RESPIRATION. 



binatton, but it gradually paffes, in the extreme veffels, into 

 more intimate combination with carbon, forming carbonic 

 acid, in confequence of which the blood pafles to the venous 

 {late ; and from this venous blood the carbonic acid is dif- 

 engaged in the lungs, and a new portion of oxygen ab- 

 forbed. 



Thefe two hypothefes have divided the opinions of phy- 

 fiologifts. They are both, however, defective : their prin- 

 ciples are not proved, and they involve fuppolitions incom- 

 patible with the laws which appear to regulate the chemical 

 actions that proceed in the animal fyftem. No proof is 

 given, in the fyftem of Crawford, of hydro-carbon being 

 communicated to the blood in the extreme veffels ; nor is it 

 eafy to imagine any fource whence this principle in an lnfu- 

 lated ftate can be derived ; for, although it has been imagined 

 by Crawford, that it may have its origin in the fohd parts of 

 the fyftem being ablorbed, this is refuted by the confidera- 

 tio'ns, that this abforption is performed not by the veins, but 

 by the lymphatics ; that it is not fufficiently uniform, nor 

 limited to carbon and hydrogen ; that to whatever extent it 

 may be carried, the blood mull, in a ftate of health, depofit 

 as much as is removed ; and that there is no evident cauie by 

 which the carbon and hydrogen can be feparated from the 

 other elements, and be brought into binary combination. 

 And, in the theory of Haifenfratz, though it were granted, 

 that a portion of oxygen is abiorbed by arterial blood, there 

 is no proof that this is combined merely with carbon, and 

 that carbonic acid, the refult of this combination, is con- 

 tained in venous blood. We have even proof that the 

 latter fuppofition cannot be juft ; for, when arterial blood 

 is expofed to carbonic acid gas until its colour is darkened, 

 it does not recover its florid hue from fubfequent expofure 

 to oxygen (Priellley, Experiments on Air, vol. iii. p. 363. 

 365.I, and is therefore not venous blood. 



Neither are the changes which thefe hypothefes fuppofe, 

 analogous to the ufual chemical operations of the animal 

 fyftem, or fufficiently connected with the purpofes which 

 the blood ferves in its circulation. They both fuppofe, that 

 the changes which the blood undergoes, depend not on 

 alterations in its compofition, ftrictly fpeaking, but on the 

 alternate communication and abilraction of a principle held 

 by it in a ftate of folution, and which appears to ferve no 

 purpofe in the animal economy, but is affumed merely to 

 account for the phenomena of refpiration. But when the 

 general facility of combination in the principles of animal 

 matter, and the tendency which the actions of the veftels 

 have to form them into ternary or quaternary compounds, 

 are confidered, there is little probability in the fuppofition of 

 the one hypothefis, — that oxygen fliould be abforbed by the 

 blood in the lungs, without immediately altering its com- 

 pofition ; that, without being attracted by any ot the other 

 principles of the blood, or influenced by the other chemical 

 changes going on in the fyftem, it fhould be merely com- 

 bined with carbon, in the proportion neceflary to form car- 

 bonic acid j and that this carbonic acid, without affecting 

 the ultimate compoiition of the blood, fhould be carried the 

 whole length of the venous circulation, and thrown out at 

 the lungs ; or in thofe of the other, that carbon and hydro- 

 gen fhould be brought into a ftate of binary combination in 

 the extreme veffels, and Ihould be held merely diffolved by 

 the venous blood, until acted on by the oxygen of the air in 

 refpiration. 



Still lefs are thefe changes connected with the known 

 changes which the blood fuflfers ; for no relation is traced 

 between the procelies of affimilation or of fecretion, and the 

 fuppofed communication of hydro-carbon, or the combina- 

 tion of carbon and oxygen, in the extreme veffels. In the 



theory of refpiration, the converfion of arterial into venous 

 blood ought to be confidered as connected with thefe pro- 

 ceffes ; and this converfion, as well as that of venous into 

 arterial blood, muft be regarded as arifing from changes in 

 the ultimate compofition of the known proximate principles 

 of the blood, and not from the alternate communication 

 and abftraction of a principle which it holds diflblved in it, 

 or in what is termed a ftate of loofe combination. Accord- 

 ing to this view of the fubject, Mr. Murray gives the fol- 

 lowing explanation of thefe phenomena. 



The blood is the fource whence the animal products are 

 formed. Its expenditure is fupplied by the chyle, a fluid 

 lefs completely animalized than the blood itfelf. The pecu- 

 liar character of animal matter, with regard to compofition, 

 is a large proportion of nitrogen, and a diminifhed propor- 

 tion of carbon. It may therefore be inferred, that in the 

 extreme veftels, where the animal folids and fluids are formed, 

 the general procefs will be the feparation from the blood of 

 thofe elements of which animal matter is compofed ; and 

 that, of courfe, carbon, which enters more fparingly into 

 its compofition, will exift in the remaining blood in an in- 

 creafed proportion. This is accordingly the general nature 

 of the converfion of arterial into venous blood. Nitrogen, 

 hydrogen, and other elements, are fpent in the formation of 

 new products, and the proximate principles of the blood, 

 probably the craftamentum chiefly, remain with an increafed 

 proportion of carbon. In this ftate it is expofed to the 

 atmofpheric air in the lungs, the oxygen of which abftra&s 

 its excefs of carbon, and forms the carbonic acid expired, 

 and this conllitutes the converfion of venous into arterial 

 blood. 



There is little reafon to fuppofe, that any combination of 

 the oxygen of the air with the hydregen of the blood takes 

 place. The fuppofition that it does, and that this is the 

 fource of the watery vapours expired, originated in the hy- 

 pothefis of Crawford, which fuppofed hydro-carbon to be 

 difengaged from venous blood. No fact has been ftated in 

 its fupport ; it is a combination which can apparently ferve 

 no purpofe in the animal economy ; for hydrogen exifts in 

 as large a proportion, (and even in a larger,) in animal as 

 in vegetable matter. And the degree of evaporation from a 

 moift furface, fo extenfive as that of the internal furface of 

 the lungs, at the temperature of 96 , is adequate to account 

 for the whole of the watery vapour expired. 



The converfion of arterial into venous blood, is thus con- 

 fidered in connection with the other chemical changes going 

 on in the fyftem, and is fubfervient to them. In the extreme 

 veftels, the condiment principles of the blood are expended 

 in the nourifhment of the fohd fibre ; in the formation of the 

 lecreted fluids ; and in the fupport perhaps of the living 

 powers. Of thefe principles, carbon is that contained in the 

 fmalleft proportion in the folids and fluids ; it is, therefore, 

 that of which there is the leaft expenditure, and confe- 

 quently it muft be prefent in a larger proportion in the 

 blood, after it has undergone thefe changes. To preferve 

 the due proportion, and prevent it from accumulating, it 

 muft hi difcharged by fome other procefs. Hence the ne- 

 ceflity of the application of oxygen to the blood in the lungs, 

 and the origin of the carbonic acid which is uniformity dif. 

 charged. We thus, too, trace the procefs of animalization 

 from the reception of the aliment to its completion. All 

 animals live directly or indirectly on vegetable matter. The 

 principal difference in the compofition of vegetable from 

 that of animal fubltances, is in the former containing a larger 

 proportion of carbon. Refpiration is the function by which 

 this difference is eftablifhed. The aliment received into the 

 ftomach, isfoon formed into a fluid capable of aflimilating 

 2 1 with 



