254 THE SOLIDS. 



Plate XXV.) Occasionally they are seen to pass in opposite 

 directions, and to decussate. I am disposed to think that 

 the one set of striae visible are rather apparent than real, and 

 are produced by the knife employed in making the section. 



It is usually easy to distinguish the superior from the 

 inferior edge of a slice of nail; the former will generally 

 appear quite smooth, while the latter will be rough and un- 

 even. (See Plate XXV.) 



Such is a brief sketch of the structure of nails * : their form, 

 position, and mode of connexion may next be considered. 



Each nail may be said to be quadrilateral and convex from 

 side to side, as it is also very generally from before back- 

 wards ; the three posterior margins are received into a groove 

 formed by a duplicature of the dermis and epidermis, the 

 anterior margin alone being free. The root and sides of the 

 nail, the former consisting of about one-fifth of its extent, are 

 intimately attached to both surfaces of the groove; the inferior 

 aspect of the body of the nail is likewise firmly adherent 

 to the derm beneath it, except for a small distance at its 

 anterior part. 



The nail, then, is attached to' the dermis by its root, and 

 by a portion of its inferior surface ; this attachment, however, 

 is scarcely to be considered as structural, since it consists of a 

 mere adaptation of the opposing surfaces of the dermis and 

 nail. The surface of the dermis upon which the nail rests, 

 it is known, is not smooth, but is raised into papillae ; to these 

 the nail adapts itself, and in this way the two become inti- 

 mately united. 



It is in this manner, also, that the longitudinal lines ob- 

 served on most nails are produced, and the appearance of 

 which has induced some observers to entertain the idea that 

 they are of a fibrous, and not a cellular constitution. 



* The first exact description of the nail and of the disposition of the 

 derm which supports it was given by Albinus (Adnotat. Acad. lib. ii. 1755, 

 p. 56.), but Schwann showed that the nail had a lamellated structure, 

 and that the lamellae are composed of epidermic scales. (Mikroskopische 

 Untersuchungen, 1839, p. 90.) 



