280 



THE NERVOUS SYSTEM 



Posterior Fissure 



its fibers pass into the brain, to the fine gray filament 

 which forms its termination in the lumbar vertebrae. 

 A cross section shows it to be composed of white fibrous 

 matter, surrounding a central core of gray cellular matter 

 (Fig. 127). The gray matter presents in section a 

 rough outline of the letter H. In the middle of the 

 cross bar, or isthmus, connecting the two sides of the let- 

 ter, is a minute chan- 

 nel which extends the 

 whole length of the 

 cord and on into 

 the brain. The ends 

 of the letter H which 

 point forward are 

 club-shaped and are 

 known as the anterior 

 horns of the gray mat- 

 ter of the cord, while 

 those pointing back- 

 ward are pointed and 

 are called posterior 

 horns. The last cut 

 through the white 

 matter nearly to the 

 surface of the cord. 

 The white matter of the cord is composed of medullated 

 fibers, running for the most part longitudinally, together 

 with fibrous connective tissue and neuroglia. These fibers 

 are arranged in several different strands, or bundles, and 

 their areas have been carefully mapped out and named. 

 In each half of the cord is an anterior, a lateral, and a pos- 

 terior column, named from their positions and separated 

 from one another by the shallow depressions seen in the 



Anterior Fissure 



Fig. 127. Diagram of a cross section of 

 the spinal cord, showing the divisions 

 of the white and gray matter. 



The right half shows the threefold division 

 of the white matter commonly described ; the 

 left half shows the physiological divisions as 

 they are at present understood. 



