84 THE FROG 



membrane without disturbing the columella, and observe a 

 large cavity, the tympanic cavity (corresponding to the 

 middle ear of man and other mammals), which communi- 

 cates with the mouth cavity by means of the Eustachian 

 tube. Clean away the muscles surrounding the auditory 

 capsule. Observe precisely where the proximal end of the 

 columella is attached; here it articulates with a small disc of 

 cartilage, the operculum, which fills a small aperture, the 

 fenestra ovalis, leading from the tympanic cavity into the 

 cavity of the auditory capsule (inner ear). With a sharp 

 scalpel carefully slice off the roof of the auditory capsule 

 in order to expose the inner ear and observe its relation to 

 the operculum. Like the ossicles of the mammalian ear, 

 the columella and operculum serve to convey sound waves 

 from the tympanic membrane to the inner ear. 



Observe within the auditory capsule a partially trans- 

 parent, slightly pigmented sac, the membranous labyrinth. 

 Distinguish the following parts : (a) an upper part, the utric- 

 ulus, which gives off (&) the three semicircular canals, 

 lying at right angles to each other and occupying the three 

 dimensions of space; and (c) a smaller inferior part, the 

 sacculus. For the probable functions of these parts con- 

 sult the text of Holmes' Biology of the Frog. Find the 

 auditory nerve; this is the eighth cranial nerve. Com- 

 parison may profitably be made with the larger membranous 

 labyrinth of an elasmobranch (skate or dog-fish). 



With a dissecting lens or the low power of the microscope 

 study prepared slides showing stained serial cross-sections 

 taken through the auditory region of the head of a small 

 frog. 



Make a diagram showing the relations of the various parts 

 of the ear of the frog. 



