594 THE NERVOUS SYSTEM 



fineness of their fibers. They are a third or a fourth of the diameter of ordinary 

 medullated fibers, measuring i.8/,i to 2.7^ instead of 14^ to 19^. Such 

 fibers are a peculiarity of the spinal nerve roots chiefly in the thoracic region, 

 but also found in the second and third sacral nerves, and constitute there 

 the nervi erigentes which pass directly to the hypogastric plexus. From the 

 hypogastric plexus branches pass upward into the inferior mesenteric ganglia 

 and downward to the bladder, rectum, and generative organs. These nerves, 

 called by Gaskell pelvic splanchnic nerves, differ from the rami viscerales of 

 the thoracic region only in not communicating with the lateral ganglia; the 

 branches which pass upward from the thoracic region to the neck, he calls 

 cervical splanchnics, and the splanchnics proper abdominal splanchnics. 



Functions. The researches of Gaskell and of Langley have done 

 much to clear up the former confusion as to the distribution and functions 

 of the sympathetic. The sympathetic nerve fibers are distributed to smooth 

 muscle, to gland cells, and to cardiac muscle. These are all organs which 

 carry on their activities either automatically or reflexly. There is no volun- 

 tary control of the function of these organs. 



The efferent sympathetic fibers supply the muscles of the vascular system, to 

 which they send the vaso-motor fibers, i.e., vaso-constrictor and cardiac aug- 

 mentor or accelerator; and vaso-inhibitory fibers, i.e., vaso-dilators. They 

 supply the muscles of the alimentary canal and of the urinogenital system. 

 The details of arrangement and functional control of these complex systems 

 have already been discussed in connection with the function of the organ or 

 part concerned. They supply the salivary, gastric, and pancreatic glands. 



According to Gaskell the functions of the main sympathetic ganglia are the 

 following: i, They effect the conversion of medullated into non-medullated 

 fibers. 2, They possess a nutritive influence over the nerves which pass from 

 them to the periphery. 3, They increase the number of fibers. 



The sympathetic ganglia are not nerve centers in the usual sense. It is 

 better to regard them merely as distributing organs in which reflexes of central 

 origin and comparatively simple type are distributed over relatively large 

 areas. These ganglia do not possess the power of reflex function. A type 

 of pseudo- reflex has been described depending on the law of neurone reaction. 

 But it is not supposed that such reflexes occur in the normal animal. 



Afferent or sensory fibers of the ordinary spinal-root ganglion cells are 

 present in the sympathetic nerves of the splanchnic region, being distributed 

 merely to the visceral region. True afferent sympathetic fibers have been 

 demonstrated. These arise from cells located in the sympathetic ganglia, and 

 pass through the rami communicantes, to end by terminal arborizations in the 

 spinal ganglia, chiefly around cells of the Dogiel type. The number and 

 significance of these afferent neurones are yet uncertain. 



