50 EPIDERMIC APPENDAGES :-NAILS, HAIR, &C. 
coherent ; and when thin sections are treated by a dilute 
solution of soda, these scales swell out again (as do also those 
of the cuticle) into globular cells. A new production is 
continually taking place in the groove of the skin in which 
the root of the nail is imbedded, and also from the whole of 
the surface beneath it; the former adds to the length of the 
nail; the latter to its thickness.—The structure of Hairs is 
essentially the same. The base of each is formed of a “ bulb,” 
which consists of a mass of epidermic cells developed from 
the vascular papilla at the bottom of the hair follicle (fig. 
8, c); and as this narrows into the “ shaft ” of the hair, a 
difference shows itself between the cortical or outer layer, and 
the medullary or pith-like substance of the interior. The 
former, which is continuous with the outer layers of the epi¬ 
dermis, is composed of flattened scales, arranged in an imbri¬ 
cated (tile-like) manner, so that the surface of the hair is 
usually marked by transverse jagged lines; the latter consists 
of cells which frequently retain their spheroidal form, like the 
inner layers of the epidermis; but in the human hair these 
cells are elongated into fibres. It is very seldom that there is 
any canal in the interior of the Hair, although irregular spaces 
are not unfrequently left by the drying-up of the fluid con¬ 
tents of the cells. The structure of Quills is essentially the 
same as that of hairs on a large’ scale; and we there see the 
difference very distinctly marked between the cortical portion 
which forms the “barrel” of the quill, and the medullary 
portion which forms the white pith-like substance of the 
stem of the feather. The Scales , where they are really epi¬ 
dermic appendages, as is the case in serpents and lizards, are 
formed upon the same pattern; and we have a good example 
of the detachment of the entire epidermis at once (reminding 
us of the casting of the shell of the crab and lobster) in the 
“ sloughing ” of the snake. 
39. The Mucous Membranes form a sort of internal skin, 
lining those cavities of the body which open on its surface; 
and the elements of which they are composed are essentially 
the same, though combined and arranged in a different 
manner, in accordance with their difference of function. The 
principal part of the thickness of every ordinary mucous 
membrane is made up, as in the skin, by the consolidation of 
areolar tissue, the fibres of which are continuous with those 
