GENERAL PHYSIOLOGY OF THE CEREBRUM. 



173 



the Rolandic area. Their form and con- 

 nections are, in general, the same as those 

 given for the small pyramidal cells. 



4. The layer of fusiform or polymor- 

 phic nerve cells. A small layer of cells 

 whose form is more irregular than that of 

 the pyramidal cells, but whose axons also 

 pass into the' medullary portion of the 

 cerebrum, while their dendrites stretch 

 externally into the layers of pyramidal 

 cells. In this layer are found also some 

 cells belonging to the second type of 

 Golgi (Martinotti cells). The axis cylin- 

 der processes from these latter cells, in- 

 stead of becoming medullated fibers of 

 the white matter of the cerebrum, pass 

 toward the external surface, to end in 

 the pyramidal or molecular layer in a 

 number of minute branches. 



5. The medulla of the cerebrum. The 

 white matter of the cerebrum begins 

 immediately below the last-named layer, 

 and consists (1) of nerve fibers which 

 originate from the pyramidal and poly- 

 morphic cells immediately exterior to it, 

 and which carry outgoing impulses from 

 that part of the cortex, and (2) of fibers 

 arising elsewhere in the cortex or in the 

 lower portions of the brain, which ter- 

 minate in the cortex and carry the 

 incoming impulses, impulses which are 

 afferent as regards that part of the cor- 

 tex. The fibers in this white matter may 

 be classified under three heads: First, 

 the projection system (A, B, C, D, and 

 E of Fig. 77), comprising those fibers, 

 afferent and efferent-, which connect the 

 cortex with underlying parts of the cen- 

 tral nervous system, the spinal cord, 

 medulla, pons, midbrain, or thalamus. 

 This great projection system emerges, for 

 the most part, through the internal cap- 

 sule and the crura of the cerebrum. 

 Certain parts of the cortex are seemingly 

 lacking in a projection system; the fibers 



Tv 



D 



I 



Fig. 76. To show the 

 structure of the cortex cere- 

 bri (Dejerine) : /, The molec- 

 ular layer; //, the layer of 

 vertical fusiform cells; III, 

 the layer of small pyramidal 

 cells; IV, the layer of large 

 pyramidal cells; V, the layer 

 of polymorphic cells. 



