Venous drainage from the cochlear labyrinth is via the posterior and anterior spiral modiolar veins that form the common modiolar vein. The common modiolar vein and the vestibulocochlear vein form the vein of the cochlear aqueduct, which empties into the inferior petrosal sinus. Venous drainage from the vestibular labyrinth is via vestibular veins that join the vein of the cochlear aqueduct and by the vein of vestibular aqueduct, which drains into the sigmoid sinus.
The internal ear, located in the temporal bone, consists of a system of chambers and canals that contain a network of membranous channels. These are referred to, respectively, as the bony labyrinth and membranous labyrinth. In places, the membranous labyrinth forms the lining of the bony labyrinth; in other places, there is a separation of the two. Within the space lined by the membranous labyrinth is a watery fluid called endolymph. External to the membranous labyrinth, i.e., between the membranous and bony labyrinths, is additional fluid called perilymph. The bony labyrinth is divided into three parts: cochlea, semicircular canals, and vestibule. The cochlea and semicircular canals contain membranous counterparts of the same shape; however, the membranous components of the vestibule are more complex in form, being composed of ducts and two chambers, the utricle and saccule. The cochlea contains the receptors for hearing, i.e., the organ of Corti, the semicircular canals contain the receptors for movement of the head, and the utricle and saccule contain receptors for position of the head.
Internal ear, ear, guinea pig, H&E 20.
In this section through the internal ear, bone surrounds the entire internal ear cavity. Because of its labyrinthine character, sections of the internal ear appear as a number of separate chambers and ducts. These, however, are all interconnected (except that the perilymphatic and endolymphatic spaces remain separate). The largest chamber is the vestibule (V ). The left side of this chamber (black arrow) leads into the cochlea (C). Just below the black arrow and to the right is the oval ligament (OL) surrounding the base of the stapes (S). Both structures have been cut obliquely and are not seen in their entirety. The facial nerve (FN ) is in an osseous tunnel to the left of the oval ligament. The com munication of the vestibule with one of the semicircular canals is marked by the white arrow. At the upper right are cross sections of the membranous labyrinth passing through components of the semicircular duct system (DS).
The cochlea is a spiral structure having the general shape of a cone. The specimen illustrated here makes 31⁄2 turns (in humans, there are 23⁄4 turns). The section goes through the central axis of the cochlea. This consists of a bony stem called the modiolus (M). It contains the beginning of the cochlear nerve (CN ) and the spiral ganglion (SG). Because of the plane of section and the spiral arrangement of the cochlear tunnel, the tunnel is cut crosswise in seven places (note 31⁄2 turns). A more detailed examination of the cochlea and the organ of Corti is provided in Plate 109.
Semicircular canal, ear, guinea pig, H&E 225.