Blood corpuscles (×400).
a a pile of red blood cells.
b red blood cells seen flatwise.
c red blood cells seen edgewise.
d white blood cells.
Blood looks like a red liquid. But if you look at it through a strong microscope, it looks like water, and millions of little red cells. These cells carry air through the body. They make the blood look red. There are also a smaller number of white cells. Blood is made of red cells, white cells, and a liquid.
82. The liquid in blood.—The liquid part of the blood is albumin, and water, with a little fat, sugar, and minerals. It is food and drink for the cells of the body. When blood is drawn from the body it soon becomes like jelly. We call the jelly a clot. When you cut your finger, a clot forms in the cut and plugs up the bleeding place. If it did not, the blood would all run out of the body and we should die.
Diagram of the heart while it is beating.
a vein entering the auricle.
b auricle.
c closed valve to keep blood from flowing
back into the auricle.
d ventricle.
e artery.
f valve to keep blood from returning to the
ventricle.
83. The heart.—The blood is held in tubes. A pump inside the body keeps it always moving. This pump is called the heart. The heart is a bag of muscle with thick sides. It is about as large as your fist. When it is full, it has the power to make itself smaller, and so it squeezes the blood out through a tube. We can feel each squeeze as a heart-beat. You can find the heart-beat just to the left of the middle of the body about two hand-breadths below the neck.
84. The heart-beat.—A man's heart beats about seventy times each minute. Boys' and girls' hearts beat much faster. Running or hard work of any kind makes the heart beat faster yet. Your heart will keep on beating until you die. It does not seem to rest at all, yet it works only while you feel it beat. Between each beat it rests while the blood is filling it again. So it really rests one half of the time.
85. Arteries.—The heart pumps the blood through a single tube. This tube opens into smaller tubes. These open into still smaller ones. You must use a strong microscope to see the finest blood tubes. The tubes reach every part of the body, and carry blood to its cells. They are called arteries. At each heart-beat a wave of blood can be felt in an artery. This wave is the pulse. It can be felt in the wrist, temples, and other places. By the pulse we can tell how often and how strongly the heart is beating.