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.
The anterior half of the sclerotic is composed of a ring of some ten to seventeen cartilaginous or bony scales which partly overlap each other.
Another cartilage or ossification, the posterior sclerotic ring, occurs within the walls of the posterior portion of the cup, and surrounds, especially in the Pici and in the Passeres, the entrance of the optic nerve.
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.
The irides are of a light orange, and the sclerotic tunics - equivalent to the "white of the eye" in most animals - which in few birds are visible, are in this very conspicuous and of a bright scarlet, giving it an air of great ferocity.
Transverse section of stem, showing the pith containing groups of sclerotic cells, the primary xylem-strands, secondary wood and phloem, pericycle and cortex.
The cortex, often sharply differentiated into sclerotic and parenchymatous zones, is bordered externally by the persistent leaf-bases.
Although not strictly forming part of the skull, allusion should be made here to the ring of sclerotic plates which has been found in many of the Stegocephalia, and which is only found elsewhere in a few Crossopterygian fishes as well as in many reptiles and birds.