896 CENTRAL XKKVOUS SYSTEM 



the vagus, while the sacral plexus is composed of the postganglionic 

 fibers from the sacral outflow. The remaining parts of the thoracico- 

 lumbar outflow undergo synapse in one of two sets of ganglia. The seg- 

 mental chain of sympathetic ganglia contain the cells of the outlying neu- 

 rons of those thoracico-lumbar paths which follow the course of the spinal 

 nerves to smooth muscles and glands located in the skin and muscles, 

 and of those which innervate the organs of the thoracic cavity. The 

 typical arrangement of this group is shown in Figs. 218 and 221. The 

 connector fiber reaches the ganglion through the white ramus and the 

 postganglionic fiber passes back to the trunk of the spinal nerve in 

 the gray ramus and follows this trunk to the peripheral structure which 

 it innervates. Many connectors do not terminate directly in the ganglion 

 of their own segment, but send collaterals forward or backward through 

 the sympathetic chain to the other ganglia, where they connect with ef- 

 fector neurons. The third group of thoracico-lumbar paths leads to the 

 organs of the abdominal viscera, including their blood vessels. The 

 connector fibers of these paths pass out over the white rami and through 

 the segmental sympathetic ganglia without interruption to terminate 

 in one of the mesenteric ganglia in connection with an effector neuron. 



The Double Innervation of the Visceral Organs. The innervation of 

 the smooth muscles and glands is peculiar in that each effector may be 

 acted upon by two neurons which effect its activity in opposite ways. 

 Impulses from one neuron tend to increase the secretion of these glands, 

 or augment the tone or degree of contraction of the smooth muscles, 

 while impulses from the other neuron set up changes in the other direc- 

 tion which inhibit or depress these activities. Because of this double 

 innervation of structures supplied by the autonomic system, the activ- 

 ity of any of them will depend on the balance which is struck between the 

 effects of these antagonistic neurons. 



The details of this arrangement may be learned in the case of any 

 particular organ from Fig. 221. 



In general it may be stated that the bulbar and sacral outflows do not 

 overlap. In the gastrointestinal tract all parts cephalad of the ileocolic 

 sphincter are innervated by the vagus and other cranial nerves, and 

 are replaced below this point by the sacral outflow. At the same time, 

 the thoracico-lumbar outflow supplies this entire tract below the cardiac 

 part of the stomach with fibers which act upon it in the opposite sense. 

 The action of the bulbo-sacral outflow on the gastrointestinal tract is 

 excitatory, except for the ileocolic, and internal anal sphincters which 

 are inhibited. It consequently favors the movement of food along the 

 digestive tract and the secretion of the digestive fluids of the pancreas 

 and salivary glands, and the emptying of the gall bladder. The thora- 

 cico-lumbar outflow, on the other hand, tends to diminish the activities 



