112 



THE THIRD DAY. 

 Fig. 33- 



-pelff' 



[chap. 



B 



■paff' 



httji 



Two Views of the membranous Labyrinth of Coldmba domestica (copied 

 from Hasse). A from the extBrior, B from the interior. 



Aor' horizontal semicircular canal, Aoj- ampulla of ditto, 'pay' posterior vertical 

 semicircular canal, yag ampulla of ditto, /)•' anterior vertical semicircular 

 canal, /;• ampulla of ditto, it utriculus, r\i recessus utriciili, v the connect- 

 ing tube between the ampulla of the anterior vertical semicircular canal and 

 the utriculus, de ductus eudolvmphaticus (recessus vestibuh), s sacculus 

 hemisphericus (this is smaller in birds than in any otiier vertebrate), cr 

 canalis reuniens, lag lagena (the dilated extremity of the cochlea), inr 

 membrane of Eeissner, which forms the boimdary between the scala 

 vestibuli and scala media, ph Basilar membrane, which forms the boundary 

 between the scala tympani and the scala media. 

 Tlie coclilea of a bird consists ( i ) of a scala vestibuli with a very small lumen, 

 which opens at one end into the perilymphatic cavity of the vestibule, and at 

 the other into the lagena (the dilated extiemity of the cochlea corresponding 

 with the cupola of mammals) ; (21 of a scala tympani, also opening into the 

 lagena at one end, and into the foramen rotundum at the other; (3) of a 

 scala media ending blindly at one end, but in communication with the mem- 

 branous vestibule at the othei', through the membranous canalis reuniens (cr). 



As in maramals, the cavity of the osseous cochlea is 

 divided lengthways by the ductus cochlearis into a scala 

 tympani ending in a foramen rotundum, and a scala vestibuli 

 ending in the cavity of the osseous vestibule, "which in its 

 turn is connected with the foramen ovale. 



The auditory nerve, piercing the osseous labyrinth in 

 various points, is distributed in the walls of the membranous 

 labyrinth. 



All these complicated structures are derived from the 

 simple primary otic vesicle by changes in its form and 

 ditferentiation of its walls. All the epiblast of the vesicle 



