THE NERVOUS SYSTEM 465 



definite septum divides the posterior column into an inner part, the column oJGoll, 

 and an outer part, the column of Burdach. 



For further variations and differences between the segments of the cord, 

 compare Figs. 323, 324 and 325. 



Fibre Tracts of the Cord 



The determination of the fibre tracts of the cord has been accom- 

 pHshed principally by two methods: (i) The myelogenetic method, 

 which is based upon the fact that the fibres of different systems ac- 

 quire their myelin sheaths at different periods of embryonic develop- 

 ment. Thus by examining cords from embryos of various ages and 

 young specimens it is possible, using a myelin stain {e.g., Weigert), 

 to distinguish different tracts by the presence or absence of myeliniza- 

 tion of their fibres. (2) The method of secondary or axonal degenera- 

 tion, based upon the fact that a fibre separated from its cell under- 

 goes degenerative changes and ultimately disappears and that the 

 cell body also usually shows certain changes (see page 143). The 

 fibres distal to the injury can be distinguished during active degenera- 

 tion by applying the Marchi stain (page 35). After their disappear- 

 ance, however, a negative picture is obtained by staining the sur- 

 rounding normal fibres (Weigert). The changes in the cell bodies 

 whose axones are injured are distinguished by applying the Nissl 

 stain (p. 39). Thus if the cord is cut at some particular level, at 

 any level above the cut all fibres present which originate from cells 

 below the cut will show degeneration ("ascending" degeneration), 

 while the cell bodies of the cut fibres will show the axonal degenera- 

 tion changes. On the other hand, at any level below the cut, 

 fibres which originate from cells above the cut will show degenera- 

 tion (''descending" degeneration), while the cell bodies of these 

 fibres, located above the cut, will exhibit axonal degeneration. 

 The indication this gives as to the direction of conduction is 

 evident when it is remembered that the impulse passes from neu- 

 rone body along the axone to its termination. (3) Atrophy (von 

 Gudden's method). This method is based upon the fact that 

 extirpation of some part of the nervous system in a young animal 

 is followed by an atrophy of parts in intimate relation therewith. 

 This method only demonstrates grosser changes than the pre- 

 ceding, but on the other hand whole conduction paths involving 

 more than one neurone relay may show changes. Other methods are 

 the method of comparative anatomy, i.e., study of the simpler nervous 



30 



