134 SPECIAL SENSES. 



that when it contracts, the pupil is directed upward and in- 

 ward, 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 ne- 

 cessarily involving the action of the oblique muscles, the 

 globe may be made to perform an immense variety of rota- 

 tions, 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. Although there has been 

 considerable discussion concerning the exact mode of action 

 of the oblique muscles, their mechanism may now be re- 

 garded as pretty well settled, at least as regards the human 

 subject. In the first place, it is sufficient for all practical 

 purposes, to assume that the superior and the inferior oblique 

 muscles act as direct antagonists to each other. The next 

 point to determine is the direction of the axis of rotation of 

 the globe with reference to the action of these muscles. The 

 most exact, recent measurements show that this axis is hori- 

 zontal, and that it has an oblique direction, from before back- 

 ward 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 infe- 

 rior recti muscles is seventy-five degrees. 1 



Given the direction of the axis of rotation and the direc- 

 tion of the superior oblique muscle, it is easy to under- 

 stand the effects of its contraction. As this muscle, passing 

 obliquely backward and forward over the globe, acts from 

 the pulley near the inner angle of the eye to its insertion 

 just behind the anterior half of the globe on its external and 

 superior surface (see Fig. 8), it must rotate the globe so as to. 

 direct the pupil downward and outward. 



The inferior oblique, passing outward and slightly back- 



1 HELMHOLTZ, Optlque physiologique, Paris, 1867, p. 41. 



