238 ROBERT S. ELLIS 



the line on the sUde. The number of cells in the line was then 

 counted under the microscope, only those cells showmg the 

 nucleolus being counted, with the exception that when the cells 

 were so disintegrated that the nucleolus would not appear in 

 the preparation, the cells were counted if they appeared to 

 belong properly to the section. Dividing this total by the 

 length of the line in millimeters gives the number of cells in a 

 line 1 mm. long on the slide, which, as has been stated, is for a 

 section 25 m thick. To correct this for shrinkage during dehy- 

 dration and embedding, the value thus obtained for 1 mm. on 

 the slide has been multiplied by the square of the percentage 

 obtained by dividing the sum of the length and breadth of the 

 section on the slide by the sum of the length and breadth of the 

 block as first cut from the cerebellum. This gives the number 

 of cells found m a line of Purkinje cells 1 mm. long and 25 fx 

 thick, this being really a surface 25 /x by 1 mm. in the cerebellum 

 as weighed. 



For comparable quantitative results this correction is neces- 

 sary because it is evident that when the cerebellum shrinks, 

 the line of Purkinje cells shrinks also, and it is necessary to correct 

 both for the change in the length of the line and for the change 

 in the thickness of the section on the slide — it being evident 

 that the 25^ thickness of the section represents a greater thick- 

 ness in the cerebellum as weighed. The sum of the length and 

 breadth of the section have been used rather than one dimension 

 alone, because careful measurements show that the section is 

 compressed in breadth to some extent when cut on the micro- 

 tome. The sum of the two measurements consequently gives a 

 more accurate indication of shrinkage. 



The number of cells found in a line 1 mm. long and in a section 

 25m thick would afford a satisfactory unit for comparing different 

 areas of the same brain; it would not do, however, as a unit for com- 

 paring different brains. If the molecular layer of the cerebellum 

 should be removed, leaving the cell bodies of the Purkinje cells 

 intact, it is evident that these would extend in a much convoluted 

 sheet over the surface of the remaining part of the cerebellum. 

 The area occupied by the Purkinje cells in a large brain is thus 

 larger than that occupied by them in a small brain. The sur- 



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