332 



NORMAL HISTOLOGY. 



closely set low narrow longitudinal ridges, that at the distal margin of the lunula 

 suddenly give place to more pronounced, but less numerous broader lineal elevations. 

 These continue as far as the distal end of the nail-bed and are then replaced by papillae. 

 Owing to strong fibrous bands and the absence of the usual layer of fatty subdermal 

 tissue, the corium of the nail-bed is closely attached to the bone. 



The epidermis underlying the nail is of especial interest in view of its genetic 

 activity. While the stratum germinativum of the skin covering the finger-tip passes 

 directly and insensibly onto the nail-bed, the entire extent of which it invests (stratum 

 germinativum unguis), the stratum corneum ends on reaching the under surface of 

 the nail-plate, the line of apposition corresponding to the narrow yellow zone which 



Nail-bed Nail-plate 



Stratum corneum 

 and 



Stratum 



germinativum 



of nail-wall 



Eponychium 

 Margin of nail 



Corium 



FIG. 378. Section across nail-wall and adjoining part of nail-plate and nail-bed. X 90. 



defines the distal boundary of the rosy area. Beneath the latter, therefore, the epi- 

 dermis of the nail-bed consists of the stratum germinativum alone, which, without 

 cornification of any of its cells, rests against the under surface of the nail. Beneath 

 the white zone, that is, within the matrix, the epidermis includes a half-dozen or more 

 layers of the usual elements of the stratum germinativum, surmounted by a like 

 number of strata of cells containing numerous granules of keratohyalin. These gran- 

 ules impart to this region of the matrix a relative opacity, which contrasts with the more 

 transparent nail-plate, thereby producing the lunula. Since the transformation of the 

 cells of the stratum germinativum into those of the nail-plate is confined to the matrix, 

 it is evident that the continuous growth of the nail takes place along the floor and 

 bottom of the nail-groove, the last formed increment of nail-substance pushing 

 forwards the previously differentiated material and thus forcing the nail towards the 

 end of the digit. As the nail leaves the groove, a part of the stratum germinativum 

 of the nail-wall blends with the epidermis and is prolonged for a variable distance over 

 the dorsal surface of the nail-plate as a delicate membranous sheet, the 'eponychium, 

 which usually ends in a ragged and abraded border. 



THE CUTANEOUS GLANDS. 



These structures include two chief varieties, the sebaceous and the sweat- 

 glands, together with certain modifications, as the ceruminous glands within 

 the external auditory canal, the circumanal glands, the tarsal and ciliary 



