126 Inside the Living Cell 



individual being examined, except that in epileptics sharp spikes are 

 observed. They may only be a kind of 'carrier wave' on which the 

 actual messages are superimposed. We are like strangers outside a 

 great power station; we hear faintly the hum of activity going on 

 inside it, but we do not know what it means. We do know that the 

 oscillations are larger and more clearly defined when we are awake 

 and giving attention, and give way to a long, slow rhythm when we 

 are asleep. 



More detailed information has been obtained by experiments with 

 animals. By means of electrodes placed on the optic nerves of a 

 frog, we can detect the electric impulses coming from the excited 

 nerve cells in the retina. They can be traced a little way into the brain 

 itself and then are lost among the innumerable fibres which join the 

 neurones. 



It is also possible to locate the region of the cortex which the 

 signals finally reach, by putting electrodes into the exposed brains 

 of patients and also by electro-encephalograph records taken on the 

 outside of the skull. It is found that the impulses produced by the 

 retina are spread over a great area of the brain. The picture on the 

 retina is enormously magnified and it would appear that millions of 

 brain cells are concerned with the vision of one instant. 



The brain of an animal, and especially human beings, is thus 

 primarily an instrument for co-ordinating large numbers of separate 

 impressions. The pattern of the impressions means much more than 

 the separate bits of which it is made up. 



When we look at a half-tone picture in a newspaper, we no doubt 

 see the separate dots, but we are only interested in the pattern of 

 light and shade which they make. The brain has a remarkable power 

 of finding the pattern in millions of diverse sense impressions and 

 there must be something basic and inherent in its structure which 

 permits it to do this If it is more concerned with the pattern of the 

 diverse sense impressions than with the details of the individual 

 impressions, it must follow that the interrelations between the 

 brain cells which receive the impressions are more important than 

 the actual impression made on each one. 



If we look at a square, it is the fact that it is a particular kind of 

 geometrical figure that we seize upon. We are indifferent to its size. 

 The actual size of the image on our retina depends on our distance 

 from the object. The projection of the square in our brain thus 

 depends on our distance from it — yet we have no difficulty in recog- 

 nizing it as the same square. It is clear that in interpreting very 

 simple experiences, the brain has to make very complicated adjust- 

 ments. However, you can only find a pattern in your sensory 



