92 THE MECHANISM OF LIFE 



two pairs of head ganglia may be called the " brain " of the 

 animal. Nerves issue from the ganglia and go to the muscles, 

 skin, and other organs of the segment, and (in general) the organs 

 in each segment are supplied with nerves that originate in the 

 two ganglia belonging to that segment. 



This is a very simple and easily understood scheme, and, strange 

 as it may appear, it is essentially the same scheme as that of the 

 nervous system of the higher animal, such as man. There also 

 the body is built up of segments, and though the latter are not 

 very evident in the adult stage, they can, easily enough, be made 

 out in the embryonic condition. Each segment contains a pair 

 of ganglia from which nerves go out to the organs of the body. 

 But the foremost segments have become coalesced to form the 

 head, and the ganglia have therefore fused together as the brain. 

 Further, all the ganglia in the trunk region have been greatly 

 enlarged, and so have coalesced to make the grey matter of the 

 spinal cord. The white matter of the brain and spinal cord 

 corresponds to the commissural nervous tracts (longitudinal and 

 transverse) of the earthworm. There are very many complica- 

 tions, and it would be wrong to press the analogy too closely; 

 still, it is a true one: the central nervous system of the higher 

 animal consists of a series of ganglia grown together to form the 

 brain and spinal cord. But although this grey matter is almost 

 continuous, we can recognise the existence of a very complex 

 system of " commissures " connecting together its centres, or 

 the positions of the original ganglia. 



We may next consider the arrangement of these nervous 

 centres and commissural nervous tracts. 



The Spinal Cord. — This is the thick strand of nervous tissue 

 lying within the tunnel formed by the cavities within the joints 

 of the vertebral column. It looks as if it were the drawn-out 

 prolongation of the medullary part of the brain, but it is really 

 to be regarded as that part of the central nervous system which 

 is situated in the trunk, while the brain is the other part situated 

 in the head. Looking at it in section (as if the spinal cord were 

 cleanly cut across), we get the general arrangement suggested 

 in the figure. The central part of the cord consists of grey 

 matter — that is, of nerve cells with their dendrites. The peri- 

 pheral part consists of nerve fibres running lengthways in the 

 cord. These fibres connect together the ganglionic matter of the 

 brain with that of the cord, and they also connect together 



