THE SYMPATHETIC NERVOUS SYSTEM 345 



branches and fine axons (Figs. 251, A ; 253). The fine axons represent the rami- 

 fications of preganglionic fibers and they degenerate when the connection 

 between the ganglion and the central nervous system is severed (Ranson 

 and Billingsley, 1918). Similar fibers pierce the capsules surrounding the 

 cells and intertwine with the intracapsular dendrites. No doubt synaptic 

 relations are established between the axonic and dendritic ramifications in 

 these plexuses. 



Another and very characteristic type of synapse is established in the peri- 

 cellular plexuses, formed by the terminal ramifications of preganglionic fibers upon 

 the surface of the cell bodies of postganglionic neurons. Huber (1899) showed 

 that fibers from the white rami branch repeatedly in the sympathetic ganglia 

 and that the branches terminate in subcapsular pericellular plexuses (Figs. 254, 

 255). 



In the sympathetic ganglia of the frog the pericellular plexus seems to be the only type 

 of synapse and there is no intercellular plexus. In the mammalian sympathetic ganglion 

 these pericellular plexuses are harder to demonstrate and are probably less numerous, while 

 the intercellular plexus is much in evidence. It is well established that one preganglionic 

 fiber may be synaptically related to several postganglionic neurons, probably in some in- 

 stances to as many as thirty or more (Ranson and Billingsley, 1918). 



COMPOSITION OF SYMPATHETIC NERVES AND PLEXUSES 



Some of the sympathetic nerves are as well myelinated as the cerebrospinal 

 nerves and present a white glistening appearance. This is true, for example, of 

 the cervical portion of the sympathetic trunk, the white rami, and the splanch- 

 nic nerves. Such white sympathetic nerves are composed at least in large part 

 of fibers running to and from the central nervous system. Other nerves like 

 the gray rami and branches to the blood-vessels are gray, because they are com- 

 posed chiefly of unmyelinated postganglionic fibers. In preceding paragraphs 

 we have shown that there are probably no association or sensory neurons in 

 the sympathetic ganglia; and, if this be true, there are no axons, arising from such 

 cells, in the sympathetic nerve trunks and plexuses. These nerves and plexuses 

 are composed of the following three kinds of fibers (Fig. 256) : (1) Preganglionic 

 visceral efferent fibers, which are of small size and myelinated, have their cells 

 of origin in the cerebrospinal axis, and terminate in the sympathetic ganglia. 

 (2) Postganglionic fibers, which are for the most part unmyelinated, have their 

 cells of origin in the sympathetic ganglia and terminate in involuntary muscle or 

 glandular tissue. (3) Visceral afferent fibers, which include myelinated fibers 

 of all sizes as well as many that are unmyelinated, have their cells of origin in 



