238 THE NERVOUS SYSTEM 



in the shape of a slender filament known as filum lerminale, 

 which is gray in color. The sacral and coccygeal nerves, having 

 taken origin from the cord in the dorsal region, pass downward 

 in the canal to find exit through the sacral and coccygeal fora- 

 mina. This collection of nerves thus passing down is known as 

 the cauda equina. 



Gross Divisions of the Spinal Cord in Section. Cross sec- 

 tion of the cord reveals the division of its substance into two 

 lateral halves connected by the anterior and posterior commissures. 

 In the center of the cord, and between these commissures, is a 

 small opening, the central canal of the cord, communicating with 

 the fourth ventricle above. This division of the substance of the 

 cord into lateral halves is effected by the two median fissures, 

 anterior and posterior. The former is the more clearly marked, 

 and is lined thoughout with pia mater. It is bounded pos- 

 teriorly by the anterior white commissure. The posterior median 

 fissure is not lined with pia mater and extends anteriorly as far 

 as the posterior gray commissure. It is to be noted that there 

 are both anterior and posterior gray commissures, but only one 

 white commissure (anterior), which is bounded posteriorly by 

 the anterior gray commissure. 



Besides the anterior and posterior median fissures there are 

 also on each side antero-lateral and postero-lateral fissures, mark- 

 ing the lines of exit of the anterior and posterior roots of the 

 spinal nerves. These are not well defined. They divide the 

 cord into anterior, posterior and two lateral columns. 



Arrangement of Gray Substance. The' disposition of the 

 gray substance in the cord (in transverse section) is somewhat 

 after the manner of the letter H, each lateral portion represent- 

 ing the anterior and posterior cornua of gray matter for that side, 

 and being connected to the corresponding portion of the other 

 side by the commissures embracing the central canal. The 

 anterior cornua are shorter and thicker than the posterior. 

 From these issue the anterior and posterior roots respectively of 



