THE NERVOUS SYSTEM 539 



either to the viscera by way of sympathetic nerves or to the body wall 

 and skin by way of the spinal nerves with which they are connected 

 through the gray rami communicantes. 



Visceral afferent fibers, the third type, with cell-bodies in the dorsal 

 ganglia, carry impulses directly from visceral muscles to the gray matter 

 of the cord. Visceral afferent fibers having cell-bodies in the sympathetic 

 ganglia have not been demonstrated, the cells of sympathetic ganglia 

 being exclusively motor. 



Most, if not all, actions of the autonomic system are reflexes mediated 

 through the brain or cord. Some intestinal reactions, however, may 

 occur after all nerve connexions with the cord and brain have been 

 severed. It is, therefore, possible that some visceral reflexes pass through 

 the intestinal plexuses only. 



Three kinds of autonomic ganglia may be distinguished, ganglia of the 

 sympathetic trunk, collateral ganglia such as the coeUac and mesenteric 

 located in the wall of stomach and intestine, and terminal ganglia hke 

 the ciliary and cardiac located in the organs which they innervate. 



Evolution of the Autonomic System 



The nervous system of coelenterates is a plexus of primitive ganglion 

 cells connected with neurosensory cells and smooth muscle fibers and 

 located between the two primary body layers. This persists as an 

 intestinal plexus in other invertebrates, from flatworms to moUuscs and 

 insects. That the intestinal plexus of vertebrates is homologous with 

 that of invertebrates has not been demonstrated beyond a reasonable 

 doubt, but may be assumed in the absence of evidence to the contrary. 

 The late ontogenetic appearance of the plexus in vertebrates does not, 

 however, harmonize with this assumption, since, in the light of the funda- 

 mental law of biogenesis, we should hardly expect the most ancient part 

 of the nervous system to be one of the last to appear in the embryo. 

 On the other hand, we may have here another example of retarded develop- 

 ment, of which there are numerous examples in ontogenesis. Moreover, 

 the relations of the myenteric and submucous plexuses of the walls of 

 the stomach and intestine resemble those of the invertebrate intestinal 

 plexuses, and both are equally autonomic in their functions. Finally, 

 in elasmobranchs, sensory ceUs in the wall of the aUmentary canal form a 

 part of the system as in invertebrates. Evidence of similar cells in 

 mammals is wanting. 



In invertebrates and vertebrates alike the evolution of the autonomic 

 system keeps pace with that of the digestive and circulatory systems. 

 Sympathetic and parasympathetic systems are recognized in arthropods; 

 but no structures are homologous with the autonomic ganglia of 

 vertebrates. 



