NERVOUS SYSTEM 



45 



are the enumerations for the myelinated fibers in the ventral 

 roots (Hatai, '03 b). 



It was found that the number of myelinated fibers in the ven- 

 tral roots diminishes from sections near the spinal cord to those 

 near the spinal ganglion. The amount of the diminution de- 

 creases with the age (body weight) of the rat. The increase in 

 the number of cells in the spinal ganglia from the small to the 

 large rats is certainly due in part to the fact that in the small 

 animals some of the smallest ganglion cells escape enumeration. 



The increase in the number of myelinated fibers in the spinal 

 roots with advancing age is due mainly to progressive myelina- 

 tion. Both roots at maturity still contain functional fibers with- 

 out myelin sheaths (Ranson, '06). 



TABLE 27 



Number of ganglion cells and number and size of myelinated root fibers in spinal 

 nerves from three levels of the spinal cord at five ages (body weights) 

 Results from Tables II, VI and VIII combined. Hatai ('02) 

 Also data on ventral root fibers from Hatai ('03 b) 



