126 Journal of Comparative Neurology. 



in each case how much each layer is reduced in thickness in the 

 sunken portions, and usually mentions whether or not the cells 

 are at the same time increased in frequency. It is possible 

 therefore to obtain for the sunken portions of each region a 

 percentage of reduction of the number of cells which is approx- 

 imately correct. These percentages are given in Table III. 



F. The next step to be taken after determining the 

 unit columns for each region is to find out now many of the 

 unit columns there are in each region. To do this, it was nec- 

 essary to know, first, the total area of the cortex, and, second, 

 the proportion of the entire area to be assigned to each region. 

 The figure adopted for the total cortical area was that obtained 

 by Donaldson,^ from a comparison of all the available material. 

 It is 2,352 sq. cm. This total was then divided among the 

 various lobes as follows •} 



Frontal Lobe 41 %• 



Parietal Lobe 21%. 



Temporal Lobe 27%. 



Occipital Lobe 10%. 



Island 1 % . 



Total, 100% = 2, 3 52 sq. cm. 



G. At this point it is possible to adopt either of two 

 methods of procedure. The absolute area of each region 

 might be found by portioning out each lobe among its various 

 regions calculated directly ; or an average unit column for each 

 lobe can be obtained — an average in which each region is rep- 

 resented according to its size, — and the total number of cells 

 for each lobe computed by means of its unit column. The lat- 



' I.e. p. 202, ff. 



'The division of the cortex into lobes is taken from Hermann Wagner (I.e. 

 p. 36), with modifieation. Wagner gives the temporal lobe as 20% and the 

 oceipital as iT^c- Formerly the ventral aspeet of the posterior portion of the 

 brain adjoining the occipital lobe was considered part of the occipital lobe, 

 while now it is included in the temporal lobe. 7% of Wagner's occipital lobe 

 was therefore transferred to the temporal lobe, making the temporal lobe 27^ 

 ftnd the occipital lobe 10^. 



