The Anterior Pyramids are two pyramidal-shaped bundles of white matter, placed one on either side of the anterior median fissure, and separated from the olivary body, which is external to them, by a slight depression. At the lower border of the pons they are somewhat constricted; they then become enlarged, and taper slightly as they descend, being continuous below with the anterior columns of the cord. On separating the pyramids below, it will be observed that the innermost fibres of the two form from four to five bundles on each side, which decussate with one another; this decussation, however, is not formed entirely of fibres from the pyramids, but mainly from the deep portion of the lateral columns of the cord which pass forwards to the surface between the diverging anterior columns. The outermost fibres do not decussate; they are derived from the anterior columns of the cord, and are continued directly upwards through the pons Varolii.

  Lateral Tract and Olivary Body. The lateral tract is continuous with the lateral column of the cord. Below, it is broad, and includes that part of the medulla between the anterior pyramid and restiform body; but, above, it is pushed a little backwards, and narrowed by the projection forwards of the olivary body.

  The Olivary Bodies are two prominent, oval masses, situated behind the anterior pyramids, from which they are separated by slight grooves. They equal, in breadth, the anterior pyramids, are a little broader above than below, and are about half an inch in length, being separated, above, from the pons Varolii, by a slight depression. Numerous white fibres (fibra arciformes) are seen winding around the lower end of each body; sometimes crossing their surface.

  The Restiform Bodies are the largest columns of the medulla, and continuous, below, with the posterior columns of the cord. They are two rounded, cord-like eminences, placed between the lateral tracts, in front, and the posterior pyramids, behind; from both of which they are separated by slight grooves. As they ascend, they diverge from each other, assist in forming the lateral boundaries of the fourth ventricle, and then enter the corresponding hemisphere of the cerebellum, forming its inferior peduncle.

  The Posterior Pyramids (fasciculi graciles) are two narrow, white cords, placed one on each side of the posterior median fissure, and separated from the restiform bodies by a narrow groove. They consist entirely of white fibres, and are continuous with the posterior median columns of the spinal cord. These bodies lie at first, in close contact. Opposite the apex of the fourth ventricle, they form an enlargement (processus clavatus), and then, diverging, are lost in the corresponding restiform body. The upper part of the posterior pyramids form the lateral boundaries of the calamus soriptorius.

  The Posterior surface of the Medulla Oblongata forms part of the floor of the fourth ventricle. It is of a triangular form, bounded on eacli side by the diverging posterior pyramids, and is that part of the ventricle which, from its resemblance to the point of a pen, is called the calamus scriptorium. The divergence of these columns, and the restiform bodies, opens to view the grey matter of the medulla, which is continuous, below, with the grey commissure of the cord. In the middle line is seen a longitudinal furrow, continuous with the posterior median fissure of the cord, terminating, below, at the point of the ventricle, in a cul-de-sac, the ventricle of Arantius, which descends into the medulla for a slight extent. It is the remains or a canal, which, in the foetus, extends throughout the the entire length of the cord.

  Structure. The columns of the cord are directly continuous with those of the medulla oblongata, below; but, higher up, both the white and grey constituents are re-arranged before they are continued upwards to the cerebrum and cerebellum.

  The Anterior Pyramid is composed of fibres derived from the anterior column of the cord of its own side, and from the lateral column of the opposite half of the cord, and is continued upwards into the cerebrum and cerebellum. The cerebellar fibres form a superficial and deep layer, which pass beneath the olive to the restiform body, and spread out into the structure of the cerebellum. A deeper fasciculus encloses the olivary body, and, receiving fibres from it, enters the pons as the olivary fasciculus or fillet; but the chief mass of fibres from the pyramid, the cerebral fibres, enter the pons in their passage upwards to the cerebrum. The anterior pyramids contain no grey matter.

  The Lateral Tract is continuous, below, with the lateral column of the cord. Its fibres pass in three different directions. The most external join the restiform body, and pass to the cerebellum. The internal, more numerous, pass forwards, pushing aside the fibres of the anterioi column, and form part of the opposite anterior pyramid. The middle fibres .ascend, beneath the olivary body, to the cerebrum, passing along the back of the pons, and form, together with fibres from the restiform body, the fasciculi teretes, in the floor of the fourth ventricle.

  Olivary Body. If a transverse section is made through either olivary body, it will be found to be a small ganglionic mass, deeply imbedded in the medulla, partly appearing on the surface as a smooth, olive-shaped eminence (fig. 239). It consists, externally, of white substance; and internally, of a grey nucleus, the corpus dentatum. The grey matter is arranged in the form of a hollow capsule, open at its upper and inner part, and presenting a zig-zag, or dentated outline. White fibres originate from the interior of this body, by the aperture in the posterior part of the capsule. They join with those fibres of the anterior column which ascend on the outer side, and beneath the olive, to form the olivary fasciculus, which ascends to the cerebrum.

  The Restiform Body is formed chiefly of fibres from the posterior column of the cord; but it receives some from the lateral column, and a fasciculus from the anterior, and is continued, upwards, to the cerebrum and cerebellum. On entering the pons, it divides into two fasciculi, above the point of the fourth ventricle. The most external one enters the cerebellum: the inner one joins the posterior pyramid, is continued up along the fourth ventricle, and, joining the fasciculi teretes, passes up to the cerebrum.

  Septum of the Medulla Oblongata. Above the decussation of the anterior pyramids, numerous white fibres extend, from behind forwards, in the median line, forming a septum, which subdivides the medulla into two lateral halves. Some of these fibres emerge at the anterior median fissure, and form a band which curves around the lower border of the olivary body, or passes transversely across it, and round the sides of the medulla, forming the arciform fibres of Rolando. Others appear in the floor of the fourth ventricle, issuing from the posterior median fissure, and form the white striae in that situation.

  Grey Matter of the Medulla Oblongata. The grey matter of the medulla, is a continuation of that contained in the interior of the spinal cord, besides a series of special deposits, or nuclei.

  In the lower part of the medulla the grey matter is arranged as in the cord, but, at the upper part, it becomes more abundant, and is disposed with less apparent regularity, becoming blended with all the white fibres, except the anterior pyramids. The part corresponding to the transverse grey commissure of the cord, is exposed to view in the floor of the medulla oblongata, by the divergence of the restiform bodies, and posterior pyramids, becoming blended with the ascending fibres of the lateral column, and thus forming the fasciculi teretes. The lateral crescentic portions but especially the posterior horns, become enlarged, blend with the fibres of the restiform bodies, and form the tuberculo cinereo of Rolando.

  Special deposits of grey matter are found both in the anterior and posterior parts of the medulla; in the former situation, forming the corpus dentatum within the olivary body, and in the latter, a series of special masses, or nuclei, connected with the roots of origin of the spinal accessory, vagus, glosso-pharyn-geal, and hypo-glossal nerves.