SEC. 12. THE PEOTECTIVE MECHANISMS OF THE EYE. 



608. 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 depres- 

 sion 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 move- 

 ment 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 cannot 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 c winking ' 

 or 'blinking,' which is so familiar as an almost typical reflex 

 movement, but which in the waking hours is repeated so regu- 

 larly, 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 of that nerve. 



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

 through the contraction of the levator palpebrce 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 of the upper eyelid, beneath the orbicularis. It is 

 governed by a branch of the third nerve ; hence injury or dis- 

 ease of this nerve is frequently the cause of a drooping of the 

 upper eyelid and an inability to open the eye fully. 



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