STRUCTURE OF HAIRS. 



473 



Fig. 210.* 



rounding skin, and the horny structure of the nail is consequently continu- 

 ous with that of the epiderma. 



HAIRS are horny appendages of the skin produced by the involution and 

 subsequent evolution of the epiderma ; the involution constituting the fol 

 licle in which the hair is enclosed, and 

 the evolution the shaft of the hair. Hairs 

 vary much in size and length in different 

 parts of the body ; in some they are so 

 short as not to appear beyond the folli- 

 cle ; in others they grow to a great 

 length, as on the scalp ; while along the 

 margins of the eyelids and in the whis- 

 kers and beard, they are remarkable for 

 their thickness. Hairs are generally 

 more or less flattened in form, and when 

 the extremity of a transverse section is 

 examined it is found to possess an el- 

 liptical or reniform outline. This ex- 

 amination also demonstrates that the 

 centre of the hair is porous and loose in 

 texture, while its periphery is dense ; 

 thus affording ground for its division 

 into a cortical and a medullary portion. 

 The free extremity of a hair is generally 



pointed, and sometimes split into two or three filaments. Its attached 

 extremity is implanted deeply in the integument extending through the 

 derma into the sub-cutaneous areolar tissue, where it is surrounded by 

 adipose cells. The central extremity of a hair is larger than its shaft, and 

 is called the root or bulb. It is usually conical in its shape. 



The process of formation of a hair by its follicle is identical with that of 

 the formation of the epiderma by the papillary layer of the derma. Plastic 

 lymph is in the first instance exuded by the capillary plexus of the follicle, 

 the lymph undergoes conversion, first into granules, then into cells, and 

 the latter are elongated into fibres. The cells which are destined to form 

 the surface of the hair go through a different process. They are converted 

 into flat scales, which enclose the fibrous structure of the interior. These 

 scales, as they are successively produced, overlap those which precede 

 and give rise to the prominent and waving lines which may be seen 

 around the circumference of a hair. It is this overlapping line that is the 

 cause of the roughness which we experience in drawing a hair, from its 

 point to its bulb, between the fingers. The bulb is the newly formed 

 portion of the hair ; its expanded form is due to the greater bulk of the 

 fresh cells compared with the fibres and scales into which they are subse- 

 quently converted in the shaft. 



* The anatomy of the skin. 1. The epiderma. 2. The rete mucosum or deep layei 

 of the epiderma. 3. Two of the quadrilateral papillary clump?, such as are seen in the 

 palm of the hand or sole of the foot; they are composed of minute conical papillae. 4. 

 The deep layer of the derma, the corium. 5. Adipose cells. 6. A sudoriparous gland 

 with its spiral duct, such as is seen in the palm of the hand or sole of the foot. ". 

 Another sudoriparous gland with a straighter duct, such as is ?een in the scalp. 8. 

 Two hairs from the scalp, enclosed in their follicles ; their relative depth in the skin is 

 preserved. 9. A pair of sebiparous glands, opening by short ducts into the follicle of 

 the hair. 



40* 



