872 



THE SENSE OF SIGHT 



(b) Upward and inward; superior rectus, inferior oblique and inter- 

 nal rectus. 



(c) Downward and outward; inferior rectus, superior oblique 

 and external rectus. 



{d) Downward and inward; inferior rectus, superior oblique and 

 internal rectus. 



Binocular Vision. — In man the movements of the eyes are bilateral, 

 each eye being moved around its center of rotation, situated 13.5 mm. 

 behind the cornea or 1.3 mm. behind the middle of the eyeball. When 

 the head is held erect and the eyes are directed to a point at the horizon, 

 their visual axes are parallel to one another. This constitutes the 

 primary position. When the eyes are moved directly upward, down- 

 ward, outward or inward, they occupy secondary positions and when 

 turned in oblique directions, tertiary positions. The movements of 

 the two eyes are correlated by a central mass of gray matter,^ the 



Fig. 472. — Diagrams to Show Homonymous and Heteronymous Diplopia. 

 In / the eyes are focused on A; the images of B fall on non-corresponding points, 

 — that is, on different sides of the foveae, and are seen double, being projected to the 

 plane of A, giving heteronymous diplopia. In // the eyes are focused on the nearer 

 point, A, and the farther point, B, forms images on non-corresponding points and 

 is seen double — homonymous diplopia — the images being projected to the focal 

 plane A. 



anatomical basis for the bilateral character of their innervation being 

 furnished by the fact that each oculomotor nerve is composed of 

 fibers derived from both nuclei and that the latter are intimately 

 connected with one another by commissural fibers. Two chief types 

 of movements may be recognized, namely : 



(a) Movements during which the visual axes of the eyes are kept practically 

 parallel to one another, no matter whether they are deviated along the vertical 

 plane of the visual field or laterally outward. Naturally, this parallelism can only 

 be maintained if the object remains at some distance from the eyes. 



(6) Movements during which the visual axes are converged in order to be able 

 to observe objects near the eyes. This convergence results invariably during near 

 vision and is therefore accompanied by the contraction of the ciliary muscle. 



Converging movements of the eyeballs directed at objects situated 

 laterally from us, may also be executed, but since these require an 



1 Bering's Law of Uniform Innervation, Hermann's Handb. der Physiol., 1879, 

 III, 343. 



