TRACTS OF THE SPINAL CORD The antero-lateral fasciculus proprius (fasciculus antero lateralis proprius) occupies the deep part of the anterior and lateral columns (Figs. 142 and 143). It embraces the anterior columna of gray matter and covers the outer surface of the center of the crescent and the base of the posterior columna. By the most lateral anterior root-fibers it is subdivided into anterior and lateral fasciculi. It approaches, but does not quite reach, the surface of the cord. Notice that it is separated from the anterior median fissure by the anterior pyramidal tract, and that the lateral pyramidal, the spino-cerebellar and spino thalamic tracts, etc., run between it and the surface of the cord. Behind, it is in relation with the lateral pyramidal tract. The antero-lateral fasciculus proprius is composed of ascending and descending fibers which are the T-branches of axones from the gray crescent. It is largely a short fiber tract, associative and commissural in function. That part situated in the anterior column, the anterior fasciculus proprius, is largely commissural, between the anterior columnT; while the lateral fasciculus proprius is chiefly associative, and connects different segments of the cord on the same side. That part of the lateral fasciculus proprius which intervenes between the posterior columna and the lateral pyramidal tract, and inter mingles somewhat with the marginal fasciculus, is called by Cajal the bundle of the dorsal horn. It rises from the cell-bodies in the substantia gelatinosa and caput of the posterior columna; its fibers are small and short. As a rule, all fibers of the fasciculus proprius which are next the gray substance are short, the longer ones run farther and farther from the gray crescent. The antero-lateral fasciculus proprius is continued in the substantia reticularis of the medulla, and the reticular formation of pons and mid-brain, constituting a short fiber tract which extends from the lower part of the cord to the basal ganglia of the cerebrum. A part of the anterior fasciculus proprius is continued into the medial longitudinal bundle (fasciculus longitudinalis medians).
The medial (posterior) longitudinal bundle is composed of an ascending and a descending strand of fibers (Figs. 142 and 143).
The ascending strand rises from the anterior columna in each segment of the spinal cord and runs upward to the motor nuclei of cerebral nerves and terminates in them. Perhaps a few fibers reach the thalamus. It ascends just ventro-medial to the anterior columna in the cord; in the medulla, it runs between the head of the anterior columna and the pyramidal decussa tion, then just lateral to the fillet decussation, after which, it takes its doros-medial position along the raphe. Its function is reflex. The descending strand is the anterior reticulo-spinal tract. It rises from all the nuclei of the reticular formation but chiefly from the nuclei centrales and the nucleus lateralis medius in the pons. Forming a part of the medial longitudinal bundle of the same side, its fibers end in the crescent as it descends the cord. Within the lateral funiculus, descends the lateral reticulo-spinal tract. This tract has the same origin and termination as the anterior reticulo-spinal tract except that it decussates. It crosses near its origin right through the medial longitudinal bundles. Both reticulo-spinal tracts extend to the lower part of the cord.
The anterior pyramidal tract (fasciculus cerebrospinalis anterior), occupies a thin area next the anterior median fissure (Figs. 142 and 143). It is the direct continuation of about io per cent. of the pyramidal tract in the medulla. It is said to be absent in 15 per cent. of human cords; in these cases there is complete decussation to the lateral funiculus, as in the cat. In the mole the whole pyramidal tract descends the anterior funiculus without division or decussation. The fibers of the anterior pyramidal tract rise from the giant cells of Betz, whose bodies are situated in the anterior central gyrus of the cerebrum. As the tract descends in the cord, the fibers decus sate through the white anterior commissure, and terminate in relation with the cells of the opposite gray crescent, probably, in the posterior columna. It reaches to the fifth sacral seg ment (Collier). Imbedded in the anterior pyramidal tract is a small strand first described by Held, the anterior tecto spinal fasciculus.