THE COVERING OF THE BODY 277 



attempt to make them stand erect, as the hairs of the lower 

 animals do in cold weather. This causes the condition known 

 as "gooseflesh." The difference between straight and curly 

 hair is due to a difference in the shape of the hair itself. Curly 

 hair is flattened; straight hair is cylindrical. When hair is 

 artificially curled it is temporarily flattened in various ways. 

 The nails are also modifications of the epidermis, designed to 

 protect the finger tips and assist in picking up small objects. 

 They are set in grooves of the epidermis and grow in length 

 from the base and in thickness from the under side. Being 

 semi-transparent they appear pink from the reflection of the 

 blood beneath them. In the lower animals, claws, horns, 

 beaks, feathers, tortoise-shell, and various other structures 

 are derived from the skin. 



238. Glands of the Skin. At the base of the hairs and in 

 other parts of the skin are certain tiny glands which pour out 

 an oil that keeps the hairs soft and the skin flexible. These 

 are called sebaceous glands. The sebaceous glands of the face 

 sometimes become clogged with dirt and are then known as 

 "blackheads." More noticeable are the sweat glands which 

 give off much water when the bodily temperature rises above 

 a certain point. Though the perspiration is noticed only 

 when the body is warm, we perspire more or less constantly 

 even in winter. This may be seen by touching a cold mirror 

 or piece of metal when a thin film of moisture will appear. 

 The sweat glands begin in the lower layers of the dermis as 

 coiled tubes closed at the ends. The water and salt they 

 excrete are taken from the blood like the material used by 

 other glands. With a good lens, the openings of the sweat 

 glands may be seen in the ridges on the finger tips. If the 

 hands are warm, a slight pressure will cause minute drops of 

 perspiration to appear from the openings. 



239. Functions of the Perspiration. Since the heat of the 

 body is derived from the oxidation of the food, its production 



