The inferior olivary body or nucleus forms the ovoid projection which extends almost the whole length of the medulla oblongata, from the lower margin of the pons Varolii to within a short distance of the level of the decussation of the anterior pyramids. It is separated from the latter by a groove through which emerge the roots of the hypoglossal nerve. On its outer margin it is separated from the line of the roots of the glossopharyngeal and pneumogastric nerves by a shallow depression. Transverse vertical and longitudinal sections of the medulla show the olive to be a highly convoluted sac of grey matter open, at its hilum, towards the mesial plane. (It has two accessory nuclei of smaller size, an internal and a posterior accessory olive, which, as they are really parts of the larger nucleus, do not call for special consideration here.) The fibres of the hypoglossal nerve pass through its substance, but do not become, as was at one time supposed, and as has been recently again affirmed by Vincenzi, connected with the olive. On its median aspect lies the interolivary stratum or fillet. Anteriorly lies the anterior pyramid, posteriorly the formatio reticularis.