Quain's elements of anatomy . erior edge, that is, allround the margin of the groovein which the nail is lodged ;in front the cuticle of thepoint of the finger becomescontinuous with the under surface of the nail a little Avay behind itsfree edge. The nail, like the cuticle, is made up of epithelial cells. The oldest andmost superficial of these are the broadest and hardest, but at the sametime very thin, and so intimately connected together that their resjoec-tive limits are scarcely discernible. They form the exterior, horny partof the nail, and cohere together in irregular layers, so as to
Quain's elements of anatomy . erior edge, that is, allround the margin of the groovein which the nail is lodged ;in front the cuticle of thepoint of the finger becomescontinuous with the under surface of the nail a little Avay behind itsfree edge. The nail, like the cuticle, is made up of epithelial cells. The oldest andmost superficial of these are the broadest and hardest, but at the sametime very thin, and so intimately connected together that their resjoec-tive limits are scarcely discernible. They form the exterior, horny partof the nail, and cohere together in irregular layers, so as to give thisj)art a lamellar structure. On the other hand, the youngest cells, whichare those situated at the root and under surface, are softer and of arounded or polygonal shape. The deepest layer dilfers somewhat fromthe others, in haA^ng its cells elongated, and arranged perpendicularly,as in the case of the epidermis. Thus the under jjart of the nail (, b) corresponds in nature with the Malpigliian or mucous layer of. HAIES. 245 fche epidermis, and the upper part (c) wifcli the horny layer. As in thecase of the epidermis, the hardened scales of the nails may be made to re-assume their cellular character by treatment with caustic alkali, and after-wards with water ; and then it is seen that they still retain their growth of the nail is effected by a constant generation of cells atthe root and under surface. Each successive series of these cells being-followed and pushed from their original place by others, they become flat-tened into dry, hard, and inseparaljly coherent scales. By the additionof new cells at the posterior edge the nail is made to advance, and bythe apposition of similar particles to its under surface it grows in thick-ness ; so that it is thicker at the free border than at the root. The nailbeing thus merely a modified and exuberant j)art of the epidermis, thequestion at one time raised, whether that membrane is continued under-neath it, lose
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Keywords: ., boo, bookcentury1800, booksubjectanatomy, booksubjecthumananatomy
