357 
PHYSIOLOGY. 
Hence the neceffity of the application of oxygen to the 
blood in the kings, and the origin of the carbonic acid' 
which is uniformly difcharged. We thus, too, trace the 
procefs of animalization from the reception of the aliment 
to its completion. All inimals live direftly or indirectly 
on vegetable matter. The principal difference in the 
composition of vegetable from that of animal fubftances, 
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, is 
foon formed into a fluid capable of affimilating with the 
blood. It is convej ed to the lungs, and lofes part of its 
carbon, or is partially animalized. It is then diftributed 
through the fyftem, and, in the extreme veflels, along 
with carbon, parts with fo much hydrogen, oxygen, 
nitrogen, and other elements, as to leave carbon pre¬ 
dominant. 
It might be fuppofed, that in any view, fuch as that 
which has been now given, there muff be forne difficulty 
in conceiving that oxygen gas ffiould combine with 
carbon with fo much facility, at a temperature much 
lower than that which is in general neceffary for their 
union, and this, too, with the intervention of the coats 
of the veflels through which the blood circulates. On 
attending, however, to the objection, it will be found to 
have no real force. Alth >ugh carbon, in its folid and 
Snfulated form, requires to be railed to a high tempe¬ 
rature to caufe it to combine with oxygen, yet, when it 
makes part of a ternary or quaternary combination, in 
which ffate its cohefion no longeroppofes the combination, 
it is abftrafted, and combined with oxygen at any natural 
temperature. It is thus that many vegetable and 
animal fubftances, when humid, are altered by expofure 
to the air, and carbonic acid formed. Blood itfelf is 
afted upon in this manner. It fuffers a change precifeiy 
fimilar to that which it undergoes in the lungs, and 
this more or lefs rapidly, and to a greater or lefs extent, 
according to the quantity of oxygen prefent, and the 
degree of agitation ufed. Arterial blood was expofed by 
Fontana to atmofpheric air for three minutes, when no 
perceptible alteration was occafioned in the purity of the 
air: they were then agitated together for three minutes: 
the volume of air was dimiuifhed, and its purity impaired. 
When oxygen gas was fubfiituted for atmofpheric air, 
the alteration was ftil! more confiderable, its purity being 
diminiffied when agitation was avoided; and, when it was 
agitated, the diminution in purity and volume was ftill 
greater. In all thefe experiments, carbonic acid was 
alfo produced. (Opufcules Phyfiques, p. 334., 5.) They 
therefore prove, that oxygen can attract carbon from 
arterial blood. With venous blood, the formation of 
carbonic acid is, according to Luzuriaga, ftill greater. 
(Diflertatio Inauguralis, p. 53, 4.) If, therefore, oxygen 
can abftraft carbon from, the principles of the blood 
under fuch circumftances, it is evident it muff do fo ftill 
more rapidly during refpiration, where the circumftances 
are to much more favourable, where there is compa¬ 
ratively a high and uniform temperature kept up, where 
the blood is expofed on an extenfive furface, and in a 
ftate of extreme divifion, and where that furface, as well 
as the air itfelf, are rapidly renewed. 
Nor can it be fuppofed, that the thin membrane which 
forms the coats of the veflels through which the blood 
circulates can oppofean obftacle to this reciprocal aftion. 
Every humid fubftance is permeable through its whole 
fubftance to elaftic fluids,' and is penetrated by them. 
Animal membrane, in a much denfer ftate than that 
which forms the coats of the extreme blood-veflels, is, 
when humid, pervious to gafeous fluids; and what is in 
point in the prefent cafe, through fuch membranes, 
when humid, oxygen can aft on blood, and communicate 
to it the florid colour, the fame as when blood is freely 
expofed to it. Thus, Prieftley found by experiment, 
that if a quantity of black blood were inclofed in a 
moiftened bladder, which was tied very clofe, on hanging 
Vol. XX. No. 1374. 
it in a free expofure to the air, it acquired “ a coating of 
a florid colour, as thick as it would have acquired if it 
had been expofed to the open air ; fo that this membrane 
had been no impediment to the aftion of the air on the 
blood.” Mr. Hunter mentions a fimilar experiment: 
“ I covered,” fays he, <e the mouths of veflels filled with 
venous blood with gold-beaters’ fkin, touching the 
furface of the blood, and the blood conftantly became of 
a florid red on the furface, and even for fome depth.” 
(Treatife on the Blood, p. 62.) Nor is there any reafon 
to believe, as has been contended, that in the living folids 
this property is fufpended, for it is one connefted merely 
with mechanical ftrufture, and the influence of humidity. 
The blood, therefore, may lie confidered, when circu¬ 
lating in veflels fo fine, as expofed to the aftion of oxygen 
nearly as if no membrane were interpofed : a part of that 
oxygen approximated to it will combine with a portion 
of its carbon ; and the carbonic acid, when formed, will, 
from its elafticity, recede and be difcharged. The whole 
aftion is purely chemical, and precifeiy the fame as that 
which is exerted between air and blood out of the body, 
favoured only by the circumftances of expofure, tempe¬ 
rature, and agitation, under which it takes place. Idem, 
p. 502. 
After adverting to the arguments by which it is fhofvn 
that no gas can pafs through the membrane? of the 
bronchial cells and pulmonary veflels from without, nor 
any fubftance pafs from within through thefe parts, to 
unite in the lungs by ordinary chemical affinity, confe- 
quently that the carbonic acid is not formed by the- 
union of carbon and oxygen in the blood ; and to the 
fafts, in which carbonic acid is formed in the lower 
animals and even in man, by the fkin, wdiere we have no 
proof of any abforption of oxygen, or paffage of it through 
the animal textures ; Mr. Ellis (in that excellent work 
fo often referred to by us) declares his opinion, that the 
carbon fuppiied in human refpiration is truly an animal 
excretion, performed by the exhalent veflels, which 
exift in fuch wonderful numbers in the lungs; confe- 
quently, that it depends primarily, like other excretions, 
on the due circulation and diftribution of the blood, and 
is more or lefs aft’efted by all its variations. The changes 
in its quantity have been found very confiderable, ac¬ 
cording as the individual was at reft or in motion, faffing 
or with a full ftomach, &c. in the experiments already 
noticed by Lavoifier and Seguin. He conceives, then, 
that the carbon, thus exhaled or excreted in the lungs; 
combines in the air-cells with the oxygen of the atmof- 
phere to form the carbonic acid expired. 
As the emiffion of carbonic acid in refpiration is carried 
on through the whole period of living aftion, and is ef- 
fential to the continuance of it, fome ulterior fource 
muff be provided, from whence its fupply may be duly 
maintained. To the organs of digeftion, aflimilation, and 
fecretion, only, we are enabled to trace it; but the mode 
in which it is reduced to that ftate in which it is after¬ 
wards expelled by the furfaces of the lungs, and fkin, 
involves a knowlege of the nature and qualities of our 
food ; of the various and fucceflive changes which it is 
made to undergo in the fyftem ; and of its diftribution by 
the blood to the different organs of fecretion, according 
to the feveral ufes which it is afterwards deftined to 
anfvver : concerning all of which fubjefts, we have of 
late fucceeded in getting rid of much error and abfur- 
dity, but have not in any inftance attained to complete 
knowledge. 
In the fame fituation that the change of colour is 
acquired by the blood, is another phenomenon difplayed ; 
viz. the prodtiftion of animal heat. That this funftion 
is connefted with refpiration requires, we think, no par¬ 
ticular proof. It is well known, that thofeanimals which 
confume moift air during refpiration, have the higheft 
temperature. Birds in particular have the molt extenfive 
breathing-organs 5 and the temperature of thefe animals 
is higher than that of any other clafs. The refpiration of 
4 Y reptiles,. 
