CHAPTER 11. 



THE STRUCTURE OF A NERVOUS SYSTEM NERVE FIBRES, 



CELLS, AND GANGLIA. 



The Nervous System in all higher animals is composed of 

 nerve fibres and nerve cells, together with an intermediate 

 basis substance in those parts where the latter units are 

 principally clustered together. The whole forms a con- 

 tinuous tissue, variously arranged and distributed through 

 the bodies of animals, and differing notably in its de- 

 velopment in accordance with the complexity of organiza- 

 tion of the creature of which it forms part. 



In all animals a certain order or plan is, howeVer, 

 recognizable in the mode of arrangement of the typical 

 elements of the nervous system. Thus, without exception, 

 we find ingoing nerve-fibres proceeding from sense organs, 

 or from other sensitive parts, to groups of nerve cells more 

 or less freely connected with one another in some * nerve 

 centre.' These cells are, in their turn, connected with 

 another set of inter-related nerve cells, situated either 

 close to or at a distance from the first ; and from this 

 second group of cells a set of outgoing nerve fibres pro- 

 ceed, which are distributed to muscles or to glands in 

 various parts of the body. Nerve elements so arranged 

 constitute the functional units of a nervous system. This 

 is the kind of mechanism by means of which * reflex 

 actions ' are brought about ; and these form the ground- 



