720 



SPECIAL SENSES. 



lime. It is probable that the eyeball is seldom moved to an angle of forty- 

 live degrees, the direction of the visual line being more easily accomplished 

 by movements of the head. 



Action of the Recti Muscles. The internal and external recti rotate the 

 globe upon a vertical axis, which is perpendicular to the axis of the eye. The 

 isolated action of these muscles, particularly of the external rectus, is often 

 illustrated in certain forms of paralysis, which have been alluded to in con- 

 nection with the history of the cranial nerves. 



The superior and inferior recti rotate the globe upon an horizontal axis, 

 which is not at right angles with the axis of the eye, but is inclined from the 

 nasal side, slightly backward. The line which serves as the axis of rotation 

 for these muscles forms an angle of about seventy degrees with the axis of 

 the globe ; and as a consequence of this arrangement, their action is not so 

 simple as that of the internal and external recti. The insertion of the supe- 

 rior rectus in such, that when it contracts, the pupil is directed upward and 

 inward, the inferior rectus directing the pupil downward and inward. 



The above represents the simple, isolated action of each pair of recti 

 muscles ; but it is easy to see how, without necessarily involving the action 



of the oblique muscles, the globe may 

 be made to perform a great variety 

 of rotations, and the line of vision 

 may be turned in nearly every direc- 

 tion, by the action of the recti mus- 

 cles alone. 



Action of the Oblique Muscles. 

 It is sufficient for all practical pur- 

 poses to assume that the superior and 

 the inferior oblique muscles act as 

 direct antagonists to each other. 

 The most exact measurements show 

 that the axis of rotation for these 

 muscles is horizontal and has an ob- 

 lique direction from before backward 

 and from without inward. The an- 

 gle formed by the axis of rotation of 

 the oblique muscles with the axis of 

 the globe is thirty-five degrees ; and 

 the angle between the axis of the 

 oblique muscles and the axis of the 

 superior and inferior recti muscles is 

 f the seventy-five degrees. 



The heavy lines represent the muscles of the eye- Given the direction of the axis of 



ball, and the fine lines, the axes of the superior , , ,. . ,-, 



and the inferior recti and the axes of the oblique rotation and the direction 01 the SU- 



perior oblique muscle, it is easy to 



understand the effects of its contraction. As this muscle, passing obliquely 

 backward and forward over the globe, acts from the pulley near the inner 



