ii6 Journal of Comparative Neurology. 



tainly less than lo %, while Hammarberg's figures yield 1.37% 

 as the correct proportion. 



B. The great discrepancy in these results makes it a mat- 

 ter of interest to look with some care both into Hammarberg's 

 methods of preparing material, making observations and tabu- 

 lating results, and also into the way in which his results have 

 been used in the present calculation. His study of the size 

 and frequency of nerve cells in the normal cortex included a 

 careful microscopic examination of twelve brains of both sexes, 

 ranging in age from a five month's foetus to middle aged adults. 

 The numbers given in his text are only those for the six adult 

 male brains he examined, and the calculations reported in this 

 paper apply, therefore, to adult male brains alone. His method 

 of preparation was to harden the brains in 95 % alcohol, stain 

 with methylene blue, clear in xylol and imbed in paraffin. The 

 shrinkage due to this process, would of course have no bearing 

 upon the number of cells in the cortex, but would, on the other 

 hand, affect the size of the cells and the thickness of the cor- 

 tex as a whole. ^ If however the separate cells shrink nearly 

 proportionately to the entire mass of the cortex, as they prob- 

 ably do, the calculation of the percentage of the entire volume 

 composed of nerve cells would also be practically unaffected. 

 Hammarberg reports that careful measurements made upon the 

 thickness of the cortex before and after the processes of hard- 

 ening, imbedding and staining, show that the shrinkage involved 

 is less than . 2 mm. The difference is so slight, he says, that it 

 cannot be detected by macroscopic measurements. Since the 

 errors due to methods of preparation are very small in absolute 

 amount, and are of such a nature that they would not modify 

 the results essentially, they have been neglected in making the 

 calculations. 



'The thickness of the cortex as given by Hammarberg is less, on an average, 

 than that determined by Donaldson, I. c, p. 206. Donaldson's average thick, 

 ness of the cortex is 3.34 mm. for the summits of the gyri, Hammarberg's 2.48 

 mm. Hammarberg's data show a very wide range of variation, amounting to 



100^, 



