150 



THE RESPIRATORY SYSTEM 



epithelium, and the hmg develops simple alveolar cavities which 

 increase the respiratory area. 



thclium 

 —Capillary 



Muscle 



Lymphatic 



Conn, i issue 



'Muscle 



Epithelium 



Fig. 92. — Wall of the lung of Neoturus. 



THE RESPIRATORY SYSTEM OF REPTILES. 



With the reptiles the lungs take on the entire burden of respiration 

 and dispense w^th the accessory skin system of amphibians. Other 

 structures associated with the mammalian lung also develop, the 

 nasal passage connecting with the pharynx and cartilaginous rings 

 about the trachea. The lungs themselves become increased in 

 respiratory capacity by horizontal and vertical folds bearing the 

 alveolae with an accompanying increase in the vascular supply. 

 (Fig. 93.) The stratified ciliated columnar epithelial lining of the 



Epithelium 

 Muscle 



Muscle-^ 



Epithelial wall "*"--- — : 



Fig. 9.'5. — Wall of lung of water snake (Xatrix). 



pharynx becomes a simple epithelium lining the respiratory surfaces 

 of the lungs. 



THE RESPIRATORY SYSTEM OF MAMMALS. 



When the mouth of a mammal is closed, air is taken in through 

 the nasal ])assages to the pharynx, thence into the larynx and on to 

 the trachea. The trachea divides into two bronchi, one to each 

 lung. From th(> bronchi, air passes through smaller branches to 



