FORMATION ANI> GROWTH OF THE NAILS. 



419 



arranged perpendicularly, as in the case of the epidermis. Thus the under part of 

 the nail (fig. 477, B) corresponds in nature with the Malpighian or mucous layer of 

 the epidermis, and the upper part (c) with the horny layer. The most superficial 

 cells of the Malpighian layer of the nail have a granular aspect, but this is due 

 according to Kolliker not to the presence of actual granules, but to the spiny 

 processes which unite them to one another. As in the case of epidermis, the 

 hardened scales of the nails may be made to reassume their cellular character by 

 treatment with caustic alkali, and afterwards with water : and then it is seen that 

 they still retain their nuclei. 



Formation and growth of the Nails. In the third month of intra-uterinelife, 

 the part of the embryonic corium which'becomes the matrix of the nail is marked 

 off by the commencing curvilinear groove, which limits it posteriorly and laterally. 



Fig. 478. LONGITUDINAL SECTION OP THE TERMINAL PHALANX OF THE THIRD FINGER OF AN EMBRYO 



OF THE THIRD MONTH. SOMEWHAT MAGNIFIED. (After BoWCD. ) 



ep, epitrichial cells, greatly multiplied at the nail-edge ep', and forming the eponychium ep" above 

 the nail formation, n. 



Fig. 479. LONGITUDINAL SECTION OF THE GREAT TOE OF AN EMBRYO OF THE FIFTH MONTH. SOME- 

 WHAT MAGNIFIED. (After Bowen.) 



ep, epitrichial layer ; ep', thickening of epitrichium at the nail edge ; ep", remains of eponychium ; 

 n, nail ; s.L, stratum lucidum. 



The epidermis on the matrix then begins to assume, in the under part of its horny 

 layer, the characters of a nail, which may, therefore, be said to be at first embedded 

 in the embryonic cuticle, forming a highly developed stratum lucidum. The nail 

 rudiment, which is preceded by and apparently formed from a layer of eleidin-cells 

 (fig. 478, n), first appears near the posterior part of the matrix, and grows forward 

 from this over the bed and backwards into the groove. After the end of the fifth 

 month it becomes free at the anterior border, breaking through the thick layer of 

 epidermis (eponychium) which covers it superficially. The remains of this layer 

 continue throughout life partly covering the lunula. At the anterior edge of the 



