- •The organ of hearing
- •The middle ear
- •The auditory ossicles
- •Semicircular canals
- •The membranous labyrinth (labyrinth membranaceus) lies inside the bony labyrinth and repeats its configurations more or less exactly.
- •Saccule and utricle
- •Duct of the cochlea
- •It is triangular in cross section. The floor is formed by the basilar membrane; the roof, by the vestibular (Reissner’s) membrane, and the outer wall by the bony wall of the cochlea.
- •The pathways of sound conduction
The pathways of sound conduction
The organ of hearing is divided into two: (1) the sound-conducting apparatus (the external and middle ear, as well as certain companentes of the internal ear (peri- and endolymph); and (2) the sound-appreciating apparatus (the internal ear).
The air waves collected by the ear pass into the external auditory meatus, hit the tympanic membrane and cause it to vibrate.→ The vibrations move the handle of the malleus fused with the membrane.→ The malleus moves the incus.→ The incus moves the stapes fitted in the fenestra vestibule leading into the internal ear.→ This movement stirs the labyrinth fluid. These fluid movements are necessary for the functioning of the highly sensitive elements of the organ of Corti. The first to move is the perilymph in the vestibule. Its vibrations in the perilymph of scala vestibule reach the apex of the cochlea and are conducted via the helicotrema to the perilymph in the scala tympani; then they descend along it to secondary tympanic membrane and close the fenestra cochleae and retund to the tympanic cavity.
From the perilymph the sound vibrations are conducted to the endolymph and through it to the organ of Corti.
The cellular body of the first neuron lies in the spiral ganglion.
The peripheral process of the bipolar cells enters the organ of Corti and ends at the receptor cells.
The central process passes as the cochlear division of the auditory nerve to its nuclei (n.dorsalis and n.ventralis).
The nuclei contain the bodies of the secondary neurons, the axon of which form the central acoustic fasciculus crosses with the fasciculae of the same name on the opposite side, forming the lateral lemniscus.
The fibres coming from the ventral nucleus form a trapezoid body and, on passing the pons, become part of the lateral lemniscus of the opposite side.
The fibres coming from the dorsal nucleus, run along the floor of the IV ventricle in the form of auditory striae, penetrate the reticular formation of the pons and, together with fibres of the trapezoid body, become part of the lateral lemniscus on the opposite side.
The lateral lemniscus ends partly in the inferior quadrigeminal bodies of the tectal lamina and partly in the medial geniculate body, where the third neurons are located.
The superior gudrigeminal body serve as the reflex center for auditory implses.