IX MUSCLES OF EYEBALL 415 



forward to their insertions into the sclerotic — one the 

 superior rectus, in the middle line above ; one, the 

 inferior, opposite it below ; and one half-way on each 

 side, the external and internal recti. The eyeball is 

 completely imbedded in fat behind and laterally ; and 

 these muscles turn it as on a cushion ; the superior rectus 

 inclining the axis of the eye upwards, the inferior down- 

 wards, the external outwards, the internal inwards. 



The two oblique muscles, upper and lower, are both 

 attached on the outer side of the ball, and rather behind 

 its centre ; and they both pull in a direction from the 

 point of attachment towards the inner side of the orbit — 

 the lower, because it arises here ; the upper, because, 

 though it arises along with the recti from the back of the 

 orbit, yet, after passing forwards and becoming tendinous 

 at the upper and inner corner of the orbit, it traverses a 

 pulley-like loop of ligament, and then turns downwards* 

 and outwards to its insertion. The action of the oblique 

 muscles is somewhat complicated, but their general ten- 

 dency is to roll the eyeball on its axis, and pull it a little 

 forward and inward. 



By means of the contraction of these several muscles 

 the eyeballs may be moved into any desired j^osition and 

 their optic axes (Fig. 126, a. a.) directed straight towards 

 any object. This mobility is largely of use in diminisliing 

 the necessity for such frequent movements of the whole 

 head as would otherwise be neces.sary. But the move- 

 ments are also chiefly of extreme importance as ensuring 

 that any object is seen as single, although there is an 

 image of it on the retina of each of the two eyes. 



6. The Protective Appendages of the Eye. — The 

 eyelids are folds of skin containing thin plates of carti- 

 lage. They are fringed at the edges with hairs, the 

 eyelashes, and contain a series of small glands called 

 Meibomian glands. Circularly disposed fibres of 

 striped muscle lie beneath the integvunents of the ej'elids, 

 and constitute the orbicularis muscle which shuts tliem. 



