INTEGUMENT 



49 



the cells of the hair sheath. Later, a connective tissue sheath is 

 formed from the corium around the lower half of the hair follicle. 

 The arrectores pile-rum are bundles of nonstriated muscle extend- 

 ing from the fibrous tissue of the corium to the connective tissue 

 sheath of the follicle. Their contractions raise the hair on end. 

 The hair grows by additions of cells from the bulb. The shaft 

 of the hair is formed of cornified cells derived from stratum 

 germinativum of the bulb which, as they are pushed out, adhere 



B 



Fig. 24. — Diagrams showing the development of hair. A, the formation of 

 the hair column from the stratum germinativum of the epidermis; B, the hair 

 at birth, b, hair bulb; c, hair column; d, corium; e, epitrichium; h, hair shaft; 

 h.c, hair canal; i.e., inner layers of epidermis; m, muscle (arrector pili) ; oe., 

 outer layer of epidermis; p, hair papilla; s.g., rudiment of sebaceous gland. 



in the form of the hair. The feathers of birds, though somewhat 

 more complicated in structure than hairs, are also composed of 

 modified epidermal cells and develop in a somewhat similar 

 manner. 



Scales. — The scaly covering of reptiles, such as snakes, is 

 composed of the stratum corneum of the epidermis, which is shed 

 as a whole when the animal molts. On the other hand, the 

 scales of fishes are primarily subepidermal structures. Of these 

 the most primitive type is represented by the place-id scale of the 

 elasmobranchs (sharks). Such a scale consists of a flattened 

 base of dentine from the center of the external surface of which 

 a spine projects. As the bony basal plate and its spine are laid 



