THE SKIN AND ITS APPENDAGES. 73 



The production of hair buds goes on until birth, the later buds 

 and hairs being thicker and stronger. After birth, new hairs are 

 constantly reproduced within the sheaths to replace the old. 

 Probably the manner in which new hairs are produced resembles 

 that of teeth, viz. : from processes of the original bud. Hairs 

 appear first on the head and then on other parts of the body. The 

 comparatively hairless condition of man must be regarded as due 

 to an arrest of development ; the hair distribution of adult man 

 corresponds to a late foetal condition of the anthropoids. Certain 

 sexual hairgrowths appear on the face, pubes and axilla at 

 puberty. Morphologically the pubic region represents the sepa- 

 rated axillary regions, and probably the explanation of sexual 

 hairs in the axilla is due to this correspondence, for there is a 

 persistent tendency towards symmetry of development in the upper 

 and lower extremities. The primitive mammary ridges, also 

 sexual structures, end at the axilla and groin. 



The Nails. — The nails are made up of the basal, stratum 

 mucosum, and stratum lucidum layers of the skin (Fig. 56), the 



termin. of corn. layer 



,stratum lucidum 



te mucosum nail fold 



^corneous layer 

 -stratum lucidum 

 nail bed * ^^rete mucosum 



Fig. 56. — Diagrammatic Section across a Kail. 



corneous layer being lost after the 4th month of foetal life. They 

 appear first in the 3rd month as fields of thickened epidermis 

 on the tips of the digits, but are afterwards shifted dorsally, 

 carrying their palmar nerves with them, so that the terminal 

 phalanx is wholly supplied from the palmar digital branches. 

 The nail of the little toe, a digit in a retrograde phase of develop- 

 ment, is frequently shaped like a claw, probably a reversion to a 

 primitive form. The nail is produced on the scattered papillae 

 (the matrix) at its root. The area of production is marked by the 

 lunule. On the nail bed, in front of the lunule, the papillae are 

 arranged in longitudinal rows. If the nail be pressed, as by the 

 boot, the lateral papillae, under the nail fold (see Fig. 56) are 



