THE NERVOUS SYSTEM 121 



which excite constriction of blood vessels, hence called 

 vasoconstrictor fibers. The less well understood are the 

 relaxers, hence called vasodilator. The two are vaso- 

 motor. Emotions may excite the activity- of these nerves. 

 The face pales with fear and flushes with shame or an- 

 ger vasoconstriction and vasodilatation. Exercise will 

 excite the dilators, cold the constrictors. It is obvious 

 that this is a form of reflex action, but of reflex action 

 controlled by higher nervous centers, neither under the 

 control of the will nor independent of the subconscious 

 brain. 



Enlarging- or decreasing the size of the intestines; con- 

 tracting the bladder or uterus, changing the amount of 

 blood which flows through the kidneys, the salivary and 

 other glands, altering the size of the pupil or of the 

 bronchioles are among the many activities of these widely 

 distributed nerves. 



The nerve fibers derived from the tenth cranial, which 

 inhibit, or slow, the heartbeat, are carried to the heart 

 muscle through the sympathetic plexus situated on the 

 aorta. They are designated bulbar autonomic fibers, as 

 distinguished from the accelerator fibers derived from 

 the upper thoracic spinal nerves and joining the same 

 cardiac plexus before distribution. 



Other bulbar autonomic fibers are carried by the 

 ninth, seventh, and third nerves. Those from the latter 

 pass to the ciliary ganglion and from it to the muscle of 

 the iris which regulates the amount of light entering 

 the pupil; or to the ciliary muscle which regulates ac- 

 commodation of the eye for near or distant vision, while 

 the similar fibers of the seventh and ninth probably 

 reach the tongue, through the chorda tympani and lin- 

 gual for the anterior two-thirds and the ninth for the 

 posterior third, and supply vasomotor fibers to those or- 

 gans. 



