VIII THE VOCAL AND RESPIRATORY ORGANS 167 



attachment to muscles by which the glottis may be opened 

 or closed. The true sound-producing organs consist of a 

 pair of elastic bands, the vocal cords, extending longitudi- 

 nally across the larynx. They may easily be seen from above 

 by spreading apart the two sides of the glottis, or from below 

 by removing the membranous floor of the laryngeal cavity. 

 Their median edges are thickened and lie near each other 

 in the middle line. Sound is produced by the expulsion of 

 air from the lungs which sets the free edges of the vocal 

 cords in vibration. Variations in the sound are caused by 

 altering the tension on the cords through the action of the 

 laryngeal muscles. The vocal apparatus of the male frog is 

 much larger than that of the female. 



The males of many species of Rana possess a pair of 

 vocal sacs situated at the sides of the pharynx. These sacs 

 are out-pocketings of the pharyngeal wall which extend 

 backward between the skin and the body. They communi- 

 cate with the mouth by small openings in the floor a short 

 distance in front of the angle of the lower jaw. Besides a 

 lining of mucous membrane they possess a muscular coat 

 which consists of fibers drawn out from the subhyoideus 

 muscle. The vocal sacs are distended during the croaking 

 of the frog through the pressure of the air in the buccal 

 cavity. They serve as resonators to reenforce the sound 

 produced by the vocal cords. They are absent in the 

 female. Their size in the males of Rana pipiens is very 

 variable ; in some of the varieties of this species they are 

 absent entirely. 



The Lungs. — The lungs are ovoid, thin-walled sacs of 

 comparatively simple structure. They are capable of great 

 distension and may be readily inflated through the glottis ; 

 they do not collapse when the body is cut open, owing to 

 the fact that the glottis under ordinary circumstances re- 



