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stream forwards and medially towards the median raphe, forming a series of concentric curves in the substance of the medulla oblongata. They cross the median plane and decussate
with the corresponding fibres of the opposite side, upon the dorsal
aspect of the pyramids. Having thus gained the opposite side of the
medulla oblongata, they immediately turn upwards and form a conspicuous strand of longitudinal fibres, which ascends close
to the median plane Infe
As we proceed up
the medulla oblon
Fasciculus longitudinalis medialis
FIG. 494.-TRANSVERSE SECTION THROUGH THE HUMAN MEDULLA OBLONGATA
gata internal arcuate fibres which first come into sight appear as coarse bundles which
FIG. 495.-TRANSVERSE SECTION THROUGH THE MIDDLE OF THE OLIVARY REGION OF THE HUMAN MEDULLA
The floor of the fourth ventricle is seen, and it will be noticed that the restiform body on each side has
now taken definite shape. Some of the descending tracts in red; ascending tracts in blue.
curve forwards in a narrow group round the central gray matter (Figs. 494 and 495). Soon, other finer bundles appear, which describe wider curves on the lateral side of the coarser
group, until a very large part of each half of the medulla is seen to be traversed by these arcuate fasciculi (Fig. 495). The internal arcuate fibres decussate in the median plane with the internal arcuate fibres of the opposite side. They then change their direction and turn upwards, and the lemniscus, as already stated, takes form and gradually increases in volume as it ascends. This great and important tract is thus laid down between the pyramid and the fasciculus longitudinalis medialis; and the consequence of this is that the latter tract is pushed still farther backwards, and, when the lemniscus is fully established, it comes to lie immediately beneath the gray matter of the floor of the fourth ventricle (Fig. 495). But the lemniscus is not in direct contact with the fasciculus longitudinalis, for a bundle of fibres, the continuation of which has been seen in the anterior funiculus of the medulla spinalis, the fasciculus tectospinalis, separates them, as well as fibres coming from sensory nuclei of the cerebral nerves which are crossing the raphe to join the medial lemniscus (Fig. 495).
It is important that we should realise at this stage the full significance of the decussation of the lemniscus and have a clear conception of the connexions of the fibres which take part in it. The funiculus posterior, which ends in the cuneate and gracile nuclei, is derived from the posterior roots of the spinal nerves. The lemniscus fibres therefore carry on the continuity of part of the posterior funiculus, the gracile and cuneate nuclei, which are thrown across its path in the lower part of the medulla oblongata, constituting merely a nodal interruption. At this point the lemniscus is transferred to the opposite side of the medulla oblongata. But it will be remembered that a large proportion of the fibres of the entering posterior nerveroots of the spinal nerves end in connexion with the cells of the posterior column of gray matter of the spinal medulla. It must not be supposed that the path represented by these latter fibres comes to a termination thereby; from these posterior column cells other fibres arise which cross, in the anterior white commissure, to the opposite side of the spinal medulla and proceed up the spinal medulla to the lateral part of the medulla oblongata. These fibres constitute the spino-thalamic tract already referred to. The practical bearing of this is that, owing to the crossing of the lemniscus medialis and lower down of the spino-thalamic tract, unilateral lesions of the medulla oblongata are apt to produce complete hemi-anesthesia; whilst unilateral lesions of the spinal medulla produce only partial hemi-anaesthesia.
The pyramid forms a massive tract in front of and quite distinct from the lemniscus medialis. The lemniscus medialis, the tecto-spinal bundle, and the medial longitudinal bundle are, in the first instance, not marked off from each other. They appear as a broad flattened band applied to the raphe. One edge of this band is directed backwards and reaches the gray matter on the floor of the fourth ventricle, while the other edge looks forwards, and is in contact with the pyramid. In the upper part of the medulla oblongata the lemniscus and the medial longitudinal fasciculus begin to draw asunder from each other. The intermediate longitudinal fibres become reduced in number and the two strands grow denserthe one on the dorsal aspect of the pyramid, and the other immediately beneath the gray matter of the floor of the fourth ventricle (Fig. 495).
The fasciculus longitudinalis medialis is largely formed of fibres homologous with those which in the spinal medulla constitute the fasciculus anterior proprius. As they are followed upwards these fibres are thrust back by the two decussations: the lower decussation pushing them behind the pyramids, and the upper decussation. displacing them still farther backwards to a position behind the lemniscus medialis.
Corpus Restiforme.-The gracile and cuneate nuclei gradually give place to the restiform body in the superior part of the posterior district of the medulla oblongata. Fibres from various quarters converge to form this great strand. It first takes shape as a thin superficial layer of longitudinal fibres, which are gathered together on the lateral aspect of the cuneate nucleus; but after that nucleus has come to an end, and as the superior part of the medulla oblongata is reached, the restiform body is seen to have grown into a massive strand, which presents a kidney-shaped or oval outline on transverse section (Fig. 495); and it ultimately enters the white central core of the cerebellum as its inferior peduncle. The fibres which build up the restiform body are the following: (1) the fasciculus spinocerebellaris [posterior]; (2) arcuate fibres coming from the nucleus gracilis and nucleus cuneatus of both
sides of the medulla oblongata; (3) external arcuate fibres coming from the arcuate nuclei; and (4) olivo-cerebellar fibres.
The fasciculus spinocerebellaris [posterior] extends upwards from the lateral funiculus of the medulla spinalis. In the lateral district of the medulla oblongata it occupies a similar position; but before the olive is reached it inclines backwards, crosses the posterior lateral furrow and passes obliquely upwards into the restiform body. As its fibres diverge backwards, they pass over and cover up the tractus spinalis of the trigeminal nerve and its nucleus, thus TRACTUS SPINALIS shutting them out from the surface. The fibres of the fasciculus spinocerebellaris, in the first instance, enter into the lateral or superficial part of the restiform body.
Bruce has shown that the fibres of the spino-cerebellar tract ultimately lie in the centre of the restiform body, forming as it were its central core, and that, in the cerebellum, they can be traced to the superior vermis.
MED AC OLIVE
Which shows in part the fibres which enter into the constitution of the restiform body.
The posterior external arcuate fibres take origin from the gracile and cuneate nuclei, and enter the superficial part of the restiform body of the same side.
The anterior external arcuate fibres proceed from the inferior portions of the gracile and cuneate nuclei of the opposite side. It can easily be determined that, after decussating in the median plane, all the internal arcuate fibres which arise from these nuclei do not enter the lemniscus medialis. A large proportion of them gain the surface by sweeping round the medial aspect of the pyramid in the anterior median fissure. Many of them gain the surface by piercing the pyramid or by passing out between it and the olive. These fibres constitute the anterior external arcuate group, and on the surface of the medulla oblongata they sweep
Detached anterior column of gray matter
backwards around it, forming a thin layer over the olive and ultimately reaching the restiform body. The anterior external arcuate fibres, as well as the spinocerebellar tract - fibres, cover over the tractus spinalis of the trigeminal nerve, which thus comes to take up a deeper position in the substance of the medulla oblongata (Figs. 495 and 496).
The other elements in the restiform body, viz., those derived from the nucleus olivaris inferior and the nuclei arcuati, have already been described.
Thus, the restiform body conveys to the cerebellum (1) fibres conveying impulses from the posterior spinal roots of the same and also from the opposite side of the medulla spinalis, the former after being interrupted in the nucleus dorsalis and the nucleus gracilis and nucleus cuneatus of the same side, the latter in the nucleus gracilis and nucleus cuneatus of the other side; and (2) fibres from the olivary and arcuate nuclei, which convey impulses from the higher regions of the brain, directly or indirectly (probably the latter) from the motor area of the cerebral cortex.
Formatio Reticularis.-Behind the olive and the pyramid is the formatio reticularis. In the medulla oblongata it occupies a position which, to a large extent, corresponds with that of the lateral funiculus in the spinal medulla. Ing transverse section it appears as an extensive area, which is divided into a lateral and a medial field by the fila of the hypoglossal nerve as they traverse the substance of the medulla oblongata to reach the surface. In the lateral portion, which lies! behind the olive, a considerable quantity of gray matter, continuous with that in the spinal medulla, is present in the reticular formation; it is, therefore, called the formatio reticularis grisea. In the medial part, which lies behind the pyramid, the gray matter is extremely scanty, and the reticular matter here is termed the
formatio reticularis alba.
The nerve-fibres which traverse the formatio reticularis run both in a transverse and in a longitudinal direction. The transverse fibres are the internal arcuate fibres. The longitudinal fibres are derived from different sources in the two fields. In the formatio grisea they represent to a large extent the fibres of the lateral funiculus of the spinal medulla, after the removal of the posterior spino-cerebellar and the lateral cerebro-spinal tracts. They consist, therefore, of the fibres of the fasciculi rubrospinalis, thalamo-olivaris, spinothalamicus, and spinocerebellaris anterior (anterolateralis superficialis) of the spinal medulla. In the formatio alba the longitudinal fibres are the tract of the lemniscus medialis, the fasciculus tectospinalis, and the medial longitudinal bundle, all of which have already been described.
Central Canal and the Gray Matter which surrounds it.—The central canal as it proceeds upwards through the closed part of the medulla, is gradually forced' to assume a more dorsal position, owing to the accumulation of fibres on its ventral aspect. (Moreover, the posterior cleft-like part of the cavity of the fœtal neural tube, which becomes obliterated in the spinal medulla by the fusion of its walls, remains patent in the medulla oblongata. Hence the central canal in the closed part of the medulla oblongata extends backwards to the roof-plate.) First the decussation of the pyramids, and then the decussation of the medial lemniscus, both of which take place in front of the canal, tend to push it backwards; and the formation of the longitudinal strands in which these intercrossings result (viz., the pyramid and the medial lemniscus), together with the continuation upwards of the funiculus anterior proprius, leads to a great increase in the amount of tissue which separates it from the anterior surface of the medulla oblongata. In the closed part of the medulla oblongata the canal is surrounded by a thick layer of gray matter, which is continuous with the basal portions of the anterior and posterior columns of gray matter of the spinal medulla. This central gray matter is sharply defined on each side by the internal arcuate fibres, which curve forwards and medially around it. Finally, the central canal opens on the dorsal aspect of the medulla oblongata into the cavity of the fourth ventricle. The central mass of gray matter which surrounds the canal in the closed part of the medulla oblongata is now spread out in a thick layer on the floor of the fourth ventricle, and in such a manner that the portion which corresponds to the basal part of the anterior column of the spinal medulla is situated close to the median plane, whilst the part which represents the base of the posterior column occupies a more lateral position. This is important, because the nucleus of origin of the hypoglossal nerve is placed in the medial part of the floor, whilst the nuclei of termination of the afferent fibres of the vagus, glossopharyngeal, and acoustic nerves lie in the lateral part of the floor. The gray matter of the ventricular floor is covered with ependyma.
Three Areas of Flechsig-In transverse sections, through the upper, open part of the medulla oblongata, the fila of the hypoglossal and vagus nerves are seen traversing the substance of the medulla oblongata. The nucleus of origin of the hypoglossal is placed in the gray matter of the floor of the fourth ventricle close to the median plane; the nucleus of the vagus is situated in the gray matter of the ventricular floor immediately to the lateral side of the hypoglossal nucleus. From these nuclei the root-bundles of the two nerves diverge from each other as they are traced to the surface and subdivide the substance of the medulla, as seen in transverse section, into the three areas of Flechsig, viz., an anterior, a lateral, and a posterior.
The anterior area, which is bounded medially by the median raphe and laterally by the hypoglossal roots, presents within its limits: (a) the pyramid; (b) the lemniscus medialis; (c) the fasciculus tecto-spinalis; (d) the medial longitudinal fasciculus; (e) the medial accessory olivary nucleus; (f) the arcuate nucleus.
the upward prolongation of the medulla oblongata, exclusive of the pyramids which are drawn forward into the basilar part.
Pars Basilaris Pontis.-This constitutes the chief bulk of the pons. It is composed of: (1) transverse fibres arranged in coarse bundles, called the fibræ pontis; (2) longitudinal fibres, gathered together in massive bundles; and (3) a large amount of gray matter, termed the nuclei pontis, which fills up the interstices between the intersecting bundles of fibres.
The fasciculi longitudinales, to a large extent, consist of the same fibres which, lower down, are gathered together in the two solid pyramidal tracts of the medulla oblongata. When the pyramids are traced upwards into the pons they are seen to