SEC. 14. THE PROTECTIVE MECHANISMS OF 



THE EYE. 



806. The eye is protected by the two eyelids, each of which 

 is strengthened and rendered firm by a curved plate of dense 

 connective tissue called the tarsus, (or incorrectly the tarsal 

 cartilage), which is larger in the upper than in the lower eyelid. 

 Elevation of the upper eyelid assisted by some depression of the 

 lower eyelid is spoken of as " opening the eye " ; depression of the 

 upper eyelid assisted by elevation of the lower eyelid is spoken of 

 as " shutting the eye." The latter movement is brought about by 

 the contraction of the orbicularis oculi, a muscle of circularly 

 disposed striated fibres placed beneath the skin of each eyelid 

 and stretching also over the adjoining bony orbit. The muscle is 

 governed by a branch of the seventh, facial nerve, and may be 

 thrown into action as part of a reflex act or of a voluntary effort. 

 When the facial nerve becomes incapable, through injury or 

 disease, of carrying motor impulses, the eye can not be shut 

 and remains widely open. There are some reasons however for 

 thinking that the motor fibres for the orbicularis, though forming 

 part of the facial nerve outside the brain, take origin within the 

 brain, not from the facial nucleus but from the hind end of the 

 third, oculo-motor nucleus. In the reflex contraction of the 

 orbicularis, known as ' winking ' or ' blinking,' which is so familiar 

 as an almost typical reflex movement, but which in the waking 

 hours is repeated so regularly, twice a minute or so, as to take on 

 almost the characters of a rhythmic automatic act, the exciting 

 afferent impulses are carried along the fibres of the fifth nerve 

 distributed to the cornea and conjunctiva, and probably, but not 

 certainly, pass some way down the ascending root ( 621) of that 

 nerve. 



The eye is opened mainly by the raising of the upper eyelid 

 through the contraction of the levator palpebrae superioris. This 

 muscle, taking origin from the back of the orbit in company with 

 the ocular muscles, is inserted into the upper surface of the tarsus 



