MOVEMENTS OF THE EYEBALL. 721 



angle of the eye, to its insertion just behind the anterior half of the globe on 

 its external and superior surface (7, Fig. 259), it must rotate the globe so as 

 to direct the pupil downward and outward. 



The inferior oblique, passing outward and slightly backward under the 

 globe, acts from its origin, at the margin of the orbit near the inner angle of 

 the eye, to its insertion, which is just below the insertion of the superior 

 oblique. This muscle rotates the globe so as to direct the pupil upward and 

 outward. 



The action of the oblique muscles seems to be specially connected with 

 the movements of torsion of the globe. It is necessary to distinct, single 

 vision with both eyes, that the images should be formed upon exactly corre- 

 sponding points on the retina, and that they should bear, for the two eyes, 

 corresponding relations to the perpendicular. Thus it is that when the head 

 is inclined to one side, the eyes are twisted upon an oblique, antero-posterior 

 axis ; as can be readily seen by observing little spots upon the iris, during 

 these movements. 



The superior oblique muscle is supplied by a single nerve, the patheticus. 

 When this muscle is paralyzed, the inferior oblique acts without its antago- 

 nist, and the eyeball is immovable, as far as the twisting of the globe, just 

 described, is concerned. When the head is moved toward the shoulder, the 

 globe can not rotate to maintain a position corresponding to that of the other 

 eye, and there is double vision. This point has already been touched upon 

 in connection with the physiology of the nerves of the eyeball and the situa- 

 tion of corresponding points in the retina. 



Associated Action of the Different Muscles of the Eyeball. It is almost 

 unnecessary to add, after the description just given of the actions of the indi- 

 vidual muscles of the globe, that their contractions may be associated so as 

 to produce a great variety of movements. There is no consciousness, under 

 ordinary conditions, of the muscular action by which the globe is rotated 

 and twisted in various directions, except that by an effort of the will the line 

 of vision is directed toward different objects. By a strong effort the axis of 

 the eyes may be converged by contracting both internal recti, and some per- 

 sons can produce extreme divergence by using both external recti ; but this 

 is abnormal. 



In looking at distant objects the axes of vision are practically parallel. 

 In looking at near objects the effort of accommodation is attended with the 

 degree of convergence necessary to bring the visual axes to bear upon iden- 

 tical points. In looking around at different objects the head is moved more 

 or less and the globes are rotated in various directions. In the movements 

 of the globes vertically the axes are kept parallel, or at the proper angle, by 

 the internal and external recti, and the superior and inferior recti upon the 

 two sides act together. In rotating the globe from one side to the other, 

 upon a vertical axis, the external rectus upon one side acts with the internal 

 rectus upon the other. In the movements of torsion upon an antero-poste- 

 rior axis there must be an associated action of the oblique muscles and 

 the recti. 



