DEVELOPMENT OF THE CEREBRAL CORTEX 117 



by no means impossible. In many instances the count varies so 

 considerably in contiguous areas of the same convolution, due 

 sometimes to the presence of blood vessels and the doubtful 

 nature of many of the cells that it is difficult to arrive at satisfac- 

 tory conclusions. As a rule, however, it appears that the deeper 

 cortex has the greater number of cells. We would naturally 

 suppose that a cortex increased in depth by increased functional 

 activity would be due either to increase in the volume of the cells 

 or separation of the cells by reason of an increase in the outgrow- 

 ing or ingrowing processes, or both together. From careful and 

 prolonged study of the motor area in the human cortex I am 

 convinced that there are great variations in different individuals 

 in the development of the largest elements — the so-called Betz 

 cells. 



Much time and study has been expended in the effort to find an 

 anatomical basis for intellectual development in brain weight and 

 in the comparative complexity of the convolutions. Such efforts 

 have so far been without result. Many milhons of cells may 

 vary greatly in development and the weight of the brain be inap- 

 preciably affected.^ Estimates based on a careful count in differ- 

 ent cortical areas of the adult human brain place the number of 

 cells in a cubic millimeter of cortical substance at about 100,000. 

 The total area of cortex in a brain weighing 1360 grams is esti- 

 mated by Donaldson at 2352 sq. cm. On that basis a cortex of 

 2.5 mm. average depth would contain nearly 6000 miUion cells 

 while a cortex averaging 3 mm. in depth would contain more than 

 7000 million cells. 



8 A note on the significance of the small volume of the nerve cell bodies in the 

 cerebral cortex of man. H. H. Donaldson, Jour. Comp. Neur., vol. 9, 1899. 



