8. The tragicus medialis ([Fig. 169], 5, 5′).
9. The conchæus externus ([Fig. 169], 9).
(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.