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 cesophagus 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 
