. Outlines of the comparative physiology and morphology of animals. Anatomy, Comparative; Physiology, Comparative. 454 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. additions to the bottom, pushing outward, not by addi- tions to the top like a tree. The follicle in which it is formed acts as a mold, determining its shape. The quills of the porcupine are only large hairs. Nails.—If we tear off a finger nail, we find beneath an exquisitely tender surface of dermis covered with a layer of very active epithelium. At the base of the nail is a pocketlike infolding, where the epithelium is espe- cially activ
. Outlines of the comparative physiology and morphology of animals. Anatomy, Comparative; Physiology, Comparative. 454 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. additions to the bottom, pushing outward, not by addi- tions to the top like a tree. The follicle in which it is formed acts as a mold, determining its shape. The quills of the porcupine are only large hairs. Nails.—If we tear off a finger nail, we find beneath an exquisitely tender surface of dermis covered with a layer of very active epithelium. At the base of the nail is a pocketlike infolding, where the epithelium is espe- cially active. Over the whole surface the epithelial cells harden into horn, but this process is especially active in the pocket (Fig. 309). Therefore the nail, by its more rapid formation in the pocket at its base, is pushed forward continuously, and, if not worn away or pared away, will grow indefinitely. In some countries, as in cX-f. Fig. 309.—Section of the end of the finger, showing how the nail is formed : c, cuticle ; ml, Malpighi- an layer; he, the horny cuticle ; b, the bone. Fig. 310.—Section through terminal joint: clcr, claw core; kc, horny cuticle ; ml, Malpighian layer; c, unhardened cuticle. Japan, they are sometimes protected and become enor- mously long, as a badge of a leisure class. Claws.—These differ from nails only in the fact that they grow on all sides instead of one side of a peculiarly shaped terminal joint of a finger or toe. The bone of the terminal joint is the claw core and determines the shape of the claw. The core is covered with dermis and with an active layer of epithelium, which hardens into. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original LeConte, Joseph, 1823-1901. New York, D. Appleton and Company
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