LECTURES IN BIOLOGICAL SCIENCES 



sory and motor centers of the cerebral cortex give us very in- 

 teresting and suggestive information on the evolution of the 

 brain in primates. I want to close by calling your attention to 

 some of this neurophysiological work— especially that by Clinton 

 Woolsey of Wisconsin on monkeys and that of Wilder Penfield 

 on man. 



"The electrophysiological technique of determining the af- 

 ferent projection areas has provided a new method for study of 

 localization of function in the cortex and, because it yields a 

 very detailed story of the relations of specific parts of peripheral 

 sensory mechanisms to specific parts of the central receiving 

 areas, it provides not only the means of determining the extent 

 of cortex concerned with a given sensory mechanism but, from 

 the detailed pattern of organization of the system, it permits 

 clear identification of homologous parts in different species." 

 (Woolsey, 1958). By tactically stimulating a peripheral part, 

 say in a toe or finger, an electrical potential is evoked in the 

 cortical receiving area, for example, in the postcentral sensory 

 area. By electrically stimulating a central part, say in the pre- 

 central motor projection area, a muscular response is evoked 

 in a peripheral part, for instance, in the finger or toe. The 

 current passes from the stimulated place in the precentral con- 

 volution down the corticospinal tract to the anterior horns of 

 the spinal cord on the opposite side and thus to the motor nerves 

 going to the hand or foot. 



In order to picture the basic organization of the pre- and 

 postcentral projection areas, it will help if you keep in mind an 

 outline drawing of a rabbit or a monkey as seen from the side. 

 The projection centers for the ends of the legs are nearest to a 

 line corresponding to the bottom of the central sulcus. The 

 centers for the head end of the animal are in the lower parts, 

 and those for the tail end are in the upper parts of the central 

 sulcus. 



Now, in our outline drawing, we make the area of each 

 anatomical part proportionate to the relative area in the cortex 

 which receives sensory connections from, or sends motor connec- 

 tions to, that particular region of the body. For example, if 

 the cortical projection area for the thumb is larger than that 

 for the big toe, we will make the thumb larger and the toe 

 smaller in our outline drawing, or cortical map. 



90 



