THE SKIN. 193 



adult which are not exposed to contact with other bodies, 

 and especially on the lips, this cuticle is thin and delicate. 

 But, whenever it is exposed to the elements or friction, it 

 becomes thicker and tougher ; for any friction, if moderately 

 applied, instead of wearing it out, causes it to grow more 

 and more ; the under skin throws out more of the matter 

 that forms the cuticle, and this latter is thickened and 

 strengthened. This is most observable in the sole of the 

 foot arid palm of the hand ; the more they are used, the 

 thicker and harder their cuticle becomes, so that the bare- 

 foot boy treads on the rough pavement without injury. 



452. Though the cuticle becomes thick and hard from 

 friction and labor, yet these must be applied cautiously and 

 gradually, otherwise the reverse will happen. When the stu- 

 dent or clerk undertakes to cut wood, or rake hay, or row a 

 boat, for several successive hours, the cuticle of his palms, 

 instead of growing thick and hard, becomes thin and sore. 

 The outer skin may be worn off, or it may separate from the 

 other, and the under skin, instead of throwing out more mat- 

 ter to be formed into cuticle, throws out a watery matter, 

 which fills a little sack between them, and forms a blister. 



453. But if this friction had been applied gradually, and 

 continued for a long time, it would have stimulated the inner 

 skin to form more and more of the outer or scarf-skin t to 

 meet the want and the pressure, instead of throwing out 

 water, and causing pain and soreness. 



454. By the gradual application of friction, the skin be- 

 comes so fortified with this thickened outer layer, that it will 

 bear very rough usage without suffering; so that the hands 

 of the stone-layer and of the mason are neither scratched nor 

 inflamed by the rough stones, nor irritated by the lime in the 

 mortar. In the same way, the hands of the blacksmith and 

 the founder become accustomed and prepared to handle very 

 hot and rough metals without being burned or suffering pain. 



455. But if one unused to labor with his hands attempts at 

 once to become a stone-layer or brick-mason, he would soon 

 find the tender skin of his hands blistered and torn. The 



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