TRACTS OF THE SPINAL CORD. 353 



by Flechsig. He found that nerve fibers when first laid down are 

 naked axones 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 fascic- 

 uli proprii of the cord are medullated and, third, the cerebellar 

 tracts. At this stage the simple automatic and coordinating mech- 

 anisms 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 pyram- 

 idal tracts being the last to receive their medullary sheaths. 

 This last occurs just before birth. Fibers of the cerebrum con- 

 cerned with the higher psychic functions of the brain become med- 

 ullated gradually, year after year, keeping pace with the mental 

 development; and the process of medullation there is not com- 

 pleted until late in life (Kaes). 



The pathological and experimental methods depend upon 

 the fact that a nerve fiber when severed from the cell-body under- 

 goes 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 de- 

 generation, above and below a destructive lesion hi the human 

 cerebro-spinal axis, the various tracts of fibers have been discov- 

 ered and many of them charted and traced from origin to termin- 

 ation. 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 section, hemi- 

 section or complete section of the spinal cord, etc. The patho- 

 logical and experimental methods are commonly called the physio- 

 logical method. 

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