THE NAILS. 157 



three or four strata of polyhedral cells, which stain deeply 

 and are perhaps analogous to those of the stratum granu- 

 losum. The innermost layer is the cuticular layer, composed 

 of a stratum of overlapping cells lying in close association 

 with the cuticle of the hair. 



The sebaceous glands are oval glandular saccules lying 

 alongside the upper part of the hair-follicles, into which they 

 empty. Some are simple or single ; others divided into a few 

 saccules. Their secretion, the sebum, is of a fatty nature, and 

 serves to oil and soften the skin and hair. In some situations, 

 as the labia minora and glans penis, they occur unassociated 

 with hairs and open directly on the surface. Sebaceous glands 

 are supported in the surrounding fibrous tissue, have short 

 outlet-ducts, and are solid masses of cells without open lumina. 

 The peripheral cells are smaller, spheroidal, and darker col- 

 ored ; the cells in the centre of the glands are much larger 

 and clearer, being infiltrated with numerous droplets of fat. 

 All the cells have well-marked nuclei. 



The nails are modifications of the epidermis, consisting 

 essentially of a much-thickened stratum lucidum. The lay- 

 ers on which the nail rests and to which it is attached form its 

 bed. The nail grows only from the posterior or proximal 

 part of the bed, or the matrix, the pale, visible convex por- 

 tion of which is the lunula. The posterior edge of the nail, 

 its root, rests in a groove formed by a fold of rete mucosum. 

 The corium beneath the nail exhibits the usual fibrous struct- 

 ure ; its surface anterior to or beyond the matrix forms longi- 

 tudinal ridges, representing papillae ; over the matrix are 

 papillae of the ordinary form. On the corium rests a stratum 

 Malpighii of the usual type, filling the depressions between 

 the ridges, and overlaid by a stratum granulosum. Upon this 

 is the body of the nail (corpus unguium), a thick, horny layer 

 derived from cells which have been transformed into keratin ; 

 it represents a highly developed stratum lucidum, and is 

 formed only over the matrix, where continual growth keeps 

 forcing it onward over the remainder of its bed. There is no 

 stratum corneum over the nails. 



