THE VASOMOTOR NERVES. 



611 



numerous nerve trunks, but especially in those distributed to the 

 skin and to the abdominal and pelvic organs. If, for instance, the 

 sciatic or the splanchnic nerve be cut, to avoid reflex effects, and 

 the peripheral end be stimulated, there will be a strong constriction 

 of the vessels, which may be detected by ocular inspection, blanch- 

 ing; by the increase in arterial pressure; or by the diminution in 

 volume of the organs. The vasoconstrictor fibers supplying these 

 two great regions arise immediately (postganglionic fibers) from 

 one or other of the ganglia constituting the sympathetic chain, or 

 from the large prevertebral ganglia (celiac ganglion, for instance) 

 directly connected with it. Ultimately, of course, they arise in the 

 central nervous system (preganglionic fiber), and it has been shown 

 that, for the regions under consideration, they all, with a few com- 

 paratively unimportant exceptions, leave the spinal cord in the 

 great outflow that takes place in the thoracic region from the second 



Fig. 251. Schema to show the path of the preganglionic and postganglionic portions 

 of a vasoconstrictor nerve fiber: a, Anterior root, showing the course of the preganglionic 

 fiber as a dotted line; d, v, dorsal and ventral branches of the bpinal nerve; r, the ramus 

 communicans; g, the sympathetic ganglion. The postgangliocic fibers in each ramus come 

 from the sympathetic ganglion with which it is connected. The preganglionic fibers enter- 

 ing at any ganglion may pass up or down to end in the cells of some other ganglion. 



thoracic to the second lumbar nerves (p. 254). In this outflow 

 they are mixed with other autonomic fibers, such as the sweat 

 fibers, pilomotor fibers, accelerator fibers to heart, pupilodilator 

 fibers, visceromo tor fibers, etc. Emerging in the anterior roots, they 

 pass to the sympathetic chain by way of the corresponding ramus 

 communicans. Having reached the chain, they end in one or other 

 of the ganglia, not necessarily in the ganglion with which the ramus 

 connects anatomically. The preganglionic fibers for the blood- 

 vessels of the submaxillary gland, for instance, enter the first 

 thoracic ganglion of the sympathetic chain, but do not actually 

 terminate until they reach the superior cervical ganglion high in the 

 neck. The postganglionic fibers arise in the ganglion in which the 

 preganglionic fibers terminate. Those destined to supply the skin 



