According to Sherrington, an anterior cerebro-spinal tract in the spinal medulla of the anthropoid apes stands in connexion with the arm-centre in the cerebral cortex. If this is the case in man it must have other connexions as well, seeing that it is carried down the spinal medulla for a considerable distance beyond the level of the spinal segments which give motor fibres to the upper limb. In the lower apes an anterior cerebro-spinal tract FIG. 489.-SECTION THROUGH THE INFERIOR END OF THE MEDULLA OBLONGATA OF A CHIMPANZEE. does not seem to exist: the whole pyramid crosses over to the opposite side of the spinal medulla in the shape of the lateral cerebrospinal tract. As we have noted, the decussating pyramidal bundles pass through the anterior column of gray matter, and cut it into two portions (Figs. 489 and 490). The basal part remains in position on the anterior and lateral aspect of the central canal, and forms part of the thick layer of gray matter which surrounds it. The detached head of the anterior column is set free; and from the large multipolar cells which lie in its midst some of the fibres of the anterior root of the first cervical nerve, and also some of the root fibres of the accessory nerve, take origin. As we proceed into the medulla oblongata another effect of the decussation of the pyramids is seen in the submergence from the surface of the strand of fibres which, in the anterior funiculus of the spinal medulla, lies to the lateral side of the anterior cerebro-spinal tract, and which receives the name of the fasciculus anterior proprius. While the decussation is going on the fasciculus proprius is thrust aside, and in the medulla oblongata, it takes up its position as a flattened band-like strand on the lateral side of the pyramid (Fig. 489). When the decussation close to the median Gray matter plane on the dorsal -Funiculus gracilis Funiculus Tractus spinalis of fifth nerve Nucleus tractus spinalis nervi trigemini Fasciculus spinocerebellaris Detached head of anterior column aspect of the pyramid, Central canal Fissura mediana anterior FIG. 490.-TRANSVERSE SECTION THROUGH THE INFERIOR END OF THE to the dorsal surface Treated by the Weigert-Pal method. The gray matter is bleached white, and the greater part of a the medullated tracts of fibres are black. strand, which is termed the medial longitudinal bundle (Figs. 494 and 495). The detached head of the anterior column of gray matter of the spinal medulla, as it is traced upwards, is observed to cling closely to its original relationship with the fasciculus anterior proprius. It is applied to the lateral side of this strand, and, gradually becoming smaller, finally disappears at the level of the inferior part of the inferior olivary nucleus. Cuneate and Gracile Fasciculi, with their Nuclei.-As the fasciculus gracilis and the fasciculus cuneatus of the posterior funiculus of the spinal medulla are traced up into the medulla oblongata they seem to increase in bulk, and in transverse sections they assume the form of massive wedge-shaped strands, quite distinct from each other. They increase in width and lose considerably in depth, and consequently the transverse diameter of the area which they occupy becomes greater. As a result of this, and also owing to the removal of the lateral cerebro-spinal tract from the lateral funiculus of the spinal medulla immediately in front, the posterior column of gray matter is gradually rotated forwards and comes to lie transversely and in the same straight line with its fellow of the FIG. 491.-SECTION THROUGH THE CLOSED PART OF THE HUMAN MEDULLA OBLONGATA IMMEDIATELY ABOVE THE DECUSSATION OF THE PYRAMIDS (Weigert-Pal specimen). opposite side (Figs. 490 and 491). The substantia gelatinosa, at the same time, becomes increased in quantity and presents a horseshoe-shaped outline in transverse section. It clasps within its concavity the somewhat reduced head of the posterior column, and forms with it a conspicuous circular mass of gray matter which lies close to the surface, and produces upon it the bulging termed the tuberculum cinereum. The basal portion of the posterior column of gray matter remains upon the dorsal and lateral aspect of the central canal, and forms a portion of the central gray mass of the closed part of the medulla oblongata; but very soon the neck of the column, which at this level is greatly reduced owing to the absence of entering posterior nerve-roots, is invaded by bundles of fibres which traverse it in different directions and convert it into a formatio reticularis. By this means the rounded head of the posterior column becomes cut off from the central gray matter, and from this point upwards it remains as an isolated gray column intimately associated with the spinal root of the trigeminal nerve. It has, in fact, become the nucleus tractus spinalis nervi trigemini. The gracile and cuneate nuclei are seen in their most typical form in sections at the level of the decussation of the pyramids (Figs. 489 and 490). The gracile According to Sherrington, an anterior cerebro-spinal tract in the spinal medulla of the anthropoid apes stands in connexion with the arm-centre in the cerebral cortex. If this is the case in man it must have other connexions as well, seeing that it is carried down the spinal medulla for a considerable distance beyond the level of the spinal segments which give motor fibres to the upper limb. In the lower apes an anterior cerebro-spinal tract FIG. 489.-SECTION THROUGH THE INFERIOR END OF THE MEDULLA OBLONGATA OF A CHIMPANZEE. does not seem to exist: the whole pyramid crosses over to the opposite side of the spinal medulla in the shape of the lateral cerebrospinal tract. As we have noted, the decussating pyramidal bundles pass through the anterior column of gray matter, and cut it into two portions (Figs. 489 and 490). The basal part remains in position on the anterior and lateral aspect of the central canal, and forms part of the thick layer of gray matter which surrounds it. The detached head of the anterior column is set free; and from the large multipolar cells which lie in its midst some of the fibres of the anterior root of the first cervical nerve, and also some of the root fibres of the accessory nerve, take origin. Funiculus gracilis Funiculus cuneatus Tractus spinalis of fifth nerve Nucleus tractus spinalis nervi trigemini As we proceed into the medulla oblongata another effect of the decussation of the pyramids is seen in the submergence from the surface of the strand of fibres which, in the anterior funiculus of the spinal medulla, lies to the lateral side of the anterior cerebro-spinal tract, and which receives the name of the fasciculus anterior proprius. While the decussation is going on the fasciculus proprius is thrust aside, and in the medulla oblongata, it takes up its position as a flattened band-like strand on the lateral side of the pyramid (Fig. 489). When the decussation is completed, this strand is seen to lie close to the median plane on the dorsal aspect of the pyramid, where it is separated from its fellow of the opposite side by the median raphe alone (Fig. 491). In the upper part of the medulla oblongata it approaches still nearer Fasciculus spinocerebellaris Detached head. of anterior columu igs. 494 and 495) al medulla as it is traced upwards, is observed to cling closely to its original relationship with the fasciculus anterior proprius. It is applied to the lateral side of this strand, and, gradually becoming smaller, finally disappears at the level of the inferior part of the inferior olivary nucleus. Cuneate and Gracile Fasciculi, with their Nuclei. As the fasciculus gracilis and the fasciculus cuneatus of the posterior funiculus of the spinal medulla are traced up into the medulla oblongata they seem to increase in bulk, and in transverse sections they assume the form of massive wedge-shaped strands, quite distinct from each other. They increase in width and lose considerably in depth, and consequently the transverse diameter of the area which they occupy becomes greater. As a result of this, and also owing to the removal of the lateral cerebro-spinal tract from the lateral funiculus of the spinal medulla immediately in front, the posterior column of gray matter is gradually rotated forwards and comes to lie transversely and in the same straight line with its fellow of the FIG. 491.-SECTION THROUGH THE CLOSED PART OF THE HUMAN MEDULLA OBLONGATA IMMEDIATELY ABOVE THE DECUSSATION OF THE PYRAMIDS (Weigert-Pal specimen). opposite side (Figs. 490 and 491). The substantia gelatinosa, at the same time, and cuneate nuclei are seen in their most typical form in nucleus appears in the form of a relatively slender mass of gray matter in the interior of the funiculus gracilis. The cuneate nucleus is a direct offshoot from that part of the base of the posterior column of gray matter which is preserved as a portion of the central gray mass. In transverse section it is seen to invade the funiculus cuneatus upon its deep aspect, and it gradually grows backwards into its substance. It presents Gracile nucleus Anterior basis-bundle Lateral cerebro-spinal a second and much smaller mass of gray matter appears in the funiculus cuneatus, superficial to the main nucleus. This is termed the accessory or the lateral cuneate nucleus (Fig. 491). Detached anterior As a series of sections is studied from below upwards, it will be noticed that the number of fibres in the gracile and cuneate funiculi rapidly decreases, until, at the level of the clava and cuneate tubercles, it is seen that these eminences are composed almost entirely of the gray nuclei, covered by a thin layer of the few remaining fibres of the two fasciculi involved. It would appear that no fibres belonging to the funiculus gracilis and funiculus cuneatus get beyond these nuclei. They all end in fine terminal ramifications around the cells of the nuclei. In the case of the funiculus cuneatus the bundles of fibres, as they pass from the surface into the subjacent gray nucleus, are very distinctly seen in transverse sections through the bulb. FIG. 492.-SECTION THROUGH THE INFERIOR PART OF THE When the medulla oblongata opens up into the fourth ventricle the gracile and cuneate nuclei are pushed laterally by the expanding ventricular floor, and the gracile nucleus soon comes to an end; but the cuneate nucleus extends upwards for a short distance farther, and terminates only when the restiform body begins to take definite shape on its of trigeminal lateral aspect. Decussatio Lemniscorum. Immediately above the level of the decussation of the pyramids another decussation of fibres takes place in the substance of the medulla oblongata Nucleus tractus spinalis nervi trigemini Decussation of lemniscus medialis Medial accessory olivary nucleus Fila of hypoglossal nerve. in the median plane, FIG. 493.-TRANSVERSE SECTION THROUGH THE CLOSED PART OF A FETAL and upon the dorsal MEDULLA, IMMEDIATELY ABOVE THE DECUSSATION OF THE PYRAMIDS. Treated by Weigert-Pal method. aspect of the pyra mids. This is the decussation of the lemniscus medialis, or the sensory decussation. so-called in contradistinction to the term "motor decussation," which is sometimes applied to the decussation of the pyramids. The fibres which take part in this decussation are called internal arcuate fibres, and they are derived from the cells of the gracile and cuneate nuclei. From the deep aspects of these nuclei these fibres |