Miscellaneous Corneal Structures
Many fishes have horny, epidermal, supporting rays, actinotrichia as well as bony or cartilaginous elements between the folds of skin that constitute the fins.
The rattle on the tail of a rattlesnake (Fig. 188) is a unique corneal apparatus. Each time, as the snake molts the outer layer of epidermis, a button, or ring of corneum, remains behind to record the fact. These rings are dry and loose enough to make a rattling noise when the thrill felt by an excited snake reaches the tip of the tail.
Horny beaks are epidermal structures characteristic of the toothless turtles, birds, and monotremes. Among birds particularly they exhibit a great variety of form, and serve a wide range of uses.
Some male birds, such as game cocks for example, also develop horny spurs upon the legs with which they settle questions of supremacy upon the avian field of honor (Fig. 189). The male jacana that strikes at its rival with outspread wings is armed with effective wing spurs.
The great sheets of “whalebone” (Fig. 190) with their frazzled edges that fill the mouth cavity of toothless whales, in the form of an elaborate mechanism for straining the myriads of small marine organisms upon which these giants feed, are not bone at all, but horny epidermal structures. Thick at the base, each plate thins out rapidly and breaks up into a long fringe of slender, closely set processes like the teeth of giant combs.
Camels and dromedaries are provided with thick corneal knee-pads to protect these heavy animals when they collapse to a kneeling posture before lying down upon the sands. Similar though smaller corneal pads of less obvious function, called “chestnuts,” are found on the inside of a horse’s leg.
The astonishingly conspicuous crimson and lilac callosities upon the seats of such monkeys as the African mandrill are still another manifestation of the epidermis, serving these interesting animals, which sit much of the time perched precariously on the branches of trees, as peripatetic sofa cushions.