TRACTS OF DESCENDING DEGENERATION 515 



the nerve fibers acquire their myelin sheath at earlier periods than others, 

 and that the different groups of fibers can therefore be traced in various 

 directions. This is known as the method of Flechsig. 



Wallerian or degeneration method. This method depends upon the fact 

 already presented that if a nerve fiber is separated from its nerve cell it wastes 

 or degenerates. It consists in tracing the course of tracts of degenerated 

 fibers which result from an injury, to any part of the central nervous system. 

 When fibers degenerate below a lesion the tract is said to be of descending 

 degeneration, and when the fibers degenerate in the opposite direction the tract 

 is one of ascending degeneration. By modern methods of staining of the cen- 

 tral nervous system it has proved comparatively easy to distinguish degener- 

 ated parts in sections of the cord and of other portions of the central 

 nervous system. Degenerated fibers have a different staining reaction 

 when the sections are treated by what are called Weigert's and Marchi's 

 methods. Accidents to the central nervous system in man have given us much 

 information as to its organization, but this has of late years been supplemented 

 and largely extended by the experiments on animals, particularly upon 

 monkeys. Considerable light has by the method of section and degeneration 

 been shed upon the path of conduction of impulses to and from the nervous 

 system. Thus we not only have embryological evidence mapping out different 

 tracts, but also confirmatory pathological and experimental observations. 



The tracts which have been made out are the following: 



Tracts of Descending Degeneration. The Crossed Pyramidal Tract. 

 This tract is situated to the outer side of the posterior cornu of gray 

 matter, figure 360, 7. It is found throughout the whole length of the spinal 

 cord; at the lower part it extends to the margin of the cord, but higher 

 up it becomes displaced inward from this position by the interpolation of 

 another tract of fibers, the direct cerebellar tract. The crossed pyramidal 

 tract is large, and may touch the tip of the gray matter of the posterior cornu, 

 but it is separated from it elsewhere. It is oval in shape on cross-section, and 

 diminishes in size from the cervical region downward. The tract is particu- 

 larly well marked out, both by the degeneration and the embryological methods. 

 The fibers are supposed to pass off as they descend, and to join the various 

 local nervous mechanisms of nerve cells and their branchings which are rep- 

 resented in the cord. The tract of degeneration may be traced upward beyond 

 the cord, in a way to be presently described. The fibers of which this tract 

 is composed are moderately large, but are mixed with some that are smaller. 



The Direct or Uncrossed Pyramidal Tract. This tract is situated in the 

 anterior column by the sides of the anterior fissure, figure 360, 10. It is smaller 

 than the crossed tract and is not present in all animals, though conspicuous 

 in the human cord and in that of the monkey. It can be traced upward to 

 the cerebral cortex, and downward as far as the mid or lower thoracic region, 

 where it ends. 



