THE CRANIAL NERVES. 221 



Distribution. From this origin the nerve-fibers pass forward and 

 emerge from the cerebrum at the inner side of the cms cerebri. The 

 nerve then passes forward, and enters the orbit through the sphenoid 

 fissure, where it divides into a superior branch distributed to the 

 superior rectus and levator palpebra muscles ; an inferior branch, 

 sending branches to the internal and inferior recti and the inferior 

 oblique muscles ; filaments also pass into the ciliary or ophthalmic 

 ganglion ; from this ganglion the ciliary nerves arise, which enter 

 the eyeball and are distributed to the circular fibers of the iris and the 

 ciliary muscle. The third nerve also receives filaments from the 

 cavernous plexus of the sympathetic and from the fifth nerve. 



Properties. Irritation of the root of the nerve produces contraction 

 of the pupil, internal strabismus, and muscular movements of the 

 eye, but no pain. Division of the nerve is followed by ptosis (falling 

 of the upper eyelid) ; external strabismus, due to the unopposed ac- 

 tion of the external rectus muscle ; paralysis of the accommodation 

 of the eye ; dilatation of the pupil from paralysis of the circular 

 fibers of the iris and ciliary muscle ; and inability to rotate the eye, 

 slight protrusion, and double vision. The images are crossed ; that 

 of the paralyzed eye is a little above that of the second, and its upper 

 end inclined toward it. 



Function. Governs movements of the eyeball by innervating all 

 the muscles except the external rectus and superior oblique, influ- 

 ences the movements of the iris, elevates the upper lid, influences the 

 accommodation of the eye for distances. Can be called into action 

 by (i) voluntary stimuli, (2) by reflex action through irritation of the 

 optic nerve. 



Fourth Pair. Patheticus. 



Origin. From nerve-cells situated in the gray matter beneath the 

 aqueduct of Sylvius, just posterior to the last nucleus of the third 

 nerve. 



Distribution. The nerve enters the orbital cavity through the 

 sphenoid fissure, and is distributed to the superior oblique muscle ; 

 . in its course it receives filaments from the ophthalmic branch of the 

 fifth pair and the sympathetic. 



Properties. When the nerve is irritated, muscular movements 

 are produced in the superior oblique muscle, and the pupil of the 

 eye is turned dozvnward and outward. Division or paralysis lessens 



