THE SKIN AXD ITS APPENDAGES 393 



manner that each layer extends a httle farther forward than the layer 

 above. The under softer portion of the nail corresponds to the stra- 

 tum germinativum of the skin and, like the latter, consists of polygonal 

 "prickle" cells and a stratum cylindricum resting upon a basement 

 membrane. In the matrix where the process of nail formation is 

 going on, this layer is thicker than elsewhere and is white and opaque 

 from the presence of keratohyahn. The convex anterior margin of 

 this area can be seen with the naked eye and is known as the lunula. 



At the junction of nail and skin, in the nail groove, the stratum 

 corneum extends somewhat over the nail as its eponychium. A simi- 

 lar extension of the stratum corneum occurs on the under surface of 

 the nail where the nail becomes free from the nail bed. This is known 

 as the hyponychium (Fig. 275). 



Growth of nail takes place by a transformation of the cells of the 

 matrix into true nail cells. In this process the outer hard layer is 

 pushed forward over the stratum germinativum, the latter remaining 

 always in the same position. 



TECHNIC 



(i) Remove two or more distal phalanges from the fingers of a new-born child 

 and fix in absolute alcohol or in formalin-Miiller's fluid (technic 6, p. 7). After 

 fixing, the bone should be carefully removed. Both longitudinal and transverse 

 sections are made, stained with haematoxylin-picro-acid-fuchsin (technic 3, p. 21), 

 and mounted in balsam. In cutting the sections it is usually best so to place the 

 block that the knife passes through volar surface first, through nail last. 



(2) The cellular elements of nail do not show well in sections. For demon- 

 strating the nail cells, boil a piece of nail in concentrated potash lye or warm it in 

 strong sulphuric acid, scrape off cells from the softened surface, and mount in 

 glycerin. 



The Hair 



The hair, like the nail, is a development of the epidermis. The 

 hair itself consists of a shaft, that portion of the hair which projects 

 above the skin, and a root, that portion embedded within the skin. At 

 its lower end the root presents a knob-like expansion, the hair bulb, 

 in the under surface of which is a cup-like depression, which receives 

 an extension of corium. This is known as the papilla. Enclosing 

 the hair root is the hair follicle. 



The Hair. — This is composed of epithelial cells arranged in three 

 layers, which from within outward are medulla, cortex, and cuticle 

 (Fig. 279). 



(i) The medulla occupies the central axis of the hair. It is absent 



