HO W ANIMALS BREATH h\ 



749 



a relation to other vertebrates similar to that of insects among the 

 invertebrates. They lead an insect-like existence; and their rapid 

 respiration is effected not, as in mammals, by minute partitioning and 

 subdivision of the lungs, but, as in insects, by extension and increase in 

 capacity. And the air-cavities in the bones and tissues bring the air, 

 as in insects, into effective contact with the capillaries of the system. 



In mammals, respiration is quite restricted to the lungs, the skin 

 performing in man only about one fiftieth part of the work. The 

 lungs are less in proportionate bulk than those of reptiles and birds ; 

 but the lack of capacity is compensated by the minute subdivision of 

 the cavity, giving immensely greater surface. The active carnivores 

 possess the largest lungs. 



The ultimate cells of the human lungs are from one two-hundredth 

 to one seventieth of an inch in diameter, and in number are about six 

 hundred million. Mammalian lungs are always partially filled with 

 air, and only by great pressure can the air be sufficiently expelled from 

 the lung-tissue to allow it to sink in water. This property has given 

 the lungs the vulgar name of " lights." 



^ig. 10. Bronchi and Lungs of Man. (Sappey.) 1. 1, summit ofthelnnjrs ; 2, 2, base of the 

 lun^s ; 3, trachea ; 1. risht bronchus ; 5. division to the upper lobe of the lnng ; <i. division 

 to the lower lobe ; 7, left bronchus; 8, division to the upper lobe ; 0, division to the lower 

 lobe : 10, left branch of the pulmonary artery : 11, right branch ; 12, left auricle of the heart. ; 

 13, left superior pulmonary vein; 14, left inferior pulmonary vein; 15, right superior pul- 

 monary vein ; 10. right inferior pulmonary vein ; IT, inferior vena cava ; 18, left ventricle of 

 the heart; 19, right ventricle. 



That portion of the contained air which in life can not be expelled 

 from the lungs is called residual air. The amount of air moved in 

 ordinary breathing employs but a small part of the breathing capacity, 



