28 THE STRUCTURE OF 



The term ganglion is, however, commonly applied to 

 any round or ovoid nodule containing nerve cells, whatever 

 its size or degree of internal complexity. Many ganglia 

 in lower animals, which are typically deserving of the 

 name as regards mere form and separateness, are also, 

 hy reason of their compound nature, true nerve centres. 

 The two terms are, therefore, to a considerable extent, 

 interchangeable. 



Fusions of ganglia may occur during the development of 

 some animals, especially if they pass Ihrough distinct 

 phases of existence, as with Insects (figs. 39-41). Similar 

 changes are also presumed, by believers in the doctrine of 

 evolution, to occur during the development of the race, since 

 in many highly organized animals we may find a large 

 compound ganglion in the place of, and doing such work 

 as falls to, two or more smaller separate ganglia in simpler 

 members of the same class of animals — for example, in 

 different forms of Crustacea (figs. 34—36). This kind of 

 fusion or coalescence of primitive ganglia attains its maxi- 

 mum in the brain and spinal cord of vertebrate animals. 



From their naked-eye appearances nerve tissues are com- 

 monly di\ided into ' grey ' and ' white ' matter. The grey 

 matter of the nervous system is, for the most part, gan- 

 glionic tissue, in which nerve cells are more or less thickly 

 clustered. The white matter, on the other hand, such as 

 we find in the brain and spinal cord, is composed of an 

 aggregate of nerve fibres. These tissues are of a soft 

 pultaceous or semifluid consistence, and are composed, in 

 the main, of water, of phosphoretted fats, and of protein 

 compounds. The amount of water varies from 75 to 85 

 per cent. It is more abundant in the grey than in the 

 white matter; more abundant in lower than in higher 

 animals ; and it likewise forms a larger proportion of tho 



