2/8 THE NERVOUS SYSTEM 



amus and from the superior corpus quadrigeminum ; the in- 

 ternal comes from the internal geniculate body. These two, 

 uniting, cross the crusta obliquely to reach the optic commis- 

 sure, or chiasm. In the commissure the fibers from the inner 

 margin of each optic tract pass to the other side of the brain, 

 and may be called commissural fibers between the internal 

 geniculate bodies. Some fibers anteriorly connect the two 

 optic nerves with each other and are not properly part of 

 the chiasm, but connect the two retinae. The outer fibers of 

 each tract pass to the nerve of the same side, while the 

 central fibers decussate in the commissure with similar fibers 

 from the other tract and pass thus- to the optic 1 nerve of the 

 opposite 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 Varolii. Its deep origin is in a nucleus just lateral to 

 the median line beneath the aqueduct of Sylvius. Here de- 

 cussation 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 sphe- 

 noidal fissure between the two heads of the external muscle 

 of the eye. The superior division is distributed to the su- 



