38 JOURNAL OF CoMPARATIVE NEUROLOGY. 
There is a general correspondence between the area of a 
cross section of the gray substance at the level of any segment 
and the area of the cross sections of all the spinal nerves be- 
longing to the segment. When, however, the volume of the 
gray substance, instead of the area, is used for the comparison, 
a disproportionately large amount of gray substance is found in 
the case of the thoracic and upper lumbar segments. This is 
interpreted as indicating a passive enlargement of the gray 
substance in these segments of the cords which have been most 
elongated. 
When the cords of immature individuals are compared 
with those from adults, several important relations are brought 
to light. In the first place, the sum of the total areas of the 
cross-sections of the cords, from one to five years, is practically 
the same (see Table VII), although during the period, a con- 
siderable growth in length has occurred. During this time, 
therefore, growth in the long axis has taken place without any 
corresponding growth at right angles to the long axis. 
The form of the cord from one to five years is nearly like 
that at maturity, the difference being that in the mature cord 
the relative enlargement of the areas of the cross-sections has 
become greater in the thoracic region, but less in the sacral and 
coccygeal (see Table IX). At maturity, the relative enlarge- 
ment of the two intumescentiae is practically the same as at the 
fifth year. From the fifth year to maturity, both the 
length and the weight of the entire cord as _ well 
as the area of the cross-sections at the level of the 
several segments are increased. The sum of the areas of 
the white substance at maturity is 98% greater than at five 
years, and that of the gray substance 23% greater (see Table 
VIII). This absolute increase must represent either enlarge- 
ment of elements already completely developed, or the develop- 
ment of elements still immature at the earlier age, or some 
combination of both of these processes. Yet the failure of the 
intumescentiae to increase in their relative area in the mature 
cord (see Table IX), or in their proportional length (RAVENEL, 
p. 350), would seem to indicate that during this period there 
