PRACTICAL DIRECTIONS. 471 
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 prep- 
aration, 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 cochlee. 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 tym- 
panic 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 ves- 
tibuli, with the stapedius muscle (p. 424) attached to it. 
(8) The internal ear (p. 424). Note the fenestra vestibuli and 
fenestra cochlez 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 semicir- 
cular canals as possible. Study the semicircular canals on demon- 
stration 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 decalci- 
fying 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. 
