ORGANS OF RESPIRATION 



VXi 



opening into the windpipe is covered by the epiglottis 

 (Greek, epi, upon; glotta, tongue), which is raised dur- 

 ing breathing and closed when food is swallowed. The 

 windpipe divides into two branches, one entering each 

 lung. Each branch is called a bronchus. The windpipe 

 and bronchi are the air passages which cany air to the 

 lungs. These passages are kept open by numerous stiff 

 cartilage rings, which, in the trachea, are not entirely 

 complete on the side of the 

 esophagus, and in the smaller 

 tubes even less so. 



On entering the lung each 

 bronchus divides into branches 

 which in turn branch out again 

 and again, until the entire lung 

 is penetrated in all its parts by 

 these passages. Finally each 

 branch ends in a small pouch- 

 like sac called an air cell. The 

 walls of the air cells are thin, 

 and the cells themselves are 

 surrounded by minute branches 

 of the blood vessels. It is esti- 

 mated that the highly folded condition of the walls of the 

 bronchi make a surface larger than the entire surface of 

 the body. All these thin walls of the lungs and blood 

 vessels are adapted to the passage of oxygen into tin- 

 blood. 



The lungs of man, then, consist of two large bronchial 

 air tubes, many brandies of the bronchi, air cells, blood 

 vessels, and a few nerves, all bound up into two definite 

 bodies (Figure 206). 



The voice box or larynx (la r' inks') is found just below 

 the opening into the windpipe and is called " Adam's 



Figure 206. — Lungs and 

 Heart. 



Note the branches of the 

 bronchus and blood vessels on 

 the right side. 



