vision. 243 



are four recti and two obliqui muscles. Tlie recti come for- 

 wards from tlie back of the orbit, to be inserted, in front of 

 the middle of the eyeball, into the sclerotic; and they are 

 named from their positions, superior, inferior, external, and 

 internal. The superior oblique muscle passes forwards from 

 the back of the orbit to the inner and upper angle of its fore 

 part ; there, becoming tendinous, it passes through a pulley of 

 fibres attached to the frontal bone, and, changing its direction, 

 turns outwards and backwards to be attached to the outer 

 part of the eyeball, behind the middle. The inferior oblique 

 muscle, springing from a point at the lower part of the inner 

 margin of the front of the orbit, takes a similar direction to 

 the tendon of the superior oblique, passing backwards and 

 outwards below the eyeball, to be inserted on its outer side. 



Fig. 124. MUSCLES OF THE EYEBALL. , Optic nerve; b, superior 

 oblique muscle ; c, pulley for the tendon of the same ; d, inferior 

 oblique muscle. The other four muscles are the four recti. 



The superior recti muscles of the two eyes always act in 

 concert, as also do the inferior recti, but the external and 

 internal recti act differently in different circumstances. In 

 turning the head, while the eyes are fixed on a stationary 

 object, or in following an object which crosses in front of us, 

 the external rectus of one eye acts in concert with the internal 

 rectus of the other; but, in turning the eyes from a distant 

 to a near object, we make their axes converge on the object 

 looked at, and the internal recti of both eyes act together. 

 These limitations of the movements of muscles which must 

 be considered voluntary are exceedingly curious, and find no 

 parallel in the muscles of the limbs. 



The use of the oblique muscles is not at first obvious; but 

 it' will be observed that when the eyes are converged, the 



