612 



THE NERVOUS SYSTEM 



(4) the temporal lobe, lying below the fissure of Sylvius. The cortical 

 portion folded under the lips of the Sylvian fissure is known as the 

 insula (lobulus insula). The average thickness of the cortex is about 

 three millimeters, but in the motor area of the frontal lobe, it may attain 

 a depth of five millimeters, while in the occipital lobe it may become 

 reduced to almost two millimeters. 



Fissure of Rolando 

 Parietal lobe 

 Parieto-occipital 



FIG. 528. LEFT LATERAL SURFACE VIEW OF CEREBRAL CORTEX IN MAN, SHOW- 

 ING THE LOBES, MAIN SULCI, AND THE LARGER FUNCTIONAL AREAS. 

 Later researches have shown that the motor area is located entirely in the pre- 

 Rolandic region. (After Oppenheim.) 



The nerve cells which enter into the formation of the gray matter 

 of the cerebral cortex present a remarkable tendency to arrange them- 

 selves in more or less well-defined layers parallel to the surface of the 

 cerebral convolutions. The number and arrangement of these layers 

 in the various lobes varies, however, with the peculiar function of each 

 of these areas. Thus, in the motor area there is a five layer type, in 

 the parietal lobe a seven layer type, in the occipital lobe a six or eight 

 layer type. The histologically different areas shade into each other by 

 insensible gradations. 



In general, it may be assumed that the nerve cells of all of these 

 layers are included in one or two physiologically distinct groups or 



