RESPIRATORY SYSTEM 769 



to the extremity of the lungs. In the chameleons the septa do not reach 

 the distal wall, consequently the chambers communicate so that the 

 bronchus enters a cavity known as the atrium. This connects with the 

 various chambers separated by the septa and these in turn open into a 

 terminal vesicle. This whole structure seems to anticipate the 

 parabronchi the small uniform sized air-tubes in the lungs of birds 

 which connect the larger secondary branches of the bronchial tubes. 

 There develops in these lizards, long, thin-walled air-sacs from the 

 caudal portion of the lung. These extend among the viscera, even into 

 the pelvic region. The air sacs are used to inflate the body. In turtles 

 and crocodiles there is no atrium and the whole lung has a spongy 

 texture. The bronchus in turtles enters on the ventral side of the lung 

 and not as in lizards in the medial. 



Inhalation and exhalation are effected partly by drawing in the neck 

 and thrusting it out again, thus decreasing and increasing the volume 

 of the thoracic cavity. The air is also swallowed into the lungs by filling 

 and then emptying the throat. 



BIRDS 



Birds have large lungs, each with nine small air-sacs. The air en- 

 ters the bronchi and passes to the air-sacs. The air is thus warmed be- 

 fore being taken into the alveoli of the lungs. It makes its exit through 

 the excurrent bronchi. A complete change of air occurs at each inspira- 

 tion and expiration. The trachea and the larger bronchi are kept open 

 by means of rings of cartilage ; the trachea is enlarged, just before it 

 divides, into a syrinx, or voice box (Fig. 443), a structure limited to 

 birds, and in no way homologous with the larynx of mammals ; the me- 

 chanics of voice production in the birds depends upon forcing the air 

 through a flexible valve, which is set into vibration. The lungs also 

 connect with visceral air-sacs, and with air-spaces in the bones. 



MAMMALS 



There are two points of view regarding the relationship of mam- 

 malian lungs to the respiratory apparatus of the lower forms of animals. 

 One view holds that the lungs are merely a further development of the 

 air-bladder of fishes, while the other insists that they are more likely to 

 be modified gill-pouches which have grown caudally into the coelom; 

 rather than opening to the exterior by growing laterally. 



The fact, however, that the pneumatic duct is dorsal in position and 

 the blood supply is arterial, makes the first view seem improbable. The 

 latter view is supported by the fact that the lungs are paired outgrowths 

 from the pharynx immediately caudal to the last gill clefts and in serial 

 order with them. The blood supply fromithe sixth arterial arch would 

 be in full accord with this view. Then, too, both in the earlier stages 



