r/OiClJC.^L DIRECTIONS. 47 1 



soft tissue (including the cartilaginous auditory meatus), and all 

 other bony parts from these, but leave them uninjured. 



(5) P^ind 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) 'l"hc middle car 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 cochlear. 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 Ijone-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 car (p. 424). Note the fenestra vestibuli and 

 fenestra cochlear 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. 'I'he semicircular canals 

 may, if desired, be exj)oscd on your own specimen, by cutting away 

 the surface of the bone in places indicated in the description (p. 

 426), and inserting fine bristles (tho.se 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. 



