870 ANATOMY [XEKVOUS SYSTEM- continuous with the spinal cord through the foramen maguum. The cerebellum lies above, and immediately behind the medulla oblongata, with which it is directly continuous. The pons lies above and in front of the medulla, with which it is directly continuous. The cere brum is the highest division, and lies above both pons and cerebellum, with both of which it is directly continuous. Several figures of the brain are given in Plate XVIII. Medulla The MEDULLA OBLONGATA rests upon the basi-occipital. oblon. it i s some what pyramidal in form, about 1^ inch long, and 1 inch broad in its widest part. It is a bilateral organ, and is divided into a right and a left half by shallow anterior and posterior median fissures, continuous with the corre sponding fissures in the spinal cord ; the posterior fissure ends above in the 4th ventricle. Each half is subdivided into elongated tracts of nervous matter. Next to, and parallel with the anterior fissure is the anterior pyramid (PL XVIII. figs. 1 and 2, P). This pyramid is continuous below with the cord, and the place of continuity is marked by the passage across the fissure of three or four bundles of nerve fibres, from each half of the cord to the opposite anterior pyramid; this crossing is called the decussation of the pyramids. To the side of the pyramid, and separated from it by a faint fissure, is the olivary fasciculus, which at its upper end is elevated into the projecting oval-shaped olivary body (PI. XVIII. figs. 1 and 2, 0). Behind the olive, and separated from it by a faint groove, is the strong tract named restiform body ; as it ascends from the cord it diverges from its fellow in the opposite half of the medulla oblongata. By this divergence the central part of the medulla is opened up, and the lower half of the cavity of the 4th ventricle is formed. Internal to the restiform body is the posterior pyramid, which is continuous with the postero-median column, and bounds the postero-median fissure. Where the restiform bodies diverge from each other, there also the posterior pyramids diverge outwards from the sides of the postero-median fissure. At the upper part of the floor of the 4th ventricle a longitudinal tract of nerve fibres, the fasciculus te?-es, ascends on each side of its median furrow (Fig. 68, 7). Slender tracts of nerve fibres, the arciform Jibres, arch across the side of the medulla immediately below the olive ; and white slender tracts emerge from the median furrow of the 4th ventricle, pass outwards across its floor, and form the stria? medidlares or acoustical, the roots of origin of the auditory nerve (Fig. 68, 8). The medulla oblongata, like the spinal cord, with which it is continuous, consists both of grey and white matter. But the exterior of the medulla is not so exclusively formed of white matter as is the outer part of the cord, for the divergence from each other of the restiform bodies and posterior pyramids of opposite sides opens out the central part of the medulla, and allows the grey matter to become superficial on the floor of the 4th ventricle. The nerve fibres which enter into the formation of the pyramids and the other tracts just described, are partly continuous below with the columns of the spinal cord, and are prolonged upwards either to the pons and cerebrum, or to the cere bellum, or they partly take their rise in the medulla oblongata itself from the cells of its grey matter. As the medulla is a bilateral organ, its two halves are united together by commissural fibres, which cross obliquely its mesial plane from one side to the other, and as they decus sate in that plane, they form a well-marked mesial band or rapJie. Further, the medulla is a centre of origin for several pairs of the more posterior encephalic nerves, and for the vaso-motor nerves. In the passage upwards through the medulla of the columns of the cord, a re-arrangement of their fibres takes place ; just as in a great central railway etation, the rails, which enter it in one direction, intersect and are rearranged before they emerge from it in the opposite direction. The fibres of the posterior median column of the cord are prolonged upwards as the posterior pyramid. The fibres of the posterior column of the cord are for the most part prolonged upwards into the restiform body, though some fibres pass to the front of the medulla to participate in the decussation of the anterior pyramids. The lateral column of the cord divides into three parts : a, the greater number of its fibres pass inwards across the anterior median fissure, to assist in forming the anterior pyramid of the opposite side, so as to produce the decussation already referred to ; b, others join the restiform body ; c, others form the fasciculus teres situated on the floor of the 4th ventricle. The ante rior column of the cord also divides into three parts : a, some fibres form the arciform fibres and join the restiform body ; b, others assist in the formation of the olivary fasciculus ; c, others are prolonged up wards in the anterior pyramid of the same side (Fig. 67). The anterior pyramid consists partly of fibres of the anterior column of the cord of the same side, partly of decus sating fibres of the ante rior commissure, partly of decussating fibres from the posterior co lumns and posterior cor- nu of grey matter, but principally of the decus sating fibres of the lateral column of the opposite side of the cord. The fibres of the anterior pyramid are prolonged through the pons to the cerebrum. Owing to the decussation of the lateral columns of the cord in the formation of the pyramids, the motor nerve fibres from one-half of the brain are trans mitted to the opposite side of the cord, so that injuries affecting one side of the brain occasion paralysis of the motor nerves arising from the opposite half of the cord. The olivary fasciculus is formed partly of fibres of the anterior column of the same side, and partly of fibres arising from the grey matter of the olive. It is continued upwards through the pons to the cerebrum. The restiform body is formed principally of fibres of the posterior column of the same side, but partly of fibres of the lateral column, and also of the arciform fibres from the anterior column, and from the grey matter of the superior and inferior olives. As the restiform body is continued upwards to the cerebellum, and forms its inferior peduncles, the arciform fibres have been called by Solly the superficial cerebellar fibres of the medulla. Through the restiform body the cerebellum is connected with the posterior, lateral, and anterior columns of the cord as well as with the olivary nuclei in the grey matter of the medulla oblongata. The posterior pyramid consists of the posterior median column of the cord, and is prolonged through the pons to the cerebrum. The fasciculus teres is formed of a small part of the lateral column of the cord, and is also prolonged through the pons to the cerebrum. Fio. C7. Diagrammatic dissection of the me dulla oblongata and pons to show the course of the fibres, a, superficial, a , deep transverse fibres of the pons; 6, b, anterior pyramids ascending at 6 through the pons; c, c, olivary bodies; cf, olivary fasciculus in the pons; d, d, anterior columns of cord; e, inner part of the right column joining the anterior pyramid ; /, the outer part going to the olivary fasciculus; y, lateral column of cord; />, the part which decussates fit t, the decussa- tion of the pyramids; I, the part which joins the restiform body ; m, that which forms the fasciculus teres ; n, arciform fibres, land 2, sensory and motor roots of fifth nerve; 3, sixth nerve ; 4, portio dura ; 5, portio intermedia; 6, portio mollisof seventh nerve; 7, glosso-pharyngeal ; 8, pneumo-gnstric; 9, spinal accessory of eighth nerve ; 10, hypo-
glosaal nerve.
