550 



VISION 



the line of vision of the left eye would describe on a plane standing at right 

 angles to the primary axis at the distance dd from the center of rotation, if 

 the eye were rotated about the several axes as given in Fig. 240. The position 

 which the horizontal meridian of the eye would have at the conclusion of the 

 movement is shown by a short, heavy line at the end of each path. The length 

 of each path corresponds to a rotation of about 50; the numbers mark the 

 successive positions of the line of vision. 



From this figure it is clear that even if the relations of the axes were in 

 fact as simple as we have supposed them to be, it would be possible to move 



FIG. 240. The extra-ocular muscles and their axes of rotation, after Fox. The left eye is shown 

 with the superior rectus removed, r.i.f., inferior rectus; r.s., superior rectus; r.e., external 

 rectus; r.i., internal rectus; o.s., superior oblique; o.i., inferior oblique. The line A A' 

 represents the line of vision ; T T' , the transverse axis of the eyeball ; D D', the axis of rota- 

 tion of the superior and inferior rectus muscles : this axis makes an angle of 63 with the line 

 of vision ; O O r , the axis of rotation of the inferior and superior oblique muscles : this axis 

 makes an angle of 35 with the line of vision. The axis for the internal and external rectus 

 muscles is perpendicular to the plane of the paper at the center of rotation, C. 



the line of vision, etc., along a vertical line only by the proper cooperation 

 of at least two muscles. The movement directly upward involves the action 

 of the superior rectus and the inferior oblique; the movement directly down- 

 ward the action of the inferior rectus and the superior oblique. The former 

 two assist each other in the rotation upward, but the one tends to roll the 

 eye outward and the other inward, so that by a compensatory action the rolling 

 can be prevented altogether. Exactly the same is true of the muscles which 

 rotate the eye downward. 



Knowing as we do that the different axes of the eye actually have a less 

 simple arrangement than that here assumed, it is evident, as Volkmann has 

 emphasized, that what are apparently the simplest movements of the eye 

 involve the simultaneous action of several muscles. 



