THE SYMPATHETIC NERVOUS SYSTEM 347 



248, 257). To each spinal nerve there runs a gray ramus from the sympathetic 

 trunk. The white rami, on the other hand, are more limited in distribution and 

 unite the thoracic and upper four lumbar nerves with the corresponding portion 

 of the sympathetic trunk. 



The white rami consist of visceral afferent and preganglionic visceral efferent 

 fibers directed from the central into the sympathetic nervous system. They 

 contribute the great majority of the ascending and descending fibers of the 

 sympathetic trunk (Fig. 257). While some of the fibers may terminate in the 

 ganglion with which the white ramus is associated, and others run directly 

 through the trunk into the splanchnic nerves, the majority of the fibers turn 

 either upward or downward in the trunk and run for considerable distances within 

 it (Fig. 250). The fibers from the upper white rami run upward, those from the 

 lower white rami downward, while those from the intermediate rami may run 

 either upward or downward. The cervical portion of the sympathetic trunk 

 consists almost or quite exclusively of ascending fibers, the lumbar and sacral 

 portions of the trunk largely of descending fibers from the white rami. The 

 afferent fibers of the white rami merely pass through the trunk and its branches 

 to the viscera. The preganglionic fibers, with the exception of those which run 

 out through the splanchnic nerves, end in the gang^a of the trunk. Here they 

 enter into synaptic relations with the postganglionic neurons. The majority 

 of the postganglionic neurons, located in the ganglia of the sympathetic trunk, 

 send their axons into the gray rami (Figs. 250, 256). 



The gray rami are composed of postganglionic fibers directed from the sym- 

 pathetic trunk into the spinal nerves. These unmyelinated fibers, after joining j 

 the spinal nerves, are distributed with them as vasomotor, secretory, and pilo- 

 motor fibers to the blood-vessels, the sweat glands, and the smooth muscle of 

 the hair-follicles. 



Especially in the cervical region there are other important branches from the 

 sympathetic trunk, which resemble the gray rami in structure and which convey 

 postganglionic fibers to certain of the cranial nerves and to the heart, pharynx, 

 the internal and external carotid and thyroid arteries, and through the plexuses 

 on these arteries to the thyroid gland, salivary glands, eye, and other structures 

 (Figs. 248, 250, 257). 



The cranial portion of the sympathetic trunk consists of three ganglia bound 

 together by ascending preganglionic fibers from the white rami. In the cat it has 

 been shown to contain few if any sensory or postganglionic fibers. The superior 

 cervical ganglion is the largest of the three ganglia and from it there are given off 



