RESPIRATORY ORGANS. 263 



matic duct and the blood supply in Polypterus. On the other hand the dorsal 

 position of the opening of the duct into the oesophagus and the arterial supply from 

 the aorta in fishes are difficult to reconcile with the conditions obtaining in the 

 tetrapoda. Favoring the gill-pouch theory are the following facts. The lungs are 

 paired outgrowths from the pharynx immediately behind the last gill cleft; the 

 blood supply can readily be derived from the branchiate condition; while the skeletal 

 supports of the larynx have the appearance of rudimentary visceral arches, and the 

 muscles of the region are modified from those of the gill arches. 



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

 spiration) or is expelled from them (expiration) 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 difficult 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. 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 verte- 

 bral column, the angle being changed accordingly as the intercostal 

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

 cavity is increased or dimininshed. Then the diaphragm (p. 135) 

 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 relaxed. When 

 it contracts it flattens, thus increasing the size of the pleural cavity 

 and drawing air in through the trachea. The abdominal muscles 

 also have their effect. Expiration is caused in part by the action of the 

 intercostal and abdominal muscles, in part by the elastic tissue and 

 smooth muscles in the lungs themselves. 



ACCESSORY RESPIRATORY STRUCTURES. 



Allusion has already been made to the pharyngeal and dermal 

 respiration of the amphibia (p. 258). There are several fishes in which 



