884 THE CENTRAL NERVOUS SYSTEM 



neurons may excite a broad field of effectors and cause a widespread 

 effect. 



FUNCTIONS OF AUTONOMIC NERVES 



The functions of the autonomic nerve fibers have been discussed in 

 connection with the structures which they supply, and we shall require in 

 this place only to review them in a general way. 



Two opposed effects may be obtained: stimulatory (augmentory) and 

 inhibitory; and these may be produced through one nerve by its being 

 stimulatory for one set of muscle fibers and inhibitory for another set 

 in the same viscus. The branches running from the inferior mesenteric 

 ganglion to the colon, for example, are augmentory (constrictor) for the 

 blood vessels and inhibitory for the muscular walls of the colon. 



The greatest interest centers on the inhibitory impulses. They are 

 best known in connection with the vagus nerve to the heart, the sympa- 

 thetic to the small intestine, and the hypo gastric to the musculature of 

 the bladder. It is interesting to compare the nature of inhibition in the 

 involuntary and voluntary systems. In the latter, it will be remembered, 

 inhibition can occur only through the intermmcial neurons and the ef- 

 fector nerve cell, stimulation of the effector nerve fiber never having 

 any other than an augmentor effect. It is quite otherwise in the involun- 

 tary nervous system, for stimulation of the effector nerve fiber, after 

 complete destruction of the effector nerve cell, is still followed by a tj^pical 

 inhibition. This, it will be remembered, may be demonstrated on the frog 

 heart by applying electric stimulation to the white crescentic line after 

 paralyzing the effector nerve cells by nicotine. The same may also be 

 shown in the case of the chorda tympani, where stimulation of the post- 

 ganglionic fibers in the hilus of the gland causes dilatation of the blood 

 vessels after paralysis of the ganglion by nicotine, vasodilatation being 

 of course a phenomenon of inhibition. 



It is a difficult matter to designate precisely which fibers in any part 

 of the involuntary nervous system are inhibitory and which augmentory. 

 Indeed, as mentioned above, one fiber may perform both functions. In 

 cases where the existence of inhibitory fibers is doubtful, great aid is 

 afforded by the use of ergotoxine, an alkaloid of ergot, which possesses 

 the remarkable property of specifically paralyzing the augmentor nerves 

 of the sympathetic system (but not of the parasympathetic) ; that is, 

 the same fibers as are stimulated by epinephrine. When a particular 

 structure is supplied with augmentory and inhibitory fibers by a com- 

 bined sympathetic nerve, electric stimulation or the application of epi- 

 nephrine usually gives only augmentory effects; after the injection of 



