192 SPECIAL HISTOLOGY. 



The downy hairs, lanugo, the eruption of which is com- 

 pleted in the 23d — 25th week, are short fine hairs, whose 

 peculiar arrangement has been noted above. They measure 

 on the bulb 0-01'", on the shaft 0-006'", at the point 0-0012 

 — 0'002"'; are pale, or almost colourless, and consist only 

 of cortical substance and a cuticle. In man, the bulb is 

 usually colourless, and often rests upon a very distinct papilla, 

 arising in the ordinary manner from the bottom of the hair- 

 sac. It has the same three layers as in the adult, and pos- 

 sesses a very well-developed epidermic investment, consisting of 

 an external root-sheath of 0-004 — 0-012'', and an inner sheath 

 of 0-006 — 0-008'", without openings. 



After their eruption, the downy hairs grow slowly to a 

 length of i — \ of a line, and in fact to a greater length in 

 the head than elsewhere. Generally the}'' remain to the end 

 of foetal life, gradually acquiring a darker colour, becoming in 

 many cases, as on the head, even blackish ; another small 

 portion falls off into the liquor amnii, is swallowed with this 

 by the foetus, and may afterwards be found in the meconium. 

 A proper shedding of the hair does not take place at all in the 

 foetus, so far as I can see, infants being born with the lanugo; 

 as little does any trace of a further formation of hair appear 

 after its complete eruption. 



The question whether the point of the hair is first formed, 

 or whether the latter is developed at once as a whole, is 

 readily solved. Hairs which are just formed, have a bulb with 

 soft cells, a horny point and an intermediate portion, in which 

 the cells are converted into horn, and are partly found passing 

 into the cells of the root, whence there can be no doubt that 

 ■we have here a whole hair. That the horny part of this hair 

 subsequently forms the point of a larger hair, is of no im- 

 portance; and as little as the hairs of the head of a newly-born 

 infant can be called points of hairs, because they subsequently 

 become the points of larger hairs, can we so denominate these. 

 Nor can it be said that the first foetal hair subsequently 

 becomes, in totality, the point of a larger hair, since the hairs 

 do not grow by the simple apposition of new elements, like the 

 bones, but by the multiplication of their lowest soft cells, some 

 of which are always retained as a reserve for cells to be newly 

 developed, whilst the others are converted into horn; whence 



