The lateral pyramidal tract (fasiculus cerebrospinalis lateralis) forms a considerable part of the lateral column of the spinal cord (Figs. 142 and 143). It is covered, superficially, by the dorsal spino-cerebellar tract in the cervical and thoracic cord; but in the lumbar and sacral cord it forms part of the surface. Its deep surface is in relation with the lateral fascicu lus proprius, its ventral border with the rubro-spinal and thal amo-spinal tracts, and its dorsal border with the marginal bun dle. The fibers composing it are axones of Betz's cells in the anterior central gyrus of the cerebral cortex. They rise with those of the anterior pyramidal tract, and the two run as one tract down through the genu and anterior two-thirds of the occipital part of the internal capsule, the middle three-fifths of the basis pedunculi, the basilar longitudinal fibers of the pons and the pyramid of the medulla. In the medulla the two tracts separate. The lateral tract, comprising four-fifths of the pyra mid, decussates with its fellow through the anterior median fissure, pierces the anterior gray columna and descends with some uncrossed fibers in the lateral column of the cord. It terminates in relation with the baso-lateral cell-bodies within the posterior columna, according to Schafer, Collier and others. The anterior tract follows the anterior median fissure as already described. Both end chiefly in the gray crescent opposite to their cortical origin. According to Marchi, io or 20 per cent. of the fibers remain uncrossed. The pyramidal tracts are the cerebral motor tracts. By them motor and inhibitory impulses are carried to the cord.
Only in the higher primates does the pyramidal tract divide in the medulla into two fasciculi: in mammals generally and in the lower monkeys it is undivided. There is one known mam mal in which it descends the anterior funiculus without decus sation, the mole. In all other mammals below primates, it decussates en masse to the opposite side; it descends the dorsal part of the lateral funiculus, near the posterior columna, in carnivora and in the rabbit; and it runs down the ventral part of the posterior funiculus in certain herbivora, such as the red squirrel, chipmunk, guinea-pig, mouse, white rat, etc. (Ran son: Am. Jour. Anat., Vol. 14, and Jour. Comp. Neurol., Vol. 24. Simpson: Jour. Comp. Neurol., Vol. 24).
Lesions.—The pyramidal tracts (especially the lateral) are involved in lateral sclerosis and in amyotrophic lateral sclerosis; and, as a consequence of it, both voluntary and inhibitory im pulses from the brain are interfered with, hence the spastic paralysis and exaggerated reflexes. The pyramidal tract may be more or less involved in insular sclerosis and in bulbar paraly sis, and the symptoms vary with the amount of sclerosis. De generation of the gray matter and of the pyramidal, spino cerebellar, marginal and posterior tracts has been demonstrated in Friedreich's hereditary ataxia, and the involvement of the pyramidal tracts explains the spastic paralysis which affects both arms and legs. In ataxic paraplegia (Gowersi) there is
diffuse sclerosis of the lateral and posterior columns of the cord. It is the degeneration in the pyramidal tracts that causes the spastic gait, incoordinated arm movements and early increase of the reflexes, observed in that affection.
Rubro-spinal and Thalamo-spinal Fasciculi.—These two tracts in close association descend the cord ventral to the lateral pyramidal tract and subjacent to the spino-cerebellar tracts. They are somewhat intermingled with the dorsal fibers of the lateral vestibulo-spinal tract, which, for the most part, is in front of them. The rubro-spinal fasciculus has been more perfectly traced and is better understood, though it is claimed that a greater number of fibers from the thalamus descend through this region than from the red nucleus. The fibers from the red nucleus constitute the rubro-spinal tract of Monakow. It extends as far as the first lumbar segment and ends in the gray crescent. Its origin in the nucleus ruber, its crossing through the ventral tegmental decussation (Foreli) and its course down the brain stem have been described. The rubro spinal tract is descending in direction. It carries coordinating impulses to the lower motor neurones presiding over locomotion (Horsley). The thalamo-spinal fasciculus probably originates in the lateral nucleus of the thalamus and possibly in the hypo thalamic nucleus. We have no positive evidence of its decus sating like the rubro-spinal tract which, lower down, it accom panies; it appears to be an uncrossed tract. The thalamo spinal tract, like the rubro-spinal, terminates in the gray matter of the cord in connection with the anterior root neurones. Manifestly its function is of a reflex nature.
The marginal fasciculus of Lissauer (f'asciculus marginalis) lies upon the apex of the posterior columna, lateral to the main bundles of the posterior root-fibers; it touches the surface, superficially; and its deep border fuses with the stratum zonale of the gelatinous substance of Rolando. The marginal tract is composed of small fibers, most of them non-medullated (Ran son), from the lateral fascicles of the posterior roots of the spinal nerves; they are axones of the small bipolar cells of the spinal ganglia. Dividing T-like as they enter the cord, their ascending and descending rami pursue a short vertical course and, then, terminate in the gelatinous substance of the posterior columna. Interspersed through the marginal tract, there are many medul lated fibers of the lateral fasciculus proprius, fibers belonging to the "bundle of the dorsal horn." The function of the mar ginal tract is unknown. It has been suggested by Ranson and others, but without supporting data, that it conveys impulses of pain and temperature. Others suggest a sympathetic func tion (Am. Jour. Anat., Vol. 16; Anat. Rec., Vol. 8, p. 119; Jour. Comp. Neurol., Vol. 24).