530 



THE NAILS AND THE HAIR. 



..1 



caustic alkalies. (3) Upon the surface of the hair is the cuticula (k), consisting 

 of laminated and non-nucleated scales arranged like the shingles on a roof (H, e). 

 The graying of the hair in late life is dependent upon a deficiency in pigment- 

 formation in the cortical structure. The silvery luster of white hair is further 

 increased by the development of numerous air-bubbles, in large number in the 

 medulla, but also in small number in the cortex, which reflect the light. Occa- 

 sionally pigment develops in the growing hair, at times not, so that, accordingly, 



it appears discolored in places and not so in 

 others. Sudden graying of the hair, of which 

 well-authenticated records exist, and which 

 has also been observed upon one side of the 

 body, was found by Landois in the case of a 

 man who during an attack of delirium tremens 

 was harassed by frightful hallucinations and 

 became gray during a single night, to be de- 

 pendent upon the presence of many air-bub- 

 bles throughout the entire medulla of the 

 blond hair, and in smaller numbers, also, in 

 the cortical structure, while the pigment was 

 preserved. These air-bubbles imparted an 

 exquisite gray luster to the hair. In rare 

 cases, intermittent graying of the hair of the 

 scalp has been observed; so that the hair ex- 

 hibited alternately light and dark curls at in- 

 tervals of about i mm. In such a case, Lan- 

 dois found the bright areas to be due to an 

 abundant development of small air-bubbles in 

 the medullary canal and the surrounding corti- 

 cal area, while the pigment was well preserved. 

 As to the development of the hair, Kolliker 

 has discovered that, first, about the twelfth or 

 thirteenth week, depressions like the finger of 

 a glove take place from the epidermis into the 

 corium. They are bounded externally by a 

 vitreous membrane, and internally are occu- 



Eied by soft homogeneous cells of the Malpig- 

 ian mucous network. As these depressions 

 subsequently enlarge downward and acquire a 

 flask-like shape, the cells, arranged axially, ac- 

 quire a rather longitudinal form and constitute 

 a conical body, rising from the bottom of the 

 recess. On this body there can be recognized 

 an inner, darker portion, the primitive hair; 

 and a thin, light, overlying cover, the inner 

 root-sheath. The outermost cells, in contact 

 with the wall of the fold, become the external 

 root-sheath. Even before this, the papilla 

 grows from below toward the hair-root ; while, 

 at the same time, the fibrous layers of the hair- 

 follicle develop externally. Later on, the apex 

 of the hair grows toward the horny layer of 

 the epidermis. Here the apex penetrates the 

 inner root-sheath, which is reflected upon the 

 constantly growing hair like a sleeve. In the 

 nineteenth week the hairs appear upon the 

 forehead and the brow; between the twenty- 

 third . and the twenty-fifth week the lanugo- 

 hair appears free, having a characteristic 

 direction or grain on all parts of the body, 



just as is the case in animals. According to Kolliker, children are born only with 

 lanugo-hair. 



Of the physical properties of the hair, its great elasticity (tension, 0.33 of its 

 length) , marked cohesion (traction of from i \ to 3 ounces) , great resistance to 

 putrefaction, as well as its high hygroscopic power, should be pointed out. The 

 last property is possessed, also, by the epidermal cells, as indicated by the pains 

 of clavi and cicatrices in damp weather. 



FIG. 182. Longitudinal Section through a 

 Hair-follicle, with the Hair in Process of 

 Change (after v. Ebner): a, external and 

 middle hair-follicle sheaths; b, vitreous 

 layer; c, hair- papillae with vascular loop; 

 d, external, e, internal root-sheath (differ- 

 entiated into Henle's and Huxley's layer); 

 /, cuticula of the inner root-sheath; g, 

 cuticula of the hair; h, young, non-medul- 

 lated hair; i, conical tip of the new hair; 

 /, hair polyp of the exfoliated hair with, k, 

 the remains of the exfoliated external 

 root-sheath. 



