ANATOMY. 
606 
named the right auricle , the other the left. They open into the 
orifices of each ventricle, called auricular orifices-, and they 
are tendinous at their opening, in the fame manner as the 
ventricles. The right auricle is'larger than the left; and 
it has two other openings united into one, and formed by 
two large veins which meet and terminate there, almoft 
in a diredt line, called vena cava fuptrior and inferior. At 
the mouth of the inferior cava we find a membrane in form 
of a crefcent, deferibed by Euftachius, and named from 
him. The left auricle is a kind of mufcular bag or refer- 
voir, of a confiderable thicknefs, and unequally fquare, in¬ 
to which the four venae pulmonares open, and which has 
a diftindt appendix belonging to it, like a third fmall au¬ 
ricle. This bag is very even on both furfaces, and is 
therefore called fnus venofus ; but, to diftinguiffi it from 
the one on the right fide, it is called fuius venofusfnifer. 
Befides the great common veffcls, the heart has vefiels 
peculiar to it/elf, called the coronary arteries and veins, be- 
caufe they in fome meafure crown the bafis of the heart. 
The coronary arteries, which are tw r o in number, go out 
from the beginning of the aorta, and afterwards fpread 
themfelves round the bafis of the heart, to the fubftance 
of which they fend numerous ramifications. The coronary 
veins are diftributed exteriorly, much in the fame manner. 
The largeft opens into the pofterior inferior part of the 
right auricle, by an orifice which is furniffied with a valve, 
fil'd deferibed by Euftachius. Befides the coronary veins, 
the heart has other anterior veins, called vence innominate. 
Some of them go into the right auricle, others end in the 
right ventricle; and there are other veins ftill fmaller, 
which are found in the fubftance of the heart, and which 
terminate in the right linus and auricle. The nerves of the 
heart, are from the par vagum and great fympathetics : 
thefe form the cardiac plexus, which fends off branches to 
the pericardium, to the roots of the great vefiels, and are 
fpent at laft on the different parts of the heart. 
The heart lies almoft tranfverfely on the diaphragm, the 
greateft part of it being in the left cavity of the thorax, 
and the apex being turned towards the bony extremity of 
the fixth true rib. The bafis is towards the right cavity ; 
and both auricles, efpecially the right, reft on the dia¬ 
phragm; but the fituation of the heart during life changes 
a little, according to the date of refpiration, and to the 
pofition of the body. 
The heart, and the parts belonging to it, are the prin¬ 
cipal inftruments of the circulation of the blood. The two 
ventricles ought to be confidered as two fyringes, fo clofely 
joined together as to make but one body, and furniffied 
with fuckers placed in contrary directions to each other, 
fo that by drawing one of them a fluid is let in, and forced 
out again by the other. When the flefiiy fibres of the ven¬ 
tricles are contracted, the two cavities are leffened in an 
equal and direCt manner; and the flefiiy fibres do the office 
of fuckers, by prefling upon the blood contained in the 
ventricles ; which blood being thus forced towards the 
bafis of the heart, prefles the tricufpidal valves againft 
each other, opens the femilunares,. and ruflies with impe- 
tuofity through the arteries and their ramifications, as 
through fo many elaftic tubes. The blood, thus puflied 
out, enters the capillary veffels, and is from thence forced 
to return by the veins to the auricles, which, like retire¬ 
ments, or antichambers, receive and lodge the blood re¬ 
turned by the veins during the time of a new contraction. 
This contraction of the heart is termed fyfole. The con¬ 
traction or fyftole of the ventricles ceafes immediately, by 
the relaxation of their flefiiy fibres ; and, in that time, the 
auricles which contain the venal blood, being contracted, 
force the blood through the tricufpidal valves into the 
ventricles, the fides of which are thereby dilated, and their 
cavities enlarged. This dilatation is termed cliafole. In 
this manner does the heart, by the alternate fyftole and 
diaftole of its ventricles and auricles, pufli the blood tliro’ 
the arteries to all the parts of the body, and receive it a- 
gain by the veins. This is called ike circulation of the blood , 
which is carried on in three different manners. The firft 
and moll univerfal kind of circulation is that by which al¬ 
moft all the arteries of the body are filled by the fyftole of 
the heart, and the greateft part of the veins evacuated by 
the diaftole. The fecond kind of circulation is through 
the coronary vefiels of the heart, the arteries of which are 
filled with blood during the diaftole of the ventricles, and 
the veins emptied during the fyftole. The third kind of 
circulation is that of the left ventricle of the heart; thro’ 
the venal duCts of which a fmall quantity of blood paffes, 
without going through the lungs, which is the courfe of 
all the remaining mafs of blood. 
There refides in the heart a kind of defire to be (Emula¬ 
ted, lo that, even when it is almoft dead, wrinkles radiating 
from a point appear in many places, and trembling mo¬ 
tions are propagated through different parts of its furface. 
Again, the heart, when torn out and cold, on being pricked, 
inflated, or irritated, contracts itfelf; and its fibres, when 
diffecled, corrugate themfelves orbicularly, when there is 
neither nerve nor artery to bring it fupplies of any kind. 
This irritability is greater, and remains longer in the heart, 
than in any other part of the body; for, by (Emulating 
the heart, its motion may be renewed at a time v'hen that 
of no other mufcle can. The heart of the foetus is moft 
irritable, as well as larger in proportion, than in adults, 
and moft tenacious of its motion, even in the cold. The 
motion of the heart appears to be innate, coming neither 
from the brain nor the foul ; it remains even when the 
heart is removed from the body, and it can neither be in- 
creafed nor retarded by the will. It is, therefore, evident, 
that the ftimulus occafioned by the venous blood driven in-r 
to the heart, caufes it to contrail. This contradEon is 
convulfive, made with great celerity, and a manifeft cor¬ 
rugation of the fibres. The blood, thus impelled from the 
fides towards the axis of the contracting heart, endeavours 
to efcape in that direction, and, by ruffling like a wedge 
between the valves, prefles their loofe edges againft the 
fides of the pulmonary artery, fo as to run freely out of 
the heart. The blood now received into the pulmonary 
artery circulates through the lungs ; for, when it has 
once entered the pulmonary artery, it cannot return to the 
heart; becaufe its valves are of fuch dimenfions, that, 
when diftended, they perfectly (hut up the opening at the 
heart ; and they are fo ftrong that they refift a much 
greater force than the contraction of the pulmonary artery. 
The pulmonary veins run into larger branches, which at laft 
terminate in four trunks ; to which it has been cuftomary 
to affix a name in the lingular, by calling them the pulmo¬ 
nary vein. Thefe enter the cavity of the pericardium, 
from whence they receive an external covering, and are 
then inferted into the corners of the left or pofterior finus, 
which is called the pulmonary fnus. In this courfe the up¬ 
per veins defeend, and the lower ones afeend. In this left 
finus the blood waits for the heart’s relaxation, when it is. 
driven into the left ventricle, in the fame manner as the 
right auricle impelled its blood into the right ventricle. 
Thus the fame blood is now arrived into the left ventricle, 
which was a little before fent from the venae cavae into the 
right auricle. This courfe of the blood, from one fide of 
the heart to the other, through the lungs, is called the 
pulmonary or leffer circulation, and was known to many of 
the antients. Again, this left ventricle being excited to 
motion by the impelled blood, from the fame irritable na¬ 
ture already mentioned, contracts, and drives its contained 
blood with a violent motion in the direction of its axis, the 
tip or cone of the heart being at the fame time drawn 
nearer to its bafis. And fince the apparatus of the mitral 
valves is here the fame as in the tricufpids, the blood now 
expanding the ring from whence they arife removes that 
valve which lay againft the mouth of the aorta, and opens 
a way for itfelf into the artery. After the contradEon of 
the heart, follows its relaxation or diaftole, in which it 
becomes empty, lax, and loft, recovers its former length, 
the ventricles recede from the feptum, and the bafis from 
the apex. But, while it is in this date, the blood in the 
auricles, having been 9 as it were, in a date of expedition, 
ruffi.es 
