MOVEMENTS OF THE EYEBALL 



701 



Given the direction of the axis of rotation and the direction of the 

 superior oblique muscle, it is easy to understand the effects of its con- 

 traction. As this muscle, passing obliquely backward and forward over 

 the globe, acts from the pulley near the inner angle of the eye to its in- 

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

 superior surface (7, Fig. 179), 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 inser- 

 tion, which is just below the inser- 

 tion of the superior oblique. This 

 muscle rotates the globe so as to 

 direct the pupil upward and out- 

 ward. 



The action of the oblique mus- 

 cles is specially connected with the 

 movements of torsion of the globe. 

 It is necessary to distinct single vis- 

 ion with both eyes, that the images 

 should be formed on corresponding 

 points on the retina, and that they 

 should bear, for the two eyes, cor- 

 responding relations to the perpen- 

 dicular. Thus it is that when the 

 head is inclined to one side, the eyes Fi s- l8 - ~ Diagram illustrating the action of 



J the muscles of the eyeball (Fick). 



are twisted on an oblique antero- 



1 The heavy lines represent the muscles of the 



posterior axis ; as Can be readily eyeball, and the fine lines, the axes of the stipe- 



seen by observing little spots on the * al recti and the axes f 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 antagonist, and the eyeball is immovable so far as the 

 twisting of the globe is concerned. When the head is moved toward 

 the shoulder, the globe can not rotate to maintain a position correspond- 

 ing to that of the other eye and there is double vision. 



Associated Action of the Muscles of the Eyeball. It is almost un- 

 necessary to add, after the description just given of the actions of the 

 individual muscles of the globe, that their contractions may be asso- 

 ciated so as to produce a great variety of movements. There is no 



