174 ELEMENTARY BIOLOGY. [XIIL 



them is -what answers to the glottis in Man. It is by their 

 vibration that the croak of the Frog is produced. 



Laterally the laryngo-tracheal chamber opens into the 

 lung of each side. The lung is a transparent oval sac, some- 

 what pointed posteriorly, which lies at the side of the 

 oesophagus in the dorsal region of the abdominal cavity. 

 It is covered by a layer of the pleuroperitoneal membrane 

 which represents the visceral layer of the pleura in the 

 higher animals. The wall of the pulmonary sac is produced 

 inwards so as to give rise to septa, which are much more 

 prominent and more numerous in the anterior than in the 

 posterior part of the lung and divide the periphery of the 

 cavity into numerous air-cells, on the walls of which the 

 ramifications of the pulmonary artery are distributed. 



The lungs are elastic, the distended lung collapsing sud- 

 denly when it is pricked, and they contain abundant muscu- 

 lar fibres. 



Inspiration is effected in the Frog by a buccal force-pump. 

 The mouth being shut and the external nostrils open, the 

 floor of the mouth is depressed, and the buccal cavity fills 

 with air. The nostrils being then shut, the hyoid, and with 

 it the floor of the mouth, is raised, the aperture of the gullet 

 being at the same time closed. Thus the air is forced 

 through the glottis and distends the lungs. 



In ordinary expiration, the elasticity of the lungs and the 

 pressure of the surrounding viscera probably suffice to expel 

 the air ; but this operation may be powerfully aided, firstly 

 by the contraction of the intrinsic muscular fibres of the 

 lungs ; secondly, by the contraction of the muscles of the 

 lateral and ventral regions of the abdominal wall ; and thirdly, 

 by the contraction of those muscular fibres which enter into 

 the diaphragm; as all these actions tend, either directly or 

 indirectly, to diminish the capacity of the lungs. 



It is essential to inspiration that the mouth should be 



