870 BINOCULAR VISION. 



the greater strength of the internal recti, the visual axes converge some- 

 what, and if prolonged, would intersect 40 cm. in front of the eye. In 

 the movements of the eye, only one, or two or even three muscles may 

 be involved. One muscle acts alone in rotation of the eye directly out- 

 ward and directly inward, namely the external rectus and the internal 

 rectus respectively. Two muscles act in rotating the eye directly upward 

 (superior rectus and inferior oblique) or directly downward (inferior 

 rectus and superior oblique). Three muscles are employed in the diag- 

 onal directions, namely for inward and upward movement the internal 

 rectus, the superior rectus, and the inferior oblique; for inward and 

 downward movement, the internal rectus, the inferior rectus, and the 

 superior oblique ; for outward and downward movement, the external 

 rectus, the inferior rectus, and the superior oblique; for outward and 

 upward movement, the external rectus, the superior rectus, and the 

 inferior oblique. 



Ruete has imitated the movements of the eyes by means of a special model 

 of the eyeballs and their muscles, and which he called the ophthalmotrope. 



The extent of movement of the eyeball decreases with age, likewise the length 

 of the eye. The mobility is less in the vertical direction than in the lateral; 

 and less upward than downward. The emmetrope and the myope can move 

 the eye further outward, the hyperope further inward. The external and 

 internal recti act most vigorously in rotation of the eye outward; the oblique 

 in rotation inward. One eye can be turned more strongly inward, if at the same 

 time the other is turned outward, than if this eye also is turned inward. In 

 near vision the right eye can be turned less toward the right, and the left eye less 

 toward the left than in distant vision. 



Both eyes are always moved simultaneously, even when one is totally 

 blind; indeed, the ocular muscles move even after the eyeball has been 

 extirpated. When the head is held erect, the movements proceed in 

 such a manner that both lines of fixation (visual axes) lie in the same 

 plane. The visual axes can diverge anteriorly to only a slight degree, 

 but they can converge to a considerable extent. When single muscles 

 are paralyzed, the position of the visual axes in the same plane is often 

 disturbed (squinting). The individual can no longer direct both visual 

 axes to one point at the same time, though each eye can be so directed 

 singly in succession. Nystagmus also occurs in both eyes simultane- 

 ously, and in the same manner. The congenital, simultaneous move- 

 ment of both eyes is termed an associated movement. E. Hering showed 

 that all ocular movements are attended with a uniformity of innervation. 

 Even with such movements in which one is apparently at rest, a move- 

 ment takes place, nevertheless, of two antagonists, as may be recognized 

 from slight to-and-fro movements. 



The nerves of the ocular muscles are the oculomotor, the trochlear, and the 

 abducens. The center is situated in the corpora quadrigemina, the cortical 

 center in the angular gyrus. 



BINOCULAR VISION. 



The conjoint action of both eyes in the visual act has the following 

 advantages : ( i ) The visual field of the two eyes is much larger than that 

 of either one. (2) The conception of depth is facilitated, as the retinal 

 images are obtained from two different standpoints. (3) A more 

 accurate estimation of the distance and the size of objects is rendered 

 possible, as a result of the estimation of the degree of convergence of the 

 two eyes. (4) Certain errors in one eye may be corrected by the other. 



