238 THE ENGINES OF THE HUMAN BODY 



manipulate and dispatch nerve messages are grouped so 

 as to form a stratum or layer on the surface of the 

 cerebral hemispheres. When the rind or cortex of the 

 cerebrum is cut into, its substance is seen to be made up 

 of white matter except a very narrow grey band or strip 

 placed just under the surface covering. This band makes 

 up what is called the cortex or cortical matter of the brain. 

 It is the presence of myriads of minute operatives which 

 gives the band its grey colour. The white matter is 

 composed of the microscopic fibres or living wires which 

 carry messages to and from the operatives. These living 

 units in the cortical stratum form the highest class of 

 nerve operatives. When we wake in the morning the 

 sheet of grey cortex on the brain becomes the screen on 

 which is lit up the cinema of the outer world. It lights 

 up and becomes our field of consciousness. The cerebral 

 operatives are thus spread out and form a thin and 

 extensive layer, varying in thickness, in some parts being 

 almost a quarter of an inch in depth, and in others little 

 more than a tenth of an inch. The operatives are spread 

 out on a wide field because of the enormous number 

 of message lines which have to be brought to and 

 from them. Their battalions are closely set, twenty units 

 deep and more, with cable cords piercing their ranks. 

 Nature has increased the extent of the cortical field by 

 throwing the surface of the cerebrum into folds or con- 

 volutions. Between the convolutions are depressions or 

 sulci (fig. 45, A). Groups or areas of convolutions are 

 separated from neighbouring groups by deep depressions 

 or fissures. Everywhere, on the convolutions and within 

 the sulci and fissures, extend the grey cortical battalions 

 of nerve operatives or cells. 



A few pages back mention was made of the brain 

 mechanism needed for an automaton motor-driver. One 

 requisite was a contrivance for registering the pictures 

 which fell on the sensitised plates of the camera, repre- 

 senting the eyes of the automaton. In fig. 45, A, the 

 situation of the sight-registering mechanism in the human 

 brain is indicated. It is placed far back on the cerebral 



