XV 



77?^ Brain and What It Does 



H 



Now we come to one of the largest collections of cells in the animal 

 body, the brain. In man. it consists of a mass of nerve cells or 

 neurones, at least 12.000 million of them, connected together by 

 very numerous branching nerve-like threads, of which the largest are 

 called axons (see Plate 15). Into it go most of the nerves coming 

 from the sense organs and out of it come many of the nerves which 

 control the muscles. We could say that the brain is like a telephone 

 exchange in which the proper circuits are connected together, but 

 this does not help very much in explaining how the information 

 provided by the senses is organized and made use of, and how 

 complicated muscle actions are controlled. 



We can easily recognize a number of different ways by which 

 muscles can be put into action. The simplest way is the involuntary 

 reflex action in which a sensation brings about an automatic in- 

 voluntary response, like the contraction of the iris when a bright 

 light is shone before the eye, or the sudden withdrawal of the hand 

 if a needle is pushed into it. These reflex actions often scarcely 

 involve the brain at all; they occur in animals in which at least the 

 higher parts of the brain have been removed. Similar to these are 

 responses which control automatic muscular rhythms, like the 

 beating of the heart, where one stage of the rhythmic process, when 

 it is completed automatically, initiates the next. Other processes, 

 like breathing, are semi-automatic. They normally look after them- 

 selves, but we can interfere with them if we want to. 



Something is known about the ways in which such automatic or 

 semi-automatic actions are controlled. The muscles themselves are 

 provided with sensory nerves which send out messages indicating 

 their state of contraction. This was discovered by Sir Charles Bell 

 as long ago as 1826. In a famous paper, which he presented to the 



