So we must have an apparatus of circulation.  We need first a central pump from which branch off large pipes, which divide into smaller and smaller branches until they reach the remotest tissues.  Through these pipes the blood must be pumped and distributed to the whole body.  Then we must have a set of return pipes by which the blood, after it has carried nourishment to the tissues, and received waste matters from them, shall be brought back to the central pumping station, to be used again.  We must have also some apparatus to purify the blood from the waste matter it has collected.

[Illustration:  Fig. 68.—­Anterior View of the Heart.

  A, superior vena cava;
  B, right auricle;
  C, right ventricle;
  D, left ventricle;
  E, left auricle;
  F, pulmonary vein;
  H, pulmonary artery;
  K, aorta;
  L, right subclavian artery;
  M, right common carotid artery;
  N, left common carotid artery.
]

This central pump is the heart.  The pipes leading from it and gradually growing smaller and smaller are the arteries.  The very minute vessels into which they are at last subdivided are capillaries.  The pipes which convey the blood back to the heart are the veins.  Thus, the arteries end in the tissues in fine, hair-like vessels, the capillaries; and the veins begin in the tissues in exceedingly small tubes,—­the capillaries.  Of course, there can be no break in the continuity between the arteries and the vein.  The apparatus of circulation is thus formed by the heart, the arteries, the capillaries, and the veins.

184.  The Heart.  The heart is a pear-shaped, muscular organ roughly estimated as about the size of the persons closed fist.  It lies in the chest behind the breastbone, and is, lodged between the lobes of the lungs, which partly cover it.  In shape the heart resembles a cone, the base of which is directed upwards, a little backwards, and to the right side, while the apex is pointed downwards, forwards, and to the left side.  During life, the apex of the heart beats against the chest wall in the space between the fifth and sixth ribs, and about an inch and a half to the left of the middle line of the body.  The beating of the heart can be readily felt, heard, and often seen moving the chest wall as it strikes against it.

[Illustration:  Fig. 69.—­Diagram illustrating the Structure of a Serous Membrane.

  A, the viscus, or organ, enveloped by serous membrane;
  B, layer of membrane lining cavity;
  C, membrane reflected to envelop viscus;
  D, outer layer of viscus, with blood-vessels at
  E communicating with the general circulation.
]

The heart does not hang free in the chest, but is suspended and kept in position to some extent by the great vessels connected with it.  It is enclosed in a bell-shaped covering called the pericardium.  This is really double, with two layers, one over another.  The inner or serous layer covers the external surface of the heart, and is reflected back upon itself in order to form, like all membranes of this kind, a sac without an opening.[32] The heart is thus covered by the pericardial sac, but is not contained inside its cavity.  The space between the two membranes is filled with serous fluid.  This fluid permits the heart and the pericardium to glide upon one another with the least possible amount of friction.[33]