THE NERVOUS SYSTEM. 155 



like the arteries, generally lie remote from the surface of 

 the body, and are well protected from injury. 



FIG. 13. THE BASE OF THE BRAIN. 



12. The Spinal Cord. The spinal cord, or "marrow," 

 is a cylindrical mass of soft nervous tissue, which occupies 

 a chamber, or tunnel, fashioned for it in the spinal column 

 (Fig. 44). It is composed of the same substances as the 

 brain ; but the arrangement is exactly reversed, the white 

 matter encompassing or surrounding the gray matter in- 

 stead of being encompassed by it. The amount of the 

 white substance is also greatly in excess of the other ma- 

 terial. A vertical fissure partly separates the cord into 

 two lateral halves, and each half is composed of two sep- 

 arate bundles of fibres, which are named the anterior and 

 posterior columns. 



13. These columns have entirelv different uses, and each 



12. The spinal cord ? Of what composed ? How divided ? Each half? 



13. Uses of these columns ? Importance of this part of the nervous system ? 

 How protected? 



