THE CORNEA. 17 



the episcleral vessels derived from the anterior ciliary. Veins corresponding 

 with the arteries run in and beneath the conjunctiva, and there is a well-marked 

 episcleral plexus of veins behind the junction of the cornea and sclerotic. The 

 veins convey their blood to the anterior ciliary and the vorticose veins of the 

 choroid. The sclerotic receives fibres from the ciliary nerves, but it is not certainly 

 known how they terminate. 



Hannover described the sclerotic as being: traversed in its thickness, opposite the f ovea cen- 

 tralis of the retina, by a strand of fibrous tissue, which unites the lamina) as it passes through 

 them (fun'triilus ticlew). According 1 to Schwalbe the strand thus described is merely connec- 

 tive tissue which accompanies the most lateral group of posterior ciliary arteries as they pierce 

 the sclerotic. 



THE CORNEA. 



The cornea, the transparent fore part of the external coat, admits light into the 

 interior of the eyeball. It is nearly circular in shape, but is slightly wider in the trans- 

 verse than in the vertical direction (about 12 mm. and 11 mm. diameter respectively) ; 

 its arc extends to about one-sixth of the circumference of the whole globe. The 

 cornea has a curvature of a smaller radius than the sclerotic : the degree of its curve 

 varies, however, in different persons, and at different periods of life in the same 

 person, being more prominent in youth than in advanced age. It is also normally 

 a trifle more curved in the vertical than in the horizontal plane. Its thickness is 

 in general nearly the same throughout, viz., 0*8 mm., excepting towards the 

 periphery, where it becomes somewhat thicker (1*1 mm.) The posterior concave 

 surface exceeds slightly in extent the anterior or convex, in consequence of the 

 latter being encroached on by the superficial part of the sclerotic ; the cornea being 

 overlapped by the sclerotic like a watch-glass by the edge of the groove into which 

 it is received (see fig. 15) ; the tissues of the two are, however, in complete 

 continuity. Around the junction a slight groove is apparent in the surface of the 

 globe (sulcits sclerce). The tissue of the cornea readily imbibes water and becomes 

 opaque after death. 



STRUCTUBE OP THE CORNEA. 



The cornea may be described as consisting of three parts a stratified epithelium 

 in front (fig. 19, 1) continuous with the epithelium of the conjunctiva ; a middle 

 part, substantia propria, or cornea proper (3), continuous with the sclerotic, com- 

 posed of modified connective tissue ; and a homogeneous elastic lamella (4), bounding 

 it behind, and itself covered with a simple layer of endothelium cells (5). 



Epithelium of the Cornea. The epithelium covering the front of the cornea 

 is of the stratified kind, the cells being in man six to eight deep (fig. 18). The 

 lowermost cells (c) are columnar, with a flattened base, where they rest on the sub- 

 stantia propria, and a rounded apex, upon which a cell of the next layer fits. To 

 the base of each columnar cell is attached a broad, flattened, strongly refracting 

 process, which projects under one of the neighbouring cells (not shown in the 

 figure). Above these columnar cells are two or three layers of polygonal cells, some 

 of the deeper of which (the fingered cells of Cleland) have projections from their 

 under surface which fit between the cells below. These polygonal cells (p) have 

 well-marked denticulations, which join one another across the intercellular spaces 

 which separate the cells. Quite superficially is a stratum of flattened scaly 

 epithelium cells, which retain their nuclei. 



The proper substance of the cornea is composed, as before said, of a modified 

 form of connective tissue, all the constituents of which have very nearly the same 



VOL. III., PT. 3. C 



