Glass 6. Mammalia. 



467 



also in the dermis. The stratum corneum is not shed all at once, but 

 in minute poi'tions. 



As a rule the greater part of the skin is covered with hair, a 

 very characteristic feature, absent in very few cases. The hairs 

 consist almost entirely of cornified cells, and each is inserted in an 

 invagination of the skin, the hair follicle. At the base of 

 each follicle there is a small hair papilla, covered by an out- 

 growth of the stratum mucosum, by the cornification of which the 

 overlying hair is formed. The rest of the hair follicle is covered 

 by outgrowths of the mucous and horny layers of the ordinary 

 epidermis, the outer and inner root-sheaths; the latter 

 is continued below into the hair, 

 the former into the stratum mucosum 

 of the papilla ; the hairs are nothing, 

 then, but well-defined, enormously- 

 developed portions of the stratum 

 corneum. In many of the thicker 

 hairs there is an inner medulla of 

 loosely-packed cells, surrounded by 

 the harder cortex; externally there 

 is a layer of thin, flattened cells, 

 the epidermis; many, especially 

 thin, hairs consist simply of cortex 

 and epidermis. In many Mammals 

 two kinds may be distinguished, 

 contour hairs and woolly 

 hairs; the latter finer and covered 

 by the others. Peculiar long stiff 

 "tactile hairs" or "whiskers" 

 {vibrissa) are often inserted on 

 certain regions of the head, es- 

 pecially on the upper lip; they are 

 well-developed and regularly ar- 

 ranged ; their follicles lie in a blood 

 space, which is in communication 

 with blood vessels. Other peculiar 

 stiff hairs, the eyelashes, are 



often present along the edges of the eyelids. Sometimes certain of 

 the hairs attain immense proportions, such as the spines of the 

 Hedgehog and Porcupine. The hairs are, for the most part, obliquely 

 implanted in the skin, and are regularly arranged, usually in small 

 groups or tufts. Smooth muscle fibres are attached to the 

 base of the follicle, they arise from the dermis, and by their 

 contraction the hair can be erected. Nerves also run to the hairs 

 (or more correctly to the hair follicles), especially to the whiskers 

 mentioned above, which are true tactile organs. 



H H 2 



Kg. 379. Longitudinal section o£ 

 a hair, and the connected hair 

 follicle; diagrammatic. a outer 

 root sheath, b connecti-ve tissue, c 

 stratum corneum, h hair, i inner root 

 sheath, r stratum Malpighii, p hair 

 papilla. — Orig. 



