124 THE INDIVIDUAL ORGANISM 



weight of the human brain, and it practically fills the larger part of the 

 cranial cavity. In degree of development, the human cerebrum is far in 

 advance of that of any other organism, and to it we owe our dominance in 

 the world. The intricate convolutions of the external surfaces of the 

 cerebral hemispheres produce a comparatively huge area that is covered 

 with gray matter to a depth of some 2 mm. This gray matter forms the 

 cerebral cortex, the seat of consciousness and all of man's higher mental 

 faculties. The cortex is an intricate structure made up of some 9,200 

 million nerve cells. In a man weighing 150 pounds, the nerve cells of the 

 brain cortex would weigh only about Mooo °f the total, but this small 

 part controls the whole. Beneath the cortex, the hemispheres consist of 

 white matter — the myelin-encased fibers that connect the cortex to the 

 rest of the nervous system or that provide connections between various 

 parts of the cortex itself. 



The cortex of gray matter, which covers the whole of the cerebral 

 hemispheres, forms the highest brain level. It is richly supplied with 

 connections with the cord and with all the other brain centers, as well as 

 having its various surface areas intimately interconnected, both by com- 

 municating fibers beneath the cortex and by adjustor neurons within it. 

 Many local areas of the cortical surface show a marked correlation with 

 various body functions and abilities. Definite motor centers are known 

 to be concerned with the voluntary movement of particular parts of the 

 body, such as the toes, foot, leg, shoulder, or fingers; other localized cen- 

 ters are concerned with such things as hearing, or the ability to under- 

 stand spoken words, to understand written language, or to form spoken 

 words, or to write. It is improbable that these centers independently con- 

 trol the whole of the functions with which they are clearly correlated, but 

 they do represent a sort of focal point in which the complex neural ac- 

 tivities necessary for these functions are correlated. Much of the func- 

 tioning of the cortex does not ordinarily affect consciousness, and only a 

 part of the cortical functioning has to do with the phenomena that we 

 designate as volition, reason, and intelligence. 



Actually, it is probable that only a small proportion of the nervous 

 impulses that enter the central nervous system ever reach the level of 

 consciousness. Many of them are properly routed and connected by 

 various synapses to produce appropriate responses without our ever 

 becoming aware of them. Some of these "routings" are permanently 

 below the level of consciousness and cannot produce a sensory impression ; 

 others we may "by taking thought" become aware of, although we do 

 not ordinarily sense them — as, for example, the position of a hand or foot 

 that is quietly at rest. Any impulse that we can sense must have reached 

 the cortex, but probably only a small proportion of even these impulses 

 are ordinarily "conscious." 



