THE CORNEA. 199 



Sulcus, a slight circular depression around the junction of the sclerotic 

 with the cornea. 



Arteries, from the Ciliary, are few and in a coarse net-work, uniting at 

 wide intervals, but forming, however, the 



Posterior Vascular Zone, or Zone of Zinn,z.n arterial zone around the 

 optic nerve entrance, formed by twigs of the short ciliary going to the 

 optic nerve to anastomose with branches of the central artery of the retina, 

 forming the only connection between the ciliary and retinal systems. 



Anterior Vascular Zone, around the cornea, formed by sub-conjunctival 

 branches of anterior ciliary vessels. 



Nerves, none have been yet found in the sclerotic. 



What is the Cornea ? The Cornea, or horny body, is the anterior trans- 

 parent projecting portion of the external tunic of the eyeball, forming about 

 one-sixth thereof. It is set into the sclerotic as a watch-crystal into its case, is 

 of ellipsoidal base, the transverse diameter being the longest, and slightly 

 more convex vertically than horizontally. Its 



Thickness, is fa inch at centre, fa inch at periphery. 

 Index of Refraction, I. 342. 



Vessels, none, except the peripheral zone, fa inch wide, formed of capil- 

 lary loops from Episcleral branches of the anterior ciliary, which termi- 

 nate at its circumference ; so that it is practically a non-vascular structure. 

 Nerves, are numerous ; 20 or 30 twigs from the Ciliary nerves form an 

 intricate plexus throughout its laminated substance. 



Describe the Structure of the Cornea. It consists of five layers, cen- 

 trally the true corneal tissue, having in front an anterior elastic lamina and the 

 conjunctival epithelium ; behind, a posterior elastic lamina and the epithelial 

 lining of the aqueous chamber. The 



Layer of Conjunctival Epithelium ^ inch thick, consists of two or three 

 layers of transparent nucleated cells. 



Anterior Elastic Lamina, also named the Anterior Limiting Layer, the 

 Membrane of Bowman or Membrane of Reichert, is -^fa^ to ^J^ inch 

 thick, a firm, elastic and transparent homogeneous membrane, consisting 

 of closely-interwoven fibrils, similar to those in the corneal tissue proper. 



True Corneal Tissue, fa inch thick, a transparent, fibrous structure, formed 

 of five connective -tissue fibrillae, united in bundles, and these in laminae, 

 between which is a semi-fluid cement, filled with Corneal Corpuscles and 

 wandering cells. 



Posterior Elastic Lamina, Membrane of Descemet, or Demours, is, like 

 the anterior elastic lamina, a structureless basement membrane, of extreme 

 thinness, resisting the action of water, alcohol or acids ; very brittle, ex- 



