MOVEMENTS OF THE EYEBALL. 135 



ward 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 up- 

 ward and outward. 



The action of the oblique muscles seems to be specially 

 connected with the movements of torsion of the globe. 1 It is 

 necessary to distinct, single vision with both eyes, that the 

 images should be formed upon exactly corresponding 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 ob- 

 served if we watch 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 antagonist, and the eyeball is immov- 

 able, as far as the twisting of the globe, just described, is 

 concerned. When the head is moved toward the shoulder, 

 the globe cannot rotate to maintain a position correspond- 

 ing to that of the other eye, and we have double vision. 

 This point has already been touched upon in connection with 

 the physiology of the nerves of the eyeball a and the discus- 

 sion of the corresponding points in the retina. 3 



Associated Action of the Different Muscles of the Eye- 

 ball. It is almost unnecessary to add, after the description 

 just given of the actions of the individual muscles of the 

 globe, that their contractions may be associated, so as to pro- 



1 We use the word torsion in a sense different from that of rotation. By 

 torsion, we mean the twisting of the eyeball upon the axis of the oblique mus- 

 cles, as a wheel would turn upon an axle, maintaining, in this way, a constant 

 and exact relation, for the two eyes, between the vertical and the horizontal me- 

 ridian. We make this explanation for the reason that torsion is a word not 

 commonly used by English writers. It corresponds to Raddrehung, in German. 



2 See vol. iv., Nervous System, p. 136. 8 See page 119. 



