SECTION XVIIi 

 THE CEREBRUM 



CHAPTER LIV 

 THE GENERAL FUNCTION OF THE CEREBRUM 



General Arrangement of the Gray Matter. The investigation of 

 the function of the brain of which the cerebral hemispheres form the 

 largest part, is usually carried on along structural, experimental 

 physiological, and clinical lines. A complete functional picture, 

 however, can only be obtained if the data derived from these sources, 

 are compiled and compared with one another. On the histological 

 side, it is of interest to note that the chief neurons of the cerebral 

 cortex are pyramidal in shape and are directed in such a way that their 

 apices are turned toward the surface and their bases toward the white 

 matter. The three poles of these cells are usually occupied by den- 

 drites, the principal one of which arises from the apex. The axon 

 is derived from a hillock in the middle of the base of the cell-body 

 and pursues a rather straight course into the white matter, giving 

 off collaterals in its course. 



While this cell is typical of the cerebral cortex, it does not exhibit 

 the same size and form in all parts of this organ. Throughout the 

 cortex, however, it is united with others to form four or five separate 

 layers which border immediately upon the central core of white matter. 



(a) The most superficial layer lies, of course, in contact with the enveloping 

 membranes of the cerebrum, i.e., with the pia mater, and is known as the plexiform 

 or molecular layer. Its thickness amounts to 0.25 mm. Besides the neuroglia 

 cells, it contains chiefly dendrites from the deeper layers and a few small cells, the 

 processes of which are directed parallel to the surface of the cortex. These proc- 

 esses terminate within this layer and do not penetrate the white matter. It 

 is believed, therefore, that their function is chiefly associative for the cells of the 

 cortex. 



(6) The layer of pyramidal cells lying directly underneath the former, is char- 

 acterized by the presence of a large number of cells possessing a pyramidal shape. 

 Campbell 1 arranges them in three layers, this classification being based upon 

 differences in their size. The inner ones are larger than the outer. As has been 

 mentioned above, their apices are directed outward and send their dendrites into 

 the molecular layer. The axon arises from the basal margin of the cell and enters 

 the white matter underneath. The dendrites of the pyramidal cells are rough and 



1 Hist. Studies on the Local, of Cort. Function, Cambridge, 1905. 

 42 657 



