130 Journal of Comparative Neurology. 



lobe is given io% of the cortex. If the entire lobe were rec- 

 oned as having a unit column of 300, instead of 700, the mis- 

 take would be only 5 ^ of the total result. The real mistake 

 could not be as much as 2%, an error which would be insignif- 

 icant in view of the magnitude of the total number. 



An inspection of the cortex shows that the mistakes in- 

 volved in assigning values to the regions within a lobe cannot 

 be very large. The importance of such mistakes would depend 

 upon the amount of the discrepancy in the unit columns for the 

 regions wrongly estimated. In the frontal lobe the number of 

 cells in the unit column varies from 310 to 592, in the parietal 

 lobe 460 to 615, in the occipital lobe from 537 to 995 and in 

 the temporal lobe does not vary at all (vide Table II). Con- 

 sidering the small percentage of the cortex which each region 

 represents, and the comparatively narrow limit of the possible 

 variation in the size of the unit columns for any lobe, any single 

 case of making a region somewhat too large or too small could 

 have but little effect on the total result. If the calculation does 

 contain a number of small errors due to this source, there is 

 every probability that they tend to balance one another in the 

 final outcome, rather than to sum themselves in either direction. 

 Neither of the chief sources of error, therefore, could cause a 

 mistake which would invalidate or even change appreciably the 

 final outcome. 



III. Calcul.'^tion of the Percentage of the Total Vol- 

 ume OF THE Cerebral Cortex of Man Composed 

 OF Nerve Cell Bodies. 



The next topic to be considered is the method employed for 

 calculating the percentage of the total volume of the cortex made 

 up of functional nerve cells. Again the unit column, whose cross 

 section is o. i mm. square on each side, and whose height is the 

 thickness of the cortex, serves as a basis for calculation. In 

 general, the method was to find the sum of the volumes of all 

 the cells in the unit column for each region, and then to calcu- 

 late what proportion of the total volume of the column the cells 



