146 THE HUMAN BODY 



the midbrain as upon a stalk. Because of the convoluted surface 

 of the cerebrum the total amount of superficial gray matter ia 

 much greater than it would be if the cerebrum were smooth. This 

 layer of gray matter is the region wherein occur those special 

 activities which set the cerebrum above the rest of the nervous 

 system. It is called the cortex cerebri, or for convenience simply 

 the cortex. 



Structure of the Cortex. The cortex cerebri consists for the 

 most part of neurons with small cell-bodies having much branched 

 processes, signifying rich synaptic connections. Many of these 

 neurons appear to be confined altogether within the cortex ; others 

 give off myelinated axons into the underlying white matter. 

 Interspersed with these small cell-bodies are others which are 

 much larger, which are pyramidal in shape, and which always 

 give off a large axon into the white matter. From their shape 

 and size these are known as large pyramidal cells. In a certain 

 region of the cortex, known as the motor area, the pyramidal 

 cells are relatively gigantic, being just at the limit of naked eye 

 visibility. 



The White Matter of the Cerebrum. This consists of myelin- 

 ated axons classified according to their course and distribution 

 into three groups. The so-called projection fibers are the axons 

 by which the cortex is brought into connection with the other 

 parts of the nervous system. These include afferent projection 

 fibers, which are the continuations within the cerebrum of the 

 various sensory paths described in previous paragraphs (see 

 p. 144), and efferent projection fibers, which convey impulses from 

 the cortex to the rest of the Body. 



At the base of the cerebrum, where it rests upon the midbrain, 

 all the projection fibers, both afferent and efferent, are crowded 

 together into a restricted space between two of the basal nuclei. 

 This region is known as the internal capsule. As the fibers emerge 

 thence into the roomy cerebrum they spread apart on their way 

 to the different parts of the cortex forming the corona radiata. 



The second group of cerebral axons are the association fibers. 

 These pass between one part of the cortex and another within 

 the same hemisphere, enabling impulses to travel freely among 

 the cortical cells. The third group of cerebral axons are the 

 commissural fibers which pass between cortical areas in opposite 



