Lateral Fasciculus Proprius (Fasciculus Lateralis Pro prius, Figs. 124 and 125) .—The whole lateral column of the spinal cord except the lateral pyramidal and dorsal spino-cerebellar tracts is continued into the lateral column of the medulla. Composed of ascending and descending axones which are commissural and associative for different segments of the spinal cord, the lateral fasciculus proprius of the medulla runs in part beneath and in part superficial to the inferior olivary nucleus; beyond the olive it is continued in the substantia reticularis grisea of the medulla and reticular formation of pons and mid-brain. Among the fibers of the lateral fasciculus proprius ventral to the olive is the s pino-olivary or triangular tract of Helwig and dorsal to the olive the thalamo-olivary bundle.
The vestibulo-spinal tract (fasciculus vestibulo-spinalis) (Fig. 13 I) rises in the nucleus of Deiters. The vestibulo-spinal fasc iculus appears at the anterior lateral sulcus near the lower end of the medulla and decends along the surface of the cord just be hind the anterior roots of the spinal nerves. It terminates in the gray cresent of the cord forming an efferent link in the vestibular arc of equilibrium and, because the nucleus of Deiters receives the fastigio-bulbar tract from the cerebellum, it also forms a segment of a long cerebello-spinal path made up of three sets of neurones—the cortical and fastigial of the cerebellum and the vestibular of the medulla. The whole path is concerned with coordination.
Ventral Spino-cerebellar and Spino-thalamic tract (fasciculus spino-cerebellaris 'ventralis) (Figs. 123 and 126).—These two tracts are combined into one throughout the cord, medulla and pons. Near the isthmus the former turns backward around the brachium conjunctivum and through the superior medullary velum and ends in the cortex of the superior vermis cerebelli while the spino-thalamic tract continues in the original direc tion to the lateral nucleus of the thalamus. The common spino-encephalic tract made up of these two bundles takes its origin from the base of the anterior columna and center of the crescent of gray matter, chiefly on the opposite side of the spinal cord and crossing through the white anterior commissure it ascends, mingled somewhat with the fibers of the above de-. scending vestibulo-spinal tract, along the lateral surface of the cord (Barker). It runs beneath the posterior lateral groove of the medulla and through the formatio reticularis of the pons, to the point of division near the isthmus rhombencephali whence the two divisions proceed to their cerebellar and thalamic terminations, as above stated. The tract is probably
reinforced in the medulla and pons by the addition of axones from the opposite terminal nuclei of common sensory cerebral nerves and, rising primarily in relation with the posterior roots of spinal nerves, it thus forms a crossed path for common sensa tions, spinal and cerebral. It conducts tactile, pain and temperature impulses (see pp. 159 and 300).
The rubro-spinal tract is the crossed descending tract of the red nucleus (Figs. 123 and 126). Running dorso-lateral to the inferior olive in the medulla it mingles with the fibers of the ventral spino:cerebellar tract. It is continued down the cord, in the lateral column, to the lateral columna and center of the gray crescent as far as the first lumbar segment (see pp. 161, 229 and 304).
Closely associated with the rubro-spinal tract in the reticular substance of the medulla there are three other fasciculi which can be identified by degeneration and myelinization. They are the thalamo-spinal fasciculis, lateral tecto-spinal fasciculus and lateral reticulo-spinal fasciculus, described on pp. 155, 235 and 3o3. Ventro-medial to these tracts the thalamo-olivary fasciculus can be seen skirting the dorsal wall of the olive near its medial end and gradually terminating it.
Longitudinal Fibers of the Posterior Columns.—The longi tudinal fibers of the posterior column form many bundles and the bundles are different in upper and lower medulla. The substantia reticularis is small.
The lower medulla contains: The funiculus gracilis, funiculus cuneatus, tractus spinalis nervi trigemini and dorsal spino cerebellar tract, named from the posterior median fissure out ward (Figs. 125 and 133). In the upper medulla are: The restiform body and the spinal tract of the fifth cerebral nerve at the surface and the tractus solitarius in the interior (Figs. x23, 124 and 133).
The funiculus gracilis is the superior end of the ascending postero-medial column (Go11's column) of the spinal cord. Near its extremity it expands and forms the clava, and then tapers off and disappears along the side of the fourth ventricle. The clava is due to the nucleus funiculi gracilis, in which the fibers of the column end. The funiculus gracilis is composed of ascending branches of the posterior roots of the spinal nerves which enter the cord below the seventh or eighth thoracic segment.