536 



CIRCULATION OF BLOOD AND LYMPH. 



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, blanching; 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 con- 

 nected 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 compara- 

 tively unimportant exceptions, leave the spinal cord in the great 



Fig. 222. 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 spiral nerve; r, the ramus 

 communicans; g, the sympathetic ganglion. The postganglionic 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. 



outflow that takes place in the thoracic region from the second 

 thoracic to the second lumbar nerves (p. 233). In this outflow 

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

 fibers, pilomotor fibers, accelerator fibers to heart, pupilodilator 

 fibers, visceromotor 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 



