SENSE ORGANS AND NERVOUS COORDINATION 



601 



and from the cerebral hemispheres, and it has become enlarged during 

 the course of evolution as the cerebral hemispheres have assumed a 

 dominant role in integrating the activities of the body. All of the sen- 

 sory impulses that go to the cerebrum, except those from the olfactory 

 organ, are relayed in the thalamus. Many motor impulses descending 

 from the cerebrum go directly to the motor nuclei and columns, but 

 some of these are also relayed in the thalamus. Other parts of the dien- 

 cephalon have not changed very much during vertebrate evolution. The 

 hypothalamus is an important center for the control of many autonomic 

 functions. Body temperature, water balance, appetite, carbohydrate and 

 fat metabolism and sleep are among the processes regulated by the 

 hypothalamus in mammals. The hypothalamus exerts its control by 

 neuronal connections with the motor nuclei and columns, and also by 

 neuronal connections with the posterior lobe of the pituitary gland. 

 Damage to it is often fatal, for so many vital processes are disturbed. 



Cerebral Hemispheres. As the cerebral hemispheres assumed the 

 dominant role in nervous integration during the course of evolution, 

 they enlarged and grew posteriorly over the diencephalon and mesen- 

 cephalon (Fig. 29.14). A layer of gray matter has developed on the sur- 

 face of the cerebrum and has formed a gray cortex which provides more 

 area for the increased number of cell bodies. Billions of cells are present 

 in the cerebrum of man. Complex folds of the cortex increase further 

 the area of the cortex. Ridges (gyri) are present with furrows (sulci) 

 between them. Parts of the cerebral hemispheres are still concerned with 

 their primitive function of olfactory integration, but their great enlarge- 

 ment is correlated with the evolution of other integration centers (Fig. 

 29.15). Afferent impulses from the eyes, ears, skin and many other parts 

 of the body are carried to the cerebral cortex by afferent internuncial 

 neurons, after being relayed in the thalamus as shown in Figure 29.8. 



Sulcu-S of 

 Rolando 



SKin sensations 



Motor coi'teoc -^ 

 Frontal lobe. ^ 



Fissxirc of 

 Sylvius 



Smell" 



Hearixig 



Temporail lobe' 



Pons- 

 Medulla.- 



Parietal 

 lobe. 



•Vision 



Occipital 

 lobe 



Cerebellum. 



Figure 29.15. Cortical areas of the human brain as seen in a lateral view. The 

 association areas of the cortex have not been hatched, 



