2 66 NORMAL HISTOLOGY. 



increment pushing before it the older already existing parts at the 

 rate of about one millimetre per week. 



The nail-bed comprises the coriurn and that portion of the 

 epidermis corresponding to the stratum Malpighii. The corium 

 consists of the usual bundles of fibre-elastic tissue, which are arranged 

 somewhat parallel to the long axis of the finger, the longitudinal bun- 

 dles being supplemented by vertical ones extending from the perios- 

 teum of the phalanx to the superficial layers. The minute elevations 



FIG. 302. 



Transverse section of child's finger, including the nail : a, connective tissue of corium ; b, longi- 

 tudinally corrugated nail-bed ; c, corneous tissue constituting body of nail ; d, its thin edge covered 

 by tissue of nail-wall (/) ; e, point where stratum Malpighii becomes continuous with nail-bed. 



which occupy the surface of the corium In transverse section are not 

 true papillae, except posteriorly over the matrix, but longitudinal 

 ridges. They are lowest behind and gradually increase in height 

 towards the front of the nail, terminating abruptly at the point where 

 the latter parts from its bed. The epithelial portion of the nail- 

 bed is principally composed of cells belonging to the stratum Mal- 

 pighii, whose numerous layers fill up the inequalities between the 

 papillae and the ridges of the corium below, and are sharply defined 

 from the substance of the nail above. The transformation of the 

 deeper cells into the horny plates of the nail takes place only over 

 the matrix, where the constantly-recurring division of the epithe- 

 lial elements furnishes material for the growth of the nail. The 

 nail-fold and the nail-wall have the same general structure as the 

 skin. 



The substance of the nail itself consists of intimately united 

 lamellae of horny epithelial cells, which possess a nucleus and 

 closely resemble the elements of the stratum lucidum ; the older 

 and most superficial layers are made up of compressed horny dry 

 scales, while those composing the last formed and hence deepest layer 

 are softer and more regularly polyhedral, resembling the cells of the 

 stratum Malpighii. 



