THE PRINCIPLES OF VISION 193 



other animals. The horizontal field is half of a circle, 

 180 degrees, while the upper and lower fields are some- 

 what less because of the supra-orbital ridges and the 

 cheek bones. The inner field of each eye, separately, 

 is less because of the nose. 



In the lower vertebrates complete decussation of the 

 optic nerves takes place, but in animals of the higher 

 order a partial decussation, or crossing over of the inner 

 portion of the optic nerves, takes place at the chiasm. 

 This causes the image of objects seen from the left field 

 to be conveyed to the right visual center, and those from 

 the right field to the left visual center. It very often 

 happens that an animal cannot see beyond the median 

 line, either to the right or to the left. This is known as 

 homonymous hemianopia (right and left, respectively). 

 Right homonymous hemianopia is due to some defect 

 of the optic nerves, tracts, or visual center supplying 

 the left half of each retina. Left homonymous hemian- 

 opia is due to some defect in the optic nerves, tracts, or 

 visual center supplying the right half of each retina. 

 When both outer fields are obhterated it is known as 

 bitemporal hemianopia, and is due to some defect of the 

 inner half of each retina or its optic nerve supply. 

 When the inner fields are obHterated (binasal hemianopia) 

 the outer half of each retina is involved. 



W^hen hemianopia occurs in both eyes, as it most al- 

 ways does, it is due to pressure or disease of the optic 

 tracts or visual center. Reaction of the pupil to light, 



13 



