NERVES. 



139 



ending near the point touched. The nerves may be stimu- 

 lated by a great variety of agents : by mechanical disturbance, 

 as in the case just cited, by heat, electricity, chemical action, 

 and in special cases by waves of light or of sound. They 

 may also be stimulated by vital changes taking place in the 

 nerve-cells or other structures in which they terminate. What- 

 ever be the nature of the stimulus, the effect is always the 

 same, viz., the origination of a nervous impulse which travels 

 rapidly through the nerve to its opposite end, and there either 

 throws into action the organ connected with it, or modifies ac- 

 tions already occurring. The nature of the nervous impulse is 

 very imperfectly understood, but it seems to be a progressive 

 molecular movement, which may be roughly compared to an 

 electric current passing through a wire. 



By means of this delicate mechanism every part of the body 

 is placed in material connection with, and is able to influence the 

 activity of, many other parts. And thus, like the soldiers of an 

 army, all the organs are enabled to act together as a unit, per- 

 forming different functions by a physiological division of labor, 

 yet co-operating for the common welfare. 



The actions of the different organs are for the most part co-ordinated by 

 a process known as reflex action, which plays a highly important part in 

 the lives of all higher animals. It is il- 

 lustrated by the diagram, Fig. 81. 



A and B are two organs (or other 

 structures) whose action is to be co-ordi- 

 nated by means of the nervous system. 

 They are not directly connected by nerves, 

 but both are connected with a nerve- 

 centre, C, which is a nerve-cell or group 

 of nerve-cells situated in one of the gan- 

 glia. If A be thrown into action, an af- 

 ferent impulse travels to C, excites the 

 nerve-centre, and causes an efferent im- 

 pulse to travel OUt to B, which is thereby FlG . SI. Diagram to illustrate reflex 



thrown into action also, or is modified in action; A and B, two organs; c, a 



, . nerve-centre; the paths of the im- 



respect to actions already going on. pulses are indicated by the arrows. 



Thus the actions of A and B are co- 



unt incited through the agency of (7; the whole chain of events constituting 



a reflex fiction. 



For example, let A be the skin and B a certain group of muscles. If 

 the skin be touched, afferent impulses travel inwards to nerve-centres 



