GENERAL ZOOLOGY 



Fig. 4.12. I'he left cerebral hemisphere of the human brain on which sensory, motor, and associa- 

 tion areas are indicated. In the sensory areas (dotted), one or many stimuli are interpreted, as 

 when we identify a typewriter by its sound alone, a lead pencil by handline; it, or the type and 

 nationality of an airplane by a synthesis of fleeting visual stimuli. In the motor areas (lined), 

 muscular activities are arranged into purposive or meaningful sequences, as in speaking. The 

 association areas (clear) are concerned with the complex correlations of memory and thought. 



is unable to use the voluntary muscles on the opposite side of the body. The 

 regions that coordinate movements of the principal parts of the body, from the 

 toes to the face muscles, are known in man. Another major division of the 

 cortex is concerned with sensory functions and contains the sensory centers 

 to which impulses are conducted from visual, auditory, and olfactory re- 

 ceptors, as well as from receptors of pressure, temperature, and taste stimuli. 

 These areas have been mapped almost completely for the human cortex (Fig. 

 4.12). The association centers of the cortex are filled with adjuster neurons 

 which are involved in the complicated pathways used in the mental activities 

 of thinking and learning. 



Intelligence depends on the degree of development of the cerebral cortex 

 and especially on the neurons of the association areas. An animal's ability 

 to profit by experience involves analysis of a situation and memory, enabling 

 the individual to react in a way that is advantageous in a new situation. 

 This ability is determined by the animal's degree of intelligence, which, in 

 turn, is limited by the number of adjustor neurons and the synapses existing 

 between them. It is known that all the neurons that an animal will ever 

 possess are present at a very early stage of its development. New synapses 

 are, however, formed throughout the life of the individual and probably de- 

 pend on the variety and intensity of the stimuli received by that individual. 

 The sensory impulses that reach the cerebral cortex, the motor impulses that 

 pass out from it, and the associations made in it constitute our so-called 



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