35 G PHYSIOLOGY. 
oxygen with carbon might take place in the courfe of the 
circulation ; (Mem. de 1 ’Acad, des Sciences, 1777, p. 191.) 
that the oxygen which difappears in refpiration may beab- 
forbed by the blood, while carbonic acid may be given 
out fully formed. This hypotbefis was afterwards endea¬ 
voured to be eftabliffied by Haffenfratz and Lagrange ; 
(Annales de Cliimie, tom. ix. p. 261.) They obferve, 
that 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 aflumes the 
dark purple colour. They conclude, therefore, that the 
florid red colour of the blood is the refult of the abforption 
of oxygen, while the dark venous colour arifes from the 
intimate combination of that oxygen with a portion of 
the carbon and hydrogen which the blood contains. 
According to this theory, oxygen is abforbed by the 
blood in the lungs, remains in the arterial blood for a 
time in a ftate of lolution, or loofe combination, but it 
gradually pafl'es, in the extreme vellels, into more in¬ 
timate combination with carbon, forming carbonic acid, 
in confequence of which the blood pafl'es to the venous 
ftate; and from this venous blood the carbonic acid is dis¬ 
engaged in the lungs, and a new portion of oxygen ab¬ 
forbed. 
Thefe two hypothefes have divided the opinions of 
phyfiologifts. They are both, however, defedive : their 
principles are not proved, and they involve fuppofitions 
incompatible with the laws which appear to regulate the 
chemical adions 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 
veflels ; nor is it eafy to imagine any fource whence this 
principle in an infulated ftate can be derived ; for, 
although it has been imagined by Crawford, that it may 
have its origin in the folid parts of the fyftem being ab¬ 
forbed, this is refuted by the confiderations, that this 
abforption is performed not by the veins, but by the lym¬ 
phatics ; 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 caufe 
by which the carbon and hydrogen can be feparated from 
the other elements, and be brought into binary combi¬ 
nation. And, in the theory of Haffenfratz, though it 
were granted that a portion of oxygen is abforbed by 
arterial blood, there is no proof that this is combined 
merely with carbon, and that carbonic acid, the refult of 
this combination, is contained 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,(Prieftley’s 
Experiments on Air, vol.iii. p.363. 365.) and is therefore 
not venous blood. 
Neither are thechanges which thefe hypothefes fuppofe, 
analogous to the ufual chemical operations of the animal 
fyftem, or fufficiently conne&ed 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, ftri6bIy fpeaking, but on 
the alternate communication and abftra&ion of a prin¬ 
ciple held by it in a ftate of folution, and which appearr 
to ferve no pnrpofe in the animal economy, but is alfumed 
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 
afiions of the veflels have to form them into ternary or 
quaternary compounds, are confidered, there is little pro¬ 
bability in the fuppofition of the one hypotbefis, that 
oxygen fiiould be abforbed b)r the blood in the lungs, 
without immediately altering its compofition; that, 
without being attrafted by any of the other principles of 
the blood, or influenced by the other chemical changes 
going on in the fyftem, it Ihould be merely combined 
with carbon, in the proportion neceffary to form carbonic 
acid ; and that this carbonic acid, without affefling the 
ultimate compofition of the blood, ffiould be carried the 
whole length of the venous circulation, and thrown out 
at the lungs; or, in tbofe of the other, that carbon and 
hydrogen ffiould be brought into a ftate of binary com¬ 
bination in the extreme veflels, and ffiould be held merely 
diflolved by the venous blood, until a£led on by the 
oxygen of the air in refpiration. 
Still lefs are thefe changes conne&ed with the known 
changes which the blood fuffers ; for no relation is traced 
between the procefles of affimilation or of fecretion, and 
the fuppofed communication of hydro-carbon, or the 
combination of carbon and oxygen, in the extreme veflels. 
In the theory of refpiration, the converfion of arterial 
into venous blood ought to be confidered as connedfted 
with thefe procefles ; and this converfion, as well as that 
of venous into arterial blood, mull 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 abftraftion of a principle 
which it holds diflolved in it, or in what is termed a 
ftate of loofe combination. According to this view of 
the fubjedi, Mr. Murray gives the following 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 peculiar chara&er of animal matter, with regard to 
compofition, is a large proportion of nitrogen, and a 
diminifhed proportion of carbon. It may therefore be 
inferred, that in the extreme veflels, 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 increafed 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 
produdls, and the proximate principles of the blood, 
probably the craffamentum chiefly, remain with an in¬ 
creafed proportion of carbon. In this ftate it is expofed 
to the atmofpheric air in the lungs, the oxygen of which 
abftrafts its excefs of carbon, and forms the carbonic acid 
expired, and this conftitutes the converfion of venous 
into arterial blood. 
There is little reafon to fuppofe, that any combination 
of the oxygen of the air with the hydrogen of the blood 
takes place. The fuppofition that it does, and that this 
is the fource of the watery vapours expired, originated 
in the hypothefis of Crawford, which fuppofed hydro¬ 
carbon to be difengaged from venous blood. No faft 
has been ftated in its lupport; 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 tem¬ 
perature of 96°, is adequate to account for the whole of 
the watery vapour expired. 
The converfion of arterial into venous blood, is thus 
confidered in connection with the other chemical changes 
going on in the fyftem, and is fubfervient to them. In 
the extreme veflels, the conftituent principles of the 
blood are expended in the nourifliment of the folid 
fibre ; in the formation of the fecreted 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 there¬ 
fore that of which there is the leaft expenditure, and 
confequently it mull be prefent in a larger proportion 
in the blood after it has undergone thefe changes. To 
preferve the due proportion, and prevent it from accu¬ 
mulating, it muft be difeharged by fome other procefs. 
, Hence 
