284 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



Polyplerus, which is looked upon as representing the ancestors of the 

 tetrapoda. Here the walls of the bladder have alveolar walls like 

 the lungs of higher vertebrates, while the pneumatic duct and the 

 blood supply are like the corresponding structures in the tetrapoda. 

 On the other hand, the dorsal position of the pneumatic duct and 

 the arterial blood supply in fishes are difiicult to reconcile with con- 

 ditions in the higher vertebrates. Favoring the other view are the 

 following facts. The lungs are paired outgrowths from the pharynx 

 immediately behind the last gill clefts and in serial order with them; 

 the blood supply from the sixth arterial arch is in full accord with 

 this view, while the skeletal supports of larynx and trachea have, in 

 the early stages and more primitive forms, the relations and appear- 

 ance of rudimentary gill arches, while the muscles of the region are 

 modified from those of the visceral arches. 



The mechanisms by which air is caused to enter the lungs (in- 

 spiration) or is expelled from them (ex^ation) differ considerably 

 in the various classes. In the amphibia air is drawn into the mouth 

 via the nares by depressing the floor of .the oral cavity. Then, the 

 nares being closed by small muscles, the contraction of the mylohyoid 

 muscle forces the air into the lungs. Expiration is affected in part by 

 the elasticity of the lungs, in part by the muscles of the body wall. 

 In most reptiles the position of the ribs is altered by the action of the 

 intercostal muscles, thus altering the size of the pleuro-peritoneal 

 cavity, to accommodate which air is drawn into and expelled from 

 the lungs. It is difiicult to understand how inspiration is effected in 

 the chelonia, but transverse muscles run ventral to the lungs, and 

 these by their contraction, expel the air. Inspiration in other 

 reptiles is not understood, expiration is effected by the muscles in 

 the walls of the lungs and by the transverse abdominal muscles. In 

 the birds the lungs are attached to the ribs and vertebrae, so that any 

 motion of the latter necessitates a change in shape and size of the 

 lungs. In addition the air sacs, as noted above, may play a part in 

 the movement of the air. 



In the mammals the ribs are hinged at an oblique angle to the 

 vertebral column, the angle being changed accordingly as the inter- 

 costal muscles are contracted or relaxed, and thus the size of the 

 thoracic cavity is increased or diminished. Then the diaphragm 

 (p. 140) also plays an important part in this alteration in size. This 

 transverse muscle forms a complete partition between pleural and 

 peritoneal cavities, projecting into the former like a dome when re- 



