Fasciculus Cuneatus (ascending postero-lateral tract, Bur dach's column).—Beginning in the middle thoracic segments (Fig. 142) the fasciculus cuneatus ascends between the entry zone and the fasciculus gracilis to the nucleus funiculi cuneati of the medulla. It acquires new fibers from every spinal nerve above its origin and grows stronger up to the first cervical nerve. In section it is wedge-shaped, being broadest at the surface; the edge of the wedge almost touches the junction of the posterior columna and posterior commissure, the posterior fasciculus proprius intervenes. Its fibers are ascending, and are branches of the axones of spinal ganglia cells, .like the fasciculus gracilis. In the nucleus funiculi cuneati all its fibers arborize and termi nate (Collier).
Descending Tracts Derived from Posterior Roots.—The descending fibers from the posterior roots of the spinal nerves arrange themselves somewhat roughly into two tracts, a postero lateral and a postero-medial (Figs. 142 and 143). These de scending radicular tracts greatly expand and multiply the ter minal relations of the posterior root-fibers.
The descending postero-medial tract (Figs. 142 and 143) has received various names at different levels. In the cervical and uppei nine thoracic segments, it is the comma tract (of Schultze), situated in the fasciculus cuneatus. Its ventral part disappears in the posterior columna above the tenth segment, but the remainder continues down the cord. Shifting its posi tion in a dorso-medial direction, it takes its place along the posterior surface in the lower thoracic segments, forming the peripheral bundle (of Hoche), and then takes up its position beside the median septum. It continues in that situation to the end of the cord, and is called, in succession, the oval tract (of Flechsig), in the lumbar segments and the septo-marginal tract (of Bruce and Muir), or the median triangular tract (of Gambault and Phillipe) in the sacral and coccygeal segments.
Fibers from the spinal nerves enter this postero-medial descend ing:tract in the cervical, thoracic, and, at least, the upper lumbar segments. After a variable course within the tract, the fibers plunge forward into the posterior columna of gray substance where they terminate.
• The descending tract (Fig. 143), situated at the posterior surface of the cord medial to the entry zone, in the lumbo-sacral region, appears to have been first described by Thiele and Horsley. It is derived from the posterior roots of the lower spinal nerves, hence it descends close to the entry zone. In section it is triangular. Its fibers terminate in the posterior columna of gray substance.
Posterior Fasciculus Proprius (fasciculus posterior pro prius).—This is made up of association fibers that connect dif ferent segments of the cord. It is a compact strand in the cornu commissural field of Marie (Figs. 142 and 143).
The cornu commissural tract (Fig. 143) is placed between the posterior columna (cornu), the posterior commissure and the posterior septum. It extends throughout the spinal cord, but is best developed in the lumbo-sacral segments. It con tains both ascending and descending fibers as do other parts of the fasciculus proprius.
Lesions in the posterior columns cause disturbances of the muscular and tactile senses, and ataxia and incoordination re sult. If the entry zone is involved there is disturbance of all kinds of common sensation, at the level of the lesion. These columns are usually involved by extension from the posterior roots in locomotor ataxia (posterior sclerosis), hence the parxs thesia, crises, loss of reflexes, disturbed equilibrium and ataxic gait.