THE AUTONOMIC NERVE SYSTEM 665 



special nerve system for the anal end of the gut and structures develop- 

 mentally connected with it. 



From the distribution of the pre-ganglionic fibers and their histologic 

 connection with the sympathetic ganglia and from the distribution and 

 histologic connection of the branching fibers from the ganglia there is 

 every reason to believe that the autonomic nerve system includes all the 

 vaso-motor, some of the vaso-inhibitor, the viscero-motor and the vis- 

 cero-inhibitor, the secreto-motor and the secreto-inhibitor nerves. 

 Structure of the Interconnecting Cords. The interconnecting cords are 

 composed of non-medullated and medullated nerve-fibers. The former are 

 the axons of cells found in the ganglia more centrally located; the latter are 

 derived from the spinal nerves, from the fibers of which, however, they differ 

 in character, being much smaller and finer. The fibers of the interconnect- 

 ing cords, as a rule, transmit nerve impulses from the more centrally to the 

 more peripherally located ganglia, and are, therefore, termed rami efferentes. 

 In the vertebral chain some of the cords transmit nerve impulses upward, 

 others downward, others again forward, to the pre-vertebral and peripheral 

 ganglia. 



Among the rami efferentes or interconnecting cords, there are some which 

 possess special interest for the physiologist, viz.: 



1. The cervical which connects the thoracic ganglia with the superior cervical 



ganglion. It is composed mainly of pre-ganglionic medullated fibers 

 derived originally from the thoracic spinal cord. 



2. The great splanchnic nerve formed mainly by the union of pre-ganglionic 



fibers which cross the vertebral chain in the region of the fifth to the 

 tenth thoracic ganglia. It connects the cells of the spinal cord with the 

 cells of the semilunar ganglion. 



3. The small splanchnic nerve formed by the union of pre-ganglionic medul- 



lated fibers that cross the vertebral chain in the region of the ninth and 

 tenth thoracic ganglia. It connects the cells in the spinal cord with 

 the renal and perhaps the superior mesenteric ganglion. 



4. The inferior splanchnic nerves formed by the union of pre-ganglionic 



medullated fibers that cross the upper lumbar chain in the region of the 

 second and third lumbar ganglia. They connect the cells in the spinal 

 cord with the inferior mesenteric ganglion. 



Afferent Sympathetic Fibers. With the foregoing groups of pre-gan- 

 glionic autonomic fibers in the thoracic and lumbar regions there is associated 

 a certain number of afferent fibers which impart a certain degree of sensibility 

 to the organs of the thoracic and abdominal regions. Though these organs in 

 the physiologic condition do not appear to be endowed with much sensibility, 

 nevertheless, in pathologic conditions they become exceedingly sensitive and 

 give rise to extremely painful sensations. The afferent nerves which mediate 

 these sensations have their origin in nerve-cells located in the ganglia of the 

 dorsal roots of the spinal nerves; from this origin they pass outward by way 

 of the white rami into nerve trunks which pass to the organs in the thoracic 

 and abdominal cavities, e.g., the cardiac plexus and the splanchnic nerves 

 and the sacral nerves to the pelvic organs. The presence of afferent nerves 

 in the splanchnics can be shown by stimulating the central end or even the 

 white rami with electric currents, a procedure that gives rise to pain as well 

 as a reflex rise of blood-pressure. The number of afferent nerve-fibers in 



