STEUCTUBE OF THE SPINAL COED. 



63 



epithelium, and that around this canal lies a very finely organized tissue, 

 the gray substance. This is, in turn, surrounded by nerVe-fibers, most of 

 which run longitudinally. These longitudinal nerve-fibers represent the 

 tracts. 



The posterior root-fibers (dorsal roots) enter the gray substance after 

 a longer or shorter course, usually in the posterior tracts, though in fishes 

 in laterally located tracts as well. After entering the gray substance the 

 fibers divide into fine terminal ramifications, which join a close net-work 

 of fibrillffi, which fills the whole posterior segment of the gray matter. 

 They probably come into contact there with the processes of smaller cells. 



Pig. 27. — Several forms of spinal ganglion-cells, showing the cell-bodies, the 

 afferent sensory nerves, and the dorsal roots {Hintere Wureel). 



The gray matter has been divided into posterior and anterior horns. A 

 better expression would be: dorsal and ventral columns, because the projec- 

 tions are really columns- which extend longitudinally throughout the whole 

 length of the cord. The dorsal columns are formed from the fiber mesh-work 

 of the posterior roots. The ventral columns are formed from the collec- 

 tion of ganglion-cells which give rise to the anterior roots. 



The development of the dorsal and ventral columns depends naturally 

 upon the number of nerve-fibers which are connected with them. The fishes 

 furnish a good example of this. In this class of vertebrates a large part of 

 the body-surface is supplied with sensory fibers, not from the spinal nerves. 



