LESSON 26.] HAIE. 77 



LESSO1ST XXYI. 



HAIR. 



420. Hair, in its origin, is an epidermic tissue, and is formed out 

 of cells ; but by the time it has acquired its full development, it has 

 become so completely modified that all trace of its origin is lost, and 

 can only be reclaimed by the judicious application of re-agents. The 

 hair originates within a follicle (a little bag), which is formed by a 

 depression of the skin, and lined by a continuation of the Epidermic 

 layer. From the bottom of this follicle there arises a cluster of cells, 

 which are Epidermic cells ; the exterior of this cluster is the bulb, 

 and the soft interior portion is the pulp. Although the follicle is ex- 

 tremely vascular, and even the bulb becomes reddened by minute in- 

 jection, yet no vessels have yet been traced to the interior of the hair. 

 The hairs of most animals present three elementary parts an exter- 

 nal cuticle, the cortical, horny, or fibrous substance, and an internal 

 medulla or pith ; these elements attain their highest development in 

 the quills, or modified hairs of the porcupine, and the English hedge- 

 hog (Erinaceus Europceus). 



Two of the three layers above indicated, may be distinguished in 

 all hairs, without any exception the cortical, and the medullary : the 

 cuticular covering is sometimes wanting. 



421. The cortical, or fibrous substance of a hair is longitudinally 

 striated (Fig. 114); it often presents dark spots, FJG 114 

 except in white hairs (in which it is transparent), 



and is more or less deeply colored. The color is 

 sometimes distributed with tolerable regularity, 

 and sometimes more concentrated in granular 

 spots. 



422. The medullary substance, or pith, is com- 

 posed of an aggregation of very large cells, which 



* & Striated appearance of 



do not possess fluid contents in that part of the the Cortical substance, 

 hair that is completely formed, but it is filled with 

 cells, usually containing pigment. 



423. The feathers of Birds are precisely analogous to the hair 

 of animals; the cortical (horny) tube of a feather, represents the 

 like tissue in the hair of an animal ; and the cellular medulla con- 

 tained within the quill is the analogue of the medulla of a hair. 



424. One striking difference exists, however, in hair, as com- 

 pared with the feather of a Bird ; it has been said that the hair-folli- 



