The Capillaries or Feeding Tubes.—These are the tiny tubes, finer than hairs, which join the smallest end branches of the arteries with the beginnings of the little veins. They are so thickly scattered in the flesh that you cannot stick it with a pin without piercing one.
They are called feeding tubes because they have such very thin walls that the food in the blood and the oxygen brought from the lungs can pass through to feed the muscles and other organs. The dead parts of the body and also the ashes of the food used up, pass from the organs into the capillaries.
The Veins or Returning Tubes.—The veins, beginning in fine branches formed by the capillaries, return the blood to the heart. The branches unite into larger and larger vessels and finally flow into one main vein, the vena cava. This extends along in front of the backbone and opens into the heart.
Why the Blood flows in only one Direction.—The heart causes the flow of the blood. It does this by squeezing together its walls so as to make the blood go out into the arteries. When once in the arteries, the blood must go forward because there are little doors at the mouths of the arteries in the heart. These doors, called valves, open in only one direction, so that the blood cannot flow backward ([Fig. 71]). There are other valves between the upper and lower cavities of the heart, preventing the blood from being pushed back into the veins.
Fig. 71 —The heart with the front part cut away to show the four chambers and valves. The arrows show the direction in which the blood flows.
The movement of the walls of the heart in and out is called the heart beat. This can be plainly felt by placing the hand on the left side of the chest. The heart beats about seventy times each minute in grown persons, but much oftener in children. At each beat a wave of blood flows along the arteries. This is known as the pulse. It may be felt at the base of the thumb, where an artery runs just under the skin.
Why the Heart sometimes beats Faster.—When we run or do hard work, the heart may beat twice as fast as when we are lying down. This is because the muscles need more oxygen to help them act. Work makes them get hungry, and they send word by the nerves to the heart to hurry along the blood to bring more oxygen from the lungs.
When germs make the body sick, the heart often beats faster because it is affected by the poison made by the germs. The doctor then feels the pulse to tell how much the body is poisoned.