the front
part of the neck. The windpipe divides into many branches. At the end
of its smallest branches are little bags or sacs. The branches and
the sacs make the two lungs. So a lung is a soft and spongy piece of
flesh, and can be blown up like a rubber bag. A frog's lung is a
single, thin bag, about half an inch across it. Each little sac of a
man's lung is like a tiny frog's lung.

[Illustration: =A frog's lung (x4).=]

=101. The diaphragm.=--The lungs fill the upper part of the body just
below the neck. They are covered by the bony ribs, and rest upon a
broad muscle. This muscle is called the _diaphragm_. It divides the
inside of the body into two parts. The upper part is the _chest_, and
holds the heart and lungs. The lower part is the _abdomen_, and holds
the stomach, intestine, and liver, and a few other parts.

[Illustration: =The parts inside the body.=

_a_ lungs.
_b_ heart.
_c_ diaphragm.
_d_ stomach.
_e_ liver.
_f_ intestine.]

=102. Breathing.=--When the diaphragm lowers itself, or the ribs are
raised, the chest is made larger. Then the air rushes through the
nose and swells out the lungs to the size of the chest. This is taking
a breath. Then the chest becomes smaller again, and blows the air out.
A man breathes about eighteen times a minute. He does not seem to rest
in breathing, but as he works only when he takes in breath, he rests
one half of the time.

=103. How air gets into the blood.=--After the blood has been around the
body through the arteries and capillaries and veins, the heart sends
every drop to the lungs before it sends it out to feed the cells again.
The blood flows through little capillaries upon the sides of the air
sacs. There the red blood cells take up some of the air, and carry it
with them. When they have a load of air, they become of a brighter red
color. The blood in the arteries on its way to the cells is bright red.

=104. How the cells get air.=--When the blood reaches the capillaries
around the cells of the body, the red blood cells give up some of the
air to the cells. Thus each cell of the body gets some air, and so it
breathes. The cells cannot reach the air themselves, and so the red
blood cells bring it to them. We breathe so as to supply the cells
with air.

=105. What burning is.=--When meat is put into a hot stove it quickly
burns, and passes off in smoke, and leaves only a little ashes. The
ashes are the mineral parts of the meat. If the f