CORNEA 



573 



The sclera is pierced also by numerous blood-vessels and nerves. 

 The longVand short posterior ciliary arteries with the ciliary nerves 

 perforate the sclera around the optic entrance ; four or five venae 

 vorticosre issue from the interior of the eyeball by piercing the sclera a 

 short distance posterior to the equator, at wide intervals from each other ; 

 whilst the anterior ciliary arteries pierce it near the corneal margin. 



Anteriorly the sclera is not only contiguous to, but is 

 directly and structurally continuous with, the cornea. This 

 is termed the corneo-scleral junction, and the faint groove on 

 the surface, which corresponds with it, receives the name of 

 the scleral sulcus. At this junction the scleral tissue slightly 

 overlaps the corneal tissue, and the line of union, when 



Vena vorticosa 



Long posterior 

 ciliary arteries 



ptic entrance 

 Short ciliary 

 arteries and 

 ciliary nerves 



FIG. 252. Diagram of the posterior aspect of the Left Eyeball. The 

 excentric position of the optic entrance is somewhat exaggerated. 

 (After Testut, modified. ) 



seen in section, is oblique. Close to this a minute canal in 

 the substance of the sclera, termed the sinus venosus sclerce 

 (O.T. canal of Schlemm), encircles the margin of the cornea. 

 Cornea. The cornea forms the anterior sixth of the outer 

 tunic of the eye. It is transparent and glass-like, and it 

 forms the window through which the rays of light gain 

 admittance into the eyeball. The curvature of the cornea is 

 more accentuated than that of the sclera, and thus it consti- 

 tutes the segment of a smaller sphere. When viewed from 

 the posterior aspect it appears circular, but when looked at 

 from the front it is seen to be slightly wider in the transverse 

 direction. This is due to the fact that the sclera overlaps it 

 to a greater extent above and below than it does at the sides. 

 The posterior concave surface of the cornea forms the anterior 



