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 j 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 



