CRANIAL NERVES. 93 



Distribution. The two roots unite to form a flattened band, the optic 

 tract, which winds around the crus cerebri to decussate with the nerve of 

 the opposite side, forming the optic chiasm. The decussation of fibres is 

 not complete ; some of the fibres of the left optic tract going to the outer 

 half of the eye of the same side, and to the inner half of the eye of the 

 opposite side ; the same holds true for the right optic tract. 



The optic nerves proper arise from the commissure, pass forward through 

 the optic foramina, and are finally distributed in the retina. 



Properties. They are insensible to ordinary impressions, and convey 

 only the special impressions of light. 



Division of one of the nerves is attended by complete blindness in the 

 eye of the corresponding side ; division of the optic tract produces loss of 

 sight in the outer half of the eye of the same side, and in the inner half 

 of the eye of the opposite side. Lesion of the anterior part of the optic 

 chiasm causes blindness in the inner half of the two eyes. 



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 filaments coming 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 lev at or palp ebr a 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 



