GENERAL PHYSIOLOGY OF THE CEREBRUM. 



183 



5. The layer of fusiform or polymor- 

 phic nerve cells. A 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 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 termi- 

 nate in the cortex and carry the incoming 

 impulses — impulses which are afferent as 

 regards that part of the cortex. The 

 fibers in this white matter may be classi- 

 fied under three heads: First, the projec- 

 tion system {A, B, C, D, and E of Fig. 82), 

 comprising those fibers, afferent and 

 efferent, which connect the cortex with 

 underlying parts of the central nervous 

 system, — the spinal cord, medulla, pons, 

 midbrain, or thalamus. This great pro- 

 jection system emerges, for the most 

 part, through the internal capsule and 

 the peduncles of the cerebrum. Second, 

 the association system. Certain parts 

 of the cortex are seemingly lacking in a 

 projection system ; the fibers arising from 

 these parts do not enter the capsule to 

 make connection with the motor and sen- 

 sory paths below, but pass to other 

 parts of the cortex, forming a part of the 

 system of association fibers. This sys- 



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Fig. 81. — Section through the cortex of the third frontal convokition (Broca's convolu- 

 tion) to show the stratification of the nerve cells: 1, The plexiform or molecular layer; 2. 

 the outer layer of pyramidal cells; 3, the granular layer; 4, the deep or inner pyramidal 

 layer; 5, the fusiform or polymorphic layer (from a camera lucida drawing by Melius). 



