CH. VIIl] FROGS, TOADS, AND NEWTS 265 



of the mouth is enlarged and air rushes in through the 

 nostrils. At the same time by the contraction of the 

 same muscles the hyoid apparatus is drawn against the 

 lungs so as to compress them. Hence the air in the lungs 

 begins to pass through the glottis directly the latter is 

 opened. 



Expiration is in part due to the muscles just mentioned, 

 but is caused chiefly by the contraction of the abdominal 

 muscles, particularly the anterior portions of those which 

 run transversely, and by the elasticity of the lungs 

 themselves. 



Injection of the air into the lungs is accomplished 

 primarily by the petrohyoid muscles : there are four of 

 these on each side of the head; the most anterior is a 

 fairly wide sheet, the other three mere slips; they rise 

 from the outer surface of the auditory capsule, pass fan- 

 wise round the floor of the pharynx and ossophagus and 

 are inserted into the mid ventral line of the pharynx and 

 the sides of the hyoid. The effect of their contraction is 

 therefore to draw the floor of the mouth and pharynx up- 

 wards and forwards, and thus forcibly to compress the air 

 contained in the mouth, driving it through the open glottis 

 down into the lungs. At the same time the muscles of 

 the lower jaw contract vigorously, and assistance may also 

 be given by the mylohyoid, geniohyoid and other muscles 

 in the floor of the mouth itself. 



The lateral margins of the nostrils are bounded by the 

 shell-shaped alar cartilages which are moveably attached 

 by their anterior ends to the nasal processes of the pre- 

 maxillary bones. 



