584 INTEGUMENTARY SYSTEM 



pattern. This pattern tends to have a definite relationship to scales when 

 present. 



3) Development of Nails, Claws, and Hoofs. Resembling and closely linked 

 to epidermal scales are the nails, claws, and hoofs of mammals. The claws 

 of reptiles and birds belong to the same category of terminal protective de- 

 vices for the digits. Nails are flattened discs of horny material, placed on the 

 dorsal surfaces of the terminal phalanges (fig. 274A, B). Claws are similar 

 and represent thickened, laterally compressed, and pointed nails (fig. 274C). 

 Hoofs are composite structures on the terminal phalanges of the digits, but, 

 unlike nails and claws, they are composed of two much-thickened nails, one 

 dorsal and one ventral. 



The distal protective device of the human digit is composed of a dorsal 

 structure, the nail plate or unguis. A formidable, horny subunguis or ventral 

 nail plate is absent, although a subungual region, consisting of an area of 

 extreme cornification of the stratum corneum of the skin, is present (fig. 

 274B). The claw of the cat or dog is similar, with the nail plate compressed 

 laterally, and the subungual cornification is greater. On the other hand, hoofs 

 possess a dorsal nail plate (unguis) and a well-developed ventral nail plate 

 (subunguis). Hoofs may be further divided into two general groups. In one 

 group are the hoofs of cows, sheep, deer, etc., which form two, nail-forming 

 mechanisms at the terminus of the digit, one dorsal and one ventral, from 

 which the dorsal and ventral nail plates arise. In the other group are the 

 hoofs of horses, donkeys, zebras, etc., which develop a dorsal, nail-developing 

 mechanism, forming the dorsal nail plate, and two ventral, nail-producing 

 structures. One of the latter generative devices gives origin to the frog and 

 the other to the ventral nail plate. Thus, embryologically, nails and claws 

 belong to one group, whereas hoofs form another. 



A better appreciation of the above-mentioned facts relative to claws, nails, 

 and hoofs can be gained by considering the development of a relatively simple, 

 terminal structure of the digit, the human finger nail. 



The nails on the terminal digits of the developing human finger begin to 

 form when the embryo (fetus) is about three months old. In doing so, a 

 thickened epidermal area arises on the dorsal aspect of the terminal end of 

 the digit. This general, thickened, epidermal area constitutes the nail field. 

 The proximal portion of the nail field then invaginates in a horizontal direc- 

 tion, passing inward into the underlying mesenchyme toward the base of the 

 distal phalanx. This invaginated epidermal material forms the nail fold or 

 groove, and it lies within the mesenchyme, paralleling the overlying epidermis 

 (fig. 274A). The nail fold, when viewed from above, is a crescent-shaped 

 aff'air with the outer aspect of the crescent facing distally; it may be divided 

 into a deeper layer, the nail matrix, and a more superficial layer. The nail 

 matrix is confined almost entirely within the nail fold or groove. The distal 

 edge of the lunula marks its greatest extension distally along the nail field. 



