104 HUMAN PHYSIOLOGY. 



Functions. Governs the sense of sight. Receives and conveys to the 

 brain the luminous impressions which give rise to the sensation of sight. 



The reflex movements of the iris are called forth by the optic nerve. 

 When an excess of light falls upon the retina the impression is carried 

 back to the tubercula quadrigemina, where it is transformed into a motor 

 impulse, which then passes outward through the motor oculi nerve to the 

 contractile fibres of the iris and diminishes the size of the pupil. The 

 absence of light is followed by a dilatation of the pupil. 



3d Pair. Motor Oculi Communis. 

 Apparent Origin. From the inner surface of the crura cerebri. 



Deep Origin. By three sets of filaments coming from the oculo-motorius 

 nucleus, which lies under the aqueduct of Sylvius; these three groups of 

 filaments are destined for the innervation of the muscles of the eyeball, the 

 sphincter pupilbe, and the ciliary muscle. By filaments coining from the 

 lenticular nucleus, corpora quadrigemina, optic thalamus ; these filaments 

 converge to form a main trunk, which winds around the crus cerebri, in 

 front of the pons Varolii. 



Distribution. The nerve then passes forward, and enters the orbit 

 through the sphenoidal 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 fibres of the iris and the ciliary muscle. The 

 3d nerve also receives filaments from the cavernous plexus of the sympa- 

 thetic and from the fifth nerve. 



Properties. Irritation of the root of the nerve produces contraction 

 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 



