62 



INTRODUCTION TO NEUROLOGY 



In the fourth type of complex reflexes (see Fig. 18, D) two dif- 

 ferent sensory paths discharge into a single center, from which a 

 final common path goes out to the effector. This mechanism 

 also provides for both allied and antagonistic reflexes. A very 

 simple apparatus for this type of reflex is found in the roof of the 

 midbrain of the lowly amphibian, the common mud puppy, 

 Necturus. Here the upper part of the midbrain roof receives 

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

 fibers from the primary acoustic and tactile centers (Fig. 20). 



OPTIC 



CENTER 



/ ACOUSTiq 

 /and TACTILE 

 / CENTER 



m nerve 



MOTOR 

 CENTER 



Fig. 20. Diagram of a cross-section through the midbrain of Necturus, 

 illustrating a single correlation neuron 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 neurons of the III nerve. 



A single neuron of the midbrain may send one dendrite down- 

 ward to receive acoustic or tactile stimuli (or both of these), and 

 another dendrite upward to receive optic stimuli. If the animal 

 receives visual and auditory stimuli simultaneously, the inter- 

 calary neuron of the midbrain may be excited by both sets of 

 stimuli. Its discharge through the axon to the motor organs of 

 response (say to the eye muscles by way of the III nerve, as in 

 Fig. 20) will be the physiological resultant of both sets of ex- 

 citations. If they reinforce each other, the discharge will be 



