226 THE SKIN. 



new growth, is gradually pushed towards the opening, and at last falls out, its 

 root-sheath having previously undergone partial absorption. WTien a hair is 

 palled out, a new one grows in its place, provided the follicle (from which the- 

 growth proceeds) remains entire. 



Growtll of hair. — On the surface of the papilla or vascular matrix, at the 

 bottom of the follicle, there is a growth of nucleated cells. The cells for the 

 most part lengthen out and unite into the flattened fibres which compose th** 

 fibrous part of the hair, and certain of them, pre\'iously getting filled with pig- 

 ment, give rise to the coloui-ed streaks and patches in that tissue ; their nuclei, 

 at first, also lengthen in the same manner, but. at last, partly become indistinct. 

 The cells next the circumference expand into the scales which form the imbri- 

 cated cuticular layer. The mediilla, where it exists, is f onned by the cells nearest 

 the centre ; these retain their primitive figure longer than the rest ; they becomn 

 coherent, and then- cavities may coalesce by destruction of their mutually adherent 

 parietes, whilst collections of granular particles of fat or pigment, and occa- 

 sionally minute air globules make their appearance between and in them and 

 around their nuclei, forming an opaque mass, which occupies the axis of the hair. 



The substance of the hair, of epidermic nature, is, like the epidermis itself, 

 quite extravascular, but. like that structure also, it is organised and subject to 

 internal organic changes. Thus, in the progress of its growth, the cells change 

 their figure, and acquire greater consistency. In consequence of then- elongation, 

 the hair, bulljous at the commencement, becomes reduced in diameter, and cylin- 

 drical above. But it cannot be said to what precise distance from the root 

 organic changes may extend. Some have imagined that the hairs are slowly 

 permeated by a fluid from the root to the point, but this has not been proved. 

 The sudden change of the colour- of the hair from dark to gi'cy, which some- 

 times happens, has never been satisfactorily exijlained : it appears, in some 

 instances at least, to be due to the development of au- between and among the- 

 cells composing the hair. 



Berthold found the rate of growth to be about half-an-inch per month. 

 This was in the hairs of the head. Erasmus Wilson states that in the beard 

 the rate of growth is J inch per week. 



Distribution and arrangement. — Haus are found on all parts of the skin 

 except the palms of the hands and soles of the feet, the dorsal surface of the' 

 third phalanges of the fingers and toes, the upper eyelids, the glans. and the inner- 

 surface of the prepuce. On the scalp they are set in groups, on the rest of the 

 skin for the most part singly. Except those of the eyelashes, which are im- 

 planted perpendicularly to the surface, they have usually a slanting dnection, 

 which is wonderfully constant in the same parts. 



Chemical nature.— The chemical composition of hair has been investigated 

 principally by Vauquelin, Scherer, and Van Laer. When treated v>'ith boiling 

 alcohol and with ether, it yields a certain amount of oily fat, which is red or 

 dark-coloured, according to the tint of the haii-. The animal matter of the hair 

 thus freed from fat, is supposed to consist of a substance yielding gelatine, and a 

 protein compound containing a large proportion of sulphm-. It is insoluble in. 

 water, unless by long boiling under pressm-e, by which it is reduced into a Aiscid 

 mass. It readily and completely dissolves in caustic alkalies. By calcination, hair 

 :y'ields from 1 to 1^ per cent, of ash, which consists of the following ingredients' 

 — viz., peroxide of iron, and according to Vauquelin, traces of manganese, siUca, 

 chlorides of sodium and potassium, sulphates of lime and magnesia, and phosphate 

 of lime. With the exception of the bones and teeth, no tis.sue of the body with- 

 stands decay after death so long as the hair, and hence it is often found preserved 

 in sepulchres, when nothing else remains but the skeleton. 



GLANDS OF THE SKIN. 



The s-adoriferous glands or swea-t-glands (figs. 155 and 156). — 

 These are seated on the under surface of the corium, and at variable 

 depths in the subcutaneous adipose tissue. They have the appearance 

 of small round reddish bodies, each of which, when examined with the 



