SYMPATHETIC NERVOUS SYSTEM. 237 



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 herves 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 will 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 

 do not return as postganglionic fibers by the gray ramus to the 

 same spinal nerve. On the contrary, the gray ramus of the twelfth 

 thoracic may consist of the postganglionic portion of autonomic 

 fibers that enter the sympathetic through a white ramus of 

 one of the higher thoracic nerves. In general, we may say 

 that there is a great outflow of autonomic fibers, including 

 vasomotor, sweat, and pilomotor fibers, in the white rami commu- 

 nicantes from the first or second thoracic to the second or fourth 

 lumbar nerves. Those of these fibers that are to be distributed to 

 the skin areas of the body head, limbs, and trunk return by way 

 of the gray rami to the various spinal nerves and are distributed with 

 these nerves, the distribution being somewhat different in different 

 animals and for the different varieties of fibers. Those fibers that 

 are distributed eventually to the blood-vessels, glands, and walls 

 of the viscera have a different course from those supplying the 

 glands, blood-vessels, and plain muscle of the head region. For 

 the head region the fibers after entering the sympathetic chain pass 

 upward along the cervical sympathetic to end in the superior 

 cervical ganglion; thence the path is continued by postganglionic 

 fibers which emerge by the various plexuses that arise from this 

 ganglion. For the abdominal and pelvic viscera the fibers (particu- 

 larly the rich supply of vasoconstrictor fibers), after entering the 

 sympathetic chain, emerge, still as preganglionic fibers, by the 

 splanchnic nerves that run to the celiac ganglion or in the branches 

 connecting with the inferior mesenteric ganglia, and then become 

 postganglionic fibers (see Fig. 105). The details of the course of 

 the vasomotor, sweat, visceromotor fibers to the different regions, 

 the cardiac fibers, etc., will be given in the appropriate sections. 



General Course of the Autonomic Fibers Arising from the 

 Brain. These fibers leave the brain in the third, seventh, ninth, 



