THE EYE AS AN OPTICAL INSTRUMENT 251 



side the optic disk; in it the retina is thinner than elsewhere 

 and so a pit is formed. This appears black, the thinned retina 

 there allowing the choroid to be seen through it more clearly than 

 elsewhere. In Fig. 82 is represented the right retina as seen from 

 the front, the elliptical darker patch about the center indicating 

 the fovea and the white circle on one side, the optic disk. The 

 vessels of the retina arise from an artery (17, Fig. 81) which runs 

 in with the optic nerve and from which branches diverge as shown 

 in Fig. 82. 



The Optic Nerves, Chiasma, and Tracts. The optic nerves 

 converge to meet in the optic chiasma (ra, Fig. 80), from which 

 the optic tracts pass to the region of the midbrain. They termi- 

 nate mainly in the anterior corpora quadrigemina, (superior col- 

 liculi) (Chap. IX) and in the corpora geniculata. The behavior 

 of the nerve-fibers in the chiasma is interesting in that part of 

 them cross to the opposite side and part continue into the tract 

 of the same side. The fibers which cross over in each optic nerve 

 are those coming from the inner half of the retina, the right half 

 of the left retina and the left half of the right retina. The effect 

 of this arrangement is to include in the right optic tract, behind 

 the chiasma, the nerve-fibers from the right halves of both retinas, 

 and in the left optic tract those from the left halves of both 

 retinas. Cutting the right optic nerve, therefore, causes total 

 blindness of the right eye, but cutting the right optic tract blind- 

 ness of the right half of each retina (hemianopia) . 



The half crossing of the optic nerve-fibers in man is correllated 

 with the fact that his eyes are so placed that most of the field of 

 vision is common to both. In mammals whose eyes are so lat- 

 erally placed that at any given moment the objects seen by the 

 two eyes are quite different, the crossing at the commissure is 

 complete; this condition obtains also in birds with the exception 

 of owls, whose eyes like those of man have their visual axes 

 parallel; in owls the crossing is only partial. It should be noted 

 that the fovea centralis, which is the center of distinct vision, has 

 nerve connections from both eyes with both optic tracts. For 

 this reason unilateral injury to the visual mechanism back of 

 the chiasma interferes practically not at all with ordinary vision, 

 and sufferers from hemianopia may be unaware of their infirmity 

 until careful examination by a physician reveals it. 



