2l8 FROM FISH TO PHILOSOPHER 



Many writers localize consciousness in the cerebral cor- 

 tex; but this localization must be qualified because sub- 

 mammalian forms that have no cortex present all the 

 evidences of conscious awareness of themselves and of 

 their environment, and even in man it can be demon- 

 strated that the brain stem contributes to conscious 

 states. It is probably more accurate to say that con- 

 sciousness is a function of the highest levels of the central 

 nervous system, but invariably includes contributions 

 from the brain stem and even from the peripheral nerv- 

 ous system. Nevertheless, there is every reason to believe 

 that consciousness in man reaches its highest integration 

 in the cortex, and neurophysiologists have long been 

 concerned with the question as to whether any particu- 

 lar region of the cortex is important for conscious activity. 



The cortex is divided longitudinally into two halves, 

 corresponding to the right and left cerebral hemispheres. 

 Most of the sensory pathways entering the cortex, as 

 well as the motor pathways leaving it, cross the central 

 nervous system at some point— so that, functionally, the 

 left hemisphere is predominantly related to the right side 

 of the body; the right hemisphere, to the left side. Al- 

 though there is only a slight difiFerence in the size of the 

 cerebral hemispheres, the left hemisphere dominates 

 cerebral activity in a right-handed person; the right 

 hemisphere, in a left-handed person. This dominance is 

 particularly evident in respect to manual skill, symbolic 

 understanding, and language. Dominance is relative, 

 however, rather than absolute, and surgical denervation, 

 or removal of one cerebral hemisphere, may be followed, 

 after prolonged training, by the development of in- 

 creased fimction on the other side. 



Each hemisphere of the cortex is somewhat arbitrarily- 

 divided into four lobes: the frontal, temporal, parietal, 

 and occipital. This division is mostly a matter of con- 

 venience in description because all parts of the cortex 

 are directly or indirectly connected with all other parts 

 by association fibers, and no region can be regarded as 



