ON THE FUNCTIONS OF THE CEREBRUM 287 



(2) The layer of small, medium, and large pyramidal cells 

 (outer cell lamina). (Average prefrontal depth, -83116 mm.) 



(3) The layer of granules (middle cell lamina). (Average pre- 

 frontal depth, -22883 mm.) 



(4) The inner layer of nerve fibres or " inner line of Baillarger," 

 containing large and frequently solitary cells (inner fibre lamina). 

 (Average prefrontal depth, -23032 mm.) 



(5) The layer of polymorphic cells (inner cett lamina). 

 (Average prefrontal depth, -30979 mm.) 



This five-layered type, though subject in different regions to 

 structural modifications which probably possess in all cases a 

 functional significance, is common to the whole cortex cerebri, 

 with the exception of the hippocampus and the pyriform lobe, 

 which parts belong to the archipallic, in contradistinction to neo- 

 pallic cortex (Elliott Smith). In the psychomotor area, for example, 

 the Betz cells lie in the inner fibre lamina, and the middle cell 

 lamina is reduced in depth almost to vanishing point. Again, in 

 the visuo-sensory area, the middle cell lamina is hypertrophied 

 and duplicated by the interposition of a special fibre layer, the 

 " line of Gennari." 



The mode of development of this five-layered type of cortex will 

 now be briefly described. 



In the foetus of four months lamination has not begun, and the 

 cortex consists solely of a superficial indifferent layer and of a 

 deeper layer of undifferentiated neuroblasts. The average depth 

 of the former is 0-154665 mm., and of the latter 0-67758 mm., a 

 total of 0-832245 mm., which is less than half the normal adult 

 general average depth of 1*90064 mm. 



For simplicity of exposition, the process of development will 

 be first described in the case of each separate layer, and the results 

 will then be summarised. 



PRIMARY CELL LAMINA OF THE CORTEX 



Lamina 2, Pyramidal Layer, Outer Cell Layer. The pyramidal 

 layer is the last cell layer of the cortex to develop during the 

 process of lamination. In a foetus of six months this layer is 

 separable from the subjacent middle cell layer owing to the less 

 differentiated condition of its cell elements, and it is at this period 

 only one-fourth of the depth to which it attains in the adult. At 



