THE HKAKT. 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 vertebras 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 witliin 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 tw^o coronary arteries, 

 the first branches are given ofi" 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 suflScient 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 



