444 THE HUMAN BODY. 



of touch (Chap. XXXV). On the palmar surface of the hand 

 the dermic papillae are especially well developed (as they are 

 in most parts where the sense of touch is acute) and are fre- 

 quently 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 papillae of the same row, but dips down 

 between adjacent rows, and thus are produced the finer ridges 

 seen on the palms. In many places the corium is also fur- 

 rowed, as opposite the finger-joints and on the palm. Else- 

 where 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 absorption of subcu- 

 taneous 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 devel- 

 oped on the top of a special dermic papilla, seated at the 

 bottom of a depression 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 stem which, in an uncut hair, tapers off to a point. The 

 stem is covered by a single layer of overlapping scales form- 

 ing the hair-cuticle ; the projecting edges of these scales are 

 directed towards the top of the hair. Beneath the hair-cuti- 

 cle comes the cortex, made up of greatly elongated cells 

 united to form fibres; and in the centre 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 fibres of the cortex. 

 All hairs contain some air cavities, especially in the medulla. 

 They are very abundant in white hairs and cause the white- 

 ness 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. 137) is a narrow pit of the dermis, 

 projecting down into the subcutaneous areolar tissue, and 

 lined by an involution of the epidermis, At the bottom of 

 the follicle is a papilla, and the epidermis, turning up over 

 this, becomes continuous with the hair. On the papilla epi- 

 dermic cells multiply rapidly so long as the hair is growing, 

 and the whole hair is there made up of roundish cells. As 



