THE HEART. 
263 
is circulated through the body. It is of a conoid form, 
with the base turned uppermost, and is opposed to the 
fourth, fifth, and sixth vertebrae of the back, from which it 
is suspended in its situation in the middle of the cavity of 
the chest, by the attachment of the venous and arterial 
trunks immediately connected with it. Its apex hangs 
loose within the cavity of the pericardium, pointing down¬ 
wards and backwards, and rather inclined towards the left 
side. It is composed of four cavities, the two uppermost 
are called auricles, from their form being somewhat like the 
ear of a dog; and two ventricles, or belly-shaped cavities, 
which occupy the substance of the heart. Although the 
heart is chiefly composed of fleshy fibres, still a tendinous 
substance is found in the middle, which seems to be the 
common medium of attachment between its auricles, ven¬ 
tricles, and vessels, one to another. 
The heart is supplied with blood by two coronary arteries, 
the first branches are given off from the aorta, or great 
artery. Its veins pour their blood into the coronary vein, 
by which it is returned into the right auricle. 
There are two orders of blood-vessels, arteries and veins; 
the former conduct the flood from the heart to all parts of 
the body, nourishes it, and returns to the heart through 
the veins. It enters the auricle on the right side, where it 
is accumulated as a reservoir, until there is sufficient to fill 
the ventricle below. The auricle then contracts, and forces 
the blood into the ventricle, which in its turn contracts, 
and drives the blood through an aperture that leads to the 
lungs. It cannot be drawn again into the auricle, because 
there is a complete valve, like that of a sucker of a pump, 
to prevent this. The blood which has thus been forced 
into the lungs traverses every portion of them, by the 
minutely ramified blood-vessels, and entering all the little 
