210 THE SKIN 



The stratum mucosum is of nearly equal thickness in all por- 

 tions of the nail body. In the nail root it is somewhat thicker and 

 forms the nail matrix of Ranvier. In this portion also is a dis- 

 tinct stratum granulosum, a layer which is absent or rudimentary 

 beneath the body of the nail. It is the presence within this layer 

 of numerous keratohyalin granules which renders the root of the 

 nail opaque and thus forms the dull white lunula which contrasts 

 with the transparent, eleidin containing, stratum lucidum, which 

 latter layer alone, covers the Malpighian layer of the nail body 

 (Unna). 



The Nail Bed. The nail rests upon a very vascular corium 

 or nail bed (matrix of authors) which is continuous with the 

 corium or derma of the skin. The nail bed at the margins of the 

 nail is provided with papillae as in other portions of the skin, but 

 beneath the body of the nail its surface is raised into longitudinal 

 ridges which possess only very minute secondary papillae. 



Nail Growth. The growth of the nail occurs almost entirely 

 in the matrix of the nail root. The cells of the stratum ger- 

 minativum of this portion, having been once formed by active 

 mitosis push obliquely forward and outward toward the nail 

 body. It is thus that the more advanced are constantly carried 

 onward toward the free border. The growth of the nail occurs 

 at the rate of about one thirty-second of an inch per week 

 (Schafer). 



Development. In the fetus the nail appears as a direct forma- 

 tion of the epidermis, which is very early evidenced by a thicken- 

 ing of the stratum lucidum in the nail area. The nail is therefore 

 at first covered by the superficial epitrichial cells of the cuticle. 

 The nail groove is rapidly formed by an invasion of the mesoblast 

 by the epidermal cells which become piled up at the margin of the 

 groove to form an excessive horny layer or eponychium. At the 

 distal extremity of the nail the superficial cells are also accumu- 

 lated into a considerable mass which forms a prominent hypony- 

 chium. Further growth of the nail pushes its distal margin for- 

 ward over the eponychium so that the border becomes free shortly 

 prior to birth. The epitrichial cells are then shed and the nail 

 body finally presents, at about the time of birth, its naked stratum 

 lucidum. 



THE HAIR, The structure of the hair will be most readily 

 appreciated if preceded by a brief introductory sketch of its 

 development. 



