VASCULARITY OF THE CEREBRAL CORTEX 209 
Wistar Institute). It may be noticed that, in all cases where 
the Wistar average is less than the Toronto one, it is the deepest 
layers which are involved, the lamina multiformis being poorer 
in the Wistar group in four of the five areas, the lamina gangli- 
onaris in three, the laminae granularis interna and pyramidalis 
in only one, and the lamina zonalis in none. It should be 
pointed out in this connection that each local group contains 
two specimens of each sex. 
Evidently the difference between the regio insularis and the 
other four regions is either of some other nature or of a much 
greater degree than their differences from each other. It is 
this area which is so much more poorly vascularized than the 
others; it is this area in which the female average exceeds that 
of the males in three laminae; and it is this area in which the 
average for the Wistar animals is poorer in four laminae than 
that for the Toronto animals. 
It should be remarked here that it is possible that some of the 
differences in the measurements of vascularity are possibly 
attributable to differences in brain weight. The situation in 
this respect is summed up by Donaldson? as follows: 
If the vascular supply grows in proportion to the brain—within the 
limits of variation of brain size in full grown rats—then a sample of a 
fixed volume of tissue from one brain will be comparable with that from 
another. 
If the increase in the volume of the entire brain is more rapid than 
that of the vascular supply, then the supply in a fixed volume of tissue 
will appear less in the larger brain—and vice-versa. 
Which of these conditions obtains, the writer is not at present 
in a position to state definitely, but he is inclined to believe that 
the second supposition is the one which represents the facts. 
This would not account, however, for the differences in the rela- 
tions seen in the different parts of the central nervous system. 
Unfortunately, the weights of the brains employed in the present 
study were not recorded, but the body weight and body length 
of the four rats from The Wistar Institute are known. From 
these data the probable brain weights may be determined from 
3 Personal communication. 
