(4) Remove the tympanic bulla and petrous bone from the rest of the
skull, by the use of bone-forceps and scalpel. Trim away all soft tissue
(including the cartilaginous auditory meatus), and all other bony parts
from these, but leave them uninjured.
(5) Find the Eustachian tube and study it (p. 423).
(6) Study specimens of the bones of the middle ear (p. 423, and Figs.
171 and 172).
(7) The middle ear should be studied on a demonstration preparation,
then dissected as follows:
Remove with bone-forceps the medial side of the tympanic bulla (the
entotympanic). Note the two cavities within the bulla, with the shelf
separating them. Observe the fenestra cochleæ. Now remove with the
forceps the shelf, first breaking through the middle part, then removing
the rest with care. Remove part also of the membranous lining of the
cavity, till the inner surface of the tympanic membrane, with the
malleus crossing it, is visible. Observe the tensor tympani muscle (p.
424) attached to the malleus by its small tendon. Next, with
bone-forceps, fracture the thick, bony portion uniting the caudal end of
the petrous with the caudal part of the ring of bone surrounding the
external auditory meatus. These two parts may then be separated with the
fingers, leaving the petrous bone on one side, the meatus, tympanum, and
malleus on the other (Fig. 170). The head of the malleus may then be
observed, with the incus attached to it. Note also the stapes, in the
fenestra vestibuli, with the stapedius muscle (p. 424) attached to it.
(8) The internal ear (p. 424). Note the fenestra vestibuli and fenestra
cochleæ and the promontory (p. 34). Study the cochlea and vestibule on a
demonstration preparation and compare with Fig. 173. Then remove the
wall of the promontory and find the cochlea. Open the vestibule and find
as many of the openings of the semicircular canals as possible. Study
the semicircular canals on demonstration preparations, consulting Fig.
173. The semicircular canals may, if desired, be exposed on your own
specimen, by cutting away the surface of the bone in places indicated in
the description (p. 426), and inserting fine bristles (those from the
sensory hairs on the face of the cat are excellent for this purpose).
The membranous labyrinth (Fig. 173) may be isolated by decalcifying the
petrous bone with ten per cent. nitric acid, then dissecting out the
labyrinth. This is an operation of considerable delicacy, but at least
one or two specimens for demonstration should, if possible, be prepared
thus and kept in the laboratory.