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."
Fig. 10. Bronchi and Lungs of Man. (Sappey.) 1, 1, summit of the lungs; 2, 2, base of the lungs; 3, trachea; 4, right bronchus; 5, division to the upper lobe of the lung; 6, division to the lower lobe; 7, left bronchus; 8, division to the upper lobe; 9, 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 pulmonary vein; 16, right inferior pulmonary vein; 17, 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,
