BINOCULAR VISION 



871 



three principal ones, namely, two horizontal and one vertical. All three 

 traverse the center of rotation at right angles to one another, allowing 

 the following movements of the eye to be executed: (a) outward or 

 inward around its vertical axis, giving rise to its average abduction 

 or adduction, (6) upward or downward around its transverse hori- 

 zontal axis, and (c) around its sagittal axis connecting its anterior 

 and posterior poles. The only movements carried on by the contraction 

 of one muscle, or rather, by the reciprocal action of a single pair of 

 muscles, are abduction and adduction. The former is accomplished by 

 the external rectus and the latter, by the internal rectus. Movements 

 upward or downward necessitate the contraction of at least two 

 muscles, the former being mediated by 

 the superior rectus and inferior oblique, 

 and the latter by the inferior rectus 

 and superior oblique. When acting 

 singly, the superior rectus draws the 

 cornea upward and inward. This ac- 

 tion is combined with that of the in- 

 ferior oblique which draws it upward rihf ^ 

 and outward, but would also, theoret- 

 ically considered, rotate the eyeball 

 outward around its sagittal axis. Quite 

 similarly, the inferior rectus, when 

 acting alone, pulls the cornea down- 

 ward, but also adducts it and should 

 rotate it outward. The superior ob- 

 lique, on the other hand, deviates the 

 cornea downward and slightly outward, 

 but should also turn it inward. 



It should be emphasized, however, FIG. 471. DIAGRAM TO SHOW 



that a rotation Of the eyeball around POINTS OF ATTACHMENT AND LINES OF 

 i. j , , i ACTION OF EXTRINSIC OCULAR ^lus- 



its antero-posterior axis does not take 



r.ext. 



>. r.ifrf 



place under ordinary conditions, 

 although the course of these four muscles might warrant us to as- 

 sume such an action. This point may be proved by first gazing at 

 the vertical filaments of an electric lamp and then resting the eyes 

 upon a uniform gray surface. The after-image of these luminous 

 lines which will then be formed, shows them in their original vertical 

 position no matter whether the eye be turned upward, downward 

 or in an oblique direction. Naturally, if the eye were actually 

 turned around its antero-posterior axis, the after-image of these 

 filaments should really assume a slanting position. Movements of 

 the eyeballs around oblique axes require the cooperation of three 

 muscles, viz.: 



(a) Upward and outward; superior rectus, inferior oblique and 

 external rectus. 



