22 ELEMENTARY BIOLOGY. [CHAP. 



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

 cations of the pulmonary vessels are distributed. 



The lungs are elastic, the distended lung collapsing sud- 

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

 cular fibres. 



It is essential to respiration that the mouth should be 

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

 their mouths open. 



Inspiration is effected 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 muscles of the flank next come into play, exer- 

 cising a pressure upon the surrounding viscera which suffices 

 to expel the air (expiration). 



The nostrils being then shut, the hyoid, and with it the 

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

 being at the same time closed, air is forced through the 

 glottis, distending the lungs. 



Expiration is doubtless aided by the contraction of the 

 intrinsic muscular fibres of the lungs ; and it may be that 

 those fibres which form the so-called diaphragm also tend 

 to diminish the capacity of the lungs. 



In addition to its principal pulmonary apparatus of re- 

 spiration, the Frog has a secondary respiratory apparatus in 

 its moist and delicate skin. A considerable amount of venous 

 blood is, in fact, constantly supplied to this organ by the 

 large cutaneous branch of the pulmo-cutaneous artery. It 

 has been experimentally ascertained that frogs in which the 

 lungs have been extirpated will continue to live and respire 

 for a considerable time, especially at a low temperature, by 

 means of the skin. 



The kidneys are elongated and flattened from side to side, 

 and are kept in position by the continuation of the peri- 



