596 THE NEEVOUS SYSTEM 



THE SPINAL CORD 



STRUCTURE 



The spinal cord (Fig. 517) consists of a considerable mass of central 

 gray matter which is surrounded by a layer of medullated nerve fibers, 

 the white matter. 



The gray matter comprises two lateral portions united by a central 

 commissure (gray commissure) in transverse section resembling the 

 letter H. Each lateral portion includes a ventral and a dorsal horn 

 with an intervening deeper portion, the central mass or 'intermediate 

 zone' of Golgi. Each horn or cornu ends in a head or caput united 

 with the intermediate portion or basis cornu by a neck or cervix cornu. 



The ventral is somewhat broader than the dorsal horn. Its cells 

 supply axons, which, after uniting into bundles, pass ventralward 

 through the white matter to form the ventral (anterior) nerve roots. 



The spinal cord may be considered as consisting of ontogenetic 

 segments whose number corresponds to the number of the spinal nerves. 

 Hence each segment contains the ventral horn cells whose axons form 

 the ventral root of the corresponding spinal nerve. 



In an entirely similar manner the dorsal horns of the gray matter 

 receive a large portion of the incoming fibers of the dorsal roots, which 

 in large part form end brushes around the cells of the dorsal horns 

 and the intermediate zone (Fig. 518). 



The dorsal roots enter through a distinct longitudinal groove, the 

 dorsolateral sulcus. At the exit of the ventral roots there is, how- 

 ever, only a broad, shallow indentation, these roots making their exit 

 in isolated bundles distributed through a vertical plane of considerable 

 width. The dorsal root fibers of each segment, on the other hand, enter 

 in a single compact mass. 



The gray matter consists of a dense tangle of nerve cells and non- 

 medullated fibers, both axons and dendrons, together with neuroglia 

 and blood-vessels. The fibers of a given area are derived not only 

 from nerve cells in their immediate vicinity, but also include many 

 processes which come from very distant regions. The gray reticulum 

 is thus supplied from fibers of the ventral and dorsal nerve roots, to- 

 gether with innumerable collaterals, not only from the root fibers, but 

 more especially from those fibers which collectively form the many 

 large tracts passing up and down the spinal cord and placing each seg- 



