592 MANUAL OF PHYSIOLOGY. 



outward, and a little backward, and are attached to the upper 

 and under surface of the eyeball respectively. These muscles 

 can slightly rotate the eye on its antero-posterior axis, the upper 

 one drawing the upper part of the eyeball inward, and the lower 

 one, as its antagonist, drawing the lower part inward, so as to 

 rotate the eyeball in an opposite direction on the same axis. 

 The internal and external recti draw the centre of the cornea 



FIG. 233. 

 offi.sup. 



r.ehf 



r.ext. r.sup. r.int 

 r.inf. 



Diagram of the direction of the action of the muscles of the eyeball, which is shown 

 by the dark lines. The axis of the rotation caused by the oblique and upper and lower 

 recti are shown by the dotted lines. The inner and outer recti rotate the ball on its 

 vertical axis, which is cut across. The abbreviated names of the muscles are affixed to 

 the lines. 



toward or from the median line respectively, directly opposing 

 one another. 



On account of the direction of the superior and inferior recti 

 being different from that of the axis of the eyeball, they draw 

 the outer edge of the cornea, not its centre, up and down respec- 

 tively, and at the same time tend to give the eyeball a slight 

 rotation in the same direction as the corresponding oblique mus- 

 cles. The tendency to rotation is counteracted by the antago- 



