344 Journal of Comparative Neurology and Psychology. 



The dorsal funiculus is subdivided into the fasciculus cu- 

 neatus and fasciculus gracilis, and the latter is again subdivided 

 into two fasciculi by a clearly marked septum. A similar sub- 

 division was found in the medulla spinalis of the white rat.^ x\s 

 in the white rat so in the guined pig the fasciculus is very late 

 in medullating. In tlie guinea pig this fasciculus presents an 

 area considerably lighter than tlie substance immediately sur- 

 rounding it, the medullated fibers in it being both small and 

 comparatively few. In the fasciculus cuneatus a tongue of 

 heavily medullated fibers passes from the level of the tip of the 

 fasciculus gracilis down the septum posterior medianus to the 

 commissura posterior (alba). On either side of the ventral por- 

 tion of this tongue is a light oval area bounded laterally by the 

 cervix columnae dorsalis. and extending from the commissura 

 posterior to the substantia gelatinosa (fig. i, plate V). This 

 is the locality of the pyramidal tract. 



A third area slightly lighter than the rest of the white sub- 

 stance is to be found in the lateral funiculus just ventro-lateral 

 to the lateral apex of the substantia gelatinosa. Possibly there 

 are in this locality some pyramidal fibers also. 



In the cervical cord of the guinea pig at birth there are, 

 then, three light areas: (i) the fasciculus gracilis ; (2) the pyra- 

 midal area in the fasciculus cuneatus along the boundary of the 

 cervix; and (3) an area containing a few fibers, around the lat- 

 eral border of the apex of the dorsal column. 



In the cervical cord the portion of the white substance im- 

 mediately surrounding the gra\- substance is much darker than 

 the white substance at the periphery of the cord, as is indicated 

 in the figures. This appearance is due to two factors, (a) the 

 great number of fibers passing between the ventral columns and 

 the white substance, these fibers seeming to radiate from the 

 ventral columns like spokes from a hub ; (b) numerous fibers 

 following the border of the gray substance; e. g., the fibers of 

 the anterior connnissure do not all pass directly to the cells of 

 the gray substance, but have to wind in and out about the edge 



1 Animal Education, p. 94. 



[52] 



