PEEIPHEEAL-NEEVE BOOTS, SPINAL GANGLIA, SPINAL COED. 'S-Ll 



cylinders from the cells of these ganglia extend peripherally in the sensory 

 nerves, and centrally into the cord. 



The central branches form what we call the posterior roots. With them 

 pass other fibers, however, which do not arise from the cells in the spinal 

 ganglion. 



A case observed by Leonowa shows how independent of each other are the 

 dorsal roots and the spinal cord in their development. The entire elementary cord 

 was missing in a monster, but the spinal ganglia were present, and from them arose 

 peripheral nerves, besides entire bundles of posterior roots, which, of course, ended 

 free in the vertebral canal. 



The ganglion-cells of the dorsal horns are smaller than those of the 

 ventral horns. For the most part they have a spindle-shaped form. Their 

 axis-cylinders divide either soon after quitting the cell-body into a fine 

 arborization like that shown in Fig. 152, g, or they enter farther into the 

 substance of the cord. They never become peripheral nerve-fibers. 



Two groups are readily distinguished by the naked eye in the posterior 

 horns, from their form and color. The group called columna vesicularis, 

 first studied by Stilling, afterward more exactly l^y Clarke, generally known 

 as Clarke's column, lies about where the -^-entral and dorsal horns come to- 

 gether (Fig. 216, s). Besides the cells, it contains a fine net-work of fibers, 

 and bundles of especially delicate fibers running longitudinally. On cross- 

 section its rounded form is distinct, and can be followed from about the 

 end of the cervical enlargement to the beginning of the lumbar enlarge- 

 ment. Single cells, however, in appearance similar to Clarke's cells, are met 

 with higher up even to the medulla oblongata. 



Still more sharply defined than the Stilling-Clarke column appears the 

 substantia gelatinosa Rolandi, in the gray matter of the posterior horn. It 

 lies near the point of the horn, and is perforated by numerous afferent fibers 

 from the posterior root. Until our staining-methods were sufficiently de- 

 veloped, the importance of this peculiar, glass-like, translucent substance 

 remained in doubt. Only in the last few years has it been discovered that it 

 contains cells of similar character to those lying in the posterior horns. 



What became of their axis-cylinders seemed especially difficult to determine 

 in a region so densely filled with axis-cylinders and collaterals. Recently, however, 

 S. EamOn y Cajal and particularly v. Lenhossek were able to show that the axis- 

 cylinders of the spindle-shaped cells (marginal cells in Fig. 219), passing around the 

 periphery of the Eolandic substance, arrive at the dorsal portion of the lateral col- 

 umn, and that those of the narrower, stellate cells of the Rolandic substance itself 

 enter the neighboring posterior eolunm and the so-called marginal zone of the poste- 

 rior horn. The latter cells possess not only one, but several processes of the his- 

 tological character of an axis-cylinder. 



