646 TEGUMENTARY ORGANS. 



panded, soft and hollow base, the apex and shaft of the hair, 

 or spine, being formed before the bulb, like the crown of the 

 tooth before its fang. They are continuous, at their base, with 

 the epidermis lining the enveloping follicles ; they are com- 

 posed of the same cells or cytoblasts, which are commonly 

 arranged in rectilineal series ; and they generally present a 

 more dense exterior laminated cortical part, inclosing a loose 

 granular medullary portion. The component cytoblasts are 

 more round and loose at the soft, dilated base of the hairs, 

 as in the rete mucosum ; and they are compressed, elongated, 

 and more compactly united, in the denser shaft of the hairs. 

 By the rectilineal arrangement of the component cytoblasts, 

 the hairs possess a fibrous structure, and greater elasticity 

 and strength, they are more permeable to the oily secretion 

 of the sebaceous follicles, and they exhibit a filamentous de- 

 composition, often seen in the spontaneous longitudinal 

 fissuring of the human hairs. The soft dilated bulbs of the 

 hairs, beneath the cutis, are alone developed, and are con- 

 fined to their follicles, in the smooth -skinned piscivorous 

 cetacea. But in the rough-skinned herbivorous species of 

 these animals, the shafts of the hairs are partly protruded 

 from their follicles, like short, hard spines, and are especially 

 developed on the upper lip, as they are also in amphibious 

 carnivora. The almost horny epidermic integument of the 

 herbivorous cetacea, has long been compared to the con- 

 tinuous horny hoofs covering the piliferous follicles and 

 their contained hair-bulbs, on the feet of solidungula and 

 ruminantia. 



Hairs are successively reproduced in the same follicles, when 

 shed periodically in mammalia, or when forcibly torn from 

 their cavities, like the teeth of crocodiles in their alveoli. The 

 hairs of mammalia grow and enlarge in their follicles, and are 

 gradually protruded through their constricted apertures, by 

 the addition of successive layers of epidermic cytoblasts, 

 lineally aggregated, to the hollow interior and base of their 

 soft, white, expanded bulb, and by the enlargement of the 

 individual cytoblasts. The constituent cells are connected 

 together, as in other epidermic parts, by the remains of 

 the soft adhesive cytoblastema, which originally afforded 

 them nutriment. By the great elongation and compres- 

 sion of the cells as they proceed oiJtwards from the bulb, 



