From the right ventricle a large vessel called the pulmonary artery passes to the lungs, and from the left ventricle a large vessel called the aorta arches out to the general circulation of the body.  The openings from the ventricles into these vessels are guarded by the semilunar valves.  Each valve has three folds, each half-moon-shaped, hence the name semilunar.  These valves, when shut, prevent any backward flow of the blood on the right side between the pulmonary artery and the right ventricle, and on the left side between the aorta and the left ventricle.

[Illustration:  Fig. 71.—­Right Cavities of the Heart.

  A, aorta;
  B, superior vena cava;
  C, C, right pulmonary veins;
  D, inferior vena cava;
  E, section of coronary vein;
  F, right ventricular cavity;
  H, posterior curtain of the tricuspid valve;
  K, right auricular cavity;
  M, fossa ovalis, oval depression, partition between the auricles formed
     after birth.
]

186.  General Plan of the Blood-vessels Connected with the Heart.  There are numerous blood-vessels connected with the heart, the relative position and the use of which must be understood.  The two largest veins in the body, the superior vena cava and the inferior vena cava, open into the right auricle.  These two veins bring venous blood from all parts of the body, and pour it into the right auricle, whence it passes into the right ventricle.

From the right ventricle arises one large vessel, the pulmonary artery, which soon divides into two branches of nearly equal size, one for the right lung, the other for the left.  Each branch, having reached its lung, divides and subdivides again and again, until it ends in hair-like capillaries, which form a very fine network in every part of the lung.  Thus the blood is pumped from the right ventricle into the pulmonary artery and distributed throughout the two lungs (Figs. 86 and 88).

We will now turn to the left side of the heart, and notice the general arrangement of its great vessels.  Four veins, called the pulmonary veins, open into the left auricle, two from each lung.  These veins start from very minute vessels the continuation of the capillaries of the pulmonary artery.  They form larger and larger vessels until they become two large veins in each lung, and pour their contents into the left auricle.  Thus the pulmonary artery carries venous blood from the right ventricle to the lungs, as the pulmonary veins carry arterial blood from the lungs to the left auricle.

From the left ventricle springs the largest arterial trunk in the body, over one-half of an inch in diameter, called the aorta.  From the aorta other arteries branch off to carry the blood to all parts of the body, only to be again brought back by the veins to the right side, through the cavities of the ventricles.  We shall learn in Chapter VIII. that the main object of pumping the blood into the lungs is to have it purified from certain waste matters which it has taken up in its course through the body, before it is again sent on its journey from the left ventricle.