Chap. 8 AN AGENT OF EVOLUTION THE BODY COVERING 131 



the outer ones form a pit or follicle sunk in the dermis; the inner ones de- 

 velop into the homy shaft which extends out as the hair. A minute papilla 

 of dermal cells containing blood capillaries and nerve endings projects into 

 a cup in the root and furnishes nourishment in this spot where growth is 

 very rapid. Sebaceous glands feed oil onto the hair, sometimes in super- 

 abundance. An involuntary muscle extends from near the base of the hair to 

 the epidermis. When this muscle contracts it pulls on the base of the hair and 

 makes it "stand up." In thickly furred animals this increases the insulating 

 power of the coat. Standing hair on the back of a dog's neck is a warning; on 

 human skin it is only "goose flesh," and no indication of danger to others, 

 meaning only that its owner is scared or chilly. It is too sparse to create any 

 insulation from the cold and is a sign of kinship to furred animals rather than 

 a protection. Above the skin a hair is composed of the dead and horny re- 

 mains of cells (Fig. 8.4). Pigment, most commonly black, is distributed along 

 the rod in varying degrees of abundance, causing the different shades of brown 

 and black hair. When the papilla of the hair does not supply materials for 

 pigment, the hairs are gray or white. Air vesicles are frequent in white hair; 

 it is an air trap, in a feeble way, like a white feather. Hairs are also like 

 feathers in being shed at regular intervals. Human hairs are among the ex- 

 ceptions in being shed irregularly; healthy human hairs of the head are esti- 

 mated to live a few years, eyelashes only a few months. A curly hair is slightly 

 flattened and shorter on one side than the other like a shaving; a straight hair 

 is a perfect cylinder. 



Claws, Nails, and Hoofs. These are all structures of cornified skin (epi- 

 dermis) (Fig. 8.5). Their development is similar to that of hairs; they are 



^ 



Fig. 8.4. Left, diagram of a human hair showing the characteristic shape of the 

 cuticular scales (F), colorless in all animals unless the hair has been dyed. Scales 

 composed of dead or cornified epithelial cells are arranged like shingles with their 

 free margins always directed toward the end of the hair. The main thread of the 

 hair (medulla, C, and cortex, D) consists of compressed remains of cells, through 

 which pigment is distributed. A, fusi or air vesicles; B, pigment granules; £, cu- 

 ticle. Center, sections of hairs from the human head showing the distribution of 

 pigment granules in hair of different colors. The color or absence of color depends 

 upon the hair's content of pigment and air. Loss of pigment makes the hair look 

 gray; when it contains much air, it is silvery white. A, cream buff; B, befza brown; 

 C, black; D white. Right, hairs from various mammals have characteristic scales; 

 hair of a star-nosed mole, percheron horse, sheep, and other. (Courtesy, Hausman, 

 Sclent. Monthly 59:195-202, 1944.) 



