. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Fig. 182. Retractile cat. (After Hesse.) side of the digital tip. Since the animal bears its weight on these cushions, the corneal layer that clothes them is considerably thickened in consequence. Usually the dominant unguis becomes laterally compressed and curved down to a point, with the result that the subunguis is much reduced. The claws of a cat are sharp and retractile within a protective sheath (Fig. 182), thus being kept unworn to be extended for u
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. Fig. 182. Retractile cat. (After Hesse.) side of the digital tip. Since the animal bears its weight on these cushions, the corneal layer that clothes them is considerably thickened in consequence. Usually the dominant unguis becomes laterally compressed and curved down to a point, with the result that the subunguis is much reduced. The claws of a cat are sharp and retractile within a protective sheath (Fig. 182), thus being kept unworn to be extended for use only in emergencies. The claws of a dog are less pointed and more exposed at all times, particularly so when it runs, for then they may come into contact with the ground, despite the presence of pads. Bats and sloths (Fig. 65c) have claws developed into elongated hooks which, al- though making locomotion on the ground awkward and difficult, are very useful when these animals hang themselves upside down from branches in trees, as is their habit. There are two striking modifications of mammalian claws, namely, hoofs and nails. In the first case the unguis thickens enormously into the shoelike hoof, which is so convex that its edges reach all the way around to the ventral side and come into contact with the ground. In the other case the unguis becomes flattened into the conspicuous nail, and the subunguis shrinks to a narrow insignificant rudiment under the projecting eaves of the nail, while the terminal pad becomes transformed into a sensitive ball, occupying the entire ventral aspect of the digital tip. (b) Hoofs. In a typical hoof (Fig. 179, e) such as that of a horse, the subunguis fills in ventrally the space between the lateral edges of the unguis, and the pad forms into a tough mass of material behind it, called the "frog," that serves somewhat the purpose of a rubber heel on a shoe. Hoofs, like the thick-soled shoes of a traffic policeman, are useful in suppo
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