The tracts on either side of the medulla oblongata which form its lower part from before backward are: the anterior pyramid, the fundamental root zone, the anterior root zone or lateral tract, the direct cerebellar tract, the posterior column, and the posterior pyramid.
The upper part of the medulla is attached to the pons Varolii, and is connected with the cerebellum by the restiform bodies, or inferior peduncles of the cerebellum. The medulla gradually- enlarges as it ascends from the cord; its upper extremity is about three-quarters of an inch from side to side, and about five-eighths of an inch from before back ward. The lower portion of the fourth ventricle is behind the upper half of the medulla oblongata, and the raphe ex tending from the median fissure of the ventricle forward to the anterior median fissure of the medulla, divides it into lateral halves. A transverse section of the medulla, seen in figure 36, or better i a section one-eighth of an inch above it, is somewhat triangular in shape, with truncated angles, and shows tlie relation of the tracts which are continued upwards from its lower portion. The relations of these tracts are on either side, beginning from the anterior median -fissure and raphe in front: the anterior pyramid; the olivary body, or olive; a triangular tra,ct between the olive and raphe, beneath the pyramid, called the fundamental root zone; the lateral tract, or anterior root zone, compressed between tlie olive and restiform bocly; and the restiform body or inferior peduncle of the cerebellum within which is seen the restiform nucleus. Figure 36.
The parts of tbe medulla which bare not been described are the anterior pyramids, tbe olive, and the restiform body. The arciform fibres and the fibres from the raphe will be described incidentally with the others above mentioned.
The anterior pyramids of the medulla emerge from the middle of the lower border of the pons Varolii as two round.ed cords, one on each side of the anterior median fissure and raphe of the medulla oblongata. They descend sde by side, separated by the anterior median fissure, to the point of decussation at the lower extremity of the Medulla where each divides into two portions: first, a small tract which continues on the same side of the anterior median fissure of the spinal cord, and is the anterior median column, or column of Tiirck; and, a large tract which breaks up into several bundles of fibres that pass backward and outward, interrupting the an terior horns of the grey columns of the cord, aud reaching the anterior surface of the posterior horns, forms the crossed pyramidal tract of the opposite side of the spinal cord.
Near the lower extremity of the olive, the pyramid gives off from its outer side a 'small tract of fibres, which arch be neath the olive and across the surface of the medulla to the restiform body. These fibres join the restiform body, pass ' to the cerebellum, and are called the arciform fibres of the medulla. They are probably associated fibres, aud do not belong to the pyramidal system which is motor in function. Immediately beneath the pons, fibres arc given off from the inner side of the pyram'd which enter the rapbe to decus sate with those from the opposite pyramid, and are continu ous on the floor of the fourth ventricle with the roots of the seventh nerve, or its nucleus of the opposite side. Below the olive other fibres enter the raphe to reach the opposite twelfth nerve, or the hypoglossal nucleus.
The anterior pyramid is bounded above externally by the olive, from which it is separated,beneath the pons,by a sulcus, and lower down by a slight groove. The lower portion of the pyramid is in relation externally- with the fundamental root zone. This zone above is buried beneath the pyramid, between the olive and raphe.
The olive is an oblong body situated on either side of the medulla, between the pyramid and the lateral tract, or anterior root zone. It is immediately beneath the pons, from which it is separated by a slight groove. This body is about half an inch in length from above downward, and one-fourth of an inch across its middle. Its upper extremity is con. 'meted with a large tract of white fibres which go to form the anterior layer of the fillet proper, and is called the olivary fasciculus of the fillet. The olivary fasciculus is related in front with the fasciculus of the fundamental root zone, and behind with that derived from the lateral tract of the medulla, or anterior root zone of the cord. The olive is covered externally by a thin layer of transverse striz, derived from the raphe, which passes to the restiform body and to join the commissure of the flocculus. These are probably cere bellar fibres from the nuclei in the floor of the fourth ventri cle, belonging to the same system as the uppermost fibres of the pons Varolii.