342 HAND-BOOK OF PHYSIOLOGY. 



generally, is effected by a constant production of cells from beneath and 

 behind, to take the place of those which are worn or cut away. Inas- 

 much, however, as the posterior edge of the nail, from its being lodged in 

 a groove of the skin, cannot grow backward, on additions being made to 

 it, so easily as it can pass in the opposite direction, any growth at its 

 hinder part pushes the whole forward. At the same time fresh cells are 

 added to its under surface, and thus each portion of the nail becomes 

 gradually thicker as it moves to the front, until, projecting beyond the 



FIG. 237. Vertical transverse section through a small portion of the nail and matrix largely 

 magnified. A, corium of the nail-bed, raised into ridges or laminae a, fitting in between correspond- 

 ing laminae 6. of the nail. B, Malpighian, and C, horny layer of nail; d, deepest and vertical cells; e, 

 upper flattened cells of Malpighian layer. (Kolliker.) 



surface of the matrix, it can receive no fresh addition from bene/ith, and 

 is simply moved forward by the growth at its root, to be at last worn 

 away or cut off. 



FUNCTIONS OF THE SKIN. 



(1.) By means of its toughness, flexibility and elasticity, the skin is 

 eminently qualified to serve as the general integument of the body, for 

 defending the internal parts from external violence, and readily yielding 

 and adapting itself to their various movements and changes of position. 



(2.) The skin is the chief organ of the sense of touch. Its whole sur- 

 face is extremely sensitive; but its tactile properties are due more espe- 

 cially to the abundant papillae with which it is studded. (See Chapter 

 on Special Senses.) 



Although destined especially for the sense of touch, the papillae are 

 not so placed as to come into direct contact with external objects; but 



