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ANATOMY
form a thick white band, the crossed pyramidal tract, that lies on the lateral side of the dorsal horn. When it reaches the apex of the pyramid in the medulla oblongata the component fibres cross to the opposite side, decussating with those of the corresponding bundle of that side and ascending in the pyramid of the opposite side (vol. i. p. 870); then passing forward through the pons, crus cerebri, and through the middle of the corpus striatum, they end in perikaryons in the cerebral cortex. These cortical cells are closely linked together by numerous commissural and association neurones, to be described hereafter. By these fibres the cortex cerebri is brought into relation with the voluntary muscles, there being but two interruptions in the circuit. (2) Other axons, ending about the same group of anterior horn cells, pass across at once to the opposite side of the spinal cord, and then ascend in the mesial part of the white tract bordering the ventral fissure. They run up in the pyramid of their own side, and otherwise follow a course comparable with those of the last set. These make the direct pyramidal tract. (3) A wide curved band of white spinal cord substance stretches from the outside of the direct pyramidal tract outside, and in front of the ventral horn, as far as the edge of the crossed pyramidal tract. This consists of the axons of shorter neurones, which begin in nerve-cells in the grey matter of the cord at different levels. Some of these axons cross in the anterior commissure to the opposite side, while others pass down in this tract to end by synapsis with the dendrites of the cells of the ventral horn; thus, although this ventrolateral association tract is apparently a continuous mass, it is made up of shorter and more varied axons than the foregoing. These three tracts agree in being descending tracts coming from nerve-cells on a higher level and in being connected with the ventral horn-cells by synapses, and thereby with the motor nerve roots arising from them. (4) Two groups of axons, mostly ascending, are closely related to these, and spring from the cells of the lateral cell column of Clarke (vol. i. p. 866), whose dendrites are contiguous with those of cells in the ventral part of the column. These axons form a thin layer on the lateral surface of the cord between the dorsal and ventral nerve roots, and they extend on the same side into the restiform body of the medulla oblongata above, by which they pass into the cerebellum. By this cerebellar tract the neurones of the cerebellum are associated with the spinal nerve system; and it has been inferred from experiment that these are connected, in some way as yet unknown, with the co-ordination of muscular actions. (5) From the cells of the ganglia above described, which form the segmental origins of the sensory fibres constituting the dorsal roots of the spinal nerves, we have already seen that T-shaped processes arise. The inner branch of each of these passes centripetally, and forms the link between the ganglion of the root and the spinal cord. The continuations of these form almost all the white matter that lies internal to the dorsal roots. Most of these fibres run upwards, but some descend to form synapses with dorsal horn-cells on a lower level. These send, as they pass along, collateral branches into the grey matter of the dorsal horn to end around the nerve-cells there. Neurones of this grey matter in turn send axons, which terminate above in tufts about the cells of the posterior columns of the medulla oblongata, and these again send axons in the fillet of the crus cerebri, to end, after interruptions in the thalamus, in tufts about some of the smaller, deeper cells , of the cerebral cortex. Thus the effect of an efferent impulse, while for the most part propagated upwards, may be to some extent continued downwards, sometimes causing reflex action by affecting the region below it. By means of these bundles of fibres in the cord, the segmental
nerves are linked together and so controlled as to act as component parts of a functional whole (Fig. 7). The spinal cord is essentially bilateral. In the embryo it is formed of the lateral thickenings of the sides of the neural canal, and no neuroblasts (embryonic nerve-cells) arise either in the ventral or dorsal median portion of the wall of the canal. Whatever fibres at a later stage cross from side to side are invasions from lateral structures. The medulla oblongata or bulb is developed from the
nerve centres. 1, Descending axons from the perikaryons of the motor centres in the cerebrum, decussating in the bulb, and united synaptically to the motor neurones in the cord ; 2, axons which descend without decussating until they reach the level of the motor neurones, then crossing on the anterior white commissure of the cord ; 3, associative neurones ; 4, associative neurones connected with the cerebellum ; 5, ascending axons from sensory cells, interrupted at the bulb ; 6, ascending axons to cerebellum, ending by synapsis with associative neurones which connect cerebellum with cerebrum (9); 7, optic fibres from neurones in retina to corpora quadrigemina ; 8, optic fibres to occipital region of cerebrum. OP, ophi thalamus ; CR, crus cerebri; DG, ganglion of dorsal root; SEN, sensory nerve. wall of that part of the neural canal which formed the hinder cranial vesicle, and lies immediately within the Bra'a foramen magnum of the skull. In it the dilated neural canal forms the fourth ventricle, which dorsally is for the most part closed by a membrane, the superior medullary velum, as well as by the cerebellum. Through the medulla pass the ascending and descending axons of the various cerebro-spinal association systems. In it are also the segmental nuclei of the nerves of the series of nine hinder cranial segments, which have become secondarily grouped into the V.-XII. nerves of Human Anatomy. The cerebellum (vol. i. p. 871) lying on the dorsal side of the ventricle has originated by two lateral ingrowths derived from the
