CHAPTER XVII 

 THE NERVOUS SYSTEM 



THK brain and the spinal cord with the nerves proceeding from 

 them constitute the nervous system of the body. The brain 

 and spinal cord being the central organs form, taken together, 

 the central nervous system. 



Nerves are afferent or efferent. Nerves carry impulses 

 , either to the central nervous system, being then called afferent 

 nerves, or from it, being then called efferent nerves. We have 

 .already met with examples of these two classes of nerves. 

 The nerves which carry to the central nervous system im- 

 pulses from the skin or from the organs of the special senses, 

 such as the eye or the ear, are afferent nerves, and, since these 

 impulses arouse in us the sensations of touch, sight, or hear- 

 ing, are sensory nerves. The nerves which carry impulses 

 from the central nervous system to the muscles are efferent 

 nerves, and, if the impulses arc such as to cause muscles to 

 contract, are motor nerves. 



Structure of Nerves. If a piece of nerve, such as may 

 be taken from a limb of a rabbit, be frayed out at its end with 

 a needle, it will be seen to consist of a number of fine fibres, 

 the nerve fibres. These separate fibres, held together by 

 a little fine connective tissue partly lying between them and 

 partly wrapping round them, make up the nerve. The nerve 

 fibres themselves vary much in size, the breadth of a medium- 

 sized one being ^^th of an inch, that is about ^th the 

 diameter of a striated muscle fibre. A nerve fibre consists of a 

 central strand of soft semi-solid protoplasmic substance called 

 tin- IK-HI axis or axis cylinder. This is covered by two sheaths. 

 Tin- inner ->h<-.ith, tin- one next to the axis cylinder, is formed 



N 



