158 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



eration) and the method of myelinization. The method of second- 

 ary degeneration depends upon the fact that, when a fiber is cut 

 off from its cell of origin, the peripheral end degenerates in a few 

 days. If, therefore, a lesion, experimental or pathological, is made 

 in the cord at any level, those fibers that are affected undergo 

 degeneration: those with their cells below the lesion degenerate up- 

 ward, and those with their cells above the lesion degenerate down- 

 ward. According to the law of polarity of conduction in the neuron 

 a descending degeneration in the cord indicates motor or efferent 

 paths as regards the brain, and ascending degeneration indicates 

 sensory or afferent paths. It is obvious that localized lesions can 

 be used in this way to trace definite groups of fibers through the cord. 

 If, for instance, one exposes and cuts the posterior roots in one or 

 more of the lumbar nerves, the portions of the fibers entering the 

 cord will degenerate, and the path of some of these fibers may be 

 traced in this way upward to the medulla. The degenerated fibers 

 may be revealed histologically by the staining methods of Weigert 

 or of Marchi. The latter method (preservation in Miiller's fluid, 

 staining in osmic acid and Miiller's fluid) has proved to be espe- 

 cially useful; the degenerated fibers during a certain period give 

 a black color with this liquid, owing probably to the splitting up 

 of the lecithin in the myelin and the liberation of the fat from its 

 combination with the other portions of the molecule.* The mye- 

 linization method was introduced by Flechsig. It depends upon 

 the fact that in the embryo the nerve fibers as first formed have 

 no myelin sheath, and that this easily detected structure is in the 

 central nervous system assumed at about the same time by those 

 bundles or trac.ts of fibers that have a common course and func- 

 tion. By this means the origin and termination of certain tracts 

 may be worked out in the embryo or shortly after birth. The 

 well-known system of pyramidal fibers, for instance, is clearly 

 differentiated in the embryo late in intra-uterine life or at birth, 

 owing to the fact that the fibers composing it have not at that 

 time acquired their myelin sheaths. Flechsig assumes that the 

 development of the myelin marks the completed structure of the 

 nerve fiber and indicates, therefore, the time of its entrance into 

 full functional activity. 



General Classification of the Tracts. The tracts that have 

 been worked out in the white matter of the cord have been classified 

 in several ways. We have, in the first place, the division into as- 

 cending and descending tracts. This division rests upon the fact 

 that the axon conducts its impulses away from the cell of origin, and 

 consequently those neurons whose axons extend upward toward the 



* See Halliburton, "The Chemical Side of Nervous Activity/' London. 

 1901; "Croonian Lectures." 



