214 THE NERVOUS SYSTEM 



chiefly white matter. It has, however, no convolutions, but is 

 subdivided by many parallel ridges, which, sending down gray 

 matter deeply into the white, central portion, give the latter 

 a somewhat branched appearance. This peculiar appearance 

 has been called the arbor vitce, or the " tree of life," from the 

 fact that when a section of the organ is made, it bears some 

 resemblance to the trunk and branches of a tree (Fig. 56, F). 

 In size, this cerebellum, or "little brain," is less than one- 

 eighth of the cerebrum. 



11. From the under surface of the cerebrum, and from the 

 front margin of the cerebellum, fibres collect together to form 

 the medulla oblongata (Fig. 57, Met), which, on issuing from 

 the skull, enters the spinal column, and then becomes known 

 as the spinal cord. From the base of the brain, and from the 

 sides of the medulla originate, also, the cranial nerves, of 

 which there are twelve pairs. These nerves are round cords 

 of glistening white appearance, and, like the arteries, generally 

 lie remote from the surface of the body, and are well protected 

 from injury. 



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

 cylindrical mass <>f soft nervous tissue, which occupies a 

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

 (Figs. 58 and 59). It is composed of the same substances 

 as the brain; but the arrangement is exactly reversed L the 

 white matter encompassing or surrounding the gray matter, 

 instead of being encompassed by it. The amount of the white 

 substance is also greatly in excess of the other material. A 

 vertical fissure partly separates the cord into two lateral halves, 

 and each half is composed of two separate bundles of fibres, 

 which are named the anterior and posterior columns. 



13. These columns have entirely different uses, and each of 

 them unites with a different portion of the nerves which have 

 their origin in the spinal cord. The importance of this part 

 of the nervous system is apparent from the extreme care taken 



11. Medulla oblongata? Cranial nerves? Their shape and position ? 



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 ? 



