94 HUMAN PHYSIOLOGY. 



of the pupil, internal strabismus, muscular movements of eye, but no pain. 

 Division of the nerve is followed by ptosis (falling of the upper eyelid), 

 external strabismus, due to the unopposed action of the external rectus 

 muscle ; paralysis of the accommodation of the eye ; dilatation of the 

 pupil from paralysis of the circular fibres 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 

 sound, and its upper end inclined toward it. 



Function. Governs movements of the eyeball by animating all the 

 muscles except the external rectus and superior oblique, 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. 



4th Pair. Patheticus. 



Apparent Origin. From the superior peduncles of the cerebellum. 



Deep Origin. By fibres terminating in the corpora quadrigemina, 

 lenticular nucleus, valve of Vieussens, and in the substance of the cere- 

 bellar peduncles; some filaments pass over the median line and decussate 

 with fibres of the opposite side. 



Distribution. The nerve enters the orbital cavity through the sphe- 

 noidal fissure, and is distributed to the superior oblique muscle ; in its 

 course receives filaments from the ophthalmic branch of the 5th pair and the 

 sympathetic. 



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

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

 downward and outward. Division or paralysis lessens the movements 

 and rotation of the globe downward and outward. The diplopia conse- 

 quent 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. 



6th Pair.* Abducens. Motor Oculi Externus. 

 Apparent Origin. From the groove between the anterior pyramidal 

 body and the pons Varolii, where it arises by two roots. 



* The 6th nerve is considered in connection with the 3d and 4th nerves, since they 

 together constitute the motor apparatus by which the ocular muscles are excited to 

 action. 



