THE AUTONOMIC NERVOUS SYSTEM 883 



same manner as the vagus. Taken together these two nerves supply the 

 musculature of the gastrointestinal tract, including the cloaca, the 

 vagus as far as the end of the small intestine, and the pelvic nerve 

 from this point on. It must of course be remembered that certain 

 muscles namely, the sphincters of the small and large intestine 

 receive their nerve supply from the sympathetic (page 882). Just as 

 structures innervated by the sympathetic are peculiarly susceptible to 

 the action of epinephrine, it has been discovered that those innervated 

 by the bulbosacral system are very susceptible to the action of acetyl- 

 choline, which is present in ergot. They are not acted on by epinephrine, 

 nor are the structures upon which this acts affected by acetyl-choline. 



AXON REFLEXES 



At this place it is convenient to consider for a moment the phenome- 

 non which has already been referred to as an axon reflex. It was dis- 

 covered that when one of the hypogastric nerves was cut and the central 

 end stimulated there was a reflex contraction of the bladder and the in- 

 ternal anal sphincter, along with vasoconstriction in the region of the rec- 

 tum and that this occurred, even after disconnecting the inferior mesen- 

 teric ganglion from the spinal cord by cutting the lumbar splanchnic nerves. 

 Injection of nicotine immediately abolished the response. It looked as 

 if reflex action was possible through the ganglion; which would justify 

 the name "sympathetic" originally given to the involuntary nervous sys- 

 tem in the belief that the ganglia were centers for local reflex actions. 

 Further investigation showed, however, that this reflex is not similar to 

 those occurring in the voluntary system, but is dependent upon the 

 presence of a collateral on internuncial fibers that run through the in- 

 ferior mesenteric ganglia to nerve cells situated peripherally on the 

 walls of the bladder and rectum. The collaterals terminate by synapsis 

 around nerve cells in the ganglion, the axons of which, as we have seen, 

 run to the bladder, the rectal blood vessels, and the internal sphincter 

 ani. The evidence for this explanation depends on the observation that 

 the axon reflex is no longer possible after the lumbar splanchnics have 

 been cut and time allowed for their fibers to become completely degen- 

 erated. 



Similar reflexes depending on collaterals have been found in the lateral 

 chain, and there can be little doubt that they are of frequent occurrence 

 throughout the whole involuntary nervous system, just as they are, 

 within the spinal cord, in the voluntary. It is because of these collaterals 

 and the fact that nerve fibers transmit impulses in both directions 

 that a stimulus transmitted through one or a limited number of connector 



