292 RESEARCHES ON CORTICAL LOCALISATION AND 



specialised from the five-layered type already referred to, and 

 consists of the following layers : 



1. The superficial layer of nerve fibres (outer fibre lamina). 



2. The layer of pyramidal cells (outer cell lamina). 

 3a. The outer layer of granules. } 



36. The middle layer of nerve fibres or " line of I (middle cell 

 Gennari " containing solitary cells of Meynert. j lamina). 



3c. The inner layer of granules. 



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

 containing solitary cells of Meynert (inner fibre lamina). 



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



The specialisation of the visuo-sensory cortex consists, there- 

 fore, in essentials, in a duplication of the third primary lamina of 

 the cortex, and in the interposition, between the double layer, of 

 a layer composed of nerve fibres. Of this triple layer, layer 3a is 

 an additional feature ; layer 36 is an exaggeration of a thin fibre 

 band, the " outer line of Baillarger," which in the adult cortex 

 lies between the second and third primary laminae, but is only 

 visible in sections prepared to show nerve fibres ; and layer 3c is 

 the original third primary lamina increased in depth. 



In congenital or long-standing blindness the depth of. layer 36 

 is decreased by nearly 50 per cent., and that of layer 3a is 

 decreased by more than 10 per cent., owing to atrophy of the 

 optic radiations. The other layers of the cortex are unchanged 

 in depth by the existence of blindness. 



These facts prove that the cortical region under consideration 

 is the projection centre for visual impressions or the visuo- 

 sensory area. 



The cortex in this region rapidly attains to mature develop- 

 ment. The second, pyramidal, or outer cell lamina thus develops 

 much earlier than is the case in the visuo-psychic and prefrontal 

 regions. For example, in infants of' one and three months re- 

 spectively, its depth is already 84 per cent, of the adult normal. 

 A further important fact is that in the normal adult this cell layer 

 of the visuo-sensory area is only about five-ninths of its depth in 

 the visuo-psychic and prefrontal regions of the cerebral cortex. 



The Visuo-psychic Region. At the periphery of the visuo-sensory 

 area, where this passes in each direction into the neighbouring 

 cortex, termed by the writer " visuo-psychic " (which word has 

 since been adopted by Campbell and by Mott), an abrupt change 



