proofs likewise already adduced, that the carbon fur- 

 nished by vegetables and by the inferior animals, as well 

 those which perspire by the skin as those which breathe by 

 lungs, depends wholly on the due circulation of their fluids, 

 and is, consequently, the result of a living action, are strong 

 presumptive evidence, that the same law obtains in the su- 

 perior animals and in man ; and seem to authorize the con- 

 clusion, that the carbon supplied in human respiration is 

 truly an animal excretion carried on by the exhalant vessels 

 of the lungs; and therefore that it primarily depends, like 

 other excretions, on the due circulation and distribution ol 

 the blood, and is more or less ailccted l)y all its variations. 

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