i8i ELEMENTARY BIOLOGY. [CHAP. 



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

 somewhat 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 pro- 

 duced 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 mus- 

 cular 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 whicli 

 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 

 shut, and it is said that frogs may be asphyxiated by keeping 

 their mouths open. 



