NERVOUS SYSTEM. 2/ 



A Model of the Cerebro-Spinal Nervous System. A 



plaster of Paris or papier-mache model of the cerebro- 

 spinal nervous system will prove very helpful at this 

 point. A study of it will show how the spinal cord as 

 snugly and safely inclosed within the spinal column. At 

 the joints between adjacent vertebras there are openings 

 through which the spinal nerves pass out without danger 

 of being crushed, or even pinched when the backbone 

 bends. It will be noted that the nerves given off in the 

 regions of the shoulders and hips are large, while in the 

 middle of the back they are small. In the middle of 

 the back only the body-wall is to be supplied with nerves, 

 while we would naturally expect large nerves for the 

 powerful muscles of the fore and hind limbs. 



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 stand, or core, of semi- 

 transparent, gray material. 



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

 material around the axis cylinder. 



