222 HUMAN PHYSIOLOGY. 



the movements and rotation of the globe downward and outward. 

 The diplopia consequent upon this paralysis 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 Pair.* Abducens. Motor Oculi Externus. 

 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 external rectus 

 muscle. Receives filaments from the cervical portion of the sympa- 

 thetic, through the carotid plexus, and spheno-palatine ganglion. 



Properties. When irritated, the external rectus muscle is thrown 

 into convulsive movements and the eyeball is turned outward. When 

 divided or paralyzed, this muscle is paralyzed, motion of the eyeball 

 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 

 eyeball is turned outward. 



Fifth Pair. Trigeminal. 



The fifth nerve consists of both afferent and efferent fibers which 

 for the most part are separate and distinct. The afferent fibers con- 

 stitute by far the major portion, the efferent fibers the minor portion of 

 the nerve. 



Origin of the Afferent Fibers. The afferent fibers have their 

 origin in nerve-cells in the Gasserian ganglion. From each cell a 

 short process develops which soon divides into two branches, one of 

 which passes centrally, the other peripherally. The centrally di- 

 rected branches form the so-called large root; the peripherally di- 

 rected branches collectively constitute the three main divisions of the 



* 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 excited to action. 



