382 CHORDATE ANATOMY 



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 parts 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 possible, therefore, that some visceral reflexes pass through the intes- 

 tinal plexuses only. 



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

 S5mipathetic trunk, collateral ganglia such as the celiac and mesenteric 

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

 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 intes- 

 tinal plexus in other invertebrates from flatworms to molluscs 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 fundamental 

 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 development, of 

 which there are numerous examples in ontogenesis. Moreover, the rela- 

 tions of the myenteric and submucous plexuses of the walls of the stomach 

 and intestine resemble those of the invertebrate intestinal plexuses, and 

 they are equally autonomic in their functions. Finally, in elasmobranchs, 

 sensory cells in the wall of the alimentary 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. 



As a system, therefore, the sympathetic of vertebrates is a new addi- 

 tion which arises late both ontogenetically and phylogenetically, Kap- 



