the heart is this:--The auricles and ventricles expand;
blood rushes into the auricles from the channels supplying them, and
distends them and the ventricles; the auricles contract and fill the
ventricles below quite full (there are no valves above the auricles, but
the force of contraction is not sufficient to return the blood to the
veins); the ventricles contract; the mitral and tricuspid valves close;
the valves leading to the arteries open; blood is forced out of the
ventricles.

THE BLOOD CHANNELS

are of two kinds--(1) The _arteries_, which lead the blood into the
circulatory system; (2) the _veins_, which lead the blood back to the
heart. The arteries divide up into branches, and these again divide into
smaller and smaller arteries. The smallest, termed _capillaries_ (Latin,
_capillus_, a hair), are minute tubes having an average diameter of
1/3000th of an inch. These permeate every part of the body. The
capillary arteries lead into the smallest veins, which unite to form
larger and larger veins, until what we may call the main streams are
reached. Through these the blood flows to the heart.

There are three main points of difference between arteries and veins. In
the first place, the larger arteries have thick elastic walls, and
maintain their shape even when empty. This elasticity performs the
function of the air-chamber of the force-pump. When the ventricles
contract, driving blood into the arteries, the walls of the latter
expand, and their contraction pushes the blood steadily forward without
shock. The capillaries have very thin walls, so that fluids pass through
them to and from the body, feeding it and taking out waste matter. The
veins are all thin-walled, and collapse when empty. Secondly, most veins
are furnished with valves, which prevent blood flowing the wrong way.
These are similar in principle to those of the heart. Arteries have no
valves. Thirdly, arteries are generally deeply set, while many of the
veins run near the surface of the body. Those on the front of the arm
are specially visible. Place your thumb on them and run it along towards
the wrist, and you will notice that the veins distend owing to the
closing of the valves just mentioned.

Arterial blood is _red_, and comes out from a cut in gulps, on account
of the contraction of the elastic walls. If you cut a vein, _blue_ blood
issues in a steady stream. The change of colour is caused by the loss of
oxygen during the passage of th