THE SKIN. 293 



ment upward, push the sliaf t of the hair and its cuticula before 

 them. The structure of an adult hair can be best studied in 

 the stiff, gray hairs of the beard. For the study of the origin 

 of the root-sheaths young hairs should be chosen. There are 

 still many points in regard to the structure of the skin and its 

 appendages which appear to be rather doubtful, owing to our 

 insufficient knowledge. The first development of the hair-fol- 

 licle takes place at the end of the third or beginning of the 

 fourth month, and it originates as a projection downward of 

 the cells of the rete mucosum. It is seen as a finger-shaped 

 collection of rete cells surrounded by the connective tissue of 

 the corium. The papilla is formed later. By the numerical 

 increase of round cells the follicle is enlarged, and the external 

 cells are pushed sideward, thus forming the external root- 

 sheath. The origin of the other parts of the hair has been 

 already described. The first hairs are always of the lanugo 

 kind that is, they are fine hairs, with a very short hair-folli- 

 cle. In certain regions the hairs always remain fine ; in other 

 parts they give place to thicker ones. In the latter case a pro- 

 longation downward of the external root-sheath takes place. 

 This forms the hair-papilla. The papilla of the first hair atro- 

 phies, the hair falls out, and its place is occupied by a thick 

 hair. The permanent hair grows to a certain length, which 

 varies in different persons and in different parts of the body. 

 If a hair has reached its proper term of existence it falls out 

 and is replaced by a new hair, which grows from the old 

 papilla. A hair ceases to be produced when no new cells are 

 formed in the hair-root. The last-formed cells become con- 

 verted into the hair proper, and form a conical or knobbed ex- 

 tremity to the lower end of the hair-shaft. 



The nails. The nail is merely a modification of the epi- 

 dermis, and differs from the stratum corneum only in being 

 harder and firmer. It is a longish, four-sided, hard, elastic, 

 transparent, dense, flat body, situated in a fold of the skin on 

 the dorsal surface of the terminal phalanges of the fingers and 

 toes. It is slightly curved in its long diameter, the convex sur- 

 face being above and the concave below. Its posterior and two 

 lateral sides are connected with the other structures of the skin ; 

 the anterior side is free. The fold of skin in which the pos- 

 terior and two lateral surfaces are imbedded increases in depth 

 from before back ward, and at the posterior margin is continued 



