830 



THE NERVE SYSTEM 



lateral from the dorsal district. An additional fissure, observed most distinctly 

 in the cervical and upper thoracic portions, termed the dorsoparamedian fissure, 

 demarcates the two principal divisions of the dorsal columns, the gracile and the 



DORSAL ROOT 



SOMATIC SENSOR 



VISCERAL SENSOR 

 VISCERAL MOTOR 



SOMATIC MOTOR 



FIG. 602. A diagram of the component _ elements in the spinal cord and the nerve roots in a trunk segment 

 to illustrate the four functional divisions of the nerve system. (After Johnston.) 



cuneate columns. The dorsoparamedian fissure is an exceedingly shallow groove 

 and is best denoted in sections of the cord by its continuance as a connective- 

 tissue (glia) septum into the substance of the dorsal column. An analogous 



8PONGIOBLAST 



GCRMINAL CELL 



GERMINAL CELL 

 DIVIDING 



SPINAL GANGLION 

 CELLS 



PERIPHERAL 

 PROCESSES 



NEUROBLAST 



AXONES OF VENTRAL 

 NERVE ROOTS 



FIG. 603. Trans-section through neural tube, early and later stages, diagrammatic. Earliest stages shown 

 on left side. On the right, the maturing neuroblasts are seen sending their axonic processes toward the 

 periphery or to other regions of the central axis, and the central processes of the spinal ganglionic cells are 

 seen to invade the dorsolateral region. 



ventroparamedian fissure is sometimes observable close to the ventral fissure, 

 demarcating the ventral (or direct) pyramidal fasciculus. 



Columns of the Cord (funiculi medullae spinalis). Each half of the spinal 

 cord is thus divided, with respect to its white substance, into three chief columns 



