EXERCISE 12

ENDOSCOPIC DISSECTION OF COCHLEA.
BASAL TURN

Basal turn of cochlea is 9 mm in length and at the level of the round window, it is 2mm in diameter.
It has two parts one subvestibular part and second spiral part.
The basal turn of cochlea consists of :
scala vestibuli,
scala tympani
scala media.
The scala vestibuli arise in the anterior external region floor of the vestibule by an eliptical orifice that measures 1mm by 2mm.
This orifice is situated 1 cm below the inferior edge of oval window
The scala tympani arises from the cul-de sac at the round window below the floor of vestibule which is formed by the bony spiral lamina.
The opening of scala tympani is 1mm and is situated 2 mm below the posterior edge of oval window

STEP A – EXPOSURE OF BASE OF THE BASAL TURN.
Once the round window membrane is removed ,the crista is lowered the structures of basal turn are clearly visualised.
At the basal turn scala tympani is larger in diameter than scala vestibuli.
The bony spiral lip, osseous spiral lamina basilar membrane are clearly visualised

SURGICAL IMPLICATIONS:
1) At the basal turn, the basilar membrane lamina width is smaller than osseus spiral lamina, Hence basilar membrane lamina is resonant site for higher frequences
2) In microscopic cochlear dissection the membrane cochlea is not seen clearly

STEP B – VISUALISATION OF THE INNER OPENING OF COCHLEAR AQUEDUCT.
At just inner side of scala tympani the inner opening of cochlear aqueduct, vertical crest and crista are seen clearly

STEP C – THE VESTIBULAR OPENING OF SCALA VESTIBULI.
The scala vestibuli arises in the anterior and external region of the floor of the vestibule by elliptical orifice measures 1mm by 2mm.
This orifice is 1cm below the anterior part of the foot plate .
The relation between saccule and scala vestibuli is observed

STEP D – RELATION BETWEEN BASAL END OF BASAL TURN AND OVAL WINDOW.
The relation of oval window basal portion of the basal turn is noted

STEP E – SCALA MEDIA..
The scala media is visualised with Reissner’s membrane, basilar membrane and stria vasularis is seen

Labelling:
1 / ST – Scala tympani
2 / SV – Scala vestibuli
3 – Bony lip
4 – Osseus spiral lamina
5 – Basilar membrane
6 – Inner opening of cochlear aqueduct
7 – Vertical crest
8 – Crista
9 – Opening of scala vestibuli
10 – saccule
RM – Reissner’s membrane
BM -Basilar membrane
SV – Stria vascularis