Allen, Association in the Guinea Pig. 349. 



It will be seen that the total increase in number of fibers, 

 all the funiculi being considered together, is 16. 81 %. The in- 

 crease in number of fibers, then, can only partially account for 

 the enlargement of the cord during this period. 



In the adult the fasciculus gracilis is notably darker than 

 the adjacent areas, whereas it was previously somewhat lighter. 

 Similarly there is a corresponding darkening in the pyramidal 

 area. The areas have been darkening by a great increase in 

 number of medullated fibers. A count of the fibers in a given 

 area of the fasciculus gracilis of both the adult and newborn 

 shows that there has been an increase of 35-5% of medullated 

 fibers in the older animal. In the newborn many fibers were 

 too small to contribute to the darkening of the light areas. 



Casual inspection shows that the fibers in the ventral funi- 

 culi are much larger than those of the dorsal fasciculi ; this 

 leads us to the conclusion that the number of fibers per unit of 

 area is greater in the dorsal funiculus than in the ventral funi- 

 culus, and a count proves this to be the case. That is, those 

 regions which show the smallest number of medullated fibers 

 in early life ultimately possess a greater number of fibers than 

 tracts which at that time are practically complete. 



In the lateral funiculus there are somewhat fewer cross sec- 

 tions of fibers at the periphery than in the ground bundle near 

 the ventral column, the ratio being 1:1.22. In the ventral 

 funiculus it is found, likewise, that at the periphery there are 

 fewer fibers than near the ventral column, the ratio being here 

 1:1.69, In both the lateral and the ventral funiculi the fibers 

 at the periphery are larger than those near the gray substance. 



Thoracic Level- 

 Little if any change occurs in the thoracic cord before the 

 fourth day, as was found to be true also in the case of the cer- 

 vical. The section at the sixth day shows much greater uni- 

 formity of medullation, and at eleven days the pyramidal tract 

 can not be distinguished from the neighboring white substance. 

 At eleven days also the fasciculus gracilis is becoming darker 

 than the surrounding regions. In the fasciculus gracilis at 



[57] 



