TISSUES OF THE BODY. 3.97 



gradually transformed into the peripheral portion of the follicle. Up to 

 this stage the development of sweat glands and hairs is identical ( 200). 

 Although at the commencement the whole aggregation of cells appears 

 solid, and of the same nature throughout, a distinction soon, makes itself 

 evident between an axial and peripheral 

 portion. From the first is formed the 

 hair and its internal root-sheath, from the 

 second the external sheath. The cells 

 of the last-named stratum are elongated 

 transversely, while those of the axial 

 portion of the rudimentary hair in- 

 crease in a longitudinal direction. This 

 is the state of the parts in the eighteenth 



wpplr nf infra iiforiiio lifa af wViinV> fimp Fi - 395. First rudiments of a hair from the 



week ol mtra-uterme lite, at wnicn time human embryo at 8ixteen weeks> 0> 6i 



the agglomeration of Cells has attained layers of the cuticle; m, m, cella of the 

 a length of 0'226-0'0451 mm. rudimentary hair; < hyaline envelope. 



Soon after, a new division in this internally somewhat club-shaped mass, 

 broad below, and more or less pointed above, commences ; the outer 

 layer, namely, with its cells, is transformed into the clear transparent 

 internal root-sheath, whilst the axial part, which becomes the bulb and 

 shaft of the hair, remains dark. At this period, also, the papilla may 

 be clearly seen. 



The true hair thus commenced is at first short, and surrounded by a 

 very strong internal root-sheath, but without any recognisable medullary 

 substance. 'It then gradually increases in length, passes between the 

 undermost cells of the epidermis, and perforates the latter either immedi- 

 ately or after turning on itself, and taking an oblique course for a certain 

 distance. 



The other hairs are developed in a manner exactly similar, but later. 

 At the end of the sixth or commencement of the seventh month, most of 

 them have made their appearance through the epidermis. The hairs, so 

 appearing by perforation of the cuticle, are thin and light-coloured. 



In regard to the regeneration of hairs it must be remembered that 

 many of the downy ones are cast off during intra-uterine life, and become 

 mixed up with the waters of the ovum. After birth, however, this 

 change of hairs increases in amount, the new appearing in the place of the 

 old. Even at an advanced age this regeneration does not cease in man. 

 Among the mammals, as is well known, a very extensive renewal of hair 

 takes place periodically. In regard, however, to these processes there 

 still exists considerable difference of opinion. 



It was Koelliker who first observed the regeneration of hairs in the 

 eyelids of the infant (fig. 396). From his statements it will be seen, in 

 the first place, that the bulb of the old hair separates from its papilla, 

 from which the rudiments of a new structure are produced in the form of 

 a conical mass (A, m). 



Above' this, consequently, lies the loosened hair (de), horny down to 

 the very bulb. This rudimentary structure (B) is transformed into hair 

 bulb (/) and shaft (bh), with inner root-sheath (g), in a manner pre- 

 cisely similar to that we have already seen in the formation of hair in the 

 embryo. The inner root-sheath of the old hair disappears from the com- 

 mencement, and the new-comer drives its point by the side of the first, 

 which is displaced, through the outlet of the follicle occupying the whole 

 of the latter as soon as its former occupant falls out. Koelliker has also 



