RESPIRATION. 247 



quantity of deteriorated blood, to enjoy the fullest intercourse 

 with the largest possible quantity of vital air ; and all the 

 mechanism of bones and muscles which I have described, 

 are only subservient to th s end. Now it has been calcula- 

 ted by Hales, that each air cell is the one hundredth part of 

 an inch in diameter, and that the amount of surface furnish- 

 ed by them, collectively, is \gual to twenty thousand square 

 inches. Other physiologists have calculated the surface to 

 be over fifteen hundred cubic feetjand Munroe states that it 

 is thirty times the surface of the numan body) 



Fig. 7. 



1. The larynx ; 2. The trachea ; 3. Right bronchia ; 4. Left 

 bronchia ; 5. Left lung divided into three lobes ; 7. Large bronchial 

 tubes ; 8. Small bronchial tubes ending in air cells or vesicles. 



13. Such is the structure of the vessel which conveys the 

 air to the blood ; let us examine how the blood gets to the 

 air. (This is effected by means of the pulmonary artery, 

 which springs from the right ventricle of the heart, divides 



