THE ANIMAL ORGANISM 



35 



of the buccopharyngeal region is accompanied by the absorption 

 of oxygen through the lining of the mouth. When these shallow 

 movements are interrupted by a deep lowering of the throat, an 

 increased volume of air is drawn in ; but since before the comple- 

 tion of this movement the external nares are closed and the 

 glottis opened, air passes from the lungs into the buccal cavity. 

 The vigorous contraction of the throat, which now follows, 

 forces the mixed air back into the lungs. One or more repetitions 

 of these deeper respiratory movements of the throat muscles 

 cause the air to surge back and forth from the lungs to the mouth, 

 after which the glottis closes, the 

 nares open, and the shallow respira- 

 tion is resumed. The closure of the 

 external nares is effected by a slight 

 forward movement of the lower jaw 

 which wedges the tuberculum pre- 

 linguale of the lower jaw between 

 the two elements forming the tip of 

 the upper jaw, the premaxillary 

 bones, spreading them and closing 

 off the nares. 



It is known that oxygen is 

 absorbed through the skin of the 

 frog throughout the entire year. 

 This rate of oxygen absorption 

 through both lungs and skin reaches its maximum during the 

 spawning season. It is interesting that a greater amount of 

 carbon dioxide is released from the skin than from the lungs. 

 During the winter, with a general reduction of respiratory 

 activity, the amount of carbon dioxide given off is at its lowest 

 level. The respiratory function of the skin is highly important 

 in the frog. A damp or moist environment is necessary for a 

 frog, since the skin cannot prevent the loss of body fluids, in a 

 dry warm atmosphere. 



The vocal cords are folds in the lateral walls of the larynx form- 

 ing thickened lips, lying parallel to the edges of the glottis. They 

 can be seen best by laying open the larynx with a cut through 

 the ventral wall (Fig. 15). In producing sounds, air is forced 

 back and forth from the lungs to the buccal cavity, thus causing 

 the vocal cords to vibrate. Since sounds can be produced with- 



Fig. 15. — View of interior of right 

 half of larynx of Rana pipiens. g, 

 margin of glottis; l, lung;vc, vocal 

 cord, attached at its ends and along 

 its length to the wall of the larynx. 



