194- 



SPECIAL HISTOLOGY. 



Fig. 76. 



lanugo, into long processes composed of cells, by which the 

 hair is raised from its papilla, whilst at the same time it 

 becomes converted into horn even in its lowermost portion. 

 AY hen these processes have attained a length of 025"', a 

 differentiation of their outer and inner cells takes place, similar 

 to that which has been already described as occurring in those 

 processes of the stratum Malpighii, in which the hairs of the 

 lanugo are developed. The outer cells, in fact, remaining 

 round and colourless, as they were before, the inner ones begin 

 to develope pigment in their interior and to elongate, becoming 

 distinguished at the same time from the former, as a conical sub- 

 stance with its point directed upwards. At first (fig. 76 A), this 



central substance is quite soft, 

 and like the layers of cells which 

 surround it externallv, dissolves 

 readily in solution of caustic soda; 

 subsequently, however, when, to- 

 gether with the process which 

 incloses it, it has elongated, its 

 elements harden, and separate 

 into two portions, an internal 

 dark pigmented, and an external 

 clear part, which are nothing else 

 than a young hair, together with 

 its inner sheath (fig. 76 B). The 

 young hair, whose point at first 

 does not project beyond its inner 

 root-sheath, now grows gradually, 

 forcing its point through the aper- 

 ture of the old sac, while at the same time its root-sheath 

 elongates, and thrusts upwards the bulb of the old hair, until 



Fig. 76. The eyelashes of a child of one year old pulled out, x 20 : A, one with 

 a process of the bull) or of the outer root-sheath, of 0-25"', in which the central cells 

 are elongated (their pigment is not represented), and are clearly defined as a cone 

 from the external ones ; B, eyelash in whose process, of 0-3'", the inner cone is 

 metamorphosed into a hair and an inner root-sheath ; the old hair is pushed up, and 

 like A and fig. 75, possesses no inner root-sheath : a, outer; b, inner root-sheath of 

 the young hair; e, pit for the papilla of the hair; d, bulb; e, the shaft of the old 

 hair; /, bulb ; g, shaft ; h, point of the young hair ; ?', sebaceous glands; k, three 

 sudoriparous canals, which in A open into the upper part of the hair-sac ; /, transition 

 of the outer root-sheath into the rete mucosum of the epidermis. 



|f d 



