ORIGIN AND SIGNIFICANCE OF THE CEREBRAL CORTEX 225 



mon mud-pirppy, Necturus) this associational mechanism is the 

 simplest possible (Fig. 2). Here the upper part of the midbrain 

 roof receives optic fibers from the optic tracts, while the lower 

 part receives lemniscus fibers from the primary acoustic and 

 tactile centers. A single neurone of the midbrain may send 

 one dendrite upward to receive optic stimuli and another den- 

 drite downward to receive acoustic or tactile stimuli (or both 

 of these).' If the animal receives visual and auditory stimuli 

 simultaneously, the intercalary neurone of the midbrain may 

 be excited by both sets of stimuli. Its discharge through the 



CPTIC 



CENTER 



MOTOR 



CENTER 



Figure 2. Diagram of a cross-section through the midbrain of Necturus, illus- 

 trating a single association neurone of the midbrain roof. One dendrite spreads 

 out in the optic center among terminals of the optic tracts; another dendrite 

 similarly spreads out in the acoustic and tactile center. The axon descends 

 to connect with the motor neurones of the third nerve. 



axon to the motor organs of response (say to the eye-muscles 

 by way of the third nerve, as in Fig. 2) will be the physiological 

 resultant of both sets of excitations. If they reinforce each 

 other, the discharge will be stronger and more rapid ; if, on the 

 other hand, they tend to produce antagonistic responses, there 

 will be an inhibition of the response or a delay until one or the 

 other stimulus obtains the mastery. 



In the human brain the corresponding structures are similar 

 in functional type, but much more complex, with many systems 

 of intercalary neurones between the different functional centers. 

 The midbrain roof (corpora quadrigemina) is differentiated into 



