NERVOUS SYSTEM. 2*] 



rabbit. The animal may be killed painlessly by shutting it in a tight 

 box with a sponge holding a teaspoonful of chloroform or ether. The 

 smaller the box, the less anesthetic necessary. A large glass jar inverted 

 over the animal is convenient. 



For a support, nail a foot of two-by-four scantling edgewise to a base 

 board. Lay the animal on this and tack out the feet. Slit and pull 

 aside the skin from the nose along the back to the base of the tail. 

 Remove the muscle along the sides of the back and neck. Between 

 the skull and the first vertebra is a space covered by a thin membrane. 

 This may be cut through with scissors. Then bone forceps may be 

 employed by inserting the point of a blade on each side of the cord and 

 cutting through the bone. In this way the whole of the dorsal portion 

 of the spinal column may be removed, exposing the spinal cord through- 

 out its entire length. With care the nerves and their roots may be 

 found. The nerves extending to the anterior limbs are easily traced, 

 but for the nerves to the posterior limbs more work is needed. 



Structure of Nerves. When we trace the sciatic nerve 

 outward, we find that it is continually subdividing. This 

 division continues until the branches are too small to be 

 seen by the naked eye. Microscopic examination shows 

 that a nerve is made up of a great number of fibers bound 

 together in a common sheath of connective tissue, as is 

 the case with muscle. When the nerve divides there is 

 ordinarily no true branching or forking, but certain of 

 the fibers simply separate from the rest, as in the separa- 

 tion of the fibers in floss silk. 



Structure of a Nerve Fiber. A single nerve fiber is 

 too small to be seen by the naked eye, being only about 

 one two-thousandth of an inch in diameter. It consists of 

 the following parts : 



1. The Axis Cylinder, a central strand, or core, of semi- 

 transparent, gray material. 



2. The Medullary Sheath is a layer of white, oily 

 material around the axis cylinder. 



