loo SCIENCE PRIMERS, [§ vi. 



will be enough to say that they act like the piece 

 of string we are speaking of. When they con- 

 tract, they pull up the ribs and thrust out the 

 sternum ; when they leave off contracting, 

 the ribs and sternum fall back to their pre- 

 vious position. 



There are many other muscles which help in 

 breathing, especially in hard or deep breathing, but 

 it will be sufficient for you to remember that in ordi- 

 nary breathing there are two chief movements taking 

 place exactly at the same time, by means of which 

 air is drawn into the chest, both movements being 

 caused by the contraction of muscles. First, the dia- 

 phragm contracts and flattens itself, making the chest 

 deeper or longer ; secondly, at the same time the ribs 

 are raised and the sternum thrust out by the contrac- 

 tion of the intercostal muscles, making the chest 

 wider. But as the chest becomes wider and longer, 

 the lungs become wider and longer too. In order 

 to fill up the extra room thus made in the lungs, 

 air enters into them through the windpipe. This is 

 inspiration. But soon the diaphragm and the inter- 

 costal muscles cease to contract; the diaphragm re- 

 turns to its arched condition, the ribs sink down, the 

 stemum falls back, and the extra air rushes back 

 again out of the lungs through the windpipe. This 

 is expiration. An inspiration and an expiration 

 make up a whole breath ; and thus we breathe some 

 seventeen times in every minute of our lives. 



43. But what makes the diaphragm and intercostal 

 muscles contract and rest in so beautifully regular a 

 fashion ? The biceps of the arm, we saw, was made 

 to contract by our will. It is not our will, however, 



