HISTOKY AND METHODS OF INVESTIGATION. 9 



through the methods of secondary degeneration and of atrophy. But a still 

 more fruitful source of knowledge is a new method based upon the study 

 of the development of the nerve-sheath. 



To Flechsig is due the merit of introducing and of exhaustively util- 

 izing this new method. In a series of communications (1872-1881), and 

 later in a larger work on the " Conducting Paths in the Brain and Spinal 

 Cord" ("Leitungshahnen im Gehirn u. Eiickenmark," 1876), he showed 

 that the different nerve-tracts, which, in sections of the central nervous 

 system of an adult, appear so similar and uniform, differ essentially in' the 

 embryonic period and that they acquire their medullary sheath at different 



Fig. 2. — For descriptionj see text. 



times. Whole "systems" in the cross-section of the spinal cord are still 

 transparent at a time when others have already become white and medal- 

 lated. The tracing of the white portions in cross-sections and longitudinal 

 sections is much easier, and gives much more reliable results than does the 

 tracing of fasciculi in fully-developed organs. 



A good idea of the peculiarities of the results of the methods thus far 

 mentioned may be had from a study qf the accompanying figures. Fig. 1 

 shows the result of a dissection, a separation of the fibers with forceps, and 

 shows the course in the cerebrum of the fibers of the corpus caUosum. 



Pig. 3 is made from a frontal section through the cerebrum of a nine- 



