The heart is a hollow organ, but the cavity is divided into two parts by a muscular partition forming a left and a right side, between which there is no communication.  These two cavities are each divided by a horizontal partition into an upper and a lower chamber.  These partitions, however, include a set of valves which open like folding doors between the two rooms.  If these doors are closed there are two separate rooms, but if open there is practically only one room.  The heart thus has four chambers, two on each side.  The two upper chambers are called auricles from their supposed resemblance to the ear.  The two lower chambers are called ventricles, and their walls form the chief portion of the muscular substance of the organ.  There are, therefore, the right and left auricles, with their thin, soft walls, and the right and left ventricles, with their thick and strong walls.

185.  The Valves of the Heart.  The heart is a valvular pump, which works on mechanical principles, the motive power being supplied by the contraction of its muscular fibers.  Regarding the heart as a pump, its valves assume great importance.  They consist of thin, but strong, triangular folds of tough membrane which hang down from the edges of the passages into the ventricles.  They may be compared to swinging curtains which, by opening only one way, allow the blood to flow from the auricles to the ventricles, but by instantly folding back prevent its return.

[Illustration:  Fig. 70.—­Lateral Section of the Right Chest. (Showing the relative position of the heart and its great vessels, the oesophagus and trachea.)

  A, inferior constrictor muscle (aids in conveying food down the
     oesophagus);
  B, oesophagus;
  C, section of the right bronchus;
  D, two right pulmonary veins;
  E, great azygos vein crossing oesophagus and right bronchus to empty
     into the superior vena cava;
  F, thoracic duct;
  H, thoracic aorta;
  K, lower portion of oesophagus passing through the diaphragm;
  L, diaphragm as it appears in sectional view, enveloping the heart;
  M, inferior vena cava passing through diaphragm and emptying into
     auricle;
  N, right auricle;
  O, section of right branch of the pulmonary artery;
  P, aorta;
  R, superior vena cava;
  S, trachea.
]

The valve on the right side is called the tricuspid, because it consists of three little folds which fall over the opening and close it, being kept from falling too far by a number of slender threads called chordae tendinae.  The valve on the left side, called the mitral, from its fancied resemblance to a bishop’s mitre, consists of two folds which close together as do those of the tricuspid valve.

The slender cords which regulate the valves are only just long enough to allow the folds to close together, and no force of the blood pushing against the valves can send them farther back, as the cords will not stretch The harder the blood in the ventricles pushes back against the valves, the tighter the cords become and the closer the folds are brought together, until the way is completely closed.