THE XKRX'OUS SYSTIO.M -107 



systems of lower forms and the correlated development or absence of 

 related parts of the nervous system, and the method of pliysiology, i.e., 

 study of the physiological effects of stimulation or extirpation of 

 various portions of the nervous system thereby indirectly demon- 

 strating anatomical pathways. 



Ascending Tracts. 



It will be recalled that various groups or systems of neurones 

 (nuclei and tracts) are linked together to form conduction paths 

 (p. 427). In general an afferent conduction path consists of (i) a 

 primary system (afferent ganglionic) whose central processes (aff'erent 

 root fibres) end in its terminal nucleus; (2) a secondary system 

 whose bodies constitute the terminal nucleus of (i) and whose axones 

 form a second ary tract and end in a secondary terminal nucleus; 

 (3) the conduction path may continue through tertiary nuclei and 

 tracts, etc. 



A . Tracts form ill i^ parts of afferent pallia! paths. 



I. Long Ascending Arms of Dorsal Root Fibres (Posterior 

 Funiculi). — The origin of these tracts — central processes of the cells 

 of the spinal gangha — has been described (page 442). The distribu- 

 tion of the posterior root fibres to the gray matter of the cord was 

 noted in connection with the study of the lumbar enlargement sec- 

 tion (page 458). The general arrangement of these fibres in the 

 dorsal funiculi remains to be noted. 



Each successive dorsal root sends its fibres into the cord next to 

 the dorsal horn and therefore lateral to the ascending fibres from the 

 next root below. Thus the fibres of the lower roots as they ascend 

 the cord are gradually pushed toward the median line until they finally 

 occupy that part of the posterior column lying near the posterior sep- 

 tum. The separation of the posterior column by a connective-tissue 

 septum into the column of Goll and the column of Burdach occurs 



Fig. 326. — Diagram of the Tracts of the Cord (Cervical Region). Ascending tracts 

 are shown on the left side, and descending tracts on the right. It will be noticed that 

 the tracts of the cord are roughly divisible into three concentric zones: (i) A zone oc- 

 cupying most of the posterior columns and the peripheral part of the lateral columns. 

 This zone comprises the principal long ascending tracts (beginnings of afferent supra- 

 segmental paths). (2) The second zone lies immediately within the first in the lateral 

 columns and also occupies the peripheral part of the anterior columns. It comprises the 

 principal long descending tracts from various parts of the brain (terminal portions of ef- 

 ferent suprasegmental paths). (3) The third zone borders the gray matter and includes 

 the ground or fundamental bundles of the cord (chiefly spinal intersegmental fibres). 



In the figure, for nu, Darlischewitschi read neucleus of medial longitudinal fasciculus. 



