THE NERVOUS SYSTEM. 109 



Kill a frog and keep it in 5 per cent formalin for three 

 or four days. Cut open the body cavity along the ventral 

 surface, lay aside the intestines, and look for the spinal 

 nerves at the back of the body cavity. 



Notice : 



a. Their number and arrangement with reference to 

 the vertebrae. 



b. Their roots, the anterior and posterior. What is the 

 size and structure of each ? On which is the ganglion ? 



c. Their distribution. To what portions of the body do 

 they extend ? 



E. Nerve Fibres. 



Materials. A frog. Chloroform. Normal salt solution. Cotton 

 batting. 



Apparatus. Bell- jar. 



Cut off a quarter of an inch of a perfectly fresh nerve, 

 and put it on a slide without any liquid. Press one end 

 against the slide, and tease out the other end by passing 

 a needle lengthwise through the fibres. Spread the fibres 

 out like a fan, add a drop of normal salt solution, and 

 cover. 



Notice : 



a. The relative size and arrangement of the nerve fibres. 



b. The connective tissue running amongst and around the 

 fibres. 



c. The primitive sheath is a thin, transparent covering on 

 the outside. 



d. The medullary sheath, also transparent, is inside the 

 primitive sheath and consists of a fatty substance. 



