586 THE NERVOUS SYSTEM 



The methods for determining the tracts in the cord and the evidence on 

 basis of which function is ascribed are briefly the following: 



The Embryological Method. It has been found that, if the development 

 of the spinal cord be carefully observed at different stages, certain groups of 

 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 

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

 which result from an injury, or lesions in 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 nervous tissue, it has proved comparatively easy to distinguish 

 degenerated parts in sections of the cord or of other portions of the central 

 nervous system. Degenerated fibers have a differential staining reaction 

 when the sections are treated by Marchi's method. Accidents resulting in 

 loss of function and in degeneration in 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 various parts of the nervous system. Thus we not only 

 have embryological evidence mapping out different tracts, but also con- 

 firmatory pathological and experimental observations. 



T'he tracts which have been made out are the following: 



Tracts of Descending Degeneration. The Lateral Pyramidal Tract. 

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

 figure 360, 7. It originates in the cerebral cortex and extends 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 

 lateral pyramidal tract is large, and may touch the tip of the gray matter of 

 the posterior gray column, 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 particularly well marked out, both by the degener- 

 ation and by 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 represented in the anterior columns of 

 the cord. The fibers of which this tract is composed are moderately large, 

 but are mixed with some that are smaller. 



The Anterior Pyramidal Tract. This tract is situated in the anterior 



