Then these particles irritate the mucous membrane, which secretes
enough mucus to imprison the intruders. Consequently there is
occasionally expulsion of gray or black mucus, which should alarm no one
under the circumstances, if feeling well. Normally the mucous membrane
secretes only enough mucus to lubricate itself, and when there is much
expulsion of mucus it means that either the respiratory or the digestive
system, or both, are being abused. At such times the sufferer should
take an inventory of his habits and correct them.
The air cells are made up of very thin membrane. So great is their
surface that if they could be flattened out they would form a sheet of
about 2,000 square feet. We can not explain satisfactorily why it is
that through their walls there is an exchange of gases, nor how the
respiratory system can act so effectively both as an exhaust of harmful
matter and a supply of necessary elements. The distribution of the blood
capillaries, so tiny that the naked eye can not make them out, is
wonderful. Under the microscope they look like patterns of delicate,
complex, beautiful lace.
The lungs are supplied with more blood than any other, part of the body.
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