CRANIAL NERVES. 105 



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. 



Deep Origin. From the gray matter of the medulla oblongata. 



Distribution. The nerve then passes into the orbit through the sphe- 

 noidal fissure, and is distributed to the external 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 

 convulsive movements, and the eyeball is turned outward. When divided 



* 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. 



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