Page 398 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

380 Veterinary Histology of Domestic Mammals and Birds

Lying within the centre of the cochlea, the coch-
VetBooks.ir sacculus, ending blindly between the meninges within lear duct is framed by rostral and caudal cartilaginous
the cranial cavity.
ledges. This arrangement gives rise to two channels, the
The utriculus gives rise to the semicircular ducts,
each bearing an expanded ampulla at one end.
scala communicate at the apex of the cochlea via the
Considerably smaller in diameter than the semicircular narrow scala vestibuli and the scala tympani. The two
canals in which they lie, the semicircular ducts occupy apical interscalar canal (canalis interscalaris apicalis),
a slightly eccentric position within the canals. These the equivalent of the helicotrema in mammals.
are attached to the periosteum of the canals by delicate The cochlear duct is separated from the scala ves-
fibres that pass through the perilymph. tibuli by the thick, folded tegmentum vasculosum.
The sensory cells of the vestibular apparatus form The basilar membrane (membrana basilaris) sepa-
the cristae ampullares (within the ampullae), the mac- rates the cochlear duct from the scala tympani. Resting
ula utriculi and crista neglecta (within the utriculus), upon the basilar membrane, the papilla basilaris car-
and the macula sacculi in the sacculus. The hair cells of ries the sensory cells of the cochlea. These cells have
the macula utriculi and the macula sacculi are in contact stereocilia that are anchored in an overlying gelatinous
with statoconia. In contrast, the cristae ampullares and layer, the tectorial membrane (membrana tectoria).
the crista neglecta are covered in a dome-shaped mem- Bipolar neurons of the cochlear ganglion form a deli-
brane devoid of statoconia. Movement of endolymph, cate network of nerve fibres at the base of the sensory
induced by inertia, stimulates the hair cells, which are cells.
connected with fibres of the vestibular ganglion and A further group of sensory cells is located within
the vestibular nerve. the lagena, forming the macula lagenae. Sensory hairs
The tubular cochlear duct (ductus cochlearis) projecting from these cells are in contact with otoconia.
emerges ventrally from the sacculus. In the chicken it The function of the macula lagenae has not been estab-
measures approximately 6 mm in length. Its blind end, lished. Auditory sensitivity in birds is greatest between
the lagena, reaches the tip of the cochlea (Figure 16.45). 1000 and 6000 Hz.

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