592 THE EYE 



THS CORWEA. 



The cornea (cornea pellucida), the transparent fore part of the 

 external coat, admits light into the interior of the eyeball. It is nearly 

 circular in shape, but is occasionally wider in the transverse direction, 

 and its arc extends to about one-sixth of the circumference of the whole 

 globe. Having a curvature of a smaller radius than the sclerotic, it 

 projects forwards beyond the general surface of curvature of that mem- 

 brane : the degree of its curve varies, however, in diflFerent persons, and 

 at diflFerent periods of life in the same person, being more prominent in 

 youth than in advanced age. Its thickness is in general nearly the 

 same throughout, viz., from ^V to -n\ of an inch, excepting towards the 

 outer margin where it becomes somewhat thinner. The posterior con- 

 cave surface exceeds slightly in extent the anterior or convex, in conse- 

 quence of the latter being encroached on by the superficial part of the 

 sclerotic ; the cornea being oveiiapped by the sclerotic (to which it is 

 joined by continuity of tissue) like a watch glass by the edge of the 

 groove into which it is received (see fig. 399). 



STRUCTUBS OF THE CORNEA. 



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

 epithelium in front (fig. 400, l), continuous with the epithelium of the 

 conjunctiva ; a middle part, substantia propria, or cornea proper (3), 

 continuous with the sclerotic, composed of modified connective tissue ; 

 and a homogeneous elastic lamella (4), bounding it behind, and itself 

 covered with a simple layer of epithelium-like cells (5). 



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

 the cornea is of the stratified kind. The lowermost cells are columnar, 

 with a flattened base, where they rest on the substantia propria, and 

 a rounded apex, upon which a cell of the next layer commonly fits. To 

 the base of each is attached 'a broad, flattened, strongly refracting pro- 

 cess, which projects under one of the neighbouring cells. Above these 

 columnar cells are several layers of more rounded cells, some of 

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

 surface, which pass between the cells below. The uppermost of tliese 

 rounded or polygonal cells present well-marked ribs and furrows, 

 similar to those described in the cuticle ; while, quite superficially, are 

 three or four strata of flattened scaly epithelium cells, which retain 

 their nuclei. Indications of division of the cells are observed both 

 amongst the columnar set and amongst the more rounded ones above 

 them (Cleland). 



The proper suhstance 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 index of refraction, so that in the perfectly 

 fresh condition it is difficult, even with the Ijest lenses, to make out 

 any structure at all. After death, however, and with the assistance of 

 reagents, it may be ascertained to consist of alternating lamella of 

 fibrous tissue (about sixty in number, Bowman), the planes of which 

 are parallel to the surfaces of the cornea. The fibres of which the 

 lamellaj are composed are nearly straight, have a definite direction in 

 each layer; and cross one another at right angles in the alternate layers 

 (fig. 400, h, d). It must, however, be understood that the latter are not 

 individually distinct, but give off frequent offsets to the layers above 



