78 LABORATORY METHODS OF 



from before backwards, and the short arm is directed 

 from side to side, joining the long arms at their mid- 

 dle points. The long arms are comma-shaped, with 

 the big end of the comma directed anteriorly, i. e., to- 

 wards the front of the cord. This end does not ap- 

 proach the surface as near as the small end of the 

 comma. The small end is called the posterior horn, 

 while the large end is called the anterior horn of the 

 cord. In the center of the transverse arm of the H. is 

 seen a small opening, called central canal, lined with 

 ciliated epithelial cells, and containing a fluid called cer- 

 ebro spinal fluid. The transverse arm of the H is called 

 the gray commissure, and the white matter imme- 

 diately in front and behind it is called respectively an- 

 terior and posterior white commissure. Some histolo- 

 gists divide the gray commissure into anterior and 

 posterior, with reference to central canal. There is a 

 narrow cleft in the white substance, beginning at the 

 anterior median periphery, and extending to within a 

 short distance of the gray commissure, called anterior 

 median fissure. A narrower but similar fissure is 

 seen on the posterior side of the gray commissure, 

 called posterior median fissure. On each limb of 

 the letter H of the cord there are three horns; an- 

 terior, posterior and lateral horns, located as their 

 names indicate. From the anterior horn emerge the 

 anterior roots, and the posterior horns receive the pos- 



