THE FROG — A REPRESENTATIVE VERTEBRATE 



407 



terrestrial vertebrates for gas exchange; the delicate respiratory mem- 

 brane in the lungs is protected against excessive desiccation, yet is acces- 

 sible to the external environment. 



Air is taken into the body through the paired external nares, 

 traverses the short nasal cavities, and enters the front of the mouth 

 through the paired internal nares (Fig. 21.11). It then passes 

 through the glottis, a slit-shaped opening in the floor of the pharynx, 

 and into a laryngotracheal chamber (comparable to the larynx and 

 trachea of higher vertebrates). Small cartilages, which are homologous 

 with parts of the visceral skeleton of fish, support this chamber, and a 

 pair of bronchi lead from its posterior corners to the lungs. The lungs 

 of frogs are simple, ovoid sacs in external shape, but their internal sur- 

 face is increased by numerous pocket-shaped folds that give them a 

 honeycomb appearance. 



Air is moved into the lungs by the pumping action of the floor of 

 the mouth. During inspiration the floor of the mouth is lowered and air 

 is drawn into the mouth and pharynx through the nasal cavities. The 

 external nares are then closed by a push of the lower jaw on the movable 

 premaxillary bones. Small valves present in the nares apparently move 

 passively. The floor of the mouth is then raised, and air is forced through 

 the open glottis into the lungs. Since the lungs contain elastic connective 

 tissue, they increase considerably in size as they fill. Expiration results 

 from the elastic recoil of the lungs and the contraction of the abdominal 

 muscles which compresses the internal viscera and lungs. Air does not 

 enter the lungs with each set of throat movements, however; the glottis 



.lusta.ch.icLn tube 



Esoph.a.^u.$ 



Externa.1 na-ris 

 '^ /Premavilla 

 ^ ''^ Internal 



ncLTlS 



o^wer law 



Glottis 

 Vocal cord 



L a.ryn^ot ra.cheal 



chamber 

 ronchus 



Figure 21.11. 

 the frog. 



A diagrammatic longitudinal section of the respiratory system of 



