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The olivary body is located on the anterior surface of the medulla lateral to the pyramid, from which it is separated by the antero-lateral sulcus and the fibers of the hypoglossal nerve. Behind, it is separated from the postero-lateral sulcus by the ventral spinocerebellar fasciculus. In the depression between the upper end of the olive and the pons lies the vestibulocochlear nerve. It measures about 1.25 cm. in length, and between its upper end and the pons there is a slight depression to which the roots of the facial nerve are attached. The external arcuate fibers wind across the lower part of the pyramid and olive and enter the inferior peduncle.
The olivary nuclei consist of the inferior olivary nucleus, the dorsal and medial accessory olivary nuclei, and the superior olivary nucleus.
The inferior olivary nucleus is the largest, and is situated within the olive. It consists of a gray folded lamina arranged in the form of an incomplete capsule, opening medially by an aperture called the hilum. Emerging from the hilum are numerous fibers which collectively constitute the peduncle of the olive. The axons, olivocerebellar fibers, which leave the olivary nucleus pass out through the hilum and decussate with those from the opposite olive in the raphé, then as internal arcuate fibers they pass partly through and partly around the opposite olive and enter the inferior peduncle to be distributed to the cerebellar hemisphere of the opposite side from which they arise. The fibers are smaller than the internal arcuate fibers connected with the median lemniscus. Fibers passing in the opposite direction from the cerebellum to the olivary nucleus are often described but their existence is doubtful. Much uncertainty also exists in regard to the connections of the olive and the spinal cord. Important connections between the cerebrum and the olive of the same side exist but the exact pathway is unknown. Many collaterals from the reticular formation and from the pyramids enter the inferior olivary nucleus. Removal of one cerebellar hemisphere is followed by atrophy of the opposite olivary nucleus.
The medial accessory olivary nucleus lies between the inferior olivary nucleus and the pyramid, and forms a curved lamina, the concavity of which is directed laterally. The fibers of the hypoglossal nerve, as theytraverse the medulla, pass between the medial accessory and the inferior olivary nuclei. The dorsal accessory olivary nucleus is the smallest, and appears on transverse section as a curved lamina behind the inferior olivary nucleus.
- Gray's Anatomy online edition 
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