CHAPTER XIX. 

 BINOCULAR VISION. 



Vision with two eyes differs from monocular vision chiefly in 

 the varied combinations of movements of the two eyeballs and the 

 aid thereby afforded in the determination of distance and size, 

 in the enlarged field of vision, and, above all, in the more exact per- 

 ception of solidity or perspective, especially for near objects. 



The Movements of the Eyeballs. — Each eyeball is moved 

 by six extrinsic muscles which are innervated through three 

 cranial nerves. The third or oculomotor nerve controls the internal 

 rectus, the superior rectus, the inferior rectus, and the inferior 

 obUque; the fourth cranial nerve (n. patheticus) innervates the 

 superior oblique alone; and the sixth cranial (n. abducens) the 

 external rectus alone. By means of these muscles the eyeballs may 

 be given various movements, all of which may be considered as 

 rotations of the ball around various axes. The common point of 

 intersection of these axes is designated as the rotation point or 

 center of rotation of the eyeball; it lies about 13.5 mms. back of the 

 cornea in the emmetropic eye. The various axes of rotation all 

 pass through this point, and we may classify them under four 

 heads: (1) The horizontal or sagittal axis, which is the line passing 

 through the rotation point and the object looked at, — the fixation 

 point. This axis corresponds practically with the line of sight, — 

 that is, the line drawn from the object looked at to the middle of the 

 fovea, and it may therefore, without serious error, be spoken of as 

 the visual axis. Rotations around this axis give a wheel movement 

 or torsion to the eyeballs. (2) The transverse axis, the line passing 

 through the rotation points of the two eyes and perpendicular 

 to 1. Rotations around this axis move the eyeballs straight up 

 or down. (3) The vertical axis, the vertical line passing through 

 the rotation point and perpendicular at this point to the horizontal 

 and transverse axes. Rotations around this axis move the eyeball 

 to the right or the left. (4) The oblique axes, under which are in- 

 cluded all the axes of rotation passing through the rotation point at 

 oblique angles to the horizontal axis. These axes all lie in the 

 equatorial plane of the eye, and rotations around any of them move 

 the eyeball obliquely upward or downward. These definitions all 

 have reference to what is known as the primary position of the 



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