STRUCTURE OF THE COATS OF THE EYEBALL 



215 



eyeball. It is covered externally with a lymphatic epithelium, while 

 internally it is lined by a layer of connective tissue containing pig- 

 ment-cells, which give it a brown appearance (lamina fusca). At the 

 entrance of the optic nerve the sclerotic is prolonged into the sheath 

 of that nerve, the bundles of which, piercing the coat, give a sieve-like 

 aspect to the part. (lamina cribrosa, fig. 257, L). 



The cornea (fig. 248) consists of the following layers enumerated 

 from before back : 



1. A stratified epithelium continuous with the epithelium of the 

 conjunctiva (i). 



2. A thin lamina of homogeneous connective tissue (membrane of 

 Boivman), iipon which the deepest cells of the epithelium rest (2). 



3. A thick layer of fibrous connective tissue which forms the 

 proper substance of the cornea (3). This is continuous laterally with the 

 tissue of the sclerotic. It is composed of bundles of white fibres 

 arranged in regular laminae, the direction of the fibres crossing one 

 another at right angles in the alternate laminae. Between the laminaB 

 lie flattened connective-tissue corpuscles, which are branched and 

 united by their processes into a continuous network ; there is of course 

 a corresponding network of cell-spaces (fig. 249, A, B). In vertical 



FIG. 249. A. CORPUSCLES OF THK RAT'S CORNEA. (From a preparation treated 

 with chloride of gold.) B. CELL-SPACES OF THE RAT'S CORNEA. (From a 

 preparation stained with nitrate of silver.) 



sections the cells appear narrow and spindle-shaped (fig. 248, c). In 

 the superficial laminae there are a few bundles of fibres which run 

 obliquely towards the surface (a). 



4. A homogeneous elastic layer (membrane ofDescemet) (fig. 248, -0. 

 This completely covers the back of the cornea, but at the angle which 

 the cornea forms with the iris it breaks up into separate fibres, which 

 are continued into the iris as the ligamentitm pectinatum, or pillars of 

 the iris. 



