250 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



nerve cells partly by the method of secondary degeneration and 

 partly by the use of nicotin, as first described by Langley and 

 Dickinson.* These authors have shown that after the use of 

 nicotin, either injected into the circulation or painted upon the 

 ganglion, stimulation of the preganglionic fiber in any part of its 

 course fails to give any response, while stimulation of the post- 

 ganghonic fiber, on the contrary, is still effective. It would seem, 

 therefore, that the nicotin paralyzes the connection (the synapse) 

 of the preganglionic fiber with the sympathetic nerve cell, and by 

 means of the local application of the drug it is possible in many 

 cases to pick out the ganglion in which the preganglionic fiber really 

 ends. For it often happens that in the sympathetic trunk a 

 preganglionic fiber will pass through several ganglia before 

 making final connections with a sympathetic cell. So far, the 

 course of these fibers has been traced most successfully in the 

 case of the nerves supplying the sweat-glands, blood-vessels, and 

 especially the erector muscles of the hairs, the so-called pilomotor 

 nerve-fibers. The visible result of stimulation in the last case 

 gives a ready means of determining the presence of the fibers. 



General Course of the Autonomic Fibers Arising from the 

 Spinal Cord — ^Thoracic Autonomics. — It has long been known 

 that the spinal nerves are connected with many of the ganglia of the 

 sympathetic chain by fine branches known as the rami communi- 

 cantes. In the thoracic and lumbar regions (first thoracic to second 

 or fourth lumbar) these rami consist of two parts, a white and a 

 gray ramus, the difference in color being due to the fact that the 

 white rami are composed almost entirely of medullated fibers, 

 while the gray rami are largely non-medullated. In the cervical, 

 lower lumbar, and sacral regions the rami consist only of the gray 

 part. Physiological experiments show that the white rami consist 

 of preganglionic fibers that arise from nerve cells in the spinal cord, 

 pass out by way of the anterior roots, enter the white ramus, and 

 thus reach the sympathetic chain. On entering this latter the fiber 

 may not end at once in the ganglion at which it enters, but may pass 

 up or down in the chain for some distance. Eventually, however, it 

 ends around a sympathetic nerve cell and the path is then con- 

 tinued by the axon from this cell as the postganglionic fiber. The 

 gray rami consist of these latter fibers, which return from the sym- 

 pathetic chain to the spinal nerves and are then distributed to the 

 areas supplied by these nerves, particularly the cutaneous areas, 

 since the skin branches are the ones that supply the sweat glands, 

 the blood-vessels, and the erector muscles of the hairs. It wdll be 

 noted that the fibers that pass from a given spinal nerve — say, the 

 twelfth thoracic — by a white ramus to enter the sympathetic chain 

 * "Proceedings, Royal Society," 1889, 46, 423. 



