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ACTION OF OCULAR MUSCLES. [Book hi. 



The six muscles therefore would seem to act as three pairs, 

 the superior and inferior rectus, the internal and external rec- 

 tus, and the inferior and superior oblique, each pair rotating 

 the eyeball round a particular axis. Calculations based on a 

 careful study of the attachments and directions of the several 

 muscles, and the results of actual observations, shew that this 

 is so, and that the movements carried out by the several pairs 

 may be more accurately described as follows. 



The superior rectus and the inferior rectus (see Fig. 160) 

 rotate the eye round a horizontal axis, which may be described 



sup 



Fig. 160. Diagram to illustrate the actions of the Muscles of the Eye. 



The eye represented is the left eye seen from above. The thick lines shew, 

 by means of the arrows, the direction in which the several muscles pull, the 

 beginning of each line also indicating the attachment of the muscle. The dotted 

 lines indicate the axis of rotation of the superior and inferior rectus and of the 

 oblique muscles. The axis of rotation of the internal and external rectus being 

 perpendicular to the plane of the paper cannot be shewn, v x represents the 

 visual axis and h x a line at right angles to it. (After Eick.) 





as one directed from the root of the nose to the temple ; it is 

 therefore not a line at right angles with the visual axis but one 

 making an acute angle (20°) with such a line. The superior 

 and inferior oblique rotate the eye round a horizontal axis 

 which may be described as one directed from the centre of the 

 eyeball to the occiput; it again is not a line at right angles to 

 the visual axis, but makes an angle, with such a line, larger 

 (60°) than the similar angle made by the inferior and superior 

 rectus, and turned in a different direction. The internal rectus 

 and external rectus rotate the eyeball round a vertical axis pass- 



