Fig. 69 —Photograph of the heart from in front with the lungs pinned aside. One fourth natural size.
The Blood Vessels.—There are four kinds of blood vessels. They are the heart, the arteries, the capillaries, and the veins. The heart lies in the chest between the lungs. It squeezes the blood into the arteries. These carry the blood to all parts of the body. It then runs into the capillaries, which are tiny tubes connecting the arteries with the veins. The veins return the blood to the heart.
The blood flows so fast that it goes from the heart down to the toes and back again in a half minute.
The Heart or Pump of Life.—When the heart stops we die, because the blood can no longer flow to carry food and oxygen to the hungry tissues. The heart is a sac with thick walls of muscle. It is shaped like a strawberry and is about as large as your fist. Its cavity is divided into four parts. The two upper ones are called auricles and the lower ones are named ventricles. The blood enters the auricles and then pours through an opening into each ventricle, from which it passes out into the arteries.
The Arteries or Sending Tubes.—The blood is sent out from the heart through the arteries leading to all parts of the body. The chief artery is the aorta. It is larger than your thumb and extends from the heart down through the body in front of the backbone. It has more than twenty branches. All of these branch again and again like the limbs of a tree until they are finer than hairs.
A large tube, the lung artery, takes blood directly from the heart to the lungs. Here it branches into more than a thousand divisions, so that the blood can take in oxygen and give off to the lungs its waste.
Fig. 70 —Arteries, the tubes carrying the blood from the heart through the body. Only the chief vessels are shown on one side.