THE AUTONOMIC NERVE SYSTEM 649 



(e) Those fibers which emerge from the spinal cord in the thoracic 

 nerves from the fifth to the tenth and which after entering the verte- 

 bral chain pass across it and turn downward and forward uniting 

 at different levels to form the greater and lesser splanchnic nerves, 

 the terminal branches of which arborize around the cells of the semi- 

 lunar, the renal, and the superior mesenteric ganglia. 



^ The lumbar ^ pre-ganglionic fibers in. accordance with their distribu- 

 tion may be divided into three groups, viz. : 



(a) Those fibers which, after pursuing a similar course to the preceding 

 terminate around the cells of the ganglia at the same level. 



(b) Those fibers which after entering, cross the vertebral chain and 

 then pass forward as the inferior splanchnic nerves to terminate 

 around the cells of the inferior mesenteric ganglion. 



(c) Those fibers which after entering, descend the vertebral chain to 

 terminate around the cells of the successive ganglia as far as the 

 third sacral. It is probable that some fibers from the lower thoracic 

 nerves also descend the lumbar chain. 



By reason of this anatomic distribution, the thoracico-lumbar pre- 

 ganglionic fibers become related to all the vertebral and the pre- vertebral 

 ganglia. From these various ganglia, as stated on page 645 post-gan- 

 glionic fibers arise which either as gray rami communicantes enter the 

 spinal nerves or as rami viscerales pass direct to their termination. In 

 either case the nerve-fibers ultimately become histologically and physio- 

 logically related to non-striated vascular and visceral muscle-fibers and 

 to epithelium of glands. The specific distributions of the post-gan- 

 glionic fibers are stated in the paragraphs relating to the ganglia them- 

 selves. A connection is thus established between the cells of the spinal 

 cord and the motor tissues by means of the pre- and post-ganglionic 

 nerve-fibers. These nerves include, therefore, all the vaso-motor (con- 

 strictor) nerves, the secreto-motor nerves for the sweat glands, the thy- 

 roid and the adrenal glands, and a large part of the viscero-motor (in- 

 hibitor) nerves. 



The thoracico-lumbar pre-ganglionic nerves together with their post- 

 ganglionic continuations together constitute the thoracico-lumbar auto- 

 nomic nerve system. 



4. The Sacral Spinal-cord Region. The pre-ganglionic nerve-fibers that 

 leave the spinal cord in this region arise from groups of nerve-cells situa- 

 ted in the gray matter between and including the levels of origin of the 

 second, the third and the fourth (?) sacral nerves. From this origin the 

 pre-ganglionic fibers emerge from the cord in association with the large 

 motor fibers composing the ventral roots of these sacral nerves and pass 

 with them to the interior of the pelvis. Here they leave the sacral 

 nerves and enter the pudendal or pelvic nerve (the nervus erigens) and 

 finally terminate around the cells of the pelvic ganglia. From these 

 ganglia post-ganglionic fibers arise which pass onward to be distributed 

 to the non-striated muscle-fibers of pelvic viscera and the blood-vessels 

 of the external generative organs. These fibers contained in the sacral 

 nerves together with their post-ganglionic continuation have collectively 

 been termed the sacral autonomic system. It may be regarded as a 



