'HUMAN STRUCTURE AND LIFE-ACTIVITIES 521 



scaly cells together constitute the horny layer, and the lower 

 cells of the epidermis form the Malpighian layer, or living 

 layer. The horny dead cells at the surface are continually 

 wearing off, and the living layer cells are by division forming 

 new cells, which are pushed toward the surface. 



In warts, corns, and callouses the horny layer becomes 

 greatly thickened. The cause is growth and division of the 

 lower living cells faster than the hard cells at the surface 

 wear off. Friction, pressure, and introduction of some 

 foreign substance through a cut or scratch are some of the 

 well-known causes of these thickenings of epidermis. 



In the ordinary healing of a cut or burn, the cells of the 

 living layer quickly form new cells to fill the gap beneath the 

 blood-clot or " scab " which forms on the surface. In ex- 

 tensive burns it is sometimes necessary to resort to skin- 

 grafting. This means taking healthy pieces of skin from 

 other persons and applying them where the epidermis has 

 been completely destroyed. The cells of these healthy bits 

 of grafted skin soon become firmly attached and by repeated 

 division grow over the injured surface. 



Nails and hairs are specialized masses of horny cells. The 

 so-called " roots " of nails and hairs are deep-lying masses of 

 living cells which grow and divide rapidly. The pit in which 

 each hair is attached is called a hair-follicle, and in it is a 

 conical elevation (papilla) from which the hair grows as cells 

 are formed and pushed outward. Oil glands (sebaceous 

 glands) open into the hair-follicles. Cutting or shaving hair, 

 contrary to popular belief, does not increase the number of 

 hairs, for the follicles are formed in embryonic life. 



(L) Examine a hair with microscope, and note that it is composed 

 of overlapping scales (cells). The center is also filled with cells. 

 The idea of barbers that hairs are hollow and require singeing to 

 prevent the escape of oil is absurd but profitable to the barber. 



Sweat-glands are abundant over the whole human skin. 

 They are most numerous on palms and soles, which also have 



