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SPECIAL SENSES 



always suddenly and powerfully drawn within the orbit when a painful 

 impression is made on the cornea. This is a matter of common obser- 

 vation by ophthalmic surgeons. 



The extent to which the line of vision may be turned by a voluntary 

 effort varies in different individuals, even when the eyes are normal. 

 In myopic eyes, the centre of rotation is deeper in the orbit than normal 

 and the extent of the possible deviation of the visual line is correspond- 

 ingly diminished. Helmholtz stated that in his own person, with the 

 greatest effort that he was capable of making, he could move the line of 

 vision in the horizontal plane to the extent of about fifty degrees, and 

 in the vertical plane, about forty-five degrees ; but he added that these 

 extreme rotations were very forced, and that they could not be sustained 

 for any considerable length of time. It is probable that the eyeball is 

 seldom moved in an angle of forty-five 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 on a vertical axis that 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 connection with the history of the cranial nerves. 



The superior and inferior recti rotate the globe on a 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 that 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, their action is not so simple 

 as that of the internal and external recti. The insertion of the superior 

 rectus is 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 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 

 direction by the action of the recti muscles 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 oblique direction from before backward and from without inward. 

 The angle 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 seventy-five degrees. 



