534 PHYSIOLOGY CHAP. 



drawn attention to the structural characters of the cerebral 

 cortex, by study of the arrangement and morphological characters 

 of the cells which constitute its various layers. By long and 

 patient comparison of the different areas of the pallium he has 

 arrived at results which are of great interest, and which can be 

 summarised as follows : 



According to Brodmann, the fundamental type of the cerebral 

 cortex, from which all the other secondary types are differentiated 

 during foetal development (from the seventh month), consists of 

 six layers which may be clearly recognised, and are formed by 

 three strata rich in cells alternating with three layers poorer in 

 cells (Fig. 268). The first and sixth of these strata are constant 

 in all cortical regions of the adult human brain and all mammals. 

 Others, on the contrary, as the second and fourth granular layers, 

 vary greatly and may disappear in many regions of the adult 



human brain ; the remaining 

 layers, the third and fifth, 

 present an intermediate grade 

 of variability. 



This structural six -layer 

 type is not permanent ; in 

 many regions it is more or 

 less transitory. The numerous 

 , , secondary structural types that 



FIG. 267. Sections of cerebral convolutions. - . 5, i 



(Baillarger.) Approximately natural size. 1, develop IrOIll it lOrill almost 



show the six layers usually seen in the cortex f-p^fha n f f V, onfivo nrrH- OY 



with the naked eye ; 2, appearance of a section nme-ientns 01 1116 entire COrtCX 



of a convolution from the neighbourhood of the Q ^g adlllt human 

 calcarine fissure. 



Ihese secondary types may 

 in their turn be grouped into two great categories : 



(a) Homotypical cortical formations, in which the structure is 

 fundamentally unchanged, the six layers persisting (Fig. 269). 

 The greater part (about three-quarters) of the cortex of the human 

 brain comes under this category. The numerous types which it 

 comprises are distinguished from each other by the varying 

 characters of the several cell layers. These characters are par- 

 ticularly the depth or thickness of the cortex, the size of the cells, 

 and, above all, the numerical richness of the cells which make up 

 the different layers. 



(6) Heterotypical cortical formations, which lose their funda- 

 mental structure during ontogenetic development, either because 

 the layers increase in number, as in the case of the cortex of the 

 calcarine fissure (Fig. 270), or because some of the original six 

 layers disappear (Fig. 271). 



We said that some nine-tenths of the whole cortex of the 

 adult human brain belong structurally to the fundamental type of 

 the six cellular layers, either because they retain it throughout life, 

 or because they exhibit it in some stage of development. The 



