754 



NEUROLOGY 



C.I. 



C2. 



C5. 



C.8. 



contains both neuroglia cells, and small nerve cells. Between the anterior an<_ 

 posterior columns the gray substance extends as a series of processes into the 



lateral funiculus, to form a net-work called the for- 

 matio reticularis. 



The quantity of gray substance, as well as the form 

 which it presents on transverse section, varies mark- 

 edly at different levels. In the thoracic region it is 

 small, not only in amount but relatively to the sur- 

 rounding white substance. In the cervical and lum- 

 bar enlargements it is greatly increased : in the latter, 

 and especially in the conus medullaris, its proportion 

 to the white substance is greatest (Fig. 665). In 

 the cervical region its posterior column is compara- 

 tively narrow, while its anterior is broad and ex- 

 panded; in the thoracic region, both columns are 

 attenuated, and the lateral column is evident; in 

 the lumbar enlargement, both are expanded; while 

 in the conus medullaris the gray substance assumes 

 the form of two oval masses, one in each half of the 

 cord, connected together by a broad gray commissure. 

 The Central Canal (canalis centralis) runs through- 

 out the entire length of the medulla spinalis. The 

 portion of gray substance in front of the canal is 

 named the anterior gray commissure ; that behind it, 

 the posterior gray commissure. The former is thin, 

 and is in contact anteriorly with the anterior white 

 commissure: it contains a couple of longitudinal 

 veins, one on either side of the middle line. The 

 posterior gray commissure reaches from the central 

 canal to the posterior median septum, and is thin- 

 nest in the thoracic region, and thickest in the conus 

 medullaris. The central canal is continued upward 

 through the lower part of the medulla oblongata, and 

 opens into the fourth ventricle of the brain; below, 

 it reaches for a short distance into the filum termi- 

 nale. In the lower part of the conus medullaris it 

 exhibits a fusiform dilatation, the terminal ventricle; 

 this has a vertical measurement of from 8 to 10 

 mm., is triangular on cross-section with its base 

 directed forward, and tends to undergo obliteration 

 after the age of forty years. 



Throughout the cervical and thoracic regions the 

 central canal is situated in the anterior third of the 

 medulla spinalis; in the lumbar enlargement it is 

 near the middle, and in the conus medullaris it 

 approaches the posterior surface. It is filled with 

 cerebrospinal fluid, and lined by ciliated, columnar 

 epithelium, outside of which is an encircling band 

 of gelatinous substance, the substantia gelatinosa 

 centralis. This gelatinous substance consists mainly 

 of neuroglia, but contains a few nerve cells and 

 fibers; it is traversed by processes from the deep ends 

 of the columnar ciliated cells which line the central 

 canal (Fig. 667). 



Th.8. 



Th.12 



L.5. 



S.2. 



Coc. 



FIG. 660. Transverse sections of the 

 medulla spinalis at different levels. 



