598 THE NERVOUS SYSTEM 



ment in communication with many other levels of both the spinal cord 

 and brain. 



The center of the gray commissure contains the central canal which 

 lies in the axis of the spinal cord and is continuous above with the ven- 

 tricles of the brain. It represents the remains of the fetal neural canal ; 

 and in the young subject is still patent, filled with cerebrospinal fluid, 

 and lined by columnar cells which are frequently ciliated. In older 

 subjects the cells of the lining epithelium have usually lost their cilia, 

 and the lumen of the canal is more or less filled by cell proliferation 

 which involves not only the lining epithelium but also the surrounding 

 glia cells. The central canal is immediately surrounded by a peculiar 

 gelatinous tissue in which are many glia cells. This mass is called the 

 substantia gelatinosa centralis. A similar area of gelatinous tissue occurs 

 near the extremity of the dorsal horns, and is called the substantia 

 gelatinosa Rolandi. The latter, however, is said to contain only a 

 scanty supply of neuroglia. 



The white matter forms a covering or shell around the central 

 gray mass. It increases in thickness from below upward. This pecu- 

 liarity is the result of the progressive influx of centripetal fibers, and 

 a corresponding contribution of centrifugal fibers, through the spinal 

 nerves of each successive segment. 



The dorsal median septum extends inward from the shallow sulcus 

 on the dorsal surface of the spinal cord to the central gray commissure, 

 and divides the dorsal mass of white matter into two white columns, 

 lying on either side of the median line, and bounded laterally by the 

 dorsal horns of gray matter and the dorsal nerve roots. The ventral 

 median sulcus in a similar manner, splits the ventral portion of white 

 matter into the two ventral white columns. This sulcus, however, 

 does not penetrate all the way to the gray commissure but leaves an 

 interval of white matter containing many transverse and obliquely 

 disposed nerve fibers. The ventral or white commissure thus formed 

 connects the two ventral columns of white matter. 



The spinal cord is thus divided into two lateral and symmetrical 

 halves by a plane passing through the ventral and dorsal median sulci 

 and the central canal. Each lateral half includes a central curved mass 

 of gray matter completely surrounded, except at the gray commissure, 

 by the white matter. The latter is subdivided into a ventral, lateral, 

 and dorsal column, each of which extends the entire length of the 

 spinal cord and is apparently (to the naked eye only) continuous above 

 with a similar column in the medulla oblongata. 



