THE FROG. 119 



p. The colujuella auris ; its head will be seen 

 as an opaque white patch in the middle of the 

 deeper layers of the tympanic membrane, now 

 laid bare. Cut away the latter around it and 

 note its relations to the Eustachiafi recess (Sect. 

 ^..^^.). Follow the cut edge of the mucous mem- 

 ■^ brane; it will be found to wrap round the 



columella, suspending it much as the mesentery 

 suspends the alimentary canal (i.e. the columella 

 is outside it). 



b. The i?iternal ea7'. Remove the skin from the top 

 of the head of a large frog which has been pre- 

 served in alcohol, and scrape the roof of the 

 auditory capsules quite clean. Isolate the capsule 

 of one side and hold it between the finger and 

 thumb of the left hand. If examined with care, a 

 couple of greyish streaks will be seen on the inner 

 side of its roof, diverging outwards. Carefully 

 slice away the bone, being especially cautious to 

 cut quite superficially along these streaks ; there 

 will thus be laid bare : 



a. The cavity of the internal ear ; enclosed on all 

 sides by the auditory capsule and filled with a 

 fluid {perilymph). 



y8. The membranous labyrinth ; a portion '-of this is 

 now visible as two delicate pigmented canals, 

 coincident in position with the divergent streaks 

 afore-named. 



c. Still holding the capsule as before, slice away, 

 bit by bit, its outer wall and the remainder of its 

 roof. A third membranous canal will be seen on 

 the outer side ; when this is liberated, transfer the 



