388 The Outline of Science 



the customary "door," automatically opening and shutting; and 

 behind this delicate folding-door are the "vocal chords" which we 

 use for speech. The windpipe divides at its base into the two 

 bronchial tubes, one for each lung, and there are ingenious 

 arrangements for dealing with dust or microbes that have got 

 past the sentinels in the nose. There is a coat of mucus to inter- 

 cept them (as the flypaper does flies), and there are countless 

 microscopic lashes or cilia which bend and straighten rhythmic- 

 ally, beating towards the entrance, and thus gradually push out 

 the intruder. If certain kinds of dangerous microbes settle on 

 them, the glands pour out large quantities of mucus, and your 

 lungs automatically blow it out at times you have a "cold" and 

 a "cough." 



In the lungs themselves the bronchial tubes branch out into 

 numbers of fine tubes as the arteries do, and each tube ends in 

 a score or more of little air-chambers. There are about six mil- 

 lions of these minute chambers (each about 1-10 of an inch long) 

 in the two lungs, and they are formed so as to give as much 

 surface as possible. If we could open them all out and piece 

 them together, we should have a total surface a hundred times 

 larger than our skin. This is the wonderful contrivance evolved 

 for bringing a large body of air into almost direct contact with 

 the blood fifteen times a minute or more. In a deep breath we 

 can take in a whole gallon of air, and even a normal breath takes 

 in two quarts. 



Nerve Messages 



But who attends to the working of this wonderful bellows 

 while we are sleeping or are concerned with other matters? It 

 is, of course, another of those automatic mechanisms which have 

 been formed in the animal body during millions of years of 

 trial and sifting. The lowest part of the brain, a part called the 

 medulla oblongata, includes a nerve-centre which is sensitive to 

 the carbonic acid in the blood and, stimulated by this, sends 



