132 77/1? HUMAN BODY. 



The diaphragm is attached to the lower edge of the 

 walls of the chest, and stretches across, separating chest 

 from abdomen, forming the floor of one and the roof of 

 the other. But it is not a flat floor. It arches up into the 

 chest, as may he seen in the figure, and is supported in 

 this position by the contents of the abdomen beneath it. 

 When we take a breath, the diaphragm contracts, 

 being partly muscle, and the top of the arch is flattened, 

 making more room in the chest. At the same time, the 

 contents of the abdomen are pressed down, and the walls 

 of that cavity expand to accommodate them. The ab- 

 domen is not made larger when we take breath. It ex- 

 pands in front just enough to make up for what it loses 

 by the flattening of its roof. 



4, But the chest is enlarged, not only by the descent of 

 its floor, but also by the raising of the ribs and breast- 

 bone. The ribs are joined to the backbone behind, and 

 connected with the breast-bone by the costal cartilages, 

 which will bend. To the ribs are attached muscles which 

 pull them up, and others which pull them down. The 

 former are called inspiratory muscles, and the latter expir- 

 atory muscles. Now, clasp your hands, and extend your 

 arms in front of you, slanting a little downward. Your 

 hands represent a portion of the breast-bone, and your 

 arms two of the ribs. Keeping your hands clasped, raise 

 your elbows as much as you can, and raise your hands a 

 little. That is the motion that the inspiratory muscles 

 give to the ribs and breast-bone. You can see that the 

 space included between them is enlarged in both direc- 

 tions. 



5, We have been speaking of the cavity of the chest. 

 The air enters the lungs. But the lungs occupy almost 



