CEREBRAL HEMISPHERES. 645 



past stimulations, or "re-collection," is exclusively the property 

 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 ac- 

 complished, 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 paraly- 

 sis known as hemiplegia. The patient is frequently fully con- 

 scious, and possesses unimpaired power of thought and memory, 

 yet he is unable to perceive the sensory impulses coming from one 

 side of his body or send voluntary impulses to the muscles 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. 



