366 ANIMAL PHYSIOLOGY. 
thoracic chamber may be said to be reserved for circulatory 
and respiratory organs which, we again point out, are so related 
that they really form parts of one system. 
The mammal’s blood requires so much aération (ventilation) 
that the lungs are very large and the respiratory system has 
become greatly specialized. We no longer find the skin or ali- 
mentary canal taking any large share in the process; and the 
lungs and the mechanisms by which they are made to move the 
gases with which the blood and tissues are concerned become 
very complicated. 
Fic. 296.—Bronchia and lungs, posterior view (Sappey). 1,1, summit of lungs ; 2, 2, base of 
lungs ; 3, trachea ; 4, right: bronchus; 5, division to upper lobe of lung: 6, division to 
lower lobe ; 7, left bronchus ; 8, division to upper lobe ; 9, division to lower lobe ; 10, left 
branch of pulmonary artery ; 11, right branch ; 12, left auricle of heart ; 13, left superior 
pulmonary vein ; 14, left inferior pulmonary vein ; 15, right superior pulmonary vein ; 16, 
aoe pases pulmonary vein ; 17, inferior vena cava ; 18, left ventricle of heart ; 19, right 
ventricle. 
Our studies of muscle physiology should have made clear 
the fact that tissue-life implies the constant consumption of 
oxygen and discharge of carbonic anhydride, and that the pro- 
cesses which give rise to this are going on at a rapid rate; so 
that the demands of the animal for oxygen constantly may be 
readily understood if one assumes, what can be shown, though 
less readily than in the case of muscle, that all the tissues are 
constantly craving, as it were, for this essential oxygen—well 
called “ vital air.” 
