The Brain and What It Does 125 



neurones deep, the so-called grey matter of the brain (to distinguish 

 it from the underlying white matter). The cerebrum in man contains 

 9,000 million cells, out of the 12,000 million present in the whole 

 brain. 



Our knowledge of what goes on in the brain is rather meagre. 

 There is undoubtedly a great deal of electrical activity going on. 

 As we have seen, it is possible, if a nerve fibre is laid bare, to detect 

 the passage of an electrical impulse or wave by placing electrodes at 

 two different points along it. 



1 r ^ 





FIG. 25. Electroencephalograph patterns of brain activity recorded through 

 the skulls of human beings (the regular marks below the tracings are time 

 signals at 1 second intervals). 



A. A normal subject. The eyes were closed between the two arrows on the 

 time line 



B. A subject who has had a brain injury. The sharp downward spikes are 

 very characteristic of certain kinds of injury 



C. A normal subject (the oscillations are less magnified vertically than in A) 



D. Same subject as C, in sleep induced^by a drug 



(Reproduced by courtesy of Dr W. F. Floyd, Middlesex Hospital Medical 

 School) 



These electrical impulses can also be detected outside the brain 

 by electrodes placed on the skin at dififerent points. With a delicate 

 electronic amplifier called an electro-encephalograph, it is then pos- 

 sible to detect oscillations or surges of electric action within the brain 

 (Fig. 25). These surges must occur on a rather large scale, i.e. to 

 produce an effect outside the skull many of the electric circuits of the 

 brain must be oscillating in unison. So far it has been difficult to 

 connect these oscillations very closely with the mental state of the 



