INSPIRATION. 



us expofure to the atmofpheric current in refpiration, ac- 

 cording to the ordinary operation of phylical laws : this, 

 therefore, feems the mod probable fource of the expired 



water. 



Lavoiiier conceived that the water is generated in the 

 lungs ; hydrogen being evolved from the venous blood, 

 and uniting with a part of the oxygen of the infpired air. 

 But this liberation of hydrogen is not fupportcd by a (ingle 

 proof, or ihadow of proof ; and we have every rcalon to 

 believe that all the oxygen confumed in breathing is em- 

 ployed to form carbonic acid. 



III. Changes produced in the Blood by Refpiration. — Soon 

 after the doctrine of the circulation had been generally re- 

 ceived, the diltindtion between arterial and venous blood was 

 pointed out, and it was underltood that this fluid. is changed 

 from the latter into the former (late in the lungs. Various 

 conjectures were reforted to, to explain the nature and 

 manner of this change. Some conddered the alteration to be 

 principally mechanical ; conceiving that the blood, while in 

 the pulmonary veflels, experiences a continual and violent 

 agitation, by means of which, its particles, before loofely 

 mixed, and confiding of feveral heterogeneous fubltances, are 

 communicated and perfectly united together, fo that the 

 whole mafs acquires an uniform confidence. Baglivi fup- 

 pofed, that the blood was rarefied, and Helvetius, that it 

 was condenfed in the lungs ; Boerhaave thought that its 

 particles acquired that peculiar organization, which he 

 deemed effential to the exidence of perfect blood. Other 

 philofophers, as Harvey, Boyle, Hales, and Haller, were of 

 opinion, that the blood emitted fome noxious or fuperfluous 

 matter in its paffage through the lungs. Some again 

 fuppofed, that the change from venous to arterial was caufed 

 by fomething imparted from the air to the blood. 



Thefc vague fpeculations were foon fuperfeded by the 

 more certain information deduced from experiment and ob- 

 fervation. We have already enumerated the proofs afforded 

 by the refearches of Lower, Cigna, Pricftley, Lavoifirr, 

 Davy, and others, that the change of the blood from ve- 

 nous to arterial is effected by expofure to the oxygenous 

 gas of the atmolphere in refpiration ; and that a fimilar 

 change, in colour at lead, is effected by fuch expofure 

 oiit of the body : the alteration being accompanied, in 

 both indancea, with a change in the compolition ol the air. 

 The proofs, that refpiration is the fource of the change in 

 colour jult alluded to, that it is (lopped in living ani- 

 mals when breathing is interrupted, and goes on again 

 when the refpiratory procefs is returned, will be found in the 

 article Linus. 



But is '. ' i i alteration in the properties of the 



blood confequcnt on refpiration, betides its change of co- 

 lour? Undoubtedly there mull be inch further alteration, 

 for life is quickly elided, if this converiion of venous into 

 arterial blond be Hopped ; and we (ball prefently ftate our 

 opinion, that carbon is exhaled from the pulmonary veflels ; 

 but chemidry has not yet (hewn us any difference ot com- 

 polition beween the two kinds of blood. 



" When (lays Mr. Murray) we examine chemically the 

 properties of arterial and venous blood, we find no other 

 difference between them than that of colour. They con- 

 tain the fame principles, and arc fubjecA to the fame changes 

 from chemical agents. The difference of colour, however, 

 points out fome difference in compolition, though it may be- 

 too (light to be difcovered by analyfie; and when we exa- 

 mine the phenomena of refpiration, which are intimately 

 j.onnc&ed with the change of venous to arterial blood, we 

 fmd, that fuch a difference mud cxiil," Syllem ol Clic- 

 inidiy, vol. iv. p. i8<j. 



Even the changes ot colour ot the blood are not welt 

 underltood. We know that this fluid is converted from 

 the fcarlet to the black date, in the courfe of its paffage 

 through the capillary veflels ot the body. But it under- 

 goes the fame change, when cxtravafated from a large artery, 

 or when confined in an arterial tube between two ligatures. 

 It never feems, however, to fuller the oppolite change, ex- 

 cept when in contact with oxygen gas, or with fome fub- 

 itanee capable of furnifhing it. 



That the paffage of the air through the lungs in refpira- 

 tion is inftrumental in converting the chyle and lymph, 

 poured from the thoracic duCt into the venous fyltem, into 

 blood ; or, that it produces fibrine from the materials jufl 

 mentioned, has been fuppofed and aiferted ; but without any 

 direct proof. We cannot, indeed, fee the chyle in the 

 blood after it has gone through the lungs ; but we know ot 

 nothing further than a mechanical admixture ; and are com- 

 pletely ignorant of the circumdances accompanying and de- 

 termining the generation of iibrine. 



IV. Theories of Refpiration. — The principal explanations 

 of the phenomena detailed in the preceding divifions of this 

 article, as exhibited in the air and blood, confequently to 

 refpiration, are detailed by Mr. Murray, in his Sydem of 

 Chemillry, vol. iv. ; where they are followed by his own view;; 

 of the fubjecl. 



Dr. Priedley, fays he, conlidered thefe phenomena as 

 owing to the difengagement of phlogifton from the blood in 

 the lungs, and its combination with the air (Philof. Traniafrt. 

 1776) ; a theory modified and rendered more compreheniive 

 by Crawford ( Experiments and Obfenations on Animal 

 Heat). Regarding hydrogen as the phlogilhc principle, 

 and fuppofing it to exiil in the blood in that date in which 

 it is difengaged from vegetable fubdances by heat, the heavy 

 inflammable air of the older chemills, the carburetted hy- 

 drogen of the modern nomenclature, he fuppofed that this 

 hydro-carbon, as it was named, is communicated to the 

 blood in the extreme veflels, by which the cpn'verfipn from 

 the arterial to the venous date is occasioned ; in the lungs, 

 he concluded, it is given out, and in its naiccnt date, or 

 its tranlition to the elatlic form, it combines, he fuppofed, 

 with the oxygen of the air, and forms the carbonic acid gas 

 and watery vapour expired, while the blood, deprived of its 

 hydro-carbon, returns to the arterial (late. The fame ex- 

 planation nearly was given by Lavoiiier ; at leall he ad- 

 vanced the ©pinion, that the carbonic acid gas and w; 

 vapour of the expired air are formed by the combinatii n - 

 carbon and hydrogen from the blood with oxygen in the 

 lungs. 



Lavoificr had alfo fuggefted, that the combination of 

 oxygen with carbon might take place in the courfe oi the 

 circulation (Mem. de l'Aead. des Sciences, 1777, p. 191.), 

 that the oxygen which difappears in refpiration may be ab- 

 forbed by the blood, while carbonic acid may be ivi n 

 fully formed. This hypotheflS wa9 afterwards emu. 

 voured to be eltablillied by Hallenfrat/. a 1.! Lagrang) 

 (Annates deChimie, torn. ix. p. 261.) The} obferve, th.a 

 venous blood expofed to oxygen acquires a vivid red colour, 

 which foon changes to a purple hue ; and that arterial blood 

 placed in vacuo, or in contact with any gas which does not 

 contain oxygen, quickly affntms the dark purple colour. 

 They conclude, therefore, that the florid red colour of the 



blood is the refult of the abforption ol' . xygen, while the 

 dark venous colour arifea from th intimate combination of 

 that oxygen with a portion of the caibon and hydrogen 



which the blood contains. According to this theory, oxy- 

 gen is ablorh-d by the blood in the lungs, remains m the 

 arterial blood for a' time in a Hate of folutiun, or loofe corn- 

 bin ll 



