442 



DEVELOPMENT AND PROPERTIES OF HAIR. 



Development of Hair. According to Kblliker, from the 12th to 13th week of intra-uterine 

 life, solid finger-like processes of the epidermis are pushed down into the chorium. The process 

 becomes flask-shaped, while the central cells of the cylinder become elongated and form a 

 conical bodv, arising as it were from the depth of the recess. It soon differentiates into an 

 inner darker part, which becomes the hair, and a thinner, clearer layer covering the former, 



the inner root-sheath. The outer cells, i.e., those lying 

 next the wall of the sac, form the outer root-sheath. 

 Outside this, again, the fibrous tissue of the chorium 

 forms a rudimentary hair-follicle, while one of the 

 papilla? grows up against it, indents it, and becomes 

 embraced by the bulb of the hair. This is the hair 

 papilla, which contains a loop of blood-vessels. The 

 cells of the bulb of the hair proliferate rapidly and thus 

 the hair grows in length. The point of the hair is 

 thereby gradually pushed upwards, pierces the inner 

 root-sheath, and passes obliquely through the epidermis. 

 The hairs appear upon the forehead at the 19th week ; 

 at the 23 rd to 25th week the lanugo hairs appear free, 

 and they have a characteristic arrangement on different 

 parts of the body. 



Physical Properties. Hair has very considerable 

 elasticity (stretching to 0'33 of its length), considerable 

 cohesion (carrying 3 to 5 lbs. ), resists putrefaction for 

 a long time, and is highly hygroscopic. The last pro- 

 perty is also possessed by epidermal scales, as is proved 

 by the pains that occur in old wounds and scars during 

 damp weather. 



Growth of a hair occurs by proliferation of the cells on 

 the surface of the hair papilla, these cells representing 

 the matrix of the hair. Layer after layer is formed, and 

 gradually the hair is raised higher within its follicle. 



Change of the Hair. According to one view, when 

 the hair has reached its full length, the process of forma- 

 tion on the surface of the hair papilla is interrupted ; 

 the root of the hair is raised from the papilla, becomes 

 horny, remains almost devoid of pigment, and is gradu- 

 ally more and more lifted upwards from the surface of 

 the papilla, while its lower bulbous end becomes split 

 up like a brush. The lower empty part of the hair- 

 follicle becomes smaller, while on the old papilla a new 

 formation of a hair begins, the old hair at the same time 

 falling out ( Unim). According to Stieda, the old papilla 

 disappears, while a new one is formed in the hair-follicle, 

 and from it the new hair is developed. According to 

 Kblliker, again, both processes obtain. 



Fig. 298. 

 Section of a hair-follicle while a hair 



284. THE GLANDS OF THE SKIN. The sebaceous 



glands (fig. 294, I, T) are simple acinous glands, which 



open by a duct into the hair-follicles of large hairs near 



is being shed, a, outer and middle their upper part ; in the case of small hairs, they may 



sheaths of hair-follicle ; b, hyaline project from the duct of the gland (fig. 299). In some 



membrane ; c, papilla, with a capil- situations, the ducts of the glands open free upon the 



lary ; d, outer, c, inner root-sheath ; surface, e.g., the glands of labia minora, glans, prepuce 



.;, cuticle of the latter ; g, cuticle of (Tyson's glands), a^id the red margins of the lips. The 



the hair ; h, young non-medullated largest glands occur in the nose and in the labia ; they 



hair ; i, tip of new hair ; I, hair- are absent only from the vola manus and planta pedis. 



knob of the shed hair, with k, the The oblong alveoli of the gland consist of a basement 



remainder of the cast-off outer root- membrane lined with small polyhedral nucleated granu- 



sheath. lar secretory cells (fig. 294, t). Within this are other 



polyhedral cells, whose substance contains numerous oil-globules ; the cells become more fatty 



as we proceed towards the centre of the alveolus. The cells lining the duct are continuous 



with those of the outer root-sheath. The detritus formed by the fatty metamorphosis of the 



cells constitutes the sebum or sebaceous secretion. [If the " oil or coccygeal-gland " of a duck 



be removed, it is found that, when the animal is submerged, it takes up between its feathers 



about the same amount of water as an intact duck ; but it retains 2 to 2 times as much water 



in its feathers {Max Joseph).] 



The sweat-glands (fig. 294, I, k), sometimes called sudoriparous glands, consist of a long 



