1296 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



Development of the nails. — The nails are developed from the epidermis. In early embryos 

 over the dorsal surface of each distal phalanx there is seen a smoother and more adherent area 

 of skin which becomes limited by folds distally as well as proximally and laterally. It is also 

 distinguished by a greater number of cell layers which later become flatter than the surrounding 

 cells. The number of cell laj-ers still further increases and at about the fifth foetal month the 

 nail proper is formed by the deeper Ij'ing cells over an area extending from the proximal fold 

 to the distal end of the lunula. The nail is pushed distally by constant formation of new cells 

 in the same way as it continues to grow throughout life. The surface epithelial cells of the nail 

 field cover the nail for some time as a thin layer, the eponychium, which later disappears except 

 a small fringe near the root. 



Growth of the Nails. — The nail grows in length and thickness by multiplication of those 

 cells of the stratum germinativum which are situated between the margo occultus of the root 

 and the distal border of the hmula. The older cells are pushed distally and toward the sur- 

 face by the deeper cells. As a result the nail becomes gradually thicker from the occult bor- 

 der as far as the distal margin of the lunula. Over the rest of the nail bed no thickening appears 

 to take place. The rate of growth is faster on the fingers than on the toes and varies with age, 

 season, and the individual. When the nail is torn off, or detached through inflammation, it 

 may be regenerated if the cells of the stratum germinativum have not been destroyed. 



Congenital hypertrophy of the nails sometimes occurs, but absence or imperfect development is 

 rarely seen. The white spots so frequently seen in the nail are caused by air between the cell 

 layers due usually to injury or impaired development. 



C. THE CUTANEOUS GLANDS 



The glands of the skin [glandulae cutis] are of two kinds: glomiform glands, 

 and sebaceous glands. The glomijorm ("skein-like") glands [glandulse glomi- 

 formes] are of four types: sudoriferous glands, ciliary glands, ceruminous glands 

 and circumanal glands. 



Fig. 1057.— Vertical Section of the Palmar Skin Showing an Isolated 



Sudoriferous Gland. (Testut.) 



1, Stratum corneum; 2, Malpighian layer; 3. corium; 4, papilla; 5, body of sudoriferous gland; 



and 6, 7, its excretory duct; 8, orifice of duct on surface; 9, sulacutaneous fat. 



8 



The sudoriferous glands [glanduhe sudorifora?] or sweat glands are modified 

 simple tubular glands which secrete the swcal [sudor]. They are found in the 

 skin of all parts of the body except that part of the terminal ])halanges covered 

 by the nails, the concave surface of the concha of the ear, the labia minora, and the 

 inferior part of the labia majora in the female and the surface of the prepuce 

 and the glans penis in the male. The number found in different parts of the body 

 varies greatly. They arc very few on the convex surface of the concha and on 



