336 THE NERVOUS SYSTEM 



ponding cerebrospinal nerves to the sympathetic nervous system, through which 

 they pass without interruption in any of its ganglia to end in the viscera. These 

 fibers are of all sizes, including large and small myelinated fibers and many which 

 are unmyelinated (Chase and Ranson, 1914; Ranson and Billingsley, 1918). 



The afferent impulses mediated by these fibers serve to initiate visceral re- 

 flexes, and for the most part remain at a subconscious level. Such general vis- 

 ceral sensations as we do experience are vague and poorly localized. Tactile 

 sensibility is entirely lacking in the viscera and thermal sensibility almost so, 

 although sensations of heat and cold may be experienced when very warm or 

 cold substances enter the stomach or colon (Carlson and Braafladt, 1915). 

 Pain cannot be produced by pinching or cutting the thoracic or abdominal 

 viscera. Acute visceral pain may, however, be caused by disease, as in the pas- 

 sage of a stone along the ureter. 



From the cerebrospinal ganglia the visceral afferent impulses are carried to the brain 

 and spinal cord by the sensory nerve roots. The relations within the cerebrospinal ganglia 

 are not entirely clear; but it seems probable that the visceral afferent impulses are conducted 

 through the ganglion by way of the two branches of the typical unipolar sensory neuron 

 (Fig. 249). Many authors believe that there are also sensory fibers which arise from cells 

 in the sympathetic ganglia and terminate in the spinal ganglia in the form of pericellular 

 plexuses (Fig. 40, C). Through these plexuses visceral sensory impulses are supposed to be 

 transmitted to somatic sensory neurons and to be relayed by them to the spinal cord. Since 

 it has not been clearly demonstrated that any sensory fibers arise from cells in the sym- 

 pathetic ganglia, this interpretation of the pericellular plexuses of the spinal ganglia must be 

 regarded as purely hypothetic. 



Langley (1903) has presented strong evidence that few if any sensory fibers arise in the 

 sympathetic ganglia. Physiologic experiments show that the visceral afferent fibers run in 

 the white rami, yet all or practically all of the fibers of a white ramus degenerate if the cor- 

 responding spinal nerve is severed distal to the spinal ganglion. Huber (1913) states that 

 "it has not been determined that the fine medullated fibers or the unmedullated fibers which 

 appear to enter the spinal ganglia from without and end in pericellular plexuses are, in 

 fact, the neuraxes of sympathetic neurones." The hypothesis that these pericellular plexuses 

 represent the termination of visceral afferent fibers is, therefore, not well supported. This 

 subject is treated in more detail in a series of papers on the sympathetic nervous system by 

 Ranson and Billingsley (1918). 



Visceral Efferent Neurons. The general visceral efferent fibers of the 

 cerebrospinal nerves take origin from cells located within the cerebrospinal axis. 

 They do not run without interruption to the structures which they innervate; 

 instead, they always terminate in sympathetic ganglia, whence the impulses, 

 which they carry, are relayed to their destination by neurons of a second order 

 (Fig. 249). This important information we owe to Langley (1900 and 1903), 

 who showed that the injection of proper doses of nicotin into rabbits prevents 



