DEVELOPMENT OF THE SKIN 581 



into the dermis, the surrounding mesenchyme forms a delicate, enveloping, 

 connective-tissue sheath around the epidermal downgrowth (fig. 272G). 



As development continues, the distal portion of the germinative stratum 

 forms a bulbous enlargement, the hair bulb. The mesenchymal rudiment of 

 the papilla pushes into this bulb at its distal end to form the beginnings of 

 the knob-like, definitive papilla of the future hair (fig. 272G). The hair rudi- 

 ment then is formed by the proliferation of the epidermal cells, immediately 

 overlying the knob-like papilla. The epithelial cells, overlying the papilla, 

 form the epithelial matrix of the bulb (fig. 272G). The cells of the matrix 

 soon produce a central core within the hair follicle, known as the hair cone 

 (fig. 272G). The latter is a conical mass of cells which extends upward 

 from the bulb into the center of the cellular material of the epidermal down- 

 growth. The hair cone thus gives origin to the beginnings of the hair shaft 

 and the inner hair (epithelial) sheath (fig. 272G). The peripheral cells of the 

 original epithelial downgrowth, which now surround the hair shaft and inner 

 hair sheath, form the outer sheath (fig. 272G). 



When the growing shaft of the hair reaches the level of the epidermal layer 

 of the skin, it follows along a hair canal or opening in the epidermal layer 

 and finally erupts at the surface of the skin. 



As the foregoing changes are effected, two epithelial growths appear along 

 the lower surface of the obliquely placed, hair follicle (fig. 272G). The upper 

 growth is the rudiment of the sebaceous gland which with certain exceptions 

 generally is associated with hair development. The lower epithelial outgrowth 

 forms the epithelial bed. This bed represents reserve epithelial material for 

 future hair generations. The arrector pili muscle arises from adjacent mesen- 

 chymal cells and becomes attached to the side of the follicle (figs. 272G; 273). 

 This muscle functions to make the hair "stand on end," so noticeable in the 

 neck-shoulder area of an angered dog. 



The first hair to be developed is known as the down hair, fine hair or lanugo. 

 In the human, the body is generally covered with lanugo by the seventh to 

 eighth fetal month. It tends to be cast off immediately before birth or shortly 

 thereafter. The lanugo corresponds somewhat to the nestling down of the 

 chick, for the replacing hairs develop from the same follicles as the down hairs 

 after the follicles have been reorganized from cells derived from the epithelial 

 bed. However, some replacing hairs appear to arise from new hair follicles. 



The hair on the face of the human female, exclusive of the eyebrows, 

 nostrils, and eyelids, and also on the neck and trunk is of the fine-haired 

 variety and resembles the lanugo of the fetus, whereas hair on the face of 

 the human male is of the fine-haired type, exclusive of the eyebrows, eyelids, 

 nostrils, and beard. Hair on various other regions of the male body may be 

 of the fine-haired or lanugo variety. 



b) Structure of the Mature Hair and the Hair Follicle. The gen- 

 eral structure of the mature hair and its follicle is as follows: The hair itself 



