Tractus pyramidalis lateralis or fasc. cerebrospinalis lateralis.
Fasciculus vestibulo-spinalis.
Fasciculus rubro-spinalis (Monakow).
Fasciculus thalamo-spinalis.
Fasciculus reticulo-spinalis lateralis.
Fasciculus tecto-spinalis lateralis.
Fasciculus spino-cerebellaris dorsalis (Flechsig).
Fasciculus spino-vestibularis (Horsley and Thiele). Fasciculus spino-cerebellaris ventralis.
Fasciculus spino-thalamicus.
Fasciculus spino-reticularis. Gowers' tract.
Fasciculus spino-tectalis.
Fasciculus spino-olivaris (Helwig).
Fasciculus marginalis (Lissauer).
Funiculus Posterior.
Fasciculus proprius posterior, the cornu-commissural tract (Marie).
Fasciculus cuneatus (Column of Burdach).
Fasciculus gracilis (Column of Goll).
Fasciculus postero-medialis descendens (comma tract (Schultze), peripheral bundle (Roche), oval tract (Flechsig), septo-marginal tract (Bruce and Muir), median triangular tract (Gombault and Phillipe).
Fasciculus postero-lateralis descendens (Thiele and Horsley). The methods of locating tracts of fibers may be summarized briefly, as follows: The embryological method was first employed successfully by Flechsig. He found that nerve fibers when first laid down are naked fibers without any insulating white substance of Schwann ensheathing them. That the medullary sheaths are developed at different times and that the medullation is nearly coincident with the beginning of function. Thus the fibers of motor and sensory nerves are first to become medullated, since life cannot be sustained without the automatic mechanism. Second, the fasciculi proprii of the cord are medullated and, third, the cerebellar tracts. At this stage the simple automatic and coordinating mechanisms are complete. Fourth, the voluntary
motor mechanism is established by the medullation of the tracts connecting the lower neurones with the cerebral cortex, the fibers of the pyramidal tracts being the last in the cord to receive their medullary sheaths. This last begins just before birth. Fibers of the cerebrum concerned with the higher psychic functions of the brain become medullated gradually, year after year, keeping pace with the mental development; and the process of medullation there is not completed until late in life (Kaes).
The pathological and experimental methods depend upon the fact that a nerve fiber when severed from the cell-body undergoes degeneration in accordance with the law of Waller. If the severed fiber be above the cell-body, the degeneration occurs above the lesion and is called ascending degeneration; but, if the degeneration extends from the lesion down the nerve fiber, the cell-body being above, then the condition is called descending degeneration, though all parts of the severed fibers really degenerate simultaneously. Thus by studying the paths of degeneration, above and below a destructive lesion in the human cerebrospinal axis, the various tracts of fibers have been discovered and many of them charted and traced from origin to termination. These investigations have been greatly aided by the study of degenerations in the brain and cord of lower animals. These degenerations are the results of definite experimental lesions, as cutting of certain posterior nerve roots, partial sec tion, hemisection or complete section of the spinal cord, etc. The pathological and experimental methods are commonly called the physiological method.