040 THE NERVOUS SYSTEM 



The fibers of the thoracicolumbar stream run by way of the white rami to 

 the sympathetic trunk, while the fibers of the cranial and sacral streams make 

 no connection with that trunk, but run directly to the sympathetic plexuses. 

 And while the thoracicolumbar preganglionic fibers terminate hi the ganglia of 

 the trunk, those of cranial and sacral origin end in the terminal ganglia. In 

 these two respects the cranial and sacral streams agree with each other and differ 

 from the thoracicolumbar outflow. Also in their response to certain drugs, 

 like atropin and adrenalin, the two former agree with each other and differ from 

 the latter. It is, therefore, desirable to group the cranial and sacral systems 

 together as the craniosacral autonomic system. This has been called by many 

 physiologists the parasympathetic system. It stands in contrast to the thoracico- 

 lumbar autonomic system to which many physiologists have unfortunately applied 

 the name "sympathetic system." The importance of recognizing these two 

 principal subdivisions is further emphasized by the fact that most of the struc- 

 tures innervated by the autonomic system receive a double nerve supply and are 

 supplied with fibers from both subdivisions. The thoracicolumbar fibers are 

 accompanied in most peripheral plexuses by craniosacral fibers of opposite func- 

 tion so that the analysis of these plexuses is greatly facilitated by subdividing 

 the autonomic system in this way. 



Visceral Reflexes. In the gastro-intestinal tract and perhaps within other 

 viscera there may be a mechanism for purely local reactions as indicated in 

 the following paragraph. With this exception the evidence strongly indicates 

 that all visceral reflex arcs pass through the cerebrospinal axis. In such an 

 arc there are at least three neurons, namely, (1) visceral afferent, (2) pregang- 

 lionic visceral efferent, and (3) postganglionic visceral efferent neurons (Fig. 249) . 



The purely local reactions which occur in the gut wall after section of all of 

 the nerves leading to the intestine are known as myenteric reflexes and must de- 

 pend upon a mechanism different from that of other visceral reflexes (Langley 

 and Magnus, 1905; Cannon, 1912). Practically nothing is known of this mech- 

 anism beyond the fact that it must be located in the enteric plexuses. Some 

 authors have assumed that within these plexuses there is a diffuse nerve net 

 similar to that found in the ccelenterates (Parker, 1919). While the evidence 

 is far from satisfactory, it may be that such a net does exist in this situation and 

 that it is responsible for these local reactions. 



