52 THE ORIGIN OF MAN 



rich in blood vessels. In the salamanders of the 

 land and in all of the tailless Amphibia, the gills 

 disappear and adult respiration is carried on 

 wholly by lungs as in the higher vertebrates. 

 While the gills are present, the air passages to 

 the lungs through the nose do not open into the 

 mouth, but assume this position as soon as the 

 gills vanish and the lungs become functional. 

 The air is then taken in through the nasal open- 

 ings and in the frogs is forced into the lungs in 

 a swallowing manner. In none of the fishes are 

 there functional ears, though there are internal 

 ones; but in the amphibians, however, there are 

 distinct organs of hearing. In the frogs these 

 are best seen where the tympanic membrane of 

 the ear drum is a more or less large circular disc 

 embedded in the outer skin on the sides of the 

 head. The cavity behind the disc or drum con- 

 nects with the back part of the mouth by a tube 

 known as the Eustachian tube which is also pres- 

 ent in all of the higher vertebrates. The first 



