The acoustic nerve consists of two parts, the nervus cochleae and the nervus vestibuli.
c. NERVUS COCHLEAE.
This nerve takes origin within the ganglion spirale cochleae. The peripherally directed fibres of these bipolar ganglion cells run to the auditory cells within the organ of Corti; the centrally directed fibres enter the brain-stem and end in two nuclei. The latter are the nucleus ventralis nervi cochleae, situated ventral and lateral to the corpus restiforme, and the nucleus dorsalis nervi cochleae, or tuberculum acusticum, which lies dorsally, although connected with the ventral nucleus. The impulses carried by these peripheral neu rones are conveyed to the higher levels by the central path including: a. Neurones passing from the nucleus ventralis to the mid-line, forming the corpus trapezoides. The path is augmented by fibres from the superior olive and from the nucleus of the corpus trapezoides. After crossing the mid-line, some fibres end within the superior olivary nucleus, while others are joined by fibres from the nucleus of the corpus trapezoides and from the superior olive of the side on which the path now runs.
The fibres form collectively the lateral fillet or lemniscus later alis, which ends within the corpus geniculatum mediate and, chiefly by collaterals, within the inferior colliculus. Some fibres extend as far as the superior colliculus. The lateral fillet receives additional fibres from a group of cells lying in the midst of the tract, known as the nucleus lemnisci lateralis.
b. Neurones passing from the nucleus dorsalis or tuberculum acusticum over the corpus restiforme and, as the superficial striae acusticae, toward the mid-line; thence cours ing deeply to cross the raphe and reach the opposite superior olive, they join the lateral fillet and finally end within the corpus geniculatum mediate.
c. Neurones arising within the lateral geniculate body and passing to the auditory centre within the cortex of the gyms temporalis superior. Fibres also run in the opposite direction, from the auditory centre to the medial geniculate body and to the inferior colliculus.