A N A T 
ruflies through the openings'of the valves of the veins, di¬ 
lates the oppofite Tides of the heart, and increafes all its 
dimensions. After the auricles have freed themfelves of 
the blood they contained, they are in like manner relaxed, 
and their oppofite Tides remove from each other. The blood 
then collected in the venae cavte and pulmonary veins fills 
the auricles by the contraction of the veins ; renders them 
long, broad, and thick, like the ventricles; and even dif- 
tends and fills the dentated precedes of the creded margin. 
Thefe motions of the right and left auricle, with the right 
and left ventricle, are not performed in that fuccedion in 
which, for the fake of method, we have here deferibed 
them ; for both the auricles are contracted, while both the 
ventricles are relaxed : To that the contraction of the au¬ 
ricles precedes the contraction of the ventricles. This faCt 
is afcertained by experiments on dying animals, and on 
animals with cold blood. Thole who have inadvertently 
taught otherwise, have not taken the advantage of making 
a lufficient number of experiments on living animals. That 
the auricle, near death, makes frequent palpitations before 
the ventricle of the heart performs one contraction, is cer¬ 
tainly true. The auricle, with its firms, forms one cavity ; 
and both are filled and both emptied in the fame inltant. 
It may be alked, Why the heart is not wearied, or be¬ 
comes painful, by fo rapid, long continued, uninterrup¬ 
ted, and violent, a mot.on ? The heart contracts, in a 
healthy perfon, about 5000 times in an hour, and never 
ceal'es its alternate contractions and dilatations from the 
firft beginning of its exiltence till death ? Why, then, does 
it not, like other mufcles, become tired, in a very few 
hours, by fuch violent motions ? How is its ftrength pre- 
ferved ? Different anlwers have been given to thefe quef- 
tions by different profeffors, founded either on a compref- 
fure of the cardiac nerves between the large arteries, or 
on an alternate repletion of the coronary arteries and cavi¬ 
ties of the heart, &c. But to Dr. Haller the fimplicity of 
nature in this matter is very confpicuous. When the au¬ 
ricle is relaxed, it is directly filled by the mufcular force 
of the continuous great vein ; and the heart contracts when 
it is irritated by the blood being driven into it from the 
auricle, and thus empties it!elf of the blood. Being freed 
from the ffimulus, it immediately refts or relaxes itfelf. 
The heart being now relaxed, the auricle is, in like man¬ 
ner, irritated by its contained blood, and, by contracting, 
fills the heart again while the incelfant aCtions of the 
heart and arteries continually urge new blood into the right 
finus and auricle. That this is the true caufe of the heart’s 
motions is proved from aCtual experiment and obfervation. 
The fuccefiive repletions and contractions made in the 
great vein, auricle, ventricle, and artery, are plainly and 
eafily feen in a weak or expiring animal; but, more efpe- 
cially, and more evidently, in thole animals which have 
but one ventricle in the heart, as tire tortoife, frog, fnake, 
fillies; and in the chick hatching in the egg, which, in- 
ffead of a heart, has only one crooked canal. Befides, it 
is confirmed from the inertia of the heart, produced by 
tying the veins ; and, from the return of its motion, when 
the ligatures are unloofed; provided thefe phenomena are 
fufficiently valid; but it is mere unequivocally corrobo¬ 
rated by injection, and by the experiment of inflating a 
frog’s heart with a fmall bubble of air, which we fee al¬ 
ternately pafs from the ventricle to the auricle and back 
again for many hours. The left ventricle firft ceafes its 
motion; then the auricle of that fide ; then the right ven¬ 
tricle; after that the right auricle; and, laft of all, the 
pulmonary veins and venae cavae. Whatever motion is in 
the venae cavse, ought to be attributed to the auricle re¬ 
pelling the blood into both thefe veins, and which the 
heart, when dead, is not capable of receiving. Dr. Haller 
believes that nothing more is requilite to produce the 
heart’s motion than a continual ffimulus applied to a very 
irritable part. For, on the approach of death, the coldnef's 
of the limbs contracts the veins, and drives the blood to the 
heart; the lungs, being impermeable for want of refpira- 
tion, tranlinit no blood to the cavities of the left fide; and 
O M Y. 607 
the heart, after it is thoroughly emptied, remains at reft. 
The velocity of the blood, at its entrance into the aorta, 
and tiie force with which it is expelled from the heart, 
have been fubjeCts of much controverfy ; and different 
anatomifts have computed them differently. To deter¬ 
mine the velocity, modern writers proceed on the follow¬ 
ing data. They iuppofe that two ounces are expelled by 
each fyliole, that each fyfiole is the third part of the whole 
puliation, and is performed in healthy perfons in part 
of a minute, that the area of the aorta is 0-4187 of an inch, 
and that two ounces of blood occupy a fpace of 3-318 cu¬ 
bic inches ; or, in other words, that a cylinder of blood, 
whole folidity is 3-318 inches, and bafe 0-4187, is expelled 
by the heart in part of a minute. Since the folidity of 
a cylinder is the prodtiCl of the area of its bafe into the 
length, its length will be equal to the folidity divided by 
the bafe; therefore, in this cafe, 3-318 divided by 0-4187 
gives 7-9245 for the length which the blood runs in 
part of a minute ; that is, 148 feet 7 inches in a minute. 
To determine the force, they fuppofe that the perpendi¬ 
cular jet from the heart is 7 feet 6 inches, and the area of 
the furface of the ventricle is 15 inches; thefe two num¬ 
bers multiplied give 1350 cubical inches, or 51 pounds of 
blood again!! which the heart aCts. The heart, therefore, 
fends a weight of 51 pounds, with the velocity of 149 fe<st 
in a minute. 
The above computation may perhaps be inaccurate; 
both from the omiflion of fome circumftances that ought 
to have been taken into the account, and from the inac¬ 
curacy of the data ; but that the heart appears to be a very 
powerful machine, is evident from the great difficulty we 
have in filling all the red blood-velfels by anatomical in¬ 
jections, and the utter impoflibihty of filling all the final ler 
ones ; yet the heart, we lee, not only gradually diftends 
all the larger, the fmaller, and even the leaff, velfels, with 
blood, but alio drives it forward through them with a 
conliderable velocity. Even in the leaft arteries, the blood- 
is urged forward by the heart with fuch a force, as to make 
the alternate motions of that powerful mufcle perceptible ; 
even in fuch animals as are fcarcely vilible to the naked, 
eye, and in the fmall embryos of infeCts. 
Moreover, that we may make a juft eftimate of the- 
heart’s force in living animals, we muff confider what great 
refinances that complex mufcle overcomes: we muff com¬ 
pute the enormous weight of the whole ntafs of blood; a 
mals perhaps of fifty pounds and upwards: for all the- 
quantity ol fluids, once ftagnant in a perfon lately drowned 
or fainting away, are eafily put into their former motion, 
by the heart alone. We muff alfo confider the great de- 
creafe of the blood’s velocity, arifing from the greater ca¬ 
pacity of the dividing branches; and yet, even in the leaff 
velfels, its velocity is very confiderable, as appears by the 
SanCtorian perfpiration feen to fly rapidly off like fmoke, 
and by the quick motion of the blood feen, by the help of' 
microfcopes, in fifties tails. FriCtions in every machine al¬ 
ways confume the greateft part of the moving forces; and 
thefe frictions will doubtlef’s be very large in the human 
body, whofe blood and juices are extremely vifeid, and. 
whofe velfels are fo fmall as fcarcely to allow more than a 
fmgle globule to pafs at a time, and even that not without 
changing its figure. All thefe refiffances being confidered, 
we may without doubt conclude that the force of the heart 
muff be extremely great, in order to preferve the motion, 
fo ftrong as we perceive it in the leaft arteries. Another 
argument of the heart’s force is, that aneurifms and arte¬ 
ries are burft, and very great weights, as well as the body 
itfelf, raifed by the force of the heart’s fyftole. 
The liquor which is contained in the arteries and their 
correfponding veins, is known by the general name of 
blood. It appears homogeneous, is coagulable, and of a 
red colour; it is redder in ftrong and well-fed animals, and 
generally yellow in fuch as are weak and ftarved. White 
ffreaks fometimes appear in it, in confequence of the chyle. 
But from various experiments it is certain, that this ani¬ 
mal liquor contains Very different ingredients. That fire 
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