AMPHIBIA 221 



and this opens widely into the saeculus, which bears a lobe, 

 the lagena, identified with the cochlea of higher vertebrates. 

 The utriculus opens into a small ductus endolymphaticus. The 

 whole labyrinth is distended by endolymph and lies in the 

 auditory capsule, which, as has been noted, is formed of 

 cartilage and of bone. The narrow space between the internal 

 ear and the capsule is occupied likewise by a fluid, the peri- 

 lymph. The auditory nerve splits into branches on entering 

 the capsule of the ear, and these end in special sensory areas 

 in the ampullae, utriculus, and sacculus. The endolymph 

 provides calcareous granules which are related to the sensory 

 cells in these regions. An important modification is related 

 to the ear externally to the capsule. The Eustachian tube 



Ductus 

 endolymphaticus 



Horizontal 

 semicircular canal ' I *3- Lagena 



Sacculus 

 FIG. 110. Auditory organ. 



represents the spiracular cleft of the fish, and it expands as a 

 cavity which penetrates in the space above the quadrate and 

 forms the cavity of the middle ear. The cavity comes into 

 close relationship with the skin and forms the tympanum or 

 drum, and to this, as has been explained, the columella auris is 

 applied ; the other end of the columella ends in the stapes. 

 The sound waves of the air, therefore, cause the thin 

 tympanum to vibrate, and the vibrations are communicated 

 to the stapes and so to the fenestra ovalis. They thus reach 

 the perilymph and, through the thin wall of the labyrinth, 

 the endolymph ; and the waves, now fluid waves, set the 

 otoliths or calcareous granules in motion and the sensory 

 cells are affected. The endolymph also, by its movements in 

 the semicircular canals, is constantly communicating to the 

 brain changes in position and direction and in equilibrium. 



Urinogenital Organs. The pronephros functions as the 

 kidney in the larva. It arises in the nephrotome mesoderm 



