he cells to breathe.

=94. Why the heart beats hard when we run.=--When we work hard, the
cells of our bodies need a great deal of food. So the heart beats much
harder, and sends them much more blood. We can feel our heart beat
when we run hard.

When the cells work they get more blood in another way. The arteries
become larger and hold more blood. Then the part looks red and feels
warm. Thus your face gets red when you run hard. This is because your
heart and arteries bring more blood to feed the working cells.

=95. Need of a strong heart.=--The heart must keep sending blood to
feed the cells. If it should stop for only a little while, the cells
would starve to death and we should die. We need strong hearts. When
we work very hard for a long time, the heart gets tired. Then the
cells do not get enough food and we feel weak all over. Boys ought not
to run and lift till they are tired out, for this hurts their hearts.

=96. What alcohol does to the blood.=--Alcohol hinders the digestion
of food. Then too little food will reach the blood, and so the cells
of the body will get too little. Alcohol does not add strength to the
body, but it takes it away. It seems to make men stronger, for it
takes away their tired feelings. But it makes them really weaker, for
it harms the blood.

=97. How alcohol affects the heart.=--Alcohol at first makes the heart
beat more strongly and quickly, but it tires it out and makes it
weaker. Then the heart pumps too little blood to the rest of the body,
and a man is weaker all over.

If a drinker tries to run or work hard, his heart may not pump enough
food to the working cells of his arms and legs. Strong drink takes
away a man's strength and makes him less able to endure a long strain.

=98. How alcohol harms the arteries.=--Alcohol causes the arteries to
become larger and to carry more blood. Then the face will be red and
the skin will become warm. This makes a person feel well, and he seems
to be helped. His blood seems to be flowing faster because his face is
red. But really it is flowing slower.

When the arteries have been made large very often, they stay large all
the time. A drinker's nose is often red from this cause.

Alcohol sometimes causes the arteries to become hard, and even to
change to a kind of bone. Then they cannot change their size to carry
just so much blood as each part needs.

=99. How tobacco affects the heart.=--Tobacco weakens all the body,
but it harms