RESPIRATION. 229 



cavity of the chest, in which the lungs are con- 

 tained, is covered by the ribs and breast-bone, and 

 is divided by a broad muscle from the abdomen. 

 This muscle, which is named the diaphragm, is 

 the great agent in breathing. There are also seve- 

 ral other muscles fixed to the ribs and other parts 

 which assist it. The lungs themselves are in two 

 divisions, lying one on each side of the chest, with 

 the heart between them. These divisions are 

 termed the right and left lobes. 



When we feel the front of our necks, we find a 

 hard body. This is a tube, and composed of carti- 

 laginous, or gristly rings, so that it is always kept 

 open, and is named the trachea, or windpipe. It 

 is through this that the air is conveyed to and 

 from the lungs. 



The windpipe opens at the back of the mouth, 

 before the entrance into the gullet, so that all our 

 food has to pass over it. How does it happen that 

 none of the food gets into it? Now and then, in- 

 deed, this does occur, and is followed by the most 

 serious consequences, and often by death. 



When we are eating fast or carelessly, a crumb 

 will sometimes slip into what we familiarly call 

 the wrong passage, that is, it gets into the wind- 

 pipe. To prevent this constantly happening, it is 

 protected in a very curious way. The opening into 

 it is a narrow slit, called the rima; the part itself 

 is named the glottis ; and over the slit is fixed a 

 moveable little body, or valve, called the epi- 

 glottis; and this is so arranged, that whenever we 

 U 



