CRANIAL NERVES. 199 



Fourth Nerve. Trochlearis. 



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

 duct 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 sym- 

 pathetic. 



Properties. When the nerve is irritated, muscular movements are pro- 

 duced in the superior oblique muscle, and the pupil of the eye is turned down- 

 ward and outward. Division or paralysis lessens the movements and rotation 

 of the globe downward and outward. The diplopia consequent upon this par- 

 alysis is homonymous, one image appearing above the other. The image of 

 the paralyzed eye is below, its upper end inclined toward that of the sound eye. 



Function. Governs the movements of the eyeball produced by the action 

 of the superior oblique muscles. 



Sixth Nerve.* Abducent. 



Origin. From nerve-cells situated beneath the upper half of the floor 

 of the fourth ventricle. 



Distribution. From this origin the nerve passes into the orbit through 

 the sphenoid fissure, and is distributed to the externa rectus muscle. Receives 

 filaments from the cervical portion of the sympathetic, through the carotid 

 plexus, and spheno-palatine ganglion. 



Properties. When irritated, the external rectus muscle is thrown into con- 

 vulsive movements and the eyeball is turned outward. When divided or 

 paralyzed, this muscle is paralyzed, motion of the eye ball outward past the 

 median line is impossible, and the homonymous diplopia increases as the 

 object is moved outward past this line. The images are upon the same 

 plane and parallel. Internal strabismus results because of the unopposed 

 action of the internal rectus. 



Function. To innervate the external rectus muscle by which the eye- 

 ball is turned outward. 



Fifth Nerve. Trigeminal. 



The fifth nerve consists of both afferent and efferent fibers which for 

 the most part are separate and distinct. The afferent fibers constitute by 

 far the major portion, the efferent fibers the minor portion of the nerve. 



*The sixth nerve is considered in connection with the third and fourth nerves since 

 they together constitute the motor apparatus by which the ocular muscles are ex- 

 cited to action. 



