334 



COLLEGE ZOOLOGY 



lungs unites readily with the hemoglobin in 

 the red corpuscles. The hemoglobin com- 

 bines with the oxygen to form a compound 

 which is then transported by the blood from 

 the respiratory organs to the capillaries, 

 where it breaks up, the oxygen being ab- 

 sorbed by the tissues. Carbon dioxide from 

 the tissues is carried to the lungs and dis- 

 charged to the outside. 



External respiration is carried on by gills 

 in most aquatic vertebrates and by lungs in 

 terrestrial vertebrates. Respiration in the 

 frog, as just described, is carried on largely 

 by the lungs, but takes place also, to a con- 

 siderable extent, through the skin. As shown 

 in Fig. 216, air passes through the nostrils or 

 external nares into the nasal cavity and then 

 through the internal or posterior nares into 

 the mouth cavity. The external nares are 

 then closed, the floor of the mouth is raised, 

 and the air is forced through the glottis into 

 a short tube, the lar\'nx, then into a very 

 short tube, the bronchus, and thence into 

 the lungs. Air is expelled from the lungs 

 into the mouth cavity by the contraction of 

 the muscles of the body wall. In addition to 

 the skin and lungs, some gaseous exchange 

 takes place through the mucous membrane 

 lining the mouth. 



The air in the mouth cavity is changed 



Hyoid- 



Figure 217. The leopard frog {Rana pipiens) is 

 one of the most common amphibians in North 

 America. Note the expanded vocal sacs between ear 

 and shoulder; these are found only in the male. 

 (Courtesy of American Museum of Natural History.) 



by throat movements. The glottis remains 

 closed, while the floor of the mouth is al- 

 ternately raised and lowered. Air is thus 

 drawn in and expelled through the nares. 



The lungs (Fig. 218) are ovoid sacs with 

 thin elastic walls. The inner surface of the 

 lungs is divided by a network of partitions 

 into many minute chambers called alveoli. 

 Blood capillaries are numerous in the walls 



Larynx (voice box) /!! 

 Glottis ^ 



Pulmonary artery 

 Pulmonary vein 



Cartilage in 

 wall of glottis 

 Bronchus 



Ventral view of larynx 



(voice box) opened to 



show vocal cords 



Cross section of lung 

 showing inner partitions 



Figure 218. Respiratory organs of the bullfrog. 



