PHYSIOLOGY. 
plete change in its nature; and in this refpefl the pulmo¬ 
nary and general capillary fyftems are the feat of the two 
mod important phenomena of the animal economy ; viz. 
t Lie change of the blood from black to red, and vice verfa. 
Each circulation evidently prefents three obje&s for our 
examination : ill, the origin ; adly, the courfe ; 3dly, the 
termination of each kind of blood. In the firft and laft 
of thefe, the mechanical phenomena of the circulation, 
and the transformation of the blood, require our confi- 
deration; in the fecond, there are only the mechanical 
phenomena to be noticed. 
General mechanical Phenomena of the two Circulations.— 
A general view of this fubjecl exhibits to us ; ill, the 
red blood departing from the lungs, forming, as it pro¬ 
ceeds, lefs numerous and more confiderable columns, in 
proportion as it draws nearer to the heart, in the cavities 
of which it is feen in larger mafles, and, as it pafles from 
thefe to the general capillary fyftem, again dividing into 
fmaller and fmaller columns ; adly, the black blood, 
accumulated, as it leaves the general capillary fyftem to 
approach the heart, into fewer and larger columns, 
exifting in the largeft mafl'es in the cavities of the heart, 
and again fucceflfively feparated into fmaller and fmaller 
dreams as it arrives at the capillary of the lungs. Thus 
the two kinds of blood circulate on the two fides in 
ftreams fo much the fmaller, as they are further from the 
heart ; and they are feen in larger columns as they come 
nearer to this vifcus. Imagine, for each of the two 
circulations, two trees united by their trunks, and 
fending their branches, the one into the lungs, the other 
into all parts of the body. Each of the two fides of 
the heart is between thefe two trunks, which it ferves to 
unite together foas to form one general canal, according 
to the defcription already given. 
Authors have generally reprefented the arteries and 
veins as forming each, by the whole of its ramifications, 
a general cone, of which the bafis is in all parts of the 
body, and the apex at the heart. This mode of confider- 
ing the fubje< 5 t is authorifed by the fa< 5 t, that the fum of 
the branches exceeds in diameter the trunks from which 
they arife : in this point of view, each half of the heart is 
placed at the apices of two cones, which would, but for 
its intervention, be united together. The pulmonary 
veins reprefent one cone, and the aorta the other, for the 
red blood ; while, for the black, the two cones are formed 
by the venae cavas and coronary veins on one fide, and 
by the pulmonary artery on the other. In each circula¬ 
tion, one of the cones (that of the lungs) is remarkable 
for its fmall extent; the other (that correfponding to all 
parts of the body) is diitinguiftied by its wide expanfion. 
Placed between thefe two cones, each fide of the heart 
mull be regarded as an agent of impulfion, driving the 
blood in one cafe towards all parts of the body, in the 
other towards the lungs. If the two cones of each cir¬ 
culation communicated together by their apices, the fides 
of the veflels compofing them would be manifeftly infuffi- 
cient to keep up the motion from the bafe of one to that 
of the other, that is, from the general to the pulmonary 
capillary fyftem, and reciprocally from that of the lung 
to that of the body at large. The courfe is obvioufly too 
long, and the vital powers of the veflels too weak; hence 
the neceflity of the heart. This confequence renders 
necefl'ary another arrangement, which we proceed to 
explain. As the red blood is deftined to move through 
a much more extenfive fpace, from the heart to the 
general capillary fyftem, than is traverfed by the black 
blood in its paffage from the heart to the pulmonary ca¬ 
pillaries, that portion of the organ which propels the 
fluid in the former cafe, muft poffefs much greater power 
than is required for the purpofe of keeping up motion in 
the latter inftance. Nature has fulfilled this objefl, by 
'compofing the ventricle of the red-blooded circulation of 
a much greater number of fibres than are poffeffed by the 
other. The auricles, which merely'‘receive the blood, 
Vol.XX. No. 1374. 
361 
and tranfmit it into the ventricles, are nearly of the fame 
thicknefs on both fides of the heart. 
Hence we may conclude, that the office performed by 
the heart in both circulations, relates merely to the me¬ 
chanical phenomena of the blood’s motion ; and that, if 
it produced any eft'eft on the compolition of the fluid, it 
could only be done by means of the motion imparted to 
it: fecondly, that, if the courfe of the two circulations 
were lefs extenfive, this intermediate impelling agent 
would not be necefl'ary. This is precifely the cafe in the 
abdominal fyftem of black blood, where the two trees, of 
which one diftributes its branches in the gaftric vifcera, 
and the other in the liver, are united by their trunks at 
the finus of the vena portarum, which occupies the poii- 
tion of the heart in the large fyftem of black blood, and 
in that of red blood. It is therefore poffible to conceive, 
ift, how the heart may be deficient, of which there are 
fome inftances in which the two great circulating fyftems 
refembled that of the abdomen in fome meafure; adly, 
how' the blood may ofcillate for a confiderable time from 
one capillary fyftem to another, although the heart, 
difeafed and enfeebled, can hardly accelerate the current 
of this fluid ; 3dly, how, after the pulfations of the heart 
have been entirely fufpended in fyncope, afphyxia, &c. 
an ofcillatory movement ftill remains, a real progreflion 
of the blood from one capillary fyftem to the other, fince 
this fluid will ftill flow in fome degree from an opened 
artery or vein. This motion is in truth very feeble, and 
cannot laft long; but that it may exift, without the 
influence of the heart, is proved by the fadt of the black 
blood being conveyed, without any organ of impulfion, 
from the inteftines to the liver; and hence we infer, that 
the ceffation of the heart’s adlion does not prove, as fome 
authors have pretended, that the blood no longer moves; 
4-thly, we know that in feveral animals of the lower 
claffes there is no heart, although diftindl veflels and 
circulating fluids exift. 
General Uj'es of the red-blooded Circulation. — This 
furnifhes the materials of all the fecretions except the 
bile. From this fource alfo the exhalents belonging to 
the ferous, cellular, cutaneous, medullary, and other, 
fyftems, derive the fluids which they pour out on their 
refpedlive furfaces : all the veflels, which convey the means 
of nutrition and growth to the various organs of the body, 
are continuous with the arteries, and confequently draw 
their fluids from the red blood. Even in thofe parts to 
which the black blood is fent, as the lungs and the liver, 
there are veflels containing red blood, manifeftly 
deftined to fulfil the purpofe of nutrition. The red 
blood communicates to all parts that general agitation or 
concuflion (fecouffe) fo necefl'ary to the performance of 
their functions, and fo clearly recognizable in the brain. 
Thus the red-blooded circulation is the moll important, 
and immediately concerned in the principal phenomena 
of the living economy. 
General Ufes of the black-blooded Circulation. —The cir¬ 
culation of black blood, on the contrary, foreign to all the 
functions, feems to be defigned merely to make good the 
lofles which the blood has undergone in the preceding. 
A confiderable portion of the red blood is expended in 
the fundlions of exhalation, fecretion, and nutrition. 
The principles which it had received in its paffage through 
the lungs, and which had given it a crimfon colour, have 
been left behind in the general capillary fyftem. The 
black blood muft then receive again in proportion to what 
the red has loft : hence a great number of matters is 
poured into the canal which contains it. Thefe are either 
interior or exterior, ill. The large trunks of the ab- 
forbing fyftem are Conftantly carrying in the lymph of the 
cellular fubftance and of the ferous furfaces, the refidue 
of the nutrition of all the organs, the fuperabundant fat, 
fynovia, and medullary matter. Every thing, which from 
the interior of the body is to be rejedled outwards, is in 
the firft place thrown into the black blood, adly. All 
4. Z which 
