278 THE NERVOUS SYSTEM 



from the other tract and pass thus to the optic nerve of the op- 

 posite side. The deep origin is indicated above. 



Course and Distribution. Each optic nerve leaves the 

 front of the optic chiasm to pass out of the cranium and enter 

 the orbital cavity by the optic foramen. Having pierced the 

 sclerotic and choroid coats of the ball it expands into the retina. 



Function. The optic nerves have no properties other than 

 the conveying to the brain of the special impressions of sight. 

 Stimulation produces neither pain nor motion. 



Third Nerve (Motor Oculi Communis) . 



Origin. The third is a motor nerve. Its apparent origin is 

 from the inner surface of the crus just in front of the pons Va- 

 rolii. Its deep origin is in a nucleus just lateral to the median 

 line beneath the aqueduct of Sylvius. Here decussation with 

 fibers from the opposite side occurs. The fibers pass forward 

 from this place through the locus niger and tegmentum to the 

 point of apparent origin. 



Course and Distribution. Having traversed the outer aspect 

 of the cavernous sinus, the third nerve divides into two branches 

 which leave the cranial cavity by the sphenoidal fissure between 

 the two heads of the external muscle of the eye. The superior 

 division is distributed to the superior rectus and levator palpe- 

 brae superioris ; the inferior separates into three branches, one 

 of which is distributed to the inferior rectus, another to the 

 internal rectus, and a third to the inferior oblique. From 

 this last a branch is given off to the lenticular ganglion to form 

 its inferior root. 



Functions. This nerve has no function other than to supply 

 motion to the parts to which it is distributed. It is insensible 

 at its root, but receives filaments from the fifth in the cavernous 

 sinus, beyond which point stimulation produces pain as well as 

 muscular contractions. The phenomena sequent upon section of 

 the nerve are suggested in its distribution, (i) There is ptosis, 



