CHAPTER XIX 



THE NERVOUS SYSTEM NEURONS, NERVE 

 IMPULSES AND REFLEX ACTION 



It has already been noted that the whole nervous system is 

 made up of nerve cells and nerve fibres, which indeed form 

 the basis of the gray and white matter, respectively. The 

 fibres and cells are not separate structures but each fibre is 

 a part of some cell; and thus it comes about that the whole 

 nervous system is composed of these units, each consisting 

 of a cell with its connected fibre or fibres. These units are 

 called neurons (one of these is pictured in Figure 11). Each 

 consists of a nerve cell with its nucleus; extending from the 

 cell are dendrites and one axis cylinder, at the distal or 

 outer end of which is a nerve ending. 



A neuron may receive an impulse through its axis cylinder 

 and send it out through its dendrites, or it may receive the 

 impulse through its dendrites and send it out through its axis 

 cylinder. In either case it is as the neurons act, each by itself, 

 and each in connection with the other neurons, that the func- 

 tions of the nervous system are performed. Some of them 

 constitute the thinking and willing part of the brain; others 

 in the cord and elsewhere are the servants of those in the brain, 

 since they carry messages to and from the brain, though they 

 have, besides, important functions in connection with reflex 

 actions. We shall consider first those which carry impulses 

 to the brain, then those that carry impulses away from it 

 and finally those concerned in reflexes. 



