Ii6 Journal of Comparative Neurology. 



The following table will show the relative increase of the 

 dorsal root fibers in the three roots at different ages. 



TABLE V. 

 Showing the Relative Increase of the Fibers at Different Ages. 



Body Weight 



Grams. VI CervicaL IV Thoracic. II Lumbar. 



As will be seen from the Table V, the relative increase of 

 the fibers in each region is quite gradual. From the same table 

 it is clear that the fibers in the cervical increase in nearly the 

 same ratio as those of the lumbar nerve, while the fibers of the 

 thoracic, increase more rapidly than the others. This rapid in- 

 crease of the number in the thoracic nerve will be discussed 

 later on. 



B. Ratios Between the Completely Formed and Immature 

 Fibers. 



The cross section of the dorsal root at birth, stained with 

 osmic acid presents two kinds of the fibers : one shows clear 

 outline and stains an intense black, while the other shows clear 

 outline and stains with the osmic less intensely, or remains 

 nearly unstained. A careful observation with high magnification, 

 however, reveals to us that the former is surrounded by a sheath 

 which contains an abundance of myelin substance, while the 

 latter possesses a sheath with less myelin substance, which sub- 

 stance sometimes appears as black dots at the side of the sheath. 

 From these facts (Wlassak '98) the present writer identified the 

 former as the completely formed fibers while the latter he con- 

 sidered as immature. The diameters, however, do not serve 

 to distinguish an immature from the completely formed fibers, 

 for the diameter of the former exceeds, in some cases, that of 

 the latter. Therefore, these distinctions can be determined only 

 by examining the fiber with high magnification, and thus de- 

 termining whether the sheath contains the full amount of the 

 myelin substance. Using these criteria as the basis of destine- 



