The outer covering of the
heart is a strong, thin membrane called the pericardium.
The superior vena cava,
aorta, and other large
blood vessles lead in and out of the heart.
Smaller blood vessels nourish the heart itself.
make up the largest part of the heart. They form
the whole lower portion. Their thick, strong
muscles pump blood throughout the body.
form the upper, smaller part of the heart. They
have thin walls, and are smaller than the
ventricles. Each atrium serves as a collecting
station for the blood that flows into the heart.
The World Book Encyclopedia
Chicago: World Book-Childcraft International, Inc., 1979