THE NERVOUS SYSTEM NERVES 



177 



greyish nerve cells, has the power to generate at produce an impulse 

 as well as to conduct it it serves as a receiver and a transmitter 

 as well as a generator. 



We may compare a nerve centre to a galvanic battery, and 

 the nerves to the wires which convey the electric current. The 

 battery generates the electric current, which flows along one of 

 the wires, and returns to the battery by the other wire, thus com- 

 pleting the circuit. The battery, like the nerve centre, generates, 

 receives, and transmits ; while the wires of the battery, representing 

 the nerves, simply conduct. 



This simile may be yet further extended, for as one wire of the 

 battery takes away the current and the other 

 brings it back, so there are nerve fibres 

 which conduct nerve impulses only to a 

 nerve centre, while other fibres convey them 

 only from the centre. 



Those nerve fibres which conduct im- 

 pulses onto to a nerve centre are called 

 afferent (Lat\a^, to ; and fero, I carry) or 

 sensory fibres?\By means of these we are 

 capable of feeling pain or of experiencing 

 any other sensation; For example, when a 

 body vibrates rapidly, and the vibrations 

 are transmitted through the air to the ear, 

 they are taken up by the auditory nerve and 

 conveyed to the brain, producing the sensa- 

 tion called sound. Thus the auditory nerve 

 is an afferent or sensory nerve. 



Those nerve fibres which convey im- 

 pressions only from a nerve centre are 

 termed efferent (Lat. e, out ; and fero). 

 They are also called motor, because their 

 office is to produce motion. 1 They origi- Fig. 163. White Nerve 

 nate in a nerve centre, and they terminate Flbres ' Ma & nified - 

 in muscular fibres. When they receive an impulse from a nerve 

 centre, they convey that impulse to the fibres they govern, causing 

 them to contract. Thus, when we hear a sharp sound behind 

 us, we quickly turn the head, or start running. The sound vibra- 

 tions, on reaching the brain, set up a disturbance in the organ. 

 The irritation is then conducted through the efferent or motor 

 nerves which supply certain muscles of the neck or limbs, and 

 causes them to contract. 



1 The terms afferent and efferent are preferable to sensory and motor, since 

 the irritation of the former does not always produce sensation ; nor does motion 

 always result from the irritation of the latter. 



