264 FROGS, TOADS, AND NEWTS [CH. vm 



sudden outward movement of the skin covering the 

 tympanic membranes. The increased pressure of air 

 within the oral cavity, acting up the Eustachian recesses, 

 forces the membrane outwards so that it becomes for a 

 moment slightly convex. The two last phases follow one 

 another in the order stated and with extreme rapidity. It 

 will thus be seen that the air in the lungs is never entirely 

 renewed, but is merely refreshed by mixing with that aspired 

 into the mouth cavity through the nostrils. The mouth itself 

 is kept firmly closed throughout. Before the introduction of 

 air into the lung is quite complete the nostrils are opened 

 again, or they may be kept slightly open from the com- 

 mencement of this phase ; in either event the elevation 

 of the floor of the mouth expels a certain amount of air 

 from the body through the nostrils. By keeping the 

 nostrils completely closed during "injection" the animal 

 can inflate itself to some extent. On the other hand if 

 the nostrils are artificially or naturally kept open during 

 the complete act of respiration the amount of air in 

 the body is diminished. 



The muscles by which the floor of the mouth is 

 lowered and pulled backwards in aspiration are the 

 sternohyoids arising from the coracoid and clavicle and 

 inserted on to the ventral surface of the body of the 

 hyoid, and the omohyoids arising from the anterior border 

 of the scapula and inserted on to the outer part of 

 the ventral surface of the body of the hyoid. The con- 

 traction of these muscles pulls the flat plate of cartilage, 

 known as the body of the hyoid, which forms the floor of 

 the mouth downwards and backwards. Thus the cavity 



