PROBLEM 4. How the Cells Are Provided with Oxygefi 



shows the increase in girth (distance 

 around) of the chest. As you read this 

 take a deep breath. Can vou feel the 

 muscular effort involved? The muscles 

 are attached to the ribs. As they contract 

 they force the breast bone up and out in 

 such a way that the cavity becomes 

 larger. You can make a very simple 

 model to demonstrate this movement of 

 the ribs by doing Exercise 4. 



This is not the only way in which the 

 chest cavity becomes larger. A thick 

 sheet of muscle called the diaphragm 

 (dye'a-fram) makes up the lower wall 

 of the chest cavity. This diaphragm com- 

 pletely separates the chest from the ab- 

 dominal cavity. There are a few large 

 holes in the diaphragm through which 

 the food pipe and some of the main blood 

 vessels pass, but the diaphragm and the 

 tubes are joined so closely that neither 

 air nor liquid can slip between them. 

 Now this sheet of muscle is dome shaped 

 when relaxed (see Fig. 229). As it con- 

 tracts it flattens. This flattening of the 

 diaphragm makes the chest cavity larger 

 from top to bottom. It occurs at the 

 same time as the raising of the ribs, so 

 that the size of the chest cavity is in- 

 creased considerably in two directions: 

 from back to front and from top to 

 bottom. 



The contraction and flattening of the 

 diaphragm and the contraction of the 

 muscles which raise the ribs and breast- 

 bone are what we call breathing move- 

 ments. With every contraction of these 

 various muscles, you breathe in. With 

 the relaxation of the rib muscles the ribs 

 once more move down and in; and as the 

 diaphragm relaxes, it arches up again. 

 The chest cavity thus becomes smaller. 



229 



This relaxation of the muscles requires 

 no effort. And as you relax the muscles 

 you breathe out. Take a deep breath 

 again. You can feel the effort of inhaling 

 and the relaxation of exhaling. 



How the lungs are filled with air. If you 

 dip the open end of a medicine dropper 

 into water while squeezing the bulb and 

 then let the bulb regain its normal size, 

 water rushes up into the dropper. The 

 water is pushed up into the dropper by 

 the force of the air pressure down on the 

 surface of the water outside the dropper. 

 Since air cannot be seen pushing any- 

 thing, you may think that the water is 

 "pulled up" into the dropper or that 

 there is a vacuum which the water must 

 fill. But these explanations have been 

 shown by many experiments to be in- 

 correct. The water is between two pres- 

 sures. There is the normal air pressure 

 on the outside, pushing against the sur- 

 face of the water in the dish, and there 

 is air pressure on the inside of the drop- 

 per. But you have squeezed much of the 

 air out of the dropper and as you relax 

 the bulb the air inside will have little 

 pressure. The outside air pressure is 

 greater and it pushes water into the 

 dropper. 



Just as water can be pushed by the 

 pressure of air, so air itself or any gas 

 can be pushed. If you put the open end 

 of the dropper into a smoke cloud you 

 would see the smoke move in just as the 

 water did. 



This is exactly what happens when 

 you breathe. While you are not breath- 

 ing in or out, the pressure of air in the 

 air sacs and the pressure of air outside 

 are equal. Air does not move in or out. 

 But as soon as you increase the size of 



