142 



THE FROG 



irregular honeycomb. All these ridges are abundantly 

 supplied 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, the mouth 



r//t^. 



Fig. 44. — The respiratory organs of the frog from the ventral aspect ; B, the laryngo- 

 tracheal chamher in longitudinal section, with the right lung. 

 ar. the arytenoid, or principal cartilage of the larynx ; 6. hy. body of hyoid ; el. 

 glottis ; /. la^. left lung ; /. tr. c. laryngo-tracheal chamber ; /. c. hv. posterior 

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

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



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 moveable premaxillse {pmx), the 

 upward processes of which (p. 42, 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 {gut) is so contracted, 

 except during the act of swallowing, as to be practically 



