238 THE NERVOUS SYSTEM 



The Spinal Cord. 



The spinal cord occupies the spinal canal and is about 

 eighteen inches long, extending from the foramen magnum 

 to the lower border of the first lumbar vertebra. Its distal 

 extremity is in the shape of a slender filament known as 

 iilum terminate, 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 foramina. 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 through- 

 out with pia mater. It is bounded posteriorly by the an- 

 terior 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 fis- 

 sures, marking 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 some- 

 what after the manner of the letter H, each lateral portion 



