The development of the human body; a manual of human embryology . ers from the nerve which supplies the muscle. Thus, in thelower half of the abdomen, the skin at any point will be sup-plied by fibers from higher nerves than those supplying theunderlying muscles (Sherrington), and the skin of the limbsmay receive twigs from nerves which are not represented atall in the muscle-supply (second and third thoracic and thirdsacral). The Development of the Nails.—The earliest indicationsof the development of the nails have been described byZander in embryos of about nine weeks as slight thicken- THE
The development of the human body; a manual of human embryology . ers from the nerve which supplies the muscle. Thus, in thelower half of the abdomen, the skin at any point will be sup-plied by fibers from higher nerves than those supplying theunderlying muscles (Sherrington), and the skin of the limbsmay receive twigs from nerves which are not represented atall in the muscle-supply (second and third thoracic and thirdsacral). The Development of the Nails.—The earliest indicationsof the development of the nails have been described byZander in embryos of about nine weeks as slight thicken- THE NAILS. I65 ings of the epidermis of the tips of the digits, these thick-enings being separated from the neighboring tissue by afaint groove. Later the nail areas migrate to the dorsalsurfaces of the terminal phalanges (Fig. 80) and thegrooves surrounding the areas deepen, especially at theirproximal edges, where they form the nail-folds (w/), whiledistally thickenings of the epidermis occur to form whathave been termed sole-plates (sp), structures quite rudi-. Fig. 80.—Longitudinal Section through the Terminal Joint op the Index-finger of an Embryo of , Epidermis; ep, epitrichium; nf, nail fold; Ph, terminal phalanx; sp, sole plate. mentary in man, but largely developed in the lower ani-mals, in which they form a considerable portion of theclaws. The actual nail substance does not form, however, untilthe embryo has reached a length of about 17 cm. By thistime the epidermis has become several layers thick and itsouter layers, over the nail areas as well as elsewhere, have 166 THE DEVELOPMENT OF THE HUMAN BODY. •S0-! sc ep- fi become transformed into the stratum corneum (Fig. 81,sc), and it is in the deeper layers of this that keratin gran-ules develop in cells which degen-erate to give rise to the nail sub-stance (n). At its first formation,accordingly, the nail is covered bythe outer layers of the stratumcorneum as well as by the epitri-chium, the two together for
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
