VISION. 



811 



gence are lost. Strabismus will then be present and the object 

 looked at will appear double : diplopia. 



Stevens has given a number of terms for the deviation of the 

 visual axis of nonparalytic origin. Thus orthophoria is a condition 

 of muscular equilibrium of the eyeballs with the least nervous effort. 

 Esophoria is a tending of the visual lines inwards. Exophoria is a 

 tending of the visual lines outward. Hyperphoria is a tending of 

 the visual lines of one eye in a direction above its fellow. 



The innervation of the muscles of the eye is derived from the 

 third, fourth, and sixth pairs of cranial nerves. 



Fig. 363. Diagram Illustrating Binocular Vision. (BECLARD.) 



The lines from the object indicate that rays from the back of the book 

 fall on coincident points of the retina, while each eye further has a special field 

 of vision. 



BINOCULAR VISION. Looking into space with one eye, one sees 

 an almost circular field. With the one eye he can look toward the 

 opposite side as far as the root of the nose permits. If he opens the 

 other eye the visual space becomes much more extended in a trans- 

 verse direction, but corresponding to a monocular field, since the two 

 monocular fields are superposed. 



"Why should any point or object be seen single and not double, 

 when the point forms not one, but two images upon the retinae? 

 The explanation accepted is that the images are as two correspond- 

 ing identical points. These points are so related to one another that 



