344 The Outline of Science 



9 

 THE NERVOUS SYSTEM AND THE BRAIN-CENTRE 



The Nervous System 



Most wonderful of all structures in the body-machine, and 

 most difficult to understand, is the telegraphic system the nerve- 

 threads or "wires" and the central stations in the brain, the spinal 

 cord, and certain other clusters of nerve-cells. A unified cluster 

 of nerve-cells is called a ganglion or nerve-centre. In the simplest 

 forms of life there is no nerve, no muscle, no mouth or stomach. 

 The microscopic unit is one single cell a bit of jelly-like living 

 matter enclosed in a more or less definite membrane. Each and 

 every part of it digests food, contributes to the movement, and 

 is sensitive to the surroundings. In the course of evolution there 

 arose larger organisms with bodies, with millions of cells bound 

 together in harmony, showing division of labour. Some cells 

 specialise on nutrition, some on reproduction, some on locomo- 

 tion, and so on. Some of these cells specialise on sensitiveness, 

 and thus arise nerve-cells. Then some specialise on one particu- 

 lar kind of sensitiveness, and there appear patches or pits in 

 the skin, one sensitive to light, another to smells, and so on. Fur- 

 ther advance unites these various centres by nerve-fibres, and at 

 last a central telegraph station, a long tract of nerve-matter, 

 connects up the various sense-centres and the muscles and glands. 

 When a backbone is evolved, the main tract of central nerve- 

 stations is enclosed in it; and, as life advances, the upper part 

 of this "spinal cord" swells into a brain and is protected by a 

 skull. 



This interesting story of the evolution of the brain and 

 sense-organs deserves to be told at greater length, but this slight 

 outline may serve at present for our understanding of the essen- 

 tial nature of the nervous system. There is, as we said previously, 



