184 THE HUMAN BODY. 



out impulses by the many nerve-fibres connected with 

 them. By such means a single nerve-fibre can act upon 

 an extended region of the Body. In other cases it seems 

 likely that a feeble nervous impulse reaching an irritable 

 nerve-cell excites changes in this comparable to those pro- 

 duced in a muscle when it is stimulated; and the cell by 

 its discharge sends on reinforced nerve impulses along its 

 other branches or one of them. 



Excitant and Inhibitory Nerves. The great majority 

 of the nerve-fibres of the Body when they convey nervous 

 impulses to a part arouse it to activity; they are excitant 

 fibres. There is, however, in the Body another very im- 

 portant set which arrest the activity of parts and which 

 are known as inhibitory nerve-fibres. Some of these check 

 the action of central nervous organs, and others the work 

 of peripheral parts. For instance taking a pinch of snuff 

 will make most persons sneeze; it excites centrally acting 

 fibres in the nose, these excite a centre in the brain, 

 and this in turn sends out impulses by efferent fibres which 

 cause various muscles to contract. But if the skin of the- 

 upper lip be pinched immediately after taking the snuff, in 

 most cases the reflex act of sneezing, which the Will alone- 

 could not prevent, will not take place. The afferent im- 

 pulses conveyed from the skin of the lip have " inhibited"" 

 what we may call the " sneezing centre;" and afford us 

 therefore an example of inhibitory fibres checking a centre. 

 On the other hand, the heart is a muscular organ which 

 goes on beating steadily throughout life; but if the branches 

 of the pneumogastric nerve going to it be excited, the beat 

 of the heart will be stopped; it will cease to work and lie in a 

 relaxed resting condition: in this we have an instance of an 

 inhibitory nerve checking the activity of a peripheral organ. 



Classification of Nerve-Fibres. Nearly all the nerve- 

 fibres of the Body fall into one of two great groups corre- 

 sponding to those which carry impulses to the centres 

 and those which carry them out from the centres. 

 The former are called afferent or centripetal fibres and the 

 latter efferent or centrifugal. Since the impulses reaching 

 the centres through the afferent fibres generally cause sen- 



