THE SYMPATHETIC SYSTEM. 211 



cated in Fig. 109 they are located in a zone of the gray matter 

 between the dorsal and ventral horns and extending from the 

 central canal to the lateral horn and the base of the dorsal horn. 

 This is the portion of the gray matter which has previously been 

 called the visceral efferent column (p. 200). As already stated, 

 the fibers from this column in lower vertebrates pass out through 

 the dorsal roots, but' in the mammals which have been most used 

 for the study of the sympathetic they pass out by way of the 

 ventral roots. 



The fibers are small myelinated fibers, usually less than 4 p 

 in diameter, which enter the ganglia of the chain and find endings 

 in relation with the cells of these or other sympathetic ganglia. 

 They may (a) end in the chain ganglion first entered, (b) run 

 through it to end in another ganglion of the chain, (c) end in one 

 of the prevertebral ganglia, or (d) in one of the peripheral ganglia. 

 During their course the fibers may give collaterals to one ganglion 

 and pass on to end in another. The method of ending of these 

 fibers is important. They pierce the capsule of the sympathetic 

 cells and their branches interlace to form more or less complex 

 plexuses or baskets immediately in contact with the sympathetic 

 cells. By means of these pericellular baskets the impulses sent 

 out from the central nervous system are transferred to the sympa- 

 thetic excitatory cells. Such endings are found in the preverte- 

 bral and peripheral ganglia as well as in the ganglia of the chain, 

 and it is believed that the great majority if not all of the excitatory 

 cells of the sympathetic are thus brought under the direct influence 

 of the central nervous system. 



(3) Sympathetic excitatory cells. The sympathetic cells have 

 in general the same forms as cells in other parts of the nervous 

 system. They may have a single process which is a neurite, or a 

 neurite and one dendrite, or a neurite and several dendrites. 

 The last is the rule -for the great majority of cells, at least in mam- 

 mals. The cell-body is surrounded by a nucleated capsule 

 which is pierced by the dendrites. Outside the capsule the 

 dendrites divide and subdivide into very delicate branches which 

 interlace with those of other cells to form a rich plexus. In most 

 cases the dendrites end within the ganglion in which the cell lies, 



