vsl' THE CENTRAL NERVOUS SYSTEM 



lopian tubes and vas deferens. Many of the fibers of the hypogastric 

 nerves are therefore those of involuntary internuncial neurons. 



The ileocolic and internal anal sphincter muscles of the intestines and 

 internal vesical sphincter receive their nerve supply from effector neurons 

 situated in the superior and inferior mesenteric ganglia, the internuncial 

 fibers arising from the thoracicolumbar region. It is possible that the 

 other sphincters of the intestinal canal — viz., the cardiac and pyloric 

 sphincters of the stomach — are similarly innervated. (Fig. 232.) 



Great aid in working out these nerve connections is received by study- 

 ing the effect of epinephrine, which acts specifically on those tissues that 

 are supplied by the sympathetic nervous system.* Epinephrine has no 

 effect on tissues innervated by the bulbar or sacral outflows, and it 

 develops its action peripherally, being indeed more potent on a dener- 

 vated organ even after all its nerves have been allowed to degenerate. 

 Advantage of this action of epinephrine has been taken in the investi- 

 gation of doubtful cases of sympathetic innervation, such as in the cere- 

 bral, coronary, and pulmonary blood vessels. The outcome of these in- 

 vestigations has been discussed elsewhere. 



THE BULBOSACRAL OUTFLOW OR THE PARASYMPATHETIC 



SYSTEM 



From the medulla oblongata arise involuntary connector neurons, 

 which are carried mainly by the vagus nerves but partly by the seventh, 

 ninth and eleventh cranial nerves to effector nerve cells situated periph- 

 erally on the structures to which the nerves run (Fig. 233). These include 

 in a general way the muscles and glands of the alimentary canal and 

 its derivatives as far as the end of the small intestine. In the small 

 intestine itself the cells of these motor neurons are those of Auerbach's 

 plexus found between the two muscular coats. In the diverticula, which 

 include the lungs and the gall bladder, the nerve cells to Avhich the 

 vagus fibers run are also situated peripherally. 



The sacral outflow occurs through the second and third sacral nerves, 

 the fibers joining to form a single nerve (the pelvic nerve or nervus 

 crigens) on each side. This passes directly to the bladder, where it 

 connects with a plexus, often called the hypogastric, which extends over 

 the bladder and neighboring portion of the rectum. The branches run 

 to connect either with the nerve cells of the ganglia of the plexus itself, 

 or with nerve cells situated on the walls of the large intestine and blad- 

 der. The pelvic nerve makes its connections with the periphery in the 



"Its action is always tin >am< as that which is produced by stimulation of tin- sympathetic nerve 

 supply, whether this effect is one ot stimulation or inhibition. 



