142 THE FROG CHAP. 



with blood-vessels fed by the pulmonary artery, the 

 blood being carried away by the pulmonary vein. 



The main substance of the lung is made of connective- 

 tissue containing elastic fibres and unstriped muscle, and 

 traversed by a network of capillaries. Its cavity is lined 

 by a layer of pavement epithelium, and its outer surface 

 is covered with peritoneum. 



Respiratory Movements. In breathing, the frog 

 depresses the floor of the buccal cavity (Fig. 45, A), and 



FIG. 44. The respiratory organs of the Frog from the ventral aspect ; B, the laryngo- 



tracheal chamber in longitudinal section, with the right lung. ( X i.) 

 ar. the arytenoid, or principal cartilage of the larynx ; b. hy. body of hyoid ; gl. 

 glottis ; 1. Ing. left lung ; 1. tr. c. laryngo-tracheal chamber ; p. c. hy. posterior 

 horn of hyoid ; r. In,",, right lung, laid open in A to show its internal surface ; 

 v. cd. vocal cord. (After Howes.) 



the mouth being kept shut, air is drawn in through the 

 nostrils. The floor of the mouth is then raised (B) by 

 muscles attached to the hyoid. At the same time the 

 anterior end of the lower jaw presses upon the movable 

 premaxillae (pmx), the upward processes of which (p. 43, 

 Figs. 8 and 9, PMX) act upon certain cartilages in 

 connection with the external nostrils in such a way as 

 to produce closure of these apertures (Fig. 45, B). The 

 gullet (gul) is so contracted, except during the act of 

 swallowing, as to be practically closed. Thus when the 

 floor of the mouth is raised, the air contained in it can 



