196 ELEMENTARY PHYSIOLOGY less. 



the square inch. At a rough estimate, the whole integu- 

 ment probably possesses not fewer than from two millions 

 and a quarter to two millions and a half of these tubules, 

 which therefore must possess a very great aggregate 

 secreting power. 



In certain regions of the skin the corneous cells of the 

 epidermis are not at once thrown off in flakes, but are at 

 first built up in definite structures known as nails and 

 hairs, which grow by constant addition to the surfaces by 

 which they adhere to the epidermis. In the case of the 

 nails, the process of growth has no limit, and the nail is 

 kept of one size simply by the wearing away of its oldest 

 or free end. In the case of the hairs, on the contrary, 

 the growth of each hair is limited, and when its term is 

 reached the hair falls out and is replaced by a new hair. 



Underneath each nail the deep or dei-viic layer of the 

 integument is peculiarly modified to form the bed of 

 the nail. It is very vascular, and raised up into 

 numerous parallel ridges, like elongated papilla} (Fig. 59, 

 B, C). The surfaces of all these are covered with growing 

 epidermic cells, which, as they flatten and become con- 

 verted into horn, form a solid continuous plate, the nail. 

 At the hinder part of the bed of the nail the integument 

 forms a deep fold, from the bottom of which, in like 

 manner, new epidermic cells are added to the base of the 

 nail, which is thus constrained to move forward. 



The nail, thus constiintly receiving additions from 

 below and from behind, slides forwards over its bed, and 

 projects beyond the end of the finger, where it is worn 

 away or cut off. 



A hair, like a nail, is composed of horny cells ; but 

 instead of being only partially sunk in a fold of the integu- 

 ment it is atfirst wholly enclosed in a kind of bag, the hair- 

 sac or follicle, from the bottom of which a papilla 

 (Fig. 60, i), which answers to a single ridge of the nail, 

 arises. The hair is developed by the conversion into horn, 

 and coalescence into a shaft, of the superficial epidermic 



