270 



AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



thin sheet of striated muscle, the panniculw carnosus, on the 

 ventral side of the neck and trunk. 



Each hair commences (in the embryo) as a thickening of the 

 Malpighian layer of the epidermis projecting towards the dermis 

 (Fig. 83). This hair-germ gradually becomes converted into a 

 hair-fotticle, at the bottom of which is a hair-papilla. A special 



.tfjSlgfttf 



* * * t?Z1*.iif4 



C:jy%? m 



Fig. 83. Six STAGES IN THE DEVELOPMENT OF HAIR (from Haddon, after 

 Wiedersheim). Sc, Hori^y layer; SM, Malpighian layer; (7,. dermis; 

 F, hair-follicle ; Dr, sebaceous glands ; CZ, shaft of hair f PZ, hair 

 sheath; HK, hair germ; P' t hair-papilla; P, commencement of ditto. 



investing sheath is formed by the dermis, which also penetrates 

 into the hair-papilla and vascularises it. The hair, developed 

 from the epidermis of the hair-papilla, exhibits an external firmer 

 cortex, covered by a scaly cuticle, and a softer internal air-con- 

 taining medulla, which, in coloured hairs, is pigmented. Small 

 branched sebaceous glands, secreting an oily substance, open into 



