CHAPTEE XV. 



The Coktex of the Porebrain and the Medulla of the Hemi- 

 spheres ; THE Commissures and the Corona Eadiata. 



A GENERAL suivey has now been attained of the outer form-relations 

 of the brain. The present chapter is intended to familiarize you with the 

 structure of the cerebral cortex and to give an insight into the mode of con- 

 nection of the cortical regions with one another and with the deeper-lying 

 structures. 



The finer structure of the cortex is known in its elements only. Knowl- 

 edge of the combinations of these elements with one another is still wanting, 

 and therewith, unfortunately, a proper understanding of the anatomical 

 basis of the great organ of the mind. There is hardly a doubt but that the 

 cerebral cortex, as a whole, may be regarded as the place where most of 

 those cerebral processes take place which arise in consciousness, that it is the 

 seat of memory, and that the voluntary acts proceed from it. 



The entire hemisphere is covered by the cortex. This has not, how- 

 ever, exactly the same structure everywhere on the convexity. Even if a 

 sort of fundamental type exists, greater or lesser difEerences in the layers, 

 in which the ganglion-cells and nerve-fibers are arranged, may, nevertheless, 

 be found, depending on the region of the brain investigated. One cortical 

 type never passes abruptly over into another. Inasmuch as the significance 

 of these anatomical variations is not understood, one region only, the frontal 

 lobe, will be considered at present. 



A dense net-work of fine, medullated fibers, mostly running parallel with 

 the surface, lies close under the pia, but is separated from it by a thicker 

 layer of neuroglia. It is the layer of tangential fibers (Fig. 150). Cells are 

 distributed in it in relatively-small quantity. Directly beneath it, however, 

 begins the layer of the pyramidal cells — cells distinctly typical of the cortex. 

 They appear first as a layer of numerous smaller elements (3, Fig. 150), 

 which passes over into the layer of the large pyramids (3, Fig. 150). All 

 of these cells send their dendrites toward the surface of the cortex and in 

 various other directions, as the apical process, lateral processes, etc., and, for 

 the most part, send their neuraxons deep down into the medullary layer. 

 The layer of the large pyramidal cells in the frontal and parietal lobes is the 

 broadest in section of any of the cortical layers of these lobes. The in- 



(327) 



