190 MANUAL OF ANIMAL BIOLOGY 



sacs as previously noted. The outer surface of the lungs is smooth, 

 but the inner lining is thrown into folds, or wrinkles, which form 

 tiny air-cavities, or alveoli, over the entire inner surface. All of 

 these open into the central cavity, and it receives the air from the 

 mouth cavity. The walls of the alveoli are richly supplied with 

 blood vessels, and it is through these walls that the necessary inter- 

 change of gases takes place. (W. f. 107.) 



Anteriorly each lung opens into the common tracheal cavity, 

 and the latter communicates directly with the mouth cavity 

 through the tiny slit-shaped opening, or glottis, in the ventral wall 

 of the mouth, as previously noted. Just below the glottis, the 

 walls of the tracheal cavity are strengthened by cartilages which 

 are in turn connected to muscles. This region constitutes the 

 voice box, or larynx. A pair of vocal cords, like elastic bands, 

 are so attached to the sides that they vibrate when the air, expelled 

 from the lungs, rushes past them. In this way sound is produced, 

 not only in the Frog, but generally in the Vertebrates. The pitch 

 of the sound can be altered by varying the tension of the cords 

 through the muscular action. 



When the respiratory movements are observed in a living Frog, 

 two distinct types can be seen. In the first type, which is confined 

 to the mouth only, there is an almost continual up and down move- 

 ment noted of the ventral muscular wall of the mouth. This 

 results in the alternate expansion and contraction of the mouth 

 cavity and a consequent intake and outflow of air through the 

 external nares. The mouth is always kept closed except when 

 feeding. In fact it is known that breathing cannot occur if the 

 mouth is forcibly kept open, and the Frog will die of asphyxiation. 

 Air coming from the lungs periodically is gradually released to the 

 exterior as stated above. Thus the mouth cavity always contains 

 air, and undoubtedly considerable respiratory interchange takes 

 place through the epidermal lining tissues. 



In the main, during the respiratory movements affecting the 

 lungs, which occur at variable periods, the external nares are closed, 

 the glottis opened, and a much heavier contraction of the ventral 

 mouth wall takes place. The imprisoned air in the mouth cavity 

 is thus forced through the glottis and into the lungs. The peri- 

 odical expiration of the air from lungs into the mouth cavity is 

 largely brought about by a contraction of the muscular body wall 

 in the fore leg region. 



