188 THE STEUCTUEE OF THE HOESE. 



which there is a special accumulation, forming a 

 rounded pad, on the under surface of the end of the 

 finger, called the bulb. In the skin over this part 

 the sense of touch is especially developed. 



The external surface is completely covered with 

 a general continuation of the skin of the rest of the 

 limb, the structure of which has been briefly de- 

 scribed at p. 176. A part of this covering has, how- 

 ever, undergone a special modification to form the 

 nail, which is a hard protecting shield for the most 

 exposed part of the finger, and the freely-projecting 

 edge of which serves many useful purposes. The 

 nail is nothing more than a flattened plate of dry, 

 hard, and horn-like epidermis, growing from a semi- 

 lunar groove in the derm and from a depressed sur- 

 face in front of this. This surface is covered, for the 

 purpose of increasing its area, with slightly raised 

 parallel longitudinal rows of papilla?, indications of 

 which may be seen in the longitudinal ridges with 

 which the surfaces of most nails are marked. The 

 nail continually grows at the base and from its in- 

 ner attached surf ace by the exudation of fresh epi- 

 dermic material, which pushes forward the older- 

 formed portion. This, if left to nature, eventually 

 wears or breaks away at the free edge. The portion 

 of the derm from which the nail grows is called the 

 " matrix." The nail of the human hand is, generally 



