69 THE FROG 
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 aper- 
ture, 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 mem- 
brane to the inner ear. 
Observe within the auditory capsule a partially transparent, 
slightly pigmented sac, the membranous labyrinth. With the aid 
of figure 93 of Holmes’ Biology of the Frog make out the 
following parts: (a) an upper part, the utriculus, which gives off 
(b) 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 consult the text of Holmes’ Biology of the Frog. 
Find the auditory nerve; this is the eighth cranial nerve. 
With a dissecting lens and with the low power of the micro- 
scope 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. 
