Thompson, Cells in the Cerebral Cortex of Man. 131 



occupy. Since each region is made up of unit columns which 

 are regarded as identical in structure, the percentage obtained 

 for the unit column is also the percentage for the entire region^ 

 (Table V). 



TABLE V. 



The Average Percentage of Volume Composed of Functional Nerve 

 Cell Bodies in each Region. 



* The explanation for the fact that the percentage for the Island (region 

 16) is so small, while its number of cells per unit column is large, is that the 

 cells, although numerous, are very small (see Table VIII). 



After having found the average percentage for each region, 

 the average percentage for each lobe was estimated by the 

 method of averaging explained above (II, G). The average 

 percentage of the volume of the entire cortex filled by nerve 

 cell bodies was then determined by averaging the percentages 

 for the separate lobes by the same method (Table VI). 



* In calculating the percentage of volume occupied by cells, no notice is 

 taken of the reduction of thickness of the cortex in the sulci, because, although 

 the absolute thickness of the cortex is lessened, the number of cells is also les- 

 sened. These two factors may not exactly balance one another. In some cases 

 the cells are crowdedcloser together in the sulci. But there are no data for deter- 

 mining to what extent this is the case. If the greater frequency of the cells 

 were a constant and marked feature of the reduction of thickness in the sulci, 

 it would no doubt have been noted. As it is, Hammarberg occasionally 

 mentions the fact that the cells are somewhat more frequent in the reduced 

 layers. Whatever the error from this source may be, it affects only the last 



three layers, and is in all probability too slight to change the result more than 



a small fraction of a per cent. 



