CHAPTER XXIX 



THE FUNCTIONAL DIVISIONS OF THE NERVOUS SYSTEM 



It is usual to describe and to refer to a nerve with regard to 

 the segment of the body in which it finds itself. So one may 

 speak of the facial (7th) nerve, or of the 2nd spinal nerve, and 

 designate by these terms well-marked structures, visible by 

 dissection. Nerves are composed of fibres formed of long 

 filaments (or axons) which are produced by cells (neurons), 

 the " bodies " and nuclei of which are situated in the brain 

 and spinal cord, or in the swellings on certain nerves, called 

 ganglia. But all the fibres of any given segmental nerve do 

 not serve the same function. The function of a nerve is to 

 conduct impulses. If the conduction is towards the brain 

 and spinal cord (which together are called the central nervous 

 system) from sense-organs, the fibres are called afferent or 

 sensory. If the conduction is from the central nervous 

 system outwards towards muscles or glands, the fibres are 

 called efferent or motor. Sense-organs may be of many 

 different kinds and appreciate various sorts of stimuli, such as 

 light, sound, pressure, vibration, pain, etc., but from the fact 

 that they do receive these stimuli they are called receptors. 

 On the other hand, muscles and glands are structures which 

 " do something," and are consequently called effectors. 



A large part of the life of an animal is taken up with adjust- 

 ing itself to different conditions, and these conditions may be 

 of two kinds. There is the outside world with which the 

 animal keeps in touch by means of its receptors at or near the 

 skin : eyes, ears, lateral-line organs, and the skin itself. These 

 are the exteroceptors. The movements which the animal 

 makes in response to the outside world are largely locomotory, 

 and brought about by the muscles of the body- wall and limbs. 



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