Thompson, Cells in the Cerebral Cottex of Man. 133 



beyond it at others. Now the assumption which is made when 

 the formula for the cone is adopted is, that if, when the cone is 

 circumscribed about the cell, the outlying portions of the cell 

 were packed into the spaces between the cell wall and the sur- 

 face of the cone, the cone would be exactly filled, A study 

 of a large number of cell outlines convinces us that this assump- 

 tion is approximately correct. The formula for the cone may 

 give a little too large a volume for some cells, and a little too 

 small a volume for others, but it is certainly nearest to correct 

 of any of the geometrical formulae, and it is probable that its 

 slight inaccuracies balance one another. 



The cells of the sixth layer, a very thick layer in most 

 parts of the cortex, are not pyramidal, but spindle-shaped. 

 Conventionalized, they have the form of two cones of the same 

 size set base to base. The volume of two cones of the same 

 size is the same as the volume of a single cone with a base 

 equal to and an altitude twice that of one of the small cones. 

 Since the measurements taken for the spindle cells are the en- 

 tire length of the cell, which is equal to twice the altitude of 

 one of the component cones, and the breadth of the cell at its 

 widest point, which is the diameter of the base of the compo- 

 nent cones, the formula for the cone holds for these cells also. 



C. Applying the formula for the cone, then, to the meas- 

 urements for the average cell in each layer of each region gives 

 the volume of an average cell for each layer. 



The number of cells in each layer of each unit column has 

 already been found in calculating the total number of cells in 

 the cortex (Tables I and VII). To find the volume of all 

 the cells in any layer of a given unit column, we multiply the 

 volume of the single cell by the number of the cells in the layer. 

 The sum of the volumes for each layer gives us the total vol- 

 ume of all the cell bodies in any unit column, expressed in cubic 

 micra. Since we know the dimensions of each unit column in 

 millimeters, it is a simple matter to calculate its volume in the 

 cubic micra. The ratio of the total volume of cell bodies in a 

 unit column to the entire volume of that column is the percent- 

 age sought for (Tables V and VIII). The way in which the per- 



