ON THE FUNCTIONS OF THE CEREBRUM 289 



Summary. The inner cell layer, therefore, appears before the 

 others, in consequence of a cleavage of the undifferentiated neuro- 

 blasts into an upper and a lower portion, and it is almost at once 

 of a depth which is about 75 per cent, of the adult normal. It 

 remains at almost a stationary depth until after birth. Its depth 

 in a child of six weeks is 82 per cent, of the adult normal. 



The middle cell layer is differentiated next in order, and is 

 almost at once about one -half of the normal adult depth. It 

 gradually increases in thickness, and at birth it has attained to a 

 depth which is 75 per cent, of the adult normal. 



The outer cell layer is the last layer of the cortex to be 

 differentiated, and at this time it is only one-fourth of the normal 

 adult depth. It gradually increases in thickness, and in an infant 

 of six weeks it has attained to a depth which is about 60 per cent, 

 of the adult normal. 



In the adult human cerebrum, as will at once be seen from 

 the measurements given on page 287, the outer cell lamina or 

 pyramidal layer, in spite of its late evolution, becomes the promi- 

 nent layer of the cortex, and in depth is greater than the depth 

 of the combined layers 3, 4, and 5, which lie below it. The 

 functional significance of the facts mentioned in this description 

 will be referred to later. 



The above account of the mode of evolution of the cortex cerebri 

 has recently been confirmed by G. A. Watson from the phylogenetic 

 aspect. This writer, whilst hitherto paying especial attention to the 

 insectivora, has carefully studied the cortex cerebri of many orders 

 of mammals. He adopts the above classification of the cortical layers, 

 but, for reasons which he details, employs a slightly different termi- 

 nology, grouping the first and second laniinje together under the term 

 *' supra-granular," and the fourth and fifth together under the term 

 " infra-granular." He remarks : 



" Regarded from the developmental aspect ontogenetic and phylo- 

 genetic ^the facts adduced support the thesis that the mammalian 

 cerebral cortex (neopallium) is built up primarily on an infra-granular 

 basis, i.e. the infra-granular portion is the earliest to appear in the 

 process of development, very quickly reaches maturity, and in the 

 adult, especially if average size of component nerve cell is taken into 

 consideration, presents remarkably little difference in absolute depth 

 in one of the lowest mammals and in the highest. 



" The granular layer may be said to be the next addition to the 



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