98 SCIENCE PRIMERS, [§ vi. 



42. Other structures besides the diaphragm assist in 

 pumping air in and out of the lungs. By the action 

 of the diaphragm the chest is alternately lengthened 

 and shortened. But if you watch anyone, and espe- 

 cially a woman, breathing, you will notice that with 

 every breath the chest rises aiid falls; the front of 

 the chest, the sternum, as you have learnt to call it, 

 comes forward and goes back ; and a little attention 

 will convince you that it comes forward during in- 

 spiration, />. while the diaphragm is descending, and 

 falls back during expiration. But this coming for- 

 ward of the sternum means a widening of the chest 

 from back to front, and the falling back of the ster- 

 num means a corresponding narrowing. So that 

 while the chest is being lengthened by the descent 

 of the diaphragm, it is also being widened by the 

 coming forward of the sternum. In inspiration the 

 lungs are expanded not only downwards, by the move- 

 ment of the diaphragm, but also outwards, by the 

 movement of the walls of the chest. 



What thrusts forward the sternum? If you were 

 to watch closely the sides of the chest of a very 

 thin person, you would be able to notice that at every 

 breathing in, at every inspiration, the ribs are pulled 

 up a little way. Now, each rib is connected with the 

 backbone behind by a joint, and is firmly fastened 

 to the sternum in front by cartilage (see Frontis- 

 piece). If you were to fasten a piece of string to 

 the middle of one of the ribs and to pull it, you 

 would find you were working on a lever, with the 

 fulcrum at the backbone, with the weight acting at 

 the sternum, and the power at the point where your 

 string was tied. Every time you pulled the string the 



