RESPONSIVENESS IN VERTEBRATES 



Fig. 4. 1 1 . The optic chiasma in higher 

 vertebrates; diagrammatic. 



retina, are adjusted. In the higher vertebrates certain reflexes following 

 sound stimuli are also adjusted in the optic lobes. The lateral and ventral 

 regions of the mesencephalon contain groups of neurons that provide for 

 numerous connections and nerve tracts over which impulses are relayed from 

 one region to another. 



In the diencephalon are found many nerve tracts connecting centers in other 

 parts of the brain with the cerebral cortex. The optic nerves and tracts over 

 which impulses are conducted from the retinas to the optic lobes form part 

 of the floor and lateral walls of the diencephalon. In the lower vertebrates 

 all the fibers from one retina cross the optic chiasma to enter the opposite 

 optic lobe. The crossing in higher vertebrates involves only the medial half 

 of the fibers of each retina; the fibers of the lateral halves do not cross (Fig. 

 4.11). Certain correlations resulting from olfactory stimuli are made in the 

 diencephalon, and impulses giving rise to pain sensations are received there. 

 Centers concerned with the regulation of many basic vital processes such 

 as sleep, water balance, and heat control are located in the hypothalamus 

 or floor of the diencephalon (p. 125). 



Among the lower vertebrates the most important parts of the telencephalon 

 are the centers for correlation of impulses transmitted from the olfactory 

 organs. The olfactory centers in mammals occupy the same relative position 

 but are overshadowed by the very great growth of the dorsal part of the 

 telencephalon to form the cerebral hemispheres. In the cerebral hemispheres, 

 as in the cerebellum, the neurons that make up the gray matter are found in a 

 continuous superficial layer known here as the cerebral cortex; there are 

 more than 9 billion neurons in the human cortex. Although the cortex is con- 

 tinuous, certain areas are known to be concerned with special functions. 

 Impulses producing movements of the voluntary muscles are conducted from 

 the motor centers of the cortex to opposite sides of the body; that is, if 

 these particular areas are destroyed in one cerebral hemisphere, the animal 



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