234 ANATOMY FOE NUESES. [CHAP. XIX. 



behind is pierced by the optic nerve. Internally it is stained 

 brown where it comes in contact with the choroid coat. The 

 cornea (derived from Latin cornu, horn, and therefore also sig- 

 nifying hard) covers the anterior sixth of the eyeball. It is 

 directly continuous with the sclerotic coat, which, however, 

 overlaps it slightly above and below, as a watch-crystal is over- 

 lapped by the case into which it is fitted. The cornea, like the 

 sclerotic, is composed of fibrous tissue, which is both firm and 

 unyielding, but, unlike the sclerotic, it has no colour, and is 



FIG. 136. THE LEFT EYEBALL IN HORIZONTAL SECTION FROM BEFORE BACK. 

 1, sclerotic; 2, junction of sclerotic and cornea; 3, cornea; 4, 5, conjunctival mem- 

 brane ; 7, ciliary muscle ; 10, choroid ; 11, 13, ciliary processes ; 14, iris ; 15, retina ; 

 16, optic nerve ; 17, artery entering retina ; 18, fovea centralis ; 19, region where 

 sensory part of retina ends ; 26, 27, 28, are placed on the lens ; 28, suspensory liga- 

 ment placed around lens; 29, vitreous humour; 30, aqueous humour in anterior 

 chamber, 



perfectly transparent : it has been aptly termed the "window 

 of the eye." Both the cornea and the anterior portion of the 

 sclerotic are covered by reflections of the mucous membrane 

 lining the eyelids. This is called the conjunctiva, and, kept 

 well lubricated by the secretions of the eye, gives the eyeball 

 its peculiar shining and glossy aspect. The sclerotic is supplied 

 with very few blood-vessels, and the existence of nerves in it is 

 doubtful; while the cornea has no blood-vessels, but is well 

 supplied with nerves. 



