PHYSIOLOGY. 
The circulation, 'is the motion by which 
the blood, setting out from the heart, is con- 
stantly carried to all parts of the body in 
the arteries, and returns to the same point 
ht the veins. The uses of this circulatory 
motion are, to submit the blood altered by 
the mixture of lymph and chyle to the con- 
tact of the atmosphere in the Inngs (respi- 
ration), to convey it to several organs in 
which various animal fluids are separated 
from it, (secretion) ; and to every part of 
the body, for supplying its growth and re- 
pairing its losses, by means of its nutritive 
particles when completely assimilated (nu- 
trition). The conveyance or transport of 
our fluids, rather than their elaboration, is 
the office of the organs of circulation. In 
this view they may be compared to those 
labourers, who in a large manufactory, from 
which various products issue, carry the ma- 
terials to the workmen employed in the ac- 
tual fabrication. As among the latter 
there are some who purify and bring to 
perfection the materials furnished by others, 
60 the lungs and secretory glands are con- 
stantly employed in separating from the 
blood all those heterogdnous matters which 
could not be assimilated to the substance of 
our organs. 
The word circulation, when used abso- 
lutely, comprehends the whole course of 
the blood, as well in the lungs, as in the ar- 
teries and veins of the body at large. The 
greater circulation is the passage of the 
blood from the leftside of the heart, through 
the arteries, to the extremities of the body, 
and its return through the veins to the right 
side of the same viscus. The lesser circu- 
lation is the transmission of the blood from 
the right to the left side of the heart, 
through the lungs. 
The course, which the blood takes, has 
been already explained in the article Ana- 
tomy. We subjoin tlie proofs and expe- 
riments, by which the facts there stated are 
supported. 
The passage of the blood through the 
heart, i. c. from the right auricle to the 
left ventricle, by the medium of the lungs, 
is manifest from the structure of the heart 
itself. The valves, which are placed at its 
various apertures, actually will not admit of 
the blood’s motion in any other direction 
than what we have described. That this 
fluid passes from the heart into tlie trunk of 
the aorta, thence into its branches, and so 
on to the most minute ramifications, is 
evinced; 1. By the effect of ligatures on 
these vessels; th« artery becomes turgid 
between the heart and the ligature, and 
empty between the ligature and its distri- 
bution. 2. By opening an artery when 
tied, above and below the ligature; the 
blood in this case flows from that open- 
ing only, which is nearest to the heart. 3. 
By ocular testimony ; the passage of the 
blood can be seen with the aid of glasses in 
frogs, fishes, &c. The passage of the blood 
through the veins, in a contrary course to 
that, in which it flows along the arteries, 
1. e. from the minute ramifications towards 
the trunks, and thence to the heart, is 
proved. 1. By the structure and disposi- 
tion of the valves, which afford an invinci- 
ble impediment to all retrograde motion. 
2. By ligatures on these vessels, which make 
the vein turgid betweeiu the extremities of 
the body and the ligature, and empty in the 
rest of its course. 3. By opening a vein, 
when tied, above and below the ligature. 
4. By microscopical observation in ani- 
mals. 
The passage of the blood from the arte- 
ries into the veins seems to flow as a corol- 
lary, from what we have stated concerning 
the proofs of its course in these two systems 
of vessels. We have shewn that the ulti- 
mate arteries are continuous with the ori- 
gins of the veins ; that tlie blood moves 
from the heart to the extremities in the 
former vessels, and that it passes from the 
extremities to the heart in the latter. The 
intermediate passage is a direct conse- 
quence of these facts. But it may be de- 
monstrated by direct proofs independently 
of this argument. If we tie the artery of a 
part, its correspondent vein receives no 
blood; if we take off the ligature the vein is 
again filled. The quantity of blood ex- 
pelled from the aortic ventricle is so consi- 
derable, that the supply can only be kept 
up by its return to the heart. We calculate 
that two ounces are sent into the aorta at 
each pulsation ; if we suppose 80 pulsa- 
tions in a minute, 9,600 ounces will be 
thrown out in an hour ; and 14,400 pounds 
in a day. The same blood, therefore, which 
the aorta received from the heart, must be 
returned to this viscus ; and the only pas- 
sage, by which it can return, is through the 
veins. Lastly, the passage of the blood 
from the arteries into the veins, may be 
proved by the direct testimony of the senses 
in living animals. The use of the micros- 
cope affords this proof in the transparent 
parts of cold blooded animals, as the me- 
sentery and web of the foot in frogs, the 
tail of fishes, &c. 
