34 GENERAL ZOOLOGY 



of reproductive and urogenital systems and the mouth of the 

 bladder. The large intestine serves to store fecal matter until 

 it can be voided. 



The frog obtains oxygen by absorption through the skin, the 

 surface of the buccal cavity, and the lining of the lungs. If the 

 alimentary canal is dissected out as shown in Fig. 12, the lungs 

 are seen as two conical sacs connected to an oblong structure, the 

 larynx, attached to the ventral surface of the wall of the pharynx. 

 The glottis, a slitlike opening, seen in the floor of the pharynx 

 behind the tongue when the mouth is opened, leads to the larynx. 

 From the internal surface of the lung, septa project inward and 

 divide the lung cavity into alveoli, all of which open into the 

 central, undivided cavity of the lung (Fig. 14). Bands of muscle 



Ai 



Fig. 14. — Cross section of lung of Rana pipiens. A, arteries; V, veins. 



are found on the inner, free edges of the septa, from which thinner 

 strands of muscle extend through the septa to the wall of the 

 lung, which likewise contains scattered muscle tissue fibers. 

 The contraction of this muscle tissue under certain conditions 

 collapses the lung and almost completely empties it of air. The 

 cells at the free edges of the septa are ciliated and among the 

 ciliated cells are found mucus-secreting goblet cells. Elsewhere 

 the alveoli are covered by a flattened layer of cells. The septa 

 are richly supplied with blood vessels, through which oxygen is 

 absorbed from the air in the lungs and from which carbon dioxide 

 is given off. 



A living frog displays two kinds of respiratory movements in the 

 throat region: (1) a regular series of shallow pulsations, and (2) 

 a deeper lowering and raising of the throat repeated several times. 

 During the period of shallow respiratory movements, the external 

 nares are open, the glottis closed, and air is drawn in and out of 

 the buccal cavity through the nasal passages. The ventilation 



