CHAPTER VI. 



RESPIRATION. 



The Close Relation between Circulation and Respira- 

 tion. Is it not a very striking fact that we take one 

 breath for every four heart beats ? That whatever quick- 

 ens the breathing also quickens the heart, so that the two 



always keep in al- 

 most the same ratio ? 

 Let us learn what 

 are the many inti- 

 mate relations of 

 the blood pump and 

 the air pump, the 

 blood system and 

 the air system, of 

 Circulation and Res- 

 piration. 



The Organs of 

 Respiration. 



1 . The lungs and 

 air tubes. 



2. The structures 

 which increase and diminish the size of the chest, princi- 

 pally the diaphragm, and the muscles acting on the ribs. 



The Parts of the Lungs. i. The Air Vesicles, an 



immense number of small sacs, which communicate with 



8 4 



Fig. 37. The Trachea and Bronchial Tubes, showing 

 Two Clusters (Alveoli)) of Air Vesicles. 



