EXCRETION AND THE EXCRETORY ORGANS 477 



vclopcd (as they are in most parts where the sense of touch is 

 acute) and are frequently compound, or branched at the tip. On 

 the front of the hand, they are arranged in rows; the epidermis fills 

 up the hollows between the papilla; of the same row, but dips down 

 between adjacent rows, and thus an; produced the finer ridges seen 

 4 on the palms. In many places the corium is also furrowed, as op- 

 posite the finger-joints and on the palm. Elsewhere such furrows 

 are less marked, but they exist over the whole skin. The epidermis 

 closely follows all the hollows, and thus they are made visible 

 from the surface. The wrinkles of old persons are due to the ab- 

 sorption of subcutaneous fat and of other soft parts beneath the skin, 

 which, not shrinking itself at the same rate, is thrown into folds. 



Hairs. Each hair is a long filament of epidermis developed on 

 the top of a special dcrmic papilla, seated at the bottom of a de- 

 pression reaching down from the skin into the tissue beneath, and 

 called the hair-follicle. The portion of a hair buried in the skin is 

 called its root; this is succeeded by a stern which, in an uncut hair, 

 tapers off to a point. The stem is covered by a single layer of over- 

 lapping scales forming the hair-cuticle; the projecting edges of these 

 scales are directed towards the iop of the hair. Beneath the hair- 

 cuticle comes the cortex, made up of greatly elongated cells united 

 to form fibers; and in the center of the shaft there is found, in 

 many hairs, a medulla, made up of more or less rounded cells. The 

 color of hair is mainly dependent upon pigment-granules lying 

 between the fibers of the cortex. All hairs contain some air cavi- 

 ties, especially in the medulla. They are very abundant in white 

 hairs and cause the whiteness by reflecting all the incident light, 

 just as a liquid beaten into fine foam looks white because of the 

 light reflected from the walls of all the little air cavities in it. In 

 dark hairs the air cavities are few. 



The hair-follicle (Fig. 143) is a narrow pit of the dermis, pro- 

 jecting down into the subcutaneous arcolar tissue, and lined by an 

 involution of the epidermis. At the bottom of the follicle is a pap- 

 illa, and the epidermis, turning up over this, becomes continuous 

 with the hair. On the papilla epidermic cells multiply rapidly so 

 long as the hair is growing, and the whole hair is there made up f 

 roundish cells. As these are pushed up by fresh ones formed be- 

 neath them, the outermost layer become flattened and form the 

 hair-cuticle; several succeeding layers elongate and form the cor- 



