GENERAL PHYSIOLOGY OF THE CEREBRUM. 



187 



granular layer, and the infragranular layer (4 and 5), comprising 

 the pyramidal and fusiform cells internal to the granular layer. 

 Comparison of the cerebral cortex in the brains of the different 

 vertebrates indicates that the supragranular cells have appeared 

 relatively late in the phylogeny of the vertebrates, and have 

 reached their greatest development in the human brain. The 

 suggestion occurs, therefore, that these cells have a different func- 

 tional significance from those in the infragranular layer. It has 

 been supposed that the supragranular cells mediate the so-called 



Fig. 84. A-D, Showing the phylogenetic development of mature nerve cells in a 

 series of vertebrates: a-e, the ontogenetic development of growing cells in a typical mam- 

 mal (in both cases only pyramidal cells from the cerebrum are shown); A, frog; B, lizard; 

 C, rat; D, man; a, neuroblast without dendrites; b, commencing dendrites; c, dendrites 

 further developed; d, first appearance of collateral branches; e, further development of 

 collaterals and dendrites. (From Ramon y Cajal.) 



higher psychical processes, which characterize man and the related 

 mammalia as compared with the lower vertebrates. The infra- 

 granular cells, on the other hand, constitute a primitive layer which 

 has obvious connections, through projection fibers, with the under- 

 lying parts of the brain and of the body at large. These cells 

 form, therefore, a mechanism through which the brain is connected 

 directly with the rest of the body, and through which the older 



