572 THE NERVOUS SYSTEM 



cortex are many tangential fibers. The second layer is composed of small 

 pyramidal cells. In parts of the brain there are here interposed what are 

 known as the vertical fusiform cells. The third layer is composed of large 

 pyramidal cells, in which, however, one also sees many small pyramidal cells. 

 The fourth layer is composed of the fusiform and polymorphous cells, beneath 

 which is the white substance. This arrangement is shown in the accompanying 

 figures, 404 and 405. The gray matter of the brain contains, however, not 

 only these layers and cells, but an infinitely rich mass of fibers, which can be 

 shown to have a certain definite arrangement. Some of the fibers are vertical, 

 passing directly up to the most superficial layers of cells; others have a hori- 

 zontal direction, dividing the gray matter into different layers. These 



FIG. 403. Scheme of Descending Conduction Pathways from the Cerebrum to Lower Nerve 



Centers. 



layers of fibers have received different names. A typical arrangement is 

 shown in figure 405. The most conspicuous fibers are those of certain 

 large triangular or pyramidal cells. 



The efferent or axone fibers from the cerebral cortex may be divided into 

 three classes: i, the projection fibers, which descend through the corona 

 radiata and internal capsule, to end in lower centers; 2, the commissural 

 fibers, which cross to the opposite cerebral hemisphere, chiefly through the 

 corpus callosum; 3, the association fibers, which pass in bundles beneath 

 the cortex, to end in other regions of the same hemisphere. 



It is by means of projection fibers and collaterals that associations are 

 made with nerve cells in the optic thalamus, tegmentum, and pons, and 

 through the latter region with tracts going to the cerebellum. 



