SCLERA 



535 



The sclera is pierced also by numerous blood-vessels and 

 nerves. The long and short posterior ciliary arteries, with 

 the ciliary nerves, perforate the sclera around the optic 

 entrance ; four or five venae vorticosse 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. The 

 region of union 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 the junction the 



Vena vorticosa 



Long posterior 

 ciliary arteries 



ptic entrance 

 Short ciliary 

 arteries and 

 ciliary nerves 



FIG. 227. Diagram of the posterior aspect of the Left Eyeball. The 

 excentric position of the optic entrance is somewhat exaggerated. (After 

 Testut, modified.) 



scleral tissue slightly overlaps the corneal tissue ; therefore 

 the line of union, when seen in section, is oblique. Close to 

 the corneo-scleral junction, 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 



