768 



THE NERVOUS SYSTEM 



The ascending branches of the dorsal root axones also give off collaterals to the 

 grey substance of the cord, thus extending the area of distribution of a given posterior 

 nerve-root to levels of the cord above the region at which the root enters. 



The greater number of the terminations of dorsal root axones within the spinal 

 cord are concerned with neurones other than those contributing ventral root-fibres. 

 The greater mass of the neurones concerned are those contributing the fasciculi proprii 

 or ground bundles of the spinal cord, or the second variety of axones composing the 

 posterior funiculus. The latter fasciculi arise from the smaller cells of the grey column. 



FIG. 570. SHOWING DISPOSITION OF THE DORSAL ROOT-FIBRES UPON ENTERING SPINAL CORD. 



(From Edinger. after Cajal.) 



A shows dorsal root axones DR. entering the spinal cord, bifurcating B. and giving off collat- 

 erals C to the neurones of the cord. B shows the terminal twigs of these axones or of their 

 collaterals displayed upon cell-bodies of the grey substance of the cord. 



They enter the surrounding white substance, bifurcate into ascending and descend- 

 ing branches, which in their turn give off numerous collaterals to the cells of the 

 grey substance of the levels through which they pass. The cell-bodies giving origin 

 to such axones are so numerous that the entire column of grey substance is sur- 

 rounded by a continuous felt-work of axones of this variety. 



The dorsal fasciculus proprius (anterior root zone of posterior column) arises 

 chiefly from cells situated in the dorsal horn (stratum zonale). Coincident with the 

 ingrowth and arrangement of the fasciculi gracilis and cuneatus many fibres of the 

 dorsal fasciculus proprius go to form both the oval bundle and the comma-shaped 

 fasciculus. Thus these two bundles are mixed, being fasciculi proprii which contain 

 caudad branches of dorsal root axones. The 'median triangle' is formed by the con- 

 tinuation of the dorsal fasciculi proprii with the oval or septo-marginal fasciculus. 



