5Q9 VERTEBRATE LIFE AND ORGANIZATION 



The impulses terminate in specific parts of the cerebral cortex which 

 have been determined by correlating brain injuries with loss of sensa- 

 tion, and by electrical stimulation during brain operations. Many human 

 brain operations can be performed under local anesthesia, and the pa- 

 tient can describe the sensations that are felt when particular regions 

 are stimulated. Impulses from the skin terminate in the gyrus that is 

 located just posterior to the central sulcus of Rolando, a promnient 

 sulcus extending down the side of each hemisphere and dividing the 

 hemisphere into an anterior frontal and a posterior parietal lobe. The 

 sensory areas of the skin are projected upside down. Impulses from 

 the head are conducted to the lower part of the gyrus whereas those 

 from the feet reach the upper part. The extent of the area receiving 

 impulses from any part of the body is proportional to the number of 

 sense organs in that part of the body. Thus the area receiving impulses 

 from the fingers is more extensive than that receiving impulses from 



the trunk. 



Impulses from the ear are carried to the temporal lobe, which is 

 separated from the frontal and parietal lobes by the lateral fissure of 

 Sylvius. Impulses from the eye are received in the occipital lobe, which 

 lies just posterior to the parietal lobe. The path of the optic fibers of 

 mammals is an exception to the generalization that afferent impulses 

 cross at some point during their ascent to the brain. Half of the fibers 

 in each optic nerve cross in the optic chiasma and end up on the opposite 

 side of the brain, but the other half do not. Thus, destruction of one 

 occipital lobe results in inability to perceive images that fall on half of 

 each retina rather than complete loss of vision in one eye (Fig. 29.11). 



Appropriate motor impulses to the striated muscles are initiated in 

 response to all of the sensory data that enters the cerebrum. The cell 

 bodies of the efferent internuncial neurons are contained in the motor 

 cortex, which lies just anterior to the sulcus of Rolando. The motor 

 cortex is subdivided, in the manner of the adjacent sensory cortex, into 

 areas associated with the different parts of the body. Fibers to the hand 

 occupy a large portion of it, tor the muscles that control finger move- 

 ments contain more motor units than do most muscles. This is correlated 

 with the intricacy of our finger movements. Most efferent internuncial 

 neurons pass directly to the motor nuclei of the brain and to the motor 

 columns of the spinal cord, crossing to the opposite side along the way 

 (Fig. 29.8). Some are relayed in a mass of gray matter, the corpus striatum, 

 situated deep within each cerebral hemisphere; others are relayed in 

 the thalamus, or at other points. 



Many association neurons interconnect the sensory and motor areas 

 of each cerebral cortex and commissural fibers extend from one hemi- 

 sphere to the other. A particularly large commissure, the corpus callo- 

 sum, can be seen in a sagittal section of the brain (Fig. 29.14). Such 

 interconnections jDcrmit the integration of the many different sorts of 

 impulses that reach the cerebrum and enable mammals to make mean- 

 ingful responses to a combination of sensory stimuli. 



The cerebral cortex of most mammals is composed almost entirely 

 of the specific sensory and motor areas just described, but in man, large 



