510 THE NERVOUS SYSTEM 



In general, it may be assumed that the nerve cells of all of 

 these layers are included in one or two physiologically distinct 

 groups or types those whose neuraxes enter the projection paths, 

 and those whose neuraxes enter the association paths ; also that 

 while these cells intermingle with each other in all portions of the 

 cortex, yet certain areas are characterized by an undue proportion 

 of one or the other type, and may accordingly be considered as 

 either projection centers or association centers. 



The larger cells belong, as a rule, to the projection centers, and 

 the peculiar type of large cell contained in a given center may 

 often be considered as characteristic of that particular area. Thus 

 the motor area contains giant pyramidal cells, the visual area has 

 the giant "solitary cells" of Meynert, and the cornu Ammonis 

 contains large bipolar spindle cells. 



The larger cells, being of Golgi's Type I, are assumed to be con- 

 nected with the projection fibres. On the other hand, the smaller 

 cells granule cells, polymorphous cells, etc. which more fre- 

 quently belong to Golgi's Type II,- are thought to supply the 

 neuraxes of association paths. Those large areas parietal lobe, 

 frontal lobe, lobulus insulae which consist in so large a part of 

 the smaller type of cells, may therefore be supposed to contain 

 the larger association centers. 



The cells in any given portion of the cortex are not only ar- 

 ranged in layers parallel to the surface of the cerebral convolutions, 

 but the passage of fibres of the corona radiata and white medulla 

 to or from their terminations within the pallium, also separates 

 the cells of the cortex into irregular rows or striations, whose axis 

 is perpendicular to the surface of the convolutions. 



IN THE MOTOR AREA the cortical cells form five tangential 

 layers, as follows : 



1. Molecular layer. 



2. Outer polymorphous cell layer. 



3. Small pyramidal cell layer. 



4. Large pyramidal cell layer. 



5. Inner polymorphous cell layer. 



The molecular layer consists of a network of fine dendritic fibres, 

 derived from the deeper layers, which are disposed in tangential 

 meshes beneath the pia mater. Occasionally small cells, apparently 

 displaced from the deeper cell layers, are scattered among these 

 fibres; they are of polymorphous form, and their processes are 

 confined to the molecular layer. The surface of the molecular 



