PHYSIOLOGY OF NERVE TISSUE. 



6 9 



fied descendants of independent, oval, pear-shaped cells the neuroblasts 

 which migrate from the medullary tube. As they approach the surface of the 

 cord their axons are directed toward the ventral surface, which eventually 

 they pierce. Emerging from the cord, the axons continue to grow, and be- 

 come invested with the myelin sheath and neurilemma, thus constituting 

 the ventral roots. 



The afferent nerve-fibers, which constitute some of the cranial nerves and 

 ; 11 the dorsal roots of the spinal nerves, develop outside of the central nervous 

 system and only subsequently become connected with it. (See Fig. 9.) 



Posterior 

 Jloot 



FIG. 9. DIAGRAM SHOWING THE MODE OF ORIGIN OF THE VENTRAL AND DORSAL 



ROOTS. 



At the time of the closure of the medullary tube a band or ridge of epithelial 

 tissue develops near the dorsal surface, which, becoming segmented, moves 

 outward and forms the rudimentary spinal ganglia. The cells in this 

 situation develop two axons, one from each end of the cell, which pass in 

 opposite directions, one toward the spinal cord, the other toward the per- 

 iphery. In the adult condition the two axons shift their position, unite, and 

 form a T-shaped process, after which a division into two branches again 

 takes place. In the ganglia of all the sensoricranial and sensorispinal 

 nerves the cells have this histologic peculiarity. 



