Thompson, Cells in the Cerebral Cortex of Man. 117 



Hammarberg's method in handling the material after it w?s 

 prepared, was as follows : He made serial sections of all the 

 gyri of the cortex, some of them \0 n, and some of them 20 ;/, 

 thick. He then recorded the thickness of each layer in each 

 region of the cortex, in tenths of a millimeter, and also the two 

 diameters of the average cell in each layer, in micra. As his 

 unit of measurement for the frequency of the cells, he used the 

 number of cells contained in a cube measuring one tenth of a 

 millimeter on a side. If his sections were cut 20 ^ thick, each 

 little cube (o. I mm.)^ of which his total mass was theoretically 

 built up, would be divided into five slices by sectioning. In 

 order to count the number of cells in his five slices and so ob- 

 tain the number of cells in any particular unit cube, he counted 

 the number of cells which appeared in corresponding areas, 

 0.1 mm., square, in five successive sections. To avoid counting 

 more than once the large cells which appear through several 

 sections, he counted in any section only those cells in which 

 nuclei were present. The frequency of the cells for each layer 

 is recorded, then, in terms of the number of cells per unit cube 

 (o. I mm.)^. The numbers which appear in the text are not 

 based on single determinations taken haphazard from some por- 

 tion of the region, but are averages made up from several enum- 

 erations and measurements, and are representative of the entire 

 region for which they stand. 



C. The division of the cortex into regions adopted in this 

 paper is determined by the number of different sets of meas- 

 urements Hammarberg found it necessary to record in order to 

 represent the entire cortex. That is to say, any portion of the 

 cortex which is uniform enough in structure to be fairly repre- 

 sented by a single average set of measurements, including the 

 thickness of the layers, the size of cells, and the frequency of 

 cells, is called a single region. In making his examinations, 

 Hammarberg followed the division of the cortex into gyri as 

 given by Ecker\ The orbital surface of the frontal lobe, for 



* In writing this paper, Hammarberg's nomenclature for the gyri has been 

 used. 



