198 HUMAN PHYSIOLOGY. 



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

 brain the nerve impulses made by ether vibrations and which give rise to the 

 sensation of light. 



The reflex movements of the iris are called forth by stimulation of the optic 

 nerve. When light falls upon the retina, the nerve impulse developed 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 fibers of the iris and diminishes the size of the pupil. The 

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



Third Nerve. The Oculo-Motor. 



Origin. From several groups of nerve-cells situated in the gray matter 

 beneath the aqueduct of Sylvius. 



Distribution. From this origin the nerve-fibers pass forward and emerge 

 from the cerebrum at the inner side of the cms cerebri. The nerve then 

 passes forward, and enters the orbit through the sphenoid fissure, where it 

 divides into a superior branch distributed to the superior rectus and levator 

 palpebrcB muscles; an inferior branch, sending branches to the internal and 

 inferior recti and the inferior oblique muscles; filaments also pass into the 

 diary or ophthalmic ganglion; from this ganglion the diary nerves arise, which 

 enter the eyeball and are distributed to the circular fibers of the iris and the 

 ciliary muscle. The third nerve also receives filaments from the cavernous 

 plexus of the sympathetic and from the fifth nerve. 



Properties. Irritation of the root of the nerve produces contraction of the 

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

 Division of the nerve is followed by ptosis (falling of the upper eyel ; d); ex 

 ternal strabismus, due to the unopposed action of the external rectus muscle; 

 paralysis of the accommodation of the eye; dilatation of the pupil from paraly- 

 sis of the circular fibers 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 second, and its upper end in- 

 clined toward it. 



Function. Governs movements of the eyeball by innervating all the 

 muscles except the external rectus and superior oblique, influences the move- 

 ments 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. 



