THE STUDY OF THE NERVOUS SYSTEM. 7 



sections are treated by the usual Weigert stain, or positive 

 pictures are obtained by means of the technique of Marchi. The 

 method is applicable wherever localized and known centers can 

 be destroyed or known tracts can be severed from their cells of 

 origin. The fibers then degenerate in a direction away from their 

 cells of origin, and the course of the fibers is brought to view by 

 selective staining of the degeneration products of their myelin 

 sheaths. Random operations may reveal the position of centers 

 or tracts not before suspected. Many operations and prepara- 

 tions are needed in order to gain complete results. The time 

 necessary for degeneration after the operation must be determined 

 by extended trial and if not accurately determined, either no 

 result or misleading results may follow. The staining technique 

 is not always reliable. A source of serious error is found in the 

 fact that fat globules resulting from the degenerative processes 

 may be carried by blood or lymph currents to situations far removed 

 from their place of origin, and when stained in those situations 

 they give very misleading pictures. The method is extremely 

 valuable as giving the gross facts regarding the course and func- 

 tional relationship of myelinated fiber tracts. With regard to 

 the place of ending of fiber tracts the method has the same limi- 

 tations as the method of Weigert, but in even greater degree 

 because the fibers may not have degenerated throughout their 

 entire length or may have passed the proper condition for staining. 

 The method is more reHable for the origin of the fibers studied, 

 or rather for the direction of the impulses which they carry, pro- 

 vided due care is taken to distinguish between Waller's degener- 

 ation and Gudden's degeneration (see p. 91) which may also 

 occur. The method requires great understanding and caution 

 in its use but promises to be one of the most important of anatom- 

 ical methods, and one which will greatly increase in usefulness. 



AlHed to the last is the study of secondary degenerations due 

 to lesions, operative or accidental, in related centers. Nature 

 performs many experiments of the greatest value to the neurol- 

 ogist, lesions in the central nervous system often offering oppor- 

 tunity through the study of primary and secondary degenerations 

 to determine pathways of impulses otherwise difficult to follow. 



