OHAPTEE V. 



RESPIRATION. 



Is it not a very striking fact that we take one breath for 

 every four heart-beats ? That whatever quickens the breath- 

 ing also quickens the heart, so that the two always keep in 

 almost 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 Respiration. 



The Organs of 

 Respiration are : 



1. The Lungs and 

 Air Tubes. 



2. The structures 

 which increase and di- 

 minish the size of the 

 chest, principally 

 the diaphragm, and 

 the muscles acting 



011 the ribs. 



The lungs con- 

 sist of : 



1. An immense 

 number of small 

 sacs, the Air Ves- 

 icles, which com- 

 municate with the 



outer air by the Bronchial Twigs, Bronchi, and Trachea. 



2. ThQ Pulmonary Capillaries, forming a thick network 



105 



Fig. 39. The Trachea and Bronchial Tubes, Showing 

 Two Clusters (Alveoli) of Air Vesicles. 



