644 MANUAL OF PHYSIOLOGY. 



of the cerebral cortex, and to it the hemispheres owe their mental 

 faculties. During our lifetime sensory impulses are continually 

 streaming into the cells of the cortex of the brain from the 

 peripheral sensory organs. Thus innumerable impressions are 

 left stored up in the nerve cells. The effect of the continuing 

 presence of these impressions in the active cells is memory, and 

 by an association, arrangement, or separation of these persisting 

 impressions, the activity of the cells gives rise to thought or 

 ideation. 



In close relation and connection with these cells of the cortex, 

 in which permanent impressions are stored and ideation is 

 accomplished, are those other groups of cells which have been 

 mentioned as being in direct communication with the lower 

 motor centres, and can by the medium of the latter execute 

 voluntary movements. 



It is a very remarkable fact, as far as the mental faculties are 

 concerned, that one side of the brain seems to be sufficient for 

 their perfect performance. Memory, consciousness and -thought 

 can all be operative in a perfectly normal way, when one side of 

 the brain is rendered incapable of performing its functions by 

 disease or injury. But this is not true as regards the reception 

 of sensory impressions or the emission of voluntary impulses. 

 The difference between the mental powers and mere motor and 

 sensory functions of the brain can be seen in those cases of 

 paralysis known as hemiplegia. The patient is frequently fully 

 conscious, and possesses unimpaired power of thought and mem- 

 ory, yet he is unable to perceive the sensory impulses coming 

 from one side of his body or send voluntary impulses to the mus- 

 cles of the paralyzed side. 



The cells which act as the immediate receivers of afferent and 

 dispensers of efferent impulses to one or other side of the body 

 are then localized to one hemisphere, and that, we have already 

 seen, is that of the opposite side. 



LOCALIZATION OF THE CEREBRAL FUNCTIONS. 

 Whether the surface of the hemispheres can be mapped out 

 into small areas, each of which is set apart for a definite duty, or 



