C, Cornea; 1, lens; v.b., vitreous body; r, retina.
The eye is always a closed vesicle, and the internal cornea is extensive.
One result of this among the Vertebrata is that the eyeball is pink in colour, since the cornea, iris and retina being transparent, the red blood contained in the capillaries is unmasked by the absence of pigmentary material.
In most cases this cornea is divided into lenticular facets corresponding to the underlying ommatidia.
Early names for it are Lac argenti and Luna cornea, the first referring to its form when freshly precipitated, the latter to its appearance after fusion.
Those of any other burrowing mammal, the retina being reduced to a mass of simple cells, and the cornea and sclerotic ("white") to a pearshaped fibrous capsule enclosing a ball of pigment.
Gelatin occurs also in the cornea and the sclerotic coat of the eye; and in fish scales, the latter containing 80% of collagen, and 20% of ichthylepidin, a substance differing from gelatin in giving a wellmarked Millon's reaction.
When the trigeminus nerve is divided (Majendie), or when its root is compressed injuriously, say Iby a tubercular tumour, the cornea begins to show points of ulceration, which, increasing in area, may bring about total disintegration of the eyeball.