236 PHYSIOLOGY OP CENTRAL NERVOUS SYSTEM. 



in the third cranial nerve and passing via the ciliary ganglion; 

 second, from the bulbar region, emerging in the seventh, ninth, and 

 tenth cranial nerves; third, from the thoracic spinal nerves (first 

 thoracic to fourth or fifth lumbar) and passing in general via the 

 ganglia of the sympathetic chain; fourth, from the sacral region 

 by way of the so-called nervus erigens supplying the descending 

 colon, rectum, anus, and genital organs. The autonomic fibers at 

 their origin in the central nervous system that is, while pregan- 

 glionic fibers are all possessed of a small medullated sheath, 

 having a diameter of 1.8 p. to 4 //. The postganglionic fiber is in 

 most cases non-medullated, but this is by no means an invariable 

 rule. In many cases the axons from sympathetic cells possess 

 distinct, although small, myelin sheaths. 



The Nicotin Method. The course of the autonomic fibers 

 has been traced in many cases to their corresponding sympathetic 

 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- 

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

 therefore, that the nicotin paralyzes the connection of the pre- 

 ganglionic 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 this 

 fiber will pass through several ganglia before making final con- 

 nections 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 Nerves. 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 communicantes. In the tho- 

 racic 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 

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



