CHAPTER IX. 

 EXTERNAL RESPIRATION. 



The Close Relation between Circulation and Respiration. 

 Is it not a very striking fact that we breathe once for 

 every four heart beats ? And that whatever quickens the 

 breathing also quickens the heart so that the two always 



keep in almost the 

 same ratio? Let us 

 try to learn why this 

 is so. 



The Organs of Res- 

 piration. i. The 



lungs and air tubes. 

 2. The organs which 

 increase and dimin- 

 ish the size of the 

 chest, chief among 

 which are the dia- 

 phragm and the mus- 

 cles acting on the 

 ribs. 



Fig. 52. 



The Trachea and Bronchial Tubes, showing 

 Two Clusters of Air Sacs. 



External Features 

 of the Lungs. The lungs are of a pinkish color; they are 

 very elastic, soft, smooth, and moist. 



The Air Tubes. The windpipe, or trachea, has in its 

 walls rings of gristle or cartilage, which keep the tube 

 always open. These rings of cartilage are not complete 



