ORGANIZATION OF HUMAN BRAIN — PENFIELD 435 



Certain general principles of organization emerge. The area of final 

 coordination and integration of neurone activity in the central nervous 

 system is evidently situated in the higher brain stem. These coor- 

 dinating circuits are essential to the very existence of consciousness. 

 Almost any interference with this portion of the nervous system, 

 either by compression or disease, produces unconsciousness. This is 

 the portion that has symmetrical connections with the gray cortex 

 of both hemispheres. It contains a converging and diverging system 

 of nerve-fiber connections which we may call the centrencephalic sys- 

 tem to indicate that it constitutes the functional center of the brain 

 or encephalon. 



The centrencephalic system occupies what may be called the old 

 brain corresponding with the rudimentary head of the central nervous 

 system of lower forms of life. 



Higher mammals, and especially man, are provided with super- 

 imposed hemispheres covered by an ever more extensive mantle of 

 gray matter, the cerebral cortex. This gray cortex is composed of 

 millions of nerve cells, or ganglion cells, and, in the case of man, the 

 cortex has so increased in extent that it is formed into deep folds or 

 fissures that convert its surface into convolutions. 



Each of the functional areas of this cortex is, in a sense, a separate 

 projection from the brain stem so that each portion makes possible new 

 and more complicated function (fig. 1). But no area of cortex is 

 mdependent and none is capable of effective function without its cor- 

 responding portion of the old brain. Indeed it seems to be through the 

 brain stem that the new capacities of each area of cortex are utilized. 



Thus it is that the sensory areas of the cerebral hemispheres are 

 no more than way stations in the several currents that carry different 

 forms of sensation into the centrencephalic integrating system, and 

 the cortical motor areas are way stations in the stream of outflowing 

 impulses that produce motor activity. Large areas of cortex may be 

 destroyed or removed without producing miconsciousness although 

 this does interfere with the function to which the injured areas were 

 devoted. 



In order to treat certain conditions such as focal epilepsy, it is some- 

 times necessary to operate under local anesthesia and to stimulate the 

 brain of conscious men and women with gentle electrical currents. 

 Wlien applied to the motor area of the cortex such stimulation pro- 

 duces crude movement because the current is conducted outward 

 through the spinal cord and nerves to the muscles. This current does 

 not enter the integrating area and the patient is surprised to dis- 

 cover that his hand, for example, has been caused to move. 



If the surgeon's electrode is applied to one of the sensory areas 

 (somatosensory, hearing, vision in fig. 2), the impulses do pass into 



