120 Journal of Comparative Neurology. 



sixth and Meynert's fifth, he calls the layer of spindle cells. His 

 six layers are then as follows: 



Layer I. Nerve fibers with a few small scattered cells. 



Layer IL Small pyramidal cells. 



Layer IIL Large pyramidal cells. 



Layer IV. Small, irregularly shaped cells. 



Layer V. Ganglion cells. 



Layer VL Spindle-shaped cells. 



E. If Hammarberg's records were complete in every detail, 

 they should, then, contain the following data upon each of the 

 sixteen regions into which the cortex is divided: first, the thick- 

 ness of each of the six layers ; second, two diameters in micra 

 for the average cells of each layer ; and third, the number of 

 cells in each layer contained in a cube measuring o. i mm., on 

 a side. But not all of these data are given for every region. 

 Sometimes the thickness of a layer is omitted ; sometimes the 

 size of the cells is merely indicated by saying that they are 

 larger or smaller than those of a corresponding layer in some 

 adjacent region ; and in some cases the frequency is not given. 

 All these omissions have been supplied as accurately as possi- 

 ble. Frequently the plates of sections accompanying the mono- 

 graph furnished the required facts ; sometimes the numbers for 

 closely similar regions were adopted ; and in some cases his 

 expressions greater or less, and more or less frequent, were 

 given a definite numerical value. Wherever such additions to 

 the original data were made, they are indicated in Tables VII 

 and VIII by enclosing the estimated number in parentheses. 



II. Calculation of the Total Number of Functional 

 Nerve Cells in the Cerebral Cortex of Man. 



By these preliminary explanations, the ground has now 

 been cleared for the calculations. The next point to be con- 

 sidered is the method adopted for making use of Hammarberg's 

 data in determining the total number of cells in the cortex. 



A. Hammarberg's record of the frequency of the nerve 

 cells was, as has been stated, kept in terms of the number of 



