3G0 P FI Y S I 
degree, have the larged: brain, although this power is not 
in the lead in proportion to the quantity of brain in that 
clafs. It is mod probable that it arifes from fome other 
principle; a principle fo connected with life, that it can, 
and does, aft independently of circulation, fenfation, 
and volition ; and is that power which preferves and re¬ 
gulates the internal machine. This power of generating 
heat is in the hig heft perfection when the hotly is in health ; 
and in many deviations from that date, we find that its 
affion is extremely uncertain and irregular ; fometimes 
ridng higher than the dandard, and at other times falling 
much below it. Indances of this we have in different 
difeafes, and even in the fame difeafe, within very diort 
intervals of time. A very remarkable one fell under my 
own obfervation, in a gentleman who was feized with an 
apoplectic fit ; and, while he lay infenfible in bed, covered 
with blankets, I found that his whole body would, in an 
indant, become extremely cold in every part, continuing 
fo for fome time ; and, as fuddenly, would become ex¬ 
tremely hot. While this was going on alternately, there 
was no fenfible alteration in his pulfe for feveral hours. 
Being fatisfied of the foregoing fafi, that animals had a 
power of generating heat, I purfued the fubjedt dill fur¬ 
ther ; not fo much with a view to account for animal heat, 
as to obferve the different phenomena, with the varia¬ 
tions or difference in the heat in different animals.” 
The formation of chyle, its entrance into the blood, 
and the aerial changes that fluid undergoes in the lungs, 
liave been now detailed. The account which fhould fol¬ 
low here of the compofition is fully anticipated in the ar¬ 
ticle Anatomy, where alfo is to be found defcriptions, 
both general and particular, of the heart, arteries, and 
■veins, and of their properties. The powers which move 
the blood being howevera matter of difpute, we mud jud 
refer to mors modern obfervations. In the article in 
queflion, the heart is fuppofed to urge the blood into the 
large arteries, and then, by mufcular contradlions, to pro¬ 
pel it into fmalleror.es through the veins, and fo to the 
heart. As Bichat is the phyflologid who has chiefly la¬ 
boured to overturn the hypothefis of the contractility of 
arteries, we fhall enter fully into his opinions. In juflice 
to him, it is firfl neceffary toinfert his excellent defcrip- 
tion of the circulation in general. 
“ All authors, (fays he,) who have written on the cir¬ 
culation flnce the time of Harvey, have defcribed it in the 
fame manner, dividing this function into two parts, the 
great and the lefs (or pulmonary) circulations, of which 
the heart is the common centre. This mode of confider- 
ing the fubjeCt does not lead us to obferve, in a general 
manner, the objects of the blood’s motion through our 
organs ; and I deem the mode, in which I explain this 
important phenomenon of the animal economy in my 
leftures, infinitely better calculated to convey a correCf 
notion of its nature. I divide the circulation into two 
parts; one of which carries blood from the lungs to all 
parts of the body, the other brings this fluid back 
again from all parts to the lungs. The firlt of thefe 
is the circulation of red blood, the fecond that of black 
blood. 
Circulation of Red Blood. —This has its origin in the 
capillaries of the lungs, where the aCtion of the air on 
the blood gives it thofecharaClers which diftinguifh it from 
the fame fluid in its black Hate. From theie capillaries 
it paffes fucceflively through the branches and trunks of 
the pulmonary veins, and the left auricle, into the left 
ventricle of the heart, which propels it into the arterial 
fyftem. The latter conveys it into the general capillary 
lyflem (the capillaries of the body at large, excepting 
thole of the lungs), which mull be regarded as the termi¬ 
nation of its courfe. Thus the red blood is conftantly 
conveyed from the capillary fyftem of the lungs to that 
of the body in general. The cavities, wdiich contain 
this blood, are all lined by a continuous membrane, 
which, being expanded over the pulmonary veins,'the left 
cavities of the heart, and the arterial fyftem, may with 
O L O G Y. 
truth be confidered to form an unbroken canal, ftrength- 
ened on its exterior, in the pulmonary veins by a°foft 
membrane, in the heart by a mufcular layer which is 
thin in the auricle, and thick in the ventricle, and in 
the arterial fyftem by a fibrous layer of peculiar texture. 
In all thefe various organs, which are added to it exter¬ 
nally, the membrane itfelf remains nearly the fame. 
Circulation of Blank Blood.— The courfe of this is in- 
verfe to that of the preceding. It begins in the general 
capillary fyftem, and here the blood which it contains 
acquires its diftinftive characters : here, if we may ufe 
the expreffion, it feems to be reproduced, probably by the 
fubtraifion of thofe principles which it had previoufly 
derived from the atmofphere in the lungs. From the 
general capillary fyftem it paffes into the veins, which 
bring it to the right cavities of the heart : thence it is 
trahlmitted through the pulmonary artery to the capil¬ 
lary fyftem of the lungs. The latter is its true termina¬ 
tion, as it is the commencement of the fyftem containing 
the red blood. One general membrane, continuous 
throughout, lines the whole pafl'ags of the black blood, 
and forms for it an unbroken canal, along which it is 
conftantly conveyed from all parts of the body to the 
lungs. On the outfide of this canal there is placed a 
loole membrane in the veins, a mufcular ftratum in the 
heart, and a peculiar fibrous tiffue in the pulmonary ar¬ 
tery : but the membrane, as in the preceding cafe, re¬ 
mains always nearly uniform, in fpite of the various or¬ 
gans which furround it externally. The folds of this 
membrane form the valves of the veins: it alfo allifts in 
the compofition of the valvular apparatus belonging to 
the right fide of the heart, of which it lines the cavities; 
as the membrane of the otbercirculation concurs in form¬ 
ing the valves, and lining the correfponding parts on 
the left fide of the heart. 
Differences of the tiuo Circulations. —The general idea 
which I have thus given of the two circulations, fliows 
plainly that they are perfectly independent of each other, 
except at their origin and termination, where the red and 
black blood are alternately changed into the oppofite 
ftates, and communicate, for this purpofe, by the capillary 
veffels. In the whole of theircourfe the two circulations 
are completely infulated. Although the two fides of the 
heart are united together in a fingle organ, they may be 
regarded as perfectly independent in their motion. There 
are in truth two hearts, a right and a left: they might 
probably perform their functions juft as well, if they were 
Separated, as in their prefent connected form. Even when 
the foramen ovale remains open after birth, we fhall prove 
afterwards that the ItruCture of its fides prevents all in¬ 
termixture of the red and black bloods, and confequently 
the two parts are (till equally independent, as far as re¬ 
gards the courfe of the blood. This complete feparation 
of the two circulations is one of their molt decided cha¬ 
racters; and it alone (hows, how preferable the point of 
view, in which I regard the circulation, is to that in 
which it is reprefented as divided into the great and 
lefs, which are manifestly confuted and identified with 
each other. 
According to the obfervations already made, the two 
circulations begin and terminate in two capillary fyftems, 
which may therefore be confidered as forming the limits 
within which the two kinds of blood circulate. In this 
point of view the lungs alone correfpond to all the other 
parts of the body. Their capillary fyftem is inoppofition 
to that of all the other organs, with the fmall exception 
of the parts from which the blood of the vena portarum 
proceeds. Thus each capillary fyftem is at once origin 
and termination ; the pulmonary is the origin of the cir¬ 
culation of red, and the termination of that of black, 
blood ; while, in the general, the former ends, and the 
latter begins. Hence arifes a grand diltinCtive character 
of the two circulations : in fact, the blood not only fol¬ 
lows an oppofite courfe at the commencement and con- 
clufion of the circulations, but it undergoes alfo a com- 
