VISION. 



791 



Optic Nerve. 



The optic nerve contains centripetal and centrifugal fibers. 

 The bundle of centripetal fibers from the second layer of retinal 

 ganglionic cells, which originate in the vicinity of the macula, go 

 into the optic tract of the same and opposite side. Those going into 

 the optic tract of the same side come from the temporal tract of 

 the macula, while the decussating fibers come from the nasal side 

 of the macula ; hence each optic tract is made up of fibers from the 

 temporal half of the retina of the same eye and from the nasal half 

 of the opposite eye. The optic tract then goes backward, passing 

 around the cerebral peduncle, and breaks up into two bundles, the 

 external and the internal. The internal bundle is connected with 

 the internal geniculate body and the posterior corpus quadrigeminum, 

 and is a part .of Gudden's commissure. It has no connection with 

 vision. The external bundle goes to the external geniculate body, 

 the pulvinar and anterior corpus quadrigeminum. The cells in the 

 external geniculate body receive the terminal arborizations, as also 

 do the cells of the pulvinar and corpus quadrigeminum. From these 

 ganglia neuraxons go through the most posterior end of the internal 

 capsule (optic radiations of Gratiolet), to end in the occipital lobe, 

 mainly in the cuneus. 



The pyramidal cells send centrifugal fibers to the external geni- 

 culate body, to the pulvinar and the anterior corpus quadrigeminum, 

 and from here new centrifugal axons go to the retina. 



Irritation of the occipital cortex in the monkey, say of the right 

 lobe, causes movement of the eyes to the opposite side, through the 

 action of the efferent fibers. 



The average dimensions of the dioptric system of the eye are 

 as follows: 



Index of refraction of air 1. 



Index of refraction of cornea, aqueous humor, and vitreous 



body 1.3365 



Total index of refraction of the crystalline lens 1.4371 



Radius of curvature of the cornea 7.829 mm. 



Radius of curvature of the anterior surface of the crystal- 

 line lens 10.0 



Radius of curvature ol the posterior surface of the crystal- 

 line lens 6.0 



Distance from the apex of the cornea to the anterior sur- 

 face of the crystalline lens 3.6 



Distance from the apex of the cornea to the posterior sur- 

 face of the crystalline lens 7.2 mm. 



Thickness of the lens - , 3.6 mm, 



mm. 



mm. 



